Carbanions as intermediates in the formation of Grignard reagents

Organometallics 2002, 21, 2119-2135

2119

Ca r ba n ion s a s In ter m ed ia tes in th e F or m a tion of

Gr ign a r d Rea gen ts

Gerard P. M. van Klink,*^,†,‡Henricus J . R. de Boer,^†Gerrit Schat,^†

Otto S. Akkerman,^†Friedrich Bickelhaupt,*^,†and Anthony L. Spek^§,|

Scheikundig Laboratorium, Vrije Universiteit Amsterdam, De Boelelaan 1083,

1081 HV Amsterdam, The Netherlands, and Bijvoet Center for Biomolecular Research,

Department of Crystal and Structural Chemistry, and Department of Metal-Mediated

Synthesis, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands

Received December 20, 2001

The reaction between certain aryl bromides RBr having substituents with group 14

elements and magnesium in THF to form the corresponding Grignard reagents RMgBr were

found to be accompanied by unusual migrations of the group 14 functionality; it partially

exchanges position with that of the original aryl bromide function. This migration is purely

intramolecular for organosilicon derivatives but both intra- and intermolecular for the

corresponding tin compounds. For example, 1-bromo-2-((trimethylsilyl)methyl)benzene (1b)

gives rise to 1-(bromomagnesio)-2-((trimethylsilyl)methyl)benzene (2b) and to its intramo-

lecular rearrangement product 1-((bromomagnesio)methyl)-2-(trimethylsilyl)benzene (3b),

whereas the trimethylstannyl analogue 1a forms, in addition to the two corresponding

Grignard reagents 2a and 3a formed by the intramolecular pathway, the intermolecular

exchange products 1-(trimethylstannyl)-2-((trimethylstannyl)methyl)benzene (4a ) and 1-(bro-

momagnesio)-2-((bromomagnesio)methyl)benzene (5′). These results, together with those from

analogous reactions of compounds such as 1-bromo-2-(trimethylstannyl)benzene (18), its

5-methyl derivative 23, 2-bromo-2′-(trimethylstannyl)biphenyl (40), and its silicon analogue

57, can be convincingly explained by invoking the intermediate formation of the highly

reactive carbanion R:^-, which may in some cases be slightly stabilized by formation of an

(intra- or intermolecular) stannate (or silicate) complex. Another serious candidate for such

migrations would have been the radical R^•, the occurrence of which is well-documented in

Grignard reactions. However, this alternative route could be excluded by producing this

radical in an unambiguous fashion from 1a and samarium(II) diiodide; under these

conditions, hydrogen abstraction by R^•strongly predominated, and minor quantities of

rearranged products were shown to be due to further reduction of R^•to R:^-.

In tr od u ction

the carbon-halogen bond. Rather, a number of steps

and intermediates are involved, and the investigations

are hampered by the heterogeneous nature of the

reaction. It is, therefore, not surprising that, despite

considerable efforts, a number of unresolved issues and

even controversies still persist. Partial consensus has

been reached on most of the steps depicted in Scheme

1.^2-5Several additional features and alternative routes

have been proposed; these will be addressed below.

Over 100 years have elapsed since the discovery by

Victor Grignard that organic halides RX react with

metallic magnesium to furnish organomagnesium ha-

lides RMgX, generally known as Grignard reagents.¹

Usually, this reaction occurs with ease and in high yield,

according to the deceptively simple overall Equation (1).

R-X + Mg f R-Mg-X

(1)

Step 1 is believed to be an outer-sphere electron

transfer from the magnesium surface to the organic

halide RX, resulting in the formation of the radical

In view of the importance of Grignard reagents as

synthetic tools, their structure and chemistry have been

intensively investigated. This also holds for their mech-

anism of formation, which, not surprisingly, does not

proceed by simple insertion of one magnesium atom into

(2) (a) Kharasch, M. S.; Reinmuth, O. In Grignard Reactions of

Nonmetallic Substances; Prentice Hall: New York, 1954. (b) Lindsell,

W. E. In Comprehensive Organometallic Chemistry, 2nd ed.; Abel, E.

W., Stone, F. G. A., Wilkinson, G., Eds.; Elsevier: Oxford, U.K., 1995;

Vol. 1, Chapter 3, pp 57-63. (c) Nu¨tzel, K. In Methoden der Organis-

chen Chemie (Houben Weyl); Mu¨ller, E., Ed.; Georg Thieme Verlag:

Stuttgart, Germany, 1973; Vol. 13/2a, pp 47-528.

* To whom correspondence should be addressed. E-mail:

G.P.M.v.K., g.p.m.vanklink@chem.uu.nl; F.B., bicklhpt@chem.vu.nl.

Fax: G.P.M.v.K., +31-30-2523615.

^†Vrije Universiteit Amsterdam.

(3) Hamdouchi, C.; Walborsky, H. M. In Handbook of Grignard

Reagents; Silverman, G. S., Rakita, P. E., Eds.; Dekker: New York,

1996; Chapter 10, pp 145-218.

^‡Department of Metal-Mediated Synthesis, Utrecht University.

^§Department of Crystal and Structural Chemistry, Utrecht Uni-

versity.

(4) Garst, J . F.; Ungva´ry, F. In Grignard Reagents, New Develop-

ments; Richey, H. G., Ed.; Wiley: Chichester, U.K., 2000; Chapter 7,

pp 185-275.

^|Address correspondence pertaining to crystallographic studies to

this author. E-mail: A.L.Spek@chem.uu.nl.

(1) Grignard, V. C. R. Hebd. Seances Acad. Sci. 1900, 130, 1322.

(5) van Klink, G. P. M. Thesis, Vrije Universiteit, Amsterdam, 1998.

Publication on Web 04/19/2002

2120 Organometallics, Vol. 21, No. 10, 2002

van Klink et al.

been ruled out in at least one case.¹⁸It is still a matter

of controversy whether R^•remains coordinated to the

magnesium surface^3,6,15or whether most of the radicals

diffuse from the magnesium surface into the solution,

where they may either perform typical radical reactions

such as hydrogen abstraction, dimerization, etc. or

return to the magnesium surface to give (additional)

RMgX.^4,7,13,16,19

Sch em e 1

One aspect of Scheme 1 has so far received much less

attention. It is conceivable that a (second) single-

electron transfer occurs to R^•with formation of the

carbanion R:^-(step 4); formally, this is accompanied by

•

oxidation of MgX to ⁺MgX. As early as 1959, Pre´vost

et al. proposed that RX was reduced to R:^-in a single

step, i.e., without the intermediate formation of radicals,

which after combination of R:^-with ⁺MgX would lead

to RMgX (step 5).²⁰However, relevant experimental

evidence for his proposal was not supplied and as in the

meantime the radical nature of the Grignard reagent

formation reaction has been unambiguously established,

such a direct formation of a carbanion is not in accord

with current thought.

A major problem is that carbanions, even if they are

true intermediates, cannot be detected by kinetics

because they are generated after the rate-determining

step (1).^7,16,19However, evidence for the transient for-

mation of carbanions was accidentally obtained from an

unprecedented rearrangement occurring during the

reaction of 1-bromo-2-((trimethylstannyl)methyl)ben-

zene (1a ) with magnesium in THF (vide infra).²¹Inde-

pendent support for a carbanionic species was provided

by the formation of crown ether cleavage products

during the synthesis of [2-(bromomagnesio)-1,3-xylylene]-

18-crown-5 from the corresponding bromide and mag-

nesium in THF.²²

anion RX^•-.^6-10Although RX^•-has been suggested to

occur as a discrete intermediate along the reaction

coordinate,^3,6such species have never been detected

directly.^7c,d,9c,11Therefore, certainly for alkyl halides,

they probably resemble a transition state rather than

an intermediate. Kinetic investigations have shown that

the outer-sphere electron transfer is the rate-determin-

ing step.^7b-fThe second stage involves a rapid dissocia-

tion of RX^•-into R^•and X^-(step 2); the latter may

combine with Mg^•+at the electron-deficient magnesium

surface^2,3,12to give the (probably surface-bound) mag-

•

nesious halide MgX. The intermediate occurrence of

radicals R^•has been well-established.^6-9,12-18In step 3,

•

R^•combines with MgX to form RMgX. The possibility

that R₂Mg might be the initially generated species has

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1994, 475, 1 and references cited therein.

Resu lts a n d Discu ssion

In tr a - a n d In ter m olecu la r Migr a tion of Sta n n yl

Gr ou p s. It was previously shown that when 1a was

allowed to react with magnesium in THF, 2a (15%), 3a

(29%), 4a (28%), and 5 (28%) were formed. These

compounds were characterized as 6a , 7a , 4a , and 8,

respectively, on quenching with chlorotrimethylgermane

(Scheme 2).²¹The ratio of these products did not change

even after keeping the primary reaction mixture at room

(17) (a) Chanon, M. Bull. Soc. Chim. Fr. 1982, 7-8, 197. (b) Chanon,

M.; Ne´grel, J . C.; Bodineau, N.; Mattalia, J . M.; Pe´ralez, E. Macromol.

Symp. 1998, 134, 13. (c) Pe´ralez, E.; Ne´grel, J . C.; Goursot, A.; Chanon,

M. Main Group Met. Chem. 1998, 21, 69. (d) Pe´ralez, E.; Ne´grel, J . C.;

Goursot, A.; Chanon, M. Main Group Met. Chem. 1999, 22, 185. (e)

Pe´ralez, E.; Ne´grel, J . C.; Goursot, A.; Chanon, M. Main Group Met.

Chem. 1999, 22, 201. (f) Bodineau, N.; Mattalia, J . M.; Thimokin, V.;

Handoo, K.; Ne´grel, J . C.; Chanon, M. Org. Lett. 2000, 2303.

(18) Bodewitz, H. W. H. J .; Blomberg, C.; Bickelhaupt, F. Tetrahe-

dron Lett. 1975, 2003.

(19) (a) Garst, J . F.; Deutch, J . E.; Whitesides, G. M. J . Am. Chem.

Soc. 1986, 108, 2490. (b) Ashby, E. C.; Oswald, J . J . Org. Chem. 1988,

53, 6068. (c) Garst, J . F.; Swift, B. L.; Smith, D. W. J . Am. Chem. Soc.

1989, 111, 234. (d) Garst, J . F.; Swift, B. L. J . Am. Chem. Soc. 1989,

111, 241. (e) Root, R. S.; Lawrence, L. M.; Whitesides, G. M. J . Am.

Chem. Soc. 1989, 111, 5405.

(20) (a) Pre´vost, C.; Gaudemar, M.; Miginiac, L.; Gaudemar, F.;

Andrac, M. Bull. Soc. Chim. Fr. 1959, 679. (b) Andrac, M.; Gaudemar,

F.; Gaudemar, M.; Gross, B.; Miginiac, L.; Miginiac, P.; Pre´vost, C.

Bull. Soc. Chim. Fr. 1963, 1385.

(21) de Boer, H. J . R.; Akkerman, O. S.; Bickelhaupt, F. Angew.

Chem. 1988, 100, 735.

(22) Markies, P. R.; Akkerman, O. S.; Bickelhaupt, F.; Smeets, W.

J . J .; Spek, A. L. J . Am. Chem. Soc. 1988, 110, 4284.

Carbanions as Intermediates

Organometallics, Vol. 21, No. 10, 2002 2121

Sch em e 2

Sch em e 3

temperature for about 2 weeks before quenching. In the

meantime, we checked that these product ratios were

not due to the establishment of a rapid equilibrium in

THF as follows: after the reaction of 1a with magne-

sium in THF had reached completion, the resulting

slurries were decanted from the excess magnesium, the

solvent was evaporated, and the residues were stirred

for 1 week in benzene or anisole, followed by evaporation

of the benzene or anisole, rapid readdition of THF, and

quenching with chlorotrimethylgermane; the ratio of

products was unchanged within the limits of error

(about 3%).

conditions provided the chloro analogue of 3a without

any products derived from tin migration.²³

Combination of 12a with ⁺MgBr yields 3a ; reaction

of 13 with ⁺MgBr followed by transformation of its aryl

bromide function to the Grignard functionality ulti-

mately furnishes 5 (5′ is the primary product, but it

disproportionates in THF to magnesium bromide and

5, which precipitates²⁴). Such metal-metal exchange

reactions were unknown for organomagnesium com-

pounds, except in reactions with strained four-mem-

bered stannacycles,^24a,25but are well-known for the more

strongly carbanionoid organolithium compounds.²⁶

Interestingly, when the reaction between 1a and

magnesium in THF was performed at 60 °C, it gave a

clearly different product distribution: 28% 6a , 4% 7a ,

The intramolecular (3a ) and intermolecular (4a and

5) migration of the original stannyl substituent of 1a

was rationalized to be initiated by the carbanion 9a

(Scheme 3), which corresponds to R:^-formed in step 4

of Scheme 1. When it attacks a tin atom, 9a will form

an ate complex; this may happen either intramolecu-

larly to give 10a or intermolecularly to give 11a . The

reaction proceeds further by cleavage of the weakest

bond, i.e., the benzylic carbon-tin bond, to afford the

carbanions 12a and 13, respectively. In line with this

reasoning, the reaction of 2-(trimethylstannyl)benzyl

chloride with magnesium under the same reaction

(23) de Boer, H. J . R. Thesis, Vrije Universiteit, Amsterdam, 1988.

(24) (a) de Boer, H. J . R.; Akkerman, O. S.; Bickelhaupt, F. J .

Organomet. Chem. 1987, 321, 291. (b) de Boer, H. J . R.; Akkerman,

O. S.; Bickelhaupt, F. In Organometallic Synthesis; Eisch, J . J ., King,

R. B., Eds.; Elsevier: New York, 1988; Vol. 4, p 396.

(25) (a) Seetz, J . W. F. L.; Akkerman, O. S.; Bickelhaupt, F. J . Am.

Chem. Soc. 1983, 105, 3336. (b) Bruin, J . R.; Schat, G.; Akkerman, O.

S.; Bickelhaupt, F. J . Organomet. Chem. 1983, 228, 13.

(26) (a) Gilman, H.; Rosenberg, S. D. J . Org. Chem. 1959, 24, 2063.

(b) Seyferth, D.; Suzuki, R.; Murphy, C. J .; Sabet, C. R. J . Organomet.

Chem. 1964, 2, 431. (c) Bott, R. W.; Eaborn, C.; Walton, D. R. M. J .

Organomet. Chem. 1964, 2, 154. (d) Maercker, A.; Sto¨tzel, R. Chem.

Ber. 1987, 120, 1695.

2122 Organometallics, Vol. 21, No. 10, 2002

van Klink et al.

Sch em e 4

lished by derivatization with Me₃GeCl, which revealed

that, in addition to the expected 19 (77%), 20 (7%) and

21 (7%) had been formed. As in the previous cases, 20

and 21 occur in the ratio 1:1, as required for intermo-

lecular exchange.

Intramolecular rearrangement is obviously not de-

tectable in this experiment, because if the primarily

formed carbanion 22 did rearrange, this would lead to

the indistinguishable 22′, both of which lead to the

identical product 19. Therefore, it was decided to

synthesize 23. Intramolecular rearrangement should

now be detectable, because the rearranged and unre-

arranged products would not be identical. Synthesis of

23 started from 2-bromo-4-methylaniline, which was

converted to the iodide 24 by a diazotization reaction;

selective lithium iodine exchange of 24 with n-butyl-

lithium, in a mixture of diethyl ether and THF at -100

°C, followed by derivatization with Me₃SnCl resulted

in the formation of 23.

34% 4a , and 34% 8, respectively, were obtained on

quenching with Me₃GeCl; note that, in accordance with

the intermolecular exchange mechanism of Scheme 3,

4a and 8 were again formed in a 1:1 ratio. The low yield

of 7a indicates that intramolecular stannyl migration

is less favored at elevated temperatures at the expense

of all other products. This can be rationalized as follows.

The intramolecular rearrangement of 9a to 10a is

favored by entropy. At higher temperature, this advan-

tage becomes less important, and the intermolecular

transformations of 9a to 2a or 11a become more

competitive.

A problem with the mechanism proposed in Scheme

3 is that one must assume that a “naked” carbanion

such as 9a is quite reactive in order to facilitate the

unusual attack on a tetraorganylstannane functionality,

while on the other hand, it has to be sufficiently long-

lived to encounter another 1a (for the formation of 11a )

rather than, for example, cleave a THF molecule, a

reaction well-known for the presumably less reactive

organolithium reagents, certainly at higher tempera-

tures.

For that reason, we also looked for other examples of

intermolecular tin exchange in the formation reaction.²¹

The preparation of phenylmagnesium bromide from

bromobenzene and magnesium in THF in the presence

of 1 equiv of benzyltrimethylstannane (14) followed by

quenching of the reaction mixture with Me₃GeCl gave

not only the expected 15 and unreacted 14 (both in 84%

yield) but also the unexpected products 16 and 17 (both

in 14% yield) (Scheme 4); note that, like 4a and 8, 16

and 17 were again obtained in a 1:1 ratio. While 15 is

the derivatization product from the expected phenyl-

magnesium bromide, 16 and 17sthe latter derived from

benzylmagnesium bromidesare formed by intermolecu-

lar exchange at the intermediate carbanion stage via

an ate complex. In an independent experiment, the

reaction mixture was deuteriolyzed to yield 12% deu-

terated toluene (the analogue of 17), 76% 14, and 12%

16 (deuteriobenzene, the analogue of 15, could not be

separated from the solvents and was not determined).

These exchange reactions must also occur during the

formation process, because phenylmagnesium bromide

was found to be inert toward 14 under these condi-

tions.²¹

Surprisingly, reaction of 23 with sublimed magnesium

in THF was extremely slow. Only after 1 month was

the reaction complete. Use of 1,2-dibromoethane as an

entraining agent did not accelerate the reaction. The

reason for this strongly reduced reactivity cannot be

ascribed to steric factors, because the non-methyl-

substituted 18 reacted within 1 day. Quenching of a

sample with Me₃GeCl revealed the formation of 25

(96%), 26 (2%), and 27 (2%); the last two are derived

from intermolecular rearrangement (Scheme 6).

Separation of the product mixture by GLC and

1

analysis by H NMR spectroscopy unambiguously con-

firmed that only 25 had been formed, and no rearranged

28 (Scheme 7). The reason for the absence of 28 is the

ring strain that would be involved in the intermediate

stannate complex 30; its trigonal-bipyramidal structure

would be highly distorted with an axial-equatorial

angle of 60° instead of the ideal 90°. The barrier for the

formation of 30 is apparently so high that an intramo-

lecular exchange does not occur. Thus, once the carban-

ionic intermediate 29 has been formed, its only option

is intermolecular stannate complexation, followed by

expulsion of 26 and recombination with ⁺MgBr to give

(after Me₃GeCl quench) 27; this should result in forma-

tion of 26 and 27 in a 1:1 ratio as experimentally

observed (Scheme 7).

We also investigated the reaction of 31 with magne-

sium in THF in order to test the above-mentioned

speculation that reaction of 23 with magnesium was

slow due to the steric hindrance of the o-trimethylstan-

nyl group (Scheme 8). Furthermore, 28 could thus be

obtained in an unambiguous way, and its data could be

used to prove its absence in the reaction of 23 with

magnesium.

Again, the reaction was slow, though slightly faster

than that of 23; it gave 33 quantitatively after 2 weeks,

as confirmed by quenching a sample with D₂O followed

by ¹H NMR spectroscopy and GCMS analysis, which

showed the quantitative formation of 32. A second

sample was derivatized with Me₃SnCl; ¹H and ¹³C NMR

spectroscopy as well as GCMS showed the quantitative

conversion of 31 to 28. Reexamination of the analytical

data for the reaction of 23 with magnesium unambigu-

ously confirmed the absence of 28 in the tin case and

thus once again underlined that intramolecular stannyl

Intermolecular migrations are not restricted to 14, as

was shown by the reaction depicted in Scheme 5. After

18 was reacted with magnesium in THF, using 1,2-

dibromoethane as entrainer, tin migration was estab-

Carbanions as Intermediates

Organometallics, Vol. 21, No. 10, 2002 2123

Sch em e 5

Sch em e 6

SnCl. While reaction of 40 with magnesium in diethyl

ether did not occur, in THF an unforeseen ring closure

was observed. Quenching of a sample with Me₃GeCl

afforded 41 in only 21% yield (Scheme 10). Besides 41,

42 was formed in 9% yield. Surprisingly, the ring-closed

9,9-dimethylstannafluorene (43) was the major product

(70% yield). Deuteriolysis of another sample gave com-

parable results (see Experimental Section).

With respect to a conceivable intramolecular rear-

rangement in the reaction of 40 with magnesium, the

situation is similar to that of 18: both would not be

discernible by simple product analysis because the

initial carbanion 45 (Scheme 11) and its intramolecular

rearrangement product would be equivalent. It was,

however, expected that the intramolecular ring closure

to give the stannate complex 46 would be facilitated by

the favorable geometric arrangement of the biphenyl

system spanning the equatorial-axial position of the

trigonal bipyramid in a five-membered ring. Indeed, this

step appears to proceed with great ease, and this is the

origin of the high yield of 43, as rationalized in Scheme

11. The difference with the previously encountered

stannyl migrations is that, in 46, it is not an arylic

carbon-tin bond which is cleaved but the methyl

carbon-tin bond dissociates with formation of 43;

simultaneously, methylmagnesium bromide is produced

migration did not occur in the reaction of 23 with

magnesium.

In another attempt to define the factors responsible

for the interplay between intra- and intermolecular

transformations, the behavior of 1-bromo-2-((tri-n-bu-

tylstannyl)methyl)benzene (34) under the conditions of

Grignard reagent formation was investigated. The

choice of 34 was based on the consideration that, by

introducing the (n-Bu)₃Sn substituent at the benzylic

position, both the intermediate radical and the carban-

ion might have longer lifetimes due to the sterically

more demanding substituent. This was expected not

only to favor reduction of the radical to the carbanion

but also to increase the extent of intramolecular rear-

rangement due to the bulkier stannyl substituent, which

would impede a bimolecular encounter. When 34 was

allowed to react with magnesium in THF, this was

indeed observed (Scheme 9). Quenching a sample of the

reaction mixture with D₂O yielded 18% [2-D]((tri-n-

butylstannyl)methyl)benzene (35) and 82% 1-[D]methyl-

2-(tri-n-butylstannyl)benzene (36).

1

in 97% yield with respect to 43, as determined by H

and ¹³C NMR spectroscopy. Apparently, in the cleavage

of 46, the greater stability of the carbanion functionality

(Ar:^-> Me:^-) is more than compensated by the favor-

able entropy due to the formation of the five-membered

ring and of two particles.

The formation of 43 is not the only anomaly in this

reaction. Another surprise was the occurrence of 42, the

derivatization product of the corresponding di-Grignard

reagent (49; see Scheme 12). While 42 is formally

analogous to 8, 17, and 21, all of which indicate the

occurrence of an intermolecular exchange process analo-

gous to 9 f 11 f 13 (Scheme 3), the corresponding

distannyl derivative 44 (Scheme 10) is missing!

Derivatization of a sample with Me₃GeCl showed

similar results: 16% 37 and 64% 38; in addition, also

10% 39 and 10% 8 were detected. The conclusion is

justified that, due to the bulky tri-n-butylstannyl group,

the possibility for a bimolecular coupling is diminished.

Furthermore, due to the longer lifetime of the interme-

diate carbanion, intramolecular rearrangement to the

more stable carbanion is enhanced.

We rationalize the formation of 42 as follows (Scheme

12). While the intermolecular attack of carbanion 45 on

the tin atom of 40 or 43 is probably impossible due to

steric crowding, the transfer of a formal methyl anion

from the ate complex 46 to 40 is sterically less demand-

ing and furnishes, besides 43, the ate complex 47. The

latter is cleaved by MgBr⁺preferentially at the aryl

carbon-tin bond, the (formal) aryl anion being more

stable than the methyl anion. The products are Me₄Sn

In an attempt to prove that it is the high ring strain

energy of the stannate complex 30 which is responsible

for the nonoccurrence of the intramolecular rearrange-

ment, and not the greater strength of the arylic carbon-

tin bond (as compared to the benzylic carbon-tin bond

in 10a ), 40 was synthesized from 2,2′-dibromobiphenyl²⁷

by monolithiation followed by derivatization with Me₃-

(27) Gilman, H.; Gaj, B. J . J . Org. Chem. 1957, 22, 447.

2124 Organometallics, Vol. 21, No. 10, 2002

van Klink et al.

Sch em e 7

Sch em e 8

Additional proof for the assigned (monomeric) struc-

ture of 43 came from an X-ray structure determination

of a single crystal. The tin atom is tetracoordinated in

a distorted tetrahedron, with a small C8a-Sn9-C9a

angle (Figure 1). The tin atom is positioned only slightly

above the biphenylene plane. The bond angles around

tin (deg) are C8a-Sn9-C9a ) 84.0(3), C8a-Sn9-C10

) 115.4(3), and C10-Sn9-C11 ) 110.5(4); selected bond

lengths (Å) are C8a-Sn9 ) 2.142(9), C9a-Sn9 ) 2.129-

(8), C10-Sn9 ) 2.133(9), and C11-Sn9 ) 2.139(10). To

our knowledge, the dibenzostannole 43 is only the

second example of a small stannacycle (SnC₄) with a

tetraorganylstannane center for which a crystal struc-

ture has been determined. Recently, the crystal struc-

ture of 1,1,2,3,4,5-hexaphenylstannacyclopentadiene was

published.²⁸Molecular models show that the formation

of an SnC₄ring must involve substantial angle strain,

and such strain has been invoked to explain the

enhanced heterolytic and homolytic reactivity of the

SnC₄ring system.²⁹

and 48. Further reaction of 48 with magnesium yields

49, which, by derivatization with Me₃GeCl, gives 42. In

support of this interpretation, Me₄Sn was identified and

1

determined by H NMR spectroscopy. While stoichiom-

etry requires the ratio Me₄Sn:42 to be 1:1, it was in fact

observed to be 1:3; this may be due to technical

problems, in particular by the volatility of Me₄Sn (bp

78 °C).

The formation of 43 is explained in terms of a

carbanion rather than a radical intermediate for two

reasons. In the first place, reaction of 2,2′-dilithiobiphe-

nyl with 2 equiv of Me₃SnCl has been reported to result

in the formation of 43; this undoubtedly must be a

carbanionoid process.³⁰Second, treatment of 2-(bro-

In this case also, the product distribution did not alter

from the initial one upon standing at room temperature

or heating at 70 °C (apart from the amount of Me₄Sn,

1

which appeared to vary between the different H NMR

measurements). To confirm the nonexistence of an

equilibrium between 43 and MeMgBr, a sample of pure

43 was mixed with MeMgBr in THF and stirred for

several hours under reflux. After it was cooled to room

temperature, the mixture was deuteriolyzed and, after

(28) Ferman, J .; Kakareka, J . P.; Klooster, W. T.; Mullin, J . L.;

Quattrucci, J .; Ricci, J . S.; Tracy, H. J .; Vining, W. J .; Wallace, S. Inorg.

Chem. 1999, 38, 2464.

(29) Davies, A. G.; Tse, M. W.; Kennedy, J . D.; McFarlane, W.; Pyne,

G. S.; Ladd, M. F. C.; Povey, D. C. J . Chem. Soc., Perkin Trans. 2 1981,

369.

(30) (a) Neugebauer, W.; Kos, A. J .; Schleyer, P. v. R. J . Organomet.

Chem. 1982, 228, 107. (b) Reich, H. J .; Phillips, N. H. J . Am. Chem.

Soc. 1986, 108, 2102.

1

standard workup, analyzed by H NMR spectroscopy

and GCMS; 43 was recovered quantitatively, and no

[2-D]-2′-(trimethylstannyl)biphenyl was observed.

Carbanions as Intermediates

Organometallics, Vol. 21, No. 10, 2002 2125

Sch em e 9

Sch em e 10

Thus 1b, the silicon analogue of 1a (Scheme 2), reacted

quantitatively on stirring for 24 h with magnesium in

THF. However, deuteriolysis followed by GCMS and

NMR spectroscopy proved that in fact two Grignard

reagents had been formed, as both 50 (22%) and 51

(67%) were obtained (Scheme 13); on prolonged standing

at room temperature before deuterolysis, the composi-

tion remained unchanged. Treatment of the Grignard

mixture with Me₃SnCl gave a mixture of 52 and 53

which, unfortunately, could not be separated or reliably

analyzed. However, 4a and 4b, the products of inter-

molecular exchange of silyl groups, were clearly absent.

Therefore, one may conclude the reaction of 1b with

magnesium leads not only to the unrearranged Grignard

reagent 2b but also to the rearranged 3b (Scheme 14);

this process occurs only during the formation reaction

and not afterward, and this again is ascribed to the

highly reactive carbanionic intermediate 9b analogous

to 9a (Scheme 3). We recently reported that in the

related case of the formation of di-Grignard reagents

from 54, only intramolecular migration of the trimeth-

ylsilyl group occurred, leading to a mixture of 55 and

56.³²

Sch em e 11

The high percentage of ring closure to 43 in the

reaction of 40 with magnesium induced us to perform

the same reaction with the silyl analogue 57. As 1b gave

intramolecular rearrangements exclusively, it was en-

visaged that 9,9-dimethyl-9-silafluorene (58) would be

obtained in even higher yield. Indeed, reaction of 57

with 2 equiv of magnesium in THF led to the formation

of 58 in 91% yield. Also in this case, again no products

resulting from intermolecular silyl migration were found

(Scheme 15).

The difference in behavior between the silyl and

stannyl derivatives was not foreseen, but in retrospect

it gives additional support to our proposal that relatively

free, and therefore highly reactive, carbanions such as

9a (Scheme 3) are responsible for the observed anoma-

lous rearrangements. In the first place, strongly car-

banionic species such as organolithium or higher organo-

alkali-metal compounds react rapidly with ethers with

proton abstraction;³³THF is particularly notorious in

this regard. Against this background, it is surprising

that the possibly even more reactive carbanions would

be sufficiently long-lived to survive in THF in order to

encounter another organotin compound, as required for

momethyl)-2′-iodobiphenyl with methyllithium yielded

fluorene; a detailed mechanistic investigation revealed

that this cyclization, too, most likely proceeded by a

carbanion route rather than via radical intermediates.³¹

In tr a m olecu la r Migr a tion of Silyl Gr ou p s. For

comparison with the rearrangements observed for or-

ganotin compounds, we also briefly investigated the

behavior of analogous silicon derivatives. Surprisingly,

in this class, only intramolecular migrations occurred.

(32) de Boer, H. J . R.; Plugge, M.; Akkerman, O. S.; Bickelhaupt,

F. Main Group Met. Chem. 2001, 24, 93.

(31) Altman, L. J .; Erdman, T. R. J . Org. Chem. 1970, 35, 3237.

(33) Wittig, G. Angew. Chem. 1958, 70, 65.

2126 Organometallics, Vol. 21, No. 10, 2002

van Klink et al.

Sch em e 12

banion on the silicon function of a second molecule is

even less likely, and only the normal reaction pattern

does occur: i.e., combination with ⁺MgX. It should,

however, be pointed out that in all reactions which lead

to the unrearranged product, the reaction may proceed

directly via steps 1-3 of Scheme 1 without the necessity

of invoking the carbanion intermediate R:^-(Scheme 1,

pathway proceeding via steps 1, 2, 4, and 5).

Non ca r ba n ion ic Rea r r a n gem en ts Ar e Un lik ely.

The evidence presented so far can be explained convinc-

ingly by invoking a highly reactive carbanion intermedi-

ate as the initiator of intra- and intermolecular migra-

tions of group 14 substituents. Nevertheless, alternative

reaction pathways might be equally attractive to ratio-

nalize the experimental results. For this reason, we

shall consider three a priori feasible possibilities which

we will briefly indicate as the dianion, the magnesium

cluster, and the radical mechanisms.

F igu r e 1. Displacement ellipsoid plot (50% probability

level) of 43. Only one out of the two crystallographically

independent molecules is shown.

The dianion mechanism involving the dianion ^-•R-

X^•-as an intermediate has been suggested in the

reduction of 2,2-diphenylcyclopropyl halides with alkali-

metal naphthalenes^4,37and in the reaction of 2-(3-

butenyl)phenyl halides with magnesium.³⁸The mech-

the intermolecular tin exchange via the ate complex 11a

(Scheme 3). Therefore, we suggested²¹that, in these

cases, the free carbanion (e.g. 9a ) is to some extent

protected against immediate reactions, such as combi-

nation with ⁺MgX (Scheme 1) or protonation, by in-

tramolecular attack at the group 14 element to give a

cyclic ate complex such as 10a . The latter forms a pool

for subsequent quasi-carbanionoid species giving rise to

the observed intra- or intermolecular exchange. Without

such intramolecular protection, the level of intermo-

lecular exchange in the tin series is much lower, as in

the case of 18 (7%; Scheme 5) or 23 (2%; Schemes 6 and

7).

This hypothesis also explains why, in the silicon

series, intramolecular migration is the only mode ob-

served. In general, pentacoordinate organosilicates are

much less stable than their stannate analogues.^30,34

More in particular, silicates carrying only carbon³⁵or

carbon and hydrogen^35c,36as ligating atoms at silicon

are much less stable than their stannate analogues and

have, with a few exceptions, been described only re-

cently. For this reason, the silicate complex 10b, analo-

gous to the stannate complex 10a (Scheme 3), will be

formed less readily and will have an insufficient lifetime

to allow an intermolecular encounter with the tetraor-

ganylsilane 1b with formation of 11b , and hence

intermolecular exchange does not occur. In the absence

of intramolecular stabilization, the attack of the car-

(34) (a) Chuit, C.; Corriu, R. J . P.; Reye, C.; Young, J . C. Chem. Rev.

1993, 93, 1371. (b) Holmes, R. R. Chem. Rev. 1996, 96, 927. (c) Deiters,

J . A.; Holmes, R. R. Organometallics 1996, 15, 3944. (d) Kost, D.;

Kalikhman, I. In The Chemistry of Organic Silicon Compounds;

Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol.

2, Part 2, Chapter 23, p 1339. (e) Bassindale, A. R.; Taylor, P. G.; Glynn,

S. J .; Taylor, P. G In The Chemistry of Organic Silicon Compounds;

Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol.

2, Part 2, Chapter 9, p 495. (f) Takeuchi, Y.; Takayama, T. In The

Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y.,

Eds.; Wiley: Chichester, U.K., 1998; Vol. 2, Part 2, Chapter 6, p 267.

(g) Hermanns, J .; Schmidt, B. J . Chem. Soc., Perkin Trans. 1 1999,

81. (h) Maggiarosa, N.; Tyrra, W.; Naumann, D.; Kirij, N. V.; Yagupol-

ski, Y. L. Angew. Chem. 1999, 111, 2392; Angew. Chem., Int. Ed. 1999,

38, 2252.

(35) (a) de Keijzer, A. H. J . F.; de Kanter, F. J . J .; Schakel, M.;

Schmitz, R. F.; Klumpp, G. W. Angew. Chem. 1996, 108, 1183; Angew.

Chem., Int. Ed. Engl. 1996, 35, 1127. (b) de Keijzer, A. H. J . F.; de

Kanter, F. J . J .; Schakel, M.; Osinga, V. P.; Klumpp, G. W. J .

Organomet. Chem. 1997, 548, 29. (c) Rot, N.; Nijbacker, T.; Kroon, R.;

de Kanter, F. J . J .; Bickelhaupt, F.; Lutz, M.; Spek, A. L. Organome-

tallics 2000, 19, 1319.

(36) (a) Hajdasz, D. J .; Squires, R. R. J . Am. Chem. Soc. 1986, 108,

3139. (b) Hajdasz, D. J .; Ho, Y.; Squires, R. R. J . Am. Chem. Soc. 1994,

116, 10751. (c) Hong, J .-H.; Boudjouk, P. Organometallics 1995, 14,

574.

(37) (a) Boche, G.; Schneider, D. R. Wintermayr, H. J . Am. Chem.

Soc. 1980, 102, 5697. (b) Boche, G.; Walborsky, H. M. In The Chemistry

of the Cyclopropyl Group; Rappoport, Z., Ed.; Wiley: Chichester, U.K.,

1987; pp 701-808.

(38) Garst, J . F.; Boone, J . R.; Webb, L.; Lawrence, K. E.; Baxter, J .

T.; Ungva´ry, F. Inorg. Chim. Acta 1999, 296, 52.

Carbanions as Intermediates

Organometallics, Vol. 21, No. 10, 2002 2127

Sch em e 13

Sch em e 14

encountered under conditions which strongly deviate

from the usual Grignard reactions, namely in metal

vapor cryosynthesis, and without the presence of polar

solvents such as ethers.³⁹Furthermore, we feel that it

would be difficult to argue that in such clusters the

carbanionoid R group should exhibit a more pronounced

carbanionic character than that of ordinary Grignard

reagents RMgX, let alone that it would exceed the

carbanionic reactivity of organolithium compounds.

The final candidate as an alternative to the carbanion

R:^-is the radical R^•, especially as radicals have been

unambiguously identified as intermediates in this reac-

tion. While the unusual group 14 rearrangements

described above appear to fit much better into a car-

banion mechanism, it was clearly desirable to prove by

direct experimental evidence that authentic R^•formed

by an alternative and unequivocal route does not give

rise to the rearranged products obtained in the reaction

of R-X with magnesium.

Sch em e 15

For that purpose, we had initially tried to create the

aryl radical 59 (Scheme 17) from 1a by photolysis of its

aryl-bromine bond. However, due to the partial cleav-

age of the benzylic carbon-tin bond, the results were

not unambiguous.²¹Similarly, experiments wherein

phenyl radicals (generated by reduction of bromoben-

zene and the “complex reducing agent” NaH/Ni(OAc)₂/

tert-amyl alcohol, by reduction of phenylmercury bro-

mide with sodium borohydride, or by photochemical

decomposition of dibenzoyl peroxide) were unsuccessful

or gave unidentified products.²³

We next considered samarium(II) iodide (SmI₂) as an

attractive reagent to react with 1a under formation of

59 without affecting the benzylic carbon-tin bond,

because SmI₂is a unique single-electron reducing agent

for a number of functional groups in organic synthesis.

Various mechanisms have been suggested for the reac-

tion of organic halides with SmI₂(Scheme 16).⁴¹Outer-

or inner-sphere electron transfer between Sm(II) and

the organic halide R-X will lead to the radical R^•

through loss of the halide ion. When R is a primary or

anism proposed includes prior reduction of an aryl group

R to the radical anion R^•-followed by a second single-

electron transfer to the σ* orbital of the R-X bond (step

1 of Scheme 1). We consider this course of events highly

unlikely because of electrostatic repulsion in ^-•R-X^•-.

This applies even more in the present cases, where both

electron transfers would have to take place to two

organic entities which are directly connected, namely

the aryl group and the halogen attached to it. Moreover,

the usual radical intermediate R^•has recently been

shown to be a more likely alternative than the dianion

in the related reaction of 2-(4-phenyl-3-butenyl)phenyl

halides with magnesium.^17f

Occasionally, organomagnesium clusters such as

17b-d

RMg_n

and RMg_nX³⁹have been suggested as inter-

mediates in the reaction between halides and magne-

sium, in particular RMg₄X.^39bRecently, it has been

considered⁴⁰that, instead of the carbanion R:^-, RMg_nX

might be responsible for the group 14 migrations

described above. However, such clusters have been

(39) (a) Smirnov, V. V.; Tjurina, L. A.; Beletskaya, I. P. In Grignard

Reagents, New Developments; Richey, H. G., Ed.; Wiley: Chichester,

U.K., 2000; Chapter 12, pp 395-410. (b) Tjurina, L. A.; Smirnov, V.

V.; Barkovskii, G. B.; Nikolaev, E. N.; Esipov, S. E.; Beletskaya, I. P.

Organometallics 2001, 20, 2449.

(40) See ref 4, p. 238.

2128 Organometallics, Vol. 21, No. 10, 2002

van Klink et al.

Sch em e 16

different amounts of free samarium metal present in

solution. To ensure that it was the samarium Grignard

reaction via the carbanion which was responsible for the

observed tin migration, the reaction was repeated in the

presence of t-BuOD or CH₃OD. Though in this case

conversion of 1a was not quantitative, it provided

convincing evidence for the origin of the rearrangement

product 60. According to GCMS, 60 showed deuterium

incorporation (t-BuOD: 78%; CH₃OD: 73%), indicating

that 60 resulted from deuteration of a carbanionic

intermediate. Since 14 did not incorporate any deute-

rium, it must originate from a radical species: i.e., 59.

Thus, finally proof was obtained that the observed

rearrangements are due to carbanionic intermediates

and that these carbanions are actual intermediates in

the formation of the Grignard reagent.

a secondary alkyl group, subsequent reaction with a

second molecule of SmI₂will afford an organosamarium

reagent which reacts with electrophilic species such as

carbonyl compounds in the same way as Grignard

reagents.

The addition of HMPA enhances the reduction of

organic halides by the SmI₂-THF system. The reduction

of aryl halides by a SmI₂-THF-HMPA solution rapidly

produces aryl radicals, but in comparison with the

outcome of the reactions of alkyl halides, further reduc-

tion to the corresponding aryl anion as the result of a

two-electron reduction is less pronounced. Since 1 equiv

of HMPA does not suffice to enhance reactivity, HMPA

is generally used as a cosolvent (5-10%). HMPA com-

plexes to Sm(II), displacing THF from the Sm(II)

coordination sphere; in this fashion, it effectively shields

the reacting organic halide from hydrogen atom donors,

thus enhancing the lifetime of the radical.^41b

When 1a was stirred in a solution of SmI₂in THF for

several days, no reaction was observed; after hydrolysis

and workup, 1a was recovered quantitatively. However,

when 1a was added to a solution of SmI₂containing 10%

HMPA as cosolvent, the reaction could be monitored

visually, as the solution color turned from the typical

purple to brown. Analysis of the reaction mixture by

GCMS revealed only the presence of benzyltrimethyl-

stannane (14); 1-methyl-2-(trimethylstannyl)benzene

(60) was not detected (Scheme 17).

When the same reaction was performed on a larger

scale in order to facilitate separation and characteriza-

tion of the reaction products, the formation of 10% 60

was observed. The formation of 60 may be explained

by further reduction of 59 to 9a , which, as in the case

of the Grignard formation reaction of 1a , is responsible

for the observed tin migrations. This further reduction

might be induced by SmI₂, which was available in

excess,^41b,cbut more likely it is the result of reduction

by samarium metal which is present in the SmI₂

solution to stabilize the SmI₂and to make it more

reactive;⁴¹ⁱin our experience, different batches of SmI₂

showed different rates of reduction, probably due to

Reaction of 1a with SmI₂in the presence of CD₃OH

confirmed that the non-rearranged 14 resulted from a

radical precursor. This assumption is made on the basis

that 14 was found to be 56% deuterated, presumably

at the arylic ortho position. The relatively low degree

of deuteration can be ascribed to the fact that the excess

of THF competes with the CD₃OH as a hydrogen donor.

A control experiment showed that 14 did not react

with a solution of SmI₂in a mixture of THF and HMPA.

However, adding bromobenzene to a mixture of 14 and

SmI₂in THF-HMPA did result in a reaction. Analysis

of the mixture by ¹H NMR spectroscopy and GCMS

revealed, besides the presence of 14 (78%), also that of

16 (22%; cf. Scheme 4). In this case, too, the relatively

high yield of 16 is explained by intermediate formation

of carbanion 9a due to a larger excess of Sm (see

Experimental Section), leading to overreduction of the

phenyl radical to the phenyl anion. Formation of toluene

was also observed, but because of its high volatility and

low yield (22% according to stoichiometry of the pro-

posed reaction mechanism), a quantitative estimate was

not feasible.

Con clu sion

The intermediate formation of reactive carbanions in

the formation of the Grignard reagent appears to be well

supported by the occurrence of inter- and/or intramo-

lecular migrations of organotin and organosilicon groups.

Organolithium compounds with their highly ionic metal-

carbon bonds exhibit striking parallels in their reaction

behavior, whereas radicals generated independently

showed a different behavior. The high percentage of

rearrangement reactions is an indication that these

anionic species are not located on a minor pathway; at

least in the cases studied here, they make a major

contribution to the overall reaction, especially when they

are stabilized as stannate complexes. The combination

of high reactivity and selectivity of the carbanion might

be explained by coordination to the (positively charged)

magnesium surface, as recently proposed for the ortho

metalation of anisole by sodium.⁴²Thus, a high steady-

state concentration of carbanions can be achieved,

resulting in an enhanced possibility to react with

neutral starting molecules.

(41) (a) Girard, P.; Namy, J . L.; Kagan, H. B. J . Am. Chem. Soc.

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Carbanions as Intermediates

Organometallics, Vol. 21, No. 10, 2002 2129

Sch em e 17

1a : ¹H NMR (CDCl₃, 200.1 MHz, reference CHCl₃7.27 ppm)

Exp er im en ta l Section

δ 0.10 (s, ²J (Sn-H) ) 53.9, 51.5 Hz, 9H, SnCH₃), 2.49 (s,

Gen er a l P r oced u r es a n d Ma ter ia ls. Manipulations in-

volving the reactions with magnesium were, unless stated

otherwise, conducted in fully sealed glassware using standard

high-vacuum techniques.^43,44The magnesium used was doubly

sublimed. Amounts of “total base” and Mg²⁺in organomagne-

sium samples were determined, after hydrolysis of a sample,

by titration with HCl and EDTA, respectively.^43,44Solvents

were distilled from LiAlH₄after predrying on KOH or in sealed

glassware, distilled from sodium-potassium alloy. The start-

ing materials 1-bromo-2-(bromomethyl)benzene (Aldrich), bro-

mobenzene (Merck), 1,2-dibromobenzene (Aldrich), 2-bromo-

4-methylaniline (Aldrich), 1,2-dibromoethane (Merck), MeMgBr

(Aldrich), SmI₂(Aldrich), Sm (40 mesh, Aldrich), CH₂I₂(J an-

ssen), HMPA (Aldrich), and Florisil (J anssen) were com-

mercially available. GLC and quantitative NMR analyses were

performed using hexamethylbenzene as internal standard.

NMR spectra were recorded on a Bruker AC 200 spectrometer

(¹H NMR, 200.1 MHz; ¹³C NMR, 50.32 MHz), on a Bruker WM

250 spectrometer (¹H NMR, 250 MHz; ¹³C NMR, 63 MHz) or

3

4

²J (Sn-H) ) 63.0 Hz, 2H, CH₂), 6.87 (ddd, J ) 8.0, 7.1 Hz, J

3

4

) 2.0 Hz, 1H, aryl H), 7.08 (dd, J ) 7.6 Hz, J ) 2.0 Hz, 1H,

3

4

aryl H), 7.14 (ddd, J ) 7.6, 7.1 Hz, J ) 1.3 Hz, 1H, aryl H),

7.47 (dd, ³J ) 8.0 Hz, ⁴J ) 1.3 Hz, 1H, aryl H); ¹³C NMR

1

(CDCl₃, 50.32 MHz, reference CDCl₃77.00 ppm) δ -8.9 (q, J

) 129 Hz, J (Sn-C) ) 332, 317 Hz, SnCH₃), 22.3 (t, J ) 132

Hz, J (Sn-C) ) 276, 264 Hz, CH₂), 122.6 (bs, J (Sn-C) ) 26

Hz, C1), 124.6 (dd, ¹J ) 164 Hz, ³J ) 8 Hz, ⁵J (Sn-C) ) 15 Hz,

1C, C5), 127.2 (dd, ¹J ) 161 Hz, ³J ) 8 Hz, ⁴J (Sn-C) ) 13 Hz,

1

3

1

3

1C, C4), 128.2 (ddd, J ) 159 Hz, J ) 6 Hz, J (Sn-C) ) 21

Hz, 1C, C3), 132.4 (dd, ¹J ) 164 Hz, ³J ) 7 Hz, ⁴J (Sn-C) ) 13

Hz, 1C, C6), 143.4 (bs, ²J (Sn-C) ) 40 Hz, 1C, C2); ¹¹⁹Sn NMR

(CHCl₃, 149.21 MHz, reference Me₄Sn 0.000 ppm) δ 8.621;

GCMS m/z (relative intensity) 334 (M^•+, C₁₀H₁₅⁷⁹Br¹²⁰Sn, 3),

319 (82), 289 (6), 229 (13), 199 (34), 169 (20), 165 (100), 150

(13), 135 (41), 120 (17), 105 (26), 90 (39), 89 (50); HRMS calcd

for C₁₀H₁₅⁷⁹Br¹¹⁸Sn 333.9378, obsd 333.9398 ( 0.0006. Anal.

Calcd for C₁₀H₁₅BrSn: C, 35.98; H, 4.53; Br, 23.94. Found: C,

36.02; H, 4.55; Br, 23.8.

on a Bruker MSL 400 spectrometer (¹H NMR, 400.1 MHz; ¹³

C

Rea ction of 1a w ith Ma gn esiu m in Dieth yl Eth er . A

mixture of 3.0 g (9.0 mmol) of 1a , 0.243 g (10.0 mmol) of triply

sublimed magnesium, and 150 mL of diethyl ether was stirred

for 72 h at room temperature. A sample was hydrolyzed and

analyzed by titration, which revealed that no reaction had

occurred. The reaction vessel was opened and the mixture

worked up, after which 1a was recovered quantitatively.

Rea ction of 1a w ith Ma gn esiu m in THF . A mixture of

3.0 g (9.0 mmol) of 1a , 0.36 g (15.0 mmol) of triply sublimed

magnesium, and 180 mL of THF was stirred for 22 h at room

temperature. During stirring, the mixture turned yellow

within a few minutes and a white precipitate slowly formed.

A sample was hydrolyzed and analyzed by titration, which

revealed the formation of 8.6 mmol of base and 8.8 mmol of

Mg²⁺. At the same time, a sample was deuterated (D₂O) and

worked up by addition of diethyl ether, separation of the

organic and aqueous fractions, and drying of the organic

fraction (MgSO₄); it was analyzed by GCMS (relative yields).

1-Deuterio-2-(deuteriomethyl)benzene, 24% yield: GCMS

m/z (relative intensity) 95 (6), 94 (M^•+, C₇H₆D₂, 82), 93 (100),

92 (25), 91 (3), 90 (3).

NMR, 100.6 MHz; ¹¹⁹Sn NMR, 149.2 MHz). GCMS analyses

were performed on a HP 5890 GC/5970 MS combination,

operating at 70 eV and equipped with a Chrompack CpSil 5CB

50 m/0.20 mm²column. The ions containing Si, Ge, or Sn

showed the expected isotope patterns, but only the m/z values

containing the most abundant isotope(s) (²⁸Si, ⁷⁴Ge, and ¹²⁰Sn)

are listed. HRMS measurements were performed on a Finni-

gan MAT 90 spectrometer (direct inlet). Elemental analyses

were carried out by Mikroanalytisches Labor Pascher, Rema-

gen, Germany.

1-Br om o-2-((tr im eth ylsta n n yl)m eth yl)ben zen e (1a ).⁴⁵

A solution of 25.00 g (100 mmol) of 1-bromo-2-(bromomethyl)-

benzene in 50 mL of diethyl ether was added dropwise under

nitrogen to 2.55 g (105 mmol) of doubly sublimed magnesium

covered with 150 mL of diethyl ether in 5 h at room temper-

ature. After the addition was completed, stirring was continued

for 1 h. The yellow solution was carefully siphoned from the

residual magnesium into another reaction flask; then it was

cooled to 0 °C followed by the addition of a solution of 19.93 g

(100 mmol) of Me₃SnCl in 100 mL of pentane. Immediately a

white precipitate formed. After the addition was completed,

the mixture was stirred for 1 h, after which 20 mL of saturated

NH₄Cl was added. The organic fraction was separated and the

aqueous fraction extracted with diethyl ether. The extracts

were combined, washed (water), dried (MgSO₄), filtered, and

evaporated to dryness. The residual oil was purified by

fractional distillation. The product was collected at 103 °C at

3 mbar to give 20.70 g (62.0 mmol, 62% yield) of a colorless

oil.

1-(Deuteriomethyl)-2-(trimethylstannyl)benzene ([D]60), 26%

yield: ¹H NMR (CDCl₃, 250 MHz, reference CHCl₃7.27 ppm)

2

δ 0.36 (s, J (Sn-H) ) 54, 52 Hz, 9H, SnCH₃), 2.43 (t, J (D-

H) ) 2 Hz, 2H, CH₂). 7.04-7.33 (m, 3H, aryl H); GCMS m/z

(relative intensity) 242 ([M - CH₃]⁺, 100), 212 (36), 135 (18),

92 (30).

2-Deuterio-1-((trimethylstannyl)methyl)benzene ([2-D]14),

18% yield: GCMS m/z (relative intensity) 257 (M^•+, 8), 242

(8), 212 (5), 165 (100), 135 (27), 120 (16), 92 (78).

1-(Trimethylstannyl)-2-((trimethylstannyl)methyl)ben-

zene (4a ),^24a32% yield.

These analyses (titration and deuteriolysis) were repeated

after 34, 108, and 300 h, but all values remained constant.

According to the mass spectra, the deuterated compounds

(43) Vreugdenhil, A. D.; Blomberg, C. Recl. Trav. Chim. Pays-Bas

1963, 82, 453.

(44) Vreugdenhil, A. D.; Blomberg, C. Recl. Trav. Chim. Pays-Bas

1963, 82, 461.

(45) Alexander, R; Attar-Bashi, M. T.; Eaborn, C.; Walton, D. R. M.

Tetrahedron 1974, 30, 899.

2130 Organometallics, Vol. 21, No. 10, 2002

van Klink et al.

contained more than 97% deuterium. After 300 h, a sample

was taken from the mixture in order to identify the white

precipitate. For that purpose, the precipitate was separated

from the yellow solution by decantation and treated with an

excess of Me₃SnCl in THF. After workup, GLC analysis of the

organic fraction revealed that 4a had been formed in 95% yield

with respect to 5.

In an independent experiment, 150 mL of a similar mixture

was derivatized with 8.0 mmol of Me₃GeCl (1.2 g) and stirred

for 1 h at room temperature; upon addition of Me₃GeCl, the

mixture turned clear and colorless within seconds. The reac-

tion vessel was opened, followed by the addition of diethyl

ether and saturated NH₄Cl. The organic fraction was sepa-

rated and the aqueous fraction extracted with diethyl ether.

The organic fractions were combined, dried (MgSO₄), filtered,

and evaporated to dryness, yielding 3.4 g of a colorless liquid.

The product distribution and yields were determined by

quantitative ¹H NMR analysis (pulse delay 15 s). The com-

pounds were isolated by preparative GLC; however, 6a and

7a could not be separated from each other.

the colorless mixture was hydrolyzed and analyzed by titra-

tion, which revealed the formation of 5.6 mmol of base and

5.6 mmol of Mg²⁺. A 130 mL amount of the mixture was

quenched with 4.9 mmol of Me₃GeCl (0.88 g); after workup,

2.3 g of a colorless liquid was isolated. The product distribution

and the yields were determined by quantitative ¹H NMR

analysis. The compounds were separated by preparative

GLC: Benzyltrimethylstannane (14) was obtained in 84%

yield.

Trimethylphenylgermane (15), colorless liquid, 84% yield:

¹H NMR (CDCl₃, 250 MHz, reference CHCl₃7.27 ppm) δ 0.28

(s, 9H, GeCH₃), 7.27-7.58 (m, 5H, aryl H); GCMS m/z (relative

intensity) 196 (M^•+, C₈H₁₄⁷⁴Ge, 4), 181 (100), 151 (20), 91 (12),

89 (13).

Trimethylphenylstannane (16), colorless liquid, 14% yield:

¹H NMR (CDCl₃, 250 MHz, reference CHCl₃7.27 ppm) δ 0.27

(s, ²J (Sn-H) ) 56, 54 Hz, 9H, SnCH₃), 7.27-7.58 (m, 5H, aryl

H). GCMS m/z (relative intensity) 242 (M^•+, C₉H₁₄¹²⁰Sn, 1), 227

(100), 197 (34), 135 (14), 120 (18).

Benzyltrimethylgermane (17), colorless liquid, 14% yield:

¹H NMR (CDCl₃, 250 MHz, reference CHCl₃7.27 ppm) δ 0.11

(s, 9H, GeCH₃), 2.21 (s, 2H, CH₂), 6.91-7.35 (m, 5H, aryl H);

GCMS m/z (relative intensity) 210 (M^•+, C₉H₁₄⁷⁴Ge, 12), 195

(12), 119 (100), 91 (51), 89 (38).

1-Br om o-2-(tr im eth ylsta n n yl)ben zen e (18). A mixture

of 8.33 g (35.0 mmol) of 1,2-dibromobenzene, 6.63 g (35.0 mmol)

of 1,2-dibromoethane, and 35 mL of diethyl ether was added

dropwise to 1.68 g (70.0 mmol) of sublimed magnesium covered

with 5 mL of diethyl ether in 2 h at 0 °C. The mixture turned

orange immediately, and slowly a white precipitate formed.

After the addition was completed, the stirring was continued

for another 1.5 h at 0 °C; then a solution of 7.19 g (35.0 mmol)

of Me₃SnCl in 50 mL of diethyl ether was added over 1 h.

Stirring was continued for 20 h at room temperature. After

workup, 10.3 g of a brown viscous oil was isolated. High-

vacuum distillation (49 °C at 0.08 mbar) afforded 3.13 g (9.8

mmol, 38% yield) of a colorless oil of 18.

4a , 28% yield.

1-(Trimethylgermyl)-2-((trimethylstannyl)methyl)benzene

(6a ),²¹15% yield.

1-((Trimethylgermyl)methyl)-2-(trimethylstannyl)benzene

(7a ),²¹29% yield.

1-(Trimethylgermyl)-2-((trimethylgermyl)methyl)benzene

(8),^24a28% yield.

1-(Tr im eth ylger m yl)-2-((tr im eth ylsta n n yl)m eth yl)ben -

zen e (6a ).⁴⁶A solution of 3.0 mmol of tert-butyllithium in 2

mL of pentane was cooled to -80 °C, and 10 mL of diethyl

ether was added. This solution was stirred for 15 min, and

then 0.5 mmol of 1a (167 mg) was added. The mixture became

immediately yellow and was stirred for 1.5 h at -80 °C.

Addition of 3.0 mmol of Me₃GeCl (462 mg) gave an almost

colorless solution and a white precipitate. The mixture was

warmed to room temperature, followed by the addition of

saturated NH₄Cl and workup. Yield: 89%.

18: ¹H NMR (CDCl₃, 250 MHz, reference CHCl₃7.27 ppm)

δ 0.38 (s, ²J (Sn-H) ) 56, 54 Hz, 9H, SnCH₃), 7.18 (m, 3H,

aryl H), 7.60 (m, 1H, aryl H); ¹³C NMR (CDCl₃, 63 MHz,

reference CDCl₃77.00 ppm) δ -7.9 (q, ¹J ) 129 Hz, ¹J (Sn-C)

) 364 Hz, SnCH₃), 126.5 (dd, ¹J ) 161 Hz, ³J ) 7 Hz, C4),

130.4 (dd, ¹J ) 161 Hz, ³J ) 8 Hz, C5), 131.8 (dd, ¹J ) 161

Solven t Dep en d en ce of th e P r od u ct Mixtu r e Obta in ed

fr om 1a w ith Ma gn esiu m . A mixture of 3.16 g (9.47 mmol)

of 1a , 0.262 g (10.8 mmol) of triply sublimed magnesium, and

180 mL of THF was stirred for 1 day at room temperature.

From the mixture, two 10 mL samples were taken and

evaporated to dryness. To each residue anisole or benzene was

added, and both mixtures were subsequently stirred for 1

week. Both mixtures were divided into two 5 mL samples. For

each solvent, one sample was quenched with Me₃GeCl, stirred

3

1

Hz, J ) 25 Hz, J ) 8 Hz, C6), 133.3 (bs, C1), 137.5 (dd, J )

161 Hz, ³J ) 8 Hz, ²J (Sn-C) ) 32 Hz, C3), 146.4 (bs, C2);

GCMS m/z (relative intensity) 305 ([M - CH₃]⁺, 100), 290 (8),

275 (7), 229 (13), 199 (40), 135 (10), 120 (9), 91 (71). Anal. Calcd

for C₉H₁₃BrSn: C, 33.80; H, 4.10. Found: C, 34.07; H, 4.12.

Rea ction of 18 w ith Ma gn esiu m . A mixture of 0.500 g

(1.56 mmol) of 18 and 0.587 g (3.12 mmol) of 1,2-dibromoeth-

ane was added to 0.112 g (4.69 mmol) of triply sublimed

magnesium covered with 10 mL of THF, and the mixture was

stirred for 1.5 h at room temperature. A sample was hydro-

lyzed and analyzed by GLC, which revealed that 18 was

completely consumed. The mixture was quenched with 0.241

g (1.56 mmol) of Me₃GeCl and worked up as usual, yielding

0.430 g of a colorless liquid. The product distribution and yields

were determined by quantitative ¹H NMR analysis. The

products were separated by preparative GLC.

1

for 1 day, and, after standard workup, analyzed by H NMR

spectroscopy, GLC, and GCMS. Product distributions were as

follows: for benzene, 18.3% 6a , 24.8% 7a , 28.6% 4a , and 28.3%

8; for anisole, 19.1% 6a , 24.4% 7a , 28.1% 4a , and 28.4% 8.

The remaining two samples were evaporated to dryness, 10

mL of THF was added, and the mixture was stirred for 1 week

and then quenched with Me₃GeCl. Analysis of the product

distribution revealed no significant differences from those

previously determined.

Rea ction of 1a w ith Ma gn esiu m a t 60 °C. A mixture of

0.194 g (0.582 mmol) of 1a , 0.0320 g (1.32 mmol) of triply

sublimed magnesium, and 10 mL of THF was stirred at 60

°C. After 24 h, the mixture was quenched with Me₃GeCl. After

workup, the product distribution and yields were determined

by ¹H NMR spectroscopy, GLC, and GCMS analysis. Yields:

34% 4a , 28% 6a , 4% 7a and 34% 8.

1-(Trimethylgermyl)-2-(trimethylstannyl)benzene (19), col-

orless liquid, 77% yield: ¹H NMR (CDCl₃, 250 MHz, reference

CHCl₃7.27 ppm) δ 0.32 (s, ²J (Sn-H) ) 55, 53 Hz, SnCH₃),

0.38 (s, 9H, GeCH₃), 7.41 (m, 4H, aryl H) ¹³C NMR (CDCl₃, 63

MHz, reference CDCl₃77.00 ppm) δ -6.3 (q, ¹J ) 128 Hz,

Rea ction of Br om oben zen e w ith Ma gn esiu m in th e

P r esen ce of Ben zyltr im eth ylsta n n a n e (14). A mixture of

0.86 g (5.7 mmol) of bromobenzene, 1.46 g (5.7 mmol) of 14,

0.14 g (5.7 mmol) of triply sublimed magnesium, and 150 mL

of THF was stirred for 26 h at room temperature. A sample of

1

SnCH₃), 0.4 (q, J ) 126 Hz, GeMe₃), 127.7 (dd, J ) 160 Hz,

³J ) 7 Hz, ³J (Sn-C) ) 47 Hz, C4,5), 134.3 (d, ¹J ) 164 Hz,

²J (Sn-C) ) 58 Hz, C3), 137.1 (d, ¹J ) 161 Hz, ³J (Sn-C) ) 43

Hz, C6), 148.4 (bs, C2), 151.0 (bs, C2); GCMS m/z (relative

intensity) 354 (68), 343 (100), 327 (13), 311 (6), 297 (3), 281

(17), 227 (16), 165 (53), 119 (53), 91 (33). Anal. Calcd for C₁₂H₂₂

GeSn: C, 40.31; H, 6.20. Found: C, 40.39; H, 5.99.

-

(46) de Boer, H. J . R.; Akkerman, O. S.; Bickelhaupt, F. Organo-

metallics 1990, 9, 2898.

Carbanions as Intermediates

Organometallics, Vol. 21, No. 10, 2002 2131

1,2-Bis(trimethylstannyl)benzene (20), colorless liquid, 7%

333.9392 ( 0.0007. Anal. Calcd for C₁₀H₁₅BrSn: C, 35.98; H,

4.53. Found: C, 36.02; H, 4.55.

yield:^{47 1}H NMR (CDCl₃, 250 MHz, reference CHCl₃7.27 ppm)

2

δ 0.34 (s, J (Sn-H) ) 53, 51 Hz, 18H, SnCH₃), 7.05-7.75 (m,

Rea ction of 23 w ith Ma gn esiu m in THF . A mixture of

0.3602 g (1.079 mmol) of 23, 0.1 mL of 1,2-dibromoethane, 100

µL of tridecane, and 5 mL of THF was added dropwise, under

an atmosphere of argon, to 0.2180 g (8.97 mmol) of magnesium

covered with 5 mL of diethyl ether over 15 min at room

temperature. The mixture was stirred for 24 h, and then a

sample was hydrolyzed and analyzed by titration, which

revealed that no reaction had occurred. After 1 week, the

mixture was deuteriolyzed and analyzed by GCMS, which

revealed that no reaction had occurred. In an independent

experiment, a mixture of 1.660 g (4.97 mmol) of 23, 0.158 g

(6.58 mmol) of Mg, and 30 mL of THF was stirred in fully

sealed glassware. After 1 day, a sample was deuteriolyzed

(D₂O) and worked up by the addition of diethyl ether, separa-

tion of the organic fraction, and drying of the organic fraction

(MgSO₄); then it was analyzed by GCMS (relative yields of

converted product (4%)).

4H, aryl H); GCMS m/z (relative intensity) 391 (65), 389 (100),

375 (2), 359 (6), 329 (6), 314 (6), 241 (34), 226 (3), 211 (21),

197 (5), 165 (41), 135 (24), 120 (9), 91 (22).

1,2-Bis(trimethylgermyl)benzene (21), 7% yield: ¹H NMR

(CDCl₃, 250 MHz, reference CHCl₃7.27 ppm) δ 0.39 (s, 18H,

GeMe₃), 7.40-7.43 (m, 4H, aryl H); GCMS m/z (relative

intensity) 299 (70), 297 (100), 283 (2), 180 (45), 150 (15), 119

(51), 89 (33).

2-Br om o-1-iod o-4-m eth ylben zen e (24). A solution of 4.55

g (65.9 mmol) of NaNO₂in 40 mL of water was added dropwise

to an ice-cooled solution of 10.67 g (57.4 mmol) of 2-bromo-4-

methylaniline in 40 mL of concentrated hydrochloric acid while

maintaining the temperature below 5 °C. After addition,

stirring was continued for 15 min and then a solution of 33.2

g (200 mmol) of KI in 100 mL of water was added over 15 min.

After it was stirred overnight at room temperature, the black

suspension was extracted with diethyl ether. The combined

organic fractions were washed with 10% NaOH (four times

with 25 mL portions), water, and saturated Na₂S₂O₃(three

times with 30 mL portions), dried (MgSO₄), filtered, and

evaporated to dryness. The residual oil was purified by

fractional distillation in the dark. The product was collected

at 91 °C at 4 mbar to give 8.89 g (29.8 mmol, 52% yield) of a

colorless oil.

2-Deuterio-4-methyl-1-(trimethylstannyl)benzene (32), 94%:

¹H NMR (CDCl₃, 200.1 MHz, reference CHCl₃7.27 ppm) δ 0.40

2

(s, J (Sn-H) ) 55.0, 52.7 Hz, SnCH₃), 2.46 (s, CH₃), 7.30 (bs,

2H, aryl H), 7.49-7.53 (m, 1H, aryl H); GCMS 257 (M^•+

,

C

₁₀H₁₅D¹²⁰Sn, 1), 242 (C₉H₁₂DSn⁺, 100), 212 (C₇H₆DSn⁺, 40),

135 (C₁₀H₁₅⁺, 15), 120 (Sn⁺, 32), 106 (C₈H₈D⁺, 10), 92 (C₇H₆D⁺,

19).

4-Methyl-1,2-bis(trimethylstannyl)benzene (26), 5.9% yield:

GCMS m/z (relative intensity) 405 ([M - CH₃]⁺, C₁₂H₂₁¹²⁰Sn₂,

80), 375 (C₁₀H₁₅Sn₂⁺, 5), 345 (C₈H₉Sn₂⁺, 5), 330 (C₇H₆Sn₂⁺, 6),

255 (C₁₀H₁₅Sn⁺, 26), 225 (C₈H₉Sn⁺, 13), 165 (C₃H₉Sn⁺, 15), 135

(C₁₀H₁₅⁺, 6), 120 (Sn⁺, 5), 105 (C₈H₉⁺, 9). These analyses were

repeated after 5, 11, and 25 days, which showed 18%, 45%,

and quantitative conversion, respectively, of 23. An aliquot was

quenched with Me₃GeCl and, after standard workup, analyzed

24: ¹H NMR (CDCl₃, 200.1 MHz, reference CHCl₃7.27 ppm)

3

4

δ 2.29 (s, 3H, CH₃), 6.82 (ddd, J ) 8.1 Hz, J ) 2.0 Hz, J )

4

5

0.6 Hz, 1H, ArH(5)), 7.47 (dd, J ) 1.0 Hz, J ) 1.0 Hz, 1H,

ArH(3)), 7.62 (d, J ) 8.1 Hz, 1H, ArH(6)); ¹³C NMR (CDCl₃,

3

50.32 MHz, reference CDCl₃77.00 ppm) δ 20.67 (qt, ¹J ) 127.3

Hz, ³J ) 4.4 Hz, CH₃), 96.82 (tq, ³J ) 9.3 Hz, ⁵J ) 1.1 Hz,

3

2

4

C1), 129.28 (ddd, J ) 9.7 Hz, J ) 3.1 Hz, J ) 1.6 Hz, C2),

129.44 (ddq, ¹J ) 160.5 Hz, ³J ) 7.2 Hz, ³J ) 5.0 Hz, C5),

133.25 (ddq, ¹J ) 164.8 Hz, ³J ) 7.4 Hz, ³J ) 2.5 Hz, C3),

1

by H NMR spectroscopy, GLC, and GCMS analysis.

4-Methyl-2-(trimethylgermyl)-1-(trimethylstannyl)ben-

zene (25), 96% yield: ¹H NMR (CDCl₃, 200.1 MHz, reference

CHCl₃7.27 ppm) δ 0.33 (s, ²J (Sn-H) ) 53.3, 50.9 Hz, 9H,

1

2

3

139.73 (dd, J ) 166.4 Hz, J ) 1.9 Hz, C6), 139.76 (dq, J )

8.4 Hz, ²J ) 6.1 Hz, C4); GCMS m/z (relative intensity) 296

(M^•+, C₇H₆⁷⁹BrI, 100), 217 (C₇H₆I⁺, 15), 169 (C₇H₆⁷⁹Br⁺, 9), 127

(I⁺, 7), 90 (C₇H₆⁺, 17).

2

SnCH₃), 0.43 (s, 9H, GeCH₃), 2.34 (s, 3H, CH₃), 7.13 (dd, J )

7.5 Hz, ⁴J ) 1.4 Hz, 1H, ArH(5)), 7.38 (d, ⁴J ) 1.4 Hz, 1H,

ArH(3)), 7.45 (d, 7.5 Hz, 1H, ArH(6)); GCMS m/z (relative

2-Br om o-4-m eth yl-1-(tr im eth ylsta n n yl)ben zen e (23).

To a solution of 5.94 g (20.0 mmol) of 24 in a mixture of 50

mL of diethyl ether and 50 mL of THF at -110 °C was added

from a syringe 12.45 mL of a 1.6 M solution of n-BuLi (19.9

mmol) in hexane. After addition, stirring was continued for 5

min. The mixture turned yellow and was subsequently quenched

with 3.98 g of Me₃SnCl (20.0 mmol, 1 M solution in pentane).

After addition, the mixture was warmed and hydrolyzed with

50 mL of a 1 M NaOH solution. The organic fraction was

separated and the aqueous fraction extracted with diethyl

ether. The extracts were combined, washed (water), dried

(MgSO₄), filtered, and evaporated to dryness. The residual oil

was purified by fractional distillation. The product was col-

lected at 108 °C at 4 mbar to give 3.90 g (11.7 mmol, 58% yield)

of a colorless oil.

intensity) 359 ([M - CH₃]⁺, C₁₂H₂₁⁷⁴Ge¹²⁰Sn⁺, 78), 329 (C₁₀H₁₅

-

GeSn, 5), 299 (5), 241 (C₉H₁₃Sn⁺, 16), 209 (13), 195 (11), 165

(C₃H₉Sn⁺, 28), 150 (6), 135 (23), 119 (45), 105 (26); HRMS calcd

for C₁₂H₂₁¹¹⁸Sn⁷⁰Ge ([M - CH₃]⁺) 352.9904, obsd 352.9893 (

0.0010. Anal. Calcd for

Found: C, 42.21; H, 6.63.

C₁₅H₂₄GeSn: C, 42.02; H, 6.51.

4-Methyl-2-(trimethylstannyl)-1-(trimethylstannyl)ben-

zene (26), 2% yield: GCMS m/z (relative intensity) 405 ([M -

CH₃]⁺, C₁₂H₂₁¹²⁰Sn₂, 80), 375 (C₁₀H₁₅Sn₂⁺, 5), 345 (C₈H₉Sn₂

,

+

5), 330 (C₇H₆Sn₂⁺, 6), 255 (C₁₀H₁₅Sn⁺, 26), 225 (C₈H₉Sn⁺, 13),

165 (C₃H₉Sn⁺, 15), 135 (C₁₀H₁₅⁺, 6), 120 (Sn⁺, 5), 105 (C₈H₉

9).

,

+

4-Methyl-1,2-bis(trimethylgermyl)benzene (27), 2% yield:

GCMS m/z (relative intensity) 328 (M^•+, C₁₃H₂₄⁷⁴Ge₂, 4), 313

(C₁₂H₂₁Ge₂⁺, 80), 297 (C₁₁H₁₇Ge₂⁺, 22), 195 (C₉H₁₃Ge⁺, 28), 119

(C₃H₉Ge⁺, 17), 105 (C₈H₉⁺, 14).

23: ¹H NMR (CDCl₃, 200.1 MHz, reference CHCl₃7.27 ppm)

2

δ 0.43 (s, J (Sn-H) ) 56.1, 53.7 Hz, 9H, ArCH₃), 2.37 (s, 3H,

3

2-Br om o-4-m et h yl-1-(t r im et h ylger m yl)b en zen e (31).

With 2-bromo-4-methylaniline (6.28 g, 21.1 mmol) as starting

material, the procedure was analogous to that described for

23, using Me₃GeCl (3.37 g, 22.0 mmol) instead of Me₃SnCl.

The product was collected at 81 °C at 3 mbar to give 3.91 g

(13.6 mmol, 64% yield) of a colorless oil.

CH₃), 7.09 (d, J ) 7.4 Hz, ArH(6)), 7.23 (d, J ) 7.4 Hz, ArH-

(5)), 7.37 (bs, 1H, ArH(3)); ¹³C NMR (CDCl₃, 50.32 MHz,

reference CDCl₃77.00 ppm) δ -7.99 (q, ¹J ) 128.7 Hz, ¹J (Sn-

1

C) ) 364.2 Hz, 348.0 Hz, SnCH₃), 20.99 (qt, J ) 126.9 Hz, J

) 4.4 Hz, CH₃), 127.32 (d, ¹J ) 159.4 Hz, ³J (Sn-C) ) 40.5

1

3

Hz, C5), 132.27 (d, J ) 162.3 Hz, J (Sn-C) ) 26.7 Hz, C3),

31: ¹H NMR (CDCl₃, 200.1 MHz, reference CHCl₃) δ 0.49

3

1

2

132.94 (d, J ) 7.8 Hz, C2), 137.13 (d, J ) 161.2 Hz, J (Sn-

C) ) 32.0 Hz, C6), 140.49 (bs, C4), 142.06 (bs, C1); GCMS m/z

(relative intensity) 319 ([M - CH₃]⁺, C₉H₁₂⁷⁹Br¹²⁰Sn, 100), 304

(C₈H₉BrSn^•+, 7), 287 (4), 199 (BrSn⁺, 43), 135 (CH₃Sn⁺, 6), 105

(C₈H₉⁺, 37); HRMS calcd for C₁₀H₁₅⁷⁹Br¹¹⁸Sn 333.9378, obsd

3

4

(s, 9H, GeCH₃), 2.31 (s, 3H, CH₃), 7.08 (d, J ) 7.5 Hz, J )

0.8 Hz, 1H, ArH(5)), 7.25 (d, ³J ) 7.5 Hz, 1H, ArH(6)), 7.37 (d,

⁴J ) 1.0 Hz, 1H, ArH(3)); ¹³C NMR (CDCl₃, 50.32 MHz,

1

reference CDCl₃77.00 ppm) δ -0.64 (qt, J ) 126.1 Hz, J )

1

3

1.8 Hz, GeCH₃), 20.81 (qt, J ) 126.9 Hz, J ) 4.5 Hz, CH₃),

127.34 (dqd, ¹J ) 158.6 Hz, ³J ) 5.0 Hz, ³J ) 4.0 Hz, C5),

130.43 (ddd, ³J ) 12.9 Hz, ²J ) 2.9 Hz, ⁴J ) 1.4 Hz, C2), 133.08

(47) (a) Eisch, J . J .; Kotowicz, B. W. Eur. J . Inorg. Chem. 1998, 761.

(b) Seyferth, D.; White, D. L. J . Organomet. Chem. 1972, 34, 119.

2132 Organometallics, Vol. 21, No. 10, 2002

van Klink et al.

(dqd, ¹J ) 163.0 Hz, ³J ) 5.0 Hz, ³J ) 1.2 Hz, C3), 134.92 (dd,

¹J ) 160.1 Hz, ²J ) 1.1 Hz, C6), 139.90 (m, C4), 140.43 (bs,

Rea ction of 34 w ith Ma gn esiu m in THF . A mixture of

2.30 g (5.0 mmol) of 34, 0.146 g (6.00 mmol) of magnesium,

and 50 mL of THF was stirred at room temperature; a white

suspension slowly formed. After 24 h, a sample was deuterio-

lyzed (D₂O) and worked up by addition of diethyl ether,

separation of the organic and aqueous fractions, and drying

of the organic fraction (MgSO₄), after which it was analyzed

by GCMS and ¹H NMR spectroscopy.

2-Deuterio-1-((tri-n-butylstannyl)methyl)benzene (35, 89%

deuterated), 18% yield: ¹H NMR (CDCl₃, 200.1 MHz, reference

CHCl₃7.27 ppm) δ 2.46 (t, ²J (D-H) ) 2.3 Hz, 2H, CH₂); GCMS

m/z (relative intensity) 326 ([M - C₄H₉]⁺, C₁₅H₂₄D¹²⁰Sn⁺, 38),

291 (C₁₂H₂₇Sn⁺, 29), 270 (C₁₁H₁₆DSn⁺, 16), 235 (C₉H₇Sn⁺, 47),

212 (C₇H₆DSn⁺, 48), 179 (100), 177 (C₄H₉Sn⁺, 97), 121 (57),

92 (36).

C1); GCMS m/z (relative intensity) 288 (M^•+, C₁₀H₁₅Br⁷⁴Ge,

3), 273 (C₉H₁₂BrGe, 55), 153 (14), 119 (C₃H₉Ge⁺, 3), 105 (C₈H₉

,

+

100), 89 (11); HRMS calcd for C₁₀H₁₅⁷⁰Ge⁷⁹Br 283.9600, obsd

283.960 ( 0.001. Anal. Calcd for C₁₀H₁₅GeBr: C, 41.74; H,

5.26; Br, 27.8. Found: C, 41.82; H, 5.33; Br, 27.4.

Rea ction of 31 w ith Ma gn esiu m . A mixture of 2.44 g

(8.47 mmol) of 31, 0.36 g (15 mmol) of magnesium, and 50 mL

of THF was stirred for 2 weeks at room temperature. A sample

was deuteriolyzed (D₂O) and analyzed by ¹H NMR spectros-

copy and GCMS. 2-Deuterio-4-methyl-1-(trimethylgermyl)-

benzene (32), 100% yield: ¹H NMR (CDCl₃, 200.1 MHz,

reference CHCl₃7.27 ppm) δ 0.54 (s, 9H, GeCH₃), 2.51 (s, 3H,

3

CH₃), 7.32-7.35 (m, 2H, ArH(3,5)), 7.55 (d, J ) 8.0 Hz, 1H,

ArH(6)); GCMS m/z (relative intensity) 211 (M^•+, C₁₀H₁₅D⁷⁴Ge,

5), 196 (C₉H₁₂DGe⁺, 100), 166 (C₇H₆DGe, 16), 106 (C₈H₈D⁺,

6), 92 (C₈H₆D⁺, 9).

1-(Deuteriomethyl)-2-(tri-n-butylstannyl)benzene (36), 82%

yield: ¹H NMR (CDCl₃, 200.1 MHz, reference CHCl₃7.27 ppm)

δ 2.38 (s, 2H, CH₂); GCMS m/z (relative intensity) 326 ([M -

C₄H₉]⁺, C₁₅H₂₄D¹²⁰Sn⁺, 57). 291 (C₁₂H₂₇Sn⁺, 5), 270 (C₁₁H₁₆

-

A second sample was derivatized with Me₃SnCl and stirred

for 1 week at room temperature. After the standard workup,

the residue was characterized by ¹H NMR and GCMS as pure

28.

DSn⁺, 37), 235 (C₉H₇Sn⁺, 6), 212 (C₇H₆DSn⁺, 100), 177 (C₄H₉-

Sn⁺, 12), 120 (Sn⁺, 27), 92 (C₇H₆D⁺, 28). Comparison with an

authentic sample (vide infra) of 1-methyl-2-(tri-n-butylstan-

nyl)benzene revealed a deuterium incorporation of 96%.

Another 10 mL of the reaction mixture was derivatized with

1.5 mmol of Me₃GeCl and after standard workup was analyzed

28: ¹H NMR (CDCl₃, 200.1 MHz, reference CHCl₃7.27 ppm)

2

δ 0.34 (s, J (Sn-H) ) 53.2 Hz, 51.0 Hz, 9H, SnCH₃), 0.42 (s,

3

9H, GeCH₃), 2.34 (s, 3H, CH₃), 7.13 (d, J ) 7.5 Hz, 1H, ArH-

1

3

(6)), 7.37 (bs, 1H, ArH(3)), 7.46 (d, J ) 7.6 Hz, ArH(5)); ¹³C

by H NMR spectroscopy and GCMS. 8 was obtained in 10%

yield.

NMR (CDCl₃, 50.32 MHz, reference CDCl₃77.00 ppm) δ -6.17

(qt, ¹J ) 128.6 Hz, J ) 1.8 Hz, ¹J (Sn-C) ) 341.8 Hz, 326.6

Hz, SnCH₃), 0.44 (qt, ¹J ) 125.4 Hz, J ) 1.9 Hz, GeCH₃), 21.30

2-((Tri-n-butylstannyl)methyl)-1-(trimethylgermyl)ben-

zene (37), 16% yield: ¹H NMR (CDCl₃, 200.1 MHz, reference

CHCl₃7.27 ppm) δ 0.43 (s, 9H, GeCH₃), 2.37 (s, 2H, CH₂);

GCMS m/z (relative intensity) 500 (M^•+, C₂₂H₄₂⁷⁴Ge¹²⁰Sn, 1),

443 (C₁₈H₃₃⁷⁴Ge¹²⁰Sn⁺, 2), 291 (C₁₂H₂₇Sn⁺, 100), 249 (13), 235

(C₈H₁₉Sn⁺, 65), 207 (15), 193 (16), 177 (C₄H₉Sn⁺, 68), 135 (21),

119 (C₃H₉Sn⁺, 32), 105 (9), 91 (7).

1

(qt, J ) 126.3 Hz, J ) 4.4 Hz, CH₃), 128.40 (ddt, J ) 157.8

3

4

Hz, J ) 5.5 Hz, J ) 4.9 Hz, J (Sn-C) ) 12.4 Hz, C4), 134.08

1

3

1

(d, J ) 157.8 Hz, J (Sn-C) ) 61.4 Hz, C6), 137.02 (qq, J )

1

3

6.4 Hz, J ) 6.4 Hz, C4), 137.83 (ddq, J ) 154.9 Hz, J ) 5.0

Hz, ³J ) 5.0 Hz, ²J (Sn-C) ) 21.4 Hz, C3), 146.97 (bs, C1),

148.06 (bs, C2); GCMS m/z (relative intensity) 359 ([M -

CH₃]⁺, C₁₂H₂₁⁷⁴Ge¹²⁰Sn, 79), 329 (C₁₀H₁₅GeSn⁺, 6), 297 (C₈H₇-

GeSn⁺, 7), 284 (C₇H₆GeSn⁺, 4), 241 (12), 165 (8), 135 (5), 119

(7), 105 (3).

1-(Tri-n-butylstannyl)-2-((trimethylgermyl)methyl)ben-

zene (38), 64% yield: ¹H NMR (CDCl₃, 200.1 MHz, reference

4

CHCl₃7.27 ppm) δ 0.14 (s, 9H, GeCH₃), 2.28 (s, J (Sn-H) )

5.7 Hz, CH₂); GCMS m/z (relative intensity) 443 ([M - C₄H₉]⁺,

C

₁₈H₃₃⁷⁴Ge¹²⁰Sn, 38), 249 (MeSnBu₂⁺, 100), 207 (35), 193 (49),

1-Br om o-2-((tr i-n -bu tylsta n n yl)m eth yl)ben zen e (34). A

solution of 10.00 g (40.0 mmol) of 1-bromo-2-(bromomethyl)-

benzene in 250 mL of diethyl ether was added dropwise under

nitrogen to 0.973 g (40.0 mmol) of doubly sublimed magnesium

covered with 50 mL of diethyl ether over 6 h at room

temperature. After the addition was complete, stirring was

continued for 1 h. The yellow solution was carefully siphoned

from the residual magnesium into another reaction flask; then

it was cooled to 0 °C followed by the addition of 13 g (40 mmol)

of (n-Bu)₃SnCl. A white precipitate slowly formed. The mixture

was stirred for 2 days, and then 20 mL of saturated NH₄Cl

was added. The organic fraction was separated and the

aqueous fraction extracted with diethyl ether. The extracts

were combined, washed (water), dried (MgSO₄), filtered, and

evaporated to dryness. The residual oil was purified by

fractional distillation. The product was collected at 78 °C at 3

mbar to give 7.24 g (15.7 mmol, 39% yield) of a colorless oil.

34: ¹H NMR (CDCl₃, 200.1 MHz, reference CHCl₃7.27 ppm)

δ 0.83-0.95 (m, 15H), 1.19-1.33 (m, 6H, CH₂CH₃), 1.37-1.54

(m, 6H, SnCH₂CH₂), 2.48 (s, ²J (Sn-H) ) 55.8 Hz, 2H, ArCH₂),

6.81-6.91 (m, 1H, ArH(5)), 7.07-7.14 (m, 2H, aryl H), 7.45-

7.49 (m, 1H, ArH(6)); ¹³C NMR (CDCl₃, 50.32 MHz, reference

CDCl₃77.00 ppm) δ 10.04 (t, ¹J ) 126.5 Hz, SnCH₂), 13.56

177 (8), 135 (27), 119 (25), 105 (4), 91 (6).

1-(Tri-n-butylstannyl)-2-((tri-n-butylstannyl)methyl)ben-

zene (39), 10% yield: ¹H NMR (CDCl₃, 200.1 MHz, reference

CHCl₃7.27 ppm) δ 2.44 (s, 2H, CH₂); GCMS m/z (relative

intensity) 361 (5), 324 (12), 291 (100), 268 (13), 235 (46), 211

(33), 177 (52), 121 (21). Complete assignment of the other

signals was not possible, due to the complexity of the spectrum.

1-Meth yl-2-(tr i-n -bu tylsta n n yl)ben zen e. For comparison

with 36 (vide supra), a solution of 1.71 g (10.0 mmol) of

2-bromotoluene in 50 mL of THF was added dropwise under

nitrogen to 0.24 g (10 mmol) of magnesium covered with 10

mL of THF. After the addition was completed, stirring was

continued for 1 h and then the mixture was quenched with

3.25 g (10.0 mmol) of (n-Bu)₃SnCl. 1-Methyl-2-(tri-n-butyl-

stannyl)benzene: GCMS m/z (relative intensity) 325 ([M -

C₄H₉]⁺, C₁₅H₂₅¹²⁰Sn, 81), 269 (C₁₅H₂₆Sn⁺, 46), 211 (C₇H₇Sn⁺,

100), 120 (Sn⁺, 24), 91 (C₇H₇⁺, 22).

2,2′-Dibr om obip h en yl.²⁷To a solution of 47.08 g (0.200

mol) of o-dibromobenzene in 400 mL of THF was added

dropwise, under an atmosphere of nitrogen, 64 mL of a 1.6 M

solution of n-BuLi (0.10 mmol) in n-hexane while the temper-

ature was maintained below -65 °C. After addition, the

mixture was warmed to 0 °C and subsequently hydrolyzed with

100 mL of a 1 M HCl solution. The organic solvents were

removed by rotary evaporation, and the residue was extracted

with diethyl ether. The combined organic layers were washed

(brine), dried (MgSO₄), filtered, and evaporated to dryness. The

brown residue was crystallized from EtOH, yielding 22.72 g

(72.8 mmol, 73% yield, lit.²⁷74%) of a white solid (mp 78.8-

1

2

1

(qt, J ) 138.4 Hz, J ) 6.0 Hz, CH₃), 20.08 (t, J ) 131.1 Hz,

ArCH₂), 27.20 (m, ³J (Sn-C) ) 54.6 Hz, CH₂CH₃), 28.85 (m,

²J (Sn-C) ) 20.4 Hz, SnCH₂CH₂), 122.41 (bs, C1), 124.30 (dd,

1

¹J ) 162 Hz, C5), 127.10 (dd, J ) 162 Hz, C4), 128.29 (dt, J

3

1

) 163.2 Hz, J ) 3.7 Hz, C3), 132. 23 (dd, J ) 163 Hz, C6),

143.86 (bs, C2); ¹¹⁹Sn NMR (CHCl₃, 149.21 MHz, reference Me₄-

Sn 0.000 ppm) δ -8.254. GCMS m/z (relative intensity) 403

([M - C₄H₉]⁺, C₁₅H₂₄⁷⁹Br¹²⁰Sn, 93), 347 (C₁₁H₁₆BrSn⁺, 12), 291

(C₇H₈BrSn⁺, 38), 235 (C₉H₇Sn₊, 55), 199 (BrSn⁺, 58), 179 (100),

177 (C₄H₉Sn⁺, 98), 120 (Sn⁺, 24), 91 (C₇H₇⁺, 21).

79.2 °C, lit.²⁷80-81 °C) which was characterized by H NMR

1

spectroscopy and GCMS as pure 2,2′-dibromobiphenyl: ¹H

NMR (CDCl₃, 200.1 MHz, reference CHCl₃7.27 ppm) δ 7.23-

Carbanions as Intermediates

Organometallics, Vol. 21, No. 10, 2002 2133

7.44 (m, 6H), 7.66-7.71 (m, 2H); GCMS m/z (relative intensity)

filtered, and evaporated to dryness. The product distribution

and yields were determined by ¹H NMR spectroscopy, GLC,

and GCMS.

310 (M^•+, C₁₂H₈⁷⁹Br₂, 25), 231 (C₁₂H₈Br⁺, 64), 152, (C₁₂H₈

100).

,

+

2,2′-Dideuteriobiphenyl, 10% yield: ¹H NMR (CDCl₃, 200.1

MHz, reference CHCl₃7.27 ppm) δ 7.07-7.40 (m, 6H, aryl H),

7.49-7.60 (m, 2H, aryl H); GCMS m/z (relative intensity) 156

(M^•+, C₁₂H₈D₂, 100), 77 (10).

2-Br om o-2′-(tr im eth ylsta n n yl)bip h en yl (40). To a solu-

tion of 6.31 g (20.2 mmol) of 2,2′-dibromobiphenyl in 125 mL

of THF was added dropwise, under an atmosphere of argon,

12.5 mL of a 1.6 M solution of n-BuLi (20.0 mmol) in n-hexane

while the temperature was maintained below -65 °C. After

addition, the mixture was stirred for 5 min and subsequently

quenched with 3.98 g of Me₃SnCl (20.0 mmol, 1 M solution in

pentane). The mixture was warmed to room temperature, and

a saturated solution of NH₄Cl in water was added. Subse-

quently, the organic solvent was removed by distillation and

the residue was extracted with diethyl ether. The combined

organic layers were washed (brine), dried (MgSO₄), filtered,

and evaporated to dryness. The residue was recrystallized from

EtOH, yielding 6.17 g (15.6 mmol, 77% yield) of a white solid

(mp 45.0-45.6 °C), which was characterized by ¹H NMR

spectroscopy and GCMS as pure 40.

2-Deuterio-2′-(trimethylstannyl)biphenyl, 12% yield: ¹H

NMR (CDCl₃, 200.1 MHz, reference CHCl₃7.27 ppm) δ 0.00

2

(s, J (Sn-H) ) 54.8, 52.4 Hz, 9H, SnCH₃), 7.34-7.40 (m, 6H,

aryl H), 7.57-7.62 (m, 1H, ArH(3′)); GCMS m/z (relative

intensity) 304 ([M - CH₃]⁺, C₁₄H₁₄D¹²⁰Sn⁺, 100), 274 (C₁₂H₉-

DSn⁺, 35), 166 (C₃H₈DSn⁺, 4), 153 (C₁₂H₇D⁺, 13), 120 (Sn⁺,

11).

9,9-Dimethyl-9-stannafluorene (43), 78% yield: ¹H NMR

(CDCl₃, 400 MHz, reference CHCl₃7.27 ppm) δ 0.557 (s,

²J (Sn-H) ) 60.63, 54.93 Hz, 6H, SnCH₃), 7.309 (td, ³J ) 7.25

Hz, ³J ) 7.00 Hz, ⁴J ) 1.10 Hz, ⁴J (Sn-H) ) 13.56 Hz, 2H,

3

4

ArH(2)), 7.426 (td, J ) 7.47 Hz, J ) 7.25 Hz, J ) 1.46 Hz,

⁵J (Sn-H) ) 6.17 Hz, 2H, ArH(3)), 7.694 (ddd, J ) 7.00 Hz,

3

40: ¹H NMR (C₆D₆, 400.1 MHz, reference C₆D₅H 7.17 ppm;

the chemical shifts and coupling constants were determined

by simulation using the PANIC program,⁴⁸a version of the

⁴J ) 1.46 Hz, J ) 0.63 Hz, J (Sn-H) ) 38.45 Hz, 37.08 Hz,

5

3

4

5

2H, ArH(1)), 7.976 (dt, J ) 7.37 Hz, J ) 1.10 Hz, J ) 0.63

Hz, ⁴J (Sn-H) ) 11.1 Hz, 2H, ArH(4); ¹³C NMR (CDCl₃, 50.323

MHz, reference CDCl₃77.00 ppm) δ -8.59 (qq, ¹J ) 131.3 Hz,

³J ) 1.0 Hz, ²J (Sn-C) ) 362 Hz, 346 Hz, SnCH₃), 122.42 (ddt,

2

LAOCOON type of programs)⁴⁹δ 0.043 (s, J (Sn-H) ) 54.74,

52.38 Hz, 9H, SnCH₃), 6.717 (ddd, ³J ) 8.07 Hz, ³J ) 7.47 Hz,

3

⁴J ) 1.80 Hz, 1H, ArH(4)), 6.88 (td, ³J ) 7.55 Hz, J ) 7.47

¹J ) 155.2 Hz, J ) 7.3 Hz, J ) 1.7 Hz, J (Sn-C) ) 38.9 Hz,

3

4

3

4

Hz, J ) 1.24 Hz, 1H, ArH(5)), 7.047 (ddd, J ) 7.55 Hz, J )

1.80 Hz, ⁵J ) 0.30 Hz, 1H, ArH(6)), 7.211 (td, ³J ) 7.39 Hz, ³J

) 7.36 Hz, ⁴J ) 1.32 Hz, 1H, ArH(5′)), 7.211 (td, ³J ) 7.57 Hz,

1

3

C4), 127.43 (dd, J ) 161.4 Hz, J ) 7.1 Hz, J (Sn-C) ) 42.3

1

3

Hz, C1), 129.10 (dd, J ) 158.6 Hz, J ) 7.4 Hz, C3), 136.24

(dd, ¹J ) 161.8 Hz, ³J ) 7.5 Hz, ³J (Sn-C) ) 44.3 Hz, C1),

140.81 (bs, C4a), 148.15 (bs, C8a); ¹¹⁹Sn NMR (CHCl₃, 149.21

MHz, reference Me₄Sn 0.000 ppm) δ -30.950 ppm; GCMS m/z

³J ) 7.29 Hz, J ) 0.62 Hz, 1H, ArH(4′)), 7.256 (dt, J ) 7.57

4

3

Hz, ⁴J ) 1.34 Hz, ⁴J ) 1.32 Hz, 1H, ArH(6′)), 7.421 (ddd, ³J )

4

5

8.07 Hz, J ) 1.24 Hz, J ) 0.3 Hz, 1H, ArH(3)), 7.574 (ddd,

³J ) 7.36 Hz, ⁴J ) 1.34 Hz, ⁵J ) 0.62 Hz, 1H, ArH(3′)); ¹³C

NMR (CDCl₃, 100.6 MHz, reference CDCl₃77.00 ppm) δ -8.46

(q, ¹J ) 128.8 Hz, ²J ) 1.1 Hz, ²J (Sn-C) ) 351.9 Hz, 336.2

Hz, C7), 124.01 (bs, J (Sn-C) ) 3.6 Hz, C2), 126.89 (dd, J )

162.6 Hz, J ) 7.8 Hz, C5), 126. 92 (ddt, J ) 162.4 Hz, J )

(relative intensity) 302 (25, M^•+, C₁₄H₁₄¹²⁰Sn) 287 (100, C₁₃H₁₁

-

Sn⁺), 272 (23, C₁₂H₈Sn⁺), 165 (5, C₃H₉Sn⁺), 120 (1, Sn⁺); HRMS

calcd for C₁₄H₁₄¹¹⁸Sn 300.0114, obsd 300.0114 ( 0.0004. Anal.

Calcd for C₁₄H₁₄Sn: C, 55.87; H, 4.69. Found: C, 55.75; H,

4.67.

4

1

2

1

2

1

3

Another sample was quenched with Me₃GeCl and, after

standard workup, analyzed by ¹H NMR spectroscopy and

GCMS. 43 was obtained in 70% yield.

7.0 Hz, J ) 0.9 Hz, C6′), 127.76 (dd, J ) 160.0 Hz, J ) 7.5

1

2

Hz, C5′), 128.90 (dd, J ) 162.5 Hz, J ) 8.9 Hz, C4), 129.17

1

(dt, J ) 167.6 Hz, J ) 4.5 Hz, C4′), 131.41 (ddt, J ) 161.7

Hz, ²J ) 8.3 Hz, J ) 1.3 Hz, C6), 132.52 (dddd, ¹J ) 165.6 Hz,

2-(Trimethylgermyl)-2′-(trimethylstannyl)biphenyl (41), 21%

yield: ¹H NMR (CDCl₃, 200.1 MHz, reference CHCl₃7.27 ppm)

δ -0.03 (s, ²J (Sn-H) ) 54.7, 52.4 Hz, 9H, SnCH₃), 0.08 (s,

9H, GeCH₃), 7.15-7.53 (m, 8H, aryl H); GCMS m/z (relative

³J ) 7.9 Hz, J ) 1.8 Hz, J ) 1.0 Hz, C3), 135.92 (ddd, J )

2

4

1

3

2

157.7 Hz, J ) 4.9 Hz, J ) 0.8 Hz, J (Sn-C) ) 35.8 Hz, 34.2

Hz, C3′), 142.10 (bs, C1), 144.93 (bs, C2), 148.91 (bs, J (Sn-

1

C) ) 29.1 Hz, 27.8 Hz, C2′); ¹¹⁹Sn NMR (CHCl₃, 149.21 MHz,

reference Me₄Sn 0.000 ppm) δ -30.321; GCMS m/z (relative

intensity) 381 ([M - CH₃]⁺, 100) 351 (6, C₁₂H₈⁷⁹Br¹²⁰Sn⁺), 287

(3, C₁₃H₁₁Sn⁺), 229 (5, C₁₂H₆Br⁺), 199 (10) 152 (6, C₁₂H₈⁺), 120

(1, Sn⁺); HRMS calcd for C₁₄H₁₄⁷⁹Br¹¹⁸Sn ([M - CH₃]⁺)

378.9297, obsd 378.929 ( 0.001. Anal. Calcd for C₁₅H₁₇BrSn:

C, 45.44; H, 4.33; Br, 20.6. Found: C, 45.51; H, 4.33; Br, 20.2.

intensity) 419 ([M - CH₃]⁺, C₁₇H₂₃⁷⁴Ge¹²⁰Sn, 51), 287 (C₁₃H₁₁

-

Sn⁺, 100), 241 (16), 165 (C₃H₉Sn⁺, 67), 152 (C₈H₈⁺, 10), 135

(CH₃Sn⁺, 12), 119 (C₃H₉Ge⁺, 18).

2,2′-Bis(trimethylgermyl)biphenyl (42), 9% yield: ¹H NMR

(CDCl₃, 200.1 MHz, reference CHCl₃7.27 ppm) δ 0.09 (s, 18H,

GeCH₃), 7.24-7.81 (m, 8H, aryl H); GCMS m/z (relative

intensity) 375 ([M - CH₃]⁺, C₁₇H₂₃⁷⁴Ge₂, 10), 241 (C₁₃H₁₁Ge⁺,

100), 226 (C₁₂H₈Ge^•+, 7), 119 (C₃H₉Ge⁺, 46), 89 (6).

Rea ction of 40 w ith Ma gn esiu m in Dieth yl Eth er . A

mixture of 0.262 g (0.662 mmol) of 40, 0.048 g (2.0 mmol) of

doubly sublimed magnesium, and 10 mL of diethyl ether was

stirred for several days at room temperature. A sample was

hydrolyzed and analyzed by titration, which revealed that no

reaction had occurred. Subsequently, the reaction mixture was

stirred for 2 days under reflux. The reaction vessel was opened

and the mixture worked up to give 40 quantitatively.

In an independent experiment, a mixture of 0.0880 g (0.22

mmol) of 40, 0.0186 g (0.78 mmol) of triply sublimed magne-

sium, and 0.6 mL of THF-d₈was stirred in a sealed NMR tube

at room temperature. After 1 h, the contents of the tube were

1

analyzed by H NMR spectroscopy. The following compounds

were observed: 2-(Bromomagnesio)-2′-(trimethylstannyl)bi-

phenyl (12.0%), 43 (40.6%), 2,2′-bis(bromomagnesio)biphenyl

(6.7%), Me₄Sn (1.2%), and MeMgBr (39.4%). These compounds

resulted in the following composition: 20.2% 2-(bromomagne-

sio)-2′-(trimethylstannyl)biphenyl, 11.2% 2,2′-bis(bromomag-

nesio)biphenyl, and 68.5% 43. All signal intensities remained

constant in time, with the exception for the signal assigned to

Me₄Sn, which varied up to 3 times its original height.

Rea ction of 40 w ith Ma gn esiu m in THF . A mixture of

2.020 g (5.1 mmol) of 40, 0.267 g (11 mmol) of doubly sublimed

magnesium, and 50 mL of THF was stirred for 1 day. Within

a few hours the mixture turned green. After settling of the

magnesium dust, the clear yellow solution was decanted into

a second reaction vessel. A sample of the crude Grignard

solution was quenched with D₂O. After hydrolysis, the solvent

was evaporated, the residue was extracted with diethyl ether,

and the organic fractions were combined, dried (MgSO₄),

MeMgBr, 97% yield (with respect to 43): ¹H NMR (THF-

d₈, 200.1 MHz, reference THF-d₇1.75 ppm) δ -1.64 (s, 3H,

CH₃); ¹³C NMR (THF-d₈, 50.32 MHz, reference THF-d₈24.00

ppm) δ -17.80 (s, CH₃).

Me₄Sn: ¹H NMR (THF-d₈, 200.1 MHz, reference THF-d₇

(48) Program PANIC, Bruker program library.

(49) Castellano, S.; Bothner-By, A. A. J . Chem. Phys. 1964, 41, 3863.

2

1.75 ppm) δ 0.09 (s, J (Sn-H) ) 54.4, 52.1 Hz, 12H, SnCH₃);

2134 Organometallics, Vol. 21, No. 10, 2002

van Klink et al.

¹³C NMR (THF-d₈, 50.32 MHz, reference THF-d₈24.00 ppm)

δ -10.84 (s, SnCH₃).

¹J (C-D) ) 17 Hz, C7), 124.9 (C4), 129.1 (C6), 129.7 (C5), 134.3

(C3), 138.4 (C2), 143.5 (C1); GCMS m/z (relative intensity) 165

(18, M^•+), 150 (100), 121 (23).

In another experiment, a mixture of 0.0830 g (0.210 mmol)

of 40, 0.0197 g (0.81 mmol) of triply sublimed magnesium, and

0.6 mL of THF-d₈was stirred in a sealed NMR tube at 5 °C.

The following distribution was established, as derived from

¹H NMR spectroscopy: 22.5% 2-(bromomagnesio)-2′-(trimeth-

ylstannyl)biphenyl, 8.8% 2,2′-bis(bromomagnesio)biphenyl, and

68.7% 43.

Rea ction of 43 w ith MeMgBr . To a solution of 0.301 g

(1.0 mmol) of 43 in 15 mL of THF was added 3.3 mL of a 3 M

solution of MeMgBr (10 mmol) in diethyl ether. The mixture

was heated under reflux for 4 h and, after it was cooled to

room temperature, deuteriolyzed (D₂O). The organic solvents

were removed by rotary evaporation, and the residue was

extracted with diethyl ether. The organic layers were com-

bined, washed (brine), dried (MgSO₄), filtered, and evaporated

to dryness. The white solid residue was characterized by ¹H

NMR spectroscopy and GCMS as pure 43.

1-Br om o-2-((tr im eth ylsilyl)m eth yl)ben zen e (1b).⁵⁰The

synthesis of 1b was performed by the procedure described by

Brandsma et al.⁵¹A solution of 41.3 mmol of 2-bromotoluene

(7.1 g) and 50.0 mmol of chlorotrimethylsilane (5.4 g) in 20

mL of THF was added to a solution of 41.3 mmol of LDA in 40

mL of THF/n-hexane (ratio 1/1) over 20 min at -100 °C. After

the addition was complete, the mixture was stirred for another

20 min at -100 °C; then it was warmed to room temperature

within 1 h. Meanwhile, the mixture became yellow while a

white precipitate formed. Stirring was continued at room

temperature for 24 h, and then 25 mL of water was added.

The organic fraction was separated, and the aqueous fraction

was extracted three times with diethyl ether. The organic

fractions were combined, washed with 2 M HCl and 1 M

NaHCO₃and finally with water until neutral. After the organic

fraction had been dried (MgSO₄), the solvents were evaporated

and the residue subjected to fractional distillation. The product

was collected at 90 °C at 9 mbar to give 4.62 g (19.0 mmol,

46% yield) of a colorless oil.⁵⁰

1b: ¹H NMR (CDCl₃, 200.1 MHz, reference CHCl₃7.27 ppm)

δ -0.01 (s, ²J (Si-H) ) 6 Hz, 9H, CH₃), 2.22 (s, 2H, CH₂), 6.70-

7.10 (m, 3H, arom), 7.32-7.50 (m, 1H, aryl H); GCMS m/z

(relative intensity) 242 (19, M^•+), 227 (15), 169 (2), 145 (7), 73

(100).

Rea ction of 1b w ith Ma gn esiu m . A mixture of 4.8 mmol

(1.6 g) of 1b, 0.58 g of triply sublimed magnesium (24 mmol),

and 60 mL of THF was stirred for 24 h at room temperature.

A sample of the colorless mixture was hydrolyzed and analyzed

by titration, which revealed the formation of 4.8 mmol of base

and 4.8 mmol of magnesium. The reaction mixture was

quenched with 2 mL of D₂O and worked up as usual, yielding

a colorless liquid. The product distribution was determined

by GCMS, and the products were separated by preparative

GLC.

2-Deuterio-1-((trimethylsilyl)methyl)benzene (50), 22% yield:

¹H NMR (CDCl₃, 200.1 MHz, reference CHCl₃7.27 ppm) δ 0.01

(s, 9H, ²J (Si-H) ) 6 Hz, CH₃), 1.99 (s, 2H, CH₂), 7.10 (m, 4H,

aryl H); ¹³C NMR (CDCl₃, 50.32 MHz, reference CDCl₃77.00

ppm) δ -1.9 (C8), 27.1 (C7), 123.8 (C5), 128.0 (C3), 128.1 (C4,

C6), 140.5 (C2) (the signal of C1 was not observed due to the

long relaxation time of C1); GCMS m/z (relative intensity) 165

(21, M^•+), 150 (11), 73 (100).

2-Br om o-2′-(tr im eth ylsilyl)bip h en yl (57). To a solution

of 9.36 g (30.0 mmol) of 2,2′-dibromobiphenyl²⁷(vide supra)

in THF at -70 °C was added dropwise, under an atmosphere

of argon, 19 mL of a 1.6 M solution of n-BuLi (30.4 mmol) in

hexane. After addition, the mixture was stirred for 15 min and

3.26 g (30.0 mmol) of Me₃SiCl was added dropwise. The

mixture was warmed to room temperature, a saturated solu-

tion of NH₄Cl in water added, and the mixture extracted with

diethyl ether. The organic fractions were combined, washed

(brine), dried (MgSO₄), filtered, and evaporated to dryness. The

residual oil was purified by fractional distillation. The product

was collected at 98 °C at 4 mbar to give 6.78 g (22.2 mmol,

74% yield) of a colorless oil.

57: ¹H NMR (CDCl₃, 200.1 MHz, reference CHCl₃7.27 ppm)

δ 0.03 (s, 9H, SiCH₃), 7.11-7.42 (m, 6H, aryl H), 7.59-7.66

(m, 2H, aryl H); ¹³C NMR (CDCl₃, 50.32 MHz, reference CDCl₃

77.00 ppm) δ 0.05 (SiCH₃), 124.36 (C2), 126.47 (C6′), 126.85

(C4′), 128.28 (C6), 128.92 (C4), 129.66 (C5), 131.60 (C5), 132.34

(C3), 134.54 (C3′), 139.10 (C2′), 144.32 (C1), 147.42 (C1′);

GCMS m/z (relative intensity) 304 (M^•+, C₁₅H₁₇⁷⁹BrSi, 1), 289

(C₁₄H₁₄BrSi⁺, 100), 273 (C₁₃H₁₁BrSi⁺, 10), 210 (41), 195 (62),

181 (11), 165 (30), 152 (13).

Rea ction of 57 w ith Ma gn esiu m . A mixture of 1.53 g

(5.00 mmol) of 57, 0.266 g (10.9 mmol) of sublimed magnesium,

and 50 mL of THF was stirred for 1 day. After hydrolysis, the

solvent was evaporated, the residue was extracted with diethyl

ether, and the organic fractions were combined, dried (MgSO₄),

filtered, and evaporated to dryness. The resulting residue was

crystallized from EtOH, yielding 0.96 g (4.55 mmol, 91% yield)

of a white solid which was characterized by ¹H NMR spec-

troscopy and GCMS as pure 58.

9,9-Dimethyl-9-silafluorene (58): mp 52.8-53.7 °C, lit.^30amp

56-57 °C; ¹H NMR (CDCl₃, 200.1 MHz, reference CHCl₃7.27

2

3

ppm) δ 0.44 (s, J (Si-H) ) 3.4 Hz, 6H, SiCH₃), 7.30 (td, J )

4

3

4

6.7 Hz, J ) 1.0 Hz, 2H, ArH(3)), 7.46 (td, J ) 7.5 Hz, J )

1.4 Hz, 2H, ArH(2)), 7.65 (d, ³J ) 6.4 Hz, 2H, ArH(1)), 7.85 (d,

³J ) 7.7 Hz, 2H, ArH(4)); ¹³C NMR (CDCl₃, 50.32 MHz,

reference CDCl₃77.00 ppm) δ -3.28 (qq, J ) 121.0 Hz, J )

2.0 Hz, J (Si-C) ) 52.3 Hz, SiCH₃), 120.78 (C6), 127.31 (C4),

130.13 (C5), 132.68 (d, J ) 159.6 Hz, C3), 138.87 (bs, J (Si-

C) ) 66.5 Hz, C2), 147.75 (m, C1); GCMS m/z (relative

intensity) 210 (M^•+, C₁₄H₁₄Si, 43), 195 (C₁₃H₁₁Si⁺, 100), 179

(4), 165 (19).

1

3

1

Rea ction of 1a w ith Sm I₂. HMPA (0.4 mL) was added,

under an atmosphere of nitrogen, to 10 mL of a commercially

available 0.1 M solution of SmI₂(1 mmol) in THF. After the

mixture was stirred for 10 min, 0.100 g (0.30 mmol) of 1 was

added; within 45 min the color of the mixture changed from

purple to yellow. The mixture was quenched with saturated

aqueous NaHCO₃, and the aqueous layer was extracted with

diethyl ether. The combined organic layers were washed

(water), dried (MgSO₄), filtered, and evaporated to dryness.

The residual oil was characterized by ¹H NMR spectroscopy

and GCMS as pure 14.

La r ge-Sca le Rea ction of 1a w ith Sm I₂. A mixture of

1.504 g (10.0 mmol) of Sm, 2.410 g (9 mmol) of freshly distilled

CH₂I₂, and 100 mL of THF was stirred at room temperature

for 2 h. The mixture turned gradually from yellow (SmI₃) to

blue-green. HMPA (11 mL, 64 mmol) was added, and the

resulting purple solution was stirred for 10 min. To the

mixture was added 1.075 g (3.22 mmol) of 1a ; the mixture

turned brown within 30 min. It was quenched with saturated

aqueous NaHCO₃, and the aqueous layer was extracted with

diethyl ether. The organic layers were combined, dried (Mg-

SO₄), filtered, and evaporated to dryness. Final purification

involved filtration through a column of Florisil using pentane

as eluent to remove residual HMPA. After evaporation of the

1-(Deuteriomethyl)-2-(trimethylsilyl)benzene (51), 67% yield:

¹H NMR (CDCl₃, 200.1 MHz, reference CHCl₃7.27 ppm) δ 0.28

2

(s, 9H, J (Si-H) ) 6 Hz, CH₃), 2.41 (bs, 2H, CH₂), 7.00-7.30

(m, 3H, aryl H), 7.35-7.48 (m, 1H, aryl H); ¹³C NMR (CDCl₃,

50.32 MHz, reference CDCl₃77.00 ppm) δ -0.1 (C8), 22.7 (t,

(50) Bott, R. W.; Eaborn, C.; Rushton, B. M. J . Organomet. Chem.

1965, 3, 448.

(51) Andrigan, H.; Heus-Koos, Y.; Brandsma, L. J . Organomet.

Chem. 1987, 336, C41.

Carbanions as Intermediates

Organometallics, Vol. 21, No. 10, 2002 2135

Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t

Deta ils for 43

solvent, 0.76 g of a colorless oil was isolated, which was

characterized by quantitative ¹H NMR spectroscopy and

GCMS.

Crystal Data

14, 90% yield: ¹H NMR (CDCl₃, 200.1 MHz, reference CHCl₃

formula

fw

C₁₄H₁₄Sn

300.96

2

7.27 ppm) δ 0.05 (s, J (Sn-H) ) 54, 52 Hz, 9H, SnCH₃), 2.32

(s, ²J (Sn-H) ) 63, 61 Hz, 2H, CH₂), 6.91-7.32 (m, 5H, aryl

H); GCMS m/z (relative intensity) 256 (M^•+, C₁₀H₁₆¹²⁰Sn, 14),

241 (C₉H₁₃Sn⁺, 17), 211 (11), 165 (100), 135 (34), 120 (18), 91

(42); identical with an authentic sample of 14.

cryst syst

space group

a (Å)

b (Å)

c (Å)

monoclinic

P2₁/c (No. 14)

17.020(2)

8.5876(8)

16.5142(12)

90

91.528(7)

90

2412.9(4)

8

60, 10% yield: ¹H NMR (CDCl₃, 200.1 MHz, reference CHCl₃

R (deg)

2

7.27 ppm) δ 0.30 (s, J (Sn-H) ) 54, 52 Hz, 9H, SnCH₃), 2.40

â (deg)

(s, ⁴J (Sn-H) ) 6 Hz, 3H, CH₃), 7.03-7.53 (m, 4H, aryl H);

GCMS m/z (relative intensity) 256 (M^•+, C₁₀H₁₆¹²⁰Sn, 2), 241

(C₉H₁₃Sn⁺, 100), 211 (29), 165 (2), 135 (9), 120 (10), 91 (8);

identical with an authentic sample of 60.²³

γ (deg)

V (Å³)

Z

D(calcd) (g/cm³)

µ(Mo KR) (mm^-1

F(000)

1.657

2.080

1184

)

Rea ction of 1a w ith Sm I₂in th e P r esen ce of t-Bu OD.

With 1a (0.963 g, 2.88 mmol) as starting material, the

procedure was analogous to that described above, but before

addition of 1a , 5 g (67 mmol) of t-BuOD was added to the

mixture. After addition of 1a , the mixture turned brown within

3 h. After standard workup (vide supra), product distribution

and yields were determined by ¹H NMR spectroscopy and

GCMS.

1a was obtained in 74% yield and 14 in 19% yield. [D]60

(78% deuterium incorporation), 6% yield: GCMS m/z (relative

intensity) 242 (C₉H₁₃D¹²⁰Sn⁺, 100), 212 (33), 135 (75), 120 (29),

116 (11), 92 (28).

crystal size (mm)

0.12 e 0.12 e 0.63

Data Collection

temp (K)

150

radiation (Å)

min, max θ (deg)

data set

Mo KR, 0.710 73

1.2, 27.5

-22 to +14; -11 to 0; -21 to +21

total, unique no. of data; R(int) 6805, 5541; 0.038

no. of obsd data (I > 2.0σ(I))

4077

Refinement

N_ref, N_par

5502, 275

0.0591, 0.1642, 1.13

0.00, 0.00

R1, wR2,^a

S

Rea ction of 1a w ith Sm I₂in th e P r esen ce of CH₃OD.

With 1a (0.918 g, 2.75 mmol) as starting material, the

procedure was analogous to that described above, but before

addition of 1a , 1 g (30 mmol) of CH₃OD was added to the

reaction mixture. After addition of 1a , the mixture turned

brown within 2 h. After standard workup (vide supra), product

max, av shift/error

min, max resd dens (e/Å³)

-0.90, 1.09

w^-1) σ²(F_o²) + (0.0349P)²+ 29.13P.

a

Cr ysta l Str u ctu r e Deter m in a tion of 43. Crystals of the

title compound are colorless. The crystal used for data collec-

tion was cut from a larger specimen and transferred in the

cold dinitrogen stream of a Nonius CAD4T diffractometer on

a rotating anode. The structure was solved by Patterson

techniques (DIRDIF⁵²) and refined on F²with SHELXL93.⁵³

Absorption correction was done with the PLATON/DELABS

procedure.⁵⁴Numerical details have been collected in Table

1. Crystallographic data (excluding structure factors) have

been deposited with the Cambridge Data Centre as Supple-

mentary Publication No. CCDC 114259. Copies of the data can

be obtained free of charge on application to the CCDC, 12

Union Road, Cambridge CB21EZ, U.K. (fax, (+44) 1223-336-

033; e-mail, deposit@ccdc.cam.ac.uk).

1

distribution and yields were determined by H NMR spectros-

copy and GCMS.

1a was obtained in 67% yield, 14 in 24% yield, and [D]60

(74% deuterium incorporation) in 9% yield.

Rea ction of 1a w ith Sm I₂in th e P r esen ce of CD₃OH.

With 1a (0.600 g, 1.8 mmol) as starting material, the procedure

was analogous to that described above, but before addition of

1a , 1 g (29 mmol) of CD₃OH was added to the reaction mixture.

After addition of 1a , the mixture turned brown within 4 h.

After standard workup (vide supra), product distribution and

1

yields were determined by H NMR spectroscopy and GCMS.

1a was obtained in 78% yield, [D]14 (56% deuterium

incorporation in 18% yield, and 60 in 4% yield.

Rea ction of 14 w ith Sm I₂. A mixture of 0.938 g (6.24

mmol) of Sm, 1.602 g (5.98 mmol) of freshly distilled CH₂I₂,

and 50 mL of THF was stirred at room temperature for 2 h.

HMPA (5 mL, 29 mmol) was added, and the resulting purple

solution was stirred for 10 min. To the mixture was added

0.512 g (2.01 mmol) of 14; then the mixture was stirred for 2

h. The mixture was quenched, and after workup, 14 was

recovered quantitatively.

Rea ction of Br om oben zen e w ith Sm I₂in th e P r esen ce

of 14. A mixture of 0.906 g (6.02 mmol) of Sm, 1.262 g (4.71

mmol) of freshly distilled CH₂I₂, and 50 mL of THF was stirred

at room temperature for 2 h. HMPA (5 mL, 29 mmol) was

added, and the resulting purple solution was stirred for 10

min. To the mixture were added 0.497 g (1.95 mmol) of 14

and, after 10 min, 0.336 g (2.14 mmol) of bromobenzene. The

mixture turned brown within 1 h. After standard workup (vide

supra), product distribution and yields were determined by

¹H NMR spectroscopy and GCMS.

Ack n ow led gm en t. Dr. B. L. M. van Baar is grate-

fully acknowledged for performing the HRMS measure-

ments and Dr. F. J . J . de Kanter for the NMR experi-

ments.

Su p p or tin g In for m a tion Ava ila ble: Details of the X-ray

crystal structure analysis of compound 43. This material is

available free of charge via the Internet at http://pubs.acs.org.

OM011083A

(52) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;

Garcia-Grande, S.; Gould, R. O.; Smits, J . M. M.; Smykalla, C. The

DIRDIF Program System; Technical Report of the Crystallography

Laboratory; University of Nijmegen, Nijmegen, The Netherlands, 1992.

(53) Sheldrick, G. M. SHELXL93: Program for Crystal Structure

Refinement; University of Go¨ttingen, Go¨ttingen, Germany, 1993.

(54) Spek, A. L. PLATON: A Multipurpose Crystallographic Tool;

Utrecht University, Utrecht, The Netherlands, 2001.

14 was obtained in 78% yield and 16 in 22% yield, identical

with an authentic sample of 16.

Article Doi

Carbanions as intermediates in the formation of Grignard reagents

DOI: 10.1021/om011083a

Source and publish data:

Authors:

Article abstract of DOI:10.1021/om011083a

Full text of DOI:10.1021/om011083a

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