β-silylcarbenes from isolable diazosilanes

Article abstract of DOI:10.1021/jo0106546

Manganese dioxide oxidation of the hydrazone derivative of tert-butyldimethylsilyl acetophenone gave 2-tert-butyldimethylsilyl-1-phenyldiazoethane (17) an isolable diazocompound. Thermal and Rh(II)-catalyzed decomposition of diazosilane 17 in cyclohexane led to 1-tert-butyldimethylsilyl-1-phenylethylene (19) as the major product. The formation of alkene 19 presumably involves (tertbutyldimethylsilyl)methylphenylcarbene (21), which undergoes preferential 1,2-silyl migration as opposed to 1,2-hydrogen migration. Thermal decomposition of 17 in cyclohexane under oxygen gave substantial amounts of tert-butyldimethylsilyl acetophenone, presumably by reaction of the intermediate carbene with oxygen. Thermal decomposition of 17 in methanol led to alkene 19 and 2-tert-butyldimethylsilyl-1-methoxy-1-phenylethane (22) as major products, along with a significant amount of trans-1-tert-butyldimethylsilyl-2-phenylethylene (20). Kinetic studies indicate that these products are not derived from acid-catalyzed decomposition of the diazocompound 17. Formation of the methyl ether product 22 suggests the involvement of a β-silyl carbocation intermediate, and solvent isotope effect studies indicate that this cation is at least partially derived from protonation of diazocompound 17 by neutral methanol. Photochemical decomposition of 17 in methanol produced the alkene 19 (97%) along with a small amount (2.4%) of the methyl ether 22. Capture of a photochemically generated carbene 21 by methanol is the proposed origin of this minor product. Geometry optimization of trimethylsilylmethylphenylcarbene (8) and carbene 21 at the HF/6-31G* computational level led to a conformation consistent with a hyperconjugative interaction between the vacant p-orbital of these carbenes and the adjacent C-Si bond. Carbenes 8 and 21 are not energy minima at the B3LYP/6-31G* level, where they rearrange to alkenes without barrier via silyl migration. These theoretical findings contrast with the proposed trapping of carbene 21 by methanol and oxygen.

Full text of DOI:10.1021/jo0106546

112
J . Org. Chem. 2002, 67, 112-118
â-Silylca r ben es fr om Isola ble Dia zosila n es
Xavier Creary* and Mark A. Butchko
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556
creary1@nd.edu
Received J une 26, 2001
Manganese dioxide oxidation of the hydrazone derivative of tert-butyldimethylsilyl acetophenone
gave 2-tert-butyldimethylsilyl-1-phenyldiazoethane (17) an isolable diazocompound. Thermal and
Rh(II)-catalyzed decomposition of diazosilane 17 in cyclohexane led to 1-tert-butyldimethylsilyl-1-
phenylethylene (19) as the major product. The formation of alkene 19 presumably involves (tert-
butyldimethylsilyl)methylphenylcarbene (21), which undergoes preferential 1,2-silyl migration as
opposed to 1,2-hydrogen migration. Thermal decomposition of 17 in cyclohexane under oxygen gave
substantial amounts of tert-butyldimethylsilyl acetophenone, presumably by reaction of the
intermediate carbene with oxygen. Thermal decomposition of 17 in methanol led to alkene 19 and
2-tert-butyldimethylsilyl-1-methoxy-1-phenylethane (22) as major products, along with a significant
amount of trans-1-tert-butyldimethylsilyl-2-phenylethylene (20). Kinetic studies indicate that these
products are not derived from acid-catalyzed decomposition of the diazocompound 17. Formation
of the methyl ether product 22 suggests the involvement of a â-silyl carbocation intermediate, and
solvent isotope effect studies indicate that this cation is at least partially derived from protonation
of diazocompound 17 by neutral methanol. Photochemical decomposition of 17 in methanol produced
the alkene 19 (97%) along with a small amount (2.4%) of the methyl ether 22. Capture of a
photochemically generated carbene 21 by methanol is the proposed origin of this minor product.
Geometry optimization of trimethylsilylmethylphenylcarbene (8) and carbene 21 at the HF/6-31G*
computational level led to a conformation consistent with a hyperconjugative interaction between
the vacant p-orbital of these carbenes and the adjacent C-Si bond. Carbenes 8 and 21 are not
energy minima at the B3LYP/6-31G* level, where they rearrange to alkenes without barrier via
silyl migration. These theoretical findings contrast with the proposed trapping of carbene 21 by
methanol and oxygen.
The effect of silicon on reactive intermediates has
silyl group (as in 1, 3, and 5) activates hydrogen toward
received much attention. Stabilization of carbocations by
adjacent silicon has been thoroughly investigated.1 The
effects of adjacent silicon on carbanions2 and radicals3
have also been described. We4 and others5 have been
interested in the chemistry of carbenes containing tri-
methylsilyl groups in the vicinity of the carbenic center.
Toward this end, carbenes 1-8 gave been generated and
the rearrangement processes that these carbenes undergo
have been examined. Two general trends have developed
from these studies. The trimethylsilyl group is quite
prone to migrate to carbenic centers, as illustrated by
the formation of 9 from 8.4a Additionally, the trimethyl-
migration to carbenic centers.
(1) (a) Lambert, J . B. Tetrahedron 1990, 46, 2677. (b) Lambert, J .
B.; Zhao, Y.; Emblidge, R. W.; Salvador, L. A.; Liu, X.; So, J .-H.;
Chelius, E. C. Acc. Chem. Res. 1999, 32, 183. (c) Lambert, J . B.; Wang,
G.; Finzel, R. B.; Teramura, D. H. J . Am. Chem. Soc. 1987, 109, 7838.
(d) Lambert, J . B.; Liu, X. J . Organomet. Chem. 1996, 521, 203.
(2) Colvin, E. W. Silicon in Organic Synthesis; Butterworth: London,
1981; pp 12-13, 21-29, and references therein.
(3) (a) Kochi, J . K.; Kawamura, T. J . Am. Chem. Soc. 1972, 94, 648.
(b) Sakurai, H.; Hosomi, A.; Kumada, M. J . Org. Chem. 1969, 34, 1764.
(c) Creary, X.; Mehrsheikh-Mohammadi, M. E.; McDonald, S. J . Org.
Chem. 1987, 52, 3254.
(4) (a) Creary, X.; Wang, Y.-X. Tetrahedron Lett. 1989, 19, 2493. (b)
Creary, X.; Wang, Y.-X. J . Org. Chem. 1994, 59, 1604. (c) Creary, X.;
J iang, Z.; Butchko, M.; McLean, K. Tetrahedron Lett. 1996, 37, 579.
(d) Creary, X.; Butchko, M. A. J . Org. Chem. 2001, 66, 1115. (e) Creary,
X.; Butchko, M. A. J . Am. Chem. Soc. 2001, 123, 1569.
(5) (a) Baird, M. S.; Dale, C. M.; Dulayymi, J . R. A. J . Chem. Soc.,
Perkin Trans. 1 1993, 1373. (b) Kirmse, W.; Konrad, W.; Schnitzler,
D. J . Org. Chem. 1994, 59, 3821. (c) Pezacki, J . P.; Loncke, P. G.; Ross,
J . P.; Warkentin, J .; Gadosy, T. A. Org. Lett. 2000, 2, 2733.
During the course of these studies the primary method
that we have used to generate carbenes involved pyrolysis
of tosylhydrazone salts 10.6 This reaction generates
diazosilanes 11 in situ, which lose molecular nitrogen
under the pyrolysis conditions. In the case of carbenes
74e and 8,4a tosylhydrazone derivatives of â-trimethylsilyl
ketones were not readily synthesized. Therefore, an
10.1021/jo0106546 CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/13/2001

â-Silylcarbenes from Isolable Diazosilanes
J . Org. Chem., Vol. 67, No. 1, 2002 113
Sch em e 1
yield of 18 therefore depends on the amount of MnO2
used in the oxidation. The mechanistic origin of this
alkyne is uncertain. Although relatively unstable, the
purple diazosilane 17 could be isolated and most of the
impurities could be removed by rapidly filtering a dilute
hexane solution through basic alumina containing added
Na2CO3. Diazosilane 17 showed a characteristic strong
absorption at 2030 cm-1 in the infrared.
Sch em e 2
Th er m a l a n d Rh (II)-Ca ta lyzed Rea ction s in Cy-
cloh exa n e. Thermal decomposition of diazosilane 17 in
cyclohexane at 78 °C in the absence of oxygen gave the
alkene 19 as the major product, along with a trace (1%)
of the alkene 20 (Scheme 4). Rhodium(II) acetate cata-
lyzed decomposition in cyclohexane at room temperature
led to the same product mixture. These products are
suggested to be derived from the carbene 21 (or the
rhodium-complexed carbenoid analogue 21-Rh ), which
preferentially migrates the silyl group to the carbene
center. This preferential migration of the silyl group, as
opposed to the typically facile migration of an adjacent
hydrogen, is suggested to be a result of a favorable
interaction of the carbene vacant orbital with the adja-
cent Si-C bond. This interaction is analogous to that
seen in â-silyl carbocations.1
Sch em e 3
Th er m a l Rea ction in Meth a n ol. It has been sug-
gested10 that certain diazocompounds give reactions that
appear to be carbene processes. However, they actually
involve concerted loss of nitrogen and simultaneous
rearrangement, i.e., carbenes are not discrete intermedi-
ates in these processes. The thermal decomposition of
diazosilane 17 in methanol was therefore examined in
an attempt to trap a thermally generated free carbene
21 since methanol is a very efficient trap for both
electrophilic and nucleophilic carbenes.11 Heating a solu-
tion of 17 in methanol at 60 °C led to the alkenes 19 and
20 along with the methyl ether 22 (Scheme 5). To rule
out a trace acid-catalyzed decomposition of 17 via a
diazonium ion, 2,6-lutidine or triethylamine was added
to the methanol solution. First-order rate constants for
the thermal decomposition of 17 in methanol are given
in Table 1, and they are independent of the amount and
nature of the base added to the methanol, even though
these solutions differ in acidity by many orders of
magnitude. The decomposition of 17 in methanol is
therefore not catalyzed by protic acid (CH3OH2+). Table
1 also gives products under various conditions, and they
are also independent of added base.
indirect method was used to generate the tosylhydrazone
salt precursors to carbenes 7 and 8. In the present study,
we wanted to develop a synthetic route that would allow
the isolation of â-diazosilanes 11 (Scheme 1). The prepa-
ration of â-diazosilanes has not been previously described
in the literature. It was also of interest to see if carbenes
12 generated from the â-diazosilanes 11 could be trapped.
This would allow us to determine if the carbene 12 was
a discrete intermediate or whether loss of molecular
nitrogen and silyl migration was a concerted process.
Reported here are the results of these studies.
Resu lts a n d Discu ssion
Syn th esis. Our initial attempts to prepare an isolable
â-diazosilane began with the readily available trimeth-
ylsilyl ketone 13.7 However, all attempts to convert this
ketone to the tosylhydrazone derivative led to desilylation
and formation of acetophenone tosylhydrazone (Scheme
2). Reaction of 13 with hydrazine also led to desilylation
and the formation of acetophenone hydrazone.
Diazosilane 17 was also thermally decomposed in
CH3OD. Figure 1 shows the olefinic region of the 1H NMR
spectrum of 20 formed by reaction of 17 in methanol as
well as the product formed in CH3OD. Only 20-d is
formed in CH3OD (as well as 22-d). This verifies that 20
is not a product of hydrogen migration in carbene 21 but
arises from a reaction with CH3OD. It is also interesting
to note the decreased amounts of 20-d and 22-d formed
in CH3OD (42% total yield) relative to CH3OH (57% total
yield).
The synthesis of an isolable â-diazosilane was finally
achieved starting with the known silyl ketone 15.8
Conversion to the hydrazone 16 was achieved without
desilylation (Scheme 3). Manganese dioxide9 oxidation of
this hydrazone gave the diazosilane 17 in reasonable
yield along with variable amounts of an overoxidized
product 18. The alkyne 18 is derived from further
reaction of the diazosilane 17 with excess MnO2, and the
(6) For
a discussion of the generation of diazocompounds and
carbenes by pyrolysis of salts of tosylhydrazones, see: (a) Bamford,
W. R.; Stevens, T. S. J . Chem. Soc. 1952, 4735. (b) Kaufman, G. M.;
Smith, J . A.; Vander Stouw, G. G.; Shechter, H. J . Am. Chem. Soc.
1965, 87, 935. (c) Creary, X. Org. Synth. 1986, 64, 207.
(7) Lutsenko, I. F.; Baukov, Y. I.; Dudukina, O. V.; Kramarova, E.
N. J . Organomet. Chem. 1968, 11, 35.
(8) Shinokubo, H.; Oshima, K.; Utimoto, K. Tetrahedron 1996, 52,
14533.
(9) Attenburrow, J .; Cameron, A. F. B.; Chapman, J . H.; Evans, R.
M.; Hems, B. A.; J ansen, A. B. A.; Walker, T. J . Chem. Soc. [London]
1952, 1094.
(10) For leading references, see: (a) Thamattoor, D. M.; J ones, M.,
J r.; Pan, W.; Shevlin, P. B. Tetrahedron Lett. 1996, 37, 8333. (b) Huang,
H.; Platz, M. S. Tetrahedron Lett. 1996, 37, 8337. (c) J ackson, J . E.;
Platz, M. S. In Advances in Carbene Chemistry; Brinker, U. H. Ed.;
J AI Press Inc.: Greenwich, CT, 1994; Vol. 1, pp 89-160. (d) Moss, R.
A. In Advances in Carbene Chemistry; Brinker, U. H., Ed.; J AI Press
Inc: Greenwich, CT, 1994; Vol. 1, pp 59-88.
(11) Kirmse, W. In Advances in Carbene Chemistry; Brinker, U. H.,
Ed.; J AI Press Inc: Greenwich, CT, 1994; Vol. 1, pp 1-57.

114 J . Org. Chem., Vol. 67, No. 1, 2002
Creary and Butchko
Sch em e 4
Sch em e 5
Sch em e 6
Ta ble 1. Ra te Con sta n ts a n d P r od u ct Ra tios for
Rea ction of 17 in Meth a n ol a t 60.0 °C u n d er Va r iou s
Con d ition s
conditions
k (s-1
)
19
22
20
CH3OH/no base
(2.5 ( 0.1) × 10-4 a 42.9 49.5
(2.6 ( 0.1) × 10-4 d 42.6 49.9
7.6
7.6
7.5
CH3OH/2,6-lutidineb (2.6 ( 0.1) × 10-4 c 42.8 49.6
CH3OH/Et3Nb
CH3OD/2,6-lutidineb (2.2 ( 0.1) × 10-4 e 57.6 36.0f 6.4g
CH3OH/hν at 23 °C
96.9
2.4
0.7
a
b
k ) 2.7 × 10-5 s-1 at 40.0 °C. 0.015 M. c k ) 2.8 × 10-5 s-1
at 40.0 °C. k ) 2.8 × 10-5 s-1 at 40.0 °C. e k ) 2.0 × 10-5 s-1 at
d
40.0 °C. f Product is 22-d. Product is 20-d.
g
As before, the alkene 19 is proposed to be derived from
silyl migration in the thermally generated carbene 21
(Scheme 6). Significantly more of the alkene 20 (7%) is
formed in methanol than in the thermal reaction of 17
in cyclohexane (1%). There are a number of reasonable
mechanisms for the formation of the alkene 20 and the
methyl ether 22. It is suggested that the â-silyl carboca-
tion 24 is the immediate precursor of the methyl ether
22 as well as the alkene 20. While catalysis by protic acid
has been ruled out as a mechanism for formation of 24,
In an attempt to distinguish between these two mech-
anisms for formation of 20 and 22, solvent isotope effects
were measured. The first-order rate constant (Table 1)
for decomposition of 17 in CH3OD (kobs(MeOD)) is smaller
than in CH3OH (kobs(MeOH)). However, our NMR method
for determination of rate constants has an error of
approximately (3.5%. Therefore the ratio of our observed
rate constant in CH3OH to that in CH3OD (1.3 ( 0.2)
has a significant error. From this value and the ratio of
the products given in Table 1, it is possible to extract
values of kMeOH/kMeOD, but this value will also have a large
error. Regardless of the error, the solvent isotope effect
is greater than 1.0. Complete decomposition of 17 in
methanol via the kcarbene process should give no solvent
isotope effect. This indicates that at least some of
products 20 and 22 arise from protonation of diazosilane
17 by methanol.
proton transfer from neutral methanol (kMeOH)
11-13 acting
as a general acid could generate the cation 24 via the
diazonium ion 23. According to this scheme, the observed
rate constants in Table 1 for disappearance of 17
(kobs(MeOH)) would be the sum of kcarbene and kMeOH. There
is also another interesting mechanistic possibility. If the
thermally generated carbene 21 has nucleophilic proper-
ties, then protonation of 21 by methanol (in competition
with silyl migration) would generate the same ion pair
24. Ion pair collapse would generate the methyl ether
22, whereas â-proton elimination would give the alkene
20 (or 20-d). In this case the observed rate constants
represent kcarbene since methanol is not involved in the
rate-determining step.
There is an alternative method, based on products
formed, for determining kMeOH/kMeOD if the products are
derived from competing kcarbene and kMeoH processes. If one
assumes that alkene 19 is derived from the kcarbene process
and 20 and 22 are derived from the kMeOH process, then
the ratio of the products formed in CH3OD to the products
(12) kMeOH is equivalent to k0 in a general acid-catalyzed scheme.
+
+
kH [H ] ) 0 under the neutral or basic conditions in Table 1.
(13) Such a protonation has been proposed for reaction of RCHN2
in methanol; see: Kirmse, W.; Rinkler, H. A. Ann. Chem. 1967, 707,
57.

â-Silylcarbenes from Isolable Diazosilanes
J . Org. Chem., Vol. 67, No. 1, 2002 115
1
F igu r e 1. Partial H NMR spectra for reaction of 17 with CH3OD and CH3OH.
formed in CH3OH gives the solvent isotope effect.14 From
the data in Table 1, the value of kMeOH/kMeOD is 1.8 ( 0.2
by this method. Previous studies15 have shown solvent
isotope effects in the range of kEtOH/kEtOD ) 3.5 for rate-
limiting protonation of diazocompounds, and the value
of 1.8 is therefore somewhat smaller than expected.
This method for calculating the solvent isotope effect
from product ratios is not based on absolute rate mea-
surements, and the calculated value of 1.8 could also be
the solvent isotope effect for protonation of carbene 21
with methanol. We therefore cannot completely exclude
protonation of carbene 21 as a source of some of the
products 20 and 22. Neither can we rule out nucleophilic
reaction of methanol with an electrophilic carbene 21 to
give the ylid 25, which could also serve as a source of
the methyl ether 22.
P h otoch em ica l Rea ction in Meth a n ol. The photo-
chemical reaction of diazosilane 17 in methanol at 23 °C
follows a somewhat different course. The silyl migration
product 19 dominates (97%), and only a small amount
(2.4%) of the methanol-trapped product 22 is observed.
This result suggests that the free ground state of carbene
21 may be largely bypassed in the photochemical decom-
position of 17. Photochemical decomposition of certain
carbene precursors has been proposed to involve excited
precursor states that bypass carbenes as discrete inter-
mediates.10c,d As such, the photochemical decomposition
of 17 in methanol may largely involve an excited state
of 17, which loses nitrogen in a concerted process along
with silyl migration. Nonetheless one sees 2.4% of the
methyl ether 22. This small amount of 22 cannot arise
from a thermal reaction of 17 in methanol. Our rate data
on the thermal reaction (k ) 3.4 × 10-6 s-1 at 23 °C)16
indicate that a maximum of 0.12% of 17 could have
reacted thermally at 23 °C during the 6 min required for
mixing of 17 with methanol and completion of the
photochemical reaction. Hence the 2.4% of product 22
represents a photogenerated intermediate that is trapped
by methanol. This intermediate (presumably a carbene)
must have a finite lifetime long enough to permit trap-
ping by methanol.
Th er m a l R ea ct ion in Cycloh exa n e u n d er O2.
When the thermal decomposition of diazosilane 17 in
cyclohexane is carried out under an oxygen atmosphere
(Scheme 7), large amounts of the ketone 15 (63%) are
formed, along with alkenes 19 (31%) and 20 (6%). There
are two reasonable mechanisms for the formation of 15.
The first involves trapping of the carbene 21 with
molecular oxygen to give the triplet biradical 26. Inter-
system crossing would generate a carbonyl oxide 27, and
ring closure would give the dioxirane 28. Subsequent
oxygen atom transfer from 27 or 28 to the diazosilane
17 would produce a second equivalent of ketone 15 via
29. This mechanism requires that carbene 21 live long
enough to undergo bimolecular trapping with oxygen.
(14) The solvent isotope effect is kMeOH/kMeOD ) [19/(20 + 22)]MeOD
[(20 + 22)/19]MeOH ) 1.95.
-
(16) This extrapolated rate constant at 23 °C is determined from
data in Table 1.
(15) Roberts, J . D.; Regan, C. M. J . Am. Chem. Soc. 1952, 74, 3695.

116 J . Org. Chem., Vol. 67, No. 1, 2002
Creary and Butchko
F igu r e 2.
Sch em e 7
the case in reactions with both methanol and with
oxygen.
Com p u ta tion a l Stu d ies. Computational studies on
carbene intermediates have been numerous in the recent
literature, and density functional methods have become
the way of choice for calculating carbene structures and
energies.4d,e,17-22 Molecular orbital calculations have now
been carried out to provide additional insights into the
structures of carbenes 8 and 21. Calculations were
carried out at both the HF/6-31G* level and the B3LYP/
6-31G* level. The geometry optimized HF/6-31G* struc-
ture of carbenes 8 and 21 both show conformations
consistent with an interaction of the carbene center with
the adjacent C-Si bond (Figure 2). Frequency calcula-
tions showed no imaginary frequencies, indicating that
these structures are indeed energy minima at the HF/
6-31G* level. The transition states, 8-TS and 21-TS, for
trimethylsilyl and tert-butyldimethylsilyl migrations to
the carbene centers have also been located at the HF/
6-31G* level, and these transition states lie 5.4 and 6.1
kcal/mol above the respective carbenes. These calculated
barriers for silyl migration are quite small for HF/6-31G*
calculated barriers, which usually overestimates the
actual barrier for migrations to carbenic centers.23
Inspection of bond lengths in 8 and 21 shows an
increase in the Si-CH2 bond lengths (1.939 and 1.944
Å) relative to the average length of the Si-CH3 bonds
(1.890 Å). This is understood in terms of hyperconjugative
electron donation from the C-Si bonds into the vacant
p-orbitals of the carbenic centers. Further evidence for
(17) Wong, M. W.; Wentrup, C. J . Org. Chem. 1996, 61, 7022.
(18) Xie, Y.; Schreiner, P. R.; Schleyer, P. v. R.; Schaefer, H. F., III.
J . Am. Chem. Soc. 1997, 119, 1370.
(19) Sulzbach, H. M.; Platz, M. S.; Schaefer, H. F. III; Hadad, C. M.
J . Am. Chem. Soc. 1997, 119, 5682.
(20) Keating, A. E.; Garcia-Garibay, M. A.; Houk, K. N. J . Phys.
Chem. A 1998, 102, 8467.
(21) Zhu, Z.; Bally, T.; Stracener, L. L.; McMahon, R. J . J . Am. Chem.
Soc. 1999, 121, 2863.
(22) Geise, C. M.; Hadad, C. M. J . Am. Chem Soc. 2000, 122, 5861.
(23) Seminario, J . M.; Grodzicki, M.; Politzer, P. In Density Func-
tional Methods in Chemistry; Labanowski, J . K., Andzelm, J . W., Eds.;
Springer-Verlag: New York, 1991; pp 419-425.
However, an alternative mechanism for formation of 15
involves the direct reaction of dioxygen with the diaz-
osilane 17 to give the triplet biradical 29. Loss of nitrogen
from 29 provides an alternative entry to the biradical 26.
This latter mechanism would require that diazosilane 17
decompose to the carbene 21 and react with dioxygen at
comparable rates. It would be quite fortuitous if this were

â-Silylcarbenes from Isolable Diazosilanes
J . Org. Chem., Vol. 67, No. 1, 2002 117
this hyperconjugative interaction comes from a compari-
son of the C-CH2 bond lengths in 8 and 21 (1.455 and
1.456 Å) with that of the â-H analogue Ph-C-CH3 (1.492
Å). The decreased C-CH2 lengths in 8 and 21 argue in
favor of this hyperconjugative interaction. The optimized
structures of 8 and 21 also have Si-CH2-C-C dihedral
angles of 106° and 107°, respectively. A dihedral angle
of 90° would provide for the optimal hyperconjugative
interaction in these carbenes.
We have not been able to locate energy minima
corresponding to carbenes 8 and 21 at the B3LYP/6-31G*
computational level. Beginning with the HF/6-31G*
geometry, attempted B3LYP/6-31G* geometry optimiza-
tions led to the rearranged olefins 9 and 19. B3LYP/
6-31G* single point calculations (using HF/6-31G* opti-
mized geometries) actually place the transition states
8-TS and 21-TS lower in energy than the corresponding
carbenes 8 and 21. Hence density functional theory
suggests that there is no barrier to rearrangement of
carbenes 8 and 21, i.e., these carbenes may not be energy
minima at the B3LYP/6-31G* level. These computational
results contrast with the HF/6-31G* calculations. The
mechanistic possibilities involving trapping of carbene
21 with methanol and dioxygen need to be further
examined in view of these computational results. How-
ever, the trapping of carbene 21 under photochemical
conditions suggests that a further evaluation of the
B3LYP/6-31G* computational results is necessary. There
is a conflict between theory and interpretation of experi-
mental results.
A conflict therefore exists between B3LYP/6-31G* theory
and our suggestion that carbene 21 must live long enough
to permit bimolecular reaction.
Exp er im en ta l Section
P r ep a r a tion of Hyd r a zon e 16. A solution of 283 mg of
hydrazine monohydrate (5.66 mmol) and 444 mg of R-silyl
ketone 158 (1.88 mmol) in 12 mL of absolute ethanol was
heated at reflux for 56 h. The mixture was then taken up into
ether, and the ether was washed with water and saturated
NaCl solution and then dried over a mixture of Na2SO4 and
MgSO4. After filtration, the ether solvent was removed using
a rotary evaporator to give 462 mg of crude hydrazone 16 (99%
yield). Data for 16: 1H NMR (CDCl3) δ 7.60 (d, J ) 8.1 Hz, 2
H), 7.38-7.28 (m, 3 H), 5.15 (bs, 2 H), 2.21 (s, 2 H), 0.94 (s, 9
H), -0.085 (s, 6 H); 13C NMR (CDCl3) δ 150.93 (s), 139.86 (s),
128.22 (d, J ) 156 Hz), 128.10 (d, J ) 154 Hz), 126.06 (d, J )
154 Hz), 26.30 (q, J ) 125 Hz), 17.09 (s), 13.51 (t, J ) 121
Hz), -4.95 (q, J ) 120 Hz); HRMS (EI) calcd for C14H24N2Si
248.1709, found 248.1705.
Oxid a tion of Hyd r a zon e 16 to Dia zosila n e 17. A mix-
ture of 394 mg of hydrazone 16 and 1.0 g of anhydrous MgSO4
in 25 mL of ether at room temperature was stirred as 1.026 g
9
of activated MnO2 was added in portions. The mixture was
stirred at room temperature for 80 min and then filtered
through a small amount of MgSO4. The ether solvent was
removed using a rotary evaporator to give 342 mg of a purple
oil that contained diazosilane 17, ketone 15, and alkyne 18 in
a 4.2:1.1:1.0 molar ratio as determined by NMR.24 A sample
of diazosilane 17 was purified by dissolving a small amount
of the crude product obtained above in 1 mL of hexane and
rapidly filtering through a short column of basic alumina/
sodium carbonate (2:1 mixture). The hexane were removed
using a rotary evaporator to give the unstable diazosilane 17,
which was stored in the dark at -20 °C and used as soon as
possible after preparation. Data for 17: 1H NMR (CDCl3) δ
7.32 (t, J ) 7.5 Hz, 2 H), 7.01 (t, J ) 7.5 Hz, 1 H), 6.93 (d, J
) 8.4 Hz, 2 H), 1.66 (s, 2 H), 0.95 (s, 9 H), 0.029 (s, 6 H); 13C
NMR (CDCl3) δ 132.9 (s), 128.8 (d, J ) 158 Hz), 123.2 (d, J )
161 Hz), 121.7 (d, J ) 156 Hz), 26.4 (q, J ) 125 Hz), 16.7 (s),
6.1 (t, J ) 121 Hz), -5.4 (q, J ) 119 Hz). Data for 18: 1H
NMR (CDCl3) δ 7.47 (d, J ) 7.5 Hz, 2 H), 7.32-7.28 (m, 3 H),
1.00, (s, 9 H), 0.19 (s, 6 H); 13C NMR (CDCl3) δ 132.0 (d, J )
161 Hz), 128.5 (d, J ) 161 Hz), 128.2 (d, J ) 159 Hz), 123.3
(s), 105.8 (s), 92.5 (s), 26.2 (q, J ) 125 Hz), 16.8 (s), -4.5 (q, J
) 121 Hz).
A final point to be considered is the ease of silyl
migration to carbenic centers. A reviewer has suggested
that, as a result of the ability of silicon to become hyper-
valent, the filled orbital of the singlet carbene may be
involved in the migration process. Stated another way,
the transition state for migration of the silicon is poten-
tially stabilized by the ability of silicon to interact with
the nonbonding orbital of the carbene as well as the
vacant carbene orbital. The Si-CH2-C-C dihedral
angles in carbenes 8 and 21 (106° and 107°) suggests a
tilt of the silicon atom toward the filled carbene orbital
and away from the ideal hyperconjugative angle of 90°.
Th er m a l Decom p osition of Dia zosila n e 17 in Cyclo-
h exa n e. A solution containing 341 mg of the diazosilane 17
in 30 mL of cyclohexane under nitrogen was heated at reflux
for 17 h during which time the purple color of 17 disappeared.
The cyclohexane was then removed by distillation (with the
last traces of solvent being removed at 15 mm pressure) to
give 299 mg of crude alkene 19.25 A pure sample of 19 was
isolated by preparative gas chromatography. Data for 19: 1H
NMR (CDCl3) δ 7.28 (t, J ) 7.5 Hz, 2 H), 7.21 (t, J ) 7.2 Hz,
1 H), 7.15 (d, J ) 8.1 Hz, 2 H), 5.83 (d, J ) 3.1 Hz, 1 H), 5.63
(d, J ) 3.1 Hz, 1 H), 0.80 (s, 9 H), 0.18 (s, 6 H); 13C NMR
(CDCl3) δ 151.9 (s), 146.3 (s), 129.8 (d of d, J ) 154, 158 Hz),
128.0 (d, J ) 160 Hz), 127.2 (d, J ) 160 Hz), 126.0 (d, J ) 161
Hz), 26.9 (q, J ) 125 Hz), 17.3 (s), -4.9 (q, J ) 120 Hz).
In a separate run, a 7 mg sample of diazosilane 17 was
dissolved in 0.6 mL of cyclohexane. The solution was placed
in a tube under nitrogen and cooled to -78 °C, and the tube
was then sealed under vacuum (0.1 mm). The tube was heated
at 78 °C for 15 h. Analysis of the contents of the tube by gas
chromatography and 600 MHz 1H NMR showed alkenes 19
and 2026 in a 99:1 molar ratio.
Con clu sion
Diazosilane 17, prepared by MnO2 oxidation of hydra-
zone 16, undergoes thermal and rhodium-catalyzed de-
composition in cyclohexane to give alkene 19, a product
of silyl migration to the carbene center. Thermal reaction
in methanol led to products derived from silyl migration
in the carbene intermediate as well as capture of a â-silyl
carbocation. This â-silyl carbocation is potentially derived
from protonation of a nucleophilic carbene intermediate
by methanol. An alternative mechanism for formation of
the â-silyl carbocation involves protonation of diazosilane
17 by neutral methanol followed by loss of molecular
nitrogen from the diazonium ion. Under photochemical
conditions, a methanol-trapped product is formed in
small amounts under conditions where protonation of
diazosilane 17 by neutral methanol is too slow to give
this product. Computational studies on carbenes 8 and
21 at the HF/6-31G* level show a substantial interaction
of the vacant carbene orbital with the adjacent C-Si bond
and a barrier to silyl migration. At the B3LYP/6-31G*
level, carbenes 8 and 21 are not energy minima but
rearrange without barrier via a silyl migration process.
(24) Fitzmaurice, N. J .; J ackson, W. R.; Perlmutter, P. J . Organomet.
Chem. 1985, 285, 375.
(25) (a) Cunico, R. F. Tetrahedron Lett. 1986, 27, 4269. (b) Goldberg,
Y.; Alper, H. Tetrahedron Lett. 1995, 36, 369.
(26) Takeuchi, R.; Yasue, H. Organometallics 1996, 15, 2098.

118 J . Org. Chem., Vol. 67, No. 1, 2002
Creary and Butchko
R h od iu m Acet a t e Ca t a lyzed Decom p osit ion of Di-
a zosila n e 17 in Cycloh exa n e. A solution of 130 mg of
diazosilane 17 in 15 mL of cyclohexane under nitrogen was
stirred as 3 mg of Rh2(OAc)4 was added. The mixture was
stirred for 5 h at room temperature, and the purple color of
17 was no longer present. The cyclohexane was removed using
a rotary evaporator to give 119 mg of crude 19. A pure sample
of 19 was isolated by preparative gas chromatography. In a
lane 17, and the solution was rapidly placed in a NMR tube
under nitrogen. The tube was maintained at room temperature
(23 °C) by air cooling and then immediately irradiated with a
Hanovia 450 W lamp for 4 min. The purple color of the
diazosilane 17 had completely disappeared after the irradia-
tion. The total time required for mixing of 17 with methanol
and completion of the photolysis was 6 min. The methanol was
then carefully removed using a rotary evaporator, and the
residue was dissolved in CDCl3 and analyzed by 600 MHz
NMR. Product ratios are given in Table 1.
1
separate run, gas chromatographic and H NMR analysis of a
Rh2(OAc)4-catalyzed reaction showed alkenes 19 and 20 in a
99:1 molar ratio.
Th er m a l Decom p osition of Dia zosila n e 17 in Meth a -
n ol. Kin etic Stu d ies. A solution containing approximately 5
mg of diazosilane 17 in 0.8 mL of the appropriate solvent
(methanol, 0.015 M 2,6-lutidine in methanol, 0.015 M 2,6-
lutidine in CH3OD, or 0.015M Et3N in methanol) was sealed
in a NMR tube under nitrogen and the tube was placed in a
constant-temperature bath at 40.0 or 60.0 °C. The amount of
17 in the tube was periodically determined by quenching the
tube in a 15 °C bath and then analyzing by 600 MHz NMR
spectroscopy (unlocked mode). The disappearance of the signal
at δ -0.04 was monitored as a function of time and the small
amount of alkyne 18 present (δ 0.12) in the sample served as
an internal standard. Rate constants were calculated using
standard least squares procedures and are given in Table 1.
Correlation coefficients were all greater than 0.999. Rate
constants in Table 1 represent an average of multiple runs.
Com p u ta tion a l Stu d ies. Ab initio molecular orbital cal-
culations were performed using the Gaussian 98 series of
programs.28 Carbene 8 showed no imaginary frequencies, while
8-TS showed one imaginary frequency.
Th er m a l Rea ction of Dia zosila n e 17 in Cycloh exa n e
u n d er O2. A solution of 6 mg of diazosilane 17 in 0.8 mL of
cyclohexane was sealed in a 5 mL glass ampule containing a
stir bar under an oxygen atmosphere. The ampule was placed
in a refluxing ethanol bath (78 °C) and stirred vigorously for
75 min. During this time the color of the diazosilane disap-
peared. The cyclohexane was carefully removed using a rotary
evaporator, and the residue was analyzed by 600 MHz NMR,
which showed 15, 19, and 20 in a 63:31:6 molar ratio.
Th er m a l Decom p osition of Dia zosila n e 17 in Meth a -
n ol. A solution containing 54 mg of diazosilane 17 in 4 mL of
methanol was heated to reflux for 6.5 h during which time
the purple color of the diazosilane disappeared. The methanol
was then removed using a rotary evaporator to give 35 mg of
a crude residue containing 19,25 20,26 and 22.27 Samples of each
product were isolated by preparative gas chromatography.
Data for 20: 1H NMR (CDCl3) δ 7.45 (d, J ) 7.2 Hz, 2 H),
7.33 (t, J ) 7.2 Hz, 2 H), 7.26-7.24 (m, 1 H), 6.89 (d, J ) 19.3
Hz, 1 H), 6.48 (d, J ) 19.3 Hz, 1 H), 0.92 (s, 9 H), 0.12 (s, 6 H).
Data for 22: 1H NMR (CDCl3) δ 7.34 (t, J ) 7.2 Hz, 2 H), 7.31-
7.25 (m, 3 H), 4.19 (d of d, J ) 7.0, 8.1 Hz, 1 H), 3.13 (s, 3 H),
1.26 (d of d, J ) 8.1, 14.4 Hz, 1 H), 1.05 (d of d, J ) 7.0, 14.4
Hz, 1 H), 0.84 (s, 9H), -0.009 (s, 3 H), -0.31 (s, 3 H); 13C NMR
(CDCl3) δ 144.2 (s), 128.4 (d, J ) 159 Hz), 127.5 (d, J ) 160
Hz), 126.7 (d, J ) 157 Hz), 82.1 (d, J ) 149 Hz), 56.1 (q, J )
140 Hz), 26.5 (q, J ) 125 Hz), 23.2 (t, J ) 119 Hz), 16.6 (s),
-5.5 (q, J ) 119 Hz), -6.3 (q, J ) 120 Hz).
Ack n ow led gm en t. The National Science Founda-
tion is acknowledged for support of this research.
Su p p or tin g In for m a tion Ava ila ble: Structures, ener-
gies, and Cartesian coordinates of carbenes 8, 21, and transi-
tion states 8-TS and 21-TS. This material is available free of
charge via the Internet at http://pubs.acs.org.
The molar ratios of products given in Table 1 were deter-
mined by 600 MHz 1H NMR analysis of separate reactions. In
a typical product analysis study, 6 mg of diazosilane 17 was
dissolved in 0.8 mL of methanol and the sample was sealed in
a tube under nitrogen. The tube was heated at 60.0 °C for 10
h, and the solvent was carefully removed using a rotary
evaporator. The residue was dissolved in CDCl3 and analyzed
by 1H NMR. Product molar ratios were determined by integra-
tion of signals for 19, 20, and 22 at δ 5.83, 6.48, and 4.19,
respectively. Estimated error is (0.5%.
J O0106546
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P h ot och em ica l Decom p osit ion of Dia zosila n e 17 in
Meth a n ol. Methanol (0.8 mL) was added to 6 mg of diazosi-
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