Electrophilic cleavage of cyclopropylmethystannanes: An experimental comparison of σ-σ and σ-π conjugation

Article abstract of DOI:10.1021/jo047822p

(Chemical Equation Presented) Cyclopropylmethyltrimethylstannanes undergo electrophilic cyclopropane cleavage in chloroform with simple inorganic electrophiles (H⁺, SO₂, I₂) in a homologous reaction to the S_E′ cleavage of allylic stannanes. The σ-σ conjugation between the carbon-tin bond and cyclopropane orbitals observed spectroscopically in the parent cyclopropylmethyltrimethylstannane is responsible for a rate enhancement of ca. 10² toward iodinolysis, relative to comparable alkyl stannanes. This acceleration is considerably less, however, than the ca. 10⁹-fold rate enhancement provided by the corresponding σ-π conjugation in allylic stannanes. Methanol-tin coordination appears to reduce the activating influence of the metal, promoting methyl cleavage over cyclopropane fission with acid and iodine. Decreased σ-σ conjugation can also explain the decreased reactivity of cyclopropyltriphenylstannane compared with its trimethyltin counterpart. Cyclopropylmethylstannanes do not undergo the synthetically useful addition of aldehydes under conditions that facilitate the corresponding reaction of allylic stannanes.

Full text of DOI:10.1021/jo047822p

Electrophilic Cleavage of Cyclopropylmethystannanes: An
Experimental Comparison of σ-σ and σ-π Conjugation†
Andrew J. Lucke and David J. Young*
School of Science, Griffith University, Nathan 4111, Brisbane, Australia
d.young@griffith.edu.au
Received December 12, 2004
Cyclopropylmethyltrimethylstannanes undergo electrophilic cyclopropane cleavage in chloroform
with simple inorganic electrophiles (H+, SO2, I2) in a homologous reaction to the SE′ cleavage of
allylic stannanes. The σ-σ conjugation between the carbon-tin bond and cyclopropane orbitals
observed spectroscopically in the parent cyclopropylmethyltrimethylstannane is responsible for a
rate enhancement of ca. 102 toward iodinolysis, relative to comparable alkyl stannanes. This
acceleration is considerably less, however, than the ca. 109-fold rate enhancement provided by the
corresponding σ-π conjugation in allylic stannanes. Methanol-tin coordination appears to reduce
the activating influence of the metal, promoting methyl cleavage over cyclopropane fission with
acid and iodine. Decreased σ-σ conjugation can also explain the decreased reactivity of cyclopro-
pyltriphenylstannane compared with its trimethyltin counterpart. Cyclopropylmethylstannanes do
not undergo the synthetically useful addition of aldehydes under conditions that facilitate the
corresponding reaction of allylic stannanes.
Introduction
Most attention has focused on the reaction of allylic
stannanes with carbon electrophiles, particularly alde-
hydes, which provides products with useful functionality
for further elaboration. The regio- and stereocourse of
these reactions depends very much on the method of
promotion, the reaction conditions, and the order of
reagent addition. A number of experimental protocols
have now been developed which allow a high degree of
control over relative and/or absolute stereochemistry and
these methodologies have been applied to the synthesis
of complex molecules bearing multiple stereogenic cen-
ters.
Allylic stannanes are highly reactive toward electro-
philic substitution reactions as a result of delocalization
of the electron-rich carbon-tin σ bond into the adjacent
π system. Substitution occurs at the γ position of the
allylic triad with concomitant allylic rearrangement (an
SE′ process). The stereochemistry of addition is dependent
on the electrophile and the steric environment at the γ
carbon. Acidolysis of unhindered allylic stannanes occurs
with anti approach of the proton as predicted for a
dominating LUMO-HOMO interaction in a concerted SE2′
process (eq 1). The corresponding reaction of electrophiles
bearing a nucleophilic appendage (e.g. SO2), however,
proceeds with syn attack as required by an SEi′ (metallo-
ene) mechanism (eq 2).
The electrophilic ring fission of cyclopropylmethylstan-
nanes (eq 3) is a logical extension to the corresponding
chemistry of allylic stannanes and would provide access
to the homologous products. The reactivity of cyclopro-
panes toward electrophiles is well-known and should be
enhanced by orbital overlap with an adjacent electron-
rich carbon-tin bond. Traylor and co-workers, who
obtained UV photoelectron spectra of cyclopropylmeth-
yltrimethylstannane and allyltrimethylstannane, pro-
vided evidence for strong σ-σ conjugation of this type.
† Dedicated to Professors C. J. M. Stirling and W. Kitching.
10.1021/jo047822p CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/02/2005
J. Org. Chem. 2005, 70, 3579-3583
3579

Lucke and Young
SCHEME 1
alternative protocol, which used N-chlorosuccinimide
(NCS) and dimethyl sulfide. This procedure is reported
to be selective for allylic and benzylic alcohols, leaving
saturated alcohols untouched. Presumably because of the
activating influence of the cyclopropane, however, these
reagents also proved effective for chlorinating 1b, albeit
over a much longer period of time (96 h) than is required
for allylic or benzylic alcohols and with some ring opening
(ca. 10%). The ring-opened product, 1-chloro-3-pentene,
was removed by careful titration with bromine followed
by flash distillation. Conversion to the corresponding
cyclopropylmethylstannanes 3 was achieved by treatment
with the appropriate triorganotin lithium species in THF.
Although chloride 2c was a 9:1 mixture of E and Z
isomers (reflecting the isomeric composition of the start-
ing alcohols), only the E isomer of stannane 3g was
isolated on treatment with Me3SnLi followed by ku¨gel-
rohr distillation.
TABLE 1. Electrophilic Cleavage of
Cylopropylmethylstannanes, 3
yield, %
entry stannane
E-N
SO2
SO2
SO2
SO2
SO2
SO2
SO2
CF3COOH CDCl3
CF3COOH CD3OD
CF3COOH CDCl3
CF3COOH CD3OD
CF3COOH CDCl3
CF3COOH CD3OD
I2
I2
I2
I2
I2
I2
I2
I2
I2
I2
solvent
CDCl3
CD3OD
CDCl3
CDCl3
CD3OD
CDCl3
CD3OD
4a
5
6
1
2
3d
3d
3e
3f
100
100
0
0
0
0
0
0
0
0
0
100
0
100
0
The electrophilic cleavage of 3 was first examined with
the inorganic electrophiles sulfur dioxide, trifluoroacetic
acid, and iodine in different solvents. Sulfinations were
performed by bubbling SO2 through a solution of 3 in
either CDCl3 or CD3OD at room temperature and then
3
4
100b
100c
80
77
100
0
5
3f
6
3g
3g
3d
3d
3f
20
23
7
8
1
9
monitoring reaction progress by H and 13C NMR spec-
10
11
12
13
14
15
16
17
18
19
20
21
22
23
100d
0
troscopy. Only triphenylstannane 3e was unreactive
under these conditions (entry 3). Cyclopropylmethyltri-
methylstannanes 3d, 3f, and 3g reacted exclusively with
ring fission (entries 1, 2, and 4-7) and reactions were
complete within 15 min. The products were identified as
trimethyltin O-sulfinates 4, 5 [E ) S(O)OSnMe3] rather
than the alternative S-sulfinates based on the S-O
stretching frequency (910-990 cm-1) in the infrared
spectrum, the characteristic downfield chemical shift of
the CHR′S(O)OSnMe3 1H NMR signal (δ 2.1 to 2.5) and
13C NMR signal (δ 59.5 to 67.5) and the large 1J(119Sn to
13CH3) ) 484-519 Hz.2 Isolated yields were g93% after
evaporation of solvent and triturating with pentane. The
R-methyl-substituted substrate 3f yielded a predomi-
nance of E-trimethyltin 3-penten-1-sulfinate, 4b, in both
solvents (E/Z ) ca. 1.7, entries 4 and 5). This product
ratio did not change throughout the course of these
reactions suggesting kinetic control, rather than post-
insertion isomerization to the thermodynamically favored
E isomer. In support of this proposition, sulfur dioxide
did not isomerize 3-penten-1-ol (Z/E ) 9.0) over several
hours under the same conditions. Sulfur dioxide cleavage
of the 2-methyl substrate 3g also gave very similar
results in CDCl3 and CD3OD, yielding a ca. 4:1 mixture
of tin sulfinates 4c and 5c (entries 6 and 7), resulting
from attack at the less substituted cyclopropane carbon.
3f
3g
3g
3d
3d
3d
3d
3e
3e
3f
90
0
10
0
100
0
CDCl3
100
0
C6D5N
CD3COCD3
CD3OD
CDCl3
100
100
100
100
100
0
100
0
100
0
0
0
CD3OD
CDCl3
CD3OD
CDCl3
CD3OD
0
100e
0
3f
3g
3g
90
0
10
0
a Percent conversion determined by 1H and 13C NMR spectros-
copy. b Z:E ) 38:62. c Z:E ) 36:65. d Z:E ) 73:27. e Z:E ) 22:78.
The energy of interaction between the bent cyclopropane
orbitals and the polarized C-Sn σ bond of the former was
1.6 eV, which can be compared with 2.2 eV for the
corresponding σ-π interaction in the latter. These re-
searchers predicted that this substantial σ-σ conjugation
might result in some new stereospecific ring-opening
reactions. We have previously communicated that cyclo-
propylmethylstannanes do indeed react with various
electrophiles to provide the corresponding ring fission
products. We now report on mechanistic and synthetic
aspects of these reactions and quantify the reactivity
difference between cyclopropylmethylstannanes and
allylic stannanes toward electrophilic cleavage.
Acidolysis of cyclopropyltrimethylstannanes 3d, 3f, and
3g with trifluoroacetic acid in CDCl3 was complete within
5 min at room temperature and proceeded exclusively
with ring fission to provide alkenes 4 and 5 (E ) H,
entries 8, 10, and 12). In contrast to the corresponding
sulfinations, however, acidolysis in CD3OD proceeded
exclusively with methyl cleavage to provide cyclopropyl-
methyldimethyltin trifluoroacetates 6 (entries 9, 11, and
13) and methane. Also in contrast to the corresponding
sulfination, acidolysis of 3f in CDCl3 (entry 10) resulted
Results and Discussion
Access to the cyclopropylmethylstannanes 3 was pos-
sible from the commercially available alcohols 1 (Scheme
1). Cyclopropylmethanols 1a and 1c (Z:E ) 15:85) were
chlorinated with hexachloroacetone (HCA) and triphen-
ylphosphine without any detectable ring fission. This
procedure is also reported to be appropriate for chlorinat-
ing the more hindered secondary alcohol 1b, but requires
a large excess of reagents, which can make isolation
difficult. To avoid this problem, we investigated an
(1) Wickham, G.; Young, D.; Kitching, W. Organometallics 1988, 7,
1187.
(2) Young, D.; Kitching, W. Organometallics 1988, 7, 1196.
3580 J. Org. Chem., Vol. 70, No. 9, 2005

Electrophilic Cleavage of Cyclopropylmethystannanes
in predominant formation of the Z rather than E ring
fission product 4b (E/Z ) 0.37). The regiochemistry of
acidolysis of 3g in CDCl3 (entry 12) favored attack at the
less substituted ring carbon (4c/5c ) 9.0) to a slightly
greater extent than was observed for sulfination.
The halogenolysis of carbon-tin bonds has been ex-
tensively investigated as an archetypal electrophilic
substitution. Iodinolysis of organostannanes can proceed
via an electrophilic or radical-chain mechanism with the
latter becoming competitive for slow reactions in nonpolar
solvents on irradiation with visible light. The iodinolyses
of cyclopropylmethylstannanes 3 were performed in the
dark and in solvents for which electrophilic substitution
has been reported to be the exclusive reaction pathway.
The regiochemistry of cleavage was as observed for
acidolysis, i.e., ring fission to yield iodides 4 and 5 (E )
I) in CDCl3 (entries 14, 20, and 22) and with substitution
at the less substituted carbon (4c/5c ) 9.0), but exclusive
methyl cleavage in coordinating solvents (entries 15-17,
21, and 23) to provide cyclopropylmethyldimethyltin
iodides 6 and iodomethane. Iodinolysis of triphenylstan-
nane 3e in either CDCl3 or CD3OD (entries 18 and 19)
proceeded exclusively with phenyl cleavage to yield 6c
and iodobenzene. While the regiochemistry of iodinolysis
was similar to that of acidolysis, the stereochemistry of
reaction of 3f in CDCl3 (entry 20) resembled that of
sulfination, providing a 3.5:1 ratio of the isomeric iodides
4b in favor of the E isomer. Again, this ratio was constant
throughout the reaction, suggesting a kinetically deter-
mined product ratio. Treatment of predominantly Z-4b
(Z/E ) 9.0, prepared by treatment of 1b with MgI2) with
excess iodine in chloroform under the same conditions
did not result in any observable isomerization over 4
days.
FIGURE 1. Pseudo-first-order rate constants as a function
of stannane concentration for the iodinolysis of 3d in dichlo-
romethane (25 °C).
TABLE 2. Relative Second-Order Rate Constants for
the Iodinolysis of Various Organostannanes at 25.0 °C
a
entry
stannane
krel
ref
1
2
3
4
5
6
Me4Sn
i-Pr4Sn
3d
allylSnPh3
(allyl)4Sn
allylSnMe3
1
9b
9b
b
9a
9a
b
0.38
79
3.5 × 107 c
ca. 5 × 109 c
>1010
a In dichloromethane, unless otherwise stated. b This work. c In
acetone.
relative to a number of other tetraorganostannanes is
presented in Table 2.
We next turned our attention to the potentially more
useful cleavage reactions involving aldehydes (Scheme
2). The corresponding reactions of allylic stannanes
generally require heat, high pressure, or Lewis acid
A second-order rate constant was measured for the
iodinolysis of 3d at 25.0 ( 0.3 °C in dichloromethane by
monitoring the disappearance of iodine at 502 nm.
Reactions were relatively rapid and required the use of
a stop-flow apparatus. The concentration of iodine after
mixing was 1.395 × 10-3 M while the concentration of
stannane was between 9.274 × 10-3 and 2.899 × 10-2
M
(5) (a) Lucke, A. J.; Young, D. J. Tetrahedron Lett. 1991, 32, 807.
(b) Lucke, A. J.; Young, D. J. Tetrahedron Lett. 1994, 35, 1609.
(6) Hrubiec, R. T.; Smith, M. B. J. Org. Chem. 1984, 49, 431.
(7) Corey, E. J.; Kim, C. U.; Takeda, M. Tetrahedron Lett. 1972,
4339.
(8) Wickham, G.; Young, D.; Kitching, W. J. Org. Chem. 1982, 47,
4884.
(9) (a) Roberts, R. M. G. J. Organomet. Chem. 1970, 24, 675. (b)
Fukuzumi, S.; Kochi, J. K. J. Phys. Chem. 1980, 84, 2254. (c) Kashin,
A. N.; Beletskaya, I. P.; Malkhasyan, A. T.; Sollov’yanov, A. A.; Reutov,
O. A. J. Org. Chem. (U.S.S.R.) 1974, 10, 2257. (d) Fukuzumi, S.; Kochi,
J. K. J. Org. Chem. 1980, 45, 2654. (e) Buchman, O.; Grosjean, M.;
Nasielski, J.; Wilmet-Devos, B. Helv. Chim. Acta 1964, 47, 1688. (f)
Buchman, O.; Grosjean, M.; Nasielski, J. Helv. Chim. Acta 1964, 47,
1679. (g) Fukuzumi, S.; Kochi, J. K. J. Am. Chem. Soc. 1980, 102, 2141.
(h) Petrosyan, V. S. J. Organomet. Chem. 1983, 250, 157. (i) Hoffmann,
R. W. Angew Chem., Int. Ed. Engl. 1982, 21, 555.
(10) McCormick, J. P.; Barton, D. L. J. Org. Chem. 1980, 45, 2566.
(11) Young, D.; Kitching W. Aust. J. Chem. 1985, 38, 1767.
(12) Negishi, E.; Swanson, D. R.; Rousset, C. J. J. Org. Chem. 1990,
55, 5406.
(13) Mangravite, J. A.; Verdone, J. A.; Kuivila, H. G. J. Organomet.
Chem. 1976, 104, 303.
(14) Patz, M.; Mayr, H. Tetrahedron Lett. 1993, 34, 3393.
(15) (a) Grignon-Dubios, M.; Dunogues, J.; Calais, R. Tetrahedron
Lett. 1981, 22, 2883. (b) Ryu, I.; Hirai, A.; Suzuki, H.; Sonoda, N.;
Murai, S. J. Org. Chem. 1990, 55, 1409. (c) Ryu, I.; Suzuki, H.; Murai,
S.; Sonoda, N. Organometallics 1987, 6, 212.
(i.e., 6.6 to 20.8 equiv). At least five pseudo-first-order
rate constants were measured at each of five concentra-
tions over this range. The rate equation for the disap-
pearance of iodine is satisfactorily described by -d[I2]/
dt ) k[CPMSn(Me)3][I2] and the plot of observed rate
constant versus stannane concentration was linear within
experimental error (Figure 1) and yielded the second-
order rate constant, k ) 4.6 ( 0.8 × 10-1 M-1 s-1
.
An attempt was made to measure the rate constant
for the corresponding iodinolysis of allyltrimethylstan-
nane. The reaction was too rapid, however, being com-
plete in less than 0.01 s at millimolar concentrations of
stannane and iodine (1:1) in dichloromethane. Thus, the
second-order rate constant is > ca. 108 M-1 s-1 (ap-
proaching diffusion control), which is consistent with the
value of ca. 3 × 107 M-1 s-1 determined for the corre-
sponding reaction of tetraallyltin with iodine in acetone.9a
A comparison of rate constants for the iodinolysis of 3d
(3) For reviews see: (a) Denmark, E. E.; Fu, J. Chem. Rev. 2003,
103, 2763. (b) Marshall, J. A. Chem. Rev. 1996, 96, 31. (c) Yamamoto,
Y.; Asao, N. Chem. Rev. 1993, 93, 2207.
(4) Brown, R. S.; Eaton, D. F.; Hosomi, A.; Traylor, T. G.; Wright,
J. M. J. Organomet. Chem. 1974, 66, 249.
(16) Castaing, M.; Pereyre, M.; Ratier, M.; Blum, P. M.; Davies, A.
G. J. Chem. Soc., Perkin Trans. 2 1979, 3, 287.
(17) Kitching, W.; Olszowy, H. A.; Harvey, K. J. Org. Chem. 1982,
47, 1893.
J. Org. Chem, Vol. 70, No. 9, 2005 3581

Lucke and Young
SCHEME 2
and iodine, but that the preferred reaction pathway for
sulfination is via a six-member cyclic transition state
even in this coordinating solvent.
A decreased activating influence of the carbon-tin
bond on cyclopropane fission was also noted on changing
from methyl to phenyl substituents on tin. Sulfur dioxide
did not react with triphenyltin derivative 3e under the
relatively mild conditions employed while iodinolysis in
chloroform resulted in exclusive phenyl cleavage in
preference to ring fission. The lower electron donating
ability of triphenyltin relative to a trialkyltin moiety has
been reported to result in a 102-fold decrease in reactivity
for the electrophilic cleavage of allylic stannanes with
acid or diarylcarbenium ions.
Iodinolysis in dichloromethane proved to be a conve-
nient reaction for quantifying the effect of σ-σ conjuga-
tion on reactivity. A comparison of the second-order rate
constants for iodinolysis of 3d and tetramethyltin (Table
2) reveals a 79-fold difference in reactivity which, while
significant, is well short of the >109 rate enhancement
provided by σ-π conjugation in the corresponding reac-
tion of allylic trialkylstannanes. It was perhaps not
surprising, therefore, that the reaction of 3d with alde-
hydes under various conditions did not mimic the corre-
sponding allylic tin chemistry. Heat or pressure in excess
of that required to promote addition of aldehydes to allylic
stannanes was insufficient to mediate reaction of 3d.
Likewise, a range of quite different Lewis acids that
catalyze the former reaction did not catalyze the latter,
which were complicated by isomerization and/or trans-
metalation. While these processes also occur in the Lewis
acid mediated addition of aldehydes to allylic stannanes,
the isomerized or transmetalated allylic metal intermedi-
ates are still reactive and add to the aldehyde.
promotion.3 We investigated the application of a number
of these experimental protocols to the reactions of 3d with
benzaldehyde or isobutyraldehyde. Neither heat (145 °C,
24 h) nor high pressure (15 kbar, 80 °C, 6 d) promoted
the addition of benzaldehyde to 3d. No reaction was
observed with 1.0 equiv of BF3‚OEt2 and benzaldehyde
(CH2Cl2, -78 to 0 °C), but isomerization to 3-butenyltri-
methylstannane was observed with 2.0 equiv of Lewis
acid or with 1.0 equiv of BF3‚OEt2 in the absence of
aldehyde. Isomerization was also observed for the at-
tempted ZnCl2 promoted addition of isobutyraldehyde to
3d (CH2Cl2, 25 °C, 7 d) and the combination of benzal-
dehyde, MgBr2‚OEt2 (1 equiv), and pressure (CH2Cl2, 17
kbar, 30 °C, 2 d). A variety of other Lewis acids were
also examined, but none promoted homoallylation of the
aldehyde.
The above results confirm the prediction of Traylor and
co-workers that σ-σ conjugation would result in en-
hanced reactivity of cyclopropylmethyltrimethylstannanes
toward electrophilic ring cleavage.4 Sulfur dioxide is a
relatively mild electrophile, yet quantitatively reacts with
3d, 3f, and 3g in under 15 min at room temperature. By
comparison, insertion into inactivated tin-alkyl bonds
can take up to several days in liquid SO2 while allylic
stannanes react with dissolved SO2 in a matter of
seconds. This latter reaction (eq 1) proceeds with syn
stereoselectivity in chloroform, consistent with a six-
member cyclic transition state. In methanol, however,
this stereoselectivity is lost, suggesting disruption of
internal O-Sn coordination. It was somewhat of a
surprise, therefore, when the reaction of sulfur dioxide
with R-methylcyclopropylmethylstannane 3f yielded the
same ratio of alkenyl sulfinates 4b, favoring the E isomer,
in both a coordinating and noncoordinating solvent. A
predominance of the E isomer was also observed for
iodinolysis of this substrate in chloroform, while acidoly-
sis under the same conditions yielded a predominance
of the Z cleavage product. It is tempting to suggest that
this dependence of stereoselectivity on electrophile re-
flects the outcome of a cyclic versus an acyclic mechanism
of addition. It seems reasonable that sulfination or
iodination in chloroform should involve internal O-Sn
or I-Sn coordination, respectively, while the stereochem-
istry of acidolysis might be controlled by steric and/or
stereoelectronic factors in an acylic transition state as is
observed for the corresponding reaction of allylic stan-
nanes.2 This explanation requires, however, that sulfi-
nation in methanol also occurs via a cyclic addition
mechanism. If this is the case, then it could also account
for the quite different regiochemical outcomes observed
for reactions of the different electrophiles in methanol.
While acidolysis and iodinolysis in this solvent proceeded
with exclusive methyl cleavage, sulfination in the same
solvent proceeded with ring fission. We suggest that
methanol-tin coordination reduces σ-σ conjugation in
cyclopropylmethyltrimethylstannanes promoting the sta-
tistically and sterically favored methyl cleavage with acid
Finally, the electrophilic cleavage of cyclopropylmeth-
ylsilanes with a variety of electrophiles has been reported
and, while no rate data are available, a qualitative
comparison of reaction conditions and yields indicates
that the corresponding stannanes are (not surprisingly)
substantially more reactive. Ring fission for both (E)-2-
methylstannane 3g and various ring-substituted silanes
occurs predominantly at the less congested unsubstituted
carbon for all electrophiles examined.
Experimental Section
1-Chloro-1-cyclopropylmethane (2a). To a flask contain-
ing 67 g (253 mmol) of hexachloroacetone at 0 °C was added
12.4 g (47 mmol) of triphenylphosphine and the suspension
was stirred for 5 min. To this solution was added 3.02 g (42
mmol) of 1-cyclopropylmethanol (1a) over 15 min, maintaining
the temperature between 0 and 10 °C. The mixture was
warmed to ambient temperature and stirred for 3 h before
flash distillation (0.1 Torr) to collect all volatile products.
Fractionally distillation provided 3.9 g (95%) of 2a: bp 87 °C
(lit.6 bp 86-88 °C); 1H NMR (CDCl3, 250 MHz) δ 3.40 (d, 2H,
J ) 7.4 Hz), 1.97 (m, 1H), 0.65-0.31 (m, 4H); 13C NMR (CDCl3,
62.8 MHz) δ 50.5, 13.7, 5.6.
Chloro(2-methylcyclopropyl)methane (2c). 2c was pre-
pared in a manner similar to that described above and was
identified by 1H and 13C NMR; see the Supporting Information
for details.
1-Chloro-1-cyclopropylethane (2b). To an ice-cold solu-
tion of 2.55 g (19 mmol) of N-chlorosuccinimide in 80 mL of
anhydrous CH2Cl2 was added 1.3 g (21 mmol) of dimethyl
sulfide dropwise. 1-Cyclopropylethanol (1.5 g, 17 mmol) was
3582 J. Org. Chem., Vol. 70, No. 9, 2005

Electrophilic Cleavage of Cyclopropylmethystannanes
then slowly added, maintaining the temperature between 5
and 10 °C for 1 h. The reaction mixture was then warmed to
ambient temperature, stirred for 62 h, and quenched with
water. The aqueous layer was extracted with ether and the
combined organic extracts were washed with brine, dried
(MgSO4), and concentrated. 1H NMR spectroscopy of the crude
product indicated ca. 5% ring-opened material, which was
carefully titrated with bromine. Subsequent fractional distil-
lation provided 1.1 g (61%) of 2b: bp 101 °C (lit.6 bp 100-102
General Procedure for the Trifluoroacetolysis of (Cy-
clopropylmethyl)stannane Derivatives, 3. To an NMR
tube containing 0.50 mmol of 3 in 0.5 mL of solvent (CDCl3 or
CD3OD) was added 0.060 g (0.52 mmol) of TFA. The NMR tube
1
was stoppered, shaken, and monitored by H NMR spectros-
copy until the reaction was complete (ca. 30 min). Products
were identified by 1H and 13C NMR; see the Supporting
Information for details.
General Procedure for the Iodinolysis of (Cyclopro-
pylmethyl)stannane Derivatives, 3. A solution of 0.5 mmol
of the (cyclopropylmethyl)stannane in 0.2 mL of solvent was
added to a clean dry 5 mm NMR tube wrapped in aluminum
foil to exclude light. To the mixture was added 0.3 mL of iodine
solution (1.6 M) in the deuterated solvent. The NMR tube was
stoppered, shaken, and monitored by 1H NMR spectroscopy
until the reaction was complete. Products were identified by
1H and 13C NMR and GCMS (if required); see the Supporting
Information for details.
1
°C); H NMR (CDCl3, 250 MHz) δ 3.38 (m, 1H), 1.57 (d, 3H,
J ) 6.4 Hz), 1.14 (m, 1H), 0.62-0.34 (m, 4H); 13C NMR (CDCl3,
62.8 MHz) δ 63.9, 24.9, 20.4, 5.6, 5.2.
General Stannylation Procedure of 1-Chlorocyclopro-
pylmethyl Derivatives 2. To a solution of trimethyltin
chloride (39.7 mmol) in THF (40 mL) was added freshly cut
lithium pieces (397 mmol). Formation of a deep green color
indicated the presence of the trimethyltin lithium. Excess
lithium was removed by filtration and the solution was cooled
in ice before drop-wise addition of 2 (33.1 mmol). The mixture
was warmed to ambient temperature, stirred for 3 h, and then
quenched with KF (10%, 15 mL). The aqueous layer was
extracted with Et2O and the combined organic extracts were
washed with saturated NH4Cl, dried (MgSO4), and concen-
trated. Short-path distillation provided products 3, which were
identified by 1H and 13C NMR; see the Supporting Information
for details.
General Procedure for the Reaction of Sulfur Dioxide
and (Cyclopropylmethyl)stannane Derivatives, 3. Sulfur
dioxide was slowly bubbled through a solution of 3 (1.0 mmol)
in CHCl3 (0.5 mL) for 15 min. The solvent was evaporated and
the product triturated with pentane and dried under high
vacuum to provide products that were identified by 1H and
13C NMR and microanalysis; see the Supporting Information
for details.
Acknowledgment. We are grateful to the Austra-
lian Research Council for financial support.
Supporting Information Available: Full experimental
and characterization data for all compounds and 13C NMR
spectra of a mixture of (Z) and (E) trimethylstannane 3-pen-
tenylsulfinate and a mixture of trimethylstannane 2-methyl-
3-butenylsulfinate and trimethylstannane 1-methyl-3-buten-
ylsulfinate. This material is available free of charge via the
Internet at http://pubs.acs.org.
JO047822P
J. Org. Chem, Vol. 70, No. 9, 2005 3583