Interaction of Trimethylsilyl and Trimethylstannyl Substituants with C-O Bonds at the γ Position. Results from X-ray Structural Studies and Solvolysis Studies

Article abstract of DOI:10.1021/jo9723265

Accurate X-ray structural studies have been carried on the γ-Me₃M (M = Si, Sn)-substituted cyclohexyl esters 21-Ns, 22-Ns, 23-Ns, 21-PNB, and 24-PNB. Analysis of the structural parameters reveals that the C(alkyl)-O(ester) bond distance is lengthened slightly compared to nonmetal- substituted analogues provided that the C-M, C-O, and intervening C-C bonds are in the W arrangement as is the case for 22-Ns, 23-Ns, and 24-Ns. However, the C-O bond distances for 22-PNB and 22-Ns, which do not have the W geometry, are not significantly lengthened. These structural effects are consistent with the presence of a percaudel interaction between the back lobe of the C-M σ bond and the C-O σ* orbital. The γ-Me₃Sn cyclohexyl nosylates 21-Ns and 23-Ns undergo unimolecular solvolysis in 97% TFE/H₂O with relative rates ca. 1:>10 000.

Full text of DOI:10.1021/jo9723265

J . Org. Chem. 1998, 63, 3943-3951
3943
In ter a ction of Tr im eth ylsilyl a n d Tr im eth ylsta n n yl Su bstitu en ts
w ith C-O Bon d s a t th e γ P osition . Resu lts fr om X-r a y Str u ctu r a l
Stu d ies a n d Solvolysis Stu d ies
Alison J . Green, Tracey Pigdon, J onathan M. White,* and J oseph Yamen
The School of Chemistry, The University of Melbourne, Parkville, Victoria 3052, Australia
Received December 30, 1997
Accurate X-ray structural studies have been carried on the γ-Me3M (M ) Si, Sn)-substituted
cyclohexyl esters 21-Ns, 22-Ns, 23-Ns, 21-P NB, and 24-P NB. Analysis of the structural parameters
reveals that the C(alkyl)-O(ester) bond distance is lengthened slightly compared to nonmetal-
substituted analogues provided that the C-M, C-O, and intervening C-C bonds are in the W
arrangement as is the case for 22-Ns, 23-Ns, and 24-Ns. However, the C-O bond distances for
22-P NB and 22-Ns, which do not have the W geometry, are not significantly lengthened. These
structural effects are consistent with the presence of a percaudel interaction between the back
lobe of the C-M σ bond and the C-O σ* orbital. The γ-Me3Sn cyclohexyl nosylates 21-Ns and
23-Ns undergo unimolecular solvolysis in 97% TFE/H2O with relative rates ca. 1:>10 000.
In tr od u ction
been demonstrated by low-temperature X-ray studies.
The C(alkyl)-O(ester) bond is significantly lengthened,
and hence weakened, by the presence of the â group 4
substituent.2,10-12 For example, the C(alkyl)-O(ester)
bond distances in the â-(trimethylsilyl) and â-(trimeth-
ylgermyl) p-nitrobenzoates 4 and 5 are 1.483(3) and
1.485(2) Å, respectively,10 which are both significantly
lengthened compared with the unsubstituted analogue
6 for which the corresponding distance is 1.473(2) Å.13
The origin of the bond lengthening in 4 and 5 is believed
to be due to σ-σ* interactions between the high-lying
C-M (M ) Si, Ge) σ orbital and the vacant low-lying
C-O σ* orbital. Consistent with this interpretation is
the absence of any significant effects on the C-O bond
distance in the gauche â-silyl p-nitrobenzoate ester 7, for
which σC-Si - σ*C-O overlap is negligible.10 The interac-
It has been well-established that group 4 metal sub-
stituents have a dramatic stabilizing effect on positive
charge at the â position.1-4 Although this property has
been known for many years, it has been best exemplified
by the elegant solvolysis studies of Lambert and co-
workers.3,5-9 For example, the relative rates unimolecu-
lar solvolysis of the â-trimethylmetal-substituted esters
1-3 are .1014:1013:1012 relative to the corresponding
unsubstituted analogues.5,6 The origin of this stabiliza-
tion is believed to be due to hyperconjugation between
the high-lying C-M σ bond and the carbenium ion p
orbital. These huge rate enhancements observed by
Lambert have their genesis in the ground state as has
tion of group 4 metal substituents with positive charge
at the γ position, although less dramatic than the â-effect,
is also well established. Since Sommer and Whitmore14
first revealed the ability of silicon to interact with positive
charge at the γ position in 1946, group 4 substituents
have been shown to have varying accelerative effects on
* To whom correspondence should be addressed.
(1) Colvin, E. Silicon in Organic Synthesis; Butterworths Mono-
graphs in Chemistry; Butterworths: Boston, 1981.
(2) White, J . M. Aust. J . Chem. 1995, 48, 1227.
(3) Lambert, J . B. Tetrahedron 1990, 46, 2677.
(4) Egorochkin, A. N. Russ. Chem. Rev. 1984, 53, 445.
(5) Lambert, J . B.; Wang, G.; Finzel, R. B.; Teramura, D. H. J . Am.
Chem. Soc. 1987, 109, 7838.
(6) Lambert, J . B.; Wang, G.; Teramura, D. H. J . Org. Chem. 1988,
53, 5422.
(7) Lambert, J . B.; Embledge, R. W.; Malany, S. J . Am. Chem. Soc.
1993, 115, 1317.
(10) White, J . M.; Robertson, G. B. J . Org. Chem. 1992, 57, 4638.
(11) Green, A. J .; Kuan. Y. L.; White, J . M. J . Org. Chem. 1995, 60,
2734.
(12) Chan, V. Y.; Clark, C. I.; Giordano, J .; Green, A. J .; Karalis,
A.; White, J . M. J . Org. Chem. 1996, 61, 5227.
(13) White, J . M.; Robertson, G. B. Acta Crystallogr. 1993, C49, 347.
(14) Sommer, L. H.; Dorfman, E.; Goldberg, G. M.; Whitmore, F. C.
J . Am. Chem. Soc. 1946, 68, 488.
(8) Lambert, J . B.; Chelius, E. C. J . Am. Chem. Soc. 1990, 112, 8120.
(9) Lambert, J . B.; Zhao, X. J . Organomet. Chem. 1996, 521, 203.
S0022-3263(97)02326-8 CCC: $15.00 © 1998 American Chemical Society
Published on Web 05/22/1998

3944 J . Org. Chem., Vol. 63, No. 12, 1998
Green et al.
on the 3-silaprop-1-yl cation 12 suggested partial bridging
with a distance of 1.82 Å between the γ-carbon and the
carbenium ion together with a high degree of positive
charge on the silicon, consistent with the percaudel
interaction.19 The carbenium ion 12 was calculated to
be 10 kcal/mol more stable than the 1-propylcarbenium
ion 13, and the C2-C3 bond distance was no longer than
the corresponding distance for the 1-propylcarbenium ion
13, indicating that the stabilizing effect of the silicon in
12 is not due to increased C-C hyperconjugation. The
γ effect of silicon in the adamantyl framework 14 was
studied by Grob et al.21 and was found to be modest by
comparison with the cyclohexyl framework; thus, the
relative rates of solvolysis of 14 and the unsubstituted
derivative 15 was only 8.6:1. It has been pointed out22
F igu r e 1. Stabilization of positive charge at the γ position
by Homohyperconjugation.
F igu r e 2. Stabilization by inductively enhanced C-C hyper-
conjugation.
the solvolysis rates of γ-disposed leaving groups, depend-
ing on the shape of the carbon framework in the system
under investigation.15-20 The stereochemical require-
ment of the group IV γ effect was first established by
Shiner et al. using a series of γ-silicon substituted
cyclohexyl p-bromobenzenesulfonates.15,16 The presence
of silicon in the γ position increases the rate of solvolysis
of the cis-cyclohexyl brosylate 8 by a factor of 452 when
both the brosylate and silicon substituents are equato-
rially disposed, so that the two substituents, with the two
intervening C-C bonds, have a “W” relationship. Ac-
celeration is not seen, however, when the C-O bond is
axial as in the trans isomer 9. It was later shown that
this acceleration is raised to 2010-fold when the γ
substituent is the trimethylstannyl group and the nucleo-
fuge is p-toluenesulfonate as in 10.20 It has been sug-
gested that during the solvolysis of 8 and 10 the γ-metal
substituent stabilizes the developing carbocation by a
through-space interaction between the back lobe of the
C-M bond and the developing carbocation p orbital.15-20
This type of participation is often referred to as percaudal
(through the tail) homohyperconjugation, and the car-
benium ion can be represented by the valence bond forms
in Figure 1; this mode of participation is supported by
the isolation of large quantities of bicyclo[3.1.0]hexane
11 among the solvolysis products of 8 and 10.15,16,20 An
that the smaller increase in solvolytic rates in the
adamantyl system may be due to the fact that in both
the silylated and unsilylated systems the carbocation
would have three â antiperiplanar C-C bonds that could
stabilize the positive charge by normal hyperconjugation,
thereby reducing the demand on homohyperconjugative
stabilization from the C-Si bond. However, a more
frequently used explanation is that the rigid carbon
framework constrains the CR-Cγ distance to be ca. 2.4
Å, which would hinder overlap between the rear lobe of
the C-Si bonding orbital and the developing carbocation
p orbital. This idea is clearly supported in the studies
of Adcock22 and co-workers on the norbornyl triflates 16
and 17 where the CR-Cγ distance is decreased. The
presence of a trimethylsilyl group at the second bridge-
head carbon of 1-norbornyl triflate increases the rate of
solvolysis by 1319, and this factor is increased to 20 486
for a trimethylstannyl group. Ab initio theoretical cal-
culations that accompany the kinetics data predict that
the CR-Cγ distance is about 2.3 Å in the parent
compounds but that this distance is reduced to 1.9 Å in
the silyl-substituted cation and to 1.8 Å in the stannyl-
substituted case.
Given the effect that γ-group 4 metal substituents have
on the reactivity of ester leaving groups toward unimo-
lecular solvolysis, we were interested in determining
whether there would be any observable ground-state
effects associated with these substituents. In particular,
we were interested to see whether the γ-metal substitu-
ent would cause any significant lengthening of the
C(alkyl)-O(ester) bond distance and to determine the
dependence of these effects (if present) upon geomety. We
chose to study a selected range of γ-trimethyl metal
cyclohexyl esters in which the metal and ester substit-
uents are constrained to have the W relationship as in
18 and also a range of γ-trimethyl metal cyclohexyl esters
in which the trimethyl metal substituent is constrained
to take the axial orientation while the ester substituent
is equatorial as typified by 19. The two basic structures
alternative suggestion is that the developing carbenium
ion in these systems is stabilized by inductively enhanced
C-C hyperconjugation (Figure 2). The electropositive
metal substituent is proposed to raise the energy of the
Câ-Cγ bond, resulting in a better energy match with the
developing carbenium ion. However, a theoretical study
(15) Shiner, V. J ., J r.; Ensinger, M. W.; Kris, G. S. J . Am. Chem.
Soc. 1986, 108, 842.
(16) Shiner, V. J ., J r.; Ensinger, M. W.; Kris, G. S.; Halley, K. A. J .
Org. Chem. 1990, 55, 653.
(17) Shiner, V. J ., J r.; Ensinger, M. W.; Rutkowske, R. D. J . Am.
Chem. Soc. 1987, 109, 804.
(18) Ensinger, M. W.; Shiner, V. J ., J r. Physical Organic Chemistry
1986, A Collection of the Invited Lectures Presented at the 8th IUPAC
Conference on Physical Organic Chemistry, Tokyo, J apan, 24-29 Aug
1986; Studies in Organic Chemistry; Elsevier Science Publishers B.
V.: Amsterdam, 1987; Vol. 31, pp 41-58.
(19) Davidson, E. R.; Shiner, V. J . J . Am. Chem. Soc. 1986, 108,
3135.
(21) Grob, C. A.; Gru¨ndel, M.; Sawlewicz, P. Helv. Chim. Acta 1988,
71, 1502.
(20) Lambert, J . B.; Salvador, L. A.; So, J .-H. Organometallics 1993,
12, 697.
(22) Adcock, W.; Clark, C. I.; Schiesser, C. H.; J . Am. Chem. Soc.
1996, 118, 11541.

Interaction of CH3Si- and CH3Sn- Substituents
J . Org. Chem., Vol. 63, No. 12, 1998 3945
Sch em e 2
F igu r e 3. Interaction of C-O σ* antibonding orbital with the
backlobe of the C-M bond 18 or with the Cγ-Câ bond 19.
Sch em e 1
1, path 2) gave the equatorial γ-silyl alcohol 22-H as the
major product in >90% yield.
The axial and equatorial γ-stannyl alcohols 21-H and
23-H were prepared according to Scheme 2. Conjugate
addition of (trimethylstannyl)lithium to 3-methyl-2-cy-
clohexenone gave the â-stannylcyclohexanone 27 in 90%
yield. The 13C δ value for the Me3Sn methyls in 27
appeared at -11.03 ppm. Given that axially disposed
cyclohexyl trimethystannyl substituents generally absorb
in the range -9 to -10 ppm and equatorial trimethyl-
stannyl substituents absorb in the range -11 to -12
ppm, the value of -11.03 ppm suggests that 27 exists
with a significant population of the conformation 27a .
The conformational A values for a trimethylstannyl
substituent and a methyl substituent are 0.95 and 1.74
kcal/mol, respectively,24 which leads to the prediction that
the axial conformation 27b should be substantially more
stable because it is less strained sterically than the
equatorial conformation 27a . It is possible that the
equatorial conformation 27a is stabilized by a percaudel
interaction between the C-Sn σ bonding orbital and the
carbonyl π* orbital. Reduction of the â-stannylcyclohex-
anone 27 with L-Selectride at -78 °C (Scheme 2, path
1) gave the axial stannylcyclohexanol derivative 21-H as
the only product detectable by NMR, suggesting that all
reaction with the bulky hydride-reducing agent occurred
from the equatorial conformation. Attempts were made
to prepare the equatorial stannylcyclohexanol 23-H by
reduction of the â-stannyl ketone 27 with sodium in wet
ether as in the preparation of the equatorial silyl alcohol.
However, this failed to give the equatorial γ-stannyl
alcohol 23-H and instead gave rise to a complex mixture
possibly resulting from reductive cleavage of the C-Sn
bond. The equatorial γ-stannyl alcohol 23-H was, how-
ever, successfully prepared as a ca. 1:1 mixture with the
axial γ-stannylcyclohexanol 21-H by reduction of 27 with
sodium borohydride (Scheme 2, path 2), separation of
21-H and 23-H was readily achieved by chromatography.
The alcohol 24-H was prepared according to Scheme
3. Conjugate addition of (trimethylstannyl)lithium to
5-isopropyl-2-cyclohexenone gave a ca. 4:1 mixture of
trans-5-isopropyl-3-(trimethylstannyl)cyclohexanone (28)
and the cis isomer 29. This mixture was not separated
but was reduced using sodium borohydride in ethanol to
afford a mixture of the alcohols 24-H and 30-H in a ratio
of ca. 1.2:1 (from reduction of ketone 28) and a smaller
amount of 31-H (from ketone 29). These structures were
tentatively assigned by comparison of their 13C NMR data
18 and 19 would allow the separation of the effects (if
any) from a percaudel-like interaction between the C-M
σ orbital and the C-O σ* orbital and inductively en-
hanced C-C hyperconjugation between the Cγ-Câ σ bond
and the C-O σ* orbital (Figure 3).
To this end, we set out to prepare the model γ-silyl-
and the γ-stannylcyclohexanols 20-H-24-H with a view
to determining the structures of their ester derivatives.
Syn th esis
The axial and equatorial γ-silyl alcohols 20-H and 22-H
were synthesized according to Scheme 1. (Trimethyl-
silyl)lithium was generated from hexamethyldisilane and
methyllithium23 in HMPA/THF and then treated with
commercially available 3-methyl-2-cyclohexenone, giving
the â-silyl ketone 25 along with a small quantity of the
pentamethyldisilanyl ketone 26. The two ketones were
readily separated by vacuum distillation. The 13C δ value
for the Me3Si methyls in 25 appeared at -4.99 ppm; this
high-field shift is consistent with an equatorially disposed
trimethylsilyl substituent, suggesting that the ketone 25
exists substantially in the chair conformation having the
silyl substituent equatorial.24 Consistent with the tri-
methylsilyl group being equatorial was the observation
that reduction of 25 with L-Selectride in THF at -78 °C
(Scheme 1, path 1) gave entirely the axial γ-silyl alcohol
20-H by approach of the bulky hydride reagent from the
face opposite the axial methyl substituent. Reduction of
the â-silyl ketone 25 with sodium in wet ether (Scheme
(23) Hudrlik, P. F.; Waugh, M. A.; Hudrlik, A. M. J . Organomet.
Chem. 1984, 271, 69.
(24) Wickham, G.; Olszowy, H. A.; Kitching, W. J . Org. Chem. 1982,
47, 3788.

3946 J . Org. Chem., Vol. 63, No. 12, 1998
Green et al.
Sch em e 3
F igu r e 4. Thermal ellipsoid plot for compound 21-P NB.
structure of 21-P NB was determined at room tempera-
ture. The structures 21-P NB, 21-Ns, 22-Ns, 23-Ns, and
24-P NB are presented in Figures 4-8, respectively,
which depict 30% ellipsoids drawn using the program
ZORTEP.25 Hydrogen atoms (which were refined isotro-
pically) are omitted for clarity. Selected bond lengths for
21-P NB, 21-Ns, 22-Ns, 23-Ns, and 24-P NB are pre-
sented in Table 1. The data for the corresponding
nonmetalated nosylate derivatives 3226 and p-nitroben-
zoate derivative 33,13 which have been reported else-
where, are included for comparison purposes. Atomic
| with those of the corresponding 5-methyl-3-(trimethyl-
stannyl)cyclohexanols.24 This mixture of alcohols unfor-
tunately was inseparable and was converted to the
corresponding mixture of p-nitrobenzoate ester deriva-
tives 24-P NB, 30-P NB, and 31-P NB. Partial separation
of these esters by chromatography followed by repeated
recrystallization gave pure crystals of the p-nitrobenzoate
ester derivative 24-P NB of the alcohol 24-H, which were
used for X-ray structural analysis (below). Hydrolysis
of 24-P NB gave pure 24-H.
The alcohols 20-H-24-H were converted to the corre-
sponding p-nitrobenzenesulfonate derivatives 20-Ns-24-
Ns by reaction with p-nitrobenzenesulfonyl chloride in
pyridine; the nosylates 21-Ns-23-Ns gave crystals suit-
able for X-ray analysis; however, the axial silicon nosylate
20-Ns consistently gave powders upon recrystallization,
while the equatorial stannyl nosylate 24-Ns decomposed
during crystallization. The axial stannyl p-nitrobenzoate
21-P NB was prepared under standard conditions and
gave crystals that were used suitable for crystallographic
analysis.
coordinates and thermal parameters and complete bond
length and bond angle listings are included in the
Supporting Information. The cyclohexane rings in all
structures have no significant distortions from the regu-
lar chair geometry. The C1-C2 and C1-C6 bond dis-
tances in all structures are on average shorter than the
remaining C-C bond distances in the cyclohexane rings;
this can be rationalized on the basis of hybridization
effects caused by the electronegative oxygen substituent
at C1; there are no systematic trends in the remaining
C-C bond distances. Comparison of the C-O bond
distances within the nosylate structures 21-Ns, 22-Ns,
23-Ns, and 32 and the p-nitrobenzoates 21-P NB, 24-
P NB, and 33, however, do reveal an interesting trend.
Molecu la r Str u ctu r es
The structures of 21-Ns, 22-Ns, 23-Ns, and 24-P NB
were determined at 130.0 K to remove unwanted thermal
effects. The p-nitrobenzoate ester 21-P NB underwent
a destructive phase change upon cooling; therefore, the
(25) Zsolnai, L. ZORTEP, An Interactive ORTEP Program; Univer-
sity of Heidelberg: Germany, 1994.

Interaction of CH3Si- and CH3Sn- Substituents
Ta ble 1. Selected Bon d Dista n ces for th e Ester Der iva tives 21-Ns, 21-P NB, 22-Ns, 23-Ns, 24-P NB, 32, a n d 33
23-Ns 33
J . Org. Chem., Vol. 63, No. 12, 1998 3947
21-Ns
21-P NB
22-Ns
M1*
M2
24-H
32a
M1b
M2
C1-O1
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C6-C1
M-C3
1.487(5)
1.519(6)
1.535(6)
1.527(6)
1.517(7)
1.520(7)
1.514(6)
2.202(4)
2.140
1.459(4)
1.504(6)
1.526(6)
1.580(11)
1.488(11)
1.521(7)
1.496(7)
2.178(5)
2.147
1.5000(16)
1.5130(19)
1.5450(19)
1.545(2)
1.527(2)
1.527(2)
1.5082(19)
1.9078(14)
1.864
1.498(5)
1.517(6)
1.526(6)
1.546(6)
1.523(7)
1.526(7)
1.514(6)
2.173(4)
2.147
1.493(5)
1.526(6)
1.532(6)
1.545(6)
1.537(7)
1.528(6)
1.507(6)
2.182(4)
2.141
1.477(3)
1.515(4)
1.535(4)
1.529(4)
1.541(4)
1.542(4)
1.511(4)
2.154(3)
2.134
1.487(2)
1.510(2)
1.534(2)
1.527(2)
1.537(2)
1.528(2)
1.511(2)
1.463(2)
1.514(2)
1.529(2)
1.537(2)
1.536(2)
1.535(2)
1.514(2)
1.468(2)
1.512(2)
1.526(2)
1.541(2)
1.539(2)
1.537(2)
1.514(2)
M-Me (av)
a
b
* There are two molecules in the asymmetric unit: M1 refers to molecule 1 and M2 refers to molecule 2. Reference 26. Reference
13.
F igu r e 5. Thermal ellipsoid plot for compound 21-Ns.
F igu r e 7. Thermal ellipsoid plot for the two independent
molecules of compound 23-Ns.
F igu r e 6. Thermal ellipsoid plot for compound 22-Ns.
The nosylate 21-Ns in which the C-Sn bond is axial has
a C-O bond distance of 1.487(5) Å, which is identical
with that observed for the nonmetalated nosylate deriva-
tive 32, which is 1.487(2) Å, whereas the two independent
molecules of the equatorial γ-stannyl nosylate 23-Ns have
C-O bond distances of 1.493(5) and 1.498(5) Å, which
appear to be lengthened. The precision with which in-
teratomic distances can be determined in the stannyl-
ated ester derivatives is limited due to the presence of
the heavy atom in these structures. Thus, although the
C-O distances in 23-Ns appear to be lengthened relative
to the axial isomer 21-Ns and the nonmetalated nosylate
32, this is only just significant, and these distances have
relatively high estimated standard deviation values. The
structure of the equatorial γ-silyl nosylate 22-Ns, how-
ever, has a C-O bond distance of 1.500(2) Å, which is
more precisely determined and, clearly, significantly
lengthened relative to 32. In all the nosylate structures,
the geometry about the sulfonate group is similar, in
particular, the S-O1-C1-H1 dihedral angle varies by
only 6°; it can be argued, therefore, that any variation
F igu r e 8. Thermal ellipsoid plot for compound 24-P NB.
in the C-O bond distances in 21-Ns, 22-Ns, and both
molecules of 23-Ns is not likely the result of significant
differences in nonbonded repulsion. The pattern of C-O
bond distances observed for the p-nitrobenzoate deriva-
tives 21-P NB, 24-P NB, and 33 are similar; the equato-
rial γ-stannyl p-nitrobenzoate structure 24-P NB (which
was well determined) revealed a C-O bond distance of
1.477(3) Å, which is significantly lengthened relative to
the nonstannylated ester 33 (1.463(2) and 1.468(2) Å).
The axial γ-stannyl p-nitrobenzoate 21-P NB has a C-O
bond distance 1.459(4), which does not differ significantly
with 33.
The observation of a slight lengthening of the C-O
bond distance in the equatorial γ-silyl 22-Ns and γ-stan-
nyl 23-Ns, 24-P NB esters relative to the nonmetalated
esters 32 and 33, whereas in the axial γ-stannyl esters
21-Ns and 21-P NB the C-O bond distances are unaf-
fected, is consistent with the presence of a percaudel

3948 J . Org. Chem., Vol. 63, No. 12, 1998
Green et al.
Ta ble 2. Rela tive Solvolysis Ra tes for 32, 21-Ns, a n d
Sch em e 4
23-Ns in 97% TF E a t 0 °C
compd
rel rate
32
1
21-Ns
23-Ns
3.1
>5.3 × 104
interaction between the back lobe of the C-M σ bonding
orbital and the σ*C-O antibonding orbital (Figure 3; 18).
This interaction, however, is not possible in the axial
stannyl derivatives 21-Ns and 21-P NB. The absence of
any effects on the C-O bond distances for the axial
stannyl derivatives 21-Ns and 21-P NB implies that the
inductive effect of the metal substituent does not result
in any significant enhancement of the σC-C-σ*C-O inter-
action in the ground state (Figure 3; 19). This result also
suggests that the stabilization of positive charge at the
γ-position by silicon and tin is very likely due in most
part to the percaudel interaction shown in Figure 1,
rather than by inductively enhanced C-C hyperconju-
gation (Figure 2).
P r od u ct An a lyses
The effects of the equatorial γ-silyl and stannyl sub-
stituents on the ground-state structures of the γ-meta-
lated esters 22-Ns, 23-Ns, and 24-P NB, although small,
are nonetheless significant. The effects that these sub-
stituents have on the solvolytic reactivities of these esters
are expected to be much larger, given that the electron
demand at an SN1 (or E1) transition state will be much
greater than the electron demand of a σ*C-O orbital in
the ground state. To test this prediction, the solvolytic
reactivities of the axial and equatorial γ-stannyl nosy-
lates 21-Ns and 23-Ns were determined and compared
with the nonmetalated nosylate ester 32.
The three compounds 21-Ns, 23-Ns, and 32 were too
insoluble in ethanol and in TFE containing more than
10% water for this study. Thus, a more thorough study
aimed at determining the molecularity of the reaction
could not be carried out. However, the products of
solvolysis were easily discernible by 1H NMR analysis of
the ca. 0.2 M solutions of each ester in CF3CD2OH after
more than 10 half-lives. 23-Ns gave exclusively 1-meth-
1
ylbicyclo[3.1.0]hexane (34), for which H NMR data had
been previously reported.28 The product resembles the
homohyperconjugatively stabilized transition state and
is convincing evidence that solvolysis proceeds not via a
ks mechanism but via a kc mechanism. The major
product from solvolysis of the axial γ-tin ester 21-Ns was
1-methylcyclohexene 35 in 70% yield in addition to small
quantities of 1-methylbicyclo[3.1.0]hexane (34) (20%) and
3-methyl-3-(trimethylstannyl)cyclohexanol (21-H) and
(23-H) (10%). The formation of these products can be
accommodated by the mechanism shown in Scheme 4.
Heterolysis of the C-ONs bond in 21-H results in the
formation of the carbenium ion intermediate 36. A small
proportion of this intermediate is captured by water to
give 3-methyl-3-(trimethylstannyl)cyclohexanols 21-H
and 23-H; the major pathway followed, however, is 1,2-
hydride migration to give the â-stannyl-stabilized car-
benium ion 37, which then undergoes destannylation to
give 1-methylcyclohexene (35). This apparently efficient
1,2 H-migration is possibly promoted by the stannyl
substituent, which is antiperiplanar to one of the â-hy-
drogens, possibly imparting some hydridic character by
a σC-Sn-σ*C-H interaction. The isolation of 20% 1-meth-
ylbicyclo[3.1.0]hexane (34) suggests some participation
by the γ-stannyl substituent; however, it is unlikely that
this participation occurs at the rate-determining step
since the activation energies for 21-Ns (23.63 kcal/mol)
and the unsubstituted nosylate 32 (23.52 kcal/mol) are
very similar. It is likely that 34 arises from a ring
inversion of the initially formed cation 36, giving the
alternative conformer 36a , which is percaudally stabi-
lized by the γ-stannyl substituent; destannylation of 36a
gives 34. The major product from solvolysis of tin-free
nosylate 32 was 4-tert-butylcyclohexene (70%), which was
Solvolysis Stu d ies
The rates of solvolysis of 21-Ns, 23-Ns, and the
nonmetalated nosylate derivative 32 were determined in
97% TFE using the NMR method reported by Creary and
J iang.27 All data used in this study gave reaction rate
constants with correlation coefficients of 0.9999 or better.
Rates for 21-Ns and 23-Ns were measured at various
temperatures in order to construct Arrhenius plots that
gave the equations (1) and (2) for 32 and 21-Ns, respec-
tively. The precision of fit for each equation was better
than r ) 0.9999, and these equations were used to predict
the rates of solvolysis for 32 and 21-Ns at 0 °C for
comparison purposes (Table 3).
ln k ) (-1.18 × 104)(1/T) + 29.23
ln k ) (-1.19 × 104)(1/T) + 30.54
(1)
(2)
For 23-Ns, the solvolysis rate was too fast to be
measured accurately even at 0 °C. The first measure-
ment was taken 2 min after solution, and the reaction
had already gone to completion. It was conservatively
estimated that at least 7 half-lives had passed, meaning
that about 99% of the ester had reacted. This gives a
minimum rate constant of at least 4.1 × 10-2 s-1, which
is more than 50 000 times faster than 32. Relative rates
for 21-Ns, 23-Ns, and 32 at 0 °C are presented in Table
3.
(26) White, J . M.; Giordano, J .; Green, A. J . Acta Crystallogr. 1996,
C52, 3204.
(27) Creary, X.; J iang, Z. J . Org. Chem. 1994, 59, 5106.
(28) Dauben, W. G.; Wipke, W. T. J . Org. Chem. 1967, 32, 2976.

Interaction of CH3Si- and CH3Sn- Substituents
J . Org. Chem., Vol. 63, No. 12, 1998 3949
Ta ble 3. Cr ysta l Da ta a n d Str u ctu r e Refin em en t Deta ils for Com p ou n d s 21-Ns, 21-P NB, 22-Ns, 23-Ns, a n d 24-P NB
24-P NB
21-Ns
22-Ns
23-Ns
21-P NB
emp formula
formula wt
C
19H29NO4Sn
C16H25NO5SSn
462.12
C16H25NO5SSi
371.52
C16H25NO5SSn
462.12
C17H25NO4Sn
426.07
454.15
T (K)
radiatn
wavelength (Å)
cryst system
130.0(1)
Mo KR
0.71069
monoclinic
P21/c
130.0(1)
Mo KR
130.0(1)
Cu KR
1.5418 Å
triclinic
P-1
130.0(1)
Mo KR
0.71069
triclinic
P-1
293(2)
Cu KR
1.5418 Å
triclinic
P-1
0.71069
monoclinic
P21/c
space grp
unit cell dimens
a
b
c
10.463(2)
7.235(2)
29.355(5)
6.948(2)
34.854(10)
8.526(2)
6.4080(10)
10.2900(10)
14.4880(10)
95.890(8)
96.730(10)
94.460(10)
939.82(18)
2
12.644(3) Å
10.245(3) Å
15.117(3)
95.49(2)°
95.07(2)°
93.92(2)°
1935.9(8)
4
7.3775(10)
7.7842(10)
19.0143(10)
81.496(10)
83.491(10)
62.008(10)
952.40(18)
2
R
â
109.91(2)
113.15(2)°
γ
vol (Å3)
Z
2089.3(8)
4
1.444
1.244
928
1898.3(9)
4
1.617
1.480
936
Dc (Mg/m3)
1.313
2.360
396
1.586
1.451
936
1.486
10.822
432
µ (mm-1
F(000)
)
cryst size
θ range
0.58 × 0.17 × 0.07 0.3 × 0.2 × 0.09
0.35 × 0.1 × 0.07 0.3 × 0.3 × 0.01
0.35 × 0.25 × 0.013
2.35-75.96
0 e h e 9
-8 e k e 9
-23 e l e 23
3
2.07-25.02
0 e h e 12
0 e k e 8
-32 e l e 32
3
2.34-25.00
-8 e h e 0
-41 e k e 9
-9 e l e 10
3
3.09-74.95
-7 e h e 7
-12 e k e 12
-18 e l e 18
3
2.21-30.05
-17 e h e 1
-12 e k e 12
-17 e l e 18
3
index ranges
intensity controls
interval (min)
160
160
160
160
160
decomposition
insignificant
analytical
0.91 and 0.78
3874
insignificant
analytical
0.97 and 0.75
3623
insignificant
analytical
0.86 and 0.67
7626
insignificant
analytical
0.95 and 0.59
9682
insignificant
analytical
0.68 and 0.09
4263
absorptn method
max min transmission
reflns collected
independent
3658
3337
3811
8712
3944
R(int)
0.0144
0.0320
0.0208
0.0541
0.0291
obsd (I > 2σ(I))
refinement method
data/restraints/param
GOF on F2
3173
2728
3577
6907
3370
full-matrix on F2
3657/0/343
1.072
full-matrix on F2 full-matrix on F2 full-matrix on F2 full-matrix on F2
3337/0/318
1.144
3811/0/318
1.070
8712/0/634
1.040
3944/0/309
1.035
final R indices [I > 2σ(I)]
R1 ) 0.0257
wR2 ) 0.0605
R1 ) 0.0334
wR2 ) 0.0641
R1 ) 0.0317
wR2 ) 0.0722
R1 ) 0.0488
wR2 ) 0.0842
R1 ) 0.0315
wR2 ) 0.0832
R1 ) 0.0336
wR2 ) 0.0847
R1 ) 0.0526
wR2 ) 0.1345
R1 ) 0.0684
wR2 ) 0.1478
R1 ) 0.0389
wR2 ) 0.1025
R1 ) 0.0480
R1 ) 0.0480
R indices (all data)
weighting scheme
w ) 1/[σ2(Fo2) + (A*P)2 + B*P] A ) 0.0287
A ) 0.0348
B ) 4.3892
SHELXL
0.0000(3)
2.081
A ) 0.0373
B ) 0.4388
SHELXL
0.0010(3)
0.001
A ) 0.0946P
B ) 3.5366
SHELXL
0.0000(3)
0.367
A ) 0.0660
B ) 0.6099
SHELXL
0.0019(3)
0.362
2
where P ) (Fo + 2Fc2)/3
B ) 2.2534
SHELXL
0.0000(2)
0.003
extinction method
extinction coeff
max shift/esd
largest diff peak and hole e Å-3
0.922 and -0.605
1.208 and -1.408 0.621 and -0.324 1.897 and -2.322 1.165 and -0.962
Ns, and 24-P NB. The crystals were flash cooled to 130.0 K
using an Oxford Cryostream cooling device. The p-nitroben-
zoate ester 21-P NB unfortunately underwent a destructive
phase change upon cooling, even when cooled slowly; therefore,
data were collected at room temperature. Unit cell dimensions
were corrected for any θ zero errors by centering reflections
at both positive and negative θ angles. The data were
corrected for Lorentz and polarization effects and for absorp-
tion (SHELX 76).30 Structures were solved by direct methods
(SHELXS-86)31 and were refined on F2 (SHELXL-97).32 Crys-
tal data and refinement details for 21-Ns, 21-P NB, 22-Ns, 23-
Ns, and 24-P NB are listed in Table 3.
(b) Syn th esis. General experimental details are as re-
ported in a previous paper.10 Nitrobenzenesulfonyl chloride
purchased from Aldrich was purified before use by column
chromatography in ether followed by 2-fold recrystallization
from chloroform-pentane. 3-Methyl-2-cyclohexenone was
purchased from Aldrich, while 5-isopropyl-2-cyclohexenone was
prepared by the method of Frank.33
accompanied by small quantities of cis- and trans-4-tert-
butylcyclohexanol 10 and 20% yield, respectively.
Con clu sion
There is small but significant C-O bond lengthening
observed in γ-Si or Sn cyclohexyl esters relative to the
corresponding nonmetalated derivatives, provided there
exists a W geometrical relationship between the C-M
bond and the C-O bond. This result is consistent with
the presence of a percaudel interaction between the back
lobe of the C-M σ bonding orbital and the C-O σ*
antibonding orbital. The high reactivity of 23-Ns toward
unimolecular solvolysis further demonstrates29 that small
lengthening of C-O bonds can manifest in a decrease in
the activation energy for heterolysis of that bond.
Exp er im en ta l Section
(a ) Cr ysta llogr a p h y. Diffraction data were recorded on
an Enraf Nonius CAD4f diffractometer operating in the θ/2θ
scan mode at low temperature (130.0 K) for 21-Ns, 22-Ns, 23-
(30) Sheldrick, G. M. SHELX76: Program for Crystal Structure
Determination, Cambridge, England, 1976.
(31) Sheldrick, G. M. SHELXS-86: Crystallographic Computing 3;
Sheldrick, G. M., Kruger, C., Goddard, R., Eds.; Oxford University
Press: Oxford, England, 1985; pp 175-189.
(32) Sheldrick, G. M. SHELXL-93: Program for Crystal Structure
Refinement. University of Gottingen, Germany, 1993.
(29) Briggs, A. J .; Glenn, R.; J ones, P. G.; Kirby, A. J .; Ramaswamy,
P. J . Am. Chem. Soc. 1984, 106, 6200.

3950 J . Org. Chem., Vol. 63, No. 12, 1998
Green et al.
Con ju ga t e Ad d it ion of Tr im et h ylst a n n yllit h iu m t o
5-Isop r op yl-2-cycloh exen on e: P r ep a r a tion of Keton es
28 a n d 29. Trimethylstannyl chloride (1.0 M in THF 34.88
mL, 0.142 mol) was added to a mixture of anhydrous tetrahy-
drofuran (50 mL) and excess lithium metal (10 g) under
nitrogen and left stirring overnight. After removal of excess
lithium, the reaction mixture was treated with 5-isopropyl-2-
cyclohexenone (3.169 g, 0.023 mol), which was added dropwise.
After the mixture was stirred for 7 h, water (150 mL) and the
mixture were left to stir for 15 min. The resulting mixture
was extracted with ether (3 × 75 mL) and the ether solution
dried (MgSO4) and then evaporated under reduced pressure
to give a mixture of cis-5- (29) and trans-5-isopropyl-3-
(trimethylstannyl)cyclohexanone (28) in an approximate ratio
of 1:4 respectively (6.25 g, 90%). 28: 13C NMR δ -10.69, 19.79,
brine (3 × 50 mL), dried (MgSO4), and evaporated under
reduced pressure to give 25 as a colorless oil (2.904 g, 58%),
which was contaminated by a small amount (ca. 10%) of
3-(pentamethyldisilanyl)-3-methylcyclohexanone (26). Puri-
fication by silica gel column chromatography gave 25 as a
colorless oil: 1H NMR (CDCl3) δ 2.32 (1H, d, J ) 12.6 Hz,
H2ax), 2.28-2.24 (2H, m, H6ax,eq), 2.06-1.92 (3H, m, H2eq,
H5ax,eq), 1.74 (1H, ddd, J ) 14.2, 12.6, 4.7 Hz, H4ax), 1.52-
1.44 (1H, m, H4eq), 0.89 (3H, s), -0.05 (9H, s); 13C NMR
(CDCl3) δ 212.58 (C1), 47.88 (C2), 41.32 (C6), 30.50 (C4), 25.55
(C3), 22.79 (C5), 18.58 (Me), -4.99 (SiMe’s). Anal. Calcd for
C
10H20OSi: C, 65.15; H, 10.93. Found: C, 64.95; H, 10.84.
3-Meth yl-3-(tr im eth ylsta n n yl)cycloh exa n on e (27).
A
solution of trimethylstannyl chloride (0.014 mol in THF 23.5
mL) was treated with lithium metal (0.09 mol) and sonicated
for 3 h. The excess lithium was removed, and the resulting
cloudy green solution was cooled to 0 °C, treated with a
solution of 3-methyl-2-cyclohexanone (1 g, 0.009 mol, in THF
5 mL), and stirred at room temperature for 12 h. The resulting
mixture was diluted with water (50 mL) and extracted with
ether (5 × 30 mL). The combined ether extracts were washed
with water (3 × 100 mL), dried (MgSO4), and evaporated under
reduced pressure to give 27 as an oil (2.4 g, 90%) that was
pure by 13C NMR. An analytically pure sample was prepared
by Kugelrohr distillation (90° C, 0.4 mmHg): 1H NMR (CDCl3)
δ 2.53 (1H, d, J ) 15.2 Hz, H2ax), 2.38-2.2 (2H, m, H6ax,eq),
2.16 (1H, dd, J ) 15.2, 2.6 Hz, H2eq), 2.07 (1H, m, H5), 1.97
(1H, m, H4eq), 1.74 (1H, m, H5), 1.57 (1H, ddd, J ) 16.0, 10.0,
5.0 Hz, H4ax), 1.15, (3H, s), 0.02, (9H, s); 13C NMR (CDCl3) δ
211.5 (C1), 53.73 (C2), 41.30 (C6), 37.04 (C5), 31.32 (C3), 26.37
(Me), 24.73 (C4), -11.03; MS M+ 276.0526 (calcd for C10H20OSn
20.03, 21.66, 31.01, 31.88, 45.20, 45.30, 45.61, 212.00. 29: 13
C
NMR δ -11.74, 19.07, 19.62, 23.53, 32.75, 45.25, 50.16, 212.00.
Sod iu m Bor oh yd r id e Red u ction of th e Isop r op yl Ke-
ton e Mixtu r e 28 a n d 29. To an ethanol (100 mL) solution
of the crude isomeric mixture of 28 and 29 (3 g, 0.0099 mol)
was added sodium borohydride (0.5 g, 0.0132 mol) in portions
with stirring at 0 °C. The reaction was left to stir at room
temperature for a further 4 h. The reaction mixture was
quenched cautiously by adding dilute hydrochloric acid (10 mL)
dropwise. Excess water (100 mL) was added, and the mixture
was extracted with ether (3 × 50 mL), the combined organic
extracts were washed with saturated sodium chloride (2 × 50
mL) and dried, and the solvent was removed under reduced
pressure to give an oily residue. In the 13C NMR of this oil,
characteristic signals were discernible at δ -11.06 (equatorial
SnMe3), 67.35 (C-OH) 24-H; -9.48 (axial SnMe3), 70.27 (C-
OH), 30-H; -11.92 (equatorial SnMe3), 71.83 (C-OH) 31-H.
The residue (2.4 g 80%) was dissolved in pyridine (10 mL) and
stirred on ice under an atmosphere of nitrogen. The mixture
was then treated with p-nitrobenzoyl chloride (1.6 g, 1.1 equiv)
and stirred for 3 h. The resulting slurry was then poured into
water (100 mL), and the aqueous suspension was extracted
with ether (3 × 50 mL). The combined extracts washed with
1 M HCl (3 × 50 mL) and aqueous NaaHCO3 (10%, 3 × 50
mL), dried (MgSO4), and evaporated down to a creamy solid
(4 g 90%). The mixture of p-nitrobenzoates 24-P NB, 30-P NB,
and 31-P NB was partially separated by dry flash chromatog-
raphy on silica gel using ether/petroleum ether as eluants.
Partially purified fractions were repeatedly recrystallized from
methanol to afford r-5-isopropyl-t-3-(trimethylstannyl)cyclo-
hexan-t-1-yl p-nitrobenzoate (24-P NB) as yellow needlelike
crystals (300 mg): mp 95-97.5 °C; 1H NMR (CDCl3) δ 8.3 (1H,
d, J ) 9 Hz), 8.3 (1H, d, J ) 9 Hz), 5.05 (1H, m), 2.2-2.0 (2H,
m), 1.95-1.75 (2H, m), 1.5-1.1 (5H, m), 0.9 (3H, d J ) 6 Hz);
13C NMR δ -11.52, 16.74, 21.09, 21.16, 26.86, 31.56, 34.19,
35.79, 42.27, 73.05, 123.36, 130.57, 136.31, 150.36, 164.21.
276.0536); IR νmax (thin film) 1707, 767 cm-1
.
tr a n s-3-Meth yl-3-(tr im eth ylsilyl)cycloh exa n ol (20-H).
A solution of L-Selectride (0.001 mol) in anhydrous THF (10
mL), stirred under nitrogen, was cooled to -78 °C and treated
with a solution of 3-methyl-3-(trimethylsilyl)cyclohexanone
(25) (0.20 g, 0.001 mol). The resulting mixture was stirred at
this temperature for 4 h and then quenched by the addition
of water (3 mL) and allowed to reach room temperature. The
resulting solution was treated with with sodium hydroxide (3
M, 0.24 mL) and 30% H2O2 (dropwise) and stirred for 1 h. The
mixture was extracted with ether (3 × 50 mL), and the
combined extracts were washed with brine (3 × 50 mL), dried
(MgSO4), and evaporated to give 20 as a colorless oil (0.15 g,
73%): 13C NMR (CDCl3) δ 68.11, 44.50, 35.00, 34.96, 26.59,
20.87, 18.98, -2.24; IR νmax (thin film) 3454, 1068 cm-1
.
tr a n s-3-Meth yl-3-(tr im eth ylsta n n yl)cycloh exa n ol (21-
H). A solution of L-Selectride (0.0018 mol) in anhydrous THF
(10 mL) was cooled to -78 °C and then treated with a solution
of 3-methyl-3-(trimethylstannyl)cyclohexanone (27) in THF (5
mL). The resulting mixture was stirred at this temperature
for 4 h and then diluted with water (3 mL) and allowed to
reach room temperature. Treatment with aqueous sodium
hydroxide (3M, 0.6 mL) was followed by dropwise addition of
30% H2O2 (0.6 mL). The mixture was stirred for a further hour
and then extracted with ether (3 × 50 mL). The combined
extracts were dried (MgSO4) and evaporated under reduced
pressure to give 21 as a colorless oil: 13C (CDCl3) δ 69.21,
49.47, 38.94, 35.46, 31.16, 29.27, 23.86, -9.69; IR νmax (thin
The ester 24-P NB (200 mg) was refluxed in methanol (20
mL) containing potassium hydroxide (2 equiv) for 2 h. The
solution was diluted with water (100 mL) and extracted with
ether (3 × 50 mL), and the combined extracts were dried
(MgSO4) and evaporated under reduced pressure, giving r-5-
isopropyl-t-3-(trimethylstannyl)cyclohexan-t-1-ol (24-H) (120
mg, 90%): 13C NMR (CDCl3) δ -11.06, 17.8, 20.80 (overlapping
i-Pr CH3’s), 27.97, 31.93, 37.65, 38.98, 41.03, 67.35.
film) 3345, 1447 cm-1
.
3-Meth yl-3-(tr im eth ylsilyl)cycloh exa n on e (25). A solu-
tion of HMPA (30 mL) and hexamethyldisilane (8.9 mL, 0.044
mol) was cooled with stirring to 0 °C. The mixture was then
diluted with cold THF (15 mL at -78 °C) and treated
immediately with methyllithium (1.4 mol solution in ether,
25 mL) followed by the addition of cold THF (100 mL, -70
°C). The resulting bright red mixture was cooled to -78 °C
and treated with a solution of 3-methylcyclohexanone (3.01 g,
0.027 mol) in THF (5 mL). The mixture was stirred at -78
°C for a further 15 min, warmed to room temperature, and
stirred for a further hour. The addition of methanol (15 mL)
was followed by extraction with petroleum (3 × 50 mL). The
combined extracts were washed with water (3 × 50 mL) and
cis-3-Meth yl-3-(tr im eth ylsta n n yl)cycloh exa n ol (23-H).
A stirred solution of 3-methyl-3-(trimethylstannyl)cyclohex-
anone (27) (0.51 g, 0.0019 mol) in ethanol (50 mL), was treated
at 0 °C with sodium borohydride (0.14 g, 0.004 mol). The
resulting mixture was stirred at this temperature for 4 h and
then quenched by the addition of 1 M HCl (5 mL). The
resulting mixture was extracted with ether (3 × 50 mL), and
the combined extracts were washed with brine (3 × 50 mL),
dried (MgSO4), and evaporated under reduced pressure to give
21-H and 23-H as a ca. 1:1 mixture (0.42 g, 84%). Separation
by flash chromatography on silica gel with ether/petroleum
eluants gave 21-H and cis-3-methyl-3-(trimethylstannyl)cy-
clohexanol (23-H): 13C NMR (CDCl3) δ 66.00, 45.38, 35.90,
35.70, 25.93, 23.50, 19.69, -11.63; IR νmax (thin film) 3357,
(33) Frank, R. L.; Hall, H. K. J . Am. Chem. Soc. 1950, 72, 1645.
1447, 1048, 762 cm-1
.

Interaction of CH3Si- and CH3Sn- Substituents
J . Org. Chem., Vol. 63, No. 12, 1998 3951
cis-3-Meth yl-3-(tr im eth ylsilyl)cycloh exa n ol (22-H). A
mixture of 3-methyl-3-(trimethylsilyl)cyclohexanone (25) (0.53
g, 0.0028 mol), water (0.3 mL), and ether (25 mL) was stirred
at 0 °C. The mixture was then treated with sodium metal
(0.29 g, 0.0127 mol) and left to stir at 0 °C for 4 h. Ethanol (1
mL) was added to destroy the excess sodium, and then water
(10 mL) was added. The organic layer was separated, dried
(MgSO4), and evaporated under reduced pressure to give 22-H
as a colorless oil (0.33 g, 65%): 13C NMR (CDCl3) δ 66.02,
40.75, 36.23, 30.21, 21.23, 19.64, 17.89, -5.26.
cis-3-Meth yl-3-(tr im eth ylsta n n yl)cycloh exyl n osyla te
(23-Ns): colorless blocks from pentane; 87% yield; mp 230-
1
236 °C dec; H NMR δ 0.00 (9H, s), 1.12 (3H, s), 1.2-1.9 (m,
8H), 4.88 (septet, J ) 3.5 Hz, 1H), 8.22 (ab quartet, 4H); 13C
δ -11.55, 19.69, 23.69, 25.74, 32.60, 35.20, 42.59, 81.52, 124.33,
128.90, 143.38, 150.2.
Kin etics. Solvents for the kinetics experiments were
purified as follows. 2,6-Lutidine was stirred over KOH for 24
h before distillation from CaH2 under nitrogen. Trifluoroeth-
anol was stirred over sodium sulfate for 10 min and then over
sodium bicarbonate for 10 min followed by distillation under
nitrogen, distilled, and stored under an atmosphere of nitrogen
and over 4 Å molecular sieves.
The method used was based on that reported by Creary and
J iang.27 The substrate (1.5 × 10-5 mol) was dissolved in 0.5
mL of a solution of 0.04 M 2,6-lutidine in 97% trifluoroethanol.
The solution was added to an NMR tube that contained an
insert tube of acetone-d6 as a standard and locking signal. The
NMR tube was kept in the temperature-controlled probe and
periodically analyzed so that 30-40 measurements were
obtained over approximately 2 half-lives. An infinity reading
was taken after at least 10 half-lives.
tr a n s-3-Meth yl-3-(tr im eth ylsta n n yl)cycloh exyl p-Ni-
tr oben zoa te (21-P NB). A stirred mixture of p-nitrobenzoyl
chloride (0.206 g, 0.0011 mol) and pyridine (0.5 mL), was
treated at 0 °C with a solution of trans-3-methyl-3-(trimethyl-
stannyl)cyclohexanol (21-H) (0.200 g, 0.0007 mol) in pyridine
(0.75 mL). The resulting mixture was stirred at room tem-
perature for 1 h and cooled to 0 °C, and excess p-nitrobenzoyl
chloride was quenched by the addition of water (0.5 mL). The
mixture was stirred for 20 min and then extracted with ether
(3 × 50 mL), washed with 10% CuSO4 solution (2 × 50 mL),
water (50 mL), and then aqueous NaHCO3 (10%, 2 × 50 mL),
dried (MgSO4), and evaporated down to a solid. Crystallization
from ether/ petroleum yielded 21-H as clear plates (0.18 g,
P r od u ct Deter m in a tion . The substrate (1.0 × 10-4 mol)
was dissolved in 0.5 mL of a solution of 0.25 M 2,6-lutidine in
97% trifluoroethanol. The solution was added to an NMR tube
and sealed. After at least 10 half-lives, the solution was
1
60%): mp 107.5-108.5 °C; H NMR (CDCl3) δ 8.26 (2H, d, J
) 9 Hz), 8.18 (2H, d, J ) 9 Hz), 5.06 (1H, m), 2.26 (1H, m),
2.08 (1H, m), 1.84 (2H, m), 1.56-1.18 (4H, m), 1.16 (3H, s),
0.14 (9H, s); 13C NMR (CDCl3) δ 164.08, 130.61, 123.44, 74.02,
45.22, 38.84, 31.60, 30.72, 29.23, 23.57, -9.74.
1
analyzed by H NMR, which showed the resonances reported
below.
Gen er a l P r oced u r e for th e P r ep a r a tion of p-Nitr oben -
zen esu lfon a te Der iva tives 20-Ns, 21-Ns, 22-Ns, a n d 23-
Ns. Nitrobenzenesulfonyl chloride (1.2 equiv) was dissolved
in pyridine (freshly distilled, 0.5 mL) at 0 °C under an
atmosphere of nitrogen as the alcohol (0.1 g) was added all at
once. The reaction was stirred at this temperature for 1 h
and left at 0 °C overnight. The reaction was diluted with cold
diethyl ether (freshly distilled; 30 mL) and filtered into a
separation flask. The solution was washed with cold cupric
sulfate solution (saturated; 3 × 10 mL) followed by cold sodium
bicarbonate (2 × 10 mL) before it was dried with magnesium
sulfate. Filtration and concentration (in vacuo) gave solids
that could usually be purified by radial chromatography and
crystallization; however, the nosylates were generally suscep-
tible to decomposition after a short exposure to the atmo-
sphere, so these esters were stored and handled, as far as
possible, under nitrogen.
23-Ns: 1-methylbicyclo[3.1.0]hexane;28 δ 0.17 (dd, J ) 8.1,
4.5 Hz, 1H, CH2 of cyclopropane ring), 0.33 (t, J ) 4.5 Hz, 1H,
CH2 of cyclopropane ring), 0.52 (s, 9H, Me3SnOR), 0.91
(quintet, J ) 4 Hz, 1H, bridgehead CH), 1.20 (s, 3H, Me), 1.48-
1.83 (m, 6H, 3 × CH2 of cyclopentane ring);
21-Ns: δ 0.47 (s, 9H, Me3SnOR, corresponds to the forma-
tion of both 1-methylbicyclo[3.1.0]hexane and 1-methylcyclo-
hexene); 1-methylcyclohexene (70%) 1.46-1.64 (m incorporat-
ing singlet at 1.6, 7H, Me and ring CH2), 1.84-1.97 (m, 4H),
5.35-5.40 (m, 1H, olefin CH); 1-methylbicyclo[3.1.0]hexane
(20%, signals as from solvolysis of 23-Ns); trans-3-methyl-3-
trimethylstannylcyclohexanol (10%) δ -0.01 (s, 9H), 1.1 (s,
3H).
32: 4-tert-butylcyclohexene (90%), δ 0.86 (s, 9H, tert-butyl
CH3), 1.1-1.35 (m, 1H, ring CH), 1.7-2.2 (m, 6H, ring CH2),
5.68 (s, 2H, olefinic CH); cis-4-tert-butylcyclohexanol (ca. 20%)
4.05 (br s, 1H); trans-4-tert-butylcyclohexanol (10%), 3.75 (m,
1H, CHOH).
tr a n s-3-Meth yl-3-(tr im eth ylsilyl)cycloh exyl n osyla te
(20-Ns): colorless powder from pentane (80%); mp 95-96 °C;
1H NMR (CDCl3) δ 8.39 (2H, d, J ) 8 Hz), 8.04 (2H, d, J ) 8
Hz), 4.8 (1H, m), 1.5-1.9 (5H, m), 1.4-1.0 (3H, m), 0.92 (3H,
s), -0.12 (9H, s); 13C NMR (CDCl3) δ -3.34, 18.66, 21.59, 24.06,
31.65, 32.90, 39.41, 82.25, 124.29, 128.84, 143.2, 150.4.
tr a n s-3-Meth yl-3-(tr im eth ylsta n n yl)cycloh exyl n osy-
la te (21-Ns): colorless blocks from pentane in 90% yield; mp
Ack n ow led gm en t. Our thanks go to the Australian
Research Council Large Grants Scheme for financial
support and to Dr. J ohn Lawlor of this department for
helpful criticisms in the preparation of this manuscript.
1
71-72 °C; H NMR δ 0.01 (s, 9H), 1.08 (s, 3H), 1.2-1.4 (m,
4H), 1.7-1.8 (m, 2H), 1.93 (br d, J ) 12 Hz, 1H), 2.10 (d, J )
12 Hz, 1H), 4.57 (septet, J ) 5 Hz, 1H), 8.23 (ab quartet, 4H);
13C δ -9.84, 23.56, 28.98, 30.82, 32.41, 38.16, 45.97, 82.81,
124.36, 128.93, 143.39, 150.52
cis-3-Meth yl-3-(tr im eth ylsilyl)cycloh exyl n osyla te (22-
Ns): colorless needles from pentane (85%); mp 92-93 °C; 1H
NMR (CDCl3) δ 8.4 (2H, d, J ) 8 Hz), 8.05 (2H, d, J ) 8 Hz),
4.9 (1H, m), 1.95 (1H, m), 1.5-1.9 (3H, m), 1.2-1.5 (4H, m),
0.95 (3H, s), -1.4 (9H, s); 13C NMR (CDCl3) δ 150.41, 143.25,
128.68, 124.88, 81.99, 37.98, 33.10, 29.47, 22.01, 19.60, 17.41,
-5.45.
Su p p or tin g In for m a tion Ava ila ble: Solvolysis data for
37 and 42, spectral data for 37 and 39, and a full listing of
bond distances, angles, dihedral angles, anisotropic displace-
ment parameters and H atom coordinates for 21-P NB, 21-
Ns, 22-Ns, 23-Ns, and 24-P NB (52 pages). This material is
contained in libraries on microfiche, immediately follows this
article in the microfilm version of the journal, and can be
ordered from the ACS; see any current masthead page for
ordering information.
J O9723265