Ground-state stereoelectronic effects involving silicon and germanium: A comparison of the effects of germanium and silicon substituents on C-O bond lengths at the β-position

Article abstract of DOI:10.1021/jo9602480

Results of low-temperature crystal structures for β-(trimethylgermyl)-substituted cyclohexyl esters and β-(trimethylsilyl)-substituted cyclohexyl esters are reported. These reveal that the trimethylgermyl substituent causes significant lengthening of este

Full text of DOI:10.1021/jo9602480

J . Org. Chem. 1996, 61, 5227-5233
5227
Gr ou n d -Sta te Ster eoelectr on ic Effects In volvin g Silicon a n d
Ger m a n iu m : A Com p a r ison of th e Effects of Ger m a n iu m a n d
Silicon Su bstitu en ts on C-O Bon d Len gth s a t th e â-P osition
Vee Yee Chan, Christopher I. Clark, J osy Giordano, Alison J . Green, Andrew Karalis, and
J onathan M. White*
School of Chemistry, The University of Melbourne, Parkville, Victoria 3052, Australia
Received February 6, 1996X
Results of low-temperature crystal structures for â-(trimethylgermyl)-substituted cyclohexyl esters
and â-(trimethylsilyl)-substituted cyclohexyl esters are reported. These reveal that the trimethyl-
germyl substituent causes significant lengthening of ester C-O bond lengths at the â position when
compared to the corresponding unsubstituted cyclohexyl esters. Comparison of trimethylgermyl
and trimethylsilyl ester C-O bond lengths suggests that the ground state effects of these two
substituents are similar.
In tr od u ction
The stabilization of positive charge at the â-position
is central to the chemistry of silicon-containing organic
compounds.1-3 This is clearly demonstrated by the en-
hanced rates of unimolecular solvolysis of â-(trimethyl-
silyl) esters compared to the corresponding silicon-free
analogues.2,4 For example, the â-(trimethylsilyl)-substi-
F igu r e 1. σC-Si-σ*C-O interaction between a C-Si bonding
orbital and a C-O antibonding orbital in the antiperiplanar
geometry.
tuted trifluoroacetate ester 1 solvolyses at a remarkable
1012 times faster than the silicon-free analogue 2, indicat-
ing that the trimethylsilyl substituent stabilizes the
intermediate cation 3 by ca. 18 kcal/mol relative to H.
The origin of this remarkable stabilization is believed to
be hyperconjugation between the C-Si bond and the
carbocation p-orbital.1-5 The C-Si bond is expected to
be particularly effective at stabilizing positive charge for
two reasons: it has a high energy bonding orbital, with
energy (and hence donor ability) similar to that of an
oxygen nonbonded pair of electrons,3 and the C-Si bond
is polarized toward the carbon atom (the difference in
the Pauling electronegativity values for C and Si is 0.7)
resulting in effective overlap with an adjacent p-orbital.
We have recently demonstrated, using low-temperature
X-ray crystallography, that the presence of a trimethyl-
silyl substituent antiperiplanar to a â disposed ester
leaving group results in lengthening and hence weaken-
ing of the C(alkyl)-O(ester) bond in the ground state.3,6
For example, the C-O bond length in the p-nitrobenzoate
ester 4 is 1.490(2) Å, which is significantly longer than
the corresponding silicon-free ester 5, which has a C-O
bond length of 1.473(2) Å. This effect is dependent upon
the geometrical relationship between the trimethylsilyl-
and the ester group; thus, whereas 4 which has an
antiperiplanar relationship between the silicon and the
ester shows significant lengthening of the C-O bond, the
ester 6 with a gauche relationship shows no significant
C-O bond lengthening. The origin of the C-O bond
lengthening was proposed to be the result of a hyper-
conjugative like σ-σ* interaction between the C-Si bonding
orbital and the C-O antibonding orbital (Figure 1); this
interaction is optimized in 4 where significant effects are
observed but significantly reduced in 6 where overlap
between the σC-Si and σ*C-O orbitals is poor.
It has been demonstrated that trimethylgermyl and
trimethylstannyl substituents have even stronger stabi-
lizing effects on positive charge at the â position,7 with
evidence to suggest that hyperconjugation is the pre-
dominant source of stabilisation. For example, the
trimethylgermyl-substituted ester 7 (with corrections for
population of the diaxial conformation) has been shown
X Abstract published in Advance ACS Abstracts, J uly 1, 1996.
(1) Colvin, E. Silicon in Organic Synthesis: Butterworths Mono-
graphs in Chemistry; Butterworth: Boston, 1981.
(2) Lambert, J . B. Tetrahedron 1990, 46, 2677.
(3) White, J . M. Aust. J . Chem. 1995, 48, 1227
(4) Lambert, J . B.; Wang, G.-T.; Finzel, R. B.; Teramura, D. H. J .
Am. Chem. Soc. 1987, 109, 7838.
(5) Lambert, J . B.; Emblidge, R. W.; Malany, S. J . Am. Chem. Soc.
1993, 115, 1317.
(6) White, J . M.; Robertson, G. B. J . Org. Chem. 1992, 57, 4638.
(7) Lambert, J . B.; Wang, G-t.; Teramura, D. H. J . Org. Chem. 1988,
53, 5422.
S0022-3263(96)00248-4 CCC: $12.00 © 1996 American Chemical Society

5228 J . Org. Chem., Vol. 61, No. 16, 1996
Chan et al.
to solvolyze by a kc mechanism at a rate 1014 times faster
than the unsubstituted ester 2, and the trimethylstannyl
esters 8 solvolyzes at .1014 faster than 2. The relative
Sch em e 1
rates of unimolecular solvolyses of â- (trimethylsilyl)-,
-(trimethylgermyl)-, and -(trimethylstannyl)-substituted
esters are 1, 102, and .ca. 102, suggesting that the ability
of group IV substituents to stabilize positive charge at
the â position increases down the group: Si < Ge , Sn.
This trend might be expected to be related to the
ionization potentials of the C-M bonds (M ) Si, Ge, Sn),
which give a measure of the effectiveness of these
substituents at stabilizing positive charge by hypercon-
jugation; these have been determined from photoelectron
spectroscopy for the Et4M compounds8 to be 10.04, 9.7,
and 8.7 eV, respectively, suggesting that a C-Si bond
has similar donor properties to an oxygen non bonded
pair,9 whereas a C-Sn bond is similar to a nitrogen lone
pair10 and the C-Ge is somewhere in between. With the
demonstrated reactivity of â-(trimethylgermyl) and -(tri-
methylstannyl) esters toward unimolecular solvolysis, we
became interested in the ground state properties of these
types of compounds; in particular, we were interested in
determining the effects that the trimethylgermyl and
trimethylstannyl substituents would have on C-O bond
lengths at the â position and comparing these with the
effects of a trimethylsilyl substituent.
Sch em e 2
butylcyclohexene oxide (14) with (trimethylgermyl)-
lithium and (trimethylstannyl)lithium7 (Scheme 1). The
trimethylgermyl- and trimethylstannyl- alcohols 10 and
12 were similarly prepared by ring opening of trans-4-
methylcyclohexene oxide (15) (Scheme 1). The trimeth-
ylsilyl alcohol 13 was prepared as previously described.11
trans-4-tert-Butylcyclohexene oxide (14) was prepared by
the method of Rickborn and Quarticci,12 while the trans-
4-methylcyclohexene oxide (15) was prepared by an
analogous procedure as described in Scheme 2; 4-meth-
ylcyclohexene was converted into a ca. 1:1 mixture of cis-
and trans-epoxides (step 1), which proved to be in-
separable. The mixture was treated with a stream of
anhydrous hydrogen chloride gas in the presence of
p-nitrobenzoyl chloride, giving the two chloroesters 16
and 17 from diaxial epoxide ring opening. Fractional
crystallization of this mixture from methanol gave the
pure chloro ester 16 (10%). The chloro ester 16 was
converted into the pure trans-epoxide 15 by treatment
with potassium carbonate in refluxing methanol. The
tert-butyl-substituted trimethylgermyl alcohol 9 was
converted to the p-nitrobenzoate ester 18 by treatment
with p-nitrobenzoyl chloride in pyridine, and the methyl-
substituted trimethylgermyl alcohol 10 was converted
into the 2,4-dinitrobenzenesulfenate ester 19 by treat-
ment with 2,4-dinitrobenzenesulfenyl chloride in meth-
ylene chloride/pyridine. The trimethylsilyl alcohol 13 was
similarly converted to the 2,4-dinitrobenzenesulfenate
ester 20. Unfortunately, all attempts to synthesize ester
derivatives of the trimethylstannyl alcohols 10 and 12
Resu lts
Syn th esis. The trimethylgermyl alcohols 9 and 10,
trimethylstannyl alcohols 117 and 12, and the trimethyl-
silyl alcohol 13 were used as model substrates for the
preparation of ester derivatives for X-ray analysis. The
trimethylgermyl alcohol 9 and the trimethylstannyl
alcohol 11 were prepared by ring opening of trans-4-tert-
(8) Schweig, A.; Weidner, U.; Manuel, G. J . Organomet. Chem. 1973,
54, 145.
(9) Bock, H.; Mollere, P.; Becker, G.; Fritz. G. J . Organomet. Chem.
1973, 61, 113.
(10) Worley, S. D. Chem. Rev. 1971, 71, 295.
(11) Kuan, Y. L.; White, J . M. J . Chem. Soc., Chem. Commun. 1994,
1195.
(12) Rickborn, B.; Quartucci, J . J . Org. Chem. 1964, 29, 2476.

Stereoelectronic Effects Involving Silicon and Germanium
J . Org. Chem., Vol. 61, No. 16, 1996 5229
resulted in elimination of the ester and the trimethyl-
stannyl groups giving the corresponding substituted
cyclohexene; therefore, attempts to obtain accurate struc-
tural information on ester derivatives of 10 and 12 were
abandoned at this point.
F igu r e 2. Thermal ellipsoid plot for 18. Ellipsoids are drawn
at the 50% probability level.
Molecu la r Str u ctu r es. The structures of 18-20
were determined at 130 K to remove unwanted thermal
effects. The structures of 18-20 and the silicon-free 2,4-
dinitrobenzenesulfenate 21, which was determined at 200
K for comparison, are presented in Figures 2-5 and were
drawn using ORTEPII13 to depict 50% ellipsoids. Hy-
drogen atoms that were refined isotropically are omitted
for clarity. Selected bond lengths, angles, and dihedral
angles for 18 are presented in Table 1, and selected bond
lengths, angles, and dihedral angles for the sulfenate
esters 19, 20 and 21 are presented in Table 2. Atomic
coordinates and thermal parameters and complete bond
length and bond angle listings have been deposited with
the Cambridge Crystallographic Data Centre.23
F igu r e 3. Thermal ellipsoid plot for 19. Ellipsoids are drawn
The structure of the trimethylgermyl 2,4-dinitroben-
zenesulfenate 19 is isomorphous with the corresponding
trimethylsilyl derivative 20; similarly, the â-(trimethyl-
germyl)-substituted p-nitrobenzoate ester 18 is isomor-
phous with the previously reported trimethylsilyl deriva-
tive 4.6 This indicates (not surprisingly) that replace-
ment of a trimethylsilyl substituent with the sterically
similar trimethylgermyl substituent has little effect on
the preferred packing in the solid state in these struc-
tures.
at the 50% probability level.
In structures 18-20 inspection of Tables 1 and 2
confirms the presence of angle strain at the C(1), C(2),
C(3), and C(6) cyclohexane carbon centers, this manifests
as a flattening of the ring in the region of the trimeth-
ylsilyl and dinitrobenzenesulfenate substituents in 20
[C(6)-C(1)-C(2)-C(3) -50.9(2)°] and a substantial open-
ing of the Si-C-C angles. As a consequence, the Si-
C(2)-C(1)-O(1) dihedral angle in 20 is reduced from 180°
to -160.83(12)°.14 Inspections of the structural param-
eters in Tables 1 and 2 indicate the presence of similar
structural distortions from the idealized chair geometry
in the trimethylgermyl-substituted esters 18 and 19.
The geometry about the S-O bond in the three
dinitrobenzenesulfenate esters as represented by the
C(1)-O(1)-S-C(11) dihedral angle is 98.6° in 20, 99.2°
in 19, and -99.5° in 21. This conformation is presumably
F igu r e 4. Thermal ellipsoid plot for 20. Ellipsoids are drawn
at the 50% probability level.
favored as it acts to minimize lone pair repulsion between
the p-type lone pairs on the sulfur and the oxygen atoms
of the sulfenate group.15 The S-O bond is essentially
coplanar with the aromatic ring in all three structures:
the O(1)-S-C(11)-C(16) dihedral angle is 0.5(2)° for 19,
0.1(2)° for 20, and 2.75(12)° for 22, allowing for maximum
delocalization of the sulfur lone pair into the electron
deficient benzene ring in all three structures. Both the
2- and 4-nitro groups are essentially coplanar with the
aromatic ring in the sulfenates 19-21; interestingly, this
results in one of the 2-nitro oxygens, O(2), coming into
very close contact with the adjacent sulfur atom.
(13) J ohnson, C. K. ORTEP. Report ORNL-3794; Oak Ridge Na-
tional Laboratories: Oak Ridge, TN, 1965.
(14) We have previously estimated the contributions that induction
and hyperconjugation make to the stabilization of positive charge by
â-silicon, on the basis of structural geometries similar to these.6 Thus,
on the basis of the data of Lambert et al.4 we estimate that with a
Si-C-C-O dihedral angle of 180° vertical acceleration of unimolecular
solvolyses of up to ca. 6.2 × 1012 (in addition to contributions from
induction of ca. 75) might be observed.
(15) Wolfe, S. Acc. Chem. Res. 1972, 5, 102.

5230 J . Org. Chem., Vol. 61, No. 16, 1996
Chan et al.
Ta ble 2. Selected Bon d Len gth s (Å), Bon d An gles (Deg),
a n d Dih ed r a l An gles (Deg) for th e
2,4-Din itr oben zen esu lfen a te Ester s 19, 20, a n d 21
19
20
(M ) Ge)
(M ) Si)
21
M-C(2)
1.992(2)
1.945
1.911(2)
1.865
M-CH3 (av)
C(1)-O(1)
C(11)-C(12)
C(12)-C(13)
C(13)-C(14)
C(14)-C(15)
C(15)-C(16)
C(11)-C(16)
C(12)-N(1)
C(14)-N(2)
N(1)-O(2)
N(1)-O(3)
N(2)-O(4)
N(2)-O(5)
C(11)-S
1.492(2)
1.411(3)
1.384(3)
1.371(3)
1.396(3)
1.380(3)
1.411(3)
1.449(2)
1.467(2)
1.235(2)
1.228(2)
1.227(2)
1.225(2)
1.750(2)
2.470(2)
1.490(3)
1.409(3)
1.383(3)
1.378(3)
1.394(3)
1.379(3)
1.413(3)
1.452(2)
1.461(2)
1.233(2)
1.229(2)
1.227(2)
1.225(2)
1.749(2)
2.473(2)
1.476(2)
1.405(2)
1.388(2)
1.369(2)
1.390(2)
1.377(2)
1.401(2)
1.442(2)
1.465(2)
1.240(2)
1.219(2)
1.220(2)
1.222(2)
1.7533(14)
2.480(2)
F igu r e 5. Thermal ellipsoid plot for 21. Ellipsoids are drawn
at the 50% probability level.
Ta ble 1. Selected Bon d Len gth s (Å), Bon d An gles (Deg),
a n d Dih ed r a l An gles (Deg) for Com p ou n d 18
Ge-C(2)
C(1)-O(1)
1.985(2)
1.485(2)
Ge-CH3 (av)
C(1)-C(2)
1.948
1.513(2)
Ge-C(2)-C(1)
O(1)-C(1)-C(2)
C(1)-C(2)-C(3)
C(3)-C(4)-C(5)
C(5)-C(6)-C(1)
111.52(10) Ge-C(2)-C(3)
107.11(11) O(1)-C(1)-C(6)
111.67(12) C(2)-C(3)-C(4)
111.68(13) C(4)-C(5)-C(6)
113.18(12) C(6)-C(1)-C(2)
115.10(10)
107.55(12)
113.21(12)
108.46(12)
113.71(12)
O(2)‚‚‚S
M-C(2)-C(1)
M-C(2)-C(3)
O(1)-C(1)-C(2)
O(1)-C(1)-C(6)
C(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(3)-C(4)-C(5)
C(4)-C(5)-C(6)
C(5)-C(6)-C(1)
C(6)-C(1)-C(2)
C(1)-O(1)-S
112.53(13)
114.77(13)
110.0(2)
104.7(2)
111.1(2)
111.4(2)
111.1(2)
109.4(2)
113.6(2)
113.7(2)
114.40(12)
100.07(9)
177.0(2)
113.6(2)
116.34(13)
110.4(2)
104.8(2)
110.6(2)
111.3(2)
110.8(2)
109.9(2)
113.5(2)
113.3(2)
114.60(11)
100.04(8)
177.0(2)
105.15(12)
110.80(12)
112.56(12)
111.86(13)
108.69(11)
112.24(12)
111.90(12)
111.33(12)
116.39(9)
100.09(6)
174.8(2)
Ge-C(2)-C(1)-O(1) -158.95(9) Ge-C(2)-C(1)-C(6) 82.38(13)
Ge-C(2)-C(3)-C(4) -78.97(14) C(1)-C(2)-C(3)-C(4) 49.5(2)
C(2)-C(3)-C(4)-C(5) -55.3(2)
C(4)-C(5)-C(6)-C(1) -55.9(2)
C(6)-C(1)-C(2)-C(3) -48.0(2)
O(1)-C(1)-C(6)-C(5) -65.5(2)
C(3)-C(4)-C(5)-C(6) 56.8(2)
C(5)-C(6)-C(1)-C(2) 52.9(2)
O(1)-C(1)-C(2)-C(3) 70.7(2)
The O(2)‚‚‚S nonbonded distances for 19-21 are re-
markably short: 2.470, 2.473, and 2.480 Å, respectively.
These are well within the sum of the van der Waals radii
of S and O, which is 3.32 Å (1.52 Å for O and 1.8 Å for
S),16 suggesting a significant stabilizing interaction be-
tween the nitro oxygen O(2) and the sulfenate sulfer as
represented by Figure 6. Consistent with this is the
observation that the O(2)-N(1) distance for 19-21
(average 1.236 Å) is significantly longer than the O(3)-
N(1) distance (average 1.225 Å). Furthermore, the
C(12)-N(1) distance for 19-21 (average 1.448 Å) is
consistently shorter than the C(14)-N(2) distance (aver-
age 1.464 Å), implying a greater degree of delocalization
of the sulfur lone pair onto the 2-NO2 oxygens than onto
the 4-NO2 oxygens. The C-C bond lengths of the
aromatic ring in 19-21 show a similar pattern of
variation and are consistent with significant delocaliza-
tion of the sulfur lone pair into the aromatic ring. The
O(2)‚‚‚S-O(1) angle in all three structures is close to
180°; this is a favorable geometry for interaction between
the 2-nitro oxygen and sulfur d and p orbitals or,
alternatively, interaction with the low-lying σ* (S-O)
orbital, which has been suggested.17
O(1)-S-C(11)
O(2)‚‚‚S-O(1)
M-C(2)-C(1)-O(1)
M-C(2)-C(1)-C(6)
M-C(2)-C(3)-C(4)
C(1)-C(2)-C(3)-C(4)
C(2)-C(3)-C(4)-C(5)
C(3)-C(4)-C(5)-C(6)
C(4)-C(5)-C(6)-C(1)
C(5)-C(6)-C(1)-C(2)
C(6)-C(1)-C(2)-C(3)
O(1)-C(1)-C(2)-C(3)
O(1)-C(1)-C(6)-C(5)
-162.16(12) -160.83(12)
80.8(2)
-75.2(2)
54.0(2)
-58.6(2)
56.8(2)
-52.4(2)
49.8(2)
82.1(2)
-76.1(2)
55.3(2)
-59.3(2)
56.8(2)
-52.7(2)
50.7(2)
-50.9(2)
66.3(2)
-69.7(2)
-2.8(3)
-4.3(3)
0.1(2)
55.2(2)
-55.8(2)
56.5(2)
-56.0(2)
52.7(2)
-52.7(2)
67.4(2)
-63.9(2)
0.2(2)
2.7(2)
2.75(12)
-95.88(10)
-49.5(2)
67.5(2)
-70.3(2)
C(11)-C(12)-N(1)-O(2) -3.6(3)
O(5)-N(2)-C(14)-C(15) -4.2(3)
O(1)-S-C(11)-C(16)
C(1)-O(1)-S-C(11)
0.5(2)
99.19(13)
98.60(13)
1.473(2) Å, but compares with the corresponding tri-
methylsilyl derivatives 4 (1.490(2) Å) and 22 (1.483(3)
Å) (mean C-O 1.487 Å).
The C(alkyl)-O(ester) bond lengths for the â-(tri-
methylsilyl)-substituted sulfenate 20 and the correspond-
ing â-(trimethylgermyl) sulfenate 19 are 1.490(3) and
1.492(2) Å, respectively; these do not differ significantly
from each other but are significantly lengthened relative
to the unsubstituted derivative 21, which has a C-O
bond length of 1.476(2) Å; more significant lengthening
might well be expected if these systems had an ideal 180°
geometry. The â-(trimethylgermyl)-substituted p-nitro-
benzoate ester 18 has a C-O bond length of 1.485(2) Å;
this is significantly lengthened relative to the unsubsti-
tuted derivative 5,18 which has a C-O bond length of
Discu ssion
The C(alkyl)-O(ester) bond lengths of the two â-(tri-
methylgermyl) esters 18 and 19 are significantly length-
ened relative to the corresponding unsubstituted esters
56 and 21; this indicates the presence of significant σC-Ge
- σ*C-O hyperconjugative-like interactions between the
high-lying C-Ge bonding orbital and the low-lying C-O
(16) Bondi, A. J . Phys. Chem. 1964, 68, 441.
(17) Kucsman, A.; Kapovits, I.; Parkanyi, L.; Argay, G. Y.; Kalman,
A. J . Mol. Struct. 1984, 125, 331.
(18) White, J . M.; Robertson, G. B. Acta Crystallogr., Sect. C 1993,
49, 347.

Stereoelectronic Effects Involving Silicon and Germanium
J . Org. Chem., Vol. 61, No. 16, 1996 5231
tr a n s-4-Meth ylcycloh exen e oxid e (15). A solution of
4-methylcyclohexene (95.64 g, 0.996 mol), acetonitrile (182 mL,
3.49 mol), hydrogen peroxide (155 mL, 30%), potassium
bicarbonate (25 g), and methanol (1.5 L) was stirred at room
temperature for 70 h. The mixture was diluted with an equal
volume of water and then extracted with chloroform (4 × 150
mL). The combined organic extracts were washed with water
(3 × 100 mL), dried (MgSO4), and evaporated down to yield a
ca. 1:1 mixture of cis- and trans-epoxides as a clear oil (78.4 g
70%): 13C NMR (CDCl3) δ 53.03, 52.18, 51.77, 51.37, 33.54,
32.45, 28.94, 27.57, 26.38, 25.17, 24.22, 23.37, 21.95, 21.57;
1H NMR (CDCl3) δ 0.84 (d, 3H, CH3), 1.2-1.9 (m, 7H), 3.2 (m,
2H).
F igu r e 6. Interaction between the 2-nitro oxygen and the
sulfur in the 2,4-dinitrobenzenesulfenates 19, 20, and 21.
antibonding orbital in these structures. The observed
lengthening (and hence weakening) of the C-O bond
lengths of 18 and 19 in the ground state is consistent
with the enhanced reactivity of â-(trimethylgermyl)
esters toward unimolecular solvolysis in solution.7 The
C-O bond lengths for 18 and 19 are, however, similar
to the corresponding silicon-substituted esters 46 and 20,
suggesting similar ground state effects of trimethylsilyl
and trimethylgermyl substituents. The relative reactivi-
ties of â-(trimethylgermyl) esters, â-(trimethylsilyl) es-
ters, and corresponding nonsubstituted esters toward
unimolecular solvolysis are ca. 1014:1012:1.2,4,7 Given that
a trimethylsilyl substituent lengthens C-O bonds at the
â position by ca. 0.017 Å relative to the corresponding
nonsubstituted esters, any extra effects of a trimeth-
ylgermyl substituent are clearly too small to be detected
by X-ray crystallography. However, â-(trimethylstannyl)
esters have relative rates of unimolecular solvolysis of
.1014 relative to nonsubstituted esters and are expected
to show very significant ground state effects; unfortu-
nately, however, these compounds are too reactive to
allow the preparation of crystals suitable for X-ray
structural analysis.
The cis/trans mixture of epoxides (72.94 g, 0.651 mol) and
p-nitrobenzoyl chloride (186.2 g, 1.00 mol) were dissolved in
chloroform (1.5 L). With constant stirring, a steady stream
of anhydrous hydrogen chloride gas was passed through the
solution over a period of 6 h. The resulting solution was then
washed with water (3 × 200 mL) to remove the excess acid
and was then dried (MgSO4). The residue was taken up in
pyridine (80 mL) and was stirred for 30 min. Water (60 mL)
was then added to the solution, which was then allowed to
stir for a further 1.5 h to decompose the excess acid chloride.
The mixture was then taken up in ether (100 mL) and washed
with hydrochloric acid (1 M, 3 × 200 mL), 10% sodium
bicarbonate (3 × 200 mL), and water (200 mL). After drying
(MgSO4), evaporation of the ether under reduced pressure gave
an ca. 1:1 mixture of the chloroesters 16 and 17 (162 g, 84%).
Fractional recrystallization of the crude product from methanol
gave pure crystals of the desired chloroester 16 (18.58 g,
10%): mp (MeOH) 100.5-101.5 °C; IR νmax 1525, 1334 (NO2),
1720 (CdO), 1273 (CO) cm-1 13C NMR (CDCl3) δ 163.4, 150.58,
;
135.45, 130.67, 123.51, 74.02, 56.78, 33.44, 29.30, 28.19, 26.78,
21.13; 1H NMR (CDCl3) δ 0.9 (d, 3H, CH3), 1.45-2.05 (m, 7H),
4.2 (m 1H), 5.3(m, 1H), 8.2(d, 2H), 8.3(d, 2H). Anal. Calcd
for C14H16NO4Cl: C, 56.50; H, 5.41; N, 4.70; Cl, 11.9. Found:
C, 56.50; H, 5.38; N, 4.65; Cl, 11.87.
The chloro ester 16 (18.0g, 0.061 mol) and potassium
carbonate (22 g in 24 mL of water) were refluxed in methanol
(200 mL) for 2 h with rapid stirring. The solution was then
taken up in an equal volume of water and extracted with
pentane (3 × 50 mL). The combined pentane extracts were
washed with water (3 × 50 mL), dried (MgSO4), and evapo-
rated under reduced pressure to give the pure trans-epoxide
Con clu sion
The presence of a â-(trimethylgermyl) substituent
causes significant lengthening of ester C-O bond lengths
in the antiperiplanar geometry, relative to the corre-
sponding unsubstituted esters. This is consistent with
the observed enhanced reactivity of â-(trimethylgermyl)
esters toward unimolecular solvolysis. The effects of
â-germanium are similar to the effects of â-silicon.
15 as a clear oil (5.3 g, 78%): IR νmax 1255 cm-1 13C NMR
;
(CDCl3) δ 53.07, 51.43, 33.65, 29.03, 24.29, 23.47, 21.66; 1H
NMR (CDCl3) δ 0.84 (d, 3H, CH3), 1.21-1.99 (m, 7H), 3.12 (m,
2H).
r -5-ter t-Bu tyl-c-2-(tr im eth ylger m yl)cycloh exa n -t-ol (9).
A suspension of lithium metal (0.35 g, 0.05 mol, 15 equiv) in
HMPA (14 mL) under nitrogen gas was sonicated until it went
dark blue in color. To this solution was added bromotrimeth-
ylgermane (1.0 mL, 0.007 mol, 2.3 equiv), whereupon the
solution developed a golden yellow color. THF (200 mL) was
then added, and the solution was left to stir for 5 h. The
mixture was cooled to 0 °C, and trans-4-tert-butylcyclohexene
oxide (14) (0.5 g, 0.032 mol) was added and then stirred
overnight. Water (100 mL) was added, then the resulting
mixture was extracted with ether (3 × 100 mL). The combined
organic extracts were washed with water (3 × 100 mL), dried
(MgSO4), and evaporated under reduced pressure to leave 9
as a white solid that was pure by NMR (0.84 g, 95%): mp 47-
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 K) for 18-20. The
unsubstituted sulfenate 21 underwent a destructive phase
change below 150 K; thus, data were collected at 200 K. Data
were corrected for Lorentz and polarization effects and for
absorption (SHELX 76).19 Structures were solved by direct
methods (SHELXS-86)20 and were refined on F2 (SHELXL-
93).21 Crystal data and refinement details for 18-21 are listed
in Table 3.
(b) Syn th esis. General experimental details are as re-
48 °C; IR νmax 3323 (OH), 823.0 s, 1233 s, (GeC) cm-1 13C NMR
;
ported in a previous paper.22
(CDCl3) δ 69.39, 40.69, 33.94, 32.21, 32.09, 27.32, 24.84, 22.54,
1
-1.59; H NMR (CDCl3) δ 0.15 (s, 9H), 0.82 (s, 9H, C(CH3)3),
(19) SHELX76: Sheldrick, G. M. Program for Crystal Structure
Determination; Cambridge, England, 1976.
(20) SHELXS-86: Sheldrick, G. M. In Crystallographic Computing
3; Sheldrick, G. M., Kruger, C., Goddard, R., Eds.; Oxford University
Press: Oxford, England, 1985; pp 175-189.
(21) SHELXL-93: Sheldrick, G. M. Program for Crystal Structure
Refinement; University of Gottingen: Germany, 1993.
(22) Green, A. J .; Kuan, Y. L.; White, J . M. J . Org. Chem. 1995, 60,
2734.
(23) The author has deposited atomic coordinates for 18-21 with
the Cambridge Crystallographic Data Centre. The coordinates can be
obtained, on request, from the Director, Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK.
0.85-2.01 (m, 8H), 4.19 (m, 1H, HCO).
r -5-Meth yl-c-2-(tr im eth ylsta n n yl)cycloh exa n -t-ol (12).
To a suspension of lithium metal (1.250 g, 0.18 mol, 17 equiv)
in THF (40 mL) was added trimethyltin chloride (26 mL, 1 M
in THF, 0.026 mol). The resulting solution was sonicated
under nitrogen gas for 3.5 h. The solution was filtered, cooled
to 0 °C, and then treated with trans-4-methylcyclohexene oxide
(15) (1.2 g, 0.011 mol). After being stirred for 15 h, the reaction
was quenched with water (100 mL) and extracted with ether
(3 × 100 mL). The combined ether extracts were dried
(MgSO4) and evaporated under reduced pressure to give the

5232 J . Org. Chem., Vol. 61, No. 16, 1996
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 18-21
Chan et al.
21
18
19
20
emp formula
formula wt
temp (K)
radiatn
wavelength (Å)
crystal syst
space grp
unit cell dimens
a (Å)
C20H31NO4Ge
422.07
130(1)
C16H24N2O5SGe
429.05
130(1)
C16H24N2O5SSi
384.53
130(1)
C16H22N2O5S
354.43
200(1)
Cu KR
1.5418
triclinic
P-1
Cu KR
Mo KR
Cu KR
1.541 80
monoclinic
P21/n
0.709 30
monoclinic
P21/c
1.541 80
monoclinic
P21/c
6.2268(8)
18.434(3)
18.681(2)
14.016(2)
12.772(4)
11.429(4)
13.941(4)
12.747(3)
11.414(4)
8.164(12)
10.968(4)
11.590(2)
111.59(2)
99.64(2)
110.13(2)
853.1(3)
2
b (Å)
c (Å)
R (deg)
â (deg)
98.568(9)
113.96(2)
113.61(3)
γ (deg)
vol (Å3)
2120.3(4)
4
1.32
2.15
888
1869.7(9)
4
1.524
1.778
888
1858.5(9)
4
1.374
2.423
816
Z
Dc (Mg/m3)
1.380
1.943
376
µ (mm-1
F(000)
)
cryst size (mm)
θ range for data collectn
index ranges
0.48 × 0.36 × 0.45
3.39-74.97
-7 e h e 7
-23 e k e 23
0 e l e 23
3
0.41 × 0.41 × 0.64
2.25-29.90
0 e h e 19
-17 e k e 0
-16 e l e 14
3
0.29 × 0.26 × 0.23
3.46-75.00
-17 e h e 17
-15 e k e 15
-14 e l e 14
3
0.46 × 0.27 × 0.12
4.36-74.84
0 e h e 10
-13 e k e 12
-14 e l e 14
3
no. intens controls
interval (min)
160
160
160
160
dec
insignificant
SHELX76
0.503, 0.329
8926
insignificant
SHELX76
0.88, 0.48
5617
insignificant
SHELX76
0.65, 0.52
8986
insignificant
SHELX76
0.81, 0.47
3765
absorptn method
max, min transmission
reflns collected
independent reflns
4361
5417
3832
3507
[R(int) ) 0.0167]
[R(int) ) 0.0284]
[R(int) ) 0.0576]
[R(int) ) 0.0116]
no. of obsd reflns
(I > 2σ(I))
refinement method
data/restraints/params
GOF on F2
4262
4242
3803
3389
full-matrix on F2
4361/0/360
0.957
full-matrix on F2
5417/0/323
1.044
full-matrix on F2
3832/0/323
1.118
full-matrix on F2
3506/0/306
1.035
final R indices [I > 2σ(I)]
R1 ) 0.0261
wR2 ) 0.0690
R1 ) 0.0280
wR2 ) 0.0702
R1 ) 0.0341
wR2 ) 0.0684
R1 ) 0.0546
wR2 ) 0.0760
R1 ) 0.0539
wR2 ) 0.1357
R1 ) 0.0541
wR2 ) 0.1359
R1 ) 0.0344
wR2 ) 0.1278
R1 ) 0.0354
wR2 ) 0.1332
R indices (all data)
weighting scheme
w ) 1/[σ2(Fo2) + (A*P)2 + B*P]
A ) 0.0398
B ) 1.321
SHELXL
0.0037(2)
0.365
A ) 0.0322
B ) 0.9241
SHELXL
0.000
0.064
0.540 and -0.416
A ) 0.0705
B ) 1.1806
SHELXL
0.0023(4)
0.035
A ) 0.102
B ) 0.1237
SHELXL
0.013(2)
-0.005
0.287 and -0.345
2
where P ) (Fo + 2Fc2)/3
extinction method
extinction coefficient
maximum shift/esd
largest diff peak and hole (e Å-3
)
0.278 and -0.336
0.866 and -0.661
crude alcohol (4.77 g) as a light yellow oil. Purification by
Kugel distillation (2 mmHg, 100 °C) yielded the pure alcohol
12 (2.97 g, 100%) as a colorless oil: IR νmax 3334 (OH), 763.0,
mL) stirred at 0 °C was treated with p-nitrobenzoyl chloride
(184 mg, 1.2 equiv). The resulting slurry was stirred at room
temperature for 5 h and then treated with water (0.5 mL) and
stirred for a further 20 min. The slurry was diluted with water
(20 mL) and extracted with ether (3 × 20 mL). The combined
organic extracts were washed with HCl (1 M, 1 × 20 mL),
saturated aqueous sodium bicarbonate (1 × 20 mL), and water
(1 × 20 mL), dried (MgSO4), and evaporated to an oil that
solidified. Recrystallization from ether/pentane gave 18 as
white blocks (254 mg, 82%): mp 91-92 °C; IR νmax 1704 (CdO),
1260 (SnC) cm-1 13C NMR (CDCl3) δ 70.28, 41.57, 33.86, 33.63,
;
27.10, 23.64, 21.22, -9.93; 1H NMR (CDCl3) δ 0.0 (s, 9H), 0.90
(d, 3H), 1.4-2.1 (m, 8H), 4.02 (m, 1H). Anal. Calcd for C10H22
OSn: C, 43.36; H, 8.01. Found: C, 42.87; H, 8.26.
-
r -5-ter t-Bu tyl-c-2-(tr im eth ylstan n yl)cycloh exan -t-ol (11).
To a suspension of lithium metal (0.625 g, 0.009 mol, 10 equiv)
in THF (40 mL) was added trimethyltin chloride (13 mL, 1 M
in THF, 0.013 mol), and the mixture was sonicated under
nitrogen gas for 2 h and then left to stir overnight. The
resulting blue solution was filtered under nitrogen and cooled
to 0 °C, and then trans-4-tert-butylcyclohexene oxide (14) (1.00
g, 0.009 mol) was added. After being stirred for 24 h, the
reaction was quenched with water (100 mL) and extracted with
ether (3 × 100 mL). The ether extracts were dried (MgSO4)
and evaporated under reduced pressure to give the crude
alcohol (2.40 g) as a light yellow oil. Purification by distillation
under reduced pressure gave the pure alcohol 11 as a waxy
1526.0, 1342.0 (NO2), 822.0, 1180 (GeC) cm-1 13C NMR
;
(CDCl3) δ 164.01, 150.29, 136.73, 130.48, 123.46, 76.04, 42.08,
1
32.24, 30.90, 29.70, 27.28, 24.52, 23.51, -1.53 (Ge(CH3)3); H
NMR (CDCl3) δ 0.19 (s, 9H), 0.97 (s, 9H, CH3), 0.8-2.4 (m,
8H), 5.4 (m 1H), 8.24 (d, 2H), 8.37 (d, 2H).
r -5-Meth yl-c-2-(tr im eth ylger m yl)cycloh ex-t-yl 2,4-Di-
n itr oben zen esu lfen a te (19). To a stirred solution of the
trimethylgermyl alcohol 10 (200 mg, 0.93 mmol) in dichloro-
methane (10 mL) in the presence of pyridine (0.5 mL) was
added 2,4-dinitrobenzenesulfenyl chloride (261 mg, 1.1 mmol),
and the mixture was then stirred for 18 h. The resulting
mixture was diluted with water (20 mL) and extracted with
dichloromethane (3 × 20 mL), and the combined organic
extracts were washed with HCl (1 M, 1 × 20 mL) and
saturated aqueous sodium bicarbonate (1 × 20 mL), dried
(MgSO4), and evaporated under reduced pressure to yield a
bright yellow solid that was recrystallized from pentane, giving
solid: IR νmax 3330 (OH), 763.0, 1235 (SnC) cm-1 13C NMR
;
(CDCl3) δ 70.77 (CO), 40.97, 33.11, 32.62, 32.21, 27.36 (CH3)3,
26.27, 23.94, -9.57 (Sn(CH3)3; 1H NMR (CDCl3) δ 0.10 (s, 9H,
Sn(CH3)3), 0.85 (s, 9H, C(CH3)3), 1.2-2.2 (m, 8H, 4.27 (1H, m).
r -5-ter t-Bu t yl-c-2-(t r im et h ylger m yl)cycloh ex-t-yl p -
Nit r ob en zoa t e (18). A solution of the 5-tert-butyl-2-(tri-
methylgermyl) alcohol 9 (200 mg, 0.73 mmol) in pyridine (1

Stereoelectronic Effects Involving Silicon and Germanium
J . Org. Chem., Vol. 61, No. 16, 1996 5233
19 as bright yellow blocks (320 mg, 80%): mp 105-107 °C; IR
and obtained as yellow blocks from methanol (85%): mp 161-
163 °C dec; 1H NMR (CDCl3) δ 9.09 (1H, br s), 8.46 (1H, br d,
J ) 11 Hz), 7.95 (1H, d, J ) 11 Hz), 3.92 (1H, m), 2.3-2.2
(2H, m), 1.7-1.3 (6H, m), 1.1-1.0 (1H, m), 0.88 (9H, s); 13C
NMR (CDCl3) δ 155.16, 144.29, 139.36, 127.59, 123.69, 121.18,
82.02, 47.49, 32.55, 31.29, 27.40, 21.19.
νmax 1511.0, 1339.0 (NO2), 815.0, 1233 (GeC) cm-1 13C NMR
;
(CDCl3) δ 155.54, 144.5, 139.6, 127.44, 123.86, 121.24, 86.58,
37.76, 32.24, 32.64, 26.68, 22.43, 21.97, -1.61; 1H NMR
(CDCl3) δ 0.09 (9H, s) , 0.9 (3H, d, J ) 7.5 Hz), 0.8-2.1 (8H,
m), 3.95 (1H, m), 7.9 (1H, d, J ) 11 Hz), 8.4 (1H, br d, J ) 11
Hz), 9.08 (1H, br s).
The same general procedure used for the preparation of 19
was employed for the preparation of the 2,4-dinitrobenzene-
sulfenate esters 20 and 21.
Ack n ow led gm en t. Our thanks go to the Australian
Research Council Large Grants Scheme for financial
support.
r -5-Met h yl-c-2-(t r im et h ylsilyl)cycloh ex-t-yl
2,4-Di-
n itr oben zen esu lfen a te (20). Prepared from the trimethyl-
silyl alcohol 13 and obtained as yellow blocks from pentane
(80%): mp 64-67 °C dec; 1H NMR (CDCl3) δ 9.11 (1H, d, J )
2.5 Hz), 8.45 (1H, dd, J ) 10, 2.5 Hz), 7.95 (1H, d, J ) 10 Hz),
4.0 (1H, m), 2.2-1.0 (8H, m), 0.92 (3H, d, J ) 7 Hz), 0.05 (9H,
s); 13C NMR (CDCl3) δ 155.4, 144.2, 139.3, 128.9, 123.9, 121.2,
85.97, 37.89, 32.26, 30.61, 26.64, 21.90, 21.54, -1.07.
Su p p or tin g In for m a tion Ava ila ble: 13C NMR spectra of
9-12, 16, and 18-21 (9 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.
tr a n s-4-ter t-Bu t ylcycloh exyl
2,4-Din it r ob en zen e-
su lfen a te (21). Prepared from cis-4-tert-butylcyclohexanol
J O9602480