Chemistry Letters Vol.32, No.12 (2003)
1159
fones 1a–g can be converted to the corresponding vinylic sulfones
1a0–g0,1a namely (E)-ꢁ-substituted allylsilanes, and then the hy-
droxide anion (generated from the reaction of DBU and H2O) at-
tacks the silyl group to produce the desilyalted allylic sulfones 2
(Scheme 1).9 An evidence to support this mechanism was found
in Entry 12 where the ꢀ-silylated (E)-vinylic sulfone 1e0 produced
by isomerization of 1e was isolated in 37% yield.
sulfones, (E)-isomer was obtained as the major product, probably
t
due to the bulkiness of the Bu and Ph groups that precludes syn-
conformation in the transition state.
In the case of ꢀ-phenylthio substituted sulfone, the contribu-
tion of the empty d-orbital of S-atom, such as ꢃC{H ! d, is still
unclear, but ꢃC{S ! ꢂꢀ interaction,11,14 as shown in the CH-
eclipsed form C (R = PhS) in Scheme 1, may be responsible to de-
crease (Z)-selectivity (Entries 14 and 15) compared with the cases
of ꢀ-methoxy (Entry 16) and ꢀ-alkyl substituted sulfones (Entries
1–9). Eventually, the relative degree of ‘‘syn-effect’’ for the ꢀ-sub-
stituents remained almost the same as that previously found in the
conversion of ꢁ-unsubstituted vinylic sulfones to the correspond-
ing allylic sulfones.1a
In conclusion, the stereochemical outcome of the desilylation
reaction of ꢀ-silylated allylic and vinylic sulfones was well ration-
alized by ‘‘syn-effect’’ which was accounted for by the ꢃ ! ꢂꢀ
interaction and/or 6ꢂ-electron homoaromaticity. In the reaction
using ꢀ-methoxy substituted vinylic sulfone, the highest (Z)-
selectivity was observed.
In the transition state of desilylation, the conformation B or C
is more favored rather than conformation A, as the hyperconjugat-
ing ability of C–Si bond is stronger than C–C or C–H bond.3 The
steric hindrance of the TMS group must be also considerable in fa-
voring the conformation B or C (Scheme 1). It should be noted that
hyperconjugation of developing anion generated by the interaction
of hydroxide anion with trimethylsilyl group becomes more effec-
tive in the conformations B and C, in both of which the developing
anion is aligned with the ꢂꢀ
orbital and other conformations
C=C
can be neglected (Scheme 1). Our recent proposal that ꢃ ! ꢂꢀ in-
teraction is the most probable explanation for the ‘‘syn-effect’’ is
well consistent with this consideration.1c At the desilylation step,
the CC-eclipsed form B might be preferred rather than CH-
eclipsed form C because hyperconjugative electron donation by
C–H bond is larger than that by C–C bond,3c,10 especially under ba-
sic conditions by induction of the negative charge aligned with the
ꢂꢀC=C orbital.11 In the case of ꢀ-methoxy substituted sulfone, CH-
eclipsed form C is much less favored due to low donor ability of
C–O bond12 compared with C–C bond, thus the highest (Z)-selec-
tivity was observed for 1h0.
References and Notes
1
a) T. Hirata, Y. Sasada, T. Ohtani, T. Asada, H. Kinoshita, H. Senda, and K.
Inomata, Bull. Chem. Soc. Jpn., 65, 75 (1992). b) A. Shibayama, T. Nakamura,
T. Asada, T. Shintani, Y. Ukaji, H. Kinoshita, and K. Inomata, Bull. Chem. Soc.
Jpn., 70, 381 (1997). c) T. Nakamura, S. K. Guha, Y. Ohta, D. Abe, Y. Ukaji, and
K. Inomata, Bull. Chem. Soc. Jpn., 75, 2031 (2002).
2
3
a) D. Cremer, J. Am. Chem. Soc., 101, 7199 (1979). b) K. N. Houk, R. W.
Strozier, N. G. Rondan, R. R. Fraser, and N. Chuaqui-Offermanns, J. Am. Chem.
Soc., 102, 1426 (1980), and references cited therein. c) E. Block, R. E. Penn, A.
A. Bazzi, and D. Cremer, Tetrahedron Lett., 22, 29 (1981).
a) K. A. Nguyen, M. S. Gordon, G. Wang, and J. B. Lambert, Organometallics,
10, 2798 (1991). b) A. Skancke, D. A. Hrovat, and W. T. Borden, J. Am. Chem.
Soc., 120, 7079 (1998). c) P. R. Rablen, R. W. Hoffmann, D. A. Hrovat, and W.
T. Borden, J. Chem. Soc., Perkin Trans. 2, 1999, 1719. d) I. V. Alabugin and T.
A. Zeidan, J. Am. Chem. Soc., 124, 3175 (2002).
i
t
In the present investigation except Pr, Bu, and Ph substitu-
ents, it is also possible to stabilize the syn-conformation at the tran-
sition state by 6ꢂ-electron homoaromaticity (another origin of
‘‘syn-effect’’) involving the developing charge at the ꢀ-position
and a pseudo p-orbital of the ꢄ-CH2 (R = R0CH2), or a lone pair
of electrons in a p-orbital of the hetero atom (Figures 1a and
1b).1,13 In the case of iPr substituent, 6ꢂ-electron homoaromaticity
is difficult to be considered, but still remarkable amount (34%) of
the sterically unfavorable (Z)-isomer was obtained (Entry 10).
Accordingly, it is clear that the ‘‘syn-effect’’ is primarily arisen
from the ꢃ ! ꢂꢀ interaction. In the case of tBu and Ph substituted
4
5
J.-B. Verlhac, M. Pereyre, and J.-P. Quintard, Tetrahedron, 46, 6399 (1990).
Preparation of 1: For 1a–f, the corresponding acylsilanes (RCOTMS) were treat-
ed with CH2=CHMgBr and CeCl3. The resulting alcohols (except for 1e) were
treated with p-TolS(O)Cl and then heated in toluene at 100 ꢁC to afford 1a–d,f.
For 1e, the alcohol obtained above was treated with SOCl2 followed by reacting
with p-TsNaꢂ4H2O in MeOH. For 1g, TMSCH=CHCH2Ts (see Ref. 15) was ep-
oxidized with mCPBA, and then treated with PhSH in the presence of neutral
Al2O3. The resulting alcohol was acetylated with Ac2O, followed by treating
with nBuLi to afford 1g. Compound 1h0 was prepared as follows:
TMSCH2OCH3 was treated with sBuLi followed by addition of p-Tol-
SCH2CHO. The resulting ꢅ-hydroxy sulfide was oxidized to sulfone with
mCPBA and then dehydrated as described for 1g by utilizing sBuLi. In some
cases, the E=Z-isomers of 1 were separated by recycle HPLC.
β
β
β
Me3Si
Ts
Me3Si
α
Ts
R
Ts
DBU
HO
DBU
γ
R
γ
α
γ
α
R
SiMe3
(Z)-1a-g
6
7
a) K. Inomata, T. Hirata, H. Suhara, H. Kinoshita, H. Kotake, and H. Senda,
Chem. Lett., 1988, 2009. b) T. Kobayashi, Y. Tanaka, T. Ohtani, H. Kinoshita,
K. Inomata, and H. Kotake, Chem. Lett., 1987, 1209.
(Z)-Selectivity of 2 obtained from (E)-1a,b, 1g and 1h0 were 83 (25 ꢁC, 10 min,
1/2 = 10/90, 86% yield), 60 (25 ꢁC, 40 min, 1/2 = 6/94, 89% yield), 50 (0 ꢁC,
7 min, 1/2 = 0/100, 86% yield) and 91% (25 ꢁC, 7 min, 1/2 = 0/100, 80%
yield), respectively;which were 7–24% lower compared with DBU and H 2O
system. Those results suggest that more basic conditions are favored to discrim-
inate the conformation B from C (Scheme 1).
(E)-1a'-g',h'
(E)-1a-g
− Me3SiOH
+ H+
− H+
Ts
+ H+
− H+
Ts
R
Ts
R
Ts
R
R
(E)-2a-h
(Z)-2a-h
HO
Me3Si
HO
Me3Si
π
π
π
π
π
π
π
π
π
H
H
H
Ts
H
Ts
H
Ts
H
H
8
9
a) T. H. Chan and W. Mychajlowskij, Tetrahedron Lett., 1974, 3479. b) H. Oda,
M. Sato, Y. Morizawa, K. Oshima, and H. Nozaki, Tetrahedron, 41, 3257 (1985).
The affinity of hydroxide anion to silicon was described: a) L. H. Sommer, L. J.
Tyler, and F. C. Whitmore, J. Am. Chem. Soc., 70, 2872 (1948). b) C. Eaborn and
D. R. M. Walton, J. Organomet. Chem., 4, 217 (1965).
σ
σ
H
σ
SiMe3
A
R
B
H
R
R
C
σC−H
→ π*C=C interaction (A) < σC−Si → π*C=C interaction (B, C)
10 a) T. Laube and H. U. Stilz, J. Am. Chem. Soc., 109, 5876 (1987). b) T. Laube
and T.-K. Ha, J. Am. Chem. Soc., 110, 5511 (1988). c) B. W. Gung and M. M.
Yanik, J. Org. Chem., 61, 947 (1996).
Scheme 1.
11 S. K. Guha, A. Shibayama, D. Abe, Y. Ukaji, and K. Inomata, Chem. Lett., 32,
778 (2003).
12 Y. Apeloig, P. v. R. Schleyer, and J. A. Pople, J. Am. Chem. Soc., 99, 5901
(1977).
13 P. v. R. Schleyer, J. D. Dill, J. A. Pople, and W. J. Hehre, Tetrahedron, 33, 2497
(1977).
14 The significance of the ꢃC{S!ꢂꢀ interaction in ꢁ-alkylthio carbonyl compounds
was pointed out: P. R. Olivato, S. A. Guerrero, Y. Hase, and R. Rittner, J. Chem.
Soc., Perkin Trans. 2, 1990, 465 and references cited therein.
15 P. Lin and G. H. Whitham, J. Chem. Soc., Chem. Commun., 1983, 1102.
+
-
+ H
Ts
-
H
Ts
H
β
Me Si
β
HO
-
HO
Me3Si -
3
α
α
γ
γ
H
H
H
H
H
δ
X
R'
R'
-electron homoaromaticity in
-heteroatom substituted system
a) 6
π
-electron homoaromaticity b) 6
-alkyl substituted system
π
in
γ
γ
Figure 1.
Published on the web (Advance View) November 17, 2003;DOI 10.1246/cl.2003.1158