Chemistry Letters 2001
807
bromide gave again the inverted substitution product (S)-6 in
methanol.
These results strongly suggest that the unrearranged products
are not derived from intermediate primary cation I1, but through
some other pathway with a less polar transition state than the σ-
participation mechanism. A possible pathway may involve a
nucleophilic reaction at the sulfonic sulfur.
In conclusion, the possibility of intermediate formation of
the primary vinyl cation during the solvolysis of either the triflate
or iodonium substrates was definitively ruled out by using a chi-
rality probe approach.
References and Notes
1
2
3
4
5
P. J. Stang, Z. Rappoport, M. Hanack, and L. R. Subramanian, “Vinyl
Cations,” Academic Press, New York (1979).
“Dicoordinated Carbocations,” ed. by Z. Rappoport and P. J. Stang,
John Wiley & Sons, Chichester (1997).
S. G. Lias, J. E. Bartmess, J. F. Liebman, J. L. Holmes, R. D. Levin,
and W. G. Mallard, J. Phys. Chem. Ref. Data, 17, 1 (1988).
Nuclear decay: S. Fornarini and M. Speranza, Tetrahedron Lett., 25,
869 (1984); J. Am. Chem. Soc., 111, 7402 (1989).
Photochemical reactions: G. Lodder, in “Dicoordinated Carbocations,”
ed. by Z. Rappoport and P. J. Stang, John Wiley & Sons, Chichester
(1997), Chap. 8.
In superacid: H. Hogeveen and C. F. Roobeek, Tetrahedron Lett.,
1971, 3343.
In concentrated sulfuric acid: L. Lucchini and G. Modena, J. Am.
Chem. Soc., 112, 6291 (1990).
M. Hanack, R. Märkl, and A. G. Martinez, Chem. Ber., 115, 772
(1982).
same conditions.8 The remarkable result of the present solvolysis
is that the rearranged product 3 retains essentially the optical puri-
ty of the substrate (R)-4. The stereochemistry of 3 is determined
as R by comparison with an authentic sample.12 This observation
definitively excludes the intermediacy of the achiral primary vinyl
cation I1 for the formation of 3, but is reasonably explained by a
σ-bond participation mechanism leading directly to the rearranged
secondary cation I2, which is trapped by water to give (R)-3 as
illustrated in Scheme 4. The substrate recovered from the reaction
in a shorter time maintained its original optical purity.
6
7
8
9
a) T. Okuyama and M. Ochiai, J. Am. Chem. Soc., 119, 4785 (1997).
b) T. Okuyama, Y. Ishida, and M. Ochiai, Bull. Chem. Soc. Jpn., 72,
163 (1999). c) T. Okuyama, K. Sato, M. Ochiai, Chem. Lett., 1998,
1177. d) T. Okuyama, H. Yamataka, and M. Ochiai, Bull. Chem. Soc.
Jpn., 72, 2761 (1999). e) T. Okuyama, S. Imamura, and Y. Ishida,
Bull. Chem. Soc. Jpn., 74, 543 (2001).
10 T. Okuyama, T. Takino, T. Sueda, and M. Ochiai, J. Am. Chem. Soc.,
117, 3360 (1995).
11 a) R. J. Hinkle and Q. A. Thomas, J. Org. Chem., 62, 7534 (1997). b)
R. J. Hinkle, A. J. McNeil, Q. A. Thomas, and M. N. Andrews, J. Am.
Chem. Soc., 121, 7437 and 10668 (1999).
12 M. Fujita, Y. Sakanishi, and T. Okuyama, J. Am. Chem. Soc., 122,
8787 (2000).
13 a) Preparation of (R)-4: Triflate (R)-4 was prepared from the (R)-
dimethylphenylsilyl derivative obtained from bromide (R)-613b via the
consecutive epoxidation and rearrangement to the silyl enol ether,13c fol-
lowed by sulfonylation.13d The product was purified by chromatogra-
phy (SiO2, hexane) and obtained as a colorless oil. The enantiomeric
excess of one sample of (R)-4 was determined as 73% and [α]20
=
–7.3 (c 1.15 CHCl3), while the other preparation gave a sample of 6D8%
ee. 1H NMR (400 MHz, CDCl3) δ 6.34 (s, 1H), 2.77–2.70 (m, 1H),
2.20–2.12 (m, 1H), 2.00–1.93 (m, 1H), 1.83–1.78 (m, 3H), 1.59–1.50
(m, 1H), 1.05–0.93 (m, 2H), 0.90 (d, J = 6.4 Hz, 3H); 13C NMR (100
MHz, CDCl3) δ 133.3, 127.6, 118.6 (q, J = 318.5 Hz), 35.6, 34.6, 32.2,
29.3, 25.5, 21.8; MS (EI) m/z (%) 258 (M+; 6), 107 (78), 79 (76), 69
(83), 55 (100); HRMS (EI) calcd for C9H13SO3F3 (M+) 258.0538,
found 258.0557. b) W. H. Perkin and W. J. Pope, J. Chem. Soc., 99,
1510 (1911); H. Gerlach, Helv. Chim. Acta, 49, 1291 (1966); H. M.
Walborsky and R. B. Banks, Bull. Soc. Chim. Belg., 89, 849 (1980). c)
I. Fleming and T. W. Newton, J. Chem. Soc. Perkin Trans. 1, 1984,
119. d) P. J. Stang and M. Hanack, Synthesis, 1982, 85. e) The ee’s of
the substrate and products were determined by gas chromatography
using complementally three chiral columns, Chrompack-Chirasil-DEX
CB, Supelco β-DEX 120, and Supelco β-DEX 325. Accuracy of the
gas chromatographic analysis was evaluated to be within ±0.5%.
14 Solvolysis was carried out in a sealed pyrex glass tube with about 2 mg
of (R)-4 in 4 mL of aqueous methanol. The reaction mixture was kept
at 140 ˚C from 4 days to a few weeks, and the products were extracted
with pentane containing tetradecane as an internal standard for analysis.
15 T. Okuyama, T. Takino, K. Sato, and M. Ochiai, J. Am. Chem. Soc.,
120, 2275 (1998).
The unrearranged product, aldehyde 5, is achiral, but some
chiral unrearranged substitution product 6 was obtained in the
presence of added bromide salt. The 4-methylcyclohexylidene-
methyl bromide (6) obtained was found to have the S configura-
tion. That is, the substitution reaction occurs largely with inver-
sion of configuration. This means that 6 is not formed from trap-
ping of the intermediate vinyl cation I1, but it must arise directly
from (R)-4 via a vinylic SN2 pathway.9,15,16
The effects of solvent composition of aqueous methanol
were also examined. The complete chirality transfer from the
substrate 4 to the rearranged product 3 was always observed.
The increased content of methanol decreases the rate of solvoly-
sis and also the fraction of the rearranged product 3. The transi-
tion state for the rearrangement must be more polar than that for
the formation of unrearranged product. In pure methanol,
Hanack et al.8 did not find any reaction of 1 taking place, but we
found small amounts of products 3 and 5 from 4. Furthermore,
16 a) M. N. Glukhovtsev, A. Pross, and L. Radom, J. Am. Chem. Soc.,
116, 5961 (1994). b) V. Lucchini, G. Modena, and L. Pasquato, J. Am.
Chem. Soc., 117, 2297 (1995). c) C. K. Kim, K. H. Hyun, C. K. Kim,
and I. Lee, J. Am. Chem. Soc., 122, 2294 (2000).