J. Am. Chem. Soc. 2000, 122, 8787-8788
8787
Scheme 1
Chirality Transfer from
4-Methylcyclohexylidenemethyl(phenyl)iodonium
Tetrafluoroborate to 4-Methylcycloheptanone during
Solvolysis: Evidence against a Primary Vinylic
Cation as Intermediate
Morifumi Fujita,* Yuichi Sakanishi, and Tadashi Okuyama*
Faculty of Science, Himeji Institute of Technology
Kamigori, Hyogo 678-1297, Japan
ReceiVed May 1, 2000
ReVised Manuscript ReceiVed July 10, 2000
Primary vinylic cations, bearing a hydrogen atom at the R
carbon, are admittedly unstable,1-3 and can only be generated
under special conditions; for example, by nuclear decay of tritiated
ethene,4 by photolysis,5 in a superacid,6 or in strong sulfuric acid.7
Vinyl cations are stabilized by â-alkyl substituents,3,8 and there
are indications for formation of primary â,â-dialkylvinyl cations
under normal conditions.9-11 Cyclohexylidenemethyl triflate was
suggested to undergo solvolysis via a transient primary vinylic
cation in aqueous alcohols at high temperature (eq 1).9 In this
rearrangements.10 The difference in migratory aptitudes of the
â-alkyl groups further complicates the analysis of the reaction.
Most of the complications are avoided by using the chiral
4-substituted cyclohexylidenemethyl derivative 1 as substrate. The
two â-alkyl moieties are electronically as well as sterically the
same. The only difference is the chirality and the primary vinylic
cation I1 formed from 1 is achiral due to the linear structure1,2 of
the cationic carbon atom. If the primary cation I1 were generated,
the chirality should be lost leading to reduced enantiomeric purity
of the products. The only products expected are enantiomeric
substitution products of unrearranged and rearranged form
(Scheme 1). Elimination products are hard to produce since seven-
membered cyclic acetylene and allene are subject to serious strain.
Optically active (R)-4-methylcyclohexylidenemethyl(phenyl)-
iodonium tetrafluoroborate (1)12,13 was prepared from (R)-4-
methylcyclohexylidenemethyl bromide which was obtained ac-
cording to a literature procedure15 (Scheme 2). The last step,
conversion of the vinylsilane to the corresponding vinyliodonium
salt is established to proceed with complete retention of config-
uration,16 and the enantiomeric excess (ee) of 1 is the same as
that of the precursor vinylsilane unless racemization or optical
resolution of 1 occurs during the workup. Crystallization of the
crude 1 was found to diminish its ee. Crystallized 1 of 69% ee17
was used for the solvolysis experiments.
contribution we report that solvolysis of the chiral 4-methylcyclo-
hexylidenemethyl iodonium salt 1 results in complete chirality
transfer to the rearranged product, 4-methylcycloheptanone (2),
which serves as evidence against the involvement of a primary
vinylic cation intermediate (Scheme 1).
We have recently shown that the solvolysis of unsymmetrically
substituted â,â-dialkylvinyliodonium salts takes place with ex-
tensive rearrangements through â-alkyl participation, and con-
cluded that primary vinylic cations are not formed in aqueous
solution.10 Nonetheless, it was suggested that primary cations may
be partially involved in less nucleophilic solvents such as 2,2,2-
trifluoroethanol (TFE) and acetic acid. The thermolysis of â,â-
dialkylvinyliodonium triflates in chloroform is also proposed to
occur via primary vinylic cations.11 The conclusions are based
on the E/Z stereochemistry of the unrearranged substitution
products obtained from geometrically isomeric substrates. How-
ever, product profiles are quite complicated due to the presence
of two different â-alkyl substituents, which leads to three pairs
of substitution products and three elimination products via
(12) [R]20 ) -20.4 (CHCl3, c ) 0.91); 1H NMR (400 MHz, CDCl3) δ
7.90 (d, J ) D7.8 Hz, 2H), 7.61 (t, J ) 7.8 Hz, 1H), 7.47 (t, J ) 7.8 Hz, 2H),
6.65 (s, 1H), 2.75-2.66 (m, 2H), 2.47-2.34 (m, 2H), 1.91-1.58 (m, 3H),
1.15-1.00 (m, 2H), 0.91 (d, J ) 6.8 Hz, 3H); 13C NMR (100 MHz, CDCl3)
δ 164.6, 134.5, 132.3, 132.2, 110.4, 91.8, 36.5, 36.1, 35.7, 35.1, 31.4, 21.0.
(13) Although the synthesis of optically active diaryliodonium salts has
recently been achieved,14 this is the first example of iodonium salts carrying
a chiral and reactive vinyl moiety.
(14) Ochiai, M.; Kitagawa, Y.; Takayama, N.; Takaoka, Y.; Shiro, M. J.
Am. Chem. Soc. 1999, 121, 9233-9234.
(15) (a) Perkin, W. H.; Pope, W. J. J. Chem. Soc. 1911, 99, 1510-1529.
(b) Gerlach, H. HelV. Chim. Acta 1966, 49, 1291-1302.
(1) Stang, P. J.; Rappoport, Z.; Hanack, M.; Subramanian, L. R. Vinyl
Cations; Academic Press: New York, 1979.
(2) Rappoport, Z., Stang, P. J., Eds. Dicoordinated Carbocations; John
Wiley & Sons: Chichester, 1997.
(16) Ochiai, M.; Sumi, K.; Takaoka, Y.; Kunishima, M.; Nagao, Y.; Shiro,
M.; Fujita, E. Tetrahedron 1988, 44, 4095-4112.
(17) Determination of the ee of the neutral organic samples was performed
using a gas chromatograph equipped with a chiral column (CHROMPACK-
Chirasil-DEX CB (i.d. 0.25 mm × 25 m)) usually within (1% in ee. The
vinylsilane of 78% ee was converted to the iodonium salt 1 by an established
procedure.16 Part of the crude 1 was converted to the vinyl bromide by treating
it with tetrabutylammonium bromide (0.1 M) in chloroform.18 The vinyl
bromide obtained was 66% excess of the S form, determined using the chiral
GC-column. This indicates that the substitution proceeds with 92% [) 100
× (66/78 + 1)/2] inversion. A sample of 1 obtained by crystallization of the
crude mixture was also converted under the same conditions to the vinyl
bromide which was of 58% ee. This means that the ee of the sample of 1 was
69% [) 58/(2 × 0.92-1)]. If the conversion of the vinylsilane to 1 would
proceed with partial loss of ee, the ee of the crystallized 1 should be lower
than 69%. On the contrary, the 69% ee of product 2 obtained in the solvolysis
experiments implies that the ee of the substrate 1 is not lower than 69% unless
enantiomeric enrichment occurs during the reaction. These results are also
consistent with complete retention of configuration in the formation of
vinyliodonium salts from the corresponding vinylsilanes.
(3) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R.
D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17, 1-861.
(4) Fornarini, S.; Speranza, M. Tetrahedron Lett. 1984, 25, 869; J. Am.
Chem. Soc. 1989, 111, 7402-7407.
(5) Lodder, G. ref 2, Chapter 8.
(6) Hogeveen, H.; Roobeek, C. F. Tetrahedron Lett. 1971, 3343-3346.
(7) Lucchini, L.; Modena, G. J. Am. Chem. Soc. 1990, 112, 6291-6296.
(8) Kobayashi, S.; Hori, Y.; Hasako, T.; Koga, K.; Yamataka, H. J. Org.
Chem. 1996, 61, 5274-5279.
(9) Hanack, M.; Ma¨rkl, R.; Martinez, A. G. Chem. Ber. 1982, 115, 772-
782.
(10) Okuyama, T.; Yamataka, H.; Ochiai, M. Bull. Chem. Soc. Jpn. 1999,
72, 2761-2769. Okuyama, T.; Sato, K.; Ochiai, M. Chem. Lett. 1998, 1177-
1178.
(11) (a) Hinkle, R. J.; McNeil, A. J.; Thomas, Q. A.; Andrews, M. N. J.
Am. Chem. Soc. 1999, 121, 7437-7438; 10668. (b) Hinkle, R. J.; Thomas,
D. B. J. Org. Chem. 1997, 62, 7534-7535.
10.1021/ja001509x CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/22/2000