J. Am. Chem. Soc. 1997, 119, 4785-4786
Acetolysis of Styryl and 1-Decenyl Iodonium Salts.
4785
Occurrence of Two-Step Mechanism via
Vinylenephenonium Ion and One-Step Inversion
Mechanism
Tadashi Okuyama* and Masahito Ochiai
Faculty of Engineering Science, Osaka UniVersity
Toyonaka, Osaka 560, Japan
Faculty of Pharmaceutical Sciences
UniVersity of Tokushima
Shomachi, Tokushima 770, Japan
ReceiVed February 21, 1997
Vinyl iodonium salts are good precursors of vinyl cations
because of a very high leaving ability of the iodonio group,
6
1
evaluated to be some 10 times as high as that of triflate.
However, a primary vinyl cation still seemed difficult to generate
from a 1-alkenyl iodonium salt. The latter salt readily undergoes
nucleophilic substitution with halide ions leading to a completely
2
inverted product in aprotic solvents. Reaction of the styryl
iodonium salt (1, R ) phenyl) is much slower than that of the
1
-alkenyl salt (1, R ) alkyl) and results in more of the
elimination product. However, prolonged reaction of (E)-styryl-
3
(
phenyl)iodonium tetrafluoroborate (1a) in acetic acid was
found to lead to mainly the retained substitution product ((E)-
a), while that of (E)-1-decenyl(phenyl)iodonium tetrafluoro-
2
Figure 1.
1
H NMR spectra of the product mixtures obtained from the
3
borate (1b) led to exclusively the inverted product ((Z)-2b).
The retained product (E)-2a was assumed from labeling experi-
ments to be formed from the intermediate vinylenephenonium
ion, while the inversion product (Z)-2 is considered to result
from a one-step in-plane (SN2) route.
reactions of 1a (a) and the R-deuterated 1a (b) in acetic acid at 70 °C
for 7 days. The spectra were recorded at 500 MHz on a Varian INOVA
500 spectrometer.
7
The R-deuterated 1a undergoes similar reaction in acetic acid
at 70 °C, resulting in the same isomeric mixture (by VPC) of
1
the monodeuterated 2a (GC MS). The H NMR spectrum of
the product mixture (Figure 1b) shows that a deuterium is
distributed equally both at the R and â positions of (E)-2a while
it is essentially located only at the original R position of the Z
isomer of 2a. The former conclusion may be obvious from the
signals for the olefinic protons (6.39 and 7.84 ppm). Both
doublet signals coalesce essentially into slightly separated triplets
coupled with the other olefinic deuterium and the integral
intensities are both about one-half that expected from the
intensity of the signal for the acetyl group. The latter conclusion
regarding (Z)-2a is less obvious. The signal for the R proton
even of the protium product (7.29 ppm) is not clearly seen
Reaction of 1b in (unbuffered) acetic acid was carried out at
5
0 °C, and only the single isomer of 2b and accompanying
4
iodobenzene were obtained in ca. 50% yield after 28 h. Any
sign of the other isomer of 2b or other products was not found
1
by VPC or H NMR. The stereochemistry of the 2b obtained
(Figure 1a), but it overlaps with the phenyl signals as the 2D
1
5
is assumed to be a Z configuration from the H NMR spectrum;
NMR clearly indicates (not shown). The spectrum of this region
of the deuterium products (Figure 1b) is simpler than that of
the protium counterpart (Figure 1a), suggesting the disappear-
ance of the R proton, if we examine the spectra closely. The
signal for the â proton (5.70 ppm) is a broad singlet due to the
deuterium coupling, and the intensity is about one-third that of
the acetyl signal. When the NMR data is examined in another
way, the relative integral intensity of the acetyl signals for (E)-
2a and (Z)-2a obtained from the deuterated substrate conforms
to the relative VPC peak area (85:15), but that of the â protons
is low, 0.34:0.11 ) 46:15. The signal intensity of the R proton
(0.35) of (E)-2a is about one-half that calculated and nearly
equal to that of the â proton (0.34).
the coupling constant between the olefinic protons (J ) 6.4 Hz)
and chemical shifts are compared with relevant data. Thus, the
substitution of 1b at the vinylic carbon occurs exclusively with
inversion of configuration.
Reaction of 1a is much slower; only 2.2% yield of 2a was
obtained after 76 h at 50 °C. The reaction at 70 °C for 7 days
gave 37% of 2a and 4.5% of 3a as well as accompanying
iodobenzene (55%). The substitution product 2a comprises two
isomers at a ratio of 85:15; the main isomer is assigned to an
1
E configuration and the minor one to Z by H NMR (Figure
6
1
a).
(
1) Okuyama, T.; Takino, T.; Sueda, T.; Ochiai, M. J. Am. Chem. Soc.
995, 117, 3360.
2) Ochiai, M.; Oshima, K.; Masaki, Y. J. Am. Chem. Soc. 1991, 113,
059.
Scrambling of the D/H isotope in (E)-2a is most reasonably
interpreted by the formation of the symmetric vinylene-
1
(
7
1
(
3) The iodonium salts were obtained as described previously. Ochiai,
(6) H NMR (CDCl3), (E)-2a: δ 2.19 (s, CH3CO), 6.39 (d, â-H, J )
M.; Sumi, K.; Takaoka, Y.; Kunishima, M.; Nagao, Y.; Shiro, M.; Fujita,
12.7 Hz), 7.84 (d, R-H, J ) 12.7 Hz). (Z)-2a: δ 2.27 (s, CH3CO), 5.70 (d,
â-H, J ) 7.1 Hz), 7.29 (d, R-H, J ) 7.1 Hz).
E. Tetrahedron 1988, 44, 4095.
(
4) Yields of the products were determined by VPC using tetradecane
(7) The R-deuterated 1a (the isotopic purity ) 98.8%) was prepared by
Lewis acid-catalyzed boron-iodine(III) exchange reaction of (E)-styryl-
boronic acid-R-d with (diacetoxyiodo)benzene. This reaction will be reported
elsewhere.
as an internal standard.
1
(
5) H NMR (CDCl3) of (Z)-2b: δ 2.14 (s, CH3CO), 4.86 (dt, â-H, J )
6
.4, 12.7 Hz), 6.98 (dt, R-H, J ) 6.4, 1.6 Hz), 0.9, 1.3, 1.5 (octyl).
S0002-7863(97)00560-X CCC: $14.00 © 1997 American Chemical Society
Okuyama
