7534
J . Org. Chem. 1997, 62, 7534-7535
temperature, it can easily be handled at normal temper-
atures and stored >10 months at -15 °C.
F a cile F r a gm en ta tion s of
Alk en yl(a r yl)iod on iu m Tr ifla tes
Salt 1a readily reacts in CDCl3 and MeOH at elevated
temperatures (60-65 °C). A new 1H NMR spectrum after
decomposition in chloroform revealed that 4-(trifluorom-
ethyl)iodobenzene was generated. New proton reso-
nances also appeared at δ 6.40 (br s), 5.60 (q), and 5.34
(q). Four components were identified by GC MS as
p-(trifluoromethyl)iodobenzene 2a , and three enol tri-
flates with M+ ) 204 (m/ z).
We then synthesized the possible vinyl triflates (3a ,
(Z)-4a and (E)-4a ) which could result from an initial vinyl
cation intermediate generated from 1a .13,14 The Z/ E
ratio of this mixture of stereoisomers (4) prepared by
addition of TfOH to 2-butyne was 4.5/1 (1H NMR). Each
of these was analyzed by GC and GCMS, and the reten-
tion times and masses all corresponded to the decom-
position products; the overall reaction is shown in eq 1.
Salt 1b also decomposes readily in CDCl3, but provides
(E)-4b as the only detected enol triflate. Contrary to
Okuyama and Ochiai’s results in which a styryl analog
containing a â-hydrogen reacted more slowly than a
decen-1-yl analog, our disubstituted styrenyl derivative,
1b, decomposes far more rapidly than dimethyl analog
1a in CDCl3 or alcoholic solvents.15 Typically, 1b was
completely decomposed at room temperature in CDCl3
for 12 h before chromatography (vide infra). Okuyama
and Ochiai’s styryl analog required 76 h at 50 °C in AcOH
to provide 2.2% of a rearranged enolacetate.15
Robert J . Hinkle* and David B. Thomas
Department of Chemistry, The College of William & Mary,
P.O. Box 8795, Williamsburg, Virginia 23187-8795
Received J une 23, 1997
The last 10-15 years have witnessed a great explosion
in the synthesis and uses of hypervalent iodine com-
pounds.1,2 Because of the labile aryliodonio moiety,
alkenyl(aryl)iodonium salts display significant reactivity
as electrophiles in copper-mediated nucleophilic substitu-
tions,3 palladium-catalyzed coupling reactions,4 and nu-
cleophilic substitution by enolates.5 Ochiai, Okuyama,
and co-workers recently quantified the nucleofugality of
the phenyliodonio moiety as 106 more reactive than the
triflate group. They also reported some elimination
product from solvolysis of trans-1-decen-1-yl(phenyl)-
iodonium tetrafluoroborate.6
In our use of â-disubstituted alkenyl(aryl)iodonium
salts in which the aryl moiety is either phenyl or contains
the 4-trifluoromethyl moiety, we observed and report
herein remarkably facile fragmentation pathways which
may occur via the intermediacy of a simple primary vinyl
cation and a phenonium ion.7-9 To the best or our
knowledge, one of the rearrangements we observe rep-
resents the first example of leaving group departure (in
absence of any exogenous nucleophile) from a primary
vinyl compound which is neither a cyclopropylidene
derivative, nor contains a â-stabilizing group such as
phenyl. Although the disubstituted substrates examined
fragment and rearrange easily, they are not prone to
direct E2 elimination because they lack hydrogen atoms
â to the iodonium moiety.
2-Methyl-1-propenyl(4-(trifluoromethyl)phenyl)iodo-
nium triflate (1a ) was synthesized10 using 4-(trifluoro-
methyl)phenyl(cyano)iodonium triflate.11 Salt 1b was
previously reported.10 The new salt 1a was characterized
1
by H NMR spectroscopy as well as IR and combustion
analysis.12 Although 1a does slowly decompose at room
* To whom correspondence should be addressed: Tel: 757-221-
1501; FAX 757-221-2715; E-mail: rjhink@chem1.chem.wm.edu.
(1) Recent reviews include: Varvoglis, A. Tetrahedron 1997, 53,
1179-1255. Zhdankin, V. V.; Stang, P. J . Chem. Rev. 1996, 96, 1123-
1178.
(2) For advances in organic synthesis, see: Varvoglis, A. Hypervalent
Iodine in Organic Synthesis; Academic: New York, 1997.
(3) Ochiai, M.; Sumi, K.; Takaoka, Y.; Kunishima, M.; Nagao, Y.;
Shiro, M.; Fujita, E. Tetrahedron 1988, 4095-4112.
(4) Hinkle, R. J .; Poulter, G. M.; Stang, P. J . J . Am. Chem. Soc. 1993,
115, 11626-11627. Moriarty, R. M.; Epa, W. R. Tetrahedron Lett.
1992, 4095-4098. Moriarty, R. M.; Epa, W. R.; Awasthi, A. K. J . Am.
Chem. Soc. 1991, 113, 6315-6316.
(5) Ochiai, M.; Shu, T.; Nagaoka, T.; Kitagawa, Y. J . Org. Chem.
1997, 62, 2130-2138.
(6) Okuyama, T.; Takino, T.; Sueda, T.; Ochiai, M. J . Am. Chem.
Soc. 1995, 117, 3360-3367.
(7) Vinyl cations containing R-alkyl groups are a well-studied class
of intermediates. See, inter alia: (a) Summerville, R. H.; Schleyer, P.
v. R. J . Am. Chem. Soc. 1974, 96, 1110-1120. (b) Summerville, R.
H.; Senkler, C. A.; Schleyer, P. v. R.; Dueber, T. E.; Stang, P. J . J .
Am. Chem. Soc. 1974, 96, 1100-1109. (c) Stang, P. J .; Rappoport, Z.;
Hanack, M.; Subramanian, L. R. Vinyl Cations; Academic: New York,
1979 and references therein.
(8) For more recent references on vinyl cations, including a primary
cation derived from hydration of acetylene, see: Bott, K. Chem. Ber.
1994, 127, 933-939. Lucchini, V.; Modena, G. J . Am. Chem. Soc. 1990,
112, 6291-6296.
(9) For NMR studies of stabilized vinyl cations, see: Siehl, H.-U.
Pure Appl. Chem. 1995, 67, 769-775. Siehl, H.-U.; Kaufmann, F.-P.;
Hori, K. J . Am. Chem. Soc. 1992, 114, 9343-9349.
(10) Hinkle, R. J .; Stang, P. J . Synthesis 1994, 393-396.
(11) Zhdankin, V. V.; Stang, P. J . Tetrahedron Lett. 1993, 34, 6853-
6856.
It was possible that adventitious acid caused the
decomposition.16 The acid-sensitivity of the decomposi-
tion pathway described herein was tested in two ways:
(a) 1.0 equiv of triflic acid was added to a CDCl3 solution
of 1a ; and (b) the CDCl3 used was filtered through basic
alumina. Compound 1a was found to be stable in the
presence of the excess acid and decomposed as described
in the alumina-filtered solvent.
The mechanism for decomposition of 1a (Scheme 1)
may involve heterolysis of the vinyl C-I bond to afford
the primary vinyl cation A and 4-CF3C6H4I, 2a . This
cation is particularly significant as it does not contain a
stabilizing group such as a cyclopropylidene or â-phenyl.
Attack of the triflate before methyl migration leads to
(12) Data for iodonium salt 1a , including a 1H NMR spectrum, is
contained in the Supporting Information.
(13) Triflate 3a was synthesized by the hindered base method:
Stang, P. J .; Treptow, W. Synthesis 1980, 283-284.
(14) See ref 7a for the preparation of (E)-4a and (Z)-4a . Isomers
were identified according to chemical shift of the vinylic protons. Also
see: Stang, P. J .; Datta, A. K. J . Am. Chem. Soc. 1989, 111, 1358-
1363.
(15) SN2 and phenonium ion reactions of vinyliodonium salts were
recently reported: Okuyama, T.; Ochiai, M. J . Am. Chem. Soc. 1997,
119, 4785-4786.
(16) Equilibration of some vinyl triflates to a thermodynamic
mixture can be accomplished in the presence of excess triflic acid.
See: ref 7b.
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