an alkenyl triflate, ligand exchange between OTf and X, and
reductive elimination of the halide,10 also seems incompatible
with the production of stereoisomers. The much higher
reactivity of a-substituted vinyl triflate 1a than b-substituted
1e, together with the E–Z isomerization of 1e may indicate an
involvement of cationic or radical species in the catalytic cycle.
In conclusion, we have disclosed a transformation of alkenyl
triflates to the corresponding halides using a ruthenium
complex and a lithium halide as a catalyst and a halide source,
respectively.
D. Kim, J. Am. Chem. Soc., 2007, 129, 2269–2274; (c) S. Schiltz,
C. Ma, L. Ricard and J. Prunet, J. Organomet. Chem., 2006, 691,
5438–5443; (d) I. Martınez, P. E. Alford and T. V. Ovaska, Org.
´
Lett., 2005, 7, 1133–1135; (e) J. L. Hubbs and C. H. Heathcock,
J. Am. Chem. Soc., 2003, 125, 12836–12843; (f) L. A. Paquette and
F.-T. Hong, J. Org. Chem., 2003, 68, 6905–6918.
4 Transformation of alkenyl triflates to iodides by treatment with
MgI2 in CS2 or cyclohexane is reported to be applicable to a certain
scope of triflates but not to simple ones such as 1-cyclopentenyl
triflate. (a) A. G. Martınez, R. M. Alvarez and A. G. Fraile,
´
Synthesis, 1986, 222–224. The halogenation of alkenyl triflates has
been reported for those having an electron-donating group at the
a-position or those derived from 1,3-dicarbonyl compounds. For
examples, see: (b) J. K. Stille and M. P. Sweet, Organometallics,
1990, 9, 3189–3191; (c) H. Tanaka, Y. Nishioka, Y. Kameyama,
S. Sumida, H. Matsuura and S. Torii, Chem. Lett., 1995, 24,
709–710; (d) H. Salim, X. Chen and Z. Rappoport, J. Phys. Org.
Chem., 2001, 14, 778–787; (e) K. Kamei, N. Maeda and
T. Tatsuoka, Tetrahedron Lett., 2005, 46, 229–232.
This work has been supported financially in part by a
Grant-in-Aid for Scientific Research, MEXT, Japan (the
Global COE Program ‘‘Integrated Materials Science’’ on
Kyoto University). We thank Central Glass Co., Ltd. for the
gift of Tf2O.
5 (a) K. Ritter, Synthesis, 1993, 735–762; (b) I. L. Baraznenok,
V. G. Nenajdenko and E. S. Balenkova, Tetrahedron, 2000, 56,
3077–3119.
6 Nickel- or copper-catalyzed halogen exchange reactions in alkenyl
halides have been reported. (a) K. Takagi, N. Hayama and
S. Inokawa, Chem. Lett., 1978, 7, 1435–1436; (b) T. T. Tsou and
J. K. Kochi, J. Org. Chem., 1980, 45, 1930–1937; (c) H. Suzuki,
M. Aihara, H. Yamamoto, Y. Takamoto and T. Ogawa, Synthesis,
1988, 236–238.
Notes and references
z A representative procedure for halogenation of alkenyl triflates
(Table 2, entry 1): to a THF solution (0.97 mL) of Ru(acac)3
(3.0 mg, 7.5 mmol) and LiBr (26 mg, 0.30 mmol) placed in an
oven-dried 20 mL Schlenk tube, was added ethylmagnesium bromide
(1.0 M THF solution, 0.030 mL, 30 mmol). After stirring for 10 min at
room temperature, 4-(4-methoxyphenyl)-1-buten-2-yl triflate (1c,
containing 9% of (E)- and (Z)-1-(4-methoxyphenyl)-2-buten-3-yl
triflates, 77.5 mg, 0.250 mmol) was added at 20 1C and stirring was
continued for 1 h. Purification by passing through a pad of silica gel
using hexane : AcOEt (10 : 1) as an eluent gave 2-bromo-4-(4-methoxy-
phenyl)-1-butene as a colorless oil (containing (E)- and (Z)-3-bromo-
1-(4-methoxyphenyl)-2-butenes, 58.3 mg, 97% yield).
7 (a) E. Shirakawa, T. Sato, Y. Imazaki, T. Kimura and T. Hayashi,
Chem. Commun., 2007, 4513–4515; (b) E. Shirakawa, Y. Imazaki
and T. Hayashi, Chem. Lett., 2008, 37, 654–655.
8 Transformation of alkenyl triflates to alkenyl metals of Sn, B, and
Si in the presence of a transition metal catalyst has been reported.
For Sn, see ref. 3. For other examples, see: (a) S. Matsubara,
J. Hibino, Y. Morizawa, K. Oshima and H. Nozaki, J. Organomet.
Chem., 1985, 285, 163–172; (b) W. D. Wulff, G. A. Peterson,
W. E. Bauta, K.-S. Chan, K. L. Faron, S. R. Gilbertson,
R. W. Kaesler, D. C. Yang and C. K. Murray, J. Org. Chem.,
1986, 51, 277–279; (c) K. Takahashi, J. Takagi, T. Ishiyama and
N. Miyaura, Chem. Lett., 2000, 29, 126–127; (d) M. Murata,
Y. Oyama, S. Watanabe and Y. Masuda, Synthesis, 2000, 778–780.
9 Stereoisomeric mixtures of 1-deuterio-1-octen-2-yl triflate of
different compositions (E:Z = 71 : 29 and 20 : 80) were transformed
to the corresponding bromide in similar isomer ratios (E : Z =
47 : 53 and 53 : 47, respectively).
1 Elimination of hydrogen halides from 1,1- or 1,2-dihaloalkanes, and
hydrometalation of alkynes followed by halogenolysis of the resulting
alkenyl metals are two major methods to prepare alkenyl halides.
2 Several methods to prepare alkenyl halides directly from ketones
are known though regioselective synthesis from dissymmetric
ketones is difficult. For examples, see: (a) R. C. Larock,
Comprehensive Organic Transformations, Wiley-VCH, New York,
2nd edn, 1999, pp. 722–724 and p. 302. For a recent example,
which is applicable to a relatively wide range of ketones, see:
(b) A. Spaggiari, D. Vaccari, P. Davoli, G. Torre and F. Prati,
J. Org. Chem., 2007, 72, 2216–2219.
10 This cycle has not been reported to be operative, which is probably
due to the reluctant reductive elimination step, which is known to
take place only under special conditions. (a) A. H. Roy and
J. F. Hartwig, J. Am. Chem. Soc., 2001, 123, 1232–1233;
(b) A. H. Roy and J. F. Hartwig, J. Am. Chem. Soc., 2003, 125,
13944–13945; (c) A. H. Roy and J. F. Hartwig, Organometallics,
2004, 23, 1533–1541.
3 Regioselective preparation of alkenyl triflates from dissymmetric
ketones through the corresponding enolates followed by trimethyl-
stannylation with the aid of a transition metal and halogenolysis is
frequently used as a reliable method for transformation from
ketones to alkenyl halides in natural product synthesis. For recent
examples, see: (a) P. B. Hurley and G. R. Dake, J. Org. Chem.,
2008, 73, 4131–4138; (b) H. Kim, H. Lee, D. Lee, S. Kim and
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5090 | Chem. Commun., 2009, 5088–5090