O. Mun˜oz-Mun˜oz, A. Rodr´ıguez-Afonso and J. R. Murguia, J. Nat.
Prod., 2003, 66, 722–724.
2 For a review on cross-coupling reactions of C(sp) atoms, see: J. A.
Marsden and M. M. Haley, in Metal-Catalyzed Cross-Coupling
Reactions, ed. A. de Meijer and F. Diederich, Wiley-VCH, Weinheim,
2nd edn, 2004, pp. 317–394.
3 For reviews, see: (a) K. Sonogashira, in Metal-Catalyzed Cross-Coupling
Reactions, ed. F. Diederich and P. J. Stang, Wiley-VCH, Weinheim,
1998, pp. 203–229; (b) K. Sonogashira, in Handbook of Organopalladium
Chemistry for Organic Synthesis, ed. E. Negishi, Wiley-Interscience, New
York, 2002, pp. 493–529.
4 For reviews, see: (a) E. Negishi and C. Xu, in Handbook of
Organopalladium Chemistry for Organic Synthesis, ed. E. Negishi,
Wiley-Interscience, New York, 2002, pp. 531–549; (b) E. Negishi and
L. Anastasia, Chem. Rev., 2003, 103, 1979–2017.
Scheme 5
5 (a) J. A. Walker, S. P. Bitler and F. Wudl, J. Org. Chem., 1984, 49,
4733–4734; (b) D. Madec, S. Pujol, V. Henryon and J. P. Fe´re´zou,
Synlett, 1995, 435–438.
shown in Scheme 5 similar to those conducted by Oshima and co-
workers gave a result consistent with the plausible catalytic
cycle.10b,h Thus, alkenyl triflate 2g was not consumed at all on
treatment with 1 equivalent of the cobalt complex generated from
Co(acac)3 and 4 equivalents of triethylsilylethynylmagnesium
bromide (1f).13 In contrast, 70% of 2g was consumed with the
use of one more equivalent (total 5 equiv.) of 1f, giving a
considerable amount of the corresponding cross-coupling product
(3fg) and the reductive homocoupling product (8) of triflate 2g.
Coproduction of 8 was completely suppressed by use of an excess
amount (8 equiv.) of 1f. The observation that 4 equivalents of 1f
was consumed to transform the precursor to some complex, which
is not yet reactive towards 2g, and that the addition of another
equivalent of 1f converted it into an active species that reacts with
2g implies that Grignard reagents first get into the catalytic cycle
and then alkenyl triflates follow as shown in Scheme 4. There is
some possibility that oxidative addition of 2 to 5 proceeds through
two successive single electron transfers, as in the case with alkyl
halides,10b,h but Oshima and co-workers proposed a non-radical
oxidative addition mechanism for alkenyl iodides in the cobalt-
catalyzed coupling with trimethylsilylmethylmagnesium chloride.8d
In conclusion, we have developed a simple, inexpensive and
widely applicable catalyst system for alkynyl–alkenyl coupling,
where alkynyl Grignard reagents undergo coupling with alkenyl
triflates in the presence of a catalytic amount of Co(acac)3 in THF.
6 (a) K. Okuro, M. Furuune, M. Enna, M. Miura and M. Nomura,
J. Org. Chem., 1993, 58, 4716–4721; (b) C. G. Bates, P. Saejueng and
D. Venkataraman, Org. Lett., 2004, 6, 1441–1444; (c) P. Saejueng,
C. G. Bates and D. Venkataraman, Synthesis, 2005, 1706–1712.
7 Terminal alkynes couple with alkenyl iodides in the presence of a Ni–Cu
catalyst. See: (a) L. Wang, P. Li and Y. Zhang, Chem. Commun., 2004,
514–515; (b) M. Wang, P. Li and L. Wang, Synth. Commun., 2004, 34,
2803–2812.
8 For examples of the use of alkenyl halides in cobalt-catalyzed cross-
coupling reactions, see: (a) G. Cahiez and H. Avedissian, Tetrahedron
Lett., 1998, 39, 6159–6162; (b) H. Avedissian, L. Be´rillon, G. Cahiez and
P. Knochel, Tetrahedron Lett., 1998, 39, 6163–6166; (c) T. Kamachi,
A. Kuno, C. Matsuno and S. Okamoto, Tetrahedron Lett., 2004, 45,
4677–4679; (d) W. Affo, H. Ohmiya, T. Fujioka, Y. Ikeda,
T. Nakamura, H. Yorimitsu, K. Oshima, Y. Imamura, T. Mizuta
and K. Miyoshi, J. Am. Chem. Soc., 2006, 128, 8068–8077. For
examples of the use of alkynyl Grignard reagents in cobalt-catalyzed
cross-coupling reactions, see: (e) A. Kuno, N. Saino, T. Kamachi and
S. Okamoto, Tetrahedron Lett., 2006, 47, 2591–2594; (f) H. Ohmiya,
H. Yorimitsu and K. Oshima, Org. Lett., 2006, 8, 3093–3096.
9 Electrophiles with an oxygen leaving group are used in cobalt-catalyzed
cross-coupling reactions. For allyl methoxides, see: (a) K. Mizutani,
H. Yorimitsu and K. Oshima, Chem. Lett., 2004, 33, 832–833; (b)
H. Yasui, K. Mizutani, H. Yorimitsu and K. Oshima, Tetrahedron,
2006, 62, 1410–1415. For ortho-acylphenyl tosylates, see: (c) T. J. Korn,
M. A. Schade, S. Wirth and P. Knochel, Org. Lett., 2006, 8, 725–728.
10 Recently the cobalt-catalyzed cross-coupling reaction of Grignard
reagents has been extensively investigated, in particular, by Oshima
and co-workers. For examples not cited in ref. 8 and 9, see: (a) Y. Nishii,
K. Wakasugi and Y. Tanabe, Synlett, 1998, 67–69; (b) K. Wakabayashi,
H. Yorimitsu and K. Oshima, J. Am. Chem. Soc., 2001, 123, 5374–5375;
(c) T. Tsuji, H. Yorimitsu and K. Oshima, Angew. Chem., 2002, 114,
4311–4313, (Angew. Chem., Int. Ed., 2002, 41, 4137–4139); (d)
K. Mizutani, H. Shinokubo and K. Oshima, Org. Lett., 2003, 5,
3959–3961; (e) T. J. Korn, G. Cahiez and P. Knochel, Synlett, 2003,
1892–1894; (f) H. Ohmiya, T. Tsuji, H. Yorimitsu and K. Oshima,
Chem.–Eur. J., 2004, 10, 5640–5648; (g) H. Ohmiya, H. Yorimitsu and
K. Oshima, J. Am. Chem. Soc., 2006, 128, 1886–1889; (h) H. Ohmiya,
K. Wakabayashi, H. Yorimitsu and K. Oshima, Tetrahedron, 2006, 62,
2207–2213.
Notes and references
{ General procedure for the cobalt-catalyzed coupling of alkynylmagne-
sium bromides with alkenyl triflates: to an alkyne (0.50 mmol) placed in a
20 mL Schlenk tube was added ethylmagnesium bromide (ca. 1.0 M THF
solution, 0.45 mmol) at room temperature, and it was stirred for 30 min. To
the mixture was added THF (0.50 mL), Co(acac)3 (2.7 mg, 7.5 mmol) and
an alkenyl triflate (0.25 mmol). After stirring at the temperature for the
time both specified in Table 2, a 0.1 M HCl aqueous solution (10 mL) was
added and the resulting mixture was extracted with diethyl ether (10 mL 6
3). The combined organic layer was washed with brine (10 mL), and dried
over anhydrous magnesium sulfate. Evaporation of the solvent followed by
purification with PTLC or column chromatography (SiO2) gave the
corresponding enyne.
11 In this coupling reaction, an excess amount of alkynyl Grignard reagent
is required for complete consumption of alkenyl triflates. A considerable
amount of RCMCMgBr was found to be transformed to RCMCCMCR
as well as some oligomers and polymers. For details, see ESI.{
12 H. P. Dang and G. Linstrumelle, Tetrahedron Lett., 1978, 19, 191–194.
See also ref. 2.
1
13 In the H NMR spectrum of each entry of Scheme 5, small peaks of
1,4-bis(triethylsilyl)-1,3-butadiyne were observed amongst a much higher
amount of broadened peaks corresponding to the Et3Si groups of some
polymeric compounds. It is likely that the diyne was produced during
the reduction of the cobalt(III) complex but was too unstable under the
reaction conditions to undergo polymerization.
1 For examples, see: (a) A. Fontana, G. d’Ippolito, L. D’Souza, E. Mollo,
P. S. Parameswaram and G. Cimino, J. Nat. Prod., 2001, 64,
131–133; (b) S. L. Iverson and J. P. Uetrecht, Chem. Res. Toxicol.,
2001, 14, 175–181; (c) N. El-Jaber, A. Este´vez-Braun, A. G. Ravelo,
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 4513–4515 | 4515