group and an electron-withdrawing group at the para position
of aryl iodides reacted with thiophenol (1 equiv) in the
presence of pyridine (1 equiv), 1 mol % of CoI2(dppe), and
Zn powder (1.5 equiv) in acetonitrile to give the correspond-
ing aryl sulfides in very good yields (entries 2-7). Under
the same reaction conditions, 4-iodoaniline and 4-iodophenol
also reacted well to give aryl sulfides in excellent yields
(entries 8 and 9). Substitution at the ortho position did not
affect the reaction. Thus, when 2-iodobenzylalcohol was
employed, the product was formed in 92% yield (entries 10
and 11). 1-Iodonaphthalene and 9-iodophenanthrene also
worked equally well for this reaction (entries 12 and 13).
Heteroaryl iodide underwent a coupling reaction with
thiophenols (entry 14). Vinyl iodide underwent the coupling
reaction with preservation of the cis stereochemistry in 88%
yield (entry 15). Sterically hindered mesityl iodide coupled
only with a modest yield of 27% (entry 16). Aryl bromides
also worked well for this coupling reaction (entries 17-22),
although in these cases 2 mol % of the cobalt catalyst was
employed. Importantly, 2-bromopyridine gave the corre-
sponding heteroaryl sulfide in good yield (entry 21), and
2-bromofuran also coupled smoothly (entry 22).
Table 3. Results of Co-Catalyzed Coupling of 4-Iodotoluene
with Thiophenols and Alkanethiolsa
By using the same protocol, we were also able to couple
aryl halides with various substituted thiophenols (Table 3,
entries 1-5). Interestingly, when 4-mercaptophenol was
employed, the coupling of the C-S bond took place very
efficiently (entry 4). 2-Isopropyliodobenzene also gave the
corresponding aryl sulfides in very good yields (entry 5).
Further, our methodology can be extended to the coupling
of various aryl halides with alkanethiols in excellent yields
(entries 6-8). It is noteworthy that in these reactions toluene
a Reaction conditions: 0.50 mmol of 4-iodotoluene, 0.50 mmol of
thiophenol or 0.50 mmol of alkanethiol, 0.0050 mmol of CoI2(dppe), 0.750
mmol of Zn, and 0.50 mmol of pyridine in 2 mL of CH3CN at 80 °C for
10 h. b Toluene was used instead of CH3CN at 70 °C.
(3) For Cu-catalyzed coupling of aryl halides and thiols, see: (a) Bates,
C. G.; Gujadhur, R. K.; Venkataraman, D. Org. Lett. 2002, 4, 2803. (b)
Kwong, F. Y; Buchwald, S. L. Org. Lett. 2002, 4, 3517. (c) Wu, Y.-J.; He,
H. Synlett 2003, 1789. (d) Bates, C. G.; Saejueng, P.; Doherty, M. Q.;
Venkataraman, D. Org. Lett. 2004, 6, 5005. (e) Deng, W.; Zou, Y.; Wang,
Y.-F.; Liu, L.; Guo, Q.-X. Synlett 2004, 1254. For other Cu-catalyzed
syntheses of thioethers, see: (f) Palomo, C.; Oiarbide, M.; Lo´pez, R.;
Go´mez-Bengoa, E.; Tetrahedron Lett. 2000, 41, 1283. (g) Savarin, C.; Srogl,
J.; Liebeskind, L. S. Org. Lett. 2002, 4, 4309. (h) Herradura, P. S.; Pendola,
K. A.; Guy, R. K. Org. Lett. 2000, 2, 2019. For Pd-catalyzed reactions,
see: (i) Mispelaere-Canivet, C.; Spindler, J.-F.; Perrio, S.; Beslin, P.
Tetrahedron 2005, 61, 5253. (j) Itoh, T.; Mase, T. Org. Lett. 2004, 6, 4587.
(k) Ferna´ndez Rodr´ıguez, M. A.; Shen, Q.; Hartwig, J. F. J. Am. Chem.
Soc. 2006, 128, 2180. (l) Murata, M.; Buchwald, S. L. Tetrahedron 2004,
60, 7397. (m) Schopfer, U.; Schlapbach, A. Tetrahedron 2001, 57, 3069.
(n) Li, G. Y. Angew. Chem., Int. Ed. 2001, 40, 1513. For Ni-catalyzed
reactions, see: (o) Cristau, H. J.; Chabaud, B.; Cheˆne, A.; Christol, H.
Synthesis 1981, 892. (p) Millois, C.; Diaz, P. Org. Lett. 2000, 2, 1705. (q)
Percec, V.; Bae, J. Y.; Hill, D. H. J. Org. Chem. 1995, 60, 6895. (r) Takagi,
K. Chem. Lett. 1987, 2221.
is much more effective than acetonitrile as the solvent.
However, the reason for the difference in product yield in
different solvents is not clear.
Even though a more detailed study is required to com-
pletely understand the mechanistic rationale of this cobalt-
catalyzed coupling sequence, a tentative pathway can be
proposed. Reduction of Co(II) to Co(I) by zinc dust initiates
the catalysis (Scheme 1). The coupling may start with a
coordination of the thiolate to the cobalt(I) center, followed
by an oxidative addition of aryl halides to Co(I). Carbon-
sulfur reductive elimination would afford the thioether and
regenerate the Co(I) catalyst. We believe that the facile
reaction of the aryl iodides and bromides, especially those
possessing electron-donating groups, is due to the coordina-
tion of thiolate to the Co center.8 Here, pyridine might also
act as a ligand in addition to being a base. However, an
alternative pathway via oxidative addition of aryl halides
to Co(I), followed by coordination of the thiolate to the
Co(III) center, cannot be completely ruled out.
(4) (a) Lindley, J. Tetrahedron 1984, 40, 1433. (b) Ley, S. V.; Thomas,
A. W. Angew. Chem., Int. Ed. 2003, 42, 5400.
(5) (a) Yorimitsu, H.; Oshima, K. Pure Appl. Chem. 2006, 78, 441. (b)
Ohmiya, H.; Wakabayashi, K.; Yorimitsu, H.; Oshima, K. Tetrahedron 2006,
62, 2207. (c) Shinokubo, H.; Oshima, K. Eur. J. Org. Chem. 2004, 10,
2081. (d) Korn, T. J.; Schade, M. A.; Wirth, S.; Knochel, P. Org. Lett.
2006, 8, 725. (e) Amatore, M.; Gosmini, C.; Perichon, J. J. Org. Chem.
2006, 71, 6130. (f) Kazmierski, I.; Gosmini, C.; Paris, J.-M.; Perichon, J.
Synlett 2006, 6, 881. (g) Korn, T. J.; Cahiez, G.; Knochel, P. Synlett 2003,
12, 1892.
(6) (a) Chang, H.-T.; Jeganmohan, M.; Cheng, C.-H. Chem. Commun.
2005, 39, 4955. (b) Chang, K.-J.; Rayabarapu, D. K.; Cheng, C.-H. J. Org.
Chem. 2004, 69, 4781. (c) Chang, K.-J.; Rayabarapu, D. K.; Cheng, C.-H.
Org. Lett. 2003, 5, 3963.
(7) (a) Shukla, P.; Hsu, Y. -C.; Cheng, C. -H. J. Org. Chem. 2006, 71,
655. See also: (b) Wang, C.-C.; Lin, P.-S.; Cheng, C.-H. J. Am. Chem.
Soc. 2002, 124, 9696. (c) Wang, C.-C.; Lin, P.-S.; Cheng, C.-H. Tetrahedron
Lett. 2004, 45, 6203.
(8) The reactivity of aryl bromides and aryl iodides with electron-donating
substituents was not found to be remarkable with this catalyst system under
reaction conditions similar to those reported earlier in our labortories. See
ref 6b.
Org. Lett., Vol. 8, No. 24, 2006
5615