DOI: 10.1002/chem.201500058
Communication
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CÀO Coupling
Cobalt-Catalyzed Intermolecular C(sp2)ÀO Cross-Coupling
Debasish Kundu,[a] Manisha Tripathy,[b] Pintu Maity,[a] and Brindaban C. Ranu*[a]
tions, their application in carbon–heteroatom bond formation
is very limited. The groups of Cheng[7a] and Tsai[7b] have report-
ed cobalt-catalyzed CÀS cross-coupling reaction in the pres-
ence of ligands to produce diaryl sulfides, whereas Chua and
co-workers[7c] demonstrated N-arylations of N-heterocycles and
Chen and co-workers[7f] reported N-arylation of benzamide by
using cobalt in the presence of chelating diamine ligands. Ya-
maguchi and co-workers also reported a few cobalt-catalyzed
cross-couplings of N-heterocycles and activated chloroare-
nes.[7d,e] However, to our knowledge, there is no report of
using Co in unactivated C(sp2)ÀO coupling. Thus, as a part of
our continuing program on inexpensive and environmentally
friendly metal-catalyzed reactions avoiding costly ligands and
additives,[5q,s,z,10a] we report herein a hitherto-unreported appli-
cation of cobalt catalysis in C(sp2)ÀO cross-coupling of vinyl,
styrenyl, and aryl halides with aryl and heteroaryl alcohols
(Scheme 1) in the presence of a catalytic amount of copper
without any ligand or additive.
Abstract: Cobalt(II)-catalyzed C(sp2)ÀO cross-coupling be-
tween aryl/heteroaryl alcohols and vinyl/aryl halides in the
presence of CuI has been achieved under ligand-free con-
ditions. In this reaction, copper plays a significant role in
transmetalation rather than being directly involved in the
CÀO coupling. This unique Co/Cu-dual catalyst system
provides an easy access to a library of aryl–vinyl, heteroar-
yl–styryl, aryl–aryl, and heteroaryl–heteroaryl ethers in the
absence of any ligand or additive.
Transition metal-catalyzed cross-coupling is a powerful tool for
carbon–carbon and carbon–heteroatom bond formation, par-
ticularly at unactivated C(sp2) centers where substitution reac-
tions are not usually successful.[1] CÀN, CÀO, and CÀS bond for-
mations are of considerable interest in organic synthesis be-
cause of their presence in a variety of biologically active mole-
cules, natural products, and pharmaceutical agents.[2] We are
especially interested in the more challenging C(sp2)–heteroa-
tom bond formation. Although C(sp2)ÀN and C(sp2)ÀS cross-
coupling reactions, catalyzed by a variety of metals[3] including
Pd,[3] Ni,[4] Cu,[5] Fe[6] and Co,[7] are well studied, the scope for
the corresponding C(sp2)ÀO couplings is limited, probably due
to the less active nature of O nucleophiles. The C(sp2)ÀO cross
couplings, especially those with vinyl halides, have been re-
ported under catalysis by Pd,[8] Cu in the presence of N-donor
ligands,[9] and Ni derivatives.[10] Recently Olofsson and co-work-
ers reported a striking protocol for the transition metal-free
synthesis of diaryl ethers through the reaction of diaryl iodoni-
um salts and phenols under ambient atmosphere.[11] However,
this reaction is limited to aryl C(sp2)ÀO bond formation.
To standardize the reaction conditions, a series of experi-
ments were performed with variation of reaction parameters
such as catalyst, solvent, base, temperature, and reaction time
for a representative coupling of 3-hydroxypyridine 1a and
(bromomethylene)cyclohexane 2a (Table 1). Our initial screen-
ings with CoCl2·6H2O, Co(OAc)2·4H2O and Co(acac)2 alone or in
presence of Zn as a reducing agent failed to initiate the reac-
tion (Table 1, entries 1–3). The use of several N-based ligands
(L1, L2, L3, L4, and L5) together with the cobalt catalyst also
gave discouraging results for this reaction (Table 1, entries 4–
8). Interestingly, the addition of 10 mol% CuI as a cocatalyst
with CoCl2·6H2O in the presence of Zn provided an encourag-
ing result (Table 1, entry 9). However, the best result was ob-
tained by using Co(acac)2 and CuI (10 mol% each) as catalysts,
Cs2CO3 as base, and N-methyl-2-pyrrolidone (NMP) as solvent
in the absence of any ligand or reducing agent (Table 1,
entry 10). The use of CuCl in place of CuI led to a lower yield
of the product (Table 1, entry 11). The reaction did not proceed
using a weaker base, K2CO3, whereas the use of K3PO4 provided
a moderate yield (Table 1, entry 12). The reaction was also un-
successful in DMSO and toluene (Table 1, entries 13 and 14).
However, DMF as solvent provided a 41% yield (Table 1,
entry 15).
The low cost, easy availability, and environmentally benign
nature of cobalt salts have made them an attractive target as
catalysts for performing cross-coupling reactions.[12,13] Very re-
cently, Co catalysts were found to be efficient for aryl CÀH
bond functionalization.[14] Although cobalt complexes have
been used as efficient catalysts for CÀC cross-coupling reac-
[a] D. Kundu, P. Maity, Prof. Dr. B. C. Ranu
Department of Organic Chemistry
Indian Association for the Cultivation of Science
Jadavpur, Kolkata-700032 (India)
Low catalyst loading (5 mol% of both Co and Cu catalyst)
led to a considerable decrease in yield (Table 1, entry 16). The
reaction did not initiate at all at room temperature (Table 1,
entry 17). The use of CuI only as catalyst, in the absence of
Co(acac)2, also failed to provide any product (Table 1, entry 18).
Thus, in a typical experimental procedure, a mixture of vinyl
halide and phenol (1:1), Cs2CO3 (2 equiv) in NMP was heated in
1008C under argon atmosphere in the presence of 10 mol%
[b] M. Tripathy
Department of Chemistry, IIT-Guwahati
Guwahati-781039 (India)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500058.
Chem. Eur. J. 2015, 21, 1 – 7
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