2
S. SHARMA AND B. R. SARKAR
aspects like facile ligand synthesis and cost pertaining to that, and the stability of the metal
complex, sensitivity to air/oxygen/aqueous environment, contribute greatly to overall
economic viability of a metal complex catalyst for large-scale applications.[2] Developmen-
tal approaches have been reported such as design of novel ligands/Pd-complexes having
myriad of donor centers (C, N, O, P, S, etc.), ligand denticity (mono-, bi-, polydentate,
etc.), use of different media to assist catalyst–product separation, etc. with varied shares
of advantages and drawbacks in their syntheses and applications. Majority of the high-
efficiency catalysts with ingenious phosphine-derived ligands reported are associated with
multistep ligand-synthesis processes (including stoichiometric steps), and hence expensive
mostly, have enhanced sensitivity to moisture and air, thus diminishing the chance of
industrial viability.[3] Among the other classes of ligands, phosphine-free ligands, typically
[
4]
functionalized N-heterocyclic carbenes/N-donor ligands were reported.
Another
important class of compounds named as Schiff’s bases have also been used extensively
for catalyzing different kinds of reactions. In a very recent report, complexes of different
Schiff’s base type N–O ligands were studied for Heck coupling reactions with promising
results. In the same context, Pd-complexes of hemilabile N–O ligands might also play
important roles in Mizoroki–Heck catalysis. In another report, new N–O ligand was
reported for Pd-catalyzed Mizoroki–Heck coupling in water presenting good promise
but with low TOF (<10 h ). As exemplified by Guo et al., Pd-complexes of these
ligands were found to have moderate to high activity under phosphine-free conditions.
Phosphine-free bispicolinate complex of palladium has also been reported by Iyer et al.
[
5]
[
6]
−
1
[7]
[8]
[
9]
for the Heck reactions, but with moderate performance only. However, Pd-complexes
of simple hemilabile N–O ligands such as picolinate along with another phosphine ligand
have been found to catalyze hydroxycarbonylation reactions and Suzuki cross-coupling
[
10]
reactions with high efficiency and stability.
We herein report the use of phosphine-
containing Pd–picolinate complex as efficient catalyst for Heck coupling reactions having
high to excellent performance.
Phosphine-containing palladium picolinate complex [Pd–Pyca complex] was obtained
[
10(a)]
by synthesis process as described elsewhere.
The complex was obtained as pale yellow
powder and was used for the Mizoroki–Heck coupling reactions after proper purification
and characterization which matched with the data in literature.
[
(10a)]
The reaction of
iodobenzene with styrene was considered as model reaction for optimization of catalyst
performance as shown in Scheme 1.
Table 1 contains the details of optimization of reaction parameters with respect to the
solvent used, base, temperature. It was observed that the catalyst showed good performance
with a relative ratio of 1:1.2 at 100 °C.
The conversion, selectivity (and hence GC yields) were calculated on the basis of
quantification by a calibrated gas chromatographic technique. The selectivity in all the
Scheme 1. Mizoroki-Heck coupling of iodobenzene with styrene using palladium(II) picolinate complex.