An
unsymmetrical pincer N-heterocyclic carbene–nitrogen–phosphine
(CNP) and its palladium complexes PdCl2(κ2-CP) (4) and [PdCl(κ3-CNP)]PF6 (5·PF
6
) were synthesized.
NMR spectra disclosed that the transformation of complex 4 structure occurred in the solution. Further NMR experimental and
single crystal structure analysis of complex 4 provided
unequivocal and structural evidence for the formation of complex [PdCl(κ3-CNP)]Cl (5·Cl) in the solution of complex 4. The catalytic performance of palladium complexes 4 and 5·PF
6
was
investigated with the direct Csp2–H arylation of
benzoxazoles with aryl bromides. Notably, aryl bromides could give
up to 97% arylation products in the presence of 0.5% complex 4. For ortho-substituted substrates, the
steric hindrance had a significant impact, but product yields could
be improved remarkably by extending reaction time. This result implied
the catalytic active species in this system was stable and kept in
a long lifetime. The flexible backbone and unsymmetrical pincer structure
with a flank N-heterocyclic carbine should be a feasible
strategy to realize an efficient catalytic transformation. This simple
catalyst system first realized the direct arylation of aryl bromides
with a catalyst loading as low as 0.25% without excessive base and
copper as assistant catalyst.
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