Convenient Entry to Mono- and Dinuclear Palladium(II) Benzothiazolin-2-ylidene Complexes and Their Activities toward Heck Coupling

Under solventless conditions, benzothiazole reacts with benzyl bromide to give near-quantitative yield
of the salt N-benzylbenzothiazolium bromide (A), which is a convenient air-stable heterocyclic carbene
precursor. Treatment of A with Pd(OAc)2 in CH3CN affords the bis(carbene) complex cis-[PdBr2(NHC)2]
(1) (NHC = N-benzylbenzothiazolin-2-ylidene). In DMSO, this reaction yields an unprecedented dinuclear
N,S-heterocyclic carbene complex, [PdBr2(NHC)]2 (2). Complex 2 undergoes bridge cleavage reactions
with CH3CN and DMF to give the mononuclear and solvated monocarbene complexes trans-[PdBr2(NHC)(Solv)] [Solv = CH3CN (3) and DMF (4)]. All compounds have been fully characterized by 1H
and 13C NMR spectroscopy, ESI or FAB mass spectrometry, and elemental analysis. The molecular
structures of A and 1−4 have been determined by X-ray single-crystal diffraction. The catalytic activities
of 1−4 toward Mizoroki−Heck coupling reactions of aryl bromides with tert-butyl acrylate are described
and compared.