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 H-1 and C-13 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.