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

Abstract

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.