Benzylidene insertion reactions in organoplatinum chemistry: mechanism and selectivity

The reaction of the cycloneophylplatinum complex [Pt(CH₂CMe₂C₆H₄)(NN)], 1, NN = 4,4'-di-t-butyl-2,2'-bipyridine (bubipy) or 2, NN = 3,4,7,8-tetramethyl-1,10-phenanthroline (phen*), with PhCHBr₂ gave the corresponding complex [PtBr₂(CH₂CMe₂C₆H₄CHPh)(NN)], 3 or 4, which are formed by formal oxidative insertion of benzylidene, PhCH, into the arylplatinum bond of 1 or 2 respectively. By monitoring the reactions by ¹H NMR spectroscopy, intermediates were detected and shown to be products of oxidative addition [PtBr(CHBrPh)(CH₂CMe₂C₆H₄CHPh)(NN)], 5 and 6, along with a side product [PtBr₂(CH₂CMe₂C₆H₄)(NN)], formed by formal bromine addition to 1 or 2. Oxidative addition also occurs with complex [PtMe₂(bubipy)] to give a single isomer of [PtBrMe₂(CHBrPh)(bubipy)]. The formation of 3 and 4 gives the first examples of selective benzylidene insertion reactions into aryl-platinum bonds, and mechanistic studies show that the reactions are retarded in the presence of LiBr. DFT calculations predict that the insertion reaction occurs synchronously or immediately following cleavage of the C-Br bond of intermediate 5 or 6, perhaps involving a shortlived benzylidene complex of platinum(iv). The potential for applications in catalysis of related reactions involving benzylidene insertion step is discussed.