Isocyanide insertion in alkyl– and vinyl–metal bonds of square-planar palladium ( II ) and platinum( II ): mechanisms and stereochemistry

Reaction of CNBut with trans-[PdX(R′)(CNBut)2](R′= Me or PhCH2) in the presence of a nucleophile L (CNBut or PPh3) gives the mono-insertion product, [PdX(CR′:NBut)(CNBut)L], and in the absence of L gives a dimer [{PdX(CR′:NBut)(CNBut)}2]. The insertion reaction in the presence of L occurs unimolecularly, being independent of the nature of L. This contrasts to the insertion reaction of CNR (R = But or p-MeC6H4) into trans-[MX(R′)(PPh3)2](M = Pd or Pt) which depends on the nature of CNR, suggesting an associative mechanism. The intramolecular insertion reaction of trans-[MBr(cis-CH:CH·CO2Me)(CNBut)2] gives [{MBr[C(trans-CH:CH·CO2 Me):NBut](CNBut}2], indicating isomerisation of the vinyl group, while insertion of CNC6H4Me-p into trans-[MBr(cis-CH:CH·CO2Me)(PPh3)2] yields trans-[MBr{C(cis-CH:CH·CO2Me):NC6H4Me}(PPh3)2] with retention of the vinyl geometry. Rapid multi-insertion of CNBut occurs with [pdX (R)(Ph2PCH2CH2PPh2)]. In contrast to CNC6H4Me-P, CNBut does not insert into trans-[PtX(cis-CH:CH·CO2H)(PPh3)2]. Various factors which influence the reaction rate and stereochemistry are described and interpretations accommodating all these features are discussed.