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

Abstract

Reaction of CNBu^t with trans-[PdX(R′)(CNBu^t)₂](R′= Me or PhCH₂) in the presence of a nucleophile L (CNBu^t or PPh₃) gives the mono-insertion product, [PdX(CR′:NBu^t)(CNBu^t)L], and in the absence of L gives a dimer [{PdX(CR′:NBu^t)(CNBu^t)}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 = Bu^t 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)(CNBu^t)₂] gives [{MBr[C(trans-CH:CH·CO₂ Me):NBu^t](CNBu^t}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 CNBu^t occurs with [pdX (R)(Ph₂PCH₂CH₂PPh₂)]. In contrast to CNC₆H₄Me-P, CNBu^t 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.