Mechanisms of rearrangements in platinacyclobutane chemistry

The mechanisms of formation, decomposition, and isomerisation of platinacyclobutane complexes [PtCl2(C3H6)L2] (L is typically pyridine) are discussed on the basis of density functional theory (DFT). The isomerisation and decomposition reactions occur through 5-coordinate intermediates [PtCl2(C3H6)L] that cannot be directly detected. These 5-coordinate complexes are predicted to have distorted square pyramidal structures, but a pinched trigonal bipyramidal (PTBP) stereochemistry is easily accessible. Both α- and β-elimination from these complexes to give hydride complexes [PtHCl2(C3H5)L] are predicted to have high activation energies. The isomerisation of [PtCl2(C3H6)L2] to the ylide complex [PtCl2{CH(L)CH2CH3)L] is instead predicted to occur after cleavage of a Pt–C bond to give an intermediate that can be considered as a corner platinated cyclopropane, leading to 1,3-hydrogen transfer without a hydridoplatinum intermediate. The reaction of [PtCl2{CH(L)CH2CH3}L] to give the alkene complex [PtCl2(CH2 = CHCH3)L] involves dissociation of the C–L bond followed by a 2,1-hydrogen transfer. The selectivity of related reactions from phenylcyclopropane follows naturally from these mechanisms.