Thermal reactions of platinacyclobutanes: Olefin and pyridinium ylide formation

Article abstract of DOI:10.1021/ic50222a025

Complexes derived from the insertion of Pt(II) into alkyl-substituted cyclopropanes react under mild conditions to produce either olefin or ylide complexes of Pt(II). The course of the reaction is dependent upon the identity of the Lewis base present, since ylide complexes are formed only when pyridine is present, whereas olefin complexes are the sole products in the presence of acetonitrile. The nature of the ring substituents is also important, as platinacyclobutanes substituted on two carbon atoms give only olefin complexes in all solvents. The isolated, thermodynamically more stable, forms of the platinacyclobutanes, PtCCRR′CR″, are not reactive, and the initial step in each case is isomerization to the more sterically constrained PtCCR″CRR′ ring system. Depending upon conditions, this complex reacts to form either the terminal olefins C=CR″CRR′ or the pyridinium ylide complexes (C₅H₅N)PtCl₂C(H)(C₅H ₅N)(CHR″CHRR′). These results are explained by a mechanism involving β elimination of a ring proton to form an intermediate π-allylplatinum hydride. If steric interactions are severe in this intermediate, immediate reductive elimination to the olefin complex occurs, whereas sterically nondemanding allyls persist long enough to allow attack by pyridine, with subsequent ylide formation.