Cyclometallation of Dialkylbis(triethylphosphine)platinum(II) Complexes: Formation of Pt,Pt-Bis(triethylphosphine)platinacycloalkanes

Article abstract of DOI:10.1021/ja00365a024

The thermal decompositions of three analogues of bis(triethylphosphine)dineopentylplatinum(II) (L2Pt2 (1)) - L2Pt2 (3), L2Pt2 (5), and L2Pt2 (7) - have been examined.Compounds 3 and 7 decompose more rapidly than 1 by a factor of ca. 10⁴ to give as products Pt,Pt-bis(triethylphosphine)-3,3-dimethylplatinacyclohexane (4) and Pt,Pt-bis(triethylphosphine)-3,3,5,5-tetramethylplatinacyclohexane (8), respectively, and 1 equiv of the corresponding alkane.Compound 5 decomposes at a rate ca. 50 times faster than 1 to yield Pt,Pt-bis(triethylphosphine)-2,4,4-trimethylplatinacyclopentane (6a), -3-methyl-3-n-propylplatinacyclobutane (6b), and -3,3-dimethylplatinacyclohexane (6c).The conversion of 3 to 4 and 5 to 6a proceeds by dissociation of triethylphosphine, intramolecular oxidative addition of a δ carbon-hydrogen bond of the alkyl groups to platinum, and reductive elimination of alkane.The decomposition of L2Pt2 (11) proceeds by β-hydride elimination rather than cleavage of a carbon-hydrogen bond and formation of platinacycloalkane.The difference in the free energies of activation for reactions which form four- and five-membered platinacycloalkanes is small (ΔΔG(excit.) ca./= 4 kcal mol^-1); that for reactions wich form four- and six-membered rings is smaller (ΔΔG(excit.) ca./= 0 kcal mol^-1).We identify these values of ΔΔG(excit.) with estimates of the strain energies of these rings, assuming the strain energy of the platinacyclohexane is small.The important conclusion from these studies is that the strain energy of the platinacyclobutane studied here is small (< 5 kcal mol^-1).