The reaction of thermally generated (ethylene)hafnocene with methylenetriphenylphosphorane: Competing reaction paths leading to Cp2HfC2H5(CH=PPh3) and a hafnacyclobutane

Article abstract of DOI:10.1021/om00130a024

Two competing reactions were observed upon treatment of bis(η-cyclopentadienyl)hafnacyclopentane (3) with methylenetriphenylphosphorane (4) at 90 °C in toluene solution. The major reaction path (60%) is characterized by loss of 1 equiv of ethylene and formation of the ethylhafnocene-substituted ylide 5. 5 was characterized by an X-ray crystal structure analysis (orthorhombic space group Pbca with Z = 8, a = 10.785 (1), b = 16.431 (2), c = 29.245 (3) A?). The hafnium-C-(ylide) bond is rather short (2.149 (6) A?). In a competing reaction sequence (40%) triphenylphosphine is set free in addition to ethylene. Methylene transfer from phosphorus to hafnium leads to the formation of the bis(η-cyclopentadienyl)hafnacyclobutane (2). Compound 2 was characterized spectroscopically and by chemical reactions. (η²-Ethylene)(methylenetriphenylphosphorane)hafnocene (7) was proposed as a common intermediate to account for the observed competition between hydrogen migration and methylene transfer in the reaction of 3 with 4.