ORGANO-TRANSITION METAL CHEMISTRY WITHOUT PHOSPHINES: THE PALLADIUM CATALYSED CYCLOTRIMERIZATION OF ACETYLENES

Abstract

The argument is presented that organo-transition metal complexes also containing phosphines and similar ligands are often too kinetically inert to act as good models for catalytic processes. A new generation of ‘lightly stabilized’ complexes are needed to probe these reactions more deeply. Some novel palladium complexes of this type, derived from the reactions of acetylenes with either Pd0 or Pd11, are described and the mechanisms of acetylene oligomerization are discussed. For Pd0 electronic factors seem to be of greatest importance while steric factors play a greater role with Pd11. The structure and properties of a new complex, (Ph3P)2Pd(Br)[(CCOOMe)4H], derived from dimethyl acetyl-enedicarboxylate, are reported as well as that of [C5Me5CHPhCH2PdCl]2, derived from but-2-yne, PdCl2, and diphenylmercury. The latter is a dihapto-cyclopentadiene complex which shows stereochemical non-rigidity in solution. Since this complex also possesses a Pd-alkyl bond it is an ideal ‘lightly-stabilized’ model and its reactions with acid, base, hydrogen, CO, and triphenylphosphine have been investigsted. This work sheds important new light on the chemistry of the Pd—C σ-bond.