Regioselective Cis Insertion of DMAD into Au–P Bonds: Effect of Auxiliary Ligands on the Reaction Mechanism

The
cis insertion of dimethyl acetylenedicarboxylate (DMAD, 2) into the Au–P bond of Au­(SAr)­(PAr′3) (1) occurs selectively at room temperature to give
adduct 3, whose X-ray crystallographic analysis exhibits
the predominant character of an alkenylphosphorus zwitterion complex.
The first-order kinetics with respect to 1 and 2 and the large negative value of ΔS⧧ indicate that the reaction proceeds through the
coordination of 2 to 1 followed by a concerted
inner-sphere nucleophilic attack of PPh3 on the coordinated
triple bond. In contrast, Au­(Cl)­(PPh3) (4)
does not react with 2 even at 60 °C. However, the
tricoordinated 16-electron complex Au­(Cl)­(PPh3)2 (5) quite successfully reacts with 2 to
produce insertion product 6, demonstrating that two PPh3 ligands are required for the reaction.