A quantum chemical study on the reaction of 1-aryl-1,2-dihydrophosphinine oxides with dimethyl acetylenedicarboxylate

A β-oxophosphorane/ylide (2a) and an oxaphosphete (3a), the product and the possible intermediate of an inverse Wittig type reaction of 1-(2,4,6-triisopropylphenyl-)1,2-dihydrophosphinine oxide with dimethyl acetylenedicarboxylate were studied by quantum chemical calculations. The reaction of the title reagents following either a traditional [4 + 2] cycloaddition protocol to afford phosphabicyclo[2.2.2]octadiene 5 or a novel route yielding eventually β-oxophosphorane/ylide 2 was evaluated by energy calculations. The mechanism for the formation of intermediate 3a2 was refined by HF/6-31G* transition state calculations. Analysis of the HOMO–LUMO orbitals of the reagents justified the reactivity experienced.