Concerted and diradical stepwise mechanisms in the Diels-Alder reactions of phosphinine and phosphinine sulfide: A DFT investigation

Computations of the concerted and diradical stepwise mechanisms of the Diels-Alder (DA) reactions on the >C=P- functionality of phosphinine and phosphinine sulfide with 1,3-butadiene at the density functional theory level B3LYP/6-311++G**//B3LYP/6-31G** give the values of energy of concert as 10.7 and 2.6 kcal mol(-1), respectively. Similarly, the DA reaction of dimethyl acetylenedicarboxylate (DMAD) with the -CH=CH-CH=P- moiety of phosphinine or its sulfide has been investigated theoretically at the same level of theory. The results reveal that in the DA reaction of phosphinine, the role of sulfur is to oxidize phosphorus to generate a phosphinine sulfide intermediate, which subsequently undergoes DA reaction with 2,3-dimethylbutadiene or DMAD by a concerted mechanism to afford the respective cycloadducts.