ORIGINS OF STEREOSELECTIVE CARBENE 1,2-SHIFTS AND CYCLOADDITIONS OF 1,2-DICHLOROETHYLIDENE - A THEORETICAL-MODEL BASED ON CBS-Q AND B3LYP CALCULATIONS

Bonneau et al. (J. Am. Chem. Sec. 1996, 118,3829-3827) have proposed t hat carbene-olefin pi-complexes can mediate the stereoselectivity of 1 ,2-dichloroethylidene (1) rearrangements. Computational studies at MP2 and CBS-Q theoretical levels confirm the experimentally observed pref erence for rearrangement of 1 to give (Z)-1,2-dichloroethylene. Studie s of this and related rearrangements have led to a model to explain th e origin of this stereoselectivity, Density functional calculations us ing Becke3LYP/6-31G theory suggest that carbene-olefin complexes do n ot exist as intermediates in the cycloadditions of 1 to ethylene or to tetramethylethylene. The experimental results can be explained by an alternative model in which 18.7% of the diazirine precursor produces a lkene product without intervention of the carbene, Becke3LYP/6-31G ca lculations on the 1,2-shift of hydrogen concerted with loss of nitroge n and on rearrangement of the diazirine to a diazo intermediate show t hat these processes are not likely to be the alternative pathways.