TRANSITION-STATE GEOMETRIES AND STEREOSELECTIVITY OF ALKYLIDENECARBENE ADDITION TO OLEFINS. AN EXPERIMENTAL AND THEORETICAL INVESTIGATION.

Article abstract of DOI:10.1021/ja00352a042

A careful investigation of the addition of unsymmetrical alkylidenecarbenes R(CH//3)C equals C:, where R equals Et, i-Pr, and t-Bu, to two unsymmetrical olefins, isobutylene and tert-butylethylene, was carried out. In all cases, the E adduct predominated over the Z adduct, with increasing stereoselectivity being observed upon going from R equals Et to R equals t-Bu in the carbene. Greater stereoselectivity was also observed with tert-butylethylene as substrate compared with isobutylene. Detailed theoretical calculations were carried out on the reaction pathways and transition-state geometries of unsaturated carbene-olefin interactions. Model studies on the H//2C equals C: plus H//2C equals CH//2 system were done both by MNDO and by standard ab initio methods at the STO-3G level, whereas the more substituted systems were evaluated by the MNDO method. These calculations indicate that attack of R(CH//3)C equals C: on (CH//3)//2C equals CH//2 leads to favored 'C//2-inward' transition states, which yield the observed E adducts.