Summary: 750 J. Am. Chem. SOC.1986,108, 750-156
Stereoselectivity of Alkylidenecarbene Addition to Olefins. 2.
Effect of Orbital Polarization in the Alkenest'
Dennis P. Fox,$Peter J. Stang,**Yitzhak Apeloig,*s and Miriam Karnia
Contribution from the Chemistry Department, The University of Utah,
Salt Lake City, Utah 84112, and the Department of Chemistry, Technion,
Israel Institute of Technology, Haifa 32000, Israel. Received September 16, 1985
Abstract: The reactions of unsymmetrical alkylidenecarbenes R(CH3)C=C:, where R = i-Pr (2) and t-Bu (3), with three
unsymmetrical olefins, 1-butene,styrene, and ethyl vinyl ether, were studied. In each case, the E olefin adduct predominated
over the Z adduct. Stereoselectivitywas found to decrease in the order ( E Z product ratios with 3at -20 "C)styrene (250:l)
>> ethyl vinyl ether (11.5:l) > tert-butylethylene (1O:l) > 1-butene (4.3:l) > isobutylene(2.0:l); with 2 stereoselectivitywith
all olefins was considerablylower. As previously predicted, the olefins with the more highly polarized R orbitals, styrene and
ethylvinyl ether, gave the most stereoselectivereactions. Polarizationof the olefinicR* orbitalswas also determined to influence
the stereoselectivityof the cycloadditions. High stereoselectivityis expected only for olefins where the R and x* orbitals are
strongly polarized in the same direction, as in styrene. The results also are discussed relative to earlier MNDO calculations.
Recently we reported a detailed experimental and theoretical
investigation of the stereoselectivityof alkylidenecarbene additions
to o1efins.l Unsymmetrical alkylidenecarbenes R(CH,)C=C:
(1-3) were found experimentally to add to two unsymmetrical
olefins, isobutylene and tert-butylethylene, to give a predominance