Organometallic compounds of group III. XXXIV. Steric factors in the carbalumination of olefins. The question of the anomalous, reduced reactivity of olefins versus acetylenes.

Article abstract of DOI:10.1016/S0022-328X(00)87127-2

In order to understand the reasons for the anomalous, reduced reactivity of olefins toward electrophilic carbalumination, compared with that of acetylenes, the reactivity, stereochemistry and regiochemistry of a series of acyclic and cyclic olefins in carbalumination with triphenylaluminum were investigated. The following substrates underwent reaction between 80 and 225°C with decreasing ease in the order: norbornadiene > cis-β-methylstyrene > trans-β-methylstyrene ～ 1,2-dihydronaphthalene ～ 1,1-dimethylindene > cis-1,2-diphenylethylene > 3,3,3-triphenylpropene > trans-1,2-diaryethylenes ～ phenylcyclopropanes. The stereochemistry of the mono- and bis-carbaluminations of norbornadiene was shown to be syn, exo. The regiochemistry observed with unsymmetrical olefins could readily be rationalized by a steric effect operative in the preferential collapse of an olefin-(C₆H₅)₃Al π-complex. Besides carbalumination, several other reactions were observed with these olefins: (1) cis, trans-isomerization of acyclic olefins; (2) metallation of vinylic carbon atoms by (C₆H₅)₃Al; (3) elimination of (C₆H₅)₂AlH from carbalumination adducts; (4) by inference from the foregoing reaction with certain systems, epimerization at sp³-hybridized carbon-aluminum bonds; and (5) decarbalumination with carbon-carbon bond scission. These side reactions were considered together with relative reactivities, stereochemistry and regiochemistry in developing energy profiles for the carbalumination of olefins and acetylenes. The reduced reactivity of olefins is thought to arise from steric factors that destabilize a π-complex-with (C₆H₅)₃Al and that cause a trapezoidal-like transition state to be of higher energy and rate-determining. The higher reactivity of acetylenes is ascribed to less steric hindrance both to π-complexation with (C₆H₅)₃Al, and to the collapse of the complex via a trapezoidal configuration. For acetylenes, it is judged that the formation of an intimate π-complex is rate-determining.