Mo-theoretical characterization of organic reaction mechanisms VII. (2 + 2) Reactions of organometallic compounds

Abstract

The concept of the triplet instability for the conventional Hartree—Fock molecular orbitals has been applied to thermal (2 + 2) reactions of organo- metallic compounds. It is found that the olefin insertions into metal—hydro- gen bonds (metal = Al, B, and Ti) proceed without radical character, while the reaction of the Ni analog must bear some biradical character. These reactions follow mechanisms distinct from those for familiar symmetry- forbidden (2 + 2) reactions. The difference in mechanism is ascribable to the advantageous participation of a vacant p orbital (for Al and B) or d symmetry orbitals (for Ti and Ni) in reactions. The catalytic cleavage of ethylene oxide and the addition of a Grignard reagent to nitrile also fall under the category of symmetry-forbidden nonradical processes. The hydrogenation of binuclear aluminum hydride, however, should possess a significant biradical character.