Metallocyclobutane dismutation

A last question that arises now is the mode of generation of the metallocarbenes. Besides the indirect formation by C-C bond rupture, in metallocyclobutane dismutation, for instance (Scheme 59a), two other possible ways are a-hydrogen elimination (Scheme 59b) and hydrogen shift in ii-adsorbed olefin (Scheme 59c) (see also Scheme 48). 

Scheme 59. Different modes of generation of metallocarbenes. (a) Metallocyclobutane dismutation (b) a-hydrogen elimination (c) a- -hydrogen shift.

These deuterium distributions were interpreted by assuming that in 11,12, and 13, rapid interconversions between a,y-diadsorbed and a,a,y-triadsorbed species, attached to the metal at the two gemdimethyl carbon atoms, were followed by metallocyclobutane dismutation. The resulting adsorbed methylene-cycloalkane may be deuterated via either n-alkyl or tertiary alkyl species. In the former case, rapid desorption would occur, with the formation of a d4 molecule, while in the latter case, a number of interconversions between cr-alkyl and 7r-olefinic cyclic adsorbed species would yield the perdeutero compound. In the case of demethylation at a tertiary center. 

Aluminium and other metal alkyls also activate tungsten and molybdenum compounds (particularly oxychlorides), to generate homogeneous or supported olefin dismutation catalysts. It is now believed that an initial M-alkyl (M = W, Mo) group is converted to a metal alkylidene group by a-hydrogen abstraction. Coordinated olefin now gives a metallocyclobutane (isolable in some cases) ... 

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