Mechanism of Formation of (.eta.3-oxocyclobutenyl)cobalt Compounds from [Co(CO)4]- and Cyclopropenium Cations
American Chemical Society
Journal of the American Chemical Society
2-Cyclopropene-1 -carbonyl chlorides 6 react with [Co(CO)4]“ in MeCN, THF, or C6H6 solution to afford (η3-oxocyclobutenyl)tricarbonylcobalt complexes 7. No η3-cyclopropenyl compounds of cobalt are produced. The reaction is shown to proceed by initial formation of a coordinatively saturated (2-cyclopropen-1-ylcarbonyl)tetracarbonylcobalt species, which then undergoes decarbonylation to afford a coordinatively unsaturated (2-cyclopropen-1 -ylcarbonyl)tricarbonylcobalt intermediate. 2H and 13C labeling studies confirm that this intermediate is the crucial precursor for ring expansion to the oxocyclobutenyl ligand. In THF or MeCN solution this intermediate is in dynamic equilibrium with a cyclopropenium cation and [Co(CO)4]-; in less polar C6H6 this equilibrium is insignificant. Evidence is presented that reactions of cyclopropenium cations with [Co(CO)4]- involve direct electrophilic attack at a CO ligand rather than at cobalt; no evidence for the presence of η3-cyclopropenyl cobalt intermediates has been obtained. In C6H6, chiral acyl chlorides 6 afford chiral oxocyclobutenyl compounds 7; in THF or MeCN only racemic products are obtained due to the dissociative equilibrium mentioned above. The effects of ring substituents on the selectivity of C-C cleavage in the ring expansion step resemble those obtained in photochemical rather than thermal ring openings of cyclopropenes. A ring expansion mechanism which involves a metal-stabilized vinylcarbene transition state is proposed; this transition state collapses to a nonplanar vinylketene species which undergoes ring closure to the oxocyclobutenyl ligand. 1H and 13C NMR data for a large number of (oxocyclobutenyl)cobalt compounds are presented.