Mechanism of Reaction of Carbon Monoxide with Phenyllithium

The product mixtures obtained by reaction between phenyllithium and carbon monoxide in diethyl ether, followed by hydrolysis, include benzophenone (1), α,α-diphenylacetophenone (2), benzil (3), α,α-diphenylα-hydroxyacetophenone (4), benzpinacol (5), α-hydroxyacetophenone (6), 1,3,3-triphenylpropane-1,2-dione (7), 1,3,3-triphenylpropan-1-one-2,3-diol (8), and benzhydrol (9). Compounds 1, 2, 6, 7, and 8 are produced in significant yields; 3, 4,5, and 9 are produced in trace quantities. Spectroscopic studies establish dilithium benzophenone dianion (18) as the first long-lived intermediate formed in this reaction; qualitative correlations between the basicity of a number of organolithium reagents and their reactivity toward carbon monoxide suggests, but does not prove, that benzoyllithium is a precursor of 18. Labeling experiments indicate that the products ultimately isolated following hydrolysis of the reaction mixture are derived from at least two pathways which compete for the initially formed 18. One involves combination of 18 with 1 equiv of phenyllithium and 1 equiv of carbon monoxide, followed by elimination of 1 equiv of lithium oxide, yielding 17, the lithium enolate of 2; a second involves combination of 18 with 1 equiv of phenyllithium and 2 equiv of carbon monoxide, yielding 22, the trilithium trianion of 8. Hydrolysis of 17 yields 2 directly. Hydrolysis of 22 yields 8; reverse aldol reactions involving 8 or its precursors generate 1 and 6. The mechanism proposed to account for the major products of the reaction of phenyllithium and carbon monoxide is outlined in Scheme III. On the basis of this scheme, plausible paths to the minor products of the reaction are proposed.