Enols evidence for

Two factors could contribute to the observed regiochemistry of base-promoted halogenation of ketones. One is a kinetic preference for formation of the less substituted enolate. Evidence for this effect can be seen in the rate constants for base-catalyzed iodination of phenyl alkyl ketones with the structure C6H5COCHR1R2, which are found to decrease with increasing alkyl substitution (Table 7.8). ... 

Wasielewski, M.R., Norris, J.R., Shipman, L.L., Lin, C.P., and Svec, W.A., Monomeric chlorophyll a enol — evidence for its possible role as the primary electron donor in photosystem I of plant photosynthesis, Proc. Natl Acad. Sci USA, 78, 2957,1981. 

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