Amides reaction with benzophenone dianion

When 1-phenyl-2,4-pentanedione is doubly deprotonated with potassium amide in ammonia, the ensuing aldol reaction with benzophenone gives the expected aldol (19). However, if sodium amide is employed to generate the dianion, the reaction product with benzophenone is the dihydro-7-pyrone (20 equation 36). This result suggests the intermediacy in the latter reaction of a dianion involving the benzyiic position. Further study of this interesting reaction is warranted. 

The feasibility of some of these radical pathways has been examined using Marcus theory to obtain rate constants for comparison with the experimental data (Eberson, 1984). For some relevant anions, including hydroxide, methoxide, t-butoxide, the anion of benzaldehyde hydrate and di-2-propyl-amide, the necessary E°(RO-/RO) values are available or can be estimated with sufficient accuracy. For the reaction of t-butoxide with benzophenone in THF, or the benzaldehyde hydrate anion with benzaldehyde in aqueous dioxan, direct electron transfers between the anion and the neutral are not feasible the calculated rate constants are orders of magnitude too low to be compatible with the observed reduction rates. Any radicals observed in these reactions must arise by some other more complex mechanism. The behaviour of an aromatic aldehyde hydrate dianion has not been examined in this way, but MNDO calculation (Rzepa and Miller, 1985) suggests that such a species could easily transfer either a single electron or a hydrogen atom to an accepting aldehyde. 

Carboxylic acid derivatives such as esters and amides undergo nucleophilic acyl substitution reactions with the ketone dianion derived fiom benzophenone, providing modest yields of the corresponding carbonyl products (equations 102 and 103). Benzhydrol is a significant by-product in these reactions. 

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