Solvation neutral enolates

Recent attention has focussed on the solvation of neutral enolates formed through the deprotonation of a-ammonio ° , a-pyridinio and a-imino carbonyl com-pounds . A trimethylammonio (MesN -) group has been shown to stabilize in water-adjacent enolates more dominantly than an unsubstituted ammonio group (HsN -). For example, a-trimethylammonio ester 35 ° is about 1000 times more acidic than the simple a-ammonio ester 36 ° . This is presumably due to the highly solvated ammonio (H3N+-) group having a lower charge effect in water than the less solvated and more... 

Even though the reaction conditions may lead to formation of the metal enolate in high yield, further reduction may occur during the quenching step of the reaction. Alcohols such as methanol and ethanol convert metal enolates to saturated ketones much faster than they react with metals in ammonia, and quenching of reduction mixtures with these alcohols will usually lead to partial or complete conversion to alcoholic product rather than to the saturated ketone. Rapid addition of excess solid ammonium chloride is the commonly employed quench procedure if ketonic products are desired,but other reagents that destroy solvated electrons before neutralization may be employed, such as sodium ben-zoate, iron(III) nitrate, - sodium nitrite, bromobenzene, sodium bromate, 1,2-dibromoethane and acetone. 

With two nitrile groups, the delocalized anion is so stable that even a weak, neutral amine (triethylamine) is sufficiently basic to deprotonate the starting material. Here double alkylation again takes place, in 100% yield note that the electrophile is good at 5 2, and the dipolar aprotic solvent DMSO (like DMF) cannot solvate the enolate anion, making it more reactive. 

Alkyl anions have been implicated as intermediates stabilized by a neutral molecule. Alkoxide ions when photolysed in a pulsed ICR spectrometer dissociate into alkanes and enolate anions The intermediate 19 in equation 25 can be represented by two possible extremes. In 19a the alkyl anion R is solvated by a ketone and inl9b the radical anion of the ketone is solvated by the radical R. The structure of this intermediate will then depend on the relative electron affinities of the alkyl group R and the ketone. Brauman and collaborators photolysed a series of 2-substituted-2-propoxides (18 with R = CH3, R" = H and R varied). For substituents R = CF3, H, Ph and H2C=CH, the C—R bond dissociation energies for homolytic fission are larger than the C—CH3 bond energy, i.e. if the intermediate complex has the structure 19b then methane would be expected to be produced. Conversely, since these R groups form more stable anions than CH3, decomposition via 19a should result in RH. The experimental observation that only RH is formed led to the conclusion that 19 is best described by the solvated alkyl anion structure 19a. 

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