Aldehyde lithium enolates aggregation

Similar information is available for other bases. Lithium phenoxide (LiOPh) is a tetramer in THF. Lithium 3,5-dimethylphenoxide is a tetramer in ether, but addition of HMPA leads to dissociation to a monomer. Enolate anions are nucleophiles in reactions with alkyl halides (reaction 10-68), with aldehydes and ketones (reactions 16-34, 16-36) and with acid derivatives (reaction 16-85). Enolate anions are also bases, reacting with water, alcohols and other protic solvents, and even the carbonyl precursor to the enolate anion. Enolate anions exist as aggregates, and the effect of solvent on aggregation and reactivity of lithium enolate anions has been studied. The influence of alkyl substitution on the energetics of enolate anions has been studied. ... 

Highly enantioselectivity assumed to originate from a mixed aggregate 171 of the trans-lithium enolate of t-butyl propionate 169 and the chiral lithium amide 170 was observed in aldol additions to various aldehydes, as exemplified in Scheme 5.55. Thus, the acylated aldols obtained with benzaldehyde formed in a diastereomeric ratio of 92 8 in favor of the anti-product, with an enantiomeric excess of 94% ee [83]. More recent studies on the structures of mixed aggregates between lithium enolates and chiral amide bases (see also Chapter 3) provided an insight in this type of enantioselective conversion. 

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