Structure and Aggregation State of Enolate Anions

Any discussion of enolate geometry must include the structure of the enolate. It is well known that metal enolates exist as dimers a or other aggregates in ether solvents S (see Section 9.2.C. for a discussion of aggregate formation with LDA).28d Jackman and Szeverenyi suggested that the lithium enolate of isobutyro-phenone exists as a tetramer (31) in THF solution but exists as a dimer (32) in DME. o Such aggregates were proposed by House et ah,3 who found that ketone enolates of groups 1 (lA), 2 (llA), and 3 (lllA) metals 

Lithium enolates of ketones exist as aggregates in solution.29-3l,34d,35 Mixed aggregates between the enolate anion and the amide base are also possible. In 1981, Seebach and co-workers confirmed by X-ray crystallography that the lithium enolates of pinacolone and cyclopentanone form a tetrameric aggregate in the solid state, and it was assumed that a similar species exited in solution. A THF solvated tetramer of lithium pinacolonate is shown (see 33), as it was reported by Seebach. Williard et al. reported the X-ray structure of 

Provided through the courtesy of Professor Dieter Seebach, E.T.H., Zurich, Switzerland. 

The solvent plays a significant role in the aggregation state of enolate anions, and this directly influences their reactivity. Streitwieser et al. found that selected lithium enolate anions have lower aggregation states in 

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D. Enolate Anions Structure and Aggregation State of Enolate Anions... 

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