Structure of lithium enolate

Fig. 1.1. Crystal structure of lithium enolate of methyl -butyl ketone in a structure containing four Li+, two enolates, and one HMDA anions, one bromide ion, and two TMEDA ligands. Reproduced from Angew. Chem. Int. Ed. Engl., 35, 1322 (1996), by permission of Wiley-VCH.

Fig. 13.2. X-ray single crystal structures of lithium enolates. TMEDA, tetramethylethylenedi-amine.

L. M. Jackman, J. Bortiatynski, Structures of Lithium Enolates and Phenolates in Solution, in Advances in Carbanion Chemistry (V Snieckus, Ed.), Vol. 1, 45, Jai Press Inc, Greenwich, 1992. 

Seebach, Dunitz and coworkers reported, in 1981226, the first crystal structures of lithium enolates of simple ketones, obtained in THF from pinacolone (3,3-dimethyl-2-butanone) and cyclopentanone. Both were arranged as tetrasolvated cubic tetramers, one THF molecule capping each lithium cation (Scheme 58A). Note that pinacolone enolate can also be crystallized, from heptane at — 20 °C, as a prismatic unsolvated hexamer exhibiting an approximate S6 symmetry and six slight it-cation interactions227,228 (Scheme 58B) or as a dimer in the presence of 2 molecules of TriMEDA29. Similarly,... 

See Section IV for structures of lithium enolates without lithium-carbon contacts. 

Scheme 3.3 Preparation methods and structure of lithium enolate-lithium halide aggregates 5.

Reaction of lithium enolate 2 with prochiral 3-buten-2-one (4) proceeds with minimal selectivity to produce nearly equal amounts of the two diastereomers of structure 540,41. 

The reaction of the enamines of cyclohexanones with a,ft-unsaluraled sulfones gives mixtures resulting from attack of the enamine at the a- and /(-carbons of the oc,/ -unsaturated sulfone. The ratio of x- and /1-adducts is dependent upon the reaction solvent, the geometry and structure of the sulfone1 4. The diastereoselectivity of these reactions is also poor. The reaction of lithium enolates of cyclic ketones with ( )-[2-(methylsulfonyl)ethenyl]benzene, however, gives bicyclic alcohols, as single diastereomers, that result from initial -attack on the oc,/ -unsaturated sulfone5. 

Experimentally, x values for gaseous lithium halides were determined as early as 1949 by molecular beam resonance experiments In solution, the quadrupolar interaction of ethyUithium and of t-butyllithium were investigated in 1964 . It was found that tetrameric and hexameric aggregates have different interactions. In the solid state x of tetrameric methyl- and ethyUithium was determined in 1965 and 1966 , and for lithium formate in 1972 . However, it was not untU Jackman started his investigations of lithium enolates and phenoxides in solution that the quadrupolar interaction was used in a systematic fashion to obtain structural information . 

Jackman and Szeverenyi were the first to systematically correlate the quadrupoiar interaction with molecular structure, i.e. with the aggregation of lithium enolates ". They noted that a tetrameric aggregate had a smaller QSC than a dimeric aggregate, ca 135 kHz compared to an estimate of 230 kHz for the dimer. The QSC of the tetramer was shown to be of the same magnitude in three different ethereal solvents. 

Interactions between sodium atoms and C = CH2 units (Na-C 270-2 287.3 pm) are apparent in the crystal structure of the enolate Li2Na4(0-C(=CH2)tBu)6.2 Pr2NH obtained from the reaction between lithium and sodium diisopropylamides and 3,3-dimethyl-2-butanone.119... 

D. Seebach, Structure and Reactivity of Lithium Enolates. From Pinacolone to Selective C-Alkylations of Peptides. Difficulties and Opportunities Afforded by Complex Structures, Angew. Chem. Int. Ed. Engl 1988, 27, 1624-1654. 

C with LDA results in the chemoselective formation of an aza-enolate D, as in the case of the analogous aldimine A of Figure 10.30. The C=C double bond of the aza-enolate D is fnms-configured. This selectivity is reminiscent of the. E-preference in the deprotonation of sterically unhindered aliphatic ketones to ketone enolates (Section 10.1.2, paragraph Stereocontrol in the Formation of Lithium Enolates ) and, in fact, the origin is the same both deprotonations occur via six-membered ring transition states with chair conformations. The transition state structure with the least steric interactions is preferred in both cases, which is the one that features the C atom in the /3 position of the C,H acid in the pseudoequatorial orientation. 

D. Seebach, Structure and reactivity of lithium enolates From pinacolone to selective C-alkyla-tions of peptides. Difficulties and opportunities afforded by complex structures, Angew. Chem., Int. Ed. Engl. 1988, 27,1624. 

Aspects of the synthesis, structure and reactivity of lithium enolates... 

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