LITHIUM DIALKYLAMIDE MIXED AGGREGATION - MNDO COMPUTATIONAL STUDY OF SALT AND SOLVENT DEPENDENCIES

Mixed aggregation of lithium diaikylamides with various LiX salts is studied by semiempirical (MNDO) computational methods. The relative stabilities of mixed aggregates of lithium diisopropylamide and lithium 2,2,6,6-tetramethylpiperidide with LiCl as well as the enolates derived from acetone, cyclohexanone, and 2,4-dimethylpentanone are described. The unsolvated forms are compared with the analogous structures solvated by tetrahydrofuran and hexamethylphosphoramide. The structural types studied include mixed cyclic dimers (R(2)NLi-LiX), trimers [(R(2)NLi)2(LiX)2], and tetramers [(R(2)NLi)(2)(LiX)(2)]; 3-rung ladders [(R(2)NLi)(2)(LiX)] and 4-rung ladders [(R(2)NLi)(2)(LiX)(2)]; two isomeric open dimer topologies corresponding to cyclic dimers fragmented at N-Li and X-Li bonds; triple ions (R(2)NLiX(-)//Li-+). A complex interplay of steric effects appears to be the dominant influence on the mixed aggregate structures and relative stabilities.