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

Mixed aggregation of lithium diaikylamides with various LiX salts is s tudied by semiempirical (MNDO) computational methods. The relative sta bilities 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 solva ted by tetrahydrofuran and hexamethylphosphoramide. The structural typ es studied include mixed cyclic dimers (R(2)NLi-LiX), trimers [(R(2)NL i)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 interpla y of steric effects appears to be the dominant influence on the mixed aggregate structures and relative stabilities.