Mono- and Dilithiation of Primary Amine/Secondary Amine Mixtures: Transamination, Solvation, and Cocomplexation

Abstract

Addition of 1 equiv of N,N,2,2-tetramethylpropanediamine (4) to 1 equiv of either lithium dibenzylamide (1) or lithium 2,2,6,6-tetramethylpiperidide (2) results in the exclusive formation of the primary amide [{Me2NCH2CMe2CH2N(H)Li}6] (5). Compound 5 was determined to be a prismatic hexamer in the solid state, with each lithium being tetracoordinated by binding to three amido anions and one chelating dimethylamino unit. In contrast, when 1 equiv of 4 is added to lithium hexamethyldisilazide (3), the sole product isolated from solution was the primary amine solvate [{(Me3Si)2NLi·H2NCH2CMe2CH2NMe2}2] (6). Similarly, reaction of 1 equiv of 1,3-diaminopropane with 3 gave the solvate [{(Me3Si)2NLi·H2NCH2CH2CH2NH2}2] (12). Crystallographic analyses of 6 and 12 revealed dimeric aggregation, with each amine η1-bonding to a metal center through the primary amine group. Addition of 1 equiv of BunLi to the amine/amide mixtures resulted in lithiation of the remaining amine to yield the mixed-anion aggregates [(1.5)2] (9), [(2.5)2] (10), and [(3.5)2] (11). These compounds adopt similar solid-state structures which can be regarded as four-rung ladders formed by the association of two mixed-anion heterodimers. Each metal center of the ladders is three-coordinate, either by bridging three amido nitrogens or by coordinating to two amido nitrogens and to one chelating dimethylamino unit.