Reactivity of ether- and amine-complexed dimers and tetramers of alkyllithiums towards triphenylmethane.

Abstract

Kinetics of Lewis base (LB) complexed primary and secondary sigma-alkyllithiums (RLi) with triphenylmethane (TPMH) are reported. RLis in which one or two LB groups (-OMe, -NMe, -NMeR) are part of the molecule form, in benzene, intramolecularly complexed tetramers, for example, 2(4), or dimers, for example, 4(2). They are used as models for their intermolecular congeners R4Li4 x 4LB and R2Li2 x 4LB (LB = NR'3, OR'2). Nonunity reaction orders in [RLi] are in line with reactions via as yet unidentified 1:1 complexes formed in an equilibrium (K(stat. corr.) approximately = 1) between aggregated RLi and TPMH. In some cases, a tetramer/dimer equilibrium mixture undergoes complexation/reaction. Reaction rates correlate linearly with calculated concentrations of the complexes. Relative rates of complexes range from 1 [prim-R4Li4 x 3LB x TPMH (presumed)] to 4250 [sec-R2Li2 x 3LB x TPMH (presumed)]. A major role in the reactivity enhancement owing to LB-induced conversion of tetramers into dimers is ascribed to increased LB participation in LB-richer dimer transition states. Amine and ether complexes have practically equal reactivities. Lithiation of TPMH by dimeric RCH2Li is retarded by a factor of 24000 if a silyl group is linked to the alpha-carbon.