A theoretical study on nBuLi/lithium aminoalkoxide aggregation in hexane and THF.

Theoretical calculations on aggregation of nBuLi/lithium aminoalkoxide superbases, such as nBuLi/LiDMAE (LiDMAE = Me(2)N(CH(2))(2)OLi) and nBuLi/LiPM (LiPM = Li-N-methyl-2-pyrrolidine methoxide) in gas phase and solution are reported. The combination of equimolar amounts of each component in hexane induced unusual reactivity of the resulting superbase, which remains misunderstood. In order to elucidate the corresponding reaction mechanisms, it is imperative to get a deeper insight into the energetics of aggregation and the effect of the medium on equilibrium constants. In the present study, we compute and compare the stability of (nBuLi)(n), (LiPM)(n), and equimolecular mixed aggregates (nBuLi:LiPM)(n) in gas phase, hexane, and THF. Calculations have been carried out at the density functional theory level (B3LYP/6-31G(d)) using continuum and discrete continuum models of solvation. Higher-level calculations (MP2/aug-ccpVQZ) have been done in some cases for test purposes. Enthalpic and entropic contributions have been discussed and were shown to play an opposite role in hexane (or gas phase) and THF. The characteristics of LiPM and mixed nBuLi/LiPM solutions are found to be significantly different from those of nBuLi solutions. These calculations are in accordance with experimental data in both hexane and THF. Further comparison of theoretical and experimental results for gas-phase Li(+)-THF and Li(+)-DME complexes has enabled a discussion on computational errors for entropic contributions in THF. The value for the release of a THF solvent molecule is proposed to be DeltaS approximately 23 eu. These results provide new insights to the aggregation of organolithium compounds in solution and will be useful for the investigation of other systems.