Lithiated tertiary carbanions display variable coordination modes: Evidence from DFT and NMR studies

Density functional calculations reveal that, whereas the reaction of 2-propyl-N,N-diisopropylbenzamide (6) with tBuLi in the presence of potentially tridentate donor ligands may result in lateral deprotonation of 6, the behavior of the Lewis base is non-trivial. The ability of N and O donor centers in the co-solvent to resist Li + coordination is found to be synonymous with interaction of lithium with the formally deprotonated carbanion center. Low-energy structures have been identified whose predicted 1H and 13C NMR spectroscopic shifts are in excellent agreement with experiment. Reaction of 2-isopropyl-N,N-diisopropylbenzamide (5) with tBuLi in the presence of bidentate Lewis base N,N,N′,N′- tetramethylethylenediamine (TMEDA) yields material that is suggested by NMR spectroscopy to be laterally deprotonated and to have the formulation 5-Li laTMEDA. In spite of the tertiary aliphatic group at the 2-position in 5, X-ray crystallography reveals that the crystalline material isolated from the treatment of 5/(-)-sparteine with tBuLi is a lateral lithiate in which amide coordination and solvation by bidentate Lewis base results in the Li + ion interacting with the deprotonated α-C of the 2-iPr group (2.483(8) A). The tertiary carbanion center remains essentially flat and the adjacent aromatic system is highly distorted. The use of a chiral co-solvent results in two diastereomeric conformers, and their direct observation in solution suggests that interconversion is slow on the NMR timescale. Two's company, three's a crowd: Tridentate ligands promote tertiary carbanion formation through benzylic deprotonation. New calculations suggest that the ligands can adopt variable denticities in solution. The alternative use of bidentate ligands N,N,N′,N′-tetramethylethylenediamine and (-)-sparteine is now shown to promote benzylic reaction, accompanied by the retention of carbanion-lithium bonding (see figure). Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.