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Lithiated Tertiary Carbanions Display Variable Coordination Modes: Evidence from DFT and NMR Studies
Author(s) -
Vincent Mark A.,
Campbell Smith Alison,
Donnard Morgan,
Harford Philip J.,
Haywood Joanna,
Hillier Ian H.,
Clayden Jonathan,
Wheatley Andrew E. H.
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201200734
Subject(s) - deprotonation , carbanion , chemistry , conformational isomerism , lewis acids and bases , denticity , crystallography , nuclear magnetic resonance spectroscopy , stereochemistry , density functional theory , tetramethylethylenediamine , medicinal chemistry , computational chemistry , crystal structure , molecule , organic chemistry , ion , catalysis
Density functional calculations reveal that, whereas the reaction of 2‐propyl‐ N , N ‐diisopropylbenzamide ( 6 ) with t BuLi 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 1 H and 13 C NMR spectroscopic shifts are in excellent agreement with experiment. Reaction of 2‐isopropyl‐ N , N ‐diisopropylbenzamide ( 5 ) with t BuLi 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 l ⋅ TMEDA. 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 t BuLi 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‐ i Pr group (2.483(8) Å). 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.