Solution phase structures of enantiopure and racemic lithium N-benzyl-N-(α-methylbenzyl)amide in THF: Low temperature 6Li and 15N NMR spectroscopic studies
Claridge TDW., Davies SG., Kruchinin D., Odell B., Roberts PM., Russell AJ., Thomson JE., Toms SM.
The antipodes of lithium N-benzyl-N-(α-methylbenzyl)amide are highly efficient enantiopure ammonia equivalents for the asymmetric synthesis of β-amino acid derivatives via conjugate addition to α,β- unsaturated esters. 6Li and 15N NMR spectroscopic studies of doubly labelled 6lithium (S)-15N-benzyl- 15N-(α-methylbenzyl)amide in THF at low temperature reveal the presence of lithium amide dimers as the only observable species. Either a monomeric or dimeric lithium amide reactive species can be accommodated within the transition state mnemonic for this class of conjugate addition reaction. This enantiopure lithium amide offers unique opportunities over achiral (e.g., lithium dibenzylamide) and C2-symmetric (e.g., lithium bis-N,N-α-methylbenzylamide) counterparts for further mechanistic study owing to the ready distinction of the various dimers formed. © 2013 Elsevier Ltd. All rights reserved.