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[摘要]：A 2:1 lithium amide/n-butyllithium aggregate is investigated as an asym. addn. template in hydrocarbon solvents. Several different chiral lithium amides (S)-i-PrNHCH(i-Pr)CH2OR1 [R1 = Me, n-Bu, Me3CSiMe2, (i-Pr)3Si] were synthesized from L-valine and tested in the asym. addn. of n-BuLi to various aldehydes R2CHO (R2 = Me3C, Ph, 2-furyl). Enantiomeric excesses up to 83% were obtained in the case of the addn. of n-BuLi to pivaldehyde at -116 癈 in pentane. 1H and 13C INEPT DOSY were utilized to characterize a new trimeric complex between 2 equiv of lithium amide and 1 equiv of lithium alkoxide. This mixed aggregate strongly indicates the possibility of product-induced chirality inhibition that is detrimental to the enantioselectivity of asym. addn. reaction. |
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