An experimental and computational study of stereoselectivity and reactivity in lewis acid promoted lithiation-substitution of tertiary amines

Abstract

Reaction of BF₃ coordinated N-ethylpyrrolidine with benzophenone, using a preformed complex of s-BuLi and (-) -sparteine for lithiation at -78 °C, affords enantio-enriched R-product (er 85:15). With a warm-cool cycle (-78° → 0 °, 2 h, →-78 °) prior to electrophile addition the enantioselectivity is reversed. Similarly, in the reaction of BF₃-indolizidine complex syn substitution (dr 97:3) occurs but with a warm-cool cycle the syn-anti ratio gets changed to 41:59. These results indicate initial syn lithiation and formation of excess of the anti intermediate on equilibration. DFT calculations at the B3LYP/6-31+G* level show the syn intermediate to be stable in the gas phase, but with the solvent continuum treatment the relative stability shifts toward the anti arrangement and it become more stable in highly polar solvents. These computations also reveal a close contact between Li and boranato fluorine, and its implications for superior effectiveness of BF₃ in promoting lithiation of amines are discussed.