Reactions of N-Chloro-N-alkylbenzylamines with Amines in Acetonitrile. Origin of Steric Effect in Imine-Forming Elimination

Reactions of N-chloro-N-alkylbenzylamines in which the alkyl group is Me, Et, i-Pr, sec-Bu, and t-Bu with MeNH₂and Et₂NH in MeCN have been studied kinetically. The eliminations are quantitative and regiospecific, producing only N-benzylidenealkylamines. The relative rates of elimination for Me/Et/i-Pr/sec-Bu/f-Bu alkyl substituents are 1/0.6/0.4/0.3/0.1 with MeNH₂and 1/0.5/0.3/0.2/0.03 with Et₂NH respectively. Comparison with published data reveals that Chaxton's |ψ| value for the imine-forming elimination decreases with the variation of the base-solvent from MeONa-MeOH to MeNH₂-MeCN but increases when the base is changed from MeNH₂ to Et₂NH. For a given base, Hammett p and Kh/kdvalues decrease and the AHP and AS* values increase with bulkier alkyl substituents. From these results, the origin of the steric effect in imine-forming elimination is attributed to the repulsive interaction between the alkyl group and the base in the transition state.