Reaction pathways and roles of N-alkylmorpholine in amine·alane transamination: A mechanistic study

The cost- and energy-effective regeneration of alane has attracted much attention because of its potential energy applications. Using N-alkylmorpholine·AlH₃ as intermediates, we have recently shown alane regeneration with improved yield, selectivity, and energy-efficiency. However, detailed interactions between AlH₃ and N-alkylmorpholine were not well-understood. Herein, we present spectroscopic and theoretical studies of the reaction pathways and roles of N-alkylmorpholine in transamination. Under reduced pressures, the 2:1 alane adduct (R₃N)₂·AlH₃ is initially transformed into its 1:1, unsaturated complex R₃N·AlH₃, which then forms an equilibrium with N-alkylmorpholine (L) to generate a mixed intermediate R₃N·AlH₃·L. Under the influence of vacuum and excess N-alkylmorpholine, R₃N·AlH₃·L loses an amine molecule (with higher volatility) to give L·AlH₃ as the final product. Using N-alkylpiperidine and 1,4-dioxane as models to mimic the coordination behaviors of the morpholine N and O atoms, we further showed the roles of N-alkylmorpholine in transamination, during which it formed the N-donating complex R₃N·AlH₃·^NL and, depending on steric hindrances of R₃N, the O-donating complex R₃N·AlH₃·^OL. These results highlight the effects of amine coordination modes and steric hindrances in the reactivity of their alane adducts and will provide information for the optimization of alane regeneration.