N-alkylpiperidine·alane compounds and their applications in alane regeneration

The synthesis, characterization, and detailed thermal properties of N-alkylpiperidine·alane compounds are described. Direct reaction of 1 equiv of N-alkylpiperidine (NMPi or NEPi, NMPi = N-methylpiperidine, NEPi = N-ethylpiperidine) with 1 equiv of γ-AlH3 readily gives the 1:1 adduct NMPi·AlH3 (or NEPi·AlH3) in good yields and purity. Attempts to prepare the related 2:1 complexes were unsuccessful; however, in situ studies by infrared spectroscopy showed the formation of (NMPi)2·AlH3 when a large excess of NMPi was present, whereas no (NEPi)2·AlH3 was observed under similar conditions. Such difference in reactivity is due to the steric effect of the ethyl group in NEPi. Under heat and vacuum, both NMPi·AlH3 and NEPi·AlH3 react with 1 equiv of LiH to form non-solvated LiAlH4 in nearly quantitative yields. However, they display dramatically different decomposition pathways without LiH or with a catalytic amount of LiH. While NMPi·AlH3 decomposes to Al metal directly, NEPi·AlH3 can be selectively decomposed to form AlH3 under certain conditions. Moreover, the transamination of (NMPy)2·AlH3 (NMPy = N-methylpyrrolidine) with NEPi has been shown to give NEPi·AlH3 in good yields. Compared to Et3N, NEPi not only extends the scope of the transamination to include a wide range of amine·alane adducts, but also improves the yield, selectivity, and energy-efficiency of the process. Combining these results with the formation of (NMPy)2·AlH3 via hydrogenation, we have established an improved regeneration pathway for AlH3 using NMPy, NEPi, and Al metal.