Reaction between lithium aluminium hydride and some amino alcohols studied by 27Al and 7Li nuclear magnetic resonance

Abstract

LiAlH₄ has been titrated with (–)-N-methylephedrine (1), (–)-quinine (2), and (–)-(S)-4-anilino-3-methylaminobutan-1-ol (3) in tetrahydrofuran and in bis-(2-methoxyethyl) ether. The titrations were followed by ²⁷Al n.m.r. The establishment of equilibrium was verified from the results of back titrations. The ion AlH₄^– was found to be present in solution when 1.5 mol equiv. of (1) or 1.2 mol equiv. of (2) had been added. Computer fittings to experimental n.m.r. data indicated that disproportionation of monoalkoxyaluminates derived from (1) and (2) with re-formation of LiAlH₄ takes place in both solvents. No sign of disproportionation was evident in the case of (3). ⁷Li N.m.r. showed that the compounds derived from LiAlH₄ and the substrates (1)–(3) complex Li⁺ strongly enough to cause slow exchange on the n.m.r. timescale.