Lithium trialkoxo[tris(trimethylsilyl)methyl]aluminates, analogues of intermediates in reductions of carbonyl compounds. Crystal structures of [Li(thf)2][AlR(OEt)3] and {Li[AlR(OPri)2(OH)]}2, R = C(SiMe3)3 and thf = tetrahydrofuran

Abstract

The organotrihydroaluminate [Li(thf)₂][(Me₃Si)₃CAlH₃] (thf = tetrahydrofuran) reacted with alcohols R′OH, (R′ = Me, Et, Prⁱ or Bu^t), PhOH, aldehydes R¹CHO (R¹ = Me, Bu^t or Ph) or ketones R¹R²CO (R¹ = R² = Me or Ph; R¹ = Me, R² = Ph) to give organotrialkoxoaluminates [Li(thf)_n][(Me₃Si)₃CAl(OR′)₃] (n = 1, R′ = Me, Prⁱ, Bu^t, CH₂Bu^t, CH₂Ph or CHPh₂; n = 2, R′ = Et or Ph; n = 4, R′ = Me), analogues of the intermediates presumed to be present in the reduction of carbonyl compounds by LiAlH₄. The structure of [Li(thf)₂][(Me₃Si)₃CAl(OEt)₃] was determined by X-ray crystallography which showed that the lithium cation and the organotriethoxoaluminate anion are linked to give a planar LiO₂Al ring. The organotrialkoxoaluminates are hydrolysed by traces of water to give organoalkoxohydroxoaluminates and the structure of one of these, {Li[(Me₃Si)₃CAl(OPrⁱ)₂(OH)]}₂, has been determined. The dimer has an Li₂Al₂O₆ framework comprising two adjacent face-sharing cubes with missing atoms at diametrically opposite corners. Terminal and bridging alkoxo groups in solutions of some of these compounds can be distinguished by NMR measurements.