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 But), PhOH, aldehydes R1CHO (R1 = Me, But 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, But, CH2But, 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.