SYNTHESIS AND SPECTROSCOPIC INVESTIGATIONS OF ALKYLALUMINUM ALKOXIDES DERIVED FROM OPTICALLY-ACTIVE ALCOHOLS - THE 1ST STRUCTURAL IDENTIFICATION OF AN OPTICALLY-ACTIVE ORGANOALUMINUM ALKOXIDE

The reaction of trialkylaluminum, R3Al (R = Me, Et, i-Bu), with optically active alcohols such as l-menthol and l-borneol in a 1:1 ratio gives high yields of R2AlOR* (OR* = l-mentholate, R = Me (1a), Et (1b), i-Bu (1c); OR* = l-borneolate, R = Me (2a), Et (2b), i-Bu (2c)) and the corresponding alkane, RH. The resulting alkoxides have been characterized by H-1 and C-13 NMR spectroscopy. The single-crystal X-ray structures of 1a,c and 2a established the dimeric structure for these compounds. 1a was assigned to the orthorhombic cell system, space group P2(1)2(1)2(1) (No. 19), with cell constants a = 10.097 (1) angstrom, b = 10.485 (1) angstrom, c = 26.920 (4) angstrom, and Z = 4 (dimers). The structure was refined to a final R = 4.5% (R(w) = 3.9%) based on 2669 observed reflections (F(o) greater-than-or-equal-to 2.5-sigma(F)). 1c was assigned to the triclinic cell system, space group P1 (No. 1), with cell constants a = 10.838 (4) angstrom, b = 12.792 (6) angstrom, c = 15.787 (7) angstrom, alpha = 81.56 (4)-degrees, beta = 83.04 (4)-degrees, gamma = 73.01 (3)-degrees, and Z = 2 (dimers). The structure was refined to a final R = 8.7% (R(w) = 8.5%) based on 4213 observed reflections (F(o) greater-than-or-equal-to 3-sigma(F)). 2a was assigned to the monoclinic cell system, space group P2(1) (No. 4), with cell constants a = 7.261 (7) angstrom, b = 14.494 (8) angstrom, c = 12.938 (7) angstrom, beta = 93.63 (7)-degrees, and Z = 2 (dimers). The structure was refined to a final R = 5.3% (R(w) = 4.4%) based on 1432 observed reflections (F(o) greater-than-or-equal-to 2.5-sigma(F)). In 1a, c and 2a, the alkoxide ligands serve as bridging units between the two dialkylaluminum moieties to give stable, planar Al2O2 rings. The behavior of the H-1 NMR spectra of 1a-c as a function of temperature has been interpreted in terms of increasing steric interaction of the l-metholate group with the methyl, ethyl, and isobutyl groups attached to the aluminum. This steric interaction gives rise to restricted rotation of the l-mentholate and/or the alkyl group and leads to nonequivalence of the protons in the ethyl and isobutyl derivatives. This does not occur for the l-borneol derivatives, since the borneal moeity cannot interact with the alkyl groups bound to the aluminum because of its orientation and rigidity.