Alkylaluminum Alkoxides Derived from Donor-Functionalized Alcohols: Factors Controlling the Formation and Redistribution of Products

The reaction of R*R*-[Me2Al(μ-elac)]2 (1) (where elac = deprotonated rac-ethyl lactate)
with Me3Al results in the quantitative formation of the Lewis acid−base adduct R*R*-[Me2Al(μ-elac·AlMe3)]2 (2). Under ambient conditions the tetranuclear dimer 2 obtained as an
oily substance changes gradually to give a white crystalline solid, which was identified as
the monomeric binuclear adduct Me2Al(elac)·AlMe3 (3). Compound 3 is also formed directly
when 2 equiv of Me3Al reacts at −78 °C with 1 equiv of elac-H in CH2Cl2 solution. In contrast,
the reaction of conjugated hydroxycarbonyl ligand, methylsalicylate (mesal-H), with 2 equiv
of Me3Al yields exclusively Me2Al(mesal)AlMe3 (4), irrespectively of the conditions applied.
The implications of the differences in products are discussed with respect to the reaction
conditions and the ligand identity. The molecular structure of compound 4 was determined
by X-ray crystallography. The structure of 4 consists of a dinuclear monomer in which an
AlMe3 molecule is complexed to the aryloxide oxygen of the monomeric dimethylaluminum
chelate complex. The geometry and orientation of the Me3Al unit and the C(12)···Al(2)
distance all indicate that there is a weak noncovalent interaction between one methyl group
of Me3Al and the metal center of the Me2Al unit. For 3 and 4 the exchange of the methyl
groups between the two aluminum centers occurs, for which ΔG⧧302K = 69.7 ± 1 kJ mol-1
and ΔG⧧328K = 77.0 ± 1 kJ mol-1, respectively, were determined, and the mechanism of
exchange reactions is discussed.