Dimethylaluminium enolates and alkoxides derived from trimethylaluminium and aromatic ketones: a synthetic, structural and theoretical investigation

Reaction of Me3Al with a series of aromatic ketones results in the precipitation of either dimethylaluminium enolates or alkoxides. In situ 1H-NMR spectroscopic studies of the reaction between Me3Al and acetophenone reveal a complex mixture of products whereas under the same conditions 2,4,6-trimethylacetophenone reacts cleanly to give the corresponding enolate. The enolate compounds [Me2AlOC(2,4,6-Me3–C6H2)CH2] (2) and [Me2AlOC(C6Me5)CH2] (4) were isolated and 2 as well as the representative alkoxide [Me2AlOCMe2Ph] (6) were characterised by X-ray crystallography. Both 2 and 6 form dimers with a central Al2O2 core. Ab initio molecular orbital calculations (HF/6-31G*) indicate that both 2 and 6 are the thermodynamic products of their reactions. For 2,4,6-trimethylacetophenone enolisation is preferred over alkylation by 4.70 kcal mol−1 whereas for acetophenone alkylation is preferred by 25.39 kcal mol−1 over enolisation. Disubstitution of the ortho positions on the aromatic ring by methyl groups results in the relative destabilisation of the alkoxide compared to the enolate due to steric crowding around the quaternary carbon atom.