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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.

The Hauser base reagents Pri2NMgCl 1 and Pri2NMgBr 2 react with a variety of enolisable ketones to yield magnesium enolates. Attempts at isolation of these enolates when THF was present in the solvent media was unsuccessful, with the exclusive precipitation of the solvated dihalide salts (MgX2 · Sx, where X=Cl or Br and S=THF, TMEDA or HMPA). Using diethyl ether as solvent media and one molar equivalent of HMPA, the halomagnesium enolate compounds [{ButC(CH2)OMgBr · HMPA}2] 3 and [Me2CHC(CMe2)OMgBr · HMPA] 5 were isolated and identified. Both 3 and 5 precipitate as mixtures with the dihalide salt [MgBr2 · (HMPA)2] 4. X-ray crystallographic studies reveal 3 to be dimeric utilizing enolate bridges, whereas 4 is a simple monomer. A molecular-orbital theoretical study (HF/6-31G*) was conducted to determine the relative bridging abilities of several model anions. The enolate anion H(CH2)CO− was determined to be a favoured bridge in preference to the halides F−, Cl− and Br−, which is consistent with the X-ray evidence. The amido anions Me2N−, (H3Si)2N− and (Me3Si)2N− are also calculated to be favoured over the chloride anion in three-coordinate dimer systems. This is contrary to the known structure of [{(Me3Si)2NMgCl · (Et2O)}2] 8 which bridges through the chloride atoms. The influence of solvent may be critical in determining which anion bridges. Solvent also plays a decisive role in the dismutation reaction of Hauser bases or halomagnesium enolates into their homoleptic components, similar to the Schlenk equilibrium for Grignard reagents.