Solvent effects and molecular rearrangements during the reaction of Hauser bases with enolisable ketones:: structural characterization of [{ButC(=Ch2)OMgBr.HMPA}2] and [MgBr2.(HMPA)2]

The Hauser base reagents (Pr2NMgCl)-N-i 1 and (Pr2NMgBr)-N-i 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 . S-x, 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 [{(BuC)-C-t(=CH2)OMgBr . HMPA}(2)] 3 and [Me2CHC(=CMe2)OMgBr . HMPA)(2)] 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)(2)N- and (Me3Si)(2)N- are also calculated to be favoured over the chloride anion in three-coordinate dimer systems. This is contrary to the known structure of [{(Me3Si)(2)NMgCl . (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. (C) 1998 Elsevier Science S.A. All rights reserved.