SOLVENT EFFECTS AND MOLECULAR-REARRANGEMENTS DURING THE REACTION OF HAUSER BASES WITH ENOLIZABLE KETONES - STRUCTURAL CHARACTERIZATION OF [((BUC)-C-T(=CH(2))OMGBR.HMPA)(2)] AND [MGBR2.(HMPA)(2)]

The Hauser base reagents (Pr2NMgCl)-N-i 1 and (Pr2NMgBr)-N-i 2 react w ith a variety of enolisable ketones to yield magnesium enolates. Attem pts at isolation of these enolates when THF was present in the solvent media was unsuccessful, with the exclusive precipitation of the solva ted 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 equivalen t 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 sal t [MgBr2 . (HMPA)(2)] 4. X-ray crystallographic studies reveal 3 to be dimeric utilizing enolate bridges, whereas 4 is a simple monomer. A m olecular-orbital theoretical study (HF/6-31G) was conducted to determ ine the relative bridging abilities of several model anions. The enola te anion H(CH2=)CO- was determined to be a favoured bridge in preferen ce 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 als o calculated to be favoured over the chloride anion in three-coordinat e 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 bridge s. Solvent also plays a decisive role in the dismutation reaction of H auser bases or halomagnesium enolates into their homoleptic components , similar to the Schlenk equilibrium for Grignard reagents. (C) 1998 E lsevier Science S.A. All rights reserved.