Barium aryloxides

Table 27 Chiral barium aryloxides-catalyzed Mannich reactions of a p.y-unsaturated ester...

Alkoxide and aryloxide complexes are moisture-sensitive, and are usually colorless or white solids with melting points substantially above room temperature. An interesting exception to this generalization is provided by the brown barium alkoxides Ba[0(CH2CH20) CH3]2 ( = 2, 3), formed from the reaction of elemental barium with a stoichiometric quantity of the polyether alcohol in THF. They are soluble in diethylether, THF and aromatic hydrocarbons, and are liquid at room temperature. 

Recently (33, 59), soluble monomeric alkoxides (aryloxides) of calcium and barium have been synthesized by the reactions of their iodides with the potassium salts of sterically hindered alcohols (phenols). 

Methoxyethanol displays numerous coordination modes (Fig. 2) but is basically a bidentate and assembling ligand [Af(0C2H40Me)n]m alkoxides are thus large oligomers (w = 9, Af = Ca m = 10, Af = Y), sometimes infinite polymers [Af = Bi, Pb(II)]. Despite the large values of the nuclearity, however, they are soluble (with the exception of lead) because of cyclic (Af = Y) or compact structures (Af = Ca) or dissociation of the polymer in solution, as observed for Bi [32]. Aryloxides are, with the exception of barium, either monomers or dimers [30,38]. 

There has been an outburst of research interest in the structures, physical properties, and chemistry of the group 2 metal aryloxides. This is particularly true for the elements strontium and barium where work has been stimulated by the possible use of metal aryloxides as precursors (either via sol-gel or MOCVD processes) for the formation of binary and ternary oxides containing these metals.2 Synthetic procedures are based on either the halide (Be) alkyl derivatives (Mg, Grignard derivatives, etc.) or the actual metallic element (Ca, Sr, Ba). Structural studies (Tables 6.14-6.18) show for the smaller elements Be, Mg, and Ca that monomeric and dimeric structural motifs dominate, with rarer examples of trinuclear clusters, e.g. [Ca3(OPh)5(HMPA)6][OPh.2HOPh].2 2 in the case of strontium and barium a more extensive cluster chemistry has been developed for small aryloxide ligands, while monomeric units with terminal aryloxides can be formed with bulky ligands and sufficient additional Lewis bases, e.g. [Ba(OC6H2Bu2-2,6-Me-4)2(THF)3].2 ... 

Fig. 13 Crystal structure of [BaSr(Odpp)4] synthesized via direct metallation under solvent-free conditions [262]. M—C(jt) interactions are shown as dashed lines. Note the decreased number of M - C(3i) interactions on the larger barium due to the presence of a terminal aryloxide ligand. Hydrogen atoms have been omitted for clarity. All non-carbon atoms are shown as shaded ellipsoids...

Shibasaki et al. also developed chiral barium catalysts prepared from barium alkoxide and optically active BINOL 3 or aryloxide 4 derivatives. These catalysts were applied to asymmetric Mannich reactions of p,y-unsaturated esters (Table 27) [101]. In this reaction, the initially formed Mannich adducts isomerized to afford aza-Morita-Baylis-Hillman-type products in moderate to good yields with good enantioselectivities. For four substrate examples, ayloxide 4 ligand worked well (entries 2—4). 

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