Pentacoordinate aluminum complex

As already discussed in this chapter, aluminum, in addition to its well-known high oxygenophilicity (Al-O = 511 3 kJ mol ), has exceedingly high affinity toward fluorine this is evident from the bond strengths in several metal-fluorine diatomic molecules Al-F, 663.6 6.3 kJ moFh Li-F, 577 + 21 kJ mol" Ti-F, 569 + 34 kJ moF Si-F, 552.7 + 2.1 kJ moF Sn-F, 466.5 + 13 kJ moF and Mg-F, 461.9 + 5.0 kJ moF [76]. Organoaluminum reagents seem, therefore, quite suitable for fluorine-assisted selective alkylation of fluoro epoxides, which also represents the experimental demonstration of the intervention of pentacoordinate chelate complexes of trialkyl-aluminums as plausible intermediates [63]. 

In a similar [4+2] reaction of a, -unsaturated esters, the aluminum catalyst complexed with the ligand S-VAPOL resulted in autoinduction , because of cooperative interaction of the product with the catalyst to generate a more selective catalytic species (Scheme 6.48) [68]. The ee% gradually increased as the reaction time lengthened. In the proposed intermediate, penta-coordinated aluminum complex 77, the cycloadduct is recognized as a complementary ligand, leading to substantial asymmetric induction. The acrylate is activated effectively within this hybridized complex which adopts pentacoordination [87]. 

Besides the already mentioned acidic aluminum chloride catalysts, alternative Friedd-Crafts catalysts such as supported acidic tin catalysts have also been developed. The tin-based catalysts were prepared by a method which closely resembled the already mentioned two-step grafting method devised for the aluminum chloride catalyst. Here, SnCU was anchored on silica materials modified with tetraalkylammonium chloride moieties obtained for example, from reaction with [3-(trimethoxysrlyl)propyl]octadecyldimethylammonium chloride, thereafter, reaction of the Lewis acid with the chloride moieties leads to formation of pentacoordinated anionic tin species forming catalytically active complexes (i.e. [R4N][SnCl5] species), associated with the surface. The supported tin catalysts were employed for condensation reactions of olefins with aldehydes forming unsaturated alcohols (Prins condensation. Scheme 5.6-2) [76]. 

The chemistry of the precipitation of aluminum hydroxides and oxides is very complex (40). When the solubility is exceeded, gelatinous precipitates, which are found to be amorphous by X-ray diffraction, usually form initially. Al MAS NMR shows the predominance of octahedraUy coordinated Al ions in these amorphous hydroxides, as are present in the crystalline trihydroxides and oxyhydroxides. However, in the amorphous materials some pentacoordinated and tetracoordinated Al ions are also found (15). As discussed above, there are many different crystalline hydroxides or oxyhydroxides, and which of them will be formed depends on the conditions (4f). Primary factors are temperature and pH, as well as aging time however, the nature of the anions present and the possible presence of organic components (42,43) also play a role. At low temperature in an excess of water, the hydroxides are preferentially formed, specifically bayerite at pH values between 5.8 and 9 or gibbsite for pH values smaller than 5.8 or larger than 9. 

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