Reactions Catalyzed by d-Block Metal Alkoxides

The d-block or transition metal alkoxides make the third category. Titanium alkoxides (methoxide, ethoxide, propoxide, isopropoxide, butoxide and isobutoxide) have been able to catalyze transesterification reaction of fatty acids (reaction 7.13) [64]. The results of Nawaratna et aV.s researches showed that the catalytic behavior of titanium alkoxides in transesterification of fatty acids depended on their steric effects meaningfully and lower steric hindrance increased the yield and selectivity of the produced ester [64]. 

Titanium alkoxides (ethoxide, isopropoxide and tert-butoxide) were also used recently for catalyzing the production of substituted indoles,with acceptable yields from cyclization of o-bromobenzylketones, 89, and primary alkyl amines or anilines, 90 (reaction 7.15) [65]. Melkonyan et al. proposed that the reaction sequence includes a titanium alkoxides catalysis reaction of ketones with amines to give imines and following intra-molec-ular cyclization into indoles, 91, using copper catalysis. Without titanium catalyst and amines the product of cyclization was almost 92. 

Bao and co-workers also reported enantioselective ring-opening ami-nolysis of epoxide, 93, with benzylamine, 94, catalyzed by the titanium isopropoxide-BINOL-water system (BlNOL=l,l -bi-2-naphthol-96) in toluene (reaction 7.16) [66]. Titanium isopropoxide-isopropanol, 96, system also used by Kim et al. for asymmetric methallylation of ketones (reaction 7.17) with high yield and enantioselectivity [67]. 

Titanium isopropoxide-TADDOLate, 99, system was used by Charette et al. for asymmetric cyclopropanation of allylic alcohols, 100, (reaction 7.18) with acceptable yield and enantioselectivity [68]. 

Schneider and Hansch used Zr(0 Bu) to catalyze the synthesis of acetone aldol adducts, 103, of aromatic aldehydes and diacetone alcohol, 102, (reaction 7.19) in moderate yields [69,70]. Moreover, they used it for rapid catalysis of Tishchenko reduction (reaction 7.20) of the P-hydroxyketones, 102, instantaneously get ahead the aldol reaction with some aliphatic aldehydes to yield the 1,3-awfi-diol monoesters, 104. 

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