Synthetic applications of titanium compounds: From PCB destruction to carbohydrate synthesis.

摘要

Titanocene(III) complexes have been shown to be useful reagents for organic synthesis: Cp{dollar}sb2{dollar}Ti{dollar}sp{lcub}rm III{rcub}{dollar}X generates radicals with great selectivity compared with traditional metallic radical species, and effectively reduces organic halides. Activated Cp{dollar}sb2{dollar}Ti{dollar}sp{lcub}rm III{rcub}{dollar} species can catalyze the reduction of aryl halides, and Cp{dollar}sb2{dollar}Ti{dollar}sp{lcub}rm IV{rcub}{dollar} byproducts are easily recovered and recycled.; Cp{dollar}sb2{dollar}Ti{dollar}sp{lcub}rm III{rcub}{dollar}BH{dollar}sb4{dollar}-pyridine can be used to reduce a wide array of aryl halides, including persistent environmental contaminants such as polychlorinated biphenyls (PCBs). These reductions are proposed to proceed by electron transfer from a Cp{dollar}sb2{dollar}Ti{dollar}sp{lcub}rm III{rcub}{dollar} (H)pyridine intermediate. A general correlation exists between the observed rates of reduction for these species and their reduction potentials.; The reaction of glycosyl halides with Cp{dollar}sb2{dollar}Ti{dollar}sp{lcub}rm III{rcub}{dollar} was studied in detail. Reactions with a series of glycosyl halides were rapid and provided the corresponding glycosyl radicals. Products formed from these radicals depended upon the ligands in the coordination sphere of titanium. If Cp{dollar}sb2{dollar}TiBH{dollar}sb4{dollar} is used for reduction, the glycosyl radical can abstract H{dollar}cdot{dollar} from BH{dollar}sb4{dollar} to give the reduced sugar species known as anhydroalditols. In the absence of a reactive ligand, the radical is captured by a second equivalent of titanium(III), forming a (glycosyl)titanium(IV) product. If heteroatomic functionality is present at C-2, then {dollar}beta{dollar}-elimination occurs to give unsaturated carbohydrates known as glycals. The scope of the glycal and anhydroalditol syntheses is described and it is proposed that glycosyl halide activation occurs by an inner sphere pathway in both cases.; In preliminary studies, (glycosyl)titanium(IV) complexes isolated in the absence of heteroatomic functionality at C-2 have been shown to be useful for C-glycosidation. In model systems, alkyltitanium(IV) complexes were used to survey potential activation systems for these compounds. Successful additions to aldehydes were accomplished by activation with ZnCl{dollar}sb2{dollar} or TiCl{dollar}sb4{dollar}, and 1,4-addition to {dollar}alpha,beta{dollar}-unsaturated ketones by CuOTf. Simple C-glycosides were successfully prepared by the addition of (glycosyl)titanium(IV) compounds to aldehydes using ZnCl{dollar}sb2{dollar} or TiCl{dollar}sb4{dollar}.