Reductive And Metathetic Coupling Reactions Mediated By Group (IV) Metal Alkoxides

Abstract

Several organic transformations are mediated by group(IV) metal alkoxides. The reactivity is based on the basic nature of alkoxide group, Lewis acidic nature of the group(IV) metals, insertion of unsaturated molecules into the M-OR bond and the reduction of M(OR)4 to low valent species. The thesis deals with insertion reactions and the reductive and metathetic coupling reactions mediated by group(IV) metal alkoxides. Titanium(IV) alkoxides and zirconium(IV) alkoxides promote insertion and metathesis of aryl isocyanates. It was observed that aryl isocyanates underwent double insertion in addition to mono insertion. At room temperature, head to tail double insertion is observed whereas at elevated temperatures, head to head double insertion occurred leading to metathesis. The reaction has also been extended to metathesis between heterocumulenes and heteroalkenes. Titanium and zirconium carry out these reactions with different efficiencies. The reasons for these differences have been sought through computational methods. New organic transformations promoted by group(IV) metal alkoxides that are reduced with Grignard reagents and silanes have been explored. Grignard reagents do show reactivity towards imines in the presence of group(IV) metal alkoxides. The reactions have been studied with stoichiometric and catalytic amounts of titanium(IV) isopropoxide and are shown to follow different pathways. Isotope labeling studies indicate that alkylated products formed in stoichiometric reactions arise due to metal-olefin intermediates. However in catalytic reactions, a metal-alkyl complex is responsible for alkylation. Titanium(IV) alkoxides when used in combination with silanes such as phenylsilane bring about the reductive coupling of imines. One of the interesting features is that this pinacol type coupling is diastereospecific.