Generation of reactive low-valent titanium species using metal--arenes as efficient organic reductants for TiCl(3): applications to organic synthesis

A comprehensive study on the use of metal-arene systems as organic reductants for TiCl(3) has resulted in an efficient method for the generation of highly reactive low-valent titanium (LVT) reagents. The activated titanium species could be prepared by refluxing a mixture of substoichiometric amounts of arenes, TiCl(3), and Li/Mg in THF or DME. Among the LVT reagents screened, TiCl(3)--Li--naphthalene--THF (reagent I) was the best for coupling of carbonyls to olefins. The reagent could carry out the McMurry olefination of both aromatic and aliphatic substrates at a lower temperature and in a much reduced time as compared to the conventional procedures. Subtle changes in the method of preparation of the LVT reagents influenced the stereoisomeric ratio of the olefins. The reagent was also useful for the synthesis of O- and N- heterocycles and vicinal diamines via intramolecular carbonyl coupling and reductive duplication of imines, respectively.