Aminotriphenolate complexes for effective epoxide activation

This thesis focuses on the synthesis of vanadium(V) and molybdnenum(VI) aminotriphenolate complexes VO(Ld,h,q,r), MoO(Lc)(TU), MoO(Lc)(U), Al(Lq,r,s)(DMF) and their use in activation of epoxides in ring opening reactions by amines and CO2 cycloadditions. Triphenolamines have been chosen as ligands because of their high versatility in metal complexation and the possibility of functionalization. Indeed, it will be pointed out that according to the metal employed and the different ligand substitutions, aminotriphenolate complexes exert a preferential activity towards a specific reaction or substrate. Moreover, the strong Lewis acidity of many aminotriphenolate complexes makes these catalysts a first choice for epoxide activation. Tert-butyl substituted vanadium(V)aminotriphenolate complex VO(Lh) gave the best performance in catalyzing ring opening of epoxides by amines, reaching complete conversion of 1,2-epoxyhexane (taken as a reference substrate) in 30 minutes, using dibutylamine as nucleophile, with 1mol% of catalyst. A wide amine and substrate scope, covering all the principal classes of amines and epoxides, has been conducted, showing excellent conversions in most cases. Chloro-substituted vanadium(V) aminotriphenolate complex VO(Ld) is the catalyst of choice for CO2 cycloaddition reactions to epoxides, demonstrating to be a highly active catalyst for coupling of both internal and terminal epoxides. High yields of cyclic carbonates have been afforded using 0.5 mol% of catalyst at 85°C in 16 hours, with a CO2 pressure of 10 bar. Interestingly, some insights into the nature of the active species involved in the catalytic cycle could be inferred from the X-ray of an unusual structure of propylene oxide coordinated to VO(Ld). The aminotriphenolate functionalization can also be designed specifically to link another catalytic moiety to the complex, in order to build a bifunctional catalyst. This is the case of complex MoO(Lc)(TU), MoO(Lc)(U), which have been synthesized and tested as bifunctional catalysts for olefin epoxidation - epoxide ring opening tandem reactions. These systems are composed by a Molybdenum centre, effective catalyst for olefin epoxidations, and a (thio)urea moiety, a well-known organocatalyst able to activate epoxides via hydrogen bonds towards nucleophilic attack. Decorating aminotriphenolate ligands with three ammonium or pyridinium salts – with halide anions as counterions – led to the synthesis of bifunctional catalysts VO(Lq,r) and Al(Lq,r,s)(DMF), which have been tested for the synthesis of cyclic carbonates starting from epoxides and CO2. In these systems, the metal catalyst and the halide salts used as co-catalysts are linked together, exploiting the proximity effect to get higher catalytic activities in respect to the binary systems.