High oxidation state molybdenum and tungsten imido alkylidene and metallacycle chemistry : catalytic asymmetric olefin metathesis and mechanistic investigation

Abstract

Enantiomerically pure molybdenum imido alkylidene complexes of the type Mo(NAr)(CHCMe₂Ph)[Ar'](THF), which contained 3,3'-diaryl (Ar' = Mes (21), Ph (22)) substituted binaphtholate ligands, were prepared and structurally characterized. Complex 21 was a trigonal bipyramidal anti alkylidene complex; both 21 and 22 were anti-THF adducts. Temperature and concentration dependent enantioselective desymmetrization of prochiral trienes by catalyst 21 afforded five- and six-membered heterocycles in >90% conversion and >90% ee. Consumption of the six-membered substrate followed pseudo first-order kinetics. The new catalyst was compatible with a variety of common functional groups. The longevity of the catalysts depended on the concentration of ethylene in solution. The resting state of the catalytic cycle, unsubstituted molybdacyclobutane 70, slowly decomposed to yield propylene. This observation suggests that β-hydride elimination can compete with bimolecular coupling of methylene complexes under some conditions. Chapter 2: Racemic and enantiomerically pure tungsten imido alkylidene complexes, W(NAr)-(CHCMe₂Ph)(Biphen) (23a, Ar = 2,6-i-Pr₂C₆H₃) and W(NAr')(CHCMe₂Ph)(Biphen) (23b, Ar' = 2,6-Me₂C₆H₃) were prepared and shown to be viable catalysts for representative asymmetric ring- closing metathesis reactions. Both catalysts were compatible with a variety of common functional groups. Intermediate tungstacyclobutane complexes were observed when a stoichiometric amount of desymmetrization substrate 62 was added to 23a. The final stable complex 78, formed between the ring-closed product and a tungsten methylene complex, was structurally characterized. Isotopic labeling experiments with ¹³C₂H₄ allowed the observation of

(cont.) unsubstituted tungstacyclobutane complexes (82), ethylene complexes (84), tungstacyclopentane complexes (86), and a heterochiral methylene dimer (85a). The tungstacyclopentane complexes catalyzed slow dimerization of ethylene to 1-butene. The observation of the methylene dimer provides the first direct evidence of a bimolecular decomposition pathway for methylene complexes. Chapter 3 Racemic and enantiomerically pure molybdenum alkylimido alkylidene complexes, Mo(NAd)(CHCMe₂Ph)(Biphen) (19d, Ad = 1-adamantyl) and Mo(NAd)(CHCMe₂Ph)[Trip]-(THF) (20d) were prepared and structurally characterized. Complex 19d was observed almost exclusively as a syn alkylidene isomer, in contrast with 20d which was observed almost exclusively as an anti-THF adduct. Complexes 19d and 20d are the only reported chiral alkylimido alkylidene complexes for enantioselective olefin metathesis reactions. Complex 19d is the first crystallographically characterized four-coordinate adamantylimido alkylidene complex in its base-free form. It offers unique reactivity and selectivity profiles in tandem AROM/RCM and AROM/CM reactions. Complex 19d is compatible with a variety of common functional groups, including boron-containing reagents. Van't Hoff analyses suggest that the bias toward syn-19d isomer is entropy-driven. Chapter 4: Solvent- and base-free molybdenum methylene complexes, Mo(NAr)(Biphen)(CH₂) (114a, Ar = 2,6-i-Pr₂C₆H₃) and Mo(NAd)(Biphen)(CH₂) (114d, Ad = 1-adamantyl) ...