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

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

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Chemistry.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2004

Author dc:creator

• Tsang, W. C. Peter, 1975-

Advisor dc:contributor.advisor

• Richard R. Schrock.

• Chemistry.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/30064

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/30064