Abstract
The catalytic partial oxidation of hydrocarbons using late transition metals is an important but challenging process. Two key steps are typically involved: C–H bond activation to form a M–C and reductive elimination to form a C–O bond. In this Dissertation, a series of rationally designed “capping arene” supported Rh(III) complexes are synthesized and used to study reductive functionalization of a Rh(III)–Me bond. This type of ligand can block an axial coordination site of the Rh(III) center, which destabilizes the complex and leads to a more facile Rh(I)/Rh(III) redox cycle. The complex (5-FP)Rh(TFA)2Me [5-FP = 1,2-bis(N-7-azaindolyl)benzene, TFA = trifluoroacetate] gives 94% yield of MeTFA in acetonitrile with AgOTf (OTf = trifluoromethanesulfonate) as an additive.
The 5-FP ligand has also been used to prevent the Rh(I) complex, (5-FP)Rh(TFA)(η2-C2H4), from undergoing undesired oxidation in the olefin hydroarylation reaction with benzene and α-olefins. This new Rh catalyst precursor achieves efficient oxidative olefin hydroarylation using either air or Cu(II) salts as oxidants. Under optimized conditions, the conversion of propylene and benzene to linear alkenyl arenes is achieved with over 13,000 turnovers without evidence of catalyst decomposition after 2 weeks at 150 °C. At a lower temperature (80 °C), a linear to branched ratio of ~18:1 has also been observed. The longevity and stability of this catalyst is an improvement on earlier systems and the potential commercialization of the process is being explored.
When investigating the olefin hydroarylation reaction with Rh catalysts, phenyl acetate is produced as a byproduct through a side reaction with benzene and Cu(II) oxidant. As phenyl acetate is a desirable precursor for phenol production, we sought to generate it selectively. A series of copper salts have been tested for the functionalization of the C–H bond of benzene. Two completing mechanistic pathways are involved in the reaction. Under the anaerobic conditions, the organometallic pathway is favored, while the reaction occurs through the radical pathway under aerobic conditions. Interestingly, the addition of a radical reagent, TEMPO ((2,2,6,6-Tetramethylpiperidin-1-yl)oxyl), can shift the reaction from the radical pathway to the non-radical mechanism under aerobic conditions. In addition, the reaction rate is greatly increased when TEMPO is used as an additive.
Hydroamination of alkenes or alkynes is one of the most straightforward methods to form C–N bonds and N-containing heterocycles. This method involves direct addition of amines to carbon-carbon multiple bonds without the formation of any by-products. Al(OTf)3 has been used as an effective catalyst for the intramolecular hydroamination of unactivated alkenes. The mechanism for this transformation has been studied. Triflic acid which is generated in situ from Al(OTf)3 is demonstrated to be the active catalyst for the hydroamination reaction. In addition, other metal triflates such as Bi(OTf)3 and Mg(OTf)2 have been shown to catalyze the hydroamination reaction through a similar reaction mechanism.
