Catalytic hydrogenation of arylphosphines by niobium aryloxide compounds
Abstract
A number of niobium aryloxide systems have been shown to carry out the homogeneous hydrogenation of arylphosphines into their saturated counterparts. We have been able to develop this reactivity to the point where synthetically useful quantities of new and existing cyclohexylphosphine ligands can be produced. This reaction can be achieved using alkyl precursors such as Nb(OAr) xR5-x (OAr = substituted aryloxide, x = 2, 3) or utilizing mixtures of the corresponding halides Nb(OAr)xCl5-x reacted with various amounts of BunLi activator. The study has allowed us to probe the importance of the structure of the catalyst precursor as well as the effect of the substrate structure. Mechanistic insight into these reactions have been obtained by monitoring the concentration of intermediates and kinetic modeling studies. No intermediates of the cyclohexadienyl, cyclohexenyl or catalytic intermediates have been detected by NMR studies in the hydrogenation of arylphosphines, but evidence of involvement of the ortho substituents on the aryloxides was found. Additional understanding into the workings of the mechanism was explored though the many variables involved in the hydrogenation. Labeling studies have shown that the hydrogenation of substrates such as PPh 3 and PPh2Pri occur in predominately all cis fashion. Further, hydrogenation of chiral alkyldiphenylphosphines brings about asymmetric induction of the intermediate products.
Degree
Ph.D.
Advisors
Rothwell, Purdue University.
Subject Area
Organic chemistry|Chemistry
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