2014 Theses Doctoral
Transition Metal Hydrides that Mediate Catalytic Hydrogen Atom Transfers
Radical cyclizations are important reactions in organic chemistry. However, they are seldom used industrially due to their reliance on neurotoxic trialkyltin hydride. Many substitutes for tin hydrides have been developed but none have provided a general solution to the problem.
Transition metal hydrides with weak M-H bonds can generate carbon centered radicals by hydrogen atom transfer (HAT) to olefins. This metal to olefin hydrogen atom transfer (MOHAT) reaction has been postulated as the initial step in many hydrogenation and hydroformylation reactions. The Norton group has shown MOHAT can mediate radical cyclizations of ɑ,ω dienes to form five and six membered rings. The reaction can be done catalytically if 1) the product metalloradical reacts with hydrogen gas to reform the hydride and 2) the hydride can perform MOHAT reactions. The Norton group has shown that both CpCr(CO)₃H and Co(dmgBF₂)₂(H₂O)₂ can catalyze radical cyclizations. However, both have significant draw backs.
In an effort to improve the catalytic efficiency of these reactions we have studied several potential catalyst candidates to test their viability as radical cyclization catalysts. I investigate the hydride CpFe(CO)₂H (FpH). FpH has been shown to transfer hydrogen atoms to dienes and styrenes. I measured the Fe-H bond dissociation free energy (BDFE) to be 63 kcal/mol (much higher than previously thought) and showed that this hydride is not a good candidate for catalytic radical cyclizations.
I have investigated the dynamics of Co(dmgBF₂)₂(H₂O)₂ under hydrogen gas to attempt to observe its hypothesized cobalt hydride. Under large pressures up to 70 atm we see two species one which we assign as the cobalt hydride and one which we assign as a ligand protonated Co(I) complex. These are supported by high pressure NMR studies of the same complexes. By varying the H₂ pressure, we can calculate the hydrogen atom donor ability of the mixture formed under H₂ as 50 kcal/mol. This makes this mixture a very good H• donor.
The Norton group has shown that vanadium hydrides have very weak V-H bonds that donate H* rapidly. However, they cannot be made catalytic under hydrogen gas. I have attempted to regenerate these vanadium hydrides by a sequential reduction then protonation of the metalloradical. With HV(CO)₄dppe this only produced hydrogen gas, presumably by one electron reduction of HV(CO)₄dppe. However, with HV(CO)₄dppf this does not readily occur and this hydride could potentially be a catalyst for radical cyclizations.
Many radical cyclizations involve vinyl (sp²) radicals. I have shown that both the CpCr(CO)₃H and the Co(dmgBF₂)₂(H₂O)₂ systems can catalytically perform metal to alkyne hydrogen atom transfers (MAHAT's) and that these reactions can be used to perform radical cyclizations very efficiently.
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More About This Work
- Academic Units
- Thesis Advisors
- Norton, Jack R.
- Ph.D., Columbia University
- Published Here
- August 12, 2014