First Row Transition Metal Hydrides Catalyzed Hydrogen Atom Transfer

The traditional reagent for H• transfer in organic chemistry is 𝓃-Bu₃SnH, which has a Sn–H bond dissociation energy (BDE) of 78.5 kcal/mol. There are, however, many disadvantages of employing 𝓃-Bu₃SnH in radical reactions. The transfer of H• from tin is necessarily stoichiometric, with 𝓃-Bu₃Sn–X being the eventual product. Overall, the tin reactions have poor atom economy; n-Bu3SnH cannot be regenerated from 𝓃-Bu₃Sn• or 𝓃-Bu₃Sn–X with hydrogen, and no general methods of regenerating the tin hydride with other hydride sources have been reported. Standard purification methods leave unacceptable levels of residual tin in the products of n-Bu3SnH reactions. Alternatives are clearly needed. Transition metal hydrides represent a class of promising reagents to replace 𝓃-Bu₃H. Due to their typically weaker M-H bonds, transition metal hydrides are often able to transfer H• to C=C and generate radicals — a reaction that 𝓃-Bu₃SnH cannot do. Furthermore, many transition-metal hydrides can be regenerated from hydrogen gas, an event that requires that the M–H BDE be over 56 kcal/mol. By combining this reaction with the H• transfer, metalloradicals can often catalyze the formation of radicals from H₂. Over the years, the Norton group has studied several transition metal hydride systems and demonstrated their applications in different scenarios. The kinetics and thermodynamics of these systems have been studies in detail, and they are shown be competent hydrogen atom donors to unsaturated organic substrates and to organic radicals. Some of these metal hydrides can be made catalytic under hydrogen pressure, thus providing an atom-economical way to effect radical reactions. Specifically, the thermodynamic properties of the chromium hydride HCpCr(CO)₃ have been carefully studied. Based on this information, I developed a Ti/Cr cooperative catalytic system featuring multiple interactions between the two metal systems. Herein are described three applications of this Ti/Cr catalytic system: anti-Markovnikov hydrogenation of epoxides ...