Hydrogen atom transfer for hydrofunctionalization of alkenes
Hydrofunctionalization by hydrogen-atom transfer (HAT) involves the addition of hydrogen and a functional group to an alkene with Markovnikov site selectivity. These reactions involve the transfer of a hydrogen atom from a metal hydride complex to the alkene to afford a carbon-centered radical intermediate, which engages with a radical acceptor to yield the functionalized product. The active metal hydride species is typically generated from a substoichiometric amount of a metal complex and a stoichiometric amount of a reducting agent such as an organosilane. HAT hydrofunctionalizations are characterized by high chemoselectivity (selective reaction of alkenes) and broad scope in the installed functional group. C–C, C–O, C–N, and C–X (halogen) can be established through appropriate choice of the radical acceptor and reaction conditions.
The accepted general mechanism of HAT hydrofunctionalization begins with the formation of a metal hydride complex. This complex transfers a hydrogen atom to the alkene substrate to afford a carbon-centered radical and reduced metal complex. In some cases, the resulting radical and metal complex undergo equilibrium collapse to an organometallic species. Separation of the metal and organic radical from their solvent cage and reaction with a radical acceptor gives the hydrofunctionalized product. Hydrogen-atom transfer occurs with Markovnikov selectivity, yielding the more stable of the two possible radicals. This mechanistic model is associated with significant uncertainty in a number of cases.