Abstract: Chapter 1 contains a thorough overview of cobalt-catalyzed hydrogen atom transfer (HAT)-initiated alkene hydrofunctionalizations with special attention given to radical–polar crossover reactions. The chapter begins with a general mechanistic discussion of metal-hydride-initiated HAT radical reactions. A historical perspective on the origins of the field is then provided, including work by bioinorganic chemists, inorganic chemists, and seminal work by Mukaiyama. Key contributions from the Carreira, Shenvi, and Herzon labs are highlighted. The second half of Chapter 1 contains an exhaustive review of all published cobalt-catalyzed HAT-initiated radical–polar crossover alkene hydrofunctionalizations to date.
Chapter 2 describes our lab’s strategy for developing a catalytic radical–polar crossover reaction under strong catalyst control. Direct conversion of tertiary allylic alcohols to epoxides or semipinacol rearrangement products could be achieved with judicious choice of cobalt(II) salen catalyst. Bifurcation of reaction pathways suggests the participation of electrophilic alkylcobalt(IV) intermediates. Evaluating the stereochemical outcomes of analogous bromohydrin expansions provided insight into which complexes promote the formation of alkylcobalt(IV) intermediates. Preliminary studies into solvent dependent radical–polar crossover hydrofunctionalizations of tertiary allylic alcohols bearing 1,1-disubstituted alkenes are described as well.
In Chapter 3, efforts that led to the development of a catalytic asymmetric HAT radical–polar crossover hydroalkoxylation are summarized. Catalyst structure-activity relationships were revealed that lead to the synthesis of a series of novel scalemic cobalt(II) salen complexes containing extended aromatic systems. Our protocol proved successful for converting a variety of cyclic tertiary allylic alcohols to the corresponding epoxides with high levels of enantioselectivity. Analysis of thermodynamic parameters and arene properties suggest that stabilizing noncovalent cation interactions within the cobalt(II) salen catalyst are essential to asymmetric induction.
Chapter 4 describes recent efforts by our lab to develop a catalytic radical¬–polar crossover variant of the Ritter reaction. Long-standing limitations to substrate scope within the field of cobalt-catalyzed HAT radical–polar crossover hydrofunctionalizations are discussed. Strategic ligand design facilitated the development of cobalt(II) salen complexes capable of efficiently engaging trisubstituted and tetrasubstituted alkenes to afford tert-alkyl acetamide products. Isotope labeling and excess water experiments identified that nucleophilic capture of electrophilic intermediates by water was competitive with the desired hydroamidation. Hydrogen evolution studies confirmed that formation of hydrogen gas is a competitive pathway that contributes to background consumption of oxidant and silane.