Development of excited-state bond homolysis as a key step for Ni catalysis

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Methods for bond construction enabling the synthesis of complex molecular scaffold are of key interest to the pharmaceutical industry. To this end, Ni catalysis has emerged as a versatile tool for the construction of C(sp2)– C(sp2), C(sp3)–C(sp2), and C(sp3)–C(sp3) bonds. The success of Ni in accomplishing these transformations lies in the ability of Ni to engage in both single- and two-electron processes – cycling through 0, I, II, and III oxidation states. As a result, in addition to canonical two-electron processes (migratory insertion, b-hydride elimination, etc.), fundamental steps such as abstractions, radical captures, and electron transfers are often encountered in Ni catalysis. Ni catalysis has also served as a fruitful platform for the integration of photochemistry in transition- metal catalysis. Recently, our group found that upon irradiation with light, aryl NiII(bpy) complexes can undergo excited-state bond homolysis to generate C(sp2) radicals. These initial stoichiometric studies demonstrate that light energy can be selectively directed to Ni to generate highly reactive intermediates from feedstock chemical precursors. We propose leveraging photoelimination from NiII as a general step to be employed in Ni catalysis. Traditional development of cross-coupling reactions focuses around achieving new outcomes from sequences of known fundamental processes. This proposal is unique as it is based on the development of a new fundamental step for Ni catalysis. Our efforts will capitalize on the interplay between single- and two- electron processes accessible to Ni to address limitations in selectivity and reactivity in the present literature. The research described herein will be comprised of three aims: (1) developing approaches for improving quantum yield of excited-state Ni bond homolysis processes, (2) explore and extend the scope of organic radical centers accessed by photoelimination, and (3) employing photoelimination as fundamental step in Ni catalysis. All three aspects will be explored concurrently and together represent an exciting new direction in the field of first-row transition metal catalysis.