Spin States Matter─from Fundamentals toward Synthetic Methodology Development and Drug Discovery

ConspectusThe potent reactivity of carbenes and nitrenes has been traditionally harnessed by the employment of a transition-metal catalyst in which the reactivity of the metal carbene/nitrene intermediates can be controlled via the judicious tuning of the metal catalyst. In recent years, progress made in this research area has unveiled novel strategies to directly access free carbenes or nitrenes under visible-light-mediated conditions without the necessity of a metal catalyst for stabilization of the carbene/nitrene intermediate. Such photochemical approaches present new opportunities to leverage orthogonal reactions with classic metal-catalyzed transformations.In this Account, we describe the major contributions from our group over the past years pushing the boundaries of light-mediated carbene and nitrene transfer reactions. In the first section, the development from purely singlet carbene chemistry toward methods that allow access to triplet carbene intermediates will be dissected. We describe how the triplet spin state of reagents provides a rich array of novel synthetic methods that build on the fundamentals of spin conservation. We lay out the different strategies in accessing the triplet spin state of carbenes (i.e., via electronic stabilization, via triplet sensitization with suitable photocatalysts, or via exploitation of geometric features of these intermediates), followed by an analysis of how the triplet spin state can be employed to leverage reactions distinct to the classic singlet carbene chemistry.The second part focuses on free nitrene intermediates, whereby both photochemical and photocatalytic strategies are analyzed and compared. We initiate with a discussion of the reactivity of iminoiodinanes as nitrene precursors in the presence of a photocatalyst or under photochemical conditions and how these two approaches result in fundamentally distinct nitrogen-based intermediates. While a nitrene radical anion is formed under photocatalytic conditions, triplet nitrene is generated under photochemical conditions. We commence with an outline of the basic reactivity of nitrene transfer reactions under both conditions, with a focus on the reaction with substrates containing double bonds. Finally, the latest developments in advanced cycloaddition chemistry beyond classic aziridination reactions are examined, with a special emphasis on the relay of the triplet nitrene reactivity to enable a Pauson-Khand-like (2 + 2 + 1) cycloaddition reaction that offers convenient access to high value bioisosteres in drug discovery.The work from our group on spin-dependent reactivities offers insight into important fundamentals in synthesis, where the spin state of the reactive intermediate will dictate the reaction outcome. We hope this may inspire others to widen the scope of applications of light-mediated carbene and/or nitrene transfer reactions, and furthermore, we anticipate that these understandings may also enable the development of advanced catalytic systems featuring triplet metal carbene/nitrene intermediates.