The discovery of chromium-photocatalyzed radical cation reactions and exploits in 1,6-enyne cycloisomerization
Stevenson, Susan Marie
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In recent years, the reemergence of photoredox catalysis has inspired exciting new prospects in the field of synthetic organic chemistry. Visible light-activated complexes of rare transition metals Ru and Ir have received considerable attention for their ability to efficiently incite these single-electron processes. In an effort to develop new and more sustainable photocatalysts, we have begun exploring synthetic applications of earth-abundant Cr-based photoredox catalysts. So far, these photooxidizing Cr complexes have been demonstrated to catalyze radical cation Diels-Alder reactions of electron-rich dienophiles. The critical roles of oxygen in this reaction have been investigated, revealing differential behavior between the Cr and Ru photocatalyst systems. Recent research has also uncovered a novel Cr-photocatalyzed radical cation [4+2] cycloaddition of electron-poor dienophiles. Remarkably, this approach provides access to [4+2] adducts of reversed regioselectivity compared to the adducts formed under conventional Diels-Alder conditions. Preliminary mechanistic results point to two competing pathways—a photochemical [2+2] cycloaddition followed by a radical cation vinylcyclobutane rearrangement, and exciplex formation followed by oxidation to generate a radical cation—that both lead to the reversed Diels-Alder products. We have also explored C–C bond migration in the cycloisomerization of oxygen-tethered 1,6-enynes. Under Pt(II) or Ir(I) catalysis, cyclic and acylic alkyl groups were found to undergo 1,2-shifts into metal carbenoids. Interestingly, this process does not appear to be driven by the release of ring strain, and thus provides access to large carbocyclic frameworks. The beneficial effect of CO on the Pt(II) and Ir(I) catalytic systems was also evaluated.