Fragment Coupling with Tertiary Radicals Generated by Visible-Light Photocatalysis

Convergent synthesis strategies in which a target molecule is prepared by a branched approach wherein two or more complex fragments are combined at a late stage are almost always preferred over a linear approach in which the overall yield of the target molecule is eroded by the efficiency of each successive step in the sequence. As a result, bimolecular reactions that are able to combine complex fragments in good yield and, where important, with high stereocontrol are essential for implementing convergent synthetic strategies. Although intramolecular reactions of carbon radicals have long been exploited to assemble polycyclic ring systems, bimolecular coupling reactions of structurally complex carbon radicals have rarely been employed to combine elaborate fragments in the synthesis of structurally intricate molecules. We highlight in this Account recent discoveries from our laboratories that demonstrate that bimolecular reactions of structurally elaborate tertiary carbon radicals and electron-deficient alkenes can unite complex fragments in high yield using nearly equimolar amounts of the two coupling partners. Our discussion begins by considering several aspects of the bimolecular addition of tertiary carbon radicals to electron-deficient alkenes that commend these transformations for the union of structurally complex, sterically bulky fragments. We then discuss how in the context of synthesizing rearranged spongian diterpenoids we became aware of the exceptional utility of coupling reactions of alkenes and tertiary carbon radicals to unite structurally complex synthetic intermediates. Our initial investigations exploit the early report of Okada that N-(acyloxy)phthalimides reductively fragment at room temperature in the presence of visible light and catalytic amounts of the photocatalyst Ru(bpy)3Cl2 to form carbon radicals that react with alkenes. We show that this reaction of a tertiary radical precursor and an enone can combine two elaborate enantioenriched fragments in good yield with the formation of new quaternary and secondary stereocenters. As a result of the ready availability of tertiary alcohols, we describe two methods that were developed, one in collaboration with the MacMillan group, to generate tertiary radicals from tertiary alcohols. In the method that will be preferred in most instances, the tertiary alcohol is esterified in high yield to give a tert-alkyl hemioxalate salt, which-without purification-reacts with electron-deficient alkenes in the presence of visible light and an Ir(III) photocatalyst to give coupled products having a newly formed quaternary carbon in high yield. Hemioxalate salts containing Li, Na, K, and Cs countercations can be employed in this reaction, whose only other product is CO2. These reactions are carried out using nearly equimolar amounts of the addends, making them ideal for coupling of complex fragments at the late stage in a synthetic sequence. The attractive attributes of the fragment-coupling chemistry that we discuss in this Account are illustrated by an enantioselective total synthesis of a tricyclic trans-clerodane diterpenoid in eight steps and 34% overall yield from commercially available precursors. We anticipate that bimolecular reactions of carbon radicals will be increasingly used for fragment coupling in the future.