Photoinduced cobalt catalysis for the reductive coupling of pyridines and dienes enabled by paired single-electron transfer

The development of methods to form C–C bonds from readily-available starting materials is essential in driving innovation of functional molecules. In this context, hydrofunctionalisation of feedstock alkenes via catalytic hydrogen atom transfer from an earth-abundant metal can be an effective and efficient approach. However, the range of amenable coupling partners for C–C bond formation is predominantly limited to strongly electrophilic radical traps. Here, we report an alternative approach: simultaneous formation of the key cobalt-hydride intermediate and the persistent radical of easily-synthesised pyridyl phosphonium salts is enabled through paired, photoinduced single-electron transfer. This facilitates selective coupling of dienes and styrenes in a traceless manner at the C4-position of a wide-range of pyridine substrates. The mildness of the method is underscored by its functional group tolerance and demonstrated by applications in late-stage-functionalisation of drugs. Based on a combination of experimental and computational studies, we propose a mechanistic pathway which proceeds through non-reversible hydrogen atom transfer from a cobalt hydride species which is uniquely selective for dienes in the presence of other olefins.