Key role of a π-π complex in diaryl cross-coupling between aryldiazonium salts and arylboronic acids using photosensitizer-free gold/photoredox catalysis

DFT and TD-DFT calculations were performed to better understand the photosensitizer-free visible-light-mediated Au-catalyzed cross-coupling between aryldiazonium salts and arylboronic acids. The pi-pi type complex between the aryldiazonium salt and the arylboronic acid, rather than either the generally accepted gold(i)-aryldiazonium salt complex or the aryldiazonium salt itself, was shown to play the role of a virtual photoinitiator. The oxidation of Au(i) to Au(iii), the key aspect of the dual gold/photoredox catalytic aryl-aryl cross-coupling, occurs via the radical addition of an aryl radical rather than an aryldiazo radical on Au(ii) species. The transmetalation of the arylboronic acid to Au(iii) species was identified as the rate-determining step, and the presence of a tetrafluoroborate anion can assist this process remarkably. The experimentally observed effect of substituents on the aryldiazonium salt and boronic acid on the reactivity was also rationalized.