1,3,2-Diazaphospholene-Catalyzed Reductive Cyclizations of Organohalides

1,3,2-diazaphospholenes hydrides (DAP-Hs) are highly nucleophilic organic hydrides serving as main-group catalysts for a range of attractive transformations. DAP hydrides can act as stoichiometric hydrogen atom transfer agents in radical reactions. Herein, we report a DAP-catalyzed reductive radical cyclization of a broad range of aryl and alkyl halides under mild conditions. The pivotal DAP catalyst turnover was achieved by a DBU-assisted σ-bond metathesis between the formed DAP halide and HBpin rapidly regenerating DAP H. The transformation is significantly accelerated by irradiation with visible light. Mechanistic investigations indicate that visible light irradiation leads to the formation of DAP dimers which are in equilibrium with the DAP radicals accelerating the cyclization. The direct use of (DAP)2 enabled a catalytic protocol in the absence of light.