Diastereoselective metal-free photoredox cyclopropanation of alkenes with dibromomethane

A metal-free photoredox cyclopropanation using commercial dibromomethane as a direct methylene source is reported. Enabled by 4CzIPN and triethylamine under visible light, this protocol accommodates diverse alkenes with good functional-group tolerance and excellent diastereoselectivity for 1,2-disubstituted substrates. Mechanistic studies support an amine-mediated halogen-atom transfer (XAT) and radical-polar crossover pathway.