Metal-free photoinduced C(sp3)-H borylation of alkanes

Boronic acids and their derivatives are some of the most useful reagents in the chemical sciences(1), with applications spanning pharmaceuticals, agrochemicals and functional materials. Catalytic C-H borylation is a powerful method for introducing these and other boron groups into organic molecules because it can be used to directly functionalize C-H bonds of feedstock chemicals without the need for substrate pre-activation(1-3). These reactions have traditionally relied on precious-metal catalysts for C-H bond cleavage and, as a result, display high selectivity for borylation of aromatic C(sp(2))-H bonds over aliphatic C(sp(3))-H bonds(4). Here we report a mechanistically distinct, metal-free borylation using hydrogen atom transfer catalysis(5), in which homolytic cleavage of C(sp(3))-H bonds produces alkyl radicals that are borylated by direct reaction with a diboron reagent. The reaction proceeds by violet-light photoinduced electron transfer between an N-alkoxyphthalimide-based oxidant and a chloride hydrogen atom transfer catalyst. Unusually, stronger methyl C-H bonds are borylated preferentially over weaker secondary, tertiary and even benzylic C-H bonds. Mechanistic studies indicate that the high methyl selectivity is a result of the formation of a chlorine radical-boron `ate' complex that selectively cleaves sterically unhindered C-H bonds. By using a photoinduced hydrogen atom transfer strategy, this metal-free C(sp(3))-H borylation enables unreactive alkanes to be transformed into valuable organoboron reagents under mild conditions and with selectivities that contrast with those of established metal-catalysed protocols.