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

Abstract

Boronic acids and their derivatives are some of the most useful reagents in the chemical sciences1, 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-activation1-3. 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(sp2)-H bonds over aliphatic C(sp3)-H bonds4. Here we report a mechanistically distinct, metal-free borylation using hydrogen atom transfer catalysis5, in which homolytic cleavage of C(sp3)-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(sp3)-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.