Olefinic C–H Amidation of Enamines with Sulfonyl Azides via Photocatalytic Radical Chain Mechanism

Abstract

The flourishing development of photocatalysis offers new opportunity for the diversified conversions of organic azides. Herein, the olefinic C–H amidation of enamines with sulfonyl azides is achieved to access 1,2-enediamine derivatives in a mild, efficient, and stereo-selective manner under visible light photocatalysis. A combined experimental and computational study suggests a photocatalytic triplet-triplet energy transfer (EnT) mediated radical chain mechanistic scenario for this reaction, which is unprecedented for the conversion of sulfonyl azides to realize C(sp²)–H amidation reactions. The substrate of β-enamino esters could undergo excitation via triplet-triplet EnT process to reach the excited state, from which the homolytic cleavage of the N–H moiety could follow to initiate an H-atom transfer (HAT) event with sulfonylazides. Thus, two key radicals, α-imino C-radical and sulfonamidyl N-radical, derived from β-enamino esters and sulfonylazides, respectively, could be produced in a photocatalytic loop. Subsequently, the formed sulfonamidyl N-radical may attack the alkenyl moiety of enamines and eventually lead to the desired C–H amidation product and regenerate the key chain-propagating sulfonamidyl N-radical. In addition, the stereo-selectivity of the desired 1,2-enediamines could be tuned to exclusively give the E-isomers under photocatalytic conditions, in which the critical roles of H-bonding interactions in the presence of formic acid are revealed.

Keywords

Visible-light catalysis / Sulfonyl azides / C(sp²)−H amidation / Radical chain mechanism / Energy transfer / DFT calculation / High E-stereoselectivity

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