Fragile intermediate identification and reactivity elucidation in electrochemical oxidative α-C(sp)-H functionalization of tertiary amines.

The direct α-C(sp)-H functionalization of widely available tertiary amines holds promise for the rapid construction of complex amine architectures. The activation of C(sp)-H bonds through electron transfer and proton transfer by oxidants, photoredox catalysis and electrochemical oxidation have received wide attention recently. In these reactions, the direct capture and identification of the key reactive radical intermediates are technically difficult due to their short life-time. Herein, an online electrochemical mass spectrometry (MS) methodology was utilized to probe the short-lived intermediates in the electrochemical oxidative α-C(sp)-H functionalization of tertiary amines. The resulting electrochemical oxidation intermediates, α-amino radical cation and iminium cation were successfully detected. Further, the α-amino C(sp) radical added to the double bond of a phenyl -styryl sulfone, yielding another C(sp) radical that leads to the final vinylation. Based on the mass spectrometric elucidation of the reactivity of the α-amino radical, a scale-up electrochemical radical vinylation methodology was established, with which a large variety of allylic amines with broad functional group tolerance were synthesized.