Efficient oxidative coupling of amines to imines under natural sunlight using a benzothiadiazole-based molecular photocatalyst†

Abstract

Developing a ‘greener’ avenue for organic synthesis is a key challenge, which must focus on energy efficiency as well as sustainability. Harnessing solar energy to chemical energy is an efficient way to utilize renewable energy resources. Herein, we report a D–A-type (donor–acceptor-type) small organic molecular photocatalyst (SOMP) “Ph-BT-Ph” with benzothiadiazole as the primary photoactive unit for oxidative coupling of amines to synthesize imines. Photocatalyst Ph-BT-Ph is synthesized using a Suzuki–Miyaura coupling reaction and thoroughly characterized by ¹H-NMR, HRMS, and cyclic voltammetry studies. Photoluminescence and lifetime studies of Ph-BT-Ph show a high excited state reduction potential (−1.37 V vs. Ag/AgCl) and longer lifetime (12.64 ns) which make it suitable for photocatalytic organic transformations. The photocatalytic activity of the catalyst has been evaluated on the direct oxidative coupling reaction of amines to synthesize imines in the presence of natural sunlight and O₂ as a green oxidant. Catalyst Ph-BT-Ph exhibits excellent photocatalytic performance under optimal reaction conditions by converting >99% amine to imine with >98% selectivity within 2 hours. This high photocatalytic efficiency has been achieved by purging oxygen only for 2 minutes and without any mechanical energy input (no stirring). Quite a moderate amount of catalyst (0.13 mol%) has been employed which results in a high catalytic turnover frequency of 381 h⁻¹. EPR spectroscopy and theoretical studies are performed to understand the reaction mechanism and to determine the active sites of the catalyst. The Ph-BT-Ph catalyst surpasses the photocatalytic efficiencies of many reported metal-free catalysts for oxidative coupling of amines. Such SOMPs, with easily tunable absorption range and well-defined energy-band positions, offer a new class of metal-free and photoactive catalysts for organic synthesis with outstanding performance under greener reaction conditions.