Dehydrogenation/(3+2) Cycloaddition of Saturated Aza-Heterocycles via Merging Organic Photoredox and Lewis Acid Catalysis

Abstract

Herein, we report a photoinduced dehydrogenation/ (3+2) cycloaddition reaction by merging organic photoredox and Lewis acid catalysis, providing a straightforward and efficient approach for directly installing a benzofuran skeleton on the saturated aza-heterocycles. In this protocol, we also describe a novel organic photocatalyst (t-Bu-DCQ) with the advantages of a wider redox potential, easy synthesis, and a low price. Furthermore, the stepwise activation mechanism of dual C(sp 3 )−H bonds was demonstrated by a series of experimental and computational studies.C ycloaddition reactions, such as Diels−Alder reaction, [3+2] cycloaddition, and [2+2] cycloaddition, are among the most significant tools for constructing various ring skeletons in the synthesis of natural products, biorelevant compounds, and functional materials (Scheme 1a, left). 1

Organic photoredox catalyzed C(sp3)–H functionalization of saturated aza-heterocycles via a cross-dehydrogenative coupling reaction

“…Based on the mechanistic experiments and our previous investigation, 28–30,38–41 we described a proposed catalytic cycle for this reaction. As shown in Scheme 3, the photocatalyst t Bu-DCQ was irradiated with visible light to deliver the excited photocatalyst *[ t Bu-DCQ] (*[ t Bu-DCQ]/ t Bu-DCQ = 1.38 V), which accepted an electron from cyclic amine 1 ( E ox = 0.74 V versus SCE in CH 3 CN) to form the reduced photocatalyst [ t Bu-DCQ ]˙ − ( III ) and the corresponding amine radical cation IV .…”

Transition‐Metal‐Free Methods for the Remote C−H Bond Functionalization of Cyclic Amines

“…In 2021, Xu, Xu and co‐workers developed an organic photoredox and Lewis acid co‐catalyzed dehydrogenative cycloaddition reaction between N ‐aryl cyclic amines and p ‐quinone esters such as 83 (Scheme 30). [63] The catalyst system of this reaction is composed of a new organic photoredox catalyst t ‐Bu‐DCQ, magnesium triflate (Mg(OTf) 2 ) and phenylboronic acid which serves as a ligand for magnesium. The reaction generates polycyclic N,O ‐acetal products in moderate to excellent yields and with excellent diastereoselectivities.…”

Advanced Synthesis Using Photocatalysis Involved Dual Catalytic System

“…In 2021, Xu et al. described a novel dehydrogenation/[3+2] cycloaddition reaction via merging organic photocatalyst, magnesium triflate and boronic acid, which enabled the construction of benzofuran‐fused N‐heterocycles more directly and economically [64] . Excitingly, in this protocol, they developed a new organic photoredox catalyst t‐Bu‐DCQ, which featured a wider redox potential than classical photoredox catalysts.…”