Rhodium‐Catalyzed CH Functionalization to Construct Annulated Molecules

Abstract

Rhodium complexes are exceptionally significant in homogeneous catalysis. The synthetic community observed the rapid burgeoning of such rhodium catalysis since the discovery of Wilkinson's catalyst. Rhodium‐catalyzed tandem C(sp 2 )H/C(sp 3 )H activation and annulation with alkenes, alkynes, arenes, and other reactive partners is a rapidly growing research field to obtain a new class of heterocycles and thereby generates potential biologically active pharmacophores. Superior reactivity of rhodium catalysts is attributed due to its good π‐acceptor character that favors the CH bond activation reaction of substrates containing a wide range of directing groups, such as amides, imines, carboxylic acid (derivatives), benzylic alcohols, ketones, various heterocycles and shown efficiency toward annulation reaction with various befitting partners; even undirected CH activations have also been achieved. This article provides a brief account of the recent developments in rhodium‐catalyzed CH activation/annulation reactions for the synthesis of new classes of fused cyclic scaffolds using suitable coupling partners. For the rhodium‐catalyzed annulation strategies, different types of cyclization reactions, namely, oxidative, redox‐neutral, photoredox, electrochemical reactions are discussed. In addition, the enantioselective synthesis of annulated molecules using chiral rhodium catalysts has also been highlighted.

Recent advances in Rh(i)-catalyzed enantioselective C–H functionalization

“…investigated in asymmetric C-H functionalization with diverse mechanisms. [32][33][34][35][36][37][38][39][40][41][42][43][44][45] From the viewpoint of substrate activation modes, Rh-catalyzed C-H functionalization could be classified into two categories: inner-sphere mechanism and outer-sphere mechanism, which differ in the mechanism of the C-H activation step (Scheme 1). 46,47 Generally, Rh(III)-catalyzed asymmetric C-H functionalization adopts the inner-sphere mechanism by taking advantage of a chiral cyclopentadienyl (Cp) ligand, 35,38,39,[48][49][50][51] whereas Rh(II) usually coordinates with a chiral carboxylic acid to achieve the effect of asymmetric C-H functionalization via the outer-sphere mechanism, 14,[52][53][54] both of which have been well reviewed elsewhere.…”