Visible‐Light‐Promoted CC Bond Cleavage: Photocatalytic Generation of Iminium Ions and Amino Radicals

Abstract

Although the use of photons as reagents has enjoyed an incredibly rich history, the use of photons of red-shifted wavelengths, particularly those extending into what is known as the visible region (i.e., wavelengths ranging from 380 nm to 750 nm), in the context of promoting synthetically attractive organic transformations has attracted considerable attention only very recently. Thanks to the pioneering work from the groups of MacMillan, Yoon, Stephenson, and others, this field has already demonstrated outstanding accomplishments and holds promise for uncovering new catalysis concepts and synthetic applications. Such efforts would most likely be fueled and accelerated in the future by the increasing demand for developing environmentally benign chemical processes with reduced energy consumption, as well as by the latest advancements in commercializing green light sources such as organic light-emitting diodes (OLEDs). Not surprisingly, as organic compounds generally cannot absorb visiblelight, the use of visible light as an effective means to initiate organic reactions must require a photosensitization strategy. Such sensitizers function as useful photocatalysts, the most widely employed example being a ruthenium(II)–polypyridine complex such as [Ru(bpy)3Cl2] (bpy= bipyridine). [8] As photocatalysts with suitable photoredox potentials and their commercial availabilities are both highly limited, it is thus readily recognizable that the success of unlocking the full potential of visible-light photocatalysis for synthetic reactions of broad utility lies in the identification of efficient methodologies for the convenient photocatalytic generation of some of the most versatile reactive species, but with the aid of only a few known sensitizers. Among the synthetic chemists arsenal, iminium ions and radicals clearly stand out as reactive intermediates of major significance. The goal that considerably motivates us is, thus, to devise and possibly implement new visible-lightpromoted photocatalysis strategies which would allow facile production of structurally robust iminium ion or radical intermediates, and preferably, in a one-pot and controllable fashion. We believe that the ability of accessing such intermediates having variable substituents or structurally editable functionalities would ensure the “freedom-of-operation” to be conveniently practiced while merging them into intended catalysis cycles. In this context, it should be highlighted that the groups of Stephenson, Xiao, and Rueping have described significant examples of oxidative C H functionalizations through in situ generation of iminium ions under visible-light photoredox conditions (Scheme 1).