The Prowess of Photogenerated Amine Radical Cations in Cascade Reactions: From Carbocycles to Heterocycles
by Morris, Scott A.; Wang, Jiang; Zheng, Nan
Cascade reactions represent a class of ideal organic reactions because they empower efficiency, elegance, and novelty. However, development of cascade reactions remains a daunting task for synthetic chemists. Radicals are known to be well suited for cascade reactions. Compared with widely used carbon-based radicals, nitrogen-based radicals, such as neutral aminyl radicals and protonated aminyl radicals (amine radical cations), are underutilized, although they are behind some notable synthetic methods such as the Hofmann-Loffler-Freytag reaction. The constraint on their usage is generally attributed to the limited number of available stable precursors. Since amine radical cations offer increased reactivity and selectivity in chemical transformations compared with neutral aminyl radicals, their generation is of utmost importance. Recently, a surge of reports has been revealed using visible light photoredox catalysis. It has been demonstrated that amines can act as an electron donor in a reductive quenching cycle while the amine itself is oxidized to the amine radical cation. Although a number of methods exist to generate amine radical cations, the photochemical formation of these species offers many practical advantages. In this Account, we discuss our journey to the development of annulation reactions with various p-bonds and electrophilic addition reactions to alkenes using photogenerated amine radical cations. Various carbocycles and heterocycles are produced by these reactions. In our annulation work, we first show that single electron photooxidation of cyclopropylanilines to the amine radical cations triggers ring opening of the strained carbocycle, producing distonic radical cations. These odd-electron species are shown to react with alkenes and alkynes to yield the corresponding cyclopentanes and cyclopentenes in an overall redox neutral process. Further development of this annulation reaction allows us to achieve the [4 + 2] annulation of cyclobutylanilines with alkynes. In our work on electrophilic addition reactions to alkenes, we reveal that photogenerated amine radical cations are capable of undergoing the electrophilic addition reactions to alkenes to form a variety of indoles and indolines. This chemistry represents a rare oxidative C-N bond-forming reaction using visible light. Conclusions drawn from observational results and proposed mechanisms are outlined in this Account. Additionally, open discussion of our successes and deficiencies in our experiences will give readers helpful insights as to how these species tend to react. The overall utility of photogenerated amine radical cations has yet to reach its full potential. With our current results, we anticipate more new transformations can still be derived from the ring opening processes of cyclopropylanilines and cyclobutylanilines under visible light photocatalysis. Additionally, since utilizing photogenerated amine radical cations in C-N bond-forming reactions has practically been absent in literature, we are confident more new reactions have yet been exploited.