Oxidative [4+2] annulation of styrenes with alkynes under external-oxidant-free conditions

Abstract

The sequenced Diels–Alder/oxidation reaction represents a powerful route for the construction of aromatic compounds in organic synthesis. The oxidative Diels–Alder reaction with H2 evolution would be a more ideal approach that can avoid the additional oxidation procedure and stoichiometric oxidant. Herein, an oxidative [4 + 2] annulation reaction of styrene derivatives with electron-rich dienophiles accompanying the H2 generation has been developed by using the synergistic merger of photoredox and cobaloxime catalyst. With respect to atom and step-economy ideals, this dual catalytic system enables the formation of high-value molecules from feedstock chemicals in a single step under room temperature.

Electrooxidation enables highly regioselective dearomative annulation of indole and benzofuran derivatives

“…Fascinated by the distinctive reactivity of indole radical cation, we anticipate that the reports for adequately studied alkene radical cation may give a hand to the development of dehydrogenative [4 + 2] annulation of indoles 65,66 . For example, in 2018, our group has demonstrated a dehydrogenative [4 + 2] annulation of styrenes with alkynes to afford a variety of six-membered aromatic rings 67 . This reaction went through the nucleophilic attack of dienophile to alkene radical cation and second nucleophilic attack from arene to in situ generated alkenyl carbon cation to afford the cyclic intermediates.…”

Design and application of aminoacridinium organophotoredox catalysts

“…Extending the application of their dual catalytic cobalt/ acridinium salt process, the same group also performed a photooxidative [4+2] annulation between aromatic ketimine derivatives and styrenes (Scheme 12). 39,40 The SET oxidation of the alkene substrate by the excited Mes-Acr-Me + catalyst thereby produces a radical cation that reacts with the nucleophilic aromatic ketimine to give a stable benzylic radical intermediate. A subsequent sequence consisting of a cyclization, oxidation and deprotonation allows to obtain highly substituted 3,4-dihydroisoquinolines with excellent trans diastereoselectivity.…”

Cross-Coupling Hydrogen Evolution Reactions

“…吴骊珠等 [14] 发现光催化 剂三(2-苯基吡啶)合铱和产氢催化剂钴肟配合物联用可 实现 N-芳基烯胺的分子内环化反应, 生成吲哚类化合 物并放出氢气. 雷爱文等利用光催化剂吖啶染料 Acr ＋ -Mes 和产氢催化剂钴肟配合物的双催化体系, 实现了苯 乙烯类化合物与富电子芳环 [15] 以及炔烃衍生物 [16] 分子 间的放氢交叉偶联反应. 在这些偶联反应中, 由于原料 或产物对氧化剂不稳定, 氧化剂的使用会导致副产物增 多甚至预期反应无法发生.…”