During the past decade, transition metal-catalyzed dehydrogenative cross-couplings have emerged as an attractive strategy in synthetic chemistry due to its high step- and atom-economy as well as the less functionalized coupling partners. However, such reactions have to always use stoichiometric amount of sacrificial oxidants such as peroxides, high-valent metals (Cu salts, Ag salts, etc.), or iodine(III) oxidants, thereby leading to possible generation of toxic wastes and making the process less desirable from a green chemistry perspective. The recently developed photocatalytic CCHE (cross-coupling hydrogen-evolution) reactions are a conceptually new type of reactions enabled by combination of photo-redox catalysis and proton reduction catalysis, wherein the photocatalyst uses light energy as the driving force for the cross-coupling and the hydrogen evolution catalyst may capture electrons and protons from the substrates or reaction intermediates to produce molecular hydrogen (H2). Thus, without use of any sacrificial oxidant and under mild conditions, the dual catalyst system may afford cross-coupling products with excellent yields and an equivalent amount of H-2 as the sole byproduct. This kind of cross-coupling delivers a greener synthetic strategy and is particularly useful for reactions that involve species sensitive to traditional oxidants. In CCHE reactions, the raw materials are directly converted into products and hydrogen, the reactions are highly atom economy, environmentally friendly, and have attractive potential industrial application prospects. In this review, recent dramatic developments of photocatalytic and electrochemical CCHE reactions are discussed via the most prominent mechanistic pathways, the types of C-C bond, C-X (heteroatom) bond, or X-X bond formations and specific reaction classes.

中文翻译：

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交叉偶联析氢反应

在过去的十年中，过渡金属催化的脱氢交叉偶联由于其高阶跃和原子经济性以及较少官能化的偶联伙伴而成为合成化学中一种有吸引力的策略。然而，此类反应必须始终使用化学计量的牺牲氧化剂，例如过氧化物、高价金属（Cu 盐、Ag 盐等）或碘 (III) 氧化剂，从而导致可能产生有毒废物并使从绿色化学的角度来看，这个过程不太理想。最近开发的光催化 CCHE（交叉偶联析氢）反应是一种概念上的新型反应，由光氧化还原催化和质子还原催化相结合，其中光催化剂使用光能作为交叉偶联的驱动力，析氢催化剂可以从底物或反应中间体捕获电子和质子以产生分子氢(H2)。因此，在不使用任何牺牲氧化剂和温和条件下，双催化剂体系可以提供具有优异产率和等量的 H-2 作为唯一副产物的交叉偶联产物。这种交叉偶联提供了一种更环保的合成策略，对于涉及对传统氧化剂敏感的物种的反应特别有用。在CCHE反应中，原料直接转化为产物和氢气，反应原子经济性高，环境友好，具有诱人的潜在工业应用前景。在这次审查中，