C–H functionalization reactions are playing an increasing role in the preparation and modification of complex organic molecules, including pharmaceuticals, agrochemicals and polymer precursors. In recent years, there have been many reports of radical C–H functionalization reactions initiated by hydrogen-atom transfer and proceeding via open-shell radical intermediates. These methods introduce strategic opportunities to functionalize C(sp³)–H bonds. Examples include synthetically useful advances in radical-chain reactivity and biomimetic radical-rebound reactions. A growing number of reactions, however, have been found to proceed via radical relay, whereby hydrogen-atom transfer generates a diffusible radical that is functionalized by a separate reagent or catalyst. The latter methods provide the basis for versatile C–H cross-coupling methods with diverse partners. In this Review, we highlight recent examples of radical-chain and radical-rebound methods to provide context for a survey of emerging radical-relay methods, which greatly expand the scope and utility of intermolecular C(sp³)–H functionalization and cross coupling.

C-H官能化反应在复杂有机分子的制备和改性中发挥着越来越重要的作用，包括药物、农用化学品和聚合物前体。近年来，有许多关于由氢原子转移引发并通过开壳层自由基中间体进行的自由基C-H官能化反应的报道。这些方法引入了 C( sp ³ )–H 键功能化的战略机会。例子包括自由基链反应性和仿生自由基反弹反应方面的综合有用进展。然而，越来越多的反应被发现是通过自由基中继进行的，由此氢原子转移产生可扩散的自由基，该自由基被单独的试剂或催化剂功能化。后一种方法为与不同合作伙伴的多功能 C-H 交叉偶联方法提供了基础。在这篇综述中，我们重点介绍了自由基链和自由基反弹方法的最新例子，为新兴自由基接力方法的调查提供背景，这些方法极大地扩展了分子间 C( sp ³ )–H 官能化和交叉耦合的范围和实用性。