Recent advance in transition-metal-catalyzed oxidant-free 4+1 annulation through C–H bond activation

4+1 annulation based reaction offers a versatile tool for the synthesis of 5-membered carbo/heterocycles. Recent advances of 4+1 annulation through transition-metal-catalyzed C–H bond activation have provided straightforward and widely applicable alternatives to the traditional methods. In particular, the redox-neutral strategies emerged in the past 5 years overcome the inherent disadvantages caused by the external oxidant which are generally required in the early protocols to regenerate the active catalyst, such as limited substrate scope, harsh reaction conditions and generation of stoichiometric by-products. Progress in oxidant-free 4+1 annulations through transition-metal-catalyzed C–H bond activation are discussed in this review until September 2017.