Radical-mediated oxidative annulations of 1,n-enynes involving C–H functionalization
The 1,n-enyne annulation reaction has emerged as one of the most powerful and straightforward tools to build carbo- and hetero-cyclic frameworks that are found in numerous natural products, pharmaceuticals and functional materials. Although the 1,n-enyne annulation methods have been well documented to date, there is a tremendous challenge with current methodologies for simultaneously incorporating external functional groups into the resulting cyclic systems. Recent advances in the radical-mediated oxidative 1,n-enyne annulation strategy involving C–H functionalization have been proven to be an ideal alternative to overcome these disadvantages. Such radical-mediated oxidative 1,n-enyne annulation can be accomplished by two different C–H functionalization modes: One proceeds through generation of the carbon-centered radicals from C–H bond direct oxidative cleavage and their subsequent addition across the CC bond or CC bond enabling the 1,n-enyne annulation; the other employs the C–H bonds as the radical acceptors to terminate the initial oxidative radical-triggered annulation of 1,n-enyne. In addition, during many annulation processes the inherent C–H bonds of 1,n-enynes were functionalized. Here, we summarize recent progress in radical-mediated oxidative annulations of 1,n-enynes involving two different conceptual C–H functionalization strategies and the inherent C–H functionalization with an emphasis on the scope, limitations and mechanisms of these different reactions.