Radical-Mediated Oxidative Annulations of 1,n-Enynes involving C-H Functionalization
The 1,n-enyne annulation reaction has been emerged as one of the most powerful and straightforward tools to build carbo- and hetero-cyclic frameworks that 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 tremendously challenge with current methodologies for simultaneously incorporating external functional groups into the resulting cyclic systems. Recent advances in radical-mediated oxidative 1,n-enyne annulation strategy involving C-H functionalization have been proven to be ideal alternative to overcome the disadvantages. Such 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 C=C bond or C≡C 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 bond of 1,n-enynes were functionalized. Here, we summarize recent progress in the 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 the mechanisms of these different reactions.