C−H Activation of Unbiased C(sp³)−H Bonds: Gold(I)‐Catalyzed Cycloisomerization of 1‐Bromoalkynes**

Abstract Selective functionalization of non‐activated C(sp 3 )−H bonds is a major challenge in chemistry, so functional groups are often used to enhance reactivity. Here, we present a gold(I)‐catalyzed C(sp 3 )−H activation of 1‐bromoalkynes without any sort of electronic, or conformational bias. The reaction proceeds regiospecifically and stereospecifically to the corresponding bromocyclopentene derivatives. The latter can be readily modified, comprising an excellent library of diverse 3D scaffolds for medicinal chemistry. In addition, a mechanistic study has shown that the reaction proceeds via a so far unknown mechanism: a concerted [1,5]‐H shift / C−C bond formation involving a gold‐stabilized vinylcation‐like transition state.