Gold‐Catalyzed Cyclization of Diynes: Controlling the Mode of 5‐endo versus 6‐endo Cyclization—An Experimental and Theoretical Study by Utilizing Diethynylthiophenes

Herein, a dual‐gold catalyzed cyclization of 3,4‐diethynylthiophenes generating pentalenoc]thiophenes through gold–vinylidenes and C?H bond activation is disclosed. Various new heteroaromatic compounds—substrate classes unexplored to date—exhibiting three five‐membered annulated ring systems could be synthesized in moderate to high yields. By comparison of the solid‐state structures of the corresponding gold–acetylides, it could be demonstrated that the cyclization mode (5‐endo versus 6‐endo) is controlled by the electronic and not steric nature of the diyne backbone. Depending on different backbones, we calculated thermodynamic stabilities and full potential‐energy surfaces giving insight into the crucial dual‐activation cyclization step. In the case of the 3,4‐thiophene backbone, in which the initial cyclization is rate and selectivity determining, two energetically distinct transition states could be localized explaining the observed 5‐endo cyclization mode by classical transition‐state theory. In the case of vinyl and 2,3‐thiophene backbones, the theoretical analysis of the cyclization mode in the bifurcated cyclization area demonstrated that classical transition‐state theory is no longer valid to explain the high experimentally observed selectivity. Herein, for the first time, the influence of the backbone and the aromatic stabilization effect of the 6‐endo product in the crucial cyclization step could be visualized and quantified by calculating and comparing the full potential‐energy surfaces.