Hexavalent/Tetravalent Tellurium-Based Chalcogen Bond Catalysis on the Cyclization of 1,6-Diyne: Mechanistic Insights...

Recently, the noncovalent organocatalysis on the alkyne has garnered considerable interest. Hexavalent tellurium-based chalcogen bond catalysis is a novel and efficient catalytic strategy in noncovalent organocatalysis, and tetravalent tellurium-based chalcogen bond catalysis has been effectively used in the metal-chloride bond activation, bromination, aza-Diels-Alder, and Povarov reactions. In this work, we perform an investigation on the intramolecular cyclization of 1,6-diyne catalyzed by hexavalent and tetravalent tellurium-based catalysts. The hexavalent and tetravalent tellurium···π chalcogen bond catalyzed [3 + 2] intramolecular cyclization of 1,6-diyne could be divided into three key steps: (i) 6-endo-dig cyclization, (ii) 5-exo-dig cyclization, (iii) proton transfer, and the 6-endo-dig cyclization is the rate-determining step. In most cases, the hexavalent tellurium-based catalysts exhibit better catalytic activity in contrast to tetravalent analogues. Introducing more electron-deficient aryl moieties into the catalyst could enhance the catalytic activity, such as benzene rings with strong electron-withdrawing groups and N-heterocycles. The catalytic activity of the hexavalent tellurium···π bond is predominantly attributed to orbital interactions, and an increase in the covalent character of the hexavalent tellurium···π interaction could also enhance the catalytic activity. The relationship between energy barriers and the lower σ*(Te-C) energy of catalysts is built, enabling the prediction of catalytic activity.