Synthesis of Cycloheptatrienes, Oxepines, Thiepines, and Silepines: A Comparison between Bronsted Acid and Au-Catalysis

The cyclization of 1-(benzyl-, oxyaryl-, thioaryl-, and silaaryl)-2-ethynylbenzenes under Bronsted acid- and Au(I)-catalysis is described. Bronsted acid catalysis favors without exception the formation of the products derived from the regioselective protonation of the alkyne to generate the most stable vinyl carbocation intermediate. This determines the type of cyclization following this initial step and therefore, the structure of the products. Contrarily, with only a few exceptions, for example, when the substrate contains a terminal alkyne moiety, Au-catalysis preferentially promotes the formation of seven-membered rings via 7-endo-dig cyclization. This reaction pathway is imposed even if a priori the natural electronic polarization in the substrates would suggest another operating mechanism. Making use of these reactivity patterns, a series of structurally differentiated dibenzo-cycloheptatrienes, -oxepines, -thiepines and -silepines have been synthesized with excellent yields and regioselectivities.