Intramolecular alkyne–dithiolium cycloaddition: a joint experimental and DFT mechanistic study

We previously by serendipity discovered a unique intramolecular alkyne–dithiolium cycloaddition, through which phenyldithiafulvenes carrying ortho -alkynyl substituents were directly transformed into complex polycyclic aromatic structures [Wang and Zhao, Org. Biomol. Chem. 2015, 13 , 9575–9579]. In this work, we carried out a joint experimental and theoretical study on the mechanisms of this type of reaction and our results showed that protic acid and oxidant (iodine) are important agents promoting the cycloaddition and subsequent elimination steps. Moreover, the degree of π-conjugation around the alkynyl group was identified to play a key role in driving the cycloaddition. Monoyne without further π-conjugation would react with the dithiolium ring through a concerted 1,3-dipolar cycloaddition transition state, whereas 1,3-butadiyne prefers to undergo a stepwise Prins-type cyclization pathway with a significantly lowered activation energy barrier as a result of the resonance stabilizing effect on the transition state provided by the additional alkynyl unit.