Why alkynyl substituents dramatically accelerate hexadehydro-Diels-Alder (HDDA) reactions: stepwise mechanisms of HDDA cycloadditions.

The hexadehydro-Diels-Alder (HDDA) reactions between suitably substituted 1,3-diynes and alkynes produce highly reactive benzynes under thermal conditions without catalysts. DFT calculations and distortion/interaction analyses show that, for the activated substrates, the stepwise diradical pathway is more favorable than the concerted [4 + 2] process. One manifestation of this mechanism is that alkynyl substituents dramatically accelerate HDDA reactions, mainly by decreasing the distortion energy required to achieve the diradical transition state.