title: Why alkynyl substituents dramatically accelerate hexadehydro-Diels-Alder (HDDA) reactions: stepwise mechanisms of HDDA cycloadditions.
abstract: 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.
authors.affiliation: Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095, United States.
authors.forename: Yong
authors.initials: Y
authors.lastname: Liang
authors.forename: Xin
authors.initials: X
authors.lastname: Hong
authors.forename: Peiyuan
authors.initials: P
authors.lastname: Yu
authors.forename: K N
authors.initials: KN
authors.lastname: Houk