Deciphering the Mechanistic Dichotomy in the Cyclization of 1-(2-Ethynylphenyl)-3,3-dialkyltriazenes: Competition between Pericyclic and Pseudocoarctate Pathways

The mechanistic aspects of the cyclization of (2-ethynylphenyl)triazenes under both thermal and
copper-mediated conditions are reported. For cyclization to an isoindazole, a carbene mechanistic pathway
is proposed. The carbene intermediate can react with oxygen, dimerize to give an alkene, or be trapped
either intermolecularly (using 2,3-dimethyl-2-butene to generate a cyclopropane) or intramolecularly (using
a biphenyl moiety at the terminus of the acetylene to form a fluorene). Density-functional theory (DFT)
calculations support a pseudocoarctate pathway for this type of cyclization. Thermal cyclization to give a
cinnoline from (2-ethynylphenyl)triazenes is proposed to occur through a pericyclic pathway. DFT calculations
predict a zwitterionic dehydrocinnolinium intermediate that is supported by deuterium trapping studies as
well as cyclizations performed using a 2,2,6,6-tetramethylpiperidine moiety at the 3-position of the triazene.