textA series of 1,2-dialkynylimidazoles (1,2-DAIs) intended as azaenediynes wherein
the nitrogen is part of a heterocyclic imidazole ring were synthesized and their chemistry
studied. The key N-alkynyl group was introduced using iodonium salts followed by a
palladium coupling to add the 2-alkynyl side chain. Upon thermolysis in neat 1,4-
cyclohexadiene, all of the 1,2-DAIs underwent an unprecedented molecular
rearrangement to cyclopentapyrazine carbenes. These carbenes participated in hydrogen
atom abstraction, cyclopropanation and C-H insertion. In benzene, all of the
cyclopentapyrazine carbenes react with benzene, by presumably first forming a
ix
norcaradiene intermediate that rearranges to the phenyl adduct without any detection of
cycloheptatriene intermediates. The carbene was also found to perform intramolecular
olefin addition and C-H insertion forming interesting three- and four-membered ring
pyrazines, yields of which were were maximized upon thermolysis in hexafluorobenzene.
The rearrangement mechanisms of 1,2-DAIs were transformed in halogenated solvents.
When compound 2.12 with an N-phyenylethynyl group was thermolyzed in
chlorobenzene, a chloroimidazopyridine product derived from the 2,3-didehydropyridine
intermediate was isolated. N-Ethynylated 1,2-DAIs, however, underwent a rearrangement
to chlorinated imidazo[1,2-a]pyridines via a electrocyclic rearrangement of an α-
chloroenamine intermediate. This rearrangement also occurred when an amine was the
nucleophile while compounds in which the 2-alkynyl side chain is sterically demanding
do not cyclize. Inclusion of the ynamine into an imidazole ring reversed the electronics of
the N-alkynyl group in a few cases, with nucleophilic attack occurring at the β carbon of
the ynamine.Pharmac