1 research outputs found
Formation of (Aza)fulvenallene, Cyanocyclopentadiene, and (Aza)fluorenes in the Thermal Rearrangements of Indazoles, Azaindazoles, and Homoquinolinic Anhydride
Flash vacuum pyrolysis (FVP) of pyrazoles and indazoles
constitutes
a valuable route to carbenes and nitrenes. In this study, we employed
M062X and CCSD(T) calculations to provide a mechanistic rationale
for the formation of fulvenallene and fluorenes from indazoles and
the corresponding formation of azafulvenallene 15, cyanocyclopentadiene 19, and azafluorenes, e.g. 45, from azaindazoles,
e.g. 12, and from homoquinolinic anhydride. The results
reveal the importance of initial tautomerization in the pyrazole moiety
of 7-azaindazole 12, which drives the mechanism toward
2-diazo-3-methylene-2,3-dihydropyridine 29 and hence
3-methylene-2,3-dihydropyridin-2-ylidene 26, followed
by Wolff-type ring contraction to 1-azafulvenallene 15. This path has a calculated activation energy ∼10 kcal/mol
lower than that for an alternate route involving ring opening to 3-diazomethylpyridine,
dediazotization, and rearrangement of 3-pyridylcarbene to azacycloheptatetraene
and phenylnitrene 24. FVP of 2,5-diphenyltetrazoles and
phenyl(pyridyl)tetrazoles leads to nitrile imines, which cyclize to
3-phenylindazoles and -azaindazoles. Nitrogen elimination from these
(aza) indazoles results in the formation of (aza) fluorenes, for which
two alternate mechanisms are described: route A by rearrangement of
(aza) indazoles to diazo(aza)cyclohexadienes and (aza)cyclohexadienylidenes
and route B by rearrangement to diaryldiazomethanes and diarylcarbenes.
Both paths are energetically feasible, but path A is preferred and
corresponds to the azafluorenes obtained experimentally