Unreactive 1-Azadiene and Reactive 2-Azadiene in Diels-Alder Reaction of Pentachloroazacyclopentadienes

Article abstract of DOI:10.1021/jo01291a011

The pentachloroazacyclopentadiene, previously assigned the 1-azadiene structure, a 2H-pyrrole, has been used as a Diels-Alder diene addend.Its structure is now validated by IR and 13C NMR studies, particularly diagnostic are the lanthanide-shifted 13C NMR spectra recorded with Eu(fod)3 and Yb(fod)3, thus eliminating both the 2-azadiene structure of a 3H-pyrrole and the 1H isomer.Also, the nearly superimposable variable-temperature 13C NMR spectra from -30 deg C to 130 deg C denote the predominance of the 1-azadiene form in the temperature range where it is found reactive.However, its reaction with styrene under various conditions does not yield the expected 1-azanorbornene but yields exclusively the 2-azabicyclo<2.2.1>hept-2-ene as shown by X-ray diffraction.Although the adduct shows ν_C=N 1568 cm^-1 and resistance to hydrolysis uncommon for an imidoyl chloride, the latter's presence is indicated by the chlorinated carbon resonances of the adduct.It appears that the 1-azadiene has undergone a chlorine <1,5>-sigmatropic shift to form the 2-azadiene prior to cycloaddition with styrene.The exclusivity of the 2-aza adduct shows that 1-azadiene is not a viable diene addend, but the utility of 2-azadienes in a Diels-Alder reaction as a one-step approach to prepare heterocycles containing an imino group is illustrated.The styrene adduct crystallizes in the monoclinic space group P2₁/n with cell constants a = 7.001 (4) Angstroem, b = 16.184 (6) Angstroem, c = 12.667 (6) Angstroem, β = 101.79 (3) deg, and ρ_calcd = 1.62 g cm^-3 for Z = 4.The structure was refined to a conventional R value of 0.053 for 1749 observed reflections.