Pseudopericyclic 1,5- versus
Pericyclic 1,4- and 1,6-Electrocyclization
in Electron-Poor 4‑Aryl-2-azabuta-1,3-dienes: Indole Synthesis
from 2<i>H</i>‑Azirines and Diazo Compounds
Transformations
of 2-azabuta-1,3-dienes, formed in Rh<sub>2</sub>(OAc)<sub>4</sub>-catalyzed reactions of diazo carbonyl compounds
with 2<i>H</i>-azirines, dramatically depend on the nature
of substituents. 4,4-Diphenyl-2-azabuta-1,3-dienes with two electron-acceptor
substituents at C<sup>1</sup> undergo thermal 1,5-cyclization to give
indoles in good yields. The increase in electron-withdrawing ability
of C<sup>1</sup>-substituents facilitates the reaction that proceeds
via pseudopericyclic 1,5-electrocyclization of 2-azabutadiene into
7a<i>H</i>-indolium ylide followed by prototropic shift.
3,4-Diphenyl-2-azabuta-1,3-dienes, resulting from reaction of 2,3-diphenyl-2<i>H</i>-azirine and diazo compounds, do not produce indoles via
1,5-cyclization due to the <i>trans</i>-configuration of
the 4-Ph-group and the nitrogen, but undergo 1,4-cyclization to 2,3-dihydroazetes.
1,6-Cyclization into 2<i>H</i>-1,4-oxazines with participation
of the oxygen of ester or amide group at C<sup>1</sup> of corresponding
2-azabuta-1,3-dienes does not take place due to kinetic and thermodynamic
reasons. Instead of this, 1,6-electrocyclization with participation
of phenyl substituent at C<sup>4</sup> of the 2-azabuta-1,3-dienes,
providing isoquinoline derivatives, can occur at elevated temperatures.
The DFT-calculations (mPWB1K/6-31+G(d,p)) confirm the dependence of
2-azabuta-1,3-diene transformation type on the nature of substituents
