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Pseudopericyclic 1,5- versus Pericyclic 1,4- and 1,6-Electrocyclization in Electron-Poor 4-Aryl-2-azabuta-1,3-dienes: Indole Synthesis from 2H-Azirines and Diazo Compounds

Novikov, Mikhail S., Khlebnikov, Alexander F., Rostovskii, Nikolai V., Tcyrulnikov, Sergei, Suhanova, Anna A., Zavyalov, Kirill V., Yufit, Dmitry S.
Journal of organic chemistry 2015 v.80 no.1 pp. 18-29
carbonyl compounds, chemical reactions, chemical structure, diazo compounds, indoles, nitrogen, organic chemistry, oxygen, temperature, thermodynamics
Transformations of 2-azabuta-1,3-dienes, formed in Rh₂(OAc)₄-catalyzed reactions of diazo carbonyl compounds with 2H-azirines, dramatically depend on the nature of substituents. 4,4-Diphenyl-2-azabuta-1,3-dienes with two electron-acceptor substituents at C¹ undergo thermal 1,5-cyclization to give indoles in good yields. The increase in electron-withdrawing ability of C¹-substituents facilitates the reaction that proceeds via pseudopericyclic 1,5-electrocyclization of 2-azabutadiene into 7aH-indolium ylide followed by prototropic shift. 3,4-Diphenyl-2-azabuta-1,3-dienes, resulting from reaction of 2,3-diphenyl-2H-azirine and diazo compounds, do not produce indoles via 1,5-cyclization due to the trans-configuration of the 4-Ph-group and the nitrogen, but undergo 1,4-cyclization to 2,3-dihydroazetes. 1,6-Cyclization into 2H-1,4-oxazines with participation of the oxygen of ester or amide group at C¹ 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 C⁴ 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.