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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.