On the Mechanism of the Thermal Conversion of Cyclopropenyl-Substituted Oxazolinones to Pyridines

Article abstract of DOI:10.1021/ja00316a042

Thermolysis or photolysis of a sample of a 3-cyclopropenyl-substituted 2H-azirine produced 2-methyl-3,4,5,6-tetraphenylpyridine in high yield.The reaction can best be rationalized by a mechanism involving formation of a nitrile ylide intermediate followed by intramolecular dipolar cycloaddition to give an azabenzvalene, which subsequently rearranges to the pyridine.The thermal chemistry of a series of cyclopropenyl-substituted oxazolinones was also investigated.These oxazolinones undergo a thermally induced 1,3-dipolar cycloreversion reaction with elimination ofcarbon dioxide to generate a nitrile ylide intermediate adjacent to the cyclopropene ring.This dipole can be trapped when the thermolysis of the oxazolinone was carried out in the presence of a reactive dipolarophile.Heating a sample of 2-phenyl-4-methyl-4-(1-methyl-2,3-diphenyl-2-cyclopropen-1-yl)-Δ²-oxazolin-5-one at 150 deg C for 24 h afforded a mixture of 2,3-dimethyltriphenylpyridine (45percent), 2,4-dimethyltriphenylpyridine (20percent), and 2,5-dimethyltriphenylpyridine (35percent).The formation of these products is proposed to involve a stepwise cycloaddition of the initially generated nitrile ylide to produce a bicyclobutyl zwitterion which can either collapse to give an azabenzvalene or undergo rearrangement to a cyclobutenyl cation. This latter species closes to produce two different aza Dewar benzenes.Reorganization of the azabenzvalene and aza Dewar benzenes gives rise to the observed pyridines.Alternate mechanisms based on a concerted intramolecular cycloaddition reaction of the nitrile ylide do not account for the observed product ratios.