The thermally induced rearrangements of 2-vinyloxirane

The kinetics of the gas phase thermolysis of 2-vinyloxirane (4) have been studied over the temperature range 270–310 °C. The racemization of chiral 4 occurs six times faster than the structural isomerization to 2,3-dihydrofuran, (E)- and (Z)-2-butenal, and 3-butenal. The butenals undergo a slow thermolysis to propene and carbon monoxide. cis-Deuterio- and trans-3-deuterio-vinyloxirane have been synthesized and their interconversion is slow. Deuterium kinetic isotope effects on mono- and dideuterio-4 suggest that for the formation of the butenals the rate determining step involves rupture of the oxirane C—O bond. The dihydrofuran is produced by thermolysis of the oxirane C—C bond. The preferred mechanistic interpretation is that a carbon–oxygen diradical serves as an intermediate for butenal formation, and that a carbonyl-ylide is involved in the formation of the dihydrofuran.The relative rates, at 307.4 °C, of cis–trans-5-isomerization:dihydrofuran formation:racemization: butenal formation for 3-deuterio-2-vinyloxirane are 1.0:0.88:40.2:5.94, respectively.