Photochemistry of methylfurans. Selectivities of ring contraction reactions

Vapor phase Hg(3P1) photosensitizations of 2-methylfuran, 3-methylfuran, and 2,4-dimethylfuran resulted mainly in the formation of, respectively, 3-methylcyclopropene, 1-methylcyclopropene, and 1,3-dimethylcyclopropene upon decarbonylation. 3-Methylfuran was also formed from 2-methylfuran. The vapor phase direct photolysis of 2-methylfuran and 3-methylfuran gave mainly the ring open isomers of the methyl cyclopropenes, although small amounts of the cyclic products were found as well. The direct irradiation also mutually transformed 2-methylfuran and 3-methylfuran in low yield. The primary reactions in the Hg(3P1) photosensitizations are the ring contraction to methyl-substituted cyclopropene carboxyaldehydes which subsequently dissociate to methyl-substituted cyclopropenes and carbon monoxide. Selective ring contraction to one of two possible cyclopropene carbonyl compounds in the mercury-sensitized reactions can be explained by assuming that it is directed by initial breakage of the weakest bond in the lowest excited state. Hückel molecular orbitals have been successfully used to predict these weakest bonds.