A comparative study on photosensitized oxidation of trans-2-vinylthiophenes with trans-3-vinylthiophenes

The photosensitized oxidation of trans-2-styrylthiophenes (trans-2-ST), trans-3-styrylthiophenes (trans-3-ST) and trans-1,2-di(2-thienyl)ethylene (trans-2-TE) were investigated using the sensitizers tetraphenylporphyrin (TPP) in a non-polar solvent and 9,10-dicyanoanthracene (DCA) in a polar solvent. The oxidation pathway is dependent on the position of the vinyl substituent in the thiophene ring. While trans-2-ST and trans-2-TE only undergo photooxidation via the singlet oxygen pathway to yield [4+2] and [2+2] products, the photosensitized oxidation of trans-3-ST proceeds exclusively via the superoxide anion pathway to produce the thiophenecarboxaldehyde and benzaldehydes. All of the above substrates may undergo auto-photooxidation via a charge-transfer complex of the substrate and oxygen.