Photochemical reaction of 2,4,4,6-tetraaryl-4H-pyrans and -4H-thiopyrans with colour change by a 1,5-electrocyclic reaction. X-Ray molecular structure of 4-methyl-2,3,6-triphenyl-2H-thiopyran

Abstract

2,4,4,6-Tetraphenyl-4H-pyran and -4H-thiopyran exhibited a photochemical colour change in the solid state. On irradiation in solution, 2,4,4,6-tetraphenyl-4H-pyran gave 1,3,5,6-tetraphenyl-2-oxabicyclo[3.1.0]hex-3-ene, while the 4H-thiopyran gave 1,3,5,6-tetraphenyl-2-thiabicyclo-[3.1.0]hex-3-ene as an initial photoproduct, followed by further transformation into 2,3,4,6-tetraphenyl-2H-thiopyran. 4-(4-Bromophenyl)-2,4,6-triphenyl-4H-pyran, 4-(4-methylphenyl)-2,4,6-triphenyl-4H-pyran, and 4-(4-bromophenyl)-2,4,6-triphenyl-4H-thiopyran also showed photochemical colour changes and finally gave both the phenyl-migrated and the (substituted phenyl)-migrated products in comparable yields in each case. 4-Methyl-2,4,6-triphenyl-4H-thiopyran showed no photochemical colour change in the solid state, but in solution it afforded 4-methyl-2,3,6-triphenyl-2H-thiopyran, whose structure was confirmed by X-ray analysis. Based on comparison with the photochemical behaviour of 2,4,4,6-tetraphenyl-1,4-dihydropyridine and 2,2,4,6-tetraphenyl-1,2-dihydro-1,3,5-triazine, a six-membered ylide was proposed as the coloured photochemical intermediate.