| Abstract: | A versatile scheme for the synthesis of geminally disubstituted cyclopentadienes is used to prepare the title compound 4 . This remarkably stable vinyl-cyclopentadiene, distinguished by its Cs symmetry, undergoes exclusively electrocyclic ring closure upon direct π–π* excitation at 254 nm. The epimeric vinyl-housenes 11 and 12 , which, for geometric reasons, are insensitive to the walk rearrangement, are suggested to be the primary photo-products. One of them, 12 , due to its syn-oriented vinyl group, undergoes spontaneous copy rearrangement to give 2-methylbicyclo3.2.1]hepta-2,6-diene ( 13 ). The other, 11 , having an anti-oriented vinyl group, can only undergo thermal return to the starting material 4 . Whereas no leakage to a 1,5-vinyl migration is discernible for the S1 state of 4 , the benzophenone-sensitized photolysis at 350 nm is shown to be governed by this rearrangement. 1-Methyl-5-vinylcyclopenta-1,3-diene ( 14 ), the unstable primary product of the sensitized photoreaction, is trapped by 4-phenyl-3H-1,2,4-triazole-3,5(4H)-dione (PTAD). |
