The Synthesis and Photoisomerization of 5‐Methyl‐5‐Vinyl‐1,3‐Cyclopentadiene

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 C s 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‐methylbicyclo[3.2.1]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 S 1 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‐3 H ‐1,2,4‐triazole‐3,5(4 H )‐dione (PTAD).