64. “The Synthesis and Photoisomerization of 5-Methyl-5-Vinyl-1,3-Cyclopentadiene”
    Burger, U.; Pamingle-Cristoforetti, E.; Bringhen, A. O.
    Helv. Chim. Acta 1988, 71, 389-394.

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 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).

DOI : 10.1002/hlca.19880710212 

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