Photochemistry of 2-methylene-6,7-benzobicyclo[3.2.2]nona-3,6,8-triene. Efficient triplet DI-π-methane rearrangement of a “free-rotor” polyene.

✍ Scribed by Zeev Goldschmidt; Andrew S. Kende

• Publisher: Elsevier Science
• Year: 1971
• Tongue: French
• Weight: 253 KB
• Volume: 12
• Category: Article
• ISSN: 0040-4039
• DOI: 10.1016/s0040-4039(01)97547-1

✦ Synopsis

Recent comparisons between the photochemistry of cyclic unsaturated ketones and their exomethylene counterparts have revealed characteristic differences in reactivity between n-r* and r -K * excited states of conjugated systems. 1 In contrast to the typical photonarrangement of 2,5-cyclohexadienones originating from their n-r l triplet states, the corresponding hydrocarbons possessing an exocyclic methylene group (e.g. 1) undergo the di-n -methane rearrange-ment2 preferentially from the arrangement of rigid bicyclic strong preference for singlet singlet excited state. Although efficient di-n-methane repolyene hydrocarbons in the triplet state has been observed, the rearrangement in 1. and in acyclic polyenes has led Swenton 3a and Zimmerman 3b to postulate that, in the latter compounds, free rotation about bonds of low n-order in the excited triplet may provide an energy dissipating mechanism pracluding efficient triplet rearrangement. 0 &% i 'R' 1 2

As an extension of earlier studies4 in the photochemistry of the bicyclo[3.2.2]trienone system zwe now report the highly regiospecific di-lr -methane rearrangement of 2-methylene-6.7-