Intramolecular [2 + 2] photocycloaddition, 4. Synthesis of [4.n]cyclophanes by [2 + 2] photocycloaddition and birch reduction. A rare fragmentation of tetramethylene radical anion

✍ Scribed by Nishimura, Jun ;Ohbayashi, Akihiro ;Ueda, Eiji ;Oku, Akira

• Publisher: Wiley (John Wiley & Sons)
• Year: 1988
• Tongue: English
• Weight: 305 KB
• Volume: 121
• Category: Article
• ISSN: 0009-2940
• DOI: 10.1002/cber.19881211120

✦ Synopsis

4.n]Metacyclophanes 6 (n = 2 -6), [4.n]paracyclophanes 7 (n = 3 -6), 4~1biphenylophanes 8 (n = 3, 4), and C2.3.2.41paracyclophane 9 were obtained from appropriate 1,2-ethano-[2.n]cyclophanes 1 -5 by Birch reduction. Some strained cyclophanes, like la, lb, and 2a, gave linear open-chain compounds 10 by fragmentation of the cyclobutane radical anions. The synthetic method and the fate of the cyclobutane radical anion are discussed.

Ring opening of cyclobutane radical anions leads to tetramethylene radical anions. which can subsequently react in situ with a proton donor, accept an electron, and finally be further protonated to give a saturated tetramethylene derivative*'. Although fragmentation of tetramethylene radical cations to an olefin radical cation and an olefin is commonly observed in mass spectrometry, this kind of cleavage is very rarely encountered in solution reactions of radical anions. Recently, we observed this rare type of radical anion fragmentation using highly strained cyclobutane-ring-containing cyclophanes.

Scheme I 2 ' u. R