Photoisomerization of dichlorocarbene adducts of 2,4-cyclohexadienones

✍ Scribed by Harold Hart; Moshe Weiner

• Book ID: 104234173
• Publisher: Elsevier Science
• Year: 1981
• Tongue: French
• Weight: 184 KB
• Volume: 22
• Category: Article
• ISSN: 0040-4039
• DOI: 10.1016/s0040-4039(01)81841-4

✦ Synopsis

Irradiation of the nn* band of the dichlorocyclopropyl enones J, and J_ resulted in ring expansion and 1,2-chlorine migration to give x and $, respectively, whereas the dihydrogen analog a gave the vinylcyclopropane-cyclopentene rearrangement product x. The photoisomerization of ally1 halides to cyclopropyl halides has attracted attention because of the mechanistic questions it raises, particularly with regard to radical vis-a-vis ionic pathways. l-3 We wish to describe here examples of the reverse process, photoisomerization of a cyclopropyl chloride to an ally1 chloride. We compare our results with those reported earlier for a related cyclopropyl system lacking the halogen substituents. 4 Addition of dichlorocarbene to hexamethyl-2,4-cyclohexadienone5 under Makosza conditions6 gave adduct i in good yield. 798 Adduct ,J was irradiated in chloroform through a uranium glass filter (A>340 nm) to give, after vpc collection of the product, 85% of J8 and 14% of ,$' (Scheme I). NMR examination of the crude photolysate showed no 2 or ,$ (no vinyl protons), but 1 Cl / hv >270 nm c1 CHs 2 (85%) 5115