Diphenyl‐ and Bis(phenylethynyl)semibullvalenes

The 2,6‐ ( 6b, c ) and 3,7–substituted 1,5–dimethylbicyclo‐[3.3.0]octadienes C 2 ‐ and C s ‐ 9b and c are brominated by N ‐bromosuccinimide in the allylic positions to afford the dibromides 7b , exo,exo ‐ 7c, 10b and c , respectively. An excess of N ‐bromosuccinimide converts 10b into the unsymmetrical tetrabromide 11 . On chromatography over silica gel, exo,exo ‐ 7c is epimerized to endo,endo ‐ 7c , while methanolysis occurs on reversed–phase chromatography yielding the dimethoxy compound 8 . Under ultrasonic irradiation, the zinc–copper couple converts the bromides 7b , exo,exo ‐ 7c, 10c , and 11 into the semibullvalenes 1b, c, 2c , and 12 which are persistent in dilute solutions when atmospheric oxygen is strictly excluded. The UV/Vis spectra of 1b, c, 2c , and 12 have been recorded during reversed–phase HPLC on a Bruker ChromScan detector. Notwithstanding the absence of a classical chromophor, 1c does absorb at no less than 450 nm.