Cycloallenes, 11. 1‐Phenyl‐1,2‐cyclononadiene: Preparation and Dimerisation

1‐Phenyl‐1,2‐cyclononadiene ( 10 ) was prepared by addition of dibromocarbene to 1‐phenylcyclooctene and treatment of the resulting 9,9‐dibromobicyclo[6.1.0]nonane 7 with methyllithium. In addition to 10 , the 9‐bromo‐9‐methylbicyclononanes cis ‐ and trans ‐ 8 as well as the bicyclopropylidene 9 were formed. The dimerisation of 10 proceeded slowly already at room temperature and gave rise to the 1,2‐bismethylenecyclobutane derivatives cis ‐ and trans ‐ 11 . From the complex mixture that resulted on thermolysis at 200°C of cis ‐ and trans ‐ 11 only their isomer 12 could be isolated. Heating of a benzene solution of tetracyanoethylene and cis ‐ and trans ‐ 11 afforded the Diels‐Alder adduct 13 of cis ‐ 11 , which rearranged to the benzodicyclononene derivative 14 on thermolysis.