Thermisches Verhalten von 1,2‐Dipropenylbenzolen

1‐ cis , 2‐ cis ‐Dipropenylbenzene ( cis , cis ‐ 1 ) isomerises thermally at 215–235° with 1st order kinetics to give trans , cis ‐ 1 and vice versa . At equilibrium 89% trans , cis ‐ and 11% cis , cis ‐ 1 are present. It is shown by thermal rearrangement of cis , cis ‐2′, 2″‐d 2 ‐ 1 that the isomerisation is attributable to aromatic [1, 7a]‐sigmatropic H‐shifts. trans , trans ‐ 1 rearranges thermally at 225–245° to yield 2, 3‐dimethyl‐1, 2‐dihydronaphthalene ( 2 ). The formation of 2 can be visualized by disrotatory ring closure followed by an aromatic [1, 5s]‐sigmatropic H‐shift. 2 is also formed when, cis , cis ‐ or trans , cis ‐ 1 are heated for 153 h at 225°. Besides 2 a small amount (3%) of 1‐ethyl‐1, 2‐dihydronaphthalene ( 5 ) is formed. The rearrangement of trans , trans ‐ 1 and trans , trans ‐2′, 2″‐d 2 ‐ 1 shows a secondary isotope effect k H / k D = 0,90.