Solvolysis and Isomerization of cis ‐ and trans ‐1‐Bromo‐1‐( p ‐anisyl)‐propene (α‐Bromoanethole) Mesomeric Vinyl Cations, Part V

The influence of a β‐methyl group on the reactivity of two stereoisomeric vinyl bromides has been studied. In 80% ethanol cis ‐( 8 ) and trans ‐α‐bromoanethole ( 9 ) undergo first order reactions leading to p ‐methoxypropiophenone ( 15 ), 1‐ethoxy‐1‐( p ‐anisyl)‐propene ( 16 ) and p ‐anisylpropyne ( 12 ). Solvolysis of the cis isomer 8 is accompanied by isomerization to the more stable trans isomer 9 which is approx. eight times less reactive than 8 . Cis ‐ trans isomerization is also observed in nitrobenzene at 150°. These results are in agreement with the unimolecular substitution‐elimination ( S N 1− E 1) mechanism which competes with cis ‐ trans isomerization at the ion pair stage. The solvolysis rate of 9 is slightly lower and that of 8 somewhat higher than the rate of α‐bromo‐ p ‐methoxystyrene ( 3c ). In the absence of other effects a β‐methyl group therefore slightly depresses the ionization rate, presumably by steric hindrance of solvation. These results confirm the negligible polar influence of a β‐methyl substituent on the stability of vinyl cations.