Mechanisms of elimination reactions. XXIL. Some cis- and trans-2-phenylcyclohexyl derivatives. The Hoffmann elimination

The products and rates of reaction of the cis and trans isomers of 2-phenylcyclohexyltrimethylammonium ion, 2-pbenylcyclohexyldimethylsulfonium ion and 2-phenyleyclohexyl p-toluenesulfonate with potassium hydroxide in ordinary ethanol have been investigated. The cis and trans isomers of the onium compounds differ in elimination reactivity (to give 1-phenylcyclobexene) by factors of 133 (ammonium) and 383 (sulfonium), trans elimination being favored over cis. A comparison of the rates of reaction with the acyclic (2-phenylethyl) analogs suggests that the cis ammonium phenylcyclohexyl isomer is abnormally unreactive, presumably because of conformational difficulties, while the trans sulfonium. isomer is abnormally reactive. The results are interpreted in terms of a concerted elimination process for the cis isomers (trans coplanar transition state), and some multistage elimination process (dipolar-ion intermediate) for the trans onium. compounds (cis elimination).