Base-catalysed and solvolytic eliminations

A number of observations, dependent on product analysis, are available for the base-catalysed reactions of a number of 3-phenylpropyl derivatives. These substrates can undergo competitive beta- and gamma-eliminations, viz- 

Using proton magnetic resonance spectroscopy, Cristol and Jarvis showed that chloro-sulphone (238) epimerises in ethanol containing sodium ethoxide at room temperature, the equilibrium slightly favouring (239). When the solution is heated under reflux, a rapid gamma-elimination occurs to yield (240). 

As epimerisation occurs much more rapidly than cyclopropane formation, the mechanism is most probably an El cB-gamma-elimination. In this case, beta-elimination is precluded, due to the difficulty of forming a double bond at a bridgehead, in accordance with Bredt s rule. The exo-chloro-sulphone (241) rapidly epimerises to give almost completely the other isomer (242) in which 

These results suggest that a 1,3-elimination can only occur if, in the carbanion, the leaving group is situated in a semi-V/ arrangement (see (236), p. 330) and not in a semi- J arrangement. This is in marked contrast to the ease of concerted 1,3-elimination from both the endo- and exo-norbornyl tosylates. Obviously more investigations are necessary and it would be of interest to study the behaviour of the above sulphones (238, 239, 241) in which the chlorine is replaced by a tosylate group. 

The solvolysis of exo-norbornyl brosylate in acetic acid gives 4% elimination products, 98% of which is nortricyclene and only 2% is norbornene. The larger yield of the cyclopropane than the olefin was quoted as support for a non-classical rather than a rapidly equilibrating pair of classical carbonium ions .  

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