Solvolytic elimination reactions. Stepwise or concerted?

Article abstract of DOI:10.1021/ja00058a018

2-Aryl-endo-2-norbornyl trifluoroacetates 11 solvolyze to give a significant fraction of elimination product. A deuterium labeling study showed that the exo hydrogen is lost exclusively when the elimination product forms. A concerted ester pyrolysis type of mechanism is therefore ruled out. The tertiary benzylic trifluoroacetate Ph(CH₃)C(CONMe₂)(OCOCF₃), 12, solvolyzes to give exclusively an elimination product. The β-CD₃ isotope effect on rate was 1.15. There is, however, a larger isotope effect (2.5) in formation of the elimination product when Ph(CH₂D)C(CONMe₂)(OCOCF₃) solvolyzes. The mechanism therefore has a minimum of two steps since the product-determining step and the rate-determining step have differing isotope effects. A concerted elimination mechanism is ruled out. The ortho-dimethyl-substituted cumyl trifluoroacetate 13 also solvolyzes to give an elimination product. The β-D₆ isotope effect of 1.61 is in the "normal" range for a secondary isotope effect in a carbocation-forming reaction. This argues against a concerted elimination mechanism. These results contrast with the recent suggestion that cumyl systems give elimination product via a concerted elimination mechanism. The analogous tertiary benzylic systems 11-13 all give solvolytic elimination products via discrete cationic intermediates.