STRUCTURAL EFFECTS IN SOLVOLYTIC REACTIONS .32. EFFECT OF INCREASING ELECTRON DEMAND ON THE RATES OF SOLVOLYSIS OF 2-ARYL-3-METHYLENE-2-NORBORNYL PARA-NITROBENZOATES - HIGH EXO-ENDO RATE AND PRODUCT RATIOS FOR TERTIARY DERIVATIVES OF THE ACCEPTED CLASSICAL 3-METHYLENE-2-NORBORNYL SYSTEM

The secondary 3-methylene-2-norbornyl system, with its low exo/endo solvolytic rate ratio of 3.9, has been proposed as a model norbornyl system which undergoes solvolysis through the intermediacy of classical carbocations, the allylic double bond of the 3-methylene group swamping out any possible a participation by the 1, 6-bonding electron pair. It follows that the more stabilized tertiary 2-aryl derivatives must also undergo solvolysis through classical cationic intermediates. Accordingly, 2-aryl-3-methylene-2-norbornyl p-nitrobenzoates, containing representative substituents in the 2-aryl ring, were synthesized and their rates of solvolysis in 80% aqueous acetone determined. These derivatives reveal high exo/endo rate ratios of 407 for p-OCH2CH2-m (5-coumaranyl), 357 for p-H, 214 for p-CF3, and 454 for 3, 5-(CF3)2. The reaction products are ≥99% exo. It follows that the high exo/endo rate and product ratios in this system cannot be the result of a participation leading to the formation of a a-bridged species but must be the result of the unique steric characteristics of the U-shaped norbornyl system. Both the 2-aryl-3-methylene-exo- and -endo-norbornyl derivatives yield almost identical values of p+: -3.79 for the exo isomers and -3.76 for the endo isomers, quite similar to the previously established values of -3.82 and -3.76 for the 2-aryl-exo- and -endo-norbornyl derivatives, respectively. © 1979, American Chemical Society. All rights reserved.