STRUCTURAL EFFECTS IN SOLVOLYTIC REACTIONS .31. HIGH EXO-ENDO RATE AND PRODUCT RATIOS IN THE SOLVOLYSES OF THE 2-METHYL, 2-PHENYL, AND 2-(5'-COUMARANYL)-EXO-5,6-TRIMETHYLENE-2-NORBORNYL PARA-NITROBENZOATES - FURTHER EVIDENCE FOR THE UNIMPORTANCE OF SIGMA-PARTICIPATION AS A FACTOR IN THE HIGH EXO-ENDO RATE AND PRODUCT RATIOS REALIZED IN THE SOLVOLYSIS OF TERTIARY 2-NORBORNYL DERIVATIVESV

Solvolyses of 2-methyl-, 2-phenyl-, and 2-(5ʹ-coumaranyl)-exo-5, 6-trimethylene-2-norbornyl p-nitrobenzoates in 80% aqueous acetone reveal high exo: endo rate ratios of 420, 118, and 286, respectively. The solvolyses of these derivatives in the presence of sodium acetate afford exclusively the exo-substituted alcohol. The secondary exo-5, 6-trimethylene-2-norborn-yl system, with its low exo: endo rate ratio, 11.2, has been proposed as a model norbornyl system which solvolyzes through the intermediacy of classical carbocations, constrained into the classical structure by the 5, 6-trimethylene bridge. Consequently, these stabilized tertiary derivatives should also undergo solvolyses through the intermediacy of classical carbocations. Yet they exhibit high exo: endo rate and product ratios, at one time considered essential criteria supporting σ-participation with the formation of a σ-bridged intermediate in the solvolysis of the exo derivative. It is clear that steric effects must make major contributions to the high exo: endo rate and product ratios observed in the solvolyses of these stabilized tertiary derivatives. © 1979, American Chemical Society. All rights reserved.