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The norbornane molecule (I) possesses a rigid structure with uncommon steric characteristics. Carbon atoms 1-6 constitute a cyclohexane structure in the higher-energy boat conformation. Moreover, the 7-methylene group not only locks this ring system into a rigid boat conformation, but the /I2 I constraint so produced accentuates the steric 6 2 crowding within the boat structure (Fig. 1). Much of the chemistry of norbornane is dominated by the far greater steric accessibility of the exo as compared to the endo position. For example, in many free radical substitution reactions the exo product is produced preferentially.' Similarly, many additions to norbornene proceed predomiA B nantly from the exo direction.2' For example, the hydroboration-oxidation of norbornene produces 99.5 per cent of exonorborneol, with only 0.5 per cent of the endo isomer.3 O.77A I.77A

proceedings of the national academy of sciences of the united states of america

THE SLOW RATES OF SOLVOLYSIS OF ENDO-5,6-TRIMETHYLENE-ENDO-(2 AND 8)-NORBORNYL TOSYLATES: STERIC HINDRANCE TO IONIZATION AND THE FOOTE-SCHLEYER CORRELATION