ION-MOLECULE REACTIONS IN THE GAS-PHASE .18. NUCLEOPHILIC-SUBSTITUTION OF DIASTEREOMERIC NORBORNEOLS, NORBORNYL ACETATES AND BENZOATES UNDER AMMONIA CHEMICAL IONIZATION

The comparative behaviour of the endo- and exo-norborneols and diastereomeric derivatives (acetates and benzoates) towards the NH3/NH4+ system was investigated. It appears that the proton affinity (PA) of the substrate relative to PA(NH3) strongly influences competition between the protonation and nucleophilic substitution processes yielding the MH+ and [M + NH4 - H2O]+ ions, respectively. Tandem mass spectrometry was used to compare collision-activated dissociation spectra of [M + NH4 - H2O]+ with those of analogous endo- and exo-norbornylamines protonated in the source. This demonstrates that an S(N)i mechanism occurs specifically for the isomeric norborneols; in contrast, for acetates and benzoates, stereospecific S(N)i and S(N)2 pathways take place for exo and endo derivatives, respectively. This particular behaviour is explained by considering the steric effect induced by the endo-H at C(6). In addition, the competitive decompositions of [M + NH4 - H2O]+ into NH4+ and [C7H11]+ daughter ions are consistent with the formation of a proton-bound complex intermediate. The observed stereochemical effects for these dauther ions are rationalized by means of arguments based on the estimated heats of formation of the transition states, which is lower for the exo-norbonyl protonated amine, consistent with anchimeric assistance, rather than a stepwise pathway which is proposed for the endo isomer.