Gas-Phase Facial Diastereoselectivity of Equatorial and Axial4-Chloro-adamant-2-y1 Cations

摘要

The acid-catalyzed addition of CH_3~(18)OH to 2-methylene-adamantanes bearing a chlorine atom in the4-equatorial (1_e) or 4-axial (1_a)position has been investigated in the gas phase, at 760 Torr, 40-120 °Ctemperature range. Two different experimental approaches were employed: (1) by adding neutral CH_3~(18)0Hto the 2-methyl-4-Cl-adamant-2-yl cation, generated by protonation of the corresponding 2-methylene-4-Cl-adamantane (the extracomplex reaction) and (2) by reaction of 2-methylene-4-Cl-adamantane withCH_3~(18)OH_2~+, generated by methylation of H_2~(O(theintracomplexreaction). The crucial role the natureof the noncovalent intermediates involved along the reaction coordinates emerges from the differencebetween the results obtained in the extracomplex and intracomplex reactions for both substratesinvestigated. The kinetic and stereochemical results indicate that the 4-Cl substituent plays a differentrole depending on its equatorial or axial orientation. Examination of the experimental results in the light ofMP2/6-31G~* theoretical calculations provides important information about the intrinsic factors governingthe facial diastereoselectivity of trigonal carbocations. The effects due to differential face solvationphenomena emerge from the comparison of the present gas-phase results with those obtained from strictlyrelated studies in solution.