Gas-phase facial diasteroselectivity of equatorial and axial 4-chloro-adamant-2-yl cations.

The acid-catalyzed addn. of CH318OH to 2-methylene-adamantanes bearing a chlorine atom in the 4-equatorial (1e) or 4-axial (1a) position has been investigated in the gas phase, at 760 Torr, in the 40-120 C temp. range. Two different exptl. approaches were employed: (1) by adding neutral CH318OH to the 2-methyl-4-Cl-adamant-2-yl cation, generated by protonation of the corresponding 2-methylene-4-Cl-adamantane (the extra complex reaction) and (2) by reaction of 2-methylene-4-Cl-adamantane with CH318OH2+, generated by methylation of H218O (the intracomplex reaction). The crucial role of the nature of the noncovalent intermediates involved along the reaction coordinates emerges from the difference between the results obtained in the extra complex and intracomplex reactions for both substrates investigated. The kinetic and stereochem. results indicate that the 4-Cl substituent plays a different role depending on its equatorial or axial orientation. Examn. of the exptl. results in the light of MP2/6-31G* theor. calcns. provides important information about the intrinsic factors governing the facial diastereoselectivity of trigonal carbocations. The effects due to differential face solvation phenomena emerge from the comparison of the present gas-phase results with those obtained from strictly related studies in soln.