Gas-Phase Facial Diastereoselective of Equatorial and Axial 4-Chloroadamant-2-yl Cations.

The acid-catalyzed addition of CH318OH to 2-methylene-adamantanes bearing a chlorine atom in the 4-equatorial(1e)or4-axial(1a)positionhasbeeninvestigatedinthegasphase,at760Torr,inthe40-120C temperaturerange.Twodifferentexperimentalapproacheswereemployed:(1)byaddingneutralCH318OH to the 2-methyl-4-Cl-adamant-2-yl cation, generated by protonation of the corresponding 2-methylene4-Cl-adamantane (the extracomplex reaction) and (2) by reaction of 2-methylene-4-Cl-adamantane with CH318OH2+,generatedbymethylationofH218O(theintracomplexreaction).Thecrucialroleofthenature of the noncovalent intermediates involved along the reaction coordinates emerges from the difference between the results obtained in the extracomplex and intracomplex reactions for both substrates investigated. The kinetic and stereochemical results indicate that the 4-Cl substituent plays a different roledependingonits equatorial or axial orientation.Examinationoftheexperimentalresultsinthelightof MP2/6-31G*theoreticalcalculationsprovidesimportantinformationabouttheintrinsicfactorsgoverning the facial diastereoselectivity of trigonal carbocations. The effects due to differential face solvation phenomenaemergefromthecomparison ofthepresentgas-phaseresultswiththose obtainedfromstrictly related studies in solution.