Gas-phase acid-induced nucleophilic displacement reactions .11. Regioselectivity in the gas-phase nucleophilic attack on O-protonated 3-methyl-2-cyclohexen-1-ol and 1-methyl-2-cyclohexen-1-ol

Our radiolytic study of the occurrence in the gas phase of concerted S(N)2' reactions on several open-chain allylic oxonium ions generated in the gas phase From the attack of gaseous GA(+) acids (GA(+) = CnH5+ (n = 1, 2), iC(3)H(7)(+) and (CH3F+) on suitable substrates is now continued with cyclic allylic alcohols, such as 3-methyl-2-cyclohexen-1-ol(1) and 1-methyl-2-cyclohexen-1-ol (2), with both MeOH and NMe(3) as neutral nucleophiles. With MeOH as the nucleophile, the substitution reaction exclusively takes place on 1 as the starting compound, whereas when the substrate is 2 it is accompanied by extensive elimination. With NMe(3), only the elimination reaction is observed in the same systems. The analysis of the isomeric distribution of the sub stitution and elimination products allows definition of the corresponding reaction patterns. As for open-chain oxonium ions, the nucleophilic attack on O-protonated 1 and 2 is preceded by significant intramolecular interconversion. Partial unimolecular dissociation of the same ionic intermediates also takes place. After careful evaluation of the extent of these side processes, it is demonstrated that the O-protonated 1 undergoes the concerted S(N)2 process with MeOH almost exclusively (greater than or equal to 99%). With O-prolonated 2, however, the concerted S(N)2' pathway (84-95%) prevails over the classical S(N)2 one (6-17%). Concomitant [1.2] (E2) and [1,4] elimination (E2') pathways involve attack of the selected nucleophiles on the oxonium ions from 1 and 2. Their relative extent (E2'/E2:1.78-1.96 (1); 1.43-1.70 (2)) appears only slightly dependent on the nature of the ionic substrate, the nucleophile (whether MeOH or NMe(3)). and the leaving group (whether H2O or MeOH). The effects of both intrinsic structural factors and experimental conditions in determining the S(N)2'/S(N)2 and E2'/E2 branchings in the selected oxonium ions is discussed and compared with related gas-phase data.