GAS-PHASE ACID-INDUCED NUCLEOPHILIC DISPLACEMENT REACTIONS .10. GAS-PHASE ACID-INDUCED S(N)2 VERSUS S(N)2 MECHANISM IN ALLYLIC ALCOHOLS

A first demonstration of the existence of the concerted S(N)2' mechani sm in the gas phase was obtained by establishing the regioselectivity of the attack of a neutral nucleophile, such as MeOH, on several allyl ic oxonium ions. These were generated in the gas phase by the reaction of radiolytically formed GA(+) acids (GA(+) = CnH5+ (n = 1, 2), iC(3) H(7)(+) and (CH3F+) with trans- (1) and cis-2-buten-1-ol (2) as well a s with 1-buten-3-ol (3). Firm evidence in favor of the concerted S(N)2 ' pathway accompanying the classical S(N)2 one in these systems was ob tained after careful evaluation of the extent of conceivable intramole cular isomerization both of the primary oxonium ions from GA(+) attack on 1-3 before nucleophilic displacement by MeOH and of their substitu ted intermediates before neutralization. The intermediacy of free ally lic ions in the nucleophilic substitution was ruled out by generating the ions by protonation of 1,3-butadiene and by investigating their be havior in exactly the same media employed in the substitution reaction s. The regioselectivity of MeOH with the ionic substrates investigated showed the occurrence of nearly equally extensive S(N)2' and S(N)2 pa thways in the oxonium ions from 1 (S(N)2' (57+/-2%) and S(N)2 (43+/-2% )) and 3 (S(N)2' (54+/-2%) and S(N)2 (46+/-2%)), whereas, with 2, the S(N)2 (66+/-2%) reaction prevailed over the S(N)2' one (34+/-2%). The role of intrinsic structural factors in determining the S(N)2'/S(N)2 b ranching in the selected oxonium ions is discussed.