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′ mechanism in the gas phase was obtained by establishing the regioselectivity of the attack of a neutral nucleophile, such as MeOH, on several allylic oxonium ions. These were generated in the gas phase by the reaction of radiolytically formed GA + acids (GA + C n H + 5 ( n = 1, 2), i C 3 H + 7 , and (CH 3 ) 2 F + ) with trans ‐ ( 1 ) and cis ‐2‐buten‐1‐ol ( 2 ) as well as 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 obtained after careful evaluation of the extent of conceivable intramolecular isomerization both of the primary oxonium ions from GA + attack on 1–3 before nucleophilic displacement by MeOH and of their substituted intermediates before neutralization. The intermediacy of free allylic ions in the nucleophilic substitution was ruled out by generating the ions by protonation of 1,3‐butadiene and by investigating their behavior in exactly the same media employed in the substitution reactions. The regioselectivity of MeOH with the ionic substrates investigated showed the occurrence of nearly equally extensive S N 2′ and S N 2 pathways 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 branching in the selected oxonium ions is discussed. The first unequivocal evidence for the hotly debated existence of a concerted S N 2′ reaction in the gas phase is provided by the regioselectivity of CH 3 OH attack on O‐protonated cis and trans allylic alcohols (right). The SN2′ mechanism accompanying the classical S N 2′ in these systems can be demonstrated after careful evaluation of the extent of intramolecular isomerization before displacement by CH 3 OH and of the substituted intermediates before neutralization. This research gives a direct insight into the intrinsic structural factors determining S N 2′/S N 2 branching in allylic compounds. magnified image