The kinetics and the stereochemistry of the protonation-induced unimolecular isomerization of (S)-(+)-1-D(1)-3-(p-tolyl)butane have been investigated in the gas phase in the 100-160 degrees C range. The process leads to the almost exclusive formation of the relevant meta isomer with complete racemization and partial 1,2-H shift in the migrating sec-butyl group. These results, together with the relevant activation parameters, point to the occurrence of low-energy, tightly bound isomeric sec-butyl cation/toluene complexes of defined structure and stability along the isomerization coordinate. The existence and the eta(1)-type structure of these low-energy intermediate species are confirmed by ab initio calculations on closely related systems at the MP2(full)/6-311++G**//HF/6-31+G** level of theory. Their role in the relevant energy surface clearly emerges from the comparison of the present results with those concerning sec-butylation of toluene carried out under comparable experimental conditions.
A first demonstration of the existence of the concerted SN2' 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,H: (n = I , 2), iC,H:, and (CH,),F+) with trans-(1) and cis-2-buten-1-01 (2) as well as with I-buten-3-01 (3). Firm evidence in favor of the concerted SN2' pathway accompanying the classical SN2 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 SN2' and SN2 pathways in the oxonium ions from 1 (SN2' (57f2%) and SN2 (43 2 YO)) and 3 (SN2' (54 + 2 YO) and SN2 (46+2%)), whereas, with 2, the SN2 (66 2 YO) reaction prevailed over the SN2' one (34 2 %). The role of intrinsic structural factors in determining the SN2'/SN2 branching in the selected oxonium ions is discussed.
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