REGIO- AND STEREOCHEMISTRY OF GAS-PHASE ACID-INDUCED NUCLEOPHILIC SUBSTITUTIONS ON CHIRAL ALLYLIC ALCOHOLS

The regio- and stereochem. of the nucleophilic attack of (S)-trans-3-hexen-2-ol (MS) and (S)-trans-4-hexen-3-ol (ES) on the corresponding O-protonated (L = H) and -methylated (L = CH3) derivs. (MSL+ and ESL+) are investigated in the gas phase at 40 C (720 Torr). The MSL+ and ESL+ intermediates are produced in the gas phase by the attack of the ionic Bronsted and Lewis acids, formed by stationary .gamma.-radiolysis of bulk CH3Cl, on the corresponding chiral alcs., i.e., MS and ES. In these systems, firm evidence in favor of the concerted SN2' pathway, accompanying the classical SN2 one, is obtained by excluding the following: (i) the isomerization of MSL+ (or ESL+) before the attack by the nucleophile NuH = MS (or ES); (ii) the isomerization of the (C6H11)2OH+ substitution intermediates before neutralization; (iii) the intermediacy of allylic cations. The regioselectivity factors [SN2'/SN2 = 1.4 (MS), 1.1 (ES)] confirm previous exptl. and theor. evidence about the prevalence in the gas phase of the SN2' pathway, over the competing SN2 one. Orientation of NuH by MSL+ (or ESL+) dets. the regiochem. of the allylic substitution. When NuH approaches the oxonium intermediate from the direction syn to the leaving moiety LOH, a front-side SN2 displacement takes places favored by preliminary proton bonding between LOH and NuH. The SN2' reaction instead follows attack on the .pi.-LUMO of the oxonium ion by the NuH juxtaposed anti to the leaving LOH group. Observation of a predominant anti-SN2' orientation provides the first exptl. basis of modern concepts pointing to Coulombic interactions as the main intrinsic factors governing the SN2' stereochem. and to solvation and ion pairing as the factors detg. the low efficiency of SN2' reactions and their preferred syn stereochem. in soln.