Ab in/tin stud,, of the pathways of migration of the double bond i+l the 3-methylthioprop-I-ene {1) and 3-metho• {2) molecules with participalion of hydroxide ion was carried out by the RHF, 6-31+G* and MP2/6-31 +G//RHF/6-31+G* methods. Conformatianal isomerism of the initial molecules and reaction products was considered. The disti~lctions are discussed in the spatial and electronic structure of intermediate carbanions stabilized (for 1 ) due zo the negative hyperconjugation. Stationaw poinls corresponding to complexes between the molecules under study and the h~droxide ion and between the corresponding carbanion and water molecule were localized on the potential energy surt:aces of lhe proton transk'r reactions. For 2, the single-stage mechanism of prototropic rearrangement involving the H atom of the hsdroxide ion was found to be more energetically preferable than the two-stage mechanism, whereas both mechanisms are expected to be equip~obable lbr I.hydroxide ion, reaction mechanism, potential energy surface, ab initio quantum-chemical calculations.Previously, 1,2 we showed that migration of the double bond in the propene molecule with participation of hydroxide ion can occur via the formation of an intermediate complex with the water molecule formed from the attacking hydroxide ion and one of the H atoms of the methyl group of the propene molecule, i.e., formall,v without exchange of the migrating proton with the medium ("intramolecularly"). This mechanism can also be realized in the case of 3-methoxyprop-I-ene. 3 In both cases, the transition states determining the activation barriers to the prototropic rearrangement lie on the energy scale below the reagents and reaction products and much lower than the system that formed comprising anion and the water molecule. This suggests thaL at least in the gas phase, this mechanism of rearrangement of the molecules of the above-mentioned compounds is more energetically preferable than the known tWo-stage mechanism 4 iiwotving separate stages of {i) proton abstraction by a base with the formation of a carbanion and ~ii) reprotonation of the carbanion by proton-containing species of the medium, Compared to ethers, organic sulfides are characterized by a much higher acidity of protons bonded to the * For Parts I--3, see Refs. I--3. ot-C atom at the substituentf In particular, isomerization of 3-methylthioprop-l-ene into l-methylthioprop-1 -erie under the action of bases occurs more readily than that of its oxygen-containing analog. 6-7 In this connection it is of interest (i) to study the possibility for a sing/e-stage 1.3-hydrogen shift involving a proton of the base to occur in the molecule of methylthiopropene ! and (ii) to compare it with analogous isomerization reaction of methoxypropene 2.
Calculation procedureCalculations were carried out following the know~l procedure, z OpUmization of geometric parameters of the struc.tares under study was performed by the restricted Hartree--Fock (RHF) method in the 6-31+G* basis set and foliowed by checking for the absence of...
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