A theoretical study of methanol vinylation reaction mechanism

Abstract

ABSTRACT:The mechanism of base-catalyzed nucleophilic addition of methanol to acetylene triple bond (vinylation) in dimethyl sulfoxide (DMSO) and methanol solution was studied using the MP2/6-311ϩϩG**//B3LYP/6-31G* calculations with solvent effects included via continuum model. The proton abstraction from methanol by nondissociated alkali in DMSO surrounding media to form alkali metal methoxides CH 3 OM (M ϭ Li, Na, K) was found to occur with a negligible activation barrier. The reasons for facilitation of base-catalyzed alcohol vinylation in the DMSO medium are discussed in the light of both poor solvation of methoxide ion and a specific coordination of reactants by nondissociated alkali in the MOH/DMSO mixtures.

Methanol vinylation mechanism in the KOH/DMSO/CH3OH/C2H2 system

“…The results of our previous simulation of methoxide-ion interaction with acetylene using the quantum-chemical MP2/6-311þþG**//B3LYP/6-31þG* approach with solvent effects included within a continuum model [11] agree with this general scheme. The activation barrier of the limiting stage was evaluated as DH ‡ ¼ 16.4 kcal/mol, while the proton transfer from methanol molecule to the carbanion formed occurs with no activation barrier.…”

“…According to our previous results [11,14] a proton transfer from the alcohol molecule to alkali metal hydroxide in the CH 3 OHAMOH (M ¼ Li, Na, K) system occurs with almost no activation barrier in both gas phase and with solvent effect included via a continuum model. Moreover, a B3LYP/6-31G* optimization of the CH 3 OHÁKOH system brings immediately to the minimum corresponding to methoxide ion and water molecule coordinated to alkali metal center.…”

“…2(d)]. The preference of the trans-distorted TS is a feature of vinylation reactions [11,24], that become apparent in predominant formation of Zproducts in nucleophilic addition to substituted acetylenes. The structure of 4 (m im ¼ 547.04i) keeps similarity with the transition state structure of free CH 3 O À attack to acetylene molecule.…”

“…The attack of the methoxide ion towards the acetylene molecule coordinated to nondissociated potassium hydroxide was also discussed in Ref. [11]. Unexpectedly, such coordination was found to have only slight influence upon the reaction profile.…”

“…The polarizable continuum model (PCM) in the integral IEFPCM formulation [21] with Almerindo et al [22]. atomic radii 1.20 (H), 1.50 (O), and 1.70 (C) combined with cationic (instead of van der Waals or atomic) radius 1.60 (K) [11] and a uniform scale factor 1.35 was used to calculate solvation energies.…”