Ion chain mechanism of catalytic alkylation of 2,6-Di-tert-butylphenol by methyl acrylate

Abstract

Phenoxides based on sterically hindered phenols and alkali metals are used in the synthesis and industrial production of a number of compounds applied in the production of antioxidants [1]. The shielding of the reaction center by the tert -butyl groups gives rise to the specific properties inherent in sterically hindered phenoxides, which are related to negative charge delocalization in the six-membered ring and the formation of a coordination bond of the metal cation with the oxygen atoms and carbon atoms of the six-membered ring. These features are responsible for the ability of potassium ( 1 ) and sodium ( 2 ) 2,6-di- tert -butylphenoxides to form supramolecular structures as contact or solventseparated ion pairs and dimers [2]. Similar structures based on alkali metal cations and radical anions have been studied previously by EPR [3]. The above data suggest that a nonsolvated cation may be formed in a matrix comprising 2,6-di- tert -butylphenol ( 3 ), methyl acrylate ( 4 ), and catalytic amounts of monomer 1 or 2 . The course of alkylation of phenol 3 with methyl acrylate in the presence of catalytic amounts of monomer 1 or 2 differs from that in the presence of the corresponding phenoxide dimers (associates). In this work, we found that, with a nearly equimolar mixture of 3 and 4 , the catalytic alkylation in the presence of 1.5‐3 mol % of monomer 1 or 2 proceeds at higher rates than those known previously (in the presence of dimers 1 or 2 ) [2‐5] and gives the alkylation product, methyl 3-(4-hydroxy-3,5-di- tert -butylphenyl)propionate ( 5 ), in up to 98% yield. The formation of the monomer species was supported by thermoelectric measurements of the molecular weights of phenoxides 1 and 2 at 115 ° C. The kinetic studies for this reaction in the range 105‐130 ° C showed that the pattern of the kinetic curve does not depend on the reaction temperature (Fig. 1), which may imply a tunneling effect in the catalytic alkylation mechanism. The monomer of phenoxide 1 is formed upon the reaction of phenol 3 with potassium hydroxide at 180‐200 ° C. On cooling to 20 ° C, the monomer is converted into dimer 1 . In the experiment, a mixture of 1 and 3 was cooled to 135‐ 120 ° C and compound 5 was simultaneously added. This gave a reaction mass in which phenoxide 1 or 2 was mainly uniformly distributed throughout the mixture of phenol 3 and acrylate 4 and the reaction between these compounds proceeded under steady-state conditions in a thermostat without stirring the reactants. By sampling the reaction mixture during the process and analyzing the samples by liquid chromatography, the contents of 3 , 5 , and dimethyl α -(4-hydroxy-3,5-ditert -butylbenzyl)glutarate ( 6 ) in the reaction mixture were determined. The yield of 6 in the experimental series did not exceed 1%. Figure 1 shows also the kinetic data for the reaction of phenol 3 with methyl acrylate in the presence of catalytic amounts of dimer 1 , which coincide with published data [2].