Steric effects etherification

Less problematic are etherifications with epoxides under acidic reaction conditions. Although we would expect the nucleophile to attack the position which most easily forms a carbocahon, this does not always happen and steric effects also seem to be important (Scheme 4.79). 

The activity of the 4-alkylpyrocatechols (la), 3-alkylpyro-catechols (lb), 3,5-dialkylpyrocatechols (Ic), 3,6-dialkylpyro-catechols (II), 4,5-dialkylpyrocatechols (III), 3,4,6-trialkyl-pyrocatechol (TV), and 4,5- and 4,6-dialkyl-2-alkoxyphenols (V and VI) was studied in isotactic polypropylene at 180 =t 0.1° C. The relative activities Ar were correlated with substituent constants and redox potentials. In the la series the activity of 4-n-alkyl derivatives exceeded that of 4-tert-alkyl derivatives. The length of the main alkyl chain in la and lb and the steric effects in the latter exerted a specific favorable influence. In agreement with these effects, 3-tert-alkyl-5-methyl derivatives are the most active compounds in Series Ic compounds with other combinations of alkyls are weaker antioxidants as are the isomeric substances II and III or compound IV. Etherification of one hydroxyl group exerts an unfavorable effect influences of substitution in 2-alkoxyphenols (V and VI) are generally the same as in the pyrocatechols Z-ZZZ. 

The activity of the 2-alkylhydroquinones (la), 2,5-dialkyl-hydroquinones (lb), 2,6-dialkylhydroquinones (Ila), and of 2,5- and 2,6-dialkyl-4-alkoxyphenols (Ic and lib) was studied in isotactic polypropylene at 180 0.1° C. The values obtained were correlated with hydroquinone (Ari) and pyro-catechol (AT2). All compounds studied (la) except the tert-octyl derivative were inferior in activity to hydroquinone. Two alkyls of Types lb and Ha exerted further but nonadditive unfavorable effects, particularly the lb type. Despite over-all low values of Ari, a weak favorable steric effect of the tert-alkyls is apparent in Type la compounds. Etherification even as acetylation of one hydroxyl group (Ic, lib), has a strong favorable effect. These compounds were the most active antioxidants of the entire hydroquinone series studied nevertheless their activity did not reach that of pyrocatechol (At2 < 1). 

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