Intensification and selectivities in complex multiphase reactions: insight into the selectivity of liquid−liquid phase-transfer-catalyzed O-alkylation of p-methoxyphenol with allyl bromide

Abstract

Studies in the preparation of aromatic ethers have proven to be quite attractive because of the extensive use of these compounds in the dyestuff, perfume, flavor, agriculture, and pharmaceutical industries. The novelties of phase-transfer-catalyzed alkylation of p-methoxyphenol with allyl bromide were studied in detail in a biphasic system with tetrabutylammonium bromide (TBAB) as the catalyst, and the selectivity of O- versus C-alkylation was compared. The effects of various parameters were studied systematically to understand the conversion patterns; product distribution; and selectivity of the desired product, 1-methoxy-4-(2-propenyloxy)benzene, which is an important perfumery compound. A mechanistic model has been proposed on the basis of the theory of mass transfer with chemical reaction in two liquid phases. The desired product is produced by the O-alkylation reaction between the quaternary 4-methoxyphenoxide and allyl bromide, which takes place instantaneously at a reaction plane in the organic film next to liquid−liquid interface, conforms to the so-called regime 4, and is an example of normal liquid−liquid phase-transfer catalysis (L−L PTC). The isomerization of the desired product to give the first byproduct, the C-alkylated derivative 4-methoxy-2-(2-propenyl)phenol, takes place in the aqueous film by a fast pseudo-first-order reaction (regime 3) and is an example of so-called inverse L−L PTC. This byproduct also has fragrance value. The second byproduct, 1-allyloxy-2-allyl-4-methoxybenzene, is formed by a slow reaction between 4-methoxy-2-(2-propenyl)phenol and allyl bromide in the bulk organic phase and conforms to the so-called regime 1. A 100% selectivity of the desired product is obtained when a stoichiometrically deficient quantity of the base is used at 30 °C. This system is a unique example of mass transfer with complex chemical reactions. It has provided insight into a practical problem with academic excitement, and it also happens to be the first reported case of a PTC reaction in which both normal L−L PTC and inverse PTC are at play.