Intensification of Rates and Selectivity Using Tri-liquid versus Bi-liquid Phase Transfer Catalysis: Insight into Reduction of 4-Nitro-o-xylene with Sodium Sulfide

Abstract

Reactions in three immiscible liquid phases are attractive, and one of the phases can be the locale of the reaction, which will have a dramatic effect on the product distribution in complex reactions. Thus, converting a bi-liquid (L−L) system into tri-liquid (L−L−L) phases is of considerable scientific and commercial interest. Such systems encounter mass-transfer resistances for transfer across two interfaces and the kinetic analysis becomes difficult. In the case of phase transfer catalysis (PTC), a majority of preparatory and industrial reactions are conducted in two phases. The L−L PTC reactions are conducted under milder conditions, using less-expensive solvents at much faster reaction rates and improved selectivities to desired products. However, the phase transfer catalyst is not recovered but treated as waste, because the quantities are very small and do not contribute much to the expensive product. L−L PTC can be elegantly modified to convert it to L−L−L PTC, to recover and reuse the catalyst and also to enhance selectivity, thereby improving profitability and environmental benefits. 3,4-Dimethyl aniline is a useful starting material for the production of riboflavin (vitamin B2) and also is used as a precursor for many drugs. It can be produced via the reduction of 4-nitro-o-xylene in toluene with aqueous sodium sulfide, using tetrabutylammonium bromide (TBAB) as a catalyst under L−L−L PTC at 80 °C. Comparison between tri- and bi-liquid phases was also conducted, and it was observed that, under L−L−L conditions, the rates of reaction of the formation of 3,4-dimethyl aniline had been dramatically enhanced. The kinetics and mechanism of complex L−L-L PTC processes have been explored in detail. The yield, based on the initial amount of reactant, was >95%, and the selectivity was 100%.