Heterogenization of an organic catalyst by adsorption on alginic acid gels

A sustainable approach focused on the practice of green chemistry was used to develop a method which improved the performances of a catalyst system based on a natural and non-toxic substance. A benchmark Michael addition reaction was performed employing 9-amino(9-deoxy)epi quinine, adsorbed on alginic acid gels by hydrogen bonds, as catalyst. Compared to conventional heterogeneization of this organic catalyst, the present approach is more straightforward and employs as support a renewable biomaterial instead of oil-derived polymers. The optimization of the adsorption protocol was carried out to obtain an active and heterogeneous system able to work under different reaction temperatures. The Michael addition reaction rate, heterogeneity, enantiomeric excess and recyclability of the catalytic system were studied. The influence of temperature, additives and the presence of water were successfully investigated. The heterogeneity of the catalyst was perfectly preserved, therefore the catalyst could be easily recovered. Two optimal conditions were disclosed, differing in reaction temperature and catalyst pre-treatment. A scale-up is performed with good results for the first three reactions cycles (conversions: 100%, 87% and 55% respectively). The enantiomeric excess is determined as 98%. The results of this project demonstrated that a green catalytic system has a great potential to be competitive with more classic heterogeneous catalysts.