Cooperative catalysis of cellulose nanofiber and organocatalyst in direct aldol reactions

Cellulose nanofibers (CNFs) are finding a wide range of applications in the forthcoming sustainable society because of their carbon-neutral renewability and superior physicochemical properties. Here, we first show a cooperative organocatalysis by combining TEMPO-oxidized cellulose nanofiber (TOCN) and proline to enhance the catalytic efficiency in a direct aldol reaction. The yields of proline-catalyzed aldol products drastically increased in the presence of catalytically-inactive TOCN. This effect was also achieved by simply adding the TOCN to the reaction conditions where various proline analogues including structurally simple pyrrolidine and piperidine were used instead of proline. TOCN was superior to physically-pulverized CNF in the organocatalytic efficiency, and the nanofibrillation of cellulose microfibrils in reaction media was essential to induce the drastic enhancement in catalytic activity. The present finding will bring a new entry in the applications of CNFs, and open up a new phase in developing highly efficient molecular transformations in green chemical industries.