Non-Classical Nitrogen Promoters for Rhodium-Catalyzed Reductive Hydroformylation: New Efficient Way to Access Polyols From Linseed Oil

Abstract

The reductive hydroformylation reaction of polyunsaturated linseed oil using a rhodium precursor associated with a nitrogen compound from five different families (pyridines, imidazoles, trialkylamines, amidines, and guanidines) has been successfully developed. The highest yield of 72% alcohols was obtained with an (Rh/1,2,4,5-tetramethylimidazole) catalytic system. This value represents the highest yield obtained in this tandem reaction of such a challenging substrate. A deep screening of different nitrogen compounds highlighted the strong influence of their pKa on the reaction kinetics. If the ligand/Rh ratio is too high, the basic character of the ligand overrides its ligating property and leads to deprotonation of the Rh hydride active species, resulting in dormant species that are inactive in the reductive hydroformylation reaction. For each family tested, except for triethylamine, a bell-shaped curve of alcohol yield versus ligand/Rh ratio was obtained, with a peak position essentially related to the pKa value of the N-ligand. This work highlights the efficiency of new families of nitrogen promoters in the HHM reaction catalyzed by rhodium complexes, and in particular in the valorization of bio-based substrates presenting challenging characteristics such as polyunsaturations.

Practical Applications: The (Rh/1,2,4,5-tetramethylimidazole) catalytic system provided the highest alcohol yield in linseed oil reductive hydroformylation. This system could be applied to other petroleum-derived olefins or bio-based substrates containing unsaturations. The value-added polyols obtained in this study have a wide range of applications in the production of lubricants, surfactants, plasticizers, or polymers.