The many roles of solvent in homogeneous catalysis - The reductive amination showcase

In the global effort to fight climate change, the design and development of novel procedures for complex chemical multi-step reactions is essential for the transformation of chemical industry. The transformation of chemical production processes from petrochemicals towards renewable, sustainable feedstock and the identification of green solvent candidates require a careful assessment of the effects of solvent on thermodynamics and kinetics. As a prime example for the many roles of solvent in chemical catalysis, the reaction mechanism of the homogeneous rhodium-catalyzed reductive amination of 1-undecanal from plant oils with diethylamine forming the long-chain tertiary N,N-diethylundecylamine is investigated. The many roles of solvent during the course of a chemical reactions become apparent when direct substrate-solvent and catalyst-solvent interactions are considered and their polarization effects. Hydrogen bond forming solvent molecules promote the enamine intermediate formation in terms of thermodynamics and kinetics by actively participating as proton transfer agents but a de-solvation penalty for polar groups compromises the overall pathway. The sophisticated bidentate phosphine (SulfoXantPhos)RhH reducing catalyst controls the regioselectivity of the reaction by dedicated ligand-substrate interactions. Its activity is critically dependent on the strength of solvent coordination. The effect of solvent on the reaction rate becomes apparent from a solvent screening of the transition state of the rate-determining step and give a perspective on solvent control of rate constants in this complex multi-step reaction. Only in presence of an appropriate solvent, the calculated Gibbs free potential energy surface becomes shallow and flat and delivers thermodynamic and kinetic parameters in good agreement with experiment.