Transition-metal catalyzed alcohol acceptorless dehydrogenation (AAD) is an atom-economical and environmentally benign alcohol oxidation approach that yields an equivalent of hydrogen gas for every ketone or aldehyde product. Incorporation of pendent alcohols or amines into the secondary ligand coordination sphere of transition metal catalysts has led to increased rates in AAD catalytic reactions. While this metal-ligand cooperative effect has been observed in several catalyst systems, the effect of tuning the proton affinity and basicity of the pendent bases on catalytic performance remains underexplored. To probe these effects, our lab has been engaged in preparing a series of biphenyl bisphosphine-type ligands (^2-XBIPHEP). Our functionalized ligand contains carboxylic acids (^2-COOHBIPHEP), which will be late-stage converted into pendent amide, alcohol, and amine groups. Our non-functionalized ‘control’ ligand contains pendent ethyl groups (^2-EtBIPHEP), which were chosen to give a comparable steric profile to our base-functionalized ligands.

In this presentation, I will discuss two distinct synthetic routes we’ve used to prepare BIPHEP-type ligands. Non-functionalized ^2-EtBIPHEP was synthesized in four steps and involves a final Cu-catalyzed Ar-Ar cross-coupling step to form the biphenyl backbone in the last step. Functionalized ^2-COOHBIPHEP was synthesized in four steps and involves a penultimate Pd-catalyzed Ar-P cross-coupling step to add two phosphine moieties to a biphenyl backbone. In this synthesis, 4,4-dimethyl-2-phenyl-2-oxazolyl protecting groups were used to protect the carboxylic acids from the basic and reducing conditions used throughout the synthesis. In the final portion of my talk, I will discuss the synthesis route and full characterization of prepared Ru(II)(^2-XBIPHEP) complexes and their initial performance results in catalytic alcohol acceptorless dehydrogenation.

Sawyer Markham

Appearances