Effect of a Substituent in Cyclopentadienyl Ligand on Iridium-Catalyzed Acceptorless Dehydrogenation of Alcohols and 2-Methyl-1,2,3,4-tetrahydroquinoline

Abstract

New iridium(III)-bipyridonate complexes having cyclopentadienyl ligands with a series of alkyl substituents were synthesized for the purpose of tuning the catalytic activity for acceptorless dehydrogenation reactions. A comparison of the catalytic activity was performed for the reaction of alcoholic substrates such as 1-phenylethanol, 2-octanol, and benzyl alcohol. The 1-t-butyl-2,3,4,5-tetramethylcyclopentadienyl iridium complex exhibited the best performance, which surpassed that of the 1,2,3,4,5-pentamethylcyclopentadienyl (Cp*) iridium catalyst in the dehydrogenation reaction of alcohols. The catalytic activity in the dehydrogenation of 2-methyl-1,2,3,4-tetrahydroquinoline was also examined. The highest efficiency was obtained in the reaction catalyzed by the same t-butyl-substituted cyclopentadienyl iridium complex.

Reusable Single Homogeneous Ir(III)–NHC Catalysts for Bidirectional Hydrogenation–Dehydrogenation of N-Heteroarenes in Water

“…Recently, in the context of reversible hydrogen-storage methods, bidirectional hydrogenation–dehydrogenation (BHD) of N-heteroarenes has been identified as an important and prospective catalytic process, and therefore a great deal of attention has been paid into this process (Figure A). − Gaseous H 2 can be loaded via hydrogenation of easily handleable unsaturated N-heteroarenes and stored therein for future use; a reverse process via dehydrogenation of the hydrogenated compounds can unload the stored H 2 gas for reutilization. With N-heteroarene platforms, many unidirectional homogeneous and heterogeneous catalysts were reported, which performed either hydrogenation or dehydrogenation under high pressure and high-temperature conditions in organic solvents. − Later, bidirectional hydrogenation–dehydrogenation using a single homogeneous and heterogeneous catalyst was developed, operating still under harsh conditions and mainly in organic solvents. − Use of water as a solvent for bidirectional hydrogenation–dehydrogenation under relatively mild conditions with just a single catalyst is a current challenge, and toward this, Albrecht and co-workers, and Fischmeister and co-workers recently discovered two significant systems based on homogeneous Ir complexes (Figure B). While both the systems performed dehydrogenation in refluxing water solution, Albrecht et al’s catalyst used 5 bar H 2 pressure at 90 °C and Fischmeister et al’s catalyst used atmospheric H 2 pressure at 80 °C for the hydrogenation process.…”

Iridium Complex Immobilized on Custom-Designed Periodic Mesoporous Organosilica as Reusable Catalyst for the Dehydrogenative Oxidation of Alcohols

“…With this background in mind, we developed a series of high-performance iridium catalysts that dehydrogenate organic molecules − and reported catalytic systems based on cooperativity between iridium and an α-pyridonate-structured ligand (Scheme ). This catalytic system contains two key elemental reactions: (1) the conversion of an alcohol to a carbonyl compound in which the iridium center acts as the hydride acceptor and the α-pyridonate ligand acts as the proton acceptor (step A) and (2) dehydrogenation by protonolysis of the iridium hydride in which the α-hydroxy­pyridine ligand acts as the proton donor (step B).…”