Synthesis and Catalytic Hydrogen Transfer Reaction of Ruthenium(II) Complex

Abstract

The catalytic hydrogenation of a ketone is a basic and critical process for making many types of alcohols used as the final products and precursors in the pharmaceutical, agrochemical, flavor, fragrance, materials, and fine chemicals industries. 1 The catalytic hydrogenation process developed by Noyori is a very attractive process. Formic acid and 2propanol have been used extensively as hydrogenation sources. The advantage of using 2-propanol as a hydrogen source is that the only side product will be acetone, which can be removed easily during the workup process. 2 Hydrogen transfer (HT) catalysis, which generates alcohols through the reduction of ketones, is an attractive protocol that is used widely. Ruthenium(II) complexes are the most useful catalysts for the hydrogen transfer (HT) of ketones. 3 In this method, a highly active catalytic system employs a transition metal as a catalyst to synthesize alcohols, and is a replacement for the hydrogen-using hydrogenation process. The most active system is based on Ru, Rh and Ir, which includes a nitrogen ligand that facilitates the formation of a catalytically active hydride and phosphorus. 4 Accordingly, this paper reports the best results for the hydrogen transfer reaction using novel Ru(II) complexes. Research examining the effectiveness of the catalysts employed acetophenone as the benchmark substrate (Scheme 1). A suspension of RuCl3·3H2O (0.331 g, 1.6 mmol) and 2,2'-bipyridine (0.5 g, 3.2 mmol) in DMF (3.0 mL) was heated under reflux for 12 hours. [PhenTPy] dissolved in 100 mL of hot ethanol was added to 0.50 g (0.96 mmol) of cis-Ru-(bpy)2Cl2, 4e dissolved in 50 mL of hot H2O. The solution was deaerated with argon for 20 minutes and heated under reflux for three hours in an argon atmosphere (Scheme 2). As shown in Figure 1 (R1, black line), a well-defined redox couple (II/II') was observed at +0.13/+0.22 V, corresponding to the Ru III /Ru II reaction. 4f The Ru complex exhibits a redox peak because dimethylsulfoxide (DMSO) is an ambidentate ligand known for linkage isomerization that is dependent on the presence of O-bound or S-bound DMSO moieties. The redox couple appearing at the lower potential indicates that ruthenium interacts with the O-bound DMSO ligand. 4g The redox peaks of the ligand, however, appeared at (I/I') and (III/III'). These were assigned to the redox reaction of the phenanthroline moiety of the PhenTPy ligand