Diruthenium(II,III) tetracarboxylates catalyzed H2O2 oxygenation of organic sulfides
Graphical abstract
Diruthenium(II,III) tetracarboxylates catalyze organic sulfide oxygenation by hydrogen peroxide under mild conditions, and the active species is likely a diruthenium(III,III) species generated upon the addition of hydrogen peroxide.
Introduction
Catalytic conversion of organic sulfides to sulfoxides and subsequently sulfones is a topic of interest in several chemical areas. In medicinal chemistry, chiral sulfoxide intermediates and products play a crucial role in pharmaceuticals such as omeprazole [1], [2], [3]. For the decontamination of chemical warfare agents such as mustard gas, selective conversion to sulfoxide largely eliminates the toxicity of the compounds, while complete oxygenation leaves the compound nearly as toxic as the initial sulfide [4]. On the other hand, complete conversion to sulfone is essential for the detoxification of V-type agents [5]. Lastly, in the field of fuel desulfurization, the oxygenation of refractory sulfides (ODS) is proposed to enable a more complete desulfurization than the traditional hydrodesulfurization (HDS) approach [6].
Bimetallic compounds have been explored as the group transfer catalysts for decades, and a prime example is the carbene transfer reactions using dirhodium compounds studied by Doyle and many others [7], [8]. In addition, dirhodium compounds have been found active in facilitating various oxidative organic transformations, such as allylic/benzylic oxidation and amine oxidation, where the redox flexibility of dirhodium species is a key [9], [10], [11], [12], [13], [14]. Diruthenium tetracarboxylates and related compounds with N,N′-bidentate ligands are highly redox active with formal oxidation states ranging from Ru2(I,II) to Ru2(III,IV) [15], [16], [17], [18], [19]. Inspired by Doyle’s success on dirhodium catalysis, our laboratory has examined organic sulfide oxygenation reactions catalyzed by diruthenium species: tert-butyl hydroperoxide (TBHP) oxygenation by Ru2(esp)2Cl [20], [21], aerobic oxygenation by Ru2(O2CR)3(CO3) [22] and H2O2/TBHP oxygenation by diruthenium(II,III) tetraamidate [23]. Reported herein are the H2O2 oxygenation facilitated by diruthenium catalysts including Ru2(OAc)4Cl (A), Ru2(esp)2Cl (B) and Ru2(3-hydroxybenzoate)4Cl (C), and related reaction kinetics. Also investigated is the activity and catalyst longevity of Na4[Ru2(hedp)2(H2O)Cl] (D, hedp = 1-hydroxyethylidenediphosphonate) [24], which is known to be stable in both the (II,III) and (III,III) states [25].
Section snippets
Preparation and characterization of Ru2(3-hydroxybenzoate)4Cl (C)
Compound C was prepared by refluxing Ru2(OAc)4Cl in methanol with 3-hydroxybenzoic acid for 24 h, and the product was authenticated using both mass spectrometry and elemental analysis. Single crystals suitable for X-ray diffraction analysis were grown via slow diffusion of hexanes into a THF solution of C, and the molecular structure of C is shown in Fig. 1. Similar to other structurally determined diruthenium(II,III) tetracarboxylates [20], [26], [27], Ru2(3-HB)4Cl displays the classical
Conclusions
Results reported herein demonstrated that diruthenium tetracarboxylates A–C are all effective in activating hydrogen peroxide in sulfide oxygenation reactions. In addition, catalysts B and C facilitate stepwise oxygenation, a feature that is potentially useful in medicinal chemistry. Using a diruthenium complex supported by bridging diphosphonates, D, it is demonstrated that (i) the active catalyst is likely to be a Ru2(III,III) species and (ii) the robustness of the (III,III) species assures
General
Solvent acetonitrile and organic reagents were purchased from Sigma Aldrich. Ru2(OAc)4Cl [44], Ru2(esp)2Cl [20] and Na4[Ru2(hedp)2(H2O)Cl] [25] were prepared according to the literature procedures. Organic sulfides were purchased from ACROS Organics. Hydrogen peroxide (30%) was purchased from Macron Fine Chemicals and standardized via iodometric titration. Samples of oxygenation reactions were analyzed by Agilent 7890A GC system equipped with a flame ionization detector. The separation of
Acknowledgement
We thank both Purdue University and the US Army Research Office (Grants DAAD 190110708 and W911NF-06-1-0305) for financial support. Leslie Villalobos was supported in part by an AGEP Supplement from the US National Science Foundation (CHE 1057621).
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