cis-Oxoruthenium complexes supported by chiral tetradentate amine (N4) ligands for hydrocarbon oxidations

We report the first examples of cis-dioxo ruthenium(vi) complexes supported by chiral N4 ligands and their stoichiometric and catalytic reactivities with alkanes and alkenes.


cis-[(Me 2 mcp)Ru III Cl 2 ]ClO 4 (2a).
The procedure was similar to that for the preparation of 1a, except that 6-Me 2 mcp, instead of mcp, was employed as ligand.
Recrystallization of the crude product by slow diffusion of diethyl ether into an acetonitrile solution afforded 2a as an orange-red crystalline solid. Yield: 32%. Anal. Calcd

Stoichiometric organic oxidations by 1e
To a 100-mL Schlenck flask was added alkene (30 mmol), tert-butanol (10 mL) and distilled water (2 mL). The mixture was degassed by three freeze-pump-thaw cycles and filled with argon. The cis-dioxoruthenium complex 1e (300 mol) was added under a positive pressure of argon, and the reaction mixture was stirred magnetically at room temperature for 30 min. To work-up, the organic products were separated from the reaction mixture by diethyl ether extraction (3  50 mL). After washing with brine (2  10 mL) and drying over MgSO 4 , the volume of the ethereal extract was reduced to about 3 mL for GC analysis and/or column chromatographic purification.
Similar experimental procedures were employed when acetonitrile was used as solvent for the alkene oxidation. The organic products (e.g. cis/trans-diols, carbonyl compounds) were identified by 1 H and 13 C NMR spectroscopy by comparing with authentic or literature data. Enantiomeric excess of the diol products was determined by chiral HPLC.
Similar procedures were employed in the oxidation of alcohols and alkanes ( Table 3 in main text), whereas acetonitrile was used as solvent.

Catalytic alkene oxidation by 1b
To a mixture of tert-butanol (2 mL) and distilled water (1 mL) containing substrate (0.5 mmol) was added catalyst 1b (1 mol%). Then, NaIO 4 (1.1 mmol, 2.2 equiv.) was added to the reaction mixture all at once. The reaction mixture was reacted at room temperature for 1 h. To work-up, any unreacted oxidant was quenched by saturated S10 Na 2 SO 3 solution (2 mL), followed by extraction with ethyl acetate (3  10 mL). The combined organic extracts were dried over MgSO 4 , and subjected to GC analysis and/or column chromatographic purification. The organic products were identified by GC by comparing with authentic sample and/or NMR spectroscopy of the isolated product.

Catalytic alkane oxidation by cis-[(N 4 )Ru II (OH 2 ) 2 ] 2+ complexes
To a mixture of substrate (0.2-0.25 mmol) and Ru catalyst (2-5 mol%) in tert-butanol (2 mL) and distilled water (2 mL) was added CAN (3 equiv.). In some entries depicted in the Tables, after the intense orange-red color of CAN had disappeared (in 10 min to 1 h), another portion of CAN (3 equiv.) was added to the system. The reaction mixture was stirred at room temperature for the time specified in each entry. To work-up, the mixture was treated with saturated Na 2 SO 3 solution (2 mL), followed by extraction with ethyl acetate (3  10 mL). Organic products were identified and quantified by GC-MS (internal standard = 1,4-dichlorobenzene) by comparing with authentic samples, or by NMR spectroscopy after purification/isolation by chromatography on silica gel. S11 Scheme S1 Preparation of 3c.   Oxidation of racemic 2-phenylhexane by "(R,R)-3c + CAN" was conducted at 0 C and compared with other ruthenium catalysts (1b, 4c-6c). Complex 3c gave the best result which afforded the tertiary alcohol product in quantitative yield based on 14% conversion (TON = 14). However, none of the catalysts gave product with significant enantiomeric excess (ee <5%).

Catalytic alcohol oxidations by cis-[Ru III (mcp)(O 2 CCF 3 ) 2 ]ClO 4 (1b) with H 2 O 2
Previously, we showed that [Ru III (Me 3 tacn)(O 2 CCF 3 ) 2 (OH 2 )]CF 3 CO 2 is an effective catalyst for the oxidation of alcohols using tert-butylhydroperoxide or hydrogen peroxide as terminal oxidant. 13,14 Prompted by these findings and the reactivity of 1e, we have examined the catalytic activities of 1b for oxidation of alcohols using H 2 O 2 as a terminal oxidant. When 35% aqueous H 2 O 2 (0.22 mL, 2.2 mmol) was added dropwise via syringe pump to a mixture of benzyl alcohol (1 mmol) and 1b (10 μmol; 1 mol%) in refluxing aqueous tert-butanol, benzoic acid (91%) and benzaldehyde (5%) were formed with 100% substrate conversion (Table S6, entry 2). However, when the reaction was conducted without dropwise addition of H 2 O 2 , the substrate conversion was markedly reduced to only 20% with benzaldehyde obtained in 78% yield (based on conversion, Table S6, entry 1).           S20). We propose that intramolecular oxidation of the ortho-methyl group has occurred via a highly reactive cis-dioxoruthenium(VI) intermediate: Similar oxygenation of aliphatic C-H bond adjacent to pyridine ring has been documented. 15