Abstract
A series of cyclometalated iridium(III) complexes that have the general formula [(C^N)2Ir(NR)(X)] (C^N = monoanionic bidentate cyclometalating ligands; NR = pyridine derivatives; X = Cl− or I−) are designed, prepared, and applied for the transformation of toluene to benzaldehyde using a clean, highly efficient, and environmentally-friendly process. The activation energies that are needed for the catalytic oxidation of toluene when using these complexes as catalysts are quite low: between 22.9 and 30.8 kcal mol−1. The catalytic frequencies (TOF) are fairly high (up to 7.0 × 102 h−1) with excellent reliability, and the turnover number (TON) can reach 4.2 × 103 after 6 h of processing time. Catalytic tests, X-ray absorption near-edge structure (XANES), and kinetic modeling are used to derive detailed insights into the characteristics of the catalysts and their effects on the reactions that are featured in the catalytic oxidation of toluene.
Highlights
The selective oxidation of toluene to benzaldehyde is an important issue in modern synthetic chemistry and pharmaceutical industry, as it provides the potential for the synthesis of fine chemicals and intermediates in an innovative fashion [1,2,3,4]
We have reported some cyclometalated iridium(III) complexes composed of benzoxazole ligand that can spontaneously release metalloradicals to promote two types of reactions: the carbon–hydrogen bond activation (CHA) and carbon–carbon bond activation (CCA) for ketones and aldehydes [5]
We have demonstrated that the cyclometalated iridium(III) complexes could directly transform toluene into benzaldehyde [6]
Summary
The selective oxidation of toluene to benzaldehyde is an important issue in modern synthetic chemistry and pharmaceutical industry, as it provides the potential for the synthesis of fine chemicals and intermediates in an innovative fashion [1,2,3,4]. We have demonstrated that the cyclometalated iridium(III) complexes could directly transform toluene into benzaldehyde [6]. Benzaldehyde was produced through the oxidation of benzyl alcohol using stoichiometric amounts of halogenated reagents or chromium salts. This noncatalytic method showed a severe drawback in that the oxidant was environmentally unfriendly; tremendous efforts have been devoted to the design of catalytic systems that use O2 as the primary oxidant [7,8,9,10,11,12,13,14,15,16,17,18,19,20]. Benzyl alcohols are commonly obtained through the halogenation of benzyl compounds; it is still a stoichiometric reaction with a highly harmful process
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