Application and Mechanistic Studies of a Water‐Oxidation Catalyst in Alcohol Oxidation by Employing Oxygen‐Transfer Reagents

Abstract

By using a dimeric ruthenium complex in combination with tert-butyl hydrogen peroxide (TBHP) as stoichiometric oxidant, a mild and efficient protocol for the oxidation of secondary benzylic alcohols was obtained, thereby giving the corresponding ketones in high yields within 4 h. However, in the oxidation of aliphatic alcohols, the TBHP protocol suffered from low conversions owing to a competing Ru-catalyzed disproportionation of the oxidant. Gratifyingly, by switching to Oxone (2 KHSO(5)⋅KHSO(4)⋅K(2)SO(4) triple salt) as stoichiometric oxidant, a more efficient and robust system was obtained that allowed for the oxidation of a wide range of aliphatic and benzylic secondary alcohols, giving the corresponding ketones in excellent yields. The mechanism for these reactions is believed to involve a high-valent Ru(V)-oxo species. We provide support for such an intermediate by means of mechanistic studies.

Artificial Photosynthesis: Molecular Systems for Catalytic Water Oxidation

“…The authors had previously managed to find support for a high-valent Ru V –oxo species by the use of HRMS for the related dimeric ruthenium complex 48 , and also in this study the methodology proved to be successful. Upon the addition of 10 equiv of the oxidant [Ru­(bpy) 3 ] 3+ to a solution containing complex 205 , it was possible to distinguish a signal by HRMS that corresponded to [Ru V O + H + ] + in positive mode, and with a isotope pattern matching the theoretically expected .…”

“…The Pourbaix diagram of complex 204 demonstrated that all three oxidation steps, from the Ru II state to Ru V , were pH-dependent with a slope of −59 mV per pH unit, indicating that PCET plays a fundamental role during the four-electron oxidation of H 2 O. 276 The authors had previously managed to find support for a high-valent Ru V −oxo species by the use of HRMS for the related dimeric ruthenium complex 48, 277 and also in this study the methodology proved to be successful. Upon the addition of 10 equiv of the oxidant [Ru(bpy) 3 ] 3+ to a solution containing complex 205, it was possible to distinguish a signal by HRMS that corresponded to [Ru V O + H + ] + in positive mode, and with a isotope pattern matching the theoretically expected.…”

Copper/Imidazolium/TEMPO-Catalyzed Aerobic Oxidation of Benzylic Alcohols­ in Water

“…[1][2][3] Aqueous reactions are especially important for oxidation processes since reactions that are possibly explosive under oxidation conditions can be avoided. [4][5][6][7] N-Heterocyclic carbenes (NHCs) represent appropriate ligands for aqueous metalcatalyzed oxidation reactions since metal complexes of NHCs show high thermal, air, and moisture stability. [8][9][10] Although water-soluble NHC complexes have been explored as catalysts in a number of C-C coupling reactions, their application as an oxidation catalyst is not previously known.…”

Catalytic Water Oxidation by Ruthenium Complexes Containing Negatively Charged Ligand Frameworks

“…[73] In aqueous phosphate buffer solutions at pH 7.2, Ru complex 37 displayed an electrochemical onset potential for H 2 0 -dicarboxylate), increased the TON to 250. In the employed photocatalytic system, addition of more photosensitizer led to sustained production of O 2 , suggesting that the decomposition of the photosensitizer was the main limitation of the studied photocatalytic system.…”

“…Initial experiments employed Ce IV as the oxidant and showed that O 2 evolution was immediately triggered upon addition of the oxidant to aqueous solutions of complex 37 72. A subsequent study suggested a first‐order dependence with respect to complex 37 using Oxone® as the oxidant 73…”