Reactivity of electrophilic Cp*Ru(NO) complex towards alcohols. (16th August 2016) - Diaz, Jason
Graphical abstract: Facile alcohol oxidation to aldehydes and ketones is achieved in stoichiometric reaction with Cp*Ru(NO)OTf2 complex, with generation of RuRu dimeric product. Alcohol binding pre-equilibrium step and kinetics of the oxidation reaction are investigated by NMR spectroscopy methods. Chelate-stabilized diol adduct is structurally characterized. Abstract: Treatment of the complex1 Cp*Ru(NO)(OTf)2 (OTf = OSO2 CF3, Cp* = η 5 -C(CH3 )5 ) with neat 2-propanol results in the rapid quantitative formation of the Ru(0) complex [Cp*Ru(μ-NO)]2 and (CH3 )2 CO. Formation of H2 and CHDCl2 is detected when the reaction between1 and 2-propanol occurs in CDCl3, indicating possible formation of a short-lived metal hydride species. Similar results are observed upon treatment of1 with ethanol and methanol, with formation of acetaldehyde and formaldehyde, respectively. The kinetics of the oxidation of 2-propanol by complex1 is studied by 1 H NMR spectroscopy in CH2 Cl2 at variable temperatures and the reaction is found to be first-order in complex1 and in 2-propanol. The kinetic isotope effect for the reaction of1 with (CD3 )2 CD-OD at −11 °C is determined to be k H / k D = 2.0 (3). A mechanism for alcohol oxidation by electrophilic ruthenium (II) complexes via a β-hydrogen elimination step is proposed. The pre-equilibrium exchange step between complex Cp*Ru(NO)(OTf)2 (1 ) and alcohol-coordinated species is examined by 19 F and 1 H NMR spectroscopy in CH2 Cl2 solution, where triflate substitution is found to be exothermic and entropically unfavorable. The synthesis and solid state structure of the chelate-stabilized complex diol salt [Cp*Ru(NO)(HO–CH2 CH2 –OH)][2OTf] are discussed.