The oxidation of alcohols into the corresponding aldehydes or ketones is one of the most important functional group transformations in organic synthesis. 1 Recently, the use of molecular oxygen as terminal oxidant has received great attention for both economic and environmental benefits, and many highly efficient systems have been developed for catalytic aerobic alcohol oxidation using copper, 2 palladium, 3 or ruthenium catalysts. 4 Of particular interest are the catalysis systems involving both transition metals and nitroxyl radicals (e.g., 2,2,6,6-tetramethyl-piperidyl-1-oxy TEMPO). 5,6 However, only a few catalyst systems, for example, small amounts of cheap metal salts, together with TEMPO, provided an efficient catalyst for aerobic oxidation of alcohols under mild conditions. 6c,h Therefore, attempting to obtain more efficient processes, chemists have paid much attention to screening various transition metals and designing new ligands while largely ignoring the advantages inherent in a nonmetal catalytic system. We are particularly interested in exploring the potential of such a transition-metal-free catalyst system for aerobic alcohol oxidations.Our research was inspired by the results of the TEMPO-Cl 2 oxidation system by Bjørsvik et al. 7a and the TEMPO-Br 2 /I 2 system by Miller et al. 7b In their procedures, NaHCO 3 or Na 2 CO 3 was used to neutralize the coproduct HX (X ) Cl, Br, I). We reasoned that if HX can be oxidized to regenerate X 2 in situ by molecular oxygen rather than being scavenged by inorganic base, a TEMPO-catalyzed process with a catalytic amount of X 2 could be established. In this communication, we report a highly efficient catalytic system without transition metal for the aerobic oxidation of a variety of alcohols.Initial investigation of TEMPO-catalyzed (1 mol %) aerobic oxidation was carried out using benzyl alcohol as substrate with 4 mol % of Br 2 and 0.5 MPa of oxygen under 100°C for 1 h. The preliminary result (8.36% of conversion) clearly indicated the role of Br 2 as an active catalyst. Prolonging the reaction time to 5 h increased the conversion to 20.4%. 8 Recognizing that the first incorporation of molecular oxygen into the reaction system has been the keystone for successful aerobic oxidations, we sought to find a cocatalyst to bridge the gap between O 2 activation and HBr reoxidation. The ready availability and unique redox property of NaNO 2 as a source of NO under acidic conditions attracted our attention. 9 Although NaNO 2 alone showed almost no activity in TEMPO-catalyzed aerobic oxidation, when 4 mol % of Br 2 and 8 mol % of NaNO 2 were both employed in TEMPO-catalyzed aerobic oxidation, a highly efficient catalyst system emerged (eq 1). 8 The quantitative oxidation of benzyl alcohol can be achieved without acid either under 0.5 MPa of oxygen at 100°C or under 0.2 MPa of oxygen at 60°C. 8 Indeed, the transition-metal-free catalyst system for aerobic alcohol oxidations exhibited extremely high selectivities and are remarkably easy to control. After systematic op...

FeCl3-TEMPO-NaNO2 catalyses the selective and mild aerobic oxidation of a broad range of alcohols to the corresponding aldehydes and ketones.

Hydrochloric acid, a very inexpensive and readily available inorganic acid, has been found to cooperate exquisitely with NaNO(2)/TEMPO in catalyzing the molecular-oxygen-driven oxidation of a broad range of alcohol substrates to the corresponding aldehydes and ketones. This transition-metal-free catalytic oxidative conversion is novel and represents an interesting alternative route to the corresponding carbonyl compounds to the metal-catalyzed aerobic oxidation of alcohols. The reaction is highly selective with respect to the desired product when carried out at room temperature in air at atmospheric pressure. Notably, the use of very inexpensive NaNO(2) and HCl in combination with TEMPO for this highly selective aerobic oxidation of alcohols in air at ambient temperature makes the reaction operationally and economically very attractive. The results of mechanistic studies, performed with the aid of electrospray ionization mass spectrometry (ESI-MS), are presented and discussed. TEMPO, TEMPOH, and TEMPO(+) were observed in the redox cycle by means of ESI-MS. On the basis of these observations, a mechanism is proposed that may provide an insight into the newly developed aerobic alcohol oxidation.