Transition-Metal-Free:  A Highly Efficient Catalytic Aerobic Alcohol Oxidation Process

Abstract

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...

Discovery and Exploitation of AZADO: The Highly Active Catalyst for Alcohol Oxidation

“…[4][5][6][7][8][9][10][11] In recent years, a stable class of nitroxyl radicals, 12) as exemplified by 2,2,6,6-tetramethyl piperidinyl 1-oxyl (TEMPO) [(1); Chart 1], has extensively been used as a catalyst for oxidation of alcohols, because TEMPO is readily available from chemical suppliers at a reasonable price, and because the method allows the use of various safe bulk oxidants, thereby offering safe and extremely efficient oxidation of alcohols with considerable operational simplicity. [13][14][15][16][17][18][19][20][21][22][23][24][25] Today, TEMPOcatalyzed alcohol oxidation has high priority not only in academic laboratories, but also in the chemical industries, particularly in the pharmaceutical industry, as an efficient, mild, and environmentally acceptable method.…”

Co2+-exchanged SAPO-5 and SAPO-34 as efficient heterogeneous catalysts for aerobic epoxidation of alkenes

“…The rapid decomposition of H 2 O 2 over the Co-catalyst at 363 K could be responsible for its low efficiency on the reaction. TEMPO used widely in the selective oxidation of alcohols [32], did not display any effect under our experimental conditions. Table 4 lists the effect of various solvents on the epoxidation of styrene with air over Co-SAPO-34.…”

β-Cyclodextrin coated Fe3O4 nanoparticles: a simple preparation and application for selective oxidation of alcohols in water

“…Obviously, all the primary benzylic alcohols tested were converted into their corresponding aldehydes in high yields and no overoxidation to acids was observed (entries 1-7). It is noteworthy that a type of heterocyclic alcohol (entries 8-10), being less active in many reported systems, worked well in the Fe 3 O 4 /SiO 2 /CM−β-CD/H 2 O/ NaClO system (entries [8][9][10]. Together with the fact that substituted groups in benzene ring lowered the reactivity, the space configuration of guest molecules was proposed to be an important factor to initiate the reaction.…”

“…1,2 Many efforts have been devoted to the field with a variety of catalysts been developed such as metal complexes, [3][4][5][6] tetramethylpiperidinyloxide (TEMPO), [7][8][9] heteropoly acids 10,11 and so on. Nevertheless, organic solvents are usually used to promote the reaction process, which is considered to be contrary to the concept of green chemistry.…”