Selective oxidations of secondary alcohols

Selective oxidation of secondary alcohols to ketones is usually performed with CrOj/HjSO, I I in acetone (Jones reagent) or with CrOjPyj (Collin s reagent) in the presence of acid-sensitive groups (H.G. Bosche, 1975 C. Djerassi, 1956 W.S. Allen, 1954). As mentioned above, a,)S-unsaturated secondary alcohols are selectively oxidized by MnOj (D.G. Lee, 1969 D. Arndt, 1975) or by DDQ (D. Walker, 1967 H.H. Stechl, 1975). 

IODOXYBENZOIC ACID (IBX)//i-Bu4NBr/CH2Cl2-H20 A MILD SYSTEM FOR SELECTIVE OXIDATION OF SECONDARY ALCOHOLS... 

Associated with BiCl3 and LiCl, RhCl3 acts as a very effective catalyst for the selective oxidation of secondary alcohols to carbonyl compounds by 02 under mild conditions. Selectivities up to 90% and turnovers numbering up to 200 have been obtained (equation 62).213... 

A scant look at the facts might suggest that the selective oxidation of primary alcohols in TEMPO-mediated oxidations can be explained solely on steric grounds. Things are not so simple, as it was found8 that the primary oxidants, that is oxoammonium salts, when used stoichiometrically, react quicker with primary alcohols when present as oxoammonium chlorides, while the reverse selectivity, that is selective oxidation of secondary alcohols, is observed when oxoammonium bromides are employed. 

Interestingly, when a Corey-Kim oxidation (Me2S/NCS) is performed with diisopropyl sulfide, instead of dimethyl sulfide, primary alcohols are selectively oxidized at 0°C, while lowering the temperature to —78°C causes the selective oxidation of secondary alcohols.34... 

Selective Oxidations of Secondary Alcohols in Presence of Primary Alcohols... 

Primary alcohols possess a considerably less congested environment than secondary ones. Therefore, it may seem contradictory that a certain oxidant could be able to perform the selective oxidation of secondary alcohols. On the other hand, the oxidation potential of aldehydes is generally higher than the one of ketones (see page 257). This means that thermodynamics usually favor the oxidation of secondary alcohols over primary ones and mild oxidants have a tendency to react quicker with secondary alcohols. Other factors that promote the selective oxidation of secondary alcohols include the intermediacy of alkyl hypohalides, which are less stable when derived from secondary alcohols, and the operation of a mechanism involving a hydride transfer, leaving a carbocation located at the a position of an alcohol that possesses a higher stability in secondary alcohols. 

Among common alcohol oxidants, Fetizon s reagent—due to its mildness—is particularly well-suited for the selective oxidation of secondary alcohols (see page 283). 

On the other hand, Fetizon s reagent is very sensitive to steric hindrance and no selective oxidation of secondary alcohols is possible in many complex substrates.4... 

In 1952, Kritchevsky et al.1 reported the selective oxidation of a secondary alcohol in the presence of a primary one with /V-bromoacetamide. In 1954, Jones and Kocher highlighted8 the importance of being able to carry out selective oxidations of secondary alcohols with /V-bromoacetamide, which was employed later by other authors for this purpose.9... 

In 1980, Stevens et al.10 reported that a plain solution of sodium hypochlorite, which is easily available as swimming pool chlorine , is able to efficiently oxidize secondary alcohols in a solution in acetic acid, while primary alcohols react very slowly. Two years later, this research team published11 a more detailed account on the ability of NaOCl/AcOH to perform the selective oxidation of secondary alcohols in the presence of primary ones. Stevens oxidant became one of the standard reagents for the selective oxidation of secondary alcohols.12... 

General Procedure for Selective Oxidation of Secondary Alcohols in Presence of Primary Alcohol, Using Stevens Protocol (Sodium Hypochlorite in Acetic Acid)... 

In 1976, Ueno and Okawara highlighted the fact that no oxidation of primary saturated alcohols to aldehydes via tin alkoxides had been reported in the literature and published a procedure for the selective oxidation of secondary alcohols.25 Interestingly, rather than performing the oxidation on pre-formed tin alkoxides, these researchers subjected a mixture of the diol and (Bu3Sn)20 in CH2C12 to the action of Br2. Regardless of the fact that no complete formation of tin alkoxides is secured and no HBr quencher is added, this method may provide useful yields of hydroxyketones during the selective oxidation of diols.26... 

Other molybdenum complexes able to catalyze the selective oxidation of secondary alcohols are ammonium molybdate in the presence of H202,30 benzyltrimethylammonium tetrabromooxomolybdate in the presence of r-BuOOH31 and molybdenum hexacarbonyl in the presence of catalytic cetylpyridinium chloride and stoichiometric t-BuOOH.32... 

Several compounds of tungsten, which is a transition metal closely related to molybdenum, are able to catalyze the selective oxidation of secondary alcohols with hydrogen peroxide as secondary oxidant. These include tris(cetylpyridinium) 12-tungstophosphate,33 peroxotungstopho-sphate (PCWP)34 and Na2W04 in the presence of a phase transfer catalyst.35 Tungstophosphoric acid is able to catalyze the selective oxidation of secondary alcohols in the presence of ferric nitrate as secondary oxidant.36... 

Cerium (TV) ammonium nitrate (CAN)37 and a cerium (TV) impregnated resin38 are able to catalyze the selective oxidation of secondary alcohols with sodium bromate (NaBrOi). Stoichiometric cerium bromate— CelBrCb h, prepared in situ from barium bromate and cerium (III) sulfate, is also able to perform selective oxidations of secondary alcohols.39... 

Other transition metal compounds able to catalyze the selective oxidation of secondary alcohols include VO(acac)2 with t-BuOOH as secondary oxidant,40 a polystyrene-supported (catecholato)oxorhenium complex in the presence of DMSO 41 and a mixture of ferric nitrate and ferric bromate that catalyzes the oxidation of secondary alcohols with air.42... 

Both, sodium bromite (NaBr02)51 and sodium bromate (NaBr03)52 are able to carry out selective oxidations of secondary alcohols in the absence of an added catalyst under properly devised experimental conditions. 

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