Oxidation catalytic procedures

Nitrogen dioxide (NO ) reacts with a wide variety of functional groups and it is the reagent of choice for a number of synthetic transformations. For example, the selective oxidation of sulfide to sulfoxide by NO forms the basis for the commercial production of dimethyl sulfoxide (from dimethyl sulfide) via a catalytic procedure (see below).250 Some representative examples of oxidative transformations carried out with NO are presented in Chart 8. 

Reactions of 1,2,4-thiadiazoles with radicals and carbenes are virtually unknown. Catalytic hydrogenations and dissolving metal reductions usually cleave the N-S bond in a reversal of the oxidative cyclization procedures used in synthesis of 1,2,4-thiadiazoles (see Section 5.08.9.4). 

Since the reagent is quite expensive, different catalytic procedures have been developed. A very useful procedure involves an amine oxide, such as morpholine-A-oxide, as the stoichiometric secondary oxidant (Scheme 10.4) [29]. 

Method C2 (typical catalytic procedure).A 5% solution of NaBH4 in 5% aqueous NaOH is added dropwise under N2 to a solution of a-bromophenylacetic acid (1.50 g, 7.0 mmol) and bis(2-thienyl) ditelluride (0.30 g, 0.71 mmol) in EtOH (40 mL) until the red colour of the ditelluride just disappears. At this point air is infioduced in the system to oxidize the catalyst back to the ditelluride. The mixture is then partitioned in ether/5% aqueous NaOH. The aqueous phase is separated, acidified with 2 M HCl and extracted with ether. The extract is dried (CaCl2) and evaporated to give phenylacetic acid (0.93 g (98%) m.p. 77°C). 

Considering that diaryl tellurides debrominate vic-dibromides, forming diarylteUurium dibromides (see Section 4.1.12.1), which in tnm can be easily hydrolysed to the corresponding tellnroxides by aqueous bases, a catalytic procedure has been provided for the oxidation of thiocarbonyl compounds. 

Method E - catalytic procedure (typical procedure). Benzaldehyde (106 mg, 1.0 mmol), methyl bromoacetate (165 mg, 1.1 mmol), triphenyl phosphite (356 mg, 1.2 mmol), dibutyl telluride (48 mg, 0,2 mmol), KjCOj (179 mg, 1.3 mmol) and THF (4 mL) are mixed and stirred at 50°C for 13 h (monitored by TLC). The reaction mixture is filtered rapidly through a small amount of SiOj with EtOAc as the eluent to remove inorganic salts and dibutyltellurium oxide. Preparative TLC with EtOAc/petroleum ether at 60-90°C (1 9) as the eluent yields 3-phenylpropenoate (160 mg (98%)). 

Butyl hydroperoxide,37 barium chlorate,38 or potassium ferricyanide39 can also be used as oxidants in catalytic procedures. Scheme 12.6 provides some examples of oxidations of alkenes to glycols by permanganate and by osmium tetroxide. 

Oxidation of cyclic ethers to lactones (1, 988). A systematic study of this reaction has been reported. In general, yields are good to excellent by either stoichiometric or catalytic procedures. No anhydrides from further oxidation are detected. The oxidation is chemoselective. Oxidation of a secondary position takes precedence over oxidation of a tertiary site. Primary positions are attacked in preference to secondary positions in the oxidation of acyclic ethers.1... 

In the noble metal-catalyzed oxidations described above vicinal diol cleavage is sometimes observed as a side-reaction but never as a main reaction. Oxidative diol cleavage usually involves stoichiometric oxidants such as periodate (Malaprade oxidation) and there is a great need for catalytic procedures employing inexpensive, clean oxidants such as O2 or H2O2. 

The copper chromium oxide (Cu/Cr = 1) has been prepared by coprecipitation of copper and chromium nitrates with ammonium hydroxide, followed by thermal decomposition in flowing nitrogen up to the final temperature (370"C), according to a previously described method (8). The apparatus and the catalytic procedure have also been described elsewhere in case of gas phase reactions (5) and liquid phase reactions (7). 

The vinyl substitution reaction often may be achieved with catalytic amounts of palladium. Catalytic reactions are carried out in different ways depending on how the organopalladium compound is generated. Usually copper(II) chloride or p-benzoquinone is employed to reoxidize palladium(0) to palla-dium(II) in catalytic reactions when methods (i) or (ii) are used for making the organopalladium derivative. The procedures developed for making these reactions catalytic are not completely satisfactory, however. The best catalytic reactions are achieved when the organopalladium intermediates are obtained by the oxidative addition procedures (method iii), where the halide is both the reoxidant and a reactant. Reviews of some aspects of these reactions have been published.u-le... 

The catalytic procedure described here allows a fast, cheap and highly selective conversion of primary alcohols into aldehydes, using sodium hypochlorite as the oxidant in a two-phase (dichloromethane-water) system. Aqueous sodium hypochlorite is buffered at pH 8.6-9.5 to ensure the presence of hypochlorous acid in the organic layer.13... 

Catalyzed oxidations.1 In catalytic procedures with Ru04, periodate or hypochlorite are generally used as the stoichiometric oxidants. The addition of acetonitrile, which is inert to oxidation but an effective ligand for lower valent transition metals, results in much higher yields. A third solvent, chloroform, also plays a significant part. The ruthenium tetroxide is generated in situ from RuCl, (H20)n or Ru02 with sodium or potassium metaperiodate sodium hypochlorite is less effective. 

Recently, an alternative to the catalytic system described above was reported [204]. The new catalytic procedure for the selective aerobic oxidation of primary alcohols to aldehydes was based on a CunBr2(Bpy)-TEMPO system (Bpy=2,2 -bipyridine). The reactions were carried out under air at room temperature and were catalyzed by a [copper11 (bipyridine ligand)] complex and TEMPO and base (KOtBu) as co-catalysts (Fig. 4.70). 

This procedure has been modified to become an effective catalytic procedure in which iV-methyl-moipholine A -oxide is used as the secondary oxidant. In this manner, ( )-stilbene has been converted into (+)-r/irco-hydrobenzoin (55% yield after two reciystallizations, >99% ee) on a one molar scale, by treatment with osmium tetroxide (0.002 mol equiv.) and iV-methylmoipholine 1 -oxide (1.2 mol equiv.) in aqueous acetone in the presence of dihydroquinidine p-chlorobenzoate (0.134 mol equiv.). The latter compound can be recovered in 91% yield. 

For the oxidation of alkenes, osmium tetroxide is used either stoichiometrically, when the alkene is precious or only small scale operation is required, or catalytically with a range of secondary oxidants which include metal chlorates, hydrogen peroxide, f-butyl hydroperoxide and N-methylmorpholine A -oxide. The osmium tetroxide//V-methylmorpholine A -oxide combination is probably the most general and effective procedure which is currently available for the syn hydroxylation of alkenes, although tetrasubstituted alkenes may be resistant to oxidation. For hindered alkenes, use of the related oxidant trimethylamine A -oxide in the presence of pyridine appears advantageous. When r-butyl hydroperoxide is used as a cooxidant, problems of overoxidation are avoided which occasionally occur with the catalytic procedures using metal chlorates or hydrogen peroxide. Further, in the presence of tetraethylam-monium hydroxide hydroxylation of tetrasubstituted alkenes is possible, but the alkaline conditions clearly limit the application. 

A highly effective catalytic method for alkynylation of epoxides has recently been reported this involves the chelation-controlled alkylation of hetero-substituted epoxides with Mc3A1 and alkynyllithiums via pentacoordinate organoaluminum complexes [82]. For instance, reaction of epoxy ether, (l-benzyloxy)-3-butene oxide (75) in toluene with PhC = CLi under the influence of catalytic MesAl (10 mol%) proceeded smoothly at 0 °C for 5 h to furnish the alkynylation product l-(benzyloxy)-6-phenylhex-5-yn-3-ol (76) in 76 % yield. The yield of the product was very low (3 %) without MeaAl as catalyst under similar conditions. This is the first catalytic procedure for amphiphilic alkylation of epoxides. The participation of pentacoordinate MesAl complexes of epoxy ethers of type 75 is emphasized by comparing the reactivity with the corresponding simple epoxide, 5-phenyl-l-pentene oxide (77), which was not susceptible to nucleophilic attack of PhC s CLi with catalytic Me3Al under similar conditions (Sch. 50). 

Macrocyclic metal complexes have recently attracted attention as dioxygen activating catalysts in oxidation reactions. A triple catalytic procedure [1,2] involving three redox systems Pd(II)/Pd(0) - benzoquinone/hydroquinone - ML° /ML was developed for the aerobic oxidation reactions. The multistep electron transfer occurs in the following way electron transfer occurs from the substrate to Pd (II), giving Pd (0), followed by another electron transfer from Pd (0) to benzoquinone. The hydroquinone thus formed, transfers electrons to the oxidized form of the metal macrocycle, which is reduced. The latter is reoxidized by electron transfer to molecular oxygen. 

Osmium tetraoxide has also been used in the oxidation of bicyclic and polycyclic dienes. Thus, oxidation of norbomadiene (26) in a stoichiometric reaction was found to yield the exo-cis diol exclusively. On the other hand, in the NMO catalytic system a mixture of the exo-cis and endo-cis products was reported. However, by use of the NMO catalytic procedure for the substituted norbomadiene 27, the exc-diol was formed exclusively at the sterically crowded unsubstituted double bond and this product was utilized in the synthesis of pentalenolactone. Somewhat surprisingly, oxidation of hexamethyl Dewar benzene (28) exclusively gave the endo-cis diol as sole product. The tricyclic compound 29 gave the usual c/s-diol oxidation product of one of the double bonds. ... 

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