Amines selective oxidation

Nickel peroxide is a solid, insoluble oxidant prepared by reaction of nickel (II) salts with hypochlorite or ozone in aqueous alkaline solution. This reagent when used in nonpolar medium is similar to, but more reactive than, activated manganese dioxide in selectively oxidizing allylic or acetylenic alcohols. It also reacts rapidly with amines, phenols, hydrazones and sulfides so that selective oxidation of allylic alcohols in the presence of these functionalities may not be possible. In basic media the oxidizing power of nickel peroxide is increased and saturated primary alcohols can be oxidized directly to carboxylic acids. In the presence of ammonia at —20°, primary allylic alcohols give amides while at elevated temperatures nitriles are formed. At elevated temperatures efficient cleavage of a-glycols, a-ketols... 

One of the exciting results to come out of heterogeneous catalysis research since the early 1980s is the discovery and development of catalysts that employ hydrogen peroxide to selectively oxidize organic compounds at low temperatures in the liquid phase. These catalysts are based on titanium, and the important discovery was a way to isolate titanium in framework locations of the inner cavities of zeolites (molecular sieves). Thus, mild oxidations may be run in water or water-soluble solvents. Practicing organic chemists now have a way to catalytically oxidize benzene to phenols alkanes to alcohols and ketones primary alcohols to aldehydes, acids, esters, and acetals secondary alcohols to ketones primary amines to oximes secondary amines to hydroxyl-amines and tertiary amines to amine oxides. 

The nickel hydroxide electrode resembles in its applications and selectivity the chemical oxidant nickel peroxide. The nickel hydroxide electrode is, however, cheaper, easy to use and in scale-up, and produces no second streams/ waste- and by-products [196], Nickelhydroxide electrode has been applied to the oxidation of primary alcohols to acids or aldehydes, of secondary alcohols to ketones, as well as in the selective oxidation of steroid alcohols, cleavage of vicinal diols, in the oxidation of y-ketocarboxylic acids, of primary amines to nitriles, of 2,6-di-tert-butylphenol to 2,2, 6,6 -tetra-rert-butyldiphenoquinone, of 2-(benzylideneamino)-phenols to 2-phenyloxazols, of 1,1-dialkylhydrazines to tetraalkyltetrazenes. For details the reader is referred to Ref. [195]. 

Finally, it is important to note that many of the anodic reactions discussed above cannot be duplicated with traditional chemical oxidants. For this reason, the anodic oxidation of nitrogen-containing compounds represents a powerful class of reactions that has the potential to open up entirely new synthetic pathways to complex molecules. From the work already accomplished, it is clear that employing such an approach is both feasible and beneficial, and that the ability to selectively oxidize amines and amides is a valuable tool for any synthetic chemist to have at their disposal. 

Recently, Choudary et al. reported the first example of catalytic N-oxidation of tertiary amines by tungstate-exchanged Mg/Al LDHs in water [113], and the halodecarboxylation of Q ,/l-unsaturated aromatic carboxylic acids to /1-bromostyrenes has also been achieved for the first time, using a molybdate-exchanged Mg/Al LDH catalyst [114] this latter catalyst was active for selective oxidation [ 115,116]. 

Peroxidases have been used very frequently during the last ten years as biocatalysts in asymmetric synthesis. The transformation of a broad spectrum of substrates by these enzymes leads to valuable compounds for the asymmetric synthesis of natural products and biologically active molecules. Peroxidases catalyze regioselective hydroxylation of phenols and halogenation of olefins. Furthermore, they catalyze the epoxidation of olefins and the sulfoxidation of alkyl aryl sulfides in high enantioselectivities, as well as the asymmetric reduction of racemic hydroperoxides. The less selective oxidative coupHng of various phenols and aromatic amines by peroxidases provides a convenient access to dimeric, oligomeric and polymeric products for industrial applications. 

The nitrosodisulfonate salts, particularly the dipotassium salt called Fremy s salt, are useful reagents for the selective oxidation of phenols and aromatic amines to quinones (the Teuber reaction). - Dipotassium nitrosodisulfonate has been prepared by the oxidation of a hydroxylaminedisulfonate salt with potassium permanganate, " with lead dioxide, or by electrolysis. This salt is also available commercially. The present procedure illustrates the electrolytic oxidation to form an alkaline aqueous solution of the relatively soluble disodium nitrosodisulfonate. This procedure avoids a preliminary filtration which is required to remove manganese dioxide formed when potassium permanganate is used as the oxidant. " ... 

An elegant four-enzyme cascade process was described by Nakajima et al. [28] for the deracemization of an a-amino acid (Scheme 6.13). It involved amine oxidase-catalyzed, (i )-selective oxidation of the amino acid to afford the ammonium salt of the a-keto acid and the unreacted (S)-enantiomer of the substrate. The keto acid then undergoes reductive amination, catalyzed by leucine dehydrogenase, to afford the (S)-amino acid. NADH cofactor regeneration is achieved with formate/FDH. The overall process affords the (S)-enantiomer in 95% yield and 99% e.e. from racemic starting material, formate and molecular oxygen, and the help of three enzymes in concert. A fourth enzyme, catalase, is added to decompose the hydrogen peroxide formed in the first step which otherwise would have a detrimental effect on the enzymes. 

In 1989, a method for the peroxysilylation of alkenes nsing triethylsUane and oxygen was reported by Isayama and Mnkaiyama (eqnation 25). The reaction was catalyzed by several cobalt(II)-diketonato complexes. With the best catalyst Co(modp)2 [bis(l-morpholinocarbamoyl-4,4-dunethyl-l,3-pentanedionato)cobalt(n)] prodnct yields ranged between 75 and 99%. DiaUcyl peroxides can also be obtained starting from tertiary amines 87, amides 89 or lactams via selective oxidation in the a-position of the Af-fnnctional group with tert-butyl hydroperoxide in the presence of a ruthenium catalyst as presented by Murahashi and coworkers in 1988 ° (Scheme 38). With tertiary amines 87 as substrates the yields of the dialkyl peroxide products 88 ranged between 65 and 96%, while the amides 89 depicted in Scheme 38 are converted to the corresponding peroxides 90 in yields of 87% (R = Me) and 77% (R = Ph). 

MAO A and B differ in primary structure and in substrate specificity [5,7]. The two isozymes, located on the mitochondrial outer membranes, have 70% homology in peptide sequence and share common mechanistic details. It is now recognized that these are different proteins encoded by different genes, but probably derived from a common ancestral gene. Crystal structures for both MAO A and B complexes with inhibitors have recently been reported [8]. Serotonin is selectively oxidized by MAO A, whereas benzylamine and 2-phenylethylamine are selective substrates for MAO B. Dopamine, norepinephrine, epinephrine, trypt-amine, and tyramine are oxidized by both MAO A and B in most species [9]. In addition, MAO A is more sensitive to inhibition by clorgyline (1), whereas MAO B is inhibited by low concentrations of L-deprenyl ((f )-( )-deprenyl) (2) [5,6cj. Development of inhibitors that are selective for each isozyme has been an extremely active area of medicinal chemistry [8]. 

In a combinatorial approach, 3,4-dimethoxybenzylalcohol (veratryl alcohol) was oxidized with Cu(S04)2 and a series of 22 amine hgands in order to screen catalytic activity for the selective oxidation towards veratryl aide-... 

AicohoJ oxidation can aJso be enantioselective. The best systems reported to date for the selective oxidation of one enantiomer of 7 to 9 depend on the naturally-occurring alkaloid sparteine as a source of chirality. Unfortunately, only one enantiomer of sparteine is available. Peter O Brien of the University of York has developed (J. Org. Chem. 2004,69, 5789) an alternative tricyclic amine 8 that is complementary to sparteine, directing oxidation toward R-7. 

Oxidation of aminesTertiary amines are oxidized by this reagent (2 equivalents) to amides in 70 95% yield. The oxidation of secondary amines is less selective amides are obtained in 35 50% yield accompanied by carboxylic acids and esters. 

The conversion of mines into oxazirane is a reasonably selective oxidation and may be carried out in the presence of functional groups, e.g. ethylenic unaaturation, winch normally react with per dds. AIwq, 1,9,6-trialkyl perhydro -triazitxea obtained from condensation of formaldehyde and primary amines can bo oxidized to oxazivaoee. 

Alkyl hydroperoxides can oxidize a variety of other nucleophilic substrates in the presence of d° metal catalysts. Thus molybdenum and vanadium catalysts have been used for the selective oxidation of tertiary amines to the corresponding JV-oxides (equations 79 and 80).225,254... 

Selenides are oxidized to selenoxides that normally suffer an in situ elimination.111 Amines are destroyed,112 although its protection as amides or carbamates prevents the reaction with Collins reagent. Lactols are very quickly oxidized to lactones,113 unless a very great steric hindrance is present.114 Tertiary lactols suffer oxidation via its opened hydroxyketone form.115 The oxidation of tertiary lactols may be slow, so that an alcohol can be selectively oxidized. 

Although primary and secondary amines are destroyed by PDC, hindered secondary amines can resist the action of PDC long enough to allow selective oxidation of alcohols.146... 

Normally, alcohols can be selectively oxidized with PDC in the presence of tertiary amines.148 Although TV-methyl tertiary amines are transformed into formamides by PDC,149 this reaction is usually slow enough so that selective oxidation of alcohols with PDC can be possible. 

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