Nickel peroxide aromatization

Isoxazolines 79, obtained from aromatic nitrile oxide cycloadditions to cyclohex-2-enone, reacted with nickel peroxide to give 3-aryl-6,7-dihydro[l] benzoisoxazol-4(5// )-ones 80. In contrast, the corresponding 2-bromocyclohex-2-enone underwent nitrile oxide cycloaddition, followed by dehydrobromination, to afford the regioisomeric 3-aryl-4,5-dihydro[l]benzoisoxazol-7(6//)-ones 81 (Scheme 1.23) (242). 

Cycloaddition of 5,6-dihydropyran-2-one with aromatic nitrile oxides leads to 3-aryl-3a,6,7,7a-tetrahydropyrano[3,4-d]isoxazol-4(47/)-ones 98. The latter react with nickel peroxide to give the corresponding dihydropyranoisoxazolones 99. Similar to 2-bromocyclohex-2-enone, 3-bromo-5,6-dihydropyran-2-one undergoes nitrile oxide cycloaddition, followed by dehydrobromination, to form regioi-someric 3-aryl-5,7-dihydropyrano 4,3-c/ isoxazol-7(4//)-ones 100 (Scheme 1.24) (242). 

A2-Thiazolines are converted into the corresponding thiazoles on treatment with nickel peroxide at room temperature. Benzothiazolines are also very easily oxidized to the corresponding benzothiazoles, the aromatization of the heteroring being the driving force of the reaction which corresponds to hydride transfer. 

The one-electron oxidation of a secondary amine results in the formation of a secondary aminium ion which on deprotonation gives an aminyl radical (Scheme 1). The nature of the final products derived from these intermediates dqiends very much on the structure of the substrate and the reaction conditions. If the amine has a hydrogen atom on the a-carbon atom the major products usually result from deprotonation at this a-position. With aromatic secondary amines, products can result from coupling of the delocalized radicals at a ring carbon atom. The formal dimerization of aminyl radicals shown in Scheme 21 is therefore not often a useful method of preparation of hydrazines. Nickel peroxide has been used to oxidize diphenylamine to tetraphenylhydrazine in moderate yield, and other secondary arylamines also give... 

Aliphatic and aromatic aldehydes have been converted to the corresponding amides with ammonia or a primary or secondary amine, NBS, and a catalytic amount of AIBN (p. 935). " In a reaction of more limited scope, amides are obtained from aromatic and a,p-unsaturated aldehydes by treatment with dry ammonia gas and nickel peroxide. Best yields (80-90%) are obtained at —25 to —20°C. In the nickel peroxide reaction the corresponding alcohols (ArCH20H) have also been used as substrates. 

Nickel peroxide, an undefined black oxide of nickel, is prepared from nickel sulfate hexahydrate by oxidation in alkaline medium with an ozone-oxygen mixture [929] or with sodium hypochlorite [930, 931, 932, 933]. Its main applications are the oxidation of aromatic side chains to carboxyls [933], of allylic and benzylic alcohols to aldehydes in organic solvents [929, 932] or to acids in aqueous alkaline solutions [929, 930, 932], and of aldehydes to acids [934, the conversion of aldehyde or ketone hydrazones into diazo compounds [935] the dehydrogenative coupling of ketones in the a positions with respect to carbonyl groups [931] and the dehydrogenation of primary amines to nitriles or azo compounds [936]. 

The unusual oxidant nickel peroxide converts aromatic aldehydes into carboxylic acids at 30-60 °C after 1.5-3 h in 58-100% yields [934. The oxidation of aldehydes to acids by pure ruthenium tetroxide results in very low yields [940. On the contrary, potassium ruthenate, prepared in situ from ruthenium trichloride and potassium persulfate in water and used in catalytic amounts, leads to a 99% yield of m-nitrobenzoic acid at room temperature after 2 h. Another oxidant, iodosobenzene in the presence of tris(triphenylphosphine)ruthenium dichloride, converts benzaldehyde into benzoic acid in 96% yield at room temperature [785]. The same reaction with a 91% yield is accomplished by treatment of benzaldehyde with osmium tetroxide as a catalyst and cumene hydroperoxide as a reoxidant [1163]. 

Of aromatic ketone hydrazones, benzophenone hydrazone is oxidized with yellow mercuric oxide [387, 390] or red mercuric oxide [387], fluo-renone hydrazone is oxidized with mercuric oxide [5SS] or nickel peroxide [935], and di-a-thienyl ketone hydrazone and phenyl naphthyl ketone hydrazones are oxidized with silver oxide or manganese dioxide [370] (equations 459-461). 

Aromatic and allylic aldehydes are converted into amides by oxidation at —20° with nickel peroxide in the presence of ammonia. At higher temperatures nitriles are formed. 

It is generally known that the processes of reversible oxidation of phenols, i.e. the conversions of phenolic systems into quinone structures and vice versa, are of great importance in biochemical reactions. The reaction partners mentioned above can serve as donors and acceptors of electrons and protons, i.e. as antioxidant systems. The conversions of phenols into cyclohexadienones are accompanied by the loss of aromaticity and in essence are not rearrangements, although the term phenol-dienone rearrangement is found in the literature. A review which summarizes in detail the oxidation reactions of phenols under conditions of halogenation, nitration and alkylation as well as radical reactions appeared . The various transformations of phenols upon oxidation with nickel peroxide were also reviewed . Therefore, only recent reports concerning the phenols-to-quinones conversions are described in this section. 

Thiazolines and 3-thiazolines are easily aromatized to the corresponding thiazoles by the action of oxidants such as nickel peroxide <94TL1379>, 2,3-dichloro-5,6-dicyano-l,4-dibenzoquinone, or t-butyl perbenzoate via the Kharasch-Sosnovsky reaction <94TL248l, 94TL6803). The aromatization of... 

The only method which can be used for monocyclic aromatic 1,2,3-triazines as well as for condensed 1,2,3-triazines is the oxidation of iV-aminopyrazoles 1, vV-aminoindazoles 3, 7 or other condensed A-aminopyrazoles. The following reagents were used for the oxidation lead(IV) acetate, 7,77,82,104 110,116 lead(IV) oxide/trifluoroacetic acid,77 106 nickel peroxide,1- 1 77 manganese(IV) oxide,77 sodium and potassium periodate,78-103 halogenating agents,79-82,102 111 and electrooxidation.112 Sodium periodate78,103 seems to be the best reagent for monocyclic 1,2,3-triazines. 

The ring synthesis of the tetrahydro-1,3-azoles is simply the formation of N,N-, N,0-or A, S-analogues of aldehyde cyclic acetals the ring synthesis of the 4,5-dihydro-heterocycles requires an acid oxidation level in place of aldehyde. A good route to the aromatic systems is therefore the dehydrogenation of these reduced and partially reduced systems. Nickel peroxide, " manganese(IV) oxide, copper(II) bromide/ base, and bromotrichloromethane/diazabicycloundecane have been used. The example shown uses cysteine methyl ester with a chiral aldehyde to form the tetrahydrothiazole. 

The further steps presumably involve another oxidation stage in order to reach R COOH products, but detailed mechanisms were not formulated. It is to be noted that a free radical type of mechanism was suggested, as was also discussed in the general review of reactions with nickel peroxide given by George and Balachandran. Free radicals in aromatic oxidations at nickel peroxide were indicated from esr experiments. 

Aromatic or allylic aldehydes can be converted into amides at -20 °C using an ethereal solution of ammonia in the presence of nickel peroxide. At higher temperatures, using a benzene solution through which ammonia is bubbled, aromatic aldehydes are converted into the corresponding nitrile in 70% yields [equation (9)]. With aliphatic aldehydes the yields are somewhat lower (40%). 

Treatment of the silyl compound, 61, above with hydrogen peroxide leads to the diol 62 which formally constitutes an oxidation as well as an electrophilic attack on the carbon atom. Classically, oxidation of nonconjugated rings to furnish their conjugated (usually aromatic) analogues is achieved by treatment with nickel(ii) peroxide however, these reactions are common and have been extensively explored for a number of different heterocyclic systems in both GHEC(1984) and CHEC-II(1996) so are not discussed further here. 

Benzoyl peroxide (dibenzoyl peroxide), (CjHjCOO>2 (mp 104-106 °C dec), and /r-nitrobenzoyl peroxide (p-02NCgH4COO)2 (mp 156 °C dec), which is synthesized from p-nitrobenzoyl chloride and sodium peroxide [229], are rarely used as oxidants, and if so, they do not offer appreciable advantages over other organic oxidation agents. The anti addition of benzoyl groups to double bonds and the benzoxylation of aromatic rings are achieved in the presence of iodine [230], and alcohols are oxidized to carbonyl compounds in the presence of nickel dibromide [231],... 

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