Oxidations with copper reagents

Hydroxyethyl)carbazole can be oxidized with copper(II) oxide-potassium hydroxide at 240°C to the carbazol-9-ylacetic acid the benzoate ester of the alcohol was cleaved to regenerate alcohol using phenyl Grignard reagent. The 9-acetic acid is also produced by alkaline hydrolysis of the corresponding nitrile. ... 

Oxidation (with such reagents as iron(III) chloride, potassium dichromate, silver oxide or nitrous acid) of 4,7-, 6,7-, and 5,6-dihydroxy-, and 5,6-dimethoxy-benzimidazoles gives the corresponding quinones. A 5-methyl group is oxidized by permanganate to carboxy (74CRV279). In the presence of copper(II)-piperidine or -dimethylamine complexes oxygen... 

Aryltrimethyl- or tri- -butylstaimanes as common Stille reagents are also successfully homo-coupled under palladium catalysed reactions in the presence of ethyl 2,3-dibromophenylpropionate (443) [26], or more conveniently by oxidation with copper(I) salts, e.g. CuCl [27], or copper(II) salts, e.g. Cu(N03)2 3H20 [28,29], which proceed smoothly at room temperature in tetrahydrofuran or DMF to afford symmetrical biaryls in excellent 3uelds. The Cu(N03)2-mediated homo-coupling works well also with diaryldimethyl(or -butyl)stannanes [30]. Moreover, the reaction can be accomplished with a catalytic amount of copper(II) chloride or manganese(II) bromide (10 mol%) in the presence of iodine as stoichiometric oxidant [31]. For example, compound 443, acting as an oxidant, converts the phenyltri-n-butylstannane (184) to biphenyl (8) in 86% yield [26], Scheme 11. 

Oxidation with Benedict s reagent (Section 25 19) Sugars that con tain a free hemiacetal function are called reducing sugars They react with copper(ll) sulfate in a sodium citrate/sodium carbonate buffer (Benedict s reagent) to form a red precipitate of copper(l) oxide Used as a qualitative test for reducing sugars... 

A cursory inspection of key intermediate 8 (see Scheme 1) reveals that it possesses both vicinal and remote stereochemical relationships. To cope with the stereochemical challenge posed by this intermediate and to enhance overall efficiency, a convergent approach featuring the union of optically active intermediates 18 and 19 was adopted. Scheme 5a illustrates the synthesis of intermediate 18. Thus, oxidative cleavage of the trisubstituted olefin of (/ )-citronellic acid benzyl ester (28) with ozone, followed by oxidative workup with Jones reagent, affords a carboxylic acid which can be oxidatively decarboxylated to 29 with lead tetraacetate and copper(n) acetate. Saponification of the benzyl ester in 29 with potassium hydroxide provides an unsaturated carboxylic acid which undergoes smooth conversion to trans iodolactone 30 on treatment with iodine in acetonitrile at -15 °C (89% yield from 29).24 The diastereoselectivity of the thermodynamically controlled iodolacto-nization reaction is approximately 20 1 in favor of the more stable trans iodolactone 30. 

Lithium dialkylcopper reagents can be oxidized to symmetrical dimers by O2 at -78°C in THF. The reaction is successful for R = primary and secondary alkyl, vinylic, or aryl. Other oxidizing agents (e.g., nitrobenzene) can be used instead of O2. Vinylic copper reagents dimerize on treatment with oxygen, or simply on standing at 0°C for several days or at 25°C for several hours, to yield LS-dienes." ... 

Cu(0) species. Alternatively, the Cu(n) species may first undergo oxidation by an external oxidant (or internal redox process) to a Cu(m) intermediate, and then undergo reductive elimination to provide the product and a Cu(i) species. Re-oxidation to Cu(n) would then, in theory, complete the catalytic cycle, but in practice, most reactions of this type have been performed with stoichiometric amounts of the copper reagent. 

Scheme 7-15 shows the significant improvement in overall stereoselectivity, due mainly to the adoption of the newly developed Sharpless asymmetric ep-oxidation. Compounds a-epoxy-67 and //-epoxy-67 can be readily obtained from 53 via the Sharpless reaction. Isomers of compounds 57 are then constructed via regioselective ring opening with a copper reagent. 

Various transition metals have been used in redox processes. For example, tandem sequences of cyclization have been initiated from malonate enolates by electron-transfer-induced oxidation with ferricenium ion Cp2pe+ (51) followed by cyclization and either radical or cationic termination (Scheme 41). ° Titanium, in the form of Cp2TiPh, has been used to initiate reductive radical cyclizations to give y- and 5-cyano esters in a 5- or 6-exo manner, respectively (Scheme 42). The Ti(III) reagent coordinates both to the C=0 and CN groups and cyclization proceeds irreversibly without formation of iminyl radical intermediates.The oxidation of benzylic and allylic alcohols in a two-phase system in the presence of r-butyl hydroperoxide, a copper catalyst, and a phase-transfer catalyst has been examined. The reactions were shown to proceed via a heterolytic mechanism however, the oxidations of related active methylene compounds (without the alcohol functionality) were determined to be free-radical processes. 

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