The use of chloranil

The chloranil dehydrogenation of A -3-ketones offers a convenient direct conversion to A -ketones. t-Butanol and xylene are the most suitable solvents. Slightly higher yields have been claimed with mixed organic acid-inert solvent systems, although somewhat lower yields (50-60%) are 

This dehydrogenation of A -3-ols to A -3-ketones undoubtedly proceeds through the A -3-ketone and the A -enol. The reaction has also been carried out using chloranil and yields of up to 75 % have been claimed.(See next section for a discussion of mechanism.) 

Enol ethers, A -3-ketones and enol acetates have also been employed in the preparation of A -3-ketones (see also sections VI-B and VI-E) and permit the use of milder conditions than do A -3-ketones.  

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Unsaturated nitriles are formed by the reaction of ethylene or propylene with Pd(CN)2[252]. The synthesis of unsaturated nitriles by a gas-phase reaction of alkenes. HCN, and oxygen was carried out by use of a Pd catalyst supported on active carbon. Acrylonitrile is formed from ethylene. Methacrylonitrile and crotononitrile are obtained from propylene[253]. Vinyl chloride is obtained in a high yield from ethylene and PdCl2 using highly polar solvents such as DMF. The reaction can be made catalytic by the use of chloranil[254]. 

Quaternization of quinoxaline is neatly avoided by the synthesis of the azirino[l,2-a]quinoxalines 71 from o-phenylenediamine and the dibromo ketones 70. The azirino compound apparently acts as a 1,3 dipole and undergoes addition with a variety of dipolarophiles to give fused quinox-alines containing a bridgehead nitrogen atom. " Thus the acetylene derivatives 72 give the pyrrolo derivatives 73. The yields are much improved by the use of chloranil in situ to oxidize the presumed dihydro intermediates 74. 

The first exanple is the use of chloranil in C—H oxidation of sulfides, leading to the in situ formation of the electrophilic thionium ion, which was trapped by 1,3-dicarbonyl conpounds in a Knoevenagel reaction. The products obtained were a-acyl-p-sulfurated carbonyl conpounds 192. This transformation and the authors proposed activation mechanism are shown in Scheme 20.37. Elimination of the sulfur-containing moiety promoted by chloranil afforded the usual Knoevenagel products 189. 

Because Pd(II) salts, like Hgtll) salts, can effect electrophilic metallation of the indole ring at C3, it is also possible to carry out vinylation on indoles without 3-substituents. These reactions usually require the use of an equiv. of the Pd(ll) salt and also a Cu(If) or Ag(I) salt to effect reoxidation of the Pd. As in the standard Heck conditions, an EW substitution on the indole nitrogen is usually necessary. Entry 8 of Table 11.3 is an interesting example. The oxidative vinylation was achieved in 87% yield by using one equiv. of PdfOAcfj and one equiv. of chloranil as a co-oxidant. This example is also noteworthy in that the 4-broino substituent was unreactive under these conditions. Part B of Table 11.3 lists some other representative procedures. 

Note It is reported that the use of chlorobenzene as solvent is essential when the reagent is to be used to detect aromatic amines [1]. In the case of steroids, penicillins, diuretics and alkaloids the reaction should be accelerated and intensified by spraying afterwards with dimethylsulfoxide (DMSO) or dimethylformamide (DMF), indeed this step makes it possible to detect some substances when this would not otherwise be possible [5,9-11] this latter treatment can, like heating, cause color changes [5,9]. Penicillins and diuretics only exhibit weak reactions if not treated afterwards with DMF [10, 11]. Steroids alone also yield colored derivatives with DMSO [9]. Tlreatment afterwards with diluted sulfuric acid (c = 2 mol/L) also leads to an improvement in detection sensitivity in the case of a range of alkaloids. In the case of pyrrolizidine alkaloids it is possible to use o-chloranil as an alternative detection reagent however, in this case it is recommended that the plate be treated afterwards with a solution of 2 g 4-(dimethyl-amino)-benzaldehyde and 2 ml boron trifluoride etherate in 100 ml anhydrous ethanol because otherwise the colors initially produced with o-chloranil rapidly fade [12]. 

Khashaba et al. [34] suggested the use of sample spectrophotometric and spectrofluorimetric methods for the determination of miconazole and other antifungal drugs in different pharmaceutical formulations. The spectrophotometric method depend on the interaction between imidazole antifungal drugs as -electron donor with the pi-acceptor 2,3-dichloro-5,6-dicyano-l,4-benzoquinone, in methanol or with p-chloranilic acid in acetonitrile. The produced chromogens obey Beer s law at Amax 460 and 520 nm in the concentration range 22.5-200 and 7.9-280 pg/mL for 2,3-dichloro-5,6-dicyano-l,4-benzoquinone and p-chloranilic acid, respectively. Spectrofluorimetric method is based on the measurement of the native fluorescence of ketoconazole at 375 nm with excitation at 288 nm and/or fluorescence intensity versus concentration is linear for ketoconazole at 49.7-800 ng/mL. The methods... 

Mahrous et al. [43] determined primaquine and other antimalarials by use of chloranilic acid for the colorimetry. Primaquine was treated with 0.2 chloranilic acid solution in acetonitrile to give a purple solution with absorption maximum at 522 nm. Beer s law was obeyed from 0.04 to 0.2 mg/mL. Analysis of pharmaceutical formulation by this method is as accurate as the official method. 

The readily available reagent diphenyl selenoxide has been used as a mild and selective oxidant in the synthesis of aporphines (and homoaporphines). When the benzylisoquinoline (13) was treated with one equivalent of the reagent at room temperature in methanol, and the product was O- methylated with diazomethane, the aporphine (14) was obtained in 80% yield. The alternative use of chloranil, which is a commonly used oxidant for catechols, yielded less than 10% of (14).20... 

Shao and coworkers [61] have demonstrated that a noncovalent charge-transfer complex of calix[4]pyrrole-chloranil can be used as a chromogenic sensor for F and H2POj. Upon addition of calix[4]pyrrole 75 to the solution of chloranil 76 in chloroform, the color changed from pale yellow to blue. This indicated the formation of a charge-transfer complex between calix[4] pyrrole and chloranil in chloroform. The Job s plot analysis showed a 1 1 complex with... 

It is evident from these considerations that the use of a less hydrophobic redox species in the O phase makes the homogeneous ET occur more favourably. Another example of the IT mechanism has been found in the ET between L-ascorbic acid in W and chloranil (with Ko = 900) in NB or DCE. This has been confirmed using potential-controlled polarography [47], potential modulated reflectance spectroscopy [46], microflow coulometry [39], ECSOW system [38] and digital simulation of cyclic voltammograms [48]. 

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