Aryldiazonium reduction

Taking advantage of the steady increasing techniques for CNT functionalization [7], several routes were explored to attach redox molecules onto SWCNTs. Ferrocene was also attached to MWCNTs by, amide coupling, 7c-stacking interactions [8], aryldiazonium reduction [8] or 1,3 dipolar cycloaddition of azomethyne ylides [9] in order to establish electrical communication between the enzyme and the electrode (Fig. 3.5). 

Another important reduction process is that of aryldiazonium salts with sulfite/bisulfite at controlled pH to produce aryUiydrazines. AryUiydrazines are important intermediates for the preparation of pyrazolones and indoles. 

Since sulphones 204 are easily available compounds one would expect that they could be used as starting materials for the preparation of sulphoxides via the selective removal of one oxygen atom from the sulphonyl group (equation 112). Up to now, there is only one example reported of a direct reduction of a sulphone to a sulphoxide. The bicyclic dideuterio sulphone 205 after 24 h treatment with three-fold excess of diisobutyl aluminium hydride in boiling dichloromethane gave the corresponding sulphoxide 206 in 36% yield (equation 113). A two-step procedure for the selective reduction of sulphones to sulphoxides, which involves an initial reaction of sulphone 204 with aryldiazonium tetrafluoroborate 207 to form aryloxysulphoxonium salt 208 and its subsequent reduction (equation 114), was alluded to by Shimagaki and coworkers and... 

Aryl aldehydes, alkane reduction, 71-72 Aryldiazonium salts, reduction of, 104 Aryl halides and triflates, reduction reaction, 32 Aryl ketone, allylation,... 

Aryldiazonium salts, usually obtained from arylamines, undergo replacement of the diazonium group with a variety of nucleophiles to provide advantageous methods for producing aryl halides, cyanides, phenols and arenes by reductive removal of the diazo group. Coupling reaction of aryldiazonium salts with phenols or arylamines give rise to the formation of azo dyes. 

The addition of aryl radicals, generated by chemical reduction of aryldiazonium salts, onto arenes in the Gomberg-Hey reaction is well established [163]. The addition of these radicals to alkenes in the Meerwein reaction is also well known [164], Aryl o-radicals generated by electrochemical reduction of aryl halides take part in similar reactions. Good yields of the products are obtained when the intermediate phenyl radical can react in an intramolecular manner. The addition step is then fast and competes successfully with further electron transfer to form the phenyl car-banion, followed by protonation. 

Scheme 1.27 Electrochemical modification of SWCNTs with aryldiazonium compounds (a) reductive coupling of preformed p-nitro-phenyldiazonium salts to SWCNTs and (b) anodic oxidation with in situ-generated dia-...

Four different aryldiazonium salts have been used to functionalize SWCNTs through electrochemical reduction. By XPS and Raman diffusion measurements, the growth of aryl chains on the sidewalls of the nanotubes was observed [178]. Electrically addressable biomolecular functionalization of SWCNT electrodes and vertically aligned carbon nanofiber electrodes with DNA was achieved by elec-trochemically addressing (reduction) of nitrophenyl substituted nanotubes and nanofibers. Subsequently, the resulting amino functions were covalently linked to DNA forming an array of DNA-CNT hybrid nanostructures (Scheme 1.28) [179],... 

An intermediate reduction of aryldiazonium salts Ar-N =N to the diazo radicals Ar-N=N also occurs when aryldiazonium salts react with KI to yield aryl iodides (Figure 5.55). Therefore, aryl radicals Ar are obtained under these conditions, too. Their fate, however, differs from that of the aryl radicals, which are faced with nucleophiles in the presence of Cu(II) (cf. Figure 5.53) or H3P02 (cf. Figure 5.54) the iodination mechanism of Figure 5.55 is a radical chain reaction consisting of four propagation steps. 

In a similar fashion ethers, acetals and formic acid may serve as efficient hydride donors towards carbonium ions, aryldiazonium salts (reduction with loss of nitrogen occurs) and pyrylium salts. A review of this chemistry is available. ... 

Aryldiazonium salts are reducible at a mercury cathode. Polarograms in acid solution show a pH-independent one-electron wave followed by a larger pH-dependent wave [194,195]. Reduction on the plateau of the first wave [196] showed n = 1 and only aryl-mercury salts were isolated, indicating the initial formation of a radical. 

Amino-5-imino-3-pyrazolidinones have been synthesized by reduction of the corresponding oximes.644,1339 Such amines form ureas and thioureas by reaction with potassium cyanate in acid solution and with phenyl isothiocyanate.644 4-Arylimino-5-imino-3-pyrazolidinones have been prepared by reaction of the 5-imino-3-pyrazolidinones with p-amino-A.-AT-diethylaniline in the presence of silver salts as oxidizing agents (eq. 172).244,519,1255,1538 4-Arylazo-5-imino-3-pyrazolidinones have most frequently been prepared by reaction of 5-imino-3-pyrazoli-dinones with aryldiazonium chlorides, as is usual for the preparation of similar compounds.591,594,920,1087,1255,1339,1539,1649 One such compound has been prepared by the Curtius rearrangement of the corresponding 4-arylazo azide.594... 

A number of catalytic conditions have been used for the reduction of 5-nitropyrimidines to amines using Pd or Raney-Ni. In the reduction of 4- and 5-nitrosopyrimidines, sodium dithionite is most commonly used. Hydrogenolysis of 5-arylazopyrimidines to form the 5-amino derivative is commonly done by catalytic hydrogenation. The 5-arylazopyrimidines are often prepared as intermediates in amination reactions. They are prepared from aryldiazonium salts, which couple with pyrimidines containing strongly electron-donating substituents [Pg.184]

A paper on new approaches to the generation of arylphosphinidenes has been published. The stable bis-azide precursor (133) upon photolysis, or vapour phase thermolysis, gave (134), obviously via the phosphinidene (135) the same product was obtained by photolysis of the phosphaketene (136). The reduction of aryldiazonium salts to arenes with triethyl phosphite or triphenylphosphine is shown to proceed by a radical-chain mechanism. The previously described photo-Arbuzov rearrangement of benzyl phosphites has been used to prepare several acyclic phosphonate nucleotide analogues, e.g. (137). ... 

Brown and Doyle have studied the reduction of aryldiazonium ions. Two mechanisms are possible. The reduction could occur via a one-electron transfer non-bonded outer-sphere mechanism where an electron is transferred from a reducing agent to the diazonium ion or via a bonded inner-sphere mechanism, where an intermediate complex undergoes homolytic cleavage (equations 52 and 53), respectively. 

Only the last method, developed by Rapoport s group [95], is a versatile, chemoselective, and high-yielding. The method is accomplished by adding an alkyl nitrite (2 eq.) to a cold acetone solution of a Pschorr s amine substrate (1 eq.) and sulfuric acid (2 eq.) followed by sodium iodide. Here the iodide serves as a one-electron reductant, which converts an aryldiazonium salt to an aryl radical [95,96]. Then, the fast intramolecular arylation takes place with formation of the phenanthrene structure. While the older Pschorr methods gave very low yields, often under 20%, the Rapoport s method introduced significant improvements as the yields were 45-71% [95]. For example, polymethoxy compound 52 was converted to phenanthrene 53 in 71% yield [95], respectively. Scheme 22. 

Cohen has shown that copper(l) salts readily induce the decomposition of the aryldiazonium salt to generate the aryl radical which subsequently reacts with Cu(l) to give an arylcopper(ll) species [99,100]. This reacts with another aryl radical to produce the key diarylcopper(III) intermediate. The latter undergoes reductive elimination of the biaryl with regeneration of Cu(l), similar to other copper-based homo-coupling reactions, see Chapter 3, [100] Scheme 26. 

A classical example of a thermal method via radicals is the Meerwein arylation of alkenes, which is based on the copper-mediated reduction of aryldiazonium salts, causing dinitrogen loss and attack of the resulting phenyl radical onto an electrophilic alkene (see Sechon 13.4.1). The reachon can be also photochemicahy induced (path a. Scheme 14.1), for example through the photodecomposition of phenyl iodides. However, these compounds absorb only weakly in the near ultraviolet (UV) and the reachon is synthehcally less prachcal. 

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