Reactions, with arenes

The crown catalyzed phase transfer of acetate ion into a nonpolar medium followed by reaction with aryldiazonium ion has also been shown to be an effective and mild method for the generation of aryl radicals [21 ]. Radicals thus generated can be reacted with deuteriochloroform or chloroform to yield deuterio- or protio-arenes. Reaction with bromotrichloromethane leads to the bromoarene and the reaction in benzene solution leads to unsymmetrical biphenyls in good yield [22]. These three processes are formulated in equation 15.7. 

Chlorination is carried out m a manner similar to brommation and provides a ready route to chlorobenzene and related aryl chlorides Fluormation and lodmation of benzene and other arenes are rarely performed Fluorine is so reactive that its reaction with ben zene is difficult to control lodmation is very slow and has an unfavorable equilibrium constant Syntheses of aryl fluorides and aryl iodides are normally carried out by way of functional group transformations of arylammes these reactions will be described m Chapter 22... 

Fluoroalkjiations are frequentiy performed indirectly using tandem reactions. Arenes react with sodium borohydride in trifluoroacetic acid to afford otherwise difficult to obtain l,l,l-trifluoro-2,2-diarylethanes. Presumably sodium borohydride reacts initially with the trifluoroacetic acid to produce the trifluoroacetaldehyde or its equivalent, which rapidly undergoes Friedel-Crafts-type condensation to give an intermediate carbinol. The carbinol further alkylates ben2ene under the reaction conditions giving the observed product. The reaction with stericaHy crowded arenes such as mesitylene and durene... 

Thermal arene exchange of tetramethylthiophene with [(/ -cymene)RuCl2]2 affords 130 (89JA8828), which on reaction with AgBE4 and excess tetramethylthiophene yields 131. The Ru—S thiophenic cluster, 132, was synthesized by reaction of 130 with (Mc3Si)2S followed by anionic metathesis and formation of the PFg salt. The coordination geometry around each ruthenium atom is pseudooctahedral. 

Arenes, on complexdQon with Cr, Fe, Mn, and so forth, acquire strongly electrophilic character, such complexes m reactions with nucleophiles behave as electrophilic tutroarenes. Synthesis of aromatic tutnles via the temporary complexanon of rutroarenes to the catiotuc cyclopentadienyhron moiety, cyarude addition, and oxidative demetalation v/ith DDQ has been reported fEq. 9.43 ... 

The I9e electron-reservoir complexes Fe Cp(arene) can give an electron to a large number of substrates and several such cases have been used for activation. After ET, the [FenCp(arene)]+ cation left has 18 valence electrons and thus cannot react in a radical-type way in the cage as was the case for 20e Fe°(arene)2 species. Thus the 19e Fe Cp(arene) complexes react with the organic halide RX to give the coupled product and the [FeCp(arene)]+ cation. Only half of the starting complex is used e.g., the theoretical yield is limited to 50% [48] (Scheme VI) contrary to the reaction with Fe°(arene)2 above. 

Although the o-xylylene complex is thermally unstable, it was characterized at — 50 °C by its 1H- and 13C-NMR spectra showing the exocyclic methylene at 5 = 5.04,4.42 ppm (JH) and 5 = 144.8 ppm (13C) using C6D5CD3 as the solvent. Its reaction with benzoyl chloride on the exocyclic carbon leaves a very acidic methylene group which transfers a proton onto the adjacent methylene unit. The double bond is benzoylated again in in situ and a di-cation of the [bis(arene)Fe]2+ type is obtained [47] Scheme VIII. 

Reaction schemes. Arenes react with mercuric trifluoroacetate according to the equation29... 

Beside these free radical reactions of sulfur dioxide, its electrophilic reactions generating sulfinates with organometallic compounds453,454 or sulfinic acids with arenes under Friedel-Crafts conditions455 are well known. To complete these three-component syntheses, the sulfinates prepared first are transformed to sulfones by reactions with appropriate electrophiles, discussed earlier in this chapter, i.e. equation 82. 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Arenes are unsaturated but, unlike the alkenes, they are not very reactive. Whereas alkenes commonly take part in addition reactions, arenes undergo predominantly substitution reactions, with the TT-bonds of the ring left intact. For example, bromine immediately adds to a double bond of an alkene but reacts with benzene only in the presence of a catalyst—typically, iron(III) bromide—and it does not affect the bonding in the ring. Instead, one of the bromine atoms replaces a hydrogen atom to give bromobenzene, C H Br ... 

Alkyl radical addition reactions to styrene chromium tricarbonyl can be accomplished using alkyl halides (10 equiv) and (TMSlsSiH (5 equiv) in the presence of AIBN in refluxing benzene, for 18 h (Reaction 66). " These reactions are believed to proceed through intermediates in which the unpaired electron is interacting with the adjacent arene chromium tricarbonyl moiety since the analogous reaction with styrene affords only traces of addition products. 

Reaction with (CH3)2Cd 6.4 Reaction with (CH3)2Zn 6.4 Reaction with Fe atoms and arenes 6.S.3.3 BioH 4 (B3Hg)2... 

Semmelhack ME, Chlenov A (2004) (Arene)Cr(Co)3 Complexes Arene Lithiation/Reaction with Electrophiles. 7 21-42... 

The transformation of arenes in the troposphere has been discussed in detail (Arey 1998). Their destruction can be mediated by reaction with hydroxyl radicals, and from naphthalene a wide range of compounds is produced, including 1- and 2-naphthols, 2-formylcinnamaldehyde, phthalic anhydride, and with less certainty 1,4-naphthoquinone and 2,3-epoxynaphthoquinone. Both 1- and 2-nitronaphthalene were formed through the intervention of NO2 (Bunce et al. 1997). Attention has also been directed to the composition of secondary organic aerosols from the photooxidation of monocyclic aromatic hydrocarbons in the presence of NO (Eorstner et al. 1997) the main products from a range of alkylated aromatics were 2,5-furandione and the 3-methyl and 3-ethyl congeners. 

Considerable attention has been directed to the formation of nitroarenes that may be formed by several mechanisms (a) initial reaction with hydroxyl radicals followed by reactions with nitrate radicals or NO2 and (b) direct reaction with nitrate radicals. The first is important for arenes in the troposphere, whereas the second is a thermal reaction that occurs during combustion of arenes. The kinetics of formation of nitroarenes by gas-phase reaction with N2O5 has been examined for naphthalene (Pitts et al. 1985a) and methylnaphthalenes (Zielinska et al. 1989) biphenyl (Atkinson et al. 1987b,c) acephenanthrylene (Zielinska et al. 1988) and for adsorbed pyrene (Pitts et al. 1985b). Both... 

The oxidation by strains of Pseudomonas putida of the methyl group in arenes containing a hydroxyl group in the para position is, however, carried out by a different mechanism. The initial step is dehydrogenation to a quinone methide followed by hydration (hydroxylation) to the benzyl alcohol (Hopper 1976) (Figure 3.7). The reaction with 4-ethylphenol is partially stereospecific (Mclntire et al. 1984), and the enzymes that catalyze the first two steps are flavocytochromes (Mclntire et al. 1985). The role of formal hydroxylation in the degradation of azaarenes is discussed in the section on oxidoreductases (hydroxylases). 

The reductive transformation of arene carboxylates to the corresponding aldehydes under aerobic conditions has already been noted. In addition, aromatic aldehydes may undergo both reductive and oxidative reactions, with the possibility of decarboxylation of the carboxylic acid formed ... 

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