Arenes substrates, aromatic

Mechanistically there is ample evidence that the Balz-Schiemann reaction is heterolytic. This is shown by arylation trapping experiments. The added arene substrates are found to be arylated in isomer ratios which are typical for an electrophilic aromatic substitution by the aryl cation and not for a homolytic substitution by the aryl radical (Makarova et al., 1958). Swain and Rogers (1975) showed that the reaction takes place in the ion pair with the tetrafluoroborate, and not, as one might imagine, with a fluoride ion originating from the dissociation of the tetrafluoroborate into boron trifluoride and fluoride ions. This is demonstrated by the insensitivity of the ratio of products ArF/ArCl in methylene chloride solution at 25 °C to excess BF3 concentration. 

This Lewis acid-catalyzed electrophilic aromatic substitution allows the synthesis of alkylated products via the reaction of arenes with alkyl halides or alkenes. Since alkyl substituents activate the arene substrate, polyalkylation may occur. A valuable, two-step alternative is Friedel-Crafts Acylation followed by a carbonyl reduction. 

As before, the exploration of the metal-free oxidative amination was a competitive process. Both Chang" and Antonchick" simultaneously discovered nearly identical I(III)-mediated aminations. They both proposed that the reactions operated by generating an electrophilic nitrogen source in situ. This new species then acted as an R2N equivalent and aminated the arene via an electrophilic aromatic substitution mechanism. This hypothesis seemed appealing, but their data could not be directly compared to ours, as neither Chang nor Antonchick performed reactions on arene substrates that could provide mixtures of regiomers (e.g., toluene). 

The arene substrates are not limited to simple benzene derivatives. A variety of het-eroarenes can also participate in alkene arylations to generate the desired coupling products. Stoichiometric oxidative coupling of aromatic heterocycles such as furan, thiophene, selenophene, A-methylpyrrole, benzofiiran and benzothiophene with a variety of alkenes, including acrylonitrile, styrene and methyl acrylate, have been extensively studied by Fu-jiwara and coworkers [8]. Furan, thiophene, selenophene and A-methylpyrrole are easily alkenylated with alkenes to give 2-alkenylated and 2,5-dialkenylated heterocycles in relatively low yields (3 6%) [8a], while the reactions of benzofuran and benzothiophene with alkenes produced a mixture of 2- and 3-alkenylated products [8b]. 

Transition metal-catalyzed Buchner reactions of arene substrates proceed via electrophilic carbenoids. In addition to cyclopropanation of the arene double bond, these a-diazoketones possessing an aromatic ring can also participate in C-H insertion reactions. As shown in the decomposition of diazomethyl ketone 53, the benzylic C-H insertion product 56 is obtained as a minor product (vide supra). The rhodium(II) acetate-catalyzed reaction of diazoketone 71 also affords cycloheptatriene derivative 73 along with the benzylic C-H insertion product, y-lactam 72, in a ratio of 1 2. Treatment of 71 with the more electron-rich rhodium(II) caprolactamate [Rh2(Cap)4] favors more C-H insertion, but the cycloaddition pathway is still significant the ratio of 73 to 72 is 1 3.5. 

The dosimeter can detect various polynuclear aromatics at the pph level after 1 hour of exposure. It has been shown that the RTF of aza-arenes can he enhanced by using mercury(II) chloride as a heavy atom (21). Also, sensitized fluorescence spectrometry with a solid organic substrate can be used to detect trace amounts of polynuclear aromatic compounds (22). 

Of a number of Tj6-arene complexes subsequently tested for reactivity toward H2, i76-C6H5CH3—Ru6C(CO)14 was converted stoichiometrically at 150°C to methylcyclohexane, and the t76-C6(CH3)6Ru-i74-C6(CH3)6 complex (cf. 58) was found to be a long-lived homogeneous catalyst for arene hydrogenation (444) in contrast to the cobalt system, extensive H- D exchange occurred in the aromatic ring and in substituent methyls of xylene substrates. 

More mechanistic insight into the C-H functionalization process of arenes is provided by theoretical and experimental studies. Multifluorinated, electron poor, aromatic substrates readily undergo CH activation with coordina-tively unsaturated rhenium complexes, attributed to the stronger C-Re bond in the product, whereas with monofluorinated analogs, the if -complex predominates (Equation (60)).61... 

The benzylic C-H activation has been effectively applied to the enantioselective synthesis of (+)-imperanene (Equation (16)).80 The key step was the Rh2(i -DOSP)4-catalyzed functionalization of the benzylic methyl C-H bond in arene 2. An impressive feature of this transformation was that both the carbenoid and substrate contained very electron-rich aromatic rings, which were compatible with the highly electrophilic carbenoids because they were still sterically protected. 

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