Diarylmethanes, oxidation

Because of Us high polarity and low nucleophilicity, a trifluoroacetic acid medium is usually used for the investigation of such carbocationic processes as solvolysis, protonation of alkenes, skeletal rearrangements, and hydride shifts [22-24] It also has been used for several synthetically useful reachons, such as electrophilic aromatic substitution [25], reductions [26, 27], and oxidations [28] Trifluoroacetic acid is a good medium for the nitration of aromatic compounds Nitration of benzene or toluene with sodium nitrate in trifluoroacetic acid is almost quantitative after 4 h at room temperature [25] Under these conditions, toluene gives the usual mixture of mononitrotoluenes in an o m p ratio of 61 6 2 6 35 8 A trifluoroacetic acid medium can be used for the reduction of acids, ketones, and alcohols with sodium borohydnde [26] or triethylsilane [27] Diary Iketones are smoothly reduced by sodium borohydnde in trifluoroacetic acid to diarylmethanes (equation 13)... 

Benzophenones are produced by the oxidation of diarylmethanes under basic conditions [6-9], The initial step requires a strongly basic medium to ionize the methane and the more lipophilic quaternary ammonium catalysts are preferred (Aliquat and tetra-n-octylammonium bromide are better catalysts than tetra-n-butyl-ammonium bromide). The oxidation and oxidative dehydrogenation of partially reduced arenes to oxo derivatives in a manner similar to that used for the oxidation of diarylmethanes has been reported, e.g. fluorene is converted into fluorenone (100%), and 9,10-dihydroanthracene and l,4,4a,9a-tetrahydroanthraquinone into anthraquinone (75% and 100%, respectively) [6]. 

Oxidation of Other Diarylmethanes. Table VII summarizes the products isolated in the oxidation of a number of other diarylmethanes. 

Anilines are readily oxidized by DIB, being converted either into azo compounds or into other products of condensation, in complex solvent-dependent reactions without much synthetic utility. Better results were obtained in some intramolecular cyclizations of 2,2-diamino-diarylmethanes and 2,2-diamino-diarylsulphones, which afforded the corresponding cyclic azo compounds (diazepines) [50,51], More generally, intramolecular cyclizations of ortho-substituted anilines are known to occur on oxidation of several substrates, according to the following scheme [52] ... 

Participation of two phenyl rings in the formation of a cyclopropane intermediate (80) was proposed for the anodic oxidation of tri- and diarylmethanes as shown below. 

The regioselectivity of 1,3-dipolar cyclization is affected by steric and electronic factors. Addition of cyclic A -oxides to propynoic esters shows regioselectivity as only the 1-carboxylic ester is obtained when the temperature is kept at 60°C. When the phosphorane (105.2) is heated in methanol, the isoxazole is formed instead of the expected diarylmethane. 

Benzylic oxidation. This combination oxidizes substituted toluenes to the corresponding acids and diarylmethanes (including fluorene) into ketones at room temperature in MeCN. Benzyl ethers such as phthalan and isochroman are converted to phthalide and 3,4-dihydroisocoumarin in quantitative yield. 

Anodic oxidation of dithioketals with /tara-iodoanisole in acetonitrile in the presence of EtsN 3HF as an electrolyte forms gem-difluoro derivatives of diarylmethane (94JOC7190) (Scheme 50). (Difluoroiodo)arenes possess a high fluorinating abihty. These substances need not be isolated pure one can employ their solutions in acetonitrile. 

Chromium(VI) oxide is also an efficient catalyst for oxidation at the benzylic position with periodic acid as the terminal oxidant in acetonitrile. Substituted toluenes with an electron-withdrawing group at the 4- or 3-position and diarylmethanes such as Ph2CH2 and fiuorene are oxidized to the respective substituted benzoic acids and ketones in excellent yields [1332], Periodic acid in the presence of catalytic CrOj can be used for the oxidation of arenes, such as naphthalenes and anthracene, to the corresponding quinones for example,... 

ArPb(OzC-CF3)2 Ar+ + Pb(02C CF3)2]. The aryl cations have been trapped with aromatic compounds to give biaryls [with certain substrates, notably poly-methylbenzenes, high yields (up to 88 %) are obtained], but with reactive aromatic substrates aryl cations are not the precursors to the biaryls and in these cases it is proposed that reaction proceeds via preliminary complex formation between the substrate and a species which contains an aryl-lead bond. Oxidative coupling of methyl-substituted benzenes by the reagent Pb(0Ac)4-CFs C02H to give biaryls and diarylmethane is also considered to involve formation of a radical cation in the primary step. A study has also been made of the plumbylation of monohalogeno-benzenes with Pb(OAc)4-CF3 COsH. ... 

Initially, this field was pioneered by Li who reported in 2007 on the formation of new carbon-carbon bonds directly starting from diarylmethanes and 1,3-dicarbonyl compounds under mild reaction conditions (80-100 °C, 5-36 h) in the presence of catalytic quantities of an inexpensive iron precursor (iron(ii) chloride, 20 mol%) and stoichiometric quantities of an oxidant (DTBP). The overall reaction can be regarded as an iron(ii) chloride-catalysed oxidative activation of a benzylic carbon-hydrogen bond followed by a cross-coupling reaction to form a carbon-carbon bond (Scheme 13.7, bottom). The mechanistic proposal is based on a single-electron transfer pathway as shown in Scheme 13.7. Initial homolysis of DTBP in the presence of the iron(ii) salt forms both tert-butoxyl radical and an iron(iii) species. Then, proton abstraction from the diarenemethane generates the two key... 

Pratt, E. E, and McGovern, T. P, Oxidation by solids. 1. Oxidation of selected alcohols by manganese dioxide, J. Org. Chem., 26, 2973, 1961 Pratt, E. E, and Van de Castle, J. E, Oxidation by solids. 11. The preparation of eithertetraarylethanes or diarylketones by oxidation of diarylmethanes with manganese dioxide,/ Org. Chem.,... 

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