Unsymmetrical benzene compounds

Photolysis at about 40°C of the /ra is-isomer of the unsymmetrical azo-compound 4 in benzene solution yields dicumyl and biphenyl as the major products, together with a little a-methylstjTene, cumene, and 2,2-diphenylpropane. All these products show polarization of their... 

Unsymmetrical diaryls may be prepared by treating an aryl diazonium salt solution with sodium hydroxide or sodium acetate in the presence of a liquid aromatic compound. Thus 2-chlorodiphenyl is readily formed from o-chloro phenyl diazonium chloride and sodium hydroxide solution (or sodium acetate solution) in the presence of benzene ... 

An interesting feature of tliese compounds is tliat, upon tlierniolysis in benzene at ao C, tlie unsymmetrical C C coupling product 2-Me2NCi-,H C-CR is fornied exclusively. Hie selectivity of tliis reaction is probably directly related to tlie structural features of tliis betemleptic aggregate [a9]. 

Arylisoxazol-5(4//)-oncs 21 react with benzene-1,2-diamines to yield 4-aryl-l,5-benzodiaze-pinones 22 by elimination of hydroxylamine from the intermediate oximes. Unsymmetrically substituted benzene-1,2-diamines are attacked at the more nucleophilic amino group. Thus, 4-methylbenzene-1,2-diamine gives 7-methylbenzodiazepinones 22f-h, whereas 4-nitrobenzene-1,2-diamine gives 8-nitro compounds 22k-n. The benzodiazepinones are accompanied by minor amounts of 2-methylbenzimidazoles 23. Selected examples are given.275... 

For example, photolysis of a suspension of an arylthallium ditrifluoro-acetate in benzene results in the formation of unsymmetrical biphenyls in high yield (80-90%) and in a high state of purity 152). The results are in full agreement with a free radical pathway which, as suggested above, is initiated by a photochemically induced homolysis of the aryl carbon-thallium bond. Capture of the resulting aryl radical by benzene would lead to the observed unsymmetrical biphenyl, while spontaneous disproportionation of the initially formed Tl(II) species to thallium(I) trifluoroacetate and trifluoroacetoxy radicals, followed by reaction of the latter with aryl radicals, accounts for the very small amounts of aryl trifluoroacetates formed as by-products. This route to unsymmetrical biphenyls thus complements the well-known Wolf and Kharasch procedure involving photolysis of aromatic iodides 171). Since the most versatile route to the latter compounds involves again the intermediacy of arylthallium ditrifluoroacetates (treatment with aqueous potassium iodide) 91), these latter compounds now occupy a central role in controlled biphenyl synthesis. 

In addition, a number of unsymmetric dipnictenes, RP=AsR, RP=SbR,62 RSb=BiR,59 and RP=BiR,63 were reported. One example, the stibabismuthene RSb=BiR (R = Bbt), was formed by the elimination of HBr from RSbH2 and RBiBr2. The stibabismuthene is stable at ambient temperature in hydrocarbon solvents in the absence of air and light. When a solution in benzene was heated, the symmetrical compounds are formed (Scheme 7).59... 

The most classical examples of atropisomerism, biphenyls, fall into this category. They form enantiomers because the two benzene rings are not coplanar and both rings are substituted unsymmetrically so that the plane passing through the pivot bond and one of the benzene rings cannot be a a plane. If we consider the conformations of biphenyls in more detail, we recognize that there are two diastereomeric conformations possible, as depicted in Scheme 3 for a compound... 

An interesting feature of these compounds is that, upon thermolysis in benzene at 80 °C, the unsymmetrical C-C coupling product 2-Me2NC6H4C=CR is formed exclusively. The selectivity of this reaction is probably directly related to the structural features of this heteroleptic aggregate [89]. 

Diorgano tellurium compounds with carbonyl groups or C — C double bonds next to the C — Te — C groups are rather labile. For example, allyl methyl tellurium decomposes in light at 60 to 80° to tellurium, 1 -butene, and 1,5-hexadiene. In benzene solution at 60° in the dark this unsymmetrical diorgano tellurium symmetrizes to dimethyl tellurium and dialkyl tellurium within 10 minutes8. 2,5-Dihydrotellurophenes deposit tellurium at 20° while photolysis of an ethanolic solution at 300 nm produced tellurium and isoprene but no cyclobutene9. 

Deuterium labelling studies have also been used to investigate the reaction of stilbenes and related compounds with amines (Lewis, 1979). It is known that tertiary amines form fluorescent exciplexes with stilbenes in nonpolar solvents and that polar solvents are necessary for chemical reaction to occur (Lewis and Ho, 1977). This suggests that radical ions are involved in product formation. When secondary amines are used, reaction occurs in solvents of widely differing polarity and this is presumably due to the acidity of the secondary N—H bond. N-deuteriated diethylamine reacts with 1,2-diphenyl-cyclobutene in benzene to give products [65], [66] and [67] incorporating deuterium (Scheme 6). For the reaction with unsymmetrically substituted... 

Triphenylphosphane is an oxophilic compound that reduces sulfoxides (R2SO), disulfane oxides (RS(O)SR), disulfane dioxides (sulfones) (RSO2SR), and trisulfane tetroxides to the corresponding sulfanes. From the products prepared by equations (62) or (63), symmetrical or unsymmetrical aryl- or alkyltrisulfanes can be obtained in high yields (refluxing in ether or benzene) (equation 64). ... 

Quinoxalinyl ketones ean be prepared by condensation of a benzene-1,2-diamine with an appropriate tricarbonyl compound yielding compounds 29. Many syntheses of quinoxaline-2-carbo.xylic acids and their derivatives arc based on the reaction of henzene-1,2-diamine with suitable tricarbonyl compounds. The formation of isomeric mixtures can prove problematic when unsymmetrical diamines arc used in this synthetic approach. 

A quite different result is obtained when the reaction conditions are modified. For example, when compound XIII is taken up in hot benzene containing a catalytic amount of -toluenesulfonic acid, not only is an oxazoline obtained in which the absolute configuration of the unsymmetrical carbon (starred atom of Scheme 14.3) in the starting aziridine is retained (only 1% inversion), but the structurally isomeric oxazoline XV is produced also. ... 

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