Isomers substituted benzene

Upon treatment with suitable cobalt complexes, methylbutynol cyclizes to a 1,2,4-substituted benzene. Nickel complexes give the 1,3,5-isomer (196), sometimes accompanied by linear polymer (25) or a mixture of tetrasubstituted cyclooctatetraenes (26). 

Table 10.3. Isomer Proportions in the Nitration of Some Substituted Benzenes ...

An earlier paper [PolG35] contained the essence of Polya s Theorem in its three-variable form, together with an informal proof of it. This paper also entered into the field of isomer enumeration by presenting a generating function for the substituted benzenes. 

There are three different substituted benzene compounds with the formula C6H4F2. Assume that the benzene rings pack similarly into their crystal lattices. If the positions of the H and F atoms are statistically disordered in the solid state, which isomer will have the least residual molar entropy ... 

In contrast to the difluorobenzenes, the ortho isomer of the dihydroxy-substituted benzenes is the most stable followed by the meta and then the para isomer. The meta isomer is 0.9 kcal/mol above the ortho isomer and the para is 2.4 kcal/mol higher. The para/ortho ratio has been measured as 6 94 over HZSM-5 catalysts.2 Although the temperature at which this measurement was made was not originally reported, the ratio is approximately invariant over the range of 250-350°C. Taking 250°C as the temperature, we obtain AG (ortho-para) =2.9 kcal/mol. 

Nametkin and co-workers hrst reported the alkylation of benzene derivatives with allylchlorosilanes in the presence of aluminum chloride as catalyst. " 2-(Aryl)propylsilanes were obtained from the alkylation of substituted benzenes (Ph—X X = H, CL Br) with allylsilanes such as allyldichlorosilane and allyltrichlo-rosilane.The yields ranged from 34 to 66% depending upon the substituents on the benzene ring, but information concerning reaction rates and product isomer distribution was not reported. 

The ratio of ortho- to the meta- and /faru-products of monoalkylbenzenes with 1 decreased as the size of the substituents on benzene ring increased. No ortho-alkylation product was found in the case of i-propylbenzene due to the sterie interaction between /-propyl and the incoming allyl groups. Sterie hindrance arising from the size of the alkyl groups at ortho positions of the substituted benzenes appeared to be the principal cause of the differences in isomer product ratio." " ... 

Reaction of rhenium atoms with alkyl-substituted arenes forms dirhenium- l-arylidene compounds (2 2) (Figure 3). The products require insertion, presumably sequential, into two carbon-hydrogen bonds of the alkyl substituent. These reactions seem highly specific and require only the presence of an alkyl-substituted benzene that possesses a CH2 or CH3 substituent. Thus, co-condensation of rhenium atoms with ethylbenzene gives two isomers (see Figure 3) in which the products arise from insertion into the carbon-hydrogen bonds of the methylene or the methyl group. The product distribution in this reaction is in accord with statistical attack at all available sp3 C-H bonds. 

Copper-catalyzed cyclopropanation of benzene and its derivatives by a diazoacetic ester yields a norcaradiene 230 which undergoes spontaneous ring opening to cyclo-heptariene 231. At the temperatures needed for successful cyclopropanation, sigma-tropic H-shifts leading to conjugated isomers of cycloheptatriene carboxylates cannot be avoided. The situation is complicated by the formation of regioisomers upon cyclopropanation of substituted benzenes, and separation of the cycloheptatriene isomers may became tedious if not impossible. 

Scheme 31. Isomer distribution [%] of Rh CFjCOO -catalyzed cyclopropanation of substituted benzenes with methyl diazoacetate at 22 °C. The numbers refer to the percentage of 1,3,5-cyelohepta-triene-7-carboxylate from the total cycloheptatriene isomers.

The vapor-phase pyrolysis of 4-hydroxy-1,2,3-triazole and its iV-methyl derivative affords methan-imine and its A-methyl analog. Analysis of the reaction path by the MNDO method shows the presence of two stable or metastable isomers, (liif)-4-hydroxy-l,2,3-triazole and its ketone protomer <89NJC551>. 4-Diazo-1,2,3-triazoles (122) thermolyze or photolyze in benzene to 4//-l,2,3-tri-azolylidenes (123) which convert benzene to 4-phenyl-1,2,3-triazoles and/or isomerize to a-diazo-nitriles (124). Intermediates (124) react with benzene via a carbene to give addition, ring expansion or substitution products (Scheme 17) <82TL5115>. The similar thermolysis of diazotriazoles in substituted benzene gives complex mixtures in which all of the components are sometimes impossible to isolate and identify <90AHC(48)65>. 

The intervention of olefin intermediates has significant effects on the stereochemistry of the hydrogenation of aromatics. Similarly to the reduction of alkenes, cis isomers are the main products during the hydrogenation of substituted benzenes,... 

Benzo[6 jthiophenes with nitro groups at the 4-, 5-, 6- or 7-positions may be synthesized by ring closure reactions of appropriately substituted benzene derivatives. Nitration of 5-acetaminobenzo[6]thiophene gives the 4-nitro derivative, which can be hydrolyzed and deaminated to yield 4-nitrobenzo[6]thiophene (equation 44). 5-Nitrobenzo[6]thiophene is conveniently available by decarboxylation of 5-nitrobenzo[6]thiophene-2-carboxylic acid, which in turn is available by a Perkin reaction of 4-nitro-2-formylphenylthioglycoIic ester (equation 45 Section 3.15.2.3). The 7-nitro isomer may be obtained similarly. 

Several isomers of benzene have been postulated. Some of them have now been synthesized by one method or another and positively identified. Evidence for some of them is best for substituted benzenes and perhaps not too sound for benzene itself. 

The formation of isomers of benzene and their characterization are more difficult than for substituted benzenes. The principal references relative to these isomers are given in the bibliography but not all of them are related to photochemistry. 

Less heavily substituted benzenes also undergo photochemical scrambling of the type observed in the vapor phase for the xylenes. Thus 1,4-difluoro benzene irradiated at 2690 A produces solely quantities of the 1,3- and 1,2-isomers.58 Upon irradiation with a less monochromatic light source, the 1,3-isomer gives 1,2 and 1,4, and the 1,2-isomer gives the 1,3 and 1,4, although in these cases there is some polymer formation.58... 

The evidence above shows quite clearly that irradiation of substituted benzenes under a variety of conditions will lead to the formation of intermediate, isolable nonaromatic isomers, most of which return thermally to the ground state of the present molecule. 

Much effort has been devoted to the synthesis of derivatives of chroman-6-ols as model compounds for studies of the chemistry of the tocopherols. Many routes concentrate on the acid-catalyzed alkylation of substituted benzene-1,4-diols by allyl alcohols or dienes. Trimethylhydroquinone and isoprene react in acetic acid in the presence of zinc chloride to give 2,2,5,7,8-pentamethylchroman-6-ol (39JOC311) isomer formation is not possible here and so the route is particularly attractive. 

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