Isomer distributions substitution

The nitration of some substituted nitrobenzenes has been studied in connection with the high o -ratios produced by [ —/ — A/] substituents. Thus nitration in sulphuric acid of 2,5-dialkyl-nitrobenzenes produces the isomer distributions shown below. As has been seen ( 9.1.3), one explanation for the occurrence of high o -ratios with [-/—A/] substituents is that the latter specifically deactivate para positions. In the... 

Methylenecyclopentane has the less substituted double bond and is the major product The reported isomer distribution is 91% methylenecyclopentane and 9% 1 methylcyclopentene ... 

This activation of the ortho position is most strikingly illustrated in the reactivity of 2,5-dimethylthiophene, which competitive experiments have shown to undergo the SnCb-catalyzed Friedel-Crafts reaction more rapidly than thiophene and even 2-methylthiophene. The influence of the reagent on the isomer distribution is evident from the fact that 2-methoxythiophene is formylated and bromi-nated (with A -bromosuccinimide) only in the 5-position. Similarly, although 3-bromo-2-methylthiophene has been detected in the bromi-nation of 2-methylthiophene with bromine, only the 5-isomer (besides some side-chain bromination) is obtained in the bromination of alkylthiophenes with A -bromosuccinimide. ° However, the mechanism of the latter type of bromination is not established. No lines attributable to 2-methyl-3-thiocyanothiophene or 2-methyl-3-chIoro-thiophene could be detected in the NMR spectra of the substitution products (5-isomers) obtained upon thiocyanation with thiocyanogen or chlorination with sulfuryl chloride. 2-Methyl- and 2-ethyl-thiophene give, somewhat unexpectedly, upon alkylation with t-butyl chloride in the presence of Feds, only 5-t-butyl monosubstituted and... 

The most accurate data on isomer distributions in alkylation of heterocycles have been obtained from the reaction of 3-n-butylpyridine with methyl radicals in acetic aeid. The ratio of the monomethyl products was determined by infrared spectroscopy and gas chromatography and is showm in (27). A small amount of 2,6-dimethyl-3-n-butylpyridine was also obtained. These ratios again show a high proportion of ortho substitution. 

Direct perfluoroalkylation of heteroaromatics occurs with RFI when sodium hydroxymethane sulfinate (Rongalite) is present. 3-Perfluoroal-kylcoumarins can be obtained (90CC1781). The distribution of isomers from substituted pyridines is compatible with a radical reaction (90TL2711). 

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. 

Partial rate factors, and hence the isomer distribution in a particular substitution reaction, are also affected by temperature. Increasing temperature has the greatest relative effect on the substitution reaction of highest AG (out of the three possible, alternative attacks on C6H5 Y),... 

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.

Both the degree and position of chlorine substitution on the biphenyl rings influence the physical-chemical and toxicological behavior of the individual PCB constituents (15). When PCBs are released into the environment, the original isomer distribution pattern of any PCB formulation may be altered as a result of specific interactions with the environment (14. 22. 23). 

The isomer distribution obtained from the oxidation of mesitylene in acetic acid, sodium acetate depends on the anode material. Graphite strongly favours nuclear substitution to side chain substitution in the ratio 23 1 while at platinum this ratio is 4 1. Oxidation of methyl benzenes in acetic acid containing tetrabutykmmonium fluoroborate and no acetate ion gives benzyl acetate as the major product since loss of a proton from the radical-cation is now faster than nuclear substitution by acetic acid as the only nucleophile present [39]. 

Ion pairs (contd.) intimate, 91, 249, 291 solvation, 45, 57, 81, 390 solvent separated, 91 Ipso substitution, 161, 169 steric effects in, 162 Isocyanate intermediates, 49, 122 Isomer distribution, 158 Isomerisation, cis- trans, 315 Isoprene, 323... 

Aminopyrazolo[3,4-d]pyrimidines 257 were converted into the corresponding 4-aryl substituted derivatives 259 via treatment with alkyl nitrites and boiling in aromatic solvent. The isomer distribution of 259 prepared by these route was that predicted for a radical intermediate (ortho, meta, and para). The structure of isomers was established by H-NMR. Unusual fragmentation products were isolated these probably result from collapse of the radical intermediate 258 (83JOC4605). Methylation of 257 takes place at either N-1 or N-2. Further methylation affords methylamino derivatives structures of the products were established by C-NMR as well as by chemical methods (75JOC1822). 

When a substituted anhydride or dianhydride is reacted with an amine (or diamine in the case of polyimides), two or three isomeric species can be fonned, respectively. In order to study the formation of different isomers and the factors that affect isomer distribution, several researchers have studied amic acid formation by both NMR techniques and chromatography. Quantitative analysis of such compositions is difficult by conventional C-NMR alone because of the complexity of the spectra. Thus, isomer analysis is typically measured by C-NMR peak height measurements, not a particularly satisfying technique. While it has proven valuable for identifying the major isomer present in two isomer mixtures, only in certain cases has C-NMR proved suitable for quantifying three isomer compositions, owing to the extreme complexity of the spectra. 

From this it is possible to calculate the overall theoretical rate ratio for acetylation of m-xylene relative to benzene, since this is one-sixth the sum of the partial rate factors (in this case 1130), and the isomer distribution if the reaction is kinetically controlled. The overall rate ratio actually is 34769 and the calculated and observed isomer distributions are listed in Table 11.2.69 In this case, and in many others, agreement is fairly good, but many cases are known where the effects are not additive.70 For example, the treatment predicts that for 1,2,3-trimethylbenzene there should be 35% 5 substitution and 65% 4 substitution, but acetylation gave 79% 5 substitution and 21% of the 4 isomer. The treatment is thrown off by steric effects, such as those mentioned earlier (p. 511), by products arising from ipso attack (p. 512) and by resonance interaction between groups (for example, 24), which must make the results deviate from simple additivity of the effects of the groups. 

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