Isomer ratio, solvent influence

Electronic effects of substituent, as well as reaction conditions (temperature, solvent) influence the ratio of isomers. The alkylation of l-alkyl-5-aryl tetrazoles with dimethyl sulfate at room temperature proceeds at the... 

Because of the different experimental conditions, the solvent/amine pair plays a crucial role in the formation of the iiii stereoisomer. Systematic study of the influence of the nature of the base, the reagent/base ratio, and the nature of the solvent (donating power), was performed with the phosphonatocavitands 12a and 12b (Table 1). The procedure is highly solvent dependent the iiii stereoisomer is the major product obtained in toluene, associated with minor amount of iiio isomer. When the reaction is run in acetone other isomers are predominant (ioio, iioo, oooi) and the iiii stereoisomer is not observed. The use of catalytic amount of this amine (0.2 equiv) [64] did not lead to any extractable compounds, and using eight equivalents of amine to trap all the HCl formed, did not change dramatically the yield and the isomer ratio (entries 5 and 6, Table 1). In the presence of triethylamine the iiii and iiio isomers were formed in 28.5% and 7% yields respectively. 

The effect of change of solvent is also quite marked, as seen from the data in Table XII.352 An effect of pH upon the isomer ratio for the reaction with 1-methoxypyridinium iodide was also observed as the pH of the initial reaction mixture was raised from 10.07 to 11.91, the 4-/2- isomer ratio fell from 1.67 to 0.49. Finally, the influence of an... 

The electronic nature of the substituent on the aryl unit actually brings about influences to the rate and the direction of the rearrangement, but it is a minor effect Other experimental conditions (solvent, reaction temperature, and reaction time) are more important for determining the rate and the isomer ratio ortho vs. para) of the rearranged product In some cases these observations are interpreted in terms of possible polar mechanisms for the aromatic Claisen rearrangement [15]. 

Purpose. An elirnination reaction is carried out using en/fEro-2,3-dibromo-3-phenylpropanoic add, the direct precursor to 2 -bromostyrene. The influence of solvents and base on the course of the elimination reaction is illustrated. You will consider factors that control the stereospedfidty of a reaction. You will explore the option of using NMR spectroscopy to establish the ds/trans isomer ratio in the 2 -bromostyrene produd. 

Toluene is moderately activated, so the Kyodai nitration of this hydrocarbon is always accompanied by more or less dinitration product. On inspecting the isomer composition of this byproduct in the course of the Kyodai nitration in the presence of montmorillonite KIO and zeolites HZSM-5 or HBEA-25, a considerable preponderance of 2,4-dinitrotoluene over the 2,6-dinitro isomer 2,4-12,6- = 5.1-9.3) has been discovered [38]. In the classical nitration based on mixed acids, the 2,4-dinitro and 2,6-dinitro isomers were obtained in about a 4 1 ratio. The nature of organic solvent was found to exert considerable influence on the regioselectivity of dinitration. Of the four solvents of varied polarity (n-C Hj, CCl CH Cl, MeCN), acetonitrile with a high dielectric constant exhibited the best result. The 2,4-/2,6-isomer ratio is enhanced up to 12 in acetonitrile, whereas these values are 5 in n-C Hj, 4.1 in CCl and 9.3 in CH Cl. ... 

HI also influence the conformational preferences of organic solutes in water. The tendency is to reduce the solvent-accessible surface area as exemphfied by MD simulations of -butane. If n-butane is transferred from the gas phase to water, there is an entropy-driven increased population of the more compact gauche conformation compared to the extended aU-trans conformation. Other interesting cases of folding of small molecules in water have been recorded in the literature. The relevance for organic reactivity is exemplified by Diels-Alder reactions in water, which occur with a significantly enhanced endo/exo isomer ratio. ... 

Looking for the influence of reaction conditions on the stereoselectivity of hydrogenations, one will of course find numerous examples reporting changing isomer ratios on variations of catalysts and solvents [39]. 

Solvent influence on isomer ratio, nitration 16, 411 Solvents, polar (s. a. Nitrobenzene) 16, 933 Sommeiet reaction 16, 818 Spiro[4,2]-l,3-heptadienes 16, 865... 

In the above discussion the problem of stereoisomeric control has been set aside. Total stereoisomeric purity is in fact difficult to achieve, and extensive mechanistic studies have been carried out to elucidate the factors which influence the (E/Z) ratio. It thus appears that, in the formation of the alkene R1,CH=CH R2, thermodynamic control leading to predominance of the ( )-isomer is promoted by elevated temperatures, apolar solvents, stabilising salts (lithium or sodium halides), carbanion stabilisation, an electron-rich phosphorus atom [—PO(OR)2], and excess base. On the other hand, kinetic control leading to a predominance of the (Z)-isomer, is promoted by low temperatures,... 

Experimental procedures are given in Expt 6.107 for o- and p-hydroxy-propiophenones (R = Et). The ortho-para ratio in the product is influenced by the nature of the alkyl residue, the temperature, the solvent and the amount of aluminium chloride used generally low temperatures favour the formation of p-hydroxyketones. It is usually possible to separate the two hydroxyketones by fractional distillation under reduced pressure through an efficient fractionating column or by steam distillation the ortho isomers, being chelated, are more steam volatile. It may be mentioned that Clemmensen reduction (cf. Sections 5.1.3, p. 476 and 6.1.1, p. 826) of the hydroxyketones affords an excellent route to alkyl phenols. 

An apparent influence of the solvent was noted in the action of aluminum chloride on the chloride of XCVI, which gave mixtures of the two ketones in which the benzanthracene derivative (XCVII) predominated. This isomer constituted about two-thirds of the total using benzene as a solvent, and a still higher proportion (about nine-tenths) in nitrobenzene solution. A difference in reaction temperatures also may have influenced these ratios. That the temperature may affect the proportion of isomers is indicated by the fact that, when cyclization with stannic chloride was conducted at room temperature instead of in the cold, a mixture of ketones resulted instead of a single ketone.107... 

Theoretical calculations on the /Fdithietane 1,3-dioxide 3 =, sy -dithietane 1,3-dioxide 4 equilibrium in the gas phase at HF/6-31G level show that the anti-isomer 3 is slightly favored (by ca. 0.27 kcal moF1) over the, sy -isomer 4. The antijsyn-ratio is 1.6, with a ry -concentration of 36%. Due to different dipole moments of the anti 3 and syn 4, the solvents of low and medium-high polarity such as carbon tetrachloride, acetonitrile, and dimethyl sulfoxide (DMSO) exert a strong influence on the antisyn interconversion, producing an increase in the ry -concentration <2001BOC57>. 

During the photocycloaddition of 88 with cyclopentene (Reaction 1), de of the major isomer 89 increased from 30% in nonpolar solvents up to 68 in a mixture of methanol and acetic acid. When prochiral enone 91 was irradia in the presence of a cyclopentene linked to the 8-phenylmenthol (Reaction the best selectivity was now obtained in nonpolar solvents. To explain this eff< it was proposed that the facial selectivity is high in every case and that diastereoselectivity depends on an s-cis s-trans ratio of the conjugated es influenced by hydrogen bonding [65]. Similar results were obtained with c... 

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