Solvent influence on isomer ratio

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... 

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>. 

Addition of amines to acetylene carboxylates 246 in ball mill was studied by Stolle and Thorwirth (Scheme 3.65) [44]. Employing solvent-free protocol in planetary ball mill in conjunction with quartz sand as inert grinding auxiliary, a number of enamines 247 were prepared in high yield (Table 3.33). No further catalysts or additives was required to accomplish reaction within 5 min. Obtained E/Z ratios vary with amine applied in the case of dicarboxylates, mainly the E-isomers have been identified. In contrast, reactions with propiolates and anilines yielded favorably the Z-isomer, whereas secondary alkyl amines furnished the E-isomer preferably. Simple stirring of DMAD and aniline in flask afforded only 80% conversion and chemoselectivity was lower (E/Zratio of 87 13) in comparison to ball milling. A common method for synthesis is the reaction in solvent, for which was found to have a strong influence on the E/Z ratio of the product, while this influence is not present in solvent-free milling. 

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. ... 

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]. 

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,... 

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... 

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... 

Asymmetric Hydrocarboxylation. The title reagent was used in the first example of an asymmetric hydrocarboxylation (eq 1). With the a-methylstyrene, the straight chain isomer was formed. The regiospecificity was much less pronounced, however, for other alkenic substrates. The influence of some reaction variables on the reaction shown in eq 1 was studied. For example, the presence of a solvent such as THF or benzene, the alcohol source, the effect of CO pressure, the effect of substitution on the phenyl ring, the PdC /DIOP molar ratio, or the presence of PPha along with DIOP, were varied to improve the optical yield. ... 

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