Pyridines solvent effects

Y. Kaibu. Busseiron KenkyU No. 64, 120-9 (1953). UV pyridine, solvent effect in oxygenated compounds. 

It is to be noted that solvent effect on the reaction is rather small e.g., a 12-fold increase in rate was observed on going from THF to dimethyl-formamide (Table V, reactions 2 and 4). Addition of pyridine or PPhj, which may take up the vacant coordination site in the acyl, slows down the reaction by less than a factor of 2 (Table V, reaction 5). Furthermore,... 

During water-gas shift in pyridine solution, they isolated [PtH(py)L2]BF4, while from water-gas shift run in acetone solution, they isolated raft -[PtF[(CO)L2]BF4. The results indicated a solvent effect. That is, it was difficult to substitute coordinated pyridine with CO, but it was easier to substitute acetone with CO, via [PtH(Solvent)L2]OH + CO <-> [PtH(CO)L2]OH + Solvent. Following this important solvent-facilitated CO addition, they proposed a nucleophilic attack of OH-on the coordinated CO, via [PtH(CO)L2]OH <-> [PtH(COOH)L2]. The next step is thermal decomposition of the species, liberating C02, via the decomposition [PtH(COOH)L2] <-> [PtH2L2] + C02. CO addition was proposed to assist in decomposing the hydride to liberate H2. A more detailed description of the catalytic cycle is provided in Scheme 19. 

The reaction rate varies with the change in the solvent composition. The catalysis of pyridine-Cu in DMSO-benzene mixed solvent is summarized in Fig. 4 (a). The rate constant of the catalyst reoxidation (k0) and the overall rate increase although the rate constant of electron-transfer (ke) decreases with the benzene content. Instead of the benzene solvent, the copolymer of vinylpyridine with styrene (PSP) was used as a polymer ligand, as shown in Fig. 4 (b). The overall rate and k0 increase with the styrene content in the PSP ligand, just as the solvent effect of benzene. Only several times amount of styrene unit to Cu ion (as polymer concentration ca. 0.1 wt% of the solvent) affects... 

Ab initio SCRF/MO methods have been applied to the hydrolysis and methanol-ysis of methanesulfonyl chloride (334). ° The aminolysis by aromatic amines of sulfonyl and acyl chlorides has been examined in terms of solvent parameters, the former being the more solvent-dependent process.Solvent effects on the reactions of dansyl chloride (335) with substituted pyridines in MeOH-MeCN were studied using two parameters of Taft s solvatochromatic correlation and four parameters of the Kirkwood-Onsager, Parker, Marcus and Hildebrand equations. MeCN solvent molecules accelerate charge separation of the reactants and stabilize the transition... 

Temperature Effects. The oxidation of 9,10-dihydroanthrafcene to anthraquinone in anhydrous pyridine solvent with benzyltrimethylammo-nium hydroxide as the base occurs over a wide temperature range (Table I). Some oxidation takes place at a temperature as low as — 20°C., but maximum anthraquinone conversions (about 70%) occur between 50° and 70°C. Above 70°C., the conversion decreases, probably as a result of thermal decomposition of the benzyltrimethylammonium hydroxide. 

Solvent Effects. The conversion of dihydroanthracene could be increased by adding water to the pyridine solvent (Table III). An 86% conversion to anthraquinone was obtained when 95% aqueous pyridine was used as the solvent. Furthermore, methanol could be substituted for the water with equivalent results. Other solvents were tried in place of pyridine (Table IV). The data indicate that 95% aqueous pyridine gave the best yields, although aniline gave nearly similar results. When acetonitrile and dimethylformamide were used, the large amounts of unreacted starting material indicate that these solvents may have deactivated the base by undergoing a hydrolysis reaction. 

We next give Tables 6, 7 and 8 to show representative values for various chemical shifts, couplings and solvent effects in and 13C NMR spectra of pyridine and its iV-oxide and protonated derivatives, in addition to those in the general section (Chapter 2.01), to show the general character of the results obtained and their susceptibility to variation of structure and media. 

The dramatic influence of solvent effects on the UV and visible spectra of certain pyridine compounds, known generally as a solvatochromic effect, has been much utilized in the expression of solvents effects. The polarity parameter. Z or ET is defined (58JA3253, B-68MI204002) from the longest wavelength charge transfer band of 1-ethyl-4-methoxycar-bonylpyridinium iodide (equation 3). 

Closely related to solvent effect, and governed by the same or similar factors, are the phenomena of dimerization via self-association, and association with other species present in solution, such as cations or anions, in definite stoichiometric proportions. Studies of such associations lead to further knowledge of such interactions involving the more complex bases in nucleic acid structures. One of the earliest workers to study dimerization systematically was Shindo (59CPB407), who examined the broad N—H stretch region 3300-2400 cm-1 in the IR spectra of a variety of substituted pyridin-2-ones and quinolin-2-ones in perfluorocarbon mulls and CC14 solution. 

Catalysts and their effects on chemical reactions aid in efficiency, effectiveness and selectivity. A recent example of current research is redox and ligand exchange reactions of the oxygenation catalyst (N,N -bis(salicylidene)ethylenediaminato)co-balt(II), Co(SALEN)2 (below), and its one-electron oxidation product, Co(salen) 2-These were investigated in DMF, pyridine, and mixtures of these solvents. Solvent effects on the potentials, the thermodynamics of cross reactions, and the distribution of Co(II) and Co(III) species as a function of the solvent composition are important considerations (Eichhorn, 1997). The results in these solvents should be compared with other work with catalysts using more environmentally benign media (Collins et al., 1998). 

One-bond coupling constants JCH may suffer from slight solvent effects. Table 3.4 shows this behavior for chloroform, whose carbon-proton coupling increases with the polarity of the medium when measured in different solvents, being 208 Hz in cyclohexane and 215 Hz in pyridine [92]. This is attributed to association between chloroform and solvents susceptible to hydrogen bonding. 

While solvent effects are small for the ring redox ( 0.2 V), the effect of donor solvents which stabilize the higher oxidation state is marked for the metal redox, especially so for those of low-spin d6/d7 and d1jd%. There is a linear relationship between E° for the Feu/I couple and the Gutmann donicity number. The Com u and Cou/I couples shift positively as the pof substituted pyridine, as solvent, decreases and the ring oxidation precedes the CoIII/n couple in noncoordinating solvents such as CH2C12. 

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