Enhancer pyridine

Nitrogen and, especially, sulfur decrease the stability of alkali and alkaline-earth ion complexes. Conversely, complexation with Ag or other heavy-metal ions and with ammonium ions is enhanced. Pyridine rings increase selectivity for sodium ions [190]. 

SERS. A phenomenon that certainly involves the adsorbent-adsorbate interaction is that of surface-enhanced resonance Raman spectroscopy, or SERS. The basic observation is that for pyridine adsorbed on surface-roughened silver, there is an amazing enhancement of the resonance Raman intensity (see Refs. 124—128). More recent work has involved other adsorbates and colloidal... 

It was first reported in 1974 that the Raman spectrum of pyridine is enhanced by many orders of magnitude when... 

Golab J T, Sprague J R, Oarron K T, Schatz G 0 and Van Duyne R P 1988 A surface enhanced hyper-Raman scattering study of pyridine adsorbed onto silver experiment and theory J. Chem. Phys. 88 7942-51... 

The value of the second-order rate constant for nitration of benzene-sulphonic acid in anhydrous sulphuric acid varies with the concentration of the aromatic substrate and with that of additives such as nitromethane and sulphuryl chloride. The effect seems to depend on the total concentration of non-electrolyte, moderate values of which (up to about 0-5 mol 1 ) depress the rate constant. More substantial concentrations of non-electrolytes can cause marked rate enhancements in this medium. Added hydrogen sulphate salts or bases such as pyridine... 

As early as 1889 Walker (320), using samples of thiazole, 2,4-dimethylthiazoie, pyridine, and 2,6-dimethylpyridine obtained from Hantzsch s laboratory, measured the electrical conductivity of their chlorhydrates and compared them with those of salts of other weak bases, especially quinoline and 2-methylquinoline. He observed the following order of decreasing proton affinity (basicity) quinaldine>2,6-dimethyl-pyridine>quinoline>pyridine>2,4-dimethylthiazole> thiazole, and concluded that the replacement of a nuclear H-atom by a methyl group enhanced the basicity of the aza-aromatic substrates. 

The protonation of the nitrogen atom of thiazole induces a large increase in reactivity of the 2-position (193, 194). This is in contrast to the pyridine series, where the reactivity of both positions adjacent to the nitrogen atom are enhanced (194). The phenylation of conjugate acid of 5-alkylthiazoles may then be considered as a preparative route to alkyl-thiazoles. The results (isomer percent and overall reactivity) are indicated in Tables III-31 (196) and 01-32 (196). 

Identify the more basic of the two nitrogens of 4 (N,N dimethylamino)pyndine and suggest an explanation for its enhanced basicity as compared with pyridine and N N dimethylanihne Refer to Learning By Modeling and compare your prediction to one based on the calculated charge and electrostatic potential of each nitrogen... 

Selectivity of propylene oxide from propylene has been reported as high as 97% (222). Use of a gas cathode where oxygen is the gas, reduces required voltage and eliminates the formation of hydrogen (223). Addition of carbonate and bicarbonate salts to the electrolyte enhances ceU performance and product selectivity (224). Reference 225 shows that use of alternating current results in reduced current efficiencies, especiaHy as the frequency is increased. Electrochemical epoxidation of propylene is also accompHshed by using anolyte-containing silver—pyridine complexes (226) or thallium acetate complexes (227,228). 

Similar disproportionation reactions are catalyzed by organic catalysts, eg, adiponittile, pyridine, and dimethyl acetamide. Methods for the redistribution of methyUiydridosilane mixtures from the direct process have been developed to enhance the yield of dimethylchlorosilane (158). 

In general, the solubility of heterocyclic compounds in water (Table 33) is enhanced by the possibility of hydrogen bonding. Pyridine-like nitrogen atoms facilitate this (compare benzene and pyridine). In the same way, oxazole is miscible with water, and isoxazole is very soluble, more so than furan. 

The more recent results of Scheme 18 can be summarized as follows the less hindered isomer is formed unless the substituent has a lone pair, the electrostatic field of which enhances the nucleophilicity of the adjacent nitrogen atom. This is true not only for pyridine and pyrazole, but probably also for CHO and C02Et. 

When the 1-position is substituted, 3- and 5-aminopyrazoles react at the C-4 carbon atom, the reactivity of which is enhanced by the amino group. Thus pyrazolo[3,4-Z ]pyridines (545) are obtained either by the Skraup synthesis or from 1,3-diifunctional compounds. Here also aminopyrazolinones have been used instead of aminopyrazoles to prepare (545 R = OH). If 1,4-ketoesters (succinic acid derivatives) are used instead of /3-ketoesters, pyrazolo[3,4-Z ]azepinones (546) are obtained. 

Intensity enhancement takes place on rough silver surfaces. Under such conditions, Raman scattering can be measured from monolayers of molecular substances adsorbed on the silver (pyridine was the original test case), a technique known as surface-enhanced Raman spectroscopy. More recendy it has been found that sur-fiice enhancement also occurs when a thin layer of silver is sputtered onto a solid sample and the Raman scattering is observed through the silver. 

Oxidations usually proceed in the dark at or below room temperature in a variety of solvents ranging from aqueous bicarbonate to anhydrous benzene-pyridine. Base is quite commonly used to consume the hydrogen halide produced in the reaction, as this prevents the formation of high concentrations of bromine (or chlorine) by a secondary process. The reaction time varies from a few minutes to 24 hours or more depending on the nature of the reagent and the substrate. Thus one finds that NBS or NBA when used in aqueous acetone or dioxane are very mild, selective reagents. The rate of these oxidations is noticeably enhanced when Fbutyl alcohol is used as a solvent. In general, saturated, primary alcohols are inert and methanol is often used as a solvent. 

A iD-Corticoids have been important intermediates since it was shown ° that substitution at C-9 enhances anti-inflammatory activity. These olefins are usually obtained from 11a- or 11)5-alcohols, and consequently several refined methods have been devised for effecting this dehydration. It is desirable that such methods be compatible with the presence of A" -3-ketone and 17-hydroxy functions. The first direct procedure for which high yields were claimed was described in a patent issued to Upjohn. According to this method, the alcohol (11a or )5) is treated first with A-bromoacetamide in pyridine, then with sulfur dioxide. Recently it has been claimed " that the A-haloamide/sulfur dioxide method gives results superior to other methods, although the methanesulfonyl chloride/sulfur dioxide procedure (see below) apparently was not compared (see also ref. 94). 

Whereas in all previously mentioned inverse cycloaddition reactions [h]-fused pyrido annelated systems are formed, some reactions are described which lead to [c]-pyridine annelated bicyclic systems. 5-(Butynylthio)pyrimidines (R = Ph, NHCOCH3) give on heating at 180°C in nitrobenzene 5-R-2,3-dihydrothieno[2,3-c]pyridines (89T803). 5-Propynyloxymethylpyrimidines also readily undergo cycloaddition into l,3-dihydrofuro[3,4-c]pyridines (89T5151) (Scheme 39). Considerable rate enhancements were observed with quaternized pyrimidinium salts. Whereas... 

The Hantsch pyridine synthesis provides the final step in the preparation of all dihydrop-yridines. This reaction consists in essence in the condensation of an aromatic aldehyde with an excess of an acetoacetate ester and ammonia. Tlie need to produce unsymmetrically subsrituted dihydropyridines led to the development of modifications on the synthesis. (The chirality in unsymmetrical compounds leads to marked enhancement in potency.) Methyl acetoacetate foniis an aldol product (30) with aldehyde 29 conjugate addition of ethyl acetoacetate would complete assembly of the carbon skeleton. Ammonia would provide the heterocyclic atom. Thus, application of this modified reaction affords the mixed diester felodipine 31 [8]. 

Trifluoromethyl-l,2,4-triazines and 1,2,4-triazin-5-ones are versatile precursors for azetidines and pyridines by cycloaddition reactions. Enhancement of reactivity was observed through the presence of the trifluor-omethyl group (88CPB3354). 

Cobalt trifluoride fluorination corresponds to the electron-transfer mechanism via a radical cation. RF groups attached to the ring enhance the stability of intermediate dienes and monoenes. Perfluoroalkyl pyridines, pyrazines, and pyrimidines were successfully fluorinated but pyridazines eliminated nitrogen. The lack of certain dienes was attributed to the difference in stability of FC=C and RFC=C and steric effects [81JCS(P1)2059]. 

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