Solvents and additives

Regardless of the solvent used, various additives have been explored in the Sonogashira reaction to increase yield, and reaction rates. Ammonium and silver salts have shown the most promise in this respect. Mori et al. [123] found that by using 2 equiv. of either tetrabutylammonium fluoride (TBAF) or tetrabutylammonium hydroxide (TBAOH), activation of the cross-coupling with 

By employing H, and Ag NMR spectroscopy, the underlying mechanism and the involved intermediates were identified [125]. According to these findings, Ag(I) ions initially interact with the alkyne moiety to form an alkyne-Ag t-complex that is subsequently transformed into the corresponding silver acetylide, eventually promoting transmetallation to the Pd center. Hence, Ag as a cocatalyst basically acts in the same manner as Cu. 

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DCHA is normally obtained in low yields as a coproduct of aniline hydrogenation. The proposed mechanism of secondary amine formation in either reductive amination of cyclohexanone or arene hydrogenation iHurninates specific steps (Fig. 1) on which catalyst, solvents, and additives moderating catalyst supports all have effects. 

Properties are furthermore determined by the nature of the organic acid, the type of metal and its concentration, the presence of solvent and additives, and the method of manufacture. Higher melting points are characteristics of soaps made of high molecular-weight, straight-chain, saturated fatty acids. Branched-chain unsaturated fatty acids form soaps with lower melting points. Table 1 Hsts the properties of some soHd metal soaps. 

Enantioselectivities were found to change sharply depending upon the reaction conditions including catalyst structure, reaction temperature, solvent, and additives. Some representative examples of such selectivity dependence are listed in Scheme 7.42. The thiol adduct was formed with 79% ee (81% yield) when the reaction was catalyzed by the J ,J -DBFOX/Ph aqua nickel(II) complex at room temperature in dichloromethane. Reactions using either the anhydrous complex or the aqua complex with MS 4 A gave a racemic adduct, however, indicating that the aqua complex should be more favored than the anhydrous complex in thiol conjugate additions. Slow addition of thiophenol to the dichloromethane solution of 3-crotonoyl-2-oxazolidinone was ineffective for enantioselectivity. Enantioselectivity was dramatically lowered and reversed to -17% ee in the reaction at -78 °C. A similar tendency was observed in the reactions in diethyl ether and THF. For example, a satisfactory enantioselectivity (80% ee) was observed in the reaction in THF at room temperature, while the selectivity almost disappeared (7% ee) at 0°C. 

Many solvents and additives have measurable transfer constants (Table 6.5). The accuracy of much of the transfer constant data in the literature is questionable with values for a given system often spanning an order of magnitude. In some cases the discrepancies may be real and reflect differences in experimental conditions. In other cases they are less dear and may be due to difficulties in molecular weight measurements or other problems. 

Table 6.5 Transfer Constants (60 °C, bulk) for Selected Solvents and Additives...

In the elaboration of a safe process it is necessary to look for those synthetic routes which do not incorporate hazardous raw materials, solvents, and additives. This is unfortunately rarely achievable. Therefore, the best way to insure higher intrinsic safety is ... 

The extraction time has been observed to vary linearly with polymer density and decreases with smaller particle size [78,79]. The extraction time varies considerably for different solvents and additives. Small particle sizes are often essential to complete the extraction in reasonable times, and the solvents must be carefully selected to swell the polymer to dissolve the additives quantitatively. By powdering PP to 50 mesh size, 98 % extraction of BHT can be achieved by shaking at room temperature for 30 min with carbon disulfide. With isooctane the same recovery requires 125 min Santonox is extractable quantitatively with iso-octane only after 2000mm. The choice of solvent significantly influences the duration of the extraction. For example talc filled PP can be extracted in 72 h with chloroform, but needs only 24 h with THF [80]. pH plays a role in extracting weakly acidic and basic organic solutes, but is rarely addressed explicitly as a parameter. 

A paint is made up of several (typically 10-15) components. These can be divided into four main groups with different functionalities binders, pigments, solvents, and additives. Not all paints have ingredients within every category. 

Solvents and additives can influence Diels-Alder reactions through a multitude of different interactions, of which the contributions to the overall rate depend uniquely on the particular solvent-diene-dienophile combination. Attempts to build a general picture are limited to the most extensively studied type (a) Diels-Alder reactions. These Diels-Alder reactions are dominated by hydrogen-bonding and solvophobic interactions. This observation predicts a very special role of water as a solvent for type (a) Diels-Alder reactions. 

Figure 6 shows the signal response in ESI of Leucine Enkephalin, a pentapeptide, as a function of solvent and additive variation. Although the peptide can be ionized in basic media, acidic pH is much more favorable. This result also confirms that formic acid and acetic acid promote ESI signals far more efficiently than TEA. 

Attempts to increase conversion by variation of the solvent, hydrogen pressure, or the catalyst and substrate concentration were unsuccessful. Coordinating solvents and additives such as amines, or coordinating anions such as halides, carboxylates. 

Chemical Reviews, 2004, Vol. 104, No. 10 Table 10. LUMO Energy Level and Reduction Potentials of Solvents and Additives... 

Despite numerous screening studies, the literature contains little evidence that homogeneous catalyst systems based on metals other than Co, Rh, or Ru have significant activity for catalytic CO reduction. As seen for the known active catalytic systems, however, properties of solvents and additives or promoters can have enormous effects on catalytic activities. Solvents and additives can serve many roles in these catalytic systems. One important function of promoters in the Rh and Ru systems appears to be that of stabilizing metal oxidation states involved in catalytic chemistry. Other... 

Remark. The procedure described in Section 2.8.3 for the synthesis of -V-phenyl imidothioesters gave poor results in the case of methylmagnesium iodide, mainly because of low stability of its addition product to phenyl isothiocyanate. A good yield could be secured by running the reaction at a lower temperature in ether as the solvent and addition of HMPA in the methylation step. Filtration on a very short column of silica (eluent petroleum ether/ethyl acetate 97.5 2.5) afforded a quite pure product in a 79% yield. 

Solvent and additives. Aprotic semipolar solvents, especially methyl propionate, give the highest ee s. But other trends have been reported [4, 12]. The purity of the solvent is important [33]. The addition of weak acids increases the ee s, especially pivalic acid in the hydrogenation of methyl ketones, while water is detrimental [1,4]. The reaction can also be carried out in the gas phase but optical yields are lower [55]. 

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