Aprotic dichloromethane

Cross-hnked polyacrylamides are a group of hydrophihc solid supports introduced primarily for preparation of biopolymers (Fig. 4). Unhke PS resins, polyacrylamides have excellent swelling capacity in both protic (water, alcohols) and aprotic (dichloromethane, dimethylformamide) solvents [88]. These beads are stable towards bases, acids, and weak reducing and oxidizing agents [89]. Predictably, conditions under which amide bonds are cleaved (i.e., sodium in liquid ammonia) [90] lead to rapid decomposition of the polymer. 

Nitrosyl halides, particularly NOCl and NOBr, can also be used as diazotization reagents undo" anhydrous conditions, as these gases (NOCl mp —59.6°C, bp —6.4°C NOBr bp 0°C) are readily soluble in many organic solvents. They can also be generated in situ in chlorinated hydrocarbons by reaction of trimethylsilyl halides with alkyl nitrites (equation 13) as shown by Weiss and Wagner . The group of Weiss found also that NW-bissUylated anilines react in aprotic dichloromethane with generation of diazonium salts and formation of the nonnucleophilic hexamethyldisiloxane (equation 14). 

We have investigated the bromo-addition of alkenes and their related compounds with BTMA Br3. Thus, we found that the reaction of alkenes with BTMA Br3 in aprotic solvents such as dichloromethane and chloroform gave 1,2-dibromo adducts in a manner of stereospecific anti-addition, and, in such protic solvents as methanol and acetic acid, gave the corresponding dibromo adducts along with considerable amounts of solvent-incorporated products in regioselective manner (Fig. 18) (ref. 29). 

The solvent and temperature effects for the Michael addition of amidoxime 7 to DMAD were probed because the reaction itself occurs without any other catalysts. As shown in Table 6.2, the reaction gave a high ratio of 8E in strongly aprotic polar solvents such as DMF and DMSO (entry 1 and 2). 8E was also found as the major product in MeCN (entry 3), dichloromethane (entry 4), and xylenes (entry 5). To our delight, the desired 8Z was obtained as the major component in methanol (entry 6). The stereoselectivity of 8Z versus 8E was better at low temperature (entry 7). A similar result was observed when the reaction was run in THF or dichlo-roethane in the presence of a catalytic amount of DABCO (entries 9 and 10). 

Fluoride ion catalyzes the hydrosilylation of both alkyl and aryl aldehydes to silyl ethers that can be easily hydrolyzed to the free alcohols by treatment with 1 M hydrogen chloride in methanol.320 The most effective sources of fluoride are TBAF and tris(diethylamino)sulfonium difluorotrimethylsilicate (TASF). Somewhat less effective are CsF and KF. Solvent effects are marked. The reactions are facilitated in polar, aprotic solvents such as hexamethylphosphortriamide (HMPA) or 1,3-dimethyl-3,4,5,6-tetrahydro-2(l //)-pyrirnidinone (DMPU), go moderately well in dimethylformamide, but do not proceed well in either tetrahydrofuran or dichloromethane. The solvent effects are dramatically illustrated in the reaction of undecanal and dimethylphenylsilane to produce undecyloxyphenyldimethylsi-lane. After one hour at room temperature with TBAF as the source of fluoride and a 10 mol% excess of silane, yields of 91% in HMPA, 89% in DMPU, 56% in dimethylformamide, 9% in tetrahydrofuran, and only 1% in dichloromethane are obtained (Eq. 164).320... 

The subsequent Claisen-Schmidt reaction was originally performed on a 10-pmol scale using 20-fold excess of both acetophenone and LiOH to achieve complete formation of the chalcone 8. This result could be verified on a small scale however, employing the same conditions on a 35-mmol scale resulted in no conversion even after 22 h, as revealed by IR spectroscopy. By cleaving a resin sample with 20% TFA in dichloromethane, only -formylbenzamide 11 was detected by HPLC. This result may be explained by the low solubility of LiOH in DME under dry/aprotic conditions. Therefore, a small amount of EtOH was added, which initiated a fast reaction (Chiu et al. 1999) and the formation of the desired chalcone 8 together with 20% of the Michael adduct 10 (Fig. 2). This was confirmed by sample cleavage from the resin and LC-MS analysis. Short reaction screening resulted in considerable im-... 

Oxygenated (e.g. alcohols, ethers, ketones and esters) Aliphatic hydrocarbons (e.g. cyclohexane, dodecane) Aromatic hydrocarbons (e.g. xylene, mesitylene) Dipolar aprotic (e.g. DMSO, DMF, NMP) Chlorinated solvents (e.g. dichloromethane)... 

For many years, prior to the development of current phase-transfer catalytic techniques, tetraalkylammonium borohydrides have been used in non-hydroxylic solvents [see, e.g. I, 2], Originally, the quaternary ammonium borohydrides were obtained by metathesis in water or an alcohol [3, 4], However, with greater knowledge of the phase-transfer phenomenon, an improved procedure has been developed in which the ammonium salt is transferred into, and subsequently isolated from, dichloromethane [5, 6], In principle, it should be possible to transfer the quaternary ammonium borohydride for use in any non-miscible organic solvent. It should be noted, however, that quaternary ammonium cations are susceptible to hydrogeno-lysis by sodium borohydride in dipolar aprotic solvents to yield tertiary amines [4]. 

Either protic (alcohols, preferentially methanol) or aprotic solvents (toluene, dichloromethane, THE) can be used, depending on the structure of the metal precursors that can generate the catalysts by a number of pathways. Metals other than palladium, for example nickel [4], can form active catalysts for alkene/CO copolymerisation, yet with largely lower productivities as compared to structurally similar palladium precursors [1]. For this reason, only Pd"-catalysed alkene/CO copolymerisation reactions are reviewed and commented in the present chapter. 

Hence bromine in dichloromethane converted 2-methylthiazole into 5-bromo-2-methylthiazole in 48% yield (76JAP76/48655). In this reaction HBr was present as a base acceptor. In its absence the yield was halved, and with an equimolar proportion of aluminium chloride the yield fell to 10%. It is evident that although 2-methylthiazole can be brominated under mild conditions, its activity is sharply reduced by protic or aprotic acids capable of complexing with the annular nitrogen (86CHE663). 

Alcohols are the solvents of choice, but other aprotic solvents such as dichloromethane can be used. At room temperature, the reaction requires an initial H2 pressure of 20 to 100 atm, but between 80 and 100°C, the reaction proceeds smoothly at 4 atm H2 90, 91). This method of hydrogenation is simple and allows reactions on any scale—... 

Oxidation of silyl tosylhydrazones (35) in an aprotic medium (dichloromethane) also results in the formation of nitriles, but in this case the conversion is apparently a true anodic process41 in which the key difference from the anodic behavior of oximes is the presence of the p-toluensulfonyl group, which facilitates cleavage of the N—N single bond (equation 20). 

The effect of complexation on redox properties was studied by cyclic voltammetry. Unbound flavin, dissolved in an aprotic solvent (dichloromethane), undergoes a two electron reduction perfectly explained by the ECE mechanism. Upon addition of cyclene ligand and coordination of flavin to the zinc ion complex, the flavohydroquinone redox state was stabilised. 

Aprotic solvents such as dichloromethane and acetone have the advantage of not possessing extractive capabilities [42], being unable to penetrate into the keratin structure. Extraction tests with conventional techniques, for dichloromethane, showed that the analytes were not found in the extracts [51]. 

This procedure offers a convenient method for the esterification of carboxylic acids with alcohols2 and thiols2 under mild conditions. Its success depends on the high efficiency of 4-dialkylaminopyridines as nucleophilic catalysts 1n group transfer reactions. The esterification proceeds without the need of a preformed, activated carboxylic acid derivative, at room temperature, under nonacidic, mildly basic conditions. In addition to dichloromethane other aprotic solvents of comparable polarity such as diethyl ether, tetrahydrofuran, and acetonitrile can be used. The reaction can be applied to a wide variety of acids and alcohols, including polyols,2 6 a-hydroxycarboxylic acid esters,7 and even very acid labile... 

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