Solvents for displacement reactions

Solvent for Displacement Reactions. As the most polar of the common aprotic solvents, DMSO is a favored solvent for displacement reactions because of its high dielectric constant and because anions are less solvated in it (87). Rates for these reactions are sometimes a thousand times faster in DMSO than in alcohols. Suitable nucleophiles include acetyUde ion, alkoxide ion, hydroxide ion, azide ion, carbanions, carboxylate ions, cyanide ion, hahde ions, mercaptide ions, phenoxide ions, nitrite ions, and thiocyanate ions (31). Rates of displacement by amides or amines are also greater in DMSO than in alcohol or aqueous solutions. Dimethyl sulfoxide is used as the reaction solvent in the manufacture of high performance, polyaryl ether polymers by reaction of bis(4,4 -chlorophenyl) sulfone with the disodium salts of dihydroxyphenols, eg, bisphenol A or 4,4 -sulfonylbisphenol (88). These and related reactions are made more economical by efficient recycling of DMSO (89). Nucleophilic displacement of activated aromatic nitro groups with aryloxy anion in DMSO is a versatile and useful reaction for the synthesis of aromatic ethers and polyethers (90). 

Polymerization and Spinning Solvent. Dimethyl sulfoxide is used as a solvent for the polymerization of acrylonitrile and other vinyl monomers, and as a reaction solvent for other polymeri7ations. Tt is also used as a solvent for displacement reactions, solvent for base-catalyzed reaclions, extraction solvent, solvent for electrolytic reactions, cellulose solvent, pesticide solvent, and clean-up solvent,... 

As a consequence of the polarity of the amide group, the lower-molecular-weight amides are relatively high-melting and water-soluble, as compared to esters, amines, alcohols, and the like. The few that are liquids, such as 7V,7V-dimethylmethanamide and l-methyl-l-aza-2-cyclopentanone, have excellent solvent properties for both polar and nonpolar substances. Therefore they are good solvents for displacement reactions of the SN type (Table 8-5). 

Dimethyl sulfoxide is a favored solvent for displacement reactions in synthetic chemistry. The rates of reaction in DMSO are many times faster than in an alcohol or aqueous medium [6]. Dimethyl sulfoxide is the solvent of choice in reactions where proton (hydrogen atom) removal is the rate determining step. Reactions of this type include olefin isomerizations and reactions where an elimination process produces an olefin. Another application that uses DMSO is its use as an extraction solvent to separate olefins from saturated paraffins [7]. Several binary and ternary solvent systems containing DMSO and an amine (e.g., methylamine), sulfur trioxide, carbon disulfide/ amine, or sulfur trioxide/ammonia are used to dissolve cellulose, and act as spinning baths for the production of cellulose fibers [8,9]. Organic fungicides, insecticides, and herbicides are readily soluble in DMSO. Dimethyl sulfoxide is used to remove polymer residues from polymerization reactors. 

Dimethyl sulfoxide is a favored solvent for displacement reactions in synthetic... 

Formamidc has been found to be a very suitable solvent for fluoride displacement reactions on optically active substrates leading to more reduced racemization that is in comparison to other aprotic solvents like tetramethylene sulfone, l,3-dimethyl-3,4,5,6-tetrahydropyrimidin-2(l/7)-one etc.146 Formamide has a high polarizability favoring SN2 reactions and a high polarity147 rendering potassium fluoride sufficiently soluble in the reaction mixture. Although the reaction rate is reduced, the less polar solvents A-methylformamide, acetamide or A-methyl-acetamide can also be used as solvents for the reaction. 

Another familiar type of sodium displacement reaction is the cleavage of ethers. This reaction is not important as a synthetic method, but it is an important side reaction which must be considered whenever ethers are used as solvents for organosodium reactions. Aliphatic ethers are essentially unreactive toward metallic sodium at moderate temperatures, but many organosodium reagents, such as phenylsodium and amylsodium, are able to cleave them by an acid-base reaction ROCH2CH2R + R"Na -> RONa + CH2=CHR + R"H... 

It has been found that the reaction at the C-Br or the C-Cl bond of l-bromo-4-chlorobutane can be controlled by changing the solvent for the reaction. For instance, the regiospeciflcity at the C-Br bond in the S 2 alkylation reaction with RZnBr in the presence of a Pd-PEPPSI-/-Pr catalyst when the DMI/THF ratio is 1/2 is 12.2 1 or 92%. DMI is dimethylimidazolidinone with an e = 37.6, whereas THF has an e = 7.5. However, when the DMI/THF = 2/1, the reaction occurs at the C—Cl bond. This means that the two reactions can be performed in one pot that is, one RZnBr reagent is used to displace bromine and, when this reaction is complete, the solvent composition is changed and a different RZnBr reagent is used to displace chlorine. Both reactions occur at room temperature, with yields of 70%. 

Although the choice of organic solvent is clearly important in phase transfer reactions, there have been few systematic studies of solvent effects. For some reactions the choice of solvent is obvious for example chloroform is both the organic solvent and the substrate for the generation of dichlor-ocarbenes, but although neat alkyl halides can be used for displacement reactions, these will not necessarily give the fastest rates. 

It also appears possible to alkylate nitrobenzene with the displacement of hydride (or its equivalent) by the use of the anion obtained from deprotonation of methyl-sulfinylmethane (dimethyl sulfoxide [( 113)280]), viz. the methylsulfinylmethide anion (dimsyl anion [CH3SO(CH2) ]. Although the details are not known, the reaction is particularly useful because Friedel-Crafts alkylation of nitrobenzene usually fails. Indeed, the reaction of nitrobenzene with Lewis acids and alkyl (or acyl) halides is so poor that nitrobenzene has occasionally been used as a solvent for the reaction of other arenes with these substrates. It is argued that nitrobenzene is so unreactive because the nitro substituent withdraws electrons, making them unavailable for an incoming electrophile, that is, the ring is deactivated toward electrophilic substitution by the electron-withdrawing substituent. 

Condensation ofDianhydrides with Diamines. The preparation of polyetherknides by the reaction of a diamine with a dianhydride has advantages over nitro-displacement polymerization sodium nitrite is not a by-product and thus does not have to be removed from the polymer, and a dipolar aprotic solvent is not required, which makes solvent-free melt polymerization a possibiUty. Aromatic dianhydride monomers (8) can be prepared from A/-substituted rutrophthalimides by a three-step sequence that utilizes the nitro-displacement reaction in the first step, followed by hydrolysis and then ring closure. For the 4-nitro compounds, the procedure is as follows. 

Reactions in liquid ammonia (cf. Chapter 3, Section III) require a certain amount of care, since the solvent is low boiling (—33 ) and its fumes are noxious. Nevertheless, with reasonable caution, the preparation of an ammonia solution of sodium acetylide can be carried out as described. The reagent so prepared can then be directly used for displacements on alkyl halides or for additions to suitable carbonyl compounds. Examples of both reactions are given. 

It is interesting to note that all the new aromatic systems, as described, undergo displacement polymerizations in DMAC solvent by the K2CO3 method, except perfluoroalkylene [10] and amide activated polymerization [9], which were performed in NMP solvent. The displacement polymerization in DMAC solvent was carried out at 155-164°C. poly(aryl ether ketones) require less reaction time (3-6 h) than other aromatic systems for synthesis of polyethers [15]. Synthesis of the fluorinated polyether as reported by Irvin et al. [16] was carried out at room temperature for 16 h (Mw = 75,000), whereas the same polymer by Mercer et al. [17] was synthesized at 120°C for 17 h (Mw = 78,970). 

Interests in the phase transfer catalysis (PTC) have grown steadily for the past several years [68-70]. The use of PTC has recently received industrial importance in cases where the alternative use of polar aprotic solvents would be prohibitively expensive [71-74]. Thus, the potential application of the phase transfer catalyzed aromatic nucleophilic displacement reactions between phenoxide or thiophenoxide and activated systems has... 

The advantage of the activated displacement polymerization is the facile incorporation of different and unconventional structural units in the polymer backbone. Most of the heteroarylene activated polyethers prepared by this route are soluble in many organic solvents. The solubility behavior of new polyethers is shown in Table 8. In contrast to many polyphenylenequi-noxalines, poly(aryl ether phenylquinoxalines) prepared by the quionoxaline activated displacement reaction are soluble in NMP. Solubility in NMP is important since it is frequently used for polymer processing in the microelectronics industry [27]. 

After extraction, the loaded solvent contains 6 g T1 zirconium as zirconium oxide with 0.2% hafnium oxide. The raffinate is left with 0.2 to 0.3 g l l of the oxides of zirconium and hafnium of this, 70-90% is hafnium oxide. This raffinate can act as a feed solution for the recovery of pure hafnium oxide. The loaded extractant, on the other hand, is subjected to a scrubbing operation with pure zirconium sulfate solution to eliminate any co-extracted hafnium. This scrubbing operation is essentially a displacement reaction ... 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

Thus solvolysis of (+)C6HsCHMeCl, which can form a stabilised benzyl type carbocation (cf. p. 84), leads to 98% racemisation while (+)C6H13CHMeCl, where no comparable stabilisation can occur, leads to only 34% racemisation. Solvolysis of ( + )C6H5CHMeCl in 80 % acetone/20 % water leads to 98 % racemisation (above), but in the more nucleophilic water alone to only 80% racemisation. The same general considerations apply to nucleophilic displacement reactions by Nu as to solvolysis, except that R may persist a little further along the sequence because part at least of the solvent envelope has to be stripped away before Nu can get at R . It is important to notice that racemisation is clearly very much less of a stereochemical requirement for S l reactions than inversion was for SN2. 

Syntheses are limited to mercuric salts of weak acids (2,110). Generally, increasing the length of the straight alkyl chain decreases the extent of decarboxylation (e.g., Ref. 133). Electron-withdrawing substituents suppress decarboxylation. For example, mercurials are not formed with Me02C, Cl, and Me(CH2)nO substituents on the a carbon (137,148,149), but some decarboxylation occurs with these on the j8 carbon (135-137). Chain decarboxylation predominated in reactions in benzene, butyric acid [R = Me(CH2)2] (150), or acetic acid (R = Me) (124). The chain reaction was also observed for R = Me(CH2)2 in the absence of solvent and in ethylacetate or heptane solution, but in these media the radical displacement reaction was dominant (2,150). When benzene was used as solvent... 

These problems can be somewhat overcome by a study of reactions in solution where much greater densities are possible than in the gas phase and fast bimolecular reaction are diffusion limited [1,28,29]. However, since coordinatively unsaturated metal carbonyls have shown a great affinity for coordinating solvent we felt that the appropriate place to begin a study of the spectroscopy and kinetics of these species would be in a phase where there is no solvent the gas phase. In the gas phase, the observed spectrum is expected to be that of the "naked" coordinatively unsaturated species and reactions of these species with added ligands are addition reactions rather than displacement reactions. However, since many of the saturated metal carbonyls have limited vapor pressures, the gas phase places additional constraints on the sensitivity of the transient spectroscopy apparatus. 

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