Dimethyl sulfoxide , hydrogen bond formation

The purpose of this research was to compare the effect on the conformational equilibrium for the hydroxylated compounds listed in Table II of changing the solvent from dimethyl sulfoxide, which is expected to minimize intramolecular hydrogen bonding, to 1,2-dichloroethane which should promote such bonds. These solvation effects on conformational equilibria were then to be compared with those of water which can serve as a hydrogen donor and hydrogen acceptor in hydrogen bond formation. As will be seen, the conformational equilibria generally appear similar for water and dimethyl sulfoxide but often different from those in 1,2-dichloroethane. 

Addition of increasing amounts of triethylamine to a benzene solution of (9a) leads to a gradual shift of the equilibrium towards the enol form (9b). This can be interpreted in terms of hydrogen-bond formation between 9-anthranol and triethylamine. In hydrogen-bond accepting solvents such as A,A-dimethylformamide (enol content 56.5 cmol/mol at 20 °C), pyridine (58 cmol/mol), and dimethyl sulfoxide (61.5 cmol/mol), the anthranol content increases further [61, 134]. 

Isotope labeling by derivative formation with deuterated reagents is useful for the preparation of analogs such as dg-acetonides, da-acetates, da-methyl ethers, dg-methyl esters, etc. The required reagents are either commercially available or can be easily prepared. (The preparation of da-methyl iodide is described in section IX-F. Various procedures are reported in the literature for the preparation of dg-acetone, da-diazometh-ane57.i63.i73 and da-acetyl chloride. ) These reactions can be carried out under the usual conditions and they need no further discussion. A convenient procedure has been reported for the da-methylation of sterically hindered or hydrogen bonded phenolic hydroxyl functions by using da-methyl iodide and sodium hydroxide in dimethyl sulfoxide solution. This procedure should be equally applicable to the preparation of estradiol da-methyl ether derivatives. 

Since the solvent properties of dimethyl sulfoxide are widely different from those of hydrocarbons and halogenated hydrocarbons, it may be difficult to compare the kinetic and thermodynamic data for the C02H group (Table 16) directly with others. However, heating the carboxylic acid (68, X = OH) in toluene affords the sp isomer almost exclusively. Probably, the observed results with the carboxylic acid derive from difficulty in the formation of a hydrogen bond owing to a steric effect, in addition to the nonplanar conformation of the carboxyl group relative to the naphthalene. 

In non-HBD solvents such as n-heptane, tetrachloromethane, diethyl ether, deuterio-tri-chloromethane, and dimethyl sulfoxide, tropolone transfers its proton to triethylamine to give an ion pair, which is in equilibrium with the non-associated reactants. There is no formation of a hydrogen-bonded complex between tropolone and triethylamine because of the fact that tropolone itself is intramolecularly hydrogen-bonded. The extent of the ion pair formation increases with solvent polarity. In polar HBD solvents such as ethanol, methanol, and water, this proton-transfer equilibrium is shifted completely towards the formation of triethylammonium tropolonate [171]. 

The concentration dependence of chemical shifts of NH protons of imidazole in various solvents has been studied. In organic solvents the position and half-width of the NH signal depends on the concentration, and with increase in concentration the line shifts downfield. This shift (A = 1 Hz for solution of imidazole in dimethyl sulfoxide) may be due to the formation of an ordinary hydrogen bond between imidazole molecules. 

The complex formation as described above also depends on the solvent used. Compound (101) and terephthalic acid have been found to assemble in a mixture of dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) with as major product the 1 2 adduct (104), even using a molar ratio 1 3 between the reactants. In DMF only the 1 3 adduct (105) has been formed. The difference in reactivity has been explained from the stronger solvating ability of DMSO in comparison to that of DMF. Bonding between the pyridyl group and the acid functionalities occurs via hydrogen bridge formation [NH OH distance in (104) is equal to 2.66... 

Multinuclear NMR spectroscopy experiments of various 1,3-dialkylimidazolium ILs dissolved in organic solvents have also pointed to the formation of floating aggregates through hydrogen bonds [81-84]. In particular, it has been demonstrated by heteronuclear NMR experiments on [C4CjIm]BF4 that contact ion pairs exist in the presence of small amounts of water and even in dimethyl sulfoxide (DMSO) solution [85]. 

Aprotic solvents do not contain —OH groups and cannot function as hydrogen-bond donors. They are unable to promote the formation of a carbocation because the leaving group would be unsolvated. Therefore aprotic solvents cannot be used in Sfjl reactions. Table 7.4 lists the aprotic solvents most commonly used for nucleophilic substitution reactions. Dimethyl sulfoxide and acetone are polar aprotic solvents dichloromethane and diethyl ether are less polar aprotic solvents. The aprotic solvents listed in the table are particularly good ones in which to carry out 8 2 reactions. Because polar aprotic solvents are able to solvate only cations and not anions, they allow for naked and highly reactive anions as nucleophiles when used with ionic nucleophiles such as Na+CN , Na+OH , and so on. 

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