Acidity in DMSO solution

Five possible isomers may coexist in the tautomeric equilibrium involving 2,6-diazidopurine 343 (Scheme 135). It was concluded (75UK1028) that diazide 343AA is the sole form in the solid state and in solutions in acetic and trifluoroacetic acids. In DMSO solution, two additional tautomers, 343AT and 343ATS were also observed. 

The order is fairly similar to that observed for acidities in DMSO solution ... 

Density functional theory has been used to calculate the P-E bond energies and orbital populations of trimethylphosphine chalcogenides and related compounds. The results indicate that Me3P=CH2 has a 7i-bond order of 0.5. The equilibrium acidities in DMSO solution and gas-phase homolytic bond dissociation energies of tributylphosphonium ylide precursors have also been determined. ... 

Obviously, to model these effects simultaneously becomes a very complex task. Hence, most calculation methods treat the effects which are not directly related to the molecular structure as constant. As an important consequence, prediction models are valid only for the system under investigation. A model for the prediction of the acidity constant pfQ in aqueous solutions cannot be applied to the prediction of pKj values in DMSO solutions. Nevertheless, relationships between different systems might also be quantified. Here, Kamlet s concept of solvatochro-mism, which allows the prediction of solvent-dependent properties with respect to both solute and solvent [1], comes to mind. 

A comparison of phenol acidity in DMSO versus the gas phase also shows an attenuation of substituent effects, but not nearly as much as in water. Whereas the effect of ubstituents on AG for deprotonation in aqueous solution is about one-sixth that in the gas phase, the ratio for DMSO is about one-third. This result points to hydrogen bonding of the phenolate anion by water as the major difference in the solvating properties of water and DMSO. ... 

Recent studies have found enhanced substituent solvation assisted resonance effects in dipolar non-hydrogen bonding solvents131. For several +R substituents acidities of phenols in DMSO are well correlated with their gas-phase acidities. The substituents include m- and p-SOMe, m- and p-S02Me, m-S02CF3 and m-N02. But there is very considerable enhancement of the effect of p-S02CF3, p-N02 and various other para-substituents in DMSO solution. 

The same goes for periodic acid. The 1.5M solutions of this acid in DMSO are explosive. Solutions of less than 0.15M are not dangerous. Between these two limits, the situation is intermediate. 

The equilibrium acidities of aniline and 26 of its derivatives have recently been measured127 in DMSO solution and they cover a range of 15 pKa units. The pKa values of anilines in DMSO are 10 units higher than the ion-pair pKa values obtained in liquid ammonia. This effect has been attributed to the greater capacity of ammonia to solvate the proton. When the pKa values determined in DMSO were compared with those derived in H2O-DMSO or EtOH-DMSO, it was evident that the latter were appreciably lower. This behaviour is different from that found for the carbon acids. The H2O-DMSO mixture can solvate nitranions better than pure DMSO, in line with the fact that nitranions are better HBA than carbanions128 (see Table 10). 

An extensive study132 has been presented on the polarizability effects of alkyl groups in RX moieties (R = Me, Et, i-Pr and i-Bu X = CH2, S, SO2, O and N) in families of weak acids and on the stabilities of adjacent anions and radicals in DMSO solution. Some of the results related to the 9-(dialkylamino)fluorenes are given in Table 12. The increases in acidity are believed to be caused by the progressive increases in anion stabilizing... 

Polymers containing pendant carbamate functional groups can be prepared by the reaction of phenyl isocyanate with poly(vinyl alcohol) in homogeneous dimethylsulfoxide solutions using a tri-ethylamine catalyst. These modified polymers are soluble in dimethyl sulfoxide, dimethylacetamide, dimethylformamide and formic acid but are insoluble in water, methanol and xylene. Above about 50% degree of substitution, the polymers are also soluble in acetic acid and butyrolactone. The modified polymers contain aromatic, C = 0, NH and CN bands in the infrared and show a diminished OH absorption. Similar results were noted in the NMR spectroscopy. These modified polymers show a lower specific and intrinsic viscosity in DMSO solutions than does the unmodified poly(vinyl alcohol) and this viscosity decreases as the degree of substitution increases. 

In relation to these works, the reaction of p-nitrophenyl esters with optically active poly( propyleneimine )(8) was studied at 25°C in DMSO solution according to the same procedure described for the case of poly-L-lysine derivatives. The poly( propyleneimine ) derivatives thus obtained have different IR and UV absorption spectra from those of the starting compounds, and show absorptions assigned to the nucleic acid bases. However, their contents determined by UV spectroscopy were substantially low as compared with the case of poly-L-lysine derivatives for (9) and (20), the base contents were below 30 and 50 %, respectively. The result was explained by a steric hindrance caused by methyl groups on the main chain of poly( propyleneimine ) ... 

Figure 4 reveals a similar pattern for acidities in DMSO solvent versus the gas phase. Once again, the small, localized anions result in solution acidities that are relatively stronger than the gas phase counterparts. The acids with large... 

DFT calculations were also used to study the NMR spectra of salicylhydroxamic acid in DMSO-dg solutions. Best fit with experimental results was observed for association with two solvent molecules. Assignments could be made for the specific structure formed in solution. Similar studies were made on other types of hydroxamic acids . 

The more acidic fluorene in tert-butyl alcohol solution, or in DMSO solution, reacts by a process that involves the carbanion in equilibrium with hydrocarbon. Thus, fluorene and 9,9-dideuteriofluorene oxidize at identical rates. We have established that the oxidation of the anion of fluorene can be catalyzed by a variety of electron acceptors (v), including various nitroaromatics (18). The catalyzed oxidation rates were found to follow the rates of electron transfer measured by ESR spectroscopy in the absence of oxygen. These results established the catalyzed reaction as a free radical chain process without shedding light upon the mechanism of the uncatalyzed reaction. 

Acidic hydrolytic products of pyoverdins, (15)-5-amino-8,9-dihydroxy-(8) and 5,8,9-trihydroxy-2,3-dihydro-l//-pyrimido[l, 2-a]quinolines-3-carboxylic acids in DMSO-d6, DMF-d7, and in 1 N DC1 solutions were characterized by 13C NMR investigations [91ZN(C)993]. 

A few years later,3 it was shown that o-iodoxybenzoic acid (36)—itself a precursor in the preparation of Dess-Martin periodinane—is able to oxidize very effectively alcohols in DMSO solution. o-Iodoxybenzoic acid—normally referred to as IBX—exists mainly as a cyclic form 37, which crystallizes as a polymer with very low solubility in most solvents with the exception of DMSO. Although, IBX (36) was already known in 1893,4 this ultracentenial reagent found very little use till very recently, when awareness about its solubility in DMSO was raised. 

Dithiocarbazic acids R1R2NNR3CSSH form planar Ni(R1R2NNR3CSS)2 (R1, R2,R3 = H, alkyl, or aryl). n.m.r. spectra of a variety of these in DMSO solution were generally consistent with NS-co-ordination for unsubstituted and 2-substituted acids and SS-co-ordination for the 3-substituted complexes. The complexes with an [NiS2N2] chromophore produce evidence for cis-trans-isomers in solution.595... 

TA systems, there is a close link between the properties of the mixture and of solutes in the mixtures. Also the impact of the solvent variation on the properties of a neutral solute cannot be overlooked. For example, the dissociation of benzoic acid in DMSO + water mixtures becomes increasingly endothermic as x2 increases, and this can be accounted for to a large extent by the exothermic transfer of undissociated benzoic acid from water to the mixtures. (Rodante et al., 1974). 

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