Acidity in DMSO

TABLE 3.6. Measured pATa Values in DMSO for Representative Functional Groups  

considerably lower than the resonance stabilized, sp hybridized hydrocarbon 

The data in DMSO also allow for an evaluation of the effect of a-alkyl groups on the stability of a carbanion. Because alkyl groups are generally categorized as electron donors, one might expect that they would destabilize a carbanion, but the data indicate that the situation is not so simple. In Table 3.7, and Af/acid values are  

Finally, Arnett et developed a different approach to determining the 

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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. ... 

Ester functions are not saponified under these ring opening conditions. However, a trans-a-acetoxy function hinders the epoxide opening reaction and a noticeable decrease in yield is observed in comparison to the cw-a-acetoxy isomer. The ring opening reaction is also dependent on the concentration of sulfuric acid. Polymer formation results when the acid concentration is too low and the reaction is markedly slower with excessive concentrations of acid. A 0.5% (vol./vol.) concentration of acid in DMSO is satisfactory. Ring opening does not occur when ethanol, acetone, or dioxane are used as solvent. 

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. 

Hojo and colleagues155,156 have carried out numerous studies of ortho-effects, in particular on the ionizations of benzoic acids in DMSO-water mixed solvents. The ortho-effects are assessed by measuring the pKa values of ortho- and para- substituted benzoic acids in solvents containing from 0 to 95% v/v DMSO and expressing the results as equation 15 ... 

The order is fairly similar to that observed for acidities 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. 

TABLE 16. x>K% values of some nitro-activated carbon acids in DMSO at 25 °C... 

While nitroalkanes are weaker acids in DMSO than in water, nitroarylmethanes show a decrease in their pKa values on increasing DMSO concentration. 

Many pK values for nitrocarbon acids are in the compilations of Palm and colleagues182. A comprehensive study of the ionization of many carbon acids in DMSO has been made by Bordwell and colleagues and many of them are nitro compounds183. 

The oxidation of thioglycolic, thiomalic, and thiolactic acids in DMSO is first-order in tetrabutylammonium-12-tungstocobaltate(III) ion. ... 

Figure 2. Gas phase acidities versus equilibrium acidities in DMSO from Ref. 31. Both axes in kcal/mol. CpH = cyclopentadiene. The lines are least squares fits for the ketones (upper line) and nitriles (lower line).

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... 

Figure 4. Gas phase acidities versus acidities in DMSO solvent. Data from Refs. 6, 27, and 64. Both axes in kcal/mol. Open squares = carboxylic acids. Open triangles = phenols. The straight line is a least-squares fit to primary alcohol acidities. CpH = cyclopentad iene.

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 . 

In conclusion, ID coordination polymer [Zn(tmbdc)(dmso)2]-2(DMSO) 2 has been synthesized with 2,3,5,6-tetramethyl-l,4-benzenedicarboxylic acid in DMSO. The structure contains ID chains formed by octahedraly coordinated Zn ions chelated by the carboxyl groups of tmbdc, rather than the 2D (4,4) nets constructed from paddle-wheel SBU of pairs Zn ions as found in [Zn2(bdc)2(dmso)2]-5DMSO 1. Analysis of the structure reveals that the steric hindrance of die four methyl groups of tmbdc determines the coordination environments of the zinc ions and the coordination modes of the carboxyls, and thus the final structures of the coordination polymers. The result also shows that DMSO is a stronger ancillary ligand and is also easier to be included in the structures of coordination polymers, compared to DMF. DMSO can be a better solvent for the syntheses of porous coordination polymers. 

Oxidation of 5,10-dialkyl-5,10-dihydrophenazines with hydrobromic acid in DMSO gives 10-alkyl-2(10/7)-phena-zinone in 52-79% yields (Equation 17) <1999JHC1057>. Depending on the alkyl substituents on C-5 and C-10 carbons, the parent phenazine is generated as the by-product. 

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]. 

System Base >Ka of Conjugate Acid in DMSO Percent 1-Butene in Total Butenes trans-2-Butene cis-2-Butene... 

The use of much-neglected bismuth derivatives for the oxidation of organic compounds has been reviewed.74 Bismuth(III) carboxylates, obtained by reaction of Bi2C>3 with pyridine mono- and di-carboxylic acids and with phthalic acid, act as catalysts for the oxidation of styrene oxide to benzoic acid in DMSO in the presence of 02.75 It is proposed that the bismuth may activate both epoxide and oxidant in a solvate, from which dimethyl sulfide evolution and elimination leads to a ketoalkoxide-bismuth complex (and hence to the initial product, 2-hydroxyacetophenone). Further oxidation to the ketoaldehyde and acid requires molecular oxygen, but is also found to be catalysed by bismuth. 

Reduction of amides. Sodium borohydride combined with methanesulfonic acid in DMSO reduces amides to the corresponding amines in 60-90% isolated yield. I he system also reduces acids and esters to primary alcohols. These reductions have been conducted with lithium aluminum hydride and with borane-tetrahydrofurane (5,48),2 hut with somewhat different selectivities. This new reagent, however, appears to be less hazardous than the latter reagent. 

Oxidation of 3-hydroxymethyl-2,3-dihydro-5H-pyrido[l,2,3-cfe][1,4] benzoxazin-5-ones with o-iodoxybenzoic acid in DMSO and with Dess-Martin periodinate in CH2CI2 afforded 3-formyl derivatives (08WOP2008/ 120003). Dehydrogenation of ethyl 10-[2-(ferf-butoxycarbonyl)-l,2,3,4,6,8a-hexahydropyrrolo[l,2-a]pyrazin-7-yl]-9-fluoro-3(S)-methyl-7-oxo-2,3-dihydro-7H-pyrido[l,2,3-de][l,4]oxazine-6-carboxylates in the presence of Pd/C with air in MeOH afforded 10-[2-(ferf-butoxycarbonyl)-l, 2,3,4-tetrahydropyrrolo[l,2-a]pyrazin-7-yl] derivatives, which then were deprotected (09BML4933). 

Oxidation of benzylic positions with o-iodoxybenzoic acid in DMSO proceeds via a similar mechanism involving the formation of benzyl radicals by SET [Eq. (89)] [164]. A variety of aromatic aldehydes were obtained in good yields with no over-oxidation to carboxylic acids. 

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