The first acidity scale to be established in a pure solvent other than water was the result of the pioneering work of Conant, Wheland, and McEwen in ether or ben~ene .~ During the past 20 years an ion-pair acidity scale covering an \"effective pKa rangefrom about 15 to 40 has been developed in cyclohexylamine (CHA),6 and similar studies in other low-dielectric-constant solvents including 1,2-dimethoxyethane (DME)7a and tetrahydrofuran (THF)7b*c have been carried out. A more limited ion-pair acidity scale has been developed in liquid NH,.7d Also, during this period, acidity scales have been established in the polar non-hydrogenbond-donor (NHBD) solvents dimethyl sulfoxide (Me$0)8 and N-methylpyrrolidin-2-one (NMP)? which have relatively high dielectric constants. The pK,'s measured in these solvents differ from ion-pair pK,'s in that they are absolute, in the sense that they are based on Me2S0 and NMP as the standard states, which allows direct comparisons to be made with H20 and gas-phase pK,'s. A truly absolute acidity scale has been established in the gas phase, which, for the first time, provides intrinsic measures of structural effects free of solvent effects.1° Our purpose in this Account is (a) to discuss briefly acidities in various solvent media, (b) to present a table of representative equilibrium acidity constants in M e 8 0 solution, and (c) to illustrate ways in which these pK, data can be used. In an accompanying Account we compare acidities in Me2S0 solution with intrinsic gas-phase acidities and discuss some of the insights into solvation effects provided thereby. Acidities in H 2 0 and Me2S0. It is important to recognize that pKa values are solvent dependent. The

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Equilibrium Acidities in Dimethyl Sulfoxide Solution

8.4.5. Nitromercuration Reactions 3878 9. Hydroamination of Alkenes and Alkynes under Microwave Irradiation 3878 * To whom correspondence should be addressed. Phone: +49 241 8

Hydroamination: direct addition of amines to alkenes and alkynes.

1,2-Amino Alcohols and Their Heterocyclic Derivatives as Chiral Auxiliaries in Asymmetric Synthesis

NMR Studies of Cyclodextrins and Cyclodextrin Complexes

The application of (S)-1,10-binaphthyl-2,20-diol as NMR chiral solvating agent (CSA) for omeprazole, and three of its analogs (lanso-, panto-, and rabe-prazole) was investigated. The formation of diastereomeric host–guest complexes in solution between the CSA and the racemic substrates produced sufficient NMR signal splitting for the determination of enantiomeric excesses by Hor F-NMR spectroscopy. Using of hydrophobic deuterated solvents was mandatory for obtaining good enantiodiscrimination, thus suggesting the importance of intermolecular hydrogen bonds in the stabilization of the complexes. The method was applied to the fast quantification of the enantiomeric purity of in-process samples of S-omeprazole. Chirality 00:000–000, 2009. VC 2009

The assignment of absolute configuration by NMR

Acidity measurements on pyridines in tetrahydrofuran using lithiated silylamines

The exact analysis of the ABC type absorption due to the —CH2OH group of 3β-acetoxy-5β,6β-oxidocholestan-19-ol provides two vicinal HCOH coupling constants for dihedral angles of 80° and 160°. A least squares fit of these and previous data in the literature to a "Karplus" relation provides values for the coefficients in such an equation. The theoretical justification for such a relation has been obtained by extended Huckel molecular orbital calculations for methanol with different dihedral angles. The practical limitations in, and applications of the relation are discussed. An effect of solvent on the vicinal coupling constants is also noted and discussed.

Stereochemical dependence of vicinal H—C—O—H coupling constants

The pKa's of 13 heterocyclic aromatic compounds have been measured in tetrahydrofuran (THF) using 13C nmr spectroscopy. The acidifying effect of a nitrogen heteroatom is seen to be quite large (4–6...

Acidity measurements in THF. V: Heteroaromatic compounds containing 5-membered rings

The spectra of tiglic and angelic acids and their methyl esters have been determined and analyzed by first-order perturbation theory. The magnitudes of the long-range coupling constants were found ...

Robert R. Fraser

Papers

Acidity measurements on pyridines in tetrahydrofuran using lithiated silylamines

Stereochemical dependence of vicinal H—C—O—H coupling constants

Acidity measurements in THF. V: Heteroaromatic compounds containing 5-membered rings

Long-range coupling constants in the n.m.r. spectra of olefines

Barriers to rotation about the sulfur-sulfur bond in acyclic disulfides