- How the sustainable solvent water unleashes the photoredox catalytic potential of ruthenium polypyridyl complexes for pinacol couplings
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By complementing laser flash photolysis with product studies in visible-LED driven syntheses, we show that the one-electron reduced forms OER of tris(2,2′-bipyridine)ruthenium(ii) and its more reactive derivative with 4,4′-dimethylated ligands exhibit a reductive power greater by 0.2 eV in water than in acetonitrile; and that this difference allows the reduction of carbonyl compounds, and thus pinacol couplings, in aqueous medium via ruthenium-based photoredox catalysis as an alternative to using more expensive and less photostable higher-energy complexes (e.g., of iridium). Ascorbate serves as sacrificial donor to access OER. SDS micelles or cyclodextrins as carriers help overcome solubility problems of less hydrophilic substrates, and more reactive water-soluble substrates can even be coupled at neutral pH, such that the mild conditions make the process fully sustainable.
- Naumann, Robert,Goez, Martin
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supporting information
p. 4470 - 4474
(2019/08/21)
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- Solvolytic Behavior of Aliphatic Carboxylates
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The leaving group abilities (nucleofugalities) of a series of aliphatic carboxylates have been obtained by determining the nucleofuge-specific parameters (Nf and sf) from solvolysis rate constants of X,Y-substituted benzhydryl carboxylates in a series of aqueous ethanol mixtures by applyication of the linear free energy relationship (LFER) equation: log k = sf (Ef + Nf). These values can be employed to compare reactivities of carboxylates with those of other leaving groups previously included in the nucleofugality scale, and also to estimate the solvolysis rates of various carboxylates. It is confirmed that the inductive effect is the most important variable governing the reactivities of halogenated carboxylates in solution. Moreover, both the Hammett correlation and the solvolytic activation parameters have revealed a strong influence of the inductive effect on the nucleofugality of alkyl-substituted carboxylates. The reaction constants (sf) indicate that carboxylate substrates with weaker leaving groups solvolyze via later, more carbocation-like, transition states, which is in accord with the Hammond postulate. In addition, due to the weaker demand for solvation of transition states that produce more strongly stabilized benzhydrylium ions, in which more efficient charge delocalization occurs, the reaction constants (sf) obtained with most of the leaving groups investigated here increase as the polarity of the solvent decreases.
- Matic, Mirela,Denegri, Bernard,Kronja, Olga
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supporting information
p. 1477 - 1486
(2015/10/05)
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- Structure-Activity Relationships in the Esterase-catalysed Hydrolysis and Transesterification of Esters and Lactones
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The Broensted exponents for the alkaline hydrolysis of alkyl esters are 1.3 and 0.4 for substitution in the acyl and alcohol portions, respectively, which is indicative of a transition state which resembles the anionic tetrahedral intermediate with a localised negative charge.By contrast, the rate of the pig liver esterase (PLE)-catalysed hydrolysis shows little dependence upon the electron-withdrawing power of substituents.The values of kcat are independent of the pKa of the leaving group alcohol suggesting rate-limiting deacylation.There is a small steric effect of α-substitution in both the alcohol and carboxylic acid residues for the enzyme-catalysed reactions but the enzyme rate enhancement factor remains high for most esters.There is no substantial ee observed for the hydrolysis of racemic esters although the kinetic data can be used for determining the regioselective hydrolysis of diesters.Unsubstituted lactones are poor substrates for PLE but derivatives with hydrophobic substituents show kcat/Km values similar to those for acyclic esters.Dihydrocoumarin undergoes transesterification catalysed by PLE, kcat increases with increasing alcohol concentration indicative of rate-limiting deacylation.There is enantioselectivity in the PLE-catalysed hydrolysis of some racemic lactones but little or none in the transesterification of racemic alcohols with dihydrocoumarin.
- Barton, Patrick,Laws, Andrew P.,Page, Michael I.
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p. 2021 - 2030
(2007/10/02)
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- PHYSICOCHEMICAL ANALYSIS AND THERMODYNAMICS OF THE EQUILIBRIA OF DIMETHYL SULFOXIDE-CARBOXYLIC ACID BINARY LIQUID SYSTEMS
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Density, viscosity, electric-conductivity, and dielectric-constant measurements were made over the temperature range 298.15 - 323.15 K on binary liquid systems formed by DMSO with acetic acid and monochloro-, dichloro-, trichloro-, and trifluoro-acetic acids.The formation of the addition products DMSO*(HA)2 and DMSO*HA was established; in the system with trifluoroacetic acid the adducts DMSO*(HA)2 and DMSO*2(HA)2 are formed (HA = the acid).By the method of seeking the minimum of the function of least squares with the variation of the variables sought over the whole range of concentrations and of values of the molar volume we determined the equilibrium constants and thermodynamic characteristics of the process of the formation of adducts and also the densities of the latter.It was shown that between the enthalpies and entropies of the processes studied a compensation effect is observed.
- Fialkov, Yu. Ya.,Bondarenko, E. S.
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p. 735 - 739
(2007/10/02)
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- Proton transfers among oxygen and nitrogen acids and bases in DMSO solution
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Rate constants for the proton-transfer reactions between conjugate acids and bases of several amines, phenols, carboxylic acids, and the solvated proton in DMSO-d6 at 20 °C have been determined by the use of NMR line-shape analysis. Equilibrium constants for the same reactions are obtained from the pKa's of the acids in dimethyl sulfoxide, some of which have been reported in earlier work and the rest obtained in the present work by use of Bordwell's indicator techniques. All of the reactions have rale constants considerably below expected diffusion-controlled limits for the proton transfers in the thermodynamically favorable direction, and several of the reactions, including the identity reactions of carboxylic acids, have kinetic deuterium isotope effects, kH/kD, between 0.8 and 1.3. For reactions of N,N-dimethylbenzylammonium ion with several phenoxides, carboxylates, and solvent, the rate constants for transfers in the unfavorable directions show a reasonable Bronsted correlation with β ≈ 1 and a reasonably constant reverse rate constant of ≈3 × 106 M-1 s-1. The data clearly indicate that the proton-transfer step is not rate-limiting in these reactions. Most likely, desolvation is involved in the rate-limiting steps, but the rate constants are not simple functions of acidities as might have been expected if hydrogen bonding of acid to solvent were the major factor involved in the solvation Other factors, particularly dispersion interactions of solvent with solutes, are discussed. We suggest that the formation of an acid-base complex with proper orientation to allow contact between the proton and the basic site is rate-determining and involves desolvation along with detailed steric interactions of the acid-base pair.
- Ritchie, Calvin D.,Lu, Shanzheng
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p. 7748 - 7756
(2007/10/02)
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