A. P. G. Kieboom

Studies on borate esters 1 : The ph dependence of the stability of esters of boric acid and borate in aqueous medium as studied by 11B NMR

The pH dependent stability of esters of boric acid and borate with glycol, glycolic acid, oxalic acid and glyceric acid as dihydroxy compounds has been studied. 11B NMR provides a suitable analytical tool for the quantitative determination and structure elucidation of the various esters in aqueous medium. The pH dependent stability of esters of boric acid and borate is formulated in a general rule of thumb: esters of boric acid and borate with dihydroxy compounds in aqueous medium show the highest stability at that pH where the sum of the charges of the free esterifying species is equal to the charge of the ester.

Studies on borate esters II: Structure and stability of borate esters of polyhydroxycarboxylates and related polyols in aqueous alkaline media as studied by 11B NMR

11B NMR data are presented for borate esters of a series of polyols and polyhydroxycarboxylates in aqueous alkaline solution. For each compound the prevailing borate mono- and diesters were identified. 11B chemical shifts and line widths together with the corresponding association constants have been determined. Empirical rules are put forward for predicting the 11B chemical shift, the relative stability and the structure of borate esters in aqueous media.

Characterization of ruthenium catalysts as studied by temperature programmed reduction

The oxidation and subsequent reduction of ruthenium have been investigated by means of temperature programmed reduction (TPR). RuSiO2 prepared by direct reduction of RuCl3SiO2 showed surface oxidation followed by “bulk” oxidation upon exposure to air at room temperature. The oxidation/reduction behavior of these catalysts differed substantially from that of RuSiO2 prepared by reduction of RuO2SiO2 obtained by calcination of RuCl3SiO2 in air. Application of higher reduction temperatures (up to 973 K) of RuCl3SiO2 was found to increase the active metal surface area of the RuSiO2 formed due to the removal of traces of chlorine from the Ru surface. Reduction of RuO2SiO2, obtained by calcination of RuCl3SiO2 in air, resulted in lower dispersed RuSiO2. The present results explain the effect of the activation procedure of RuSiO2 catalysts on the activity for benzene hydrogenation. Measurement of H2 desorption by means of the TPR apparatus gave rapid and useful information about the Ru metal dispersion.

Enolisation and isomerisation of monosaccharides in aqueous, alkaline solution

Abstract The initial behaviour of a series of monosaccharides in aqueous, alkaline medium has been studied by using u.v. spectroscopy. An absorption band at 310 nm is observed which may be due to the enediol-anion species as well as to β-elimination intermediates. Comparison of u.v. kinetic data with data from both H/D exchange and degradation reactions shows that the 310-nm band is mainly due to the enediol-anion species. Reaction rate constants have been determined for both the formation and the conversion of the enediol anion by using the overall reaction scheme: sugar + -OH α sugar anion α enediol anion → β-elimination intermediates → products. Calcium (II) and carbonate ions promote the formation of the 310-nm band. Traces of oxygen cause an induction period, due to a rapid oxidation of the enediol anion. The rate of enediol-anion formation is strongly dependent on the configuration of the sugar, whereas the overall rates of conversion of the enediol anion are comparable for the different sugars. For the enolisation and the isomerisation reactions, a molecular picture is presented on the basis of the principle of least motion. It is proposed that the enediol anion is formed through a rate-determining, intramolecular proton-shift.

Hydrogenation and hydrogenolysis in synthetic organic chemistry

The major aim of this book is to provide preparative organic chemists with the insight and the know-how necessary to apply catalytic hydrogenation and hydrogenolysis to synthetic problems. Several texts on hydrogenation and hydrogenolysis are available, but the authors feel that many chemists will welcome a book in which more attention has been paid to the mechanistic background of these reactions and its relation to synthetic problems. In this book a special effort has been made to present the various types of hydrogenation and hydrogenolysis reactions from both a mechanistic and a preparative point of view. After a short general introduction concerning catalyst systems and reaction conditions, hydrogenation and hydrogenolysis are discussed separately. The chapters have been organized according to a logical arrangement of the various bonds which can be reduced with hydrogen in the presence of a catalyst system. Reaction rate, selectivity, and steric course of the hydrogen addition are dealt with in relation to the reaction mechanism. Numerous synthetically interesting examples exemplify these aspects as well as the scope and limitations of the reactions.

Pt/C oxidation catalysts. Part 1. Effect of carrier structure on catalyst deactivation during the oxidation of glucose 1-phosphate into glucuronic acid 1-phosphate

Abstract In the platinum catalyzed liquid phase oxidation of glucose 1-phosphate into glucuronic acid 1-phosphate, a strong catalyst deactivation is observed. This deactivation, caused by the presence of excess oxygen leading to a complete surface coverage and a distortion of the surface structure, can be prevented by the use of a catalyst carrier with such particle geometry that a ‘built-in’ diffusion barrier is provided.

Substituent effects in the hydrogenolysis of benzyl alcohol derivatives over palladium

Abstract A kinetic study of substituent effects in the liquid-phase hydrogenolysis of benzyl oxygen compounds has been made using carbon-supported palladium as the catalyst. The reaction is zero order in substrate and is progressively inhibited by its product. Proton catalysis is essential for the hydrogenolysis of benzyl alcohols and alkyl benzyl ethers. For the hydrogenolysis of benzyl alcohols and 2-aryl-3-methyl-2-butanols the effects of substituents in the aromatic ring may be expressed in terms of a Hammett-Yukawa relationship with ϱ = −0.37 and −1.43, respectively. α-Substituents retard the rate of hydrogenolysis of benzyl alcohol. The order of reactivity for different leaving groups is: OH, OAlkyl ⪡ OAryl + OHAlkyl + OH 2 , OAc 3 . It is concluded that the hydrogenolysis reaction involves hydride attack at the benzylic carbon displacing the leaving group as its anion. For primary alcohols an S N 2 mechanism is operative, whereas tertiary alcohols show s N 1 type character. The adsorbed state is discussed on the basis of deuterolysis experiments.

Hydrogenation of d-fructose and d-fructose/d-glucose mixtures

Abstract d -Fructose and d -fructose/ d -glucose mixtures have been hydrogenated in water at 60–80° and 20–75 atm. of hydrogen with Ni, Cu, Ru, Rh, Pd, Os, Ir, and Pt severally as catalysts. The selectivity for the formation of d -mannitol from d -fructose as well as the selectivity for the hydrogenation of d -fructose in the presence of d -glucose with Cu/silica as the catalyst are substantially higher than those for the other catalysts. With Cu/silica as the catalyst, the hydrogenation of d -fructose is first order with respect to the amount of catalyst and the hydrogen pressure, whereas a shift from first- to zero-order kinetics occurs on going from low ( m ) to high (0.8 m ) concentrations of d -fructose. d -Fructose is preferentially hydrogenated via its furanose forms, presumably by attack of a copper hydride-like species at the anomeric carbon atom with inversion of configuration. Preferential adsorption of pyranose with respect to furanose forms occurs, whereas the furanose forms show a much higher reactivity. The mechanism proposed for the copper-catalysed hydrogenation reaction explains both the enhanced yield of d -mannitol from boric esters of d -fructose and the diastereoselectivity of the hydrogenation of seven other ketoses.

Catalytic dehydrogenation of reducing sugars in alkaline solution

Abstract Aldoses in alkaline medium under the catalytic action of platinum or rhodium are converted into aldonic acids with high selectivity and with concomitant evolution of hydrogen gas. The dehydrogenation reaction has been studied for 25 different mono- and di-saccharides, and is generally applicable for reducing sugars. The influence of several reaction variables has been studied, leading to an adsorption model in which both the negatively charged O-1 and the close contact of H-1 with the catalyst surface are considered to be driving forces for the transfer of hydride from C-1 of the sugar to the catalyst.

Selective hydrogenation of citronellal to citronellol over Ru/TiO2 as compared to Ru/SiO2

Abstract The hydrogenation of citronellal was investigated with Ru/TiO2 and Ru/SiO2 catalysts, prepared from different precursors at various activation temperatures. Ru/TiO2 was the more active catalyst reaching maximum carbonyl selectivity when activated at 773 K, while Ru/SiO2 became selective at much higher activation temperatures, but at the cost of activity. Both phenomena are related to the occurrence of H-spillover on TiO2 supported catalysts and to the absence of contaminating C1 on catalysts activated at high temperature.