André Gerstmans

Friedel−crafts acylations with modified clays as catalysts

Friedel-Crafts acylations are very effectively catalyzed by clays modified through exchange of the interlamellar cations or through impregnation of metal chlorides. The acidic K10 montmorillonite exchanged with iron(III) gives outstanding results in quantitative (98–100%) acylation of anisole (15 min), mesitylene (15 min), and p-xylene (3 h) at 140–160 °C. Clays impregnated with zinc chloride, such as the K10 montmorillonite, or Japanese acidic clay show near-equivalent catalytic performance.

Clay-supported reagents IV. A novel coupling of thiols into disulphides, via thionitrite intermediates using a clay-supported nitrosation reagent.

Abstract Thiols are converted into symmetrical disulphides, under very mild conditions, by bentonite-supported Fe(NO 3 ) 3 .9 H 2 O.

The theoretical interpretation of sodium-23 NMR chemical shifts and quadrupolar coupling constants, as supported by new experimental evidence

Abstract The changes in 23N chemical shifts and linewidths for Na+ with the composition of binary solvent mixtures of THF with amines: aniline, pyridine, pyrrolidine, piperidine, propylamine, and i-propylamine, are analyzed in terms of successive displacement steps involving the species (Na+)4THF, (Na+)3THF,LA, (Na+)2THF,2LA, (Na+)THF,3LA, and (Na+)4L4. Each of these discrete solvates is fully characterized by its individual chemical shift and linewidth. Under the reasonable assumption of the equality of their molecular volumes, these two quantities are related to one another by the very simple relation: (Δv 1 2 ∗ ) 1 2 ∞ δ . This correlation obtains because changes in chemical shifts arise solely from variations of the paramagnetic part of the shielding constant, as determined by electronic transfer from the ligand into the 3p orbitals on Na+ Quadrupolar relaxation originates in modulation of the direction of the electrostatic field gradient, which is set up permanently in all cases (inclusive of pure solvents), by the reorientational diffusion of the solvates. Therefore, observed linewidths should be normalized to unit viscosity, if they are to be related to chemical shift changes, which are found to be approximately additive.

Chelation of Na+ cations in nonaqueous solvents. Structural requirements and methodological considerations

Intrinsic formation constants for complexes of Na+ ions with a series of polyamines have been determined from23Na NMR measurements in binary mixtures of tetrahydrofuran (THF) and amines. The results show in a totally unambiguous manner a regular decrease in the magnitude of the chelate effect with the number of atoms intervening between the nitrogen ligators. The key assumption, tetracoordination of the cation, is critically examined, and experimental evidence is adduced in its support.

Solid acid catalyzed reaction of aminals with methyl 3-aminocrotonate

Solid acid catalyzed reaction of aminals 1, with methyl 3-aminocrotonate 2 affords dimethyl 4-aryl-2,6-dimethyl-l,4-dihydropyridine-3,5-dicarboxylates 3. 2-Methyl-4//-pyrido[ 1,2-a]pyrimidin-4-one 4_ is formed from 2-aminopyridine moiety of the aminals. Recently attention has been focused on the use of solid acids in organic syntheses as environmentally acceptable alternatives to conventional Brönsted and Lewis acids. Clays and zeolites are effective catalysts for a wide variety of organic reactions (1-3). The prowess of acid-treated montmorillonite clay (K10) and ZF520 zeolite, as strong Brönsted acidic catalysts, has been shown in carbon-carbon and carbon-nitrogen bond formation processes, such as addition and cyclization reactions (4-6). Azines (4) derived from aromatic aldehydes and iV-benzylideneanilines (6) undergo clay-mediated cyclocondensation reaction with methyl 3-aminocrotonate to give symmetrically substituted 1,4-dihydropyridines, which are highly effective calcium antagonists (7). Condensation reaction between aromatic aldehydes and 2-aminopyridine leads to the formation of aminals instead of the expected Schiff bases (8-11). The easy access to aminals I prompted us to examine their reaction with methyl 3-aminocrotonate 2. The presence of an acidic catalyst is a prerequisite for the success of the reaction. Solid acids tested include K10 montmorillonite, kaolinite, ZF520 zeolite and silica gel. The acid strength of a solid surface is defined as its proton-donating ability, quantitatively expressed by Hammett Ho function (Ho = -3 to -6 for kaolinite and Ho = -6 to -8 for acid treated montmorillonite) (12). K10 montmorillonite is a mesoporous solid having pores with the majority in the 60-100 Ä diameter range (13). The Brönsted acidity in zeolites is linked to the Si/Al ratio (14). ZF520 is a highly acidic Y zeolite (Si/Al = 20). Besides microporosity of the faujasite structure (15) with diameters around 8 Ä there is a secondary porous system in the range of 50-200 Ä. Vol. 5, No. 1, 1999 Solid acid catalyzed reactin of aminals with methyl 3-aminocrotonate

Direct NMR studies of ionic solvation

The nuclear magnetic resonance of quadrupolar ions provides a wealth of information about solvation thermodynamics and dynamics. In this contribution, we consider the local ordering induced by an ion, surrounding itself with neutrals, solute or solvent molecules, in competition with its counter-ion. Examples include determination of the solvent electron-accepting, or hydrogen bond donor, abilities, anionic activation, and hydrophobic interactions involving Q+ cations, preferential solvation, and cation chelation, including that by calciproteins. Our approach is to describe the first solvation shell of an ion such as Na+ in specific terms, i.e. there are solvent molecules truly coordinated to the ion. Beyond this coordination sphere, we treat the solvent as a continuum, whose effects are estimated through bulk parameters such as the macroscopic viscosity η or the dielectric constant e.