Computational advances in organic chemistry: Molecular structure and reactivity. Edited by C. Ögretir and I. G. Csizmadia,NATO ASI Series C,Vol, 330.

We here report an AM 1 study for the reaction of different types of radicals with the acetone enolate ion 1 and other ambident nucleophiles. The theoretical reactivity order determined is phenyl > methyl > tert-butyl ? bicyclo-[2.2.1]hept-1-yl. The activation energy for the coupling of aliphatic radicals is ascribed mainly to nonbonded interactions. Considering possible solvent effects, we propose the same order of reactivity in solution. The coupling reaction of phenyl radical with 1 is an exothermic process with nonactivation energy in the gas phase. Any energy barrier in solution is here ascribed to desolvation of the anion. The regiochemistry of the coupling of radicals with ambident nucleophiles such as 1 , phenoxide, or phenylamide anions is also explained on a theoretical basis. © 1992 John Wiley & Sons, Inc.     

Flat rank of automorphism groups of buildings

The flat rank of a totally disconnected locally compact group G, denoted flat-rk(G), is an invariant of the topological group structure of G. It is defined thanks to a natural distance on the space of compact open subgroups of G. For a topological Kac-Moody group G with Weyl group W, we derive the inequalities alg-rk(W) ≤ flat-rk(G) ≤ rk(|W|₀). Here, alg-rk(W) is the maximal Z-rank of abelian subgroups of W, and rk(|W|₀) is the maximal dimension of isometrically embedded flats in the CAT0-realization |W|₀. We can prove these inequalities under weaker assumptions. We also show that for any integer n ≥ 1 there is a simple, compactly generated, locally compact, totally disconnected group G, with flat-rk(G) = n and which is not linear.     

Structural and spectroscopic studies of silylated cyclo- and bicyclosilanes

A number of cyclo-and bicyclosilanes have been prepared and structurally characterized by X-ray crystallography and Raman spectroscopy. 1,1,4,4- and 1,1,3,3-tetrakis(trimethylsilyl)octamethylcyclohexasilanes were found to exhibit unusual twist- and twisted boat-conformations. The UV absorption properties of all compounds were studied and found to show absorption maxima red shifted compared to the parent compound dodecamethylcyclohexasilane. Dedicated to Prof. Mitsuo Kira on the occasion of his reception of the Wacker Silicon Award 2005 and in recongnition of his numerous outstanding achievements in organosilicon chemistry.     

Fluorimetric determination of acidity constants of naphthoic and anthroic acids

A fluorimetric method of determining the acidity constants (pK_a values) of some naphthoic and anthroic acids is presented, based on the changes in fluorescence of solutions of these acids in water on changing the pH. The method is useful in the absence as well as in the presence of excited-state proton transfer. In the latter case, when the excited-state protonation occurs in the same pH region as the ground-state dissociation, resolution of the two processes can be accomplished by addition of a suitable quencher such as iodide. The method permits the accurate determination of the pK_a value of fluorescent compounds, even when they are poorly soluble in water, because of the high sensitivity of the spectrofluorimetry.     

Synthèse des (R)- et (S)-O-isopropylidène-1,2-glycérols. Détermination de la pureté optique

Synthesis of the (R)- and (S)-Glycerol Acetonides. Determination of the Optical Purity The optical purity of (R)-glycerol acetonide ( 1 ) and (S)-glycerol acetonide ( 1 ′) has been determined with great accuracy by gas chromatography after derivatization. The optical purity of (S)-glycerol actonide prepared from D -mannitol was > 99.4% while for (R)-glycerol acetonide obtained from L -serine it was 95%.     

Physikalische und spektroskopische daten von trifluormethylselenol und derivaten

The characterization of the compounds CF₃SeX (X = H, Cl, Br, CN, CF₃, SeCF₃) is completed by the report of melting points, boiling points, enthalpies of vaporization and entropies of vaporization. Ultraviolet and mass spectra are presented and discussed. An improved synthesis for CF₃SeH is reported.     

Zum Mechanismus der Photochemie des Benzfurazans

Mechanistic studies on the photochemical reactions of benzfurazan . From other works it is known that irradiation of benzfurazan ( 1 ) in methanol gives the carbaminacid-ester 4 , whereas in benzene the azepinederivative 3 is obtained (Scheme 1). The compounds 5–8 (Scheme 2) have been proposed as intermediates. In our investigations we detected and characterized by means of UV.- and IR.-spectroscopy the two species 5 and 8 . Irradiation of 1 with 350 nm light at room temperature in a strongly polar solvent (e.g. H₂O) yields exclusively 5 (Fig. 1) with a quantum yield of 0.48. In non polar solvents (e.g. hexane) 5 isomerizes in a second photochemical step to 8 (quantum yield 0.43) (Fig. 3). Thermally, 5 can be converted back to 1 . The rate constant for this reaction at room temperature is 2 · 10^–5s^–1. The transformation 5 → 8 was also investigated at low temperature. There was no direct evidence for any intermediates of the type oxazirene ( 6 ) or nitrene ( 7 ). However, the formation of azepine 3 upon irradiation of 5 in benzene suggests as intermediate the nitrene 7 which could be converted into 8 in a fast thermal reaction (Scheme 3).     

Enhanced fluorescence from o-phthalaldehyde and fluorescamine fluorophores using Triton and β-cyclodextrin

Mexiletine— and lysine hydrochloride—o-phthalaldehyde and mexiletine hydrochloride—, cysteine—, cysteamine—, homocysteine— and lysine hydrochloride—fluorescamine derivatives were subjected to Triton and β-cyclodextrin enhancement treatments. Of several classical fluorescence-enhancing reagents tested (Triton, β-cyclodextrin, sodium dodecyl sulphate, Brij), Triton provided the best results, followed by β-cyclodextrin. Increases in fluorescence emission by a factor of up to about 10 (mexiletine—fluorescamine—Triton X-100) were observed, with a generally negligible influence of the enhancing reagents on the excitation and emission maxima. Fluorescence enhancement by the addition of a suitable reagent solution to the final analyte solution may, in specific instances, enhance the detectability of native or chemically induced fluorophores.