Helmut Knoll

Rate constants of the thermal cis-trans isomerization of azobenzene dyes in solvents, acetone/water mixtures, and in microheterogeneous surfactant solutions

Rate constants kiso of the thermal cis-trans isomerization of four 4,4’-nitro-aminoazobenzenes with different amino groups have been determined in homogeneous aprotic solvents and polyglykol oligomers, primarily by means of conventional flash photolysis. The rate constants have been correlated with polarity (according to λmax from UV/Vis absorption spectra of the trans isomers) and bulk viscosity of the solvents. Qualitative conclusions about the influence of varying concentrations of water with respect to polarity and hydrogen bonding on kiso- and λmax-values in acetone/water mixtures were derived. Based on these results the data from microheterogeneous solutions have been interpreted. In microheterogeneous water/surfactant solutions kiso-values of selected azo dyes were strongly dependent on the concentrations of SDS, Triton®X-100, C12EO8 in water, and varied with the composition of bicontinuous microemulsions of Igepal® CA-520/ heptane/water. The large spread of isomerization rate constants is in part due to varying microviscosity. Replacement of H2O by D2O in aqueous surfactant solutions produced surprisingly large kinetic solvent isotope effects.

Spectroscopic studies on the mechanism of photolysis of Pt(N3)2(P(C6H5)3)2

Pt(N3)2(P(C6H5)3)2 has been irradiated in different media between r.t. and 10 K mainly with the 351, 364 nm lines of an argon ion laser. The photoreactions taking place have been studied by following the changes of the νas(N3) IR band. The intensity decrease of this band indicates that a photoredox reaction between the metal and azide ligands has occurred partly via dd band absorption. An azide containing intermediate was detected, which was stable in the dark but also disappeared upon further irradiation. It has been assigned to the trans form of the parent complex. A different intermediate, PtN3(P(C6H5)3)2, was detected by laser flash photolysis with 308 nm, having λmax=460 nm and a lifetime of (75±25)10−6 s in CH2Cl2, and (210±60)10−6 s in 2-MeTHF, respectively. The azide free photoproduct Pt0P(C6H5)3)2(2-MeTHF)n was stabilized in 2-MeTHF at 133 K showing an absorption band with λmax=409 nm.

Photochemistry and photocatalytic reactions of mixed-ligand azido transition metal complexes

Abstract Mechanistic details of the photolysis of mixed-ligand azido metal(II) complexes 1 (M = Pd(II), Pt(II), Ni(II); X-X = 2 PR 3 , R 2 P(CH 2 )nPR 2 , R-N=CH-CH=N-R ) are discussed with respect to formation of intermediate isomerization products, metal(I) complexes and azidyl radicals, respectively. Coordinatively unsaturated metal(O) complexes formed as the ultimate products in the course of photoreaction (1) exhibit catalytic activity. Thus, oxidative addition has been observed with photochemically generated platinum(O) and palladium(O) species whereas photoinduced cyclotrimerization of alkynes has been detected in the presence mixed-ligand azido nickel(II) complexes. cyclotrimerization of alkynes. Under similar aspects we have been able to prove the advantages of photocatalysis in oxygenation reactions of olefins in the presence of various metallo porphyrins [3,36–38] and metal acetylacetonates [39], respectively. A more comprehensive review on homogeneous photocatalysis in organic synthesis is going to be published elsewhere [40].

Synthesis of fluorescent derivatives of the antibiotic moenomycin A.

Moenomycin has been highly selectively converted into amino derivative 3b. The primary amino group of this compound can be used to attach various fluorescent labels to moenomycin, through conversion of isothiocyanates into thioureas and squaric acid diesters into diamides. The application of some of the derivatives is outlined. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Methyl radical addition to methyl ethyl ketone

From relative rates of acetone formation in the azomethane sensitized decomposition of methyl ethyl ketone at 290 °C, log10(k/cm3mol−1s−1)=5.3±0.4 for the methyl radical addition to methyl ethyl ketone has been determined.AbstractИз относительных скоростей образования ацетона при разложении метилэтилкетона при 290°C, сенсибилизированным азометаном, была определена константа скорости присоединения метильного радикала к метилэтилкетону. lg10к(см3 моль−1сек−1)=5,3±0,4.

Spectrophotometric studies of the photolysis of diazido-bis(phosphine)-metal(II) complexes

Abstract The photolysis of mixed ligand complexes of the type ML n (N 3 ) 2 (MNi, Pd, Pt; L = phosphane and diphosphane ligands, respectively; n = 1, 2) was monitored by electronic spectroscopy in absorption, emission and excitation. Initial quantum yields of these complexes were measured in CH 2 Cl 2 and EtOH in dependence on the irradiation wavelength. Photochemically formed M O I n fragments of Pd and Pt are distinguished by an intensive emission detected at 77 K. No emission was observed with respect to the appropriate Ni O fragments. An initial emission at 615 nm detected by irradiating Pd(PPh 3 ) 2 (N 3 ) 2 at 77 K is explained by assuming a photochemical cis/trans isomerization Quenching experiments have been performed by using Ru(bpy) 3 Cl 2 , Os(bpy) 3 Cl 2 , Re(CO) 3 (phen)Cl and Cr(bpy) 3 (ClO 4 ) 3 as sensitizers and the mixed-ligand complexes under discussion as quenchers.

On the displacement reaction CH3+CH3 COCOCH3→CH3 COCH3+CH3 CO

AbstractThe radical displacement reaction(0)

Kinetics of carbonyl triplet reactions in water

CH_3 + CH_3 COCOCH_3 \to CH_3 COCH_3 + CH_3 \dot CO