Ulrich Flörke

From Structural Models of Galactose Oxidase to Homogeneous Catalysis: Efficient Aerobic Oxidation of Alcohols

Two reaction pathways are catalyzed by the dinuclear copper(II)-phenoxyl complex 1-a functional model for the metalloenzyme galactose oxidase-by the oxidation of primary and secondary alcohols with dioxygen (air) in homogeneous solution to their corresponding aldehydes or ketones and/or 1,2-glycols (oxidative C-C coupling). The reduction product formed is H2 O2 , not water.

FUNKTIONALISIERTE OCTA-(PROPYLSILSESQUIOXANE)(3-XC3H6)8(SI8O12) MODELLVERBINDUNGEN FUR OBERFLACHENMODIFIZIERTE KIESELGELE

Functionalyzed octa-(propylsilsesquioxanes) [(3-XC3H6)8(Si8O12)] have been prepared by the following methods: (a) Hydrolisation of corresponding functionalyzed propyltrialkoxysilanes with X = -Cl, -SH. (b) Hydrosilylation of allyl compounds of octa-(hydridosilsesquioxans) [H8(Si8O12)] with X = -p-(CH3)OC6H4, -C6H5, -OC4H9, , , -CN, -OSi(CH3)3, -C6F5, -OC6H5, -SO2C6H5, -Si(CH3)3. (c) Nucleophilic substitution of the chlorine atoms of octa-[(3-chloropropyl)-silsesquioxane] [3ClC3H6)8(Si8O12)] with X = -I, -SCN, -P(C6H5)2, -SCH3. 1H, 13C, 29Si, 31P NMR, IR, UV/vis und melting point data are reported. The single crystal X-ray structure of (3-ICP3H6)8(Si8O12) is given.

Lactide polymerisation with air-stable and highly active zinc complexes with guanidine-pyridine hybrid ligands.

The synthesis of zinc complexes of guanidine-pyridine hybrid ligands [Zn(DMEGpy)Cl(2)] (C1), [Zn(TMGpy)Cl(2)] (C2), [Zn(DMEGqu)Cl(2)] (C3), [Zn(TMGqu)Cl(2)] (C4), [Zn(DMEGpy)(CH(3)COO)(2)] (C5), [Zn(TMGpy)(CH(3)COO)(2)] (C6), [Zn(DMEGqu)(CH(3)COO)(2)] (C7), [Zn(TMGqu)(CH(3)COO)(2)] (C8), [Zn(DMEGqu)(2)(CF(3)SO(3))][CF(3)SO(3)] (C9) and [Zn(TMGqu)(2)(CF(3)SO(3))][CF(3)SO(3)] (C10) is reported. These zinc complexes were completely characterised and screened regarding their activity in the ring-opening polymerisation of D,L-lactide. They proved to be active initiators in lactide bulk polymerisation, and polylactides with molecular weights (M(w)) up to 176,000 g mol(-1) could be obtained. They combine high activity with robustness towards moisture and air. The influence of reaction temperature and of the anionic component of the zinc salt on the activity of the catalyst, as well as the occurrence of undesired side reactions, was investigated. By correlating these findings with the structural study on the zinc complexes we could deduce a structure-reactivity relationship for the zinc catalysts. This study was accompanied by DFT calculations. The bis-chelate triflate complexes C9 and C10, supported by quinoline-guanidine ligands L3 and L4, exhibit by far the highest reactivity. Systematic comparison of these complexes with their mono-chelate counterparts and their bis-guanidine analogues allows the attributes that promote polymerisation by neutral guanidine ligand systems to be elucidated: accessibility to the zinc centre and Lewis acidity.

Phenolate Hydroxylation in a Bis(μ-oxo)dicopper(III) Complex : Lessons from the Guanidine/Amine Series

A new hybrid permethylated-amine-guanidine ligand based on a 1,3-propanediamine backbone (2L) and its Cu-O2 chemistry is reported. [(2L)CuI(MeCN)]1+ complex readily oxygenates at low temperatures in polar aprotic solvents to form a bis(mu-oxo)dicopper(III) (O) species (2b), similar to the parent bis-guanidine ligand complex (1b) and permethylated-diamine ligand complex (3b). UV-vis and X-ray absorption spectroscopy experiments confirm this assignment of 2b as an O species, and full formation of the 2:1 Cu-O2 complex is demonstrated by an optical titration with ferrocene-monocarboxylic acid (FcCOOH). The UV-vis spectra of 1b and 2b with guanidine ligation show low-intensity visible features assigned as guanidine pi --> Cu2O2 core transitions by time-dependent density functional theory (TD-DFT) calculations. Comparison of the reactivity among the three related complexes (1b-3b) with phenolate at 195 K is particularly insightful as only 2b hydroxylates 2,4-di-tert-butylphenolate to yield 3,5-di-tert-butylcatecholate (>95% yield) with the oxygen atom derived from O2, reminiscent of tyrosinase reactivity. 1b is unreactive, while 3b yields the C-C radical-coupled bis-phenol product. Attenuated outer-sphere oxidative strength of the O complexes and increased phenolate accessibility to the Cu2O2 core are attributes that correlate with phenolate hydroxylation reactivity observed in 2b. The comparative low-temperature reactivity of 1b-3b with FcCOOH (O-H BDE 71 kcal mol(-1)) to form the two-electron, two-proton reduced bis(mu-hydroxo)dicopper(II,II) complex is quantitative and presumably precedes through two sequential proton-coupled electron transfer (PCET) steps. Optical titrations along with DFT calculations support that the reduced complexes formed in the first step are more powerful oxidants than the parent O complexes. These mechanistic insights aid in understanding the phenol to bis-phenol reactivity exhibited by 2b and 3b.

Dihydroisocoumarins from fungi : Isolation, structure elucidation, circular dichroism and biological activity

Five known and three new dihydroisocoumarins were isolated from different fungi. The new isocoumarins are 5-chloro-6-hydroxymellein, 5-chloro-4,6-dihydroxymellein and 5,6-dihydroxymellein. The absolute configuration of these secondary metabolites was confirmed by CD measurements and in two cases by X-ray structure analysis.

New Mono‐ and Dimeric Members of the Secalonic Acid Family: Blennolides A–G Isolated from the Fungus Blennoria sp.

Blennolides A-G (2-8), seven unusual chromanones, were isolated together with secalonic acid B (1) from Blennoria sp., an endophytic fungus from Carpobrotus edulis. This is the first reported isolation of the blennolides 2 and 3 (hemisecalonic acids B and E), the existence of which as the monomeric units of the dimeric secalonic acids had long been postulated. A compound of the proposed structure 4 (beta-diversonolic ester) will need to be revised, as its reported data do not fit those of the established structure of blennolide C (4). Other monomers, the blennolides D-F (5-7) seem to be derived from blennolides A (2) and B (3) by rearrangement of the hydroaromatic ring. The heterodimer 8, composed of the monomeric blennolide A (2) and the rearranged 11-dehydroxy derivative of blennolide E (6), extends the ergochrome family with an ergoxanthin type of skeleton. The structures of the new compounds were elucidated by detailed spectroscopic analysis and further confirmed by an X-ray diffraction study of a single crystal of 2. The absolute configurations were determined by TDDFT calculations of CD spectra, including the solid-state CD/TDDFT approach. Preliminary studies showed strong antifungal and antibacterial activities of these compounds against Microbotryum violaceum and Bacillus megaterium, respectively. They were also active against the alga Chlorella fusca and the bacterium Escherichia coli.

A review on the chemistry, coordination, structure and biological properties of 1-(acyl/aroyl)-3-(substituted) thioureas

This review provides an overview of the chemistry, structure and potential applications of 1-(acyl/aroyl)-3-(mono-substituted) and 1-(acyl/aroyl)-3,3-(di-substituted) thioureas, with general formula R1C(O)N(1)HC(S)N(3)R2R3. In recent years, the title compounds have found extensive applications as ligands in coordination chemistry. The effect that nitrogen substituents exert on the intra- and intermolecular hydrogen-bonding interactions is discussed, including their role on the coordination properties displayed by these ligands. Novel applications of transition metal complexes bearing 1-(acyl/aroyl)-3-(mono- and di-substituted) thioureas are introduced. Biological aspects are also highlighted. As recently demonstrated, high-throughput screening assay and structure–activity analyses are feasible for this class of compounds. The chemical versatility of 1-(acyl)-3-(substituted) thiourea molecules and the derived metal complexes, together with the possibility of determining detailed structural properties, join biological applications in a promising interdisciplinary approach. The bibliography includes 382 references with emphasis on the literature appearing after 2007. GRAPHICAL ABSTRACT

Absolute structural elucidation of natural products—A focus on quantum‐mechanical calculations of solid‐state CD spectra

In this review article we examine state-of-the-art techniques for the structural elucidation of organic compounds isolated from natural sources. In particular, we focus on the determination of absolute configuration (AC), perhaps the most challenging but inevitable step in the whole process, especially when newly isolated compounds are screened for biological activity. Among the many methods employed for AC assignment that we review, special attention is paid to electronic circular dichroism (CD) and to the modern tools available for quantum-mechanics CD predictions, including TDDFT. In this context, we stress that conformational flexibility often poses a limit to practical CD calculations of solution CD spectra. Many crystalline natural products suitable for X-ray analysis do not contain heavy atoms for a confidential AC assignment by resonant scattering. However, their CD spectra can be recorded in the solid state, for example with the KCl pellet technique, and analyzed possibly by nonempirical means to provide stereochemical information. In particular, solid-state CD spectra can be compared with those calculated with TDDFT or other high-level methods, using the X-ray geometry as input. The solid-state CD/TDDFT approach, described in detail, represents a quick and reliable tool for AC assignment of natural products.

Synthesis and characterization of copper(II), nickel(II) and cobalt(II) complexes with novel thiourea derivatives

A novel series of thiourea derivatives, namely, N,N-diphenyl-N′-(4-phenyl-benzoyl)thiourea (HL1), N,N-diphenyl-N′-(4-chloro-benzoyl)thiourea (HL2) and N,N-di-n-propyl-N′-(4-chloro-benzoyl)thiourea (HL3), and its metal complexes has been prepared and characterised by elemental analysis, i.r. spectroscopy, 1H-n.m.r. spectroscopy, mass spectrometry and single crystal X-ray diffraction. The ligand coordinates to NiII, CuII and CoII in a bidentate manner yielding essentially neutral complexes of the type cis-[ML2]. N.m.r. spectra and single crystal X-ray diffraction analysis revealed the presence of a distorted tetrahedral coordination ML2 complex.

An effectively diamagnetic oximato-bridged asymmetric dinuclear copper(II) complex with a Cu(II)I bond

Abstract A new bis(oximato)-bridged dinuclear copper(II) complex containing an axial Cu(II)I bond with the formula [LCu(PyA) 2 Cu(I)]ClO 4 ·CH 3 CN ( 1 ) (where L = 1,4,7-tritrimethyl-1,4,7-triazacyclononane; PyA = monoanion of pyridine-2-aldoxine) has been characterized by IR, UV-Vis, variable temperature (81–283 K) magnetic susceptibility measurements. 1 crystallizes in the monoclinic system space group P2 1 n , with cell constants a= 8.304(2), b = 27.410(7), c = 13.975(5) A β = 93.48(2)° V = 3175(2) A and Z = 4 . The structure consists of oximato-bridged CuIICuII(I) monocations and non-coordinated perchlorate anions. Both copper ions have a distorted square-pyramidal geometry with a Cu ⋯ Cu separation of 3.45 A and Cu(II)I bond length of 2.74 A. The copper centres are very strongly antiferromagnetically coupled, the singlet-triplet splitting being greater than 1000 cm−1.