Kai Seehafer

Enantio- and regioselective iridium-catalyzed allylic hydroxylation.

The first enantioselective allylic hydroxylation to prepare branched allylic alcohols directly is described. Bicarbonate was used as nucleophile in conjunction with new single component Ir-catalysts, which are stable to air and water. Excellent regio- and enantioselectivities have been achieved with a representative set of substrates.

Porous Polymers Based on Aryleneethynylene Building Blocks

Porous conjugated polymers are synthesized by metal-catalyzed coupling reactions. The progress for porous polymers when planar or tetrahedral building blocks are connected by alkyne units into novel materials is highlighted. The most prominent reaction for the buildup of the microporous alkyne-bridged polymers is the Sonogashira reaction, connecting alkynes to aromatic iodides or bromides. The availability of the building blocks and the potency of the Sonogashira reaction allow preparing a large variety of intrinsically porous polymeric materials, in which rigid struts connect multipronged centers. The microporous polymers are used as catalysts and as storage materials for gases and sensors. Postfunctionalization schemes, understanding of structure-property relationships, and the quest for high porosity are pertinent.

Identification of White Wines by using Two Oppositely Charged Poly(p-phenyleneethynylene)s Individually and in Complex.

We present a simple array composed of an anionic and a cationic poly(para-phenyleneethynylene) (PPE), together with an electrostatic complex between the two of them. The individual PPEs and the PPE complex were employed in the sensing of white wines at pH 13; the complex was also successfully employed as a sensor element at pH 3. The sensing mechanism is fluorescence quenching. Thirteen different wines were differentiated by this chemical tongue, which consists of four elements. The fluorescence quenching is not induced by the major components of the wines. Compounds such as acids, sugars, and alcohols alone do not quench the fluorescence, but rather the colored tannins and other polyphenols contained in wine are the main quenchers. However, the major constituents of wine significantly modulate the quenching of the PPEs by the tannins.

Polyelectrolyte Complexes Formed from Conjugated Polymers: Array-Based Sensing of Organic Acids

One fluorescent, positively charged poly(para-phenyleneethynylene) (PAE 1) forms electrostatic complexes with five negatively charged pyridine- or benzothiadiazole-containing poly(para-aryleneethynylene)s (PAE 2-PAE 6). The PAE 2-PAE 6 are less fluorescent in water and act as quenchers for PAE 1 in their electrostatic complexes C 1-C 5; the PAE-complexes (2 μm) were exposed to thirteen different carboxylic acids (50 mm) in buffered aqueous solution. The fluorescence responses of the small library of electrostatic PAE-complexes towards the acids was analyzed; discrimination of all of the thirteen acids was achieved. The investigated acids include acetic, butyric, tartaric, maleic, lactic, sorbic, oxalic, aspartic, and citric acids. A random, simple, ad-hoc library of electrostatic polymer complexes, C 1-C 5, therefore discerns the thirteen carboxylic acids in water.

Truxene-Based Hyperbranched Conjugated Polymers: Fluorescent Micelles Detect Explosives in Water

We report two hyperbranched conjugated polymers (HCP) with truxene units as core and 1,4-didodecyl-2,5-diethynylbenzene as well as 1,4-bis(dodecyloxy)-2,5-diethynylbenzene as comonomers. Two analogous poly(para-phenyleneethynylene)s (PPE) are also prepared as comparison to demonstrate the difference between the truxene and the phenyl moieties in their optical properties and their sensing performance. The four polymers are tested for nitroaromatic analytes and display different fluorescence quenching responses. The quenching efficiencies are dependent upon the spectral overlap between the absorbance of the analyte and the emission of the fluorescent polymer. Optical fingerprints are obtained, based on the unique response patterns of the analytes toward the polymers. With this small sensor array, one can distinguish nine nitroaromatic analytes with 100% accuracy. The amphiphilic polymer F127 (a polyethylene glycol-polypropylene glycol block copolymer) carries the hydrophobic HCPs and self-assembles into micelles in water, forming highly fluorescent HCP micelles. The micelle-bound conjugated polymers detect nitroaromatic analytes effectively in water and show an increased sensitivity compared to the sensing of nitroaromatics in organic solvents. The nitroarenes are also discriminated in water using this four-element chemical tongue.

Water-Soluble Poly(p-aryleneethynylene)s: A Sensor Array Discriminates Aromatic Carboxylic Acids

A chemical tongue consisting of 11 elements (four poly(p-aryleneethynylene)s (PAE) at pH 7 and pH 13, and seven electrostatic complexes formed from oppositely charged poly(p-aryleneethynylene)s at pH 7) discriminate 21 benzoic and phenylacetic acid derivatives in aqueous solution. The mechanism of discrimination is the fluorescence modulation of the PAEs, leading to quenching or fluorescence turn-on. The PAEs alone at both pH values and the tongue, consisting of the complexes only, discriminate the 21 acids with 92% (PAEs at pH 7), 95% (PAEs at pH 13), and 99% (complexes at pH 7) reliability after linear discriminant analysis (LDA). A sensor field with all 14 elements, according to LDA, discriminates all of the 21 acids with 100% accuracy.

Synthesis and Biological Properties of Novel Brefeldin A Analogues

New brefeldin A (1) analogues were obtained by introducing a variety of substituents at C15. Most of the analogues exhibited significant biological activity. (15R)-Trifluoromethyl-nor-brefeldin A (3), (15R)-vinyl-nor-brefeldin A (5), their epimers 4 and 6 as well as (15S)-ethyl-nor-brefeldin A (2) were prepared from the key building blocks 12 or 24 by Julia-Kocienski olefination with tetrazolyl sulfones and subsequent macrolactonization. The vinyl derivative 5 allowed analogues to be synthesized by hydroboration and Suzuki-Miyaura coupling. The following biological properties were assessed: (a) inhibition of cell growth of human cancer cells (NCI), (b) induction of morphological changes of the Golgi apparatus of plant and mammalian cells, and (c) influence on the replication of the enterovirus CVB3. Furthermore, conformational aspects were studied by X-ray crystal structure analysis and molecular mechanics calculations, including docking of the analogues into the brefeldin A binding site of an Arf1/Sec7-complex.

Poly(aryleneethynylene) Tongue That Identifies Nonsteroidal Anti-Inflammatory Drugs in Water: A Test Case for Combating Counterfeit Drugs

We report a sensor array composed of a highly fluorescent positively charged poly(para-phenyleneethynylene) P1 and its complex C with a negatively charged pyridine-containing poly(para-aryleneethynylene) P2 (quencher) at pH 10 and pH 13; a sensor field composed of four elements, P1 (pH 10), P1 (pH 13), C (pH 10), and C (pH 13), results. The elements of this small sensor field experience either fluorescence turn on or fluorescence quenching upon exposure toward nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin, ibuprofen, diclofenac, or naproxen. The combined responses of the sensor field are analyzed by linear discriminant analysis (LDA). All of the NSAIDs were identified and discriminated, and the sensing mechanism, hydrophobic versus electrostatic, was discussed.

Polymer/Peptide Complex-Based Sensor Array Discriminates Bacteria in Urine

A negatively charged poly(para-phenyleneethynylene) (PPE) forms electrostatic complexes with four positively charged antimicrobial peptides (AMP). The AMPs partially quench the fluorescence of the PPE and discriminate fourteen different bacteria in water and in human urine by pattern-based fluorescence recognition; the AMP-PPE complexes bind differentially to the components of bacterial surfaces. The bacterial species and strains form clusters according to staining properties (Gram-positive and Gram-negative) or genetic similarity (genus, species, and strain). The identification and data treatment is performed by pattern evaluation with linear discriminant analysis (LDA) of the collected fluorescence intensity data.

Pyridine-based poly(aryleneethynylene)s: a study on anionic side chain density and their influence on optical properties and metallochromicity

We report the Pd-catalyzed synthesis of six new water soluble, alternating poly(p-phenyleneethynylene-p-pyridinyleneethynylene) (abcb-alternating) copolymers and one poly(p-pyridinyleneethynylene). These poly(aryleneethynylene)s (PAEs) have a degree of polymerization Pn of 10 to 16 repeat units, with polydispersities ranging from Mn/Mw = 1.2–2.6. The attachment of two to six carboxylate groups per repeat unit renders the PAEs watersoluble. Six of the seven PAEs share the exact same backbone structure; they are only distinguished by an increasing number of solubilizing carboxylate groups. Optical properties, emissive lifetimes and quantum yields are reported, revealing the influence of anionic side chain density and distribution on the fluorophore. Protons and metal cations affect the optical properties of these PAEs. The emission of aqueous solutions of these PAEs is quenched upon acidification and by both mercury and lead salts. LogKSV values of up to 5.0 for mercury ions and 5.4 for lead ions are reported.