Beiträge zur chemie des bors LX zur reaktion von borhalogeniden mit trimethyl-stannyldiphenylphosphin-metall-pentacarbonylen: Methylhalogenstannyldiphenylphosphin-metall-pentacarbonyle

The stannylphosphine-metal pentacarbonyls (CO)₅M·PPh₂SnMe₃ (M = Cr, Mo, W) methylate BCl₃ and yield the monochloro compounds (CO)₅M·PPh₂SnMe₂Cl. However, up to two of the methyl groups are replaced during the reaction with BBr₃ in benzene solution, the products being (CO)₅M·PPh₂SnMe₂Br and (CO)₅M·PPh₂SnMeBr₂.     

Development and test of an L-band magnetron

An L-band relativistic magnetron has been designed, built, and tested at approximately 1.1 GHz. The authors have obtained a peak RMS power of 2.4 GW in one waveguide, and a total of 3.6 GW from two waveguides. The overall efficiency reached 9%, compared with the input power. In contrast with previous S- and X-band magnetron results, the L-band magnetron impedance does not collapse at the end of the pulse, and microwave generation tends to last as long as high voltage is applied. This implies that large total energies can be produced by applying long high-voltage pulses     

Scalar coupling across [C-H···F-C] interactions in (σ-aryl)-chelating post-metallocenes

The nature and importance of C-H···F-C interactions is a topical yet controversial issue, and the development of spectroscopic methods to probe such contacts is therefore warranted. A series of Group 4 bis(benzyl) complexes supported by (σ-aryl)-2-phenolate-6-pyridyl [O,C,N-R(1)] ligands bearing a fluorinated R(1) group (CF(3) or F) in the vicinity of the metal has been prepared. The X-ray crystal structure of the CF(3)-substituted Hf derivative features intramolecular C-H···F-C and Hf···F-C contacts. All complexes have been characterized by multinuclear NMR spectroscopy. The (1)H and (13)C NMR spectra of [M(O,C,N-CF(3))(CH(2)Ph)(2)] derivatives display coupling (assigned to (1h)J(HF) and (2h)J(CF) for Ti; (3)J(HF) and (2)J(CF) (through M···F) for Hf and Zr) between the benzyl CH(2) and CF(3) moieties. [(1)H,(19)F]-HMBC NMR experiments have been performed for the M-[O,C,N-R(1)] complexes and their [O,N,C] counterparts, revealing significant scalar coupling across the C-H···F-C interactions for Ti-[O,C,N] and [O,N,C] species.     

Photoinduced Fe-Cp bond cleavage and insertion reactions of strained silicon- and sulphur-bridged [1]ferrocenophanes in the presence of transition-metal carbonyls

The photochemical reactions of the moderately strained sila[1]ferrocenophane [Fe(eta-C(5)H(4))(2)SiPh(2)] (1) and the highly strained thia[1]ferrocenophane [Fe(eta-C(5)H(4))(2)S] (8) with transition-metal carbonyls ([Fe(CO)(5)], [Fe(2)(CO)(9)] and [Co(2)(CO)(8)]) have been studied. The use of metal carbonyls has allowed the products of photochemically induced Fe-cyclopentadienyl (Cp) bond cleavage reactions in the [1]ferrocenophanes to be trapped as stable, characterisable products. During the course of these studies the synthesis of 8 from [Fe(eta-C(5)H(4)Li)(2)TMEDA] (TMEDA=N,N,N',N'-tetramethylethylenediamine) and S(SO(2)Ph)(2) has been significantly improved by a change of reaction solvent and temperature. Photochemical reaction of 1 with excess [Fe(CO)(5)] in THF gave the dinuclear complex [Fe(2)(CO)(2)(mu-CO)(2)(eta-C(5)H(4))(2)SiPh(2)] (9). The analogous photolytic reaction of 8 with [Fe(CO)(5)] in THF gave cyclic dimer [Fe(eta-C(5)H(4))(2)S](2) (10) and [Fe(2)(CO)(2)(mu-CO)(2)(eta-C(5)H(4))(2)S] (11), with the former being the major product. Photolysis of 1 with [Co(2)(CO)(8)] afforded the remarkable tetrametallic dimer [(CO)(2)Co(eta-C(5)H(4))SiPh(2)(eta-C(5)H(4))Fe(CO)(2)](2) (13). The corresponding photochemical reaction of 8 with [Co(2)(CO)(8)] gave a trimetallic insertion product in high conversion, [Co(CO)(4)(CO)(2)Fe(eta-C(5)H(4))S(eta-C(5)H(4))Co(CO)(2)] (14). These reactivity studies show that UV light promotes Fe-Cp bond cleavage reactions of both of the [1]ferrocenophanes 1 and 8. We have found that, whereas the less strained sila[1]ferrocenophane 1 requires photoactivation for Fe-Cp bond insertions to occur, the highly strained thia[1]ferrocenophane 8 undergoes both irradiative and non-irradiative insertions, although the latter occur at a slower rate. Our results suggest that such photoinduced bond cleavage reactions may be general and applicable to other related strained organometallic rings with pi-hydrocarbon ligands.     

Enantioselective separation in capillary electrophoresis using a novel mono-6-propylammonium-β-cyclodextrin as chiral selector

The chiral resolving ability of a novel single-isomer cationic β-cyclodextrin (CD), mono-6^A-propylammonium-6^A-deoxy-β-cyclodextrin chloride (PrAMCD), as a chiral selector in capillary electrophoresis (CE) is reported in this work for the enantioseparation of hydroxy, carboxylic acids and amphoteric analytes. The effect of chiral selector concentration on the resolution was studied. Good resolutions were achieved for hydroxy acids. Optimum resolutions were obtained even at 3.5 mM CD concentration for carboxylic acids. The electrophoretic method showed good linearity and reproducibility in terms of migration times and peak areas, which should make it suitable for routine analysis. In addition, baseline chiral separation of a six-acid mixture was achieved within 20 min. PrAMCD proved to be an effective chiral selector for acidic analytes.     

Organoclays from alkaline-treated acid-activated clays

The cetyltrimethylammonium hydroxide (C16TMAOH) solution was proposed for the preparation of organoclays. Montmorillonite clay was acid activated at different acid/clay (a/c) (in mass) ratios, then treated with alkaline (sodium hydroxide) solution before being reacted with C16TMAOH solution. The acid activation caused a reduction in the number of cation exchange sites, and hence improved the exfoliation of the silicate sheets at higher pH values. The basal spacing increased significantly from 2.20 to 4.01 nm, and depended on the a/c ratios. The acid-activated clays with a/c ratios greater than 0.3 adsorbed significant amounts of C16TMA cations with a basal spacing of 4.01 nm compared with the non-acid-activated montmorillonite (2.51 nm). Meanwhile, the treatment of NaOH solution yielded clays with similar properties to that of the raw used clay. The XRF data, FT-IR, and ²⁹Si MAS-NMR techniques confirmed that the resulting amorphous silica during the acid activation was dissolved, and accompanied by a dramatical reduction in the surface areas. Similar amounts of C16TMA cations were adsorbed, i.e., close to 1 mmol g^?1, with a single basal spacing of 2.52 nm, independently of the treated acid-activated clays. The in-situ powder XRD studies revealed that an increase of the basal spacing to 4.20 nm was observed at intermediate temperatures ranging from 50 to 150 °C for organo-acid-activated clays with basal spacing of 4.01 nm, while a continuous decrease of the basal spacing was observed for organoclays with a basal spacing of 2.52 nm. At higher temperatures greater than 250 °C, the decomposition of the surfactant occurs, and the basal spacing decreases to a value of about 1.4 nm.     

Concerted Ring Opening and Cycloaddition of Chiral Epoxy Enolsilanes with Dienes

Silyl‐triflate‐catalyzed (4+3) cycloadditions of epoxy enolsilanes with dienes provide a mild and chemoselective synthetic route to seven‐membered carbocycles. Epoxy enolsilanes containing a terminal enolsilane and a single stereocenter undergo cycloaddition with almost complete conservation of enantiomeric purity, a finding that argues against the involvement of oxyallyl cation intermediates which have been previously proposed for these types of reactions. Reported are theoretical and experimental investigations of the cycloaddition mechanism. The major enantiomers of the cycloadducts are derived from S_N2‐like reactions of the silylated epoxide with the diene, in which stereospecific ring opening and formation of the two new C? C bonds occur in a single step. Calculations predict, and experiments confirm, that the observed small losses of enantiomeric purity are traced to a triflate‐mediated double S_N2 cycloaddition pathway.     

Organocatalytic Mitsunobu reactions

A catalytic Mitsunobu reaction system is described in which the azo reagent is used as an organocatalyst and iodosobenzene diacetate is used as the stoichiometric oxidant. In this system, iodosobenzene diacetate oxidizes the formed hydrazine byproduct to regenerate the azo reagent. Yields obtained in the catalytic reactions using a variety of carboxylic acids and alcohols were slightly lower than those obtained from corresponding stoichiometric reactions. Both primary and secondary alcohols can be used as substrates in this reaction system, with the secondary alcohols affording products with inverted stereochemistry at the carbinol center.     

Utilization of the highly environment-sensitive emission properties of rhenium(I) amidodipyridoquinoxaline biotin complexes in the development of biological probes

We report the synthesis and characterization of luminescent rhenium(I) amidodipyridoquinoxaline biotin complexes [Re(CO)3(dpqa)(L)](PF6) (dpqa = 2-(n-butylamido)dipyrido[3,2-f:2',3'-h]quinoxaline; L = 4-(biotinamidomethyl)pyridine (py-4-CH2-NH-biotin) (1), 3-(N-((2-biotinamido)ethyl)amido)pyridine (py-3-CO-NH-en-NH-biotin) (2), 4-(N-((6-biotinamido)hexanoyl)aminomethyl)pyridine (py-4-CH2-NH-cap-NH-biotin) (3)), and their biotin-free counterpart [Re(CO)3(dpqa)(py)](PF6) (py = pyridine (4)). Upon irradiation, these complexes exhibited intense triplet metal-to-ligand charge-transfer (3MLCT) (dpi(Re) --> pi(dpqa)) emission in fluid solutions at 298 K and in alcohol glass at 77 K. However, the emission became much weaker in aqueous buffer, probably due to the interactions of water molecules with the amide substituent of the dpqa ligand. These properties render the complexes good candidates as luminescent probes for hydrophobic media, such as the substrate-binding sites of proteins. The avidin-binding properties of the new biotin complexes have been studied by 4'-hydroxyazobenzene-2-carboxylic acid (HABA) assays, emission titrations, and competitive association and dissociation assays. Most importantly, the complexes showed a profound increase in emission intensities upon binding to avidin. Additionally, we found that the fluorescence of anthracene was quenched by these rhenium(I) complexes, and the 3MLCT emission of the complexes was also quenched by anthracene. On the basis of these findings, new homogeneous assays for biotin using these complexes, avidin, and anthracene-labeled avidin have been designed.