Pre´paration chimique et structure cristalline de l'iodate d'argent AgIO3

A new chemical preparation of silver iodate AgIO₃ is given. The crystal structure has been determined by single-crystal X-ray diffraction methods and refined to a finalR value of 0.051 for 1280 independent reflections. The unit cell is orthorhombic witha = 7.265(2), b = 15.17(4), c = 5.786(2)A?,Z = 8. The space group isPbc2₁. IO₃ groups and AgO₆ octaedra form a compact bidimensional framework perpendicular to theb axis.     

Reactions of (Z)-3-aryl-3-chloropropenals with nucleophiles: stereoselective formation of (E)-vinylogous esters, (E)-vinylogous amides, and vinamidinium salts

The highly stereoselective conversions of (Z)-3-aryl-3-chloropropenals to (E)-3-alkoxy-3-arylpropenals, to (E)-3-aryl-3-morphorlinopropenals, and to vinamidinium salts are reported. The stereochemical assignments were based on 2D-NMR experiments.     

Reactivite des carbanions benzyliques : II. Stereochimie d'alcoylation des ethyl et isopropyl-9 lithio-10 dihydro-9,10 anthracenes

A detailed study of the stereochemistry of alkylation of 9-ethyl- and 9-isopropyl-10-lithio-9,10-dihydroanthracenes by alkyl halides (R′ = Me, Et, i-Pr) is reported. The reaction is under kinetic control. cis-9-Ethyl-10-isopropyl-9,10-dihydroanthracene has been found more stable than the trans isomer. If we take into account the stereoelectronic factor, which favors axial attack, and the main steric interactions in the transition state, stereochemistry seems to be mainly controlled by the competition between axial-axial and peri interactions.     

Low-coordinate iron(II) amido complexes of beta-diketiminates: synthesis, structure, and reactivity

The synthesis, structure, and reactivity of a series of low-coordinate Fe(II) diketiminate amido complexes are presented. Complexes L(R)FeNHAr (R = methyl, tert-butyl; Ar = para-tolyl, 2,6-xylyl, and 2,6-diisopropylphenyl) bind Lewis bases to give trigonal pyramidal and trigonal bipyramidal adducts. In the adducts, crystallographic and (1)H NMR evidence supports the existence of agostic interactions in solid and solution states. Complexes L(R)FeNHAr may be oxidized using AgOTf, and the products L(R)Fe(NHAr)(OTf) are characterized with (19)F NMR spectroscopy, UV/vis spectrophotometry, solution magnetic measurements, elemental analysis, and, in one case, X-ray crystallography. In the structures of the iron(III) complexes L(R)Fe(NHAr)(OTf) and L(R)Fe(OtBu)(OTf), the angles at nitrogen and oxygen result from steric effects and not pi-bonding. The reactions of the amido group of L(R)FeNHAr with weak acids (HCCPh and HOtBu) are consistent with a basic nitrogen atom, because the amido group is protonated by terminal alkynes and alcohols to give free H(2)NAr and three-coordinate acetylide and alkoxide complexes. The trends in complex stability give insight into the relative strength of bonds from three-coordinate iron to anionic C-, N-, and O-donor ligands.     

Synthesis and characterization of platinum diimine bis(acetylide) complexes containing easily derivatizable aryl acetylide ligands

New Pt(II) diimine bis(acetylide) complexes where the diimine is a substituted bipyridine or phenanthroline and the arylacetylide is 4-ethynylbenzaldehyde have been prepared in good to excellent yields. Spectroscopic characterization supports a square planar coordination geometry with cis-alkynyl ligands, and the crystal structure of one of the complexes, Pt(phen)(Ctbd1;CC(6)H(4)CHO)(2) (1), confirms the assignment. The new diimine bis(acetylide) complexes exhibit an absorption band ca. 400 nm that corresponds to a Pt(d) --> pi diimine charge transfer transition and are brightly emissive in fluid solution, with excited state lifetimes in the range 100-800 ns. Correlation of diimine substituent with lambda(max) for the 400 nm absorption band gives strong support to the MLCT assignment. Complex 1 undergoes electron transfer quenching, showing good Stern-Volmer behavior with a variety of oxidative and reductive quenchers. Quenching studies conducted with DNA nucleosides (A, T, C, G) were also investigated. Silyl-protected adenosine and guanosine were found to quench the luminescence of 1 better than similarly protected cytidine or thymidine. Since the former are the more easily oxidized bases, the results suggest that the Pt(II) diimine bis(acetylide) complexes are more powerful photooxidants than photoreductants with regard to electron transfer to DNA bases.     

Surface Raman characterization of cinchonidine-modified polycrystalline platinum in ethanol: effects of temperature and comparison with 10,11-dihydrocinchonidine

The adsorption of the chiral modifier cinchonidine on platinum in ethanol as a function of temperature has been studied with surface-enhanced Raman spectroscopy (SERS). The temperature range chosen was from 30 to 70 °C, within which both the activity and selectivity of cinchonidine-modified Pt catalysts have been shown to change dramatically. Platinum surfaces were modified with 260 μM cinchonidine in ethanol, and examined both in pure ethanol and in the modifying solution itself. Adsorbed cinchonidine under pure ethanol was found to partially desorb as the temperature was raised, accompanied by an increase in the average tilt of the quinoline group with respect to the surface. In contrast, the presence of solution-phase cinchonidine resulted in an increase in the cinchonidine surface coverage and average tilt as temperature was raised. In a previous study [J. Mol. Catal. A 212 (2004) 277] we showed that hydrogen causes a dramatic enhancement in the SERS response of adsorbed cinchonidine. This was attributed to a conversion of cinchonidine to 10,11-dihydrocinchonidine on the Pt surface and a more flat orientation of the quinoline group. In both pure ethanol and in 260 μM cinchonidine, the presence of hydrogen causes a significant decrease in the alkaloid SERS bands at temperatures above 40 °C. In addition, the average tilt of the quinoline group increases significantly at these elevated temperatures. The temperature-dependence of 10,11-dihydrocinchonidine adsorption was also investigated, and is almost identical to that observed for cinchonidine in the presence of hydrogen. This lends further support to the conclusion that cinchonidine is being hydrogenated on the Pt surface in the presence of hydrogen. The significant changes observed on the cinchonidine-modified Pt surface above 40 °C correlate well with reported decreases in enantioselectivity and turn-over frequency at similar temperatures during ethyl pyruvate hydrogenation.     

High-speed imaging of the ignition of ethanol at engine relevant conditions in a rapid compression machine

The rapid compression machine (RCM) is a great tool for investigating fuel properties under engine relevant conditions (high-pressures, low temperatures). The most common diagnostics is measuring the pressure over time and determining the ignition delay time (IDT). In this study, for the first time, the OH* luminescence of ethanol/air mixture is measured within an RCM experiment at 15 and 20?bar for Φ?=?0.5. Combining the common pressure measurements with the simultaneously recorded high-speed images (up to 74.5?kHz framerate) gives a first insight into understanding the ignition modes and the corresponding pressure traces. At 74.5?kHz, in contrast to findings in literature, the ethanol ignition did not show to be purely homogeneous. Four different propagating fronts of OH* luminescence have been recorded. Besides a flame kernel and a detonation-like ignition front two further fronts prior to main ignition have been observed. The propagating speeds of the fronts have been determined and depend on the overall IDT.     

Multi C−H Functionalization Reactions of Carbazole Heterocycles via Gold-Catalyzed Carbene Transfer Reactions

Herein we describe a multiple C−H functionalization reaction of carbazole heterocycles with diazoalkanes. We show that gold catalysts play a distinct role in enabling a multiple C−H functionalization reaction to introduce up to six carbene fragments onto molecules containing multiple carbazole units or to link multiple carbazole units into a single molecule. A one-pot stepwise approach enables the introduction of two different carbene fragments to allow orthogonal deprotection and straightforward derivatization.     

Rhodium-Catalyzed C−H Methylation and Alkylation Reactions by Carbene-Transfer Reactions

In this combined computational and experimental study, the C−H functionalization of 2-phenyl pyridine with diazoalkanes was investigated. Initial evaluation by computational methods allowed the evaluation of different metal catalysts and diazoalkanes and their compatibility in this C−H functionalization reaction. With these findings, suitable reaction conditions for the C−H methylation reactions were quickly identified by using highly reactive TMS diazomethane and C−H alkylation reactions with donor/acceptor diazoalkanes, which is applied to a broad scope on alkylation reactions of 2-aryl pyridines with TMS diazomethane and donor/acceptor diazoalkane (51 examples, up to 98 % yield).