Structural and bonding characteristics in transition metal-silane complexes

Transition metal-silane complexes containing metal-eta 2-H-Si coordination display different structural and bonding characteristics in comparison to other sigma-complexes, such as dihydrogen and alkane (agostic) complexes. The different characteristics can be related to the strong sigma*-accepting properties of the eta 2-silane ligand(s) because of the weaker H-Si sigma bond. Various examples of metal-silane complexes have been reviewed and their structural stabilities have been systematically discussed. Silyl-hydride complexes having substantial silyl-hydrido interactions have also been emphasized.     

Electronic-to-vibrational energy transfer reactions.X* + CO (X = O,I and Br)

The vibrational distribution of CO produced from the following two electronic-to-vibrational energy transfer reactions:

have been determined by means of infrared resonance absorption measurements employing a cw CO laser. The CO molecules formed in both reactions were found to be vibrationally excited up to the limits of available electronic energies carried by the excited atoms. A similar result was also observed in the Br(4²P_{$\text{1}\text{2}$}) + CO reaction, in which absorption occurred only in the 1 → 2 band. For the O^* + CO reaction the efficiency of E → V energy transfer was determined to be 16%. Our present results were found to be inconsistent with the impulsive (half-collision) model.     

Fluorescent sensors for the enantioselective recognition of mandelic acid: signal amplification by dendritic branching

Novel chiral bisbinaphthyl compounds have been synthesized for the enantioselective fluorescent recognition of mandelic acid. By introducing dendritic branches to the chiral receptor unit, the fluorescence signals of the receptors are significantly amplified because of the light-harvesting effect of the dendritic structure. This has greatly increased the sensitivity of the sensors in the fluorescent recognition. Study of the three generation sensors demonstrates that the generation zero sensor is the best choice for the recognition of mandelic acid because of its greatly increased fluorescence signal over the core and its high enantioselectivity. This sensor is potentially useful for the high throughput screening of chiral catalysts for the asymmetric synthesis of alpha-hydroxycarboxylic acids.     

Ultrafast collective dynamics in the charge-density-wave conductor K(0.3)MoO(3)

Low-energy coherent charge-density wave excitations are investigated in blue bronze (K(0.3)MoO(3)) and red bronze (K(0.33)MoO(3)) by femtosecond pump-probe spectroscopy. A linear gapless, acousticlike dispersion relation is observed for the transverse phasons with a pronounced anisotropy in K(0.33)MoO(3). The amplitude mode exhibits a weak (opticlike) dispersion relation with a frequency of 1.67 THz at 30 K. Our results show for the first time that the time-resolved optical technique provides momentum resolution of collective excitations in strongly correlated electron systems.     

Concise synthesis of acyl migration-blocked 1,1-difluorinated analogues of lysophosphatidic acid

Lysophosphatidic acid (LPA, 1- or 2-acyl-sn-glycerol 3-phosphate) is an important phospholipid mediator produced by activated platelets and by ovarian cancer cells. Efforts to understand LPA signaling through G-protein-coupled receptors are hampered by the facile acyl migration that results in equilibration to a mixture of the 1- or 2-acyl species under physiological conditions. We describe a new and efficient route to enantiomerically homogeneous lysophospholipid analogues from D-mannitol 1,2:5,6-bis-acetonide to give two 1,1-difluorodeoxy analogues of (2R)-acyl-sn-glycerol 3-phosphate. These compounds are migration-blocked analogues of the labile sn-2 LPA species. The (19)F NMR of diastereotopic fluorines of the difluoromethyl group shows an unexpected solvent dependence.     

Kinetics of the reaction of C6H5 with HBr

The rate constant for the reaction of phenyl radical with hydrogen bromide has been measured with the cavity-ring-down method at six temperatures between 297 and 523 K. The Arrhenius expression for the H abstraction reaction can be effectively given by: . The values of these parameters are similar to those for the H + HBr reaction, but are in sharp contrast to those for alkyl radical reactions. The gross difference between the alkyl radical reactions and the phenyl and H-atom reactions could be rationalized in terms of the inductive effects of these radicals as measured by Taft's σ* (polar) constants. © 1993 John Wiley & Sons, Inc.     

Synthesis and electrospray ionization mass spectra of AZT/d4T boranophosphates

New anti-HIV prodrugs, conjugates of AZT and d4T with boranophosphates, were prepared by the H-phosphonate method. Their structures were determined by negative ion electrospray ionization mass spectrometry (ESI-MS) in conjunction with tandem mass spectrometry (MS/MS). The fragmentation pathways were investigated, and most of the fragment ions contained the boranophosphate or phosphinate group.     

An investigation of mononuclear and binuclear palladium(II) complexes of egta (egta = glycine,N,N′-(1,2-ethanediylbis(oxy-2,1-ethanediyl)bis[N-carboxymethyl]) by 1H-, 13C- and 15N-n.m.r. methods

1:1 and 2:1 palladium(II) complexes of egta^4– (egta^4– = glycine, N,N-(1,2-ethanediylbis)(oxy-2,1-ethanediyl)bis[N-carboxymethyl]) were prepared by 1:1 and 2:1 addition of K₂PdCl₄ to K₄egta, and examined by ¹H-, ¹³C- and ¹⁵N-n.m.r. methods. The 1:1 complex, [Pd(egta)]^2– in solution, utilizes a square-planar coordination comprised of two nitrogen and two glycinato carboxylate donors of egta^4–, leaving two glycinato carboxylates pendant. The complex has a cis-(R,S) stereochemistry which places both pendant carboxylates below the PdN₂O₂ square plane and the tether backbone of egta^4– in the up, up sense above the same plane. The cis-(R,S) assignment was assisted by computer simulations of the ¹³C-n.m.r. spectrum for four possible isomers. Only cis-(R,S) and trans-(R,R) calculated ¹³C-spectra were compatible with the observed ¹³C-n.m.r. pattern. The HH NOESY spectrum of [Pd(egta)]^2– detects long range coupling of the backbone –OCH₂CH₂O– linkage with both coordinated and pendant glycinato CH₂ moieties. The cis-(R,S) isomer's tortional movements allow such contacts whereas a trans-(R,R) isomer does not. The 2:1 complex, [Pd₂(egta)(H₂O)₂] in solution has an extended-chain structure with each palladium(II) center coordinated in the mer-iminodiacetate-like coordination with two bound glycinato-functionalities.     

Photolysis of Isoniazid

The UV light irradiation of isoniazid (I) in methanol four products, isonicotinic acid (II), isonicotinamide (III), N, N′-bis(isonicotinic acid)hydrazide (IV) and isonicotinaldehyde isonicotinyl hydrazone (V), in ethanol three products, (III), (IV) and acetaldehyde isonicotinyl hydrazone (VI) were isolated and identified. Also, the photoreaction mechanism of isoniazid in methanol and ethanol were discussed.     

Chiral metallacyclophanes: self-assembly, characterization, and application in asymmetric catalysis

[structure: see text] A family of chiral metallacyclophanes has been readily assembled based on robust Pt-acetylide linkage and characterized by a variety of spectroscopic techniques and X-ray crystallography. The steric congestion around the chiral dihydroxy groups in rigid metallacyclophane 4 prevents their reactions with Ti(O(i)()Pr)(4) to form active catalysts for enantioselective diethylzinc additions to aromatic aldehydes. In contrast, chiral dihydroxy groups in more flexible unclosed metallacyclophane 5 are effective ligands for enantioselective catalytic diethylzinc additions to aromatic aldehydes.