An ab initio study of the geometry and rotational barrier of 4-phenylimidazole

The molecular design of several synthetic artificial enzymes, which mimic the action of the serine protease-chymotrypsin, incorporates the phenylimidazole molecular fragment to play the role of the His-57 residue in the native enzyme active site. Study of these artificial enzymes by molecular modeling techniques requires accurate torsional force field parameters for the phenylimidazole interring bond. This, in turn, requires accurate characterization of the barrier to rotation around this bond. Previous semiempirical calculations of this rotational barrier have neglected geometry optimization of the molecule at the points along the rotational pathway. The 4-phenylimidazole rotational barrier (5.6 kcal mol^–1] presented here was obtained by full ab initio geometry optimization at the 3–21G level at each of the points along the rotational pathway.     

Nitrogen Donors in Organometallic Chemistry and Homogeneous Catalysis

Homogeneous catalysis has been responsible for many major recent developments in synthetic organic chemistry. The combined use of organometallic and coordination chemistry has produced a number of new and powerful synthetic methods for important classes of compounds in general and for optically active substances in particular. For this purpose, complexes with optically active ligands have been used, most of them coordinating through phosphorus. More recent developments have highlighted the use of “nitrogen-donors”, particularly as they are easily obtained from the “chiral pool”. However, the remarkable achievements in this area have been based on an empirical approach. This article attempts to bridge the gap between the synthetic and the coordination chemist. The first section discusses the rates of formation and dissociation of complexes with nitrogen donors, their relationship to the rates of product formation, and presents the factors which induce homolytic cleavage of M? C bonds. It also provides a summary of the main types of organometallic complexes formed by metal centers coordinated to nitrogen donors and their reactivity patterns. The second section highlights the most significant, homogeneously catalyzed reactions involving complexes with nitrogen ligands. Foremost among them are the asymmetric aspects of hydrogenation (particularly those involving boranes as reducing agents), hydrosilylation, cyclopropanations, Diels-Alder reactions, aldol condensations, alkylation of aldehydes, conjugate addition reactions, Grignard cross-coupling reactions, allylic alkylations, oxidation reactions, olefin epoxidations, and di-hydroxylation of olefins.     

Transition-Metal Complexes with Bidentate Ligands Spanning trans-Positions. Part XVII. Some platinum(II) complexes with anionic ligands such as methoxide,formate, carboxylate,hydride, and borohydride,and the X-ray crystal structure of [PtHCl( )] (PP = 2,11-bis(diphenylphosphinomethyl)benzo[c]phenanthrene)

The complexes trans-[PtXY( 2 ] (X = H or Me; Y = OMe, OCHO, CO₂H, and BH₄; 2 = 2,11-bis{bis[3-(trifluoromethyl)phenyl]phosphinomethyl}benzo[c]phenanthrene) were prepared, and their decompositions to trans[PtHX( 2 )] were studied. Some binuclear hydrido-bridged complexes, e.g.[( 2 )HPt(μ-H)PtH( 2 )]⁺, were also obtained. The preparation of complexes trans-[PtHX( 28 )₂] (X = H or Me, 28 = bis[3-(trifluoromethyl)phenyl]benzylphosphine) is also reported. The X-ray crystal structure of trans-[PtHCl 1 )] ( 1 = 2,11-bis(diphenylphosphinomethyl)benzo[c]phenanthrene) was carried out.     

The preparation of cis- and trans-[PtH(C6Cl5)(PEt3)2 and a study of the “hydride-donor capacity” of the complexes trans- [PtH(C6X5(PEt3)2] (X = F and Cl)

The preparations of cis- and trans-[PtH(C₆Cl₅)(PEt₃)₂] by thermal decomposition of cis- and trans-[Pt(OCHO)(C₆Cl₅)(PEt₃)₂], respectively, are reported. Also described are cis- and trans-[Pt(SnCl₃)(C₆Cl₅)(PEt₃)₂], obtained by treating SnCl₂ with cis- and trans-[PtCl(C₆,Cl₅)(PEt₃)₂], respectively. It is shown that while trans- [PtH(C₆Cl₅)(PEt₃)₂] does not form hydride-bridged complexes in the presence of trans-(PtH(MeOH)(PEt₃)₂]⁺, the corresponding complex trans-[PtH(C₆)(PEt₃)₂] reacts with the same solvento complex, in methanol, giving labile [(PEt₃)₂HPt(-μH)Pt(C₆F₅)(PEt₃)₂]⁺.     

High-Yield Syntheses of Sodium,Potassium, and Thallium Hydrotris[3,5-bis(trifluoromethyl)pyrazolyl]borates and the X-ray crystal structure of {hydrotris[3,5-bis(trifluoromethyl)pyrazolyl]borato} thallium(I)

An improved synthesis of 3,5-bis(trifluoromethyl)pyrazole ( 1 ) is described. This compound was used for the high-yield syntheses of the tris(pyrazoly1)borates Nd[HB(3,5-(CF₃),-pz)₃] ( 2a ) and the corresponding potassium salt, 2b , starting from 1 and NaBH₄ and KBH₄, respectively. A convenient route to the corresponding thallium(I) salt, 2c , using thallium(I) acetate and either 2a or 2b in CHCI₃, is also described. The sodium ( 3a ), potassium ( 3b ), and thallium ( 3c ) salts of bis(pyrazolyl)borate [H₂B(3,5-(CF₃)₂-pz)₂]^? were also prepared. The above pyrazolylborates were characterized by ¹H-, ¹³C-, ¹⁹F-, and ¹¹B-NMR spectroscopy. The X-ray crystal structure of the thallium derivative 2c was determined. The compound crystallizes in the monoclinic space group P2₁/m with a = 8.248(9) Å, b = 15.034(12) Å c = 9.243(8) Å, β = 100.10(7)°, Z = 2. The Tl-atom adopts a pyramidal geometry with respect to the three N-atoms. However, two TI–N distances (2.725(7) Å) are longer than the third (2.675(10) Å).     

Supramolecular chirality control by solvent changes. Solvodichroic effect on chiral porphyrin aggregation

The mode of aggregation of amphiphilised porphyrin derivatives bearing a chiral functionality is strongly affected by solvent composition; this results in the tuning of the supramolecular chirality of the porphyrin aggregates.     

1H and 13C NMR Structural Studies of Amiloride in Cryosolvents

Amiloride hydrochloride is a potent inhibitor of several cellular ion transport systems and has been used as a diuretic drug and a probe of ion transport processes. ¹H and ¹³C NMR studies have been carried out at a proton field strength of 400 MHz in DMSO/CH₂Cl₂ and DMF cryosolvents over a temperature range from 20 to –55°C in order to investigate the conformations of the hydrochloride and its free-base form in solution. Resonances for all protons have been assigned. The results of double resonance and NOE experiments suggest that the structures in solution are most likely planar and consistent with previously reported computational results.     

Transition Metal Complexes with Bidentate Ligands Spanning trans-Positions. XII. Steric effects in the kinetics and mechanism of substitutions at hydride and methyl bisphosphine platinum (II) complexes

Ligand substitution reactions on square-planar platinum (II) complexes of the types trans-[PtRXL₂], trans-[PtR(4-PADA)L₂][BF₄], trans-[PtRX( ) and trans-[PtR(4-PADA)( )][BF₄] R=H, Me; X=Cl^?, I^?; L=PEt₃, bis(3-trifluoromethyl-phenyl)benzylphosphine ( 4 ), =the trans-spanning 2,11-bis[bis(3-trifluoro-methylphenyl)phosphinomethyl]benzo [c]phenanthrene ( 3 ); 4-PADA (=pyridine-4-azo-4′-(N, N-dimethyl)aniline have been studied at 30° using stopped-flow and conventional spectrophotometry, methanol as solvent, and 2.5 × 10^?2 M ionic strength (LiClO₄ as supporting electrolyte). 4-PADA was used as indicator ligand, as its absorption spectrum differs from those spectra where it is complexed. The expected steric effects of the bulky ligands, especially of 3 , on the rates and mechanisms of all the reactions studied are small. All reactions take place by the usual two-term rate law. Noteworthy, for the complexes with the bulky ligands 3 and 4 , the direct reaction path with the entering nucleophile is predominant. There is no preference for a solvent or dissociative path. The reactivity order for the hydride complexes is trans-[PtHX (PEt₃)₂]<trans-[PtHX( 4 )₂]<trans-[PtHX ( 3 )]. However, for the corresponding methyl complexes, there is some retardation by ligand 3 , probably due to an interaction between the methyl group and the hydrocarbon moiety of 3 , which inhibits the fluxional behavior of this ligand. The results have some relevance for the understanding of olefin-insertion reactions of hydride complexes containing these phosphine ligands.     

Bianchi I cosmology in the presence of a causally regularized viscous fluid

We analyze the dynamics of a Bianchi I cosmology in the presence of a viscous fluid, causally regularized according to the Lichnerowicz approach. We show how the effect induced by shear viscosity is still able to produce a matter creation phenomenon, meaning that also in the regularized theory we address, the Universe is emerging from a singularity with a vanishing energy density value. We discuss the structure of the singularity in the isotropic limit, when bulk viscosity is the only retained contribution. We see that, as far as viscosity is not a dominant effect, the dynamics of the isotropic Universe possesses the usual non-viscous power-law behaviour but in correspondence to an effective equation of state, depending on the bulk viscosity coefficient. Finally, we show that, in the limit of a strong non-thermodynamical equilibrium of the Universe mimicked by a dominant contribution of the effective viscous pressure, a power-law inflation behaviour of the Universe appears, the cosmological horizons are removed and a significant amount of entropy is produced.