A tetracoordinated rhodium aminyl radical complex

A [16 + 1] valence electron configured rhodium aminyl radical complex could be synthesized and characterized in detail by pulse EPR spectroscopy and DFT calculations. The unpaired electron is delocalized over the metal center and two adjacent nitrogens. H-abstraction reactions from thiols and triethylsilane show that the spin density is predominantly localized on both nitrogens.     

Redox‐Triggered Reversible Interconversion of a Monocyclic and a Bicyclic Phosphorus Heterocycle

Molecules which change their structures significantly and reversibly upon an oxidation or reduction process have potential as future components of smart materials. A prerequisite for such an application is that the molecules should undergo the redox‐coupled transformation within a reasonable electrochemical window and lock into stable redox states. Sodium phosphaethynolate reacts with two equivalents of dicyclohexylcarbodiimide (DCC) to yield an anionic, imino‐functionalized 1,3,5‐diazaphosphinane [ 3 a ]^?. The oxidation of this anion with elemental iodine causes an intramolecular rearrangement reaction to give a bicyclic 1,3,2‐diazaphospholenium cation [ 6 ]⁺. This umpolung of electronic properties from non‐aromatic to highly aromatic is reversible, and the cation [ 6 ] ⁺ is reduced with elemental magnesium to reform the 1,3,5‐diazaphosphinanide anion [ 3 a ]^?. Theoretical calculations suggest that phosphinidene species are involved in the rearrangement processes.     

Approaches to Carbocyclic Sialidase Inhibitors

Summary.  The crucial role played by carbohydrates in many physiological processes has made this class of compounds an interesting target for drug design. Consequently mimicking carbohydrates has been one of the most rapidly growing fields in synthetic organic chemistry in recent years, and particularly intense focus has been devoted to sialic acids and sialic acid metabolizing enzymes, including sialidases. Inhibition of the latter enzyme from influenza virus can be regarded as one of the most successful examples of structure-based drug design and high affinity inhibitors based on neuraminic acid have been developed. There is an ongoing search for inhibitors with improved physicochemical properties and among them, carbocyclic systems, where the ring oxygen of the carbohydrate is replaced by carbon, have become the center of interest. This review intends to give a brief overview over the structures and synthetic approaches which surfaced in the last decade. E-mail: hansjoerg.streicher@uni-konstanz.de Received June 17, 2002; accepted June 21, 2002     

Gaschromatographische Untersuchung der Adsorptionseigenschaften von Silicalit und ZSM-5

High silica molecular sieves (silicalite, ZSM-5) were tested as adsorbents for gas chromatographic trace analysis. Therefore the retention behaviour of low-boiling organic compounds (hydrocarbons, halogenated hydrocarbons, amines, alcohols and ethers) on these materials was investigated. The specific retention volumes at different temperatures have been determined and elution orders and peak shapes were studied. The retention data allow a simple calculation of the breakthrough volumes (dynamic adsorption capacity) and the chromatographic characterisation of the adsorbents. Both nitrogen and oxygen containing compounds could not—or at least unreproducibely—be eluated up to 300°. The elution order and the peak shape of compounds with the same number of carbon atoms but different geometric and electronic structure (e.g.n-butenes;n-hexane, cyclohexane, benzene) can be explained by the action of exclusion effects and different diffusion barriers. On the basis of calculated breakthrough volumes we conclude that silicalite should be useful in the preconcentration of both saturated C₃–C₆ hydrocarbons and C₁–C₂ chlorinated hydrocarbons from gaseous streams.     

Olefins as steering ligands for homogeneously catalyzed hydrogenations

Iridium(I) complexes containing a (5H-dibenzo[a,d]cyclohepten-5-yl)-phosphane (tropp(R); R = phosphorus-bound substituent = Ph, Cyc) as a rigid, concave-shaped, mixed phosphane olefin ligand were prepared and tested as catalyst precursors in the hydrogenation of imines. With the complex [Ir(tropp(Cyc))(cod)]OTf, turnover frequencies (TOFs) of >6000 h(-1) were reached in the hydrogenation of N-phenyl-benzylidenamine, PhN==CHPh. Lower activities (TOF>80 h(-1)) are observed with N-phenyl-(1-phenylethylidene)amine, PhN==CMePh. Chiral tropp-type ligands were prepared in few simple steps. Monosubstitution of the olefinic unit in the dibenzo[a,d]cycloheptenyl moiety with (R)- or (S)-mentholate gave mixtures of diastereomers that could be separated and isolated in enantiomerically pure form. Iridium(I) complexes with these ligands are rare examples of side-on bonded enolether complexes. In catalytic imine hydrogenations, complete conversion (>98 %) was reached in all cases (conditions: p[H(2)] = 50 bar, T = 50 degrees C, t = 2 h, substrate/catalyst 100:1). The best enantiomeric excess (ee = 86 % S isomer) was reached with PhN==CMePh as substrate and the R,R form of the (10-menthyloxy-5H-dibenzo[a,d]cyclohepten-5-yl)diphenylphosphane ligand. The iridium(I) complex containing the same phosphane gave a 60 % ee (S isomer) with the enamide N-(1-phenylvinyl)acetamide as substrate (conditions: p[H(2)] = 4 bar, T = 50 degrees C, t = 18 h, substrate/catalyst = 50:1). These reactions constitute the first examples in which chiral olefins have been used as steering ligands in catalytic enantioselective hydrogenations.