A wide variety of monomeric and oligomeric, donor-substituted 1,1,4,4-tetracyanobutadienes (TCBDs) have been synthesized by [2+2] cycloaddition between tetracyanoethylene (TNCE) and donor-substituted alkynes, followed by electrocyclic ring opening of the initially formed cyclobutenes. Reaction yields are often nearly quantitative but can be affected by the electron-donating power and steric demands of the alkyne substituents. The intramolecular charge-transfer (CT) interactions between the donor and TCBD acceptor moieties were comprehensively investigated by X-ray crystallography, electrochemistry, UV-visible spectroscopy, and theoretical calculations. Despite the nonplanarity of the new chromophores, which have a substantial twist between the two dicyanovinyl planes, efficient intramolecular CT interactions are observed, and the crystal structures demonstrate a high quinoid character in strong donor substituents, such as N,N-dimethylanilino (DMA) rings. The maxima of the CT bands shift bathochromically upon reduction of the amount of conjugative coupling between strong donor and acceptor moieties. Each TCBD moiety undergoes two reversible, one-electron reduction steps. Thus, a tri-TCBD derivative with a 1,3,5-trisubstituted benzene core shows six reversible reduction steps within an exceptionally narrow potential range of 1.0 V. The first reduction potential E(red,1) is strongly influenced by the donor substitution: introduction of more donor moieties causes an increasingly twisted TCBD structure, a fact that results in the elevation of the LUMO level and, consequently, a more difficult first reduction. The potentials are also strongly influenced by the nature of the donor residues and the extent of donor-acceptor coupling. A careful comparison of electrochemical data and the correlation with UV-visible spectra made it possible to estimate unknown physical parameters such as the E(red,1) of unsubstituted TCBD (-0.31 V vs Fc+/Fc) as well as the maxima of highly broadened CT bands. Donor-substituted TCBDs are stable molecules and can be sublimed without decomposition. With their high third-order optical nonlinearities, as revealed in preliminary measurements, they should become interesting chromophores for ultra-thin film formation by vapor deposition techniques and have applications in opto-electronic devices.

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The kinetics and the mechanism of the selective reduction of nitric oxides (NO x ) by hydrogen is studied on a trimetallic Pt± Mo±Co/-Al 2 O 3 catalyst under oxidising conditions. This system is interesting in view of an exhaust gas control of power plants or lean-burn cars. It can be shown that the NO dissociation is the crucial step, dominating the overall reaction behaviour and that it depends on temperature and on the partial pressure of H 2 . With increasing temperatures the reaction reveals an autocatalytic behaviour resulting in bistability and hysteresis. At higher temperatures, where no bistability is found, the NO/H 2 as well as the competing O 2 /H 2 reaction occur only above a certain critical partial pressure of H 2 . The kinetics of the NO/H 2 / O 2 reaction are established using a modi®ed Langmuir±Hinshelwood model (T1428C±1608C, y O2 >4%) which takes into account the critical H 2 partial pressure. The model describes the experimental data within AE15%. The determined activation energies are: 63 kJ/mol for the NO x consumption, 77 and 45 kJ/mol for the N 2 and N 2 O formation, respectively, and 130 kJ/ mol for the O 2 /H 2 reaction. Adsorption enthalpies are determined to À59 kJ/mol for the adsorption of H 2 , À77 kJ/mol for the adsorption of NO and À97 kJ/mol for the adsorption of O 2 . An interesting feature of the reaction is the enhancement of the NO/H 2 reaction by oxygen for low partial pressures of O 2 . This appears to be the ®rst study where a promoting effect of oxygen for the NO/H 2 reaction is found on a platinum supported catalyst. #

We report the synthesis and properties of two series of homologous donor-acceptor (D-A) chromophores in which N, N-dimethylanilino (DMA) or N,N-dihexylanilino (DHA) donors and dicyanovinyl acceptors are separated by up to four CϵC triple-bond spacers or up to three C=C doublebond spacers. The intramolecular charge-transfer (CT) interactions of the new D-A oligoynes and the known all-trans D-A oligoenes were investigated by X-ray crystallography, electrochemistry, UV/Vis spectroscopy, and theoretical calcu-

Synthesis. -The synthesis of the title compounds is based on a [2 + 2] cycloaddition followed by electrocyclic ring opening of the cyclobutene intermediate. -(MICHINOBU, T.; BOUDON, C.; GISSELBRECHT, J.-P.; SEILER, P.; FRANK, B.; MOONEN, N. N. P.; GROSS, M.; DIEDERICH*, F.; Chem. Eur. J. 12 (2006) 7, 1889-1905; Lab. Org. Chem., ETH-Hoenggerberg, CH-8093 Zuerich, Switz.; Eng.) -Nuesgen 25-095

A new class of nonpeptidic inhibitors of the Zn II -dependent metalloprotease neprilysin with IC 50 values in the nanomolar activity range (0.034 ± 0.30 mm) were developed based on structure-based de novo design (Figs. 1 and 2). The inhibitors feature benzimidazole and imidazo [4,5-c]pyridine moieties as central scaffolds to undergo H-bonding to Asn542 and Arg717 and to engage in favorable p-p stacking interactions with the imidazole ring of His711. The platform is decorated with a thiol vector to coordinate to the Zn II ion and an aryl residue to occupy the hydrophobic S1' pocket, but lack a substituent for binding in the S2' pocket, which remains closed by the side chains of Phe106 and Arg110 when not occupied. The enantioselective syntheses of the active compounds ()-1, ()-2, ()-25, and ()-26 were accomplished using Evans auxiliaries (Schemes 2, 4, and 5). The inhibitors ()-2 and ()-26 with an imidazo[4,5-c]pyridine core are ca. 8 times more active than those with a benzimidazole core (()-1 and ()-25) ( Table 1). The predicted binding mode was established by X-ray analysis of the complex of neprilysin with ()-2 at 2.25-resolution (Fig. 4 and Table 2). The ligand coordinates with its sulfanyl residue to the Zn II ion, and the benzyl residue occupies the S1' pocket. The 1H-imidazole moiety of the central scaffold forms the required H-bonds to the side chains of Asn542 and Arg717. The heterobicyclic platform additionally undergoes p-p stacking with the side chain of His711 as well as edge-to-face-type interactions with the side chain of Trp693. According to the X-ray analysis, the substantial advantage in biological activity of the imidazopyridine inhibitors over the benzimidazole ligands arises from favorable interactions of the pyridine N-atom in the former with the side chain of Arg102. Unexpectedly, replacement of the phenyl group pointing into the deep S1' pocket by a biphenyl group does not enhance the binding affinity for this class of inhibitors.1. Introduction. ± In the preceding paper [1], we described a new class of inhibitors of the metalloprotease neprilysin with a central 1H-imidazole platform, featuring IC 50 values (IC 50 : concentration of inhibitor at which 50% V max is observed) in the low micromolar range. The de novo design of these compounds was based on the X-ray crystal structure of NEP complexed with phosphoramidon (Protein Data Bank (PDB) file name 1DMT) [2]. For the design of the second-generation inhibitors, we reverted to an unpublished X-ray crystal structure [3] of NEP complexed with the inhibitor thiorphan [4]. During the analysis, we carefully compared the active sites of the two

Chiral and achiral push-pull chromophores have been prepared by cascades of sequential [2+2] cycloadditions of tetracyanoethene (TCNE) and tetrathiafulvalene (TTF) to different oligoynes. Thermal [2+2] cycloaddition of TCNE to donor-substituted alkynes, followed by electrocyclic ring-opening of the initially formed cyclobutenes, affords donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs). Similarly, TTF reacts with electron-deficient CϵC bonds to give the corresponding buta-1,3-diene derivatives, 1,2-bis(1,3-dithiol-2-ylidene)ethanes. Thus, achiral [AB]-type oligomers were synthesized from N,N-dialkylanilino (DAA)-substituted tetraynes and hexaynes and chiral [AB]-type oligomers from alkyne-substituted 1,1Ј-binaphthalenes. The [AB]-type oligomers exhibit complex conformational equilibria in solution, as revealed by 1 H and 13 C NMR spectroscopy. Therefore, the circular dichroism (CD) spectra of the chiral [AB]-type oligo-

A series of monomeric and oligomeric donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) with various topologies have been synthesized by means of thermal [2+2] cycloaddition between tetracyanoethylene (TCNE) and donor-substituted alkynes, followed by retro-electrocyclization. One-electron-reduced and -oxidized stages of the donor-substituted TCBDs were generated by chemical methods. The obtained radical anions and radical cations were studied by using electron paramagnetic resonance/electron nuclear double resonance (EPR/ENDOR) spectroscopy, supported by density functional theory (DFT) calculations. The extent of pi-electron delocalization in the paramagnetic species was investigated in terms of the EPR parameters. Despite favorable molecular orbital (MO) coefficients, the EPR results suggest that in radical anions the spin and charge are confined to the electron-withdrawing TCBD moieties on the hyperfine EPR timescale. The observed spin localization is presumably caused by an interplay between the nonplanarity of the studied pi systems, limited pi-electron conjugation, and very likely counterion effects. In radical cations, an analogous spin and charge localization confined to the electron-donating N,N-dialkylaniline moieties was found. In this case, an efficient electron delocalization is disabled by small MO coefficients at the joints between the donor and acceptor portions of the studied TCBDs.