No abstract

During the past 10 years iron-catalyzed reactions have become established in the field of organic synthesis. For example, the complex anion [Fe(CO)3 (NO)](-) , which was originally described by Hogsed and Hieber, shows catalytic activity in various organic reactions. This anion is commonly regarded as being isoelectronic with [Fe(CO)4 ](2-) , which, however, shows poor catalytic activity. The spectroscopic and quantum chemical investigations presented herein reveal that the complex ferrate [Fe(CO)3 (NO)](-) cannot be regarded as a Fe(-II) species, but rather is predominantly a Fe(0) species, in which the metal is covalently bonded to NO(-) by two π-bonds. A metal-N σ-bond is not observed.

A density functional for dynamical correlation, to be used in connection with wavefunctions of the complete-active-space self-consistent ® eld type, is described, and ® rst applications to the series of two-and four-electron atomic ions as well as to the H2 potential curve are given. IntroductionDensity functional theory (DFT) has been very successful for both an economic and an accurate description of electronic structure in atomic, molecular and solid-state physics (for example [1]); electron correlation e ects are implicitly included while formally retaining an independent-particle Hartree± Fock (HF)-like picture. The HF picture, on the other hand, is just the starting point for an explicit electron correlation treatment with the wavefunction-based quantum-chemical ab initio methods (for example [2]); in contrast with DFT, a systematic improvement towards the exact results is possible, but at the expense of a heavy computational burden caused by long con® guration interaction (CI) expansions. However, long CI expansions are usually related to dynamical correlation e ects which have to do with the short-range description of the electron correlation hole (electron± electron cusp), while static (longrange) correlation e ects caused by near-degeneracies of ground and low-lying excited states of the same symmetry very often can e ciently be covered by quite short optimized expansions (of the multicon® guration self-consistent ® eld (MCSCF) or complete-active-space self-consistent ® eld (CASSCF) type). Since de® ciencies of DFT, if any, are likely to occur just with the description of near-degeneracies which are special to individual molecules and not easily covered by a universal functional, the idea of a CI± DFT coupling combining the advantages of the two approaches seems to be rewarding.

The crystal structures of hydroxynitrile lyase from Manihot esculenta (MeHNL) complexed with the native substrate acetone and substrate analogue chloroacetone have been determined and re®ned at 2.2 A Ê resolution. The substrates are positioned in the active site by hydrogen-bond interactions of the carbonyl O atom with Thr11 OG, Ser80 OG and, to a lesser extent, Cys81 SG. These studies support a mechanism for cyanogenesis as well as for the stereospeci®c MeHNLcatalyzed formation of (S)-cyanohydrins, which closely resembles the base-catalyzed chemical reaction of HCN with carbonyl compounds.

Novel columnar liquid crystals 5a–f bearing a dibenzo[18]crown[6] central unit have been prepared from bromodialkyloxybenzenes 1 and tetrabromodibenzo[18]crown[6] 2 via Suzuki coupling reactions. Differential scanning calorimetry (DSC) revealed that the length of the alkyl chains in the liquid‐crystalline compounds 5 influenced their mesomorphic properties. Whereas 5a–d gave fan‐shaped textures only upon rapid cooling, 5e and 5f with C9 and C10 side chains formed stable columnar textures upon slow cooling. A hexagonal columnar mesophase of 5a is apparent from X‐ray diffraction studies. Complexation of 5 with potassium significantly shifted the clearing points to higher temperatures and also increased the mesophase stability in the cooling cycle.

The structure and function of hydroxynitrile lyase from Manihot esculenta (MeHNL) have been analyzed by X-ray crystallography and site-directed mutagenesis. The crystal structure of the MeHNL-S80A mutant enzyme has been refined to an R-factor of 18.0% against diffraction data to 2.1-Å resolution. The threedimensional structure of the MeHNL-S80A-acetone cyanohydrin complex was determined at 2.2-Å resolution and refined to an R-factor of 18.7%. Thr11 and Cys81 involved in substrate binding have been substituted by Ala in site-directed mutagenesis. The kinetic measurements of these mutant enzymes are presented. Combined with structural data, the results support a mechanism for cyanogenesis in which His236 as a general base abstracts a proton from Ser80, thereby allowing proton transfer from the hydroxyl group of acetone cyanohydrin to Ser80. The His236 imidazolium cation then facilitates the leaving of the nitrile group by proton donating.

The effect of different substituents, such as bromo, chloromethyl, hydroxymethyl, formyl, acetyl, carboxy, and acylated hydroxymethyl and ammonium groups, on the furan ring of substrates in gold-catalyzed phenol synthesis has been investigated. The furan ring was also replaced by different heterocycles, such as pyrroles, thiophenes, oxazoles, and a 2,4-dimethoxyphenyl group; gold catalysis then delivered no phenols, but occasionally other products were obtained. [Ru(3)(CO)(12)] also catalyzed the conversion of 1 at a low rate, [Os(3)(CO)(12)] failed as a catalyst, and with [Co(2)(CO)(8)] the alkyne complex 19 can be obtained, it does not lead to any phenol but reacts with norbornene to give the product of a Pauson-Khand reaction. Efforts to prepare vinylidene complexes of 1 provided the only evidence for these species; in the presence of a phosphane ligand with ruthenium an interesting deoxygenation to 22 was observed. The phenol 2 c was converted to the allyl ether, a building block for para-Claisen rearrangements, and to the aryl triflate, a building block for cross-coupling reactions.

Monomeric ferrocene bis-imidazoline bis-palladacycles (FBIP) have recently been reported to be efficient bimetallic catalysts in different sorts of asymmetric reactions by the cooperation of two Pd(II) centers. A crucial parameter regarding the efficiency of reactions catalyzed in a bimetallic mode isin generalthe intermetallic distance of both catalytically relevant metal centers. In this article we describe the structural elucidation of the monomeric FBIP catalyst type (usually generated in situ from a catalytically inactive dimeric chloride bridged precatalyst) by X-ray crystal structure analysis. Two dicationic monomeric complexes are compared to a neutral complex. The solid-state structures reveal varying Pd–Pd distances ranging from 3.15 to 5.27 Å for the doubly charged complexes, whereas for the neutral complex a distance of 3.28 Å has been found. This variability is supposed to be one of the key advantages of a ferrocene backbone in a bimetallic catalyst system, since the Fe–Cp bonds allow the bimetallic complex to readily open and close like a pair of scissors, employing just a few degrees of rotational freedom. In addition, on the basis of the nature of the reported catalyst species, we suggest that a permanent switch among neutral and mono- and dicationic catalyst species by a Brønsted acid such as acetic acid might facilitate different elementary steps in a catalytic cycle. By DFT calculations we have found that weak d8–d8 interactions contribute to short Pd–Pd distances but are less important than dispersive interactions, which can even overcome the Coulombic repulsion of two cationic palladium centers.