Dicarbonyl Systems Research Articles

Methylborabenzene ligands in binuclear iron carbonyl derivatives: High spin states and iron–iron multiple bonding

The experimentally known methylborabenzene iron carbonyl (C5H5BCH3)2Fe2(CO)4 and its decarbonylation products (C5H5BCH3)2Fe2(CO)n (n = 3, 2) have been studied by density functional theory for comparison with their cyclopentadienyl analogs. The lowest energy (η6-C5H5BCH3)2Fe2(CO)4 structures are the experimentally known singlet doubly bridged cis-(η6-C5H5BCH3)2Fe2(CO)2(μ-CO)2 and the corresponding trans isomer similar to the corresponding (η5-C5H5)2Fe2(CO)2(μ-CO)2 system. Also the triplet triply bridged (η6-C5H5BCH3)2Fe2(μ-CO)3 is the lowest energy tricarbonyl structure similar to the cyclopentadienyl system. However, significant differences between the methylborabenzene and cyclopentadienyl derivatives are found in the dicarbonyl systems. A singlet (η5-C5H5)2Fe2(μ-CO)2 structure with a short FeFe distance of ∼2.1 Å was previously found to be the lowest energy structure in the cyclopentadienyl system. An analogous methylborabenzene structure (η6-C5H5BCH3)2Fe2(μ-CO)2 is found as the lowest energy singlet structure. However, an unsymmetrical quintet (η6-C5H5BCH3)FeFe(CO)2(η6-C5H5BCH3) structure is predicted to lie ∼22 kcal/mol below this singlet structure. The existence of this low-energy, high-spin (η6-C5H5BCH3)2Fe2(CO)2 structure makes (η6-C5H5BCH3)2Fe2(μ-CO)3 disfavored with respect to disproportionation into (η6-C5H5BCH3)2Fe2(μ-CO)4 + (η6-C5H5BCH3)2Fe2(μ-CO)2 unlike its cyclopentadienyl analog.

Unsaturated Binuclear Cyclopentadienylosmium Carbonyl Derivatives: Comparison with Their Iron Analogues

The binuclear cyclopentadienylosmium carbonyls Cp2Os2(CO)n (Cp = η5-C5H5; n = 4, 3, 2) have been studied by density functional theory (DFT). The unbridged Cp2Os2(CO)4 structure found experimentally is predicted to be of lower energy than the doubly bridged structures cis- and trans-Cp2Os2(CO)2(μ-CO)2, in contrast to the analogous iron system. However, the energy differences are rather small (∼5 kcal/mol). The Cp2Os2(CO)3 system differs from its iron analogue in that the triplet structure Cp2Os2(μ-CO)3 analogous to the known, stable Cp2Fe2(μ-CO)3 lies at least ∼12 kcal/mol above the lowest energy singlet structure. Thus the three lowest lying Cp2Os2(CO)3 structures are all singlet structures, which are either unbridged or doubly bridged. The iron and osmium dicarbonyl systems Cp2M2(CO)2 (M = Fe, Ru) are analogous in that the lowest energy structures are doubly bridged with short M≡M distances suggesting the formal triple bonds required to give both metal atoms the favored 18-electron configuration. The pro...

Nitrocompounds as useful reagents for the synthesis of dicarbonyl derivatives

The reaction of functionalized nitroalkanes with electrophiles such as Michael acceptors and aldehydes is one of the most exploited procedures for the synthesis of new carbon-carbon bonds. Conversion of the nitro group in the adduct into a carbonyl derivative usually provides a rapid entry to dicarbonyl systems that are amenable to further synthetic transformation into a plethora of important targets.

Reactions of a modified lysine with aldehydic and diketonic dicarbonyl compounds: an electrospray mass spectrometry structure/activity study

The phenomenon known as non-enzymatic glycation is described as the reaction of reducing sugars with basic amino groups of proteins and nucleic acids, as well as with simple amines, without enzyme mediation. Non-enzymatic model glycation reactions that make use of low-molecular-weight compounds make an important contribution in the elucidation of glicated processes in vitro and in vivo. Four alpha-dicarbonyl compounds, aldehydic (glyoxal, methylglyoxal and phenylglyoxal) and ketonic (diacetyl), were reacted with the modified amino acid N(alpha)-acetyl-L-lysine (AcLys) in an attempt to establish structure/activity relationships for the reactivity of alpha-dicarbonyls with the amine compound. Electrospray ionization mass spectrometry (ESI-MS) combined with tandem mass spectrometry (MS/MS) and collision-induced dissociation (CID) was used to identify and characterize reagents, intermediates and reaction products. The formation of dicarbonyl-derived lysine dimers was observed exclusively. Especially, attention is drawn to alkyl- (asymmetrical dicarbonyl systems) and carboxyl- (glyoxal system) substituted imidazolium ions, at ring position 2. The main differences observed in the reactions studied were related to the reactivity with the diimine intermediate. This intermediate can react either with a non-hydrated dicarbonyl molecule at the aldehydic carbonyl, or with a mono-hydrated one at the ketonic carbonyl, particularly for asymmetrical dicarbonyls. For 2-carboxyl-substituted imidazolium ion (glyoxal reaction), besides the usual keto-enol rearrangement from the diol group, an alternative reaction pathway (proton abstraction) appears to contribute also for the imidazolium ring-closure process. Moreover, the formation of imidazolium ring structures can depend on several factors, namely, the presence (or absence) of electron donor substituents at the formed diol, the degree of stability of the new electrophile generated and/or the equilibrium concentration of the non- and mono-hydrated dicarbonyl forms in solution, the last being particularly important for asymmetrical dicarbonyls. The results reported reveal the complexity of reactivity as well as the diversity of imidazolium molecular structures.

Carbonylation of four-membered ruthenium and osmium metallacycles incorporating an orthometallated phenolic function: New acylruthenium and arylosmium complexes

The solution reaction of Ru(QL 1)(PPh 3) 2(CO)Cl ( 3) and Os(QL 1)(PPh 3) 2(CO)Br ( 4) with carbon monoxide at one atmosphere pressure has respectively afforded the orange acylruthenium system Ru(QL 2)(PPh 3) 2(CO)Cl ( 5) and the yellow arylosmium dicarbonyl system Os(QL 3)(PPh 3) 2(CO) 2Br ( 6) in excellent yields. (QL 1 is C 6H 2O-2-CHNHC 6H 4Q( p)-3-Me-5, QL 2 is C 6H 2(CO-1)O-2-CHNHC 6H 4Q( p)-3-Me-5 and QL 3 is C 6H 2OH-2-CHNC 6H 4Q( p)-3-Me-5 and Q is Me, OMe and Cl.) It is proposed that in the case of 3 a dicarbonyl complex similar to 6 is formed as an intermediate which rapidly undergoes aryl migration with concomitant phenolato coordination furnishing 5. The stability of 6 is consistent with the greatly diminished ability of osmium in promotion of migratory reactions. In the reaction 4 → 6 the Os–O(phenolato) bond is cleaved and the Schiff base moiety undergoes iminium-phenolato → imine-phenol tautomerization. The observed cis geometry of 6 may arise by a concerted route involving edge displacement of the halide ligand. The crystal and molecular structure of 5(Q = Cl) has revealed the presence of a distorted octahedral RuC 2P 2OCl coordination sphere and a highly planar acyl chelate ring characterized by a Ru–C distance of 2.013(4) Å. In the hydrogen bonded zwitterionic iminium-phenolato ring the N ⋯ O distance is 2.561(6) Å. The acyl complexes of type 5 display an MLCT band near 500 nm which is absent in 6. The Schiff base C N stretch in 5 (∼1630 cm −1) is significantly higher than that in 6 (∼1600 cm −1) which displays two strong C O stretches near 2020 and 1940 cm −1 ( cis-Os(CO) 2 configuration). A single 31P NMR signal occurs in both 5 and 6 near 37 and −6 ppm, respectively ( trans-M(PPh 3) 2 configuration). The voltammetric reduction potentials of the M III/M II couple is observed near 1.0 and 0.8 V vs. SCE in 5 and 6, respectively. Both are significantly higher than those in parent complexes ( 3 and 4) due to stabilization of the bivalent state upon carbonylation.

Elemental fluorine : Part 18. Selective direct fluorination of 1,3-ketoesters and 1,3-diketones using gas/liquid microreactor technology

1,3-Ketoesters and 1,3-diketones react with fluorine gas, using Durham multichannel modular microreactor technology, on a preparatively useful scale. High conversions and yields of monofluorinated products were obtained. A consideration of the mechanism of fluorination of dicarbonyl systems provides a rationale of the scope and limitations of the use of microreactor technology for such direct fluorination processes. We conclude that it is especially appropriate for the fluorination of diketones and ketoesters that exist mainly in the enol form at equilibrium and/or undergo rapid keto-enol interconversion. A one-stage route to fluoroketones from ketoesters was achieved by allowing the crude product mixture obtained from the fluorination of the ketoesters to stand for a period of time before reaction work-up.

DESIGN AND SYNTHESIS OF SOME P-SUBSTITUTED STYRYLISOXAZOLE CARBOXYLIC ACID DERIVATIVES AS ANTI-INFLAMMATORY AGENTS

A series of 5-(p-substitutedstyryl)-isoxazole-3-carboxylic acids (4a-g), carboxylic acid ethyl esters (3a-g) and their corresponding amides (6-29) as well as their hydroxamic acid analogues (5a-g) were synthesized and evaluated for their anti-inflammatory activity. The 1,3dicarbonyl systems 2,4-dioxo-6-arylhex-5-enoic acid ethyl esters (2a-g) were the key intermediate for synthesis of the target isoxazole derivatives. The synthesis of these intermediate was achieved through the reaction of chalcones (1a-g), prepared via aldol condensation of the respective substituted benzaldehyde with acetone in alkaline medium, with diethyloxalate. The purity of the synthesized derivatives was determined by thin layer chromatography (TLC) in addition to the microanalyses and their structures were confirmed by different spectroscopic means. The anti-inflammatory activity of twenty-one compounds [(3a,e,f and g); (4a-g); (5a-g); 22; 24 and 25] were assessed, at a dose of 100 mg/kg, by carrageenan-induced paw edema in rats in comparison to indomethacin as a reference drug. The ulcerogenicity of the compounds 3g, 4g and 5g as representatives of the anti-inflammatory active compounds (ester, carboxylic acid and hydroxamic acid) and indomethacin as reference drug was examined under scan electron microscope, after 24 hours of administration of single dose (100 mg/kg) in rats. Significant anti-inflammatory activity was displayed by most of the target derivatives. p-Nitro and p-methoxystyryl isoxazole carboxylic acid derivatives (4e and 4f) revealed 75% inhibition of inflammation after 1 hr reflected the rapid onset of action of these derivatives, which may sustained up to 5 hr as in case of 4e. The anti-inflammatory activity of 5-(p-nitro and pdimethylaminostyryl)-isoxazole-3-carboxylic acid ethyl esters (3e and 3g) being favorably comparable with indomethacin in terms of potency and ulcerogenic liability. The rational behind the synthesis of these compounds and the structure activity relationship are discussed.

ChemInform Abstract: A Novel Oxidative Dimerization of Acetylenes to Conjugated Dicarbonyl Systems Using Iron(III) Perchlorate.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Characterization of clay-based enterosorbents for the prevention of aflatoxicosis.

Appropriate chemical interventions that can block, retard, or significantly diminish foodborne exposure to aflatoxins are high priorities. A practical and effective approach to the aflatoxin problem has been the dietary inclusion of a processed calcium montmorillonite clay (HSCAS). HSCAS acts as an enterosorbent that rapidly and preferentially binds aflatoxins in the gastrointestinal tract resulting in decreased aflatoxin uptake and bioavailability. In mechanistic studies, we have shown that the intact dicarbonyl system in aflatoxin is essential for optimal sorption by HSCAS. Evidence also suggests that aflatoxins react at multiple sites on HSCAS clay surfaces (especially those within the interlayer region). Due to conceivable risks associated with the dietary inclusion of nonspecific binding agents, all aflatoxin enterosorbents should be tested in sensitive animal models for efficacy, safety, and the potential for nutrient interactions.

Dietary clay in the chemoprevention of aflatoxin-induced disease.

Aflatoxins are harmful by-products of mold growth and, though invisible to the naked eye, are potentially fatal. The aflatoxin problem is long-standing and inextricable. Concerns about the aflatoxins originate from the strong implications of their involvement in disease and death in humans and animals, yet scientists and clinicians are still seeking ways to effectively deal with these dangerous and elusive chemicals. Safe, practical, and effective strategies for the detoxification of aflatoxin-contaminated food and feed are highly desirable. A simple and effective approach to the chemoprevention of aflatoxicosis has been to diminish or block exposure to aflatoxins via the inclusion of HSCAS clay in the diet. HSCAS clay acts as an aflatoxin enterosorbent that tightly and selectively binds these poisons in the gastrointestinal tract of animals, decreasing their bioavailability and associated toxicities. Further studies to delineate the molecular mechanisms of action have shown that the dicarbonyl system of aflatoxin is essential for tight binding by HSCAS. In these studies, adsorption data was fitted to multiple isotherm equations including the Langmuir, multi-Langmuir, general Freundlich, Langmuir-Freundlich, Toth and various transforms. Information derived included: the Gibbs standard free energy change of adsorption, enthalpy of adsorption, capacity, affinity, and heterogeneity coefficient. Computer modeling was also utilized to provide additional structural information and insight into the mechanism. Evidence suggests that aflatoxins may react at multiple sites on HSCAS particles, especially the interlayer region, but also at edges and basal surfaces. Since clay and zeolitic minerals comprise a broad family of functionally diverse chemicals, there may be significant hidden risks associated with their indiscriminate inclusion in the diet. All aflatoxin binding agents should be rigorously tested, paying particular attention to their effectiveness and safety in aflatoxin-sensitive animals and their potential for interactions with critical nutrients.

Reactions of the cycloheptatrienyl complexes [MX(CO) 2( η-C 7H 7)] (M=Mo, X=Br; M=W, X=I) with CNBu t: X-ray crystal structure of [WI(CO) 2(CNBu t) 2( η3-C 7H 7)

Reaction of [MX(CO) 2( η 7-C 7H 7)] (M=Mo, X=Br; M=W, X=I) with two equivalents of CNBu t in toluene affords the trihapto-bonded cycloheptatrienyl complexes [MX(CO) 2(CNBu t ) 2( η 3-C 7H 7)] ( 1, M=Mo, X=Br; 2, M=W, X=I). The X-ray crystal structure of 2 reveals a pseudo-octahedral molecular geometry with an asymmetric ligand arrangement at tungsten in which one CNBu t is located trans to the η 3-C 7H 7 ring. Treatment of 2 with tetracyanoethene results in 1,4-cycloaddition at the η 3-C 7H 7 ring to give [WI(CO) 2(CNBu t ) 2{ η 3-C 9H 7(CN) 4}], 3. The principal reaction type of the molybdenum complex 1 is loss of carbonyl and bromide ligands to afford substituted products [MoBr(CNBu t ) 2( η 7-C 7H 7)] 4 or [Mo(CO)(CNBu t ) 2( η 7-C 7H 7)]Br. Reaction of [MoBr(CO) 2( η 7-C 7H 7)] with one equivalent of CNBu t in toluene at 60°C affords [MoBr(CO)(CNBu t )( η 7-C 7H 7)], 5, which is a precursor to [Mo(CO)(CNBu t )(NCMe)( η 7-C 7H 7)][BF 4], 6, by reaction with Ag[BF 4] in acetonitrile. In contrast with the parent dicarbonyl systems [MoX(CO) 2( η 7-C 7H 7)], complexes of the Mo(CO)(CNBu t )( η 7-C 7H 7) auxiliary, 5 and 6, do not afford observable η 3-C 7H 7 products by ligand addition at the molybdenum centre.

Recent Developments in the Cycloaddition Reactions of o-Benzoquinones

o-Benzoquinones display multiple reaction profiles in cycloaddition reactions. With electron rich dienes, benzodioxin structures resulting from a [3,3] sigmatropic rearrangement of the initially formed spirocyclic compounds are obtained. 6-Aryl and 6,6-dialkyl fulvenes give rise to bicyclo[2.2.2] adducts, whereas 6,6-tetramethylenefulvene undergoes rearrangement to cyclopent-1-enyl cyclopentadiene prior to cycloaddition to the dicarbonyl system leading to benzodioxins. The bicyclo[2.2.2]octene diones derived from o-benzoquinones undergo a number of synthetically useful transformations.

On the nature of the irreversible inhibition of histidine ammonia lyase by cysteine and dioxygen

The irreversible inhibition of histidine ammonia lyase by l-cysteine and dioxygen has been reexamined. After denaturation and consecutive digestion of the inhibited enzyme by trypsin and endoproteinase Glu-C, the generated chromophoric system ( λ max = 340 nm) remained intact and was isolated in an octapeptide containing amino acids 138–145, as previously described (Hernandez, D.; Stroh, J. G.; Phillips, A. T. Arch. Biochem. Biophys. 1993, 307, 126). Conducting the inhibition in the presence of 18O 2 did not result in the incorporation of the heavy isotope into the isolated octapeptide. Total hydrolysis followed by amino acid analysis of the octapeptide revealed the presence of one arginine in addition to those expected from the deduced sequence (G 3SVAD). 1H NMR spectroscopy of the octapeptide confirmed the presence of the amino acids GSVAD and showed no signals for olefinic or aromatic protons. To account for the excess mass of the octapeptide, we propose an oxidative degradation of the dehydroalanine prosthetic group, followed by reaction of the resulting dicarbonyl system with a nearby arginine residue.

ChemInform Abstract: Reactions of Cyclic Anhydrides. Part 10. Reactions of o‐Aminothiophenol with α,β‐Unsaturated Dicarbonyl Systems.

AbstractThe reaction of o‐aminothiophenol (I) with the various maleanilic or fumaranilic acids or esters (II) affords the benzothiazines (III) in good to excellent yields irrespective of the configuration of the compounds (II) and irrespective of whether an acid or an ester is brought to reaction.

Reactions of o-aminothiophenol withαβ-unsaturated dicarbonyl systems. Facile synthesis of benzothiazines and benzothiazepines

When treated with maleanilic/fumaranilic acids or esters or the corresponding imides or isoimides, o-aminothiophenol furnishes benzothiazines, whereas treatment of o-aminothiophenol with acrylic acids/esters furnishes benzothiazepines. The orientational preferences in these Michael-type reactions are discussed.

A new synthetic method for the preparation of [(η 5-C 5H 5)Fe(CO)(L)(un)]BF 4 (L = CO, PPh 3; un = unsaturated hydrocarbon) complexes and reduction of the η 2-acetylene complexes

The reaction of (η 5-C 5H 5)Fe(CO) 2I with AgBF 4 in CH 2Cl 2 generates the reactive intermediate [(η 5-C 5H 5)Fe(CO) 2] + in solution which, when mixed with 2 or 3 equivalents of an olefinic or acetylene ligand produces η 2-olefin and η 2-acetylene complexes in ca. 70% yield. The analogous reaction of (η 5-C 5H 5)Fe(CO)(PPh 3)I with AgBF 4 in CH 2Cl 2 produces [(η 5-C 5H 5)Fe(CO)(PPh 3)] + in solution which also reacts readily with olefins and acetylenes to yield new π-complexes. The dicarbonyl complexes are generally more stable than the phosphine-substituted complexes. The η 2-acetylene complexes in both systems can be reduced with hydride reagents to yield the corresponding neutral σ-vinyl compounds. Although unstable in the dicarbonyl system, these vinyl compounds in the phosphine system are quite stable and show no tendency to isomerize or thermally decompose to a metal hydride as previously established for the analogous alkyl derivatives. Attempts to prepare cyano-substituted olefin complexes lead instead to new complexes in which the iron bonds to these ligands through the nitrogen lone-pair.

Structures of the Excited States of Benzil and Related Dicarbonyl Molecules

Nonhindered aromatic α-dicarbonyl molecules show abnormal spectroscopic behavior when compared to other molecules containing carbonyl groups. Taking benzil as a typical example, its photoabsorption and emission spectra cannot be accounted for by the normal Jablonski scheme. The spectral data are consistent with a mechanism involving molecular reorientation. It is postulated that photoexcitation of benzil causes it to change from its normal skew configuration into one in which the dicarbonyl system becomes coplanar while it is in the excited singlet state. As a consequence photoemission (both fluorescence and phosphorescence) takes place from this configuration, resulting in a ground-state molecule with the same nuclear configuration as the excited state; relaxation then occurs to the normal configuration. Formation of a trans configuration is most likely to occur on photoexcitation, but a cis configuration cannot be ruled out. The formation of a coplanar dicarbonyl system must involve movement of the phenyl groups into positions out of the dicarbonyl plane. Additional support for this mechanism is obtained from the energetics of the various processes.

SOME ASPECTS OF TRITERPENE BITTER PRINCIPLE BIOSYNTHESIS

The triterpene bitter principles are reviewed in order to understand their biogenesis. A derivative of 3–hydroxy lanost–24–ene–11–one is suggested as the source of the cucurbitacins. The limonoids, meliacins and Simarubaceae bitter principles are all considered to be derived from 3α–hydroxy–8β–methyl–(13α, 17βH, 20αH) lanosta–14,24–diene. This has its side chain degraded to a furan in the limonoids and meliaçins. Biological Baeyer Villiger oxidation of ring A and D is considered to occur in the limonoids and of ring B, C, and/or D in the meliacins. With the Simarubaceae bitter principles the side chain may be degraded, but this is entirely lost by lèavage of the 12,17–β–dicarbonyl system.