Microwave-assisted synthesis of (η-arene)tricarbonylchromium complexes

We have undertaken a study of the microwave-assisted synthesis of (η⁶-arene)tricarbonylchromium complexes. Under microwave irradiation, the reactions of hexacarbonylchromium with arenes gave high yields of (η⁶-arene)chromium tricarbonyl complexes.     

Lithiation of tricarbonylchromium complexes with polyaromatic carbo-and heterocyclic ligands. DFT study

The reactions of tricarbonylchromium complexes of polyaromatic carbo-and heterocyclic derivatives with BuⁿLi was studied by the density functional theory. The kinetic and thermodynamic factors for controlling the direction and selectivity of metallation were calculated for the model biphenylenetricarbonylchromium complex. Both approaches indicate that lithiation occurs exclusively at the aromatic ring bonded to the transition metal, which agrees with experimental data, while the selectivity inside this ring is determined more exactly by the thermodynamic factor. The solvation effects were simulated for the lithium salt of the tricarbonylnaphthalenechromium complex in which the lithium atom is localized in position 1 of the coordinated ring. The simulation showed the stable coordination of the lithium atom with two THF molecules, and the addition of the next THF molecule is thermodynamically unfavorable. The results of calculation of the relative energies for all possible THF-solvated lithium salts of the tricarbonylchromium complexes of biphenyl, naphthalene, biphenylene, and dibenzothiophene indicate that the difference in energies Δ E ≤ 1 kcal mol⁻¹ corresponds to the experimentally observed absence of selectivity, while the difference more than 2.5 kcal mol⁻¹ corresponds to the selectivity of the reaction. No additional coordination of the lithium atom to the free electron pair of the heteroatom was observed for the sulfur-containing dibenzothiophene complex. Similar calculations show that double metallation in the dibenzothiophene complex occurs at positions 1 and 4. The developed approach enables one to predict the direction and selectivity of metallation reactions of transition metal complexes with different arenes. Dedicated to Academician N. S. Zefirov on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1993–2003, September, 2005.     

Chemical Chromogenic Sensors Based on Calixarenes: Syntheses and Recognition Properties for Alkylamines

Nitration of 1,3-distal dialkylated calix[4]arenes wasinvestigated in order to prepare chromogenic sensor compounds. There were three kinds of separableproducts, which came from the nitration and oxidation of the phenol moieties. The coneconformation of calix[4]arenes containing a quinone structure was first determined. And the effectof the structure of reactants for the nitration and oxidation is also discussed. Dinitratedcalix[4]arenes possess recognition ability for the alkylamines, which can be as the chromogenicsensors for determining the alkylamines. The recognition behavior was due to the protontransfer between alkylamine and calix[4]arenes, and the resulting ammonium phenoxide wasstabilized by the phenoxide oxygen atom and the carbonyl at the lower rim with hydrogen bonding.     

Reactivity of (eta(6)-arene)tricarbonylchromium complexes toward additions of anions, cations, and radicals.

A computational and experimental study of additions of electrophiles, nucleophiles, and radicals to tricarbonylchromium-complexed arenes is reported. Competition between addition to a complexed arene and addition to a noncomplexed arene was tested using 1,1-dideuterio-1-iodo-2-((phenyl)tricarbonylchromium)-2-phenylethane. Reactions under anionic and cationic conditions give exclusive formation of 1,1-dideuterio-1-((phenyl)tricarbonylchromium)-2-phenylethane arising from addition to the complexed arene. Radical conditions (SmI(2)) afford two isomeric products, reflecting a 2:1 preference for radical addition to the noncomplexed arene. In contrast, intermolecular radical addition competition experiments employing ketyl radical addition to benzene and (benzene)tricarbonylchromium show that addition to the complexed aromatic ring is faster than attack on the noncomplexed species by a factor of at least 100,000. Density functional theory calculations using the B3LYP method, employing a LANL2DZ basis set for geometry optimizations and a DZVP2+ basis set for energy calculations, for all three reactive intermediates showed that tricarbonylchromium stabilizes all three types of intermediates. The computational results for anionic addition agree well with established chemistry and provide structural and energetic details as reference points for comparison with the other reactive intermediates. Intermolecular radical addition leads to exclusive reaction on the complexed arene ring as predicted by the computations. The intramolecular radical reaction involves initial addition to the complexed arene ring followed by an equilibrium leading to the observed product distribution due to a high-energy barrier for homolytic cleavage of an exo bond in the intermediate cyclohexadienyl radical complex. Mechanisms are explored for electrophilic addition to complexed arenes. The calculations strongly favor a pathway in which the cation initially adds to the metal center rather than to the arene ring.     

Mononuclear and binuclear tricarbonylchromium complexes of aryl-substituted [2.2]paracyclophanes

The complexation reactions of monoaryl- and diaryl-substituted [2.2]paracyclophanes with (NH₃)₃Cr(CO)₃ have been studied. The aromatic rings of [2.2]paracyclophane are more favorable for coordination than aryl substituents. This leads to the regioselective formation of the corresponding mono- or binuclear tricarbonylchromium complexes. In some cases, the tricarbonylchromium group is coordinated to the aryl ring of the substituent to form (in low yields) the corresponding mononuclear complex or binuclear complexes with both the aromatic ring of paracyclophane and the aryl ring of the substituent involved in coordination. The structure of such complex, namely, [4-(η⁶-2,4,6-trimethylpheny)-11-16-η⁶-[2,2]paracyclophane]bis[tricarbonylchromium(0)] was confirmed by X-ray diffraction study. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 142–150, January, 1998.     

One‐Pot C−H Functionalization of Arenes by Diaryliodonium Salts

A transition‐metal‐free, mild, and highly regioselective synthesis of nitroarenes from arenes has been developed. The products are obtained in a sequential one‐pot reaction by nitration of iodine(III) reagents with two carbon ligands, which are formed in situ from iodine(I). This novel concept has been extended to formation of aryl azides, and constitutes an important step towards catalytic reactions with these hypervalent iodine reagents. An efficient nitration of isolated diaryliodonium salts has also been developed, and the mechanism is proposed to proceed by a [2,2] ligand coupling pathway.     

The Chemical Behavior of Multibridged [2n] Cyclophanes

Multibridged [2_n] cyclophanes comprise a novel class of aromatic compounds, in which two benzene rings are connected by three up to a maximum of six ethano bridges. Compared to classical nonannelated arenes, their most distinct chemical property is the ease with which, particularly the higher-bridged homologs, take part in addition reactions (Diels-Alder additions, hydrogenations, and ionic additions). On the other hand, the typical regenerative behavior of aromatic molecules is not fully suppressed in the [2_n]cyclophanes as evidenced by such typical electrophilic aromatic substitution reactions as bromination, Friedel-Crafts acylation, and nitration. Besides these reactions in which the benzene rings are either destroyed or retained, reactions at and with the ethano bridges, e.g. their cleavage, isomerization, and functionalization can also occur.     

Silylarene Hydrogenation: A Strategic Approach that Enables Direct Access to Versatile Silylated Saturated Carbo‐ and Heterocycles

We report a method to convert readily available silylated arenes into silylated saturated carbo‐ and heterocycles by arene hydrogenation. The scope includes alkoxy‐ and halosilyl substituents. Silyl groups can be derivatized into a plethora of functionalities and find application in organic synthesis, materials science, and pharmaceutical, agrochemical, and fragrance research. However, silylated saturated (hetero‐ ) cycles are difficult to access with current technologies. The yield of the hydrogenation depends on the amount of the silica gel additive. This silica effect also enables a significant improvement of a previously disclosed method for the hydrogenation of highly fluorinated arenes (e.g., to all‐cis‐C₆H₆F₆).     

As Nice as π: Aromatic Reactions Activated by π-Coordination to Transition Metals

π-Coordination of aromatic molecules to metals dramatically alters their reactivity. For example, coordinated carbons become more electrophilic and C−H bonds of coordinated rings become more acidic. For many years, this change in reactivity has been used to trigger reactions that would not take place for uncoordinated arenes, however, there has been a recent resurgence in use of this technique, in part due to the development of catalytic reactions in which π-coordination is transient. In this Minireview, we describe the key reaction chemistry of arenes coordinated to a range of transition metals, including stereoselective reactions and industrially relevant syntheses. We also summarise outstanding examples of catalytic processes. Finally, we give perspectives on the future direction of the field, with respect to both reactions that are stoichiometric in activating metals and those employing catalytic metal.     

Studies in negative ion mass spectrometry: XIV—Electron attachment reactions of a series of η6-arenetricarbonylchromium(O) complexes

Electron attachment reactions of a series of (η⁶-arene)tricarbonylchromium(O) complexes have been examined in the gas phase. The electron capture process has been shown to be influenced by the structure of the η⁶-arene ligand and its substituents. Whereas (η⁶-benzene)- and (η⁶-mesitylene)tricarbonylchromium(O) undergo dissocative electron capture, or reductive decarbonylation, yielding [M? CO]^?˙ ions of highest abundance in their negative ion mass spectra, [η⁶-(2,2-dimethylindan-1,3-dione)]tricarbonylchromium(O) forms a molecular negative ion which undergoes sequential CO eliminations and finally a demetallation to give the arene radical ion. A localization of charge on the coordinated arene ligand is proposed for the formation of [M]^?˙ in this case. (η⁶-Methylbenzoate)tricarbonylchromium(O) also forms a molecular negative ion by secondary electron attachment which decomposes by simultaneous and consecutive eliminations of up to four CO molecules. The elucidation of a mechanism and sequence for these CO eliminations has been achieved by synthesizing and examining the negative ion mass spectrum of [η⁶-(C₆H₅·¹³CO₂Me)]Cr(CO)₃. The first CO loss in the principal fragmentation pathway occurs solely from the –Cr(CO)₃ group of [M]^?˙. The effect of para substituent groups on the stabilities of molecular negative ions and their fragmentations has been ascertained using a series of para-substituted (η⁶-methylbenzoate)tricarbonylchromium(O) compounds containing the groups NH₂, OH, OCH₃, CL and COOMe. The stabilities of the [M]^?˙ ions have been rationalized in terms of the Hammett and Taft parameters σ_P, σ_I, σ_R^P, σ_P^O and σ_R^O. The overall electronic substituent effect transmitted to the carbonyl groups of the –Cr(CO)₃ unit involves both resonance and inductive components. In this series of compounds the stability of [M]^?˙ decreases as the electron withdrawing capacities of the para substituents increase.     

Conformationally Selective Synthesis of Mononitrocalix[4]arene in Cone or Partial Cone

Nitro‐substituted calixarenes in a cone and a partial cone conformation were prepared selectively using distinct synthetic routes. The selective nitration of tris‐ or penta‐substituted phenols of calix[4]arene or calix[6]arene provided mononitrocalix[n ]arenes (n  =  4, 6). Subsequent addition of ethylene glycol (EG) moieties to mononitrocalix[4]arene provided tetraEGylcalix[4]arene in locked partial cone conformation. By an alternative route – initial addition of EG moieties to the non‐derivatized calix[4]arene followed by the uncontrolled nitration – provided mononitro‐ and dinitro‐tetraEGylcalix[4]arenes locked in the cone conformation. These nitrocalix[4]arenes with locked cone or partial cone conformation are useful building blocks for further assembly of supramolecular systems, especially in the area of material sciences.     

Efficient and Practical Arene Hydrogenation by Heterogeneous Catalysts under Mild Conditions

An efficient and practical arene hydrogenation procedure based on the use of heterogeneous platinum group catalysts has been developed. Rh/C is the most effective catalyst for the hydrogenation of the aromatic ring, which can be conducted in iPrOH under neutral conditions and at ordinary to medium H₂ pressures (<10 atm). A variety of arenes such as alkylbenzenes, benzoic acids, pyridines, furans, are hydrogenated to the corresponding cyclohexyl and heterocyclic compounds in good to excellent yields. The use of Ru/C, less expensive than Rh/C, affords an effective and practical method for the hydrogenation of arenes including phenols. Both catalysts can be reused several times after simple filtration without any significant loss of catalytic activity.     

Substituent effects in π-(tricarbonylchromium)arenes: I. 1H NMR Spectroscopy of alkylsubstituted π-(tricarbonylchromium)benzenes

Some thirty substituted π-(tricarbonylchromium)benzenes, including some crowded alkylsubstituted complexes, e.g. (l',l'-diethylpropyl)-, (1'-t-butyl-2',2'dimethylpropyl)-, 1,2-di-t-butyl- and 1,2,4-tri-t-butyl-π-(tricarbonylchromium)benzenes, have been prepared. ¹H NMR spectra have been recorded from acetone solutions of the complexes. Using the chemical shift increments of the aromatic protons on complex formation of C₆H₅CH-t-Bu₂, as a reference for the eclipsed conformation with the substituent staggered with respect to the carbonyl ligands, the contribution of this conformer to the conformational equilibrium of the other monoalkylsubstituted complexes has been estimated, ¹H NMR spectroscopy turns out to be a useful probe in conformation analysis of π-(tricarbonylchromium)arenes.     

Organometallic polymers. XXIV. Synthesis and copolymerization behavior of h6-(2-phenylethyl acrylate) tricarbonylchromium

The monomer h⁶-(2-phenylethyl Acrylate) tricarbonylchromium (PEAC) was synthesized and copolymerized in solution at 70°C with styrene, methyl acrylate, acrylonitrile, and 2-phenylethyl acrylate, azobisisobutyronitrile being used as the initiator. In ethyl acetate solvent the relative reactivity ratios were crudely approximated. The copolymers all exhibited binodal molecular weight distributions, the high molecular weight node probably resulting from branching. The h⁶-benzenetricarbonylchromium unit decomposed when exposed to sunlight or ultraviolet light both in air and under N₂. In air, Cr₂O₃ was initially formed within copolymer films. Several unsuccessful attempts were made to prepare h⁶-(2-phenylethyl acrylate)tricarbonylmolybdenum, (III), the molybdenum analog of PEAC, from arenes and both Mo(CO)₆ and (CH₃CN)₃Mo(CO)₃. These reactions are described.     

Heteroatom‐Free Arene‐Cobalt and Arene‐Iron Catalysts for Hydrogenations

75 years after the discovery of hydroformylation, cobalt catalysts are now undergoing a renaissance in hydrogenation reactions. We have evaluated arene metalates in which the low‐valent metal species is—conceptually different from heteroatom‐based ligands—stabilized by π coordination to hydrocarbons. Potassium bis(anthracene)cobaltate 1 and ‐ferrate 2 can be viewed as synthetic precursors of quasi‐“naked” anionic metal species; their aggregation is effectively impeded by (labile) coordination to the various π acceptors present in the hydrogenation reactions of unsaturated molecules (alkenes, arenes, carbonyl compounds). Kinetic studies, NMR spectroscopy, and poisoning studies of alkene hydrogenations support the formation of a homogeneous catalyst derived from 1 which is stabilized by the coordination of alkenes. This catalyst concept complements the use of complexes with heteroatom donor ligands for reductive processes.     

The synthesis of new 1,3-oxazolidines and 1,3-oxazinanes containing (η<Superscript>6</Superscript>-arene)tricarbonylchromium group based on condensation between aldehydes and amino alcohols

The condensation reactions of β- and γ-amino alcohols containing phenyl or (η⁶-arene) tricarbonylchromium substituent with formaldehyde, acetaldehyde, benzaldehyde, and (η⁶-benzaldehyde)tricarbonylchromium were studied. The resulting 1,3-oxazolidine and 1,3-oxazinane products were isolated in a pure form and identified by different physicochemical methods. The effect of (η⁶-arene)tricarbonylchromium moiety on the reaction process was demonstrated.     

Electrophilic substitution reactions in 4H-benzo[d,e,f]carbazole

The behavior of 4H-benzo[d,e,f]earbazole and its N-aeetyl derivative in various electrophilic substitution reactions (e.g. nitration, nitrosation, bromination, Friedel-Crafts acylations) has been investigated, and a number of new derivatives of this heterocycle (some of which are to be tested as potential carcinogens and enzyme-inducers) have been synthesized.     

Metal- and acid-free,TBN-mediated direct CH nitration of arenes

A metal-free, acid-free, direct and eco-friendly nitration methodology has been developed. A tert-butyl nitrite efficiently promotes the direct CH nitration of electron-rich arenes with good to excellent yields and regioselectivities. The practical utility of this protocol has been demonstrated in gram-scale nitration of crown ether and mono nitration of Sesamin. A plausible free radical mechanism is proposed based on detailed experimental observations.     

Eco-Friendly Methodology for the Formation of Aromatic Carbon–Heteroatom Bonds by Using Green Ionic Liquids

A new sustainable method is reported for the formation of aromatic carbon–heteroatom bonds under solvent-free and mild conditions (no co-oxidant, no strong acid and no toxic reagents) by using a new type of green ionic liquid. The bromination of methoxy arenes was chosen as a model reaction. The reaction methodology is based on only using natural sodium bromine, which is transformed into an electrophilic brominating reagent within an ionic liquid, easily prepared from the melted salt FeCl₃ hexahydrate. Bromination reactions with this in-situ-generated reagent gave good yields and excellent regioselectivity under simple and environmentally friendly conditions. To understand the unusual bromine polarity reversal of sodium bromine without any strong oxidant, the molecular structure of the reaction medium was characterised by Raman and direct infusion electrospray ionisation mass spectroscopy (ESI-MS). An extensive computational investigation using density functional theory methods was performed to describe a mechanism that suggests indirect oxidation of Br⁻ through new iron adducts. The versatility of the methodology was successively applied to nitration and thiocyanation of methoxy arenes using KNO₃ and KSCN in melted hexahydrated FeCl₃.     

Hydrogenation of arenes, alkenes and alkynes catalyzed by a sol–gel entrapped mixture of [Rh(cod)Cl]2 and Na[HRu3(CO)11]

A sol–gel entrapped 1:3 mixture of [Rh(cod)Cl]₂ and Na[HRu₃(CO)₁₁] catalyzes the hydrogenation of various unsaturated substrates by two distinguishable mechanisms. Under 13.8 bar H₂ and 20 °C methylated arenes react rapidly to give cycloalkane derivatives. XRD and TEM studies showed that under these conditions the hydrogenation proceeds without the generation of free metal particles. The hydrogenation of non-methylated arenes, as well as that of alkenes and alkynes, require a temperature of 80–120 °C at which the entrapped complexes form metallic nano-particles of 3–5 nm. Chloroarenes are also hydrodechlorinated at 120 °C, but require a hydrogen pressure of ≥25 bar. At both temperature ranges the catalysts are reusable at least four times. The high efficiency of the hydrogenation process at 20 °C is rationalized by a synergistic effect between the two different metal atoms of the combined catalyst. This may be related to a remote control model through a hydrogen spillover mechanism.