Monocyclic Arenes Research Articles

Photoredox-Enabled Dearomative [2π + 2σ] Cycloaddition of Phenols.

Dearomative photocycloaddition of monocyclic arenes is an appealing strategy for comprehending the concept of "escape from flatland". This brings the replacement of readily available planar aromatic hydrocarbon units with a 3D fused bicyclic core with sp3-enriched carbon units. Herein, we outline an intermolecular approach for the dearomative photocycloaddition of phenols. In order to circumvent the ground-state aromaticity and to construct conformationally restrained building blocks, bicyclo[1.1.0]butanes were chosen as coupling partners. This dearomative approach renders straightforward access to a bicyclo[2.1.1]hexane unit fused to a cyclic enone moiety, which further contributed as a synthetic linchpin for postmodifications. Mechanistic experiment advocates for a plausible onset from both the reactants, depending on the redox potential.

Mechanistic and Synthetic Studies of Biaryl Birch Reductions

AbstractThe Birch reduction of biaryls generally converts one of the two arenes into a cyclohexa-1,4-diene. Biaryls are more reactive than monocyclic arenes under the Birch conditions. Unlike the reduction of monocyclic arenes, biaryl reduction proceeds through two consecutive electron transfer steps before the protonation of the dianion intermediate. The biaryl reductions and subsequent alkylations in one pot rapidly increase the molecular complexity and thus have been used in the synthesis of natural products and drug-like molecules.1 Introduction2 The Physical Organic Chemistry of the Birch Reduction of Biaryls3 Biaryls as the Mediators of Electron Transfer4 Methods for the Dissolving-Metal Reduction of Biaryls5 Intercepting the Biaryl Reduction Intermediates with Electrophiles6 Synthetic Applications of the Dissolving-Metal-Mediated Reductions of Biaryls7 Outlook

Facile synthesis of multifunctional poly(ionic liquid)s as an advanced three-way catalyst for the efficient production of lignite-derived arenes

The precise synthesis of multifunctional poly(ionic liquid)s (MFPILs) featuring basic sites, mesoporous channels, and hydrophobicity continues to be a substantial challenge. Moreover, the directional conversion of lignite to monocyclic arenes (MCAs) is highly attractive but particularly challenging on account of its geometrical complexity and chemical inertness. The efficiency of lignite directional conversion to MCAs over solid bases is substantially decreased by the presence of water molecules leading to the deactivation and leaching of basic sites. Herein, the novel solid bases, i.e., MFPILs with dibasic sites, unique hydrophobicity, and abundant mesoporous channels, were successfully fabricated by a radical multicomponent cross-linked copolymerization for the first time. Even more excitedly, the resulting MFPILs were demonstrated to be very efficient for directional catalytic hydroconversion of extraction residue from Dahuangshan lignite (DLER) into MCAs with 32.3 wt% yield, which is by far the highest value reported for lignite-derived MCAs by solid bases under similar conditions. The synergistic catalysis, i.e., hydrophobic microenvironment, mesoporous confinement, and synergistic effect of dibasic sites, for active hydrogen transfer of DLER conversion into MCAs over MFPILs is confirmed by characterization methods. The work not merely sheds substantial light on developing the novel solid bases, but also exhibits a potentially powerful catalyzing strategy to synthesize MCAs.

Dearomative Ring Expansion of Polycyclic Arenes

Benzocycloheptenes constitute a common structural motif embedded in many natural products and biologically active compounds. Herein, we report their concise preparation from non-activated polycyclic arenes using a two-step sequence involving dearomative [4+2]-cycloaddition with arenophile in combination with palladium-catalyzed cyclopropanation, followed by cycloreversion-initiated ring expansion. The described strategy provides a working alternative to the Buchner reaction, which is limited to monocyclic arenes. Overall, this methylene-insertion molecular editing approach enables rapid and direct conversion of simple (hetero)arenes into a range of substituted (aza)benzocycloheptatrienes, which can undergo a myriad of downstream functionalizations.

Dearomative Ring Expansion of Polycyclic Arenes.

Benzocycloheptenes constitute a common structural motif embedded in many natural products and biologically active compounds. Herein, we report their concise preparation from non‐activated polycyclic arenes using a two‐step sequence involving dearomative [4+2]‐cycloaddition with arenophile in combination with palladium‐catalyzed cyclopropanation, followed by cycloreversion‐initiated ring expansion. The described strategy provides a working alternative to the Buchner reaction, which is limited to monocyclic arenes. Overall, this methylene‐insertion molecular editing approach enables rapid and direct conversion of simple (hetero)arenes into a range of substituted (aza)benzocycloheptatrienes, which can undergo a myriad of downstream functionalizations.

Luminescence of Aromatic Compounds During Ultrasonic Treatment of Tb2(SO4)3 Suspension in Commercial Gasoline.

The spectral-luminescent properties of a suspension of terbium sulfate in commercial gasoline under sonication are studied. The following emitters are identified from the luminescence spectrum: *Tb3+ ions in crystals, electronically excited molecules of arenes (benzene, toluene, and xylenes), and polycyclic aromatic hydrocarbons (PAHs) in the liquid phase of the suspension. The study of sonotriboluminescence in gasoline-heptane and heptane-xylene-PAH synthetic mixtures shows for the first time that there is an effective luminescence activator in gasoline, that is, terphenyl molecules. It has been established that these molecules have the highest luminescence yield among all sonotriboluminescence emitters found. This is provided by the transfer of the excitation energy from monocyclic arene molecules primarily excited during the sonication. In this case, the primary excitation of aromatic hydrocarbon molecules in commercial gasoline during the ultrasonic treatment of suspensions occurs under the impact of charged particles/electrons generated during electrical discharges initiated by collision and destruction of microcrystals. This process is similar to radioluminescence excitation in liquid scintillators which can be considered commercial gasoline. The formation of *Tb3+ is due to separation and recombination processes of charges that populate the excited states of luminescence centers in microcrystals electrified during tribodestruction under sonication of the suspension.

Extraction–Chromatographic Determination of the Total Concentration of Monocyclic Arenes С 6 –С 9 in Wastewater

To control the technogenic pollution of water bodies, we proposed to determine the total concentration monocyclic arenes C6–C9 (cAr, mg/L), which are the most toxic and readily water-soluble hydrocarbons. In contrast to the widely used indicator “petroleum products,” the value of сAr directly characterizes the toxicity of the water under study. We developed a procedure for the extraction–chromatographic determination of cAr in wastewater, which does not require the evaporation of the extract or the preparative separation of arenes from other hydrocarbons. The cAr values were calculated from the total areas of arene peaks identified in the chromatogram of a purified hexane extract. To take into account the loss of arenes during sample preparation, the calibration dependence was constructed using multicomponent aqueous solutions with known cAr values (simulating solutions), by passing them through the same operations as the samples of the test water. The lower limit of the determined values of cAr is 0.1 mg/L; the errors in determining cAr do not exceed 15 rel %; the duration of analysis of a single sample is 2 h. The procedure was tested in the analysis of various types of wastewater, verified, and patented.

Precise Fabrication of Novel Multifunctional Poly(Ionic Liquid)s: Synergistic Catalysis of Dibasic Sites, Hydrophobic Microenvironment, and Mesoporous Confinement in Ultra-Efficient Production of Lignite-Based Monocyclic Arenes

Precise Fabrication of Novel Multifunctional Poly(Ionic Liquid)s: Synergistic Catalysis of Dibasic Sites, Hydrophobic Microenvironment, and Mesoporous Confinement in Ultra-Efficient Production of Lignite-Based Monocyclic Arenes

Chemical Equivalent of Arene Monooxygenases: Dearomative Synthesis of Arene Oxides and Oxepines.

Direct epoxidation of aromatic nuclei by cytochrome P450 monooxygenases is one of the major metabolic pathways of arenes in eukaryotes. The resulting arene oxides serve as versatile precursors to phenols, oxepines, or trans-dihydrodiol-based metabolites. Although such compounds have an important biological and chemical relevance, the lack of methods for their production has hampered access to their utility. Herein, we report a general arenophile-based strategy for the dearomative synthesis of arene oxides. The mildness of this method permits access to sensitive monocyclic arene oxides without any noticeable decomposition to phenols. Moreover, this method enables direct conversion of polycyclic arenes and heteroarenes into the corresponding oxepines. Finally, these studies provided direct connection between simple aromatic precursors and complex small organic molecules via arene oxides and oxepines.

Chemoenzymatic Synthesis of (-)-Ribisins A and B from Dibenzo[b,d]furan.

cis-Dihydrodiols, derived from monocyclic aromatic compounds, are valuable chiral pool intermediates for the synthesis of cyclic natural products. A drawback of this approach, to the synthesis of polycyclic secondary metabolites, is that additional rings must be annulated. To date, relatively few chiral natural products have been synthesized from polycyclic arene cis-dihydrodiols. Fungal metabolites, (-)-ribisins A and B, have now been obtained by functional group manipulation of a tricyclic arene metabolite, obtained from toluene dioxygenase-catalyzed regioselective and stereoselective cis-dihydroxylations of dibenzo[b,d]furan. The synthetic sequences were marginally shorter than the alternative routes, using monocyclic arene cis-dihydrodiols, and required no carbon-carbon bond-forming reactions.

Breaking the Limit of Lignin Monomer Production via Cleavage of Interunit Carbon–Carbon Linkages

Summary Conversion of lignin into monocyclic hydrocarbons as commodity chemicals and drop-in fuels is a highly desirable target for biorefineries. However, this is severely hindered by the presence of stable interunit carbon–carbon linkages in native lignin and those formed during lignin extraction. Herein, we report a new multifunctional catalyst, Ru/NbOPO4, that achieves the first example of catalytic cleavage of both interunit C–C and C–O bonds in one-pot lignin conversions to yield 124%–153% of monocyclic hydrocarbons, which is 1.2–1.5 times the yields obtained from the established nitrobenzene oxidation method. This catalyst also exhibits high stability and selectivity (up to 68%) to monocyclic arenes over repeated cycles. The mechanism of the activation and cleavage of 5–5 C–C bonds in biphenyl, as a lignin model adopting the most robust C–C linkages, has been revealed via in situ inelastic neutron scattering coupled with modeling. This study breaks the conventional theoretical limit on lignin monomer production.

Properties of residual marine fuel produced by thermolysis from polypropylene waste

Thermal degradation of waste plastics with the aim of producing liquid fuel is one of the alternative solutions to landfill disposal or incineration. The paper describes thermal conversion of polypropylene waste and analysis of produced liquid fuel that would satisfy ISO 8217-2012 requirements for a residual marine fuel. Single pass batch thermolysis processes were conducted at different own vapour pressures (20-80 barg) that determined process temperature, residence time of intermediates what resulted in different yields of the liquid product. Obtained products were stabilized by rectification to achieve required standard flash point. Gas chromatography and 1H NMR spectrometry show aliphatic nature of the liquid product where majority of the compounds are isoalkanes and isoalkenes. Only lightest fractions boiling up to a temperature of 72 oC have significant amount of n-pentane. Distribution of aromatic hydrocarbons is not even along the boiling range. The fractions boiling at a temperature of 128 oC and 160 oC have the highest content of monocyclic arenes – 3.16 % and 4.09 % respectively. The obtained final liquid residual product meets all but one requirements of ISO 8217-2012 for residual marine fuels.DOI: http://dx.doi.org/10.5755/j01.ms.21.2.6105

Arene production by W2C/MCM-41-catalyzed upgrading of vapors from fast pyrolysis of lignin

W2C/MCM-41-catalyzed upgrading of vapors from fast pyrolysis of lignin was investigated with the aim of arene production. The experiments were conducted in a micro pyrolyzer-gas chromatography/mass spectrometer (P-GC/MS). A series of W2C/MCM-41 catalysts with different catalyst loading amount (Si/W) were prepared and the activity, selectivity, and stability of the catalysts were investigated. Associated with the analysis results of P-GC/MS for the pyrolysis vapors, the optimal condition for producing arenes was found. Besides, possible mechanisms for lignin conversion with the as-prepared catalysts were discussed. In the presence of W2C/MCM-41 (Si/W=50), the arenes yield is about 20% of volatile product and the selectivity for monocyclic arenes is over 85% at 750°C. The yield of arenes increases significantly with the increase of catalyst-to-lignin mass ratio (C/L). According to the run tests, the conversion of arenes decreases slightly by 1% or 2% after each run, indicating that the catalyst features good stability under the experimental conditions. Thus, W2C/MCM-41 catalysts effectively catalyzed the formation of monocyclic arenes in the fast pyrolysis process.

Long-Lived Radical Cation Salts Obtained by Interaction of Monocyclic Arenes with Niobium and Tantalum Pentahalides at Room Temperature: EPR and DFT Studies

The 1:3 reactions of the alkoxy arenes 1,4-(MeO)2 C6 H4 and 1,4-F2 -2,5-(MeO)2 C6 H2 with TaF5 in chloroform at 40-50 °C resulted in formation in about 35 % yield of the long-lived radical cation salts [1,4-(MeO)2 C6 H4 ][Ta2 F11 ] (2 a) and [1,4-F2 -2,5-(MeO)2 C6 H2 ][Ta2 F11 ] (2 b), respectively. The non-alkoxy-substituted [arene][M2 X11 ] [M=Ta, X=F: arene=C6 H5 Me (2 c), 1,4-C6 H4 Me2 (2 d), C6 H5 F (2 e), C6 H5 NO2 (2 f); M=Nb, X=F: arene=C6 H5 Me (4 a), 1,4-C6 H4 Me2 (4 b), C6 H5 F (4 c), C6 H5 NO2 (4 d); M=Ta, X=Cl: arene=1,4-C6 H4 Me2 (5)] were obtained from the 3:1 reactions of MX5 with the appropriate arene in chloroform at temperatures in the range 40-90 °C. Compounds 2-5 were detected by EPR spectroscopy (in CHCl3 ) at room temperature, and their gas-phase structures were optimized by DFT calculations. Formation of the M(IV) species [MX4 (NCMe)2 ] [M=Ta, X=F (3 a); M=Nb, X=F (3 b); M=Ta, X=Cl (3 c)] was ascertained by EPR spectroscopy on solutions obtained by treatment of the reaction mixtures with acetonitrile. Non-selective reactions occurred upon combination of 1,4-F2 -2,5-(MeO)2 C6 H2 with AgNbF6 (in CH2 Cl2 ) and 1,4-(MeO)2 C6 H4 with SbF5 .

Substituent effects on the dehydration of arene hydrates in aqueous solution

Rate constants have been determined by UV spectrophotometry at 25 °C for the acid‐catalyzed dehydration of different types of monocyclic arene hydrates including those substituted at the 1‐, 2‐ or 3‐positions. General acid catalysis was not observed, and linear plots of pseudo‐first‐order rate constants for dehydration against hydronium concentration were obtained. A Hammett plot of the second‐order rate constants for acid‐catalyzed dehydration, kH (M−1s−1), of unsubstituted‐ (8a), 3‐substituted (8b, 8c, 8d, 8e) and 1‐substituted‐benzene hydrates (14f and 14h) shows an excellent correlation with σ+ values and yields a large negative ρ‐value of –6.5. The results are consistent with rate‐determining formation of a benzenium ion in which direct mesomeric interaction with the substituent occurs, presumably permitted by the coplanar arrangement of the diene and carbocation centre in the intermediate. Data points for 2‐substituted arene hydrates (13f, 13g, 13h, 13i) deviate negatively from the Hammett plot as direct mesomeric interaction with the substituent is not possible in the corresponding benzenium intermediates. Copyright © 2013 John Wiley & Sons, Ltd.

Chemoenzymatic synthesis of monocyclic arene oxides and arene hydrates from substituted benzene substrates

Enantiopure cis-dihydrodiol bacterial metabolites of substituted benzene substrates were used as precursors, in a chemoenzymatic synthesis of the corresponding benzene oxides and of a substituted oxepine, via dihydrobenzene oxide intermediates. A rapid total racemization of the substituted benzene 2,3-oxides was found to have occurred, via their oxepine valence tautomers, in accord with predictions and theoretical calculations. Reduction of a substituted arene oxide to yield a racemic arene hydrate was observed. Arene hydrates have also been synthesised, in enantiopure form, from the corresponding dihydroarene oxide or trans-bromoacetate precursors. Biotransformation of one arene hydrate enantiomer resulted in a toluene-dioxygenase catalysed cis-dihydroxylation to yield a benzene cis-triol metabolite.

ChemInform Abstract: Enantiomeric Excess and Absolute Configuration Determination of cis- Dihydrodiols from Bacterial Metabolism of Monocyclic Arenes.

ChemInform Abstract: Enantiomeric Excess and Absolute Configuration Determination of cis- Dihydrodiols from Bacterial Metabolism of Monocyclic Arenes.

Long‐Lived Radical Cations of Monocyclic Arenes at Room Temperature Obtained by NbF5 Acting as an Oxidizing Agent and Counterion Precursor

Long‐Lived Radical Cations of Monocyclic Arenes at Room Temperature Obtained by NbF₅ Acting as an Oxidizing Agent and Counterion Precursor

Long‐Lived Radical Cations of Monocyclic Arenes at Room Temperature Obtained by NbF5 Acting as an Oxidizing Agent and Counterion Precursor

Salt of the earth: Radical cation salts of monocyclic arenes, including benzene, have been obtained in a reaction where niobium pentafluoride behaves both as an oxidizing agent (conversion into NbF4) and fluoride acceptor (to afford the counterion [Nb2F11]−, see picture). Anion–π-electron density interactions, as revealed by experimental and computational studies, are crucial in providing unprecedented inertness to the radical cations.

Chemical and Electrochemical Reduction of Polyarene Manganese Tricarbonyl Cations: Hapticity Changes and Generation of Syn- and Anti-Facial Bimetallic η4,η6-Naphthalene Complexes

(Eta6-naphthalene)Mn(CO)(3)(+) is reduced reversibly by two electrons in CH(2)Cl(2) to afford (eta4-naphthalene)Mn(CO)(3)(-). The chemical and electrochemical reductions of this and analogous complexes containing polycyclic aromatic hydrocarbons (PAH) coordinated to Mn(CO)(3)(+) indicate that the second electron addition is thermodynamically easier but kinetically slower than the first addition. Density functional theory calculations suggest that most of the bending or folding of the naphthalene ring that accompanies the eta6 --> eta4 hapticity change occurs when the second electron is added. As an alternative to further reduction, the 19-electron radicals (eta6-PAH)Mn(CO)(3) can undergo catalytic CO substitution when phosphite nucleophiles are present. Chemical reduction of (eta6-naphthalene)Mn(CO)(3)(+) and analogues with one equivalent of cobaltocene affords a syn-facial bimetallic complex (eta4,eta6-naphthalene)Mn(2)(CO)(5), which contains a Mn-Mn bond. Catalytic oxidative activation under CO reversibly converts this complex to the zwitterionic syn-facial bimetallic (eta4,eta6-naphthalene)Mn(2)(CO)(6), in which the Mn-Mn bond is cleaved and the naphthalene ring is bent by 45 degrees . Controlled reduction experiments at variable temperatures indicate that the bimetallic (eta4,eta6-naphthalene)Mn(2)(CO)(5) originates from the reaction of (eta4-naphthalene)Mn(CO)(3)(-) acting as a nucleophile to displace the arene from (eta6-naphthalene)Mn(CO)(3)(+). Heteronuclear syn-facial and anti-facial bimetallics are formed by the reduction of mixtures of (eta6-naphthalene)Mn(CO)(3)(+) and other complexes containing a fused polycyclic ring, e.g., (eta5-indenyl)Fe(CO)(3)(+) and (eta6-naphthalene)FeCp(+). The great ease with which naphthalene-type manganese tricarbonyl complexes undergo an eta6 --> eta4 hapticity change is the basis for the formation of both the homo- and heteronuclear bimetallics, for the observed two-electron reduction, and for the far greater reactivity of (eta6-PAH)Mn(CO)(3)(+) complexes in comparison to monocyclic arene analogues.