Direct synthesis of phenols by iron-catalyzed biphasic oxidation of aromatic hydrocarbons with hydrogen peroxide

The hydroxylation of a series of aromatic hydrocarbons with hydrogen peroxide, catalyzed by iron complexes with pyrazine-3-carboxylic acid N-oxide, was investigated, operating in a biphasic reaction medium.

The new catalyst showed a high selectivity to the corresponding phenols, minimizing the over-oxidation reactions to polyoxygenated derivatives and tars which, along with dimers formation, are the major limitations of the classical Fenton’s reagent for a practical synthetic application.

In the case of alkylbenzenes, the competitive side chain oxidation at the benzylic positions also occurred. Electron rich substrates, such as anisole, were oxidized with very poor selectivity.

The reactions were carried out in a biphasic system that allows a convenient recovery and recycling of the catalyst by phase separation techniques. The catalyst showed a complete retention of activity after six consecutive reaction cycles.

The new catalyst appears as a promising tool for the direct synthesis of phenols, in alternative to the conventional multi-step methods.     

Highly Strained Oxygen-Doped Chiral Molecular Belts of the Zigzag-Type with Strong Circularly Polarized Luminescence

Herein we report a two-directional cyclization strategy for the synthesis of highly strained depth-expanded oxygen-doped chiral molecular belts of the zigzag-type. From the easily accessible resorcin[4]arenes, an unprecedented cyclization cascade generating fused 2,3-dihydro-1H-phenalenes has been developed to access expanded molecular belts. Stitching up the fjords through intramolecular nucleophilic aromatic substitution and ring-closing olefin metathesis reactions furnished a highly strained O-doped C₂-symmetric belt. The enantiomers of the acquired compounds exhibited excellent chiroptical properties. The calculated parallelly aligned electric ( μ ) and magnetic ( m ) transition dipole moments are translated to the high dissymmetry factor (|g_lum| up to 0.022). This study provides not only an appealing and useful strategy for the synthesis of strained molecular belts but also a new paradigm for the fabrication of belt-derived chiroptical materials with high CPL activities.     

Alignment and Dynamic Inversion of Planar Chirality in Pillar[n]arenes

Pillar[n]arenes are symmetrical macrocyclic compounds composed of benzene panels with para-methylene linkages. Each panel usually exhibits planar chirality and prefers chirality-aligned states. Because of this feature, pillar[n]arenes are attractive scaffolds for chiroptical materials that are easy to prepare and optically resolve and show intense circular dichroism (CD) signals. In addition, rotation of the panels endows the chirality of pillar[n]arenes with a dynamic nature. The chirality in tubular oligomers and supramolecular assemblies sometimes show time- and procedure-dependent alignment phenomena. Furthermore, the CD signals of some pillar[n]arenes respond to the addition of chiral guests when their dynamic chirality is coupled with host–guest properties. By using diastereomeric pillar[n]arenes with additional chiral structures, the response can also be caused by achiral guests and changes of the environment, providing molecular sensors.     

Synthesis of 3-aryl-3,4-dihydroisocoumarins by regioselective domino ‘[3+3] cyclization/lactonization’ reactions of 1,3-bis-(silyloxy)-1,3-butadienes with 1-hydroxy-5-silyloxy-4-en-3-ones

The [3+3] cyclization of 1,3-bis(silyloxy)-1,3-butadienes with 1-hydroxy-5-silyloxy-4-en-3-ones afforded 6-(2-aryl-2-chloroethyl)salicylates, which were transformed into 3-aryl-3,4-dihydroisocoumarins by silica gel-mediated lactonization.     

Synthesis of 5-(2-aryl-2-haloethyl)salicylates by the first domino ‘[3+3] cyclization/ring-cleavage’ reactions of 1,3-bis(silyloxy)-1,3-butadienes with 3-acetyl-5-aryl-4,5-dihydrofurans

5-(2-Aryl-2-haloethyl)salicylates are efficiently prepared by the first domino ‘[3+3] cyclization/ring-cleavage’ reactions of 1,3-bis(silyloxy)-1,3-butadienes with 3-acetyl-5-aryl-4,5-dihydrofurans.     

A NMR, X-ray, and DFT combined study on the regio-chemistry of nucleophilic addition to platinum(II) coordinated terminal olefins

The series of platinum complexes [PtCl(η²-CH₂CH-C₆H₄-X)(tmeda)](ClO₄) (X = H, 1b; 4-OMe, 1c; 3-OMe, 1d; 4-CF₃, 1e; 3-CF₃, 1f; 3-NO₂, 1g; tmeda = N,N,N′,N′-tetramethyl-1,2-ethanediamine) has been considered. In the styrene complex (1b) both solution (NMR) and solid state (X-ray) data indicate a significant difference in the Pt-C bond lengths (the longer bond being that involving the olefin carbon atom carrying the phenyl ring). Such a difference increases when X is an electron donor group (EDG, 1c) and decreases when X is an electron withdrawing group (EWG, 1d-g). The attack of a nucleophile (MeO⁻) to the substituted carbon (Markovnikov type, M) is by far the most favoured in the case of unsubstituted (1b) or EDG-substituted (1c) styrenes. The presence of an EWG (compounds 1d-g) levels off the probability of M and anti-M type of attack. DFT calculations on 1b,c and 1e were also performed. The NLMO analysis reveals the crucial role of the interaction between the filled π orbital of the olefin and the empty d orbital of platinum; the carbon with greater electron density becoming less susceptible of nucleophilic attack.     

Tuning the selectivity of ruthenium nanoscale catalysts with functionalised ionic liquids: Hydrogenation of nitriles

Ruthenium nanoparticles prepared in nitrile-functionalised ionic liquids (ILs) display unusual selectivities toward the hydrogenation of nitrile containing aromatic compounds. In particular, a selective catalytic hydrogenative coupling of nitriles was observed. In this transformation, nitrile groups are exclusively hydrogenated in the presence of arenes, which are typically hydrogenated by ruthenium nanoparticles in non-functionalised ILs. The catalyst material was characterised by means of TEM and EDS analysis. Furthermore, molecular species formed during the catalytic process were characterised by MS-analysis of the gaseous phase and the ionic liquid phase by ESI/MS Q-TOF.     

Cross-coupling of arene–gold(III) complexes

The reactivity of electron-deficient arene–gold(III) complexes toward nucleophilic aromatic and heteroaromatic counterparts has been studied. 1-Methylindole proved to be the best reaction partner while trimethoxybenzenes did not react. The ancillary ligand on gold also influenced the reactivity in the order PPh₃>P^tBu₃>IPr. An oxidative cross-coupling starting from the corresponding gold(I) complexes in presence of hypervalent iodide oxidants was also studied.     

Encapsulation of Triphenylene Derivatives in the Hexanuclear Arene Ruthenium Metallo‐Prismatic Cage [Ru6(p‐PriC6H4Me)6(tpt)2(dhbq)3]6+ (tpt = 2,4,6‐tri(pyridin‐4‐yl)‐1,3,5‐triazine,dhbq = 2,5‐dihydroxy‐1,4‐benzoquinonato)

A large cationic triangular metallo‐prism, [Ru₆(p‐PrⁱC₆H₄Me)₆(tpt)₂(dhbq)₃]⁶⁺ ( 1 )⁶⁺, incorporating p‐cymene ruthenium building blocks, bridged by 2,5‐dihydroxy‐1,4‐benzoquinonato (dhbq) ligands, and connected by two 2,4,6‐tri(pyridin‐4‐yl)‐1,3,5‐triazine (tpt) subunits, allows the permanent encapsulation of the triphenylene derivatives hexahydroxytriphenylene, C₁₈H₆(OH)₆ and hexamethoxytriphenylene, C₁₈H₆(OMe)₆. These two cationic carceplex systems [C₁₈H₆(OH)₆⊂ 1 ]⁶⁺ and [C₁₈H₆(OMe)₆⊂ 1 ]⁶⁺ have been isolated as their triflate salts. The molecular structure of these systems has been established by one‐dimensional ¹H ROESY NMR experiments as well as by the single‐crystal structure analysis of [C₁₈H₆(OMe)₆⊂ 1 ][O₃SCF₃]₆.     

A novel aza-Prins-Friedel-Crafts reaction for the synthesis of 4-arylpiperidines

Aldehyde, homoallylic amine and arene undergo smooth cyclization in the presence of BF₃·OEt₂ to afford 4-arylpiperidines in good yields and with high trans-selectivity. This is the first report on the preparation of 4-arylpiperidines via aza-Prins-Friedel-Crafts reaction.