Metal-assisted reactions. Part 9. Rapid replacement of phenolic hydroxyl groups by hydrogen.

Conversion of phenolic ethers to hydrocarbons by catalytic transfer hydrogenation is reported.     

A new method for determining positions of phenolic hydroxyl groups through silylation and application of H(Si)C triple-resonance NMR experiments

Two-dimensional triple-resonance H(Si)C NMR experiments have been applied at natural abundance to assign ¹³C NMR signals in silylated phenols. The method showing its great potential in determining positions of hydroxyl groups is widely applicable to signal assignment and structure elucidation of synthetic and natural phenolic compounds.     

Chemoselective aminomethylation of bifunctional substrates: carbonyl versus phenolic hydroxyl,carbonyl versus pyrazole and pyrrole versus phenolic hydroxyl as competing activating groups

Chemoselectivity in the Mannich reaction for three different types of bifunctional substrates has been investigated. 1-Hydroxy-2-naphthalenylethanone affords either phenolic Mannich bases at high pH (free amines), or ketonic Mannich bases at low pH (amine hydrochlorides), whereas the use of N,N-dimethylmethyleneiminium chloride as a preformed dimethylaminomethylation reagent gave the phenolic Mannich base. 1-Aryl-3-(1H-pyrazol-1-yl)-1-propanones undergo aminomethylation at position 4 of the pyrazole ring, and not at the methylene group α to the carbonyl function, regardless of the reaction conditions. 4-(2,5-Dimethyl-1H-pyrrol-1-yl)phenol is aminomethylated chemoselectively on the pyrrole ring under mild reaction conditions.     

The assignment of IR absorption bands due to free hydroxyl groups in cellulose

Interpretation of the IR hydroxyl absorption bands in cellulose has been limited to the inter- and intramolecularly hydrogen-bonded hydroxyl groups in the crystalline form. This paper attempts to assign IR frequencies due to ‘free‘ or non-hydrogen bonded hydroxyl groups by using a curve fitting method. The almost completely methylated cellulose derivatives of tritylcellulose (previously used in related studies) exhibited small IR bands due to hydroxyl groups. The IR bands were assumed to appear under stereohindered conditions and thus resulted in a mixture of bands which included the contribution of free hydroxyl groups. The curve fitting method deconvoluted the IR bands into three bands in the OH stretching region: they were interpreted in terms of free or hydrogen bonded hydroxyl groups. The assignments were confirmed by comparison of an almost completely methylated derivative with partially methylated derivatives having different degrees of substitution. In addition, intramolecular hydrogen bonds involving OH at the C-3, C-2 and C-6 positions were shown to be easily formed, even between extremely small numbers of unsubstituted hydroxyl groups present, and thus cause perturbation of the specific deconvoluted band. This revised version was published online in August 2006 with corrections to the Cover Date.     

Crystal structure and tautomerism in 4-phenylisoxazoles with two potential hydroxyl groups at positions 3 and 5

The crystal structure of 4-phenyl-3-hydroxyisoxazol-5-one semihydrate exhibits an exceptional hydrogen bonded polymeric structure with a unit cell of 8 molecules. The hydrogen bonds stretch out, using the oxohydroxy groups in positions 3 and 5 in the direction of one axis and along a perpendicular direction the layers are stitched together by water molecules. The layers are stitched by using four hydrogen bridges of water molecules, the heterocyclic ring nitrogen as well as the oxygen at position 5. Tautomerism of this moiety in solution is discussed, in light of some new dialkylation products. The state in which these products exist in solution depends on the solvent. A zwitterionic tautomer is present in ether. In some alkylation conditions, the predominant dialkylation product is the N,N-disubstituted betaine (Anhydro-2,2-dialkyl-3(5)-oxo-5(3)hydroxy-4-phenylisoxazolonium hydroxide). Study of tautomerism in polar and protic solvent is unreliable owing to associations and dissociation phenomena.     

Interaction of phenolic antioxidants and hydroxyl radicals

Based on pulse radiolysis of aqueous solutions of four phenolic antioxidants including green tea polyphenols, quercetin, caffeic acid and sinapic acid the rate constants for reactions of OH and the antioxidants were determined. And green tea polyphenols and quercetin are the strongest antioxidants.     

Spectroscopic investigation of phenolic groups ionization in the vipoxin neurotoxic phospholipase A2: comparison with the X-ray structure in the region of the tyrosyl residues

The neurotoxin vipoxin is the major lethal component of the venom of Vipera ammodites meridionalis, the most toxic snake in Europe. It is a complex between a toxic phospholipase A2 (PLA2) and a non-toxic protein inhibitor (Inh). Tyrosyl residues are involved in the catalytic site (Tyr 52 and 73) and in the substrate binding (Tyr 22). Spectroscopic studies demonstrated differences in the ionization behavior of the various phenolic hydroxyl groups in the toxic PLA2. The tyrosyl side chains of the enzyme can be classified into three groups: (a) three phenolic hydroxyls are accessible to the solvent and titrate normally, with a pKeff = 10.45; (b) three residues are partially 'buried' and participate in hydrogen bonds with neighboring functional groups. They titrate anomalously with a pKeff = 12.17; (c) two tyrosines with a pKeff = 13.23 are deeply 'buried' in the hydrophobic interior of PLA2. They became accessible to the titrating agent only after alkaline denaturation of the protein molecule. The spectroscopic data are related to the X-ray structure of the vipoxin PLA2. The refined model was investigated in the region of the tyrosyl side chains. The accessible surface area of each tyrosyl residue and each phenolic hydroxyl group was calculated. A good correlation between the spectrophotometric and the crystallographic data was observed. The ionization behavior of the phenolic groups is explained by peculiarities of the protein three-dimensional structure and the participation of tyrosines in the catalytic site hydrogen bond network. Attempts are made to assign the calculated pKeff values to individual residues. The high degree of 'exposure' on the protein surface of Tyr 22 and 75 is probably important for their function as parts of the substrate binding and pharmacological sites.     

Analysis of phenolic acids and flavonoids in honey

Honey is rich in phenolic acids and flavonoids, which exhibit a wide range of biological effects and act as natural antioxidants. The analysis of polyphenols has been regarded as a very promising way of studying floral and geographical origins of honeys. This review surveys recent literature on determination of these active compounds in honey. The analytical procedure to determine individual phenolic compounds involves their extraction from the sample matrix, analytical separation and quantification. We pay particular attention to sample pre-treatment and separation techniques (e.g., high-performance liquid chromatography and electrophoresis).     

Spectroscopic detection of free “lone-pair” groups or terminal methanol molecules in methanol solutions

When basic aprotic solvents are added to methanol they become hydrogen bonded, and there is a consequent growth in non-bonded lone-pairs, (LP)_free. Although corresponding non-bonded OH groups, (OH)_free, have been detected for alcohols and for water, using overtone infrared spectroscopy, no different spectroscopic evidence for (LP)_free groups has previously been reported. We have found that unique OH stretching bands develop when strongly basic solvents such as dimethylsulphoxide are added to methanol. Band maxima assigned to (LP)_free groups occur at 3440 cm^?1 in the fundamental and 6790 cm^?1 in the overtone region. These are at considerably higher frequencies than those for bulk methanol (3340 cm^?1 and 6600 cm^?1) showing that the hydrogen bond is weakened in this unit, as expected. Proton resonance shifts for the OH protons of methanol on adding basic aprotic cosolvents are reported, and explained in terms of these results.     

Spectroscopic properties of orthometalated platinum(II) bipyridine complexes containing various ethynylaryl groups

Neutral orthometalated platinum(II) complexes of the deprotonated 6-phenyl-2,2'-bipyridine ligand (bearing a trialkoxygallate, tolyl, ethynyltrialkoxygallate, or ethynyltolyl substituent) and a sigma-bonded Cl, ethynyltolyl, or ethynyltrialkoxygallate coligand have been prepared by a stepwise procedure based on copper-promoted cross-coupling reactions. The X-ray structure of the [2-(p-tolyl)ethynyl][4-{2-(p-tolyl)ethynyl}-6-phenyl-2,2'-bipyridyl)]platinum(II) complex revealed a coplanar arrangement of all residues bound to platinum, although the tolylethynyl groups exhibit position-dependent bending in the solid state. The complexes exhibit charge-transfer absorption in the visible region. All except two of the complexes also exhibit charge-transfer emission, typically from an excited state that has a submicrosecond lifetime at room temperature in deoxygenated dichloromethane solution. In accordance with the presence of a carbometalated polypyridine ligand, the emitting state is assumed to have a mixture of metal-to-ligand charge-transfer (MLCT) and intra-ligand charge-transfer (ILCT) character. However, spectral comparisons and electrochemical data suggest that the emissive state also exhibits interligand charge-transfer (LLCT) character when an electron-rich ethynylaryl group is bound to platinum. In keeping with altered orbital parentage in the latter systems, the emission occurs at longer wavelength. The excited-state lifetime is also shorter, evidently due to vibronic interactions. The decay is so efficient when an ethynyltrialkoxygallate group binds to platinum that there is no detectable emission in fluid solution, although the complexes do emit in a frozen glass. The excited states are subject to associative (exciplex) quenching by Lewis bases, but the admixture of ILCT and/or LLCT character diminishes efficiency, except for relatively strong bases like dimethyl sulfoxide and dimethylformamide.     

Synthesis and Fe(III)‐complexation ability of polyurethane bearing kojic acid skeleton in the main chain prepared by polyaddition of aliphatic hydroxyl groups without protection of phenolic hydroxyl groups

Polyaddition of a kojic acid dimer and diisocyanates yielded polyurethane with metal‐coordination ability owing to the phenolic hydroxyl groups of kojic acid. Although the kojic acid dimer contains two phenolic and two aliphatic hydroxyl groups, 1,5‐diazabicyclo[4.3.0]non‐5‐ene catalyzed polymerization proceeded through highly selective reactions of the aliphatic hydroxyl groups without any protection of the phenolic hydroxyl groups. The resulting polymers complexed with FeCl₃, and specific colorizations were observed. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Regioselectivity among six secondary hydroxyl groups: selective acylation of the least reactive hydroxyl groups of inositol

We report the first, general and selective acylation of the least reactive hydroxyl group among six secondary hydroxyl groups of inositol in high yield, using very cheap and easy-to-make H(2)SO(4)-silica as the catalyst, providing easy access to bioactive molecules.     

Synthesis and characterization of phenolic hydroxyl terminated phenylquinoxaline oligomers

The synthesis of phenylquinoxaline oligomers of predictable molecular weight and with phenolic hydroxyl functional end groups is reported. The synthetic procedure utilizes 4-hydroxybenzil as a substituted monofunctional monomer in a conventional PPQ synthesis to both control molecular weight and introduce the functional end groups. The molecular weights of the oligomers were determined by derivatization of the hydroxyl end groups with 3-(trimethylsilyl)propionyl chloride, followed by ¹H-NMR analysis. Comparison of the integration of the trimethylsilyl resonance to the aromatic resonances allowed calculation of the number average molecular weight. A plot of log(η) vs. log(M_n) correlated well with that reported for high molecular weight PPQ. The end groups influenced the solubility behavior of the oligomers in chlorinated solvents, possibly due to hydrogen bonding. The oligomers were chain-extendable to high molecular weight, confirming their suitability for utilization in copolymerizations to prepare phenylquinoxaline-based block copolymers.