Versatile Iridicycle Catalysts for Highly Efficient and Chemoselective Transfer Hydrogenation of Carbonyl Compounds in Water

Cyclometalated iridium complexes are shown to be highly efficient and chemoselective catalysts for the transfer hydrogenation of a wide range of carbonyl groups with formic acid in water. Examples include α‐substituted ketones (α‐ether, α‐halo, α‐hydroxy, α‐amino, α‐nitrile or α‐ester), α‐keto esters, β‐keto esters and α,β‐unsaturated aldehydes. The reduction was carried out at substrate/catalyst ratios of up to 50 000 at pH 4.5 and required no organic solvent. The protocol provides a practical, easy and efficient way for the synthesis of β‐functionalised secondary alcohols, such as β‐hydroxyethers, β‐hydroxyamines and β‐hydroxyhalo compounds, which are valuable intermediates in pharmaceutical, fine chemical, perfume and agrochemical synthesis.     

Synthesis of heavily substituted 1,2-amino alcohols in enantiomerically pure form

[reaction: see text] A simple and convenient methodology for the preparation of optically pure 2-amino-2-aryl-1,1-diphenylethanols is presented. Allylamine was found to produce the ring-opening of triaryloxiranes in a regioselective and a stereospecific fashion. Removal of the allyl protecting group provided the free 1,2-amino alcohols in enantiomerically pure form.     

Synthesis and crystal structure of the <Emphasis Type="Italic">N</Emphasis>-8-(diphenyl-hydroxy-2-aminomethylpyridine)borane

Reaction of 2-aminomethylpyridine with diphenylborinic acid in a mixture of CHCl₃-diethyl ether gives diphenyl-(2-aminomethylpyridine-N,N)borane, a cyclic adduct that in presence of environmental moisture yields the N-8-(diphenyl-hydroxy-2-aminomethylpyridine)borane. The two compounds were characterized by NMR and infrared spectroscopy. The structure of N-8-(diphenyl-hydroxy-2-aminomethylpyridine)borane was confirmed by X-ray diffraction. This adduct crystallizes in monoclinic P2₁/n space group with unit cell dimensions: a = 13.193(2) (Å), b = 5.913(7) (Å), c = 19.604(2) (Å), = 90.295(3), V = 1529.3(3) ų, Z = 4. The BN distance is 1.645(3) Å and lies in the range of a dative bond. The NH and OH groups are involved in intermolecular hydrogen bonds formation and they add to the stabilization of the acyclic adduct in the solid state.     

Electrochemical characterization of praseodymium centers in Pr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zr<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript> zirconias using electrocatalysis and photoelectrocatalysis

The voltammetry of nanoparticles and scanning electrochemical microscopy are applied to characterize praseodymium centers in tetragonal and monoclinic zirconias, doped with praseodymium ions (Pr _x Zr_1−x O₂), prepared via sol–gel routes. Doped zirconia nanoparticles were synthesized by a sol–gel liquid-phase route and characterized by different techniques, including X-ray diffraction powder pattern, ultraviolet–visible diffuse reflectance spectroscopy, infrared spectroscopy, and transmission electron microscopy (TEM). Gels annealed at around 400 °C yielded tetragonal Pr _x Zr_1−x O₂ phases. The monoclinic forms of Pr-doped ZrO₂ were obtained by annealing at temperatures higher than 1,100 °C. TEM micrographs proved that the size of the nanoparticles produced was dependent on their crystalline form, around 15 and 60 nm for tetragonal and monoclinic, respectively. The electrochemical study confirmed that a relatively high content of praseodymium cation was in the chemical state (IV), i.e., as Pr⁴⁺, in both zirconia host lattices. The catalytic and photocatalytic effects of Pr⁴⁺ centers located in the monoclinic zirconia lattice on nitrite reduction and oxygen evolution reaction were studied.     

Synthesis, characterization, and crystal structures of n-alkyldiorganodithiophosphates RS2P(OC6H4)2

Treatment of KS₂P(OC₆H₄)₂ (1) with five n-alkyl halides afforded the stable n-alkyldithiophosphate compounds RS₂P(OC₆H₄)₂ (2–6). Single crystal X-ray analysis revealed that the PS₂ fragment in 1 displays an isobidentate coordination pattern; compounds 2–4 display a monodentate binding pattern. The crystal packing of title compounds is stabilized by cation-π, weak hydrogen bonding C–H---O, as well as aromatic non-bonded interactions.     

Development and validation of an HPLC-DAD analytical procedure for quality control of damiana (Turnera diffusa), using an antioxidant marker isolated from the plant

Damiana is a shrub used as a dietary supplement and as an additive in several foods. This report presents the first validated analytical method for QC of products made from damiana. Through bioassay-guided fractionation, the compound showing the best antioxidant activity, determined using 2,2-diphenyl-1-picryl-hydrazyl assay in TLC, was purified and identified as 8-C-beta-[6-deoxy-2-O-(alpha-1-rhamnopyranosyl)-xylo- hexopyranos-3-uloside] (Cl50 5.56 g/mL). This is the only natural source from which this compound has been reported, and has been found in all native damiana samples analyzed. Therefore, it was used as a biomarker to develop an HPLC-diode-array detection analytical procedure using the first-order derivative of the chromatograms. Separation was achieved using an AccQ Tag C18 150 x 3.9 mm (4 microm) column, with a gradient of methanol and 0.1% trifluoroacetic acid in water. The method was shown to be accurate, sensitive, and reproducible, providing a useful quantitative means to apply QC to damiana products.     

EPR Equilibration Measurement of the C–H Bond Enthalpy in the Methylene Groups of 1,3,5-Trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene (Irganox)

On the basis of previous and current studies, it has been possible to evaluate and compare the bond dissociation energies of the phenol (O–H) bond and of the methylene and methyl (C–H) bonds in Irganox (trade name) by means of electron paramagnetic resonance records of galvinoxyl–irganoxyl hydrogen transfer reaction equilibria. The bond dissociation enthalpy of the C–H methylene and methyl bonds in Irganox was found to be 339.0 ± 3.0 kJ/mol, very close to the previously found value of 340.7 ± 3.0 kJ/mol for the O–H bond of some benzyl–phenolic antioxidant.