Showing papers in "Journal of The Chemical Society-perkin Transactions 1 in 2000"

Multi-step organic synthesis using solid-supported reagents and scavengers: a new paradigm in chemical library generation

Abstract: 1 Review 1.1 Overview 1.2 Solid-phase organic synthesis (SPOS) 1.3 Beyond conventional solid-phase organic synthesis 1.4 The review 1.5 Some definitions 1.6 Solid-supported reagents and scavengers 1.7 Multi-step use of solid-supported reagents and scavenging reagents 1.8 Conclusions and future perspective 2 Introduction to the tables 2.1 Organisation of tables (reagent and catalyst section) 2.2 Representative data entry (reagent and catalyst section) 2.3 Organisation of tables (scavenging agents section) 2.4 Representative data entry (scavenging agents section) 3 Tables of reagents and catalysts 4 Tables of scavengers 5 Other relevant reviews 5.1 Insoluble polymers 5.1.1 Structure and physical properties (insoluble polymers) 5.1.2 General (insoluble polymers) 5.1.3 Reactions (insoluble polymers) 5.1.4 Miscellaneous (insoluble polymers) 5.2 Soluble polymers 5.2.1 General (soluble polymers) 5.2.2 Reactions (soluble polymers) 5.3 Inorganic solids 5.3.1 Structure and physical properties (inorganic solids) 5.3.2 General (inorganic solids) 5.3.3 Reactions (inorganic solids) 5.3.4 Miscellaneous (inorganic solids) 5.4 Miscellaneous supports 5.4.1 Structure and physical properties (miscellaneous supports) 5.4.2 General (miscellaneous supports) 5.4.3 Reactions (miscellaneous supports) 5.4.4 Miscellaneous (miscellaneous supports) 5.5 Purification strategies 5.5.1 Various supports (purification strategies) 5.5.2 Insoluble polymers (purification strategies) 6 Acknowledgements 7 Abbreviations 8 References 1 Review

Imines, enamines and oximes

Abstract: Covering: January 1997 to December 1998Continuing the coverage in Contemp. Org. Synth., 1997, 4, 517 p

Advances in indium-catalysed organic synthesis

Abstract: The use of indium based catalysts in organic synthetic transformations was investigated. Indium(III) complexes acted as efficient Lewis acid catalysts in organoindium chemistry. Catalytic reactions in water and aqueous media were studied where indium exhibited unique activity due to its high coordination number and fast coordination-dissociation equilibrium.

The reactions of ozone with tertiary amines including the complexing agents nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA) in aqueous solution

Abstract: Using the stopped-flow technique, the rate constants of the reaction of ozone with a number of amines have been determined. While the protonated amines do not react with ozone, the free amines react with rate constants of around 106 dm3 mol−1 s−1 in the case of tertiary and secondary amines, while primary amines react more slowly. Mono-protonated EDTA reacts only with k=1.6×105 and mono-protonated 1,4-diazabicyclo[2.2.2]octane (DABCO) with k = 3.5 × 103 dm3 mol−1 s−1. In aqueous solution, tertiary amines react with ozone mainly by forming the aminoxide and singlet dioxygen [O2(1Δg)] and to a lesser extent the secondary amine and the corresponding aldehyde, a reaction which can be partially suppressed by tert-butyl alcohol. These data suggest that O-transfer [aminoxide plus O2(1Δg)] is in competition with an electron transfer which leads to the amine radical cation and an ozonide radical. In water, the latter gives rise to ˙OH which further reacts with the amine (and ozone). The amine radical cation deprotonates at a neighbouring carbon. The resulting radical adds dioxygen. Subsequent elimination of O2˙− and hydrolysis of the Schiff-base thus formed leads to the secondary amine and the corresponding aldehyde. In its reaction with ozone, O2˙− yields further ˙OH. Their reaction with the amines leads to the same intermediate as the free-radical pathway of ozone does, i.e. induces a chain reaction. This is interfered with by tert-butyl alcohol at the OH-radical stage. When complexed to Fe(III), EDTA reacts only very slowly with ozone (k = 330 dm3 mol−1 s−1). This explains why EDTA is not readily removed by ozonation in drinking-water processing.

Tautomerism of 2-hydroxynaphthaldehyde Schiff bases

Abstract: A UV–Vis spectroscopic study based on the recently developed chemometric approach for quantitative analysis of undefined mixtures is performed on a series of donor and acceptor substituted Schiff bases of 2-hydroxynaphthaldehydes. In CCl4 solution all compounds preferentially exist as the phenol tautomer independent of the nature of the respective substituent. With increasing polarity the tautomeric equilibrium is shifted towards the quinone form. In CHCl3 and, especially, ethanol a clear distinction between the effect of donors (stabilization of the quinone form) and acceptors (stabilization of the phenol tautomer) is evident. Ab initio calculations including solvent effects via the polarized continuum model of solvation as well as the supermolecule approach are used to rationalize the experimental findings.

Efficient visible light sensitisation of water-soluble near-infrared luminescent lanthanide complexes

Abstract: Fluorexon (4′,5′-bis[N,N-bis(carboxymethyl)aminomethyl]fluorescein) forms stable, water-soluble complexes with lanthanide ions. The complexes with neodymium(III), erbium(III) and ytterbium(III) display sensitised near-infrared lanthanide luminescence upon excitation of the fluorexon with visible light. The photosensitisation efficiency is very high (0.5–1.0) as a result of lanthanide-enhanced intersystem crossing in the sensitising chromophore and rapid intracomplex energy transfer. The overall luminescence quantum yields, however, are low (2 × 10−4–5 × 10−3) due to nonradiative deactivation of the excited lanthanide ion.

Synthesis of heterocycles by radical cyclisation

Abstract: Covering: July 1998 to June 2000. Previous review: J. Chem. Soc., Perkin Trans. 1, 2000, 1.

Reaction of ascorbic acid with S-nitrosothiols: clear evidence for two distinct reaction pathways

Abstract: Ascorbate reacts with S-nitrosothiols generally, in the pH range 3–13 by way of two distinct pathways, (a) at low [ascorbate], typically below ≈1 × 10−4 mol dm−3 which leads to the formation of NO and the disulfide, and (b) at higher [ascorbate] when the products are the thiol and NO. Reaction (a) is Cu2+-dependent, and is completely cut out in the presence of EDTA, whereas reaction (b) is totally independent of [Cu2+] and takes place readily whether EDTA is present or not. For S-nitrosoglutathione (GSNO) the two reactions can be made quite separate, although for some reactants the two reactions overlap. In reaction (a), ascorbate acts as a reducing agent, generating Cu+ from Cu2+, which in turn reacts with RSNO forming initially NO, Cu2+ and RS−. The latter can then play the role of reducing agent for Cu2+, leading to disulfide formation. Ascorbate will initiate reaction when the free thiolate has initially been reduced to a very low level by the synthesis of RSNO from a large excess of nitrous acid over the thiol. Reaction (b) is interpreted in terms of nucleophilic attack by ascorbate at the nitroso-nitrogen atom, leading to thiol and O-nitrosoascorbate which breaks up, by a free-radical pathway, to give dehydroascorbic acid and NO. A similar pathway is the accepted mechanism in the literature for the nitrosation of ascorbate by nitrous acid and alkyl nitrites. The rate constant for the Cu2+ -independent pathway increases sharply with pH and analysis of the variation of the rate constant with pH identifies a reaction pathway via both the mono- and di-anion forms of ascorbate, with the latter being the more reactive. As expected the entropy of activation is large and negative. Some aspects of structure–reactivity trends are discussed.

Polystyrene and polymethacrylate resin-supported Jacobsen’s alkene epoxidation catalyst

Abstract: Polystyrene and polymethacrylate-based resin supported Jacobsen’s chiral Mn salen complexes have been prepared. The resins are of defined molecular structure and morphology, and the complexes have been attached primarily in a pendant fashion. The loadings of Mn(III) are in the range ≈0.08–0.35 mmol g−1 to maximise the likelihood of site-isolation. The polymer-supported complexes have been used as enantioselective catalysts in the epoxidation of 1,2-dihydronaphthalene, indene, 1-phenylcyclohex-1-ene and 1-phenyl-3,4-dihydronaphthalene using m-chloroperbenzoic acid as the oxidant and 4-methylmorpholine N-oxide as the co-oxidant. Though the activities of the polymer catalysts are reduced relative to the soluble homogeneous analogue, the catalysts are sufficiently active to be useful. The corresponding reduction in enantioselectivity is more significant, and is both substrate and polymer resin dependent. However, in the case of 1-phenylcyclohex-1-ene and a macroporous polymethacrylate-based resin the enantioselectivity is equivalent to that of the soluble complex (91–92% ee). This is the first report of a polymer-supported analogue of Jacobsen’s catalyst being as selective as the homogeneous species. The catalysis data is discussed in detail in the context of the design of the polymer-supported system, and the existing data already available in the literature. Attempts have also been made to recycle the polymer catalysts with and without re-loading of Mn. In fact the level of leaching of Mn is very low, but the catalysts show a very rapid fall off in both activity and selectivity in the first and second cycles. Overall therefore it seems that the intrinsic stability of the chiral Mn(II) salen complex itself is too low to allow viable recycling, and the development of other more stable supported chiral metal salen complexes for use in other enantioselective reactions seems a better future option. p

The crystal and liquid structures of N,N-dimethylthioformamide and N,N-dimethylformamide showing a stronger hydrogen bonding effect for C–H⋯S than of C–H⋯O

Abstract: The crystal structures of N,N-dimethylthioformamide and N,N-dimethylformamide were determined at 90 ± 2 K from single crystal X-ray diffraction data. Both compounds comprise planar molecules, a consequence of the π-electron delocalization over the N–C–S and N–C–O entities, respectively. In N,N-dimethylthioformamide, almost linear, 175.4(7)°, C–H⋯S cooperative hydrogen bonding between the thioformyl groups connects the molecules in helix-shaped chains with an intermolecular C⋯S distance of 378.10(7) pm. The two crystallographically independent molecules in N,N-dimethylformamide form four-membered centrosymmetric rings held together by C–H⋯O intermolecular interactions, two via the formyl protons, C⋯O 329.41(9) pm, and two involving methyl protons, C⋯O 341.41(9) pm. The structures of both liquids were studied at room temperature by large angle X-ray scattering in transmission mode and for N,N-dimethylthioformamide also in reflection geometry. The structure of liquid N,N-dimethylformamide can, despite the stronger hydrogen bond acceptor properties of the oxygen atom, be described without hydrogen bonding. This apparent anomaly with more significant effect of hydrogen bonding in both crystal and liquid forms of N,N-dimethylthioformamide than N,N-dimethylformamide is discussed using results from theoretical calculations on single molecules. Mulliken population analyses indicate a lower positive charge and thus weaker hydrogen-bond donor properties of the formyl than of the thioformyl hydrogen atom. Raman and infrared spectra of the solids and the liquids are used for discussions of the hydrogen bonding effects.

Palladium(II)-catalyzed oxidation of terminal alkenes to methyl ketones using molecular oxygen

Abstract: Palladium(II) acetate catalyzes the aerobic oxidation of terminal alkenes in toluene into the corresponding methyl ketones in the presence of a catalytic amount of pyridine using propan-2-ol as a reductant and molecular oxygen as an oxidant. Two catalytic cycles sharing a Pd(II)–OOH species are proposed. One is the formation of a Pd(II)–H species in the oxidation of propan-2-ol to acetone, followed by reaction with molecular oxygen to give a Pd(II)–OOH species, and the other is peroxypalladation of an alkene with the Pd(II)–OOH species produced to afford a methyl ketone in the presence of H2O2 produced by the former catalytic cycle.

Catalytic cyclopropanation of electron deficient alkenes mediated by chiral and achiral sulfides: scope and limitations in reactions involving phenyldiazomethane and ethyl diazoacetate

Abstract: Phenyldiazomethane reacts with electron deficient alkenes in the presence of catalytic amounts of transition metal catalyst [Rh2(OAc)4 was better than Cu(acac)2] and catalytic amounts of sulfide to give cyclopropanes. Pentamethylene sulfide was found to be superior to tetrahydrothiophene and the optimum solvent was toluene. Under these optimised conditions a range of enones were cyclopropanated in high yields. Cyclic enones and acrylates were not successful in this process. The use of the chiral 1,3-oxathiane derived from camphorsulfonyl chloride in 2 steps in this process furnished cyclopropanes in good yield and very high enantiomeric excess (>97% ee). The absolute stereochemistry of cyclopropane 10 was proven by X-ray analysis and the origin of the stereochemical induction has been rationalised. Extension of this work to include diazoesters was partially successful. Again pentamethylene sulfide was found to be superior to tetrahydrothiophene, but this time both Rh2(OAc)4 and Cu(acac)2 were found to be equally effective. Enones, fumarates and unsaturated nitro compounds worked well but simple acrylates and unsaturated aldehydes were not effective substrates. Control experiments were conducted in which the stabilised ylide was isolated and reacted with the less successful substrates and, whilst unsaturated aldehydes still gave low yields, simple acrylates gave high yields of the corresponding cyclopropane. The use of the chiral 1,3-oxathiane was not successful with these more stable diazo compounds.

Oxidation of flavonols with Cu(II), Fe(II) and Fe(III) in aqueous media

Abstract: Reactions of flavonols, 3-methoxyflavones, and flavones with Cu2+, Fe3+, and Fe2+ in acidic aqueous solutions with three different organic modifiers are investigated. Complexation may be followed by an oxidation of flavonols, yielding additional peaks in HPLC chromatograms of the reaction mixtures. The products are synthesized with high yields and isolated in mg amounts. They are characterized as 2-(hydroxybenzoyl)-2-hydroxybenzofuran-3(2H)-ones, using two-dimensional NMR techniques and PBI-NCI and -EI-MS. The electrochemical behaviour of the compounds is further investigated with cyclic voltammetry.

Interaction of novel cationic meso-tetraphenylporphyrins in the ground and excited states with DNA and nucleotides

Abstract: The syntheses and aggregation properties of novel cationic meso-tetraphenylporphyrins substituted in the para-positions with -CH2(pyridinio)+ (P11), -CH2N+(CH3)3 (P22), -CH2P+(n-butyl)3 (P33), -CH2P+(phenyl)3 (P44), -CH2S+(CH3)2 (P55) and -CH2SC(NH2)2+ (P66) groups are described. Their use as photosensitizers and their interactions with DNA and nucleotides were studied by optical methods and their properties were compared with those of anionic meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and cationic meso-tetrakis(4-N-methylpyridyl)porphyrin (TMPyP). P11 and P22 formed stable complexes with calf thymus DNA in phosphate buffer (Ka ≈ 106 M−1; outside stacking binding mode) and with some nucleotides in methanol (Ka ≈ 103–104 M−1). P33–P66 aggregated readily in aqueous solution due to their more hydrophobic nature. The cationic porphyrins TMPyP and P11–P66 sensitized the decomposition of guanosine 5′-monophosphate (GMP). The rates of GMP decomposition were found to be greater with cationic porphyrins P11–P66 than with anionic TPPS, presumably because of Coulombic attraction between the positively charged porphyrins P11–P66 and the anionic GMP. In oxygen-free conditions, GMP decomposition was initiated by interaction of the singlet (P11, P22) or triplet (P11–P66) excited states of the porphyrins with GMP. In the presence of oxygen, GMP is decomposed predominantly via singlet oxygen mechanism.

Microwave-mediated Biginelli reactions revisited. On the nature of rate and yield enhancements

Abstract: The microwave-mediated Biginelli dihydropyrimidine synthesis was reinvestigated using a purpose-built commercial microwave reactor with on-line temperature, pressure, and microwave power control. Transformations carried out under microwave heating at atmospheric pressure in ethanol solution show no rate or yield increase when the temperature is identical to conventional thermal heating. In the case of superheating by microwave irradiation at atmospheric pressure the observed yield and rate increases are rationalized as a consequence of a thermal (kinetic) effect. Under sealed vessel conditions (20 bar, 180 °C) the yield of products is decreased and formation of various byproducts observed. The only significant rate and yield enhancements are found when the reaction is performed under “open system” conditions where the solvent is allowed to rapidly evaporate during microwave irradiation. However, the observed rate and yield enhancements in these experiments are a consequence of the solvent-free conditions rather than caused specifically by microwave irradiation. This was confirmed by control experiments of the solventless Biginelli reaction under microwave and thermal heating.

Regioselective reaction of 5,15-disubstituted porphyrins with organolithium reagents—synthetic access to 5,10,15-trisubstituted porphyrins and directly meso-meso-linked bisporphyrins

Abstract: The reaction of organolithium reagents with 5,15-disubstituted porphyrins, followed by hydrolysis of the intermediate anion to give a porphodimethene and oxidation with DDQ allows the facile preparation of 5,10,15-tri- and 5,10,15,20-tetrasubstituted porphyrins with mixed meso substituent types Easily available 5,15-disubstituted porphyrins react almost quantitatively with sterically undemanding organolithium reagents (PhLi, BuLi) to give the respective 5,10,15-trisubstituted porphyrins (A2B-type) When bulkier reagents are used (eg Bus or But) the number and yield of side products increases considerably Thus, the regioselectivity of the nucleophilic attack (meso versus β) depends on the steric hindrance of the LiR reagents The 5,10,15-trisubstituted porphyrins can in turn be used for another cycle of nucleophilic attack, hydrolysis, and oxidation giving rise to 5,10,15,20-tetrasubstituted porphyrins of the A2B2- or A2BC-types In contrast, reaction of 5,15-disubstituted porphyrins with LiR and direct oxidation of the intermediate anion without a hydrolysis step gives directly linked (5,5′) bisporphyrins allowing the preparation of bisporphyrins with mixed meso substituent pattern In addition, the crystal structures of two 5,10,15-trisubstituted porphyrins (25, 28) are reported Both compounds exhibit moderately nonplanar macrocycles as a result of the sterically hindered meso substituents

Catalytic asymmetric epoxidation of stilbene using a chiral salen complex immobilized in Mn-exchanged Al-MCM-41

Abstract: Manganese-exchanged Al-MCM-41 modified by the chiral salen ligand [(R,R)-(−)-N,N′-bis(3,5-di-tert-butylsalicylidene)cyclohexane-1,2-diamine] can be used as an enantioselective heterogeneous epoxidation catalyst using iodosyl benzene as oxygen donor. Epoxidation of (Z)- and (E)-stilbene is studied in detail and experiments are described that demonstrate that the reaction is wholly catalysed heterogeneously. Similar enantioselectivity is observed for the oxidation of (Z)-stilbene to the (E)-epoxide using homogeneous (77.5% ee) or heterogeneous (70% ee) catalysts. The effect of temperature, solvents and donor ligands on the yield and enantioselection are discussed.

NMR diffusion spectroscopy for the characterization of multicomponent hydrogen-bonded assemblies in solution

Abstract: NMR diffusion measurements on 10 different multicomponent hydrogen-bonded assemblies, viz. the three single rosettes SR1–SR3 ( 13· 2a3, 13· 2b3, 13· 2c3) the double rosettes DR1–DR5 ( 3a3· 2a6, 3b3· 2b6, 3c3· 2a6, 3d3· 2a6, 3e3· 2a6), and DR6 ( 4a3· 16), and the tetrarosette TR ( 53· 2a12) are described. Some of the above rosettes have been previously identified as well-defined assemblies (viz. SR1, DR1–DR3, and TR) using established characterization techniques (1H NMR spectroscopy, X-ray diffraction, and MALDI-TOF MS after Ag+-labeling). The diffusion coefficients of these assemblies were studied and used as a reference for the identification of three new assemblies ( DR4–DR6), the characterization of which could not be established unequivocally using other characterization tools. A good correlation was found between the experimental and calculated diffusion coefficients when DR1 was used as a reference. A relatively good correlation was obtained between the effective hydrolytic radii calculated from the diffusion data and those extracted from gas phase-minimized structures with SR1 and DR2 being exceptions. The diffusion measurements show that assembly DR4 is a thermodynamically stable species, while assemblies DR5 and DR6 are less stable and only present to a minor extent.

The self-association of the black tea polyphenol theaflavin and its complexation with caffeine

Abstract: Caffeine is found in both coffee and tea whilst polyphenols are present in a wide variety of foods and beverages. Theaflavin and its gallate esters are polyphenolic molecules which can be isolated from black tea infusions. The theaflavin family of polyphenols contribute to the taste and colour of tea, and their complexation with caffeine is thought to be largely responsible for the formation of tea cream, a precipitate that forms as tea cools. The self-association of theaflavin and caffeine was studied using nuclear magnetic resonance methods (chemical shift changes and self-diffusion constants on dilution) and it is shown that caffeine forms stacks of molecules (Ks = 7.9 l mol−1 at 300 K), while theaflavin forms stable dimers (Ks = 230 l mol−1). The theaflavin monomer consists of a planar benzotropolone ring system, with the two flavan rings approximately orthogonal to this plane, and stacked against each other. In the dimer, two benzotropolone rings align with an antiparallel geometry. Two molecules of caffeine bind to one molecule of theaflavin in a strictly sequential manner, with first and second association constants of 11.9 and 16.5 l mol−1, respectively. It is proposed that the first caffeine inserts between the two flavan rings, and the second then binds to the newly liberated flavan surface.

Novel methods for the synthesis of three-, four-, five-, six- and seven-membered, saturated and partially unsaturated carbocycles

Abstract: Reviewing the literature from May 1998 to April 1999. Previous Review: J. Chem. Soc., Perkin Trans. 1, 1998, 983.

Supramolecular motifs in metal complexes of Schiff bases. Part 5. Zinc(II)-assisted self-assembly of some bis-N,N- and N,O-bidentate Schiff bases and chiral packing modes in solid state

Abstract: Zinc(II)-assisted self-assembly of a new bis-N,O-bidentate Schiff base ligand, (N-salicylidene-4,4′-diaminodiphenyl)methane (L16) with two chelating sites linked by a spacer group (-C6H4CH2C6H4-), afforded in high yield the double-helical dinuclear complex (L16 : ZnII = 2∶2). Single-crystal X-ray analyses demonstrated clearly that the two ZnII centers have a distorted tetrahedral (Td) coordination sphere with two-wrapped ligands. The analogous Schiff base, bis[4-(2-pyridylmethyleneaminophenyl]methane (L17) was also designed to self-assemble in the presence of metal ions, leading to a triple-helical dinuclear (L17∶ZnII = 3∶2) supramolecular motif. Each zinc ion has six-coordinate octahedral geometry with six nitrogens from three ligands. These unprecedented helical motifs in the solid state and solution seem to be induced by the geometrical preference for octahedral or tetrahedral coordination mode of the ZnII ion and the interligand π-stacking interactions between the spacer groups of L16 and L17. Electrospray mass spectrometry proved a very useful characterisational tool in detecting the distribution of supramolecular species in solution. Use of a N,N′-bis(2-pyridylmethylene)-1,4-diaminobenzene (L35) with a rigid phenylene spacer in its center resulted in the one-dimensional zigzag polymeric structure ((L35∶ZnII)∞ = (1∶1)∞) where the multiple π-stacking interactions operate between the aromatic rings of linking ligands. Each zinc ion is octahedrally coordinated by two N,N-bidentate arms of two different ligands and two cis oxygens from N,N-dimethylformamide as coordinating solvent.

Nickel(0)-catalyzed asymmetric cross-coupling reactions of allylic compounds with arylboronic acids

Abstract: Optically active oxazolinylferrocenylphosphines have been found to work quite effectively as chiral ligands in nickel(0)-catalyzed cross-coupling reactions of allylic compounds with arylboronic acids, which are known to behave as “hard” nucleophiles. The expected coupling products have been obtained in good yields with moderate enantioselectivities (up to 53% ee). This is the first example of asymmetric allylic substitution using organoboron compounds.

Antioxidant properties of anthocyanins and tannins: a mechanistic investigation with catechin and the 3′,4′,7-trihydroxyflavylium ion

Abstract: Plant polyphenols act as antioxidants mainly by trapping reactive oxygen species and by regenerating endogenous membrane-bound α-tocopherol (vitamin E). In both processes polyphenols are oxidized. Hence, knowledge of the oxidation mechanisms of polyphenols is important for an understanding of their antioxidant activity at the molecular level. This work focuses on anthocyanins (pigments) and flavanols (tannins), two important classes of polyphenols which are both relatively abundant in human diet. The oxidation of the 3′,4′,7-trihydroxyflavylium ion (1) and catechin (2), respectively taken as models for anthocyanins and tannins, has been investigated. From kinetic data and partial product analysis, the mechanisms for the reactions of 1 and 2 with sodium periodate and DPPH, a H atom-abstracting radical, are proposed. Both polyphenols are shown to form o-quinone intermediates upon H atom abstraction and subsequent radical disproportionation. In the case of 2, the quinone and a second molecule of antioxidant quickly couple to form dimers. By contrast, 1 is extensively degraded into coumarins by repeating sequences of oxidation–solvent addition, which consume several equivalents of oxidants. In aqueous solutions, 1 is typically a mixture of coloured and colourless forms. The latter (chalcones) are also shown to take part in the antioxidant activity. p

Palladium(II)-catalysed oxidation of alcohols under an oxygen atmosphere in a fluorous biphase system (FBS)

Abstract: Palladium(II) acetate catalyses the aerobic oxidation of alcohols into the corresponding aldehydes and ketones in the presence of a catalytic amount of a novel perfluoroalkylated-pyridine as a ligand using molecular oxygen in a fluorous biphase system (FBS) composed of toluene and perfluorodecalin. This catalytic system is applicable to various benzylic and aliphatic alcohols. The fluorous phase containing the active palladium species is easily separated and can be reused several times without a significant loss of catalytic activity.