Showing papers in "Journal of The Chemical Society-perkin Transactions 1 in 1997"

Efficient cyclopropanation of C60 starting from malonates

Abstract: The direct treatment of C60 with malonates in the presence of CBr4 and diazabicyclo[5.4.0]undec-7-ene (DBU) leads to an efficient conversion to methanofullerenes, which is demonstrated with the syntheses of monoadducts and highly symmetric hexaadducts in good yields.

Isolation and characterisation of C70O

Abstract: C70O has been isolated in ca. 0.5% yield by HPLC separation of the soot extract obtained from a novel arc-discharge reactor for fullerene production. The reactor (DC conditions) can be operated continuously for 24 h, employs a rotating cathode and an anode consisting of continuously-fed strips cut from a carbon sheet. The 13C NMR spectrum of C70O shows that two isomers (1,2-epoxy[70]fullerene and 5,6-epoxy[70]fullerene) are present in a ratio of ca. 43∶57 and all 37 peaks corresponding to each isomer have been identified. These are the first [70]fullerene derivatives in which 5,6-addition is preferred over 1,2-addition, this preference probably deriving from the considerable strain that accompanies bridging with a single atom; the greater curvature across the 1,2-positions compared with the 5,6-positions therefore disfavours the former. Differential polarisabilities of the electrons of the 1,2- and 5,6-bonds, a factor producing a variation in reactivity order according to the attacking reagent (but not considered hitherto in the context of fullerene chemistry) may also be significant. The epoxides are unstable towards EI mass spectrometry, in contrast to epoxides possessing additional addends, and this may reflect the reduced cage strain present in the latter.

Thienothiophenes. Part 2.1 Synthesis, metallation and bromine→lithium exchange reactions of thieno[3,2-b]thiophene and its polybromo derivatives

Abstract: Methods for the large-scale synthesis of thieno[3,2-b]thiophene [including a catalytic vapour-phase reaction (at 550 °C) between 2-(2-thienyl)ethanol and carbon disulfide], its 2-carboxylic acid and its 3,6-dibromo and 2,3,5,6-tetrabromo derivatives are reported. With 2 mol equiv. of butyllithium thieno[3,2-b]thiophene gives its 2,5-dilithiated derivative and its 3,6-dibromo derivative gives the 3,6-dilithiated compound. By quenching with suitable electrophilic reagents these dilithiated compounds have been converted into various 2,5- or 3,6-disubstituted thieno[3,2-b]thiophenes, respectively. Likewise 2,3,5,6-tetrabromothieno[3,2-b]thiophene has been converted into 2,5-disubstituted 3,6-dibromothieno[3,2-b]thiophenes.

Synthesis of functionalised fluorescent dyes and their coupling to amines and amino acids

Abstract: A series of novel functionalised benzophenoxazinones, analogues of Nile Red, is prepared and their fluorescence properties evaluated. The ring system is prepared by reaction of 5-diethylamino-2-nitrosophenol with 1,6-dihydroxynaphthalene followed by alkylation of the 2-hydroxy group with 6-bromohexanoic acid derivatives. Subsequent ester cleavage under a variety of conditions gives the acid 6. Replacement of the 9-diethylamino group with the N-ethyl-(3-sulfonylpropyl)amino group is carried out to increase water solubility and the resulting dye 14 has similar fluorescence properties. The acid 6 is coupled to a range of amino compounds.

Tetra-n-propylammonium perruthenate (TPAP)-catalysed oxidations of alcohols using molecular oxygen as a co-oxidant

Abstract: Tetra-n-propylammonium perruthenate (TPAP) has been used as an efficient catalyst for the conversion of primary and secondary alcohols to the corresponding aldehydes and ketones under very mild aerobic conditions.

Metamorphosis of caffeine hydrate and anhydrous caffeine

Abstract: The phase stability, interconversion and physicochemical characterisation of caffeine (1,3,7-trimethylpurine-2,6-dione) hydrate and anhydrous caffeine relate to the strength of the available hydrogen-bonds. A hydrogen-bonded T-branched spine of hydrate molecules establishes a weak lattice bond for the caffeine hydrate which, under ambient conditions, transforms to a β-anhydrous phase which in turn, at 155 °C and with an enthalpy of 3.6 kJ mol–1, converts to a trigonal phase α-anhydrous caffeine. The anhydrous phases are stabilised by weak CH3 to CO hydrogen bonds.

Improved procedure for Juliá–Colonna asymmetric epoxidation of α,β-unsaturated ketones: total synthesis of diltiazem and TaxolTM side-chain

Abstract: Poly-L-leucine catalyses the asymmetric epoxidation of enones 1–6 efficiently in a non-aqueous medium to provide the epoxy ketones 7–12 (70–91% yield; 80 to 95% ee). The strategy was used to make diltiazem 16 and the TaxolTM side chain 23 in single enantiomer form.

Solute–solvent and solvent–solvent interactions in binarysolvent mixtures. Part 6. A quantitative measurement ofthe enhancement of the water structure in2-methylpropan-2-ol–waterand propan-2-ol–water mixtures by solvatochromicindicators

Abstract: A preferential solvation model that takes into account the enhancement of the structure of water when small amounts of alcohol are added is proposed and tested for solvatochromic indicators in binary mixtures of 2-methylpropan-2-ol and propan-2-ol with water. The model modifies an equation previously developed for solvatochromic indicators in binary solvent mixtures. The new model includes a correction term for the effect of the enhancement of the water structure on the measured solvatochromic property that allows a quantitative estimation of this effect. It is shown that the enhanced water structure is more polar and has a larger hydrogen-bond donor capability, but a smaller hydrogen-bond acceptor ability, than the common water structure.

The electrochemical reduction of indigo dissolved in organic solvents and as a solid mechanically attached to a basal plane pyrolytic graphite electrode immersed in aqueous electrolyte solution

Abstract: The electrochemical properties of solid materials such as organic compounds, irrespective of their electrical conductivity, can be studied via the use of submicron sized particles mechanically attached to electrode surfaces immersed in aqueous media containing different electrolytes. In this study the reduction of solid indigo in buffered and non-buffered aqueous media has been investigated. Data are compared to those obtained from a voltammetric study of indigo dissolved in dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF) and in pyridine and the interpretation of results is facilitated by in situ EPR, in situ UV–VIS spectroscopy and atomic force microscopy (AFM) studies. For the reduction of solid indigo two distinct types of reduction processes, ‘surface-type’ and ‘bulk-type’, have been observed. The latter process has been found to be associated with the reductive dissolution of indigo. The ‘surface-type’ responses which occur in a microphase at the electrode/solid/solution interface are reversible 2H+-2e- reduction and oxidation processes with a corresponding 60 mV shift in half wave potential per pH unit in buffered electrolyte media over the pH range 4 to 11. The ‘bulk-type’ reduction process is proposed to be associated with the electroinsertion of cations into the solid indigo particles, thereby producing a reduced material and causing the loss of material from the electrode surface under convective flow conditions. The electrochemical solubilization of indigo occurred in non-buffered aqueous media even at neutral pH values.

Oxygen activation by metal complexes and alkyl hydroperoxides. Applications of mechanistic probes to explore the role of alkoxyl radicals in alkane functionalization

Abstract: The mechanism of the oxidation of cycloalkanes by tertiary alkyl hydroperoxides catalysed by iron(III) dichlorotris(2-pyridylmethyl)amine [FeIIICl2(TPA)]+ and by the acetate bridged (µ-oxo) di-iron complex [Fe2III(TPA)2O(OAc)]3+ has been investigated. Product studies do not support oxidation via a high valent iron–oxo intermediate (formally FeVO), but are consistent with a mechanism involving hydrogen atom abstraction from the alkane by alkoxyl radicals derived from the hydroperoxide. In the presence of a large excess of tert-butyl hydroperoxide, the oxidation of cyclohexane yields cyclohexanone, cyclohexanol and tert-butylcyclohexyl peroxide in more than stoichiometric amounts and, in the case of the mono-iron catalyst, one equivalent of cyclohexyl chloride. Replacement of Me3COOH by hydroperoxides, which could yield tert-alkoxyl radicals having much shorter lifetimes than the tert-butoxyl radical prevents oxidation of the cycloalkane. The products obtained with these hydroperoxide mechanistic probes are those derived from the fast unimolecular reactions (generally β-scissions) of the corresponding alkoxyl radicals. The inapplicability of dimethyl sulfide as a mechanistically diagnostic trap for the putative FeVO intermediate and the value of di-tert-butyl hyponitrite as a non-iron-based source of tert-butoxyl radicals are discussed.

Polymer supported perruthenate (PSP): a new oxidant for clean organic synthesis

Abstract: A polymer supported perruthenate reagent has been prepared and used in the conversion of primary and secondary alcohols to aldehydes and ketones, respectively, affording pure products without the need for conventional work-up procedures.

Palladium-catalysed heteroannulation with acetylenic compounds: synthesis of benzofurans1

Abstract: A detailed study of the heteroannulation of o-iodophenol with acetylenic substrates through palladium–copper catalysis leading to the synthesis of the 2-substituted benzofurans 21–29 is reported. An acylic compound 30 has been isolated and proved to be an intermediate in the synthesis of the benzofurans. Some of the benzofurans have been transformed into biologically active compounds.

Enantioselective synthesis of α-hydroxyphosphonates throughasymmetric Pudovik reactions with chiral lanthanoid and titaniumalkoxides

Abstract: The Pudovik reaction of aromatic aldehydes with diethyl phosphite catalysed by either tartrate-modified titanium(IV) alkoxides or binaphthol-modified lanthanoid alkoxides has been examined. The enantioselectivity for the Pudovik reaction catalysed by tartrate-modified titanium(IV) alkoxide 1 was strongly dependent upon the solvent used. The electronic nature of the carbonyl affected the enantioselectivity for the Pudovik reaction with the La-Li-BINOL complex 2.

Phosphorus(III) ligands with fluorous ponytails

Abstract: A series of phosphorus(III) compounds incorporating one or more C6F13 ‘ponytails’ have been prepared as a pool of ligands for coordination and catalytic studies under the fluorous biphase regime. Solubility studies indicate that only ligands with three or more ponytails are preferentially soluble in perfluorocarbon solvents.

Direct preparation of diacetals from 1,2-diketones and their use as 1,2-diol protecting groups

Abstract: A range of 1,2-diketones have been evaluated as potential protecting groups for trans-1,2-diols via 1,2-diacetal formation. The procedure is especially useful in oligosaccharide and natural product synthesis.

Peroxyhydrolysis of nerve agent vx and model compounds and related nucleophilic reactions

Abstract: The exceedingly toxic agent VX [O-ethyl S-(diisopropylaminoethyl) methylphosphonothioate], 1a, and the midly toxic model compound O,S-diethyl methylphosphonothioate, 1b, react with HO- to give parallel P–S and P–O bond cleavages; the P–O cleavage of VX produces relatively unreactive but very toxic anionic phosphonothioate. Peroxyhydrolysis of 1a,b with HO2- involves quantitative P–S cleavage at rates 30–40 times that with HO- giving the corresponding phosphonate and sulfonate ions and disulfide as nontoxic products. In reaction of 1b with HO2- in H218O, oxygen in these final products is not derived from water and HO2- exclusively displaces the thiolate ion at phosphorus. Reaction of 1b with HSO5- gives the same products, but via oxidatively promoted attack of H218O on phosphorus. Kinetic and isotopic labelling results on reactions of 1a,b and a range of related compounds with HO2-, HO- and RO- and an oximate ion are interpreted in terms of concerted SN2(P) reactions rather than stepwise reactions with formation of a trigonal bipyramidal (TBP) intermediate. Product selectivity depends on the relative basicities of the anionic nucleophile and the leaving anions.

Solvent effects on Diels-Alder reactions. the use of aqueous mixtures of fluorinated alcohols and the study of reactions of acrylonitrile

Abstract: Rate, endo/exo, regio- and diastereo-facial selectivities of several Diels–Alder reactions were measured in a series of fluorinated alcohol–water mixtures, whose solvophobicity has been determined by means of the solvophobic power (Sp) parameter. Solvophobicity is the main factor influencing the reaction rate, although in some reactions hydrogen bond donating (HBD) ability may also play a role. Both solvophobicity and HBD ability are important to account for changes in endo/exo selectivity. HBD ability is the main factor responsible for the changes in regio- and diastereo-facial selectivities, induced by the reaction medium. On the other hand, the kinetic rate constants and endo/exo selectivity of the reaction of acrylonitrile with cyclopentadiene, as well as the regioselectivity of the reaction of acrylonitrile with isoprene, have been determined in 23 reaction media. The analysis of the results using empirical solvent parameters show that the reaction rate depends on solvophobic, HBD and dipolarity interactions, whereas endo/exo selectivity is influenced by solvophobic and dipolarity interactions, and the regioselectivity only by HBD effects.

Palladium-catalysed asymmetric allylic substitution: synthesis ofα- and β-amino acids

Abstract: Methodology has been established for the formation of enantiomerically enriched α-amino acids using palladium-catalysed allylic amination. The formation of enantiomerically enriched allylamines has been achieved with high enantioselectivity. Oxidative cleavage of the allylamines provides arylglycine and glutamic acid derivatives. Additionally, enantiomerically enriched β-amino acids have been prepared in high enantiomeric excess. Palladium-catalysed asymmetric allylic substitution is used as the key synthetic transformation.

EPR study of paramagnetic rhenium(I) complexes (bpy>>·>>–>>)Re(CO)3X relevant to the mechanism of electrocatalytic CO2 reduction†

Abstract: In situ reduction of (bpy)Re(CO)3Cl, bpy = 2,2′-bipyridine, at a platinum cathode under CO2 atmosphere in acetonitrile or acetone has produced a series of distinct EPR spectra which are attributed to (bpy˙–) containing species and which invariably reveal the interaction of the unpaired electron with the metal nuclei (185,187Re, I = 5/2). The EPR investigation of various model complexes (bpy˙–)Re(CO)3X, generated by electrochemical reduction under argon of diamagnetic precursors (bpy)Re(CO)3X, X = Cl–, CF3SO3–, CH3O–, H–, tetrahydrofuran, CH3CN, CO, HCO2–, HCO3– and CH3C(O)– has been used to interpret the results from the reactions under CO2 atmosphere.

Novel chemoselective and diastereoselective iron( III )-catalysed Michael reactions of 1,3-dicarbonyl compounds and enones

Abstract: Iron(III) chloride hexahydrate catalyses the Michael reaction of 1,3-dicarbonyl compounds with α,β-unsaturated ketones under mild and neutral conditions with extraordinary efficiency. The chemoselectivity of this FeIII-catalysed process is superior to that of the classic base-catalysed Michael reaction, since the latter suffers from various side reactions, namely drawbacks such as aldol cyclisations and ester solvolysis. Excellent yields and chemoselectivities together with the environmentally friendly nature of FeIII catalysis makes this an important alternative to classic base catalysis. Moreover, the reaction procedure is reasonably easy: FeIII catalysis does not require inert or anhydrous conditions, and in most cases no solvent is needed. In terms of diastereoselectivity, the FeIII-mediated reaction may also prove superior to a base-catalysed one. In at least one example, FeIII catalysis forms a diastereoisomer as the major kinetic product, which is disfavoured in the base-mediated Michael reaction, where a thermodynamic mixture is obtained. The relative configuration of the diastereoisomeric Michael products has been determined for two examples by synthesis and structure elucidation of the cyclic aldol derivatives.

Selective mono-N-methylation of primary aromatic amines by dimethylcarbonate over faujasite X- and Y-type zeolites

Abstract: The reaction of dimethyl carbonate (DMC) with different primary aromatic amines has been investigated under batch conditions (autoclave) in the presence of Y- and X-type zeolites. Operating at 120–150 °C, highly selective mono-N-methylations are observed for anilines even when they are deactivated by either electronic effects or steric hindrance (G–C6H4NH2, G = p-NO2, p-CN, o-CO2CH3 and 2,6-dimethylaniline); typical selectivities for the formation of the corresponding mono-N-methyl derivatives [ArNH(CH3)] are in the range 92–98%, at a substrate conversion of 72–93%. A synergic effect between the reactivity of DMC (acting both as a methylating and as a reversible methoxycarbonylating agent) and the dual acid–base properties of zeolites is considered to be responsible for the unusually high selectivity observed; accordingly, a reaction mechanism is discussed, involving carbamates (ArNHCO2CH3) and N-methylcarbamates [ArN(CH3)CO2CH3] as intermediates. The reaction is an example of a synthesis with low environmental impact: it couples the use of a non-toxic methylating agent (DMC, in place of the highly toxic methyl halides or dimethyl sulfate) with eco-friendly catalysts (zeolites) in a waste-free process.

An in-depth study of the biotransformation of nitriles into amidesand/or acids using Rhodococcus rhodochrousAJ2701

Abstract: A variety of aliphatic, aromatic and heterocyclic nitriles have been readily hydrolysed into the corresponding amides and/or acids under very mild conditions using Rhodococcus sp. AJ270. The nitrile hydratase involved in this novel nitrile-hydrolysing microorganism efficiently hydrates most nitriles tested, irrespective of the electronic and steric effects of the substituents, to form the amides. Conversion of amides into acids catalysed by the associated amidase is rapid and efficient in most cases. Substrates bearing an adjacent substituent (which may be an ortho substituent on an aromatic nitrile, an adjacent heteroatom in a heterocyclic ring or a geminal substituent in an α,β-unsaturated nitrile) undergo slow hydrolysis of the amides allowing efficient amide isolation. The scope, limitations and reaction mechanism of this enzymatic process have been systematically studied. A molecular size of >7 A diameter and the presence of functions capable of metal complexation near to the nitrile inhibit hydrolysis.

Synthesis of glycopeptide sequences of repeating units of the mucins MUC 2 and MUC 3 containing oligosaccharide side-chains with core 1, core 2, core 3, core 4 and core 6 structure

Abstract: An efficient synthesis of glycosylamino acid building blocks containing core 1, core 2, core 3, core 4 or core 6 mucin core oligosaccharide structures linked O-glycosidically to threonine has been developed. These building blocks 6, 10, 16, 24 and 30 can be used directly for coupling reactions in a glycopeptide synthesis. In a multiple-column solid-phase synthesis, they have been used to prepare different series of glycopeptides. Decapeptide sequences have been synthesized from repeating units of the mucins MUC 2 and MUC 3 in which different threonine residues are each systematically glycosylated with an oligosaccharide of core 1, core 2, core 3, core 4 or core 6 structure. Glycopeptides are substrates for the study of the biosynthesis of the saccharide side-chains of mucins.

Deracemization of (±)-2,3-disubstituted oxiranes via biocatalytic hydrolysis using bacterial epoxide hydrolases: kinetics of an enantioconvergent process

Abstract: Asymmetric biocatalytic hydrolysis of (±)-2,3-disubstituted oxiranes leading to the formation of vicinal diols in up to 97% ee at 100% conversion was accomplished by using the epoxide hydrolase activity of various bacterial strains. The mechanism of this deracemization was elucidated by 18OH2-labelling experiments using a partially purified epoxide hydrolase from Nocardia EH1. The reaction was shown to proceed in an enantioconvergent fashion by attack of OH– at the (S)-configured oxirane carbon atom with concomitant inversion of configuration. A mathematical model developed for the description of the kinetics was verified by the determination of the four relative rate constants governing the regio- and enantio-selectivity of the process.

Thermodynamically-controlled cyclisation and interconversion of oligocholates: metal ion templated ‘living’ macrolactonisation

Abstract: A series of ester-linked macrocyclic oligomers (dimer–pentamer) of cholates equipped with a variety of recognition and reporter elements has been prepared (a) by conventional irreversible chemistry, and (b) using methoxide-catalysed transesterification under reversible equilibrium conditions. Templating by metal ions under these equilibrium conditions is also demonstrated. These results demonstrate the feasibility of creating a dynamic combinatorial library of receptors.

Synthesis, spectroscopy and electrochemistry of phthalocyaninederivatives functionalised with four and eight peripheraltetrathiafulvalene units

Abstract: Metal-free phthalocyanine derivatives 2 and 14 bearing eight and four peripheral tetrathiafulvalene (TTF) units, respectively, have been synthesised, and their solution electrochemistry and optical spectroscopy have been studied. The compounds display redox properties arising from the TTF and from the phthalocyanine groups. 1H NMR and UV–VIS spectroscopic studies in solution show that aggregation is strongly solvent dependent. Quenching of the fluorescence of the phthalocyanine core by the TTF units was observed. The X-ray crystal structure of 4,5-bis(hexylthio)-4′,5′-bis(hydroxymethyl)-TTF 11, which was synthesised during the course of this work, has been determined. The hydroxy groups of 11 engage in intermolecular (and interstack) hydrogen bonds. Computer modelling studies on phthalocyanine derivatives 2 and 14 are reported.

Intramolecular O-H ? ? ? O hydrogen bonds assisted by resonance. Correlation between crystallographic data and 1 H NMR chemical shifts

Abstract: A number of crystal structures of molecules where the π-conjugated · · · O C–CC–OH · · · β-diketone enol group is found to form intramolecular O–H · · · O hydrogen bonds and for which 1H NMR spectroscopic data were known are discussed. Five of these structures, determined by X-ray diffraction techniques, are reported and the other 42 were retrieved from the Cambridge Structural Database. It is shown that all the descriptors of hydrogen-bond strength [d(O · · · O) shortening, increased enolic 1H NMR chemical shift, δ(OH), and increased π-delocalization of the hydrogen-bonded heteroconjugated fragment] are mutually and linearly intercorrelated according to the rules defined by RAHB (resonance-assisted hydrogen bonding). Such a model is found to be of general applicability to all intramolecular O–H · · · O bonds observed in a variety of molecules of different complexity embedding the simple β-diketone enol fragment and to be extensible to other hydrogen-bonded conjugated compounds such as · · · OC–C C–CC–OH · · · δ-diketone enols and · · · OC –CN–OH · · · α-keto-oximes. The proton chemical shifts, δ(OH), measured in CDCl3 solutions are found to depend strongly on the O · · · O contact distances going from 8.6–10.1 ppm for weak non-resonant [2.59 ⩽ d(O · · · O) ⩽ 2.64 A] to 14.9–19.0 ppm for the strongest resonant hydrogen bonds [2.41 ⩽ d(O · · · O) ⩽ 2.55 A]. Comparison with 1H NMR data obtained in the solid-state shows a strictly similar dependence of δ(OH) on d(O · · · O), irrespective of the very different experimental conditions and in spite of the fact that solution and solid-state values concern intramolecular and intermolecular hydrogen bonds, respectively.

Insertion of ethyl diazoacetate into N–H and S–H bonds catalyzed by ruthenium porphyrin complexes

Abstract: Ruthenium porphyrin complexes catalyze insertion of ethyl diazoacetate into sulfur–hydrogen and nitrogen–hydrogen bonds under mild conditions and with reasonable to very good yields

Metal-catalysed reactions of imines with ethyl diazoacetate leading to aziridines

Abstract: The metal-catalysed aziridination of imines with ethyl diazoacetate as the carbene fragment donor using various Lewis acids as the catalyst has been investigated. The catalytic properties of different Lewis acid complexes have been tested and it has been found that both main-group complexes, such as BF3·OEt2, early- and late-transition metal complexes, such as TiCl2(O-Pri)2, Cu(OTf)2 and Zn(OTf)2 and rare-earth metal complexes, such as Yb(OTf)3, can catalyse the formation of aziridines. The aziridination gives mainly the cis-aziridines as the major diastereoisomer, but the selectivity is dependent on the substrate, catalyst and solvent. Zn(OTf)2 and Yb(OTf)3 have been shown to be general catalysts for the formation of various aziridines using different imines and a variety of reaction conditions. Both Zn(OTf)2 and Yb(OTf)3, as well as some of the other Lewis acids, in combination with various chiral ligands, have been tested as catalysts for the formation of optically active aziridines, but only low ees are obtained. The Zn(OTf)2- and Yb(OTf)3-catalysed reactions have been investigated for imines having both electron-donating and -withdrawing substituents, and in reactions containing diethyl fumarate as a trapping reagent, in attempts to obtain insight into the mechanism of the aziridination.

Glycosylation of silybin

Abstract: Silybin glycosides—23-O-β-glucoside 2b, β-galactoside 3b, β-lactoside 4b and β-maltoside 5b—have been synthesized by different methods (Helferich glycosylation, Lewis acid catalysis). Separation of two silybin diastereoisomers in the form of acetylated monoglycosides has been achieved for the first time. These new silybin glycosides are 4–30 times more water-soluble, and their hepatoprotectivity is increased compared with that of the parent compound silybin 1.