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Donald Bethell

Publications and source records attributed to Donald Bethell.

12 recordsLinked to original sources
PubMed2007

Electrophilic reactions of skin-sensitizing sultones.

Reactions of the strong skin sensitizer hexadec-1-ene-1,3-sultone with sodium hydroxide, sodium methoxide, sodium metabisulfite, sodium butanethiolate, n-butylamine, and aniline have been investigated, and the reaction products have been identified. Most of the nucleophiles studied react by nucleophilic addition (Michael type addition) to the double bond at the 2-position, although in most cases the final products result from further reactions of the initital adducts. The findings are considered together with those reported by Meschkat, Barratt, and Lepoittevin for reactions of hex-1-ene-1,3-sultone and hexane-1,3-sultone, and the implications of the two sets of findings for the mechanism of skin-sensitizing action are discussed. It is concluded that nucleophilic attack at the 3-position of the alk-1-ene-1,3-sultones occurs only with those nucleophiles, which either have very low reactivity in nucleophilic addition or are unable to give rise to thermodynamically stable products via initial reaction at the 2-position. It is further concluded that the observed differences in electrophilic reactivity between alk-1-ene-1,3-sultones and alkane-1,3-sultones are not large enough to rationalize the differences in skin sensitization properties between the two types of sultone. It is suggested that the differences in specificity arise because the alk-1-ene-1,3-sultones act as Michael type electrophiles whereas the alkane sultones act as SN2 electrophiles. It is suggested that the reason for the greater potency of the alk-1-ene-1,3-sultones may be the ability of the initial C2 adducts with protein to undergo further reactions by substitution at C3, leading to cross-linking and consequent perturbation to the protein tertiary structure.

Butylamines↗
PubMed2006

Thermal gating of the single molecule conductance of alkanedithiols.

The temperature dependence of the single molecule conductance (SMC) of alpha,omega-alkanedithiols has been investigated using a scanning tunnelling microscopy (STM) method. This is based on trapping molecules between a gold STM tip and a gold substrate and measuring directly the current across the molecule under different applied potentials. A pronounced temperature dependence of the conductance, which scales logarithmically with T(1), is observed in the temperature range between 293 and 353 K. It is proposed the origin of this dependence is the change in distribution between molecular conformers rather than changes in either the conduction mechanism or the electronic structure of molecule. For alkanedithiols the time averaged conformer distribution shifts to less elongated conformers at higher temperatures thus giving rise to higher conductance across the molecular bridges. This is analysed by first calculating energy differences between different conformers and then calculating their partition distribution. A simple tunnelling model is then used to calculate the temperature dependent conductance based on the conformer distribution. These findings demonstrate that charge transport through single organic molecules at ambient temperatures is a subtle and highly dynamic process that cannot be described by analysing only one molecular conformation corresponding to the lowest energy geometry of the molecule.

Journal Article↗
PubMed2005

Effect of structure of the redox molecular sieve TS-1 on the oxidation of phenol, crotyl alcohol and norbornylene.

A range of crystalline TS-1 samples with different morphologies as well as the corresponding TS-1 precursor structures have been synthesised using hydrothermal crystallisation. The materials have been characterised using powder X-ray diffraction, IR and Raman spectroscopy and electron microscopy. The materials were used as catalysts for the oxidation of crotyl alcohol, phenol and norbornylene and, in particular, the reactivity of the precursor structures was contrasted with crystalline TS-1. The oxidation of crotyl alcohol, selected as a relatively non-reactive substituted alkene, did not require the TS-1 structure for reactivity and TS-1 precursor structures are active, although crystalline TS-1 was found to be more reactive than the precursor structures. In contrast, phenol hydroxylation is only catalysed by crystalline TS-1. The reaction of phenol is observed to occur only on the exterior surface of large TS-1 crystallites. With smaller crystallites of TS-1, i.e. the size range of interest for catalysis, the rapid subsequent reaction of hydroquinone makes it difficult to determine whether reaction occurs solely on the exterior of the crystallites or at sites within the porous structure. Hence it is suggested that this reaction has limited scope as a probe reaction for the reactivity of sites within the crystallites. It is, however, feasible that phenol hydroxylation is a viable probe reaction for TS-1 type structural units. Norbornylene was studied as an example of a reactant too large to enter the internal pore structure of TS-1 and hence only reaction at pore mouths and external surface sites was possible. Larger TS-1 crystallites were more active for this substrate than suggested by surface area considerations. The results are discussed in terms of the selection of model reactions for the study of TS-1 catalysts.

Butanols↗
PubMed2004

Catalytic asymmetric heterogeneous aziridination of styrene derivatives using bis(oxazoline)-modified Cu2+-exchanged zeolite Y.

The copper-catalysed aziridination of styrene derivatives using either Cu(OTf)2 as a homogeneous catalyst or copper-exchanged zeolite Y modified as a heterogeneous catalyst with bis(oxazoline) using [N-(p-nitrophenylsulfonyl)imino]phenyliodinane (PhI=NNs) as nitrene donor is described and discussed. For the homogeneously catalysed reaction virtually all the substituted derivatives gave lower yields of the aziridine with lower ee when compared with styrene, although 4-chlorostyrene did give enhanced ee. For the heterogeneous catalysed reaction the styrene derivatives often gave enhanced yield, particularly when the substituent was in the 4-position. Particularly high ee was observed for 2-chlorostyrene (95%) and 4-chlorostyrene (94%) and in general the ee observed for the heterogeneously catalysed reaction with the 2- and 4-substituted derivates was significantly higher than that for the equivalent homogeneously catalysed reaction. With copper-exchanged zeolite Y the leaching of Cu2+ into solution during reaction was much lower for the substituted styrene derivatives (0.4-7% of total Cu) than with styrene itself (10.3% of the total Cu). The effect of reaction time on aziridine yield reveals an S-shaped profile that is accentuated by the position of the substituent and this effect is observed for both the homogeneously catalysed pathway and the heterogeneously catalysed reaction. A series of competitive experiments in which styrene was co-reacted with equivalent quantities of 2-, 3- and 4-chlorostyrene revealed that the reaction of styrene enhanced by the other substrate, but the rate of reaction of the substituted styrene was decreased. Co-reaction of 2- and 4-chlorostyrene reveals interesting competitive effects, which are discussed in terms of the nature of the active site.

Aziridines↗
PubMed2004

Molecular wire formation from viologen assemblies.

The adsorption behavior of viologen alpha,omega-dithiols (viologen dithiols) on gold has been investigated. At short exposures, a low-coverage phase consisting of flat-lying molecules has been determined by STM and IR spectroscopy. In contrast, multilayer films are formed after long adsorption times. Single molecular wires could be formed between a gold STM tip and a surface with a low coverage of the adsorbed dithiols, and their electrical behavior was investigated. Molecular conductivity was determined either by the repeated measurement of I(s) curves or by recording I-V curves for different tip-sample separations. These methods concurred in producing a value of (0.5 +/- 0.1) nS for the single-molecule conductivity of the alpha,omega-viologen dithiol molecule HS-6V6-SH. The high conductivity of HS-6V6-SH, as compared to that of HS-C12-SH, may be related to the low-lying LUMO, which provides a barrier indentation for electron transport in a two-step electron-transfer mechanism.

Journal Article↗
PubMed2004

Chloromethyl chlorosulfate: a new, catalytic method of preparation and reactions with some nucleophiles.

The reaction of liquid (gamma-) SO3 with CH2Cl2 at room temperature leads to SO3 insertion into the C-Cl bonds, giving the useful chloromethylating agent chloromethyl chlorosulfate (CMCS). The process is very slow but becomes rapid on addition of catalytic quantities of trimethyl borate. The product mixture consists almost entirely of CMCS and the product of further sulfation, methylene bis(chlorosulfate)(MBCS), in a ratio of ca. 2 : 1, but typical yields of CMCS, isolated by distillation, are only 30-35%. The catalysed reaction in the homogeneous liquid phase at -45 degrees C has been followed as a function of time and of reactant concentration by 1H nmr spectroscopy. It is observed that, besides CMCS and MBCS, three additional, transient products (designated A, B and C) are formed. Products A, B and C decompose slowly at -45 degrees C but much more rapidly if the reaction mixture is raised to room temperature, giving additional CMCS and MBCS. From an analysis of the SO3 balance, it is inferred that products A, B and C arise from the reaction of one molecule of CH2Cl2 with respectively two, three and four molecules of SO3; they are suggested to be chloromethyl chloropolysulfates. By measuring initial rates of CMCS formation or total CH2Cl2 consumption, it is shown that the reaction is first order in the catalyst and roughly third order in SO3. A mechanistic scheme is proposed in which SO3 forms equilibrating zwitterionic molecular complexes with CH2Cl2. of 1 : 1, 2 : 1 and higher stoichiometries. The boron-containing catalyst can activate these complexes towards nucleophilic attack at carbon by the negatively charged oxygen of another zwitterion. An analogous mechanism can be written for the conversion of CMCS into MBCS by SO3 in the presence of trimethyl borate. CMCS reacts rapidly with anionic nucleophiles, such as halide or acetate ions (X-), in homogeneous solution of their tetrabutylammonium salts in CD3CN, or in a two-phase system (CDCl3/H2O) using alkali-metal salts in conjunction with a phase-transfer catalyst. In both situations the products (ClCH2X) arise by rapid nucleophilic displacement of the chlorosulfate moiety; this then more slowly liberates chloride ion, which converts further CMCS into CH2Cl2. The reactivity of CMCS has been compared with that of MBCS and methyl chlorosulfate (MCS) in competitive experiments; the reactivity order is MCS > MBCS > CMCS >> CH2Cl2. Evidence is also presented suggesting that, in contrast to the halide nucleophiles, reaction of CMCS with sodium phenoxide in tetrahydrofuran solution leads to nucleophilic displacement of the sulfur-bound chloride.

Borates↗
PubMed2004

Wiring nanoparticles with redox molecules.

Gold nanoparticles were used to make electrical contact to redox-active organic molecules. Viologen based dithiols were self-assembled from solution on Au(111) for use as tethers to attach nanoparticles to a conducting substrate. The topography and electrical properties of the resulting films were investigated by STM and STS and the orientation of these linkers was investigated by FTIR. Surface coverage increased with increasing reaction time, resulting in a change of film orientation from a flat to a more upright standing conformation. Gold nanoparticles attached to these self-assembled films were characterised by STM. It was possible to isolate a single redox-active molecule in an alkanethiol matrix and by subsequent attachment of a single gold nanoparticle the electrical properties of single wired molecules could be investigated. This method allowed the measurement of the conductivity of single molecules connecting a nanoparticle to the substrate chemically, thus forming stable electrical contacts at both ends.

Journal Article↗
PubMed2003

Redox state dependence of single molecule conductivity.

Spontaneous formation of stable molecular wires between a gold scanning tunneling microscopy (STM) tip and substrate is observed when the sample has a low coverage of alpha,omega-dithiol molecules and the tunneling resistance is made sufficiently small. Current-distance curves taken under these conditions exhibit characteristic current plateaux at large tip-substrate separations from which the conductivity of a single molecule can be obtained. The versatility of this technique is demonstrated using redox-active molecules under potential control, where substantial reversible conductivity changes from 0.5 to 2.8 nS were observed when the molecule was electrochemically switched from the oxidized to the reduced state.

Journal Article↗
PubMed2003

The decomposition of diazo-compounds induced by nucleophiles. The decomposition of 9-diazofluorene in the presence of hydroxide or alkoxide ions.

9-Diazofluorene, on treatment with stoichiometric or substoichiometric amounts of quaternary ammonium hydroxide or methoxide or of potassium t-butoxide in solution in aqueous or alcoholic dimethyl sulfoxide or acetonitrile at 30 degrees C, decomposes with evolution of nitrogen to yield fluorenone azine [bis(fluorenylidene)hydrazine] in almost quantitative yield. Studies are reported of the identity of the minor by-products, together with an examination of the kinetics of the reactions and additional spectroscopic experiments. The general rate equation is v = k[FIN2]3/2[Nu-]1/2, where Nu- represents the nucleophile. It is concluded that the oxyanions, most probably after nucleophilic attack on 9-diazofluorene under these basic conditions to generate a new anionic species, are capable of transferring an electron to the diazo-compound. This initiates decomposition of further diazo-molecules in a process of electron-transfer chain catalysis that bears some similarities to, but has also some differences from, that encountered using cathodic initiation. In support of this interpretation it is found that reactions can be accelerated by the introduction of carbon acids (fluorene, 9-phenylfluorene) into the reaction mixtures. The nature of the initiation, propagation and termination steps of the chain mechanism are discussed.

Journal Article↗
PubMed2002

The fate of sulfur-bound hydrogen on formation of self-assembled thiol monolayers on gold: (1)H NMR spectroscopic evidence from solutions of gold clusters.

It is demonstrated that thiols can adsorb to gold without losing hydrogen. Dodecyl sulfide-capped gold clusters have been prepared and subjected to ligand exchange reactions in perdeuterated benzene by addition of dodecanethiol and subsequently dodecyl disulfide. It is shown by 1H NMR spectroscopy that dodecanethiol molecules are readily taken up as ligands producing characteristic broad signals corresponding to the alpha-methylene and S-H protons, with chemical shifts close to those found for thiol in solution; these signals are absent in spectra of thiolate-capped clusters. Addition of excess disulfide to such clusters capped with both dialkyl sulfides and thiols leads to the appearance of sharp signals for free dialkyl sulfide and intact thiol. Amounts of thiols up to 50% of the ligand shell are, however, taken up by the clusters under rapid and irreversible loss of hydrogen.

Journal Article↗