Jonathan E. H. Buston

Rotaxane-encapsulated cyanine dyes: enhanced fluorescence efficiency and photostability

Two isomers of a cyanine dye α-cyclodextrin rotaxane have been synthesised in aqueous solution, and structurally characterised by 2D NMR spectroscopy; they exhibit enhanced fluorescence and photostability, compared with the free dye.

Enhanced chemical reversibility of redox processes incyanine dye rotaxanes

When a cyanine dye is encapsulated inside the cavity of α-cyclodextrin, by rotaxane formation, its one-electron oxidation and reduction become reversible, due to a thousand-fold increase in the kinetic chemical stability of the oxidised and reduced forms of the chromophore.

STUDIES INTO THE ASYMMETRIC MEISENHEIMER REARRANGEMENT

Abstract Oxidation of a variety of N-allyl prolinol derivatives gave amine N-oxides with complete diastereoselectivity. On warming, these amine N-oxides undergo [2,3]-Meisenheimer rearrangement to give O-allyl hydroxylamines, albeit with low diastereoselectivity. Attempts to promote asymmetric N-oxidation of N-allyl tertiary amines with a variety of asymmetric oxidants produced only racemic N-oxides.

On the structure and reaction with pyridine of o-methoxyphenyllead acetates

Abstract Pyridine catalyses the ligand redistribution of o -methoxyphenyllead triacetate to di( o -methoxyphenyl)lead diacetate. Characterisation of these two lead (IV) compounds by single-crystal X-ray diffraction demonstrates a weak interaction between the lead atom and the methoxy oxygen atom leading to an 8-coordinate lead atom.

Sonoelectrochemically Enhanced Electrocatalytic Processes: The Pb(IV) Catalyzed Cleavage of 1,2‐cis‐Cyclopentanediol at Graphite and Glassy Carbon Electrodes

In this study, the electrocatalytic (EC′-type) carbon–carbon bond cleavage of cis-1,2-cyclopentanediol by Pb(IV) benzoate generated in situ is shown to be possible under extremely fast mass transport conditions. Based on the comparison of data obtained experimentally and by the numerical simulation of sonoelectrochemical limiting currents with commercially available software (Digisim 2.0), excellent agreement with the theoretical model of a planar diffusion layer can be achieved and allows the straightforward prediction and optimization of sonoelectrochemcial processes involving homogeneously coupled chemical steps. The ultrasonic cleaning of the electrode surface strongly affects the Pb deposition process at glassy carbon and basal plane pyrolytic graphite electrodes, but is not efficient enough to prevent electrode fouling which accompanies the anodic process at positive potentials. The overall rate and efficiency of the electrochemical process is enhanced due to fast mass transport in the presence of ultrasound.

Water-soluble macrocyclic receptors for small neutral guests

Two new water-soluble dicationic spiro-cyclophanes, 1 and 2, with CH2 and (CH2)2 links between phenolic oxygens, have been prepared and characterised crystallographically. The binding properties of these cyclophanes have been compared with those of seven other water-soluble macrocycles: Diederich-type cyclophanes with (CH2)3 and (CH2)4 links, (3 and 4), Stoddarts cyclophane (5), α-cyclodextrin (6a), 2,6-dimethyl-α-cyclodextrin (6b), 2,3,6-trimethyl-α-cyclodextrin (6c) and β-cyclodextrin (7). The affinities of all nine macrocycles for 1,6-hexanediol (8) and hexa-2,4-diyne-1,6-diol (9) in D2O were investigated by 1H NMR. The strongest binding was observed between 9 and 2,6-dimethyl-α-cyclodextrin (6b) (K=1.2×103M-1). The alkyne diol 9 also forms a 1:2 complex with β-cyclodextrin (7) (K1=18 M-1, K2=4 M-1). The small cyclophanes 1 and 2 exhibit very little affinity for these hydrophobic guests. This unexpected finding was explained by an analysis of their crystal structures: 1 has an open cavity with an internal van der Waals radius of ca. 1.2–1.7 A; it binds nitromethane and ethyl acetate in the solid state by C–H···π interactions, but is slightly too narrow to accommodate an alkyne, and 2 has a collapsed cavity.

Rate enhancing ligands for lead(IV)-mediated arylations

The level of acceleration of the reactions of aryllead(IV) tricarboxylates with β-dicarbonyl reagents in the presence of several catalysts has been determined; in the case of 1,10-phenanthroline, almost a thousandfold increase in rate over the uncatalysed reaction was observed.

207Pb NMR chemical shifts of lead tetracarboxylates

207Pb NMR spectroscopy was used to investigate 20 lead(IV) tetracarboxylates both in solution and in the solid state by cross‐polarization magic angle spinning (CP/MAS) NMR. There is generally good correlation between isotropic chemical shifts in the solid state and those in solution. Multiple CP/MAS NMR isotropic resonances are observed for three of the compounds and are interpreted in terms of multiple molecules in the crystallographic asymmetric unit. From an analysis of the range of, and trends in, the NMR chemical shifts, circumstantial evidence is presented for augmentary coordination by Lewis basic ortho‐aromatic substituents of the carboxylate groups. The 207Pb chemical shift anisotropy (CSA) parameters were extracted from analysis of the spinning sideband manifolds of the CP/MAS spectra. The CSA spreads are small in comparison with others reported for 207Pb, and all have positive skew, in contrast to the typical case for lead(II) compounds. Thirteen of the 20 CSA analyses performed show effectively axial CSA tensors. A simple shielding model which rationalizes this typical CSA pattern is presented, based on an analysis of geometric information derived from the two known single‐crystal x‐ray structures of lead(IV) carboxylates. Copyright

Detection of aryllead(IV) carboxylates and their solvent adducts by ESI-mass spectrometry

Abstract The successful detection for the first time of some aryllead(IV) carboxylates by electrospray ionisation mass spectrometry (ESI-MS) is reported; in the presence of excess carboxylate ligand, these compounds gave a reliable [M+Na] + signal. These conditions appear to be generally suitable for the observation of organolead derivatives, and may find wider application for the reliable detection of other heavy main group and transition organometallic compounds. Furthermore, under these conditions, direct evidence for the formation of solvent (MeCN) and pyridine adducts for several different aryllead(IV) carboxylates was obtained, confirming the existence of facile ligand equilibration in lead(IV) carboxylates.

207Pb NMR chemical shifts of aryllead tricarboxylates

207Pb NMR spectroscopy was used to investigate the nature of aryllead(IV) tricarboxylate species in solution. The 207Pb chemical shifts of p‐Br‐ and p‐MeO‐aryllead tricarboxylates were found to depend on both electronic (as measured by the pKa of the parent acid) and steric effects of the neighbouring carboxylic acid ligands.