Mechanistic insight into the lanthanide(III) salt catalysed monoacylation of symmetrical diols from structural models

Model studies are presented that suggest the mechanism of the lanthanide(III) salt catalysed mono acylation of symmetrical diols proceeds via chelation of the diol and the anhydride to the lanthanide salt, followed by an 'intramolecular' acyl transfer.     

Oxalate-bridged complexes of dimolybdenum and ditungsten supported by pivalate ligands: ((t)BuCO(2))(3)M(2)(mu-O(2)CCO(2))M(2)(O(2)C(t)Bu)(3). Correlation of the solid-state, molecular, and electronic structures with Raman, resonance Raman, and electronic spectral data

The compounds ((t)BuCO(2))(3)M(2)(mu-O(2)CCO(2))M(2)(O(2)C(t)Bu)(3) (M(4)OXA), where M = Mo or W, are shown by analysis of powder X-ray diffraction data to have extended lattice structures wherein oxygen atoms from the oxalate and pivalate ligands of one M(4)OXA molecule are linked to metal atoms of neighboring molecules. Raman, resonance Raman, electronic absorption (2-325 K in 2-MeTHF), and emission spectra are reported, together with corresponding spectra of the mu-O(2)(13)C(13)CO(2) isotopomers. To aid in the assignment, the Raman spectra of K(2)C(2)O(4).H(2)O and K(2)(13)C(2)O(4).H(2)O have also been recorded. The visible region of the electronic spectra is dominated by intense, fully allowed MLCT transitions, M(2) delta to oxalate pi*, which show pronounced thermochromism and extensive vibronic progressions associated with the oxalate ligand at low temperatures. With excitation into these charge-transfer bands, strong resonance enhancement is seen for Raman bands assigned to the oxalate nu(1)(a(g)) and, to a lesser extent, nu(2)(a(g)) modes. Electronic structure calculations for the model compounds (HCO(2))(3)M(2)(mu-O(2)CCO(2))M(2)(O(2)CH)(3), employing density functional theory (gradient corrected and time-dependent) with the Gaussian 98 and ADF 2000 packages, predict the planar oxalate D(2h) configuration to be favored, which maximizes M(2) delta to oxalate pi* back-bonding, and indicate low barriers (<8 kcal mol(-1)) to rotation about the oxalate C-C bonds.     

Liquid chromatographic determination of nystatin in pharmaceutical preparations

A rapid, reversed-phase liquid chromatographic method was developed for the assay of nystatin in the bulk drug and a variety of dosage forms. Analysis was performed on a Symmetry C18 reversed-phase column using a mobile phase of methanol-water-dimethylformamide (DMF; 55 + 30 + 15, v/v/v), with detection by UV at 305 nm. Quantitation is based on the sum of the peak areas of the 2 major isomers of nystatin. The linearity of the assay was determined for a concentration range of 0.05 to 0.2 mg/mL (correlation coefficient > 0.999). Accuracies and precision showed good reproducibility.     

Cooperative diastereoselectivity of palladium- and platinum-promoted intramolecular [4 + 2] Diels-Alder cycloaddition reactions of 3,4-dimethyl-1-phenylphosphole

The complexes [(DMPP)2M(CH3CN2)]X2 (DMPP = 3,4-dimethyl-1-phenylphosphole; M = Pd, Pt; X = BF4-, NO3-, ClO4-) react with 2 equiv of the dienophiles N,N-dimethylacrylamide (DMAA), 2-vinylpyridine (VyPy), and diphenylvinylphosphine (DPVP) to form bis-[4 + 2] Diels-Alder cycloaddition products. The [M(DMPP)2(DMAA)2]2+ and [M(DMPP)2(VyPy)2]2+ complexes form exclusively as the cis-geometric isomers, whereas for [M(DMPP)2(DPVP)2]2+, both cis- and trans-geometric isomers are formed. The two Diels-Alder cycloadditions occur sequentially, and the absolute configuration of the first reaction influences the absolute configuration of the second. In all cases, recemic mixtures of the (R,R) and (S,S) diastereomers are formed; none of the meso (R,S) diastereomer is observed. New complexes were characterized by elemental analyses, physical properties, infrared spectroscopy, 1H, 1H(31P), 13C(1H), and 31P(1H) NMR spectroscopy, and, in most cases, X-ray crystallography.     

The thermal decomposition of dicyclopentadienyldimethyltitanium(IV)

Thermal decomposition of (C₅H₅)₂Ti(CH₃)₂ in alkane or benzene solution yields primarily methane, with only traces of ethane. Methane is produced by hydrogen abstraction from both the cyclopentadienyl rings and from the methyl groups, but never from the alkanes or benzene. A scheme of decomposition is suggested involving two paths, each with a two stage formation of methanes. A very much slower ethane formation competes with the second stage. Strong evidence exists for slow hydrogen exchange between the eyelopentadienyl rings and the methyl groups prior to significant production of ethane. Decomposition in diethyl ether yields methane, much of it arising through hydrogen abstraction from the ether. Decomposition in CCl₄ and C₂Cl₄ yields mainly methane but also some methyl chloride and ethane. Both diethyl ether and C₂Cl₄ almost block methane formation through ring attack.     

Nuclear shielding calculations on the primary structure of the gellan polysaccharide

Ab initio, GIAO and IGLO, nuclear shielding calculations are performed on each of the four pyranose ring residues of the tetrasaccharide repeating unit in a single chain of the gellan polysaccharide, [→ 3)- -Glcp-(1 → 4)-β- -GlcpA-(1 → 4)-β- -Glcp-(1 → 4)-- -Rhap(1 _n. The results provide an insight into the effects of the changing primary molecular electronic structure on the calculated ¹³C, ¹⁷O and ¹H shieldings. In particular, the observed trends in the calculated isotropic (σⁱ) shielding values are rationalised withi the framework of the localised molecular orbital shielding contributions and Mulliken population analyses (MPA).     

Applications of electrospray ionization mass spectrometry to neutral organic molecules including fullerenes

The use of electrospray ionization mass spectrometry (ESI/MS) for the detection of neutral organic molecules becomes possible by their derivation with specific ESI/MS tagging reagents that have either proton or metal ion binding sites. We used the neutral crown ether group in several reagents to attach a metal binding site to substrate molecules. Application of this method to steroids, amino acids, vitamin D, fatty acids, and fullerenes is described. Besides characterization, tagged molecules can be used for studying organic reactions by ESI/MS. This work demonstrates that ESI/MS provides a unique window on fullerene solution chemistry. ESI/MS is not only an excellent tool for the analysis of biopolymers but is also useful for studying the organic chemistry of small neutral molecules.     

On the use of mixtures of organotin species for catalytic enantioselective ketone allylation--a detective story

In the presence of enantiopure MTBH(2)(monothiobinaphthol, 2-hydroxy-2[prime or minute]mercapto-1,1[prime or minute]-binaphthyl; 0.2 eq.) quantitative allylation of ArC([double bond]O)Me takes place with impure Sn(CH(2)CH[double bond]CH(2))(4)(prepared from allyl chloride, air-oxidised magnesium and SnCl(4)) to yield tert-homoallylic alcohols in 85-92% ee. In the same process highly purified, or commercial, Sn(CH(2)CH[double bond]CH(2))(4) yields material of only 35-50% ee. The origin of these effects is the presence of small amounts of the compounds, EtSn(CH(2)CH[double bond]CH(2))(3), ClSn(CH(2)CH[double bond]CH(2))(3) ClSnEt(CH(2)CH[double bond]CH(2))(2) in the tetraallyltin sample and the presence of traces of water (which inhibits achiral background reactions). All the triallyl and diallyl species enhance the stereoselectivity in the catalytic allylation reaction, the chlorides more so than the ethyl compound. Hydrolysis of ClSnEt(CH(2)CH[double bond]CH(2))(2) affords crystallographically characterised Sn(4)(mu(3)-O)(mu(2)-Cl)(2)Cl(2)Et(4)(CH(2)CH[double bond]CH(2))(4). Reaction of this latter compound with MTBH(2) leads to the most potent catalyst.     

An NMR study of the freezing of emulsion-containing drops

Various nuclear magnetic resonance (NMR) techniques were used to monitor the freezing behaviour of suspended 2-mm-diameter drops. The drops were composed of hydrocarbon oils emulsified in either water or water/sucrose mixtures. As such they were good model systems for the study of spray freezing, sharing structural similarities with potential products such as ice cream. In particular, simple 1H NMR spectroscopy was used to monitor and individually quantify the freezing or solidification behaviour of the various constituent species of the drops. In addition, the effect of freezing on the emulsion droplet size distribution (and hence emulsion stability) was also measured based on NMR self-diffusion measurements. The effect of freeze/thaw cycling was also similarly studied. The nucleation temperature of the emulsion droplets was found to depend on the emulsion droplet size distribution: the smaller the droplets, the lower the nucleation temperature. Emulsion droplet sizing indicated that oil-in-sucrose-solution emulsions were more stable, showing minimal coalescence, whereas oil-in-water emulsions showed significant coalescence during freezing and freeze/thaw cycling.     

Micellar molecular weights and hydration of ethoxylated anionic and cationic surfactants

Summary Further work on the micellar properties of ethoxylated anionic and cationic surfactants is reported. Micellar molecular weights in water obtained from light scattering and corrected for charge effects, increased with increase in hydrocarbon chain length but decreased with increase in ethoxy group number. Micellar size of the ethoxylated surfactants was smaller than usual for paraffin chain salts. Micellar weights decreased 18% as the temperature increased from 25 to 45 °C. Addition of 0.1M. electrolyte increased micellar weight four fold. Analysis of the hydrodynamic data showed the ethoxylated micelles were hydrated with two molecules of water per ethoxy group. The hydrocarbon chain had little effect on hydration.

---

Zusammenfassung Es werden weitere Ergebnisse über micellare Eigenschaften äthoxylierter anionischer und kationischer oberflächenaktiver Stoffe berichtet. Micellare Molekulargewichte wurden ermittelt durch Lichtstreuung und für Ladungseffekte korrigiert. Diese nahmen mit der Länge der Kohlenwasserstoffkette zu, mit der Zahl der Äthoxy-Gruppen dagegen ab. Die micellare Größe der äthoxylierten oberflächenaktiven Stoffe war geringer als üblich bei Paraffinkettensalzen. Das Micellargewicht ging bei einem Temperaturanstieg von 25° auf 45° um 18% zurück. Wenn 0.1M. Elektrolyt hinzukam, steigerte sich das Micellargewicht um das Vierfache. Eine Analyse der hydrodynamischen Daten zeigte, daß die äthoxylierten Micellen einen Hydratgehalt von zwei Wassermolekülen per Äthoxy-Gruppe hatten. Die Kohlenwasserstoffkette hatte kaum Auswirkungen auf die Hydration.

---

---

With 7 figures and 2 tables