Rosa M. Toscano

One-flask transformation of secondary amines to nitrones by oxidation with hydrogen peroxide mediated by triscetylpyridinium tetrakis oxodiperoxotungsto-phosphate (PCWP). Some mechanistic considerations

Abstract Acyclic and cyclic secondary amines are oxidized to nitrones by H 2 O 2 /PCWP system in water/chloroform under phase transfer catalysis conditions. The different acidities of the protons of the carbon atoms α to the nitrogen might be responsible for the identity and ot the stereochemistry of the formed nitrone. The presence of an aromatic ring such as benzene directly linked to the nitrogen represents a limitation, since in this case many oxidation products are observed. As far as the mechanistic aspects are concerned, it is suggested that the oxidation process might be started by a nucleophilic attack of the amine to the peroxidic oxygens of the peroxometal complex or by a single electron transfer from the amine to the oxidant.

Recognition of Achiral and Chiral Ammonium Salts by Neutral Ditopic Receptors Based on Chiral Salen-UO2 Macrocycles

A mononuclear (M20) and a dinuclear (M40) uranyl chiral macrocyclic complex, incorporating both a salen unit containing two phenyl rings linked to a chiral diimine bridge and the (R)-BINOL unit, behaves as an efficient ditopic receptor for achiral and chiral quaternary ammonium salts. Binding affinities in chloroform solution have been measured for 1:1 complexes of many quaternary salts encompassing tetramethylammonium (TMA), tetraethylammonium (TEA), tetrabutylammonium (TBA), and acetylcholine (ACh), as well as trimethylanilinium (TriMAn), benzyltrimethylammonium (BnTriMA), (alpha-methylbenzyl)trimethylammonium and pyrrolidinium cations. The anion of the salt is bound by the hard Lewis acidic uranyl site, with an increasing binding efficiency on increasing the anion hardness (I(-) < Br(-) < Cl(-)), whereas CH-pi or pi-pi attractions by binapthyl moiety, or the salicylaldehyde unit, or the phenyl rings of diimine bridge ensure the recognition of the cation partner. Optimized structures of receptor-anion-cation ternary complexes obtained by MM calculations are supported by 2D-ROESY NMR measurements.

Pair of Diastereomeric Uranyl Salen Cavitands Displaying Opposite Enantiodiscrimination of α‑Amino Acid Ammonium Salts

A pair of diastereomeric salen cavitands and their uranyl complexes combine a chiral (R,R) salen bridge and an inherent chiral tris-bridged quinoxaline cup within the same molecule. Whereas the free ligands show a preference for the same enantiomer of an α-amino acid pair, the corresponding UO(2) complexes display opposite enantiodiscrimination and exceptionally high enantioselectivities (K(D)/K(L) = 26.4).

Insulin mimesis of vanadium derivatives. Oxidation of cysteine by V(V) oxo diperoxo complexes

Kinetics of the oxidation of cysteine to cystine by four V(V) oxo diperoxo complexes [VO(O2)2L] possessing insulin mimetic activity, where L = oxalate(oxa), picolinate (pic), bipyridil (bipy), phenanthroline(phen), were performed in water at 10 degrees C by the UV or stopped-flow technique. 51V NMR spectra indicate that oxa undergoes a total ligand dissociation differently from pic, bipy and phen which hold their ligands also in solution. The observed reactivity is deeply affected by the identity of the ligand. The process seems to require coordination of the cysteine to the metal, followed by oxidation within the coordination sphere. In this respect phen and bipy make the coordination of cysteine much easier than oxa and pic. It is suggested, also on the basis of some preliminary observations concerning the oxidation of C6H5CH2SH, that the oxidation process is triggered by an electron transfer step. The rate of this step would be higher for oxa and pic than for phen and bipy. The observation that the oxidative ability of these vanadium peroxo complexes is dependent upon the nature of the ligands might match the analogous finding that their insulin mimetic activity is also modulated by the ligand identities.

Laurencianol, a new halogenated diterpenoid from the marne alga

Abstract The structure of an anitmicrobial dibromomochloroditerpene alcohol, laurencianol, has been determined by spectral and X-ray crystallographic techniques. The compound has been isolated from the Mediterranean red alga barLaurencia obtusa .

Sensing of linear alkylammonium ions by a 5-pyrenoylamido-calix(5)arene solution and monolayer using luminescence measurements†

Covalent chemisorption of a p-chloromethylphenyltrichlorosilane monolayer on silica substrates was achieved. A cone 5-pyrenoylamido-calix[5]arene bearing a 12-aminododecyl moiety at the lower rim was further covalently bonded, producing a 5-pyrenoylamido-calix[5]arene-based monolayer on silica. The surface chemical characterization of this hybrid material was carried out by X-ray photoelectron spectroscopy. The sensing properties of this pyrenoylamido-calix[5]arene system were probed by both NMR and luminescence measurements in solution. The optical recognition properties of the functional monolayer were similarly studied at room temperature by emission measurements. This system demonstrates to have significant recognition properties for the linear alkylammonium ions. As a result, n-dodecylammonium ions can be detected at ppm levels. The adopted synthetic procedure provided evidence to be useful in transferring molecular properties to a solid state device.

The relative reactivity of thioethers and sulfoxides toward oxygen transfer reagents: the case of dioxiranes

Abstract In the competitive oxidation of thioethers and sulfoxides by electrophilic reagents the reactivity ratio may be small enough to cause the formation of mixtures of products. This has been studied in the reaction of dimethyldioxirane, DMD, and trifluoromethyl-methyldioxirane, TMMD. In particular TMMD oxidizes both thioethers and sulfoxides whereas DMD attacks only thioethers. By applying the logic which has been used to develop mechanistic probes based on such competitive oxidations, the unrealistic conclusion that TMMD is less electrophilic than DMD would be reached. On the contrary we show also by means of Hammett plots on substituted sulfoxides that DMD and TMMD are both electrophilic oxidants toward sulfoxides and that TMMD is less selective than DMD being a stronger oxidant.

A New Labile Bromoterpenoid from the Red Alga Laurencia Majuscula: Dehydrochloroprepacifenol

Abstract A new labile brominated chamigrene, dehydrochloroprepacifenol, was isolated from the red alga Laurencia majucula. The structure of this compound was determined by its spectral properties and low-temperature X-ray diffraction analysis.

A surface-confined OMnV(salen) oxene catalyst and high turnover values in asymmetric epoxidation of unfunctionalized olefins

Given the widespread importance of chiral epoxides in chemical, biochemical and agricultural fields, an efficient synthesis that avoids wasteful use of the catalyst and assures its total recovery and reuse and high turnover values is highly desirable. We report on a thermally and temporally robust covalent monolayer of (salen)Mn(III) molecules on silica functionalized substrates that resulted in an active heterogeneous catalyst for enantioselective epoxidation of 6-ciano-2,2-dimethyl-chromene, chosen as a model alkene, in high yields and high turnover numbers. No catalyst leaching was observed thus allowing continuous recycling. Experimental XPS evidence for the formation of the surface confined OMnV(salen) oxene species has been obtained.

Oxidation of N,N-benzylalkylamines to nitrones by Mo(VI) and W(VI) polyperoxo complexes

Oxidation of N,N-benzylalkylamines in chloroform by Mo(VI) and W(VI) polyperoxo complexes (PPC) of general formula Q+3{PO4[MO(O2)2]4}3− (Q+ = onium ion) yield the corresponding nitrones as oxidized products quantitatively. Only in the case of N,N-benzylmethylamine the formation of nitrone is accompanied by 25% of benzaldoxime. Oxidation of N,N-benzyltertbutylamine and N,N-benzylisopropylamine follows second order kinetics. This finding does not disqualify the hypothesis that the reaction might occur by a rate determining nucleophilic attack of the amine onto the peroxide oxygens leading, through a Bartlett-type transition state, to the probable formation of the corresponding hydroxylamine, which then is converted to nitrone in a faster step. Under this respect PPC behavior seems related to that of the corresponding anionic monuclear oxidants Q+[MO(O2)2L]−) (L = anionic ligand). On the other hand PPC toward N,N-benzylmethylamine behave similar to neutral mononuclear oxidants, MO(O2)2L, since in both cases the formation of an oxidant-substrate association complex appears a probable event along the reaction coordinate. However, whereas for the neutral mononuclear oxidants this oxidant-substrate adduct seems to react further toward external amine molecules through a Bartlett-type transition state, for PPC such an adduct seems to evolve to products through unimolecular events akin enzymatic processes.