Gaetano A. Tomaselli

Oxidations with peroxotungsten complexes: rates and mechanism of stoichiometric olefin epoxidations

Abstract The stoichiometric epoxidation reactions of a series of olefins with WO-(O2)2HMPT and MoO(O2)2HMPT have been studied in dichloroethane at 40 °C. The data obtained point out the superior oxidizing ability of W(VI) over Mo(VI). However, the two peroxo groups in WO(O2)2HMPT appear to react with the substrates at different rates, the transfer of the first peroxide oxygen being faster than of the second one, contrary to that observed in the behavior of MoO(O2)2HMPT. The crystal structure of WO(O2)2HMPT·H2O, determined by diffractometric techniques, does not show any significant difference compared to that reported for MoO(O2)2HMPT·-H2O, indicating that it might be difficult to correlate the solid state structures with the different behaviors experimentally observed in solution. In spite of the different features shown by the two peroxo complexes, the data collected in this investigation suggest that an identical mechanism of oxygen transfer from the two oxidants is operating. Moreover, this mechanism should not involve the occurrence of substrate-oxidant intermediates.

A Versatile Strategy for Signal Amplification Based on Core/Shell Silica Nanoparticles

The design of fluorescent chemosensors for biologicallyrelevant chemical species has important impacts in many ap-plications, and for this reason it has been the subject ofactive research in many laboratories worldwide. The ach-ievements in this wide research topic have promoted enor-mous steps forwards, for example, in the field of cell biology,thanks to the comprehension of the role of different chemi-cal species in many biological processes. Recently, research-ers have been moving from molecular chemosensors basedon two communicating units, a receptor and a dye, towardmore complex and sophisticated structures, and have triedto push further the limits of sensitivity and selectivity. Manydifferent solutions have been proposed but, among them,sensing systems based on nanoparticles are certainly one ofthe most interesting and promising.

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.

Oxidation of alkynes by dioxiranes

Abstract In situ generated or isolated dimethyldioxirane (1a) and methyl(trifluoromethyl)dioxirane (1b) efficiently afford oxidation of alkynes, most likely via oxirene intermediates, which rearrange into ketene or α,γ-unsaturated carbonyls, or else are further oxidized to α,β-dicarbonyls. Diphenylacetylene and phenylacetylene yield mostly ketene derived products, whereas 8-hexadecyne (an internal dialkyl alkyne) gives 9-hexadecen-8-one (both trans and cis) as the main product. Reaction of cyclodecyne (a conformationally rigid cycloalkyne) with isolated 1b affords cis-bicyclo[5.3.0]decan-2-one (15) and cis-bicyclo[4.4.0]decan-2-one (16), which derive from the oxirene by stereoselective 1.5- and 1,6-transanular insertion, respectively.

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.

Kinetics and mechanism of the tungsten-catalyzed oxidation of organic sulphides and alkenes by hydrogen peroxide

Abstract The oxidation of a series of phenyl-substituted arylmethyl sulphides and of cyclohexene and 1-methylcyclohexene by hydrogen peroxide in the presence of catalytic amounts of WO 5 · HMPT · H 2 O and W(CO) 6 has been studied in ethanol (HMPT = hexamethyl phosphoric acid triamide). The reactions afford the corresponding sulphoxides and epoxides in quantitative yield. A subsequent ethanolysis of the oxirane ring occurs in solution. Kinetic studies indicate that the reaction is first-order in the substrate and in the catalyst, whereas a zero-order in hydrogen peroxide is found. The identity of the catalytic efficiency of the two W(VI) species employed suggests that HMPT ligand is displaced in solution. The substituent effect on the oxidation rates of arylmethyl sulphides follows the Hammett free-energy relationship (ϱ ≈ −1). Sulphides are much more reactive than cyclohexene which, in turn, is 2.5-fold less reactive than 1-methylcyclohexene. Reaction rates are remarkably enhanced by addition of CH 3 SO 3 H, whereas added HMPT at relatively high concentration inhibits the oxidation. The data are discussed in the light of the results of previous studies on metal-hydrogen peroxide systems, pointing out similarities and differences. The conclusion is reached that W(VI)—H 2 O 2 is a particularly efficient oxidant.

The relevance of acid-base equilibria in the catalytic oxidations by tungsten and molybdenum peroxo complexes

Abstract The catalytic behaviour of the two peroxo complexes MO(O 2 ) 2 HMPT· H 2 O (M = W(VI), Mo(VI); HMPT = hexamethylphosphoric acid triamide) in the oxidations by hydrogen peroxide in ethanol has been examined. Rate data, obtained in a model reaction, i.e. the oxidation of organic sulphides to the corresponding sulphoxides, indicate that in ‘neutral’ ethanol the Mo(VI)-catalyzed system is more efficient than the W(VI) one. Spectroscopic ( 1 H NMR), potentiometric and kinetic experiments suggest that this is mainly due to the much stronger acidity of the W(VI)-peroxo complex which is almost completely dissociated into its anionic form, and which is a poor electrophilic oxidant. In fact, when the anions are neutralized upon addition of methanesulphonic acid, the neutral peroxotungsten complex appears to be a much more effective oxidizing agent than the structurally similar peroxomolybdenum compound. An additional advantage of the W(VI)-catalyzed system may be found in the reduced affinity of W(VI)-peroxo species, as compared with Mo(VI), for neutral ligands. Therefore HMPT, even at the highest catalytic concentrations employed, is completely removed from the coordination sphere of tungsten.

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.

Spectroscopic and structural properties of some molybdenum and tungsten polyoxoperoxo complexes. A comparison with mononuclear complexes

Abstract Polyoxoperoxo complexes (PPC), either generated in situ by addition of H 2 O 2 to heteropoly acids or isolated, appear to be more versatile oxidants of organic substrates than mononuclear peroxo complexes of the same metals, namely Mo and W, both anionic or neutral. The comparison of some spectroscopic properties of a series of PPC with those of simple peroxo complexes indicates that no major differences between the two families of oxidants are observed. Thus, the remarkable electrophilic reactivity of PPC, which are effective epoxidizing agents, in contrast to anionic simple peroxo complexes, may be explained by a more effective charge delocalisation in the former peroxo complexes. Moreover, 17 O-NMR data concerning the line widths of the peroxo oxygens of PPC, seem to indicate a high degree of ion pairing of these species which should serve to reduce their anionic character. The structure of a Mo(VI)—PPC has been determined by diffractometric analysis and compared with that ofa W(VI) derivative already reported in the literature. Also such a comparison does not reveal appreciable differences in the solid state structures of PPC.