Marco Lucarini

Transition metal salts catalysis in the aerobic oxidation of organic compounds. Thermochemical and kinetic aspects and new synthetic developments in the presence of N-hydroxy-derivative catalysts

Abstract The catalytic system formed by Mn(II) and Co(II) or Cu(II) nitrates has shown to be particularly effective for the oxidation of ketones and aldehydes by molecular oxygen under mild conditions. The process has a general character, but it is particularly selective for the oxidation of alkyl–aryl ketones to aromatic carboxylic acids, alkyl-cyclopropyl ketones to cyclopropane carboxylic acids and cycloalkanones to dicarboxylic acids. Mn salt plays a key role in this catalysis, which catalyses the enolisation of the carbonyl compound and initiates a free-radical redox chain with oxygen by an electron-transfer process. Co and Cu salts alone are inert, but they exalt the catalytic activity of the Mn salt, being more effective in the decomposition of the hydroperoxides. The same metal salt complexes, associated with TEMPO revealed particularly effective for the aerobic oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones under mild conditions (with air or oxygen at room temperature and atmospheric pressure); the mechanism of the catalysis is discussed. Thermochemical and kinetic investigations by EPR spectroscopy, concerning N-hydroxy compounds, have allowed to evaluate the Bond Dissociation Enthalpies (BDE) of several OH bonds and the absolute rate constants for the formation of the phthalimide-N-oxyl (PINO) radical from N-hydroxyphthalimide (NHPI) and for the hydrogen abstraction from several CH bonds by the PINO radical. The thermochemical and kinetic results have allowed us to explain the opposite catalytic behaviour of the two nitroxyl radicals, TEMPO and PINO, the former being an inhibitor of free radical processes, the latter a promoter of free-radical chain, and to develop new selective processes concerning the aerobic oxidation of alcohols, amines, amides, silanes and the substitution of heteroaromatic bases.

Water-Soluble Gold Nanoparticles Protected by Fluorinated Amphiphilic Thiolates

The preparation and the properties of gold nanoparticles (Au NPs) protected by perfluorinated amphiphiles are described. The thiols were devised to form a perfluorinated region close to the gold surface and to have a hydrophilic portion in contact with the bulk solvent to impart solubility in water. The monolayer protected clusters were prepared, in an homogeneous phase using sodium thiolates because of the low nucleophilicity of the alpha-perfluorinated thiols, and fully characterized with (1)H, (19)F NMR spectrometry, IR and UV-vis absorption spectroscopies, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). Au NPs with core diameters ranging from 1.6 to 2.9 nm, depending on the reaction conditions, were obtained. Water-soluble NPs (MPC-F8-PEGs) were obtained with the thiol HS-F8-PEG ending with a short poly(ethylene glycol) unit (PEG-OMe 550), whereas thiols with shorter PEG chains give rise to NPs insoluble in water. MPC-F8-PEGs undergo an exchange reaction with amphiphilic alkyl thiols. ESR investigations, using a hydrophobic radical probe, indicate that the MPC-F8-PEG monolayer shows a greater hydrophobicity compared to the analogous hydrogenated monolayer.

Free radical intermediates in the inhibition of the autoxidation reaction.

The autoxidation of organic materials is a detrimental radical chain process often leading to their rapid deterioration unless they are protected by preventive and/or chain breaking antioxidants. The properties of the more important family of the latter ones, that one of phenols, are illustrated in this tutorial review. A short outline of diarylamine antioxidants is also given. We describe simple experimental methods employed for the determination of the two parameters more useful for estimating the inhibiting power of antioxidants, that is the kinetic rate constant for their reaction with the chain carrying peroxyl radicals, kinh, to give persistent phenoxyl or aminyl radicals and the bond dissociation enthalpy BDE(X–H) (X = O, N) of the bond cleaved in the inhibition process. The dependence of these parameters on the number and nature of the substituents is discussed and, in the case of phenols, a simple rule allowing to predict with reasonable accuracy the BDE and kinh values, from their structure. The effect of solvent polarity on the antioxidant power is also described. Finally, the information on the mechanism of reaction between phenols and peroxyl radicals provided by both experiments and theoretical calculations are examined. Because of difficulties associated with the analysis of non-homogenous systems all the reported results refer to homogenous solution in which experimental data can be analysed by means of more reliable and complete treatments.

Dynamic aspects of cyclodextrin host-guest inclusion as studied by an EPR spin-probe technique

Major changes in the EPR parameters of benzyl tert-butyl nitroxide and other dialkyl nitroxides on complexation by cyclodextrins (CDs) make these radicals particularly suitable probes for studying inclusion phenomena, because the signals for the free and included species are well separated. Furthermore, selective line broadening of the EPR spectra enables the kinetics of inclusion to be investigated [Eq. (a)].

Unraveling Unidirectional Threading of α-Cyclodextrin in a [2]Rotaxane through Spin Labeling Approach

We present here the results of a CW-ESR investigation of a double spin labeled α-cyclodextrin-based [2]rotaxane that is characterized by the presence of nitroxide labels both at the wheel and at the dumbbell. This was accomplished by synthesizing a spin labeled α-CD (the wheel) that was mechanically blocked on a thread containing a nitroxide unit by a Cu(I) catalyzed azide-alkyne cycloaddition (CuAAC). Both ESI-MS analysis and NMR spectroscopy were used to provide evidence of the threading processes. Because of the unsymmetrical structure of both the wheel and the axle, two different geometrical isomers could be predicted on the basis of the orientation of the CD along the thread. By combining molecular dynamic calculations and information extracted from the CW-ESR spectra, we were able to determine the geometrical nature of the isomer that was isolated as the only species. The ESR spectra showed J-coupling between the two mechanically connected nitroxide units and were analyzed by a model assuming three main molecular states. The intramolecular noncovalent nature of spin exchange was confirmed by reversibly switching the magnetic interaction on-off by changing the pH of the solution in the presence of a competing macrocyclic host.

Nitroxide Radicals as Hydrogen Bonding Acceptors. An Infrared and EPR Study.

The equilibrium constants for the formation of hydrogen-bonded complexes with phenol, benzyl alcohol and diphenyl amine of a persistent nitroxide radical (TEMPO) have been measured for the first time by IR spectroscopy in solution. By making use of the data obtained with IR measurements it was possible to quantitatively determine the effect of hydrogen bonding on the nitrogen hyperfine-splitting constant. On this basis it is shown that EPR spectroscopy can be used as an alternative to IR spectroscopy for the determination of the thermodynamic parameters of hydrogen bond formation with nitroxides by following the dependence of the experimental EPR nitrogen hyperfine splitting of these radicals upon changing the nature of the solvent. The experimental data obtained from both IR and EPR spectroscopy indicate that nitroxides are versatile hydrogen bond acceptors giving hydrogen bonds of strength similar to that of ethers or esters. The corresponding Abrahams beta 2H values have been determined as 0.46.

Medium effects on the antioxidant activity of dipyridamole.

Very strong medium effects have been observed when testing the antioxidant activity of dipyridamole (DP) in different media such as benzene, tert-butanol, methanol solutions and egg yolk lecithin unilamellar and multilamellar vesicles. Actually, dipyridamole behaves as a very poor antioxidant in benzene while its ability to inhibit the lipid peroxidation reaction increases with increasing solvent polarity, being the highest in lipid vesicles. This behavior can not be rationalized on the basis of the classical chain breaking mechanism which operates in the case of phenolic and amine antioxidants and involving the transfer of a hydrogen atom to peroxyl radicals. An explanation is instead given in terms of an electron transfer reaction which leads to the oxidation of DP by the chain carrying peroxyl radical to give the dipyridamole cation radical, DP+*, and the peroxyl anion LOO-, and whose rate constant is expected to increase in strongly polar media. EPR and electrochemical data supporting this interpretation have been collected.

Increasing the Persistency of Stable Free-Radicals: Synthesis and Characterization of a Nitroxide Based [1]Rotaxane

A persistent paramagnetic [1]rotaxane based on beta-cyclodextrin showing an increased persistency under reductive conditions has been prepared and characterized for the first time.

Electron spin resonance imaging of polymer degradation and stabilization

Abstract We present the application of one-dimensional (1D) and two-dimensional (2D) electron spin resonance imaging (ESRI) for the study of photo- and thermal degradation of polymers containing hindered amine stabilizers (HAS). The method is based on the formation of stable nitroxide radicals derived from HAS during polymer treatment, and on encoding spatial information in the ESR spectra via magnetic field gradients. The imaging technique allowed nondestructive profiling of the HAS-derived nitroxide radicals. The intensity profile in the sample depth was deduced by 1D ESRI, and the spatial variation of the ESR line shapes (‘spectral profiling’) was determined by 2D spectral-spatial ESRI. Application of this approach to the photodegradation of polypropylene and to the photo- and thermal degradation of poly(acrylonitrile–butadiene–styrene) will be described. The method can be used to detect spatial heterogeneities in the degradation process and to identify morphological domains that are selectively degraded.

Self-Organization of Mixtures of Fluorocarbon and Hydrocarbon Amphiphilic Thiolates on the Surface of Gold Nanoparticles

Self-assembled monolayers composed of a mixture of thiolate molecules, featuring hydrocarbon or perfluorocarbon chains (H- and F-chains) terminating with a short poly(oxoethylene) (PEG) moiety, are the most extreme example of surfactant immiscibility on gold nanoparticles reported so far. The phase segregation between H-chains and F-chains and the consequent, peculiar folding of PEG chains are responsible for the increased affinity of a selected radical probe for the fluorinated region, which increases as the size of the fluorinated domains decrease, independently of the shape of such domains. This feature has been revealed by ESR measurements and an in silico innovative multiscale molecular simulations approach in explicit water. Our results reveal an underlying mechanism of a transmission of the organization of the monolayer from the inner region close to the gold surface toward the external hydrophilic PEG region. Moreover, this study definitively proves that a mixed monolayer is a complex system with properties markedly different from those characterizing the parent homoligand monolayers.