Roberto Tassinari

Oxidation of benzene by molecular oxygen catalysed by vanadium

Abstract Vanadium compounds can efficiently catalyse the oxidation of benzene to phenol with molecular oxygen. The reaction is truly catalytic only in the presence of a reducing agent able to recycle the oxidised catalyst. Among the reducing agents tested, ascorbate afforded the highest selectivity and reactivity to phenol. With other reductants, such as dithioalcohols, high selectivity (>96%) was obtained toward more oxidised products (i.e. hydroquinone). The reactions were performed in the two-phase system formed by mixing water/acetonitrile/benzene which allows a good separation between reactant and product. The reaction rate and product selectivity depend upon the type of ligand interacting with vanadium as well as upon the ascorbate/V molar ratio. ESR measurements confirmed that V IV species is rapidly formed in the reaction medium after the initial addition of V III Cl 3 . Moreover, the observed initial induction period of benzene oxidation can be related to the formation of oligomeric V IV species, as assessed by ESR analysis, rather than to the slow formation of V–ascorbate complex.

Direct synthesis of phenols by iron-catalyzed biphasic oxidation of aromatic hydrocarbons with hydrogen peroxide

The hydroxylation of a series of aromatic hydrocarbons with hydrogen peroxide, catalyzed by iron complexes with pyrazine-3-carboxylic acid N-oxide, was investigated, operating in a biphasic reaction medium. The new catalyst showed a high selectivity to the corresponding phenols, minimizing the over-oxidation reactions to polyoxygenated derivatives and tars which, along with dimers formation, are the major limitations of the classical Fenton’s reagent for a practical synthetic application. In the case of alkylbenzenes, the competitive side chain oxidation at the benzylic positions also occurred. Electron rich substrates, such as anisole, were oxidized with very poor selectivity. The reactions were carried out in a biphasic system that allows a convenient recovery and recycling of the catalyst by phase separation techniques. The catalyst showed a complete retention of activity after six consecutive reaction cycles. The new catalyst appears as a promising tool for the direct synthesis of phenols, in alternative to the conventional multi-step methods.

Ligand effect on the iron-catalysed biphasic oxidation of aromatic hydrocarbons by hydrogen peroxide

Abstract The hydroxylation of benzene and toluene with hydrogen peroxide, catalysed by iron complexes with nitrogen ligands in a biphasic reaction medium, was investigated. After testing a series of pyridine and pyrazinecarboxylic acid derivatives, it was found that the ligand markedly affects the catalyst efficiency and the selectivity based on the oxidant. In the case of toluene, the ligand has a tuning effect on the catalyst activity, driving the oxidation preferentially to the ring or to the methyl group. The complex with pyrazine-3-carboxylic acid N -oxide was selected as the most efficient catalyst and appears to be a promising tool for the direct synthesis of phenols. The mechanism of the aromatic hydroxylation is also discussed, in comparison with the conventional Fenton’s reagent.

Substrate specificity and stereoselectivity of hydrolytic enzymes from Brevibacterium imperiale B222

Four different hysrolytic enzymes were isolated and partially purified from Brevibacterium imperiale B222 cells. The stereoselectivity of each enzyme was assayed by using the nitrile, amide and esters derivatives of 2-aryloxypropionic acid. Within the cellular pool of hydrolytic enzymes, a non-stereoselective nitrile hydratase, a stereoselective amidase and two partially stereoselective esterases of opposite enantiomeric preference were found.

Partial purification of glutaryl-7-ACA acylase from crude cellular lysate by reverse micelles

Glutaryl-7-ACA acylase was partially purified from the cellular lysate ofPseudomonas sp. NCIMB 40409 by means of reverse micelles-water two-phases extractions. The tetrameric enzyme can be solubilized inside the reverse micelles formed by anionic (Aerosol OT, AOT) and cationic (tetradecyltrimethylammoniumbromide, TDAB) surfactants with retention of the enzymatic activity. With TDAB reverse micelles system, the acylase was partially extracted from the aqueous phase and, after backward transfer into a second water phase, a twofold purification factor was achieved. On the other hand, with the AOT micellar system, in conditions were most of the proteins but acylase, were extracted by the organic micellar solution, a sixfold increase of the specific activity of the acylase remaining in the aqueous phase was obtained.