Francesca d'Acunzo

A Mechanistic Survey of the Oxidation of Alcohols and Ethers with the Enzyme Laccase and Its Mediation by TEMPO

The oxidation of alcohols and ethers by O2 with the enzyme laccase, mediated by the stable N-oxyl radical TEMPO, affords carbonylic products. An ionic mechanism is proposed, where a nucleophilic attack of the oxygen lone-pair of the alcohol (or ether) onto the oxoammonium form of TEMPO (generated by laccase on oxidation) takes place leading to a transient adduct. Subsequent deprotonation of this adduct α

A study of the oxidation of ethers with the enzyme laccase under mediation by two N–OH–type compounds

The oxidation of ethers of various structure by O2 with two laccase/N–OH systems (i.e., HBT, 1-hydroxybenzotriazole, and HPI, N-hydroxyphthalimide) is described. The process affords carbonylic products in reasonable-to-good yields. The oxidation is carried out by the intermediate N–O˙ species of the mediators, through a radical H-atom abstraction (HAT) route that represents an elaboration of the HAT route followed with benzyl alcohols. Alternative mechanisms have been considered and dismissed. A comparison with a similar oxidation of ethers by the laccase/TEMPO system, reported previously, is made. A clear-cut specialisation of the mediator vs. the substrate emerges, i.e. the laccase/HBT system is more competent for the oxidation of ethers, whereas the laccase/TEMPO system was most proficient in the oxidation of benzyl alcohols.

The radical rate-determining step in the oxidation of benzyl alcohols by two N–OH-type mediators of laccase: the polar N-oxyl radical intermediate

Determination of the effect of substituents in the aerobic oxidation of X-substituted benzyl alcohols by laccase, with mediation by HPI or HBT, confirms the H-atom abstraction from the benzylic C–H bond as the rate-determining step (HAT route), and supports a polar nature for the N-oxyl radical intermediate originating from the two N–OH mediators.

Tertiary : secondary : primary C–H bond relative reactivity in the one-electron oxidation of alkylbenzenes. A tool to distinguish electron transfer from hydrogen atom transfer mechanisms

Data of tertiary: secondary: primary C–H bond relative reactivity (TSP selectivity) for a number of electron transfer (ET) and hydrogen atom transfer (HAT) reactions of alkylbenzenes have been critically reviewed and in a few cases supplemented by additional experiments. The resulting picture indicates that there are significant differences in TSP selectivity between ET and HAT reactions. When the HAT mechanism is operating the reactivity order tertiary > secondary > primary C–H bond is always observed. This order never holds in reactions occurring by an ET mechanism where, generally, the secondary C–H bond is the most reactive one and the tertiary centre can be either more or even less reactive than the primary one. Whatever the possible reasons for these differences, it turns out that TSP C–H bond selectivity determinations can afford useful information with respect to the distinction between ET and HAT mechanisms in the oxidations of alkylbenzenes. To check this conclusion a study of TSP selectivity in the oxidation of alkylbenzenes promoted by metalloporphyrins and by microsomal cytochrome P-450 has been carried out, which has allowed us to assign a HAT mechanism to these reactions, in full accord with previous attributions.

Steric effects and selectivity in the benzylic hydroxylation by metalloporphyrins and by the fungus Mortierella isabellina

Abstract The role of steric effects in the biomimetic and enzymatic benzylic hydroxylation is assessed by comparing inter- and intramolecular selectivity of these reactions.

A first study on copolymers of a methacrylate containing the 2-(hydroxyimino)aldehyde group and OEGMA. RAFT polymerization and assessment of thermal and photoresponsive polymer behavior

We report the first synthesis of 4-[(hydroxyimino)aldehyde]butyl methacrylate and its RAFT copolymerization with oligo(ethylene glycol) monomethylether methacrylate. For comparison, 6-(hydroxyimino)hexyl methacrylate, containing a free oxime group, and 6-oxohexyl methacrylate, with an aldehyde group, are also synthesized and subjected to RAFT copolymerization with OEGMA. DP(GPC) values of all polymers are in the 18–33 range with PDI < 1.2. Interestingly, the thermal behavior of aqueous solutions of the 2-(hydroxyimino)aldehyde-containing copolymers exhibit a lower cloud point than their analogs, which comprise only a CN(OH) or CHO functionality. This effect can be ascribed to the simultaneous presence of a hydrogen donor and acceptor in the 2-(hydroxyimino)aldehyde group. Moreover, we present evidence for the thermally reversible E/Z photoisomerism of the 2-(hydroxyimino)aldehyde group (HIA) both in HIABMA and in its copolymers.

2-(Hydroxyimino)aldehydes: Photo- and Physicochemical Properties of a Versatile Functional Group for Monomer Design

In the context of our research on stimuli-responsive polymers bearing the 2-(hydroxyimino)aldehyde (HIA) group, we have explored the photochemical behavior and physicochemical properties of a number of HIAs. Interpretation of the experimental data is supported by quantum mechanical calculations. HIAs are expected to undergo photoisomerization, chelate metal ions, yield hydrogen-bonded dimers or oligomers, exhibit relatively low pKa s, and form >C=NO. radicals through OH hydrogen abstraction or oxidation of the oximate ion. Besides the well-established E/Z oxime photoisomerism, we observed a Norrish-Yang cyclization resulting in cyclobutanol oximes, to our knowledge not previously described in the literature. The acidity, bond dissociation enthalpies, and electrochemical properties of the HIAs are compared with literature data of simple oximes. The results are discussed in relation to the many potential applications for HIAs, with emphasis on the synthesis of novel HIA-containing responsive polymers.

OXIDATION OF ISOPROPYLBENZENE BY IRON TETRAPHENYLPORPHYRIN : EVIDENCE FOR THE INTERACTION OF THE CUMYL RADICAL WITH OXYGEN DONORS

Evidence is presented for an unprecedented oxygen transfer from oxygen donors to a cumyl radical in the oxidation of isopropylbenzene catalysed by tetraphenyl metalloporphyrins.