Substituent effect on iron phthalocyanines as cyclohexene oxidation catalysts

Two iron phthalocyanines peripherally octasubstituted either with electron-withdrawing isobutylsulfonyl moities or electron-donating isobutoxy moieties were designed to investigate the effect of the substitution pattern on their oxidation catalytic activity, and were then tested in oxidation of cyclohexene as a reaction model. For both catalysts, the main product of oxidation was 2-cyclohexen-1-ol which is an allylic oxidation product. The electron-withdrawing isobutylsulfonyl substituted iron phthalocyanine 1exhibited better catalytic activities than the electron-donating isobutoxy substituted iron phthalocyanine 2.

The Over-Riding Role of Autocatalysis in Allylic Oxidation

In this paper we aim to highlight the need to consider the possible role of autocatalysis in oxidation reactions when using molecular oxygen as the terminal oxidant. Oxygen in its ground state is a diradical, and depending on the reaction conditions, it can initiate oxidation through radical pathways through mechanisms which do not require the presence of a catalyst. Consequently, we contrast the oxidation of benzyl alcohol with oxidation of α-pinene. For benzyl alcohol oxidation the initial reaction is the oxidative dehydrogenation to form benzaldehyde, a non-radical process; but the subsequent over-oxidation to benzoic acid is a radical process. In this case the role of the autocatalysed reaction can be minimised. With α-pinene, the oxidation reaction is via radical pathways and now the autocatalysed reaction can be dominant and, indeed, can be the preferred pathway for the formation of high yields of the desired verbenone product. Graphic Abstract

SELENIUM AS A VERSATILE REAGENT IN ORGANIC SYNTHESIS: MORE THAN ALLYLIC OXIDATION

: For many years since its discovery, Selenium has played the role of a bad boy who became a hero in organic transformations. Selenium dioxide, for instance, is one of the most remembered reagents in allylic oxidations, having been applied in the synthesis of several naturally occurring products. The main goal of this review is to show the recent advances in the use of classical and new selenium reagents in organic synthesis. As demonstrated through around 60 references discussed in this study, selenium can go even forward as a versatile reagent. We bring a collection of selenium reagents and their transformations that are still hidden from most synthetic organic chemists.

Ensemble Effects of Explicit Solvation on Allylic Oxidation

Umbrella-sampling density functional theory molecular dynamics (DFT-MD) has been employed to study the full catalytic cycle of the allylic oxidation of cyclohexene using a Cu(II) 7-amino-6-((2-hydroxybenzylidene)amino)quinoxalin-2-ol complex in acetonitrile to create cyclohexenone and H$_2$O as products. In comparison to gas-phase DFT, the solvent effect is observed as the rate determining allylic H-atom abstraction step has a free energy barrier of 12.1 $\pm$ 0.2 kcal/mol in solution. During the cycle, the explicit solvation and ensemble sampling of solvent configurations reveals important dehydrogenation and re-hydrogenation steps of the -NH$_2$ group bound to the Cu-site that are essential to catalyst recovery. This work illustrates the importance of ensemble solvent configurational sampling to reveal the breadth of processes that underpin the full catalytic cycle.<br>

Ensemble Effects of Explicit Solvation on Allylic Oxidation

<div>Umbrella-sampling density functional theory molecular dynamics (DFT-MD) has been employed to study the full catalytic cycle of the allylic oxidation of cyclohexene</div><div>using a Cu(II) (E)-6-amino-7-((2-hydroxybenzylidene)amino)quinoxalin-2-ol complex in acetonitrile, which creates the desired cyclohexenone and H 2 O as products. In comparison to prior study using gas-phase DFT, a significant solvent effect is observed on the rate determining allylic H-atom abstraction step (which has a free energy barrier of 12.1 ± 0.2 kcal/mol). During the cycle, the explicit solvation and ensemble sampling of solvent configurations reveals an important dehydrogenation and re-hydrogenation step of the -NH 2 ligand that is essential to catalyst recovery. This work illustrates the importance of ensemble solvent configurational sampling to reveal the breadth of processes that underpin the full catalytic cycle.</div>

Ensemble Effects of Explicit Solvation on Allylic Oxidation

<div>Umbrella-sampling density functional theory molecular dynamics (DFT-MD) has been employed to study the full catalytic cycle of the allylic oxidation of cyclohexene</div><div>using a Cu(II) (E)-6-amino-7-((2-hydroxybenzylidene)amino)quinoxalin-2-ol complex in acetonitrile, which creates the desired cyclohexenone and H 2 O as products. In comparison to prior study using gas-phase DFT, a significant solvent effect is observed on the rate determining allylic H-atom abstraction step (which has a free energy barrier of 12.1 ± 0.2 kcal/mol). During the cycle, the explicit solvation and ensemble sampling of solvent configurations reveals an important dehydrogenation and re-hydrogenation step of the -NH 2 ligand that is essential to catalyst recovery. This work illustrates the importance of ensemble solvent configurational sampling to reveal the breadth of processes that underpin the full catalytic cycle.</div>

Cascade Biocatalysis Designed for the Allylic Oxidation of α-Pinene

A biocatalytic cascade system using a cocktail of oxidoreductase enzymes (2-1B peroxidase and M120 laccase) was designed for the allylic oxidation of (+)-α-pinene into value-added products (e.g., verbenol and verbenone). The oxidative transformation involved a two-step process as follows: (+)-α-pinene was (i) oxidized on the allylic position with H2O2 mainly assisted by 2-1B peroxidase leading to verbenol as the principal reaction product, and (ii) directed to verbenone in the presence of M120 laccase responsible for further oxidation of verbenol to verbenone. The reaction environment was ensured by the acetate buffer (0.1 M, pH = 5). Optimum values for the experimental parameters (e.g., concentration of 2-1B peroxidase, M120 laccase, and H2O2) were set up. The biocatalytic cascade process was monitored for 24 h in order to evaluate the process pathway. Maximum performance under optimum conditions was reached after 5 h incubation time (e.g., 80% (+)-α-pinene conversion and 70% yield in verbenol). Therefore, the developed biocatalytic cascade system offered promising perspectives for (+)-α-pinene valorization.