Simulation and optimization of a supercritical extraction process for recovering provitamin A

In this work, a simulation procedure of a supercritical extraction process was developed through the use of the commercial simulator HYSYS^TM (Hyprotech Ltd.), adapting the existing units to the operating conditions typical of the supercritical extraction process. The objective is to recover provitamin A (β-carotene) from palm oil (esterified) using carbon dioxide/ethanol as the supercritical mixed solvent. This example characterizes the problem for recovering high added value product from natural sources, as the palm oil, which is desired by the market. Owing to the fact that esterified palm oil is a complex mixture, made by several components, in order to characterize this system in the simulator, it was necessary to create hypothetical components using the UNIFAC (universal function-group activity coefficients model) group contribution, because they are not present in a conventional database and, then, their physical properties must be estimated and/or predicted before the simulation. The optimization was carried out in each simulation for each equipment, in terms of operating conditions (temperature and pressure), in order to obtain the maximum recovery of carotenes. According to the results, it was possible to concentrate carotenes through two cycles of supercritical extraction with high yield. Furthermore, ethyl esters (biodiesel) were also obtained, as a byproduct of the proposed process, which can also be used as an alternative fuel, with the important characteristic that it is renewable.     

Recent efforts directed to the development of more sustainable asymmetric organocatalysis

In line with the principles of "green" chemistry, organocatalysis seeks to reduce energy consumption and to optimize the use of the available resources, aiming to become a sustainable strategy in chemical transformations. Nevertheless, during the last decade diverse experimental protocols have made organocatalysis an even "greener" alternative by the use of friendlier reaction conditions, or via the application of solvent-free methodologies, or through the design and synthesis of more selective catalysts, or via the development of multicomponent one-pot organocatalytic reactions, or by the recycling and reuse of organocatalysts, or by means of the application of more energy-efficient activation techniques, among other approaches. In this feature article we review some of the remarkable advancements that have made it possible to develop even more sustainable asymmetric organocatalyzed methodologies.     

Degradation and stabilisation of poly(ethylene-stat-vinyl acetate): 1 - Spectroscopic and rheological examination of thermal and thermo-oxidative degradation mechanisms

The degradation of ethylene vinyl acetate (EVA) copolymers was compared with low density polyethylene (LDPE), poly(vinyl acetate) (PVAc) and poly(vinyl chloride) (PVC) using FTIR, UV-visible and fluorescence spectroscopy as well as thermal and rheological analyses. Thermal, thermo-oxidative and photo-oxidative studies were conducted. Thermo-oxidation below 180 °C shows more similarities between EVA and LDPE. The luminescence spectra of degraded EVA and LDPE were almost identical but very different to that of PVAc. UV-vis analysis showed that the polyenes present in aged PVC were unlikely to be the same species responsible for the observed colour formation in aged EVA. It is suggested that they are polyconjugated carbonyl products. Rheological analysis also showed the evolution of crosslinking reactions during thermo-oxidation. FTIR studies after thermal degradation in inert conditions 290 °C showed complete loss of the ester functionality and associated lactone formation along with some evidence for ketonic and unsaturated carbonyl groups. Degradation in air at 180 °C, however, revealed that loss of the ester group was not so marked, with PVAc exhibiting the greatest stability. This was in line with the induction time to onset of autocatalytic carbonyl growth at 180 °C; the latter showed an apparent exponential decrease with increasing vinyl acetate content up to 28% w/w. Fluorescence analysis produced trends that complemented those of carbonyl index; the time to decomposition of initial fluorescent α,β-unsaturated carbonyl species coincided with the time to onset of carbonyl growth. Furthermore, the rate of formation of the new fluorescent species produced in EVA, and LDPE was similar to that of carbonyl growth. These new fluorescent species are therefore likely to be di- or tri-carbonyl products.     

Asymmetric Synthesis of β2‐homo‐tert‐Leucine via Radical Addition to Enantiopure N‐Fumaroylhexahydrobenzooxazolidin‐2‐one

β‐Amino acids are key structural elements in unnatural peptides, peptidomimetics, and many other physiologically active compounds. In view of their importance, we have developed an efficient synthetic route that provides highly enantiomerically enriched (R)‐ and (S)‐H‐β²‐h^tLeu‐OH via highly diastereo‐ and regioselective addition of tert‐butyl radical to enantiomerically pure N‐fumaroyloxazolidinones, followed by removal of the chiral auxiliary, Curtius rearrangement, ester hydrolysis, and catalytic hydrogenolysis.     

Looking for treasure in stereochemistry-land. A path marked by curiosity, obstinacy, and serendipity

Over the past 40 years, much of my research has evolved around various topics of conformational analysis and asymmetric synthesis. This Perspective describes some of my salient contributions in eight different areas of organic stereochemistry: (1) conformational analysis of six-membered rings, (2) evaluation of stereoelectronic interactions in (1)J(C-H) one-bond coupling constants in six-membered rings, (3) eclipsed conformation in cis-2-tert-butyl-5-(tert-butylsulfonyl)-1,3-dioxane, (4) determination of enthalpic and entropic contributions to ΔG°(CH(2)Ph) and ΔG°(t-Bu), (5) study of the "attractive gauche effect" in O-C-C-O segments, (6) examination of salt effects on conformational equilibria, (7) asymmetric synthesis of β-amino acids, and (8) asymmetric organocatalysis and "Green" chemistry. It will be appreciated that a basic understanding of the principles of physical organic chemistry has been essential in all projects. Furthermore, curiosity, enthusiasm, obstinacy, and paying attention to unexpected observations will lead to many new (stereo)chemical discoveries.     

Competition between electron and phonon excitations in the scattering of nitrogen atoms and molecules off tungsten and silver metal surfaces

We investigate the role played by electron-hole pair and phonon excitations in the interaction of reactive gas molecules and atoms with metal surfaces. We present a theoretical framework that allows us to evaluate within a full-dimensional dynamics the combined contribution of both excitation mechanisms while the gas particle-surface interaction is described by an ab initio potential energy surface. The model is applied to study energy dissipation in the scattering of N(2) on W(110) and N on Ag(111). Our results show that phonon excitation is the dominant energy loss channel, whereas electron-hole pair excitations represent a minor contribution. We substantiate that, even when the energy dissipated is quantitatively significant, important aspects of the scattering dynamics are well captured by the adiabatic approximation.     

A search for single-electron transfer (set) mechanisms in the reaction of 1,3-dithianyl- and 1,3-oxathianyllithiums with primary halides

The reaction of 1, 3-dithianyl-, 2-methyl-1, 3-dithianyl- and oxathianyllithium with 6-halo-1-hexenes affords the products expected from a S_N2 (rather than SET) mechanism.