Sergiu Coseri

Phthalimide‐N‐oxyl (PINO) Radical, a Powerful Catalytic Agent: Its Generation and Versatility Towards Various Organic Substrates

The last two decades represents a “start line” for the worldwide chemists, to develop new oxidizing methods, to replace the “old‐fashioned” ones, which are expensive, pollute the environment, and proceed in harsh conditions. One of the best candidates to satisfy the present global needs is N‐hydroxyphthalimide (NHPI), which can be used as a catalytic reagent successfully in a wide range of organic transformations. In this article, a review of the most frequently used methods to transform the NHPI into its nitroxyl radical correspondent, and the use of this powerful catalytic agent into various organic transformations, are presented.

Isocyanates in polyaddition processes. Structure and reaction mechanisms

Abstract The multiple possible addition processes of isocyanates, in accordance with a new proposed general classification based on the interatomic bond type changes is presented. Starting from the latest achievement of the physico-chemical methods, new aspects regarding the isocyanates structure and different hydroxy compound association forms as part of the addition processes were revealed. Some basic reaction mechanisms presented in the current research were re-evaluated in accord with recent quantum chemical and kinetic data. The intramolecular hydrogen bonding in the symmetrical diols was found to determine a significant difference between their two –OH group reactivities. Recent kinetic and other reactivity measurements, extended to polyfunctional systems reported in some macromolecular synthesis, were presented. The perspective of these findings, in order to obtain in the near future an acceptable mathematical model of the polyaddition process by involving the multiple new synergetic effects observed in different real complex reaction mixtures, is analysed.

Oxidized cellulose—Survey of the most recent achievements

The functionalization and particularly the oxidation of cellulose is an intriguing and challenging topic due to the presence of multiple reactive sites, which can undergo specific reactions. The variety of the oxidizing agents used to improve the selectivity and yields of these transformations is illustrated by the steadily growing of the number of publications and patents reported. This paper is focused on the most selective agents for cellulose oxidations, i.e., sodium periodate and stable or non persistent nitroxyl radicals, emphasizing on the most recent developments reported so far.

Natural Products and their Analogues as Efficient Anticancer Drugs

Every year about 7 million people die from of various types of cancer, making this disease responsible for 12.5% of deaths worldwide. Consequently, there is an overwhelming demand to develop new, more potent and effective, anticancer drugs. Natural products represent the most valuable source with inexhaustible reserves, in which the researchers work could identify novel active agents that may serve as the leads and scaffolds for transformation into desperately needed efficacious drugs. This paper is devoted to reviewing some of the most outstanding achievements in the application of natural products as models and precursors for anticancer agents published in the post 2000 literature. Particular emphasis is placed on the chemical modification of active principles isolated from natural products, in hope of obtaining the desirable derivatives for the treatment of different types of cancer, including pancreatic, gastric, breast, colon cancers and leukemia.

Mild and Selective Oxidation of Cellulose Fibers in the Presence of N-Hydroxyphthalimide

The oxidation reaction of regenerated cellulose fibers mediated by N-hydroxyphthalimide (NHPI) and various cocatalysts at room temperature for different time intervals and various amounts of low concentration sodium hypochlorite solution has been investigated to produce oxidized cellulose (OC), a biocompatible and bioresorbable polymer. The results revealed that the nonpersistent phthalimide-N-oxyl (PINO) radical generated in situ from NHPI in both, metallic or metal-free systems, is a powerful agent in this kind of transformation. Moreover, the reaction converts highly selectively C(6) primary hydroxyl groups to carboxylic groups under mild reaction conditions and shorter reaction times than previously reported. The amounts of negatively charged groups in OC were determined by means of potentiometric titration. Further characterization of the products were accomplished by using Fourier transform infrared spectroscopy/attenuated total internal reflection spectroscopy (FT-IR/ATR), environmental scanning electron microscopy (ESEM), and X-ray and energy-dispersive X-ray (EDX) spectroscopy. Notably, water retention values of the oxidized fibers increased by 30% in comparison with the original nonoxidized sample, as a result of the introduction of hydrophilic carboxylate groups.

Green synthesis of the silver nanoparticles mediated by pullulan and 6-carboxypullulan

Unoxidized and carboxylated pullulan (obtained by pullulan oxidation using TEMPO-sodium hypochlorite-sodium bromide system) have been used as mediators for the silver nanoparticles formation (AgNPs), under environment-friendly conditions: using aqueous solutions, room temperature and notably, by using both mediators as reducing and stabilizing agents. The formation of AgNPs was first screened by measuring the surface plasmon resonance peak in the range of 380-440 nm using UV-vis spectroscopy. The morphology of the synthesized silver nanoparticles was determined by TEM, which indicated that the AgNPs differ on shape and thickness of the polymer shell by varying the silver nitrate concentration, different size and shape of AgNPs was achieved. The presence of elemental silver and the crystalline structure of the AgNPs were confirmed by EDX and XRD analyses. Moreover, the possible functional groups of pullulan (oxidized pullulan) responsible for the reduction and stabilization of AgNPs were evaluated using FT-IR. The results showed that both, pullulan and 6-carboxypullulan could be successfully used as reducing as well as capping agents for the AgNPs synthesis which shows potential antimicrobial activity against Gram positive and Gram negative bacteria.

Comparison study of TEMPO and phthalimide-N-oxyl (PINO) radicals on oxidation efficiency toward cellulose

Regenerated cellulose fibers, type viscose, have been oxidized with sodium hypochlorite and catalytic amounts of sodium bromide by using two different protocols: first, involving the presence of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) and second, employing N-hydroxyphthalimide (NHPI). The reactions were carried out at room temperature and pH=10.5 for 2.5h. Viscose oxidized samples were analyzed and compared in terms of the negative charged groups content, as determined by potentiometric titration and methylene blue adsorption, morphologies and crystallinities changes, as well as changes in the degree of polymerization. The highest content of the carboxylic groups and the best preservation of the morphology and molecular weight of the original material have been found in the case of using NHPI/anthraquinone as oxidation mediators. TEMPO-mediated oxidation leads to the highest depolymerization and cause significant degradation of the cellulosic material.

Cellulose: To depolymerize… or not to?

Oxidation of the primary OH groups in cellulose is a pivotal reaction both at lab and industrial scale, leading to the value-added products, i.e. oxidized cellulose which have tremendous applications in medicine, pharmacy and hi-tech industry. Moreover, the introduction of carboxyl moieties creates prerequisites for further cellulose functionalization through covalent attachment or electrostatic interactions, being an essential achievement designed to boost the area of cellulose-based nanomaterials fabrication. Various methods for the cellulose oxidation have been developed in the course of time, aiming the selective conversion of the OH groups. These methods use: nitrogen dioxide in chloroform, alkali metal nitrites and nitrates, strong acids alone or in combination with permanganates or sodium nitrite, ozone, and sodium periodate or lead (IV) tetraacetate. In the case of the last two reagents, cellulose dialdehydes derivatives are formed, which are further oxidized by sodium chlorite or hydrogen peroxide to form dicarboxyl groups. A major improvement in the cellulose oxidation was represented by the introduction of the stable nitroxyl radicals, such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). However, a major impediment for the researchers working in this area is related with the severe depolymerisation occurred during the TEMPO-mediated conversion of CH2OH into COOH groups. On the other hand, the cellulose depolymerisation represent the key step, in the general effort of searching for alternative strategies to develop new renewable, carbon-neutral energy sources. In this connection, exploiting the biomass feed stocks to produce biofuel and other low molecular organic compounds, involves a high amount of research to improve the overall reaction conditions, limit the energy consumption, and to use benign reagents. This work is therefore focused on the parallelism between these two apparently antagonist processes involving cellulose, building a necessary bridge between them, thinking how the reported drawbacks of the TEMPO-mediated oxidation of cellulose are heading towards to the biomass valorisation, presenting why the apparently undesired side reactions could be turned into beneficial processes if they are correlated with the existing achievements of particular significance in the field of cellulose conversion into small organic compounds, aiming the general goal of pursuing for alternatives to replace the petroleum-based products in human life.

One-shot carboxylation of microcrystalline cellulose in the presence of nitroxyl radicals and sodium periodate

Correction for ‘One-shot carboxylation of microcrystalline cellulose in the presence of nitroxyl radicals and sodium periodate’ by Sergiu Coseri et al., RSC Adv., 2015, 5, 85889–85897.

Synthesis, characterization and solution behaviour of oxidized pullulan

Various amounts of carboxyl groups were introduced at C-6 of the non-ionic, water soluble polysaccharide, i.e. pullulan, by applying the well-established TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), sodium hypochlorite/sodium bromide oxidation protocol, varying the reaction time. The oxidized products, more water-soluble than pullulan, were further characterized by FT-IR, (1)H NMR and (13)C NMR techniques, in order to assess the degree of oxidation. The absolute molecular weight measurements performed using a multiangle laser light scattering molecular weight analyzer, reveals a sharp drop of the molecular weight of the samples oxidized for longer reaction times. The second virial coefficients (A2), increased from unoxidized pullulan to the oxidized samples. Dynamic light scattering (DLS) measurements provide zeta potentials and hydrodynamic radius for all studied samples. The viscosity of the initial and oxidized pullulan dilute aqueous solutions was studied in detail. All oxidized samples except the highest oxidized pullulan sample (OxPu8) showed strong polyelectrolyte behaviour, whereas this effect is less pronounced for OxPu8 due to the high degradation of the chains. The intrinsic viscosity and the interaction parameter were determined at 25 °C as a function of solvent ionic strength according to Wolf approach. The dependence of these parameters on the salt concentration follows Boltzmann sigmoid model.