Cyclopentadienyl molybdenum dicarbonyl η-allyl complexes as catalyst precursors for olefin epoxidation. Crystal structures of Cp′Mo(CO)2(η-C3H5) (Cp′ = η-C5H4Me, η-C5Me5)

The complexes Cp′Mo(CO)₂(η³-C₃H₅) [Cp′ = η⁵-C₅H₅ (1), η⁵-C₅H₄Me (2), η⁵-C₅Me₅ (3)] have been prepared, structurally characterised by X-ray diffraction (2, 3), and tested as catalyst precursors for the epoxidation of olefins at 55 °C. Complex 1 gave a turnover frequency (TOF) of 310 mol mol_Mo⁻¹ h⁻¹ in the epoxidation of cis-cyclooctene with tert-butylhydroperoxide (TBHP, in decane) as oxidant, and 1,2-epoxycyclooctane was obtained quantitatively within 6 h. A similar result was obtained for complex 2, while the TOF for 3 was about one order of magnitude lower, suggesting a possible activity dependence on the ring substituents. For 1 the use of 1,2-dichloroethane as solvent increased the initial reaction rate to 361 mol mol_Mo⁻¹ h⁻¹, with no decrease in epoxide selectivity. Under these conditions the reaction rates for other olefins increased in the order 1-octene < trans-2-octene < cyclododecene < (R)-(+)-limonene < cis-cyclooctene, and, with the exception of limonene, the corresponding epoxide was the only product. For 1 the selective epoxidation of cis-cyclooctene could also be achieved in aqueous solution, using TBHP or H₂O₂ as oxidants, which gave epoxide yields of 99% and 27% at 24 h, respectively. The possibility of facilitating catalyst recycling by using ionic liquids as solvents was investigated.     

Synthesis of reduction sensitive comb‐like polyurethanes using click chemistry

The synthesis and characterization of new highly biodegradable multipropargyl and comb‐like copolyurethanes have been successfully achieved by the combination of step‐growth polymerization and click chemistry. This work discloses a simple, accurate, and straightforward method to generate brush polymers with an excellent control of the branch nature, distribution, and density in the final materials. The introduction of controlled branching has led to a great variety of polymers, whose hydrophilic/lipophilic characters were adjusted as required. Furthermore, their stiffness, semicrystalline nature, and degradability were fine‐tuned by appropriate selection of the monomers and their feed ratios. The presence of propargyl functional groups in the (co)polymers was confirmed by the presence of a thermal degradation step in the range from 450 °C to 455 °C. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3888–3900     

Aminocatalysis: Beyond Steric Shielding and Hydrogen‐Bonding

The stereochemical outcome of most aminocatalytic transformations is determined by the steric‐shielding or hydrogen‐directing approach. However, in recent years several reactions have appeared in which the stereochemical outcome is beyond these two approaches. This Concept article will highlight such reactions and postulate that the stereochemical outcome can be accounted for by electrostatic interactions between the catalyst‐bound substrate and the reagent in the transition state.     

A green approach toward oleic‐ and undecylenic acid‐derived polyurethanes

Naturally occurring oleic and undecylenic acids were used as raw materials for the synthesis of novel polyurethanes (PUs). The application of environmentally friendly thiol‐ene additions to 10‐undecenoate and oleate derivatives was studied with the goal of obtaining renewable diols. The resulting monomers were then polymerized with 4,4′‐methylenebis (phenylisocyanate), in N,N‐dimethylformamide solution using tin (II) 2‐ethylhexanoate as catalyst, to produce the corresponding thermoplastic PUs (TPUs). Also, ultrasound irradiation has been tested to improve the synthesis of PU. Under these conditions, TPUs were obtained in high yields (80–99%) with weight‐average molecular weights in the 36–83 kDa range. The chemical structures of PUs were assessed by FTIR and NMR spectroscopy. The thermal and mechanical properties of the synthesized TPUs have been studied and they showed a clear dependence on the structure of the parent diol. MTT test was carried out to asses the potential cytotoxicity of the prepared PUs, indicating no cytotoxic response. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Numerical and experimental study of the Ti6Al4V macrostructure obtained by Nd:YAG laser

Titanium and its alloys (Ti6Al4V) have been widely used in the biomedical field; nevertheless, they should be subject to specific surface treatments, before being implanted, in order to improve bio-integration. Although laser processing is a useful technique for this purpose, different aspects of the basic mechanisms of this process are still in progress, with special emphasis on the modeling structure formation on the irradiated surface. For this research, the finite element method was used to study the generation of a macrostructure on the Ti6Al4V surface using a Nd:YAG laser. The temperature profiles, estimated during the extremely high heating and cooling rates caused by the output power of the laser beam, allowed us to analyze, among other things, the melting depth and the heat affected zone, in order to optimize the process. Moreover, the experimental results (SEM data) were positively compared with the numerical model, and a relationship of the crater profile formation (depth to width ratio) was determined.     

Isomeric sugar effects on thermal phase transition of aqueous PNIPA solutions,probed by ATR-FTIR spectroscopy; insights to protein protection by sugars

To illuminate the impacts of sugar concentration and stereochemistry on protein protection we used attenuated-total-reflectance Fourier-transform infrared-spectroscopy (ATR-FTIR) to study the effects of four aldohexoses on poly-N-isopropylacrylamide (PNIPA) phase transition. Protein stability in aqueous solutions is essential in numerous fields, predominantly biotechnology and food science. Saccharides protect proteins against thermal denaturation, but the mechanisms are still debatable. We therefore studied the effect of sugar concentration and stereochemistry on the LCST phase transition of PNIPA as a model for protein cold renaturation, using ATR-FTIR. The transition temperature of PNIPA, as observed by both the shift in amide II peak and its area, revealed the following order: galactose > glucose > mannose > talose, i.e., galactose is the most kosmotropic of the four isomers and talose is the least. We concluded that the soluting-out effect exerted on the polymer by these sugars positively correlates with the sugar hydration number governed by sugar stereochemistry.