Rodrigo César da Silva

Terpene‐Derived Bifunctional Thioureas in Asymmetric Organocatalysis

Chiral hydrogen‐bond donors have played a crucial role in the progress of asymmetric catalysis as an essential tool in modern organic synthesis. Among these donors, thioureas have been found to be one of the most powerful systems in organocatalytic approaches. Within the last three years, a new generation of bifunctional thiourea catalysts has emerged as an effective way to promote highly demanding chemical transformations, frequently with success rates that are intriguingly higher than with other traditional thioureas. This new class of organocatalysts incorporates a bulky, rigid, and chiral terpene skeleton as one of the thiourea substituents, thus providing an additional and effective rigidifying (entropic) effect on the electrophile activation profile of thioureas. Structural integrations, either with additional Lewis basic nucleophile‐activating motifs or with amino groups for enamine catalysis, have enabled their implementation in a wide repertoire of stereocontrolled catalytic processes. This Review charts the development of these new catalysts, as well as their applications in a wide variety of reactions and reaction sequences, with the hope of encouraging further progress in the field of thiourea‐catalyst discovery and of bringing attention to the countless possibilities that are opened up by the tremendous success of terpene‐derived thiourea catalysts.

Basic-functionalized recyclable ionic liquid catalyst: A solvent-free approach for Michael addition of 1,3-dicarbonyl compounds to nitroalkenes under ultrasound irradiation

A task-specific ionic liquid (TSIL) has been introduced as a recyclable catalyst in Michael addition. A series of nitroalkenes and various C-based nucleophiles were reacted in the presence of 30mol% of recyclable basic-functionalized ionic liquid. Good to excellent yields were obtained in 30min under ultrasound irradiation.

Iron(III) chloride catalyzed glycosylation of peracylated sugars with allyl/alkynyl alcohols

Neste trabalho, o emprego de cloreto ferrico como um eficiente catalisador em reacoes de glicosilacao de acucares na presenca de alcoois alilicos e propargilicos e descrito. Os glicosidios correspondentes foram obtidos com rendimentos de moderados a bons. Este novo procedimento apresenta maior seletividade quando comparado a metodos classicos encontrados na literatura. As principais caracteristicas desse metodo simples incluem condicoes de reacao nao perigosas, quantidade de catalisador baixa, bom rendimento e seletividade anomerico elevada. In this work, the use of ferric chloride as an efficient catalyst in glycosylation reactions of sugars in the presence of allyl and alkynyl alcohols is described. The corresponding glycosides were obtained with moderate to good yields. This new procedure presented greater selectivity when compared to classic methods found in the literature. Principal features of this simple method include non-hazardous reaction conditions, low-catalyst loading, good yields and high anomeric selectivity.

Organocatalyzed Asymmetric Vinylogous Addition of Oxazole-2(3H)-Thiones to α,β-Unsaturated Ketones: An Additive Free Approach for Diversification of Heterocyclic Scaffold

A straightforward organocatalyzed asymmetric addition of oxazole-2(3H)-thiones to α,β-unsaturated ketones is described. This additive-free Michael reaction in the presence of chiral cinchonine-derived primary amines as catalysts has proven to be highly effective for a wide range of cyclic and acyclic enones, leading to the Michael adducts in very good yields and excellent enantioselectivities. The absolute configuration (R) of compound 5j was unambiguously assigned by X-ray diffraction analysis. Furthermore, experimental and theoretical studies were performed and a mechanism is presented and discussed for this novel reaction.

Effect of Thermal Aging at Low Temperature on the Mechanical Properties and Corrosion Resistance of LDX 2404 Duplex Stainless Steel

It is well known that when duplex stainless steels (DSS) are subjected to temperatures ranging from 300 to 1000 °C they may undergo precipitation of several phases, which can seriously impair their mechanical properties and corrosion resistance. The present work studied the effect of thermal aging (up to 2000 h) at 475 °C on the corrosion and mechanical properties of the newly developed 2404 DSS. The evaluation was based on potentiodynamic polarization in 3.5% NaCl solution and on Charpy tests. The pitting corrosion resistance was found to decrease significantly with aging time at 475 °C. In addition, Charpy tests revealed that after 100 h of thermal aging the material becomes brittle due to the spinodal decomposition of ferrite.

Corrosion Behavior of Fe-Mn-Si-Cr-Ni-Co Shape Memory Stainless Steel in Highly Oxidizing Medium

A study was conducted on the corrosion behavior and characteristics of the passive oxide film of Fe-Mn-Si-Cr-Ni-(Co) shape memory stainless steels (SMSS) in a concentrated nitric acid (HNO3) solution, based on potentiodynamic polarization, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analyses. The results indicated that Fe-Mn-Si-Cr-Ni-(Co) SMSSs exhibit a passive behavior similar to that of 304L austenitic stainless steel (304L SS). However, unlike 304L SS, their high silicon (Si) content renders them insensitive to intergranular attack in highly oxidizing environments. The XPS analysis also indicated that Si appears to be the main element responsible for the high protectiveness afforded by the passive film formed on Fe–Mn–Si–Cr–Ni–Co SMSS.

Sequential organocatalytic -hydroxy and Mukayama addition: A perspective for the synthesis of Orlistat

a Performed with 1 (0,05 mmol) 2 (0,5mmol), 3 (1,5 mmol), 4 (0,05mmol) in 0,25 mL of solvent. b Determined by chiral stationary phase HPLC. When the reaction was performed in THF as solvent, the β-hydroxy carbonyl compound could be obtained with 90% ee and 56% yield (Entry 3). Decreasing the reaction temperature to -10 o C and -20 o C (entries 5 and 6) furnished desired product in 96% and 91% ee respectively. The Mukayama aldol reaction with silyl enol ether were carried out using 50 mol % of TBAF, figure 2.