Microwave Activation of Processes in Mesopores: The Thiourea Electrooxidation at Mesoporous Platinum

In situ microwave activation is applied to the electrochemical oxidation of thiourea at low surface area (polished polycrystalline) and at high surface area (electrodeposited mesoporous platinum coated) platinum microelectrodes. The one electron oxidation of thiourea to formamidine disulfide is monitored as a function of the different activation parameters. In the absence of microwaves (ambient temperature, low mass transport) increasing the surface area (roughness) increases the thiourea oxidation response predominantly due to adsorption effects. In the presence of high microwave intensities, high mass transport and thermal effects further increase the oxidation current at mesoporous platinum. The most effective thickness of the mesoporous platinum film for the thiourea oxidation process is estimated as 3 μm independent of electrode diameter, temperature, or mass transport effects.     

New catalytic effect of thiourea on the oxidative cyanation of N-aryltetrahydroisoquinolines

Thiourea itself has been introduced as a mild and efficient organocatalyst for the oxidative α -cyanation of N-aryltetrahydroisoquinolines (THIQs) with trimethylsilyl cyanide (TMSCN), giving the corresponding products in good to excellent yields. Experimental investigations demonstrated that thiourea acts as a radical initiator by abstracting hydroxyl radical (OH) from tert-butyl hydroperoxide (TBHP) directly instead of non-covalent hydrogen bondings (H-bondings) activation. The use of thiourea as a radical initiator offers a new avenue for innovative chemical transformations in organocatalyzed radical chemistry.     

Alkylation of ethylenethiourea with alcohols: a convenient synthesis of S-alkyl-isothioureas without toxic alkylating agents

The alkylation of ethylenethiourea with alcohols and aqueous acids (HCl, HBr, and HI) allows the synthesis of the respective S-alkyl-isothioureas in high yield and purity. Consistently high yields (91-98%) were obtained with 56% HI, the yields for 48% HBr (48-93%) and 37% HCl (36-85%) were lower and varied with the type of alcohol. The method is a convenient low-cost alternative to the use of alkyl iodides and an easy access to the S-tert-butyl isothiourea.     

GRAFT COPOLYMERIZATION OF VINYL MONOMERS ONTO STARCH INITIATED BY TRANSITION METAL-THIOUREA REDOX SYSTEMS

In this paper, the capabilities of grafting acrylonitrile (AN) onto starch initiatedby Fe(III)-TU, V(V)-TU, Cr(VI)-TU, Mn(VII)-TU redox systems were compared in thepresence of sulfuric acid of different concentrations. It was shown that the grafting capabili-ty of Mn(VII)-TU is the highest in these initiating systems. Using Mn (VII-TU as initia-tor, the effects of various acids (HClO_4, H_2SO_4, HNO_3, HCl) on the graft copolymerizationof acrylonitrile onto starch were discussed, and the capabilities of graft copolymerizationof methyl methacrylate (MMA), acrylamide (AM), acrylic acid (AA) onto starch wereinvestigated. The experimental results show that the order of the influences of differentacids is HClO_4>H_2SO_4>HNO_3>HCl, and the order of grafting capabilities of differentmonomers grafted onto starch is MMA>AN>AM>AA. The structure and morphology ofgraft copolymers were studied with infrared spectroscopy and scanning electron microscopy.The size, shape and roughness of surface of the grafted starch granules are changed aftergrafting.     

Thiourea to bicyclic scaffolds: highly regio- and stereoselective routes to dithiazolopyrimidines

Microwave-activated solvent-free Michael addition of 3-imino-1,4,2-dithiazoles to 4-arylidene-5(4H)-oxazolones furnished isolable adducts regio- and diastereoselectively, which underwent ring transformation to yield the target dithiazolopyrimidines. Alternatively, the similar conjugate addition of methanesulfinylmethylisothioureas to 4-arylidene-5(4H)-oxazolones diastereoselectively afforded Michael adducts, which underwent ring transformation followed by heterocyclization via deoxygenative demethylation with thionyl chloride to yield the same products dithiazolopyrimidines regio- and diastereoselectively.     

Spectral studies on VO ions in potassium thiourea bromide single crystals using EPR and optical absorption spectroscopy

Electron paramagnetic resonance (EPR) studies have been carried out on VO²⁺ ions doped in single crystals of ferroelectric material, potassium thiourea bromide (PTB) at room temperature and in the temperature range 103–343 K on X-band MW frequency. An isotropic octet spectrum characteristic of VO²⁺ ion was obtained due to the fast re-orientation of the VO²⁺ in PTB lattice, which exhibits glassy nature at certain range around room temperature. The temperature dependant EPR spectra of VO²⁺ ions in this host lattice has been attributed to the occurrence of multiple phase transitions due to the combined environment effects of KBr and thiourea materials in the single crystal. From the optical absorption spectrum, the crystal field splitting parameter Dq, tetragonal parameters Ds and Dt have been evaluated and discussed.     

Solvent and thiourea adsorption/intercalation effects on the solid-state electrochemistry of α-phase nickel hydroxide nanoparticles

Nanoparticles of α-phase nickel hydroxide were synthesized by a single-step hydrothermal method using urea as the hydrolytic agent. Precipitated powders were of pure turbostratic α-phase as confirmed by x-ray diffraction profile. The ageing of α-Ni(OH)₂ in 1.0 M alkali solutions is investigated for pure non-intercalated α-Ni(OH)₂ and thiourea intercalated/absorbed α-phase nanomaterials. The α-Ni(OH)₂ powder immobilized on the surface of graphite electrodes shows a gradual α→β phase transformation with continuous voltammetric cycling, and the concentration gradient of water that exists in the layered-double-hydroxide-like interlayers of α-phase and the solution was shown to play a crucial role on the high electrochemical activity of this phase nickel hydroxide. To understand the role of water in the ageing process, concomitant entries of non-aqueous solvents like ethanol and acetonitrile along with thiourea were effected. Cyclic voltammetric measurements of thiourea-treated α-Ni(OH)₂ samples revealed that hydroxyl ion influx during the anodic oxidation depends on the counter flux of solvent molecules, and if the intercalated the solvent is acetonitrile, then the electrochemical activity of α-Ni(OH)₂ reduced drastically; Q _a/Q _c>1 for water as solvent in the interlayers α-Ni(OH)₂ and Q _a/Q _c<1 for ethanol and acetonitrile as solvents. The α-phase gets stabilized in the presence of thiourea with water and ethanol as co-intercalates. Transmission electron microscope images of α-Ni(OH)₂ and thiourea-treated samples show a change in particle size and morphology. Elemental CHNS analysis confirms the presence of sulphur in the thiourea intercalated samples.     

联萘酚多氢键硫脲催化合成1,4-二氢吡啶衍生物

本文合成了三种具有多氢键的联萘酚轴手性硫脲催化剂,并将其用于催化合成1,4-二氢吡啶衍生物。结果显示,所合成的新型多氢键硫脲均表现出较好的催化作用,能有效地提高1,4-二氢吡啶衍生物的收率和对映选择性。所有新化合物结构均经过¹H NMR、¹³C NMR、IR、熔点等表征所确认。     

多级孔芳香骨架材料的合成及固载硫脲催化剂的研究

合成了一种多级孔芳香骨架材料(PAF-70); 使用由氨基修饰过的单体, 应用该合成策略得到了同样具有窄分布介孔的含有氨基活性位点的PAF材料, 并通过硫脲单体与其氨基活性位点的反应, 将硫脲基团引入PAF-70材料中, 获得了含有硫脲催化位点的材料(PAF-70-thiourea). 氮气吸附-脱附测试结果显示, PAF-70存在孔径分布较窄的介孔, 介孔孔径为3.8 nm, 与模拟计算值(约3.7 nm)吻合. 热重分析结果表明, PAF-70具有很高的热稳定性. PAF-70在大多数溶剂中可以稳定存在, 具有良好的化学稳定性. 将PAF-70-thiourea作为催化剂, 应用在N-溴代琥珀酰亚胺(NBS)氧化醇类的反应中, 其表现出较高的催化活性、 较高的稳定性和广泛的底物适用性. 与含有相同硫脲催化位点的金属有机框架(MOF)材料(IRMOF-3-thiourea)作为催化剂对比, 进一步证实PAFs材料非常适合作为催化有机反应的固载平台.     

Preparation of (1R,4S)-4-hydroxycyclopent-2-en-1-yl acetate via Novozym-435® catalyzed desymmetrization of cis-3,5-Diacetoxy-1-cyclopentene

Photooxidation of cyclopentadiene has been carried out in methanol using white light of LED lamp, rose bengal as photo initiator, and compressed air at 0?°C. Under conditions of [thiourea] ? [cyclopentadiene], the consumption of thiourea follows a pseudo-first-order reaction kinetics with half life of 75?±?10?min; corr. coeff. r?=?0.989. Slow addition of the monomer and maintaining excess thiourea concentration in reaction mass improves the yield. cis-3,5-Dihydroxy-1-cyclopentene is acetylated without isolation to obtain cis-3,5-Diacetoxy-1-cyclopentene of high purity (>99%) with overall isolated yield of 30%. Desymmetrization of the diacetate to (1R,4S)-4-hydroxycyclopent-2-en-1-yl acetate has been carried out via enzymatic transesterification with methanol in methyl tert-butyl ether (MTBE) at 5?°C using Novozym-435^®. The enantiomerically pure monoacetate (e.e. >99%) was obtained in 95% isolated yield. The recovered enzyme was reused for more than 10 times without loss in yield and selectivity. The entire protocol does not require purification of final product by chromatography.