MTO and OsO4: an efficient catalytic couple for mild H2O2-based asymmetric dihydroxylation of olefins

A novel and robust system for osmium-catalyzed asymmetric dihydroxylation of olefins by aqueous H2O2 with methyltrioxorhenium (MTO) as electron transfer mediator (ETM) has been developed. The MTO is catalyzing the H2O2 oxidation of the chiral ligand to its mono-N-oxide, which in turn reoxidizes OsVI to OsVIII. Thus the (DHQD)2PHAL plays a dual role serving as the chiral inductor as well as the tertiary amine generating the N-oxide required for the recycling of osmium. The present catalytic system gives vicinal diols in good isolated yields and high enantiomeric excess (up to 99 % ee).     

ChemInform Abstract: Quantum-Mechanical ab initio Investigation of the Transition-Metal Compounds OsO4, OsO3F2, OsO2F4, OsOF6, and OsF8

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

An electrophilic cleavage procedure for the asymmetric dihydroxylation: direct enantioselective synthesis of cyclic boronic esters from olefins

A variation within the osmium-catalysed asymmetric dihydroxylation (AD) of olefins is described that yields cyclic boronic esters from olefins in a straight-forward manner. This process represents the first real product alteration in asymmetric dihydroxylation, since all previous protocols lead to free diols exclusively. A protocol based on the Sharpless AD conditions (for enantioselective oxidation of prochiral olefins) was developed that gives cyclic boronic esters with excellent enantiomeric excesses (ee's). Some of the ee's are higher than those reported for conventional AD. The unprecedented role of phenyl boronic acid on the course of the AD reaction was investigated in detail. PhB(OH)2 does not interfere with the chiral ligand, leaving the enantioselective step of olefin oxidation intact. The main role of the boronic acids-apart from protecting the diol products against potential overoxidation-relies on removing the diol entity in an electrophilic cleavage, which is in contrast to the conventional hydrolytic cleavage of the AD protocols. Thus, a mechanistically new cleavage for enantioselective dihydroxylation reactions is introduced within the present work.     

Chemo- and periselectivity in the addition of [OsO2(CH2)2] to ethylene: a theoretical study

Quantum chemical calculations by using density functional theory at the B3LYP level have been carried out to elucidate the reaction course for the addition of ethylene to [OsO2(CH2)2] (1). The calculations predict that the kinetically most favorable reaction proceeds with an activation barrier of 8.1 kcal mol(-1) via [3+2] addition across the O=Os=CH2 moiety. This reaction is -42.4 kcal mol(-1) exothermic. Alternatively, the [3+2] addition to the H2C=Os=CH2 fragment of 1 leads to the most stable addition product 4 (-72.7 kcal mol(-1)), yet this process has a higher activation barrier (13.0 kcal mol(-1)). The [3+2] addition to the O=Os=O fragment yielding 2 is kinetically (27.5 kcal mol(-1)) and thermodynamically (-7.0 kcal mol(-1)) the least favorable [3+2] reaction. The formal [2+2] addition to the Os=O and Os=CH2 double bonds proceeds by initial rearrangement of 1 to the metallaoxirane 1 a. The rearrangement 1-->1 a and the following [2+2] additions have significantly higher activation barriers (>30 kcal mol(-1)) than the [3+2] reactions. Another isomer of 1 is the dioxoosmacyclopropane 1 b, which is 56.2 kcal mol(-1) lower in energy than 1. The activation barrier for the 1-->1 b isomerization is 15.7 kcal mol(-1). The calculations predict that there are no energetically favorable addition reactions of ethylene with 1 b. The isomeric form 1 c containing a peroxo group is too high in energy to be relevant for the reaction course. The accuracy of the B3LYP results is corroborated by high level post-HF CCSD(T) calculations for a subset of species.     

Flash Chemistry Extensively Optimized: High‐Temperature Swern–Moffatt Oxidation in an Automated Microreactor Platform

The generally accepted benefits of small lateral dimensions of microreactors (1 μm to 1 mm) enable a different way of performing synthetic chemistry: Extremely short contact times in the millisecond range can circumvent the need for performing highly exothermic and fast reactions at very low temperatures. In order to fully exploit this technology, such fast processes need to be redesigned and investigated for optimal reaction conditions, which can differ drastically from the ones traditionally applied. In a comprehensive study, we optimized the selective Swern–Moffatt oxidation of benzyl alcohol to benzaldehyde by varying five experimental parameters, including reaction time and temperature. Employing an ultrashort mixing and reaction time of only 32 ms, the optimal temperature was determined to be 70 °C, approximately 150 °C higher than in the conventional batch conditions. This remarkable difference shows both the potency of continuous‐flow chemistry as well as the urgency of a paradigm shift in reaction design for continuous‐flow conditions.     

The Use of a Microreactor for Rapid Screening of the Reaction Conditions and Investigation of the Photoluminescence Mechanism of Carbon Dots

A microreactor is applied and reported, for the first time, in the field of research of carbon dots (CDs), including rapid screening of the reaction conditions and investigation of the photoluminescence (PL) mechanism. Various carbonaceous precursors and solvents were selected and hundreds of reaction conditions were screened (ca. 15 min on average per condition). Through analyzing the screened conditions, tunable PL emission maxima, from about 330 to 550 nm with respectable PL quantum yields, were achieved. Moreover, the relationship between different developmental stages of the CDs and the PL properties was explored by using the microreactor. The PL emission was observed to be independent of the composition, carbonization extent, and morphology/size of the CDs. This study unambiguously presents that a microreactor could serve as a promising tool for the research of CDs.     

一种新手性配体的合成及其锇配合物的催化作用

以1,4-二氯-2,3-二氮杂萘为原料,经两步反应合成了一种新的金鸡纳生物碱衍生物手性配体2,其结构经1H NMR,13C NMR和MS表征。OsO4-2用于催化烯烃的不对称双羟化反应,化学产率91%~94%,e.e.值93%~99%;用于催化烯烃的不对称氨羟化反应,收率45%~63%,e.e.值83%~95%。     

MCM－41介孔分子筛在精细有机合成非均相催化中的应用*

本文综述了近年来介孔分子筛MCM－41在精细有机合成非均相催化中的应用,特别是手性合成等领域的研究进展。     

A QM/MM study of the asymmetric dihydroxylation of terminal aliphatic n-alkenes with OsO4.(DHQD)2PYDZ: enantioselectivity as a function of chain length

The dihydroxylation of terminal aliphatic n-alkenes catalyzed by OsO4(DHQD)2PYDZ ((DHQD)2PYDZ=bis(dihydroquinidine)pyridazine) has been computationally studied by the hybrid QM/MM IMOMM(Becke3LYP:MM3) method. The cases of propene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, and 1-decene have been considered. A systematic treatment for the large number of possible conformations of the longer chain alkenes has been defined and applied, leading to the selection of approximately 1700 conformations to be computed. The IMOMM calculations of the transition states formed between these conformations and the catalyst generate enantiomeric excesses that closely resemble the experimental data of related systems, specifically in the preference for the R isomer and in its dependence on the chain length of the substrate. The selectivity increases sharply with the elongation of the short-chain alkenes until a ceiling value is reached, with further elongations having little effect on the enantiomeric excess (ee). These results are rationalized through the partitioning of the total energy of selected conformers, a process that leads to the identification of the most relevant regions of the catalyst and the characterization of the interactions critical for selectivity.     

Excited states of OsO4: A comprehensive time‐dependent relativistic density functional theory study

A large number of scalar as well as spinor excited states of OsO₄, in the experimentally accessible energy range of 3–11 eV, have been captured by time‐dependent relativistic density functional linear response theory based on an exact two‐component Hamiltonian resulting from the symmetrized elimination of the small component. The results are grossly in good agreement with those by the singles and doubles coupled‐cluster linear response theory in conjunction with relativistic effective core potentials. The simulated‐excitation spectrum is also in line with the available experiment. Furthermore, combined with detailed analysis of the excited states, the nature of the observed optical transitions is clearly elucidated. It is found that a few scalar states of ³T₁ and ³T₂ symmetries are split significantly by the spin‐orbit coupling. The possible source for the substantial spin‐orbit splittings of ligand molecular orbitals is carefully examined, leading to a new interpretation on the primary valence photoelectron ionization spectrum of OsO₄. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Highly Effective Water Oxidation Catalysis with Iridium Complexes through the Use of NaIO4

Exceptional water oxidation (WO) turnover frequencies (TOF=17 000 h^?1), and turnover numbers (TONs) close to 400 000, the largest ever reported for a metal‐catalyzed WO reaction, have been found by using [Cp*Ir^III(NHC)Cl₂] (in which NHC=3‐methyl‐1‐(1‐phenylethyl)‐imidazoline‐2‐ylidene) as the pre‐catalyst and NaIO₄ as oxidant in water at 40 °C. The apparent TOF for [Cp*Ir^III(NHC)X₂] ( 1 X , in which X stands for I ( 1 I ), Cl ( 1 Cl ), or triflate anion ( 1 OTf )) and [(Cp*‐NHCMe)Ir^IIII₂] ( 2 ) complexes, is kept constant during almost all of the O₂ evolution reaction when using NaIO₄ as oxidant. The TOF was found to be dependent on the ligand and on the anion (TOF ranging from ≈600 to ≈1100 h^?1 at 25 °C). Degradation of the complexes by oxidation of the organic ligands upon reaction with NaIO₄ has been investigated. ¹H NMR, ESI‐MS, and dynamic light‐scattering measurements (DLS) of the reaction medium indicated that the complex undergoes rapid degradation, even at low equivalents of oxidant, but this process takes place without formation of nanoparticles. Remarkably, three‐month‐old solution samples of oxidized pre‐catalysts remain equally as active as freshly prepared solutions. A UV/Vis feature band at λ_max=405 nm is observed in catalytic reaction solutions only when O₂ evolves, which may be attributed to a resting state iridium speciation, most probably Ir–oxo species with an oxidation state higher than IV.     

Synthesis,Structure, and Oxidation Catalysis of a Hybrid Compound of Bisubstituted Molybdovanadophosphate

A new compound, Na(HINA)₂(NH₄)₂[PMo₁₀V₂O₄₀] · 14H₂O ( 1 ) (INA = isonicotinic acid) was synthesized and characterized structurally by elemental analysis, IR spectroscopy, and TG. Bisubstituted molybdovanadophosphate anions have typical Keggin anion structure and are linked weakly by Na⁺ ions into an anionic chain along crystallographic b axis. Protonated INA cations form another supramolecular cationic chain also along crystallographic b axis. Two chains, water molecules, and ammonium ions form the 3D supramolecular architecture through hydrogen bonds. The compound has an excellent catalytic activity to starch oxidation. The detailed study gives optimum reaction condition.     

稀土金属在不对称催化中的应用:从均相催化到异相催化

稀土金属的配位数较高,可通过容纳大型手性配体,构筑手性环境,催化不对称反应的定向发生,在工业生产特别是制药工程中具有重要应用价值.本文以Henry反应、Mannich反应和Strecker反应为例,总结回顾了稀土金属催化剂在此类反应中的设计思路、性能特点与应用前景,旨在展现稀土金属催化剂兼具融合均相催化与异相催化的优势...     

A Versatile Co(bisalen) Unit for Homogeneous and Heterogeneous Cooperative Catalysis in the Hydrolytic Kinetic Resolution of Epoxides

Bisalen—very cooperative : A novel styryl‐substituted, unsymmetrical bisalen is shown to be a versatile precursor to both soluble and insoluble cooperative catalysts for the hydrolytic kinetic resolution of epoxides. The unique bisalen motif ensures all catalysts derived from it have the necessary molecular‐level ligand pairing needed for cooperative bimetallic catalytic reactions (see scheme).

     

Encapsulation of a Single Metal Nanoparticle with Tunable Size in a Monodisperse Polymer Microcapsule

A versatile approach to fabricate monodisperse poly[styrene‐co‐(divinyl benzene)] (PS‐co‐DVB) microcapsules that contain a single gold nanoparticle (AuNP) has been demonstrated. Using the PS‐co‐DVB microcapsule as a microreactor, aqueous HAuCl₄ and NaBH₄ solutions are subsequently infiltrated. The size of the resulting AuNP inside of the PS‐co‐DVB microcapsules is easily tunable by controlling the repeated infiltration cycles of aqueous HAuCl₄ and NaBH₄. PS‐co‐DVB microcapsules that contain a single silver and palladium nanoparticle are also obtained by following a similar protocol.