A Heterogeneous cis-Dihydroxylation Catalyst with Stable, Site-Isolated Osmium-Diolate Reaction Centers

Boosting Thermal Stability of Volatile Os Catalysts by Downsizing to Atomically Dispersed Species

Os-based catalysts present remarkable catalytic activity; however, their use has been limited by the undesirable side reactions that generate highly toxic and volatile OsO₄ even at room temperature. Herein, we demonstrate that the thermal stability of Os-based catalysts can be dramatically improved by downsizing Os nanoparticles (NPs) into atomically dispersed species. We observed that Os NPs were converted into OsO₄ after calcination at 250 °C followed by sublimation, whereas single Os sites retained their structure after calcination. Temperature-programmed oxidation analysis confirmed that Os NPs started to undergo oxidation at 130 °C, whereas atomically dispersed Os preserved its state up to 300 °C. The CO oxidation activity of the atomically dispersed Os catalyst at 400 °C (100% conversion) was stably preserved over 30 h. By contrast, the activity of Os NP catalyst declined drastically. This study highlights the unique catalytic behavior of atomically dispersed catalysts, which is distinct from that of NP-based catalysts.

Efficient catalyst reuse by simple dissolution in non-conventional media

This feature article is a description of the achievements made on the development of attractive sustainable approaches to synthetic organic chemistry, namely, catalyst reuse by simple dissolution in water and ionic liquids and asymmetric transformations induced by readily available chiral ionic liquids.

Ionic liquids as a convenient new medium for the catalytic asymmetric dihydroxylation of olefins using a recoverable and reusable osmium/ligand

The use of room-temperature ionic liquids (RTILs) in the Sharpless catalytic asymmetric dihydroxylation (AD) as a cosolvent or replacement of the tert-butanol was studied in detail by screening 11 different RTILs. The AD reaction is faster in 1-n-butyl-3-methylimidazolium hexafluorophosphate [C(4)mim][PF(6)] as a cosolvent than in the conventional system of tert-butanol/H(2)O. For the range of six substrates tested, comparable or even higher yields and enantiomeric excess (ee) were found using [C(4)mim][PF(6)] or 1-n-octyl-3-methylimidazolium hexafluorophosphate [C(8)mim][PF(6)] compared to the conventional solvent system. Due to high affinity of the catalytic osmium/quiral ligand system to the ionic liquid, the use of ionic liquid/water (biphasic) or ionic liquid/water/tert-butanol (monophasic) solvent systems provides a recoverable, reusable, robust, efficient, and simple system for the AD reaction. Using 1-hexene and [C(4)mim][PF(6)] as RTIL it was possible to reuse the catalytic system for 9 cycles with only a 5% of yield reduction from the first cycle, allowing an overall yield of 87%, TON = 1566, and with similar ee. Additionally, for each cycle, after extraction of the reaction mixture with diethyl ether, the osmium content in the organic phase (containing the AD product) and in the aqueous phase was in the range of the detection limit (<or=3%, <or=7 ppb) and 3-6% of initial amount, respectively. In contrast, the ionic liquid phase retained more than 90% of the osmium content of the previous cycle.

Markedly enhanced recyclability of osmium catalyst in asymmetric dihydroxylation reactions by using macroporous resins bearing both residual vinyl groups and quaternary ammonium moieties

Markedly enhanced recyclability of osmium catalyst in asymmetric dihydroxylation has been achieved by using osmylated macroporous resins bearing both residual vinyl groups and quaternary ammonium moiety.

A QM/MM Study of the Asymmetric Dihydroxylation of Terminal Aliphaticn-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.

Ionic liquid-immobilized catalytic system for biomimetic dihydroxylation of olefins

A biomimetic catalytic system for dihydroxylation of olefins consisting of OsO4, N-methylmorpholine, and a flavin has been immobilized in an ionic liquid. This immobilized catalytic system is highly efficient for dihydroxylation with 30% aqueous H2O2 and it can be reused (at least 5 times) without loss of activity.

[3+2] versus [2+2] addition of metal oxides across C=C bonds. Reconciliation of experiment and theory

The reaction of a metal oxide with a double bond is the initial step in the osmium-catalyzed cis-dihydroxylation of olefins. The mechanism of the addition of osmium tetraoxide and rhenium(VII) oxides across C=C bonds was controversial. Early work indicated a [3+2] addition, and later kinetic studies suggested an initial [2+2] addition, whereas recent quantum-chemical calculations showed the [3+2] addition to be favored. Experiment and theory have now become reconciled. In this Account, we discuss recent contributions to the mechanistic debate as well as future challenges.

A trifunctional catalyst for one-pot synthesis of chiral diols via Heck coupling-N-oxidation-asymmetric dihydroxylation: application for the synthesis of diltiazem and taxol side chain

A heterogeneous bifunctional catalyst composed of OsO4(2-)-WO4(2-) and a trifunctional catalyst comprising PdCl4(2-)-OsO4(2-)-WO4(2-), designed and prepared by an ion-exchange technique using layered double hydroxides (LDH) as an ion-exchanger and their homogeneous bifunctional analogue, K2OsO4-Na2WO4 and trifunctional analogue, Na2PdCl4-K2OsO4-Na2WO4, devised for the first time are evaluated for the synthesis of chiral vicinal diols. These bifunctional and trifunctional catalysts perform asymmetric dihydroxylation-N-oxidation and Heck-asymmetric dihydroxylation-N-oxidation, respectively, in the presence of Sharpless chiral ligand, (DHQD)2PHAL in a single pot using H2O2 as a terminal oxidant to provide N-methylmorpholine oxide (NMO) in situ by the oxidation of N-methylmorpholine (NMM). The heterogeneous bifunctional catalyst supported on LDH (LDH-OsW) displays superior activity to afford diols with higher yields over the other heterogeneous catalysts developed by the ion exchange on quaternary ammonium salts covalently bound to resin (resin-OsW) and silica (silica-OsW) or homogeneous catalysts in the achiral dihydroxylation reactions. The LDH-OsW and its homogeneous analogue are found to be very efficient in performing a simultaneous asymmetric dihydroxylation (AD)-N-oxidation of a wide and varied range of aromatic, cyclic, and mono, di-, and trisubstituted olefins to obtain chiral vicinal diols with higher yields and ee's using H2O2. Further, the use of OsO4(2-)-WO4(2-) catalysts as such or in the supported form offers a simplified procedure for catalyst recycling, which shows consistent activity for a number of cycles. In this process, Os(VI) is recycled to Os(VIII) by a coupled electron transfer-mediator (ETM) system based on NMO-WO4(2-) using H2O2, leading to a mild and selective electron transfer. The one-pot biomimic synthesis of chiral diols is mediated by a recyclable trifunctional heterogeneous catalyst (LDH-PdOsW) consisting of active palladium, tungsten, and osmium species embedded in a single matrix. This protocol, which provides prochiral olefins and NMO in situ by Heck coupling and N-oxidation of NMM, respectively, required for the AD, unfolds a low cost process. We extended the present method to the one-pot synthesis of trisubstituted chiral vicinal diols with moderate to excellent ee's by AD of trisubstituted olefins that are obtained by in situ Heck arylation of disubstituted olefins. The heterogeneous trifunctional catalysts offers chiral diols with unprecedented ee's and excellent yields in the AD of prochiral cinnamates, which are obtained in situ from acrylates and halobenzenes for the first time. The new variants such as LDH support and Et3N*HX inherently composed in the heterogeneous multicomponent system and slow addition of H2O2 facilitates the hydrolysis of osmium monogylcolate ester to subdue the formation of bisglycolate ester to achieve higher ee's. Without resorting to recrystallization, the chiral diols of cinnamates thus synthesized with 99% ee's and devoid of osmium contamination are directly put to use in the synthesis of diltiazem and Taxol side chain with an overall improved yield to demonstrate the synthetic utility of the trifunctional heterogeneous catalyst. The high binding ability of the heterogeneous osmium catalyst enables the use of equimolar ratio of ligand to osmium to give excellent ee's in AD in contrast to the homogeneous osmium system in which the excess molar quantities of the expensive chiral ligand to osmium are invariably used. Further, the XRD, FT-IR, UV-vis DRS, and XPS studies indicate the retention of the coordination geometries of the specific divalent anions anchored to LDH matrix in their monomeric form during the ion exchange and after the reaction.

Microencapsulation of osmium tetroxide in polyurea

[reaction: see text] Osmium tetroxide has been microencapsulated in a polyurea matrix using an in situ interfacial polymerization approach. These microcapsules have been effectively used as recoverable and reusable catalysts in the dihydroxylation of olefins

The Os/Cu–Al-hydrotalcite catalysed hydroxylation of alkenes

A new Os/Cu-Al-hydrotalcite-like catalyst is described which, with N-methylmorpholine oxide as co-oxidant, heterogeneously catalyses the hydroxylation of olefins to give diols selectively and in high yield.

Catalytic asymmetric dihydroxylation of olefins using a recoverable and reusable OsO(4)2- in ionic liquid [bmim][PF6]

The use of the solvent systems water/ionic liquid or water/ionic liquid/tert-butanol provides a recoverable, reusable, robust and simple system for the asymmetric dihydroxylation of olefins, based on the immobilization of the osmium-ligand catalyst in the ionic liquid phase.