Regioselective Friedel-Crafts Acylation Reaction Using Single Crystalline and Ultrathin Nanosheet Assembly of Scrutinyite-SnO2.

Peculiar physicochemical properties of two-dimensional (2D) nanomaterials have attracted research interest in developing new synthetic technology and exploring their potential applications in the field of catalysis. Moreover, ultrathin metal oxide nanosheets with atomic thickness exhibit abnormal surficial properties because of the unique 2D confinement effect. In this work, we present a facile and general approach for the synthesis of single crystalline and ultrathin 2D nanosheets assembly of scrutinyite-SnO2 through a simple solvothermal method. The structural and compositional characterization using X-ray diffraction (Rietveld refinement analysis), high-resolution transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and so on reveal that the as-synthesized 2D nanosheets are ultrathin and single crystallized in the scrutinyite-SnO2 phase with high purity. The ultrathin SnO2 nanosheets show predominant growth in the [011] direction on the main surface having a thickness of ca. 1.3 nm. The SnO2 nanosheets are further employed for the regioselective Friedel-Crafts acylation to synthesize aromatic ketones that have potential significance in chemical industry as synthetic intermediates of pharmaceuticals and fine chemicals. A series of aromatic substrates acylated over the SnO2 nanosheets have afforded the corresponding aromatic ketones with up to 92% yield under solvent-free conditions. Comprehensive catalytic investigations display the SnO2 nanosheet assembly as a better catalytic material compared to the heterogeneous metal oxide catalysts used so far in the view of its activity and reusability in solvent-free reaction conditions.

An efficient base-free N-arylation of imidazoles and amines with arylboronic acids using copper-exchanged fluorapatite.

N-Arylation of imidazoles and amines with arylboronic acids was accomplished with copper-exchanged fluorapatite (CuFAP) in methanol at room temperature. The products N-arylimidazoles and N-arylamines were isolated in good to excellent yields. A variety of arylboronic acids were converted to the corresponding N-arylimidazoles and N-arylamines, demonstrating the versatility of the reaction.

Nanocrystalline MgO for asymmetric Henry and Michael reactions.

Nanomaterials with their three-dimensional structure and defined size and shape are considered to be suitable candidates for proper alignment with prochiral substrates for unidirectional introduction of reacting species to induce an asymmetric center. We herein report the design and development of a truly recyclable heterogeneous catalyst, nanocrystalline magnesium oxide, for the asymmetric Henry reaction (AH) to afford chiral nitro alcohols with excellent yields and good to excellent enantioselectivities (ee's) for the first time. Bronsted hydroxyls are the sole contributors for the ee, while they add on to the activity in AH. It is demonstrated that the hydrogen bond interactions between the -OH groups of (S)-(-)-binol and the -OH groups of MgO are essential for the induction of enantioselectivity. Further, to prove the above hypothesis, we have successfully carried out another reaction, asymmetric Michael reaction (AM) with nanocrystalline MgO. The reusable and suitably aligned nanocrystalline MgO-catalyzed AH and AM reactions afforded chiral products with comparable ee's to that of the homogeneous system.

Design and evolution of copper apatite catalysts for N-arylation of heterocycles with chloro- and fluoroarenes.

We report the preparation of recyclable heterogeneous catalysts, copper-exchanged fluorapatite, and copper-exchanged tert-butoxyapatite by incorporating basic species F-/tBuO- in apatite in situ by coprecipitation and subsequent exchange with Cu(II). These basic copper catalysts catalyzed N-arylation of imidazoles and other heterocycles with chloroarenes and electron-poor fluoroarenes in good to excellent yields. Synthesis and characterization of some of the intermediates of the catalytic cycle gave some insight into the mechanism of the very important organic transformation. The necessity of basic sites for the activation of C-Cl and C-F bonds in the N-arylation of heterocycles with haloarenes is well-established.

Bifunctional nanocrystalline MgO for chiral epoxy ketones via Claisen-Schmidt condensation-asymmetric epoxidation reactions.

Design and development of a truly nanobifunctional heterogeneous catalyst for the Claisen-Schmidt condensation (CSC) of benzaldehydes with acetophenones to yield chalcones quantitatively followed by asymmetric epoxidation (AE) to afford chiral epoxy ketones with moderate to good yields and impressive ee's is described. The nanomagnesium oxide (aerogel prepared) NAP-MgO was found to be superior over the NA-MgO and CM-MgO in terms of activity and enantioselectivity as applicable in these reactions. An elegant strategy for heterogenization of homogeneous catalysts is presented here to evolve single-site chiral catalysts for AE by a successful transfer of molecular chemistry to surface metal-organic chemistry with the retention of activity, selectivity/enantioselectivity. Brønsted hydroxyls are established as sole contributors for the epoxidation reaction, while they add on to the CSC, which is largely driven by Lewis basic O2-sites. Strong hydrogen-bond interactions between the surface -OH on MgO and -OH groups of diethyl tartrate are found inducing enantioselectivity in the AE reaction. Thus, the nanocrystalline NAP-MgO with its defined shape, size, and accessible OH groups allows the chemisorption of TBHP, DET, and olefin on its surface to accomplish single-site chiral catalysts to provide optimum ee's in AE reactions.

Synthesis of surface organopalladium intermediates in coupling reactions: the mechanistic insight.

The design and synthesis of surface transient organometallic intermediates on nanopalladium supported on layered double hydroxides is conceived and developed for the first time. The formation of only one STO intermediate in all the Heck-, Suzuki-, Sonogashira-, and Stille-type coupling reactions during their reaction sequences and the excellent isolated yields of each of the coupling products from the corresponding organometallic complexes not only validate the mechanism but also demonstrate the evolution of the single-site heterogeneous catalyst.

The first example of direct oxidation of sulfides to sulfones by an osmate molecular oxygen system.

Osmate-exchanged Mg-Al layered double hydroxides catalysed the delivery of two oxygen atoms simultaneously via a 3 + 1 cycloaddition to sulfide to form sulfone directly for the first time, reminiscent of 3 + 2 cycloaddition in asymmetric dihydroxylation reactions.

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.

Benzylation of aromatic compounds with different crystallites of MgO.

Magnesium oxide nanocrystals are shown to be selective catalysts for the benzylation of aromatics with benzyl chloride, in the order xylene > toluene >> benzene. Normal polycrystalline MgO is not as effective as the nanomaterials. However, at the highest surface area, normally most reactive MgO samples (aerogel prepared AP-MgO) were poorer catalysts than a hexagonal platelet form (CP-MgO) of lower surface area. During the catalysis, the MgO is partially converted to MgCl(2), but crystal shape differences are not washed out. Rate data, differential thermal analysis, and consideration of crystal shapes and faces suggest that the CP-MgO form is most catalytically active because of optimal adsorption, molecular trafficking, and desorption energies. The AP-MgO apparently adsorbs the reactants too strongly, and thereby this hinders the catalytic process. These results serve as another example of where nanocrystalline shape plays an important role.

Layered double hydroxide supported nanopalladium catalyst for Heck-, Suzuki-, Sonogashira-, and Stille-type coupling reactions of chloroarenes.

Layered double hydroxide and Merrifield resin supported nanopalladium(0) catalysts are prepared by an exchange of PdCl(4)(2-) followed by reduction and well characterized for the first time. The ligand-free heterogeneous layered double hydroxide supported nanopalladium (LDH-Pd(0)) catalyst using the basic LDH in place of basic ligands indeed exhibits higher activity and selectivity in the Heck olefination of electron-poor and electron-rich chloroarenes in nonaqueous ionic liquids (NAIL) over the homogeneous PdCl(2) system. Using microwave irradiation, the rate of the Heck olefination reaction is accelerated, manifold with the highest turnover frequency ever recorded in the case of both electron-poor and electron-rich chloroarenes. The basic LDH-Pd(0) shows a superior activity over a range of supported catalysts, from acidic to weakly basic Pd/C, Pd/SiO(2,) Pd/Al(2)O(3), and resin-PdCl(4)(2-) in the Heck olefination of deactivated electron-rich 4-chloroanisole. The use of LDH-Pd(0) is extended to the Suzuki-, Sonogashira-, and Stille-type coupling reactions of chloroarenes in an effort to understand the scope and utility of the reaction. The catalyst is quantitatively recovered from the reaction by a simple filtration and reused for a number of cycles with almost consistent activity in all the coupling reactions. The heterogeneity studies provide an insight into mechanistic aspects of the Heck olefination reaction and evidence that the reaction proceeds on the surface of the nanopalladium particles of the heterogeneous catalyst. TEM images of the fresh and used catalyst indeed show that the nanostructured palladium supported on LDH remains unchanged at the end of the reaction, while the XPS and evolved gas detection by TGA-MS of the used catalyst identify ArPdX species on the heterogeneous surface. Thus, the ligand-free nanopalladium supported on LDH, synthesized by the simple protocol, displays superior activity over the other heterogeneous catalysts inclusive of nanopalladium in the C-C coupling reactions of chloroarenes.

A new bifunctional catalyst for tandem Heck-asymmetric dihydroxylation of olefins.

A new bifunctional catalyst consisting of active palladium and osmium species anchored on silica gel through a mercaptopropyl spacer and a cinchona alkaloid respectively has been prepared for the first time and used in the heterogeneous tandem Heck-asymmetric dihydroxylation of olefins to afford diols with excellent yields and enantiomeric excesses (ee's) in presence of N-methylmorpholine N-oxide or K3Fe(CN)6 as cooxidants.

Catalytic asymmetric dihydroxylation of olefins with reusable OsO(4)(2-) on ion-exchangers: the scope and reactivity using various cooxidants.

Exchanger-OsO(4) catalysts are prepared by an ion-exchange technique using layered double hydroxides and quaternary ammonium salts covalently bound to resin and silica as ion-exchangers. The ion-exchangers with different characteristics and opposite ion selectivities are specially chosen to produce the best heterogeneous catalyst that can operate using the various cooxidants in the asymmetric dihydroxylation reaction. LDH-OsO(4) catalysts composed of different compositions are evaluated for the asymmetric dihydroxylation of trans-stilbene. Resin-OsO(4) and SiO(2)-OsO(4) designed to overcome the problems associated with LDH-OsO(4) indeed show consistent activity and enantioselectivity in asymmetric dihydroxylation of olefins using K(3)Fe(CN)(6) and molecular oxygen as cooxidants. Compared to the Kobayashi heterogeneous systems, resin-OsO(4) is a very efficient catalyst for the dihydroxylation of a wide variety of aromatic, aliphatic, acyclic, cyclic, mono-, di-, and trisubstituted olefins to afford chiral vicinal diols with high yields and enantioselectivities irrespective of the cooxidant used. Resin-OsO(4) is recovered quantitatively by a simple filtration and reused for a number of cycles with consistent activity. The high binding ability of the heterogeneous osmium catalyst enables the use of an equimolar ratio of ligand to osmium to give excellent enantioselectives in asymmetric dihydroxylation in contrast to the homogeneous osmium system in which excess molar quantities of the expensive chiral ligand to osmium are invariably used. The complexation of the chiral ligand (DHQD)(2)PHAL, having very large dimension, a prerequisite to obtain higher ee, is possible only with the OsO(4)(2-) located on the surface of the supports.