Development of High-Performance Catalysts toward Green Organic Syntheses

 

Background and Objectives of Our Research

Evolution of environmentally-acceptable organic synthesis is an ultimate goal of the present-day chemistry. The promising approach to “Green & Sustainable Chemistry” is to replace conventional methods employing toxic and/or hazardous stoichiometric reagents by atom-efficient alternatives [1]. Catalysis is one of the leading principles to achieve high selectivity and minimization of waste production.

The research in the Design of High-Performance Catalysts group is directed towards the development of highly-functionalized heterogeneous catalysts for atom-efficient, low-waste processes for versatile intermediates for the syntheses of pharmaceutical and agrochemicals. Especially, heterogeneous catalysts have the advantages of being operationally simple as well as enabling unprecedented reactions based on specific ensemble sites within a regular arrangement of surface atoms, e.g., metal cations and hydroxyl groups. Creating precise architectures of active metal species on solid surfaces is one of the most important challenges in creating highly-functionalized heterogeneous catalysts.

 

Current Research

Our group have developed highly-functionalized metal catalysts using unique properties of inorganic and organic materials, i.e., hydroxyapatite, montmorillonite, hydrotalcite, giant metal cluster, and dendrimer as macroligands of metal species that makes a pivotal contribution to developing environmentally-acceptable organic syntheses including selective oxidations using molecular oxygen as an oxidant and carbon-carbon bond-forming reactions.

We strive to obtain scientific insights for the catalyst and catalytic reactions in combination with application- oriented research. These goals are pursued via five research topics:

 

1. Development of transition metal-based heterogeneous catalytic methods for clean and selective oxidations of hydrocarbons using molecular oxygen as the sole oxidant.

2. Application of immobilized metal species in organic synthesis: development of highly selective and efficient carbon-carbon bond-forming reactions under solvent-less or aqueous conditions.

3. One-pot syntheses based on the multificntional catalysis by several active sites.

4. Preparation of nano-sized metal clusters with narrow size distribution and unique oxidation states.

5. Development of high performance nanoreactors using dendritic materials for the functional group transformations.

 

The following describes the development of hydroxyapatite-bound transition metal catalysts as a typical example.

 

Bone-supported catalysts;

 Hydroxyapatites (HAP), Ca₁₀(PO₄)₆(OH)₂, possess Ca²⁺ sites surrounded by PO₄^3- tetrahedra parallel to the hexagonal axis, as shown in Figure 1 [2], and are of considerable interest in many areas because of their ion-exchange ability, adsorption capacity, and acid-base properties. The chemical composition of hydroxyapatites can be varied from the stoichiometric to the Ca-deficient form. Here, we present two new classes of hydroxyapatite- bound Pd complexes designed with strict compositional and structural control. Both stoichiometric and Ca-deficient hydroxyapatites are employed, and the catalysts exhibit specific novel functions as heterogeneous Pd catalysts. The precise construction of Pd species described here also represents a great contribution to modern palladium chemistry.

Synthesis and characterization of hydroxyapatite-bound palladium complexes [3]

Hydroxyapatites were synthesized from Ca(NO₃)₂×4H₂O and (NH₄)₂HPO₄ by precipitation method. Selecting appropriate Ca/P molar ratios gave the stoichiometric hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ (Ca/P=1.67, HAP-0) and the Ca-deficient hydroxyapatite Ca₉(HPO₄)(PO₄)₅(OH) (Ca/P=1.50, HAP-1). Treatment of the HAP-0 and the HAP-1 with an acetone solution of PdCl₂(PhCN)₂ yielded the hydroxyapatite-bound Pd complex, PdHAP-0 and PdHAP-1, respectively.

The characterization of the PdHAPs using physicochemical methods such as X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and inductively coupled plasma (ICP) analysis revealed that the palladium species is immobilized by adsorption on the HAP surface. Further, the result of Pd K-edge X-ray absorption near-edge structure spectra of both PdHAPs confirmed the divalent state of all Pd species. A monomeric Pd species was evidenced by the absence of peaks around 2.5 A in the Fourier transform (FT) of k³-weighted extended X-ray absorption fine structure (EXAFS) data for the PdHAPs (Fig. 2A and 2B).


The inverse FT of the peaks around 1-2 A for the PdHAP-0 was well fitted using Pd-Cl and Pd-O shells, whereas the best fit for the PdHAP-1 was achieved using only a Pd-O shell. It was conclusively established that a monomeric PdCl₂ species was grafted by chemisorption on the HAP-0 surface (Fig. 3A), and a monomeric Pd^II phosphate complex surrounded by four oxygens was formed at a Ca-deficient site of the HAP-1 (Fig. 3B). Two unique monomeric Pd species with intrinsically different surroundings can be created on the solid surfaces through precise control of the Ca/P ratios of the parent hydroxyapatites.

Alcohol oxidation using molecular oxygen

The oxidation of alcohols into carbonyl compounds is one of the most pivotal functional group transformations in organic synthesis [4]. The PdHAP-0 proved to be an effective heterogeneous catalyst for the aerobic oxidation of a wide variety of alcohols such as benzylic, allylic, aliphatic, and heterocyclic alcohols, giving the corresponding ketones and aldehydes in excellent yields. To highlight the applicability of the present protocol, a 250 mmol scale reaction of 1-phenylethanol was undertaken using 4 x 10^-4 mol % of the Pd catalyst without organic solvents. The oxidation proceeded smoothly and the turnover number (TON) of acetophenone based on Pd approached 236,000 (scheme. 1). This TON value is three orders of magnitude larger than those previously reported for any catalyst systems under an atmospheric O₂ pressure.

The FT EXAFS for the recovered PdHAP-0 exhibited a single peak at approximately 2.5 A due to the formation of Pd metal, as shown in Figure 2C. TEM also revealed the presence of Pd nanoparticles with a diameter of ca. 40 A having a narrow size distribution (Fig. 4). The diameters of the generated Pd nanoparticles can be controlled upon acting on the alcohol substrates used. Oxidation of alcohols is proposed to occur primarily on low-coordination sites within a regular arrangement of the Pd nanoparticles by performing calculations on the palladium crystallites [5].  

ICP analysis of the filtrate confirmed that no leaching of Pd species occurred during the above oxidations, and then the PdHAP-0 could be reused without loss of the catalytic activity and selectivity. It is noteworthy that the above oxidations hardly occurred in the presence of the PdHAP-1, and that there were no structural changes around the Pd^II center as confirmed by EXAFS (Fig. 2D).

 

Carbon-carbon bond-forming reactions

The Heck coupling reaction has received considerable attention due to its enormous synthetic potential to form new carbon-carbon bonds. Commercial applications have, however, been limited by the low TONs and relatively short catalyst lifetime. We found that the PdHAP-1 was an outstanding catalyst for the Heck reaction. For example, as listed in Table 1
,
the TON based on Pd reached 47,000 for 24 h in the case of bromobenzene with styrene (entry 1). Moreover, the PdHAP-1 was applicable to the Suzuki coupling reactions with a TON approaching 40,000 for 4 h in the reaction between bromobenzene and phenylboronic acid (entry 4). The recovered PdHAP-1 had an original monomeric Pd^II structure and was recyclable with retention of its catalytic activity. By contrast, the PdHAP-0 catalyst was less effective in the Heck reaction, and gave a poor TON in the case of bromobenzene with styrene under the above reaction conditions due to the formation of Pd⁰ particles with a diameter of ca. 50 A. It can be concluded that the high catalytic activity of the PdHAP-1 is ascribable to the exceptional robust structure of monomeric Pd^IIspecies under the reaction conditions.

Further applications

Recently, we also found that the PdHAP-0 acted as an efficient heterogeneous catalyst for the deprotection of N-benzyloxycarbonyl group from amino acids using molecular hydrogen, and the dehydrogenation of indolines into indoles that have served as versatile intermediates for the synthesis of pharmaceuticals and agrochemicals [6].

The cation-exchange ability of hydroxyapatites enables an equimolar substitution of Ru³⁺ for Ca²⁺ on their surface, which gives a monomeric Ru³⁺ phosphate species (RuHAP) [7]. As shown in Figure 5, this RuHAP could efficiently catalyze the oxidation of alcohols, amines, and silanes under an atmospheric pressure of O₂ [8]. Furthermore, treatment of the RuHAP with an aqueous solution of AgX (X = SbF₆^-, TfO^-) readily afforded hydroxyapatite-bound cationic Ru complexes having potentially vacant coordination sites [9]. Such cationic RuHAP proved to be promising heterogeneous Lewis acid catalysts that promote Diels-Alder and aldol reactions under mild and neutral reaction conditions (Fig. 6).

Summary

A novel approach to catalyst design of metal species using hydroxyapatite as a macroligand and their excellent catalytic performances for aerobic alcohol oxidation and C-C bond-forming reactions were demonstrated. These catalytic systems can offer significant benefits in achieving simple and clean organic syntheses since the organic syntheses described here have following advantages; 1) easy preparations of the catalysts, 2) high catalytic activity under mild conditions, 3) simple work-up procedures, and 4) a reusability of harmless catalysts.

We expect that our immobilizing protocol based on the inorganic crystals will offer an attractive route for the design of high-performance catalysts at the atomic and molecular level, and thus have continued to create nanostructured heterogeneous catalysts with the aim of realizing environmentally-benign chemical processes.

 

References:

1)       For example, Trost, B. M. Science 1991, 254, 1471; Sheldon, R. A. Chemtech 1994, March 38; Clark, J. H. Green Chem. 1999, 1, 1; Anastas, P. T. and Warner, J. C. Green Chemistry; Theory and Practice, Oxford Press. 1998.

2)       Elliot, J. C. Structure and Chemistry of the Apatites and Other Calcium Orthophosphates, Elsevier, 1994.

3)       Kaneda, K. et al. J. Am. Chem. Soc. 2002, 124, 11572.

4)       Sheldon, R. A.; Kochi, J. K. Metal Catalyzed Oxidations of Organic Compounds, Academic Press: New York, 1981.

5)       Kaneda, K. et al. J. Am. Chem. Soc. In press.

6)       Kaneda, K. et al. Tetrahedron Lett. 2003, 44, 4981; Tetrahedron Lett. 2003, 44, 6207.

7)       Kaneda, K. et al. J. Am. Chem. Soc. 2000, 122, 7144.

8)       Kaneda, K. et al. Chem. Commun. 2001, 461; New J. Chem. 2002, 26, 1536.  

9)       Kaneda, K. et al. J. Am. Chem. Soc. 2003, 125, 11460.