Current Organic Chemistry

ISSN: 1385-2728

Current Organic Chemistry
Volume 10, Number 13, September 2006

Contents

Heterogeneous Catalysts in Organic Synthesis
Guest Editor: Árpád Molnár


Editorial Pp. 1511


Asymmetrically Modified Nickel Catalyst for the Enantio-Differentiating Hydrogenation of Prochiral Ketones Pp. 1513-1531
T. Osawa, T. Harada and O. Takayasu
[Abstract]


Heterogeneous Catalytic Enantioselective Hydrogenation of Activated Ketones Pp. 1533-1567
Mihály Bartók
[Abstract]


Heterogeneous Catalytic Asymmetric Hydrogenation of the C=C Bond Pp. 1569-1583
A. Tungler, É. Sípos and V. Háda
[Abstract]


Supported Palladium Catalysts in Heck Coupling Reactions – Problems, Potential and Recent Advances Pp. 1585-1601
K. Köhler, S.S. Pröckl and W. Kleist
[Abstract]


Regioselective Electrophilic Aromatic Substitution Reactions over Reusable Zeolites Pp. 1603-1625
Keith Smith and Gamal A. El-Hiti
[Abstract]


Solid Base Catalysts in Organic Synthesis Pp. 1627-1637
François Figueras, Mannepalli Lakshmi Kantam and Boyapati Manoranjan Choudary
[Abstract]


Meerwein-Ponndorf-Verley Reduction over Heterogeneous Catalysts Pp. 1639-1654
G. K. Chuah, S. Jaenicke, Y.Z. Zhu and S. H. Liu
[Abstract]


Selective Syntheses on Heterogenized Metal Complexes Pp. 1655-1680
Ágnes Zsigmond and Ferenc Notheisz
[Abstract]


Developments and Applications of Supported Liquid Phase Catalysts Pp. 1681-1695
Fengyu Zhao, Shin-ichiro Fujita and Masahiko Arai
[Abstract]


Organic Transformations over Silica Materials Modified by Covalently Bonded Surface Functional Groups Pp. 1697-1726
Árpád Molnár and Bulcsú Rác
[Abstract]




Abstracts
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Editorial

This volume of Current Organic Chemistry is dedicated to the topic of the use of heterogeneous catalysts in organic synthesis. Catalysis, in general, plays a crucial and dominant role in chemistry, which may be best illustrated by the fact that about 90% of all the chemical processes applied in industry are catalytic transformations. Since the early 1990s, with the advent of green chemistry, there has been a strong driving force to replace homogeneous (soluble) catalysts with heterogeneous (solid) catalysts in chemical processes. Organic synthesis is an important field also benefiting from the achievements of catalysis research. This has been reflected in the tremendous development in the application of heterogeneous catalysts in organic synthesis in recent years. This special issue brings together experts of a few selected, important fields to highlight major achievements.

The first three chapters in this volume are about asymmetric hydrogenations. The first review is a contribution from Tsutomu Osawa and his Toyama University group. They present a comprehensive summary of the application of asymmetrically modified Ni catalyst in the enantio-differentiating hydrogenation of prochiral ketones. In the second chapter Mihály Bartók (University of Szeged) writes about the enantioselective hydrogenation of activated ketones over cinchona-modified Pt catalysts. In the third chapter Antal Tungler and his group (Budapest University of Technology and Economics) review asymmetric hydrogenation of the C=C bond focusing on the use of chirally modified Pd catalysts.

C–C coupling reactions is a burgeoning field represented here by a paper written by Klaus Köhler and co-workers (Technische Universität München) evaluating the use of heterogeneous Pd catalysts in the Heck reaction.

Two contributions disclose the use of versatile solids, successfully applied in a wide range of organic transformations. The review by Keith Smith and Gamal A. El-Hiti (University of Wales Swansea) focuses on their results with respect to regioselective electrophilic aromatic substitution reactions over reusable zeolites. The sixth chapter by François Figueras from the Institut de Recherches sur la Catalyse, Villeurbanne, and Mannepalli L. Kantam and Boyapati M. Choudary from the Indian Institute of Chemical Technology, Hyderabad, is about the application of solid base catalysts in organic synthesis focusing primarily on the use layered double hydroxides (hydrotalcites).

The heterogeneous Meerwein-Ponndorf-Verly reduction, which has attracted special attention resulting in the development of various active solids, is treated accordingly in a review by Gaik K. Chuah and his group from the National University of Singapore.

The next two papers represent specific approaches. The eighth chapter, provided by Ágnes Zsigmond and Ferenc Notheisz (University of Szeged) summarizes their results on the use of immobilized homogeneous complexes in selective syntheses. Masahiko Arai and his group from Hokkaido University, Sapporo are the contributors of the ninth chapter. They review the developments and applications of supported liquid phase catalysts.

The final chapter in this volume is a contribution from myself and Bulcsú Rác (University of Szeged). This review surveys methods to prepare mesoporous silica materials and their derivatization by catalytically active functional groups. The main emphasis is on the catalytic application of the functionalized solids in synthetic transformations.

It has been a pleasure to be involved as a Guest Editor with this issue of Current Organic Chemistry and I look forward to future involvement with this series. I would like to thank all the authors contributing to this issue for their efforts in making this issue interesting and informative. I hope you enjoy reading it.

Árpád Molnár
Department of Organic Chemistry
University of Szeged
Szeged
Hungary


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Asymmetrically Modified Nickel Catalyst for the Enantio-Differentiating Hydrogenation of Prochiral Ketones
T. Osawa, T. Harada and O. Takayasu

The state of the art for the enantio-differentiating hydrogenation over an asymmetrically modified nickel catalyst is reviewed along with the background development of the catalyst. An enantioselectivity up to 98% was obtained for the hydrogenation of β-functionalized ketones. The durability of the enantioselectivity was significantly improved using an in-situ-modified nickel powder catalyst (80-90% enantioselectivity for over 20 runs). Recent mechanistic investigations based on surface science are also summarized for the discussion of the enantio-differentiating model.


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Heterogeneous Catalytic Enantioselective Hydrogenation of Activated Ketones
Mihály Bartók

The manuscript gives an overview mainly on the results performed at the Department of Organic Chemistry the University of Szeged on the enantioselective hydrogenation of activated ketones of various structures on Pt/alumina catalysts modified with cinchona alkaloids. Relevant results by other research groups are also referred too. The review covers the literature till the end of 2004.


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Heterogeneous Catalytic Asymmetric Hydrogenation of the C=C Bond
A. Tungler, É. Sípos and V. Háda

The heterogeneous catalytic asymmetric hydrogenation of the C=C bond resulting in optically active product has been a challenge since decades. Such products can arise in enantio- and in diastereoselective reactions depending on whether the prochiral compound is hydrogenated with a chirally modified catalyst or the substrate itself contains the source of chirality in the form of a chiral moiety. The common step in these reactions is the stereodifferentiating hydrogen uptake on the surface of the metal, first of all on that of the Pd.

In our laboratory, experiences were collected in the last two decades for both reaction types. The asymmetric C=C hydrogenation of α,β-unsaturated ketones was studied with modified catalysts in the presence of chiral auxiliaries, like the (S)-proline. The latter turned out to be a diastereoselective reaction. Beside the well-known cinchona alkaloids a vinca alkaloid and diphenyl methyl pyrrolidine methanol also were applied as chiral modifiers. The afforded e.e. (enantiomeric excess) values were in most cases moderate, which could be explained among others with the relatively low reaction rates. In comparison with the Pt/cinchona system, large amount of modifier was necessary for the Pd catalyst, and this resulted in the lower reaction rates.

The asymmetric hydrogenation of the C=C bond was studied with optically active dehydroamino acid amides, prolineamides of picolinic, nicotinic and pyrrole acetic acids and pyridoin in diastereoselective reactions.


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Supported Palladium Catalysts in Heck Coupling Reactions – Problems, Potential and Recent Advances
K. Köhler, S.S. Pröckl and W. Kleist

The review reports the general approach, problems, potential and recent advances in heterogeneously catalyzed Heck reactions. It demonstrates the advantages and limitations for practical applications at present. A literature overview is given separately for substrates that are easier to activate (aryl iodides, activated aryl bromides) and for non-/deactivated aryl bromides and aryl chlorides during the period between 2001 and 2005. New approaches and strategies for the activation of aryl bromides and especially chlorides by heterogeneous catalysts are discussed. The argumentations clearly exceed simple separation and reuse arguments. Particular attention is given to the relation between homogeneous and heterogeneous catalysis from the mechanistic point of view. Very recent reports show clear success in the understanding of corresponding reaction mechanisms. Palladium species dissolved from the support are proven to be responsible for high activity and selectivity in Heck reactions by supported catalysts (Pd on activated carbon, oxides, polymers and in zeolites). The careful choice of optimum catalyst and reaction conditions are crucial and allowed the development of simple heterogeneous catalysts that activate even deactivated aryl chlorides with high yields within few hours of reaction time.


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Regioselective Electrophilic Aromatic Substitution Reactions over Reusable Zeolites
Keith Smith and Gamal A. El-Hiti

Zeolite catalysts can play an important role in the development of greener organic syntheses through their abilities to act as heterogeneous catalysts, support reagents, entrain by-products, avoid aqueous work-ups and enhance product selectivities via shape-selectivity. We have shown that zeolites under modest conditions can have advantages in para-regioselective nitration, alkylation, acylation, methanesulfonylation and halogenation of aromatic compounds. They can also assist in the electrophilic rearrangement of aryloxiranes. The zeolites can be easily removed from reaction mixtures by filtration and regenerated by heating and can then be reused several times to give the same selectivity.


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Solid Base Catalysts in Organic Synthesis
François Figueras, Mannepalli Lakshmi Kantam and Boyapati Manoranjan Choudary

The basic and catalytic properties of hydrotalcites, supported fluorides and mixed oxides are reviewed, with a special emphasis on C-C bond forming reactions. Relatively strong basicity can be induced on hydrotalcites by calcination followed by rehydration. In that case the shift from Lewis to Bronsted basicity has strong influence on the catalytic properties. The basicity is further enhanced by introduction of t-butoxide or fluoride anions in place of hydroxyls, while chloride anions inhibit activity. Excellent yields have been reported, by fine-tuning of basic sites of the hydrotaclites, supported fluorides and mixed oxides as exemplified in aldol, Henry, Knoevenagel, Michael, trans-esterification Wadsworth- Emmons and N-oxidation reactions. In many cases the catalysts can be recycled.


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Meerwein-Ponndorf-Verley Reduction over Heterogeneous Catalysts
G. K. Chuah, S. Jaenicke, Y.Z. Zhu and S. H. Liu

The Meerwein-Ponndorf-Verley (MPV) transfer hydrogenation of aldehydes and ketones to the corresponding alcohols is distinguished by its exceptional chemoselectivity. This reaction is particularly suited for the reduction of unsaturated aldehydes and ketones, compared to catalytic reductions with molecular hydrogen using noble metal catalysts. The MPV reaction has traditionally been carried out using homogeneous catalysts like aluminium or titanium alkoxides. However, over the past two decades, an increasing number of reports on heterogeneous catalysts for the MPV reaction have been published. This interest is stimulated by the advantages of heterogeneous over homogeneous catalysis, particularly for larger-scale applications. The article provides a critical review of the recent literature on heterogeneous catalysts for the MPV reaction. The materials include hydrotalcites, metal oxides such as magnesium oxide, zirconia, silica and alumina, alkoxides of Al, La, Zr and Hf grafted to high surface area mesoporous materials, and zeolites. In particular, zeolite beta has been found to be a highly stereoselective catalyst in the MPV reduction of 4-tert butylcyclohexanone, giving predominantly cis-4-tert butylcyclohexanol rather than the thermodynamically stable trans-alcohol. The activity was improved with the incorporation of other metals such as Ti, Sn and Zr. These materials show good tolerance to moisture, lack of leaching and ease of regeneration, thus making them useful catalysts in MPV reduction. The activity of a catalyst depends on its ligand exchange capability with the reductant with a high ligand exchange rate enabling the use of catalytic amounts.


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Selective Syntheses on Heterogenized Metal Complexes
Ágnes Zsigmond and Ferenc Notheisz

The preparation and application of heterogenized metal complexes is a relatively new field of research resulting in a new way of catalysis, that we cannot realize either with traditional homogeneous or heterogeneous catalysts. The heterogenized metal complexes can successfully combine the excellent performance (high activity and selectivity) of homogeneous catalysts with the easy separation and recycling of the heterogeneous catalysts. Additionally, in several cases the immobilization of metal complexes leads to enhanced stereo-, chemo- and enantioselectivity, interpreted by a mechanism related to the “confinement concept”. This review gives a summary of the different ways of preparation of heterogenized metal complexes on inorganic matrices and shows some applications of such catalysts in synthetic organic chemistry.


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Developments and Applications of Supported Liquid Phase Catalysts
Fengyu Zhao, Shin-ichiro Fujita and Masahiko Arai

Supported liquid phase catalyst (SLPC) is one of effectively heterogenized homogeneous catalysts using organometallic complexes as active components, which are dissolved in a small quantity of liquid phase dispersed in the form of isle or film on the surface of supports. The SLPC has successfully been applied for several chemical transformations and this article will review recent results with respect to the preparation and catalytic performance, the applicability to continuous flow operations, and the capability of multifunctional catalysis.


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Organic Transformations over Silica Materials Modified by Covalently Bonded Surface Functional Groups
Árpád Molnár and Bulcsú Rác

The methods to prepare mesoporous silica materials and their derivatization by catalytically active functional groups covalently anchored to the surface are summarized. A short description of the major characterization techniques is also given. The transformations induced by solids with anchored basic functions mainly with various amino groups including oxidations, C–C forming reactions, and enantioselective alkylations are treated in details. A similar comprehensive analysis is given for processes catalyzed by anchored sulfonic acid groups. The important reactions studied are esterification, various condensations, protection of functional groups, and Friedel–Crafts reactions. The results of comparative studies are treated separately. The two final subsections are devoted to the application of bridged polysilsesquioxanes, also called periodic mesoporous organosilanes (PMO’s) and cooperative acid–base catalysis.