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- Herausgeber: Wiley-VCH
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Written by experts in the field, this is a much-needed overview of the rapidly emerging field of cooperative catalysis.
The authors focus on the design and development of novel high-performance catalysts for applications in organic synthesis (particularly asymmetric synthesis), covering a broad range of topics, from the latest progress in Lewis acid / Br?nsted base catalysis to e.g. metal-assisted organo catalysis, cooperative metal/enzyme catalysis, and cooperative catalysis in polymerization reactions and on solid surfaces. The chapters are classified according to the type of cooperating activating groups, and describe in detail the different strategies of cooperative activation, highlighting their respective advantages and pitfalls. As a result, readers will learn about the different concepts of cooperative catalysis, their corresponding modes of operation and their applications, thus helping to find a solution to a specific synthetic catalysis problem.
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Table of Contents
Cover
Related Titles
Title Page
Copyright
Preface
References
Acknowledgments
List of Contributors
Chapter 1: Lewis Acid–Brønsted Base Catalysis
1.1 Introduction
1.2 Lewis Acid–Brønsted Base Catalysis in Metalloenzymes
1.3 Hard Lewis Acid–Brønsted Base Cooperative Catalysis
1.4 Soft Lewis Acid–Brønsted Base Cooperative Catalysis
1.5 Conclusion
References
Chapter 2: Lewis Acid–Lewis Base Catalysis
2.1 Introduction
2.2 Lewis Acid and Lewis Base Activation
2.3 Addition to Carbonyl Compounds
2.4 Condensation Reactions
2.5 Morita-Baylis-Hillman Reactions
2.6 Epoxide Openings
2.7 Cyclization Reactions
2.8 Polymerizations
2.9 Conclusions and Outlook
References
Chapter 3: Cooperating Ligands in Catalysis
3.1 Introduction
3.2 Chemically Active Ligands Assisting a Metal-Localized Catalytic Reaction
3.3 Redox-Active Ligands Assisting Metal-Based Catalysts
3.4 Summary
References
Chapter 4: Cooperative Enamine-Lewis Acid Catalysis
4.1 Introduction
4.2 Reactions Developed through Cooperative Enamine-Lewis Acid Catalysis
4.3 Conclusion
Acknowledgment
References
Chapter 5: Hydrogen Bonding-Mediated Cooperative Organocatalysis by Modified Cinchona Alkaloids
5.1 Introduction
5.2 The Emergence of Highly Enantioselective Base Organocatalysis
5.3 Hydrogen Bonding-Based Cooperative Catalysis by Modified Cinchona Alkaloids
5.4 Conclusion and Outlooks
Acknowledgments
References
Chapter 6: Cooperation of Transition Metals and Chiral Brønsted Acids in Asymmetric Catalysis
6.1 General Introduction
6.2 Cooperative Catalysis of Palladium(II) and a Brønsted Acid
6.3 Cooperative Catalysis of Palladium(0) and a Brønsted Acid
6.4 Cooperative Catalysis of a Rhodium Complex and a Brønsted Acid
6.5 Cooperative Catalysis of a Silver Complex and a Brønsted Acid
6.6 Cooperative Catalysis of a Copper Complex and a Brønsted Acid
6.7 Cooperative Catalysis of an Iridium Complex and a Brønsted Acid
6.8 Cooperative Catalysis of an Iron Complex and a Brønsted Acid
6.9 Perspective
References
Chapter 7: Cooperative Catalysis Involving Chiral Ion Pair Catalysts
7.1 Introduction
7.2 Chiral Cation-Based Catalysis
7.3 Chiral Anion Based Catalysis
7.4 Synopsis
References
Chapter 8: Bimetallic Catalysis: Cooperation of Carbophilic Metal Centers
8.1 Introduction
8.2 Homobimetallic Catalysts
8.3 Heterobimetallic Catalysts
8.4 Synopsis
Acknowledgments
References
Chapter 9: Cooperative H2 Activation by Borane-Derived Frustrated Lewis Pairs
9.1 Introduction
9.2 Mechanistic Considerations
9.3 General Considerations
9.4 Hydrogenation of Imines
9.5 Hydrogenation of Enamines and Silylenol Ethers
9.6 Hydrogenation of Heterocycles
9.7 Hydrogenation of Enones, Alkylidene Malonates, and Nitroolefins
9.8 Hydrogenation of Unpolarized Olefins and Polycyclic Aromatic Hydrocarbons
9.9 Summary
Abbreviations
References
Chapter 10: Catalysis by Artificial Oligopeptides
10.1 Cooperative Catalysis by Short Peptides
10.2 Cooperative Catalysis by Supramolecular Systems
10.3 Cooperative Catalysis by Nanosystems
10.4 Conclusions
References
Chapter 11: Metals and Metal Complexes in Cooperative Catalysis with Enzymes within Organic-Synthetic One-Pot Processes
11.1 Introduction
11.2 Metal-Catalyzed
In situ
-Preparation of an Enzyme's Reagent (Cofactor) Required for the Biotransformation
11.3 Combination of a Metal-Catalyzed Racemization of a Substrate with a Stereoselective Biotransformation Toward a Dynamic Kinetic Resolution
11.4 Combinations of Metal Catalysis and Biocatalysis Toward “Consecutive” One-Pot Processes without Intermediate Isolation
11.5 Summary and Outlook
References
Chapter 12: Cooperative Catalysis on Solid Surfaces versus Soluble Molecules
12.1 Introduction
12.2 Tuning Cooperativity of Acid–Base Bifunctional Groups by Varying the Distance Between Them in a Soluble-Molecule Platform
12.3 Acid–Base Bifunctional Catalysts on Two-Dimensional Surfaces: Organic–Inorganic Materials
12.4 Cooperative Catalysis on Surfaces versus Soluble Molecular Platforms for Kinetic Resolution of Racemic Epoxides
12.5 Depolymerization of Biomass Polymers via Cooperative Catalysis on Surfaces
12.6 Conclusions
References
Chapter 13: Cooperative Catalysis in Polymerization Reactions
13.1 Introduction
13.2 Cooperative Effects for the Polymerization of Lactide and Other Cyclic Esters
13.3 Polymerization Reactions of Vinyl Monomers with Frustrated Lewis Pairs
13.4 Zinc-Based Cooperative Catalysis of Epoxide/CO
2
Copolymerization
13.5 Cooperative Mechanism of Epoxide/CO
2
Copolymerization by Salen-Type Complexes
13.6 Summary
References
Index
End User License Agreement
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Guide
Cover
Table of Contents
Preface
Begin Reading
List of Illustrations
Figure 1.1
Figure 1.2
Figure 1.3
Figure 1.4
Scheme 1.1
Figure 1.5
Scheme 1.2
Figure 1.6
Figure 1.7
Scheme 1.3
Figure 1.8
Figure 1.9
Scheme 1.4
Figure 1.10
Scheme 1.5
Scheme 1.6
Scheme 1.7
Figure 1.11
Figure 1.12
Scheme 1.8
Scheme 1.9
Figure 1.13
Scheme 1.10
Scheme 1.11
Figure 1.14
Scheme 1.12
Figure 2.1
Figure 2.2
Figure 2.3
Figure 2.4
Figure 2.5
Figure 2.6
Scheme 2.1
Scheme 2.2
Scheme 2.3
Scheme 2.4
Scheme 2.5
Scheme 2.6
Scheme 2.7
Figure 2.7
Figure 2.8
Scheme 2.8
Scheme 2.9
Scheme 2.10
Scheme 2.11
Scheme 2.12
Scheme 2.13
Scheme 2.14
Scheme 2.15
Scheme 2.16
Scheme 2.17
Scheme 2.18
Scheme 2.19
Scheme 2.20
Scheme 2.21
Scheme 2.22
Scheme 2.23
Scheme 2.24
Scheme 2.25
Scheme 2.26
Scheme 2.27
Scheme 2.28
Scheme 2.29
Scheme 2.30
Scheme 2.31
Scheme 2.32
Scheme 2.33
Scheme 2.34
Scheme 2.35
Scheme 2.36
Scheme 2.37
Scheme 2.38
Scheme 2.39
Scheme 2.40
Scheme 2.41
Scheme 3.1
Scheme 3.2
Scheme 3.3
Scheme 3.4
Scheme 3.5
Scheme 3.6
Scheme 3.7
Scheme 3.8
Scheme 3.9
Scheme 3.10
Scheme 3.11
Scheme 3.12
Scheme 3.13
Figure 3.1
Scheme 3.14
Scheme 3.15
Scheme 3.16
Scheme 3.17
Scheme 3.18
Scheme 3.19
Scheme 3.20
Scheme 3.21
Scheme 3.22
Scheme 3.23
Scheme 3.24
Scheme 3.25
Scheme 3.26
Scheme 3.27
Scheme 3.28
Scheme 3.29
Scheme 3.30
Scheme 3.31
Scheme 3.32
Scheme 3.33
Scheme 3.34
Scheme 3.35
Scheme 3.36
Scheme 3.37
Scheme 3.38
Scheme 3.39
Scheme 3.40
Figure 4.1
Figure 4.2
Figure 4.3
Figure 4.4
Figure 4.5
Scheme 4.1
Scheme 4.2
Scheme 4.3
Scheme 4.4
Scheme 4.5
Scheme 4.6
Scheme 4.7
Scheme 4.8
Scheme 4.9
Scheme 4.10
Scheme 4.11
Scheme 4.12
Scheme 4.13
Scheme 4.14
Scheme 4.15
Scheme 4.16
Scheme 4.17
Scheme 4.18
Scheme 4.19
Scheme 4.20
Scheme 4.21
Scheme 4.22
Scheme 4.23
Scheme 4.24
Scheme 4.25
Scheme 4.26
Scheme 4.27
Scheme 4.28
Scheme 4.29
Scheme 4.30
Figure 5.1
Scheme 5.1
Scheme 5.2
Scheme 5.3
Scheme 5.4
Scheme 5.5
Scheme 5.6
Scheme 5.7
Scheme 5.8
Scheme 5.9
Figure 5.2
Scheme 5.10
Scheme 5.11
Scheme 5.12
Scheme 5.13
Scheme 5.14
Figure 5.3
Scheme 5.15
Scheme 5.16
Scheme 5.17
Scheme 5.18
Scheme 5.19
Scheme 5.20
Scheme 5.21
Scheme 5.22
Scheme 5.23
Scheme 5.24
Scheme 5.25
Scheme 5.26
Scheme 5.27
Scheme 6.1
Figure 6.1
Scheme 6.2
Scheme 6.3
Scheme 6.4
Scheme 6.5
Scheme 6.6
Scheme 6.7
Scheme 6.8
Scheme 6.9
Scheme 6.10
Scheme 6.11
Scheme 6.12
Scheme 6.13
Scheme 6.14
Scheme 6.15
Scheme 6.16
Scheme 6.17
Scheme 6.18
Scheme 6.19
Scheme 6.20
Scheme 6.21
Scheme 6.22
Scheme 6.23
Scheme 6.24
Scheme 6.25
Scheme 6.26
Scheme 6.27
Scheme 6.28
Scheme 6.29
Scheme 6.30
Scheme 6.31
Scheme 6.32
Scheme 7.1
Figure 7.1
Scheme 7.2
Scheme 7.3
Scheme 7.4
Scheme 7.5
Scheme 7.6
Scheme 7.7
Scheme 7.8
Scheme 7.9
Scheme 7.10
Scheme 7.11
Scheme 7.12
Scheme 7.13
Scheme 7.14
Scheme 7.15
Scheme 7.16
Scheme 7.17
Scheme 7.18
Scheme 7.19
Scheme 7.20
Scheme 7.21
Figure 8.1
Figure 8.2
Figure 8.3
Figure 8.4
Figure 8.5
Figure 8.6
Figure 8.7
Figure 8.8
Figure 8.9
Figure 8.10
Figure 8.11
Figure 8.12
Figure 8.13
Figure 8.14
Figure 8.15
Figure 8.16
Figure 8.17
Figure 8.18
Figure 8.19
Figure 8.20
Figure 8.21
Figure 8.22
Figure 8.23
Figure 8.24
Figure 8.25
Figure 8.26
Figure 8.27
Figure 8.28
Figure 8.29
Figure 8.30
Figure 8.31
Figure 8.32
Figure 8.33
Figure 8.34
Figure 8.35
Figure 8.36
Figure 8.37
Figure 8.38
Figure 8.39
Figure 9.1
Figure 9.2
Figure 9.3
Figure 9.4
Figure 9.5
Figure 9.6
Figure 9.7
Figure 9.8
Figure 9.9
Figure 9.10
Figure 9.11
Figure 9.12
Figure 9.13
Figure 9.14
Figure 9.15
Figure 9.16
Figure 9.17
Figure 9.18
Figure 9.19
Figure 9.20
Figure 9.21
Figure 9.22
Figure 9.23
Figure 9.24
Figure 9.25
Figure 9.26
Figure 9.27
Figure 9.28
Figure 9.29
Figure 9.30
Figure 9.31
Figure 9.32
Figure 9.33
Figure 9.34
Figure 9.35
Figure 9.36
Figure 9.37
Figure 9.38
Figure 9.39
Figure 9.40
Figure 9.41
Figure 9.42
Figure 9.43
Figure 9.44
Figure 9.45
Figure 10.1
Figure 10.2
Figure 10.3
Figure 10.4
Figure 10.5
Figure 10.6
Figure 10.7
Figure 10.8
Figure 10.9
Figure 10.10
Figure 10.11
Figure 10.12
Figure 10.13
Scheme 11.1
Scheme 11.2
Scheme 11.3
Scheme 11.4
Scheme 11.5
Scheme 11.6
Scheme 11.7
Scheme 11.8
Scheme 11.9
Scheme 11.10
Scheme 11.11
Scheme 11.12
Scheme 11.13
Scheme 11.14
Scheme 11.15
Scheme 11.16
Scheme 11.17
Scheme 11.18
Scheme 11.19
Scheme 11.20
Scheme 11.21
Figure 12.1
Figure 12.2
Figure 12.3
Figure 12.4
Figure 12.5
Figure 12.6
Figure 12.7
Figure 12.8
Figure 12.9
Figure 12.10
Figure 12.11
Figure 12.12
Scheme 13.1
Scheme 13.2
Figure 13.1
Scheme 13.3
Figure 13.2
Figure 13.3
Scheme 13.4
Scheme 13.5
Scheme 13.6
Figure 13.4
Scheme 13.7
Figure 13.5
Figure 13.6
Figure 13.7
Figure 13.8
Scheme 13.8
Figure 13.9
Scheme 13.9
Scheme 13.10
Figure 13.10
Figure 13.11
Scheme 13.11
Scheme 13.12
Figure 13.12
Scheme 13.13
Figure 13.13
Scheme 13.14
Figure 13.14
Figure 13.15
Scheme 13.15
Figure 13.16
Scheme 13.16
Scheme 13.17
Figure 13.18
Scheme 13.19
List of Tables
Table 3.1
Table 12.1
Table 12.2
Table 12.3
Table 13.1
Table 13.2
Table 13.3
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Edited by René Peters
Cooperative Catalysis
Designing Efficient Catalysts for Synthesis
Editor
Prof. Dr. René Peters
Universität Stuttgart
Institut für Organische Chemie
Pfaffenwaldring 55
70569 Stuttgart
Germany
www.peters.oc.uni-stuttgart.de
Cover
The title picture was designed based on an idea commonly developed by Prof. René Peters and his (former) Ph.D. ox{students} Melanie Mechler, Carmen Schrapel, Dr. Manuel Weber and Marcel Weiss.
All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Library of Congress Card No.: applied for
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A catalogue record for this book is ox{available} from the British Library.
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The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
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Preface
The field of asymmetric catalysis has witnessed an amazing progress during the last decades. Even so, technical scale applications are still largely limited to few catalytic asymmetric reaction types [1]. From a technical point of view the large majority of traditional catalytic asymmetric methodologies is not proficient enough in terms of various fundamental aspects such as catalytic activity, substrate scope, selectivity, and cost efficiency.
In order to develop asymmetric catalysts of considerably improved activity, selectivity, and general applicability, the research field of cooperative catalysis is currently intensively studied by a large number of research groups worldwide, following the seminal marks of pioneers in that field like E. J. Corey, Eric Jacobsen, Ryoji Noyori, Masakatsu Shibasaki, or Hisashi Yamamoto to mention just a few. Their research strategy has mimicked the catalytical principles used by Nature to design artificial tailor-made catalysts: like Nature's catalysts – enzymes – these artificial catalyst systems make use of the synergistic and often sophisticated interplay of two or more functional groups. By simultaneous activation of the reactants using different catalyst functional groups cooperative catalysts can decrease the energy of the transition states of the rate-limiting steps to a much greater degree compared to either functional group working independently. Cooperative catalysts can thus notably accelerate and precisely control a chemical reaction, at the same time reducing the amount of side products and accordingly the production of waste. Dual/multiple activation catalysts consequently very often accomplish higher efficiencies than conventional monofunctional catalysts in terms of reactivity, substrate scope, regio-, diastereo- or enantioselectivity and potentially also cost-efficiency. Cooperative catalysis is arguably the most promising strategy to realize high reactivity and selectivity in chemical transformations. It thus appears likely that the different strategies of cooperative catalysis will streamline organic synthesis in general and will in the future also enable a growing number of technical scale applications for catalytic asymmetric C–C, C–N and C–O bond formations. Cooperative catalysis is hence expected to significantly strengthen asymmetric catalysis as a key technology for our society.
Like mentioned, cooperative catalysis makes use of two or even more functional groups present in a catalytic system, which simultaneously work in concert to accelerate and control a chemical reaction. In the definition utilized in most chapters of this book these activating functional groups might be part of the same bi- or multifunctional catalyst entity or of two or more separate (co)catalyst molecules. This implicates that terms like bi(multi)functional catalysis, dual (multiple) activation catalysis, contemporaneous dual catalysis, synergistic catalysis or catalyzed catalysis are all covered by the general title of this book – ‘Cooperative Catalysis’. Examples for cascade catalysis will thus usually (with some exceptions where suitable) not be presented, because in cascade catalysis the different activating catalyst functionalities do not collectively team up in a way that they decrease the energy of the same transition state by their simultaneous action. An exception has, e.g., been made for Chapter 11 , in which the intriguing cooperation of enzymes and metal(–complexe)s is described, albeit both catalysts do not activate the substrates simultaneously.
The present book is considered to provide an overview of the most intensively studied concepts of cooperative catalysis, their historical development, their mode of operation and important applications. Advantages of these concepts, and sometimes also pitfalls that need to be overcome in the future, are described and illustrated. A central but not limiting aspect of this book is asymmetric catalysis. The book is subdivided in 13 chapters – each one written by scientific experts in the corresponding field – and classified by the types of the activating principles. It needs to be mentioned though that the transition between different concepts is often floating. For example, the areas of bimetallic catalysis and Lewis acid/Brønsted base catalysis are to a certain degree related concepts and it sometimes depends on your standpoint which classification might be preferred. To avoid a large overlap, this book thus contains a chapter about bimetallic catalysis with carbophilic Lewis acids, but there is no additional chapter for aza- or oxophilic bimetallic catalysts, as the arguably most important systems are already discussed in the chapter about Lewis acid/Brønsted base catalysis. In addition, as theoretically almost every traditional catalytic activation principle may be combined with another one in a cooperative sense, a huge variability appears to be possible. For that reason the title of some chapters specifies only one of the activating principles.
Summing up the most important – often complementary – concepts of cooperative catalysis in one book is expected to support the further development of this important field by both sharpening and extending our perception. It is not very risky to predict that the future of catalysis will be cooperative! Emil Fischer described a related vision already more than 100 years ago, when he stated: If we wish to catch up with Nature, we shall use the same methods as she does, and I can foresee a time in which physiological chemistry will not only make greater use of natural enzymes but will actually resort to creating new synthetic ones [2].
René Peters Universität Stuttgart, 2014
References
1. H. U. Blaser, E. Schmidt,
Asymmetric Catalysis on Industrial Scale
, Wiley-VCH, 2004.
2. E. Fischer:
Synthesen in der Purin- und Zuckergruppe
In
Les Prix Nobel en 1902
(ed. P. T. Cleve, C.-B. Hasselberg, K.-A.-H. Morner), P.-A. Norstedt & Fils, 1905.
Acknowledgments
René Peters sincerely thanks all authors of this book for their valuable contributions. Moreover, he is very grateful to Dr. Anne Brennführer of Wiley-VCH for her suggestion to edit a book about cooperative catalysis and for her excellent support during its preparation. In addition, the editor is indebted to Dr. Waltraud Wüst of Wiley-VCH for her very valued help during the whole editing process. René Peters also gratefully acknowledges the generous financial funding of his research on cooperative catalysis by the Deutsche Forschungsgemeinschaft (DFG, PE 818/3-1, PE 818/4-1, PE 818/6-1). He warmly thanks his former and present coworkers for their high commitment and enthusiasm.
List of Contributors
Li Deng
Brandeis University
Department of Chemistry
Waltham, MA 02454-9110
USA
Yongming Deng
Miami University
Department of Chemistry and Biochemistry
Oxford, OH 45056
USA
Liu-Zhu Gong
University of Science and Technology of China
Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemistry
Hefei 230026
China
Katharina Gratzer
Johannes Kepler University
Institute of Organic Chemistry
Altenbergerstrasse 69
Linz
Austria
Harald Gröger
Bielefeld University
Faculty of Chemistry
Universitätsstr 25
Bielefeld
Germany
Hansjörg Grützmacher
ETH Zürich
Laboratorium für Anorganische Chemie
Vladimir-Prelog-Weg 1
Zürich
Switzerland
Yu-Ping He
University of Science and Technology of China
Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemistry
Hefei 230026
China
Alexander Katz
University of California
Department of Chemical and Biomolecular Engineering
Berkeley, CA 94720
USA
Naoya Kumagai
Microbial Chemistry Research Foundation
Institute of Microbial Chemistry
Laboratory of Synthetic Organic Chemistry
3-14-23, Kamioosaki
Shinagawa-ku
Tokyo 141-0021
Japan
Xiaojie Lu
Brandeis University
Department of Chemistry
Waltham, MA 02454-9110
USA
Fabrizio Mancin
University of Padova
Department of Chemical Sciences
via Marzolo 1
Padova
Italy
Christina Moberg
KTH Royal Institute of Technology
Department of Chemistry
Organic Chemistry
Stockholm
Sweden
Michael M. Nigra
University of California
Department of Chemical and Biomolecular Engineering
Berkeley, CA 94720
USA
Johanna Novacek
Johannes Kepler University
Institute of Organic Chemistry
Altenbergerstrasse 69
Linz
Austria
Jan Paradies
University of Paderborn
Institute for Organic Chemistry
Warburger Strasse 100
Paderborn
Germany
René Peters
Universität Stuttgart
Institut für Organische Chemie
Pfaffenwaldring 55
Stuttgart
Germany
Leonard J. Prins
University of Padova
Department of Chemical Sciences
via Marzolo 1
Padova
Italy
Bernhard Rieger
Wacker Chair of Macromolecular Chemistry
Department of Chemistry
Technische Universität München
Lichtenbergstr 4
Garching b. München
Germany
Paolo Scrimin
University of Padova
Department of Chemical Sciences
via Marzolo 1
Padova
Italy
Masakatsu Shibasaki
Microbial Chemistry Research Foundation
Institute of Microbial Chemistry
Laboratory of Synthetic Organic Chemistry
3-14-23, Kamioosaki
Shinagawa-ku
Tokyo 141-0021
Japan
Mónica Trincado
ETH Zürich
Laboratorium für Anorganische Chemie
Vladimir-Prelog-Weg 1
Zürich
Switzerland
Sergei Vagin
Wacker Chair of Macromolecular Chemistry
Department of Chemistry
Technische Universität München
Lichtenbergstr 4
Garching b. München
Germany
Hong Wang
Miami University
Department of Chemistry and Biochemistry
Oxford, OH 45056
USA
Mario Waser
Johannes Kepler University
Institute of Organic Chemistry
Altenbergerstrasse 69
Linz
Austria
Marcel Weiss
Universität Stuttgart
Institut für Organische Chemie
Pfaffenwaldring 55
Stuttgart
Germany
Malte Winnacker
Wacker Chair of Macromolecular Chemistry
Department of Chemistry
Technische Universität München
Lichtenbergstr 4
Garching b. München
Germany
Hua Wu
University of Science and Technology of China
Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemistry
Hefei 230026
China
Chapter 1Lewis Acid–Brønsted Base Catalysis
Masakatsu Shibasaki and Naoya Kumagai
1.1 Introduction
From the synthetic point of view, organic synthesis via catalytic processes offers many benefits. Catalysis frequently obviates the excessive use of the activating reagents and associated tedious purification processes, thereby offering more environmentally benign synthetic processes. Furthermore, the specific activation mode of a catalyst allows for highly chemoselective transformations that are seldom achieved by noncatalytic processes. Over the past two decades, the concept of cooperative catalysts has evolved and subsequently rapidly advanced as the most finely refined class of artificial catalysts for preparative chemistry [1]. The cooperative catalysts exhibit two catalytic functions simultaneously to achieve a dual activation mode to specific substrate(s) (Figure 1.1). The obvious advantage of this activation strategy is not only the significant enhancement of the reaction rate due to intramolecularity or a proximity effect but also the broadened scope of the applicable reactions following the synergistic activation of otherwise unreactive substrate sets.
Figure 1.1 Schematic representation of the Lewis acid–Brønsted base cooperative catalysts.
In this chapter, cooperative catalysts that exhibit Lewis acid and Brønsted base activation modes are reviewed. While recent interest in artificial catalysts focuses on the efficient production of enantioenriched building blocks [2], herein only asymmetric Lewis acid–Brønsted base cooperative catalysts are covered. Metal-based asymmetric cooperative catalysts that display transition-metal catalysis are described in other chapters [3]. In this chapter, the focus is on the reactions promoted by the effective coupling of an in situ generated active nucleophile by a Brønsted base and an electrophile activated by a Lewis acid.
1.2 Lewis Acid–Brønsted Base Catalysis in Metalloenzymes
The essence of Lewis acid–Brønsted base catalysis is the manifestation of two different catalytic functions in a synergistic manner. This often occurs via two different catalytic sites in near proximity – referred to as two-center catalysis. Two-center catalysis involving a Lewis acid and a Brønsted base is largely exploited in metalloenzyme reactions [4, 5]. A typical biological degradation reaction, such as urea hydrolysis promoted by urease, utilizes dinickel two-center cooperative catalysis (Figure 1.2) [4b, 6]. Two Ni(II) cations are located in near proximity at the active site of urease, and one Ni(II) cation is coordinated by urea to electrophilically activate the urea carbonyl. Another Ni(II) cation (Ni hydroxide) functions as a Brønsted base with the aid of the adjacent histidine side chain to produce a nucleophilically active Ni hydroxide. The synergistic activation of both the nucleophile and electrophile provides significantly accelerated hydrolysis. Urea generally does not readily undergo simple basic hydrolysis in organic synthesis, but with the cooperative catalysis of a dinickel active site the reaction rate is enhanced by a factor of 1014. An artificial model of this cooperative hydrolysis has been achieved with a dicopper catalyst comprising a low molecular weight ligand and Cu(II) cations [7].
Figure 1.2 Proposed activation mode in urease.
This type of Lewis acid–Brønsted base cooperative catalysis is operative also in enantioselective carbon–carbon bond-forming processes in biological contexts. Class II aldolase, a Zn-dependent metalloenzyme, illustrates this (Figure 1.3). The aldolase efficiently promotes the enantioselective aldol reaction of dihydroxyacetone phosphate (DHAP) and various aldehydes under virtually neutral conditions [8]. DHAP coordinates to a Zn(II) cation in a bidentate manner to increase the acidity of the α-proton, which is deprotonated by the adjacent glutamic acid-73 residue as a Brønsted base. This cooperation enables the catalytic generation of an active Zn-enolate, which is integrated into the following aldol addition to an aldehyde that is activated by the tyrosine-113 residue by hydrogen bonding. These naturally occurring macromolecular catalytic machineries have inspired chemists to mimic the cooperative activation strategy in artificial catalyst design.
Figure 1.3 Proposed activation mode in Zn-dependent class II aldolase.
Obviously, an inevitable drawback in enzymatic catalysis is its strict substrate specificity at the expense of extraordinary rate enhancement. Artificial cooperative catalysts follow a somewhat loose three-dimensional design of two catalytic functions to acquire both rate enhancement through synergistic activation and sufficient substrate generality to showcase the synthetic utility.
1.3 Hard Lewis Acid–Brønsted Base Cooperative Catalysis
1.3.1 Cooperative Catalysts Based on a 1,1′-Binaphthol Ligand Platform
1.3.1.1 Heterobimetallic Catalysts
A series of hard Lewis acid–Brønsted base cooperative heterobimetallic catalysts utilizing 1,1′-binaphthol and its derivatives as a chiral bidentate ligand were developed by Shibasaki et al. [9] (Figure 1.4). Depending on the nature of the central metal cation [rare earth metal (RE) or group 13 metal (M(13))], two general types of cooperative catalysts are generated [10]. By combining RE and alkali metals (M(1)), heterobimetallic catalysts of the general formula RE-M3-tris(1,1′-binaphthoxide) (type 1) are formed. Following the initial identification of La-Li3-tris(1,1′-binaphthoxide) (RE = La, M(1) = Li, abbreviated as LLB) in the first report on the catalytic asymmetric nitroaldol reaction [10a–12] (Scheme 1.1), several heterobimetallic catalysts emerged by changing the combination of RE (Y, La, Pr, Sm, Yb) and M (Li, Na, K) to promote a wide range of catalytic asymmetric transformations (Figure 1.5) [13–26].1 Irrespective of the combination, a highly symmetrical architecture of RE-M3-tris(1,1′-binaphthoxide) is maintained (based on laser desorption/ionization time-of-flight mass spectrometry data). Some of the heterobimetallic catalysts, such as LSB (RE = La, M(1) = Na), PrSB (RE = Pr, M(1) = Na), NdSB (RE = Nd, M(1) = Na), and EuSB (RE = Eu, M(1) = Na), were unequivocally characterized by X-ray crystallographic analysis [10b, 13, 27].
Figure 1.4 Two types of Lewis acid–Brønsted base cooperative heterobimetallic catalysts based on 1,1′-binaphthol and its derivatives as a chiral ligand platform.
Scheme 1.1Seminal nitroaldol reaction promoted by the heterbimetallic catalyst LLB.
Figure 1.5Schematic representation of the utility of RE-M(1)3–tris(1,1′-binaphthoxide) cooperative catalysts in catalytic asymmetric transformations.
Although these complexes have a chiral center at the central RE, a 1,1′-binaphthol unit existed only in the Λ configuration, presumably because of the higher thermodynamic stability. Biphenyldiols were also exploited to constitute similar catalyst architecture for some reactions. The essence of this catalytic system is the cooperative function of RE as the Lewis acid to activate electrophiles and M(1)-1,1′-binaphthoxide as the Brønsted base to activate pronucleophiles, allowing for the subsequent facilitated bond formation in the chiral environment. The coordination number of RE generally ranges from 6 to 12 [28]. Hence, the central RE of these complexes is not coordinatively saturated, and it is anticipated that it accepts the additional coordination of electrophiles. Coordination to the RE center of these complexes has been of interest [29], and direct evidence to prove the coordination of Lewis basic electrophiles to RE has been reported by Walsh et al. in a series of NMR and crystallographic studies [30]. Differences in RE–M(1) combinations lead to a series of complexes with slightly different metal–oxygen bond lengths, covering a broad range of catalytic asymmetric transformations (Figure 1.5). La is most frequently identified as the best RE, presumably because La has the largest ionic radius and is prone to functioning more as a Lewis acid to activate electrophiles. The exceptionally wide variety of reactions presented in Figure 1.5 is indicative that these heterobimetallic cooperative catalysts are one of the most successful classes of asymmetric catalysts known. A reaction mechanism based on Lewis acid–Lewis acid cooperative catalysis in which M(1)serves as a Lewis acid has also been proposed for the aza-Michael reaction, Corey–Chaykovsky epoxidation, and cyclopropanation [21, 25].
It is worth highlighting the direct aldol reaction with LLB (RE = La, M(1) = Li) because this specific reaction was the first to be demonstrated by this heterobimetallic cooperative catalyst and because of the sustained topic in the field of Lewis acid–Brønsted base cooperative catalysis. In 1997, Shibasaki et al. reported the first example of the direct aldol reaction, in which nucleophilically active enolate species were generated in situ and the thus-formed enolate was integrated into the following aldol addition in an enantioselective manner [20]. At that time, a commonly accepted catalytic asymmetric aldol reaction was the chiral Lewis acid-catalyzed Mukaiyama aldol reaction in which a preformed (preactivated) enol silyl ether was used as an active enolate [31]. The obvious advantage of the direct aldol strategy is the elimination of the redundant preactivation step in a separate operation, thereby offering a more operationally simple protocol without the undesired waste derived from stoichiometric amounts of reagents used for preactivation [32]. Cooperative functions of the Lewis acid and Brønsted base of LLB are crucial to electrophilically activate aldehydes 1 while generating the active enolate from ketones 2 in a catalytic manner, enabling the smooth enantioselective aldol reaction in an asymmetric environment of 1,1-binaphthyl walls [20a]. LLB modified by KOH was later found to exhibit superior catalytic activity to afford the aldol adducts 3 in moderate to high enantioselectivity (Scheme 1.2a) [20b]. In contrast to the requisite excess amount of ketones 2 to drive the reaction efficiently, the α-hydroxyketones 4 emerged as particularly suitable substrates, and high conversions were obtained with 2 equiv of 4 to afford anti products preferentially with high enantioselectivity (Scheme 1.2b) [20c]. These early works stimulated research into the direct aldol reaction. Today, the term “direct aldol” is widely accepted in the chemical community, and a number of achievements have been reported in both metal-based catalysis and organocatalysis [33]. The heterobimetallic catalysts of RE-M(1)3-tris(1,1′-binaphthoxide) architecture continue to be a topic of interest in catalysis and in the construction of metal complexes. Further explorations using Ce(III)/Ce(IV) or an actinide, for example, U(IV), as a central metal [34, 35], and Cs or Zn as peripheral metals, have been reported [36].
Scheme 1.2 (a, b) Direct catalytic asymmetric aldol reaction of unmodified ketones promoted by (S)-LLB·KOH cooperative catalyst.
Another type of heterobimetallic catalysts with the general formula M(13)-M(1)-bis(1,1′-binaphthoxide) incorporating group 13 metals (Al, Ga) has been investigated by Shibasaki et al. [9] (Figure 1.4, type 2 (right side)). In 1996, Al-Li-bis(1,1′-binaphthoxide) (M(13) = Al, M(1) = Li, abbreviated ALB) was designed on the basis of the concept of Lewis acid and Brønsted base catalysis, assuming that Al(III) and Li-phenoxide synergistically function as the Lewis acid and Brønsted base, respectively [37, 38]. The architecture bearing a tetracoordinated Al(III) and pendant Li cation was unequivocally determined by X-ray crystallographic analysis. (R)-ALB was identified as a particularly effective catalyst for the asymmetric conjugate addition of malonates to cyclic enones, in which a cyclic enone is activated by Al and an active carbanion is generated by Li-phenoxide in close proximity (Figure 1.6). The addition of an achiral alkali metal alkoxide significantly enhanced the catalytic efficiency [11g, 39], allowing for the completion of the reaction with as little as 0.1 mol% of catalyst on a >1 kg scale [11i, 40, 41]. The use of polymeric 1,1′-binaphthol led to the development of immobilized ALB catalyst, which could be used iteratively [42]. The scope of M(13)-M(1)-bis(1,1′-binaphthoxide) catalyst was expanded to the analogous cooperative catalyst (R)-GaLB, where M(13) and M(1) are Ga and Li, respectively, promoting the ring-opening reaction of meso-epoxide by tert-butylthiol [43, 44].
Figure 1.6 Schematic representation of the utility of M(13)-M(1)–(1,1′-binaphthoxide) cooperative catalysts in catalytic asymmetric transformations.
1.3.1.2 Cooperative Catalysts Based on Linked-BINOL
Connecting two 1,1′-binaphthol units at the 3 position provides an intriguing tetraol chiral ligand referred to as linked-BINOL [45–47] (Figure 1.7). Originally, this ligand was specifically designed to prevent the formation of an undesirable polymeric complex via intermolecular infinite coordination of 1,1-binaphthols to metal cations. Indeed, complexation of (S,S)-O-linked-BINOL with Ga(OiPr)3 and nBuLi afforded a monomeric Ga-Li-linked-BINOL complex, which showed higher stability than the corresponding complex GaLB prepared from the 1,1′-binaphthol complex [45a]. X-ray crystallographic analysis of Ga-Li-linked-BINOL revealed that the architecture was analogous to that of M(13)-M(1)-bis(1,1′-binaphthoxide). Similar cooperative catalytic function is anticipated, as demonstrated by high catalytic performance in the asymmetric ring opening of meso-epoxides with p-methoxyphenols. The combination of O-linked-BINOL and La(OiPr)3 afforded monometallic La-O-linked-BINOL, which is an air-stable and storable catalyst, effective for the asymmetric conjugate addition of malonates [48, 49]. The dual function of La(III) as a Lewis acid and La-phenoxide as a Brønsted base has been proposed [50]. The related La-NMe-linked-BINOL exhibited superior catalytic efficiency in the asymmetric conjugate addition of β-keto esters [51]. Interestingly, treatment of O-linked-BINOL with Et2Zn afforded a trinuclear Zn complex as precatalyst. The latter emerged as particularly effective for the catalytic generation of Zn-enolate from 2′-methoxy α-hydroxyacetophenone 6. The catalytic asymmetric aldol reaction of 6 with (S,S)-O-linked-BINOL/Et2Zn catalyst afforded the syn adduct 7 preferentially (Scheme 1.3) [20c, 52], which is complementary to the anti-selective reaction promoted by (S)-LLB·KOH (Scheme 1.2b). Cold-spray ionization mass spectroscopy analysis indicated the formation of Zn-rich species containing 6. Zn-phenoxide functions as a Brønsted base to generate Zn-enolate, while an aldehyde is electrophilically activated by a neighboring Lewis acidic Zn(II). A similar activation mode is operative in catalytic asymmetric direct conjugate addition [53] and Mannich-type reactions of 6 [54]. The combination with Y[N(TMS)2]3 or In(OiPr)3 expanded the scope of the direct Mannich-type reaction [55, 56]. The cooperative use of nucleophilic tertiary amines in a La-O-linked-BINOL system was also explored [57].
Figure 1.7Schematic representation of the utility of linked-BINOL-based cooperative catalysts in catalytic asymmetric transformations.
Scheme 1.3Direct catalytic asymmetric aldol reaction of unmodified α-hydroxy ketone 6 promoted by Et2Zn/(S,S)-O-linked-BINOL cooperative catalyst.
1.3.2 Cooperative Catalysts Based on a Salen and Schiff Base Ligand Platform
Metal–salen and related metal–Schiff base complexes are commonly accepted as one of the most successful classes of organometallic entities. In this regard, these complexes have been utilized in several fields of chemistry other than asymmetric catalysis. The focus of this section is on the use of these complexes in the context of Lewis acid–Brønsted base cooperative asymmetric catalysts [58, 59]. Other applications of these complexes are beyond the scope of this section [60].
Meticulous mechanistic studies conducted by Jacobsen et al. revealed a second-order dependence of the reaction rate on the concentration of the catalyst in a series of mononuclear metal–salen complexes [61]. Specifically, the cooperative activation of a nucleophile (Cr-azide) and an electrophile (epoxide) is postulated in the catalytic asymmetric ring-opening reaction of epoxides with azide promoted by two molecules of monomeric Cr(N3)–salen complexes (Figure 1.8, compare with Chapter 13 in this book on cooperative catalysis in polymerization reactions). These kinetic data are in accordance with the bimolecular cooperative catalysis in an intermolecular manner [62], and the strategic linking of two metal–salen units has been systematically explored to render the cooperative catalysis intramolecularly. Systematic investigation of the position and the length of linker revealed that a dimeric Cr(N3)–salen complex 8 tethered by a pimelate diester linkage (n = 5) at the aromatic group produced the highest catalytic efficiency, accelerating the reaction by two orders of magnitude compared with a monomeric Cr(N3)–salen catalyst (Figure 1.9) [63]. The beneficial effect of the covalently linked dimeric Al(Cl)–salen complexes is also observed in the catalytic asymmetric conjugate addition of in situ generated HCN to α,β-unsaturated imides [64], in which the second-order rate dependence on the catalyst was observed in a monomeric Al(Cl)–salen complex [61f, 65, 66].
Figure 1.8 Postulated bimolecular bimetallic mechanism in ring-opening reaction of epoxides promoted by two monometallic Cr(N3)–salen complexes.
Figure 1.9 Covalently linked dimeric metal–salen complexes.
Among the catalytic asymmetric reactions promoted by metal–salen complexes, the Co–salen-catalyzed hydrolytic kinetic resolution of racemic epoxides is of prime importance from a synthetic standpoint (Scheme 1.4) [61c,d,e]. From the mechanistic point of view, one Co(OAc)–salen complex functions as a Lewis acid to activate epoxides and another molecule of Co(OAc)–salen complex functions as a Brønsted base to nucleophilically activate H2O (or alcohols or phenols), constituting a perfect example to elicit the power of Lewis acid–Brønsted base cooperative catalysis. Because of the broad synthetic utility of the reaction, a number of strategies have been developed to covalently or noncovalently link the monomeric Co–salen complexes to enhance the catalytic efficiency (Figure 1.10) [67]. Jacobsen et al. reported that the dimeric Co(OAc)–salen complex 8 tethered by suberic ester (n = 6) outperformed the monomeric complex in the intramolecular cyclization of epoxyalcohol [68]. The same research group developed the highly active dendrimeric catalyst 9 [69] and oligomeric catalyst 10 [70] which contain multiple Co–salen units in one molecule. In particular, 10 (X = nbs (3-nitrobenzenesulfonate), Y = CH2, Z = H, n = 1–3) promoted the hydrolytic kinetic resolution of racemic propylene oxide with as little as 0.0004 mol% catalyst loading.
Scheme 1.4 (a) Hydrolytic kinetic resolution of terminal epoxides and (b) hydrolytic desymmetrization of meso-epoxides.
Figure 1.10 Various strategies for the construction of multimetallic Co–salen complexes.
Alcohols and phenols can be used as nucleophiles, and generally excellent yields and enantioselectivity are achieved. The beneficial effect of oligomeric catalyst 10 (X = OTf, Y = O, Z = H, n = 1–4) over the corresponding monomeric catalyst was also observed in asymmetric intramolecular ring-opening reactions of oxetanes [71]. Coates et al. developed a well-designed binaphthyl-embedded dimeric Co–salen catalyst 11 for the asymmetric polymerization of racemic terminal epoxides [72]. Homochiral catalyst afforded highly isotactic (99%) polyethers, and unreacted epoxides were obtained in high enantiopurity (compare with Chapter 13 in this book on cooperative catalysis in polymerization reactions). Remarkably, treatment of the racemic epoxides with the racemic catalyst allowed the polymerization to proceed in a highly isoselective manner to afford both the S and R purely isotactic polymers. Wezenberg and Kleij [73] utilized a calix[4]arene scaffold for the construction of a dimeric Co–salen complex 12, which was applied to the hydrolytic kinetic resolution of terminal epoxides. Kinetic analysis showed an intramolecular cooperative pathway. The catalyst had greater stability than the monomeric complex, but the overall catalytic efficiency was not significantly enhanced. Noncovalent interactions also have been exploited to construct multimetallic Co–salen complexes [74]. Co–salen complexes 13 immobilized on gold colloids, developed by Belser and Jacobsen [75], enabled repetitive use in the kinetic resolution of racemic epoxides without any loss of reactivity and enantioselectivity. The cooperativity is operative in the catalyst on gold colloids, and significant rate enhancement was observed. Hong et al. reported an intriguing dimeric Co–salen catalyst assembled through hydrogen bonding. A monomeric Co–salen unit armed with pendant urea functionalities formed a homodimeric supramolecular complex 14 [76]. Complex 14 (X = OTs, Ar = 4-CF3C6H4) exhibited significant rate enhancement in kinetic resolution of racemic epoxides with as little as 0.03 mol% catalyst loading under solvent-free conditions. The related Co–salen complex was also utilized in anti-selective catalytic asymmetric nitroaldol (Henry) reactions, in which the Lewis acid–Brønsted base cooperative function of two Co–salen units to activate both aldehydes and nitroalkane was crucial [77, 78]. With the combined use of 14 [X = O2CC6H3-3,5-(CF3)2, Ar = 3,5-(CF3)2C6H3] and a substoichiometric amount of N-ethylpiperidine, the anti-nitroaldol adduct was obtained in high enantioselectivity (Scheme 1.5).
Scheme 1.5 anti-Selective catalytic asymmetric nitroaldol reaction promoted by dimeric Co–salen complex 14.
The introduction of multiple metal cations in a Schiff base scaffold is an attractive strategy for devising a cooperative catalyst. Kozlowski et al. designed a dinucleating Schiff base ligand bearing two 1,1′-binaphthol units and four phenolic hydroxyl groups (Scheme 1.6) [79]. Formation of the heterobimetallic catalyst 15 comprising the Schiff base ligand Ni(II) and two Cs cations was confirmed by X-ray crystallographic analysis. It promoted the asymmetric conjugate addition of dibenzyl malonate to cyclic enones. Zhu et al. reported the heterobimetallic Ti-Ga–salen complex 16 prepared from the parent salen ligand, GaMe3, and Ti(OiPr)4 (Scheme 1.7) [80]. The order of metal addition (GaMe3 first) was crucial for the heterobimetallic complex 16, which was successfully utilized for the asymmetric ring-opening reaction of meso-epoxides with thiols and selenols.
Scheme 1.6 Catalytic asymmetric conjugate addition of dibenzyl malonate to cyclic enones promoted by Ni-Cs–Schiff base heterobimetallic catalyst 15.
Scheme 1.7 Catalytic asymmetric ring-opening reaction of meso-epoxides with thiols and selenols promoted by Ti-Ga–salen heterobimetallic catalyst 16.
Shibasaki and Matsunaga et al. developed a series of hetero- and homobimetallic cooperative catalysts utilizing a dinucleating chiral Schiff base scaffold 17(Figure 1.11) [59c, 81]. Introduction of additional phenolic hydroxyl groups on the aromatic ring of the parent salen ligand provides additional sites for metal coordination. On the basis of the coordination chemistry of this class of ligands [82], the N2O2 inner cavity is expected to preferentially incorporate a transition metal, and an oxophilic RE having a larger ionic radius is located in the O2O2 outer cavity. As a chiral diamine unit to link two coordinating aromatic groups, rigid cyclic diamines 18 and 20, or axially chiral diamines 19 and 21, were selected. Cu-Sm–17 [M1 = Cu, M2 = Sm(OAr)] initially emerged as a highly effective Lewis acid–Brønsted base cooperative catalyst in the syn-selective asymmetric aza-nitroaldol (aza-Henry) reaction, in which N-Boc imine and nitroalkane were synergistically activated by Cu(II) and Sm-phenoxide, respectively (Figure 1.11a) [83]. Simple catalyst tuning by changing the incorporated metals from Cu-Sm to Pd-La permitted the use of aldehydes as electrophiles, leading to an anti-selective asymmetric nitroaldol (Henry) reaction (Figure 1.11b) [84]. Although the combination of transition metals and REs with chiral Schiff base 17 produced a variety of cooperative catalysts [85–87], the generation of uniform catalytically active species by precisely placing two different metals into two distinct cavities is not a simple task.
Figure 1.11 Schematic representation of the utility of bimetallic complexes of Schiff base 17 in catalytic asymmetric transformations.
To further expand the utility of 17 as a platform for bimetallic catalysts, incorporation of two identical metal cations was investigated. A homodinuclear Ni2–17 catalyst (M1 = M2 = Ni), prepared from 17 bearing 1,1′-binaphthyldiamine 19 as chiral diamine and 2 equiv of Ni(OAc)2·4H2O, was a bench-stable powder and identified as a particularly useful catalyst for a wide range of asymmetric reactions, for example, Mannich reactions [88], conjugate addition reactions [89], amination [90], desymmetrization of meso-anhydrides [91], and aldol-type reaction of isothiocyanato oxindols (Figure 1.11e, h–j, m–p) [92]. Replacing the diamine unit from 19 to 21 proved beneficial and improved the stereoselectivity in the Mannich reaction of 1,2-dicarbonyl compounds (Figure 1.11k) [93]. The corresponding Co and Mn homobimetallic catalysts Co2–17 [M1 = M2 = Co(OAc)] and Mn2–17 [M1 = M2 = Mn(OAc)] were also readily prepared from metal acetates. They found their utility in asymmetric conjugate addition reactions [94] (Figure 1.11f, g, l). Productive interplay of Lewis acid function and Brønsted base function deployed in the designed bimetallic catalysts culminated in remarkably broad utility, and these catalytic reactions have been successfully applied to the enantioselective synthesis of natural products [95].
1.3.3 Cooperative Catalysts Based on a Ligand Platform Derived from Amino Acids
α-Amino acids are some of the most abundant homochiral materials available in nature. In 2000, Trost et al. developed a ProPhenol ligand platform (S,S)-22 derived from L-proline, a pentadentate C2-symmetric ligand bearing two hydroxyl groups, one phenol, and two pyrrolidyl groups (Figure 1.12) [96]. Compound 22 can incorporate two different metal cations to synergistically offer a hard Lewis acid function and a Brønsted base function in an asymmetric environment. The utility of 22 was initially found with its dinuclear Zn complex 23 (M1 = M2 = Zn, X = Et, Ar1 = Ph, R1 = Me, R2 = H) in the direct catalytic asymmetric aldol reaction of aromatic methyl ketones (Figure 1.12a), which was conveniently prepared with 2 equiv of Et2Zn. Quantitative analysis of ethane evolution indicated that one ethyl–Zn bond remained. This dinuclear Zn complex 23 was a remarkably effective catalyst in direct asymmetric aldol reactions when using various aldol donors such as acetone [97], α-hydroxyl ketones [98], acetylenic ketones [99], or vinyl ketones (Figure 1.12b–e) [100]. Nitromethane also served as a suitable pronucleophile to enable an asymmetric nitroaldol reaction [101]. As mentioned above, the direct aldol reaction is the representative example in Lewis acid–Brønsted base cooperative catalysis. Zn alkoxide of 23 functioned as a Brønsted base to generate Zn-enolate, which was coupled with an aldehyde activated by another Zn cation that functioned as a Lewis acid (Scheme 1.8). A similar type of proline-incorporated chiral ligand 24 was reported by Da et al., which afforded the corresponding dinuclear Zn complex to promote the direct aldol reaction, but with inferior catalytic efficiency (Scheme 1.9) [102]. The bimetallic catalytic system of complex 23 was particularly suitable for generating a nucleophilically active Zn-enolate from α-hydroxy ketones, allowing for the direct Mannich reaction [103] and the conjugate additions (Figure 1.12g–j) [104]. For the conjugate addition of α-hydroxy ketones to nitroolefins, a heterobimetallic complex 23 (M1 = Zn, M2 = Mg, X = nBu, Ar1 = Ph, R1 = H, R2 = Me) prepared from equimolar amounts of Et2Zn and nBu2Mg gave the best performance. In contrast to the broad utility of dinuclear Zn complex 23 in the direct aldol reaction, the corresponding dinuclear Mg complex 23 (M1 = M2 = Mg, X = nBu, Ar1 = Ph, R1 = H, R2 = Me) was the superior catalyst for the direct asymmetric aldol reaction of ethyl diazoacetate (Figure 1.12k) [105]. The diastereoselective transformation of the diazo group in the product highlights the synthetic utility. A bulkier aromatic group at the tertiary alcohol of 22 was beneficial for higher yield and enantioselectivity in the catalytic asymmetric desymmetrization of meso-diols (Figure 1.12l) [106]. Although a stoichiometric amount of Me2Zn was required, the dinuclear Zn complex broadened the scope of its utility in asymmetric alkynylation of aldehydes (Figure 1.12m) [107]. The synthetic utility of a variety of catalytic asymmetric carbon–carbon bond-forming reactions was demonstrated by the enantioselective synthesis of a number of natural products [108].
Figure 1.12 Schematic representation of the utility of bimetallic complexes of ProPhenol 22 in catalytic asymmetric transformations.
Scheme 1.8 Direct catalytic asymmetric aldol reaction of α-hydroxy ketones 6 with dinuclear Zn complex 23.
Scheme 1.9 Direct catalytic asymmetric aldol reaction of aromatic ketones 2 with dinuclear Zn complex prepared from ligand 24.
Shibasaki et al. developed a heterobimetallic catalytic system comprising an amide-based ligand 25 bearing two phenolic hydroxyl groups [109, 110], RE, and alkali metal (Figure 1.13). A Nd/Na heterobimetallic catalyst was designed on the basis of a transition state analysis for an anti-selective nitroaldol (Henry) reaction [78] in which Na-phenoxide functioned as a Brønsted base to generate nitronate, and the Nd cation functioned as a Lewis acid to electrophilically activate an aldehyde (Scheme 1.10) [111]. Intriguingly, this catalyst functioned as a heterogeneous catalyst in tetrahydrofuran (THF). It was readily prepared by mixing 25 (X = H, Y = OH, Z = F, R = iBu), NdO1/5(OiPr)13/5, and NaHMDS through self-assembly. Its unique nature was exploited to produce a recyclable Nd/Na catalyst confined in a multiwalled carbon nanotube with enhanced catalytic efficiency [112]. The nitroaldol reaction offers a rapid access to enantioenriched vicinal amino alcohols, and the synthetic utility of the catalyst system culminated in the enantioselective synthesis of zanamivir (Relenza), a clinically used anti-influenza drug [113]. A similar catalytic system with Yb/K cations permits the use of N-Boc imines to promote the nitro-Mannich (aza-Henry) reaction [114]. The combination of La and Ag cations with amide-based ligand 25 (X = OH, Y = H, Z = H, R = iPr or tBu) emerged as an effective catalyst for the asymmetric Conia-ene reaction [115].
Figure 1.13 Schematic representation of the utility of the heterobimetallic catalysts derived from amide-based ligand 25 in catalytic asymmetric transformations.
Scheme 1.10 anti-Selective catalytic asymmetric nitroaldol reaction promoted by Nd/Na heterobimetallic catalyst using amide-based ligand 25.
1.4 Soft Lewis Acid–Brønsted Base Cooperative Catalysis
Potential pronucleophiles with high pKa values not only mandate the use of a strong Brønsted base, which can trigger undesirable side reactions, but also retard the catalytic turnover through proton transfer, namely, the protonation of an intermediary adduct with a protonated catalyst, to regenerate the active catalyst. Therefore, an ingenious mechanistic trick to overcome the pKa problem is required to broaden the scope of the direct catalytic asymmetric carbon–carbon bond-forming reaction. The exploitation of the specific soft–soft interaction of a soft Lewis acid catalyst and a soft Lewis basic pronucleophile is a particularly effective strategy to reinforce the chemical toolbox for direct catalytic asymmetric carbon–carbon bond-forming reactions, largely using carbon pronucleophiles with low pKa values.
A very early example, reported in 1986 by Ito, Sawamura, and Hayashi, used a Au(I)-based soft Lewis acid–Brønsted base cooperative catalyst 26 (Scheme 1.11) [116, 117]. In the aldol reaction of isocyanoacetate 27 as pronucleophile, ferrocene-based bisphosphine catalyst 26 bearing a Au cation as a soft Lewis acid and a tertiary amine as a Brønsted base generated the nucleophilically active enolate, which was coupled with aldehyde 1 to afford the isoxazole 28. The scope of the reaction was expanded by applying this class of catalysts to functionalized aldehydes and α-substituted isocyanoacetates with structural modification of the catalyst [118]. The thus-obtained functionalized oxazoles offered direct access to α-amino-β-hydroxy carboxylic acid derivatives. This catalytic protocol has been utilized in the enantioselective synthesis of biologically active compounds [119].
Scheme 1.11 anti-Selective catalytic asymmetric aldol reaction of isocyanoacetate 27 promoted by soft Lewis acid–Brønsted base cooperative catalyst 26.
Shibasakiet al. revealed the broad utility of soft Lewis acid–Brønsted base cooperative catalysis by demonstrating a series of catalytic asymmetric transformations (Figure 1.14) [120]. The use of a soft Lewis acid is particularly advantageous to specifically activate soft Lewis basic pronucleophiles in the presence of hard electrophiles. The chemoselective coordination of soft Lewis basic pronucleophiles to a soft Lewis acid significantly enhanced the deprotonative activation of high-pKa pronucleophiles with a mild Brønsted base. Although the deprotonative activation of nitrile-based pronucleophiles is generally difficult because of their high pKa values, cooperative use of soft Lewis acid copper decorated with chiral bisphosphine ligand (R,R)-Ph-BPE and Brønsted base Li-phenoxide allowed for the smooth generation of a nucleophilically active α-cyanocarbanion. With the cooperative catalyst, direct addition of allyl cyanide proceeded efficiently with ketimines and ketones to produce tetrasubstituted stereogenic centers (Figure 1.14a, b) [121].
Figure 1.14 Schematic representation of the utility of the soft Lewis acid–Brønsted base cooperative catalyst in catalytic asymmetric transformations.
This catalysis is also valid for other soft Lewis basic pronucleophiles, such as thioamides 29, leading to the direct aldol [122], Mannich [123], and conjugate additions (Figure 1.14c–e) [124]. Whereas the use of latent enolates in the carboxylic oxidation state has been a long-standing problem because of their reluctant enolization, the exploitation of soft–soft interactions enabled efficient enolization of thioamides, leading to the development of these useful reactions. In particular, the direct catalytic asymmetric aldol reaction of thioamides 29 is an intriguing example demonstrating the high chemoselectivity via soft–soft interactions. Aldehydes are inherently more prone to enolization than thioamides 29, and self-condensation proceeds extensively under simple Brønsted basic conditions (Scheme 1.12). In contrast, with the soft Lewis acid–Brønsted base cooperative catalyst, chemoselective activation of thioamides 29 allowed preferential enolization of thioamides 29 to afford the desired aldol product 30 exclusively.
Scheme 1.12 Direct catalytic asymmetric aldol reaction of thioamide promoted by soft Lewis acid–Brønsted base cooperative catalyst.
The scope of this catalytic system was productively expanded to soft Lewis acidic electrophiles such as α,β-unsaturated thioamides [125–128] and N-thiophosphinoyl ketimines (Figure 1.14f–l) [129–131]. α,β-Unsaturated carboxylic acid derivatives and ketimines are generally poor electrophiles and are rarely used as substrates in asymmetric catalysis. The soft Lewis acid–Brønsted base cooperative catalysts has found significant potential in the promotion of the reactions of these poor electrophiles by installing soft Lewis acid functionalities, enabling the simultaneous activation of pronucleophiles and electrophiles to significantly enhance the reaction. Ag(I) as a soft Lewis acid functioned best in the chemoselective activation of α-sulfanyl lactones to facilitate the efficient enolization, promoting the direct aldol and Mannich reactions (Figure 1.14m, n) [132]. This new avenue within activation modes in asymmetric catalysis has paved the way for the enantioselective reactions of a previously neglected class of substrates. These catalysts have been applied to the enantioselective synthesis of therapeutic agents [133].
1.5 Conclusion
Over the past three decades, the arsenal of chemical tools has been substantially reinforced by the great number of asymmetric catalysts, which has allowed the efficient production of a wide variety of enantioenriched compounds. In view of the need for sustainable chemistry, including environmentally friendly chemical processes, asymmetric catalysis is a key methodology that continues to attract growing attention. An issue that requires particular attention is the overall reaction efficiency – the pursuit of atom economy of reactions to avoid the excessive
