Polymeric Chiral Catalyst Design and Chiral Polymer Synthesis

Shinichi Itsuno

• Publisher: Wiley
• Year: 2011
• Print ISBN:
  9780470568200
• Edition: 1
• eBook ISBN:
  9781118063958
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

This book reviews chiral polymer synthesis and its application to asymmetric catalysis. It features the design and use of polymer-immobilized catalysts and methods for their design and synthesis. Chapters cover peptide-catalyzed and enantioselective synthesis, optically-active polymers, and continuous flow processes. It collects recent advances in an important field of polymer and organic chemistry, with leading researchers explaining applications in academic and industry R & D.

Chapter 1

An Overview of Polymer-Immobilized Chiral Catalysts and Synthetic Chiral Polymers

Shinichi Itsuno

1.1 Introduction

Polymer-immobilized chiral catalysts and reagents have received considerable attention in regard to organic synthesis of optically active compounds [1]. Use of polymer-immobilized catalysts has become an essential technique in the green chemistry process of organic synthesis. They can be easily separated from the reaction mixture and reused many times. It is even possible to apply the polymeric catalysts to the continuous flow system. Not only the practical aspect but also particular microenvironment created in the polymer network has sparked a fascination with their attractive utilization in organic reactions, especially in stereoselective synthesis. In some cases, the polymer-immobilized catalyst accelerates the reaction rate. In other cases, the polymer-immobilized chiral catalyst realizes higher stereoselectivity compared with its low-molecular-weight counterpart. These examples clearly show that the design of the polymeric catalyst is very important for understanding the efficient catalytic process. Chiral polymer synthesis that is directed toward the novel immobilization method of chiral catalysts also should be developed.

Most support materials used for the chiral catalyst have been cross-linked polystyrene derivatives, mainly because of their easy preparation. Various kinds of reactions have been used for the introduction of functional groups into the side chain of the polymer. However, there are so many different types of synthetic polymers, including both organic and inorganic polymers, which may be used as support material. Each polymer would provide a specific microenvironment for the reaction if it was precisely designed. Although the choice of solvent in organic reaction is limited, the choice of polymer network structure may be almost infinite. The most suitable polymer network for each reaction may be easily found.

Although a substantial amount of work has been carried out using side-chain functionalized polymers for the preparation of a polymeric catalyst, only a limited number of investigations have been performed to elucidate the use of main-chain functional polymers. Recently, some main-chain chiral polymers including helical polymers have been successfully applied to a chiral catalyst in various kinds of asymmetric reactions. Because of the importance of main-chain chiral polymers in an asymmetric catalyst, this book also focuses on the synthesis of polymers that have main-chain chirality. Polymerization of enantiopure monomers simply produces optically active polymers. Although most enantiopure monomers involve a chiral carbon center, polymerization of some monomers consists of chiral heteroatoms such as silicon and phosphorous, which also have been studied. Asymmetric polymerization by means of a repeated asymmetric reaction between prochiral monomers has been applied to obtain optically active polymers. Several types of main-chain chiral polymers have been prepared by asymmetric polymerization.

Helicity is an important factor in characterizing a chirality of macromolecules. Helical synthetic polymers have gained increasing interest on the basis of recent progress in asymmetric polymer synthesis [2–4]. Efficient induction of the main-chain helical sense to macromolecules, such as poly(methacrylate)s [5], poly(isocyanate)s [6, 7], poly(isocianide)s [8], poly(acetylene)s [9], poly(quinoxaline-2,3-diyl)s [10, 11], and polyguanidines [12], has been achieved. Other types of chiral polymers such as chiral dendrimers and hyperbranched polymers are also involved. Major application of these chiral polymers should be focused on the polymeric asymmetric catalyst.

1.2 Polymeric Chiral Catalyst

Synthetic chiral polymers include (1) polymers possessing side-chain chirality (Scheme 1.1), (2) polymers possessing main-chain chirality (Scheme 1.2), (3) dendritic molecules containing chiral ligands (Scheme 1.3), and (4) helical polymers (Scheme 1.4). The use of polymeric chiral catalysts in asymmetric synthesis is an area of considerable research interest, and it has been the subject of several excellent reviews during the last decade. [13–21]

Scheme 1.1 Polymer having a side-chain chiral ligand.

Scheme 1.2 Polymer containing a main-chain chiral ligand.

Scheme 1.3 Periferally modified chiral dendrimer.

Scheme 1.4 Helical polymer catalyst.

Polymeric catalysts obviously have considerable advantages over the corresponding low-molecular-weight counterparts. They can be easily separated from the reaction mixture, which can be reused many times. The catalyst stability is usually improved in the case of a polymeric catalyst. Catalyst immobilization on a polymer sometimes results in the site isolation effect, which is also important when the catalyst molecule has a tendency to be aggregated to each other. Immobilization of the catalyst can prevent the aggregation of catalysts. The insolubility of the polymeric catalysts usually facilitates their separation from the reaction mixture. The application of the polymeric catalyst to the continuous flow system becomes possible when the insoluble polymer is used. Although many heterogeneous reactions using the polymeric catalyst suppress the reactivity, in some cases, even higher stereoselectivity with sufficient reactivity in the asymmetric reaction is obtained by using well-designed polymeric chiral catalysts. The conformational influence of the polymeric chiral catalysts sometimes becomes a very important factor in the asymmetric reaction.

1.2.1 Polymers Having a Chiral Pendant Group

Polymer-immobilized chiral catalysts and reagents have received considerable attention in the organic synthesis of optically active compounds. A typical example of a polymeric catalyst is the polymer-immobilized catalyst. The achiral polymer chain possesses the chiral ligand as a side-chain pendant group. In most cases, polystyrene or cross-linked polystyrene has been used as the polymer support. Because phenyl groups in polystyrene can be easily modified to introduce functional groups, various kinds of chiral ligands are attached to the polystyrene supports (Scheme 1.5). Polyethylene fibers [22], polymeric monoliths [23, 24], poly(2-oxazoline) [25], polyacetylene [26], poly(ethylene glycol) [27], and poly(methylmethacrylate) [28] have also been developed.

Schem...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• Foreword
• Contributors
• Chapter 1: An Overview of Polymer-Immobilized Chiral Catalysts and Synthetic Chiral Polymers
• Chapter 2: Polymer-Immobilized Chiral Organocatalyst
• Chapter 3: Asymmetric Synthesis Using Polymer-Immobilized Proline Derivatives
• Chapter 4: Peptide-Catalyzed Asymmetric Synthesis
• Chapter 5: Continuous Flow System using Polymer-Supported Chiral Catalysts
• Chapter 6: Chiral Synthesis on Polymer Support: A Combinatorial Approach
• Chapter 7: Synthesis and Application of Helical Polymers with Macromolecular Helicity Memory
• Chapter 8: Poly(isocyanide)s, Poly(quinoxaline-2,3-diyl)s, and Related Helical Polymers Used as Chiral Polymer Catalysts in Asymmetric Synthesis
• Chapter 9: C2 Chiral Biaryl Unit-Based Helical Polymers and Their Application to Asymmetric Catalysis
• Chapter 10: Immobilization of Multicomponent Asymmetric Catalysts (MACs)
• Chapter 11: Optically Active Polymer and Dendrimer Synthesis and Their Use in Asymmetric Synthesis
• Chapter 12: Asymmetric Polymerizations of N-Substituted Maleimides
• Chapter 13: Synthesis of Hyperbranched Polymer Having Binaphthol Units via Oxidative Cross-Coupling Polymerization
• Chapter 14: Optically Active Polyketones
• Chapter 15: Synthesis and Function of Chiral π-Conjugated Polymers from Phenylacetylenes
• Chapter 16: P-Stereogenic Oligomers, Polymers, and Related Cyclic Compounds
• colour plates
• Index