Bifunctional Molecular Catalysis

Table of Contents

1. Frontmatter

2. Multimetallic Multifunctional Catalysts for Asymmetric Reactions

   Masakatsu Shibasaki, Motomu Kanai, Shigeki Matsunaga, Naoya Kumagai

   Abstract

   The utility of several types of multimetallic asymmetric catalysts is reviewed. The appropriate design of multidentate chiral ligands provides homobimetallic, heterobimetallic, and polymetallic catalysts upon complexation with metal cations. Cooperative work of multiple catalytically active sites in the asymmetric multimetallic catalysts allows for enhanced catalytic activity and stereoselectivity over monometallic catalysts. Facile systematic tuning of the asymmetric multimetallic catalysts by manipulation of multidentate ligand structure and combination of metal cations enables the generation of diverse set of catalysts having distinct three-dimensional and catalytic properties, boosting the optimization process to identify the best catalyst for a specific reaction of interest.

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3. Bifunctional Transition Metal-Based Molecular Catalysts for Asymmetric Syntheses

   Takao Ikariya

   Abstract

   The discovery and development of conceptually new chiral bifunctional molecular catalysts based on the metal/NH acid–base synergy effect are described. The chiral bifunctional molecular catalysis originally developed for asymmetric transfer hydrogenation of ketones is applicable in enantioselective hydrogenation of polar functionalities as well as practical oxidative reactions including aerobic oxidation of alcohols. The structural modification and electronic fine-tuning of the protic amine chelating ligands are crucial to develop unprecedented catalytic reactions. The present bifunctional transition metal-based molecular catalyst offers a great opportunity to open up new fundamentals for stereoselective molecular transformations.

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4. Bond Activation by Metal-Ligand Cooperation: Design of “Green” Catalytic Reactions Based on Aromatization-Dearomatization of Pincer Complexes

   Chidambaram Gunanathan, David Milstein

   Abstract

   We have developed a new mode of bifunctional catalysis based on metal–ligand cooperation, involving aromatization–dearomatization of pyridine- and acridine-derived pincer complexes. This type of metal–ligand cooperation is involved in the recently discovered environmentally benign reactions of alcohols, catalyzed by PNP and PNN pincer complexes of ruthenium, including: (a) dehydrogenation of secondary alcohols to ketones, (b) dehydrogenative coupling of primary alcohols to form esters and H₂, (c) unprecedented amide synthesis: catalytic coupling of amines with alcohols, with liberation of H₂, (d) direct synthesis of imines from alcohols and amines with H₂ liberation, (e) direct conversion of alcohols to acetals with H₂ liberation, (f) selective synthesis of primary amines from alcohols and ammonia, and (g) hydrogenation of esters to alcohols under mild conditions. These reactions are very efficient, proceed under neutral conditions, and produce no waste.

5. Shvo’s Catalyst in Hydrogen Transfer Reactions

   Madeleine C. Warner, Charles P. Casey, Jan-E. Bäckvall

   Abstract

   This chapter reviews the use of ShvoB1;apos;s catalyst in various hydrogen transfer reactions and also discusses the mechanism of the hydrogen transfer. The Shvo catalyst is very mild to use since no activation by base is required in the transfer hydrogenation of ketones or imines or in the transfer dehydrogenation of alcohols and amines. The Shvo catalyst has also been used as an efficient racemization catalyst for alcohols and amines. Many applications of the racemization reaction are found in the combination with enzymatic resolution leading to a dynamic kinetic resolution (DKR). In these dynamic resolutions, the yield based on the starting material can theoretically reach 100%. The mechanism of the hydrogen transfer from the Shvo catalyst to ketones (aldehydes) and imines as well as the dehydrogenation of alcohols and amines has been studied in detail over the past decade. It has been found that for ketones (aldehydes) and alcohols, there is a concerted transfer of the two hydrogens involved, whereas for typical amines and imines, there is a stepwise transfer of the two hydrogens. One important question is whether the substrate is coordinated to the metal or not in the hydrogen transfer step(s). The pathway involving coordination to activate the substrate is called the inner-sphere mechanism, whereas transfer of hydrogen without coordination is called the outer-sphere mechanism. These mechanistic proposals together with experimental and theoretical studies are discussed.

6. Liquid-Phase Selective Oxidation by Multimetallic Active Sites of Polyoxometalate-Based Molecular Catalysts

   Noritaka Mizuno, Keigo Kamata, Kazuya Yamaguchi

   Abstract

   The catalytic oxidation is an area of the key technologies for converting petroleum-based feedstocks to useful chemicals such as diols, epoxides, alcohols, and carbonyl compounds. Many efficient homogeneous and heterogeneous oxidation systems based on polyoxometalates (POMs) with green oxidants such as H₂O₂ and O₂ have been developed. This chapter summarizes the remarkable oxidation catalyses by POMs with multimetallic active sites. The multifunctionality of multimetallic active sites in POMs such as cooperative activation of oxidants, simultaneous activation of oxidants and substrates, stabilization of reaction intermediates, and multielectron transfer leads to their remarkable activities and selectivities in comparison with the conventional monometallic complexes. Finally, the future opportunities for the development of shape- and stereoselective oxidation by POM-based catalysts are described.

7. Bifunctional Acid Catalysts for Organic Synthesis

   Pingfan Li, Hisashi Yamamoto

   Abstract

   The concept of bifunctional acid catalysis is very helpful for inventing new catalytic asymmetric reactions. Compared with single functional acid catalysts, cooperative effect of two acid components has the potential to fine tune the reactivity as well as the selectivity of desired reaction pathways. This chapter focuses on some representative examples on the recent developments of bifunctional acid catalysis, including combined acid catalysis and other cooperative acid catalysis.

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8. Bifunctional Phebox Complexes for Asymmetric Catalysis

   Jun-ichi Ito, Hisao Nishiyama

   Abstract

   Chiral bifunctional rhodium complexes bearing chiral bis(oxazolinyl)phenyl ligand (phebox) catalyzed asymmetric reactions were described. Meridional C ₂-symmetric environment around the metal center in the chiral phebox ligand is a crucial structure factor for determining the catalytic performance in terms of reactivity and selectivity. The phebox–Rh chloro complex serves as a mild Lewis acid catalyst for asymmetric allylation, hetero-Diels–Alder, and Michael reaction. Although the catalytic precursor, Rh(III) complex, with octahedral geometry is a quite stable molecule, it can be readily activated by hydrosilane to generate the putative Rh(I) species, which is a key intermediate in asymmetric hydrosilylation of alkenes and conjugate reduction of α,β-unsaturated carbonyl compounds. The Rh enolate species generated in situ can be used for the C–C bond formation reaction, asymmetric reductive aldol reactions, and direct aldol reactions.

9. Backmatter