Ionically-tagged Transition Metal Catalysts

1.1 Introduction

Ionic liquids are liquids consisting solely of ions. Those with melting points below 100 °C are often called room temperature ionic liquids but within this chapter simply the term 'ionic liquids' will be used. Their interesting physical/ chemical properties stimulated the investigation of a great number of research and even technical applications. Their use in synthesis has been summarized in an excellent two-volume book edited by Wasserscheid and Welton.

The concept of ionically-tagged transition metal catalysts develops on the interface of homogeneous and heterogeneous catalysis. It aims to combine positive features of both worlds. The primary function of ionic tags is to enable catalyst recycling. On the other hand, appropriate reaction media offer the possibility of maintaining catalysis in the liquid phase and so benefit from high catalytic activities of homogeneous catalysts. Furthermore, the effect of ionic tags often goes beyond a simple anchor for immobilization and recycling of catalyst. Many reactions are enhanced as a result of an ionic tag installed into the catalyst structure compared to the unmodified catalyst. This effect led Lombardo and Trombini to postulate a concept of electrosteric activation. This means the stabilization of a transition state of a catalytic reaction by electrostatic and steric interaction with ionic tags.

Suitable reaction media for ionically-tagged catalysts can be ionic liquids, supercritical liquids, water or combinations of these. Use of biphasic set-ups is also attractive. This chapter gives an overview of enantioselective catalysis with transition metal based catalysts having an ionic moiety. Ionic catalysts used in water are, however, excluded as they are extensively covered in Chapter 7.

In this chapter, 1-butyl-3-methylimidazolium and 1-ethyl-3-methylimida-zolium cations are denoted as [bmim] and [emim]; 1-butyl-2,3-dimethylimi-dazolium and 1-hexyl-2,3-dimethylimidazolium as [bdmim] and [hdmim]; 1-butylpyridinium cation is denoted as [bpyr]; N-ethyl-3-methylpiccolinium cation is denoted as [epic]. The bis(trifluoromethyl sulfonyl)imide anion is denoted as NTf2, and triflate and tosylate as OTf and OTs.

1.2 Achiral Ionically-tagged Transition Metal Complexes

Highly polar ligands are well known in the coordination chemistry of transition metals. Such complexes were, at first, only used in water. Several reviews and books give a good overview of synthesis and applications of these hydrophilic ligands. However, it was only in 1996 when Chauvin and co-workers suggested that ligands bearing ionic moieties can be used for immobilization of metal complexes in ionic liquids. They noticed that rhodium-catalyzed hydrogenation of pent-1-ene was five times faster in [bmim]PF6 than in acetone. Cationic rhodium complexes were well retained in ionic liquids, which could be reused. However, in rhodium-catalyzed hydroformylation, catalyst leaching was observed. After extraction of organic products, the ionic liquid was reused but part of the rhodium complex was extracted to the organic phase too. Use of mono-and trisulfonated triphenylphosphines as ligands to rhodium completely prevented catalyst leaching to the organic phase.

The concept was then further developed in a number of ways. Various cationic phosphine ligands were tested in rhodium-catalyzed hydrogenations and hydroformylations in ionic liquids or biphasic conditions. Several, structurally different, cations were successfully applied (Figure 1.1). Wasserscheid and Olivier-Bourbigou showed that guanidinium ion is particularly suitable for immobilization of rhodium complexes. van Leeuwen and co-workers attached two imidazolium moieties to Xantphos skeleton. In this way they created ligand 3, which showed excellent recycling results and very little leaching.

Ruthenium-arene catalysts were modi?ed by appending imidazolium moiety as well (Scheme 1.1). Imidazolium ionic tags are designed by their similarity with the most common type of ionic liquids. Therefore, it is also the most frequent type of ionic tag. However, it has also a disadvantage, because the proton in position 2 is acidic; therefore, an imidazolium ion with an alkyl group substituted position 2 is often used.

Palladium-catalyzed Suzuki and Stille couplings as well as Heck reaction were successfully performed with various ionically-tagged catalysts (Figure 1.2). These catalysts have good catalytic activities and at the same time significantly reduced metal leaching from ionic liquid.

The usefulness of the ionic-tagging strategy for catalyst immobilization was also amply demonstrated in ruthenium-catalyzed olefin metathesis. Both Grubbs catalyst of the first and second generation (8–13) have been modified with ionic tags. Usually imidazolium ions have been selected. Figure 1.3 shows various ways in which ionic tags can be introduced into metathesis catalysts.

Achiral Mn-Schiff base 14 was used as an epoxidation catalyst, or porphyrin 15 derivatized with four pyridinium moieties was used for the oxidation of styrene (Figure 1.4).

1.3 Chiral Transition Metal Catalysts with Ionic Tag

Chiral transition metal catalysts can be immobilized through ionic forces in ionic liquids. This is enabled by charge carried by a metal ion. Numerous examples of this approach have been described. Such systems, however, often suffer from considerable metal leaching during work-up operations. The introduction of an auxiliary ionic moiety into the ligand has been suggested as a possible solution to this problem. The installation of an ionic tag into the ligand structure, indeed, helps prevent metal leaching. Catalytic activities are usually comparable or better than that of unmodified catalysts.

1.3.1 Catalysts for Enantioselective Reduction

Enantioselective hydrogenations are one of the most widely employed asymmetric transformations. Expensive rhodium, ruthenium and iridium complexes are the most active hydrogenation catalysts. This motivated great effort for the synthesis of immobilized catalysts. Also, the concept of ionically-tagged catalysts has been developed and tested on hydrogenation reactions. Several researchers therefore tried to immobilize rhodium and ruthenium complexes with the help of ionic tags. An important factor is also good solubility of hydrogen ionic liquids. However, prototypical hydrogenation catalysts, Rh-BINAP complexes, are too air-sensitive and therefore have not been used in ionic liquids. Rhodium...