Chiral catalytic application

Abstract The unique and readily tunable electronic and spatial characteristics of ferrocenes have been widely exploited in the field of asymmetric catalysis. The ferrocene moiety is not just an innocent steric element to create a three-dimensional chiral catalyst enviromnent. Instead, the Fe center can influence the catalytic process by electronic interaction with the catalytic site, if the latter is directly coimected to the sandwich core. Of increasing importance are also half sandwich complexes in which Fe is acting as a mild Lewis acid. Like ferrocene, half sandwich complexes are often relatively robust and readily accessible. This chapter highlights recent applications of ferrocene and half sandwich complexes in which the Fe center is essential for catalytic applications. 

The attractive (80) features of MOFs and similar materials noted above for catalytic applications have led to a few reports of catalysis by these systems (81-89), but to date the great majority of MOF applications have addressed selective sorption and separation of gases (54-57,59,80,90-94). Most of the MOF catalytic applications have involved hydrolytic processes and several have involved enantioselec-tive processes. Prior to our work, there were only two or three reports of selective oxidation processes catalyzed by MOFs. Nguyen and Hupp reported an MOF with chiral covalently incorporated (salen)Mn units that catalyzes asymmetric epoxidation by iodosylarenes (95), and in a very recent study, Corma and co-workers reported aerobic alcohol oxidation, but no mechanistic studies or discussion was provided (89). 

In summary, the examples given above demonstrate that immobilization of metal salts in a block copolymer micellar system followed by a reduction step is a suitable method to synthesize stable colloids with small particle sizes and narrow size distributions. Moreover, such systems are very interesting for catalytic applications because they offer the possibility of designing tailored catalysts for special demands and can be easily tuned by the choice and combination of different polymer block types and lengths, different types of the metal precursor and of the reduction method used. Additional introduction of further functionalities such as charges or chiral groups could make these catalyst systems even more versatile and effective. 

For the further discussion of the catalytic applications it is important to note the acronym system which we apply to designate each ligand easily and unequivocally. According to the general label R/PR 2/PR", R denotes the substituent in the paro-position to the chiral a-chain (when R = H, it is omitted), PR 2 stands for the group in the ortho-position and finally PR" for the phosphino group in the a-chain itself (see Fig. 1.4.4). The abbreviations PbA and PbB refer to the regioisomeric phobane skeletons (PbA = phosphabicyclo[3.3.1]nonane, PbB = phosphabicyclo[4.2.1]nonane), Ind stands for indane and the (R,R)-2,5-dimethylphospholane unit is abbreviated to (R,R)-DMP . 

The same synthetic strategy as in the synthesis of planar-chiral ferrocenes was applied to the preparation of rheniumtricarbonyl 14, which has also been studied as a catalyst in aryl transfer reactions [21], Subsequently, this chemistry has been extended, and various catalytic applications of cyrhetrenes 15, 16 (AAPhos), and related derivatives have recently been demonstrated [22]. 

Another catalytic application of chiral quaternary ammonium salts is their use... 

P-Menthylphosphetane 76, in which the chiral menthyl group was introduced on the phosphorus atom, is a highly hindered, chiral, and electron-rich monodentate ligand. It is expected to provide good activity in asymmetric catalytic applications and has been reported for specific applications in organometallic catalysis <2000CPB315>. 

Much activity continues to be centered around the preparation of enantioenriched epoxides using chiral Co(III)-, Mn(III)- and Cr(III)-salen complexes, particularly in the area of innovative methods. A recent brief review <02CC919> focuses on the synthesis, structural features, and catalytic applications of Cr(III)-salen complexes. In an illustrative example, Jacobsen and coworkers <02JA1307> have applied a highly efficient hydrolytic kinetic resolution to a variety of terminal epoxides using the commercially available chiral salen-Co(III) complex 1. For example, treatment of racemic m-chlorostyrene oxide (2) with 0.8 mol% of catalyst 1 in the presence of water (0.55 equiv) led to the recovery of practically enantiopure (> 99% ee) material in 40% yield (maximum theoretical yield = 50%). This method appears to be effective for a variety of terminal epoxides, and the catalyst suffered no loss of activity after six cycles. 

As discussed in section 3.2, hybrid frameworks provide a unique opportunity to create interesting enantiomerically pure (homochiral), porous networks. One of the motivations for so doing is the possible applications of such networks in the area of chiral separations, which was first demonstrated in 2000 by Rosseinsky and co-workers.83 In more recent work, a homochiral network based upon nickel benzene-1,3,5-tricar-boxylate showed a modest enantiomeric excess (ee) of 8% for the adsorption of a simple naphthol derivative.84 In general, it is found that the enantiomeric discrimination depends upon the relative sizes of the cavities and the sorbate molecules, with better selectivity being found when the size match is close. It also appears that ee values are higher for catalytic applications than chiral separations, as described in the following section. 

Many other reports of ligand libraries for specific catalytic applications have been reported. Among them, Gilbertson and co-workers reported a chiral phosphine library, tested in the rhodium-catalyzed asymmetric hydrogenation of an enamide (158,159), and a similar library for the palladium-catalyzed allylation of malonates (160, 161) Hoveyda and co-workers (162, 163) reported a chiral Schiff base library, screened in the titanium-catalyzed opening of epoxides with (TMSCN) (trimethyl silyl cyanide) ... 

Formation of the palladium(II) complexes can be achieved using standard protocols and resulting in square planar complexes with mer (pincer) chelate structure. The six-membered, very flexible metallacycles featuring alkyl linker chains display chiral puckering that would make the use of chiral analogues difficult in asymmetric catalytic applications. 

The reaction of chiral lithiated ferrocenylamines with dialkylthiuramdisulfides leads to the corresponding (dialkylthiocarbamoyl)thioferrocenes [144]. For catalytic applications, sulfur and selenium derivatives with one [140, 141, 145] or two [146—148] chalcogen substituents have been prepared (see Sect. 4.5.3.1). The technique is essentially the same as for the chiral phosphines lithiation of enantiomerically pure lV,iV-dimethyTl-ferrocenylethylamine, followed either by the addition of a disulfide or a diselenide to the monosubstituted compounds, or by... 

Catalytic reactions have the advantage over the methods discussed so far in that the chiral catalyst need not be added in stoichiometric amounts, but only in very small quantities, which is important if not only the metal (very often a precious one) but also the chiral ligand are expensive. Among the ferrocenes, phosphines are by far the most important catalysts for stereoselective reactions, and are covered in Chapter 2 of this book. We will therefore focus here mainly on the catalytic applications of chiral ferrocenes not containing phosphine groups. Only recently, some progress has been made with such compounds, mainly with sulfides and selenides, and with amino alcohols in the side chain (for this topic, see Chapter 3 on the addition of dialkyl zinc to aldehydes). 

Catalysis by zeolites is a rapidly expanding field. Beside their use in acid catalyzed conversions, several additional areas can be identified today which give rise to new catalytic applications of zeolites. Pertinent examples are oxidation and base catalysis on zeolites and related molecular sieves, the use of zeolites for the immobilization of catalytically active guests (i.e., ship-in-the-bottle complexes, chiral guests, enzymes), applications in environmental protection and the development of catalytic zeolite membranes. Selected examples to illustrate these interesting developments are presented and discussed in the paper. 

Coordination chemistry and catalytic applications of metal complexes with chiral tertiary phosphines... 

Jacobsen reported in 1990 that Mnm complexes of chiral salen ligands (41) were the most efficient catalysts available for the enantioselective epoxidation of alkyl- and aryl-substituted olefins.118 This stimulated a rapid development in the chemistry and applications of chiral SB complexes, which offer promising catalytic applications to several organic reactions, such as enantioselective cyclopropanation of styrenes, asymmetric aziridination of olefins, asymmetric Diels-Alder cycloaddition, and enantioselective ring opening of epoxides.4,119... 

The individual chemical species with chiral catalytic properties, such as complex, organometallic compounds, organic ligands or molecules, anchored or grafted into the channels of microporous and mesoporous materials, and some microporous compounds possessing chiral channels or their pore structures composed of the chiral motifs, all promise further development and potential application in microporous chiral (asymmetric) catalysis and separations. It is an important frontier direction in the zeolite catalytic field at present. Therefore, the synthesis and assembly of chiral microporous compounds and materials are of particular interest for researchers engaged in porous materials. This is a research field in rapid development. 

Another catalytic application of chiral ketene enolates to [4 + 2]-type cydizations was the discovery of their use in the diastereoselective and enantioselective syntheses of disubstituted thiazinone. Nelson and coworkers described the cyclocondensations of acid chlorides and a-amido sulfones as effective surrogates for asymmetric Mannich addition reactions in the presence of catalytic system composed of O-TM S quinine lc or O-TMS quinidine Id (20mol%), LiC104, and DIPEA. These reactions provided chiral Mannich adducts masked as cis-4,5 -disubstituted thiazinone heterocycles S. It was noteworthy that the in situ formation of enolizable N-thioacyl imine electrophiles, which could be trapped by the nucleophilic ketene enolates, was crucial to the success of this reaction. As summarized in Table 10.2, the cinchona-catalyzed ketene-N-thioacyl-imine cycloadditions were generally effective for a variety of alkyl-substituted ketenes and aliphatic imine electrophiles (>95%ee, >95%cis trans) [12]. 

In this chapter, we attempt to summarize the recently developed chiral dendrimer catalysts with their chiral catalytically active species located either at the core or at the periphery of the dendritic macromolecular supports. The discussion will also be focused on dendrimer effects and the development of new methodologies for the recovery and reuse of chiral dendrimer catalysts, with special emphasis on their applications in enantioselective synthesis. The published data have been classified according to the type of reachon in each of the following three sections. 

As might be expected, there are few reports of catalytic applications of chiral 6-ring chelates. The homologue of PROPHOS (19) is much less effective in asymmetric hydrogenation of enamides (25). A number of aromatic a-aminoethylaryl diphenylphosphines have been prepared, but none show promise in catalysis (26). 

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