Polymeric asymmetric epoxidation

Extensive studies of stereoselective polymerization of epoxides were carried out by Tsuruta et al.21 s. Copolymerization of a racemic mixture of propylene oxide with a diethylzinc-methanol catalyst yielded a crystalline polymer, which was resolved into optically active polymers216 217. Asymmetric selective polymerization of d-propylene oxide from a racemic mixture occurs with asymmetric catalysts such as diethyzinc- (+) bomeol218. This reaction is explained by the asymmetric adsorption of monomers onto the enantiomorphic catalyst site219. Furukawa220 compared the selectivities of asymmetric catalysts composed of diethylzinc amino acid combinations and attributed the selectivity to the bulkiness of the substituents in the amino acid. With propylene sulfide, excellent asymmetric selective polymerization was observed with a catalyst consisting of diethylzinc and a tertiary-butyl substituted a-glycol221,222. ... 

Spectacular achievements in catalytic asymmetric epoxidation of olefins using chiral Mnm-salen complexes have stimulated a great deal of interest in designing polymeric analogs of these complexes and in their use as recyclable chiral catalysts. Techniques of copolymerization of appropriate functional monomers have been utilized to prepare these polymers, and both organic and inorganic polymers have been used as the carriers to immobilize these metal complexes.103... 

One of the first attempts to extend polymer-assisted epoxidations to asymmetric variants were disclosed by Sherrington et al. The group employed chiral poly(tartrate ester) hgands in Sharpless epoxidations utilizing Ti(OiPr)4 and tBuOOH. However, yields and degree of stereoselection were only moderate [76]. In contrast to most concepts, Pu and coworkers applied chiral polymers, namely polymeric binaphthyl zinc to effect the asymmetric epoxidation of a,/9-unsaturated ketones in the presence of terPbutyl hydroperoxide (Scheme 4.11). 

Asymmetric Epoxidation with Polymeric Cinchona-PTCs 63... 

A polymeric binaphthyl zinc complex has been used for related epoxidation reactions <1999JOC8149>.A bimetallic samarium-based Lewis acid complex catalyzes the nucleophilic epoxidation of unsaturated carbonyl compounds very efficiently <2002JA14544>. The use of amino acids has organocatalysts for asymmetric epoxidations of enones and enals has been investigated <2005OL2579, 2005JA6964>. 

An important and significant amount of work has also been done on the asymmetric polymerization of epoxides. Much of this work was carried out with catalysts derived from diethylzinc and diethylmagnesium. The asymmetry was introduced by using optically active alcohols in the synthesis of the catalyst species. Alternately, optically active monomers were polymerized using optically inactive catalysts. A detailed discussion of these studies is beyond the scope of this chapter and the reader is referred to reviews of the subject [9, 14]. 

The reaction of Grignard reagents with epoxides, at first glance, appears to be an effective method for a two-carbon homoligation. However, there are several competing reactions that limit the use of this method. These deleterious reactions are formation of halohydrins, rearrangements, polymerization of the epoxide, and in asymmetric epoxides, regioselectivity issues. 

A poly(bmaphthyl metallosalen complex) 128 (Scheme 3.36) was prepared and used as a catalyst for the asymmetric epoxidation of alkene [72]. Although enantioselectivities obtained by using the polymeric catalyst were low, this represented a new type of polymeric chiral complex based on the main-chain hehcity. 

Polymer-supported chiral (salen)Mn complexes 131 were also used in other asymmetric epoxidation reactions (Scheme 3.37). For example, cis-P-methylstyrene 132 was efficiently epoxidized with uj-CPBA/NMO in the presence of the polymeric catalyst [73]. For most of the tested substrates, the enantioselectivities... 

Sharpless asymmetric epoxidation was also conducted by using polymer-supported catalysts. Some very interesting phenomena were observed when methoxy PEG (MeO-PEG) -supported tartrate 147 was used as the polymeric chiral ligand (Scheme 3.43). In the epoxidation of 148 under Sharpless epoxidation conditions, 2S,3S -trans 149 with 93% ee was obtained using 147 (MW = 750), while (2R,3R)-trans 149 with 93% ee was obtained using 147 (MW = 2000) [80]. More recently, Janda studied the precise effects of the molecular weight of the PEG chain on the... 

Asymmetric epoxidation of unfunctionalized aUcenes catalyzed by chiral Mn(III)(salen) complexes has proven to be a useful solution-phase reaction [88]. To simplify product isolation and to avoid degradation of the Mn(salen) complex through formation of i-oxo-manganese(lV) dimers by spatial redistribution, the polymer-supported catalyst 112 was prepared by co-polymerization of complex 113, styrene 58, and divinylbenzene as a cross-linker (Scheme 20) [89]. As a stoichiometric oxidant, a combination of meta-chlor-operbenzoic acid (mCPBA) and N-methyl-morpholine N-oxide (NMO) in acetonitrile was used. Yields and rates of conversion were satisfactory for the epoxidation of styrene 58 and of methyl styrene, but only low enantioselectivities were obtained. Nevertheless, the catalyst retained its efficiency in terms of yields and enantioselectivities after repetitive use. Similar results have been described by other researchers [90]. 

The National Laboratory at Los Alamos has been actively engaged over the past several years in research in the applications of SC-CO2 as a synthetic solvent. The work of Tumas (61-66) and co-workers as detailed in a later chapter of this volume profiles the performance of reactions such as polymerization of epoxides, oxidation of olefins and asymmetric hydrogenations in supercritical systems. In each of these cases the reactions proceeded without compromise when compared to conventional solvent systems, and superior performance was reported in the asymmetric hydrogenation reactions. 

Stereospecific ring-opening polymerization of epoxides or episulfides with two asymmetric carbon atoms in the ring... 

Stereoselectivity and stereoelectivity in the polymerization of racemic a-olefins have probably the same origin as in the polymerization of epoxides (Section 3.2.2.), auid are determined by the chiral character of the catalysts used. Chiral catalytic centers of a given configuration attack prevailingly one face of the double bond of the monomer and centers of the opposite configuration attack prevailingly the other face. In the case of chiral olefins, the two diastereofaces of a monomer molecule have different reactivities, and whether the re-re face or the si-si face of this molecule is more reactive depends on the type of chirality of the asymmetric carbon atom. For instance, on the basis of the investigation on Pt-complexes (179), the si-si face is the more reactive face in an (S)-olefin. Thus chiral catalytic centers which attack the si-si face may prefer the (S)-antipode, whereas those which attack the re-re face may prefer the (R)-antipode (Scheme 23). 

In all these molecules, the carbon atom where the potential handedness resides is labeled with an asterisk. Asymmetric catalj4ic transformations of most of these prochiral substrates will be discussed later. In particular, we will see the basic mechanisms of hydrogenation of 3.42 and 3.43, asymmetric epoxidation of 3.44, stereoselective polymerization of propylene, and asymmetric dihydroxylation of 3.46. Asymmetric isomerization of 3.47 is one of the critical steps in a homogeneous catalyst-based industrial manufacturing process for L-menthol. 

Zheng and coworkers [35] disclosed a new approach to soluble linear polymeric salen ligands with main-chain chirality for the Mn-catalyzed asymmetric epoxidation of olefins. The chiral polymer ligands (16) were prepared by the... 

Figure 4.13 Chiral polymeric salen catalysts for asymmetric epoxidation of olefins.

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