Polymer bond-forming reactions

Single and double carbon-carbon bond forming reactions, and metathesis (ROMP) reactions, can be used to produce polymers. However, many other... 

Copolymeriziation of polystyrene-bound dicyanoketene acetal (DCKA) and ethylene glycol dimethacrylate (EGDMA) yielded a polymer (41) with high n -acidity. It was found to be an effective and completely recyclable catalyst in the high yielding carbon-carbon bond-forming reaction of dimethylacetals with silylated nucleophiles (Scheme 4.26) [118]. 

Enzymes that belong to the class of hydrolases are by far the most frequently-applied enzymes in polymer chemistry and are discussed in Chaps. 3-6. Although hydrolases typically catalyse hydrolysis reactions, in synthetic conditions they have also been used as catalysts for the reverse reaction, i.e. the bond-forming reaction. In particular, lipases emerged as stable and versatile catalysts in water-poor media and have been applied to prepare polyesters, polyamides and polycarbonates, all polymers with great potential in a variety of biomedical applications. 

Polymer-Supported Catalysts in Carbon-Carbon Bond-Forming Reactions... 

The bulk polymeric format, characterised by highly cross-linked monolithic materials, is still widely used for the preparation of enzyme mimic despite some of its evident drawbacks. This polymerisation method is well known and described in detail in the literature and has often be considered the first choice when developing molecular imprinted catalysts for new reactions. The bulk polymer section is presented in three subsections related to the main topics covered hydrolytic reactions, carbon-carbon bond forming reactions and functional groups interconversion. 

The use of this phenomenon to control carbon-carbon bond-forming reactions relies on R being rapidly converted into another transient radical which, in the case of a polymerisation, occurs by repetitive addition to a monomer double bond to give the propagating polymer radical, P Thus, the PRE prevents dead polymer (P—P ) formation and the dormant concentration of P—T remains effectively constant. It follows that the excess of T ensures that reversible termination and addition of P to monomer are dominant reactions allowing all polymer chains to grow practically simultaneously (Section 10.5.4). 

Methyl methacrylate (MMA) is one of the most important monomers [80-82]. It forms the basis of acrylic plastics and of polymer dispersion paints. The traditional production is by the formation of acetone cyanohydrin, elimination of water and hydrolysis of the nitrile group, followed by the ester formation. In the carbon-carbon bond forming reaction large amounts of excess HCN and ammonium bisulfate are left as waste. Although these problems have been addressed there is still much room for improvement. In particular the number of reaction steps should be reduced and, in order to achieve this, cyanide should be avoided. The building block to replace it is CO. 

The synthesis of an 88-membered combinatorial library 24 based upon the natural product 25 was reported by Nesterenko et al. in their search for small molecules that selectively induce apoptosis in cancer cells. The key amide bond-forming reaction was performed in parallel using a polymer-supported carbodiimide as the dehydrating agent. 

A novel type of polymer-supported Lewis acid, a microencapsulated Lewis acid catalyst was investigated by Kobayashi [117]. Sc(OTf)3 was immobilized on to polystyrene by microencapsulation—Sc(OTf)3 is physically enveloped by polystyrene and stabilized by the interaction between the jr-electrons of benzene rings and vacant orbitals of the Lewis acid. This microencapsulated catalyst was used successfully in several Lewis acid-catalyzed carbon-carbon bond-forming reactions (imino aldol, aza Diels-... 

This chapter covers the polymerization of alkenes with homogeneous and heterogeneous catalysts based on group 4 metals, including the underlying reaction principles and the relationship between catalyst structure and polymer properties. Applications of related complexes in C-C bond-forming reactions in organic synthesis are covered in Chapter 00125. The use of transition metal catalysts in polymer synthesis is more widely discussed in chapter 11.06. 

It is generally accepted that an enzymatic reaction is virtually reversible, and hence, the equilibrium can be controlled by appropriately selecting the reaction conditions. On the basis of this view, many hydrolases, which are enzymes catalyzing a bond-cleavage reaction by hydrolysis, have been employed as catalysts for the reverse reaction of hydrolysis, leading to polymer production by a bond-forming reaction. 

Insertion of the monomer, bonded to the metal in rf-cis fashion, into the metal-polymer bond forms a new 7t-allyl polymer end with the substituent at the anti-position. Successive insertion of the new monomer with if-cis coordination would produce cis- 1,4-polybutadiene. Insertion of if-trans-co-ordinated monomer into the metal-polymer bond leads to trans-1,4-polybu-tadiene via syn zr-allyl intermediates. The above anti Tt-allyl polymer end is often equilibrated with the thermodynamically more favorable syn zr-allyl structure via n-a-n rearrangement. Thus, the ratio of cis-1,4 and trans-1,4 repeating units of the polymer produced depends on the relative rates of the two reactions C-C bond formation between the monomer and the polymer end, and anti to syn isomerization of the zr-allyl end of the growing polymer. If the anti-syn isomerization of the anti zr-allyl polymer end occurs more rapidly than the insertion of a new monomer, the polymer with trans-1,4 units is formed even from 7j4-ds-coordinated monomer. The polymerization catalyzed by Ti, Co, or Ni complexes shows high cis-1,4 selectivity, while that with low monomer concentration results in increase of the trans content of... 

S. Erase, F. Lauterwasser, R. E. Ziegert, Recent advances in asymmetric C-C and C-heteroatom bond forming reactions using polymer-bound catalysts, Adv. Synth. Gated. 345 (2003) 869. 

In recent years, catalytic asymmetric Mukaiyama aldol reactions have emerged as one of the most important C—C bond-forming reactions [35]. Among the various types of chiral Lewis acid catalysts used for the Mukaiyama aldol reactions, chirally modified boron derived from N-sulfonyl-fS)-tryptophan was effective for the reaction between aldehyde and silyl enol ether [36, 37]. By using polymer-supported N-sulfonyl-fS)-tryptophan synthesized by polymerization of the chiral monomer, the polymeric version of Yamamoto s oxazaborohdinone catalyst was prepared by treatment with 3,5-bis(trifluoromethyl)phenyl boron dichloride ]38]. The polymeric chiral Lewis acid catalyst 55 worked well in the asymmetric aldol reaction of benzaldehyde with silyl enol ether derived from acetophenone to give [i-hydroxyketone with up to 95% ee, as shown in Scheme 3.16. In addition to the Mukaiyama aldol reaction, a Mannich-type reaction and an allylation reaction of imine 58 were also asymmetrically catalyzed by the same polymeric catalyst ]38]. 

Catalysts which lead to cis polymer show a significantly higher stereoselectivity in the bond forming reaction. When (ir-allyl nickel iodide)2 modified with TiCl is employed, the 1, 2 deuterium stereochemistry is 70% dl, and 30% meso as revealed by analysis of succinic anhydride. In addition, monomer isomerization is extensive, and could account for a large fraction of the meso structures which are formed, j ke use of (ir-allyl nickel trifluoroacetate)2 as catalyst led to a similar result (32% meso), accompanied by little if any monomer isomerization. Thus, it appears that in reactions to form cis polymer, some, but not always all, of the stereochemical information present in the starting diene is preserved in the polymer. In contrast, none of the initial diene stereochemistry can be detected in the trans polymer. 

Keywords Lewis acids, Polymer-supported catalysts, Rare earth triflate, Combinatorial synthesis, Carbon-carbon bond-forming reactions... 

Thus, a micro encapsulation technique has been shown to be quite effective for binding catalysts to polymers. Utilizing this technique, unprecedented polymer-supported, microencapsulated rare earth Lewis acids have been prepared. The catalysts thus prepared have been successfully used in many useful carbon-carbon bond-forming reactions. In all cases, the catalysts were recovered quantitatively by simple filtration and reused without loss of activity. This new technique for binding nonpolymer compounds to polymers will be applicable to the preparation of many other polymer-supported catalysts and reagents. 

Recently, polymer-supported lanthanide catalysts have beem of great interest, and these topics are discussed elsewhere. Use of lanthanide catalysts in solid-phase organic synthesis is now well-recognized [107]. There have also been many transformations other than carbon-carbon bond-forming reactions in organic synthesis using lanthanide triflates as catalysts, and all these will be reviewed in the near future. 

The Knoevenagel reaction [3] is one of the most important C-C bond-forming reactions available to synthetic chemists. It is widely used in the synthesis of important intermediates or end-products for perfumes [4], pharmaceuticals [5], e. g. antihypertensive and calcium antagonists [6], and polymers [7]. The reaction is catalyzed by bases, acids, or catalysts containing acid-base sites [8], e. g. bases such as ammonia, primary and secondary amines and their salts [1], and Lewis acids such as CUCI2 [9], ZnCl2 [10], and Sml3 [11]. 

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