Cyclization polymer-analogous

The synthetic route represents a classical ladder polymer synthesis a suitably substituted, open-chain precursor polymer is cyclized to a band structure in a polymer-analogous fashion. The first step here, formation of the polymeric, open-chain precursor structure, is AA-type coupling of a 2,5-dibromo-1,4-dibenzoyl-benzene derivative, by a Yamamoto-type aryl-aryl coupling. The reagent employed for dehalogenation, the nickel(0)/l,5-cyclooctadiene complex (Ni(COD)2), was used in stoichiometric amounts with co-reagents (2,2 -bipyridine and 1,5-cyclooctadiene), in dimethylacetamide or dimethylformamide as solvent. 

In order to ensue a clear presentation of the results the authors decided to segregate both synthetic principles All synthetic strategies developed from the multifunctional condensations of Stille and Marvel were assigned to this general type of reaction. At the same time the first multistep sequences (polymer-analogous cyclization of poly(methyl vinyl ketone) and polyacrylonitrile) are used as point of reference for the classification of the other type of synthesis (stepwise procedures). 

In this paper, we shall discuss, first by a polymerization of unsaturated side-groups (side-chains), second by the polymer-analogous condensation of suitable functional groups, third by ring-closures (cyclization) via electrocyclic reactions and, fourth by cyclization via electrophilic substitution reactions. 

Aromatization experiments show some evidence for relatively small aromatic substructures (naphthalene, anthracene, tetracene) and this independently demonstrates the success of the polymer-analogous cyclization process. The occurence of up to 20% non-cyclized vinylic side-groups is an indication of the statistical nature of the ring-closure reaction [58]. 

Besides the thermal condensation, many publications describe the use of acid catalysts (CF3COOH [77], POCI3, polyphosphoric acid [78]) to carry out the polymer analogous cyclization. The occurrence of soluble products shows that the intramolecular cyclization is greatly favored over an intermolecular condensation step (crosslinking). 

The above examples of a stepwise synthesis of a ladder polymer involve the formation of single-stranded polymers via polymerization of suitable monomers to functionalized precursors. These consist of substituted poly(ethylene)- or (polyacetelyne)-type macromolecules, from which attempts are made to carry out a defined polymer-analogous cyclization reaction. 

The synthetic sequence to methylene-bridged poly(phenylene)s 71 represents the first successful employment of the stepwise process to ladder-type macromolecules involving backbone formation and subsequent polymer-analogous cyclization. As shown, however, such a procedure needs carefully tailored monomers and reaction conditions in order to obtain structurally defined materials. The following examples demonstrate that the synthesis of structurally defined double-stranded poly(phenylene)s 71 (LPPP) via a non-concerted process is not just a single achievement, but a versatile new synthetic route to ladder polymers. By replacing the dialkyl-phenylenediboronic acid monomer 68 by an iV-protected diamino-phenylenediboronic acid 83, the open-chain intermediates 84 formed after the initial aryl-aryl cross-coupling can te cyclized to an almost planar ladder-type polymer of structure 85, as shown recently by Tour and coworkers [107]. 

This methodology has also been extended to 4-acetoxystyrene, which was successfully polymerized with a Cu(I) catalyst with bipyridine as the ligand (22). The resultant 2700 Da bromide terminated polymer was then successfully converted to the azide and cyclized using analogous reaction conditions as used for styrene cyclization (Figure 11). The deprotection of poly(acetoxystyrene) was carried out in a 1.5 M solution of KOH in a 1 1 water / methanol solution for 12 hours at room temperature. This transformation was verified by the complete loss of the acetate protons in the H-NMR (2.15 ppm). 

Efforts to prepare ladder-type polymers in a multistep process were first undertaken in the late 1930s. Marvel and coworkers [1,2] described the first attempt at the polymer-analogous cyclization of poly(methyl vinyl ketone) (Scheme 2, aldol condensation of the acetyl side groups). However, a degree of conversion (cyclization) only up to 86% is possible on purely statistical grounds. 

Other polymer-analogous precursors employed include polyacrylonitrile (oxidative or nonoxidative cyclization [3-5]), poly( 1,2-butadiene) or poly(3,4-isoprene) [6,7], and polyalkinylacetylenes [8-11] prepared by the polymerization of butadiynes. The unsaturated side... 

Scheme 2 Polymer-analogous cyclization of poly(methyl vinyl ketone) according to Marvel et al. [1,2].

Scheme 6 Polymer-analogous cyclization of open-chain LPPP precursors.

Table 14.2 The Influence of the Substitution at the Methylene Bridge on the Course of the Polymer-Analogous Cyclization Reaction...

For the preparation of ladder polymers and copolymers of the PPP type, as described in detail, the classical synthetic methods were adopted as the polymer-analogous cyclization of a suitably substituted single-stranded precursor polymer. In spite of frequently expressed doubts regarding the power of this method [351, it was possible to attain the impressive proof that it really does lead to defect-free band structures after careful selection of reaction centers and reaction conditions. The effect of this was a renaissance for the classical synthetic routes for ladder polymers [36-391 (see Section II.C). 

The polymer-analogous cyclization is accompanied by a remarkable change in the absorption properties. The colorless intermediate is converted to the deep yellow planar ladder polymer connected with this is a distinct bathochromic shift of the longest wavelength absorption maximum. The polymer possesses an absorption band with well-defined vibrational fine structure and a sharp absorption edge of the 0-0 transition (see Table 14.4 and Fig. 14.3). 

Fig. 14.3 Absorption spectra of the polymer-analogous cyclization products to angularly annelated polyacenes according to ---) Chmil and Scherf [36], (-) Tour and Lamba [37,38], and (- - -) Goldfinger and Swager [39].

The absorption behavior of the resulting cyclization products (see Fig. 14.3), however, was unusual, with Amax 340 nm (shoulder at 364 nm), which was scarcely different from that of the open-chain precursor polymer (apart from a weak salient in the range above 400 nm). This absorption behavior, compared with the results of Scherf and Chmil and the calculations of Toussaint and Bredas, is difficult to reconcile with the postulated conjugated ladder structure, even considering the somewhat different substitution pattern in the peripheral substituents. It is rather the result of an incomplete polymer-analogous cyclization. 

The synthesis of these ladder-type polyphenylenes 2 and 3, as first accomplished in our group by Ullrich Scherf [120, 121], was realized via suitably functionalized polyphenylene precursors 2a and 3a, which were then transformed into the target ribbon via a polymer-analogous Friedel-Crafts cyclization [122, 123] (Scheme 2). 

The intramolecular Heck reaction of polymer bound aryl halides such as 84 affords indole analogs 85 after cleavage of the final product from the resin with TFA <96TL4189>, Other notable uses of the Heck cyclization include a synthesis of an antimigraine agent <96TL4289>, and thia-tryptophans <96T14975>. 

It is necessary to point out that while various types of polyrotaxane have been conceived (Table 1), to date, only polyrotaxanes of Types 4, 5, 6, 7, 9, 10 and 11 have been reported. Polyrotaxanes of Types 8 and 12 are worth study this might provide more interesting information about the relationship between properties and structure. In addition to those discussed so far, other potential preparation approaches have also been conceived but have not been applied. These methods are simply summarized and demonstrated via those for the side-chain polyrotaxanes of Type 10 (Figure 18). They are (i) chemical conversion, (ii) polymerization of rotaxane monomers, (iii) clipping (cyclization in the presence of preformed polymer), and (iv) grafting. The corresponding methods for other types of polyrotaxanes in Table 1 are analogous [6-8, 12]. 

Furans have also been incorporated (76USP3979367) into the main chains of polymers by modification of ethylene-carbon monoxide interpolymers in analogous fashion to the pyrroles (Section 1.11.4.1.1). The interpolymer was simply treated with acid in a solvent, and magnesium sulfate was added to react with the water formed in the cyclization. As with the pyrroles, the furan units were introduced to improve thermal processing. 

A second enzyme (of mass 100 kDa) is needed for activation of phenylalanine. It is apparently the activated phenylalanine (which at some point in the process is isomerized from l- to D-phenylalanine) that initiates polymer formation in a manner analogous to that of fatty acid elongation (Fig. 17-12). Initiation occurs when the amino group of the activated phenylalanine (on the second enzyme) attacks the acyl group of the aminoacyl thioester by which the activated proline is held. Next, the freed imino group of proline attacks the activated valine, etc., to form the pentapeptide. Then two pentapeptides are joined and cyclized to give the antibiotic. The sequence is absolutely specific, and it is remarkable that this relatively small enzyme system is able to carry out each step in the proper sequence. Many other peptide antibiotics, such as the bacitracins, tyrocidines,215 and enniatins, are synthesized in a similar way,213 216 217 as are depsipeptides and the immunosuppresant cyclosporin. A virtually identical pattern is observed for formation of polyketides,218 219 whose chemistry is considered in Chapter 21. 

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