Linear polymer-supported synthesis

In contrast to phthalocyanines (tetra- or octasubstituted) in which the isoindoline units carry all the same substituents, reports of phthalocyanines with lower symmetry, which have been prepared by using two different phthalonitriles, have rarely appeared. This is due to the problems which are associated with their preparation and separation. For the preparation of unsymmetrical phthalocyanines with two different isoindoline units four methods are known the polymer support route,300 " 303 via enlargement of subphthalocyanines,304 " 308 via reaction ofl,3,3-trichloroisoindoline and isoindolinediimine309,310 and the statistical condensation followed by a separation of the products.111,311 319 Using the first two methods, only one product, formed by three identical and one other isoindoline unit, should be produced. The third method can be used to prepare a linear product with D2h symmetry formed by two identical isoindoline units. For the synthesis of the other type of unsymmetrical phthalocyanine the method of statistical condensation must be chosen. In such a condensation of two phthalonitriles the formation of six different phthalocyanines320 is possible. 

Kunitake, T., Enzyme-like Catalysis by Synthetic Linear Polymers, Chap. 4 in Polymer-Supported Reactions in Organic Synthesis, P. Hodge and D. C. Sherrington, eds., Wiley, New York, 1980. 

Polymeric supports can also be used with advantage to form monofunctional moieties from difunctional (Hies. Leznoff has used this principal in the synthesis of sex attractants on polymer supports (67). Starting from a sheap symmetrical diol he blocked one hydroxyl group by the polymer. Functionalization of cross-linked polymers is mostly performed by chloromethylation (65). A very promising method to introduce functional groups into crosslinked styrene-divinylbenzene copolymers is the direct lithiation with butyllithium in presence of N,N,N, N -tetramethyl-ethylenediamine (TMEDA) (69, 70). Metalation of linear polystyrene with butyl-lithium/TMEDA showed no exchange of benzylic hydrogen and a ratio of attack at m/p-position of 2 1 (71). In the model reaction of cumene with amylsodium, a kinetic control of the reaction path is established. After 3h of treatment with amyl-sodiuni, cumene is metalated 42% in a-, 39% m-, and 19% p-position. After 20h the mixture equilibrates to affort 100% of the thermodynamically more stable a-prod-uct (72). 

Cheng-Ting et al. [13] developed a simple and efficient method for the synthesis of benzimidazole linked quinoxalinones (i) on soluble polymer support using microwave conditions. The key steps involved in the implemented linear synthesis are the acid catalyzed condensation of 4-fluoro-3-nitrobenzoic acid with polymer... 

Structural analysis of linear polymers molecularly dissolved in a suitable solvent using and solution phase NMR spectroscopy is long established [87-89]. Not surprisingly therefore when a linear soluble polymer is used as a support in solid phase synthesis and solution phase NMR spectroscopy can be a powerful tool in following the chemical synthesis on the support [90]. Figure 15.3.58, for example, shows a series of H NMR spectra of dissolved linear polymer samples taken at various stages in the solid phase synthesis of oligoethers on soluble polystyrene [91]. The various chemical steps Fig. 15.3.59 are clearly demonstrated. 

The catalyst activity decreased with increasing polymer crystallinity. A high regioselectivity of the catalyst in the hydrosilylation of alkenes towards formation of the linear products was achieved due to the favorable microporous structure of the polyamide supports with pore size of 10-20. The stereoselectivity of the reaction can be reversed by a proper choice of donor functions in a polymer support, for example the traditional cis-selectivity of Rh catalysts in hydrosilylation of phen-ylacetylene was changed to trans-selectivity by use of a 2,5-py instead of a 2,6-py moiety. The polyamide-supported catalysts showed high stability through 6-9 synthesis runs [25]. 

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