Benzofuran, asymmetric polymerization

An interesting aspect of the benzofuran cationic polymerization was uncovered by Natta, Farina, Peraldo and Bressan who reported in 196160,61 that an asymmetric synthesis of an optically active poly(benzofuran) could be achieved by using AlCl2Et coupled with (-)j3-phenylalanine, (+)camphorsulphonic acid or with (-)brucine. The optical activity was definitely due to the asymmetric carbon atoms in the polymer chain, indicating that at least some of the polymer s macromolecules possessed a di-isotactic structure, v/ z.62 ... 

Farina, M., and G. Rressan Optically Active Polymers Some New Results and Remarks on the Asymmetric Polymerization of Benzofuran. Makromolekulare Chem. 61, 79 (1963). 

The first asymmetric polymerization of cyclic monomers was achieved by Natta et al. [196] in 1961. Polymers of high optical activity and molecular weight have been obtained in good yield from benzofuran (LXXIII) which is free from chiral sites [196,197], using complex cationic catalysts obtained from monoethylaluminum dichloride and an optically active co-monomer (-)-]3-phenylalanine, (-)-brucine or (+)-10-camphorsulfonic acid at very low temperature. 

Following these theoretical considerations the first asymmetric polymerization of cyclic monomers was performed in the case of benzofuran [71, 72] in the presence of chiral cationic initiators obtained from Lewis acids and optically active Lewis bases (Table XVIII). With AICI3 and j3-phenylalanine the polymer fractions showed essentially constant rotatory power with varying molecular weight, thus indicating an asymmetric growth process. If some polymer is initially present, higher values of asymmetric induction are ob-... 

In the case of benzofuran and naphthofuran, asymmetric polymerization takes place in the presence of mixed catalysts such as AlCl2-(C2H5) and (-)-/3-phenylalanine/or (—) brucine, [17, 18], AICI3 -(-)-menthoxytriethyl tin system [19]. 

Takeda, Y., Y. Hayakawa, T. Fueno, and J. Furukawa Studies on the Mechanism of the Stereospecific Polymerization Asymmetric-induction Polymerization of Benzofuran by Use of Optically Active Organo-stannic Compounds. Makromolekulare Chem. 83, 234 (1965). 

An equimolar binary system consisting of aluminum trichloride and (—)-menthoxytriethyltin, -germanium, or-silicon is effective for asymmetric cationic polymerization of prochiral benzofuran (Scheme 69) (158). 

This supports the hypothesis that, at least in the case of the polymerization of benzofuran, the optical activity is due to steric control of the propagation effected by an asymmetric catalyst however, such a control becomes less effective as the polymerization proceeds probably because of modifications of the structure of the catalytic complex. 

The earliest report of a reaction mediated by a chiral three coordinate aluminum species describes an asymmetric Meerwein-Poimdorf-Verley reduction of ketones with chiral aluminum alkoxides which resulted in low induction in the alcohol products [1]. Subsequent developments in the area were sparse until over a decade later when chiral aluminum Lewis acids began to be explored in polymerization reactions, with the first report describing the polymerization of benzofuran with catalysts prepared from and ethylaluminum dichloride and a variety of chiral compounds including /5-phenylalanine [2]. Curiously, these reports did not precipitate further studies at the time because the next development in the field did not occur until nearly two decades later when Hashimoto, Komeshima and Koga reported that a catalyst derived from ethylaluminum dichloride and menthol catalyzed the asymmetric Diels-Alder reaction shown in Sch. 1 [3,4]. This is especially curious because the discovery that a Diels-Alder reaction could be accelerated by aluminum chloride was known at the time the polymerization work appeared [5], Perhaps it was because of this long delay, that the report of this asymmetric catalytic Diels-Alder reaction was to become the inspiration for the dramatic increase in activity in this field that we have witnessed in the twenty years since its appearance. It is the intent of this review to present the development of the field of asymmetric catalytic synthesis with chiral aluminum Lewis acids that includes those reports that have appeared in the literature up to the end of 1998. This review will not cover polymerization reactions or supported reactions. The latter will appear in a separate chapter in this handbook. 

A compound such as benzofuran is a monomer with no asymmetric center. In a formal sense, its oxygen-containing five-membered ring may be considered as a vinyl ether. Upon polymerization two asymmetric centers form from the two vinyl carbon atoms. The product is optically active although its exact structure and its optical purity is not known. 

Cyclic olefins afford optically active polymers by asymmetric synthesis polymerization. The first example was the polymerization of benzofuran (265) with AlEtCl2 or AICI3 in the presence of optically active cocatalysts such as p-phenylalanine and 10-CSA the polymer is considered to possess the chiral erythro- or threodiisotactic structure (266a or 266b)... 

An example of an asymmetric induction from optically inactive monomers is an anionic polymerization of esters of butadiene carboxylic acids with (/ )-2-methylbutyUithium or with butyUithium complexed with (—)methyl ethyl ether as the catalyst. (This type of polymerization reaction is described in Chap. 4) The products, tritactic polymers exhibit small, but measurable optical rotations [78]. Also, when benzofuran, that exhibits no optical activity, is polymerized by cationic catalysts like aluminum chloride complexed with an optically active co catalyst, like phenylalanine, an optically active polymer is obtained [77]. 

Similar results have been obtained by polymerization of a or j3-naphthofuran (LXXIV) [200]. Like benzofuran, these monomers are not dissymmetric and it is only when the molecule has reacted that a sequence of asymmetric carbon atom is formed along the main chain they polymerize very easily in presence of aluminum chloride or titanium tetrachloride and j3-phenylalanine the use of c/-alanine or /-histidine as co-catalyst always leads to polymers with no optical activity both for benzo- and naphthofurans. Polynaphto-furan can be crystallized by thermal treatment, in contrast to polybenzofuran and its structure, at least partially, must be considered isotactic. This last work dates back to 1966. Since that time, no other results have been published in this field. 

Even if, in some cases, particularly with benzofuran and a-naphthofuran, polymeric materials with rather high optical rotation have been obtained, nothing can be said at present about enantiomeric purity of the macromolecules and about the extent of asymmetric induction achieved. 

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