Benzofuran polymerization

Benzamido-cinnamic acid, 20, 38, 353 Benzofuran polymerization, 181 Benzoin condensation, 326 Benzomorphans, 37 Benzycinchoninium bromide, 334 Benzycinchoninium chloride, 334, 338 Bifiinctional catalysts, 328 Bifiinctional ketones, enantioselectivity, 66 BINAP allylation, 194 allylic alcohols, 46 axial chirality, 18 complex catalysts, 47 cyclic substrates, 115, 117 double hydrogenation, 72 Heck reaction, 191 hydrogen incorporation, 51 hydrogen shift, 100 hydrogenation, 18, 28, 57, 309 hydrosilylation, 126 inclusion complexes, oxides, 97 ligands, 19, 105 molecular structure, 50, 115 mono- and bis-complexes, 106 NMR spectra, 105 olefin isomerization, 96... 

The furan ring can be made to polymerize through one or both of its double bonds and the polymers obtained will therefore have dihydro- and tetrahydrofuran rings in their backbone. This situation occurs when furan, the alkylfurans, benzofuran and some dihydrofurans are treated with suitable initiators and is discussed in the first section of this chapter. 

Furan derivatives with an aromatic system fused on one of the ring s double bonds, such as benzofuran, naphthofuran etc., can be polymerized cationically through the other ring s double bond. In these polymerizations the complications encountered with furan and alkylfurans [see Section III-A-l-c] are absent because only one unsaturation is available for propagation, the other being tied up in the benzene system... 

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 ... 

During incineration of 1 in the polymeric matrix debromination/hydrogenation occur in addition to cyclization process. Tetrabrominated dibenzofuran isomers are the most abundant products formed in the temperature range between 300° or 400° (Figure 6 shows Br-composition at 300° - 800°C). Incineration at 400°C gives tetrabromo-benzofurans in yields up to 13 % (Fig. 6). Besides of PBDF, brominated dibenzodioxins are also formed, but to a much lesser extent (30-90 ppm) (ref. 11). 

Polymerization of 1 -benzofuran with an optically active initiator gives the optically active polymer, poly[(27 ,35)-2,3-dihydro-l-benzofuran-2,3-diyl], containing predominantly one type of stereorepeating unit. 

Occupational exposure to 2,3-benzofuran may occur in several energy-related industries. 2,3-Benzofuran is part of the naphtha fraction of coal distillates and exposure is possible in coke production and coal gasification facilities (see Chapter 4). Exposure may also occur during the polymerization process used to produce coumarone-indene resin. 2,3-Benzofuran was not included in the NIOSH National Occupational Hazard Survey or the National Occupational Exposure Survey. However, the naphtha fraction of coal tar is considered in the NIOSH (1978) evaluation of occupational hazards associated with coal gasification. 

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

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). 

Preparation of addition polymers having the oxolene (dihydrofuran) functionality can be envisioned to occur in two possible ways (Scheme 13). Both, in fact, have been observed (77MI11102). Whereas furan (53) or its derivatives do not homopolymerize under free radical conditions, 1 1 alternating copolymers possessing the 1,4-structure are produced with maleic anhydride (50). Intermediate formation of a CT complex between monomers (50) and (53) is believed to be necessary before polymerization can occur. On the other hand, cationic polymerization is quite facile. The outcome is straightforward with benzo[f>]furan derivatives, producing 1,2-polymers. Optically active poly(benzofurans) are formed when the cationic polymerizations are conducted in the presence of a chiral anion. 

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). 

Benzofurans and dihydrobenzofurans have been prepared on polymeric supports by the palladium-mediated reaction of 2-iodophenols with dienes or alkynes (Entries 1 and 2, Table 15.9). This reaction is closely related to the synthesis of indoles from 2-iodoanilines, and probably proceeds via an intermediate palladacycle (Figure 15.3). Benzofuran and isobenzofuran derivatives have also been prepared on cross-linked polystyrene by intramolecular addition of aryl radicals to C=C double bonds and by intramolecular Heck reaction. 

The phenolics include anthocyanins, anthraquinones, benzofurans, chromones, chromenes, coumarins, flavonoids, isoflavonoids, lignans, phenolic acids, phenylpropanoids, quinones, stilbenes and xanthones. Some phenolics can be very complex in structure through additional substitution or polymerization of simpler entities. Thus xanthones can be prenylated and flavonoids, lignans and other phenolics can be glycosylated. Condensed tannins involve the polymerization of procyaninidin or prodelphinidin monomers and hydrolysable tannins involve gallic acid residues esterified with monosaccharides. As detailed in this review, representatives of some major classes of plant-derived phenolics are potent protein kinase inhibitors. 

In the last case the presence of preformed optically active or optically inactive poly-benzofuran during the polymerization, the other conditions being the same, yields polymers having higher optical activity (79). 

The investigations done to clarify this feature of the polymerization of benzofuran, have revealed an increase in the optical activity of the monomer unit at the beginning of the polymerization such an increase has been attributed (35) to the action exerted by the already formed polymer on the catalyst. Subsequently, the optical activity of the monomer unit keeps constant for a certain period and finally it decreases, though to a limited extent. 

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. 

II. 2.1.4. Cyclic Olefin Polymers Benzofuran (16) gives an optically active polymer by cationic polymerization with AlEtCl2 or A1C13 in the presence of an optically active cocatalyst such as P-phenylalanine and 10-camphorsulfonic acid [12,48-50], The optically active polymer is considered to have an erythro- or threodiisotactic structure with no plane of symmetry. Initiator systems of AlCl3/(-)-menthoxytriethyltin, -germanium, and -silicon also give an optically active polymer [51,52],... 

As 2-vinylfuran rapidly polymerizes even in a nitrogen atmosphere in the presence of a stabilizer, yields obtained for these Diels-Alder reactions were very low. In fact, when the more stable 5-(4-nitrophenyl)-2-vinylfuran lb reacted with DMAD, the yield of the aromatized cycloadduct, dimethyl 2-(4-nitrophenyl)benzofuran-4,5-dicarboxylate 4b, was 50%. The 4-nitrophenyl group not only deactivated the vinylfuran for oxidation and polymerization, but also deactivated the diene system toward cycloadditions, and the reaction was successful only when conducted in boiling xylene. The decrease in reactivity of the reactive diene may account for the relatively low yield of methyl 2-(4-nitrophenyl)benzo-furan-4-carboxylate 6b obtained in a similar reaction with MP (73-AJC1059). 

Polymerization behavior of 4 -methylenespiro[2-benzofuran-2,2 -(l,3-dioxola-ne)] (102) is rather complicated. Han et al. [95] claimed that the resulting polymer consists of two structural units (103 and 104), but Pan et al. [96] later proposed a structure containing ketone units (105) instead of keto-ester units 103. 

Although obtained only in low yields upon troublesome chromatography of product mixtures that contained much polymeric material, the tricyclic benzofuran and benzothiophene lactones (61) were shown to be isolable products from attempted Diels-Alder reactions on the allene ester precursors shown in Equation (32) <85JCS(Pl)747>. Although it was noted in the case of the two thiophenes that the tricyclics appeared to be forming from a precursor (presumably a dihydro form) on the chromatographic column, it was not possible to convert the crude suspected cycloaddition adducts directly into the aromatics by dehydrogenation with DDQ. Complex mixtures were obtained instead. It is possible that the actual dienophiles in these Diels-Alder reactions are alkynes. In a related study, the bis-lactone (62) was also obtained (Equation (33)) <86H(24)88l>. 

The soundness of the theory was indirectly corroborated by Natta s group. The Italian workers started their investigation with a different purpose (120, 123). At one point of their research the polymerization of benzofuran was studied with conventional optically inactive Lewis acid catalyst (AlEtCl2) at low temperatures. The polybenzofuran, whose... 

There are a few examples of polymers based on vinylbenzofurans. Vinyldibenzofuran 324 has been patented for use in copolymer formulations with other vinyl arenes, used to prepare light-emitting devices <2004USP6803124>. Benzofuran 325 was developed as one of four polymerizable monomers that contain a built-in antioxidant. The polymerization process was transition metal catalyzed <2003MM8346>. Benzofuran 326 also contains the styrene substructure, but there are few examples of its polymerization. Poly(2,3-benzofuran) films were synthesized by anodic oxidation on stainless steel in the presence of boron trifluoride etherate. The films had good thermal stability and conductivity of lO Scm <2005MI1654>. 

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. 

Some years ago a benzothiophene that exhibited some activity as a tubulin polymerization inhibitor and appeared to bind weakly to the colchicine binding domain of tubulin was reported. This finding stimulated the synthesis of several benzofuran and indole analogues, Fig. (17). 

Very common Cl fragments bound to a polymeric support are carbenoids. The simplest carbenoid is CO itself, which can be used in transition metal-catalyzed insertions between benzofuran-3yl palladium and a nucleophile such as trifluoroethanol [161]. One of the most common carbenoids is isonitrile, which is used in a large number of multicomponent reactions. Diazoalkanes have often been used as carbene precursors, since they can be readily obtained from support-bound anions and sulfonyl azides [252]. Commonly, diazoalkanes are converted transi-... 

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