1.3- Pentadiene asymmetric polymerization

The polymerization of trans-1,3-pentadiene, 149, in a chiral channel inclusion complex with enantiomerically pure perhydrotriphenylene affords an optically active polymer, 150 (236). Asymmetric polymerization of this monomer guest occurs also in deoxycholic acid inclusion complexes (237). 

Treatment of 1.3-pentadiene with tribenzylaluminum/tetrabenzyltitanium/(—)-menthol also gives optically active polymers. Interestingly, a change of optical rotation of the polymer sample is observed when the same catalyst is used but the order of mixing the catalyst components is inverted56. Neodymium(III) chloride modified with optically active sulfoxides in the presence of alkylaluminums as catalyst precursor is also used for asymmetric polymerization of 1,3-pentadiene57. 

These studies were followed by the asymmetric polymerization of similar trans- and cis-l,3-pentadienes within the channels of two steroids of deoxy-cholic acid (DCA) and apocholic acid (ACA) [12-14]. The packing of the two... 

Starting from these substituted dienes, asymmetric syntheses of optically active polymers are possible, since the chirality of the asymmetric carbon atoms of the main chain is determined by the local environment. Indeed these syntheses have been successfully carried out with many of them. The synthesis of optically active polysorbates by Natta et al. dates back to 1960 (228), and is the first example of an asymmetric synthesis of homopolymers. A conclusive proof of the asymmetric induction was obtained by oxidative degradation of the polymers to succinic acid derivatives (229). This synthesis, as well as those performed with trans 1,3-pentadiene (230), 1-phenyl-butadiene (231), and l-phenyl-4-methy1-butadiene (146) have been carried out using optically active initiators. A new kind of asymmetric polymerization was obtained by Farina et al (232) by y-irradiation of trans-1,3-pentadiene included in (-)perhydrotriphenylene (XIX). 

More recently, asymmetric polymerizations have been reported for cis and trans 1,3-pentadiene (233) and for trans-2-methy1-1,3-pentadiene (234) included in deoxycholic acid (XX). The optically active polymers obtained from these monomers possess an essentially trans-isotactic structure. 

Very little information is available on the polymerization ofO.A. butadiene derivatives, in contrast with the asymmetric polymerization of diene hydrocarbons, like trans-1,3-pentadiene, in the presence of an optically active complex catalyst in heterogenous phase, (see ref. [4] and Part 11(1)) subsequently carried out by G. Natta and coworkers. 

Let us consider a case where the selectivity by propagating end-type asymmetric polymerization of the monomer does not lead to pseudo asymmetry of the main chain e. g. 1-methylbutadiene (i.e. 1,3-pentadiene). 

One of the most important results of inclusion polymerization is the synthesis of optically active polymers from nonchiral compounds. Asymmetric polymerization of /ra 5-pentadiene in PHTP has been reported. The optical purity of the polypentadiene is about 7%. DCA and ACA, as natural hosts, induce a greater asymmetric polymerization. The cis and rra/i5-2-methyl-pentadiene gave the highest asymmetric polymerization values [88]. The optical rotatory power disappeared with temperatures higher than 70°C, indicating that this process is reversible and results from a conformational transformation. 

Diene Polymers Polymerization of a 1,3-diene yields a polymer having true asymmetric centers in the main chain and ozonolysis of the polymer gives a chiral diacid compound (12) whose analysis of optical purity discloses the extent of chiral induction in the polymerization (Scheme 11.2) [12,35-39], The polymerization of methyl and butyl sorbates methyl and butyl styrylacrylates and methyl, ethyl, butyl, and /-butyl 1,3-butadiene-1-carboxylates using (+)-2-methylbutyllithium, butyllithium/(-)-menthyl ethyl ether, butyllithium/menthoxy-Na, butyllithium/bomeoxy-Na, butyllithium/Ti((-)-menthoxy)4, and butyllithium/bomyl ethyl ether initiators [35-37] and that of 1,3-pentadiene in the presence of... 

Chiral crystals generated from non-chiral molecules have served as reactants for the performance of so-called absolute asymmetric synthesis. The chiral environments of such crystals exert asymmetric induction in photochemical, thermal and heterogeneous reactions [41]. Early reports on successful absolute asymmetric synthesis include the y-ray-induced isotactic polymerization of frans-frans-l,3-pentadiene in an all-frans perhydropheny-lene crystal by Farina et al. [42] and the gas-solid asymmetric bromination ofpjp -chmethyl chalcone, yielding the chiral dibromo compound, by Penzien and Schmidt [43]. These studies were followed by the 2n + 2n photodimerization reactions of non-chiral dienes, resulting in the formation of chiral cyclobutanes [44-48]. In recent years more than a dozen such syntheses have been reported. They include unimolecular di- r-methane rearrangements and the Nourish Type II photoreactions [49] of an achiral oxo- [50] and athio-amide [51] into optically active /Mactams, photo-isomerization of alkyl-cobalt complexes [52], asymmetric synthesis of two-component molecular crystals composed from achiral molecules [53] and, more recently, the conversion of non-chiral aldehydes into homochiral alcohols [54,55]. 

Polymer-supported TADDOL-Ti catalyst 79 prepared by chemical modification was poorly active in the Diels-Alder reaction of 3-crotonoyloxazolidinone with cyclo-pentadiene (Eq. 24) whereas polymeric TADDOL-Ti 81 prepared by copolymerization of TADDOL monomer 80 with styrene and divinylbenzene had high activity similar to that of the soluble catalyst. In the presence of 0.2 equiv. 81 (R = H, Aryl = 2-naphthyl) the Diels-Alder adduct was obtained in 92 % yield with an endolexo ratio of 87 13. The enantioseleetivity of the endo product was 56 % ee. The stability and recyclability of the catalyst were tested in a batch system. The degree of conversion, the endolexo selectivity, and the enantioseleetivity hardly changed even after nine runs. Similar polymer-supported Ti-TADDOLate 82 was prepared by the chemical modification method [99]. Although this polymer efficiently catalyzed the same reaction to give the (2R,2S) adduct as a main product, asymmetric induction was less than that obtained by use of a with similar homogeneous species. 

Farina, M. Audisio, G. Natta, G. A new kind of asymmetric synthesis. The radiation polymerization of tran.s-l,3-pentadiene ineluded in optieally active perhydro-triphenylcne. J. Am. Chem. Soc. 1967, 89, 5071. 

Stereoregularities of the resulting polymers depend on the sizes of the host channels. Moreover, the space effect in chirality was observed in asymmetric inclusion polymerization of trans- or cis-2-methy 1-1,3-pentadiene by using a pair of hosts, deoxycholic acid and apocholic acid. We obtained optically active polymers with predominant absolute configurations (R). Optical yields varied with the polymerization conditions and the hosts. A maximum optical yield of the trans monomer was 36% m the channel of apocholic acid. 

A detailed simulation of the way in which the local interior environment of the channel of DCA induces direct asymmetric induction in the polymerization pathway of 2-methyl-trans-l,3-pentadiene was reported [17]. 

Asymmetric synthesis polymerization of 1,3-dienes with solid matrices has been reported." This was first attained by using optically active (R)-(-)-trans-anti-trans-flnti-trans-perhydrotriphenylene (256) matrix for y-ray irradiation polymerization of trans-1,3-pentadiene to afford isotactic poly-trans-254. °° Deoxyapocholic ° ° and apocholic acids (257 and 258) are also effective as optically active matrices. Matrix polymerization tended to result in higher optical purity. The highest value of optical purity so far reported is 36% for the polymerization of (Z)-2-methyl-1,3-butadiene with 258 as a matrix. 

Asymmetric synthesis polymerization is known also for an allene monomer. (i )-2,3-Pentadiene (259) gives an optically active polymer with a unique structure. The polymer is obtained by polymerization with n-allylnickel iodide, and four triad sequences are possible for the polymer, among which only one (260) is chiral. 

DCA and apoCA can serve as effective host components for asymmetric inclusion polymerization of prochiral monomers such as 1-substituted butadienes. We reported previously the preparation of optically active polymers with extremely high specific optical rotation of arbitrary sign from (E)- or (Z)-2-methyl-l,3-pentadiene by inclusion polymerization in the canals [7,12-14]. Moreover we have found that butadiene derivatives with polar groups such as cyano or carbomethoxy group can be polymerized to yield optically active polymers. The [ajp values of the resulting polymers were much higher than those of polymers obtained by other known polymerization method. 

The first OA polybutadienes were prepared by Natta and coworkers [201,203] who polymerized alkyl sorbates (LXXV), jS-styrylacrylic acid and 1,3-pentadiene in the presence of OA catalyst and it was the first example of asymmetric synthesis of a homopolymer. Alkylesters of sorbic acid and /J-styrylacrylic acid, which do not contain asymmetric carbon atoms, have been polymerized to OA polymers using metal-organic catalysts containing an alkyl group like (i )-2-methylbutyllithium, in toluene, following the scheme (LXXV) [202] ... 

Typical type B monomers are 1,3-dienes substituted at position 1, like 1,3-pentadiene and sorbates. 1,4 Polymerization of these monomers gives units containing one asymmetric carbon atom directly bound to four different groups, the configuration of which is independent of position along the chain. Even the single monomeric unit exists in two enan-... 

Only the first type is of interest for the preparation of high molecular weight optically active polymers and is the sole method capable of giving a remarkable enantiomeric purity, provided asymmetric induction during monomer insertion is high enough. After the first example appeared in 1960 of the asymmetric induction polymerization of several sorbates [61], other optically active polymers have been obtained following this method (Table XVII). Successful experiments were in fact carried out with 1,3-pentadiene in the presence... 

Probably the first example of asymmetric induction in the solid-state reaction of a chiral was that of polymerization. Farina et al [26, 34] polymerized by 7-irradiation trans.trans-pentadiene included in channels of a resolved sample of perhydrotriphenylene, and obtained an optically active product. 

There is no doubt that these principles are applicable too to other systems which undergo topochemically controlled polymerizations, such as the diacetylenes [33] and, to a lesser extent perhaps, to mixed crystals (to give copolymers) and to inclusion complexes, in urea for example. Trans, fraws-pentadiene in urea has been reported to give a stereoregular polymer [42]. Further, it is known that crystals of the urea channel complexes have chiral structures. Thus we expect that this polymerization in a single crystal would give rise to an asymmetric polypentadiene, and therefore would provide a further example of absolute synthesis. 

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