Poly cyclisation

The isoprenoid polyenes famesyl acetate, geranyl acetate and squalene underwent oxidative poly cyclisation to bis-, tris- and penta-tetrahydrofurans with RuO /aq. Na(IO )/CH3CN-EtOAc [185]-[188]. This oxidative polycyclisation of squalene with RuO was shown to lead to the cis-threo-cis-threo-trans-threo-trans-threo-trans penta-tetrahydrofuranyl diol product, this configuration being determined by 2D-NMR (Fig. 3.14) [185]-[188] cf mech. Fig. 1.8 [185]. 

It was found that the amount of chlorine that could be removed (84-87%) was in close agreement to that predicted by Flory on statistical grounds for structure Figure 12.10(a). It is of interest to note that similar statistical calculations are of relevance in the cyclisation of natural rubber and in the formation of the poly(vinyl acetals) and ketals from poly(vinyl alcohol). Since the classical work of Marvel it has been shown by diverse techniques that head-to-tail structures are almost invariably formed in addition polymerisations. 

A number of polymers are capable of fulfilling these demanding requirements. Typically negative photoresists are based on cyclised poly(l,4-isoprene). These polymers are prepared by dissolving poly(l,4-isoprene) in an appropriate solvent and subjecting it to thermal degradation. This is followed by treatment with acid to produce the cyclised material (see Reaction 8.8). 

Coleman and Sivy also used an infrared transmission cell to undertake degradation studies under reduced pressure on a series of poly(acrylonitrile) (ACN) copolymers [30-33]. Thin films prepared from a polymer were mounted in the specially designed temperature-controlled cell mounted within the infrared spectrometer. The comparative studies were made on ACN copolymers containing vinyl acetate [30,32], methacrylic acid [30,31] and acrylamide [30,33]. The species monitored was the production of the cyclised pyridone structure. This was characterised in part by loss of C=N stretch (vC = N) intensity at 2,240 cm-1 accompanied by the appearance and increase in intensity of a doublet at 1,610/1,580 cm-1. 

A research team from Bloemfontein (South Africa) have also taken advantage of the Julia and Colonna oxidation in elegant research aimed at the synthesis of optically active flavonoids. Bezuidenhoudt, Ferreira et al. have oxidised a range of chalcone derivatives using poly-(L)-alanine in the three phase system to afford optically active epoxides 4 which were readily cyclised to target compounds of the dihydroflavinol type 5, (Scheme 3) [16]. 

Stereoregular c/.v-poly(butadiene) compositions may explode when heated at 337—427°C/1—0.01 pbar, presumably owing to exothermic cyclisation. 

The cyclopolymerisation of unsymmetrical a, co-diolefins such as 2-methyl-1,5-hexadiene in the presence of catalysts such as Cp2 ZrMc2 M(Me)0 x, [Cp2 , ZrMe]+ [B(C6F5)4] or [Cp2 ZrMe]4 [McB(C6 F5)2] yields highly regiore-gular cyclopolymers [501]. The perfectly head-to-tail linked monomeric units in the formed poly[methylene-l,3-(l-methylcyclopentane)] arises from the chemo-selective insertion of the less hindered terminus of 2-methyl-1,5-hexadiene into the active Mt—P bond, followed by cyclisation involving the insertion of the disubstituted olefinic bond (Figure 3.50) [497]. The insertion of the disubsti-tuted olefinic bond is made easier by its intramolecular nature. 

The dialkyne/carbon dioxide copolymerisation is controlled by the relative rate of inter- and intramolecular cyclisations of the dialkyne the former is favoured when the number of methylene groups in the monomer R C C (CH2)X C = C—R is equal to 3,4 or 5 (x 3—5), but the intermolecular cyclisation of the dialkyne is favoured to effect 1 1 cycloaddition copolymerisation of the dialkyne and CO2 to a poly(2-pyrone) when the number x has other values [91 96]. 

The electrophilic component in these co-cyclisation reactions can bear other functionality. For example, reaction of isobutenyl dichloride 10 with the appropriate linear dithiols gives good yields of polythiacycles such as 11, 12 and 13.11 A related reaction allows the direct introduction of a keto group into such poly-thia macrocycles. The electrophilic component in this case is 1,3-dichloropropanone, 14. The reaction conditions (Cs2C03, DMF, 60 °C), are insufficiently basic to induce a Favorskii reaction. Furthermore, under the aprotic, basic conditions, thioketal formation cannot take place, allowing successful co-cyclisation to be undertaken, such as in the formation of 15 (Scheme 3.4). Similar reactions allow the preparation of 16,17 and 18.11... 

It has been suggested that the anthraquinone 22 may result from the cyclisation of an enediyne rather than a poly-P-ketoacyl heptaketide. If this or any other a non-aldol pathway is involved in the formation of tiie 1,8-disubstituted naphthalene structures 23a and 23b, then the mode F and S classification criteria would not be applicable. 

The reaction of mono- and poly-alcohols catalyzed by solid acids has been widely investigated. An important application is the synthesis of five membered cyclic ethers starting from di- or triols. Several authors described such cyclisation reactions, starting from 1,2,4-butanetriol (clay) [1], 1,2,5-pentatriol (pentasile, mordenite, erionite) [2]. Linear ethers like dimethyl ether are formed from methanol (modified aluminosilicate, zeolites) [3,4] or MTBE from methanol and i-butene (zeolite, resin) [5,6] The yields of the desired products are often quite high, e g over 90 % in the case of 1,2,4-butanetriol to 3-hydroxy-tetrahydrofiiran and about 60 % in the case of dimethyl ether. The reactions are either carried out in the presence of water as slurry process [1,2] at 150 - 200 °C or at temperatures > 300 °C in the gas phase with a fixed bed catalyst [2-4]... 

Scheme 14.2 Divergent synthesis of poly(propylene imine) dendrimers (A and B) and possible side reactions resulting in defects and chain termination (C missed Michael additions D cyclisation).

Other applications for clayzic include the preparation of benzothiophenes by cyclisations of phenylthioacetals (normal catalysts can cause extensive poly-... 

The oxidation stability of poly( butylene isophthalate-co-terephthalate) copolyesters has been shown to decrease steadily with isophthalate unit content [26]. As expected, GC-MS analyses show that oxidation takes place mainly in the butylene units, through the same mechanism as before. Small-molecule products of a copolyester containing 25 mol% isophthalate included THF, butyrolactone, 3-buten-2-one, 2-propenal, and various other cyclised and carbonyl fragments, along with acetic acid. As has been observed for most polyesters, thermal and thermo-oxidative reactions occur simultaneously, and the lower stability of butylene-isophthalate units is most probably responsible for the lower overall stability of copolymers containing this structure, even under the oxidation conditions used. 

This means that water molecules evolve within the adhesive layer and must be removed to avoid the formation of large voided areas. To circumvent this drawback, research was focused on the synthesis of fully cyclised heterocyclic polymers, which are intrinsically thermoplastic, even though the Tg can be extremely high. Polyphenylquinoxalines, poly(isoimides), and poly(ether-imides) have been extensively studied to prepare structural adhesive films and pastes. 

Figure 19 Acetylene-terminated polyimide precursors synthesised by reacting BTDA 18 with less than the stoichiometric balance of 3,3 -(l,3-phenylenedioxy) bisbenzeneamine 44 and the complement to stoichiometry of 3-ethynylbenzeneamine 45 as end-capping agent. Low molecular weight polyamic acid 46 is then chanically cyclised to poly(isoimide) 47 or thermally cyclodehydrated to give polyimide 48.

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