Cis- and trans- 1,3-pentadienes

It is noteworthy that, at variance with bromination and chlorination which generally occur without isomerization of the disubstituted double bond, fluorine addition to the 1,2-bond of cis- and trans-1,3-pentadienes gives mainly the trans-adduct 13, besides smaller amounts of compounds 14-16 (equation 24). 

Later on, product distribution studies15 of the ionic addition of chlorine to conjugated dienes, and in particular to cyclopentadiene, 1,3-cyclohexadiene, cis,cis-, trans,trans-and c ,fraws-2,4-hexadienes, and cis- and trans-1,3-pentadienes have supplied the first stereochemical data, showing that the stereochemistry of 1,4-addition is predominantly syn, although to an extent smaller than that of bromine addition. Moreover, the 1,2-addition is generally non stereoselective, except for the addition to the 3,4-bond of cis-and trans-1,3-pentadienes where the attack is 89-95% anti. Finally, appreciable amounts of cis- 1,2-dichlorides were obtained from the two cyclic dienes, whereas 2,4-hexadienes showed a preference for anti 1,2-addition, at least in the less polar solvents (carbon tetrachloride and pentane). On the basis of all these results the mechanism shown in equation 29 was proposed. 

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. 

Figure 19.7. Cyclic batch elution chromatography obtaining high product purity and high throughput by using incomplete resolution (overlapping bands) and recycling the mixed fraction (mf) to the feedstock (a) Control of band separation and cut points determines fractional impurities t mij mr and Ami /mr2-<4l)> (b) Chromatogram for separation of pure ds- and trans- 1,3-pentadiene. Components 1, isoprene 2, trans- 1,3-pentadiene 3, cis-l,3-pentadiene 4, cyclo-pentadiene. Component 1 is eluted at almost the same time as component 4 of the...

The 1,4 polymerization of trans-1.3-pentadiene has been studied by Natta, Porri and coworkers. Their results also show that the cis polymerization occurs with catalysts of ionicities in the middle region, while the trans structures come in the more ionic regions. The methyl group at the end of the diene systems shifts the transition points... 

Inomata 211) studied the H-NMR spectra of poly(penta-1,3-diene) and concluded that with hexane as polymerization medium the polymers were about 49% cis-1,4 and 40% trans-1,4 enchained. The polymer derived from the cis monomer had 12% of 1,2-units which were exclusively trans that from the trans monomer had some 10 % of 1,2-units, two thirds of which were trans. Aubert et al.2I6) made a more extensive study of pentadiene polymers using both1H and 13C-NMR spectroscopy and modified the cis and trans-1,4 methyl resonance assignments made by Inomata 2U). 

Let us examine separately the case of the cis and trans isomers of pentadiene. For the trans isomer the cis conformation is permissible so that one cannot assume a priori that this isomer won t coordinate to Ti by the two double bonds. This hypothesis, however, can be easily rejected by the following considerations. If the steric situation around Ti during the polymerization were to permit the coordination of the trans isomer of pentadiene by the two double bonds, in the cis conformation, butadiene or isoprene should also coordinate the same way. In this case, however, cis-1,4 units should be obtained both from butadiene and isoprene, and not 1,2 and 3,4, respectively, as observed. It seems reasonable to conclude, therefore, that the trans isomer of pentadiene coordinates to Ti by the vinyl group only, as butadiene or isoprene, before it is incorporated as a cis-1,4 unit. 

However, as the temperature rises this is rapidly converted to the TT-allylic complex. Similar complexes of conjugated dienes involving complexing of only one double bond have been observed with trans-1,3-pentadiene, cis,trans-2, i-hexadiene, and 1,3-cyclooctadiene. 

Trcns-1,3-pentadiene and isoprene copolymerize with Ti(On-Bu)4/ AlEta [293]. Isoprene units in the copolymer chain are predominantly of 3,4 structure while the pentadiene units are mixed of cis and trans 1,4, 1,2 and 3,4 structure composition was independent of Al/Ti ratio. Kinetic measurements were not reported but reactivity ratios (Table 26) show somewhat greater reactivity of the pentadiene and a tendency for alternation. 

The pH-independent reaction of cyclopentadiene oxide 12 in water is clearly different from that of 13.97 This reaction yields 33% of 3-cyclopentenone, 35% of cis-2,4-pentadienal and 35% of a mixture of cis and trans 1,2- and 1,4-diols (Scheme 28). When the reaction is carried out in D20 instead of H20, no deuterium is incorporated into the ketone product. Thus, 1,2-hydrogen migration is required for this reaction just as it is in the rearrangement of arene oxides to phenols. The mechanisms of product formation in this reaction are not fully understood. The observation that the diol mixture is similar to that from the acid-catalyzed hydrolysis of 12 suggests that an allylic carbocation may be involved in the diol-forming reaction. Ketone (96) and dienal (97) products are potentially formed either by stepwise or concerted mechanisms, and there is insufficient evidence to rule out either one. There is a significant normal salt effect on this pH-independent... 

FIGURE 14.2-6 Preparation of pure cis- and rrans-l. 3-pentadiene. (ti) Analysis of the feedstock. Column diameter = j in., 6 m long packed with 80-100 mesh Chromosorb P coated with 10% 2.2 oxydipropio-nitrile temperature 30°C. (1) isoprene (2) rrans-1,3-pentadiene (3) c/s-l,3-pentadiene (4) cyclopenta-diene. (b) Preparative chromatogram. Column diameter = 4 cm, 4 m long packed with 20% squalane on 60-80 mesh Chromosoib P temperature 35°C, helium flow-rate 3 L/min (5 cm/s). Injection 4 mL every 20 min. The isoprene of one injection is eluted at about the same time as the cyclopentadiene of the previous injection, (c) Analysis of the trans-l,3-pentadiene prepared (purity 99.8%, cyclopentadiene —1.5 ppm). Reproduced from the Journal of Chromatographic Science by permission of Preston Publications, Inc. 

The pol)nners obtained from trans-1,3-pentadiene with lanthanide catalysts have a cis-1,4 content of about 60-80%, and trans-1,2 content of 20-33%... 

The presence of two bulky substituents at C-1 position of a diene prevents the adoption of s-cis conformation of the diene and decreases the reaction rate. For example, 4-methyl-l,3-pentadiene is about 1000 times less reactive than trans-1,3-pentadiene toward tetracyanoethene [36]. 

Radiation polymerization of cis- and tram-1-3-penta-diene " trans-2 -methyl-l,3-pentadiene, tram-3-methyl-1,3-pentadiene, 4-methyl-1,3-pentadiene, cis,cis- and cis, tram-2,4-hexadiene was carried out. Obtained products were characterized by Ir spectrometry, NMR and X-ray analysis. It was found that the polymers obtained have head-to-tail tram-1,4 stmctures and in some cases show crystallinity and stereoregularity. 

The other type of steric effect has to do with interactions between diene substituents. Adoption of the s-cis conformation of the diene in the transition state brings the m-oriented 1- and 4-substituents on the diene close together, trans-1,3-Pentadiene is 10 times more reactive than 4-methyl-1,3-pentadiene toward the very reactive dienophile tetracyanoethylene. This is because of the unfavorable interaction between the additional methyl substituent and the C-1 hydrogen in the s-cis conformation. ... 

French researchers [38c] have investigated the /zetero-Diels-Alder reaction of methylglyoxylate and glyoxal monoacetal with 2-methyl-1,3-pentadiene in a microwave oven under various reaction conditions (Table 4.9). The microwave (MW) irradiation does not affect the diastereoisomeric ratio of adducts trans/cis = 70 30) but dramatically reduces the reaction time. The glyoxal monoacetal, for instance, gives 82 % adducts after 5 minutes when submitted to irradiation with an incident power (IP) of 600 W in PhH and in the presence of ZnCL (Table 4.9, entry 1), while no reaction occurs if carried out for 4h at 140 °C in sole PhH. Similarly, methylgloxylate in water at 140 °C gives 82% adducts after 3h, whereas microwave irradiation reduces the reaction time to 8 minutes (Table 4.9, entry 5). 

According to the conformational energy minima, isotactic trans- 1,4-poly (1,3-pentadiene)73,74,90 - 94 and trans-, 4-poly(2-methyl-1,3-pcntadicnc)95 are characterized by chains in the conformation (A trans A+T)n (tl symmetry) and chain axes c values of 4.85 and 4.82 A, respectively. The conformation (A cisA 1 T) with s(2/l) symmetry characterizes the chains in the structures of isotactic m-l,4-poly(l,3-peiiladiene)96 98 and cis-1,4-poly (2-methyl-1,3-pentadiene).85... 

Furthermore, in the addition to the 3,4-bond of 1,3-pentadienes, the anti stereoselectivity observed with both bromine and chlorine has been attributed to a tightly bridged bromonium ion intermediate involving less charge dispersal in the vinyl group. In support of this hypothesis, it has been noted that bromine addition to the terminal double bond of the 1,3-pentadienes occurs without isomerization of the internal cis or trans double bond15. 

On the other hand, l-ethoxy-l,3-pentadiene (44a) and l-(trimethylsilyloxy)-1,3-pentadiene (44b), as mixtures of four isomers, gave low yields of 45a and [16], respectively, besides a complex mixture of products including cis and trans-46, head-to-head dimers of 4 [7a, 17] (see Sect. 4.1.1) (Scheme 9). 

The combination of cis-trans isomerism with iso-syndio and erythro-threo dispositions gives complex stractures as exemplified by the 1,4 polymers of 1-or 4-monosubstituted butadienes, such as 1,3-pentadiene (72, 73), and 2,4-pentadienoic acid (74, 75) and of 1,4-disubstituted butadienes, for example, sorbic acid (76). This last example is described in 32-35 (Scheme 6, rotated Fischer projection). Due to the presence of three elements of stereoisomerism for each monomer unit (two tertiary carbons and the double bond) these polymers have been classed as tritactic. Ignoring optical antipodes, eight stereoregular 1,4 structures are possible, four cis-tactic and four trans-tactic. In each series (cis, trans) we have two diisotactic and two disyndiotactic polymers characterized by the terms erythro and threo in accordance with the preceding explanation. It should be noted that here the erythro-threo relationship refers to adjacent substituents that belong to two successive monomer units. 

The all-trans-all-isotactic and all-trans-all-syndiotactic structures for the 1,4-polymerization of 1,3-pentadiene are shown in Fig. 8-6. In naming polymers with both types of stereoisomerism, that due to cis-trans isomerism is named first unless it is indicated after the prefix poly. Thus, the all-trans-all-isotactic polymer is named as transisotactic l,4-poly(l,3-penta-diene) or isotactic poly( -3-methylbut-l-ene-l,4-diyl). 

At least four applications of this technique can be cited. Quantum yields for triplet formation in benzene108 and fluorobenzene109 have been estimated by comparing the phosphorescence yields of biacetyl produced by sensitization to that produced by direct irradiation. Intersystem crossing yields of a number of organic molecules in solution have been obtained by measuring the quantum yield with which they photosensitize the cis-trans isomerization of piperylene (1,3-pentadiene) and other olefins.110 As will be discussed later, the triplet states of... 

A plot of the effect of ionicities of the catalyst and the structures which are produced and the requirements for the production of cis polybutadiene, cis polyisoprene and cis pentadiene is shown in Fig. 14. Cis and trans poly-1.3-dienes are produced according to the balance of ionicities of the catalyst. 

The reactivities of dienes in the Diels-Alder reaction depend on the number and kind of substituents they possess. The larger the substituents are, or the more of them, at the ends of the conjugated system, the slower the reaction is likely to be. There also is a marked difference in reactivity with diene configuration. Thus trans- i, 3-pentadiene is substantially less reactive toward a given dienophile (such as maleic anhydride) than is cis-1,3-pentadiene. In fact,... 

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