1,3-Pentadiene, trans isomer

In the presence of certain ZNCs, an equilibrium exists between cis- and tra 5 -l,3-penta-diene. Here, the cw-l,4-polypentadiene is formed from trara-l,3-pentadiene or from a mixture of the cis and trans isomers. 

As with the Aratani catalysts, enantioselectivities for cyclopropane formation with 4 and 5 are responsive to the steric bulk of the diazo ester, are higher for the trans isomer than for the cis form, and are influenced by the absolute configuration of a chiral diazo ester (d- and 1-menthyl diazoacetate), although not to the same degree as reported for 2 in Tables 5.1 and 5.2. 1,3-Butadiene and 4-methyl- 1,3-pentadiene, whose higher reactivities for metal carbene addition result in higher product yields than do terminal alkenes, form cyclopropane products with 97% ee in reactions with d-men thy 1 diazoacetate (Eq. 5.8). Regiocontrol is complete, but diastereocontrol (trans cis selectivity) is only moderate. 

By polymerizing the trans isomer of 1,3-pentadiene two different types of crystalline cis-1,4 polymers have been obtained, one with an isotactic, the other with a syndiotactic structure. The isotactic polymer was obtained by homogeneous systems from an aluminum alkyl chloride and a cobalt compound, the syndiotactic one by homogeneous systems from an aluminum trialkyl and a titanium alkoxide. Some features of the polymerization by Ti and Co catalysts are examined. IR and x-ray spectra, and some physical properties of the crystalline cis-1,4 polymers are presented. The mode of coordination of the monomer to the catalyst, and possible mechanisms for the stereospecific polymerization of pentadiene to cis-1,4 stereoisomers are discussed. 

The cis-1,4 syndiotactic polypentadiene has been obtained by soluble catalysts prepared from an aluminum alkyl chloride and a cobalt compound. Only the trans isomer of pentadiene has been polymerized by these catalysts. However, mixtures of the cis and trans isomers can be used, the cis isomer remaining seemingly unaltered. The presence of the cis isomer was found to have no appreciable influence on the stereospecificity when its percentage in the mixture of the two isomers is lower than about 30%. For higher percentages a decrease in the stereoregularity of the syndiotactic polymer obtained was observed. 

Shortly after the cis-1,4 syndiotactic polypentadiene was obtained and characterized, it was observed that the homogeneous systems prepared from an aluminum trialkyl and a titanium tetralkoxide can polymerize pentadiene to polymers predominantly cis-1,4. Both the cis and trans isomers of pentadiene are polymerized by these systems. However, while the crude polymers obtained from the cis isomer were found to be amorphous by x-ray examination, those obtained from the trans isomer were found to be crystalline. The type of crystallinity of these polymers appeared different from that of the polymers... 

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. 

The Co catalysts polymerize only the trans isomer of pentadiene. 

This monomer is usually obtained as a mixture of the cis and trans isomers both of which have been polymerized with coordination type catalysts. Polymerization of the cis form is considered to be preceded by isomerization, since those catalysts which do not isomerize the cis monomer (e.g. cobalt salt—organo aluminium halide) selectively polymerize the trans isomer. A kinetic study of the polymerization of cis 1,3-pentadiene using Ti(OBu-n)4/AlEt3 (Al/Ti = 1.3—6) as catalyst has been published [267]. This gives a polymer containing ca. 73% cis 1,4 15—16% trans 1,4 and 11—12% 3,4 microstructure. 

Zirconocene complexes (with s-cis geometry) of isoprene, 2,3-dimethylbutadiene, and 3-methyI-I,3-pentadiene are reported to give exclusively 1 1 addition with carbonyl substrates even if these are used in excess and the reaction temperature is fairly high (ca. 100 C). On the contrary, zirconocene complexes of 5-ci5-butadiene, 1,3-pentadiene, and 2,4-hexadiene ca, 1 1 mixture of the s-cis and s-trans isomers) easily accept (equation 53) 2 equiv. of either butanal or 3-pentanone, at low temperature ca. 30 C) in high yields (95%). This can be exploited for the stepwise insertion of two different electrophiles... 

Fig. 2.2 [64] shows chromatograms of commercial-grade 1,3-pentadiene, obtained on two columns connected in series, the first column containing a saturated solution of silver nitrate in ethylene glycol and the second containing chloromaleic anhydride reacting with a trans isomer. As a result of the reaction the relative content of the irons isomer decreases. The pseudo-first-order rate constant varied from 2.0-10 to 1.4 10 sec (the half-time of transformation changed from 30 to 100 sec) [64]. 

Fig. 2.2. Chromatograms of 1,3-pentadiene on a composite column containing silver nitrate (column 1) and a column reactor containing chloromaleic anhydride (column 2) at40°C [64]. Carrier gas (helium) flow-rate (A) 77ml/min (B) 11.4ml/min. First peak area (trans isomer) (A) 60% (B) 31%. From ref. 64.

If 1,3-pentadiene, either the cis or the trans isomer, dissolved in a river water sample, is irradiated, isomerization occurs (p. 213) producing a photostationary state (Fig. 6.13). The same steady-state (60% trans) was achieved starting with either isomer and reflects a balance between the cis trans and the trans cis processes. The composition of the photostationary state observed with different water samples and humic acids are summarized in Table 6.11. Solutions were adjusted to be optically matched each showing an absorbance of 0.20 at 366 nm. A small effect of wavelength is observed with smaller proportions of the trans isomer observed at 313 nm compared to 366 nm. 

Pentadiene Dissolved in % trans Isomer at Steady State ... 

PEI-RhCl3 and PEI-RUCI3 complexes exhibit different catalytic activity with regard to a mixture of the cis- and trans isomers of pentadiene [61]. The former complex is more selective for pentene (0.94) than the latter. A quantitative aniline yield results from the reduction of nitrobenzene in the presence of PEI complexes with Ni(II), Co(ll), Sn(II), Pd(II) and Rh(III) [61]. The reduction proceeds rapidly at 20- 70°C and at 1-25 atm pressure of H2 both in a solvent and without. Besides aniline, cyclohex-ylamine is produced by further hydrogenation of the aromatic ring in the presence of the PEI-Rh(III) complex. Polymer-metal catalysts do not lose their catalytic stability after repeated application. For example, when eight hydrogenation reactions were catalyzed with PEI-Pd(II), in each case a 100% product yield of aniline was attained. 

Plan Because the compound is named pentene and not pentadiene or pentatriene, we know that the five-carbon chain contains only one carbon carbon double bond. Thus, we be n by placing the double bond in various locations along the chain, remembering that the chain can be numbered from either end. After finding the different unique locations for the double bond, we consider whether the molecule can have cis and trans isomers. 

Mixture with trans isomer. - Radical also generated from 1,4-pentadiene and t-butoxyl and ethoxyl radicals. ) INDO calculations of spin densities. ) da/dr = 2.7 10 TK from 103 to 182 K. ) Assignment to particular hydrogens uncertain. ") du/dr=1.5-10- TK froml03tol82K. ... 

With these eatalysts a mixture of cis- and tm y-l,3-pentadienes in a wide range could be polymerized. But the polymer obtained from the trans isomer is more clean and crystalline. In the titanium catalyst the Al/Ti ratio plays an important role for the molecular weight. With an increasing Al/Ti ratio, the molecular weight of the polymer deereases [370,371]. An optimal values is Al/Ti = 7. 

The cis and trans isomers of piperylene, i.e., cis- and /m 5-l,3-pentadienes, differ markedly in their reaction with MA. The trans-isomer readily forms a Diels-Alder adduct with MA at 35-70 C (see Chapter 4), while the c/5-isomer reacts very reluctantly even at 100 C to yield a resinous product accompanied by a small amount of adduct.Dioxane and toluene... 

Piperidine, isomerization catalyst, 483, 485 Piperylene, cis and trans isomers MA Diels-Alder reaction, 106 see also 1,3-pentadiene... 

The isomer distributions were measured at <25% butadiene conversion. T 1C = trans/ cis ratio of 1,4-hexadiene 3-MeP=3-methyl-l,4-pentadiene. 

The isomer distribution of the nickel catalyst system in general is similar qualitatively to that of the Rh catalyst system described earlier. However, quantitatively it is quite different. In the Rh system the 1,2-adduct, i.e., 3-methyl-1,4-hexadiene is about 1-3% of the total C6 products formed, while in the Ni system it varies from 6 to 17% depending on the phosphine used. There is a distinct trend that the amount of this isomer increases with increasing donor property of the phosphine ligands (see Table X). The quantity of 3-methyl-1,4-pentadiene produced is not affected by butadiene conversion. On the other hand the formation of 2,4-hexadienes which consists of three geometric isomers—trans-trans, trans-cis, and cis-cis—is controlled by butadiene conversion. However, the double-bond isomerization reaction of 1,4-hexadiene to 2,4-hexadiene by the nickel catalyst is significantly slower than that by the Rh catalyst. Thus at the same level of butadiene conversion, the nickel catalyst produces significantly less 2,4-hexadiene (see Fig. 2). 

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