Trans-l,4-Hexadienes

Compression molded (150°C for 3 minutes press chilled with cold water immediately thereafter) samples of poly(trans-l,4-hexadiene) (14) and poly(5-methyl-l,4-hexadiene) were examined with a General Electric (XRD-3) X-ray unit. Transmission Laue X-ray photographs were taken using nickel filtered copper X-radiation. Samples were stretched to four times of their original lengths to obtain oriented fibers. The fiber patterns were obtained in a flat plate film holder with the specimen to film distance standardized at 5 centimeters. X-ray diffraction patterns were similarly obtained for the hydrogenated sample of poly(5-methyl-l,4-hexadiene). 

We have reported earlier (14) that during the polymerization of trans-l,4-hexadiene with a Et3Al/6-TiCl3 catalyst (Al/Ti atomic ratio = 2) at 25°C, a major portion of the consumed monomer was converted to isomerized products, thereby accounting for the relatively low conversion to isotactic 1,2-polymer (Figure 1). The relative amounts of the hexadiene isomerization products were in the following order cis-2-trans-4-hexadiene> trans-2-trans-4-hexadiene> 1,3-hexadiene > 1,5-hexadiene >cis-2-cis-4-hexadiene. 

In contrast to the spectrum of isotactic trans-l,4-hexadiene polymer (Figure 5), the 300 MHz -H-NMR spectra of the 5-methylhexadiene polymer in both CCI4 and CgDg solutions exhibit only one peak for its backbone methylene protons. As in the case of cis-l,4-hexadiene polymer (14), the backbone methylene protons were not resolvable. The absence of a doublet for the methylene protons in these polymers does not necessarily preclude the possibility that they are isotactic. 

To clarify the tacticity problem, trans-l,4-hexadiene and 5-methyl-l,4-hexadiene polymers were examined by X-ray diffraction. Fiber diagrams were obtained from samples stretched to four times their original lengths. Eight reflections from the poly(trans-1,4-hexadiene) fiber pattern may be interpreted on the0basis of a pseudo-orthorhombic unit cell with a = 20.81 + 0.05 A b =... 

Silylene addition to dienes was first reported in 1968 (Scheme 17) (68JA3438). The l-silacyclopent-3-ene produced is again thought to result from a silirane intermediate, as a concerted 1,4-cycloaddition can be ruled out. Thermally generated silylene SiH2 does not add stereospecifically to trans,trans- and cis,trans-2,4-hexadiene but gives a mixture of the two silacyclopentenes (6 1 and 2 1 ratio) in each case (Scheme 18) (74JA6198). 

For the competitive epoxidation of cis- and fra s-2-octenes with 4a, the ratio of the formation rate of czs-2,3-epoxyoctane to that of the trans isomer is >3x10 , which is much larger than the ratios (1.3-11.5) reported for other stereospecific epoxidation systems. The epoxidation of 3-substituted cyclohexenes, such as 3-methyl-l-cyclohexene and 2-cycIohexen-l-ol, showed an unusual diastereoselectivity the corresponding epoxides were formed highly diastereoselectively with the oxirane ring trans to the substituents anti configuration) (Eq. 4.6). In addition, the more accessible but less nucleophilic double bonds in noncon-jugated dienes, such as frfl is-l,4-hexadiene, (R)-(-F)-limonene, 7-methyl-l,... 

Figure 1. Formation of the absorption spectra of trsLns-trans-2,4-hexadiene-l,6-dial and 2,4,6,8,10-dodecapentaenedial during uv irradiation of pure benzene in the pres-ence of air...

Hexaazadecalin photoelectron spectra, 3, 543 2,4,12,20,22,30-Hexaaza-1,5,13,29-tetrathia[l,2-30]-[5.2.3.2]paracyclophane nomenclature, 1, 27 Hexabenzof 18]crown-6 complexes, 7, 742 c/s, trans-2,4-Hexadiene synthesis, 1, 431 trans, trarts-2,4-Hexadiene synthesis, 1, 431 Hexahelicene... 

We have recently studied the addition of SiHj and Sip2 to trans-2- rans-4-hexadiene and its cis-cis and cis-trans isomers. A steric effect which decreases the reactivities of these hexadienes in comparison with that of butadiene is again observed. The relative reactivities of trans-l-transA-hexadiene toward silylenes in comparison with those of butadiene are around 0.6, while the corresponding values for c/s-2-c/s-4-hexadiene are only around 0.3. 

In this paper we wish to report the hydrogenation of cis-1,4-polybutadiene, cis-l,4-polyisoprene, cis-l,4-poly(2,3-dimethylbutadiene), and trans-l,4-poly(2,4-hexadiene) which were prepared with lanthanide polymerization catalysts. The physical and mechanical properties of these hydrogenated polymers are discussed. 

Polydienes such as 1,2-polybutadiene (3.125) 3,4-polyisoprene (3.126) 1,2-poly(trans-l,3-pentadiene) (3.127) and l,2-poly(l,4-hexadiene)s (3.128) under UV irradiation may undergo photocyclization according to the following mechanisms [795, 798, 799] ... 

In MeOH, l,4-dimethoxy-2-cyclohexene (379) is obtainejl from 1,3-cydo-hexadiene[315]. Acetoxylation and the intramolecular alkoxylation took place in the synthesis of the naturally occurring tetrahydrofuran derivative 380 and is another example of the selective introduction of different nucleo-philes[316]. In intramolecular 1,4-oxyacetoxylation to form the fused tetrahy-drofurans and tetrahydropyrans 381, cis addition takes place in the presence of a catalytic amount of LiCI, whereas the trans product is obtained in its absence[317]. The stereocontrolled oxaspirocyclization proceeds to afford the Irons product 382 in the presence of Li2C03 and the cis product in the presence of LiCl[ 318,319]. 

As we have indicated with our arrows, the mechanism of the uncatalyzed Cope rearrangement is a simple six-centered pericyclic process. Since the mechanism is so simple, it has been possible to study some rather subtle points, among them the question of whether the six-membered transition state is in the boat or the chair form. ° For the case of 3,4-dimethyl-l,5-hexadiene it was demonstrated conclusively that the transition state is in the chair form. This was shown by the stereospecific nature of the reaction The meso isomer gave the cis-trans product, while the ( ) compound gave the trans-trans diene. If the transition state is in the chair form (e.g., taking the meso isomer), one methyl must be axial and the other equatorial and the product must be the cis-trans alkene ... 

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