High-trans

Rather more success has been achieved by trans-polyoctenamer, which was introduced by Hiils in 1980 under the name Vestenamer. The two initial grades had trans -contents of approx. 80 and approx. 62%. The polymers have a low molecular weight, and above the T, which increases with increasing transcontent once the latter exceeds 30% and is about 60°C with the high trans-grades, the polymer on its own has a very low melt viscosity. Below the melting... 

Conditions under which the reaction is directed solely toward the formation of the sulfide 46 (yield of up to 89.5%) have been reported (79ZOR1554) Liquid ammonia is used as a solvent, whereas sulfide ions are generated by ammonium sulfide formed directly in the reaction mixture from ammonia and hydrogen sulfide. The sulfide 46 possesses the Z,Z-configuration, providing evidence for a high trans stereoselectivity of the reaction (79ZOR1554). 

The above described approach was extended to include the 1,3-dipolar cycloaddition reaction of nitrones with allyl alcohol (Scheme 6.35) [78]. The zinc catalyst which is used in a stoichiometric amount is generated from allyl alcohol 45, Et2Zn, (R,J )-diisopropyltartrate (DIPT) and EtZnCl. Addition of the nitrone 52a leads to primarily tmns-53a which is obtained in a moderate yield, however, with high ee of up to 95%. Application of 52b as the nitrone in the reaction leads to higher yields of 53b (47-68%), high trans selectivities and up to 93% ee. Compared to other metal-catalyzed asymmetric 1,3-dipolar cycloaddition reactions of... 

Due to the presence of residual double bonds, the polymer could be cross inked with regular agents. TPR is a linear polymer with a high trans configuration. It is highly amorphous at normal temperatures and has a Tg of about 90°C and a density of 0.85. 

Allenols can be converted into terminal vinylepoxides by a Pd(0)-catalyzed insertion of aryl or vinyl halides [81]. The reactions take place with high trans-selectivity and in good yields (Scheme 9.17a). Chiral 2,3-allenols, which can be easily... 

The cyclization of ro-[2-(4-bromo-2-butenyloxy)phenyl]alkanals18, which are prone to 1.4-elimination at the stage of the metalorganic intermediate, constitutes a severe test to the viability of this method. Nevertheless, the benzodihydrooxepin derivatives are formed with good yield and high trans(anti) selectivity18,... 

This 1,2-asymmetric induction has been attributed to stcric and stcrcoclectronic factors. Similarly, the cuprate additions to 4-alkylcyclopentenones l7 -19, and 4-alkylcyclohexcnones16 b-18 proceeded with very high trans diastereoselection. The copper iodide catalyzed addition of propylmagnesium bromide to 4-methyl-2-cyclohexenone gave a trans/cis ratio of 80 20, whereas the addition to 5-methyl-2-cyclohexenone produced a transjcis ratio of 93 72 3-Silyloxy system 3 gave the trans-adduct 4 on treatment with butylcopper-boron trifluoride reagent20. 

The reversal of the stereoselectivity is attributed to the ability of chlorotrimethylsilane to trap the initially formed cuprate-enone complex, thereby suppressing equilibration of the diastereomeric complexes. The copper-catalyzed 1,4-addition of Grignard reagents to 5-substituted 2-cyclo-hexenone also proceeded with very high trans diastereoselectivity22. 

The addition of a-(acylamino) esters to 3-aryl-2-propenoates, with sodium ethoxide in ethanol or sodium hydride in benzene as base, is a frequently ultilized procedure9-" A The initial Michael adducts cyclize to 3-aryl-5-oxo-2-pyrrolidinecarboxylic acids with modest to high trans diastereoselectivities 10°. 

The structures of the two tri(n-butyl)phosphine complexes of this type have been determined (Figure 2.58), again showing the high trans-influence of PR3 compared with Cl (Figure 2.58). 

Since thiourea has a high trans-influence, the ammonia trans to it is replaced, repetition of the sequence causing the formation of yellow needles of Pt(tu)4Cl2. 

A combination of neutral and anionic ligand displacements is shown in Eq. (55) 115) it was suggested that the high trans influence of Me3Si leads to Pt—Cl bond weakening and hence the ionic product rather than cis-Me3SiPtCl(diphos) [which is a stable compound 69, 71)]. 

The magnitude of J(Pt—P) for some phosphine-substituted SiPt complexes depends on the covalency of the Pt—P bond (136). Such a bond is relatively ionic in a system where it lies trans to a ligand of high trans influence and J(Pt—P) is then small. The coupling in cis complexes decreased in the order Cl > C > Si and since a comparable situation has been observed for methyl-Pt compounds it is suggested that the effect is one of a- rather than tr-interaction. 

A more practical catalytic system is based on the use of air-stable [Fe(TTP)Cl] as catalyst, which can efficiently catalyze the olefination reactions at 80°C in excellent yields and high trans-selectivity [71] (Scheme 18). 

Both substituents at the Sn atoms are highly trans- mA, and the Sn-Sn bond length of 2.8123(9) A is quite normal for the Sn-Sn single bond. 51-K features an ESR signal (g = 2.0069) together with the satellites, hfccs of which were... 

Such representation with a formal Sn-Sn bond order of 1.5 and highly trans-btrA substituents is suggestive of the diminished Sn-Sn hybridization resulting from the poor 5p -5p -orbitals overlap to form weak tt-bonds. The anion radical sodium salt 51-Na, prepared similarly by the reaction of chlorostannylene 52 with sodium an-thracenide in THF, exhibited structural and spectral features identical to those of the above-described potassium derivative 51-K (Scheme 2.39). ... 

In any consideration of stereoselectivity in radical addition to acylic substrates, interpretation of the results is complicated by the knowledge that alkenes may be converted, at least in part, into their geometrical isomerides by traces of bromine (or of HBr, i.e. by Br, cf. p. 315). This may, however, be minimised by working at low temperatures, and by using a high concentration of HBr. Thus addition of liquid HBr at -80° to cis 2-bromobut-2-ene (67) was found to proceed with high TRANS stereoselectivity, and to yield (68) almost exclusively ... 

To account for this very high TRANS stereoselectivity, it has been suggested that addition proceeds via a cyclic bromonium radical (71), analogous to the cyclic bromonium cations involved in the polar addition of bromine to alkenes (p. 180) ... 

In fact, by fractional precipitation we found that the fractions with the highest molecular weights are sterically very uniform and contain more than 97 % of cis-1,4 double bonds (Table IV). Lower molecular weight fractions, on the other hand, have relatively high trans-1,4 contents. We therefore take the view that the rate of crystallization is determined mainly by the high molecular weight fractions. 

Thus, this polymerization catalyst yields more random copolymers with styrene, while still maintaining a low vinyl content and high trans-1,4 content. 

Comparison of Strain Induced Crystallization Behavior of NR with High Trans SBR s from Ba-Mg-Al and Ba-Li Catalysts... 

Figure 19 shows the temperature dependence of the percent crystallinity for high trans SBR, prepared with a Ba-Li catalyst and containing 757. trans-1,4 content with 14 wt.7. styrene, at 3 extension ratios. The percent crystallinity that develops is temperature dependent, there being an increase in the amount of crystallinity with a decrease in temperature. However, the amount of crystallinity that develops is essentially independent of strain. The amount of crystallinity that develops at room temperature, regardless of the level of strain, is extremely small ( 9) ... 

Figure 20 shows the percent crystallinity as a function of temperature and extension ratio for high trans SBR (227. styrene, 877. trans) prepared with a Ba-Mg-Al catalyst. A... 

Figure 17. X-ray diffraction patterns for high trans-SBi (15% styrene, 85%...

In observing the time dependent changes in birefringence and stress-optical coefficient, for elongated samples at 25 C, it was found that the rate of crystallization of high trans SBR s was very much faster, some 10 times more rapid, than that for NR (8). This is consistent with the reported rates of isothermal crystallization for NR (2.5 hours at -26°C) and for 807. trans-1,4 polybutadiene (0.3 hours at -3°C) in the relaxed state (12). 

The main conclusions of the strain induced crystallization behavior of high trans polybutadiene based rubber and natural rubber are (1) the rate of crystallization is extremely rapid compared to that of NR (2) the amount of strain induced crystallization is small compared to that of NR, especially at room temperature and (3) for the high trans SBR s relative to NR, crystallization is more sensitive to temperature at low extension ratios, and crystallization is less sensitive to strain. 

Two additional properties that may depend on the strain induced crystallization behavior of NR are green strength and building tack. A comparison of the performance of the experimental high trans SBR s with NR was, therefore, carried out. 

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