Disubstituted ethylenes asymmetric

Complications arising from other types of isomerism. Positional and geometrical isomerism, also described in Sec. 1.6, will be excluded for simplicity. In actual polymers these are not always so easily ignored. Polymerization of 1,2-disubstituted ethylenes. Since these introduce two different asymmetric carbons into the polymer backbone (second substituent Y), they have the potential to display ditacticity. Our attention to these is limited to the illustration of some terminology which is derived from carbohydrate nomenclature (structures [IX]-[XII]) ... 

Subsequently, aliphatic 1-substituted, 1,1-, and 1,2-disubstituted ethylenes were investigated (3) using RhH(CO)(PPh3)3 in the presence of (-)DIOP (diisooctylphthalate) (4) as the chiral ligand, and asymmetric induction was shown to occur during or before the formation of a rhodium-alkyl intermediate (R-[Rh]). This formation appears to be practically irreversible under the reaction conditions used (5). As another consequence of this important feature, regioselection in hy-droformylation is likely to occur during or before the formation of the rhodium-alkyl intermediates. 

Fig. 4. Enantiofaces preferentially attacked in asymmetric hydroformylation of 1,1-disubstituted ethylenes with the Rh/(—)-DIOP catalytic system...

As already mentioned, in the cases in which a very large excess of a single chiral product is formed, the enantiomeric excess gives a good indication of the enantioface which reacts preferentially. If we take this into consideration, it appears from the results of the asymmetric hydroformylation of 1,1-disubstituted ethylenes that opposite enantiofaces can preferentially react when Pt or Rh is used together with the same chiral ligand ((-)-DIOP or (-)-CHIRAPHOS)52) (Table 8). The same is true when styrene is used as a substrate 28,52) (Table 7). 

These results show that, at least for the hydroformylation of 1,1-disubstituted ethylenes and styrene, the geometry of the complexes, including the chirality of the metal atom and possible differences in the prevailing conformation of the ligands, is relevant in determining the asymmetric induction. 

Table 12. Comparison between experimental results and predictions in the asymmetric hydroformyiation of 1,1-disubstituted ethylenes by different catalytic systems... 

The same ligand can also give opposite results when used with different metals (platinum or rhodium). In some cases 1,1-disubstituted ethylenes and styrene converted in the presence of platinum and rhodium complexes modified with the same chiral ligand give opposite results, while with other alkenes [( /Z)-2-butenes] this is not observed. Thus, at least in some cases, the overall metal center geometry, and not solely the ligand configuration, is relevant for the direction of asymmetric induction. 

Several possibilities exist for ditacticity in macromolecules formed by polymerizing 1,2-disubstituted ethylenes of the type RHC=CHR, structures (XVIII)-(XX). It can be seen that each unit contains two different asymmetric carbon atoms in the chain. The original definitions of tacticity have been extended to include these modifications and Newman s (1956) definitions of erythro and threo structures. Structures (XVIII)-(XX) illustrate ifereo-diisotactic, erythro-diisotactic, and disyndiotactic poisoners, respectively. 

Stereospecific polymerization of mono- or 1,1-disub-stituted ethylenes. Historical developments. In a polymerization of mono- or 1,1-disubstituted ethylenes, including the 1,2-polymerization of butadiene, macromolecular chains are generated that have a backbone consisting exclusively of carbon atoms, and, in general, every second carbon atom is asymmetric. ... 

The 7 -symmetric complex ethylene bis(tetrahydroindenyl) titanium l,T-binaphth-2,2 -dithiolate has been used to catalyze the asymmetric hydrogenation of unfunctionalized trisubstituted olefins.1680 The kinetic resolution of racemic disubstituted 1-pyrrolidines via asymmetric reduction has been described.1681... 

Quite a considerable number of papers deal with the effect of structure of olefinic substrates on their reactivity in the catalytic hydrogenation 65). Lebedev 66) attempted a generalization of the problem. His conclusion that the rate of hydrogenation of olefins decreases in the order monosubstituted - symmetric disubstituted - asymmetric disubstituted - trisubstituted tet-rasubstituted ethylene derivatives is called the Lebedev rule. Campbell 67) supplemented it by demonstrating that the rate of hydrogenation decreases with the number and size of substituents on carbon atoms of the double bond, cis isomers are usually hydrogenated more quickly than trans isomers, and olefins containing the terminal double bond are more reactive than those with the double bond inside the chain. 

With mono- or asymmetrically dialkylated ethylene oxides one obtains, in general, after hydrolysis of the products of alkylation, 2-substituted or 2,2-disubstituted primary alcohols. Only one alkyl group of the trialkyl alanes reacts with the epoxide function (124), e.g.,... 

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