Ethylene, 1,2-disubstituted

For disubstituted ethylenes, the presence and type of tacticity depends on the positions of substitution and the identity of the substituents. In the polymerization of a 1,1-disubstituted ethylene, CH2=CRR, stereoisomerism does not exist if the R and R groups are the same (e.g., isobutylene and vinylidene chloride). When R and R are different (e.g., —CH3 and —COOCH3 in methyl methacrylate), stereoisomerism occurs exactly as in the case of a monosubstituted ethylene. The methyl groups can be located all above or all below the plane of the polymer chain (isotactic), alternately above and below (syndiotactic), or randomly (atactic). The presence of the second substituent has no effect on the situation since steric placement of the first substituent automatically fixes that of the second. The second substituent is isotactic if the first is isotactic, syndiotactic if the first substituent is syndiotactic, and atactic if the first is atactic. 

The polymerization of 1,2-disubstituted ethylenes, RCH=CHR, such as 2-pentene (R = — CH3, R = —C2H5), presents a different situation. Polymerization yields a polymer structure II in which there are two different stereocenters in each repeating unit. Several possibilities of ditacticity exist that involve different combinations of tacticity for the two stereocenters. Various stereoregular structures can be defined as shown in Fig. 8-2. Diisotactic structures occur when placement at each of the two stereocenters is isotactic. 

The stereocenters in all three stereoregular polymers are achirotopic. The polymers are achiral and do not possess optical activity. The diisotactic polymers contain mirror planes perpendicular to the polymer chain axis. The disyndiotactic polymer has a mirror glide plane of symmetry. The latter refers to superposition of the disyndiotactic structure with its mirror image after one performs a glide operation. A glide operation involves movement of one structure relative to the other by sliding one polymer chain axis parallel to the other chain axis. 

It should be noted that other polymer structures can be postulated—those where one substituent is atactic while the other is either isotactic or syndiotactic or those where one substituent is isotactic while the other is syndiotactic. However, these possibilities are rarely observed since the factors that lead to ordering or disordering of one substituent during polymerization generally have the same effect on the other substituent. An exception is the formation of hemiisotactic polypropene where isotactic placements alternate with atactic placements [Coates, 2000]. 

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In this representation the X indicates the substituent other bonds involve only hydrogens. This formalism also applies to 1,1-disubstituted ethylenes in whicli the substituents are different. With these symbols, the isotactic, syndiotactic, and atactic structures shown in Fig. 1.2 are represented by structures [VI]-[VIII], respectively ... 

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]) ... 

The only physical property which has been studied for substituted vinylidene sets is the nmr chemical shift of the vinylidene proton in substituted ethylenes and in tra s-l,2-disubstituted ethylenes. The first attempt at correlating chemical shift data for substituted ethylenes with the Hammett equation appears to be the work of Banwell and Sheppard (53), who reported a correlation of A2 values with the or constants, the A2 values being defined by the equation... 

Chemical shifts of cis protons in substituted ethylenes and in trans-1,2-disubstituted ethylenes have been correlated with the Hammett equation in... 

We may now proceed to an analysis of the significance of the d values. If the transition state were close to the product, then the P values of the disubstituted ethylenes should be small, because in the product, the substituent is bonded to an sp hybridized carbon atom. Thus, it is incapable of resonance interaction. As the observed values of p for the trans-disubstituted dienophiles are very large, we conclude that the transition state is closer to reactants than to products. The cis-disubstituted dienophiles show a much smaller value of P than do the trans compounds. It therefore seems likely that the transition state for the CIS compounds will be closer to product than is the transition state for the trans compounds. The values of p for the reaction of the tnws-disubstituted dienophiles with 9,10-dimethylanthracene, while large, are much smaller than... 

Nenajdenko et al. described the first example of addition of a 1,2-dication to C-C mutiple bonds. The only S-S dication found to participate in this reaction was the highly strained dication 115 derived from 1,4-dithiane. The reaction with alkenes 119 proceeded under mild conditions and led to derivatives of dithioniabicyclo[2.2.2]octane 120 as shown in Equation (33) and Table 21 <1998JOC2168>. This reaction was sensitive to steric factors and proceeded only with mono and 1,2-disubstituted ethylenes. Only alkenes conjugated with aromatic or cyclopropane moieties underwent this reaction. For the 1,2-disubstituted alkenes used in this study, the relative configuration of substitutents at the double bond was preserved and only one diastereomer was formed (see entries 2 and 3). 

Reaction with alkenes is sensitive to steric factors - in the case of dication 49, only reaction with mono- and 1,2-disubstituted ethylenes afforded identifiable reaction products. Only alkenes conjugated with aromatic or cyclopropane moiety undergo this reaction. In the case of 1,2-disubstituted alkenes, the relative configuration of substitutents at the double bond is preserved and only one diastereomer is formed. 

The addition of trialkylsilyl radicals to 1,2-disubstituted ethylene derivatives is subject to a steric effect [49], This shows itself in the greater Ee0 value for Et3Si addition to RCH=CHR compared with that for the addition of the same radical to CH2=CHR. The contribution of... 

No effect of this type is manifested for the addition of alkyl radicals to the same alkenes. Evidently, the steric effect involved in the addition of trialkylsilyl radicals to 1,2-disubstituted ethylene derivatives is due to the repulsion between the carbon and silicon atom, caused by the large size of the silicon atom in the reaction center of the transition state. 

This regioselectivity is practically not influenced by the nature of subsituent R. 3,5-Disubstituted isoxazolines are the sole or main products in [3 + 2] cycloaddition reactions of nitrile oxides with various monosubstituted ethylenes such as allylbenzene (99), methyl acrylate (105), acrylonitrile (105, 168), vinyl acetate (168) and diethyl vinylphosphonate (169). This is also the case for phenyl vinyl selenide (170), though subsequent oxidation—elimination leads to 3-substituted isoxazoles in a one-pot, two-step transformation. 1,1-Disubstituted ethylenes such as 2-methylene-1 -phenyl-1,3-butanedione, 2-methylene-1,3-diphenyl- 1,3-propa-nedione, 2-methylene-3-oxo-3-phenylpropanoates (171), 2-methylene-1,3-dichlo-ropropane, 2-methylenepropane-l,3-diol (172) and l,l-bis(diethoxyphosphoryl) ethylene (173) give the corresponding 3-R-5,5-disubstituted 4,5-dihydrooxazoles. 

The aliphatic mono-olefins present a particularly complicated picture at first sight. Only a few of them will give high polymers by cationic catalysis most of these are either 1,1-disubstituted ethylenes, or ethylenes with a single branched substituent, such as 3-methylbutene-1 and vinylcyclohexane. The reasons why ethylene, propene, and the n-butenes do not give high polymers have been set out in detail [71]. Briefly, the ions derivable directly from all of these are either primary or secondary, e.g.,... 

In this section we shall determine the effect of nonbonded interactions on the physical properties of related molecules. As our model compounds we will utilize difluoroethylene, a molecule having substituents bearing only lone pairs and dicyano-ethylene, a molecule bearing substituents with both filled and unfilled MO s. In particular, we focus on the three possible isomers of a disubstituted ethylene ie. the 1,1, trans-1,2- and cis- 1,2-isomers. The three possible isomers are shown below. 

An alternative approach to 1,2-disubstituted ethylenes is within the framework of open shell-open shell interactions. Specifically, the dissection used in this theoretical approach is shown below for the model system 1,2-difluoroethylene ... 

According to Koopmans theorem28, the first ionization potential of a disubstituted ethylene corresponds to the negative of the pi HOMO energy. From the previous discussion, the pi HOMO energy is predicted to vary in the following order ... 

It should be pointed out that the predicted higher ionization potential and lower electron affinity of a cis 1,2-disubstituted ethylene relative to the corresponding trans isomer is based upon the assumption that substantial pi nonbonded inter-... 

Ionization Potentials (IP) for various disubstituted ethylenes are shown in Table 24. As can be seen, the IP s for 1,2 cis and trans isomers are usually very close but the IP s of 1,1-disubstituted ethylenes relative to their 1,2-disubstituted counterparts are consistently higher, in agreement with our predictions. 

We now consider the effect of nonbonded interaction on the energies of pi lone pair MO s in cis and trans disubstituted ethylenes. Our model systems are again cis and trans 1,2-difluoroethylene. The consequence of the interaction of the pi lone pair electrons in the cis and trans geometries is shown in Fig. 36. We distinguish the following three hypothetical cases and work out their consequences. 

As can be seen, the ng MO of the tram isomer is lower in energy than the ns MO of the cis isomer, a result consistent only with the presence of strong through space and through bond interaction of the two Cl atoms. This is an important result insofar as it indicates that four electron overlap repulsion is greater for tram than for cis 1,2-disubstituted ethylenes. 

Linear correlations between the energy of the LUMO of a molecule and its electron affinity are known39. Since the energy of the LUMO of the disubstituted ethylenes varies, according to our model, in the order, molecular electron affinities will follow the order ... 

Experimental electron affinity data of disubstituted ethylenes is not abundant but Table 25 lists the A s for some typical examples. As can be seen, the A of the 1,1-isomer is larger than the A s for the 1,2-isomers, in accord with our prediction. 

We first consider the relative electronic transition energies in cis and trans 1,2-disubstituted ethylenes. From Fig. 28 we can clearly see that the pi HOMO-LUMO energy gap is larger for the case of the cis isomer relative to the trans isomer. Hence, the mr transition is expected to occur at shorter wavelengths in cis 1,2-disubsti-tuted ethylenes. 

Table 26. Ultraviolet spectral data for cis and trans 1,2-disubstituted ethylenes...

Scheme 6.86 displays the generation of 417 from the cephalosporin triflate 416 and the formulas of the trapping products with ethylene, monosubstituted ethyl-enes, 1,1-disubstituted ethylenes and 1,1-dimethylallene [155], It was shown for two such reaction partners (styrene and phenyl vinyl thioether) that the exchange of the... 

Scheme 6.86 Generation of 4-methoxybenzyl (6R,7R)-8-oxo-7-(phenylacetamido)-l-aza-5-thiabicyclo[4.2.0]octa-2,3-diene-2-carboxylate (417) from the cephalosporin triflate 416 and products of the trapping of 417 by ethylene, monosubstituted ethylenes, 1,1-disubstituted ethylenes and 1,1-dimethylallene, according to Elliott and co-workers.

Scheme 6.87 Trapping products of cephalosporin derivative 417 with cycloalkenes and 1,2-disubstituted ethylenes.

Table 7. Comparison of NMR and IR solvent dependence for J13 r °f disubstituted ethylenes...

Benzocyclobutenone was first prepared from 1 -bromobenzocyclobutene by hydrolysis followed by chromium trioxide oxidation.3 More recent procedures involve hydrolysis of 1,1-dichloro- or 1,1-dimethoxybenzocyclobutene which in turn have been obtained through cycloaddition of the appropriate 1,1-disubstituted ethylenes to benzyne generated either from anthranilic acid through diazotization5 6 or from bromobenzene through sodium amide treatment.7.3 Benzocyclobutenone has... 

Regiochemistry of the obtained product leading to 1,1-disubstituted ethylenes is contrary to that observed in the addition of sodium butyltellurolate to aryl acetylenes, which produces exclusively the Z isomer resulting from an anti addition. ... 

For polymers obtained fiom disubstituted ethylenes CHA=CHB, Price (45, 103) considered four elementary states and developed a statistical scheme with eight conditional probabilities. The four elementaiy states correspond to the monomer units 00, 01, 11, 10, the first digit referring to substituent A, the second to B, 61. The knowledge of the sequences of two monomer units (four-center sequences) is required for the testing of such a scheme. 

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