Ethylene electron-rich

Seml-empiricai and ab initio calculations have been used to study the l,2 cycloaddition of 02( Ag) to aminoethylene and ethylene. Electron-rich olefins react via a zwitterionic intermediate and the total reaction paths were determined using Fakul s IRC theory.The same group has examined... 

FIGURE 6 4 Electro static potential maps of HCI and ethylene When the two react the interaction is between the electron rich site (red) of ethylene and electron poor region (blue) of HCI The electron rich region of ethylene is associ ated with the tt electrons of the double bond and H IS the electron poor atom of HCI... 

FIGURE 9 3 Electro static potential maps of eth yiene and acetylene The region of highest negative charge (red) is associated with the TT bonds and lies between the two carbons in both This electron rich re gion IS above and below the plane of the molecule in ethylene Because acetylene has two TT bonds a band of high electron density encir cles the molecule... 

Copolymers of VF and a wide variety of other monomers have been prepared (6,41—48). The high energy of the propagating vinyl fluoride radical strongly influences the course of these polymerizations. VF incorporates well with other monomers that do not produce stable free radicals, such as ethylene and vinyl acetate, but is sparingly incorporated with more stable radicals such as acrylonitrile [107-13-1] and vinyl chloride. An Alfrey-Price value of 0.010 0.005 and an e value of 0.8 0.2 have been determined (49). The low value of is consistent with titde resonance stability and the e value is suggestive of an electron-rich monomer. 

The addition of maleic anhydride can occur by excitation of either dienone or the anhydride. It is tempting to ascribe the 4,5-adduct (264) to a reaction between the excited dienone (260) and unexcited maleic anhydride by analogy with the observed major products of ethylene addition [cf. (261), (262)]. The 6,7-adducts (265) and (266) would then imply that these cycloadditions proceed by way of excited maleic anhydride which adds preferentially to the more electron-rich y,5-double bond of the groundstate dienone. 

For the ordinary Diels-Alder reaction the dienophile preferentially is of the electron-poor type electron-withdrawing substituents have a rate enhancing effect. Ethylene and simple alkenes are less reactive. Substituent Z in 2 can be e.g. CHO, COR, COOH, COOR, CN, Ar, NO2, halogen, C=C. Good dienophiles are for example maleic anhydride, acrolein, acrylonitrile, dehydrobenzene, tetracya-noethylene (TCNE), acetylene dicarboxylic esters. The diene preferentially is of the electron-rich type thus it should not bear an electron-withdrawing substituent. 

The electrophilic addition of HBr to ethylene is only one example of a polar process there are many others that vve ll study in detail in later chapters. But regardless of the details of individual reactions, all polar reactions take place between an electron-poor site and an electron-rich site and involve the donation of an electron pair from a nucleophile to an electrophile. 

Ethylene disulfonyl-1,3-butadiene (43) is an example of an outer-ring diene with a non-aromatic six-membered heterocyclic ring containing sulfur. It is prepared by thermolysis of sulfolenes in the presence of a basic catalyst. It is very reactive [43] and even though it is electron-deficient, it readily reacted with both electron-rich and electron-poor dienophiles (Equation 2.15). 

Nucleophilic attack with electron-rich arenes and ethylene derivatives at C-7 of 5-methoxyfuroxano[3,4-d -pyrimidine 245 leads to 7-substituted 6,7-dihydro-5-methoxyfuroxano[3,4-r/]pyrimidines 246 (Equation 47) <2003JP0431>. 

Absolute rates for the addition of the methyl radical and the trifluoromethyl radical to dienes and a number of smaller alkenes have been collected by Tedder (Table l)3. Comparison of the rate data for the apolai4 methyl radical and the electrophilic trifluoromethyl radical clearly show the electron-rich nature of butadiene in comparison to ethylene or propene. This is also borne out by several studies, in which relative rates have been determined for the reaction of small alkyl radicals with alkenes. An extensive list of relative rates for the reaction of the trifluoromethyl radical has been measured by Pearson and Szwarc5,6. Relative rates have been obtained in these studies by competition with hydrogen... 

Most likely the cobalt catalyst is HCo(CO)2(L), which has a very electron rich metal centre and dissociation of CO does not occur under the reaction conditions. The first step is a reaction of the cobalt hydride with ethylene oxide forming a hydroxyethylcobalt species, which does not require dissociation of... 

In 1971, Coulson at DuPont reported the first example of an OHA reaction catalyzed by soluble Rh and Ir complexes [5]. Secondary amines such as dimethyl-amine, pyrrolidine and piperidine were effectively added to ethylene, while primary amines, ammonia and heavier olefins were essentially unreactive (see Equation 6.3). IrCl3-3H20 proved to be an equally effective catalyst precursor in these reactions. It is probable that, under the conditions employed in this study, the Rh(III) and Ir(III) salts are reduced to monovalent, electron-rich species such as 3 (see Equation 6.6). 

Historically, NHC complexes were investigated for the first time as catalysts and discussed as catalytic intermediates in the dismutation of electron rich tetraamino-ethylenes [Eq, (49)] Mixtures of two differently substituted olefins were reacted in the presence of rhodium(I) complexes and the products obtained showed mixed substitution patterns. Starting from Wilkinson s catalyst [(Ph3P)3RhCl], NHC complexes are formed as intermediates which could be isolated and used as even more active catalysts. In this first example, however, the NHC actively participates in... 

There are a number of other mechanisms by which alkenes can undergo photochemical f2 + 2) cycloaddition, one of which works well for electron-rich alkenes and electron-acceptor sensitizers. The pathway is through the radical cation of the alkene, which attacks a second, ground-state alkene molecule and then cydizes and accepts an electron to give the product cyclobutane. Typical of this group of reactions is the formation of 1,2-dialkoxycydobutanes from alkoxy-ethylenes with drcyanonaphthalene as sensitizer 12.78). 

When an alkyl or aryl ketone, or an aryl aldehyde, reacts with an alkyl-substituted ethylene, or with an electron-rich alkene such as a vinyl ether, the mechanism involves attack by the (n,n triplet state of the ketone on ground-state alkene to generate a 1,4-biradical that subsequently cyclizes. The orientation of addition is in keeping with this proposal, since the major product is formed by way of the more stable of the possible biradicals, as seen for benzophenone and 2-melhylpropene (4.64). As would be expected for a triplet-state reaction, the stereoselectivity is low, and benzophenone gives the same mixture of stereoisomers when it reacts with either trans or... 

Cyano-substituted ethylenes react in a different way with aliphatic ketones. The orientation of photochemical cycloaddition (4.661 is the opposite of that found for electron-rich alkenes, and the reaction is highly stereoselective (4.69) in the early stages. These processes involve the formation and subsequent decay of an excited complex (exciplex) from the (n,n ) singlet state of the ketone and the alkene. Aryl ketones undergo intersystem crossing so efficiently that such a singlet-state reaction is rarely observed, but the reaction of a benzoate ester with an electron-rich alkene 14.70 rnay well be of this type, with the ester acting as electron-acceptor rather than electron-donor. 

These hybridisation variations are caused by anisotropy within the chemical bonds. This is due to the non-homogeneous electronic distribution around bonded atoms to which can be added the effects of small magnetic fields induced by the movement of electrons (Fig. 9.12). Thus, protons on ethylene are deshielded because they are located in an electron-poor plane. Inversely, protons on acetylene that are located in the C-C bond axis are shielded because they are in an electron-rich environment. Signals related to aromatic protons are strongly shifted towards lower fields because, as well as the anisotropic effect, a local field produced by the movement of the aromatic electrons or the ring current is superimposed on the principal field (Fig. 9.12). 

The electron-rich diene system reduced the electrophilicity of the catalyst and favored the polymerization of ethylene at the expense of the propylene. The mast important role of diene complexes is this of shifting the catalyst ionicity toward more anionic character. 

It is also pertinent that there are two HOMO-LUMO interactions possible between butadiene and ethylene, one in which die HOMO is diat of die diene, which acts as die electron donor, and one in which the HOMO is that of the olefin, which would be die electron donor. A normal Diels-Alder reaction is one in which die diene is electron rich and acts as the electron donor and the... 

Over the last decade, noncovalent bonding interactions between appropriate Jt-elec-tron-deficient and Jt-electron-rich recognition sites have been exploited in our laboratories for the synthesis of catenanes and rotaxanes. 271 In the example illustrated in Figure 10, the bis(hexafluorophosphate) salt 11-2PF6 was treated 28 with trons-bis(pyridine)ethylene (12) in the presence of the previously formed macrocyclic polyether bis-p-phenylene-34-crown-10 (13). The resulting [2]catenane 14-4PF6 was isolated in 43 % yield after counterion exchange. 

Electron-rich bifunctional vinyl ethers (e.g. ethylene glycol divinyl ether) react with electron-poor alkenes (e.g. TCNE) to produce cyclobutanes in good yields via tetramethylene zwitterion intermediates. In some cases, cyclobutanes reacted with the solvent (MeCN) to yield tetrahydropyridines.9 Trifluoromethanesulfonimide is an... 

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