Ethene radical cation

Ethene Cation Radical s-frans-1,3-Butadiene Cation Radical... 

Pi cation radicals, that is cation radicals in which the SOMO is a pi type orbital, are obviously key intermediates in the ET chemistry of alkenes and alkynes. The ethene cation radical, which has the novel twisted structure shown in Scheme 5, provides an excellent example of a significant structural adjustment which can accompany the loss of an electron [8]. 

The cation-radicals of ethenes, which are the primary products of one-electron oxidation, differ in their reactivity from the corresponding neutral compounds. This widens the possibilities of syntheses. Primary oxidation of ethenes (photochemically, with salts of transition metals or ammo-niumyl salts) makes it possible to obtain cation-radicals, which initiate reactions that are unusual for ethenes in an uncharged state. For instance, the cation-radical of phenylvinyl ether initiates head-to-head cyclodimerization as shown in Scheme 7.19 (Ledwith 1972, Farid and Shealer 1973, Kuwata et al. 1973). 

The generation of active radicals as a result of bond breakage makes cation-radicals useful as syn-thons. For example, arylsulfenamide cation-radicals may be used as sources of sulfenyl radicals. The reaction of 4 -nitrobenzenesulfenanilide with Lewis acids, such as BF3 and AICI3, leads to the formation of sulfonamide cation-radical. The latter appears to be an active sulfenyl transfer species. In the presence of anisol, ethenes, or ethynes, it gives phenylthiyl derivatives (Benati et al. 1990, Gross and Montevecchi 1993). 

The actual E of a particular system is often not as informative as is the comparison of the delocalized system with a reference system having localized orbitals. Figure 4.11 shows the reference system for allyl a double bond separated by an imaginary barrier from a p orbital that may have 0 (cation), 1 (radical), or 2 (anion) electrons. The molecular orbital description of that system, then, is simply a sum of the HMOs of the double bond and of the p orbital. Again, it does not matter whether we are talking about the cation, radical, or anion in Figure 4.11. The HMOs of the reference system are simply those of ethene (E = a -H /3, = a — /8) superimposed on the one HMO for an isolated p orbital (E = a). 

The cation-radical Diels-Alder reactions of cis- and fran5 -l,2-(diaryloxy)ethenes with butadienes are stereospecific, in agreement with a concerted cycloaddition mechanism. " Tris(4-bromophenyl)aminium hexachloroantimonate catalyses the two-step, non-stereospecific cation-radical Diels-Alder reaction of cis- and traui-prop-l-enyl aryl ethers with cyclopenta-1,3-diene in CH2CI2 solution. 

The calculation of an activation barrier for the reactions (21) and (22) must not necessarily be considered as an error of the method. For example, the MINDO/3 calculated activation barrier for the attack of a methyl radical on ethene 137-138) which is comparable to the former reactions was confirmed by experiments 139). In contrast to a free proton (Eq. (20)) the methyl radical as well as the ethyl cation possess steric space need. For this reason, the calculation of repulsive interactions which are able to overcome the attractive forces at certain distances cannot be seen without doubt as faulty. 

Other systems studied include the spiroheptadiene system (13) in which the cyclopropane moiety lies in the nodal plane of the butadiene HOMO [1 2,9 10]bis-methano[2.2]paracyclophane (15), in which two cyclopropane moieties are joined with two benzene rings in such a way that tertiary-tertiary cyclopropane bonds lie parallel to the aromatic n system 7-methylenequadricyclane radical cation (16 ), in which a pair of cyclopropane groups lie orthogonal to an olefinic moiety and the 7-spirocyclopropanenorbomadiene radical cation (18 ), in which a pair of nonconjugated ethene groups and a cyclopropane moiety are joined. 

No discussion about strained-ring radical cations would be complete without the valence isomers quadricyclane (15 +) and norbornadiene, (16 +) 15 features two adjacent rigidly held cyclopropane rings, whereas 16 contains two ethene n systems well suited to probe through-space interactions.Molecular orbital considerations suggest the antisymmetric combination of the ethene n orbitals (16) or cyclopropane Walsh orbitals (15) as respective HOMOs of the two parent molecules. The radical ions have different state symmetries and their SOMOs have different orbital symmetries. 

Most technically important polymerizations of alkenes occur by chain mechanisms and may be classed as anion, cation, or radical reactions, depending upon the character of the chain-carrying species. In each case, the key steps involve successive additions to molecules of the alkene, the differences being in the number of electrons that are supplied by the attacking agent for formation of the new carbon-carbon bond. For simplicity, these steps will be illustrated by using ethene, even though it does not polymerize very easily by any of them ... 

The 1,2-hydrogen shift isomers of neutral (singlet and triplet) thiazole and its radical cation have been investigated87 by a combination of mass spectrometric experiments and hybrid density functional theory calculations. An unexpected isomerization of A-aryl-3-amino-4-nitroisothiazole-5(2//)-imines (71) to 2-(benzothiazol-2-yl)-2-nitro-ethene-1,1-diamines (72) has been reported.88... 

The formation and cleavage of cyclobutane systems have been discussed in Sect. 3.1 and 4.4. The structure of the intermediates is of major interest. The cyclobutane radical cation has been calculated by several groups. Bauld and coworkers [342] modeled the cycloaddition of ethene radical cation to ethene by the MNDO method. At this level of theory an unsymmetrical structure with one long one-electron C—C o-bond is of lowest energy (Scheme 10, type Q. 

The 71 subunits, ethene and benzene, differing in both size as well as topology, but containing the same tetra-substitution pattern, seem to approach identity15,16. ESR spin delocalization data of their organosilicon radical cations as well as additional information from own structures of sterically overcrowded molecules and many more registered in the Cambridge Structural Database stimulate speculation that both tetra-substituted molecules... 

Mass spectral peaks are often seen corresponding to loss of small, stable molecules. Loss of a small molecule is usually indicated by a fragment peak with an even mass number, corresponding to loss of an even mass number. A radical cation may lose water (mass 18), CO (28), C02 (44), and even ethene (28) or other alkenes. The most common example is the loss of water from alcohols, which occurs so readily that the molecular ion is often weak or absent. The peak corresponding to loss of water (the M-18 peak) is usually strong, however. 

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