Ethylene fragmentation

The generalized Woodward-Hoffmann rule suggests that a synchronous addition of disulfonium dications at the double C=C bond of alkenes would be a thermally forbidden process and so would be hardly probable. Simulation of the frontal attack by ethylene on l,4-dithioniabicyclo[2.2.0]hexane 115 gave no optimal structure of an intermediate complex. On the other hand in the lateral approach of the reactants, orbital factors favor attack of the double bond by one of the sulfonium sulfur atoms of the dication. This pattern corresponds to SN2-like substitution at sulfur atom as depicted in Figure 5. Using such a reactant orientation, the structure of intermediate jc-complex was successfully optimized. The distances between the reaction centers in the complex, that is, between the carbon atoms of the ethylene fragment and the nearest sulfur atom of the dication, are 2.74 and 2.96 A, respectively. 

The pi MO s of the three conformations of 1,3-butadiene can be derived from union of the pi MO s of two ethylene molecules in the appropriate geometry. The dissection of 1,3-butadiene into two ethylenic fragments is illustrated below for the cis conformer ... 

We first consider the union of the ethylenic fragments A and B in a cis and trans geometry. The interaction diagrams for these unions are shown in Fig. 3. We distinguish two types of interactions ... 

Fig. 3. The pi MO s of both cis and trans butadiene as constructed from two ethylenic fragments...

We now consider the relative stability of 1,3,5-hexatriene in its aU-fnzns and all-cis planar conformations. 1,3,5-hexatriene can be viewed, theoretically, as the result of the union of a central ethylenic fragment A and two ethylenic fragments B and... 

The substitution reaction of CP with methyl chloride, 2-chloroethyl radical, and allyl chloride has been treated by several different ab initio theoretical models. Depending on the method, the intrinsic barrier for the 5ivr2 process in allyl chloride is 7-11 kcalmoP higher than the barrier for the 5ivr2 reaction of methyl chloride. The reaction of CP with the 2-chloroethyl radical involves an intermediate complex, which is best described as an ethylene fragment flanked by a resonating chloride anion-chloride radical pair. There are many other points of interest. 

In contrast, the nitro and ethylenic fragments in trani-4-nitrostilbene form the united conjugation system. Such a conjugation is a necessary condition for the whole-contour delocalization of an unpaired electron in arylethylene anion-radicals. Whether this condition is the only one or there is some interval of allowable strength for the acceptor is a question left to future experiments. 

If the nitro group is located at the ethylene fragment, one-electron transfer initiates dimerization of the developing anion-radicals. a-Nitrostilbene, w-methyl-co-nitrostyrene, and a-nitro-p-ferrocenylethylene give anion-radicals, which dimerize spontaneously. It is interesting to compare reactions of cyclooctatetraene dipotassium (C8HgK2) with a-nitro and a-cyano ferrocenylethylenes (Todres and Tsvetkova 1987, Todres and Ermekov 1989 Scheme 3.4). 

It is also worthwhile to compare the ferrocenyl ethylene (vinylferrocene) anion-and cation-radicals. For the cyano vinylferrocene anion-radical, the strong delocalization of an unpaired electron was observed (see Section 1.2.2). This is accompanied with effective cis trans conversion (the barrier of rotation around the -C=C- bond is lowered). As for the cation-radicals of the vinylferrocene series, a single electron remains in the highest MO formerly occupied by two electrons. According to photoelectron spectroscopy and quantum mechanical calculations, the HOMO is mostly or even exclusively the orbital of iron (Todres et al. 1992). This orbital is formed without the participation of the ethylenic fragment. The situation is quite different from arylethylene radical cations in which all n orbitals overlap. After one-electron oxidation of ferrocenyl ethylene, an unpaired electron and a positive charge are centered on iron. The —C=C— bond does not share the n-electron cloud with the Fe center. As a result, no cis trans conversion occurs (Todres 2001). 

Thus, it appears that the focal point of the reaction has transferred. The presence of a styryl (not a methoxyl) group protects the nitro group from reduction. For some reason or other, the styryl group causes a shift of excess electron density from the nitro to the ethylene fragment. 

The reaction of 1,3-butadiene with ethylene to give cyclohexene is an allowed reaction if the ethylene fragment approaches the butadiene fragment from one face, preserving a plane of symmetry as indicated in 4. The Hiickel-type arrangement of the p-orbitals shown in 4 involves six electrons and so should express some aromatic stabilization. 

An alternative mechanism for the Diels-Alder reaction involves a stepwise pathway (Reaction 4.3). Here, one end of the butadiene and ethylene fragments approach each other, passing through transition state 6, to form a diradical intermediate 7. In a second distinct chemical step, the second new o-bond is formed through transition state 8 to give cyclohexene. 

In our view, it is this circumstance which accounts for the chemical and physical similarities between thiophene and benzene, as well as between pyrazine and 1,2,5-thiadiazole, which have frequently been pointed out in this review and in the chemical literature. In a formal sense the sulfur atom having two pd orbitals is like an ethylenic fragment with two p orbitals. We therefore propose that a sulfur-containing heterocycle be termed quasiethylenic with its cyclic counterpart in which the sulfur atom is replaced by two doubly bonded carbon atoms. 

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