Symmetry-allowed processes

The reaction of carbon atoms with A-unsubstituted aziridines leads to alkenes and hydrogen cyanide (72IA3455), probably via extrusion from the initially formed adduct (285). The fragmentation does not appear to be concerted, although this would be a symmetry-allowed process, since only about half the alkene formed retains the aziridine stereochemistry in the case of cM-2,3-dimethylaziridine. 

Dithiadiazolyl radicals are typically prepared by reduction of the corresponding cations with SbPh3. They are unstable with respect to isomerization to the 1,2,3,5-isomers both in solution " and in the solid state. The isomerization is a photochemically symmetry-allowed process, which is thermally symmetry forbidden. A bimolecular head-to-tail rearrangement has been proposed to account for this isomerization (Scheme 11.1). This rearrangement process is conveniently monitored... 

The Diels-Alder Reaction. A Symmetry Allowed Process 273... 

The photochemical cycloaddition of a carbonyl compound 1 to an alkene 2 to yield an oxetane 3, is called the Patemo-Buchi reaction - This reaction belongs to the more general class of photochemical [2 + 2]-cycloadditions, and is just as these, according to the Woodward-Hofmann rules, photochemically a symmetry-allowed process, and thermally a symmetry-forbidden process. 

Within the isolobal formalism, the conversion of 47 to 48 is a symmetry-allowed process, if it were to proceed as a concerted reaction (50). Structure 47 represents a transoid-2-meta.Wa-1,3-butadiene. In the bonding description, complex 48 represents formally a 1-metalla-bicyclo[1.1.0]butane. Therefore, the conversion of 47 to 48 represents a thermally allowed, concerted [ 2a + 2S] ring closure, in analogy to the pericyclic ring opening of bicyclo[1.1.0]butanes to give trans,trans-, 3-butadienes. 

There is an obvious analogy in orbital energy and symmetry between D3h and C4v structures since the Cs structure also has much the same pattern, it is evident that any interconversion of PHS is a symmetry-allowed process. The high-lying non-bonding molecular orbital (see Fig. 26) has its electron density on the exterior of the molecule, making it prone to attack by Lewis acids. 

On the other hand in a photochemical transformation by a disrotatory process, one electron is promoted from jt to n orbital and so the o, n and it orbitals of cyclobutene would correlate with /. /2 and /3 orbitals of butadiene. Thus the first excited state of cyclobutene, since it correlates with the first excited state of butadiene, therefore, the process would be a photochemically symmetry allowed process. 

It resembles both a cycloaddition and a [1, 5] sigmatropic shift of hydrogen. It is a symmetry allowed process shown as follows ... 

In recent years some applications of retro-ene reactions in the generation of thio-carbonyls have appeared. The retro-ene reaction is a symmetry-allowed process which resembles both [2 + 4] cycloreversion and a [1,5] sigmatropic shift of hydrogen. It usually requires high temperatures, therefore FVT has become very widespread in these reactions. 

Radical 47 can also undergo a transformation to afford the 1,2,3,5-analogue 48 (Equation 4) <1987CC69>. This isomerism is a photochemically symmetry-allowed process, providing radical 48 in quantitative yield. This controlled... 

Interestingly, the radical cation 138 can be generated also by light-induced isomerization of cyclooctatetraene radical cation (140). The conversion of the red non-planar ion 140 (4n - 1 n electrons) upon irradiation with visible light had been observed previously [395], but the blue photo-product had not been recognized as the cyclic conjugated species 138 with 4n + 1 n electrons. This interconversion is one of only a few orbital symmetry allowed processes documented in radical cation chemistry [393]. 

The second mechanism is a concerted (Sch. 7) rearrangement involving symmetry-allowed processes. The third mechanism is a mixed mechanism, involving one concerted process leading to radicals and followed by radical combination to give the ODPM rearrangement products. 

The classical theory suggests the [jts2 + n2] cyclization reaction of a singlet olefin and a ground-state olefin is a prototypical symmetry-allowed process [116]. The concerted collapse of the olefin pairs or their exciplexes produces cyclobutanes. The evidence comes from the stereochemistry of the cycloadducts. It has been... 

A cycloaddUion reaction lA one in which t wo unsaturated tnoleruies add to one another, yieldintr o r jc product. A wfth electrocydw reaction, cycloadditions are controlled t>y the orbital symmetry of the reactants. Symmetry allowed processes often lahe place readily, but symsnetry-disallowod processes take place with great dilTiculty, if at all, and then only by non-concerted pathways. 1 a look at two examples to see how they differ. 

Note that for the inverse electron demand, the LUMO of the diene ( T3) and HOMO of the ethene (-tt) are also symmetry matched for supra-supra interaction, the symmetry allowed process (Fig. 8.29). 

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