Electrocychc reaction

New photochromic dyes with electrocycHc reactions have been proposed on the basis of 1,5-electtocycHzation of heterogenous pentadienyl—anions (124). StiH newer are investigations into the photocycHzation of 2,4,6-tri-isoptopylbenzophenones for vinyl polymers ia the glassy state (133). 

A second process that occurs concurrently with the dissociation— redistribution process is an intermolecular rearrangement by which cyclohexadienone groups move along a polymer chain. The reaction maybe represented as two electrocycHc reactions analogous to a double Fries rearrangement. When the cyclohexadienone reaches a terminal position, the intermediate is the same as in equation 8, and enolization converts it to the phenol (eq. 9). 

Photochromism Based on Electrocyclic Reactions. The most common general class of photochromic systems involves reversible electrocychc reactions. Within this general class, the most weU-studied compounds are the indolino spiropyrans and indolino spiroxa2ines. 

In real space, that is, in a chemical view, electrocycHc reactions are governed by the Woodward-Hoffmann rules of orbital symmetry conservation. According to Fukui only the frontier orbitals are regarded here to decide whether a given reaction is permitted or not The highest occupied orbital must be examined for a thermal process, which applies to the imaginary formation of a carbon nanotube from a graphene layer. (The first unoccupied orbital would have to be considered for a photochemical process-which is not the case here.)... 

As related to Diels-Alder reaction this also represents a 6jr electron electrocychc reaction. Like Diels-Alder reaction the ene reaction is reversible. For example, the produce) 1-pentene of the ene reaction between ethene and propene, gives back ethene and propene on decomposition at 400° C. 

The guiding principle regarding how electrocychc reactions occur was provided by R. B. Woodward and R. Hoffmann. The rule is based on the number of electrons in the tt bonding system of the molecule. That number of electrons can be expressed as either 4 or 4 + 2, where... 

Scheme 8. Enantioselective Photoreactions in TADDOL Inclusion Compounds with a Cou-marin, a Methacryl Anilide, and an Oxocyclohexenyl-carboxamide. In the first case, the packing of the coumarin molecules in the mixed crystal is such that the double bonds are predisposed for the (2+2) cycloaddition. In the second example, a photochemical electrocychc reaction is followed by a sigmatropic H shift. The third reaction is an intramolecular (2+2) cycloaddition with dia- and enantioselective formation of three new stereogenic centers. There are several more reactions of this type, described in the literature [54], and the Toda group has determined the crystal structures of a number of inclusion compounds to show the correlation between the crystal packing and the configuration of the photoproducts. EMastereoselective solid-phase reactions of chiral guests in TADDOL-host lattices have also been described by the...

Ring expansion can be drawn in various ways. There is a direct route from the neutral amine, or its anion, that doesn t look very convincing, or you can ionize one of the chlorides first and open the cyclopropyl cation in an electrocychc reaction. However you explain it, this is a good way to make 3-substituted pyridines. 

The reaction with excess bromoketone starts the same way but the oxyallyl cation is intercepted by one of the benzene rings in a four-electron conrotatory electrocychc reaction like the Nazarov reaction (p. 927 of the textbook). 

After your experience with cycloadditions and sigmatropic rearrangements, you will not be surprised to learn that, in photochemical electrocychc reactions, the rules regarding conrotatory and disrotatory cycUzations are reversed. 

Formal insertions into aromatic C-H bonds often proceed by another mechanism. This is also true in intramolecular cases, and the formation of carbazole from biphen-2-ylnitrene, a weU-studied example, probably occurs by an electrocycHc reaction followed by 1,5-hydrogen shift The high yielding synthesis of indoles from azidocinnamates proceeds similarly by cycHzation of a vinyl nitrene (Scheme 6.32). 

The existence of this parallel can be best demonstrated graphically in Fig.3, correlating the similarity index with the activation energies for the first three members of the series of electrocychc reactions, for which the experimental values are available [95]. 

In contrast to electrocychc reactions for which the parallel between the Woodward-Hoflfmann rules and the least motion principle is entirely straightforward, the situation is more complex for cycloadditions and sigmatropic reactions, where the values of similarity indices for alternative reaction mechanisms are the same so that the discrimination between the allowed and forbidden reaction paths is impossible. The origin of this insufficiency was analyzed in subsequent studies [96-98], in which we demonstrated that the primary cause lies in the restricted information content of... 

According to the Woodward-Hoffmann rules for electrocychc reactions, a 6k electrocychzation is thermally allowed in a disrotatory manner and photochemically allowed in a conrotatory manner. However, in the present context of synthesis of aromatic compounds as final products, which are devoid of any stereocenters, the stereochemical aspects of the substituents in the intermediate dihydroaromatic compound should not matter. The photochemical 6x electrocychzation of c/s-stilbene derivatives followed by oxidation of the dihydroaromatic intermediate provides access to angularly fused polycyclic aromatic compounds (Scheme 16.2) [4]. 

These stereochemical observations puzzled chemists for many years, until Woodward and Hoffmann developed their theory describing conservation of orbital symmetry. This single theory is capable of explaining all of the observations. We will first apply this theory to explain the stereochemical outcome of electrocyclic reactions taking place under thermal conditions, and then we will explore electrocychc reactions taking place under photochemical conditions. 

Now let s apply this approach to thermal electrocychc reactions of it systems containing only four tt electrons, rather than six it electrons. Once again, the configuration of the product is dependent on the configuration of the reactant. 

We have seen that the stereochemical outcome of an electrocychc reaction depends on whether the reaction is performed under thermal conditions or under photochemical conditions. 

Woodward-Hoffmann mles for electrocychc reactions are summarized in Table 2.1. 

Hexadienes 9 on direct irradiation undergo both cw-trani-isomerization and stereospecific electrocychc reaction [18]. 

Now let us apply this theory to the electrocychc reaction of hexatriene ... 

What you have learned about electrocyclic reactions can be summarized by the selection rules hsted in Table 28.1. Tliese are also known as the Woodward-Holfinann rules for electrocychc reactions. The rales show that the mode of ring closure depends on the number of conjugated double bonds in the reactant and on whether the reaction is carried out under thermal or photochemical conditions. And once you know the mode of ring closure, you can determine the products of an electrocyclic reaction. 

In Summary Conjugated dienes and hexatrienes are capable of (reversible) electrocyclic ring closures to cyclobutenes and 1,3-cyclohexadienes, respectively. The diene-cyclobutene system prefers thermal conrotatory and photochemical disrotatory modes. The triene-cyclohexadiene system reacts in the opposite way, proceeding through thermal disrotatory and photochemical comotatory rearrangements. The stereochemistry of such electrocychc reactions is governed by the Woodward-Hoffmann rules. 

The thermal cyclization of cM-l,3,5-hexatriene to yield 1,3-cyclohexadiene is an example of an electrocychc reaction. C-1 and C-6 of the original polyene are linked by a O bond in the cyclic product. This reaction is known as the Cope rearrangement. 

Classify each of the following pericychc reactions as an electrocychc reaction, a cycloaddition, or a sigmatropic shift. 

As a general rule, the electrocychc reaction leading to ring opening is favored when the electron density of the pyran part is reduced. The decrease of the electron density on the C4-09 bond leads to the stable TCC form in the case of protonation on 09 (Fig. 8b) and to a decrease of the barrier height to reach CCC upon application of an external electric field that withdraw electron density from that bond (Fig. 8d). 

Ohkura, K., Nishijima.K., Sakushima, A., and Seki, K., Photochemistry of 9,10-dihydro-l,3,5,7-tetramethyl-9-methylenecylooctapyrimidine-2,4-dione synthesis of novel ring systems through electrocycHc reactions, Heterocydes, 53,1247, 2000. 

Darcy, P.J., Heller, H.G., Strydom, P.J., and Whittall, J., Photochromic heterocyclic fulgides. 2. Electrocychc reactions of (jB)-a-2,5-dimethyl-3-furylethylidene (alkyl-substituted methylene)-suc-cinic anhydrides,/. Chem. Soc., Perkin Trans. 1, 202,1981. 

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