Photochemical electrocyclic reactions

Electrocyclic Reaction of Radicals Metal Catalysed Electrocyclic Reactions Photochemical Electrocyclic Reactions... 

One can also carry out electrocyclic reactions photochemically. The excited butadiene now has an electron in a tt MO that was empty in the ground state. This MO was the lowest unoccupied MO (LUMO) of ground-state butadiene, pictured in Fig. 14-19. One can see that the step to the next-higher MO of butadiene has just introduced one... 

Thermal and photochemical electrocyclic reactions are particularly useful in the synthesis of alkaloids (W. Oppolzer, 1973,1978 B K. Wiesner, 1968). A high degree of regio- and stereoselectivity can be reached, if cyclic olefin or enamine components are used in ene reactions or photochemical [2 + 2]cycloadditions. 

A striking illustration of the relationship between orbital symmetry considerations and the outcome of photochemical reactions can be found in the stereochemistry of electrocyclic reactions. In Chapter 11, the distinction between the conrotatory and the disrotatory mode of reaction as a function of the number of electrons in the system was... 

Scheme 13.1. Some Examples of Photochemical Cycloaddition and Electrocyclic Reactions...

Scheme 13.1 lists some example of photochemical cycloaddition and electrocyclic reactions of the type that are consistent with the predictions of orbital symmetry considerations. We will discuss other examples in Section 13.4. 

The latter product is an example of a product formed by a concerted, photochemically allowed, electrocyclic reaction. A hydrogen-atom migration from a cyclopropyldimethyl... 

The most striking feature of electrocyclic reactions is their stereochemistry. For example, (2 ,4Z,6 )-2,4,6-octatriene yields only c/s-5,6-dimethyl-l,3-cyclo-hexadiene when heated, and (2 ,4Z,6Z)-2,4,6-octatriene yields only trnns-5,6-dimethyl-l,3-cyclohexadiene. Remarkably, however, the stereochemical results change completely when the reactions are carried out under what are called photochemical, rather than thermal, conditions. Irradiation, or photolysis,... 

Thermal and photochemical electrocyclic reactions always take place with opposite stereochemistry because the symmetries of the frontier orbitals are always different. Table 30.1 gives some simple rules that make it possible to predict the stereochemistry of electrocyclic reactions. 

Thermal and photochemical cycloaddition reactions always take place with opposite stereochemistry. As with electrocyclic reactions, we can categorize cycloadditions according to the total number of electron pairs (double bonds) involved in the rearrangement. Thus, a thermal Diels-Alder [4 + 2] reaction between a diene and a dienophile involves an odd number (three) of electron pairs and takes place by a suprafacial pathway. A thermal [2 + 2] reaction between two alkenes involves an even number (two) of electron pairs and must take place by an antarafacial pathway. For photochemical cyclizations, these selectivities are reversed. The general rules are given in Table 30.2. 

Thiophenes continue to play a major role in commercial applications as well as basic research. In addition to its aromatic properties that make it a useful replacement for benzene in small molecule syntheses, thiophene is a key element in superconductors, photochemical switches and polymers. The presence of sulfur-containing components (especially thiophene and benzothiophene) in crude petroleum requires development of new catalysts to promote their removal (hydrodesulfurization, HDS) at refineries. Interspersed with these commercial applications, basic research on thiophene has continued to study its role in electrocyclic reactions, newer routes for its formation and substitution and new derivatives of therapeutic potential. New reports of selenophenes and tellurophenes continue to be modest in number. 

Electrocyclic reactions were first described by Woodward and Hoffmann in their classic series of articles. One very interesting aspect of such reactions is, that for a given conjugated polyene photochemical transformation leads to the opposite stereochemical outcome than the thermal one314). 

Electrocyclic reactions can be brought about by heat, by ultraviolet irradiation and sometimes by use of metal catalysts. The thermal reaction is generally not reversible and as written above cyclobutenes have been converted to 1, 3 dienes by heating between 100° and 200°C. But the photochemical conversion can be carried out in either direction. Generally 1, 3 dienes can be converted to cyclobutenes rather than the reverse because the dienes because of n electrons are strong absorbers of light at the used wavelengths. 

The simplest example of an electrocyclic reaction involving 4n electron system is the thermal opening of cyclobutenes to 1,3 butadienes. The reaction can be done thermally or photochemically and under either conditions, it is stereospecific. 

There seems to be no great difference in the free energy between acyclic triene and the cyclic diene. This is because of smaller strain in the six-membered ring as compared with the four-membered one. On the other hand in 6n electron system in electrocyclic process there is more efficient absorption in the near regions of u.v. spectrum. This is why under both thermal and photochemical conditions, the (1, 6) electrocyclic reactions are reversible. Side reactions are more frequent in reversible. Side reactions are more frequent in reversible transformations of trienes than in dienes. The dehydrogenation of cyclic dienes to aromatic compounds may also occur in the thermal process. On heating cyclohexadiene yields benzene and hydrogen. 

Many other examples of contrasting behaviour have been discovered. For example all-cis-cyclodecapentaene (VII) photochemically equilibrate at low temperatures with trans 9, 10 dihydronapthalene by a conrotatory six electron electrocyclic reaction but it is converted thermally into cis-9, 10 dihydronaphthalene by disrotatory closure. 

Thermal and photochemical electrocyclic reactions are both stereospecific, with the two processes giving rise to stereospecific reactions in the opposite sense. 

For the photochemical electrocyclic reaction of the diene, irradiation promotes one electron from n2 to n and the disrotatory mode of reaction gives the cis isomer (Scheme 8.4). 

Enamides 163 undergo photochemical conrotatory six-electron electrocyclic reactions to yield the dihydro intermediate 164, which in turn yields the fraws-fused cyclic product 165 (equation 105) by a (l,5)-suprafacial hydrogen shift. Several natural product syntheses like that of benzylisoquinoline and indole type alkaloids can be achieved by this type of photocyclization (equations 106163, 107164, 108165 and 109166). 

Figure 14.6. (ia) Orbital correlation diagram for the photochemical electrocyclic reaction of butadienes. (b) Orbital correlation diagram for for the photochemical electrocyclic reaction of hexa-trienes. Solid lines and S, A denote correlation for conrotatory motion dashed lines and S, A denote correlation for disrotatory motion. 

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