Cyclobutene ring opening, photochemical

The experimental results on the photochemical cyclobutene-to-butadiene ring opening are not as straightforward as for the thermal reaction. For simple alkyl-cyclobutenes, the photolysis must be done in the vacuum ultraviolet (<200 nm) because of the high energy of the absorption maximum. Cyclobutene ring opening... 

The cyclohexadiene-hexatriene system seems to be less complicated than the cyclobutene-butadiene system. Cyclohexadiene undergoes photochemical electrocyclic ring opening ... 

The two reactions can be separately carried out. The cyclobutene must be thermodynamically unstable with respect to the diene. Therefore in a thermal process only a ring opening of cyclobutene is observed. But under photochemical conditions dienes are more efficient in absorbing radiant energy than are simple alkenes. This is why ring closure of dienes can be carried out in high yields photochemically. 

The principal route of formation of fumarate and maleate is protonation of one of the methoxycarbonyl groups of the bicyclo[4.2.0]oct-2,4,7-triene, followed by proton loss, leading to rearomatization and enolization. The formation of fumarate and maleate in the absence of acid is ascribed to a photochemical 1,3 H-shift followed by opening of the new cyclobutene ring. 

In every case, Ri and R2 are trans to each other in the major product. R2 is always cis to the cyclobutene ring in the final product. The initial cycloaddition to the benzene ring occurs syn ring opening proceeds disrotatory and yields a boat-shaped all-cis cyclooctatriene. The photochemical 4-rr ring closure is also disrotatory it occurs in such a manner that the five-membered ring is trans to the... 

Conversely, the photoinduced ring-opening of cyclobutenes allows the construction of medium-sized hetero- or carbocycles, starting from bicyclic frameworks. Alkenes can undergo thermal (metal-assisted) or photochemical [2 + 2]-cycloaddition to a variety of alkynes the subsequent two-carbon ring-enlargement can serve as a versatile method for the preparation of hydroazulenes and dioxacyclooctadiene derivatives (Scheme 9.44) [74, 75]. 

In this reaction, light of appropriate energy is used to selectively excite 1,3-cyclohepta-diene. The diene closes to a cyclobutene by a disrotatory motion. Although the product, because of its strained cyclobutene ring, is much less stable than the reactant, it is unable to revert back to the diene by an allowed pathway. It does not absorb the light used in the reaction, so the photochemically allowed disrotatory pathway is not available. A conrotatory opening is thermally allowed but results in a cycloheptadiene with a trans double bond. Such a compound is much too strained to form. Therefore, the product can... 

Similarly, thermal (at 150°C) electrocyclic opening of cyclobutenes forms conjugated butadienes this mode of reaction is favoured by relief of ring strain. However, the reverse ring closure is not normally observed. Photochemical ring closure can be affected, but the stereospecificity is opposite to that of thermal ring opening. 

The correlation diagrams for the conrotatory process show that there is a good correlation between the bonding orbitals v 1 and v 2 of butadiene and ct- and TT-orbitals of cyclobutene (Fig. 8.40). Thus, the ring opening of cyclobutene to butadiene or the reverse reaction is thermally allowed and occurs by the conrotatory process. The reaction proceeds with conservation of orbital symmetry. The photochemical conrotatory process in this case will be symmetry forbidden. 

Thus, conrotatory ring closing of butadiene to cyclobutene is symmetry allowed under thermal conditions. However, under photochemical conditions the LUMO becomes HOMO and the disrotatory ring opening is symmetry allowed (Fig. 8.48). 

The criss-cross cycloadduct (169) is formed on irradiation of the enone (170) in ethanol solution. The cyclohexadienyl ring of the enone (171) undergoes photochemical ring opening when irradiated at 254 nm in cyclohexane. The products from this process are the enone (172) and the cyclobutene derivative (173), formed by secondary irradiation. A minor process, that of a-cleavage. 

One report has appeared of a system in which the first-formed cyclobutene adds a second molecule of alkyne photochemically, leading eventually to a benzene derivative as a result of electrocyclic ring-opening and a retro-Diels-Alder process (equation 55). 

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