1,3,5-hexatriene derivatives electrocyclization reactions

Why is the thermal stability of diarylethene derivatives enhanced by replacing phenyl groups with furan or thiophene groups In molecular orbitals calculation, the photochromic reaction is treated as a typical electrocyclic reaction between hexatriene and cyclohexadiene. The thermal reaction proceeds disrotatorily and the photoreaction, conrotatorily. Disrotatory cyclization of A to B requires an increase in free energy larger than 138 kJ/mol, and hence no thermal ring-closure occurs in the case of either phenyl- or furan-substituted molecules (see... 

Correlation diagrams can be constructured in an analogous fashion for the disrotatory and conrotatory modes for interconversion of hexatriene and cyclo-hexadiene. They lead to the prediction that the disrotatory mode is an allowed process while the conrotatory process is forbidden. This is in agreement with the experimental results on this reaction. Other electrocyclization reactions can be analyzed by the same process. Substituted derivatives of polyenes obey the orbital symmetry rules, even in cases where the substitution pattern does not correspond in symmetry to that of the orbital system. It is the symmetry of the participating orbitals, not of the molecule as a whole, that is crucial to the analysis. 

The proposal that the wavelength dependence of the product yields from irradiation of Z-14 is due largely to selective excitation of specific conformers is supported by the observation that the UV spectrum of the compound can be simulated accurately by a 9 1 combination of the spectra of Z-2-rcrr-butyl- (Z-131) and Z-2,5-di-fert-butyl-l,3,5-hexatriene (Z-15), which serve as model compounds for the cZi and cZc conformers of Z-14, respectively . Both Z-15 and the perfluorinated ,Z, -4,5-dimethyl-2,4,6-octatri-ene derivative (135) are thought to adopt preferred helical cZc conformations, and undergo highly selective electrocyclic ring closure to the corresponding cyclohexadienes 132 and 136, respectively upon irradiation (equations 49 the reaction of Z-15 the... 

In an interesting reaction involving ring expansions of cyclobutenones based on hexatriene electrocyclizations, the lithiated species 95 underwent conversion to the fused thiophene derivative 96 upon treatment with the cyclobutenone 97 <04JA1624>. 

The case of butadiene-cyclobutene interconversion, which one might expect to provide a straightforward example illustrating the stereoselectivity of photochemical electrocyclization, is actually quite complex, especially when substituted systems are involved. We first consider experimental outcomes from the photolysis of butadiene and substituted derivatives, as well as the reverse reaction, the photochemical ringopening reactions of cyclobutenes. We then examine the 1,3,5-hexatriene system in the same way. 

The general strategy illustrated in Scheme 16.3 is used in the synthesis of indane and tetralin derivatives (Scheme 16.4) [6]. Pd(0)-catalyzed double Heck reaction yielded the hexatriene framework, which on heating underwent disrotatory electrocyclization to yield the dihydroaromatic derivative. Dehydrogenation was carried out either with Pd/C or elemental sulfur to yield the aromatic... 

The reaction of hex-3-ene-1,5-diyne derivatives (enediynes) to p-benzyne, the Bergman cyclization, is related to the electrocyclic ring closure of hexatriene. Following the discovery of a new class of potent antitumor antibiotics that possess an enediyne unit, this reaction has received considerable attention in recent years. Although the reaction has been investigated using semiempirical methods, the use of more advanced methods for the study of a closed-shell species reacting to a diradical is clearly desirable. 

---