Photochemical Reactions of Thiophenes

Novel Intramolecular Photocyclization of α-Arylthiophene Derivatives

“…Considering these experimental results and previous reports in the literature, ,, the reaction seems to proceed mainly via the singlet excited state of 1a followed by the formation of noncharge-separated reactive species. A plausible reaction pathway is proposed in Scheme .…”

“…Considerable attention has been paid to the photochemical behavior of thiophene derivatives in a wide range of scientific fields, for example, light emitting materials, photochromic compounds, organic photovoltaic devices, and physical organic chemistry . However, synthetic use of the photoreaction of thiophenes is quite limited, and has mostly consisted of the simple rearrangement between regioisomers, electrocyclization, and [2 + 2] addition. − In addition, a few examples of [4 + 2] addition have also been reported . In the course of our study on the photoreaction of 5-membered heteroaromatic compounds, we found that α-arylthiophenes linked with an alkene moiety through a three-atom spacer cyclized to give cyclobutene-fused perhydrothiapentalene-type compounds under UV (>300 nm) irradiation.…”

“…A part of 1a undergoes reversible formation of 3a . The formation of the Dewar thiophene type intermediate is postulated in the photoisomerization of 2-substituted thiophene to 3-substituted thiophene. ,,− Some of these intermediates were trapped and characterized. − The thiirane ring-opening and successive cyclization are known under thermal conditions . The details of the mechanism should be disclosed in a further investigation.…”

Mechanistic Analysis of an Isoxazole–Oxazole Photoisomerization Reaction Using a Conical Intersection

“…The photochemistry of the isomerization of five-membered heterocyclic molecules has been the subject of intense research for many decades. − Traditionally, five mechanisms have been proposed to rationalize the experimental findings of such photochemical transformations: (1) the internal cyclization–isomerization mechanism; (2) the ring contraction–ring expansion mechanism; (3) the van Tamelen–Whitesides mechanism; (4) the zwitterion–tricycle mechanism; (5) the fragmentation–readdition mechanism (Scheme S1, Supporting Information). Nevertheless, these proffered mechanisms still retain many illogical problems, because these earlier mechanisms did not consider the photoreaction pathways from the excited state to the ground state. As a result, they cannot offer a good explanation of the available experimental observations. − …”

“… The mechanisms of the three reaction pathways for the photochemical transformation of 3,5-dimethylisoxazole ( 1 ) in its first singlet excited state (π→ π*) have been determined using the CASSCF (11-orbital/14-electron active space) and MP2-CAS methods with the 6-311G­(d) basis set. These three reaction pathways are denoted as (i) the internal cyclization-isomerization path (path A), (ii) the ring contraction-ring expansion path (path B), and (iii) the direct path (path C).…”