Computational investigation of the photochemical deoxygenation of thiophene- S-oxide and selenophene- Se-oxide

Abstract

CASSCF and multireference MP2 calculations were carried out on thiophene-S-oxide (TO) and selenophene-Se-oxide (SeO), comparing the energies of the ground state to the first two electronically excited singlet and triplet states, using constrained optimizations and multiple fixed S-O or Se-O distances. For both molecules, one of the two triplet states smoothly dissociates to yield O((3)P) with little or no barrier. Single point calculations are consistent with the same phenomenon occurring for dibenzothiophene-S-oxide (DBTO). This provides an explanation for the inefficient unimolecular photochemical dissociation of O((3)P) from DBTO despite a phosphorescence energy below that of S-O dissociation, i.e., that S-O scission probably occurs from a spectroscopically unobserved triplet (T2) state.

Thiophene S-Oxides

“…Otherwise, in those cases, where the thiophene S-oxide does not exhibit a CH 3 substituent on the ring system, furans are often the main products along with (deoxygenated) thiophenes (Scheme 39). This has been noted with phenyl-substituted (96, 160) and tert-butyl substituted thiophene S-oxides (73,143,153) as well as with 3,4-dibenzylthiophene S-oxide (158)(Scheme 40) [92][93][94][95]. Different mechanisms have been forwarded for this photochemical formation of furans.…”

“…Different mechanisms have been forwarded for this photochemical formation of furans. A viable mechanism involves a cyclic oxathiin, where the first step within the photochemical reaction is initiated by the homolytic ring cleavage α to the sulfoxy group [92][93][94]. A rearrangement of thiophene Soxides to produce furans can also proceed thermally as found by Thiemann et al [18] in the transformation of thiophenophane S-oxide 110 to furanophane 111 (Scheme 26) and by Thiophene S-Oxides http://dx.doi.org/10.5772/intechopen.79080…”

Kinetic and mechanistic studies of reactive intermediates in photochemical and transition metal-assisted oxidation, decarboxylation and alkyl transfer reactions

“…99 Similarly, recent calculations by Jenks also suggest that the observed excited triplet of thiophene S-oxide is too low in energy for deoxygenation. 100 Others suggest that free O( 3 P) is never formed, and that an oxaziradine intermediate is responsible for the observed chemistry. [101][102][103] For example, the cleavage of DNA resulting from photolysis of 3-amino-1,2,4-benzotriazine 1,4-dioxide is attributed to an oxaziridine intermediate, that is believed to have been trapped by photolysis in the presence of p-anisidine.…”

Determination of sulfoxide bond dissociation enthalpies of dibenzothiophene S‐oxide derivatives with computational methods

“…As shown in Scheme , S–O bond scission from any excited singlet state to O( 3 P) and DBT would be a spin‐forbidden process. Additionally, the fluorescence spectrum of DBTO indicates a S 1 energy of about 85 kcal mol −1 , and thus, the spin‐allowed cleavage to generate O( 1 D) and DBT from S 1 is thermodynamically unfavorable by roughly 35 kcal mol −1 . Similarly, the spectroscopic triplet state (T 1 ) of DBTO lies roughly 11–15 kcal mol −1 below the BDE leading to DBT and O( 3 P) .…”

“…More recently, a study employing multireference methods to examine the excited states of thiophene S‐oxide found a dissociative triplet state (T 2 ) leading directly to O( 3 P) . This suggests the mechanism of deoxygenation for DBTO proceeds by intersystem crossing from S 1 to T 2 followed by cleavage of the S–O bond.…”