Photochemistry of Thiophen‐2(5H)‐ones

Abstract

Dedicated to Prof. Dr. 0. E. Polunsky on the occasion of his 70th birthday (4.XI. 88)Mechanistic evidence for the light-induced ring opening of thiophen-2(5H)-ones 1 in alcohols affording c@-unsaturated mercapto esters 2 is presented. Regio-and stereochemical aspects of the ring closure of alkenylthio (type 3) radicals 15 and 17 to S-heterocycles 16 and 18, of 3-thiahex-5-enyl radicals 4 to (tetrahydrothien-3-yl)methyl radicals 6 and of (2,3-dihydrothien-3-yl)methyl radicals 30 (type 7, but-3-enyl radicals) to cyclopropanemethyl radicals 29 are discussed. Irradiation (A. 350 nm) of 1 in cyclohexane in the presence of 2,3-dimethylbut-2-ene affords [2 + 21 cycloadducts 14 albeit in very low yields.We have recently reported [ 1-71 that on irradiation thiophen-2(5H)-ones 1 behave different from the corresponding 0-heterocycles. While furan-2(5H)-ones exhibit a typical enone-like behaviour yielding cyclodimers, [2 + 21 cycloadducts with alkenes or photoreduction products in alcohols from their triplet state [S] [9], the unsaturated thiolactones 1 undergo ring opening in alcohols to give a$-unsaturated mercapto esters 2 from the excited singlet state. Mercapto esters 2 undergo consecutive light-induced S-H homolysis to afford alkenylthio radicals 3 which in reacting with alkenes or alkynes proved to be useful synthetic intermediates for one-pot syntheses of thiolanes, 2,3-dihydrothiophenes as well as bi-and tricyclic thiolane derivatives via intermediates 4-7 (Scheme 1). Such -Scheme 1 <s -KC P 300 nm

UV-induced unimolecular photochemistry of 2(5H)-furanone and 2(5H)-thiophenone isolated in low temperature inert matrices

“…While 2(5H)-furanones exhibit a typical enone-like behaviour, giving rise to cyclic dimers, [2 + 2] cycloadducts with alkenes or photoreduced products (e.g., saturated lactones) in alcoholic solutions via triplet state [16,18], the unsaturated thiolactones undergo ring opening to a,b-unsaturated mercapto esters via singlet state. On the other hand, it has also been shown that, like furanones, thiophenones can also undergo [2 + 2] photocycloadditions with alkenes in cyclohexane solution [11]. Interestingly, in all above mentioned studies, furanones and thiophenones were involved in cross-reactions with the solvent, whereas the photochemical behaviour of the isolated compounds (either in gaseous phase or in inert matrices), as far as we know, was not reported hitherto.…”

“…Margaretha and co-workers [6][7][8][9][10][11][12][13][14][15][16][17] reported that upon irradiation 2(5H)-thiophenones behave quite differently from the corresponding oxygen-based heterocycles. While 2(5H)-furanones exhibit a typical enone-like behaviour, giving rise to cyclic dimers, [2 + 2] cycloadducts with alkenes or photoreduced products (e.g., saturated lactones) in alcoholic solutions via triplet state [16,18], the unsaturated thiolactones undergo ring opening to a,b-unsaturated mercapto esters via singlet state.…”

ChemInform Abstract: Synthesis and Photochemistry of 5,5‐Dimethyl‐1H‐pyrrol‐2(5H)‐one and of Some N‐Substituted Derivatives.

“…We had shown that both 3(2H)-and 2(5H)-furanones l a and 2a and 3(2H)-thiophenone l b exhibit photochemistry typical for cyclopent-2-enone itself and that 2(5H)-thiophenones, e.g. 2b, behave differently in undergoing light-induced ring-opening reactions in hydroylic solvents [6] [7]. Finally, while lH-pyrro1-3(2H)-one l c is photoinert [8], the corresponding N-(ethoxycarbonyl) derivative Id again exhibits typical enone behavior [9].…”