Kinetics of intramolecular alkyl radical attack on sulfur in disulfides and thioesters

Abstract

The 4-(alkyldithio) butyl radicals 8a and c, and the 4-(pheny1dithio)butyl radical 8b, generated from the corresponding esters of Nhydroxypyridine-2( 1 H ) -thione, undergo fast exo-cyclisation by SHi attack at sulfur. Similarly, the 5-(a1kyldithio)pentyl radical 8d undergoes 1.6-ring formation. The rate constants for cyclisation were determined by photolysis of the radical precursors in the presence of appropriate thiols. Butyl and pentyl radicals bearing o-acetylthio or a-benzoylthio substituents also undergo ring closure but much more slowly. The kinetics of these intramolecular SH2 reactions are discussed and compared with those for the intermolecular attack of hexyl radicals on diphenyl disulfide and on dibutyl disulfide.Although ring formation by intramolecular homolytic substitution at a hetero atom is of synthetic utility14 and may have biological significance, 5 , 6 this type of process has received much less attention than have cyclisations involving intramolecular radical addition to multiple bonds. Recent reports include investigations of the attack of carbon-centred radicals on sulfur in sulfides, 1 , 2 , 8 -1 di~ulfides,~.' ' 9 ' sulfoxides,' 3 3 1 thioesters,' 3*1 8-20 sulfinyl esters 2 1 and S-sulfonates,22 on selenium in ~e l e n i d e s , ~. ~. ~ 5 3 2 3 and on oxygen in peroxides 24 and sulfonates.22 Intramolecular displacements of carbon-centred radicals by attack of iminyl radicals on sulfides and selenides have also been described.25In the first reported study of intramolecular radical substitution at sulfur, Kampmeier found that the radical 1, derived from the corresponding iodide, gave only dibenzothiophene 2 by substitution in the exo mode,26 and the same behaviour was observed later for radicals of the general type 3.17,27,28 No products derived from the radical 5 generated by endo substitution could be detected. Exclusive exo substitution was 2 SMe 1 3 endo f 5 6a R = But, n = 1 b R = P h , n = 1 c R = B u , / I = 1 d a = B u ' , n = 2 4 S 7a R = But, n = 1 b R=Ph, n = l c R=Bu, n = 1 d R = B d , n = 2 11 12 a R=Bu',n = 1 c R=Bu, n = 1 b R = P h , n = 1 d R=Bu',n = 2 13 Paper 4/00839A

Electrochemical oxidative C–H/S–H cross-coupling between enamines and thiophenols with H2 evolution

“…At the same time, concomitant cathodic reduction of a proton leads to hydrogen evolution. It should be noted that a mechanism involving the homolytic substitution pathway 11 could not be completely excluded.…”

Formation of Cyclic Sulfinates and Sulfinamides through Homolytic Substitution at the Sulfur Atom

“…[10,11] Sulfur is particularly wellsuited, and several reactions have been reported that employ homolytic substitution on this particular atom. A variety of different functions have been introduced (sulfides, [12][13][14] disulfides, [15] thioesters, [16][17][18] and sulfoxides; [14,19,20] sulfones seem not to be prone to such processes [14,21] ), but so far none in which an additional heteroatom was present in the ring next to the sulfur atom, despite the increased potential offered by the heterocyclic adducts.…”

Chemistry of β-(Acyloxy)alkyl and β-(Phosphatoxy)alkyl Radicals and Related Species: Radical and Radical Ionic Migrations and Fragmentations of Carbon−Oxygen Bonds

“…26 On treatment with tributylstannane and a radical initiator, acyl selenides 27,28 provide convenient sources of acyl radicals, 29 as do certain classes of acyl sulfide. [30][31][32] Alternatively, acyl tellurides 33 and acylcobalt(III) 34 derivatives can provide acyl radicals on white light photolysis. Qingwei Yao was born in northern Shanxi, Peoples Republic of China, in 1963 and received his college education at Shanxi Medical College (1979− 1983).…”