(a) (b) isomerization requires a kinetically and thermodynamically feasible reaction pathway for the overall catalytic process to proceed with a reasonable economy. This work is, to our knowledge, the first example of the efficient catalytic isomerization under benign conditions using thiols as molecular catalysts with a catalytic TON of >2,500 and TOF of 1.1 s -1 under the limiting conditions. The reactions were initiated either by thermal radical activation or photoinduced electron transfer. To unveil the enigma of the difference in the reactivity, we carried out a DFT study to clarify the role of thiyl radicals.