For the classical ionic process, the reaction can be regarded as a 1,2-addition of the thiol unit to the α,β-unsaturated carbonyl unit. The experimental ∆Ea ‡ value (44 kJ mol -1 , Figure ) has a consistency with previously reported values for enzymatic processes. However, the obtained adduct intermediates are thermodynamically stable, and the adduct will work as a "pitfall" state for the overall process (Figures ). In addition, under non-radical conditions, the 1,2-adducts are highly stable against radical activation, as demonstrated by the use of two different molecular catalysts (Figure ). As such, the non-radical reaction remained stoichiometric, not catalytic, as shown in Figure , and C-S bond formation took place with good product yield. Thus, the relatively unstable intermediates in the radical pathways may result in plausible thermodynamics for catalysis, while the high reactivity of the thiyl radicals toward maleate at the early stage of thiyl insertion could be rationalized by the molecular orbitals (MOs). As shown in Figure , charge transfer (CT) from the bonding π-orbital of maleate to the singly occupied molecular orbitals (SOMOs) of the thiyl radicals will expedite the insertion reaction by weakening the π-bonding. In this regard, the smaller gap between the bonding π-orbital of maleate and lowerlying SOMO of the thiyl radical will be suitable for efficient insertion. Thus, electron-rich thiolates give stable adducts with an electron-deficient maleate, while an electron-deficient thiyl radical gives the adduct with maleate as a less stable intermediate, (a) (b) with a kinetically less demanding process of S-C bond formation via CT from the bonding ΜΟ of maleate to the SOMOs of thiyl radicals, realizing efficient catalytic isomerization leading to fumarate. The spin densities of the reaction intermediates are delocalized over the molecules (Figure ), rationalizing the suppressed reactivity toward polymerization.