While stronger bases like NEt3 (pKa = 9.0, DMSO) gave minimal conversion to P1 (8%) (Table , Entry 4), the weaker base pyridine (pKa = 3.4, DMSO) (Table , Entry 5) resulted in 85 PA%. This significant improvement can be partly attributed to N1b's limited solubility, a consequence of pyridine's reduced propensity to generate a soluble thiolate intermediate, resulting in improved chemoselectivity driven by inefficient mass transfer. Additionally, E1a's complete consumption may be explained by N-activation (e.g., Hbond, PCET, PT) that was likely inhibited by stronger bases under the previous conditions tested. N-activation of basic heterocycles such as pyridine is ubiquitous, supporting this hypothesis (vida infra). To test this further, various solvents were evaluated in the absence of pyridine and successfully formed product (Table , Entries 6-10). MeCN (Table , Entry 10) performed best, giving P1 in high conversion with trace P2 and disulfide (P3). The absence of exogenous base and low organic solubility allowed P1 to precipitate from the reaction as an HI salt, which could then be collected through filtration in high yield (91%). Control/Mechanistic Experiments. After successful optimization, we performed standard photochemical control experiments. To our surprise, in the absence of light, we observed identical performance to the standard conditions (Table and). This unexpected result suggested a change in mechanism to that occurring under Miyake's reported conditions. To gain mechanistic understanding that might also help guide substrate scope evaluation, we performed additional control experiments to determine key reaction parameters enabling the transformation (Table ). We first wanted to understand how protonation state affects this reaction. As previously discussed, no product was observed when using sodium thiolate N1a instead of thiophenol N1b (Table , Entry 2). By contrast, the reaction proceeds with N1a when using pyridinium salt E1b (Table , Entry 3). Analogous to Entry 2, no reaction is observed when combining N1b and E1b (Table , Entry 4), indicating a basic component is required. Furthermore, using sub stoichiometric quantities of N1b (1 equiv of a 9:1 mixture of N1a:N1b) gave complete conversion (Table 5). These data indicated that catalytic to stoichiometric equivalents of proton enabled this transformation, while zero or super stoichiometric is inhibitory. We next sought to understand structural features that might enable this reaction. Using a thiophenol was important since switching N1b (pKa~10, DMSO) to benzyl mercaptan (N2) (pKa = 15.3, DMSO) results in no reaction (Table ). To probe for electrophile activation, E1a was swapped for electronically similar 4-iodonitrobenzene (E2) using pyridine as an exogenous base (Table , Entry 7), resulting in no product formation. 2b,g Taken together, these control experiments support that the reaction requires a suitably acidic nucleophile to activate a basic electrophile via proton transfer before the desired reaction can occur. Activation of basic heterocycles through a nitrogen atom is a well precedented strategy to promote nucleophilic attack of a heteroarene. Pyridine activation is the most well documented, with common approaches including N-methylation, N-oxidation, N-coordination (TM, LA, etc.), Nprotonation, general N-functionalization, and H-bond activation. To confirm the hypothesis that protonation was occurring versus H-bonding, we performed 1 H NMR titration experiments with pyridine and thiophenol N1b (Figure ). Our results show an absence of discrete downfield peak shifts for pyridine when 1 equivalent of N1b was reached. Instead, pyridine experienced gradual downfield peak shifting (0.015-0.04 ppm) as equivalents of N1b was increased. These data suggested that a proton transfer equilibrium occurring between the reaction partners as opposed to formation of a discrete H-bound intermediate. . Control experiments elucidating key reaction parameters and general mechanistic features. #= experiment number, N= nucleophile, E= electrophile, T= temperature in °C, Add.= additive, X= no reaction, ✓= conversion to product. General conditions: N (1.1 equiv), E (1.0 equiv), MeCN (0.5M), 18h. Finally, we set out to probe if the reaction proceeds through a two-electron SNAr or electron transfer (ET) akin to a radical-nucleophilic aromatic substitution (SRN1) (Table , Entries 8-10). 14a Examples of thermal or light-initiated ET reactions between substrates of similar oxidation potential are rare but known. 14a Performing the reaction with 3-iodopyridine (E3), a suitable substrate for radical coupling, resulted in no reaction (Table , Entry 8). 14b Additionally, 4fluoro-2-aminopyridine (E4), a poor substrate for radical coupling, yielded product (Table , Entry 9). Finally, adding a radical trap (TEMPO) did not impact the reaction or result in any TEMPO derived products (Table , Entry 10). 14c Taken together, these data support a two-electron mechanism akin to an SNAr.