Scheme 2. Synthesis of compound library 4{1-5,1-10} and heatmap of the reaction yields according to LC-MS (A -for trifluoroborates 2, B -for carboxylic acids 3). For aromatic substrates having a singly defined electrophilic center (namely, 1{1}, 1{3}, and 1{5}), only one monoalkylation product 4 was formed (Figure ). For pyrimidine derivatives 4{2,j}, regiospecific formation of the C-4 alkylation products was observed, which is in full accordance with the previously reported data. For pyridine-derived compounds 4{4,j}, all possible isomers (i.e., C-2, C-4, and C-6) were formed. In most cases, C-4 substitution occurred predominantly (ca. 50%), and the corresponding products 4a{4,j} (в сапорті змінив) were isolated in pure form after reverse phase HPLC. At the same time, separation of C-2 and C-6 isomers appeared to be more challenging, so that their mixtures with varied ratios containing up to 40% of C-2 isomer (according to 1 H NMR spectroscopy) could be obtained. Somewhat higher efficiency of carboxylic acids 3 as the radical source as compared to trifluoroborates 2 might be rationalized through the radical generation mechanism. According to literature data, watersoluble trifluoroborate salts 2 readily undergo direct oxidation to corresponding reactive species 6 (Figure ). In contrast, the main route for carboxylic acids 3 involves in single electron oxidation of corresponding carboxylate anion 5 by reactive species generated in the Ag + /S2O8 2-system. An alternative pathway, albeit less probable, includes hydrogen atom transfer from carboxylic acid to SO4 -radical with subsequent decarboxylation generates radical 6. At the acidic reaction conditions, concentration of carboxylate anions 5 is relatively low, which results in low steady-state concentration of corresponding radicals 6 and their slow release for the reaction with protonated heterocycle 7. This might lead to suppressing undesired second-order radical side reactions (e.g., dimerization or disproportionation) and therefore to higher product yields. On the other hand, low concentration of the radical species can be disadvantageous in the Minisci reaction with substrates sensitive to oxidation (e.g., benzothiazole 1{5}). In this case, efficient formation of alkyl radicals 6 from trifluoroborates 2 overcomes slower competitive side reactions of the heterocycle 1 and enhances the yield of desired product 4.