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Hydroxyl radicals were generated radiolytically in N2O-saturated aqueous solutions of thiourea and tetramethylthiourea. The rate constant of the reaction of OH radicals with thiourea (tetramethylthiourea) has been determined using 2-propanol as well as NaN3 as competitors to be 1.2 × 1010 dm3 mol-1 s-1 (8.0 × 109 dm3 mol-1 s-1). A transient appears after a short induction period and shows a well-defined absorption spectrum with λmax = 400 nm (ε = 7400 dm3 mol-1 cm-1); that of tetramethylthiourea has λmax = 450 nm (ε = 6560 dm3 mol-1 cm-1). Using conductometric detection, it has been shown that, in both cases, OH- and a positively charged species are produced. These results indicate that a radical cation is formed. These intermediates with λmax = 400 nm (450 nm) are not the primary radical cations, since the intensity of the absorbance depends on the substrate concentration. The absorbance build-up follows a complex kinetics best described by the reversible formation of a dimeric radical cation by addition of a primary radical cation to a molecule of thiourea. The equilibrium constant for this addition has been determined by competition kinetics to be 5.5 × 105 dm3 mol-1 for thiourea (7.6 × 104 dm3 mol-1 for tetramethylthiourea). In the bimolecular decay of the dimeric radical cation (thiourea, 2k = 9.0 × 108 dm3 mol-1 s-1; tetramethylthiourea, 1.3 × 109 dm3 mol-1 s-1), formamidine (tetramethylformamidine) disulfide is formed. In basic solutions of thiourea, the absorbance at 400 nm of the dimeric radical cation decays rapidly, giving rise (5.9 × 107 dm3 mol-1 s-1) to a new intermediate with a broad maximum at 510 nm (ε = 750 dm3 mol-1 cm-1). This reaction is not observed in tetramethylthiourea. The absorption at 510 nm is attributed to the formation of a dimeric radical anion, via neutralization of the dimeric radical cation and subsequent deprotonation of the neutral dimeric radical. The primary radical cation of thiourea is deprotonated by OH- (2.8 × 109 dm3 mol-1 s-1) to give a neutral thiyl radical. The latter reacts rapidly with thiourea, yielding a dimeric radical, which is identical to the species from the reaction of OH- with the dimeric radical cation. The dimeric radical cations of thiourea and tetramethylthiourea are strong oxidants and readily oxidize the superoxide radical (4.5 × 109 dm3 mol-1 s-1 for thiourea and 3.8 × 109 dm3 mol-1 s-1 for tetramethylthiourea), phenolate ion (3 × 108 dm3 mol-1 s-1 for tetramethylthiourea), and even azide ion (4 × 106 dm3 mol-1 s-1 for thiourea and ∼106 dm3 mol-1 s-1 for tetramethylthiourea). With O2, the dimeric radical cation of thiourea reacts relatively slowly (1.2 × 107 dm3 mol-1 s-1) and reversibly (2 × 103 s-1).

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How can catalytic reactions be discovered? Here, a two-dimensional screening strategy for reaction discovery is described. For this purpose, the investigation of single mechanistic steps is merged with combinatorial screening. As a showcase, application to the field of visible light photocatalysis allowed for the discovery of three unexpected cyclization reactions. Extensive mechanistic analysis by advanced spectroscopic and computational tools enabled insights into the underlying molecular processes. In particular, a significantly endergonic sensitization event could be discovered and substantiated by transient absorption spectroscopy.