The change from the temperature independence of the primary (1°) H/D kinetic isotope effects (KIEs) in wild-type enzyme-catalyzed H-transfer reactions (ΔE a = E aD – E aH ∼ 0) to a strong temperature dependence with the mutated enzymes (ΔE a ≫ 0) has recently been frequently observed. This has prompted some enzymologists to develop new H-tunneling models to correlate ΔE a with the donor–acceptor distance (DAD) at the tunneling-ready state (TRS) as well as the protein thermal motions/dynamics that sample the short DADTRS’s for H-tunneling to occur. While extensive evidence supporting or disproving the thermally activated DAD sampling concept has emerged, a comparable study of the simpler bimolecular H-tunneling reactions in solution has not been carried out. In particular, small ΔE a’s (∼0) have not been found. In this paper, we report a study of the hydride-transfer reactions from four NADH models to the same hydride acceptor in acetonitrile. The ΔE a’s were determined: 0.37 (small), 0.60, 0.99, and 1.53 kcal/mol (large). The α-secondary (2°) KIEs on the acceptor that serve as a ruler for the rigidity of reaction centers were previously reported or determined. All possible productive reactant complex (PRC) configurations were computed to provide insight into the structures of the TRS’s. Relationships among structures, 2° KIEs, DADPRC’s, and ΔE a’s were discussed. The more rigid system with more suppressed 2° C–H vibrations at the TRS and more narrowly distributed DADPRC’s in PRCs gave a smaller ΔE a. The results replicated the trend observed in enzymes versus mutated enzymes and appeared to support the concepts of different thermally activated DADTRS sampling processes in response to the rigid versus flexible donor–acceptor centers.

Substituent effects on the temperature dependence of primary kinetic isotope effects, characterized by ΔE a = E aD − E aH , for two series of the title reactions in acetonitrile were studied. The change from ΔE a ≈ 0 for a highly rigid system to ΔE a > 0 for systems with reduced rigidities was observed. The rigidities were controlled by the electronic and steric effects. This work replicates the observations in enzymes and opens a new research direction that studies structure−ΔE a relationship.

Photodeoxygenation of dibenzothiophene S‐oxide (DBTO) is believed to produce ground‐state atomic oxygen [O(3P)] in solution. Compared with other reactive oxygen species (ROS), O(3P) is a unique oxidant as it is potent and selective. Derivatives of DBTO have been used as O(3P)‐precursors to oxidize variety of molecules, including plasmid DNA, proteins, lipids, thiols, and other small organic molecules. Unfortunately, the photodeoxygenation of DBTO requires ultraviolet irradiation, which is not an ideal wavelength range for biological systems, and has a low quantum yield of approximately 0.003. In this work, benzo[b]naphtho[1,2‐d]selenophene Se‐oxide, benzo[b]naphtho[2,1‐d]selenophene Se‐oxide, dinaphtho[2,3‐b:2’,3’‐d]selenophene Se‐oxide, and perylo[1,12‐b,c,d]selenophene Se‐oxide were synthesized, and their ability to utilize visible light for generating O(3P) was interrogated. Benzo[b]naphtho[1,2‐d]selenophene Se‐oxide produces O(3P) upon irradiation centered at 420 nm. Additionally, benzo[b]naphtho[1,2‐d]selenophene Se‐oxide, benzo[b]naphtho[2,1‐d]selenophene Se‐oxide, and dinaphtho[2,3‐b:2’,3’‐d]selenophene Se‐oxide produce O(3P) when irradiated with UVA light and have quantum yields of photodeoxygenation ranging from 0.009 to 0.33. This work increases the utility of photodeoxygenation by extending the range of wavelengths that can be used to generate O(3P) in solution.

Oxidation of thiols yield sulfenic acids, which are very unstable intermediates. As sulfenic acids are reactive, they form disulfides in the presence of thiols. However, sulfenic acids also oxidize to sulfinic acids (−SO 2 H) and sulfonic acids (−SO 3 H) at higher concentrations of oxidants. Hydrogen peroxide is a commonly used oxidant for the oxidation of thiols to yield sulfenic acids. However, hydrogen peroxide also oxidizes other reactive functional groups present in a molecule. In this work, the reaction intermediates arising from the oxidation of sterically hindered thiols by aryl chalcogen oxides, dibenzothiophene S -oxide (DBTO), dibenzoselenophene Se -oxide (DBSeO), and dibenzotellurophene Te -oxide (DBTeO), were investigated. Photodeoxygenation of DBTO produces triplet atomic oxygen [O( 3 P)], which has previously shown to preferentially react with thiols over other functional groups. Similarly, aryl selenoxides have also shown that they can thermally react selectively with thiols at room temperature to yield disulfides. Conversely, aryl telluroxides have been reported to oxidize thiols to disulfides thermally with no selectivity toward thiols. The results from this study demonstrate that sulfenic acids are an intermediate in the oxidation of thiols by DBTeO and by photodeoxygenation of DBTO. The results also showed that the oxidation of thiols by DBSeO yields sulfonic acids. Triptycene-9-thiol and 9-fluorotriptycene-10-thiol were for the thiols used in this oxidation reaction. This work expands the list of oxidants that can be used to oxidize thiols to obtain sulfenic acids.

The cover image is based on the Research Article Visible light‐induced photodeoxygenation of polycyclic selenophene Se‐oxides by Satyanarayana M. Chintala, John C. Throgmorton, Peter F. Maness and Ryan D. McCulla, https://doi.org/10.1002/poc.4144.