In this ACS meeting, we also report the temperature dependence of KIEs (represented by ΔE_a = E_aD - E_aH) for several apparent hydride transfer reactions of NADH analogues with chloranil (2,3,5,6-tetrachloro-1,4-benzoquinone) and DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) in acetonitrile. The observed KIEs at 25°C > 8, with ΔE_a values > 1.6 kcal/mol for the reactions with chloranil. For the reactions of DDQ with different NADH analogues, both KIE and ΔE_a values are smaller; KIE ranges from 4 to 6 and ΔE_a from 1.1 to 1.6 kcal/mol. These results are interpreted in terms of the electron-proton-electron sequential transfer mechanism within the framework of the vibration-assisted activated H-tunneling (VA-AHT) model. In this study, we report the KIE and ΔE_a values for reactions involving another efficient one-electron/hydride acceptor, tetracyanoethylene (TCNE), in acetonitrile. These reactions are believed to follow the same electron-proton-electron transfer mechanism. As a result, the KIE reflects the proton transfer step between the radical cation of the NADH analogues and the radical anion of TCNE. The KIEs range from 4 to 6, and the ΔE_a values range from 1.4 to 1.6 kcal/mol (across different NADH analogues), which are similar to the results observed for the reactions with DDQ. Notably, the ΔE_a values for these reactions are generally higher than those for one-step hydride transfer reactions. According to the VA-AHT model, this difference can be explained by the hydride transfer occurring within a tighter charge-transfer complex, while the proton transfer takes place within a looser complex consisting of two radical ions that are relatively well solvated and separated.

Jessica Sager

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