Proton‐Coupled Electron Transfer in Hydrogen and Hydride Transfer Reactions

Abstract

ET. [7][8][9]12,13 The inner-sphere character in organic ET reactions is established by their high sensitivity to steric effects. 14 As the H DA value increases further, the ET process is coupled with the subsequent PT process. This is generally referred to as proton-coupled electron transfer (PCET). [15][16][17][18][19][20][21][22][23][24] What distinguishes PCET from classic HAT is that the proton and electron are transferred between two different (noninteracting) orbitals. In particular, PCET plays an important role in enzymatic reactions such as those of lipoxygenases, which are mononuclear nonheme iron enzymes. [25][26][27] The key step of the lipoxygenase reactions is PCET from a substrate to ferric hydroxide cofactor (Fe(III)-OH) to produce Fe(II)-OH 2 and a radical intermediate substrate, when an electron goes to the metal center and a proton is transferred to the OH ligand. 28,29 When PCET is a concerted process whereby a proton and an electron are transferred simultaneously, such a PCET process may be merged into the one-step HAT process. 30 Thus, there has been long-standing ambiguity as to the mechanistic borderline where a sequential PCET pathway is changed to a one-step HAT pathway or vice versa. Understanding HAT reactions certainly requires knowledge of the thermodynamics and kinetics of the overall HAT, ET, and PT steps.Considering only the two-electron reduction of A, the reduction and protonation give nine species at different oxidation and protonation states as shown in Scheme 2.3. Each species can have an interaction with a variety of metal ions (M n þ ), and such an interaction can control each ET and protonation step, as well as their combined step (hydrogen transfer) in Scheme 2.3. [31][32][33][34][35] The binding of M n þ to radical anions of electron acceptors results in a substantial increase in the ET rate. 31-35 This is defined herein as metal ion-coupled electron transfer (MCET) in analogy to PCET. 36 The binding of M n þ with A . À can also be combined with other noncovalent interactions such as hydrogen bonding and p-p interaction. 36 The initial PCET and MCET processes are followed by the second PCET and MCET processes to afford AH 2 as the two-electron reduced species of A with two protons. 36 SCHEME 2.3 MECHANISTIC BORDERLINE BETWEEN ONE-STEP HAT AND SEQUENTIAL PCETAmong a variety of hydrogen donors, dihydronicotinamide adenine dinucleotide (NADH) and analogues have attracted particular interest, because NADH is the most important source of hydrogen and hydride ion in biological redox reactions. 37-40 If HAT from NADH and analogues to hydrogen atom acceptors (A) occurs in a one-step manner, NAD . and AH . would be the only detectable radical products. In the case of sequential electron and proton transfer, however, NADH . þ and A . À would also be detected as the intermediates for the hydrogen transfer reaction. The radical intermediates such as NADH . þ , NAD . , and the corresponding analogues are involved in a variety of thermal and photoinduced ET reactions of NADH and analo...

Strategies for the Direct Catalytic Valorization of Methane Using Heterogeneous Catalysis: Challenges and Opportunities

“…This type of activation occurs when the metal site is not accessible and methane activation is dominated by the interaction with the surrounding basic ligands. Typically, oxygen atoms at the coordination sphere (or at the surface for heterogeneous catalysts) are responsible for the hydrogen atom abstraction (HAA) , from methane, hence activating the C–H bond. This “fake” C–H bond activation mechanism results in the formation of “free” methyl radicals (Scheme ).…”