Kinetics and Mechanism of the Oxidation of 10-Methyl-9,10-dihydroacridine by Chromium(VI,V,IV):  Electron vs Hydrogen Atom vs Hydride Transfer

Abstract

The oxidation of AcrH2 to AcrH+ by hydrogen chromate ions in H2O/MeCN (4:1, v/v) is a chain reaction that is strongly inhibited by oxygen. The initiation reaction between AcrH2 (or AcrD2) and H2CrO4 forms AcrH2 •+ (or AcrD2 •+) and occurs by a one-electron mechanism, k H = k D = 4.6 × 102 L mol-1 s-1 (25 °C). The Cr(V) produced along with AcrH• (from the acid ionization of AcrH2 •+) are chain-carrying intermediates. The propagating reaction between AcrH2 and Cr(V), k = 1 × 108 L mol-1 s-1, is of key importance since it is a branching reaction that yields two chain carriers, AcrH• and CrO2+, by hydrogen atom abstraction. The same partners react competitively by hydride ion abstraction, to yield Cr3+ and AcrH+, k = 1.2 × 107 L mol-1 s-1, in the principal termination step. The reaction of CrO2+ and AcrH2, k H = 1.0 × 104 and k D = 4.8 × 103 L mol-1 s-1, proceeds by hydride ion transfer. The Cr2+ so produced could be trapped as CrOO2+ when O2 was present, thereupon terminating the chain. AcrH2 itself reacts with Cr2O7 2- + H+, k = 5.6 × 103 L2 mol-2 s-1, but this step is not an initiating reaction. Two successive electron-transfer steps are believed to occur between Cr2O7 2- and AcrH2, yielding CrIV + CrVI + AcrH+.

Evidence for the involvement of CrII and free radicals as intermediates in the reduction of by saccharides, alcohols and hydroxyacids

“…In O 2 -saturated solutions (1.26 mM) and [Cr VI ] 0 6 0.01 mM, reactions (9) and (11) can be neglected and Cr II and S Å intermediates formed in reactions (7) and (8) should be rapidly trapped by O 2 [27]. The proposed mechanism is in accordance with the previous observation that O 2 has not kinetic effect on this reaction [1][2][3][4], because when [Cr VI ] 0 P 0.1 mM (as usually employed in the kinetic measurements), both Cr II and S Å react with HCrO À 4 faster than they do with O 2 [6,27,28].…”

Proton‐Coupled Electron Transfer in Hydrogen and Hydride Transfer Reactions

“…[39][40][41][42][43][44][45][46][47] The mechanistic borderline between one-step and sequential pathways in HAT reactions of NADH analogues has been clarified using the triplet excited states of a series of tetrazines ( 3 R 2 Tz * ) as hydrogen atom acceptors as described below. 37-40 If HAT from NADH and analogues to hydrogen atom acceptors (A) occurs in a one-step manner, NAD .…”

“…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. [39][40][41][42][43][44][45][46][47] The mechanistic borderline between one-step and sequential pathways in HAT reactions of NADH analogues has been clarified using the triplet excited states of a series of tetrazines ( 3 R 2 Tz * ) as hydrogen atom acceptors as described below. Understanding HAT reactions certainly requires knowledge of the thermodynamics and kinetics of the overall HAT, ET, and PT steps.…”

Similar substituent effects in the oxidations of primary aliphatic alcohols with dichromates and halochromates of heterocyclic bases

“…Generally, these reagents are capable of effecting chemospecific, regiospecific, and stereospecific oxidations in highly sensitive systems. Kinetic and mechanistic studies on chromium(VI) oxidations continue to be of interest [5][6][7], and similar investigations with these reagents are numerous. So are those dealing with the structure-reactivity relationships operating in these oxidations [8,9].…”