Kinetic, thermodynamic and mechanistic studies on the reduction of carbenium ions by NAD(P)H analogues

Abstract

Hydride transfer mechanisms of the reductions of xanthylium ion by NAD(P)H analogues (i.e. BNAH, HEH and AcrH2) were investigated. Both the kinetic observations and an analysis of thermodynamic driving forces for each mechanistic step in all the possible mechanisms indicate that the reductions are initiated by a rate‐determining electron transfer, followed by a fast hydrogen atom abstraction. The mechanism of the reductions of 9‐phenylxanthylium and triphenylmethylium ions by BNAH were also investigated and are similarly discussed. © 1997 John Wiley & Sons, Ltd.

Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications

“…Nicotinamide radical cations are quite acidic, like the hydrocarbon radical cations discussed above, and the closed shell (reduced) nicotinamides are very poor acids, highlighting the typical preference of these compounds to undergo PCET rather than ET or PT. Cheng et al have presented thermochemical data for aqueous NADH351 and for 10-methyl-9,10-dihydroacridine (AcrH 2 ) in DMSO352 and MeCN (Figure 9),353 although questions have been raised about some of these results 354. For AcrH 2 in MeCN, there is reasonable agreement between Cheng’s values and those reported earlier by Savéant and Neta, but there is a substantial disagreement in the values for BNAH 355.…”

“…Square schemes showing the PCET thermochemistry of 10-methyl-9,10-dihydroacridine (AcrH 2 ) in (a) DMSO and (b) MeCN from Cheng;352,353 see text. Values above horizontal arrows give p K a values; numbers beside vertical arrows give electrochemical potentials vs. Cp 2 Fe +/0 ; numbers bisecting diagonal lines are BDFEs in kcal mol −1 ; and numbers along the steep diagonals are hydride affinities.…”

Moving Protons and Electrons in Biomimetic Systems

“…Hydride transfer is another kind of PCET, although not always grouped that way. Like other PCET reactions, H – transfer reactions can be rationalized using thermodynamic cycles 55 (e.g., Figure 6 56 ). The thermodynamics of the discrete H + , H • or e – steps are often better known in aprotic media, though some data are available in aqueous solutions for NADH models.…”

Infrared Spectra of H+(H2O)5-8 Clusters: Evidence for Symmetric Proton Hydration

“…12 Numerous theoretical calculations 2,13-22 concerned the energetics of the association reaction H + (H 2 O) n-1 + H 2 O f H + -(H 2 O) n and the isomeric interconversion between H 3 O + (H 2 O) n-1 and H 5 O 2 + (H 2 O) n-2 . While the protonated water dimer has been treated at all (including the highest available) levels of theory, [13][14][15][16] larger clusters were only accessible to reduced or approximated theoretical treatments. [16][17][18][19][20][21] This was unfortunate as protonated water clusters are presumed to potentially exist either as…”