Electrochemically Controlled One‐Electron Oxidation Coupled to Consecutive Hydrogen Atom Transfer of Caffeine

Abstract

Caffeine (CAF) undergoes a one‐electron oxidation in acetonitrile to form a cation radical, with variable scan rate CV experiments indicating that the lifetime of the cation radical improves as the trace water content of the solvent is decreased. Electrochemical oxidation (and chemical oxidation with NOSbF6) of CAF in CH3CN leads to the generation of the protonated CAF cation as the long‐term product in high yield, whose structure is confirmed by single‐crystal X‐ray crystallography and NMR spectroscopy. The protonated cation is able to be electrochemically reduced back to CAF under electrolysis conditions. The formation of the protonated cation involves the initial one‐electron oxidation of CAF to form the cation radical, which undergoes a hydrogen atom abstraction reaction. Digital simulations of the CV data show that the rate and equilibrium constants for the hydrogen atom abstraction step are kf=1.0×102 L mol−1 s−1 and Keq=1.0×102.

Anodic Oxidation of Ethynylferrocene Derivatives in Homogeneous Solution and Following Anodic Deposition onto Glassy Carbon Electrodes

“…That exhibited at the top, reversed at 1 V, shows a second anodic peak, this one resulting from oxidation of the amine group. Consistent with previous studies of amine oxidations in nonaqueous solvents, it is irreversible, E pa =0.83 V ( ν =0.4 V s −1 ) and, under these conditions, may be expected to abstract an H‐atom from solvent or trace moisture to give the corresponding ammonium salt 8‐H + . Since the ferrocenyl‐based redox couple for 8‐H + is expected to appear at a potential that is more positive than that of 8 , the reversible wave for the follow‐up product appearing at E 1/2 =0.15 V (dashed line in Figure ) is assigned to the couple [ 8‐H + ] 0/+ .…”

“…[23] One also notes that the E 1/2 potential is more positive (by 90 mV) when the electrolyte anion is [B(C 6 F 5 ) 4 ] À , which is consistent with comparative values for 1 in homogeneous solution (60 mV positive shift of E 1/2 ) and the general trends of larger anion electrolytes in solvents of lower polarity. [21] Smaller positive shifts in E 1/2 (30 mV for surface-bound 1; 0 mV in homogeneous solution) were measured when the scans were recorded in the more polar solvent nitromethane (ɛ = 36), in which ion-pairing effects are less important.…”

“…Consistent with previous studies of amine oxidations in nonaqueous solvents, [16][17][18] it is irreversible, E pa = 0.83 V (ν = 0.4 V s À 1 ) and, under these conditions, may be expected to abstract an H-atom from solvent or trace moisture to give the corresponding ammonium salt 8-H + . [19][20][21] Since the ferrocenyl-based redox couple for 8-H + is expected to appear at a potential that is more positive than that of 8, the reversible wave for the follow-up product appearing at E 1/2 = 0.15 V (dashed line in Figure 8) is assigned to the couple [8-H + ] 0/ + . This assignment was confirmed by the observation that addition of an equivalent of HCl to the THF solution of 8 shifted the reversible wave from À 0.01 V to 0.15 V. Subsequent addition of NaOH shifted the wave back to the original value of À 0.01 V. Relevant to the possible surface deposition of 8 is the anodic behavior observed for the lithium acetylide species 8-Li.…”

Electrochemical Oxidation of Theophylline in Organic Solvents: HPLC‐PDA‐ESI‐MS/MS Analysis of the Oxidation Products

“…As for caffeine, by far the most thoroughly studied methylxanthine (also in organic solvents,) the few reported studies on TPh were mainly carried out in water at different pH . Major differences compared to the electrochemical behavior of caffeine were firstly evidenced, as the formation of little amounts of a dimeric form of TPh insoluble in water: a one‐electron one‐proton anodic oxidation of TPh was there supposed, and two oxidative routes were proposed for the electro‐generated radical: dimerization or further oxidation to uric acids and degradation derivatives.…”

“…[12] If few studies on the chemically induced oxidation of TPh have been reported in literature, also the electrochemical oxidation has been sparingly studied, [7,13] and more recent electrochemical studies mainly aimed at analytical application. [14][15] As for caffeine, by far the most thoroughly studied methylxanthine (also in organic solvents, [7,[16][17][18][19] ) the few reported studies on TPh were mainly carried out in water at different pH. [20] Major differences compared to the electrochemical behavior of caffeine were firstly evidenced, as the formation of little amounts of a dimeric form of TPh insoluble in water: [13] a one-electron one-proton anodic oxidation of TPh was there supposed, and two oxidative routes were proposed for the electro-generated radical: dimerization or further oxidation to uric acids and degradation derivatives.…”