Abstract:The electrochemical behavior of 4-trifluoromethylacetophenone (TFMA) has been studied in aqueous (2 ≤ pH ≤ 10), micellar, and ethanolic solutions using polarographic techniques. A slow hydration process of the carbonyl group has been observed by means of polarography and UV–visible spectrophotometry. Besides the well-known pH dependence of the electroreduction of aromatic ketones, C—F bond cleavages concomitant to the ketone reduction occur under electrolysis conditions in alkaline medium (pH = 10) at a very n… Show more
“…With the exception of BP−CHO and BP−NO 2 , the 2nd waves of the free bases are generally chemically irreversible and smaller (about half) compared with the 1st-e waves. This observation is consistent with the fact that as the 2-e reduced form of a free base diffuses out of the electrode, it reacts with free base diffusing toward the electrode yielding the pinacol dianion . Meanwhile, as will become apparent by the ensuing discussion, the reversible reduction waves of BP−CHO and BP−NO 2 do not have the same origin as the waves of the rest of the free bases, as they involve reduction of the substituents.…”
Section: Resultssupporting
confidence: 66%
“…The pyridinum ring of 2 is aromatic and planar, but there is no such guarantee for [ 1 -X] • , for which PM3 calculations show that the geometry of N is pyramidal (Scheme 2). Species 2 conforms to the typical 1-e reduced form of a ketone, and should lead to pinacolization, which has not been observed …”
In anhydrous CH3CN a series of nine 4-(4-substituted-benzoyl)-N-methylpyridinium cations (substituent: -OCH3, -CH3, -H, -SCH3, -Br, -Ctbd1;CH, -CHO, -NO2, and -(+)S(CH3)2) demonstrate two chemically reversible, well-separated one-electron (1-e) reductions in the same potential range as other main stream redox catalysts such as quinones and viologens. Hammett linear free energy plots yield excellent correlation between the E(1/2) values of both waves and the substituent constants sigma(p)(-)(X). The reaction constants for the two 1-e reductions are rho(1) = 2.60 and rho(2) = 3.31. The lower rho(1) value is associated with neutralization of the pyridinium ring, and the higher rho(2) value with the negative charge developing during the 2nd-e reduction. Structure-function correlations point to a purely inductive role for substitution in both 1-e reductions. The case of the 4-(4-nitrobenzoyl)-N-methylpyridinium cation is particularly noteworthy, because the 4-nitrobenzoyl moiety undergoes reduction before the 2nd reduction of the 4-benzoyl-N-methylpyridinium system. Correlation of the third wave of this compound with the 2nd-e reduction of the others yields sigma(p)(-NO)2*- = -0.97 +/- 0.02, thus placing the -NO2-* group among the strongest electron donors. Solvent deuterium isotope effects and maps of the electrostatic potential (via PM3 calculations) as a function of substitution support that 2-e reduced forms develop H-bonding with proton donors (e.g., CH3OH) via the O-atom. The average number of CH3OH molecules entering the H-bonding association increases with e-donating substituents. H-bonding shifts the 2nd reduction wave closer to the first one. This has important practical implications, because it increases the equilibrium concentration of the 2-e reduced form from disproportionation of the 1-e reduced form.
“…With the exception of BP−CHO and BP−NO 2 , the 2nd waves of the free bases are generally chemically irreversible and smaller (about half) compared with the 1st-e waves. This observation is consistent with the fact that as the 2-e reduced form of a free base diffuses out of the electrode, it reacts with free base diffusing toward the electrode yielding the pinacol dianion . Meanwhile, as will become apparent by the ensuing discussion, the reversible reduction waves of BP−CHO and BP−NO 2 do not have the same origin as the waves of the rest of the free bases, as they involve reduction of the substituents.…”
Section: Resultssupporting
confidence: 66%
“…The pyridinum ring of 2 is aromatic and planar, but there is no such guarantee for [ 1 -X] • , for which PM3 calculations show that the geometry of N is pyramidal (Scheme 2). Species 2 conforms to the typical 1-e reduced form of a ketone, and should lead to pinacolization, which has not been observed …”
In anhydrous CH3CN a series of nine 4-(4-substituted-benzoyl)-N-methylpyridinium cations (substituent: -OCH3, -CH3, -H, -SCH3, -Br, -Ctbd1;CH, -CHO, -NO2, and -(+)S(CH3)2) demonstrate two chemically reversible, well-separated one-electron (1-e) reductions in the same potential range as other main stream redox catalysts such as quinones and viologens. Hammett linear free energy plots yield excellent correlation between the E(1/2) values of both waves and the substituent constants sigma(p)(-)(X). The reaction constants for the two 1-e reductions are rho(1) = 2.60 and rho(2) = 3.31. The lower rho(1) value is associated with neutralization of the pyridinium ring, and the higher rho(2) value with the negative charge developing during the 2nd-e reduction. Structure-function correlations point to a purely inductive role for substitution in both 1-e reductions. The case of the 4-(4-nitrobenzoyl)-N-methylpyridinium cation is particularly noteworthy, because the 4-nitrobenzoyl moiety undergoes reduction before the 2nd reduction of the 4-benzoyl-N-methylpyridinium system. Correlation of the third wave of this compound with the 2nd-e reduction of the others yields sigma(p)(-NO)2*- = -0.97 +/- 0.02, thus placing the -NO2-* group among the strongest electron donors. Solvent deuterium isotope effects and maps of the electrostatic potential (via PM3 calculations) as a function of substitution support that 2-e reduced forms develop H-bonding with proton donors (e.g., CH3OH) via the O-atom. The average number of CH3OH molecules entering the H-bonding association increases with e-donating substituents. H-bonding shifts the 2nd reduction wave closer to the first one. This has important practical implications, because it increases the equilibrium concentration of the 2-e reduced form from disproportionation of the 1-e reduced form.
“…In general, chemical or electrochemical reduction of ketones yields alcohols . More specifically, in aprotic media electrochemical reduction gives primarily pinacol dianions through radical-radical, radical-ketone or ion-ketone coupling . In the presence of proton donors such as acetic acid, two-electron (2-e) reduction also yields carbinols. 2g, The same reactivity pattern is demonstrated by pyridine-based aromatic ketones.…”
Section: Introductionmentioning
confidence: 94%
“…More specifically, in aprotic media electrochemical reduction gives primarily pinacol dianions through radical-radical, radical-ketone or ion-ketone coupling . In the presence of proton donors such as acetic acid, two-electron (2-e) reduction also yields carbinols. 2g, The same reactivity pattern is demonstrated by pyridine-based aromatic ketones. For instance, one-electron (1-e) reduction of 4-acetylpyridine is followed by dimerization leading to the corresponding pinacol, while 2-e reduction at pH = 6.5 leads to methyl-4-pyridylmethanol …”
In anhydrous CH 3 CN, 4-benzoyl-N-methylpyridinium cations undergo two reversible, well-separated (∆E 1/2 ∼ 0.6 V) one-electron reductions in analogy to quinones and viologens. If the solvent contains weak protic acids, such as water or alcohols, the first cyclic voltammetric wave remains unaffected while the second wave is shifted closer to the first. Both voltammetric and spectroelectrochemical evidence suggest that the positive shift of the second wave is due to hydrogen bonding between the two-electron reduced form of the ketone and the proton donors. While the one-electron reduction product is stable both in the presence and in the absence of the weak-acid proton donors, the two-electron reduction wave is reversible only in the time scale of cyclic voltammetry. Interestingly, at longer times, the hydrogen bonded adduct reacts further giving nonquaternized 4-benzoylpyridine and 4-(R-hydroxybenzyl)pyridine as the two main terminal products. In the presence of stronger acids, such as acetic acid, the second wave merges quickly with the first, producing an irreversible two-electron reduction wave. The only terminal product in this case is the quaternized 4-(Rhydroxybenzyl)-N-methylpyridinium cation. Experimental evidence points toward a common mechanism for the formation of the nonquaternized products in the presence of weaker acids and the quaternized product in the presence of CH 3 CO 2 H.
“…This sequence is inverted for the protons of the alcohol function (Figure 2). [15] The d,l/meso ratio of 9.5 for the two diastereomeric pinacols 7 is quite high. Bewick et al [16,17] and Stocker et al [18,19] have postulated that this diastereoselectivity can be explained by ion-pair formation between the anion-radical and BuN ϩ from the supporting electrolyte.…”
Section: Electrolysis Of the Model Compoundmentioning
Electroreducible amphiphilic aromatic ketones derived from D‐glucose and D‐glucofuranurono‐6,3‐lactone (D‐glucurone) have been synthesized by Schmidt condensation and reaction of the unprotected lactone with the appropriate substrates, respectively. The macroscale electrolyses of the glucose derivatives, performed in an aprotic solvent (DMF), yield the pinacols possessing two glycosidic side chains. Under the same conditions of electrolysis with the D‐glucurone derivative, the glyosidic carbon‐oxygen bond is cleaved. The use of a redox mediator (couple anthracene−•/anthracene) has demonstrated that a glucosidic bond can be reduced by a homogeneous electron transfer. In the presence of a proton donor the expected D‐glucuronic pinacol is obtained. The radical‐radical coupling involves the formation of two chiral centers. The diastereo‐ and the enantioselectivity of the reaction have been studied by 1H‐ and 2H‐NMR spectroscopy, respectively.
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