Electrogenerated bases: the role of weak electroinactive proton donors and the effect of electrocatalysis on the redox behavior of azobenzene

Abstract

acid, 97278-21-2; 2-methyl-4-(2-naphthyl)butanoyl chloride, 97278-22-3; 3-methyl-Cketo-THP, 40154-19-6; 3,4-dimethyl-4-hydroxy-THP, 97278-23-4; 3,4-dimethyl-l,2-dihydrophenanthrene, 97278-24-5; 2,2'-diacetylbiphenyl, 24017-95-6; (R*,R*)-2,2'-bis(l-hydroxyethyl)biphenyl, 18152-34-6; (R*,R*)-2,2'-bis(l-hydroxyethyl)biphenyl, 18152-33-5; 2,2'-bis(l-bromoethyl)biphenyl, 10, 9,10, 2, 2,.The stepwise electrochemical reduction of PhN=NPh in DMF-O.l M (CH3),NPFB can give, depending upon proton availability, three anionic reduction products. The basicities of these electrogenerated bases increase in the order PhN=NPh-. < PhNHN-Ph < PhN-N-Ph. Water (pKaMezSo = 31.6), which is capable only of protonating PhN-N-Ph, causes the reversible PhN=NPh-./PhN-N-Ph redox couple to shift to more positive potential. From the magnitude of the potential shift and the reported value of pKaDm(H2O) = 34.7, pKaDMF(PhNHN-Ph) is estimated to be 38.1. Ph2C=NNH2 and PhNHNHPh both protonate PhN-N-Ph irreversibly. The PhNHN-Ph that is produced in this reaction is subsequently consumed in an electrocatalytic cycle which involves PhN=NPh as the oxidant, the less easily oxidized PhNHN-Ph as the reductant, and proton transfer from PhNHNPh to the conjugate base of the added proton donor as the rapid, irreversible follow-up reaction. The anodic peak for the irreversible oxidation of PhNHN-Ph is discernible only at scan rates in excess of about 1 V/s at -51 "C. Protonation of PhNHN-Ph by both diethyl malonate (pKaMefiO = 16.4) and fluorene (pKaM@ = 22.6) and of PhN=NPh-. by diethyl malonate is irreversible and affords PhNHNHPh and the conjugate base of the added proton donor as the principal electroactive products.T h e electrochemical reduction of azobenzene (PhN= N P h ) in aprotic media is reported t o occur in two successive one-electron steps.14 Although PhN=NPh--is quite stable in the absence of added proton donors and can be reoxidized quantitatively on t h e cyclic voltammetric time scale, the lifetime of t h e corresponding dianion is sufficiently short in most solvent-electrolyte systems so as t o preclude a cyclic voltammetric anodic peak for its redoxidation. Rapid reaction of t h e dianion involves its protonation by acidic impurities such as water and is generally believed t o afford h y d r a z o b e n z e n e ( P h N H N H P h ) or its conjugate base as t h e principal p r~d u c t .~ Because of t h e basicity of PhN-N-Ph and the ease with which it can be prepared in known amount, electrogenerated PhN-N-Ph has been used widely as a strong base in electroorganic and diagnostic electrochem-(1) Sadler, J. L.; Bard, A. J. J. Am. Chem. SOC. 1968, 90, 1979. (2) Aylward, G. H.; Garnett, J. L.; Sharp, J. H. Chem. Commun. 1966, (3) Aylward, G. H.; Garnett, J. L.; Sharp, J. H. Anal. Chem. 1967,39, (4) (a) Boto, K. G.; Thomas, F. G. A u t . J. Chem. 1971,24,475. (b) (5) Lipsztajn, M.; Krygowski, T. M.; Galus, Z. J. Electroanal. Chem. (6) Herlem, M.; Van Amerongen, G. Anal. Lett. 1980, 13, 549. (7) Iverson, P. E.; Lund, H. Tetrahedron Lett. 1969, 3...

An electrochemical alternative strategy to the synthesis of β-lactams

“…The direct addition of N-heterocyclic carbene to benzaldehyde, in imidazolium-based ionic liquids containing bases (NR 3 ), has been reported and discussed by many authors [5,59,62]. The protonexchange reaction (Scheme 4, reaction (1)) suggests that, in RTIL 2a, N-heterocyclic carbene (1-butyl-3-methylimidazoliun cation: pK a = 22-23 in DMSO and H 2 O [5]) may be considered a weaker base than the EGB [63].…”

Some reactions of electrochemically prepared anions of DMF and DMSO

“…The main compounds were separated on a column of silica starting with pentane as eluent using increasing polarity of the eluent. (6), 163 (19), 162 (23), 150 (8).…”

“…The DMF-anion deprotonates acetophenone 1 and 1 À reacts with the electrophile Me 2 NCH@O by addition to the carbonyl group and loss of dimethylamine to 3-phenyl-3-ketopropionaldehyde (6); this aldehyde is attacked by 1 À to 1,5-diphenylpenta-3-ol-1,5-dione (7) which loses water to 1,5-diphenylpent-2-ene-1,5-dione (8). A Michael addition of DMF-anion to 8 produces 2.…”

“…The reduction of the first probase, azobenzene, to the radical anion or anion, was used in a Wittig reaction to deprotonate a benzyltriphenylphosphonium salt in the presence of benzaldehyde with the formation of stilbene (Z + E) [4]. Several papers on the use and properties of probases have been published [5][6][7][8]. In some cases the the substrate may act as a probase [9].…”

Effect of hydroxylic additives on the electrochemical reduction of some azobenzenes

“…In fact, under normal conditions the second reduction step of azobenzene is irreversible owing to rapid reaction of the dianion, Azb 2À with protic impurities. However, in very dry DMF the reaction is reversible [27,28] and when trace amounts of water are present, a reversible protonation of the dianion is detected [28][29][30]. Weak interactions between the anion radical of azobenzene, Azb ÅÀ , and hydroxylic additives such as water and alcohols have been detected [31].…”