Abstract
The reactions of electrochemically generated <i >o</i>-quinones from oxidation of catechol and 4-methylcatechol as Michael acceptors with N-methylaniline as nucleophile have been studied using cyclic voltammetry. Voltammetric responses show that oxidation of catechols is followed by Michael addition of N-methylaniline. The reaction products are believed to be diphenylamine derivatives of catechols that undergo electron transfer at more negative potentials than the catechols. The observed homogeneous rate constants <svg style="vertical-align:-3.39069pt;width:37.487499px;" id="M1" height="16.450001" version="1.1" viewBox="0 0 37.487499 16.450001" width="37.487499" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(.017,0,0,-.017,.062,12.163)"><path id="x28" d="M300 -147l-18 -23q-106 71 -159 185.5t-53 254.5v1q0 139 53 252.5t159 186.5l18 -24q-74 -62 -115.5 -173.5t-41.5 -242.5q0 -130 41.5 -242.5t115.5 -174.5z"/></g><g transform="matrix(.017,0,0,-.017,5.944,12.163)"><path id="x1D458" d="M480 416q0 -21 -18 -41q-9 -11 -17 -7q-20 9 -42 9q-62 0 -140 -78q23 -69 88 -192q17 -31 27 -42t20 -11q16 0 62 46l17 -20q-64 -92 -119 -92q-35 0 -70 66q-41 73 -84 187q-36 -30 -62 -61q-27 -115 -35 -172q-41 -8 -78 -20l-6 6l140 612q7 28 0.5 34t-37.5 7l-34 1
l5 26q38 4 74 13.5t57 17t25 7.5q12 0 4 -32l-104 -443h2q35 38 97 93q39 35 65.5 56t62 41.5t58.5 20.5q19 0 30.5 -10t11.5 -22z"/></g> <g transform="matrix(.012,0,0,-.012,14.5,16.25)"><path id="x6F" d="M257 449q92 0 154 -65t62 -158q0 -112 -67 -175t-150 -63q-98 0 -158.5 66.5t-60.5 154.5q0 59 21 106.5t54.5 75.5t71 43t73.5 15zM244 416q-48 0 -81 -47t-33 -128q0 -96 38 -158t99 -62q51 0 82 43.5t31 139.5q0 91 -36 151.5t-100 60.5z"/></g><g transform="matrix(.012,0,0,-.012,20.509,16.25)"><path id="x62" d="M152 404l81 35q23 10 41 10q81 0 139 -61.5t58 -149.5q0 -107 -74.5 -178.5t-176.5 -71.5q-71 0 -147 36v555q0 48 -9.5 59.5t-56.5 15.5v23q79 10 145 35v-308zM152 374v-258q0 -20 6 -35q7 -19 30 -37t58 -18q63 0 99.5 50t36.5 137q0 82 -43.5 131t-108.5 49
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q-28 12 -46 22.5t-38.5 27t-30.5 38.5t-10 49q0 54 42.5 92t109.5 38q48 0 88 -18q6 -15 13 -50.5t9 -55.5z"/></g> <g transform="matrix(.017,0,0,-.017,31.538,12.163)"><path id="x29" d="M275 270q0 -296 -211 -440l-19 23q75 62 116.5 174t41.5 243t-42 243t-116 173l19 24q211 -144 211 -440z"/></g> </svg> for Michael addition were estimated by digital simulation and show that the reactivity of 4-methyluinone is considerably less than <i >o</i>-quinone. The oxidation potentials of the desired products drastically depend on the solution pH and their structures.
Highlights
Catechol is one of the common building blocks in organic synthesisand is produced in industrial scales as the precursor of pesticides, perfumes, and pharmaceuticals [1]. e catechol skeleton occurs in a variety of natural products specially the antioxidant [2]. e most well-known characteristic of the catechols is that they can be oxidized mainly due to their antioxidant activity and low oxidation potentials [3]. e products of oxidation are the corresponding reactive and electron-de cient o-quinones
Is nding, changes in voltammetric responses, and the previously reported papers [5] are indicative of an ECE mechanism for the electrode reaction. e nomenclature EE is used for an electron transfer reaction and CC for a chemical reaction and their order is indicative of the order of reactions. en ECE mechanism consists of an electron transfer reaction (E) followed by a chemical reaction (C) that the product of this reaction is electroactive to and undergoes the electron transfer (E) at the electrode surface. is reaction mechanism has some diagnostic criteria which are obtained by voltammetric study at various scan rates or experiment time scale [12]
Voltammetric study at various scan rates (Figure 2(I)) shows that by augmentation of scan rate the height of both anodic and cathodic peaks increases, but the increase in C1 height is more than expected for diffusion controlled electrode reaction
Summary
Catechol is one of the common building blocks in organic synthesisand is produced in industrial scales as the precursor of pesticides, perfumes, and pharmaceuticals [1]. e catechol skeleton occurs in a variety of natural products specially the antioxidant [2]. e most well-known characteristic of the catechols is that they can be oxidized mainly due to their antioxidant activity and low oxidation potentials [3]. e products of oxidation are the corresponding reactive and electron-de cient o-quinones. E products of oxidation are the corresponding reactive and electron-de cient o-quinones. Considering their instability they can be produced only by in situ methods and mild oxidation of catechols. One of the most successful in situ generations of reactive o-quinones species is the electrochemical oxidation [4]. Ere are many reports on electrooxidation of catechols to produce o-quinones as reactive intermediates in many useful homogeneous reactions [5]. E Michael additions of aliphatic, aromatic, and benzylic amines to electrochemically generated o-quinone have been reported previously [6–8]. E reaction products of aromatic amines with o-quinones have been reported to be diphenylamine derivatives or their related quinonic forms. Diphenylamine is a parent compound of many derivatives, which are used for the production of dyes, perfumery, pharmaceuticals, photography chemicals, and further smallscale applications [9]. We would like to examine the electrochemical oxidation of two catechol derivatives in the presence of N-methylaniline in kinetic and electrochemical details
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