Anodic substitution of aromatic

Scheme 59 Anodic substitution of aromatic compounds with electrogenerated...

Anodic Side Chain Substitution of Aromatic Compounds. 159... 

Anodic conversion of aromatics proceeds in most cases by le-transfer to the anode to form a radical cation (34) (Scheme 9). Oxidation is facilitated by extension of the 7T-system ( 1/2 vs. Ag/Ag+ benzene 2.08 V, pyrene 0.86 V) and by electron donating substituents ( 1/2 vs. Ag/Ag+p-phenylenediamine —0.15 V). Oxidation potentials of polycyclic aromatics and substituted benzenes are collected in Ref [140-142]. 

Anodic side chain substitution is a competing reaction to nuclear substitution of aromatic compounds. In side chain substitution, the first formed acidic radical cation is deprotonated at the a-carbon atom of an alkyl group to form a radical. This is further oxidized to a benzyl cation, which reacts with a nucleophile (Scheme 9, path d). The factors that influence the ratio of nuclear to side chain substitution have been described in 5.4.1. 

Nuclear and side chain substitution in aromatics or substitution of a -hydrogen in alkylamines is — in most cases — best rationalized by postulating radical cations as intermediates. For anodic nuclear substitution of aromatics, especially for acyloxylation, cyanation or bromination a ECnECb3 -mechanism is assumed 37,4 9,50,226,227). jc-oxidation of the aromatic to the radical cation 28, which reacts with a nucleophile Nu, e.g., acetate, cyanide, alkoxide, followed by a second electron transfer and deprotonation (Eq. (98) ) ... 

The preparative scope of anodic side chain substitution is shown in Table 6. Table 6. Anodic side-chain substitution of aromatics... 

The same workers590,591 also showed that, in the Pd(II)-catalyzed acetoxylation of substituted arenes, a complete reversal of the usual pattern of isomer distribution for electrophilic aromatic substitution or anodic oxidation of aromatics is observed. To explain these results it was suggested that acetoxylation by Pd(OAc)2 takes place via the following addition-elimination sequence ... 

For the nucleophilic substitution of aromatic CH bonds, electrochemistry has a clear advantage over chemical conversions. While at the anode, nucleophiles like OR, OCOR, Br, and CN can be introduced simply and in one step [60], chemically multistep procedures are necessary to achieve the same goal. 

A novel O-deprotection protocol involving anodic cleavage of aromatic ether (LXXXV) was effectively used in the total synthesis of the natural products alkannin and shikonin [Eq. (42)]. The electrolysis was conducted at a carbon anode in MeCN/ H2O with LiC104 as electrolyte, giving an 80% yield at 50% conversion [97]. Although both naphthoquinone tautomers were initially formed after trapping of the radical cation by water and loss of CH2O, the tautomer with alkyl substitution at the quinone double bond was more thermodynamically stable. 

The anodic methoxylation of aromatic compounds such as naphthalene [41], anthracene [42], alkylbenzenes [31,43], phenols [44-46], anisoles [33,47-54] and other alkoxyben-zenes [53], methoxynaphthalenes [33], methoxyanthracenes [50,54], inden-l-ones [55], / a/r/-substituted anilides [56] and heterocyclic compounds, such as furans [57], thiophenes [58], and pyrroles [59], has received considerable attention. 

Beck et al. [250] conducted infrared reflection absorption spectroscopic studies on anodic over-oxidation of polypyrrole in the presence of nucleophiles such as II2O, OH", CH3OH, CHjO", Br" and CN". The formation of carbonyl groups occurred at the three-position and hydroxyl groups at the four-position via a di-cation intermediate in aqueous medium, but a bromine substitution at the four-position in the presence of Br nucleophile. Multi-methoxylation, similar to that reported in anodic oxidation of aromatics [256], occurred when the experiment was conducted in methanolic solution with 0.5 M KF as supporting electrolyte according to the following equations... 

In the oxidation of aromatic substances at the anode, radical cations or dications are formed as intermediates and subsequently react with the solvent or with anions of the base electrolyte. For example, depending on the conditions, 1,4-dimethoxybenzene is cyanized after the substitution of one methoxy group, methoxylated after addition of two methoxy groups or acetoxylated after substitution of one hydrogen on the aromatic ring, as shown in Fig. 5.55, where the solvent is indicated over the arrow and the base electrolyte and electrode under the arrow for each reaction HAc denotes acetic acid. 

This chapter deals with anodic oxidation of saturated hydrocarbons, olefins, and aromatic compounds. Substituted hydrocarbons are included, when the substituents strongly influence the reactivity. Anodic functional group interconversions (FGI) of the substituents are covered in Chapters 6, 8-10 and 15. 

I 5 Anodic Reactions of Alkanes, Alkenes, and Aromatic Compounds Tab. 14 Anodic side chain substitution... 

The conversion of aromatic compounds comprises coupling, nuclear and ben-zylic substitution, and in some cases, addition. Homo- and in a more limited scope, heterocoupling is achieved for unsubstituted and substituted aromatic compounds in direct or indirect anodic processes. Chemically, there is a limited variety of expensive oxidation reagents available, but a large scope of transition... 

Anodic oxidation of o-amino substituted aromatic Schilf bases (38 and 40) to imidazole derivatives 39 and 41 were carried out in CH3CN-O.I mol/1 Et4C104 solution with addition of pyridine as a base, using controlled potential electrolysis and a divided cell [72] (Scheme 22). 

Anodic substitution reactions of aromatic hydrocarbons have been known since around 1900 [29, 30]. The course of these processes was established primarily by a study of the reaction between naphthalene and acetate ions. Oxidation of naphthalene in the presence of acetate gives 1-acetoxynaphthalene and this was at first taken to indicate trapping of the acetyl radical formed during Kolbe electrolysis of... 

Reactions between aromatic hydrocarbon radicabcations and cyanide ions, with few exceptions, give low yields of nuclear substitution products [76], In some cases, better results have been obtained by anodic oxidation of the aromatic compound in an emulsion of aqueous sodium cyanide and dichloromethane with tetra-butylammonium hydrogen sulphate as a phase transfer agent [77, 78]. Methoxy-benzenes give exceptionally good yields from reactions in acetonitrile containing tetraethylammonium cyanide, sometimes with displacement of methoxide [79, 80]... 

Anodic oxidation of l,3-diaryl-5-methyl-A2-pyrazoline-5-carboxylic acids in CH3CN-Et4NBF4 proceeded with decarboxylation to the aromatized pyrazoles in high yield.414 Similarly, electrochemical oxidation of N-acetyl-2,3-substituted A4-pyrroline-2-carboxylic acids in water-tetrahydrofuran (3 1) containing KOH forms the corresponding pyrroles (80-98%).415... 

Anodic substitution in the aromatic nucleus, the side chains of aromatic compounds, and the ally lie position of double bonds. 

Several industrial processes make use of anodic substitution in the side chain of alkyl aromatic compounds. Thus, aromatic aldehydes or aldehyde dimethyl acetals are generated at graphite electrodes in methanol. A typical example is the formation of 4-methoxybenzaldehyde (anisaldehyde) dimethyl acetal starting from 4-methoxytoluene [4] ... 

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