Alvin Ronlán

Anodic oxidation of phenolic compounds. Part III. Anodic hydroxylation of phenols. A simple general synthesis of 4-alkyl-4-hydroxycyclo-hexa-2,5-dienones from 4-alkylphenols

The oxidation of simple, monohydric phenols at a lead dioxide anode in aqueous sulphuric acid has been studied. The effects of current density, electrolysis time, pH, concentration of phenol, and method of anode preparation on conversion and product distribution have been investigated, and optimal conditions for anodic hydroxylation of simple phenols have been deduced. In all cases studied the hydroxy-group entered the 4-position: thus 4-substituted phenols gave 4-substituted 4-hydroxycyclohexa-2,5-dienones, and phenols without substituents at C-4 gave p-benzoquinones. The former reaction provides a simple and efficient synthesis of these cyclohexa-2,5-dienone derivatives. A mechanism involving hydrolysis of an anodically generated phenoxonium ion is suggested. Evidence is presented which indicates that the phenoxonium ion is formed by ‘chemical’ oxidation with anodically generated lead dioxide. The lead dioxide anode is superior to carbon, nickel, and platinum anodes for hydroxylation.

Anodic oxidation of phenolic compounds. Part II. Products and mechanism of the anodic oxidation of hindered phenols

2,6-Di-t-butyl-4-alkylphenols undergo anodic hydroxylation giving high yields of the corresponding 4-hydroxy-cyclohexa-2,5-dienones providing that the medium is buffered to avoid acid-catalysed dealkylation and quinone formation. The use of nucleophiles other than water affords a general synthesis of 4-substituted cyclohexa-2,5-dienones.

Anodic oxidation of phenolic compounds. Part 5. Anodic methoxylation of phenols. A simple synthesis of quinones, quinone acetals, and 4-methyl-α-methoxycyclohexa-2,5-dienones

The products and yields obtained on anodic oxidation in methanol of a series of phenols (1a–h) have been investigated as a function of anode material, anode potential, methanol concentration, supporting electrolyte, temperature, and substituents. ortho- and para-Methoxylated and dimeric products were observed. However, experimental conditions were found which, in favourable cases, allow selective formation of para-methoxylated products (isolated as a para-dienone or para-benzoquinone), ortho-methoxylated products (isolated as an ortho-dienone or the Diels–Alder adduct of this dienone), or dimeric products. The methoxylated products are formed via nucleophilic attack of methanol on an anodically generated phenoxylium ion; the dimeric products are probably formed by dimerisation of anodically generated phenoxyl radicals.

Anodic functionalisation in synthesis. Part 1. Methoxylation of methyl-substituted benzene and anisole derivatives, and the synthesis of aromatic aldehydes by anodic oxidation

The anodic oxidation in methanol of a series of alkylanisoles and hydroquinone ethers, and of p-xylene has been studied. It was established that the initial step in the product formation always involves direct anodic oxidation of the aromatic compound to the corresponding cation radical. If the electrolysis is conducted under conditions where formation of methoxyl radicals also takes place (platinum anode, supporting electrolyte NaOMe) nuclear methoxylation dominates. Substitution in the nucleus of the aromatic compound presumably occurs either by nucleophilic attack of a methoxide ion on the anodically generated cation radical or by a coupling reaction between the latter and an anodically generated methoxyl radical. If the electrolysis is conducted under conditions where formation of methoxyl radicals does not take place (carbon or platinum anode with supporting electrolyte LiBF4) side-chain oxidation of p-alkylanisoles and p-xylene, with the formation of benzyl ethers, benzaldehyde dimethyl acetals, or ortho-esters of benzoic acids becomes important. This side-chain oxidation occurs via nucleophilic attack of methanol on a benzyl cation formed by deprotonation–anodic oxidation of the initially formed cation radical. Phenol ether cation radicals without p-alkyl substituents under these conditions either dimerise or undergo nuclear methoxylation via nucleophilic attack of methanol on the cation radical.

Coupling of phenols via an anodically generated phenoxonium ion

Anodic oxidation of 2,6-di-t-butyl-p-cresol (I) in the presence of phenol, anisole, or 2,6-di-t-butylphenol gives the unsymmetrically coupled cyclohexa-2,5-di-enones (VIII), (IX), and (X)via a mechanism involving the phenoxonium ion (III).

Electrochemistry of biphenylenes. Observation of biphenylene cation radicals and dications

The biphenylene ring system imparts greatly enhanced stability to cation radicals and dications as compared to other simple ring systems such as the biphenyl system.

Anodic aromatisation of a dihydrophenanthrene. A novel dehydrogenation

Anodic oxidation of 3,4,3′,4′-tetramethoxybibenzyl or 2,3,6,7-tetramethoxy-9,10-dihydrophenanthrene in acetonitrile results in dehydrogenation with the formation of the corresponding phenanthrene.