Para-coupling

The combination of carbon-centered radicals usually involves head-to-head (a,a ) coupling. Exceptions to this general rule occur where the free spin can be delocalized into a n-system. The classic example involves the triphenylmethyl radical (13) which combines to give exclusively the a-para coupling product (26), Scheme I.8).27 This chemistry is also seen in cross reactions of 13 with other tertiary radicals.146... 

The anodic polymerization of aniline can occur by a radical cation coupling mechanism analogous to that shown in Scheme 1, with coupling occurring between the N of one molecule and the para-position of another (Structure 4)21,22 However, a variety of other mechanisms have also been proposed,21 and it is likely that their relative rates depend upon the conditions (solvent, potential, pH, etc.) employed. The links between monomers are therefore not exclusively between the N and para-position (head-to-tail coupling). Head-head (-N=N-) and tail-tail (para-para) coupling occur more often as the pH is increased.71... 

Hz for the ortho, meta and/or para coupling constants of p-nitroanisole, two dinitrotoluenes and 2,4-dinitrochlorobenzene. Both reports indicate that other aromatic compounds investigated showed even less solvent dependence. 

Parallel oxidations either electrochemically [117] or with iron([Il) salts [119] in water have also been made using l-alkyl-7-hydroxy-6-methoxytetrahydroisoquinO line salts as substrates. 4-Methy phenol is oxidised at a carbon anode in alkaline solution to give Pummerer s ketone 25 by the ortho-para coupling of two phenoxy radicals [120],... 

It is actually possible with a closer view to see additional splitting of all of the aromatic proton signals and this is due to long-range coupling between the protons meta to each other, i.e. H-1 and H3 and H2 and H4, which can occur in aromatic systems and can be up to 3 Hz. Para coupling can also occur but it is only ca 1 Hz. 

We normally do not see i, 4- (para) coupling as the J values are very smail for five-bond coupling. 

Only para coupling was observed during the alkaline ferricyanide oxidation of 4-hydroxy-3-(3-hydroxybenzoyl)benzoic acid. 7-Hydroxy-9-oxoxanthene-2-carboxylic acid (515) was obtained in 21% overall yield from methyl 4-hydroxybenzoate (78JCS(P1)876). The synthesis utilizes a photochemical Fries rearrangement of methyl 4-(3-methoxybenzoyloxy)benzoate to prepare the benzophenone (514 Scheme 189). A similar route was used to prepare 2-hydroxy-2 -methoxybenzophenones, which undergo intramolecular cyclization with loss of methanol on treatment with base. 

Aromatic compounds. The magnitude of the coupling constants due to coupling between protons attached to the aromatic ring varies in the order o> mp p para coupling constants are in fact frequently not discernible in the spectrum. 

The ortho coupling of phenoxy radicals is noted in the formation of l,l -bi-2-naphthol from 2-naphthol by oxidation with iron(m) chloride (Expt 6.10). In the case of hindered 2,6-dialkylphenols, a very wide range of oxidants have been found to be variously successful in effecting para coupling further oxidation then occurs to give an extended quinone. 

Narciclasine is therefore biosynthetized from O-methylnorbelladine by para-para coupling, three of the 3H atoms present in the corresponding position in the fed precursor being retained completely at positions 2,1, and 4a of 376. Half of the 3H activity originally present in one of the two positions ortho to the phenolic hydroxy group in the precursor is lost from position 4 of 376. Furthermore, [3H]oxocrinine was specifically incorporated into narciclasine and haemanthamine (102). 

The use of 2 -nitro or 2 -N-acetylamino substituents leads to cleavage at C-l. The trifluoroacetylation of amino group, however, changed the course of the reaction and the products were not morphinandienones but neospirodienones. This change of the reaction course may be initiated by rearrangement of the intermediate morphin-andienone-type cation. The addition of methanol to the reaction system as a nucleophile, however, makes it possible to trap the intermediate cation before its rearrangement. But the para-para coupling pattern is retained 10). 

Asymmetric phenolic oxidative coupling.1 Oxidation of the chiral oxazolidine 1 with VOF3 results in efficient para-para coupling to a spirodienone 2. The trans-... 

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