Para-dienone

When = H, the dienone (2), usually known as the orf/zo-dienone, rapidly tautomerizes to the 2-(prop-2-enyl)-phenol (3). This overall process is known as the orf/zo-Claisen rearrangement. If H, the dienone (2) undergoes a Cope rearrangement to give a 4-(prop-2-enyl)-cyclohexa-2,5-dienone (4), usually termed the para-dienone. The para-dienone (4) rapidly tautomerizes to the para-substituted phenol (5), and this overall transformation is termed the para-Claisen rearrangement (Scheme 3.22). 

Experiments with 14C-labelled substrates also demonstrated conclusively the intramolecularity of the rearrangement. The generally accepted scheme involves the formation of the dienone LXXXV which can lose a hydrogen atom if R = H, to form the ortho product or if R H further rearrangement to LXXXVI occurs with subsequent formation of the para product, viz. 

Early kinetic work127 showed that the formation of both ortho and para products was a first-order process and that the rates of reaction were insensitive to added acid or base and to change of solvent. The activation parameters were of the same order of magnitude for both reactions and the suggestion was made that both had a similar rate-determining step. Schmid et a/.128 showed that the formation of a dienone intermediate in the para rearrangement was also reversible since the radioactivity from allyl 2,6-dimethyl-4-allyl-y-14C phenyl ether LXXXVII became uniformly distributed in the y carbon atoms of the O- and C-allyl groups... 

The dienone, which is prepared essentially as described by Benedikt7 and Calo,8 monobrominates a wide range of primary, secondary, and tertiary aromatic amines almost exclusively in the para-position. The procedure described is of general synthetic utility for the preparation of para-brominated aromatic and heteroaromatic amines in high yields and frequently in a high state of purity. The submitters have used this technique to para-brominate many compounds in quantities... 

The presence of two substituents at C-4 also strongly influences the regios-electivity as shown in the cycloaddition of dienone 13 with isoprene (2) (Equation 3.1). In violation of the para-rule for Diels-Alder reaction, only metfl-adduct was obtained [19,20]. 

Methylanisole. The competition between ortho and ipso attack [analogous to that depicted in (83)] applies to the simultaneous nitration and demethyla-tion of 4-methylanisole. The identification of 4-nitro-4-methylcyclohexa-2,5-dienone as the metastable intermediate in charge-transfer nitration (Kim et al., 1993) is particularly diagnostic of the ipso adduct (84) that is also apparent in the electrophilic nitration of 4-methylanisole (Sankararaman and Kochi, 1991). The common bifurcation of nitration pathways resulting from para (ortho) and ipso attack on the various aromatic donors, as noted above, indicates that the activation step leading to the Wheland intermediate and... 

Cyclohexadienones 61 and 64 are readily available from monoprotected hydro-quinones or para-substituted phenols, respectively. Conjugate additions to these symmetrical dienones result in desymmetrization of the prochiral dienone moieties, providing access to multifunctional chiral synthons in two steps from the aromatic precursors (Scheme 7.17) [72]. 

Accordingly, carbon-carbon bonds involving positions ortho or para to the original phenols, or ether linkages may be formed. The reactive dienone systems formed as intermediates may, in some cases, be attacked by other nucleophilic groupings (see Section 10.10), extending the range of structures ultimately derived from this basic reaction sequence. 

Coupling ortho to the phenol group in the tetrahydroisoquinoline and para to the phenol in the benzyl substituent then yields the dienone salutaridine, found as a minor alkaloid constituent in the opium poppy. Only the original benzyl aromatic ring can be restored to aromaticity, since the tetrahydroisoquinoline fragment becomes coupled para to the phenol function, a position that is already substituted. 

In the next step (R)-reticuline is first transformed to the dienone salutaridine by regioselective para-ortho oxidative coupling, catalyzed by a stereo- and regioselective... 

Electrochemical oxidation of alkyl aryl ethers results in oxidative dealkylation and coupling of the intermediate radicals. Electro-oxidation of alkyl (4-fluorophenyl) ethers in the presence of a hydrogen fluoride double salt leads to 4,4-difluorocyclohexa-2,5-dienone in 50% yield (Table 10).182 In the electrochemical oxidation of methyl tetrafluorophenyl ethers with a hydrogen atom at the para position, coupled products 6 arc obtained.183 If the para position in the substrate is occupied by a fluorine substituent, then no reaction occurs. 

Androsta-l,4,6-triene-3,17-dione 246, when treated with ordinary acid catalysts such as acetic anhydride-para-toluenesulfonic acid, gives the meta phenolic product 247 (Scheme 5.88). However, under HF SbF5 catalysis at 50°C the phenolic product 248 is obtained in 75% yield855 (Scheme 5.88). The mechanism of the above discussed phenol-dienone and dienone-phenol rearrangement has been investigated in detail.856 857... 

In their early studies, Pinhey et al. showed that, when the reaction of a phenol with an aryllead triacetate is performed in the presence of pyridine, only the C-arylated dienones 35 and 36 are formed in a nearly quantitative yield (90% ortho para ratio of 4 1) (Equation (38)). Under their classical conditions (phenol organolead triacetate base in a ratio 1 1 3), the reaction of methylated phenols only proceeded in high yield when both ortho-positions are substituted. There is a preference for attack ipso to a methoxyl group compared to a methyl group.45 453... 

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