Phenols from cyclohexadienones

Palladium-carbon Hydrogenolytic C-cleavage Phenols from cyclohexadienones... 

A variety of oxidation products result from hindered phenols, ranging from cyclohexadienones to quinones as shown in Table 6.9 (refs.109-114). 

The smooth removal of alkyl groups to form phenols from substituted cyclo-hexadienones can be achieved by heating with alkalies Alkyl groups other than CHj are split off by acids from alkyl-substituted cyclohexadienones. Examples are cyclohexadienones having the following sp -node C(C2Hj)2 C(BrX2HBrj C(Br)CH(OR)R - >. 

Photoreactions of 2,5-cyclohexadienones Phenols from 4-bromo-2,5-cyclohexadienones... 

Polymerization Mechanism. The mechanism that accounts for the experimental observations of oxidative coupling of 2,6-disubstituted phenols involves an initial formation of aryloxy radicals from oxidation of the phenol with the oxidized form of the copper—amine complex or other catalytic agent. The aryloxy radicals couple to form cyclohexadienones, which undergo enolization and redistribution steps (32). The initial steps of the polymerization scheme for 2,6-dimethylphenol are as in equation 6. 

ZINCKE - SUHL Cyclohexadienone synthesis Synthesis of cyclohexadienones from phenols by Fnedel Crafts alkylafion... 

Lead tetrafluoride, generated in situ from lead dioxide and hydrogen fluonde, can replace benzylic hydrogen by fluonne [3] Under sirmlar conditions phenol is simultaneously oxidized to 4,4-difluoro-2,5-cyclohexadienone [4 (equations 3 and 4)... 

Structural analogues of the /]4-vinylketene E were isolated by Wulff, Rudler and Moser [15]. The enaminoketene complex 11 was obtained from an intramolecular reaction of the chromium pentacarbonyl carbene complex 10. The silyl vinylketene 13 was isolated from the reaction of the methoxy(phenyl)-carbene chromium complex 1 and a silyl-substituted phenylacetylene 12, and -in contrast to alkene carbene complex 7 - gave the benzannulation product 14 after heating to 165 °C in acetonitrile (Scheme 6). The last step of the benzannulation reaction is the tautomerisation of the /]4-cyclohexadienone F to afford the phenol product G. The existence of such an intermediate and its capacity to undergo a subsequent step was validated by Wulff, who synthesised an... 

The last step of the reaction is the keto-enol tautomerization from T 4-cyclohexadienone intermediates (15) to aromatic products (16). Such a step is accompanied with a considerable gain in energy about 80 kJ mol 1 for vinylcarbenes [29], (where a phenol system is formed by the tautomerization step), and about 175 kJ mol 1 for phenylcarbenes [25] (where a naphtol system is produced). The energy barrier for such step should be lower than 40 kJ mol 1 according to previous calculations on similar systems [42],... 

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]. 

As can be seen, the pathway from the cyclohexadienone to the ortho-rearranged phenol does not imply a jump over an energy barrier (34 kcal/mol), but a tunneling from the vibrational energy levels located at E(vo) = 0 kcal/mol and E(vi) = 3.9 kcal/mol (4.4 kcal/mol for the deuterated compound). Analogous results have been reported for the photo-Fries rearrangement of 2,4-dimethoxy-6-( ara-tolyloxy)-v-triazine (28) to give 2,4-dimethoxy-6-(2-hydroxy-5-meth-ylphenyl)-v-triazine (29) (Scheme 11) [43]. 

Substrates with doubly bonded nitrogen-atom functionalities, e.g. the C=N-R (imino, oxime) group, are usually cleaved by dioxirane to give the corresponding carbonyl product" . A particular case represents the DMD oxidation of the nitronate ions, generated from nitroalkanes" or nitroarenes. For example, the nitronate anion 16 (equation 13) affords initially the cyclohexadienone on oxidation with DMD, which subsequently tau-tomerizes to the phenol as the final product. An exception is the DMD oxidation of an... 

Another synthesis of a bridged hydrocarbon takes advantage of high electron release from the para-position of phenolate anions, which may be used to transform the phenol moiety into a substituted cross-conjugated cyclohexadienone system (S. Masamune, 1961, 1964). 

Since OH is strongly electrophilic, the OH group directs OH into its ortho- and para-positions [reactions (51)—(53)]. One of the ortho-positions is occupied by the somewhat bulky carboxylate group which renders reaction (51) less likely than reaction (52). An addition to the meta-position can be largely neglected. Upon oxidation of the OH-adduct radicals, cyclohexadienones are formed [reactions (54)-(56)] which either decarboxylate [reaction (57)] or rearrange into the corresponding phenols [reactions (58) and (59) e.g., Bausch et al. 1976]. Product yields from hydroxybenzoic acids are compiled in Table 3.5 from salicylic acid in Table 3.6. 

Scheme 10. Cyclohexadienone formation from PIDA-mediated oxidative coupling of phenolic substrates.

Cyclohexadienones and quinone methides from phenolic antioxidants and UV absorbers... 

Regioselective reaction of phenols. Reaction of phenols with chloroform in aqueous alkaline solution catalyzed by P-cyclodextrin results in virtually complete attack at the para-position by dichlorocarbene to give, after hydrolysis, 4-hydroxybenzaldehydes. If the para-position is substituted, 4-(dichloromethyl)-2,5-cyclohexadienones are obtained as the major product. The selectivity results from formation of a ternary complex from P-cyclodextrin, chloroform, and the phenol. ... 

Singular examples to form aromatic ethers are a base-catalyzed, multistep, one-pot reaction of aryl methyl ketones with the appropriate fluorinated arylidenemalonitriles, the mercury acetate assisted synthesis of pentahalophenylvinyl ethers from vinyl acetate and the corresponding phenol, and the radical displacements in aryloxycyclohexadienones (e.g., 27) by halophenols. 2,3-Dichloro-5,6-dicyanohydroquinone (28) and products such as 29 are readily formed when cyclohexadienone 27 is treated with different phenols. 

Complexes (160) (see equation 35) undergo conversion to phenols upon thermolysis. Alternatively, treatment with AICI3 under a CO atmosphere gives an j)" -iron cyclohexadienone complex thermolysis or treatment with CuCl2 affords a phenol as well. Complexes (162), derived from nucleophihc addition to ( ) -pentadienyl)Fe(CO)3+ cations (see equation 37), can be converted to vinyl cyclopropanes by using CAN (equation 41). ... 

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