Cyclohexadienones aromatization

Reaction with the electrophilic peroxodisulfate occurs preferentially at the para position, leading to formation of a cyclohexadienone derivative 5, which loses a proton to give the aromatic compound 6. Subsequent hydrolysis of the sulfate 6 yields 1,4-dihydroxybenzene 3 ... 

The basic principle of all diazotizations of aromatic amines with a hydroxy- or a sulfonamido group in the 4-position relative to the amino group involves a deprotonation of the OH or NH group, respectively, after diazotization of the amino group. There is also a case of a deprotonation of a CH group in the 4-position of an aniline derivative, namely in the diazotization of 4-aminophenylmalononitrile (2.41) which, by the sequence of steps shown in Scheme 2-23, yields 3-diazo-6-dicyanomethylene-1,4-cyclohexadienone (2.42), as found by Hartzler (1964). This product can also be represented by a zwitterionic carbanion-diazonium mesomeric structure. 

For all QM complexes, the ring signals in both and 13C JH NMR spectra indicate the lack of aromaticity. The carbonyl group appears in the 13C JH NMR spectrum in the range of 18418-20116ppm, which is the region observed for 2,5-cyclohexadienones and quinones.38... 

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],... 

It is seen from Table 7 that all these carbenes in solution react nonstereospeci-fically, indicating triplet character. For some carbenes such as CF3CH, PhCH, (CH3)sSiC(C02C2H5), 2,6-di-t-butyl-carbena-cyclohexadienone and 4,4-dimethyl-carbena-cyclohexadienone, the degree of non-stereospecifity is quite small. Other carbenes usually contain 7t-bonds, aromatic rings or heavy atoms as functional groups. 

If the Ddtz benzannulation reaction is conducted with ori/zo-disubstituted aryl-carbene complexes, the final aromatization step is blocked and cyclohexadienones can be isolated (Figure 2.34) [356,378,379]. 

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]. 

Cyclohexadienone (keto form not aromatic) Phenol (enol form aromatic) ... 

The photochemical rearrangements of 2,4-cyclohexadienones are of three types (1) ring fission to a cis-diene-ketene ( ) expulsion of a heteroatom from the 6-position with concomitant aromatization of the... 

Aromatic enols, that is, phenols, are generally more stable than their ketone tautomers. The pH-rate profile for the enolization reaction of 2,4-cyclohexadienone to parent phenol is shown in Fig. V.44 The rate constant kK of the reverse reaction was determined at pH = 1 by measuring the rate of isotopic exchange and correcting for isotope effects to determine the enolization constant Kb = = 5.4 x 1012, pA E = -12.73. The dashed line in... 

This cation cannot yield the product of electrophilic aromatic substitution by loss of a proton from the ring but can lose a proton from oxygen to give a cyclohexadienone derivative. 

The classical Claisen rearrangement is the first and slow step of the isomerization of allyl aryl ethers to orlho-a ly lated phenols (Figure 14.46). A cyclohexadienone A is formed in the actual rearrangement step, which is a [3,3]-sigmatropic rearrangement. Three valence electron pairs are shifted simultaneously in this step. Cyclohexadienone A, a nonaromatic compound, cannot be isolated and tautomerizes immediately to the aromatic and consequently more stable phenol B. 

Transient 4-bromo-2,5-cyclohexadienone intermediates are also clearly observed in the aqueous bromination of phenol1045. Addition products are occasionally noticed with less activated aromatic systems. Thus, in the chlorination of para-xylene in acetic acid, dichloroxylenes have been found to form directly via an ipso mechanism1046. 

Some metals show such a pronounced tendency to adopt rj4-butadi-ene coordination that this may override the driving force to aromaticity. Selected complexes of d8 M(CO)3 (M = Fe, Ru) fragments illustrating this point are included in Figure 6.29. Phenol is far more stable than its keto tautomer (cyclohexadienone), which may be stabilized, however, by... 

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