Rearrangements dienone

Finally, it should be noted that there are various reactions which can be called phe-nol-dienone rearrangements. They proceed upon halogenation, nitration and alkylation of phenols as well as in the course of radical reactions of phenols186. 

The driving force in the overall reaction (the dienone-phenol rearrangement) is of course creation of an aromatic system.136 It may be noted that 50 and 51 are arenium ions (p. 502), the same as those generated by attack of an electrophile on a phenol.137 Sometimes, in the reaction of a phenol with an electrophile, a kind of reverse rearrangement (called the phenol-dienone rearrangement) takes place, though without an actual migration.I3B An example is... 

As just described, Zimmerman has reported one instance of a dienone rearrangement which definitely does not fit Chapman s general picture. Schuster has provided two reports410,411 of cross-conjugated cyclohexadienones which eliminate radical species. With the trichloro-methyl-substituted ketone 34, both cleavage to the cresol and rearrangement to lumiproduct are quenched by dienes.411 Stern-Volmer quenching plots indicate that the rate at which the excited triplet reacts exceeds 10° sec"1 for both reactions.412... 

Three matters remain to be considered prior to proceeding to further photochemistry. These deal with the questions whether the excited state undergoing transformation is actually the n-ir singlet or triplet, whether the precise chronology of events is as outlined in the dienone rearrangement mechanisms, and whether this sequence is general. 

Similarly, many A-norsteroids 244 have been subjected to phenol-dienone rearrangement in HF—SbF5 medium854 [Eq. (5.312)]. 1H NMR spectroscopic studies at low temperature confirms the formation of 0-protonated intermediates (Scheme 5.87), which subsequently rearrange to diprotonated precursors of the dienones 245. 

Zimmerman 103) applied the well known rearrangement of 4,4-disubstituted cyclohexa-2,5-dienones (e.g. the 4,4-diphenyl derivative tives the 3,4-disubstituted phenols l)). Using acetic anhydride and a small amount of concentrated sulfuric acid, 97.5 % of 3,4-diphenylphenolacetate is formed, which in turn is hydrolyzed to give 3,4-diphenylphenol. This is one of the easiest synthesis of this phenol. 2-Bromo-4,4-diphenylcyclohexa-2,5-dienone rearranges by a similar route to give a 49 % yield of 2-bromo-3,4-diphenylphenolacetate and 47 %2-bromo-4,5-diphenylphenolacetate U4). A [1.2]-phenyl shift of 4-methoxy-2,3,4,6-tetraphenyl-2,5-dienone in acetic anhydride and zinc chloride gives rise to 4-methoxy-2,3,5,6-tetraphenylacetate (m.p. 284 °C, 85% yield)115 . 

All photoreactions which have been mentioned in the previous sections, are effected by ra — ir excitation. Alternatively, -ir- - v excitation was frequently found more convenient to selectively conduct single reaction steps in the dienone and bicyclohexenone field. Conversion of the initially formed Tr,ir singlet excited state to the lower energy n,ir excited state should be very rapid in general. However, quantitative studies of the product distribution from the bicyclohexenone 33a (Chart 5) showed that the relative amounts of products 36a and 38a, both formed from the same spirocyclic precursor, dienone 34a, varied somewhat when light of wavelengths was employed that corresponded to either absorption band. Apparently, the dienone rearrangement may compete to some extent with this internal conversion. 

A is exothermic. An additional advantage is that one has a wider margin to select spin-state specific photochemical transformations. Representative examples include ds-trans-olefin isomerization (Eq. 49), olefin dimerization (Eq. 50), oxetane formation (Eq. 51), dienone rearrangement (Eq. 52), di-vr-methane rearrangement (Eq. 53), azoalkane denitrogenation (Eq. 54a, b), and photocyclization (Eq. 55). 

Phenol-dienone rearrangement The reverse of the dienone-phenol rearrangement. 

Zincke-Suhl reaction. Phenol-dienone rearrangement of p-cresols by addition of carbon tetrachloride in the presence of aluminum chloride, with formation of 4-methyl-4-trichloromethylcyclo-hexa-2,5-dienone. 

It is generally known that the processes of reversible oxidation of phenols, i.e. the conversions of phenolic systems into quinone structures and vice versa, are of great importance in biochemical reactions. The reaction partners mentioned above can serve as donors and acceptors of electrons and protons, i.e. as antioxidant systems. The conversions of phenols into cyclohexadienones are accompanied by the loss of aromaticity and in essence are not rearrangements, although the term phenol-dienone rearrangement is found in the literature. A review which summarizes in detail the oxidation reactions of phenols under conditions of halogenation, nitration and alkylation as well as radical reactions appeared . The various transformations of phenols upon oxidation with nickel peroxide were also reviewed . Therefore, only recent reports concerning the phenols-to-quinones conversions are described in this section. 

Phenol-azaaromate complexes 377 Phenol-benzonitrile complex 177, 178 Phenol-dienone rearrangements 651-655 Phenol-formaldehyde ratio 1457 Phenol-formaldehyde resins 606, 608, 626-628, 1457 functionahty in 1457-1460 Phenolic acids 965-967... 

Figure 11. Reaction course of the Type A dienone rearrangement. ...

More generally, making use of reactant electronic structures as derived from Scheme 1.1, it proved possible to write detailed mechanisms for a variety of n-ji and other photochemical reactions. However, in the case of the Type A dienone rearrangement, we still need to understand the driving force for (i,(i-bonding of the dienone triplet. 

---