1.4- Cyclohexadiene ring quinones

Nucleophile addition to a quinone methide is formally a Michael addition reaction.153 However, an important difference between 1,6-addition of nucleophiles to / -quinone methides and conventional Michael addition reactions is the aromatization of the cyclohexadiene ring that accompanies this addition. The effect of aromatic ring formation on the thermodynamic driving force for 1,6-addition of water to p-1 has been evaluated by comparing the thermodynamics... 

A second example from the same group is the synthesis of an elaborate diethynyltriphenylene derivative (Scheme 7 Table 8,entries 12,13) [58].Zn/Pd-promoted homocoupling of a 4-iodo-l,2-dialkoxybenzene furnishes the desired tetraalkoxybiphenyl, an electron-rich aromatic system. Iron trichloride-catalyzed Friedel-Crafts arylation of the biphenyl derivative with dimethoxy-benzene furnishes an unsymmetrical triphenylene derivative. Deprotection, oxidation, and subsequent Diels-Alder reaction with cyclohexadiene is followed by catalytic hydrogenation and reoxidation. TMS-CC-Li attack on the quinone delivers the alkyne modules, treatment with SnCl2 aromatizes the six-mem-bered ring, while KOH in MeOH removes the TMS groups cleanly to give the elaborate monomer. 

The net effect of this cyclohexadiene/phenyl ring insertion at the carbonyl group is to cause an increase in the overall equilibrium constant for the addition of solvent water, from A dd = 2.3 x 103 for hydration of formaldehyde154 to A dd = 4.0 x 107 for hydration of the p-quinone methide l,3 so that Kj = AT. dd/Aiadd = 1.7 x 104 for transfer of the elements of water from formaldehyde hydrate to 1 (Scheme 43). We have proposed that the relatively small driving force of 6 kcal/mol for this transfer of water from CH2(OH)2 to 1 represents the balance between larger opposing effects3 ... 

The 18 7r-electron structure has the stability of an aromatic ring according to the Hiickel 4n + 2 rule, while the others may be considered to be active like the semiquinone or quinone. So, when an oxidizing agent can be found to oxidize the aromatic molecule, a structure is formed which may abstract a hydrogen atom, by which it is reduced again. A mixture of nitrobenzene and cyclohexadiene-1,4 suited this purpose, and complexes of phthalocyanine and tetraphenylporphyrin were found to catalyze the oxidative dehydrogenation of cyclohexadiene by nitrobenzene t00> ... 

Except under extreme conditions, oxidation of the benzene ring requires the presence of strongly electron-donating groups such as hydroxyl or amino. These groups are simultaneously oxidized. The best known products of this oxidation process are the quinones benzo-l,4-quinone (p-benzoquinone, cyclohexadiene-l,4-dione, 10) and benzo-l,2-quinone (o-benzoquinone, cyclohexadiene-l,2-dione, 11). 

Quinoline Two condensed aromatic rings in which one CH unit is replaced by a nitrogen atom. Quinone The ortho- orpara-d carbonyl derivative of cyclohexadiene. Also known as benzoquinone. 

A double diene synthesis occurs with benzoquinone. With 2 molecules of cyclohexadiene, for instance, this affords the 1,4,5,8-diethanoanthracene ring system the resulting l,4,4a,5,8,8a,9a,10a-octahydro-l,4,5,8-diethanoanthra-quinone readily loses four hydrogen atoms, to give the 1,4,5,8-tetrahydro compound and, when that is heated, it spits off ethylene to afford anthraquinone ... 

Theoretical studies have been carried out on the conrotatory and disrotatory reaction pathways of hexa-l,3,5-triene to cyclohexadiene, and the effect of solvent and salt effects on the rates of the electrocyclic ring closure of (IZ, 3Z, 5 )-l,2,6-triphenylhexa-l,3,5-triene has been determined. An ab initio study of the electro-cyclization of (Z)-hexa-l,3,5-triene and its hetero-substituted analogues has been undertaken.The involvement of a lone pair on the nitrogen or oxygen atom appears to facilitate the interaction between the terminal atoms that bond to each other to close the ring. It has been reported that the intramoiecular aza-Diels-Alder reaction of an a./S-unsaturated hydrazones to a quinone, as in (197), is followed by an unprecedented rearrangement in which the aminoaryi moiety formally undergoes a 1,2-shift to yield benzo- or pyrido-[fc]acridine-6,11-dienes (198). 

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