Aromatic rings 1,4-cyclohexadienes

The cyclization of the enediynes 110 in AcOH gives the cyclohexadiene derivative 114. The reaction starts by the insertion of the triple bond into Pd—H to give 111, followed by tandem insertion of the triple bond and two double bonds to yield the triene system 113, which is cyclized to give the cyclohexadiene system 114. Another possibility is the direct formation of 114 from 112 by endo-rype. insertion of an exo-methylene double bond[53]. The appropriately structured triyne 115 undergoes Pd-catalyzed cyclization to form an aromatic ring 116 in boiling MeCN, by repeating the intramolecular insertion three times. In this cyclization too, addition of AcOH (5 mol%) is essential to start the reaction[54]. 

Birch reduction (Section 11.11) Reduction of an aromatic ring to a 1,4-cyclohexadiene on treatment with a group I metal (Li, Na, K) and an alcohol in liquid ammonia. 

Path e. Attack of a nucleophile on 13. In some cases, the products of such an attack (cyclohexadienes) have been isolated (this is 1,4 addition to the aromatic ring), but further reactions are also possible. 

One 7i-bond of an aromatic ring can be converted to a cyclohexadiene 1,2-diol by reaction with enzymes associated with P. putida A variety of substituted aromatic compounds can be oxidized, including bromobenzene, chlorobenzene, " and toluene. In these latter cases, introduction of the hydroxyl groups generates a chiral molecule that can be used as a template for asymmetric syntheses. " ... 

A solution of lithium, sodium, potassium or calcium in liquid ammonia can reduce a wide variety of unsaturated groups. Thus when aromatic rings are reduced by such metals in liquid ammonia, non-conjugated cyclohexadienes are produced. The reaction is called Birch reduction. 

As already stated above, the aromatic rings are changed to non conjugated cyclohexadienes. 

According to the method of Taber12 compound 7 is first reduced with lithium in a liquid ammonia/THF/EtOH mixture at -78 °C. This leads to 33 with concomitant formation of the alcohol at the six-membered ring and the 1,4-cyclohexadiene from the aromatic ring. 

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... 

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> ... 

Cyclohexane, cyclohexene and cyclohexadiene are used as hydrogen sources in the hydrogenation of alkenes to alkanes, when they are themselves oxidized to benzene. In these reactions, the driving force is the formation of the aromatic ring. 

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. 

The Birch reduction is the organic reduction of aromatic rings by sodium in liquid ammonia invented by Arthur Birch. The reaction product is a 1,4-cyclohexadiene. The metal can also be lithium or potassium and the hydrogen atoms are supplied by an alcohol such as ethanol or tertbutanol. Sodium in liquid ammonia gives an intense blue color. 

The first step of a Birch reduction is a one-electron reduction of the aromatic ring to a radical anion. Sodium is oxidized to the sodium ion Na. This intermediate is able to dimerize to the dianion. In the presence of an alcohol the second intermediate is a free radical which takes up another electron to form the carbanion. This carbanion abstracts another proton from the alcohol to form the cyclohexadiene. 

Annuitization of cyclohexenes and cyclohexadienes. This reaction can be effected in refluxing CC14 by treatment with Se02 (1 eq.) and PPSK (10 cquiv. 10,437 11,427 12,54.3). Note that a cyclohexane fused to an aromatic ring is not affected. 

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