Conjugated cyclohexadienes

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. 

Reduction of benzenoid hydrocarbons with solvated electrons generated by the solution of an alkali metal in liquid ammonia, the Birch reaction [34], involves homogeneous electron addition to the lowest unoccupied 7t-molecular orbital. Protonation of the radical-anion leads to a radical intermediate, which accepts a further electron. Protonation of the delocalised carbanion then occurs at the point of highest charge density and a non-conjugated cyclohexadiene 6 is formed by reduction of the benzene ring. An alcohol is usually added to the reaction mixture and acts as a proton source. The non-conjugated cyclohexadiene is stable in the presence of... 

Reduction of substituted benzenes with sodium (or lithium) in liquid ammonia in the presence of a proton source (such as methanol, ethanol, etc.) leads to a substituted, non-conjugated cyclohexadiene as a result of 1,4-addition of hydrogen (the Birch reduction).16 With benzene the product is cyclohexa-1,4-diene as a result of the following mechanistic pathway. 

Reactions of 99 with 1-substituted dienes gave a mixture of two different cyclohexadienes 103 and 104, resulting from the non-regioselective sulfinyl elimination from the unstable adducts 102. The optical purities of the conjugated cyclohexadienes 103 (ee >96%) are very high regardless of the catalyst used,... 

Aromatization." Nonconjugated cyclohexadienes such as y-terpinene (1) can be aromatized in excellent yield at room temperature by exposure to KMn04 in benzene in the presence of an equimolar amount of dicyclohexyl-18-crown-6. Conjugated cyclohexadienes are recovered unchanged after simitar treatment. 

Similar decomposition patterns can be proposed for secondary radicals derived from 2 in the initiating hydrogen abstraction Step 1. Thermal aromatization of 6-ring cyclic dienes containing one exocyclic and one endocyclic double bonds is a facile process at 550°-600°C. The reaction involves fast double bond isomerization to a conjugated cyclohexadiene, followed by dehydrogenation (30, 31,32). 

The synthesis of dihydrocostunolide (11) involves a light-induced rearrangement of the conjugated cyclohexadiene (9) to its isomer, the conjugated triene (10). ... 

Diene)tricarbonyliron complexes have found use as synthons for the preparation of functionalized dienes. Substituted 4-vinylcyclohexene derivatives are isomerized by pen-tacarbonyliron into a mixture of conjugated cyclohexadiene tricarbonyl iron complexes . When the 4-vinyl cyclohexene 90 was refluxed with 1.2 equivalents of fclCOjs in di-n-butyl ether, a 3 1 mixture of cyclohexadiene isomers 91 and 92 was acquired in 75% overall yield (equation 48). 

However, a slightly different mechanism was postulated by Kovacic et al. [3,15-16] for the polymerization of benzene by catalyst-oxidant systems. According to Kovacic et al, benzene can be oxidized to a radical-cation by a variety of acid catalysts with or without oxidants, but will react on the substrate so that polymer chain growth occurs according to a stair-step mechanism, in which the radical-cation is delocalized over the whole chain (11) before forming a new chain of non-conjugated cyclohexadiene moieties (12) which after oxidation yields the PPP (Scheme 6.4)... 

The proposed synthetic route to TM 2.8 starts with anisole, which on methy-lation and Birch reduction affords a derivative of non-conjugated cyclohexadiene TM 2.8g. The importance and mechanism of this reaction are discussed in Sect.5.4. Chemoselective hydrogenation of one C=C bond is possible since the double bond in enol ether C=C-OMe is less reactive. TM 2.8f affords ester-aldehyde TM 2. 8e on ozonolysis (Scheme 2.20). 

The cyclopentadienylcobalt complexes CpCo(L)n are also good catalysts for co-cyclotrimerization of diynes with alkenes [79a]. Recently, it has been shown that the similar catalytic activity is exhibited by the ruthenium complex 106, which efficiently catalyzed cycloaddition of diynes with cyclic alkenes to the conjugated cyclohexadienes 174 (Scheme 77) [112]. 

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