Cyclohexadiene thermodynamic acidity

Conjugated or skipped dienes and trienes undergo ionization with alkali amides in liquid ammonia. In the cases of allylbenzene, heptatriene, indene, and cyclopentadiene, the equilibrium RH + MNH2 RM +NH3 is probably completely on the right side (with M = Na or K), but the thermodynamic acidities of cyclohexadiene and pentadiene are probably very close to that of ammonia, so that metallation with alkali amides (even potassium amide) may be incomplete. 

Cyclohexadienol was prepared by Rickborn in 1970 from reaction of the epoxide of 1,4-cyclohexadiene with methyl lithium.100 A hydrate of naphthalene, 1-hydroxy-1,2-dihydro-naphthalene was prepared by Bamberger in 1895 by allylic bromination of O-acylated tetralol (1-hydroxy-l,2,3,4-tetrahydronaphthalene) followed by reaction with base.101 Hydrates of naphthalene and other polycylic aromatics are also available from oxidative fermentation of dihydroaromatic molecules, which occurs particularly efficiently with a mutant strain (UV4) of Pseudomonas putida.102,103 The hydrates are alcohols and they undergo acid-catalyzed dehydration to form the aromatic molecule by the same mechanism as other alcohols, except that the thermodynamic driving force provided by the aromatic product makes deprotonation of the carbocation (arenonium ion) a fast reaction, so that in contrast to simple alcohols, formation of the carbocation is rate-determining (Scheme 6).104,105... 

The function of the alcohol in the metal -NH3 reduction is to provide a proton source that is more acidic than ammonia to ensure efficient quenching of the radical anion and pentadienyl anion species. Furthermore, the presence of alcohol represses the formation of the amide ion NH2 , which is more basic than RO M and is capable of isomerizing the 1,4-cyclohexadiene product to the thermodynamically more stable conjugated 1,3-cyclohexadiene. 

The diene ligands transform to the cisoid form because of the thermodynamic stability of the complex. Because the bond distances of Cj—and C2—Cy in the butadiene ligand are 1.45 and 1.46 A, respectively, the double bond character between C2—C3 and the bond order alternation is still insignificant [43]. This may be due to the strong K-acidity of three carbonyl ligands which reduces n-back donation from iron to the butadiene ligand. (Cyclohexadiene)tricarbonyliron complexes are also noteworthy. Fe(l,3-cyclo-hexadiene)(CO)3 was prepared by the reaction of 1,3-cyclohexadiene with Fe(CO)5 [44] (eq (13)) and more stable Fe(l,4-cyclohexadiene)(CO)3 was also prepared [45]. 

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