Cyclohexanone Wolff-Kishner reduction

Methyl- or 2-ethyl-benzo[Z> ]thiophenes are conveniently prepared by treatment of 2-benzo[6]thienyllithium with the appropriate alkyl sulfate <70AHC(11)177). Clemmensen or Wolff-Kishner reductions of the 2-acylbenzo[Z>]thiophenes are useful, but since acylation produces a mixture of the 2- and 3-acyl isomers (Section 3.14.2.4), these must be separated. Cyclization of phenyl phenacyl sulfide with hydrofluoric acid leads exclusively to 2-phenyl-benzo[6]thiophene, and 3-phenylbenzo[6]thiophene can be rearranged to the 2-isomer in hydrofluoric acid (Section 3.15.2.3.2). Aromatization of 2-cycIohexenylbenzo[6]thiophene, obtained by condensation of the 2-lithio reagent with cyclohexanone, gives 2-phenyl-benzo[6]thiophene, and the reaction is adaptable to the 2-(l-naphthyl) derivative also. 

Propose a mechanism for both parts of the Wolff-Kishner reduction of cyclohexanone the formation of the hydrazone, then the base-catalyzed reduction with evolution of nitrogen gas. 

Cyclohexene is converted to cyclohexanol upon treatment with aqueous acid (acid-catalyzed hydration). Cyclohexanol is oxidized to cyclohexanone upon treatment with a strong oxidizing agent. Upon treatment with hydrazine in acid-catalyzed conditions, cyclohexanone is converted into the corresponding hydiazone. A WolfF-Kishner reduction then gives cyclohexane. 

Modified Wolff-Kishner methods work well on ordinary cyclobutanones. Generally, the hydrazones are made at fairly mild reaction temperatures, base is added and the cyclobutane is distilled directly from the hot reaction mixture.237,272 279-284 The hydrazone can also be isolated prior to the reduction.285 The relatively facile reduction of cyclobutanones which occurs before that of larger ring ketones can be utilized in the selective reduction of cyclobutanones in the presence of, for example, a cyclohexanone, i.e. pentaspiro[3.1.3.3.3.3]heneicosane-5,11,19-trione was reduced in a stepwise manner to pentaspiro[3.1.3.3.3.3]heneicosane-5-one.237... 

As mentioned previously, DMSO as the reaction medium provides significant enhancement of Wolff-Kishner reaction rates and this allows the use of much lower temperatures to effect reductions. In 1962 Cram et al. introduced the use of r-butoxide in dry DMSO for the successful reduction of preformed hydrazones at room temperature. Using this process, benzophenone hydrazone (15) afforded an 88% yield of diphenylmethane (16), along with 11% of benzophenone azine (17) as side product (equation 5). However, maximum success requires very slow addition (i.e. over 8 h) of the hydrazone to the reaction solution, otherwise yields of reduced products are decreased and azine formation augmented. Thus, addition of (15) over 0.5 h in the above reaction lowered the yield of (16) to 72%, while the yield of (17) was increased to 22%. - Other successful reductions reported - include hydrazones of benzaldehyde (67%), camphor (64%) and cyclohexanone (80%). 

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