Benzophenone Wolff-Kishner reduction

A mixture of benzophenone (1.84 g, 10 mmol) and 80% hydrazine hydrate (1 g, 20 mmol) in toluene (15 mL) was taken in an Erlenmeyer flask and placed in a commercial microwave oven operating at 2450 MHz frequency. After irradiation of the mixture for 20 min, (monitored by TLC) it was cooled to room temperature, extracted with chloroform and dried over anhydrous Na2S04. Removal of solvent gave the benzophenone hydrazone in 95% yield. For the Wolff-Kishner reductions, a mixture of hydrazone 3a (5 mmol) and KOH (2 g) were taken in an Erlenmeyer flask and placed in a microwave oven. Irradiation for 30 min and usual workup gave the corresponding diphenylmethane in 95% yield. 

Alternatively, benzene could have been subjected to Friedel-Crafts acylation with benzoyl chloride to give benzophenone. Clemmensen or Wolff-Kishner reduction of benzophenone would then furnish diphenylmethane. 

Wolff-Kishner reduction converts benzophenone to diphenylmethane. 

Benzophenone can be reduced to diphenyhnethane by the Wolff-Kishner reduction, for example. 

The reactivity of anions in dimethyl sulfoxide greatly exceeds that of hydroxylic solvents of comparable dielectric constant. The use of dimethyl sulfoxide as solvent allows both the Wolff-Kishner reduction and the Cope elimination (s. Synth. Meth. 10, 671 12, 910) to be run at room temp., the latter in dry dimethyl sulfoxide and tetrahydrofuran.— E Benzophenone hydrazone added at 25° to a soln. of sublimed K-ferf-butoxide in dimethyl sulfoxide diphenylmethane. Y 90%. F. e. s. D. J. Cram, M. R. V. Sahyun, and G. R. Knox, Am. Soc. 54, 1734 (1962). 

Hydrazone synthesis from a mixture of benzophenone and hydrazine hydrate in toluene was also observed (Gadhwal et al., 1990). 95% Yield of the hydrazone was obtained within 25-30 min. These hydrazones were later reduced to corresponding hydrocarbons by Wolff-Kishner reduction. 

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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