Benzaldehyde Wolff-Kishner reduction

The carbonyl reactivity of pyrrole-, furan-, thiophene- and selenophene-2- and -3-carbaldehydes is very similar to that of benzaldehyde. A quantitative study of the reaction of Af-methylpyrrole-2-carbaldehyde, furan-2-carbaldehyde and thiophene-2-carbaldehyde with hydroxide ions showed that the difference in reactivity between furan- and thiophene-2-carbaldehydes was small but that both of these aldehydes were considerably more reactive to hydroxide addition at the carbonyl carbon than A-methylpyrrole-2-carbaldehyde (76JOC1952). Pyrrole-2-aldehydes fail to undergo Cannizzaro and benzoin reactions, which is attributed to mesomerism involving the ring nitrogen (see 366). They yield 2-hydroxymethylpyrroles (by NaBH4 reduction) and 2-methylpyrroles (Wolff-Kishner reduction). The IR spectrum of the hydrochloride of 2-formylpyrrole indicates that protonation occurs mainly at the carbonyl oxygen atom and only to a limited extent at C-5. 

Huang-Minton Wolff-Kishner reduction (1,1197). Reduction of 3,4,5-trimethoxy-benzaldehyde ... 

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%). 

---