ZINC-PROMOTED REACTIONS .3. THE REDUCTION OF ALPHA,BETA-UNSATURATED KETONES IN ACETIC-ACID

The zinc-promoted reduction of 1,3-diphenylpropenone, 1, 4-phenyl-3-buten-2-one, 2, and 3-penten-2-one, 3, have been reinvestigated in AcOH. The reduction of 1 in neat AcOH gave 1,3-diphenylpropene, 4, 1,3-diphenylpropan-1-ol, 5, 1,3-diphenylpropane, 6, and 1,3,4,6-tetraphenylhexane-1,6-dione, 12, in comparable yields, accounting for about 90% of the product. In the presence of co-reagents like LiCl, HCl, and CF3CO2H, the predominant products was 4, and a more scattered product distribution was observed. In addition to compounds 4-6, and 12, 1-acetoxy-1,2-diphenylcyclopropane, 1,2-diphenylcyclopropene, 1,2,4,5-tetraphenylbenzene, and two dimeric polyenes were formed in minor amounts. Instead, the reduction of 2 gave ketonic compounds only, 4-phenylbutan-2-one, being the predominant product in all the reactions performed in anhydrous AcOH. Under the classical Clemmensen procedure, i.e. in 7 M HCl, the reductions of 1 and 2 were diverted towards the formation of dimeric ketones. The reduction of 3 resulted exclusively in the saturation of the double bond. The mechanisms possibly involved and, in particular, the chemoselective reduction of the carbon-carbon double bond, are discussed.