Question

10.74 The following compounds exhibit tautomerism, with a particu- ly high enol concentration. Compound 1 exhibits an enol concen- tration for (CH3)2CHCHO. Compound 2 exhibits an enol concentration of 95% (Acc. Chem. Res. 1988, 21, 442-449): of 9.1%, as compared with the enol concentration of 0.014% 2 (a) Draw the enol of compound 1 and offer an explanation for the high enol concentration, relative to (CH3)2CHCHO b) Draw the enol of compound 2, and explain why the enol is favored over the aldehyde in this case.

Answer 1

There is no extra stabilization for this enol form Enol content is more than the isobutyraldehyde because it is stabilized by the extended conjugation. There are steric hindernce in the keto-form because of the proximity of the ortho methyl and C = 0. The rotation of the C = 0 is restricted. But in enol form C-OH is fixed and there is no rotation possible. The two phenyl ring can be orthogonal to each other so that ortho interaction is minimum.