Ring‐chain tautomerism of aldehyde n‐methylated semicarbazones

Ring‐chain tautomerism of thirty‐six aldehyde semicarbazones was studied by nmr spectro‐scopy. The 2,4‐dimethylsemicarbazones of benzaldehyde, p ‐tolualdehyde, p ‐anisaldehyde, and P ‐chlorobenzaldehyde exist as chain forms in DMSO‐d 6 , but as both ring and chain forms in deuteriotriiluoroacetic acid. Unlike these semicarbazones, the aldehyde semicarbazones of N ‐unsubstituted semicarbazide, 2‐methylsemicarbazide, and 4‐methylsemicarbazide did not cyclo‐isomerize in either DMSO‐ d 6 or deuteriotrifluoroacetic acid. These results indicate that protona‐tion of the C=N nitrogen atom and steric hindrance of the 2‐methyl group with an aromatic group at the C=N carbon atom in the chain form cause cycloisomerization to ring isomers.