A NMe in Acylamidines and Imides

If an amide is an amine possessing contiguous carbonyl, and a vinylogous amide interposes a double bond, acylamidines in some sense combine both features. Two types of cyclic acylamidine have been investigated, with somewhat contradictory results. Since no iminol form has ever been detected, no data can provide A(OMe). 

The only imides for which we have useful data are the 3,5-dioxo-l,2,4-triazoles 33-36 [24], whose measured pK values are those of the dominant tautomer throughout so the effect of replacing NMe by NH is unambiguous. Unfortunately, they are probably atypical the low value of A(NMe) 0.1 probably results from repulsion by the flanking carbonyl groups of solvent proton acceptors which would otherwise provide solvation. Open-chain imides typically take up the E,Z-conformation [25] in which solvation of NH will certainly be stronger, but for which A (NMe) is unknown. 

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