Nucleophilic substitution reactions of N-alkoxyimidoyl fluorides by carbon nucleophiles

Abstract

Nucleophilic substitutions of N-alkoxybenzoimidoyl fluorides [p-ClArC(F)=NOR; R = CH3, i-Pr] by enolate-type ions have been performed to produce compounds that can exist in two tautomeric forms: the imine form{p-ClArC(Y)=NOR [Y = CH(CN)2, CH(CN)(CO2Et), CH(CO2Et)2]}or the enamine form {p-ClArC(NHOR)=C(R1)(R2) [R1, R2 = CN, CO2Et]}. These compounds display varying ratios of imine–enamine tautomerizm in chloroform: the diester compound exists almost solely in the imine form, the dicyano compound exists solely in the enamine form, and the cyano-ester compound exists in both tautomeric forms. Comparison of the imine–enamine equilibrium in chloroform of the cyano-ester compound to an analogue where the p-ClAr group has been replaced by a methyl group reveals that the aromatic ring has a significant effect on the imine–enamine distribution. The former exists in both the imine and enamine form, while the latter exists solely in the enamine form. Complete NMR characterization data for the enamine form of CH3C(NHOCH3)=C(CN)(CO2Et) is also reported. The acidity of p-ClArC(NHOR)=C(CN)2 is demonstrated by a high degree of dissociation of the enamine proton in DMSO. Substitution reactions of p-ClArC(F)=NOi-Pr by Grignard reagents (PhMgBr, CH3MgBr, and PhCCMgBr) have also been performed to produce single geometric isomers of p-ClArC(Ph)=NOi-Pr, p-ClArC(CCPh)=NOi-Pr, and p-ClArC(CH3)=NOi-Pr. A modified synthesis of N-alkoxyimidoyl fluorides, which prevents a competing reaction of the product with the solvent, is reported.Key words: N-alkoxyimidoyl fluoride, enolate ion substitution, Grignard substitution, imine–enamine tautomerizm, carbon acid.