Activation of Electrophilicity of Stable Y?Delocalized Carbamate Cations in Intramolecular Aromatic Substitution Reaction: Evidence for Formation of Diprotonated Carbamates Leading to Generation of Isocyanates

摘要

Although cations with three heteroatoms, such asnmonoprotonated guanidine and urea, are stabilized by Y-shapednconjugation and such Y-conjugated cations are sufficiently basicnto be further protonated (or protosolvated) to dications innstrongly acid media, only O-monoprotonated species have beenndetected in the case of carbamates even in magic acid. We foundnthat the trifluoromethanesulfonic acid-catalyzed cyclization ofnarylethylcarbamates proceeds to afford dihydroisoquinolones innhigh yield. In strong acids, methyl carbamates are fully Omonoprotonated,nand these monocations do not undergo cyclization even under heating. But, as the acidity of the reactionnmedium is further increased, the cyclization reaction of methyl phenethylcarbamates starts to proceed as a first-order reaction,nwith a linear relationship between rate and acidity. The sign and magnitude of the entropy of activation ΔS⧧ were found to bensimilar to those of other AAc1 reactions. These results strongly support the idea that further protonation of the O-protonatedncarbamates is involved in the cyclization, but the concentration of the dications is very low and suggests that the rate-determiningnstep is dissociation of methanol from the diprotonated carbamate to generate protonated isocyanate, which reacts with thenaromatic ring. Therefore, O-protonated carbamates are weak bases in sharp contrast to other Y-shaped monocations.