Functionalized pyrroleьvьЄ enols, 2-(1-hydroxy-2.2-dicyanoethenyl)-1-methylpyrroles, at heating (75–135°C) unexpectedly readily rearranged in high yield into 3-isomers. Evidently the migration of the enol fragment involves a mesomeric zwitterion formed as a result of an intra- and intermolecular autoprotonation of the pyrrole ring by the acidic enol hydroxy group. Under similar conditions no migration of the ethenyl moiety occurred in 2-(1-hydroxy-2-carbamoyl-2-cyanoethenyl)-1-methylpyrroles. The quantum-chemical calculations (MP2/6-311G**) show a clear-cut distinction in the relative stability of 2- and 3-isomers of 1H- and 1-methylhydroxyethenylpyrroles: in the former case the 2-isomer is more stable, whereas in the 1-methyl-substituted compound, the 3-isomer.
在加热(75-135°C)条件下,2-(1-羟基-2.2-二
氰基
乙烯基)-1-甲基
吡咯的官能化
吡咯烯醇意外地容易重新排列成 3-异构体,产量很高。显然,烯醇片段的迁移涉及到
吡咯环上的酸性烯醇羟基在分子内和分子间自质子化形成的中间体齐聚物。在类似条件下,2-(1-羟基-2-
氨基甲酰基-2-
氰基
乙烯基)-1-甲基
吡咯中的
乙烯基没有发生迁移。量子
化学计算(MP2/6-311G**)表明,1H- 和 1-甲基羟基
乙烯基吡咯的 2-异构体和 3-异构体的相对稳定性有明显的区别:在前者中,2-异构体更稳定,而在 1-甲基取代的化合物中,3-异构体更稳定。