THERMAL ISOMERIZATION OF SOME TRISUBSTITUTED PYRROLES

The pyrolysis (at 500-600°) of 1,2,5-trisubstituted pyrroles produces the corresponding 2,3,5-trisubstituted pyrroles as the major products. When the 1 substituent is different from the 2,5 substituents, a mixture of the isomeric 2,3,5-trisubstituted pyrroles is produced. Thus the pyrolysis of 1-pheny 1-2,5-dimethylpyrrole yields 2,5-dimethy 1-3-phenylpyrrole and 3,5-dimethy 1-2-phenylpyrrole in equivalent amounts; 1-methyl-2,5-diphenylpyrrole yields 3-methyl-2,5-diphenylpyrrole and 2-methyl-3,5-diphenylpyrrole in equivalent amounts; and 1-benzyl-2, 5-dimethylpyrrole yields 3-benzyl-2,5-dimethylpyrrole and 2-benzyl-3,5-dimethylpyrrole, the 3-benzyl isomer predominating. The pyrolysis of 2-benzyl-2,5-dimethyl-2H-pyrrole produces the same relative yields of isomers that were obtained from the pyrolysis of 1-benzyl-2,5-dimethylpyrrole. These data are regarded as evidence for a pyrrolenine intermediate in the thermal isomerization reaction. © 1968, American Chemical Society. All rights reserved.