Tautomerism and Atropisomerism

Tautomerism has been investigated in 3,5-dimethyl-2-(2 -pyridyl)pyrrole and 3,5-di-/ /t-butyl-2-(2 -pyridyl)pyrroles in the gas phase, in their alcohol complexes, and as dimers 2004PCP3938 . The compounds exist preferentially in the normal ry -conformation, which in the dimethyl compound is energetically favored over the /f-conformation by 4.3 kcal moP with a rotational barrier of 11.3 kcal mol , and are more stable than their tautomers in the gas phase. Tautomerism is observed in pyrrole-2,5-diacetic acid and its diethyl ester with the pyrrole form being favored for the free acid and the pyrrolidinediylidene tautomer for the diester 2003NJC1353 . 

Tautomerism has been observed for 2-(2 -pyridyl)indoles using NMR and UV-fluorescence techniques 1998JOG4055 . Molecular orbital calculations indicate that the indole is about 40 kcal mol more stable than its tautomer. 

Finally, bilirubin and biliverdin congeners with propionic acids replaced by o-carboxyphenyl groups exhibit diastereomerism due to axial chirality about the carbon-carbon single bond linking the o-carboxyphenyl group to a pyrrole ring 2002T7411 . 

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