Reactivity of pyrrole pigments. Part 5: Electrophilic substitution—Nitration and bromination—Of some pyrromethenones and 5-arylmethylene-3,4-dimethyl-3-pyrrolin-2-ones

摘要

Some 5-arylmethylene-3,4-dimethyl-3-pyrrolin-2-ones react with both bromine and nitronium tetrafluoroborate (NO2BF4) to give 5-(aryl)nitromethylene-3-pyrrolin-2-ones and 5-(aryl)bromomethylene-3-pyrrolin-2-ones, respectively. The use of bromine in methanol affords 5-(aryl)bromomethyl-3,4-dimethyl-5-methoxy-3-pyrrolin-2-ones. Whereas pyrromethenones react mainly on the pyrrole ring, ethyl 3,4-demethyl-5-(3,4-dimethyl-5-oxo-3-pyrrolin-2-yl)methylene-1H-pyrrole-2-carboxylate reacts as the aryl derivatives, however, with bromine in methanol the addition of two methoxy groups at the exocyclic double bond takes place. 3,4-Dimethyl-5-(2-pyridylmethylene)-3-pyrrolin-2-one does not react with bromine or NO2BF4, but reacts as the aryl derivatives with bromine in methanol. The reactivity patterns are in agreement with the theoretical ones obtained from MINDO/3 calculations, using theFukuifrontier orbital model. The obtained results are used to explain the reactivity of rubins (biladienes-a,c) and verdins (bilatrienes-a,b,c) in front of electrophiles.