engleski

Photochemical transformations of pyrrole-stilbenes

In continuation of our interest on photochemistry of heterocyclic compounds we introduced the pyrrole moiety to the beta-position of o-divinylbenzene (1a). The irradiation of pyrrole (1a) gives dimeric products 4 by regiospecific intermolecular addition of the pyrrole to the double bond. No intramolecular formation of a bicyclic product was observed, as we expected on analogy with furan derivatives. We proposed that the formation of dimeric products 4 occurs via photoinduced electron transfer, followed by proton transfer and radical recombination. Gilbert reported similar intermolecular photoaddition of pyrrole to styrene and stilbene. The dipyrrole derivative 2a gives a mixture of stereoisomers 6 in 40 % yield, besides traces of minor intramolecular product 7a. When the 5 position in the dipyrrole 2b is blocked by a methyl group the intramolecular photoproducts 7 and 8 are formed. The irradiation of properly substituted styrylpyrrole (1b) gives bicyclic structure 5a that is transformed into the wanted 5b. Irradiation of 3a gives also the intramolecular bicyclic structure. The identification of the products as well as the mechanism of the reaction will be discussed.

pyrrole; photochemistry

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