PHOTOCHEMICAL REACTIVITY OF HALOFURAN AND HALOTHIOPHENE DERIVATIVES IN THE PRESENCE OF ARYLALKENES AND ARYLALKYNES

The photochemical reactions of 5-iodothiophene-2-carbaldehyde (4a), 2-acetyl-5-iodothiophene (4b), 5-bromofuran-2-carbaldehyde (3a), and 5-iodofuran-2-carbaldehyde (3b) with arylalkenes 5 (styrene), 7 (2-vinylthiophene), 10 (2-vinylfuran), 12 (4-methyl-5-vinylthiazole), and 16 (benzofuran) are reported. All of the reactions give the corresponding substitution products as a cis-trans mixture. The photochemical reaction of 4a and 4c (methyl 5-iodothiophene-2-carboxylate) with arylalkynes is also reported: in this case the reaction of 4a and 4c with phenylacetylene (18) furnishes the substitution products (19 and 26, respectively) deriving from an attack on the alkyne moiety, while the reaction with 2-ethynylthiophene (20) and 2-ethynylfuran (23) furnishes a mixture deriving from the attack both on the alkyne and on the heterocyclic ring. The ratio between these two products can be modified by changing the concentrations of the reagents. The mechanism of these reactions is discussed on the basis of photochemical and electrochemical properties of the reagents in terms of an electron-transfer process. The experimental results are explained in terms of ΔG values, and they are in agreement with the formation of both a solvent-separated pair and a contact radical ion pair. © 1990, American Chemical Society. All rights reserved.