Photoinduced cycloadditions of N-methyl-1,8-naphthalenedicarboximides with alkynes

Abstract

Photoinduced cycloadditions of N-methyl-1,8-naphthalenedicarboximide 1 with phenylacetylenes 2a– 2c , cyclopropylacetylene 2d , diphenylacetylenes 2e– 2f and 1-phenylpropyne 2g were investigated. In the case of phenylacetylenes 2a , 2b and cyclopropylacetylene 2c , photoreaction with 1 takes place at the naphthalene C(1) C(2) bond to give the cyclobutene products. For 4-methoxyphenylacetylene 1c , the cyclobutene 3c is obtained together with the 4-benzo[ a]thebenidinone 4c derived from a primary oxetene product formed by [2+2] addition of the imide carbonyl with the alkyne. Similar to 2c , photocycloaddition of 1 with 2e and 2f gave the cyclobutenes 7e , 7f , 8f and the 4-benzo[ a]thebenidinone products 9e , 9f and 10f , respectively, derived from the corresponding oxetenes. Photoreaction of 1 with 2g gave cyclobutene 7g and benzo[ a]thebenidinone 9g . Sensitization experiment and internal heavy atom effect study showed that these reactions proceed from the ππ* singlet excited state of 1 . Estimation of the free energy change for electron transfer between 1 1* and the alkynes and the calculation of charge and spin density distribution in the anion radical of 1 and the cation radical of the alkynes suggested that the cyclobutene products are formed by direct [2+2] cycloaddition of 1 1* with the alkyne, while the formation of the oxetene products is the result of electron transfer interaction between 1 1* and the alkyne. The regioselectivity in the oxetene formation is accounted for by charge and spin density distribution in the anion radical of 1 and the cation radical of the alkyne.