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

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