TRAPPING OF CYCLOPENTANEDIYL AND TRIMETHYLENEMETHANE TRIPLET DIRADICALS WITH THE NITROXIDE 1,1,3,3-TETRAMETHYL-1,3-DIHYDROISOINDOLIN-2-YLOXYL

Nitroxide trapping constitutes a convenient and effective alternative to dioxygen for the detection of triplet diradical intermediates. Thus, photolysis of the azoalkanes 1a-c in the presence of the nitroxide 1,1,3,3-tetramethyl-1,3-dihydroisoindolin-2-yloxyl produced the bisalkoxyamines 3a-c by trapping of the transient triplet diradicals 5a-c. The resulting bis-adducts 3a-c were fully characterized, and their regio- and stereochemistry were established on the basis of spectral and X-ray data for trans-3a and trans-3b. The novel bis-azoalkane 1d was prepared and its photochemical loss of nitrogen studied in the presence of the above nitroxide or dioxygen as scavengers. In the case of the nitroxide, the tetrakis-adduct 3d was obtained, tentatively assigned in view of its thermal instability, while with dioxygen the stable bis-peroxide 4d was isolated and rigorously characterized. Instead of concurrent double denitrogenation to afford the high-spin non-Kekule species 5d or its low-spin quinoid diradical 5d', stepwise loss of dinitrogen and trapping is proposed to be the pathway to these products.