Phototransformations of Bridgehead-Substituted Dibenzobarrelenes Steady-State and Laser Flash Photolysis Studies

Article abstract of DOI:10.1021/jo00234a003

The photochemistry of bridgehead-substituted dibenzobarrelene derivatives 1a-f has been investigated by steady-state photolysis, product analysis, and laser flash photolysis. 4b- or 8b-substituted dibenzosemibullvalenes or products derived thereof were formed in the course of irradiation of 9-(hydroxymethyl)-, 9-ethyl-, and 9-benzyl-substituted dibenzobarrelenes. 9-Cyclopentyl-substituted dibenzobarrelene upon irradiation gave the corresponding 4b-substituted dibenzosemibullvalene and dibenzocyclooctatetraene, whereas from the photolyzates of 9-isopropyl- and 9-cyclohexyl-substituted dibenzobarrelenes the corresponding dibenzocyclooctatetraenes were obtained as the only isolable products.The 337.1-nm laser flash photolysis of 1a-f in benzene results in the formation of their triplets in high yields (Φ_T = 0.7-1.0).The triplets possess short lifetimes (0.03-2.2 μs) and are readily quenchable by oxygen, di-tert-butyl nitroxide, and ferrocene (k_g^T = (0.2-8) X 10⁹ M^-1s^-1).In spite of high Φ_T's, some of the dibenzobarrelene substrates are reluctant to undergo the barrelene-semibullvalene rearrangement under steady-state irradiation; this appears to be related to the unusually short lifetimes of their triplets; that is, decay modes other than those eventually leading to dibenzosemibullvalenes are available to these triplets.