A laser flash photolysis study of some aromatic keto-epoxides. Characterization of ylides and their precursors

Abstract

Long-lived transient species characterized by broad spectral absorption with maxima at 500–600 nm and by lifetimes in the range 2–100 µs are observed at room temperature upon laser flash photolysis (337.1 nm) of benzene solutions of trans-3-benzylidenechromanone epoxide (8), trans,cis-3-anisylideneflavonone epoxide (9), and trans-6-methoxyaurone epoxide (10). These are assigned ylide structures (4) on the basis of (a) spectral similarity with stable, coloured species formed upon irradiation in methylcyclohexane glass at 77K and (b) reactivity towards dipolarophiles (tetramethylethylene, dimethyl acetylenedicarboxylate, and maleic anhydride), methanol, and water. The rate constants for reactions of the ylides with these reagents are in the range 4.5 × 10⁴–1.1 × 10⁸ dm³ mol ¹ s¹. The formation of the ylides is mediated by triplets of aryl ketone chromophores and the intersystem crossing efficiencies are 0.8–0.9 in benzene. Quenching studies with 1-methylnaphthalene and 2,5-dimethylhexa-2,4-diene (typical quenchers for aromatic carbonyl triplets) give estimates for the lifetimes of the triplet precursors at 0.5–18 ns (in benzene).