Comparison of Isoxazole and Azirine as precursor to triplet vinyl nitrene

In recent years scientists are more interested in studying reactive intermediates like triplet carbenes and triplet nitrenes. Those molecules have unpaired electrons and have high spin properties. By considering the magnetic properties of these organic molecules scientists attempt to build organic magnets. Even though they have extensively studied reactive intermediates like carbenes, less attention has been paid to investigations of nitrenes, specially the vinylnitrenes. Several research groups have proposed triplet vinylnitrene formation from different precursors. Isoxazoles, azirine derivatives and vinyl azides are the some of these precursors to triplet vinylnitrenes.

In this work, I studied the photoreactivity of 3,5-diphenylisoxazole (1) and 3-benzoyl-2-phenylazirine (2) as precursors to triplet vinylnitrene. Irradiation of both compounds in acetonitrile solution with 308 nm laser lead to give triplet vinylnitrene (4) with lifetime of 1.7-1.8 µs. Triplet vinylnitrene (4) shows an abroad absorption with λ_max at ∼ 350 nm and 415 nm which trails further out towards 500 nm. Furthermore 2 forms ylide (5) which absorbs with λ_max 460 nm with life time of 13 µs. In oxygenated-saturated solution I observed an efficiently trapping of 4 with oxygen and formed a peroxide radical which intersystem cross to decay the singlet surface. Then this peroxide radical auto oxidizes to give benzoic acid and benzamamide as photoproducts.

Calculations (B3LYP/6-31+G(d) illustrate that T₁ of 1 is located 63 kcal/mol above the S₀ of 1. Comparatively, T₁ of 2 is located 65 kcal/mol above the S₀ of 2. The calculated transition state energy barrier to form 4 from 1 is 12 kcal/mol above T₁ of 1. Similarly the transition state energy barrier to yield 4 form 2 is only 2 kcal/mol above T_1K of 2. Spin density calculations show that 4 has 1,3-carbon iminyl biradical character which explains the efficient reactivity of 4 with oxygen. In addition to the triplet reactivity, 2 shows singlet reactivity, that yields formation of 5. The transition state for cleaving carbon, carbon bond in 2 to form 5 is located 44 kcal/mol above the S₀ of 2. Internal conversion of S_1A to a vibrationally hot S₀ of 2 forms 5.