No abstract

Thermolysis of azoperester 5 affords γ-azo radical 14, which cyclizes to hydrazyl radical 15 at a rate of 1.3 × 109 M-1 s-1 at 110 °C. Our experimental results are consistent with loss of a methyl radical from 15 to afford 2-pyrazoline 9, which is oxidized in situ by the starting 5 to pyrazole 6. This unusual and endothermic β-scission can be rationalized if the odd electron in 15 is better aligned with the CH3−C bond than with the weaker t-Bu−N bond. The fact that 5-endo cyclization of 14 is 5 × 107 faster than that of the analogous olefinic radical 30 led us to carry out ab initio calculations on simplified structures. ΔH ⧧ for methyl radical addition to diimide is only 0.84 kcal/mol lower than for addition to ethylene and the exothermicity is only 3.5 kcal/mol greater. However, the smaller CNN than CCC bond angle leads to a ΔH ⧧ 13.3 kcal/mol lower for 5-endo cyclization of 4,5-diazapenten-1-yl than for the analogous 4-pentenyl radical. Photolysis of 5 selectively cleaves the azo group, producing γ-perester radical 20. This species undergoes intramolecular attack on the peroxide linkage to form lactone 23 at a rate of 1.5 × 104 s-1 at 22 °C. The cyclization rate of 20 is slow enough that 5 could be used as a photochemical bifunctional initiator, but cyclization of 14 to the azo group is so rapid that this radical would only rarely attack a monomer.