Calorimetric and equilibrium studies on some stable nitroxide and iminoxy radicals. Approximate oxygen-hydrogen bond dissociation energies in hydroxylamines and oximes

Abstract

The crude mixture (30 ml), consisting mainly of 1,4-and 1,5cyclooctadiene, was converted to a mixture of 1,3-and 1,5-cyclooctadiene by treatment for 12 hr at 140°with a solution of sodium (20 g) in tert-amyl alcohol (150 ml).The dienes were recovered from the purified silver nitrate complexes by steam distillation and distillation at atmospheric pressure in purity greater than 99% by glpc on a silicone column. Each sample was identified by ir and nmr spectra: cycloocta-1,3-diene, bp 141.0°, Xmax 227 nm(<""6160); cycloocta-1,4-diene, bp 143.0°; cycloocta-1,5-diene, bp 148.5°.Cycloocta-1,3,5-triene was prepared according to Jones.49 It was freed of bicyclo[4.2.0]octa-2,4-diene by treatment with maleic anhydride according to the procedure of Cope, et al.i0Cycloocta-1,3,6-triene was prepared by the procedure of Sanne and Schlichting.51 The substance was separated from accompanying cycloocta-1,3,5-triene by distillation in a 1-m spinning band column.Any bicyclo[4.2.0]octa-2,4-diene was removed by shaking at 50°with maleic anhydride.cis-and /nms-Hexa-1,3,5-triene were prepared according to Hwa, DeBenneville, and Svens.52 (49) W. O.

Organocatalytic Oxidations Mediated by Nitroxyl Radicals

“…[84] To explain their results they proposed a catalytic cycle in which a key step was the reaction of the nitroxyl radical with a copper(II)/phenanthroline/methanol ternary complex, to afford the corresponding hydroxylamine, copper(I) and formaldehyde according to Equation (14). Regeneration of the nitroxyl radical occurs by reaction of the hydroxylamine with a second equivalent of the copper(II) phenanthroline complex as shown in Equation (15). They further proposed that copper(II) was regenerated by reaction of copper(I) with oxygen, to complete the catalytic cycle.…”

“…Two groups [13,14] have recently determined the BDE of the O À H bond in NHPI and mixed acylalkylhydroxylamines (substituted hydroxamic acids), and compared them (see Table 1) with that of the O À H bond in TEMPOH which was already known. [15] The chemistry of nitroxyl radicals was extensively investigated in the 1960s and 1970s by Perkin [16 -19] and others [20] and the higher reactivity of acylalkylnitroxyls …”

Garlic: Source of the Ultimate Antioxidants—Sulfenic Acids

“…The O À H BDEs of sulfenic acids are among the weakest known and comparable to those of hydroxylamines, such as TEMPO-H (70.7 kcal mol À1 ; TEMPO = 2,2,6,6-tetramethylpiperidin-1-oxyl). [19,20] To provide additional insight into the reactions of sulfenic acids with peroxyl radicals, we also calculated the transitionstate (TS) structures (Figure 2) and associated activation energies of some representative reactions (Table 2). Two TS structures were identified: one with a cisoid geometry with respect to the oxygen atoms between which the H atom is being transferred (Figure 2 a) and one with a transoid geometry (Figure 2 b).…”

Spectral and kinetic measurements on a series of persistent iminoxyl radicals

“…24 The O-H bond strength in 8H was assumed to be representative and has previously been measured calorimetrically as 80·9 kcal mol Ϫ 1 . 25 For radicals 5 and 1 at 25°C, we calculate K eq = 6·9ϫ 10 Ϫ 6 and 3·1ϫ 10…”

“…24, 25 However, attempts to locate the transition state for H-transfer in the system were unsuccessful (see Table below). …”