On the Reaction of Aminoxyls with Dioxiranes

Abstract

In the reactions of dimethyldioxirane (1a) and methyl(trifluoromethyl)dioxirane (1b) with 2,2,6,6‐tetramethylpiperidinyl‐1‐oxyl (2) (TEMPO) in acetone, the corresponding methoxyamine 1‐methoxy‐2,2,6,6‐tetramethylpiperidine (5) is produced in ≥98% yield, both in air and under N2, and in the absence or presence of a hydrocarbon (adamantane). Kinetic experiments show that aminoxyl 2 triggers the radical decomposition of the dioxirane, in addition to scavenging methyl radicals derived therefrom. The reactions of an aminoxyl less prone to oxidation, namely 1,2‐dihydro‐2‐methyl‐2‐phenyl‐3H‐indol‐3‐one‐1‐oxyl (4), with dioxiranes 1a and 1b in acetone have also been studied. In these cases, not only is the corresponding methoxyamine 8a produced (yield 12−16%), but quinoneimine‐N‐oxides 10 (yield 12−21%) and 11 (yield 18−19%) are also formed. Furthermore, significant amounts (8−14%) of the amine 9 (the product of deoxygenation of 4) can be isolated. These observations provide useful information concerning the free‐radical mechanism that follows the initial attack by the aminoxyl at the dioxirane.

Efficient Oxidation of Adamantanes by Sodium Nitrite with Molecular Oxygen in Trifluoroacetic Acid

“…An involvement of radical species in those oxidation reactions was then examined by adding TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) as a radical scavenger 11 to a solution for oxidation of 1a. In the oxidation of 1a by O 2 -NaNO 2 , as the amount of TEMPO increased, the yield of product 3a decreased (Scheme 3), while in the electrochemical oxidation of 1a addition of 0.2-1.0 equivalent of TEMPO did not show any effect on the reaction (Scheme 4).…”

“…The results are summarized in Table 2, in which the ratios of products 3a-c almost depended on the number of bridgehead hydrogens of 1a-c in the competitive oxidation reactions by O 2 /NaNO 2 (Entries 1-3), whereas the competitive oxidation reactions between 1a-c by electrochemical method afforded 3a-c with 1.1-1.3 ratio of products 3a-c, which were not dependent on the number of bridgehead hydrogens of 1a-c (Entries 4-6). An involvement of radical species in those oxidation reactions was then examined by adding TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) as a radical scavenger 11 to a solution for oxidation of 1a. In the oxidation of 1a by O 2 /NaNO 2 , as an amount of TEMPO increased, the yield of product 3a decreased (Scheme 3), while in the electrochemical oxidation of 1a addition of 0.2-1.0 equiv of TEMPO didn't show any effect to the reaction (Scheme 4).…”

Efficient Oxidation of Adamantanes by Sodium Nitrite with Molecular Oxygen in Trifluoroacetic Acid.

“…An involvement of radical species in those oxidation reactions was then examined by adding TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) as a radical scavenger 11 to a solution for oxidation of 1a. In the oxidation of 1a by O 2 /NaNO 2 , as an amount of TEMPO increased, the yield of product 3a decreased (Scheme 3), while in the electrochemical oxidation of 1a addition of 0.2-1.0 equiv of TEMPO didn't show any effect to the reaction (Scheme 4).…”

Dioxiranes: from oxidative transformations to chemiluminescence

“…120 Generally speaking, nitroxides [e.g., tetramethylpiperidyloxyl (TEMPO)] are not convenient traps for the radical intermediates formed during dioxirane decomposition, because they are able to react directly with 1a and 1b thus inducing their consumption in a radical chain process; for TEMPO, this is illustrated in Scheme 8. 124 When the dioxirane 1a was made to react (0 ± 56 8C) with a spin trap, N-tert-butyl-C-phenylnitrone 15, the formation of the nitroxide 16 was detected based on the EPR signal (a triplet of doublets with splitting constants at the N and H atoms equal to 13.5 and 2 ê, respectively) 125 (Fig. 3).…”

“…Even when the concentration of the dioxirane 1a is relatively low (0.03 mol litre 71 ) and the temperature is moderate (20 8C), the induction period in the reaction performed under argon decreases to *150 min. 124 It comes as no surprise that the dioxirane decomposition under an atmosphere of O 2 , which inhibits the radical chain process, strictly follows first-order kinetics irrespective of the initial dioxirane concentration or temperature (30 ± 49.5 8C). 132 e. Isomerisation of dimethyldioxirane to methyl acetate Although arguments in favour of the radical mechanism of decomposition of the dioxirane 1a are quite forcible, the existence of a parallel channel of the decomposition of 1a, namely, its isomerisation to MA via a biradical intermediate, can also be considered to be proven.…”