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Abstract:The dimethyldioxirane oxida-served. Even for these alkenes, which are proposed radical mechanism. The selection of a-methylstyrene, rruns-cyclo-prone to radical reactions, the previously tive hydroxylation of ( -)-2-phenylbutane octene, and 1 -vinyl-2,2-diphenylcycloestablished electrophilic concerted mech-by dimethyldioxirane gave only ( -)-2-propane gave, under all reaction condi-anism applies, rather than the recently phenylbutan-2-01 with complete retention tions employed, the corresponding epoxof configuration and no loss of optical ides in high yields. No radical products purity. Thus, a radical-chain oxidation is also discounted in the oxygen insertion infrom allylic oxidation, from translcis isoto hydrocarbon C-H bonds for dioximerization, or from cyclopropylcarbinyl ranes. rearrangement (radical clock) were ob-

The powerful methyl(trifluoromethyl)dioxirane (1b) was employed to achieve the direct oxyfunctionalization of 2,4-didehydroadamantane (5), spiro[cyclopropane-1,2'-adamantane] (9), spiro[2.5]octane (17), and bicyclo[6.1.0]nonane (19). The results are compared with those attained in the analogous oxidation of two alkylcyclopropanes, i.e., n-butylcyclopropane (11) and (3-methyl-butyl)-cyclopropane (14). The product distributions observed for 11 and 14 show that cyclopropyl activation of alpha-C-H bonds largely prevails when no tertiary C-H are present in the open chain in the tether; however, in the oxyfunctionalixation of 14 cyclopropyl activation competes only mildly with hydroxylation at the tertiary C-H. The application of dioxirane 1b to polycyclic alkanes possessing a sufficiently rigid framework (such as 5 and 9) demonstrates the relevance of relative orientation of the cyclopropane moiety with respect to the proximal C-H undergoing oxidation. At one extreme, as observed in the oxidation of rigid spiro compound 9, even bridgehead tertiary C-H's become deactivated by the proximal cyclopropyl moiety laying in the unfavorable "eclipsed" (perpendicular) orientation; at the other end, a cyclopropane moiety constrained in a favorable "bisected" orientation (as for didehydroadamantane 5) can activate an "alpha" methylene CH2 to compete effectively with dioxirane O-insertion into tertiary C-H bonds. Comparison with literature reports describing similar oxidations by dimethyldioxirane (1a) demonstrate that methyl(trifluoromethyl)dioxirane (1b) presents similar selectivity and remarkably superior reactivity.

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The challenging hypothesis of a "biphilic" (i.e., electrophilic vs nucleophilic) character for dioxirane reactivity, which envisages that electron-poor alkenes are attacked by dioxiranes in a nucleophilic fashion, could not be sustained experimentally. Rate data, which estimate Hammett "rho" values for the epoxidation of 3- or 4-substituted cinnamonitriles X x Ph-CH=CH-CN, unequivocally allow one to establish that dioxiranes epoxidize electrophilically even alkenes carrying electron-withdrawing groups. The greater propensity of methyl(trifluoromethyl)dioxirane TFDO (1b) to act as an electrophilic oxidant with respect to dimethyldioxirane DDO (1a) parallels the cathode reduction potentials for the two dioxiranes, as measured by cyclic voltammetry. A simple FMO approach for alkene epoxidation is helpful to conceive a likely rationale for the greater oxidizing power of TFDO as compared to DDO.

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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.