A novel simplified procedure affords consistently 0.09-0.10 M solutions of distilled dimethyldioxirane in acetone: other than control of the reaction temperature below 15°C and vigorous mechanical stirring, no other precautions are mandatory.The importance of isolated dimethyldioxirane (Eq. 1) for preparative oxidation chemistry, as witnessed by the surge of activity2) since its isolation3) in 1985, demands a convenient method of isolation of this efficient oxidant. Presently, we provide a much simplified version of a recently reported procedure4), which should be of general interest and utility. Me (Eq. 1) H20. NaHC03 CH,COGH, + KHSO, pH Z 7 4 . 5 -1 O o C MeKey features of the new version are that even normal water (not distilled from EDTA) and commercial-grade acetone (directly from the metal canister) can be employed, that the high-efficiency

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Ace. Chem. Res. 1989,22, 205-211 205 unenriched while the cis-trans isomer and 10 must be enriched. This was confirmed for 612. Photolysis at 10 "C of cis-612 gives p = 0, 90, 128% (130%) for cis-612, trans-(i12, and lol2, respectively, whereas photolysis of t r~n s -6 ,~ gives ( 3 = 68, 2, 131% for the same three compound^.^^ This confirmation strongly supports the above model.In summary, the physical basis for the difference in 13C enrichment among the products of biradical decay is the inherent tendency of HFC and SOC to produce different products, and this in turn comes from the R dependences of the different ISC mechanisms shown in Figure 2.Conclusion. Through a combination of product studies, isotope effects, and transient absorption kinetics including the effect of biradical chain length and substituent, solvent, temperature, and magnetic field, we have made progress in elucidating the relation of the rates and product distributions to the spin interactions in the biradicals. Probably the most surprising result is the profound effect exerted by extremely small interactions such as the S-T gap, SOC, and HFC upon the dynamics and product distribution of triplet biradicals.

The hitherto unknown monomethyl derivative If and the parent 1,2-dioxetane lg have been prepared and fully characterized. The influence of the degree and pattern of methyl substitution of the complete set of 1,2-dioxetanes la-g on the activation parameters (AH*, AS*, and AG') and on the excitation yields ( and 5) have been determined. It was found that (1) the thermal stability increases with the degree of methylation, (2) the pattern of methylation does not alter appreciably thermal stability, (3) triplet n,ir* states are preferentially energized, and (4) the triplet and singlet excitation yields increase with the degree of methylation. These experimental results are compared with thermochemical estimates and rationalized in terms of the diradical hypothesis and energy surface crossings. The present findings are most consistent with the merged mechanism, in which the activated complex starts out on the ground-state diradical energy surface and crosses over to the carbonyl excited state surfaces, yielding excited carbonyl fragments.Numerous recent results1 of substituent effects in the thermolysis of 1,2-dioxetanes have been interpreted in favor of a diradical2 rather than a concerted3,4 mechanism (Scheme I). A series of ris/Zra ns-3,4-dialkyl-lc and 3,3-dialkyl-l,2-dioxetaneslf reveal that 3,3-interactions are more important than 3,4-inter-

Nonmetal oxidation catalysts have gained much attention in recent years. The reason for this surge in activity is 2-fold: On one hand, a number of such catalysts has become readily accessible; on the other hand, such catalysts are quite resistant toward self-oxidation and compatible under aerobic and aqueous reaction conditions. In this review, we have focused on five nonmetal catalytic systems which have attained prominence in the oxidation field in view of their efficacy and their potential for future development; stoichiometric cases have been mentioned to provide overview and scope. Such nonmetal oxidation catalysts include the alpha-halo carbonyl compounds 1, ketones 2, imines 3, iminium salts 4, and nitroxyl radicals 5. In combination with a suitable oxygen source (H2O2, KHSO5, NaOCl), these catalysts serve as precursors to the corresponding oxidants, namely, the perhydrates I, dioxiranes II, oxaziridines III, oxaziridinium ions IV, and finally oxoammonium ions V. A few of the salient features about these nonmetal, catalytic systems shall be reiterated in this summary. The first class entails the alpha-halo ketones, which catalyze the oxidation of a variety of organic substrates [figure: see text] by hydrogen peroxide as the oxygen source. The perhydrates I, formed in situ by the addition of hydrogen peroxide to the alpha-halo ketones, are quite strong electrophilic oxidants and expectedly transfer an oxygen atom to diverse nucleophilic acceptors. Thus, alpha-halo ketones have been successfully employed for catalytic epoxidation, heteroatom (S, N) oxidation, and arene oxidation. Although high diastereoselectivities have been achieved by these nonmetal catalysts, no enantioselective epoxidation and sulfoxidation have so far been reported. Consequently, it is anticipated that catalytic oxidations by perhydrates hold promise for further development, especially, and should ways be found to transfer the oxygen atom enantioselectively. The second class, namely, the dioxiranes, has been extensively used during the last two decades as a convenient oxidant in organic synthesis. These powerful and versatile oxidizing agents are readily available from the appropriate ketones by their treatment [figure: see text] with potassium monoperoxysulfate. The oxidations may be performed either under stoichiometric or catalytic conditions; the latter mode of operation is featured in this review. In this case, a variety of structurally diverse ketones have been shown to catalyze the dioxirane-mediated epoxidation of alkenes by monoperoxysulfate as the oxygen source. By employing chiral ketones, highly enantioselective (up to 99% ee) epoxidations have been developed, of which the sugar-based ketones are so far the most effective. Reports on catalytic oxidations by dioxiranes other than epoxidations are scarce; nevertheless, fructose-derived ketones have been successfully employed as catalysts for the enantioselective CH oxidation in vic diols to afford the corresponding optically active alpha-hydroxy ketones. To date, no catalytic...