Oxygen atom transfer to olefins

We point out that the mechanism sketched in path A of Scheme 11 is in agreement with the kinetic and spectroscopic data collected from several research groups. On the other hand, a series of contradictions was encountered in fitting the experimental data into the mechanism proposed in path B. Furthermore, several other papers have appeared in the last decade, based on both experimental results and theoretical calculations, supporting an epoxidation mechanism involving a direct oxygen atom transfer to olefins. For selected examples, see References 34, 145-155. 

The Darzens reaction is the base-promoted generation of epoxides XIII from aldehydes (or ketones) XI and alkyl halides XII, the latter carrying an electron withdrawing group, for example the carbonyl, nitrile, or sulfonyl, in the a-position (Scheme 6.83) [188, 189]. It is, formally, addition of a carbene to the C=0 double bond (Scheme 6.83, path B) and thus complements oxygen atom transfer to olefins... 

The organic substrates in Chart 8 can be divided into two main categories in which (i) the oxidation of olefins, sulfides, and selenides involves oxygen atom transfer to yield epoxides, sulfoxides, and selenoxides, respectively, whereas (ii) the oxidation of hydroquinones and quinone dioximes formally involves loss of two electrons and two protons to yield quinones and dinitrosobenzenes, respectively. In order to provide a unifying mechanistic theme for the seemingly disparate transformations in Chart 8, we note that nitrogen dioxide exists in equilibrium with its dimeric forms, namely, the predominant N—N bonded dimer 02N—N02 and the minor N—O bonded isomer ONO—N02 (equation 88). 

Oxygen-atom transfer to PPh3, and to olefins to give epoxide, has been accomplished via (Ph3P)2Pt02 in the presence of electrophiles such as benzoyl chloride (90) transfer occurs from a peroxyben-zoatobisphosphine intermediate (Ph3P)2Pt(OOCOPh)Cl (cf. 6 in Scheme 1), which is converted to the benzoate derivative. 

Peroxo and alkylperoxo metal conqilexes are presumed intermediates in the homogeneous and heterogeneous catalytic epoxidations of olefins. Both solid and soluble versions containing high valent, d transition metal ions activate peroxides heterolytically, thereby facihtating oxygen atom transfer to electron-rich substrates... 

The oxidation of olefins,251 sulfides,252 and selenides253 (denoted as D) involves oxygen-atom transfer from nitrogen dioxide to yield epoxides, sulfoxides, and... 

Two extreme epoxidation modes, spiro and planar, are shown in Fig. 9 [33, 34, 53, 54, 76-85]. Baumstark and coworkers had observed that the epoxidation of cis-hexene of dimethyldioxirane was seven to nine times faster than the corresponding epoxidation of tran.y-hexene [79, 80]. The relative rates of the epoxidation of cisitrans olefins suggest that spiro transition state is favored over planar. In spiro transition states, the steric interaction for cw-olefm is smaller than the steric interaction for fran -olefm. In planar transition states, similar steric interactions would be expected for both cis- and trans-olefms. Computational studies also showed that the spiro transition state is the optimal transition state for oxygen atom transfer from dimethyldioxirane to ethylene, presumably due to the stabilizing interactions... 

The formation of methylperoxy intermediates—i.e., the product of a formal insertion of O2 into the metal-methyl bond—was substantiated by the observation of epoxidation of allylic alkoxides (Scheme 6), in analogy to the proposed mechanism for the Sharpless epoxidation utilizing tert-butylhydroperoxide (TBHP). A similar oxygen atom transfer from a coordinated alkylperoxide to olefin was also postulated for the epoxidation of olefins with TBHP catalyzed by Cp Mo(0)2Cl [31]. The use of organomolybdenum oxides in olefin epoxidafion cafalysis (albeit not with O2) has recently been reviewed [32]. 

Scheme 13 Oxygen atom transfer from chromium to olefins yields variety of products...

Read s group (86, 87) are studying some interesting co-oxidation reactions catalyzed particularly by RhCl(PPh3)3 in benzene at 20°C. Terminal olefins and triphenylphosphine are converted by 02 to the corresponding methyl ketones and the phosphine oxide. Radical chain processes were not detected, and such reactions normally do not yield methyl ketone products. Net oxygen-atom transfers were considered to result via the mechanism shown in Scheme 3. 

J. P. Collman, T. Kodadek, S. A. Raybuck, J. I. Brauman, L. M. Papazian, Mechanism of oxygen atom transfer from high valent iron porphyrins to olefins Implications to the biological epoxidation of olefins by cytochrome P-450, /. Am. Chem. Soc. 107 (1985) 4343. 

Oxygen atom transfer reactions from metal-oxo species to phosphines (forming phosphine oxides), alkylsulfides (forming sulfoxides), olefins (forming epoxides), and other substrates depend on the relative X=0 bond dissociation energies in the metal-oxo starting material and the oxidized product the reactivity scale based on bond dissociation energies was proposed.62... 

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