Oxygen atom transfer alkenes

The epoxidation of electron-deficient alkenes, particularly a,P-unsaturated carbonyl compounds, continues to generate much activity in the literature, and this has been the subject of a recent concise review <00CC1215>. Additional current contributions in this area include a novel epoxidation of enones via direct oxygen atom transfer from hypervalent oxido-).3-iodanes (38), a process which proceeds in fair to good yields and with complete retention of... 

The reactions of a number of ozonides and alkenes in the presence of BF3 Et20 resulted in the formation of E- and Z-l,2-dioxolanes in 12-70% yields (Scheme 30) <83JA6279>. 2,5-Dimethyl-thiophene endoperoxide can act as a sulfur- or oxygen-atom transfer agent in analogous reactions to those shown by furan endoperoxides <86JA2775>. 

For practical reasons the two-electron oxidations of such prototypical substrates as alkenes, sulfides, amines and phosphines with peroxynitrous acid have been smdied since this type of oxygen atom transfer reaction is similar to the corresponding oxidations with peroxyformic acid, where both experimental and computational data are available The oxidative reactions (equations 3-6) of these key substrates can... 

Early studies of these complexes focused primarily on the investigation of their oxygen-atom-transfer reactivity. Relatively little success was achieved, however. The peroxo fragments in these complexes exhibit nucleophilic character, and, therefore, they are generally ineffective oxidants for reactions with synthetically interesting electron-rich substrates such as alkenes and sulfides. Most of the known reactivity involves electrophilic substrates that, in many cases, insert into the Pd-0 bond of peroxopalladium(II) species (Fig. 4) [105,118]. 

Chromium(III) salen will form chromium(V) derivatives in much the same way as porphyrin complexes do. By reaction of iodosylbenzene with chromium(III)salen oxochromium(V) complexes are obtained (equation 74) which will effect oxygen atom transfer to phosphines and alkenes such as norbornene (equation 75) in stoichiometric and catalytic systems.1270 The... 

Many compounds have been synthesized, characterized, and studied as models for proposed intermediates in various homogeneous catalytic reactions. Here we discuss two examples. Complex 2.4 is proposed as a model that shows the mode of interaction between an organic hydroperoxide and high-valent metal ions such as Ti4+, V5+, and Mo6+. This type of interaction is considered to be necessary for the oxygen atom transfer from the hydroperoxide to an alkene to give an epoxide (see Chapter 8). 

It is clear that in the asymmetric epoxidation of allyl alcohols, coordination by the OH functionality to the titanium center plays a crucial role. Such a coordination has a favorable effect on the entropy of activation, that is, the rate constant (see Question 14 of Chapter 8). It also helps to orient only one of the two possible enantiofaces for a facile oxygen atom transfer. With alkenes that do not have any such functional groups, the titanium tartarate system gives poor enantioselectivities. The precatalyst that has been found to exhibit re-... 

The proposed mechanism for epoxidation with 9.38B as the catalyst is shown in Fig. 9.8. The oxidation state of manganese in 9.38A and 9.38B is three. The oxygen donor NaOCl or PhIO oxidizes Mn3+ to Mn5+, and an oxo complex such as 9.39 is produced. Reaction of 9.39 with the alkene produces the chiral epoxide and regenerates 9.38B. While there is enough evidence for the basic mechanism and the involvement of a catalytic intermediate such as 9.39, there is some controversy about the details of the oxygen atom transfer from 9.39 to the alkene. 

The oxygen atom transfer from a peroxy acid to an alkene is facilitated by electron-donating substituents on the carbon-carbon double bond and electron-withdrawing groups on the peroxy acid.16 Further, conclusions which can be made from the experimental data collected over the years, include ... 

Within this general context, the formation of an acylperoxymanganese(III) species and its decomposition to an oxomanganese(V) species which undergoes oxygen atom transfer to alkenes has been investigated.694... 

Dioxiranes are extremely useful reagents for the epoxidation of alkenes under neutral conditions. Since the oxygen atom transfer to the alkene regenerates the initial ketone, this epoxidation is catalytic. Dioxiranes are easily generated by the action of an oxone (potassium persulfate) on a ketone (usually acetone) either in a biphasic mixture or in a homogeneous aqueous organic solution. 

Porphyrin hgands also allow the preparation of Cr complexes. For example, reaction of [(TPP)CrCl] with iodosylbenzene provides [(TPP)Cr(0)Cl], a six-coordinate complex, which is stable in solution for several hours. Its reaction with alkenes involves oxygen atom transfer and yields alcohols and epoxides. Another Cr porphyrin complex is the nitride [(TPP)CrN]. A crystal-stmcture determination has confirmed the square pyramidal structure with a very short bond distance to the apical lutrido ligand (Cr N, 156.5 pm). 

Cr salen also reacts with iodosylbenzene and yields cationic Cr 0x0 complexes. These catalyze oxygen atom transfer to alkenes. ... 

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