Alkenes Alkyl peroxides

Metal alkyl peroxides can be used for the epoxidation of electron-deficient alkenes such as enones. The use of a combination of diethylzinc, oxygen, and A-methylephedrine gave epoxides in very high yield and generally high enantio-selectivity (Figure 11.8). " ... 

Co(ni) alkyl peroxides have been prepared and used by Mimoun and coworkers in the hydroxylation of hydrocarbons with this metal a Haber-Weiss type of reactivity is suggested. Square-planar Pt(II) complexes, of the type [(dppe)Pt(CF3)(solv)], used by Strukul in the epoxidation of alkenes and in Baeyer-Villiger oxidations of ketones (Schemes 8 and 9), are effective catalysts also in the direct hydroxylation of aromatics with hydrogen peroxide. The reactivity increases in the presence of electron releasing substituents in the aromatic ring. Ortho and para derivatives are practically the only products observed and interesting selectivity toward the ortho products has been detected (equation 85). 

It is possible to obtain anti-Markovnikov products when HBr is added to alkenes in the presence of free radical initiators, e.g. hydrogen peroxide (HOOH) or alkyl peroxide (ROOR). The free radical initiators change the mechanism of addition from an electrophilic addition to a free radical addition. This change of mechanism gives rise to the anh-Markovnikov regiochemistry. For example, 2-methyl propene reacts with HBr in the presence of peroxide (ROOR) to form 1-bromo-2-methyl propane, which is an anh-Markovnikov product. Radical additions do not proceed with HCl or HI. 

The alkylation of Si surfaces is an interesting subject. It is reported that the immersion of H-Si (111) in organic solutions of 1-alkenes, diacyl peroxide, etc. produced alkyl-monolayers on Si.281 The formation of organic monolayers on H-Si (111) via electrochemical reduction of diazonium compounds in aqueous solutions is also reported.291... 

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

All these data are in favor of a homolytic mode of oxygen transfer from Vv alkyl peroxides to hydrocarbons, and the mechanism suggested in Scheme 4, based on that of oxidation by Vv-peroxo complexes (Scheme 2), was tentatively attributed to a biradical V17 — OR—O species which can add to arenes and abstract hydrogen atoms from alkanes. It is probable that the absence of a releasable coordination site adjacent to the triangular alkyl peroxide group in (22) precludes the possibility of the alkene coordination to the metal and therefore prevents its heterolytic epoxi-dation. 

If we consider the d0 metal-N,JV-dialkylhydroxylamino complexes (79), (80) and (81) as valid models for the reactive but unstable alkyl peroxide species Mo02(OOR)2, VO(OOR)3 or V203(00R)4, and Ti(OOR)4 presumably involved in catalytic oxidations, the low activity of vanadium and titanium for the epoxidation of simple alkenes could be interpreted by the fact that these alkenes cannot displace the O.O-bonded alkyl peroxide groups in the coordinatively saturated Vv- and Tiiv-alkyl peroxide species, whereas allylic alcohols can displace the alkyl peroxide groups by forming bidentate allylic alkoxides as in equation (75).162... 

Since the metal-alkene association preceding the peroxymetalation reaction in mechanism (B) is a pure Lewis acid/Lewis base interaction, it would be expected that compounds having alkylperoxy groups bonded to a Lewis acid center should be active for the epoxidation of alkenes. This is indeed found for boron compounds, which are active as catalysts for the epoxidation of alkenes by alkyl hydroperoxides.246,247 Isolated boron tris(alkyl peroxides), B(OOR)3, have been shown to epoxidize alkenes stoichiometrically, presumably through alkylperoxyboration of the double bond (equation 76).248... 

Novel vanadium(V)-alkyl peroxide complexes with the general formula VO(OOBu )(ROPh-salR ) (197 ROPhsalR = tridendate N-(2-oxidophenyl)salicylidenaminato Schiff base ligand) were found to epoxidize alkenes stoichiometrically with high selectivity.620... 

The heterolytic reactivity of complex (197) contrasts with the homolytic reactivity of the vanadium(V) (dipicolinato)( alkyl peroxide) (22), and is presumably due to the lower binding ability of the OBu oxygen atom in (197), shown by NMR, which allows the coordination of alkene to the metal. 

A catalytic asymmetric oxidation of mono-, di-, and tri-substituted alkenes using a chiral bishydroxamic acid (BHA) complex of molybdenum catalyst in air at room temperature leads to good to excellent selectivity. It has been suggested that the Mo-BHA complex combines with the achiral oxidant to oxidize the alkene in a concerted fashion by transfer of oxygen from the metal peroxide to the alkene.78 The chiral BHA-molybdenum complex has been used for the catalytic asymmetric oxidation of sulfides and disulfides, utilizing 1 equiv. of alkyl peroxide, with yields up to 83% and ees up to 86%. An extension of the methodology combines the asymmetric oxidation with kinetic resolution providing excellent enantioselectivity (ee = 92-99%).79... 

The reversal of orientation in the presence of peroxides is called the peroxide effect. It occurs only with the addition of HBr to alkenes. The peroxide effect is not seen with HC1 because the reaction of an alkyl radical with HC1 is strongly endothermic. 

The O -linolenic peroxo compound (w -3-alkyl peroxide) can be degraded to aldehydes and alkanes with some alkenes (B IV) depending on the valence state of the metal ions (Fe) in the cell (43,44). 

An obvious extension of the application of metal alkyl peroxides for epoxidation of electron-deficient alkenes is to use chiral ligands on the metal for asymmetric epoxidations. However, this extension has only very recently met with success. 

An alkyl peroxide can be used to reverse the order of addition of H and Br to an alkene. The akyl peroxide contains a weak oxygen-oxygen single bond that is readily broken homolytically in the presence of light or heat to form radicals. A radical (also called a free radical) is a species with an unpaired electron. 

An alkyl peroxide has the same effect on the addition of HBr to an alkyne that it has on the addition of HBr to an alkene (Section 4.10)—it reverses the order of addition because the peroxide causes Br to become the electrophile. 

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