Olefins into Metal-Oxygen Bonds

Insertion of Olefins into Metal-Oxygen Bonds 

The insertion of olefins into the metal-oxygen bonds of isolated alkoxo, phenoxo, or hydroxo compounds has been observed directly in a few cases. As will be noted in Chapter 11, a hydroxy or alkoxyalkyl group can be formed by insertion of an olefin into a metal-oxygen bond or by attack of hydroxide or an alkoxide on a coordinated olefin. Many studies described in Chapter 11 imply that this type of compound is formed by nucleophilic attack on a coordinatively saturated olefin complex, and this reaction has been proposed as the C-0 bond-forming step during oxidations of olefins catalyzed by palladium complexes. However, Henry provided some of the first evidence that the C-0 bond forms by insertion of olefins into metal-hydroxo and -alkoxo complexes under certain reaction conditions.  

A similar experiment with an unsaturated alcohol reported by Stoltz also implies that syn oxypalladation occurs. Again, the observation of deuterium in the product implies that the alkoxyalkylpalladium intermediate contains the palladium cis to the alkoxo group and trans to the deuterium label. The stereochemical outcome of catalytic cyclizations reported by Wolfe also imply that cis oxypalladation occurs during certain transformations, as shown in Equation 9.82. 

Finally, reaction of the discrete rhodium-alkoxo complex containing a tethered olefin shown in Equation 9.83 has been shown to undergo insertion of the alkene into the metal-alkoxo bond, followed by p-hydrogen elimination. This mechanism has also been shown to occur by a stereochemical labeling study in which the stereochemistry of the olefinic product reveals the mode of addition. The stereochemistry of the product from reaction of the alkoxo olefin complex containing deuterium in the trans position of the coordinated alkene shows that a syn addition of the rhodium and oxygen occurred across the carbon-carbon bond.  

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I. Insertion of Olefins into Metal-Oxygen Bonds... 

An alternative mechanism (Scheme 7) was proposed by Mimoun, which involves olefin coordination to the metal.112-116 The insertion of the alkene into the metal-oxygen bond is thought to be the rate-limiting step of this reaction mechanism. 

An alternative mechanism for oxygen transfer was proposed by Mimoun [21-25]. In this mechanism (eq. (11)) initial coordination of the olefin to the metal is followed by its rate-limiting insertion into the metal-oxygen bond giving a pseudocyclic dioxometallocyclopentane (Structure 3). The latter decomposes to the epoxide and the metal alkoxide. 

The formation of an intermediate organometaUacycle, a metalla-2,3-dioxo-lane, is a central part of the mechanism proposed in the pioneering work of Mimoun and co-workers (1970) for the olefin epoxidation with Mo diperoxo complexes (Fig. 4). The initial step of the suggested mechanism is the coordination of the olefin with the d° metal. The second step is the formation of the organometaUacycle upon cycloinsertion of the coordinated olefin into a metal-oxygen bond of the metal diperoxo complex. Note that metalla-2,3-dioxolanes of late transition metals are known [73,74]. The final step is the cycloreversion of the organometaUacycle, yielding the oxirane. 

A few final comments should be made on the insertions of substrates containing C-C multiple bonds into the bonds between a transition metal and an electronegative heteroatom. First, insertions of olefins into related thiolate and phosphide complexes are as rare as insertions into alkoxo and amido complexes. Reactions of acrylonitrile into the metal-phosphorus bonds of palladium- and platinum-phosphido complexes to give products from formal insertions have been observed, and one example is showm in Equation 9.90. However, these reactions are more likely to occur by direct attack of the phosphorus on the electrophilic carbon of acrylonitrile than by migratory insertion. Second, the insertions of alkynes into metal-oxygen or metal-nitrogen covalent bonds are rare, even though the C-C ir-bond in an alkyne is weaker than the ir-bond in an alkene. 

Reaction of a-diimines with vinyl acetate (VAc) unfortunately does not result in polymerization reactions. The low coordination ability to a cationic Pd complex for this functional olefin is due to an inaeased sterical hindrance. The, compared to ethene, higha n(C=C) frontier orbital (HOMO) energy should promote a n-coordination to the metal. Unfortunately, effective a-coordination via the oxygen atom is also possible and is assumed to be a severe restriction to the incorporation. Furthermore, the insertion rate of VAc into metal-alkyl bonds is reduced compared to ethene and can occur either in a... 

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]. 

The second insertion in the productive cycle of Fig. 14 would involve the chelated acyl complex (5). Again it might have been difficult to convert this to the olefin complex (6) in the nickel case as the strong chelate Ni-oxygen bond has to be weakened. However, for nickel it seems that (5) is replaced with the five-coordinated acyl complex 5a by uptake of one additional CO. However, 5a is not amenable for ethylene uptake as a first step in the insertion of ethylene into the metal-acyl bond since ethylene will have to replace the more strongly bound CO (>10 kcal mol ). It is thus Ukely that the CO/ethylene polymerization cycle is blocked by a species such as 5a or the four-coordinated chelate (5) of Fig. 14. 

The direct attack of the front-oxygen peroxo center yields the lowest activation barrier for all species considered. Due to repulsion of ethene from the complexes we failed [61] to localize intermediates with the olefin precoordinated to the metal center, proposed as a necessary first step of the epoxidation reaction via the insertion mechanism. Recently, Deubel et al. were able to find a local minimum corresponding to ethene coordinated to the complex MoO(02)2 OPH3 however, the formation of such an intermediate from isolated reagents was calculated to be endothermic [63, 64], The activation barriers for ethene insertion into an M-0 bond leading to the five-membered metallacycle intermediate are at least 5 kcal/mol higher than those of a direct front-side attack [61]. Moreover, the metallacycle intermediate leads to an aldehyde instead of an epoxide [63]. Based on these calculated data, the insertion mechanism of ethene epoxidation by d° TM peroxides can be ruled out. 

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