Olefin insertions into alkoxides

Allylic alcohols are isomerized via direct interaction of the ruthenium atom with alcohol. /3-Elimination of ruthenium hydride from metal alkoxide yields a ruthe-nium-enone species C which undergoes insertion of the olefinic moiety into the Ru-H to form an oxyallylic intermediate D. As a result, the hydrogen atom shifts from the a- to y-position of the allylalcohol. Protonolysis of the oxyallylic species leads to a saturated carbonyl compound and cationic unsaturated species, [CpRu(PPh3)2] A. 

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 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]. 

On the other hand, the other process without involving the formation of r)3-allylic complexes may operate as an alternative route in the course of the C-O bond cleavage. One is the SN2 type attack of a ligand bound to the metal such as hydride, alkyl or alkoxide on the terminal carbon of the allylic entity. The process is followed by elimination of the OX group (acetate or alkoxide) in a concerted manner as shown in Eq. 4. The other mode of cleavage is insertion-elimination type as shown in Eq. 5. The process proceeds by insertion of the olefinic moiety of the allylic entity into the M-Y bond, such as hydride, alkyl, or alkoxide ligand followed by P-elimination of the acetate or alkoxide moiety. 

The first catalytic insertion of a C=C bond into an acyl CO bond was attained with Rh catalysts. Thus, the ort/io-substituted olefinic phenol esters with the quinoline chelating group (70) were converted into the )3-alkoxy ketones (72). The stabilization of the intermediate Rh-alkoxide by chelation (71) is a prerequisite for the reaction to occur. 

The insertions of ketones and imines into metal hydrides have also been studied, although they have been studied less extensively than the insertion of olefins into metal hydrides. This elementary reaction is relevant to one of the mechanisms for the hydroge nation of ketones and imines. An example of the insertion of a ketone into a ruthenium hydride is shown in Equation 9.55. Hie alkoxide product is stable toward reductive... 

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. ... 

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