Alkenes, metal induced

Oxidation of Alkenes Metal Induced Formation of an Allylic Carbon-Oxygen Bond"... 

In summary, transifion-metal-catalyzed alkene-polymerization reactions highlight the metal-induced electrophilic activation of C—C n bonds to form carbo-cation-like alkene complexes. Considerations involving substituent pi-donor or pi-acceptor strength (i.e., tendency toward carbocation formation) will be useful in similarly rationalizing polymerization reactions (4.105) for more general alkenes. 

None of these difficulties arise when hydrosilylation is promoted by metal catalysts. The mechanism of the addition of silicon-hydrogen bond across carbon-carbon multiple bonds proposed by Chalk and Harrod408,409 includes two basic steps the oxidative addition of hydrosilane to the metal center and the cis insertion of the metal-bound alkene into the metal-hydrogen bond to form an alkylmetal complex (Scheme 6.7). Interaction with another alkene molecule induces the formation of the carbon-silicon bond (route a). This rate-determining reductive elimination completes the catalytic cycle. The addition proceeds with retention of configuration.410 An alternative mechanism, the insertion of alkene into the metal-silicon bond (route b), was later suggested to account for some side reactions (alkene reduction, vinyl substitution).411-414... 

The unique antagonistic features of the (butadiene)zirconocene isomers 3a/5a have been used as a probe for the elucidation of organometaUic reaction mechanisms. In some cases it was possible to distinguish between mechanistic alternatives by simply allowing the isomeric substrates 3 and 5 to compete for a reagent. An example is as follows. Transition metal-induced C—C coupling between a conjugated diene and an olefin can occur by two basic ly different types of reaction sequence. Either a new C— C bond can be formed by olefin insertion into a metal-carbon bond of a (o--allyl)M-type intermediate (24) (95), or, alternatively, the alkene may... 

The cis ligand alkene then inserts into the M—H bond (hydrometallation) of (6) to afford an alkyl metal complex (7), which interacts with another molecule of alkene to induce the coupling of the alkyl and silyl ligands and to regenerate the active catalyst (5). This last reaction is the rate-determining step of the catalytic cycle. 

The intermediates generated from transition metal induced cleavage of methylenecyclopropanes are, in contrast to other trimethylenemethane equivalents, capable of undergoing [3 -I- 2] cycloaddition not only with activated, i.e. electron-deficient alkenes, but also with unsaturated, even nonstrained, hydrocarbons. Some of the reactions summarized in this section are also briefly discussed in Sections 2.2.2.2. and 2.2.2.3.I. because homodi- and homooligomerization of the methylenecyclopropane, as well as [2-1-2] cycloadditions, may efficiently proceed as side reactions of the [3 -f 2] cycloadditions. 

Cyclopropenes may undergo [2 + 2] cycloaddition to themselves, to form dimers, or to other alkenes the reactions may be brought about by photolysis, metal catalysis, Lewis acid catalysis, or simply by heat. Due to the difficulties that can be encountered in completely purifying some cyclopropenes, it is possible that some thermal reactions are actually initiated by small quantities of impurity. The following sections include those reactions occurring under thermal and Lewis acid catalysis, Those brought about by photolysis are covered in Section Ll.6.2.3.1,1. metal-induced reactions are dealt with in Section 1.1.6.3.3. 

Manceron and Andrews have investigated the alkali metal-induced intermolecular hydrogen transfer to the C=C bond of an alkyne to form an alkene in solid argon at 15 K (equation 111). 

Oxidative coupling is a reaction like that shown in Eq. 6.4S in which the metal induces a coupling reaction between two alkene ligands to give a me-talacycle. The formal oxidation state of the metal increases by two units ... 

Carbopalladation Reactions. The transition metal-induced addition of carbon nucleophiles to unactivated alkenes is an attractive area of research. Although the addition of stabilized carbon nucleophiles or an alkoxycarbonyl group across the C=C bond of an unactivated olefin was initially achieved in the presence of stoichiometric amount of Pd salts, such as Pd(OAc)2 or PdCl2(CH3CN)2, more recently this reaction has been achieved catalytically. 

Scheeren et al. reported the first enantioselective metal-catalyzed 1,3-dipolar cycloaddition reaction of nitrones with alkenes in 1994 [26]. Their approach involved C,N-diphenylnitrone la and ketene acetals 2, in the presence of the amino acid-derived oxazaborolidinones 3 as the catalyst (Scheme 6.8). This type of boron catalyst has been used successfully for asymmetric Diels-Alder reactions [27, 28]. In this reaction the nitrone is activated, according to the inverse electron-demand, for a 1,3-dipolar cycloaddition with the electron-rich alkene. The reaction is thus controlled by the LUMO inone-HOMOaikene interaction. They found that coordination of the nitrone to the boron Lewis acid strongly accelerated the 1,3-dipolar cycloaddition reaction with ketene acetals. The reactions of la with 2a,b, catalyzed by 20 mol% of oxazaborolidinones such as 3a,b were carried out at -78 °C. In some reactions fair enantioselectivities were induced by the catalysts, thus, 4a was obtained with an optical purity of 74% ee, however, in a low yield. The reaction involving 2b gave the C-3, C-4-cis isomer 4b as the only diastereomer of the product with 62% ee. 

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

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