Elimination from metal alkyl complexes

P-Eliminations (Equation 10.1) are the most common type of elimination reaction from transition metal complexes, and p-hydrogen eliminations from metal-alkyl complexes are the most common type of p-elimination reactions. p-Hydrogen elimination from aUcoxo and amido complexes has also been observed in a few cases. p-Alkyl elimination, p-aryl elimination, p-aUcoxide elimination, and p-chloride elimination have also been observed and have been studied carefully because of their importance as side reactions in catalytic chemistry. Although P-hydrogen elimination from metal-alkyl complexes occurs almost exclusively by migratory de-insertion pathways, p-hydrogen ehmmation from alkoxides has been shown to occur by several different pathways. 

The reductive elimination from metal-alkyl-hydride complexes L MR(H) (M = Ir, R = Cy or M = Rh, R = Ph) giving the alkane or arene occurs with inverse kinetic isotope effects which were taken into account by a preequilibrium with the a-alkane or r -arene complex. These reactions represent the microscopic reverse of oxidative addition of an alkane or arene C-Fl bond on electron-rich 16-electron metal centers... 

One of the most defining characteristics of the late metal a-diimine polymerization systems is the uniquely branched polyolefins that they afford. This arises from facile p-hydride elimination that late transition metal alkyl complexes undergo. The characteristics of the isomerization process have been the subject of much investigation, particularly with the more easily studied Pd(II) a-diimine system. The process is initiated by P-hydride elimination from the unsaturated alkyl agostic complex 1.17, followed by hydride reinsertion into olefin hydride intermediate 1.18 in a non-regioselective manner (Scheme 5). In doing so, the metal center may migrate... 

A further possibility of inducing the elimination of alkanes from transition metal alkyl complexes is photolysis [398,424-427]. Two examples of photolytic a-eliminations leading to non-heteroatom-substituted alkylidene complexes are shown in Figure 3.7. 

The carbene complexes can also be formed by direct oxidative addition of ze-rovalent metal to an ionic liquid. The oxidative addition of a C-H bond has been demonstrated by heating [MMIM]BF4 with Pt(PPh3)4 in THF, resulting in the formation of a stable cationic platinum carbene complex (Scheme 15) (189). An effective method to protect this carbene-metal-alkyl complex from reductive elimination is to perform the reaction with an imidazolium salt as a solvent. 

From the practical point of view //-hydride elimination might also be an obstacle. In reactions that involve metal-alkyl complexes as early intermediates one has to block //-elimination to increase the lifetime of the intermediate and enable subsequent transformations on the complex. A reaction, which proved elusive partially for this very reason, is the coupling of alkyl halides. A set of conditions, which allowed for the Negishi coupling of primary alkyl halides and even tosylates with alkyzinc halides is shown in Equation 1.5.20 The recent work of Fu and others showed that the careful... 

Figure 3.12 Generic equation for [S-hydride elimination from a transition metal-alkyl complex.

The resulting extraordinary stability of NHC-metal complexes has been utilized in many challenging applications. However, an increasing number of publications report that the metal-carbene bond is not inert [30-38]. For example, the migratory insertion of an NHC into a ruthenium-carbon double bond [30], the reductive elimination of alkylimidazolium salts from NHC alkyl complexes [37] or the ligand substitution of NHC ligands by phosphines [36,38] was described. In addition, the formation of palladium black is frequently observed in applications of palladium NHC complexes, also pointing at decomposition pathways. 

Although it is now almost fifty years since Speier and his colleagues first announced the chloroplatinic acid-catalyzed hydrosilation of olefins, we are still far from complete control of the chemistry. A particular problem is the suppression of double bond migration. A solution of this problem will require a more detailed understanding of the factors affecting the relative rates of P-hydride elimination from an alkyl group and of the reductive elimination of Si-H from a platinum silyl hydride complex. Another factor which is poorly understood is suppression of the irreversible reduction of the platinum catalyst to Pt° metal. Both of these problems can greatly increase costs of production of certain products. 

This mechanism is quite general for this substitution reaction in transition metal hydride-carbonyl complexes [52]. It is also known for intramolecular oxidative addition of a C-H bond [53], heterobimetallic elimination of methane [54], insertion of olefins [55], silylenes [56], and CO [57] into M-H bonds, extmsion of CO from metal-formyl complexes [11] and coenzyme B12- dependent rearrangements [58]. Likewise, the reduction of alkyl halides by metal hydrides often proceeds according to the ATC mechanism with both H-atom and halogen-atom transfer in the propagation steps [4, 53]. 

Oxidative additions are a special class of insertion reactions. In addition to the categories mentioned in Section 10, which covers this topic, insertions of alkylidenes, silylenes, etc., into M-H bonds fall into an ambiguous domain they are insertion reactions of the unsaturated species into the M-H bond, yet oxidative additions at the C, Si, etc., atom. A similar ambiguity exists regarding the reverse reactions, namely /i-hydride and a-hydride eliminations from element-alkyls compounds to yield hy-drido-olefin and hydrido-alkylidene complexes, respectively. The former reaction is a reverse insertion if the product is viewed as an olefin complex, but an oxidative addition if it is viewed as a three-membered metallocycle. The latter reaction is a reverse insertion if the alkylidene is viewed as neutral, but an oxidative addition of a C-H bond to the metal centre. The tautomerization of phosphorous acid and of dialkylphosphites ... 

Experimental evidence and computational analysis point to a mechanism in which the alkene (or alkyne) carbons and the M-H bond must be nearly coplanar to react. Once the metal alkene complex has achieved such geometry, 1,2-insertion can occur. During insertion, the reactant proceeds through a four-center transition state. 14The reaction involves simultaneous breakage of the M-H and C-C n bonds, as well as the formation of an M-C a bond and a C-H bond at the 2-position of the alkene (or alkyne). The result is a linear compound, L M(CH2CH3), in the case of ethene insertion. The reverse reaction, (3-elimination, follows the same pathway starting from a metal-alkyl complex with an open coordination site. 

The reverse of alkene insertion. 0 elimination, represents the chief pathway for decomposition of a transition metal alkyl complex.tEq. 15.451. The process begins with deinsertion of the alkyl ligattd to yield a metal hydrido alkene complex, which then eliminates the alkene.The decomposition process cun be thwarted by designing alkyl complexes in which 0 elimination is not possible either because the ligands have no hydrogens on carbon atoms 0 to the metal [e.g.. PhCH,. CH, or (CH,),CCH,J or the 0 hydrogen is too far from the mclal to allow deinsertion to occur (e.g.. C=CH). Note also that 0 elimination cannot lake place unless there is a vacant site for... 

All of the steps in this mechanism have precedent in stoichiometric reaction chemistry. It is assumed that alkene adds before silane, but these steps may be reversed in some cases. The reversibility of two steps in the mechanism accounts for both the observed isotopic exchange between the alkene and silane, and the accompanying isomerization of alkenes. The fact that hydrosilylation occurs with retention of configuration at silicon266 is consistent with this mechanism, since oxidative addition of silane to a metal center is known to proceed in a cis manner and with retention199,267. The product-releasing step (elimination from an alkyl/silyl complex) has recently been observed in the thermal decomposition of an iron alkyl/silyl derivative (equation 100)268. Hydrosilylation as catalyzed by Co2(CO)8 appears to proceed by a somewhat different pathway8,35,262. 

Kinetic studies of C-H reductive elimination from the alkyl-hydrido complexes bearing a d metal center have been reported [80-82], Similarly to the reactions of d metal complexes, the reductive elimination proceeds via an alkane-coordinate intermediate, as supported by the observation of an inverse kinetic isotope effect. Representative data are as follows = 0.75 for Cp2W(H)(Me) in... 

Transition-metal-silyl complexes are also formed by the reactions of metal-alkyl complexes with silanes to form free alkane and a metal-silyl complex. Two examples are shown in Equations 4.114 and 4.115. ° The synthesis of silyl complexes by this method has been accomplished with both early and late transition metal complexes. The formation of metal-silyl complexes from late-metal-alkyl complexes resembles the hydrogenolysis of metal-alkyl complexes to form metal hydrides and an alkane. The mechanisms of these reactions are discussed in Chapter 6. In brief, these reactions with late transition metal complexes to form silyl complexes typically occur by a sequence of oxidative addition of the silane, followed by reductive elimination of alkane. An example of this is shown in the coupling of 1,2-bis-dimethylsilyl benzene with a dimethyl platinum(II) complex (Equation 4.114). Similar reactions occur with d° early metal complexes by a a-bond metathesis process that avoids these redox events. For example, the reaction of Cp ScPh with MesSiH, has been shown to proceed through this pathway (Equation 4.115). ... 

Cleavage of the C-C bond to the 3-carbon of late-metal-alkyl complexes is slower and less common than that of d early metal complexes. However, a few examples of 3-aU yl elimination from late metal complexes are known. The reversible intramolecular insertion of an olefin into the platinum-alkyl complex described in Chapter 9 involves an early example of 3-alkyl elimination from a late-metal-alkyl complex. In addition, mild 3-aIkyl elimination has been reported to occur from a ruthenacylobutane complex. In this case the product is a stable alkyl allyl species. - ... 

Manganese.—Elimination of transition-metal hydride from metal alkyls and addition of metal hydrides to alkenes are usually considered to be cA-processes. Since acylmanganese compounds undergo stereospecific reversible decarbonylation, thermal decomposition of (eryrAro-2,3-dimethylpentanoyl)(pentacarbonyl)manga-nese(i) should allow the determination of the stereochemistry of elimination of [MnH(CO)8] (Scheme 4). However, both the erythro and a mixture of the erythro and threo acyl complexes decompose thermally to give the same mixture of cis- and trans-3-methylpent-2-ene and 3-methylpent-l-ene under conditions which do not isomerize these alkenes. It is suggested that the mechanism involves interconversion of... 

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