A bond metathesis reaction

A careful investigation of the reaction kinetics and detailed trapping experiments allow the conclusion that in this case a a-bond metathesis reaction mechanism applies. The polymerization reaction of PhSiH3 by CpCp Hf(SiH2Ph)Cl has been monitored by H-NMR spectroscopy. The data k(75 °C) = 1.1(1) x 10-4 M 1 s AH = 19.5(2) kcal mol" AS = -21(l)euandkH/fcD = 2.9(2) (75 °C) are in good agreement with the proposed mechanism with a metallacycle as transition state [164],... 

Third, in analogy with the discussion presented above, oxidative addition represented by Pattern 11 may not be readily observable, and oxidative addition must also proceed mostly through more complex processes, such as that shown in Scheme 1.6 [31], More readily observable are various types of a-bond metathesis reactions of d° Cp2Zr(IV) species (Pattern 13). 

Scheme 1.69. Various examples of a-bond metathesis reactions of three-membered zirconacycles.

Scheme 1.71. a-Bond metathesis reaction of zirconacyclopentanes with alkyllithiums. 

In this equilibrium (a a bond metathesis reaction ), the scandium-hydrogen bond in to(pentamethylcyclopentadienyl)scandium hydride is replaced by a scandium-carbon bond in phenylto(pentamethylcyclopendienyl) scandium... 

Effectively, this is another example of the addition of a functional aromatic compound to an alkene, as the Murai reaction, but the mechanism is different. Alkyl substituted pyridine derivatives are interesting molecules for pharmaceutical applications. The a-bond metathesis reaction is typical of early transition metal complexes as we have learnt in Chapter 2. 

Neutral dimethyl metallocenes of group 4, Cp2MMe2, are effective catalysts for the dehydropolymerization of primary silanes, via a a bond metathesis pathway.234 Given the topological similarities between the four centered transition states for olefin insertion into M-C bonds and a bond metathesis reactions, it was postulated that activation of these metallocenes by B(C6F5)3 might enhance dehydropolymerization of silanes in... 

Chain transfer reactions in homogeneous olefin polymerisation systems with metallocene catalysts, which terminate individual polymer chains, in some instances can also terminate the polymerisation kinetic chain. For example, chain transfer with the monomer in propylene oligomerisation or polymerisation, which involves a bond metathesis reactions between the Mt-C species of the growing polymer chain and the C H species of methyl [scheme (45)] or vinyl [scheme (46)] groups in the monomer, gives rise to temporally inactive metal allyl or metal-vinyl species respectively [177, 241, 264] ... 

The gas-phase reactions of the cationic Irm complexes follow a previously unreported mechanism for their observed a-bond metathesis reactions. Previous discussions had considered a two-step mechanism involving intermolecular oxidative addition of either [Cp Ir(PMe3)(CH3)]+ or [CpIr(PMe3)(CH3)]+ to the C-H bond of an alkane or arene producing an Irv intermediate, followed by reductive elimination of methane, or a concerted a-bond metathesis reaction sim-... 

An overview of recent work from the Takats laboratory deals with divalent Ln(Tp Bu Me) complexes and preparation, molecular structure, NMR characteristics and reactivity of [Yb( x-H)(TptBu,Me)]2. Reactions of this compound with Lewis bases and Lewis acids, such as the perfluoroaryl boranes, a-bond metathesis reactions generally involving acidic H-X bonds, but also an example of C-Si bond cleavage, and insertion chemistry, primarily involving alkynes have been described. The molecular structures of representative complexes have been determined by X-ray crystallography.110... 

Bergman et al. have suggested [39a,b] two mechanisms for the a-bond metathesis reaction by cationic iridium (III) complexes (see review [39c], i.e., a two-step addition-elimination mechanism ... 

In general, a-bond metathesis reactions have been studied for complexes of... 

The Energies (Relative to Reactants, in Kilocalories per Mole) of a-Bond Metathesis Reactions Calculated at the DFT Level by Ziegler et al. 

Scheme 13 Generalized mechanism for the alkene insertion and a-bond metathesis reactions catalyzed by lanthanocenes...

The redistribution reactions mentioned above indicate that the opening of a coordination site not only promotes oxidative addition and possibly a-bond metathesis reactions, but — difG ndy from carbon compounds — also induces migrations of silicon substituents with the concomitant formation of silylene complex intermediates. It was pointed out by Tilley and co-workers that the transfer of a silyl substituent from silicon to platinum is easier in three- than in four-coordinated... 

Apparent oxidative addition-reductive elimination sequences can in fact be a-bond metathesis reactions. These are best recognized for d early metal complexes such as Cp2ZrRCl or WMe because oxidative addition is forbidden in these cases. (The oxidative addition product would unambiguously exceed the maximum permitted oxidation state for the metal.) In a reaction of such a complex with H2 (Eq. 6.42), the metal therefore cannot follow mechanism a of Eq. 6.43. Instead a concerted process (path b of Eq. 6.43) is believed to operate. Path b probably goes via formation of an intermediate H2 complex... 

An alkane RH can also be activated by addition across a metal heteioatom bond, as shown by Wolczanski Sadow and HUey have seen a numb of a-bond metathesis reactions of methane with Cp2 ScR (R = alkyl). 

Once the pre-catalyst formed, this species reacts with a hydrosilane via a a bond metathesis reaction to yield the active copper hydride species. As shown in Scheme 3, the formation of a cr bond between the copper and hydrogen atoms occurs through transmetalation [46, 72, 110] passing by a 4-center transition state. 

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