U-bond metathesis

The added primary amine may facilitate the cleavage of the Ti-N bond of the key intermediate 107 through the coordination to the titanium center followed by u-bond metathesis. Such an intramolecular exchange process is expected to be facile. The amine exchange product is 106, which can then be rapidly converted to 105 and the corresponding silylated amine to complete the catalytic cycle. 

The subsequent decomposition of the intermediate metallacycle results in the formation of a new olefin. The u-bond metathesis [43] in which a a bond interacts with a transition-metal complex and is broken... 

In the complex a2, the olefin has a choice of inserting into the Rh-B bond or the Rh-H bond. The latter process (I.l.A, a2 a5 a8) requires higher barriers than the former (I.l.B) discussed above. The insertion into the Rh-B bond (mechanism (II.2)) is preferred to the Rh-H bond (mechanism (II. 1)) also in the u-bond metathesis pathway. 

Because lanthanide catalysts do not have the appropriate accessible oxidation states for reaction by a sequence of oxidative addition and reductiye elimination, an alternative mechanism is followed by these catalysts. These hydrosilylations are thought to occur by a version of a Qhalk-Harrod mechanism in which the C-Si bond is formed by u-bond metathesis instead of reductive elimination. As shown in Scheme 16.9, conversion of the alkyl precursor to a metal hydride is followed by insertion of the alkene to form a yttrium alkyl complex. Reaction of this alkyl complex with the silane generates the alkylsilane and the metal hydride in a u-bond metathesis process. 

The essential aspect of the u-bond metathesis mechanism is the concerted exchange of a metal-ligand a-bond with one of an incoming substrate where the reaction proceeds via a [2o-+2o-] cycloaddition, as illustrated in the transitions state in Figure 14.6. The key concept is that CT-bond metathesis is a one-step reaction with two u-bonds breaking (M-C and C -H) and two u-bonds forming (M-C and C-H). Therefore, there is no change involved in the oxidation state of the metal center. This reaction mechanism has been proposed for the d° and d°P metal compounds, and has been recently reviewed for transition metals. [58,59]... 

For such a synergistic nonadditive H-migration process, we refer to this reaction mechanism as agostic interactions assisted u-bond metathesis. Given its concerted nature, this... 

Mainly Th and U have been studied. The complexes [MCp 2R2] are the most common ones, when R lacks 3-H atoms (otherwise, they decompose by 3-elimination). They undergo hydro-genolysis by a-bond metathesis and CO insertion driven by the oxophilicity of the metal. They are efficient olefin hydrogenation and polymerization catalysts. 

A natural question to ask is the cause of the reactivity difference between the thorium and uranium complexes. We will examine the electronic properties of the reactants, products, and transitions in order to understand the similarities and differences. The natural populations of 6d and 5f for all species are depicted in Figure 14.10 the 5f population at the metal center roughly keeps consistent along the reaction pathways. See color plate section. The higher 5f populations for U reflect the extra two unpaired electrons at the uranium center, which does not participate in the reaction directly. Thus, there is no oxidation state of the metal center consistent with the proposed a-bond metathesis and experimental observations. 

Nb and Ta alkyhdenes readily undergo metathesis reactions (see Alkene Metathesis) with carbonyl functionalities or imines to form alkene and M=0 or M=NR bonds. Reactions 0f(Me3SiCH2)(ArN=)Ta(At-CHSiMe3)(/u.-Ji Ji -PT2-tacn)Li (PT2-tacn = anionic triazacyclononane, 29) with Ph2C=0 give [(Me3SiCH2)(RN=)Ta(/u-0)(/u.-J7 J7 -PT2-tacn)Li]2. ... 

The metathesis of metal-metal triple bonds with carbon-carbon triple bonds directly connects the chemistry of inorganic, organometallic, and organic multiple bonds. This reaction is synthetically very useful. Krouse and Schrock prepared the series of /t-a,[Pg.256]

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