Metathesis, with hydrogenation

Reaction of dpp-bian with Mg in THF for 30 min reflux gives complex 87 (Ar = 2,6-diisopropylphenyl) which undergoes oxidative addition via m-bond metathesis with PhC=CH to give the black alkynyl amido complex 88. The insertion reaction of 88 with Ph2CO in EtzO yields complex 89. Unexpectedly, hydrogen abstraction to give the radical anion occurs simultaneously with ketone insertion.268... 

Since group 4 derived species are of particular interest as catalysts for olefin polymerization and epoxidation reactions, the thermal stability of surface metal-alkyl species, as weU as their reactivity towards water, alcohols and water, deserve some attention. On the other hand, mono(siloxy) metaUiydrocarbyl species can be converted into bis- or tris(siloxy)metal hydrides by reaction with hydrogen [16, 41, 46-48]. Such species are less susceptible to leaching and can be used as pre-catalysts for the hydrogenolysis of C-C bonds, alkane metathesis and, eventually, for epoxidation and other reactions. 

Niobia-supported MTO has been prepared either by the deposition of sublimed MTO onto the support, or by the impregnation of the support by a solution of MTO, and has been well characterised [54]. A large variety of oxidation reactions were efficiently performed with niobia-supported MTO, such as olefin metathesis catalysis [53,54], reactions of ethyl diazoacetate, heteroatom oxidation (amine and phosphine oxidations) and olefin epoxidation with hydrogen peroxide [55] (Scheme 13). 

Ring-closing alkyne metathesis (RCAM) leads to alkyne macrocydes in excellent yields [36]. When combined with hydrogenation using a Lindlar-type catalyst, it is a convenient pathway to macrocydic Z-cydoalkenes. Fiirstner and coworkers utilized this strategy for numerous total syntheses of natural products [37], epothilone A being one of them [38]. 

Figure S.9 Approaches to tetraloop compounds with different size and structure of the loops via heterodimerization with tetra-tosylurea 4 followed by olefin metathesis and hydrogenation.

Ring-Closing Metathesis in Tandem with Hydrogenation. 99... 

A similar approach was performed using the alkene metathesis catalyst RuCl2(= CHPh)(PCy3)2 [76]. It was known that this complex reacts with hydrogen in THF to give ruthenium hydride complexes capable of catalytic alkene hydrogenation [77] (Scheme 32). 

The predominant chain termination mechanisms are (i) /3-hydride elimination see Polymerization), giving a chain with a terminal alkene (ii) reaction with hydrogen, frequently used to control chain length (iii) chain transfer to monomer by <7-bond metathesis see a-Bond Metathesis)-, and (iv) chain transfer to cocatalyst (equation 4). 

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