Hydride species, early studies

Industrially this diene is made the same way as ethylidenenorbomene from butadiene and ethene, but now isomerisation to 2,4-hexadiene should be prevented as the polymerisation should concern the terminal alkene only. In both systems nickel or titanium hydride species react with the more reactive diene first, then undergo ethene insertion followed by (3-hydride elimination. Both diene products are useful as the diene component in EPDM rubbers (ethene, propene, diene). The nickel hydride chemistry with butadiene represents one of the early examples of organometallic reactions studied in great detail [22] (Figure 9.14). 

Early Studies on Rhenium-Carbonyl-Hydride Clusters and Related Species... 

The first site of protonation in a dinitrogen complex, based on mechanistic studies of complexes, must be the dinitrogen itself, yielding the diazenido species (N2H). If protonation occurs at the metal then reaction proceeds no further, or results in the loss of coordinated dinitro-gen. The formal insertion of dinitrogen into a metal-hydride bond (a popular proposal in the early chemical nitrogen-fixation literature) is unknown. 

The importance of the OH group in catalysis is well established, and much n.m.r. work has been directed towards the study of the distribution and types of OH groups on a number of surfaces, and the relevance to catalytic activity. Early work on this subject has been reviewed by Duncan and Dybowski.1 N.m.r. studies of H2 species in clays and metal hydrides have been reviewed by Derbyshire.2... 

It should be emphasized that the results of these kinetic studies do not represent only the TBAF-promoted coupling of alkenylsilanols. Analyzed in the context of the spectroscopic studies and reported observations - which reveal that sile-tanes, silanols, and fluorosilanes, silyl hydrides, and heterocyclic silanes all form related species when mixed with TBAF - the mechanism deduced by the kinetic studies likely represents the mechanism of cross-coupling of all of these species. These mechanistic results are therefore relevant to a significant body of work, encompassing everything from the early fiuorosilane cross-coupling systems to the more recently developed TBAF-promoted coupling of pyridyl-, thienyl-, and benzylsilanes. 

---