Hydrosilylation and dehydrogenative

Hydrosilylation and Dehydrogenative Silylation of Carbon-Carbon Multiple Bonds... 

Scheme 14.1 Organometallic intermediate competitive hydrosilylation and dehydrogenative silylation of alkenes.

Scheme 14.2 Competitive hydrosilylation and dehydrogenative silylation of ethylene and 1-hexene.

Use of a hydrosilane instead of molecular hydrogen in combination with a transition metal has opened the door leading to hydrosilylation and dehydrogenative silylation of unsaturated bonds. Thus, replacement of hydrogen by a hydrosilane is a reasonable strategy to improve serious issues in hydroformylation of alkenes. Along this line, some types of silylcarbonylation were extensively studied by Murai and his co-workers. However, the silicon moiety always attaches to the oxygen atom of incorporated GO molecule (Scheme 1). 

Iridium complexes with O-donor ligand environments, Ir(triso)(ol)2 [triso = tris(diphenyloxophosphoranyl)methanide ol = C2H4, cyclooc-tene), catalyze the hydrosilylation and dehydrogenative silylation of ethylene with triphenylsilane.35 Diphenylmethylsilane can also be used in the... 

A less explored area of transition metal catalysis involves bond formation between Group 14 elements and nitrogen. In direct analogy to previously discussed areas of research, silicon-nitrogen bonds can be formed by dehydrocoupling, hydrosilylation, and dehydrogenative silylation. The compounds produced are valuable for use in organic synthesis or as polymer precursors to silicon nitride ceramics. 

Figure 22-4 Mechanism for the hydrosilylation and dehydrogenative silylation of 1-alkenes catalyzed by cationic palladium complexes Pd represents [(phen)Pd]+. The palladium alkene complex A is the resting state of the cycle. Cycle I denotes the hydrosilylation cycle, Cycle II describes the dehydrogenative silylation reaction.

Direct observation of seven intermediates involved in the hydrosilylation and dehydrogenative silylation of phenylacetylene by an lr"-NHCtype catalyst allowed the elucidation of both the hydro- and dehydrogenative silylation mechanisms (Scheme 5) [ 12], Analysis of fragmentation patterns produced by MS/MS experiments provided insight into the connectivity of species [1] and [111] where the mass assignment itself was ambiguous. Use of the modified substrate 4-aminophenylacetylene, which is visible by ESl-MS, confirmed formation of the products and indicated the dominant mechanism under each set of reaction conditions. 

Schemes. Proposed mechanisms for the hydrosilylation and dehydrogenative silylation of phenylaceylene by an iridium catalyst, m/z values are given for cationic species observed by ESI(+)-MS.

Ruthenium carbonyl is reported to catalyze both the hydrosilylation (addition) and the dehydrogenating hydrosilylation (substitution) of olefins. With trifluoromethylethylene, the ratio of hydrosilylation and dehydrogenating hydrosilylation depends on the nature of die silane HSiEt3 gives specifically (E)-1 -triethylsilyl-2-trifluoromethylethylene... 

Subsequent extensive synthetic and catalytic studies have shown Aat silylative coupling of alkenes with vinyl-substituted silicon compounds proceeds (similarly to the hydrosilylation and dehydrogenative silylation reactions) via active intermediates containing M-Si (silicometallics) and M-H bonds (where M = Ru, Rh, Ir, Co, Fe). The insertion of alkene into M-Si bonds and vinylsilanes into M-H bonds, followed by elimination of vinylsilane and ethene, respectively, are the key steps in this new process [9]. 

The latter reaction was revealed as a side-reaction of the hydrosilylation occurring especially in the presence of Fe and Co-triad complexes and this made the basis for an alternative to the Chalk and Harrod concept of the hydrosilylation known as the Seitz and Wrighton mechanism [8,15]. The key step of this mechanism involves insertion of an alkene into a metal-silicon bond (equation 2). Concurrent insertion of olefin into the M-H and M-Si bonds can potentially lead to a complex containing a-alkyl and a-silylalkyl ligands. Competitive P-H transfer from the two ligands to the metal is a decisive step for alternative hydrosilylation and dehydrogenative silylation [16]... 

Although the Co2(CO)g-catalyzed ring-opening sUylformylation of epoxides gives -siloxyaldehydes, the products tend to undergo furthCT reactions such as formylation, hydrosilylation and dehydrogenative sUylation undo" the reaction conditions (140 °C and 50 atm of In order to suppress such secondary reactions, the use of excess... 

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