Reactions of Silylene Complexes

A second type of reactive metal-silicon bond involves multiple bonding, as might exist in a silylene complex, LnM=SiR2. The synthesis of isolable silylene complexes has led to the observation of new silicon-based reactivity patterns redistribution at silicon occurs via bi-molecular reactions of silylene complexes with osmium silylene complexes, reactions have been observed that mimic proposed transformations in the Direct Process. And, very recently, ruthenium silylene complexes have been reported to be catalytically active in hydrosilylation reactions. 

The reactivity of metal-silylene complexes is more limited than the reactivity of carbene complexes. The cationic base-stabilized ruthenium-silylene complex in Equation 13.37 does not react with olefins or alkynes to undergo [2-1-2] addition reactions. However, a related complex did undergo [2-1-2] addition reactions with isocyanates, as shown in Equation 13.46. Other reactions of silylene complexes are distinct from those of carbene complexes or those of other conventional organometallic compounds. For example, the reaction of the silylene hydride with an acetylene generates a p-silylvinylarene complex... 

With the stable donor adducts of silylene complexes, valuable model compounds are now available for reactive intermediates which otherwise cannot be observed directly. For example, a side reaction occurring in the hydrosilation process [61 -63], is the dehydrogenative coupling of silanes to disilanes. This reaction could be explained in terms of a silylene transfer reaction with a coordinated silylene as the key intermediate. 

Data obtained from collision-induced dissociation experiments did not allow for a distinction of the isomeric metal-silene and-silylene species however, structure-specific ion-molecule reactions of the complexes with labeled ethene were used to clearly differentiate between the metal silene and the silylene. In this intriguing study, Jacobson and coworkers also bracketed the bond dissociation energies of the isomeric ions. 

Alcohols are also bases toward silylenes. Complexes of alcohols with Mes2Si and other hindered silylenes are observed spectroscopically when a 3-MP matrix containing the silylene and 5% of 2-propanol or 2-butanol is annealed113. Melting of the matrix results in rapid reaction of the complex to give the O—H insertion product (equation 92). These results imply that formation of an acid-base complex is the probable first step in the reaction of silylenes with alcohols and perhaps with many other reagents259. 

Reactions of silylenes with lone-pair donors such as alcohols and ethers are also quite rapid (ca 10% as fast as addition to rr-bonds). For such reactions, the silylene-donor complexes have been spectroscopically detected (Section V.A). Even when rearrangement to stable products is facile, as in the case of alcohols, the second, nucleophilic, step is believed to have a higher barrier than the electrophilic complex-forming step. 

In some cases, insertion of coordinately unsaturated metals into Si—H bonds has been used for the synthesis of silylene complexes, as in the reaction of equation 101. 

Insertion of a silylene into a P-P single bond was observed for the reaction of silylene 85 with the diphosphaetidin complex 163. The tungsten complex of l,2,4,3-azadiphosphasilol-5-ene, 164, was obtained in high isolated yields (Equation 14) <2005CC4842>. 

In addition to participating in [2 + l]-cycloaddition reactions, divalent reactive intermediates can form ylides in the presence of carbonyl or other Lewis basic functionalities.108 These ylides participate in cycloaddition or other pericyclic reactions to furnish products with dramatically increased complexity. While carbenes (or metal carbenoids) are well known to participate in these pericyclic reactions, silylenes, in contrast, were reported to react with aldehydes or ketones to form cyclic siloxanes109,110 or enoxysilanes.111,112 Reaction of silylene with an a,p-unsaturated ester was known to produce an oxasilacyclopentene.109,113,114 By forming a silver silylenoid reactive intermediate, Woerpel and coworkers enabled involvement of divalent silylenes in pericyclic reactions involving silacarbonyl ylides115 to afford synthetically useful products.82,116,117... 

The reaction of hydrosilylmanganese complexes with ethylenebis(phos-phine) platinum or tetrakis(phosphine)platinum offers a convenient method for the synthesis of silylene-bridged heterometallic complexes [Eq. (18)].45... 

Key questions regarding the reactivity of silylene complexes concern their potential role in metal-catalyzed transformations. For the participation of intermediate silylene complexes in a catalytic cycle, low-energy chemical pathways must exist for the conversion of simple silanes to silylene ligands via activation processes at the metal center. Most probably, a key step in such silylene-forming processes would be the a-migration of a group from silicon to the metal. In search of such a reaction, we prepared the methyl silyl complex shown in Eq. 3. This complex is quite... 

In Sect. 2.3, generation of silylene complexes of transition metals was discussed on the basis of the reactivity of disilanyl-transition-metal complexes. The formation of silylene species in the presence of a catalytic amount of transition metals is also involved in the reactions of hydrodisilanes, which may readily form disilanyl complexes through oxidative addition of the Si-H bond prior to the activation of the Si-Si bond. Platinum-catalyzed disproportionation of hydrodisilanes affords a mixture of oligosilanes 89 up to hexasilane (Eq.45) [83]. The involvement of silylene-platinum intermediate was proven by the formation of a l,4-disila-2,5-cyclohexadiene derivative in the reaction of the hydrodisilane in the presence of diphenylacetylene. 

The chemistry of silylene complexes has become much more developed in the past 10 years as silylene complexes have been found to be important intermediates in reactions such as the dehydrogenative coupling of hydrosilanes, redistribution reactions, and the Direct Process for the production of simple chlorosilanes. For reviews of this work, see References 43, 44 and 294. Several complexes have now been prepared in which there is a silene or disilene ligand (R2C=SiR2 or R2Si=SiR2 species) but, since all these contain several Si-C bonds, they will not be discussed further here. 

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... 

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