Oxidative addition of silanes

The oxidative addition of the Si-H bond in silanes to transition metal complexes draws many parallels with tlie oxidative addition of dihydrogen. It occurs readily and is a step in the most common mechanism of the hydrosilylation of olefins, alkynes, and ketones by late 

In other cases, low-valent metal centers react with silanes to form -n -silane complexes. 

This process occurs with less electron-rich metal centers and with metal centers that are less able to e q)and their coordination number. For example, photolysis of (MeCp)Mn(CO)3 in the presence of a silane leads to dissociation of one CO ligand and coordination of the silane to form (MeCp)Mn(CO)j( q -HSiPh3), as shown in Equation 6.17.  

The mechaiiism of the oxidative addition of silanes has been studied in detail. Like the addition of the reaction of silanes occurs by donation of electron density in the H-Si CT-orbital into a metal orbital of u-symmetry and back-donation of electron density from a metal orbital of tr-symmetry into the H-Si a -orbital. Reactions of optically active silanes occur with retention of configuration in the product, the metal occupies the position that the hydrogen occupied in the starting silane (Equation 6.18). Moreover, the oxidative addition has been shown by femtosecond flash photolytic and infrared spectroscopic methods to occur with prior formation of a silane complex, perhaps in parallel with the formation of complexes from slipping of the Cp ring or those resulting from coordination of the triethylsilane through ethyl C-H bonds. 

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The oxidative addition of silanes (with silicon-hydrogen bonds) to coordinatively unsaturated metal complexes is one of the most elegant methods for the formation of metal-silicon bonds. Under this heading normally reactions are considered which yield stable silyl metal hydrides. However, in some cases the oxidative addition is accompanied by a subsequent reductive elimination of, e.g., hydrogen, and only the products of the elimination step can be isolated. Such reactions are considered in this section as well. 

When hydrosilane is used instead of hydrogen in the Rh-catalysed carbonylation of alkynes, the silylformylation of alkynes occurs to give the / -silyl-a,/ -unsaturated aldehydes 71 [38,39]. Oxidative addition of silane to Rh generates the silylrhodium hydride 69. It should be noticed that insertion of alkyne to the Rh-SiR.3 bond, but not... 

Hu S, Farrell GJ, Cook C, Johnston R, Butkey TJ. Rearrangement of rf-CpMn(CO)2(HSiEt3) a missing step in the energy surface for the oxidative addition of silane to CpMn(CO)2 (heptane). Organometallics 1994 13(11) 4127—4128. 

A typical method of formation of a transition metal-silicon bond is the oxidative addition of silane derivatives, involving either Si-H or Si-Si bond cleavage, to a low valent, coordinatively unsaturated transition metal complex. Various kinds of transition metal silyl or silylene complexes have so far been synthesised via this method. However, in contrast to the rich chemistry of the late transition metal complexes with an M-Si bond, a limited number of syntheses of Group 6 metal complexes with an M-Si bond have been reported. ... 

All of the steps in this mechanism have precedent in stoichiometric reaction chemistry. It is assumed that alkene adds before silane, but these steps may be reversed in some cases. The reversibility of two steps in the mechanism accounts for both the observed isotopic exchange between the alkene and silane, and the accompanying isomerization of alkenes. The fact that hydrosilylation occurs with retention of configuration at silicon266 is consistent with this mechanism, since oxidative addition of silane to a metal center is known to proceed in a cis manner and with retention199,267. The product-releasing step (elimination from an alkyl/silyl complex) has recently been observed in the thermal decomposition of an iron alkyl/silyl derivative (equation 100)268. Hydrosilylation as catalyzed by Co2(CO)8 appears to proceed by a somewhat different pathway8,35,262. 

Careful analysis of the crystal structures of type of Cp(CO)2Mn(H-SiR3) complexes indicates that the Mn-Si distance becomes considerably longer as the oxidative addition proceeds while the Mn-H distance becomes a little longer and r(Si-H) remains about same [81]. Based on the structural analysis a reaction trajectory for oxidative addition of silane to a metal was proposed. The H atom of the Si-H unit approaches Mn, and the Si-H unit then pivots, increasing the Mn-Si interaction. This is similar to the trajectory for the interaction of an agostic C-H bond with a metal atom, as mentioned earlier in this chapter, and is consistent with theoretical work [83]. 

The classic Chalk-Harrod mechanism is shown in Scheme 16.6, and the modified Chalk-Harrod mechanism " is shown in Scheme 16.7. In the Chalk-Harrod mechanism, oxidative addition of silane occurs to form a silyl hydride complex. Migratory... 

In the Chalk-Harrod mechanism, oxidative addition of silane occurs to a Pt(0) complex. The ancillary ligands on the active catalyst when reactions are initiated with "Speier s catalyst" are unknown, but are likely to be the olefin substrate. The ancillary ligands on the active form of Karstedt s catalyst or the related PtfCOD) complex could be the original diene ligands or the olefin substrate. Details on the mechanism of oxidative additions of silanes are provided in Chapter 6. In brief, this reaction occurs by coordination of silane to an open site to form a silane a-complex, followed by cleavage of the Si-H bond to form a silyl hydride species. 

It is now accepted that alkene hydrosilylation catalyzed by several systems occur by the modified Chalk-Harrod medTanism. Wrighton s studies on tlie insertion of ethylene into the M-Si bond of (CO) CoSiMe3 led to die conclusion that this photocatalytic process occurs by the mechanism in Scheme 16.8. Conversion of the p-silylalkyl product of insertion to the free organosilane is thought to occur by a sequence of oxidative addition of silane... 

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