Silyl exchange

Pure cis-71 could be isolated in 65% yield and was characterized by means of x-ray structure analysis (Fe-Si 2.270(1)/2.272(1) A). This ds-conformer is exclusively formed. The formation of 71 requires a photoinduced silyl exchange followed by a decarbonylation reaction and further steps of an oxidative addition to the metal [176]. 

In cycle B, complex 118 is derived from 113 by oxidative addition of R3SiH and then interacts directly with alkyne to form 119 and then 120. Insertion of GO into the Rh-vinyl carbon bond in 120 leads to 117. The rest of the cycle is shared with A. It is difficult to conclude unequivocally which of these cycles dominates silylformylation, because such observable steps as formation of 11 and silyl exchange discussed in Scheme 5 require far longer time than that sufficient for silylformylation. 

Bertrand and coworkers showed, however, that apparent 1,2-silyl migrations in aromatic carbenes occurred by intermolecular silyl exchange (equation 10)38. 

Although the silyl exchange should be typically intramolecular according to this associated mechanism, significant exchange between growing chains occurs during GTP. In order to account for this experimental observation. Quirk and coworkers proposed a dissociative mechanism, which relies on the dissociation of the pentacoordinat siliconate (49), followed by the Michael addition of the released enolate (50) onto the monomer... 

The phosphine-alane is expected to displace PPh at Ni, but no reaction takes place in the absence of substrate due to strong intramolecular P Al interactions. Lewis bases such a triethylamine are able to split the head-to-tail phosphine-alane dimer and consistently, further increase catalytic activity. The precise role of the Lewis acid moiety in the catalytic cycle remains unknown. It is supposed to interact with the methyl group at Ni and to facilitate methyl/silyl exchange. The key active species 36a could not be characterized, but its Lewis base adduct 36b was identified by NMR (Figure 19). 

Fig. 2. Possible reaction profile of the dyotropic silyl-silyl exchange.

The allyl-silyl exchange reactions are clearly more complicated than the 100% intramolecular rearrangements of silyl silylmethyl ethers. Nevertheless, the present author believes that they are closely related. It is rather striking that both processes (in the fluorenyl series) have nearly identical activation parameters. Solvent effects are small in both cases. Thus, it appears reasonable to assume similar transition states, namely rate-determining coordination between silicon and oxygen. For the fluorenyl substrate a mechanistic dichotomy evolves (Scheme 3). The... 

A series of alkyl silylmethyl ethers 36 were synthesized (17, 39, 40) in the expectation that thermolysis should afford benzyl-silyl exchange products 38 as shown in Eq. (8). 

Rate Data for Benzyl-Silyl Exchange Reactions... 

The title compound was prepared in order to test the effect of replacing the stationary carbon atom in allyl silylmethyl ethers (Section III) by nitrogen. Thermolysis at 170°C leads within several hours to nearly quantitative formation of 1,5-hexadiene and hexamethyldisiloxane 48). Although no mechanistic studies have been initiated, allyl-silyl exchange followed by a-elimination according to Scheme 4 is one of several possibilities. 

Scheme 4. Possible decomposition mechanism involving allyl-silyl exchange.

Rate Data for Dyotropic Silyl-Silyl Exchange in Hydroxylamines... 

Since the activation parameters are quite similar to those of the silyl-silyl exchange in silyl silylmethyl ethers (Section II), the authors suggest that the rearrangements in the hydroxylamine series may also occur via a dyotropic mechanism (51). Furthermore, insensitivity to solvent polarity appears to rule out the intermediacy of ions or zwitterions. The possibility of an asymmetric transition state was discussed (51). 

Although the above evidence points to a dyotropic exchange, it might be mentioned that the thermolysis of 7V-phenyl-iV,0-bis(trimethylsilyl)hydroxylamine (43) at 100°C (16 hours) results in phenylnitrene formation [Eq. (11)], as shown by trapping experiments (53). This is reminiscent of Scheme 4. Possible silyl-silyl exchange in... 

The erbium compound decomposes upon heating in hexane with formation of a pink solution, which contains the lithium salt of the homoleptic anion [Er CH(SiMe3)2 4] . This complex could not be prepared independently via chlo-ride-silyl exchange from [Li(THF)4][Er CH(SiMe3)2 3C1] and excess LiCH(SiMe3)2. 

The formation of methysilanes implies that the formation of the bis-silyl complex from the hydrogenosilane is not a simple methyl/silyl exchange but instead a more complex process. The experimental findings suggest the mechanism depicted in Scheme 1 (the products obtained are indicated in bold). 

The ability of NHC to activate silylated nucleophiles led to further investigations and useful procedures. Song reported the formation of silyl enol ethers by an NHC-catalyzed silyl exchange reaction that transferred a silyl group from a silyl ketene acetal to a ketone. When aldehydes, rather than ketones, were used as substrates the NHC catalysts promoted a Mukaiyma aldol reaction to give p-hydroxy esters and ketones in good yields. ... 

Hlina J, Mechtler C, Wagner H, Baumgartner J, Marschner C (2009) Multiple silyl exchange reactions a way to spirooligosilanes. Organometallics 28 4065... 

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