Silanes alcoholysis

Silylformylation of 1-alkynes is not affected by the presence of small amounts (2 to 3 molar equiv.) of water or methanol in the reaction mixture. The corresponding products are formed in yields similar to the reaction without these additives.This information implies that the silylformylation of alkyne is not affected by the presence of a protic functional group, such as hydroxy and amino groups in alkyne. In fact, 2-propyn-l-ol 54 smoothly gives 55, in which the hydroxy group remains intact (Equation (15)). (Z)-Selectivity in 55 is drastically improved by the reaction in the absence of EtsN or by use of Rh2(pfb)4 as the catalyst. The Rh complex is known to work as an active catalyst for alcoholysis of hydrosilane." " In particular, since Rh2(pfb)4 shows high activity for alcoholysis of EtsSiH in the absence of it is quite notable that the product derived from silane alcoholysis is not detected at... 

Doyle and co-workers have employed Rh2(pfb)4 as a highly selective catalyst for the room temperature synthesis of silyl ethers from alcohols and triethylsilane.159 The selectivity of the catalyst is demonstrated by reactions of olefinic alcohols, in which hydrosilylation is not competitive with silane alcoholysis when equimolar amounts of silane and alcohol are employed. High yields (>85%) of triethylsilyl ethers are obtained from reactions of alcohols such as benzyl alcohol, 1-octanol, 3-buten-l-ol, cholesterol, and phenol. Tertiary alcohols are not active in this system. 

In 1992, Crabtree and co-workers reported the first nickel catalyst effective for silane alcoholysis.161 The complex, [Ni(tss)]2 (tss = salicylaldehyde thiosemicarbazone), bears a ligand that contains O and N donor groups and a semicarbazide sulfur. Alcoholysis of Et3SiH with ethanol or methanol occurs at room temperature in 50% dimethyl sulfoxide-benzene. However, the reaction is inhibited in the presence of strong donor ligands, H2, or atmospheric pressure of CO. 

The side-on (r)2) bonding in M r 2-H2 and other a-complexes has been termed non-classical, on analogy to the 3-center, 2-electron bonding in non-classical carbocations and boranes (Fig. 2). One of the first questions raised when H2 complexes were discovered is whether they would be important in catalytic reactions. As will be shown below the answer is an emphatic yes, as exemplified by the elegant asymmetric catalytic hydrogenation systems of Nobel-laureate Ryoji Noyori. Also, the mechanism of catalytic silane alcoholysis directly involves two different a complexes M(r 2-Si-H) and M(r 2-H2). In both of these systems, the crucial step is heterolytic cleavage of the H H and/or Si-H bond, the primary subject of this review. 

If this reaction is carried out in the presence of alcohols, homogeneous catalysis of silane alcoholysis occurs, as will be discussed below. Heterolysis of Et3SiH in the highly electrophilic complex cis-Re(CO)4(PR3)(ri2 -HSiEt3)] [A] (R = Ph, Cy) occurs (Scheme 8) (85) much... 

Such formation of [CpFe(CO)(PPh3)(H2)]+ from hydrolysis of [CpFe(CO) (PPh3)(HSiR3)]+ has previously been observed and employed for catalytic silane alcoholysis discussed below. 

Brookhart and co-workers found that the cationic a-silane complex [CpFe(CO)(PR3)(HSiEt3)]+ was observable by NMR at RT but only in the presence of excess silane (sacrificial removal of trace H20 as Et3SiOH) and could not be isolated as a solid (122). The [CpFe(CO)(PR3)] fragment catalyzed silane alcoholysis in the presence of the BArF counterion (123). Although rapid deactivation of the catalyst occurred with ethanol as substrate, phenol reacted continuously with turnover numbers up to 80 min-1. It was proposed (Scheme 9)... 

The mechanism of silane alcoholysis has been the focus of discussions over many years.10a 14 15 17 25 30 A general mechanism is outlined below. This mechanism begins with oxidative addition of the metal into the Si-H bond to form either a r 2 complex (la) or a silyl hydride (lb) (Figure 5). The alcohol then coordinates to the silicon forming a new complex (II) which can lose silyl ether to form a metal dihydrogen complex (HI). The catalyst is regenerated when another silane displace molecular hydrogen from the catalyst. There are several minor variations of this mechanism however this basic mechanism is believed to hold for many catalytic systems. 

Table 12. The Evaluation Silane Alcoholysis of Prochiral Silanes with Chiral Alcohols in the First Addition. Entry Silane Alcohol Ligand Ratio of Isomers...

If this reaction is carried out in the presence of alcohols, however, homogeneous catalysis of silane alcoholysis occurs with high efficiency and selectivity ... 

Transition metals have already established a prominent role in synthetic silicon chemistry [1 - 5]. This is well illustrated by the Direct Process, which is a copper-mediated combination of elemental silicon and methyl chloride to produce methylchlorosilanes, and primarily dimethyldichlorosilane. This process is practiced on a large, worldwide scale, and is the basis for the silicones industry [6]. Other transition metal-catalyzed reactions that have proven to be synthetically useful include hydrosilation [7], silane alcoholysis [8], and additions of Si-Si bonds to alkenes [9]. However, transition metal catalysis still holds considerable promise for enabling the production of new silicon-based compounds and materials. For example, transition metal-based catalysts may promote the direct conversion of elemental silicon to organosilanes via reactions with organic compounds such as ethers. In addition, they may play a strong role in the future... 

The importance of a Si-H-M interaction in reaction intermediates was demonstrated in the studies of silane alcoholysis by an Ir complex (Eq. 2.29) [88]. Kinetic and mechanistic studies of silane alcoholysis catalyzed by [IrH2S2(PPh3)2]SbF6 (S = solvent) suggest that an unstretched silane 27 is an active intermediate. In this system the Ir(III) center carries a positive charge making the metal electrophilic. A Si-H bond coordinated to the electrophilic metal center would be activated without oxidative addition. The result is enhanced sensitivity to nucleophilic attack by... 

Silane Alcoholysis. Triethylsilane reacts with alcohols in the presence of metal catalysts to give triethylsUyl ethers. The use of dirhodium(n) perfluorobutyrate as a catalyst enables regioselective formation of monosilyl ethers from diols (eq 6). ... 

Additional Silane Alcoholysis. The direct sUylation of alcohols with triethylsilane (eq 26) continues to be an interesting, if somewhat underused, method to TES protect alcohols. Recent works have demonstrated that this process is promoted by a number of catalysts including PdCl2, a Au(I) catalyst, and the Lewis acid B(C6F6)3 (eq 23). 

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