Pentafluorosilicates

One of the first indications that higher valent silanes could be useful donors in palladium-catalyzed cross-coupling reactions was reported by Kumada and Tamao, when they observed that the dipotassium salt of pentafluorosilicate 70 could transfer its vinylphenyl group for the palladium-catalyzed coupling with iodobenzene at high temperature (Scheme 6). ... 

A zwitterionic hexacoordinate pentafluorosilicate (175) was recently reported by Tacke193.175 was prepared in the same manner as pentacoordinate zwitterionic complexes (cf Section II.A.3), as shown in equation 45. The crystal structure of 175 showed that one water molecule is present for each molecule, and that the structure is dominated by N—H-F and O—H-F hydrogen bonds. The Si—C bond length (1.943-1.946 A) is distinctly longer than in the corresponding pentacoordinate zwitterion (9), while the Si—F bonds (1.685-1.740 A) are comparable to those in the pentacoordinate analog. The solid state 29 Si NMR of 175 is also compatible with hexacoordination, with 529Si= -177.2 ppm. 

As discussed in Section n, allylsilane normally serves as a nucleophile which reacts with an electrophile. However, pentafluorosilicates occasionally behave like an electrophile... 

The ion is usually made by attack of HF on hydrous silica and is stable even in basic solution. Although the salts that crystallize are normally those of the SiF -ion, the pentafluorosilicate ion is found in compounds such as [Ph4As][SiF5]. Typical reactions producing SiFj salts are... 

The following mechanism is suggested for the cross-coupling of alkenylsilanes. Nucleophilic attack of a fluoride ion to the silicon atom of alkenylsilanes is first assumed to afford a pentacoordinate silicate and enhance both nucleophilicity of the silicon-substituted carbon and Lewis acidity of silicon to assist transmetalation effectively through a four-centered transition state (Scheme 2). Lewis acidity on silicon is critical as evidenced by the fact that hexacoordinate pentafluorosilicates that are fully coordinated and thus should have sufficient nucleophilicity are much less effective for the cross-coupling reaction (Eq. 2, vide supra). 

Early work involving simple alkenes employed the catalyst dichloro (y )-AUV-dimethyl-l-[(S)-2-(diphenylphosphino)ferrocenyl]ethylamine palladium(II), 1 (often abbreviated to Pd-Cl2[(i )-(S)-PPFA]). When applied to the hydrosilylation of norbornene with trichlorosilane, it yielded an adduct which was converted to a pentafluorosilicate and thence to the alcohol 2 in 25% overall yield and with 50% ee26. 

Trichlorosilanes are readily transformed to the pentafluorosilicate compounds (100) and (101) by reaction with excess potassium fluoride. The C—Si bond of the hexavalent organosilicon species is converted into a C—X (X = halogen or heteroatom like hydroxy and alkoxy) or a C—C bond. Thus, through the hydrosilylation reaction net transformations of alkenes and alkynes to halides, ethers and alcohols, as well as the construction of carbon frameworks, is easily achieved (Scheme 18). ... 

To enhance the polarity of a C—Si bond, Kumada and Tamao introduced five fluorine atoms on silicon and demonstrated that pentafluorosilicates underwent the palladium-catalyzed... 

At the same time, synthetic methods using the efficacy of nucleophilic agents in the activation of various silicon-element bonds, such as Si-H, Si-O, Si-N, and Si-C, were developed. The activation of these bonds was used as a tool for reduction of carbonyl compounds (271-273), Michael condensations (274,275), and formation of heterodienes by cleavage of the Si-N bond (276). The activation of the Si-C bond has received particular attention, as illustrated by the cleavage of the Si-allyl bond (277) and the use of pentafluorosilicates as synthetic intermediates (278). 

The pioneering work of Klanberg and Muetterties on pentafluorosilicates SiF, RSiF and R2SiF3 suggested the possibility of pseudorotation at silicon65. Recently, there have been a number of studies on pseudorotation of pentacoordinate silicon species. 

FIGURE 1. Trigonal bipyramidal structures of pentafluorosilicate ion and of organofluoro-silicates... 

Primary alcohols. l-Alkenes can be converted into primary alcohols by the sequence formulated in equation (I), which involves hydrosilylation, conversion to a pentafluorosilicate, and finally cleavage with m-chloroperbenzoic acid (c/, N-Bromosuccinimide, this volume). The choice of solvent is important in this... 

Aspects of the rhodium-catalysed hydroformylation of olefins have been reviewed. " Copper(ii) acetate catalyses the highly stereoselective solvolysis of -alkenylpentafluorosilicates to -alkenyl ethers under an atmosphere of air. Since the pentafluorosilicates can be obtained via hydrosilylation of acetylenes, the sequence represents a regio- and stereo-selective transformation of acetylenes into carbonyl precursors in moderate yield. The reaction of vinylmercurials and mercury carboxylates catalysed by palladium(ii) acetate provides a stereospecific route to enol carboxylates, which are valuable precursors of specific enolates. ... 

Asymmetric hydrosilylation of alkenes, catalysed by ferrocenylphosphine-palladium complexes, can be utilized in a synthesis of optically active alcohols from alkenes (Scheme 1). The initial optically active silane adducts are converted to alkyl pentafluorosilicates (1) which are then cleaved oxidatively, with retention of configuration, using peracid (c/. 3,132). Complex (2) was the best of several hydrosilylation catalysts examined using this, alcohols with enantiomeric purities of approx. 50% were obtained from norbornene and styrene as the alkenes. 

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