Fluoride ions, pentacoordinated silicon

Several aromatic and heterocyclic acyl trimethylsilanes have been used as acyl anion equivalents by treatment with fluoride ion (Scheme 81, path a)23 133 154b160191192. Provided that the acyl substituent is electron-withdrawing, and that there are no aryl substituents on the silicon atom, acyl anions can be trapped by various electrophiles in moderate to good yields indeed, acyl anions and pentacoordinate silicon anionic species have both been detected in gas-phase reactions of acyl silanes with fluoride ion193. 

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). 

To our surprise, however, ( )-l-octenyl(trifluoro)silane (n = 3) did not give the coupled product to any extent. We assumed in this case an unreactive hexacoordinate silicate was preferentially produced by the reaction with 2 mol of TASF, whereas with the mono- or difluorosilane, coordinatively unsaturated pentacoordinate silicate was assumed to be produced and underwent transmetallation effectively. Therefore, we considered that the transmetallation proceeded through a four-centered transition state, wherein silicon should be hexacoordinated [Eq.(8)]. The corresponding hexacoordinated silicate had to liberate one fluoride ion to be incorporated into the transmetallation. 

The Pd-catalyzed couphng of vinylsilanols with aryl iodides can also be achieved efficiently by using a base instead of a fluoride ion source (Scheme 10.211) [550]. The formation of an arylpalladium siloxide bearing a pentacoordinated silicon center might accelerate the transmetalation step. 

Because of the available pentacoordinate state for silicon, fluoride ion and neutral fluorine in C—F bonds coordinate much more strongly to tetravalent silicon than to carbon, in spite of the comparable bond strengths of Si—F and C—F (132 kcal/mol). Therefore, most of the fluoride-ion-mediated reactions proceed via high valent silicate intermediates. The synthetic potential of the fluoride-ion-mediated reactions has been exploited in a number... 

Attack of the base, at the silicon atom, is postulated as the first step in the reaction sequence with subsequent dissociation of the pentacoordinated intermediate in the rate-determining step the carbanion thus liberated would rapidly react with benzaldehyde. Because the C-Si bond is not enough polarized, activation is required by fluoride ions to form a pentacoordinated silicate which is assumed to be a soft nucleophile or generates a nucleophilic anion species (ref. 27) ... 

Although the nitrogen of the N = S = N group has low nucleophilicity, an intramolecular cyclization can occur in the presence of a catalyst (fluoride ion). Thus, heating of l-pentafluorophenyl-3-trimethylsilyl-l,3-diaza-2-thiaallene 170 in acetonitrile in the presence of cesium fluoride leads to 4,5,6,7-tetrafluoro-2,l,3-thiadiazole 171 (90JFC(50)359) (Scheme 157). It is assumed that 171 is formed via the intermediate pentacoordinated silicon anion, which is further transformed into the N-nucleophile with elimination of trimethylfluorosilane. 

These systems permit studies of the intrinsic properties of pentacoordinate silicon in the absence of solvation. The fluoride ion affinity of fluorotrimethylsilane is estimated to be 40.5 5 kcal mol" whereas that of tetramethylsilane is evaluated at less than 23 kcal mol -1. 

These observations were rationahzed by the assumption that a pentacoordinate silane is necessary for the cross-coupHng. Both mono- and difluorosilanes are efficient fluoride ion acceptors (from TASF), thereby accessing a pentacoordinate state (Scheme 7.47). The remaining coordination site on sihcon would presumably be occupied by the hahde from the arylpalladium halide complex to allow for a four-centered transmetallation transition state. This last coordination site would not be accessible with a trifluorosilane because it would readily accept two fluoride ions from the promoter, forming an unreactive coordinatively saturated siliconate, and thereby leaving no site for palladium halide complexation. 

Allylsilanes are intrinsically nonnucleophilic. We believe that addition of fluoride ion to an allylsilane forms a silicate anion, which can react at either end of the tc-system. Thus from a mechanistic viewpoint, substitution of an allylsilane under nucleophilic conditions is more appropriately considered an addition-elimination reaction with the terms Se2 and Se2 aptly representing the elimination process. For recent evidence of pentacoordinate silicate intermediates in fluoride ion catalyzed allylation using trimethylallylsilane, see a) M. Kira, M. Kobayashi and H. Sakurai, Tetrahedron Lett.. 28. 4081 (1987) b) M. Kira, K. Sato and H. Sakurai. J. Am. Chem. Soc.. 110, 4599 (1988) c) T. Hayashi, Y. Matsumoto, T. Kiyoi, Y. Ito, S. Kohra, Y. Tominaga and A. Hosomi, Tetrahedron Lett., in press d) H. Sakurai, Lewis Acid Character and Selective Reactions of Pentacoordinate Silicon Compounds, this volume. 

More recently, Majetich et al. have proposed pentacoordinate allylsilicate intermediates formed from fluoride ion addition to silicon as an ambient nucleophilic species. However, nothing had been known so far about pentacoordinate allylsilicates, before we have initiated the present study, whereas a number of pentacoordinate organosilicates have been well characterized. It is an interesting problem whether the fluoride-catalyzed reactions actually involve free allyl anions or not. The author will discuss this problem in the next section somewhat in detail. In this section the author will describe preparation of well-defined pentacoordinate allylsilanes and their reactions which have been published recently. ... 

Transition-metal catalyzed cross-coupling reactions have been demonstrated to be an extremely powerful tool for the construction of carbon-carbon bonds in regio- and stereoselective manners. These metal catalysts enable the transmetallation of carbon-silicon bonds into other carbon-metal bonds from which further reactions can proceed, leading to carbon-carbon bond formation. The use of palladium catalyst in the activation of the carbon-silicon bond is particularly noteworthy various important transformations have thus been developed. The presence of fluoride ions will make the reaction more facile, pentacoordinate organofluorosiUcates being presumably formed under these conditions . ... 

Swain et al. [79] compared the hydrolysis behavior of triphenylmethyl fluoride (TMF) and triphenylsilyl fluoride (TSF) under similar conditions. They found that, whereas the hydrolysis of TMF was consistent with a positively charged carbonium ion intermediate, the hydrolysis of TSF was not consistent with a positively charged siliconium ion intermediate but rather an intermediate in which silicon is less positively charged than in the original molecule. They concluded that pentacoordinate intermediates are easy pathways for displacements on silicon that are not available for carbon which cannot expand its valence to include more than eight electrons. 

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