Hexacoordinate silicate

Compounds 9-16 were found to undergo an acidic reaction in aqueous solution.21 Possibly, these zwitterionic A5S/-silicates behave as Lewis acids reacting with the Lewis base OH- to generate protons (2 H20 OH- + H30+). This interpretation is supported by the observation that the related compounds 6 and 7 react with [HNMe3]F in aqueous solution to yield the ionic A6S/-silicates 25 and 26, respectively (Scheme 4 the Lewis bases F-and OH- are isoelectronic).24 The identity of these hexacoordinated silicon... 

The coordination state of the silyl enol ether in the transition state strongly influences the diastereoselectivity (synlanti). If a ligand is sterically demanding, like phosphoramide 33, a boat-like transition state with a pentacoordinated silicate is formed and affords the syn product in the reaction of trichlorosilyl enol ether with benzaldehyde. In contrast, the less hindered ligand 34 gave the anti product through a chair-like transition state with a hexacoordinated silicate (Scheme 25). 

However, Corriu and coworkers66 postulate a hexacoordinate intermediate (or transition state) in the hydrolysis of organic silicates, with the rate-determining step in the reaction being the coordination of water to a pentacoordinate intermediate formed by initial nucleophilic attack. 

The possibility of ligand substitution reactions in pentacoordinate silicates Sil I31 2 and Sil FfF via hexacoordinate intermediates was studied by Fujimoto, Arita and Tamao73. Attack on each of these silicates by F or hydride produced qualitatively similar reaction pathways, leading to stable hexacoordinate intermediates, without significant breaking of the bond between silicon and the leaving group. It was concluded that a nonconcerted displacement mechanism via a hexacoordinate intermediate is likely. 

The Strecker reaction of silyl cyanide (H3SiCN) with benzaldehyde A-methylimine (PhCH=NMe), catalysed by an axially chiral 2,2 -bipyridine A,A -dioxide has been explored computationally as a model for the corresponding reaction using TMSCN, PhCH=NCH2Ph, and a biquinoline dioxide.70 The non-catalysed reaction is found to be concerted (via a five-membered ring TS), whereas the catalysis is stepwise, via a hexacoordinate hypervalent silicate. 

The mechanism A very detailed mechanistic study of this phosphoramide-catalyzed asymmetric aldol reaction was conducted by the Denmark group (see also Section 6.2.1.2) [59, 60], Mechanistically, the chiral phosphoramide base seems to coordinate temporarily with the silicon atom of the trichlorosilyl enolates, in contrast with previously used chiral Lewis acids, e.g. oxazaborolidines, which interact with the aldehyde. It has been suggested that the hexacoordinate silicate species of type I is involved in stereoselection (Scheme 6.15). Thus, this cationic, diphosphoramide silyl enolate complex reacts through a chair-like transition structure. 

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. 

Based on the above observations, we suggest the mechanism shown in Scheme 10-1. Transmetallation of a pentacoordinate silicate with R-PdL -X proceeds in a way so that R-PdL -X can coordinate to the Si atom of the coordinatively unsaturated silicate to give a coordinatively saturated hexacoordinate silicate. Interaction of the bivalent palladium with the C= C bond followed by C-Pd bond formation would produce a /S-cationic silicate... 

Considering the thermodynamic parameters of the reaction, it has been determined that the silicic acid with a hexacoordinated atom of silicon H2[Si(OH)6] formed in die process of reaction displaces ammonium ion from phenolates, so that this acid is stronger than catechol. This is in agreement with our experimental data at the beginning of the reaction process the decrease in the pH of the reaction mixture from 7.16 to 5.85 was observed before the pH became constant. 

Therefore, this thermodynamic process can be described in the following way the first stage is replacing the ammonium ion by catechol from silicates the second stage is replacing ammonium ion by silicic acid with a hexacoordinated atom of silicon from catecholates. 

In obtained salts, silicon cannot be present as a silicate, because this would be replaced by salts of stronger acids, such as H2CO3, H3PO4, and catechol. So we assumed, based on this fact and considering the thermodynamics of formation of (C6Hs02)6Si2(NH4)4, that humic acids with silica form compoimds with a hexacoordinated atom of silicon. Ammonium phosphate and water can be present in the product composition as admixtures from the extraction solution. 

Three doubly deprotonated xylitol molecules act as 1,2-diolato ligands in the hexacoordinate silicate [Si(Xylt2,3H 2)3] 27 (O Fig. 6) which is the hydroxyunethyl derivative of the thre-itolato complex 19 (O Fig. 4) and, as a consequence, shows the same stabilizing intramolecular hydrogen bonds [33]. 

It should be noted that apart from the high pressure modifications silicates with hexacoordinated silicon atoms are stable at normal pressure (Table 6). 

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