Silicates, pentacoordinated

Allylic transposition is observed in fluoride-induced reactions of allyl(trifluoro)silanes50 and allyl(trichloro)- and allyl(trialkoxy)silanes in the presence of hydroxylic promoters, e.g., 1,2-benzenediol51,52. Pentacoordinated silicates are believed to be involved53. 

The Hiyama coupling offers a practical alternative when selectivity and/or availability of other reagents are problematic. Hiyama et al. coupled alkyltrifluorosilane 74 with 2-bromofuran 73 to give the corresponding cross-coupled product 75 in moderate yield in the presence of catalytic Pd(Ph3P)4 and 3 equivalents of TBAF [65]. In this case, more than one equivalent of fluoride ion was needed to form a pentacoordinated silicate. On the other hand, alkyltrifluorosilane 74 was prepared by hydrosilylation of the corresponding terminal olefin with trichlorosilane followed by fluorination with C11F2. This method provides a facile protocol for the synthesis of alkyl-substituted aromatic compounds. 

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). 

Several mechanisms for the peroxide oxidation of organosilanes to alcohols are compared. Without doubt, the reaction proceeds via anionic, pentacoordinate silicate species, but a profound difference is found between in vacuo and solvated reaction profiles, as expected. In the solvents investigated (CH2CI2 and MeOH), the most favorable mechanism is addition of peroxide anion to a fluorosilane used as starting material or formed in situ, followed by a concerted migration and dissociation of hydroxide anion. In the gas phase, and possibly in very nonpolar solvents, concerted addition-migration of H2O2 to a pentacoordinate fluorosilicate is also plausible. ... 

In 1992, R.M. Laine (University of Michigan, Ann Arbor) announced the development of a process that transforms sand and other forms of silica into reactive silicates that can be used to synthesize unusual silicon-based chemicals, polymers, glasses, and ceramics. The Lame procedure produces pentacoordinate silicates directly from low-cost raw materials—silicon dioxide,ethylene glycol, and an alkali base. The mixture is approximately a 60 1 ratio of silica gel, fused silica (or sand) to metal hydroxide and ethylene... 

A large upfield shift of 29Si chemical shift from 28 (S 10.66) to 29 (S - 72.45) strongly supports the structure of a pentacoordinate silicate. 

An interesting case of simultaneous exchange of two fluorides has been reported recently22. In compounds 2g and 2h there is one tetra- and one pentacoordinate silicon. Fluoride transfer from the pentacoordinate silicate to the silane was found to be inter-molecular, with a large negative entropy of activation. It was concluded that exchange takes place simultaneously between two complexes, through a cyclophane-like transition... 

The 29 Si chemical shifts for dinuclear pentacoordinate silicates have been compiled in Table 7. 

Similar condensation of phenyltriethoxysilane with a spirocatechol yielded the first macrocyclic tetrasiliconate, a tetraanion containing four pentavalent silicons (equation 24a)58b. NMR evidence showed that only one meso-stereomer (C2h symmetry) was formed in solution out of the four possible diastereomers. The 29Si chemical shifts of the two unique silicons in DMSO-dg solution were —86.3 and —86.9 ppm, respectively, consistent with a pentacoordinate silicate anion. 

Direct addition of H- and other nucleophilic anions to primary, secondary and tertiary alkylsilanes formed pentacoordinate silicate anions (equation 26). Interestingly, the parent silicate, Sills. could not be prepared in this way from silane and hydride65,68. Sills was prepared from alkylhydridosilicates by a hydride transfer reaction (equation 27). The reducing activity of alkylhydridosilicates toward various substrates via hydride transfer in the gas phase was discussed in detail in a previous review67. 

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. 

In the presence of a Lewis acid or fluoride ion, imines react with allylsilane to yield the homoallylic amines with high stereoselectivity119,120. Thus, treatment of N-galactosylaldimine 81 with allylsilane in the presence of excess of SnCU yields the corresponding allylated product 82 (equation 54). It is noted that aliphatic aldimines do not react under these conditions. Fluoride ion promoted crotylation of aldimines proceeds in a regiospecific and diasteroselective manner121. A pentacoordinate silicate moiety is involved in this reaction. 

Substitution on silicon did not appear to affect the reaction pathway. These reactions are proposed to proceed via a pentacoordinate silicate intermediate. 

Nakayama and Tanaka found that the chlorination of 2-silylfuran 109 with sulfuryl chloride gave the corresponding 2-chloro-3-silylfuran 110 and 2-chlorofuran 111 as shown in equation 75184. 1,2-Migration of a pentacoordinate silicate group was assumed to be involved in this reaction (equation 76). 

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). 

Although trimethyl(vinyl)silane undergoes the coupling reaction with aryl halides in the presence of TASF and a palladium catalyst as described in Section 1 (Eq. 3), those having an aliphatic substituent on vinyl fail to couple with aryl iodides under similar conditions, probably because they are not capable of affording pentacoordinate silicates efficiently owing to the electron-donating nature of the substituent. To assist the formation of the pentacoordinated intermediates, the methyl on silicon was replaced by fluorine [13]. The cou-... 

As we have seen, pentacoordinate silicate TASF is one of the best activators for organosilicon compounds in the palladium-catalyzed cross-coupling... 

The stereoselective allylation of aldehydes was reported to proceed with allyltrifluorosilanes in the presence of (S)-proline. The reaction involves pentacoordinate silicate intermediates. Optical yields up to 30% are achieved in the copper-catalyzed ally lie ace-toxylation of cyclohexene with (S)-proline as a chiral ligand. The intramolecular asymmetric palladium-catalyzed allylation of aldehydes, including allylating functionality in the molecules, via chiral enamines prepared from (5)-proline esters has been reported (eq 15). The most promising result was reached with the (S)-proline allyl ester derivative (36). Upon treatment with Tetrakis(triphenylphosphine)palladium(0) and PPh3 in THF, the chiral enamine (36) undergoes an intramolecular allylation to afford an a-allyl hemiacetal (37). After an oxidation step the optically active lactones (38) with up to 84% ee were isolated in high chemical yields. The same authors have also reported sucessful palladium-catalyzed asymmetric allylations of chiral allylic (S)-proline ester enamines" and amides with enantiomeric excesses up to 100%. 

In the absence of organosilanes, TASF itself reacts as a coupling partner and methylates various aryl halides and vinyl iodides [Eq.(27)] [24]. This was the first example which demonstrated that a structurally well-characterized pentacoordinated silicate could undergo the coupling reaction. 

Assuming that transmetallation occurs between an jy -allylpalladium complex and a bulky pentacoordinate silicate (right-hand side of Scheme 10-11), regiocontrol of the coupling might be feasible. However, the configurational equilibrium of the resultant r] ... 

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... 

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