Pentacoordinate silicate

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 study of compounds containing pentacoordinate silicon atoms currently represents one of the main areas of research in silicon chemistry. This is evident from the numerous reviews and proceedings published on this topic in recent years.112 Most of the pentacoordinate silicon compounds described in the literature are either salts with A5.S7-silicate anions or neutral silicon complexes with a 4+1 coordination to silicon. This review deals with a completely different class of pentacoordinate silicon compounds zwitterionic A S /-silicatcs. These molecular compounds contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom. 

The pentacoordinate silicon compounds 50,43 51,43 and 529 are spirocyclic zwitterionic A557-silicates with an Si04C skeleton. They contain two ethane-l,2-diolato(2-) ligands or two meso-oxolane-3,4-diolato(2-) ligands. 

The pentacoordinate silicon compounds 81, 8,54 82,54 83,54 and 8455 are spirocyclic zwitterionic A5S7-silicates with an Si04C skeleton. The chiral zwitterions contain two diolato(2—) ligands that formally derive from aceto-hydroximic acid and benzohydroximic acid (tautomers of acetohydroxamic acid and benzohydroxamic acid). 

The spirocyclic pentacoordinate silicon compounds 85,56 86,39 and 8739 are zwitterionic A5S7-silicates with an S702N2C skeleton. The chiral zwitterions... 

The spirocyclic pentacoordinate silicon compounds 90,45,58 91 45,58 and 9259 are zwitterionic A5Si-silicates with an Si05 skeleton. The chiral zwitter-ions contain two 2-methyllactato(2-) or two benzilato(2-) ligands. In these molecules, two five-membered SiC>2C2 rings are connected by the silicon spirocenter. Compounds 90 and 91 represent isoelectronic analogs of the zwitterionic A5Sz-silicates 58 and 59 (0/CH2 replacement see Section III,D). 

The pentacoordinate silicon compounds 94,23 95,23 96-98,60 99,60,61100,61,62 101-103,60 104,61,62 105,62 106,62 and 10761,63 are monocyclic zwitterionic A5S7-silicates with an Si02FC2 skeleton. The chiral zwitterions each contain one bidentate diolato(2-) ligand that formally derives from 1,2-dihydroxy-benzene, salicylic acid, glycolic acid, oxalic acid, benzohydroximic acid (tautomer of benzohydroxamic acid), 2-methyllactic acid, or (S)-mande-lic acid. 

Most of the zwitterionic compounds studied so far are chiral, with a chiral A5S/-silicate skeleton. Most of them have been isolated as racemic mixtures and in some cases as enantiomerically pure compounds, some of the optically active compounds being configurationally stable in solution. With these experimental investigations, in combination with computational studies, a new research area concerning the stereochemistry of molecular pentacoordinate silicon compounds has been developed. 

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. 

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. 

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... 

By this method also the zwitterionic silicates 9-15 were obtained The geometry at silicon in these compounds is TBP, like in anionic and neutral pentacoordinate silicon complexes. A typical crystal structure is shown in Figure 5 for compound 9. This structure apparently also exists in solution (CD3CN), as the 29Si chemical shift for 9 in this solvent (—122.9 ppm) compares well with the solid state CP-MAS shift of -121.0 pm28 31. 

A new class of pentacoordinate zwitterionic silicates (52-70) has been developed and reported by Tacke and his coworkers28-31 46-52 (cf Section II.A.3). These are generally high melting crystalline solids, which are almost insoluble in nonpolar organic solvents, and only slightly soluble in polar solvents. 

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. 

Fluxional behavior was demonstrated for pentacoordinate alkylhydridosilicates in the gas phase, by the reaction of deuterium-labeled alkylsilicate with carbon dioxide65. The monodeuteriated silicate was first obtained in the flow tube by D- addition to a neutral alkylsilane, followed by H-/D- transfer to CO2 (equation 28). A statistical distribution of HC02 /DCC>2 (3 1 ratio) indicated complete scrambling of the hydrogens and deuterium in the silicate, presumably by a Berry pseudorotation mechanism. 

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. 

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