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Siliconate anions, pentacoordinated form

Corriu and coworkers37 showed that alkoxysilanes R Si(OR )4-n [in particular RSi(OMe)3] give better yields for the DDB cyclization than do chlorosilanes. The reaction of anionic pentacoordinated silicon complexes [RSi(02CgH4-0)2] Na+ with DDB and subsequent LiAlPLi reduction give 1-R-diphenylsiloles (R = Me, Ph)38. Bis(silacyclopentadien-l-yl)alkanes were formed from DDB and a, a>-bis(dihalomethylsilyl)alkanes39. [Pg.1972]

Although anionic, pentacoordinate silicon species have received attention, the isolation of siliconates 194, 103, 195, 196 (Figure 28) by Perozzi, Martin and colleagues (155) has provided an opportunity to study in detail both structure and stereomutation of a pentacoordinate silicon compound (155, 321). This point will be discussed further in Sect. V-C. The crystal structure of 195 shows that the silicon is pentacoordinated with the fluorine and the two carbon atoms forming the equatorial plane of a somewhat distorted trigonal bipyramid (321). It is essentially the same as that proposed for similar siliconates on the basis of NMR data obtained in the solution phase (155). [Pg.163]

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. [Pg.351]

Acyl silanes are extremely sensitive towards nucleophiles and nucleophilic bases for example, alcoholic solutions of benzoyl triphenylsilane containing a trace of aqueous hydroxide ion rapidly produce triphenylsilanol and benzaldehyde . Three reasonable mechanisms may be conceived for this reaction (Scheme 60) Sat2 displacement at the silicon atom (path A) nucleophilic attack at the carbonyl carbon atom followed by Brook rearrangement, initially to give a hemiacetal (path B) and nucleophilic attack at the silicon atom to form a pentacoordinate silicon anionic intermediate, followed by migration of the nucleophile to the carbonyl group and subsequent Brook rearrangement (path C). [Pg.1632]

Three types of halogenosilanes (235, 236 and 237) with pentacoordinate silicon atom are considered. The first comprises anions of type 235, the second is a neutral species containing a Lewis base (D) (236) and the last is a cation formed and stabilized by a chelate ligand of a Lewis base character (237). [Pg.235]

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. [Pg.1370]

From these observations, Woerpel and Cleary proposed a mechanism to account for allylic silane formation (Scheme 7.23).85 Silacyclopropane 94 is formed from cyclohexene silacyclopropane 58 through silylene transfer. Coordination of the Lewis basic benzyl ether to the electrophilic silicon atom86-88 generates pentacoordinate siliconate 95 and increases the nucleophilicity of the apical Si-C bond.89 Electrophilic attack by silylsilver triflate 96 forms silyl anion 97. Intramolecular deprotonation and elimination then affords the silylmethyl allylic silane. [Pg.200]

In summary, we conclude that in contrast to phosphinomethanides A, anionic phosphines of type B can only form intra- or intermolecular phosphorus silicon donor-acceptor complexes under particularly favorable circumstances. These are seen primarily in suitable ligand geometries. Multidentate phosphine ligands which strongly impose coordination numbers higher than four to silicon seem to be especially suitable. In accord with previous findings [4], chlorine atoms as further silicon substituents favor the phosphine coordination additionally. With some caution we also state that hypervalent silicon with phosphorus donor atoms seems to prefer hexacoordination over pentacoordination. [Pg.456]

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) ... [Pg.82]

It is also notable that the spirosilane Si(-0C6H40)2 is much less reactive than the salt Na + PhSi(-0C6H40)2 towards phenylmagnesium bromide implying that the latter ion is not readily formed from the tetravalent molecule in this system. Pentacoordinated anionic silicon species are seen from these results to be susceptible to nucleophilic attack and may therefore reasonably be considered as possible intermediates in nucleophilic substitutions at silicon. [Pg.1274]

TBAF-assisted Trifluoromethylation, Perfluoroalkylation, and Polyfluoroalkoxylation. TBAF combines with TMSCF3 to form nucleophilic trifluoromethyl anion equivalents. The precise structure of this nucleophile is unknown, although it is likely that a pentacoordinate silicon intermediate is involved. The TBAF/TMSCF3 combination leads to smooth trifluoromethylation of aldehydes,ketones, esters, imines, nitroso compounds, a-ketoesters, Q -ketoamides (eq and... [Pg.464]

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. [Pg.186]

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. ... [Pg.210]


See other pages where Siliconate anions, pentacoordinated form is mentioned: [Pg.1360]    [Pg.1632]    [Pg.336]    [Pg.205]    [Pg.280]    [Pg.496]    [Pg.1265]    [Pg.79]    [Pg.1360]    [Pg.21]    [Pg.565]    [Pg.568]    [Pg.1634]    [Pg.156]    [Pg.159]    [Pg.181]    [Pg.1243]    [Pg.1260]    [Pg.63]    [Pg.115]    [Pg.1634]    [Pg.115]    [Pg.330]    [Pg.92]    [Pg.168]    [Pg.60]    [Pg.82]   
See also in sourсe #XX -- [ Pg.1265 ]




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Pentacoordinate silicon

Pentacoordinated anions

Pentacoordinated silicon

Pentacoordination

Silicon anions, pentacoordinate

Siliconates pentacoordinated

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