Pentavalent silicon anion

Of particular interest are the reactions reported by Sullivan, DePuy, and Damrauer (107). In the gas phase, pentavalent silicon anions, including silicon anions with five carbon substituents, have been generated by reaction of anions with substituted silanes. For example, direct addition of F" or of the allyl anion occurs, leading to anions formulated as pentacoordinate species (Scheme 21). 

These findings suggest that the formation of pentavalent silicon anions may take place even in the case of a silicon atom bearing carbon ligands. 

Recent theoretical49 and experimental50 studies have converged on the reactions of the potent hydride ion with silane, and in particular on the gas-phase synthesis and characterization of the archetype pentavalent silicon anion, SiHj. Ab initio calculations show that H- should react at the silicon atom of SiH4 without a barrier to form SiHj, and at the hydrogen periphery to form SiH3 and H2. The calculations further predict... 

Cyclization reactions of vinyl- and alkynylsilanes have been reviewed100. The course of the reaction of the cyclohexenone derivative 184 depends on the catalyst employed ethylaluminium dichloride gives solely the product 185 of 1,6-addition, whereas tetrabuty-lammonium fluoride yields a mixture containing 69% of the 1,4-adduct 186 and 31% of the bridged compound 187 (equation 89)101. Intramolecular addition reactions of allylic silanes102 may also be catalysed by Lewis acids (equation 90) or fluoride ions, and in this case an allyl anion or a pentavalent silicon intermediate may be involved (equation 91). Such reactions are exemplified by the formation of a 1 5 mixture of the diastereomers 189 and 190 when the cyclohexenone derivative 188 is treated with ethylaluminium dichloride (equation 92). In the presence of fluoride anion the ratio of the isomers is reversed103. 

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. 

Returning to the introductory sentence of this section, it may be noted that hypervalent silicon hydrides have been prepared as anions in the gas phase and their ion chemistry has been investigated by Squires and co-workers [238]. Thus, the synthetic methods for the precursor anions are known and it is perhaps only a question of time for pentavalent silicon radicals to be generated by NRMS as transient species in the gas phase. 

The final focal point of silicon chemistry to be mentioned in this introduction deals with the continuing evolution of the Brook rearrangement. At its inception, the Brook rearrangement involved the intramolecular C O silicon migration of a-oxidosilane 99 to an a-silyloxy anion 101 via the intermediacy of pentavalent siliconate 100 (eq 23). ... 

Silicon is also more electropositive (Pauling eleetronegativity index 1.8) than carbon (2.4), and the silicon-carbon bond is polarized Si -C . Tetravalent silicon, a weak Lewis acid, is thus essentially attacked by anions (F, etc.) to form hyper-valent silicon derivatives. This phenomenon is used for the sterie control of reactions that proceed in the coordination sphere of silicon. For instance, fluorides and catecholate anions, among others, catalyze the stereoselective reaction between an allylsilane and an aldehyde leading to the corresponding homoallylic alcohols via an anionic intermediate of pentavalent silicon ... 

In 2000, Kagan et al. presented the addition of trimethylsilylcyanide (TMSCN) (99) to aldehydes catalyzed by the monolithium salt of (5)-(-)-BINOL (106) [118]. The proposed mechanism includes a hypervalent silicon intermediate. In an initial attack of the anionic chiral catalyst, a pentavalent silicon complex is formed (100). The complex acts as Lewis acid and is supposed to coordinate the carbonyl group of the aldehyde (101) to a hexavalent species (102). The next step is the enantioselective transfer of cyanide (103), followed by elimination of the anionic catalyst. Consequently, the TMS cyanohydrin (104) is formed (Scheme 7.18). 

Treatment of hydrosilanes with potassium hydride is a known route to silyl anions and has been used to generate silacyclopentadienide anions. Reinvestigation of the reaction of KH with C4Ph4SiMeH has shown that the dominant pathway is simple addition of the hydride anion to the silicon centre to give the pentavalent silicate [Ph4C4SiMeH2] K+ (129)189 (equation 75). The Si—H coupling constants in 129 (Jsi-H = 192.5 and... 

A similar approach, even though initiated by a Brpnsted acid, is the organocatalytic approach put forward by List et al. [50]. They used their chiral disulfonimide 36 to induce VMARs stereoselectively. With aromatic aldehydes as the substrates, selectivities up to 98 2 er can be obtained. Additionally, they were able to perform bis-VMARs with triene 40. As mentioned earlier, even though a chiral Brpnsted acid is used, the transition state also includes a hypervalent silicon species (42), which contains the disulfonimide anion and the coordinated aldehyde in a pentavalent, trigonal bipyiamidal geometry (Scheme 2.130). 

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