Pentavalent silicon

The silyl group directs electrophiles to the substituted position. That is, it is an ipso-directing group. Because of the polarity of the carbon-silicon bond, the substituted position is relatively electron-rich. The ability of silicon substituents to stabilize carboca-tion character at )9-carbon atoms (see Section 6.10, p. 393) also promotes ipso substitution. The silicon substituent is easily removed from the c-complex by reaction with a nucleophile. The desilylation step probably occurs through a pentavalent silicon species ... 

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. 

Extensive mechanistic investigation of the ring expansion 33 —> 34 was performed in order to differentiate between a ring-opening reaction to give a silyl radical 39 (path a), followed by the 6-endo cyclization, or a pentavalent silicon transition state 40 (path b). It was clearly demonstrated that the ring expansion proceeds via a pentavalent silicon transition state (Scheme 6.10) [16]. 

In the phosphonium iodide and chloride salt catalyzed TMSCN addition on aldehydes and ketones, a double activation should exist. Not only the activation of the ketones or aldehydes with the phosphonium cation is necessary, but also the activation of the TMSCN by the soft Lewis base [I] or the harder Lewis base [Cl], which can form a pentavalent silicon intermediate [121]. 

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. 

Crucial for the success of the Hiyama Coupling is the polarization of the Si-C bond. Activation of the silane with base or fluoride ions (TASF, TBAF) leading to a pentavalent silicon compound is a first necessary step. 

Lithium chloride is a convenient catalyst for cyanosilylation of a range of ketones and aldehydes by trialkylsilyl cyanide, under solvent-free conditions the silylated cyanohydrin product can be directly distilled out.266 As little as a microequivalent of catalyst proved effective. Evidence for nucleophilic chloride, generating a pentavalent silicon (65) as reactive species, is presented. 

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. 

Trialkylsilyl protection of carboxylic acids and amines is rare owing to hydrolytic lability. Nevertheless, synthetically useful silicon protecting groups have been developed for these functional groups in which the requisite stability is achieved by incorporating the silicon atom into a 2-(trimethylsilyl)ethyl substituent. The principle is illustrated [Scheme 1.9] by the reaction of 2-(trimethylsilyl)ethyl esters with tetrabutylammonium fluoride the pentavalent siliconate intermediate fragments with loss of ethylene and fluorotrimethylsilane14-15 to liberate a carboxylic arid as its tetrabutylammonium salt. 

In polymerizations of cyclic siloxanes, because of the higher coordination number of silicon the active centre can be formed by the decomposition of the transition complex with pentavalent silicon [208]... 

Pentavalent silicon adducts (78,79) can be isolated from the reaction of diaUcyl- or diaryl-dichlorosilanes with (1) (R = Et, i-Pr R = Me), whereas when Me3SiI is reacted with(l), ionic imidazolium salts are formed (77). Unfortunately, no information regarding the structure of these compounds is available. A crystalline carbene-silylene adduct 1,2-C6H4[N(R)]2C-Si[N(R)]2C6H4-l,2 (80, R = CH2-t-Bu) has... 

The hypervalent silicon derivative 100, formed from the interaction of the more basic secondary amine group of the catalyst 99 with MesSiCN (Scheme 3.28), was assumed by the authors to produce the transition state 101 by interaction with the carbonyl compound (since the nucleophilicity of the cyano group of 100 is enhanced by electron donation from the pentavalent silicon) immediate silyla-tion gives the corresponding cyanohydrin trimethylsilyl ether 98 and restores the catalyst. 

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. 

Trivalent siliconium ion (I, 7) and pentavalent silicon species (6) were proposed as condensation intermediates. Gas-phase mass spectrometric reactions of TEOS indicate a surprising stability of both trivalent and five-coordinate [(OEt)4SiOH]+ species (8), but the trivalent species was incapable of initiating condensation, at least under these conditions. Although the gas-phase studies may not directly correlate with solution behavior, they do suggest that three- and five-coordinate species are at least possible. 

Stereoselective introduction of C2-units and the ring enlargement reaction of (3-oxa-2-silacyclopentyl)methyl radicals into 4-oxa-3-silacyclohexyl radicals via a pentavalent silicon-bridging radical 01YGK589. 

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... 

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 ... 

Corriu and his group have synthesized the first stable pentavalent silicon derivative (below). This compound also reacts with aldehydes leading to homoallylic alcohols regio- and stereoselectively via a transition state in which silicon is hexacoor-dinate. It is the coordination of the allyl and aldehyde that generates the regio- and stereoselectivity. In this way, the allylsilane Z leads to the syn isomer (scheme below), whereas the allylsilane E reacts to produce the anti isomer. 

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). 

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