Silicon hypervalence

Owing to the silicon hypervalency and the cage structure, silatranes are of considerable interest from the standpoint of molecular electrochemistry. However, little work has been devoted to electrochemistry of silatranes279-281. 

The kinetics of nucleophilic substitution at the silicon atom assisted by uncharged nucleophiles have been studied by Corriu et at. (248-251). Hydrolysis of triorganochlorosilanes induced with HMPA, DMSO, and DMF was used as the model. The reaction proceeded according to the third-order kinetic law, first order with respect to the nucleophile, the silane, and the silylation substrate. Very low values of activation enthalpy and high negative entropy of activation were observed (Table VI). These results were taken as evidence for the intermediacy of silicon hypervalent species (249,251) however, they are also perfectly consistent with... 

Keywords Benziodoxoles Hypervalency Hypervalent Hypervalent boron Hypervalent bromine Hypervalent heterocycles Hypervalent iodine Hypervalent silicon Hypervalent sulfur Iodine heterocycles... 

The hypervalent silicon hydride anion, SiHj (cf. SiFs below), has been synthesized as a reactive species in a low-pressure flow reactor ... 

On the other hand, the fluorine-induced addition of the diastereomeric silyl-subsliluted sulfides 36 A and 36B to benzaldehyde proceeds without loss of stereochemical information and with retention of configuration32. Since, however, the anionic reagent 35A/35B is known to be configurationally labile, the observed retention of configuration in the fluorine-induced desi-lylative hydroxy alkylation lends experimental evidence to the notion that these reactions proceed via hypervalent silicon species rather than anionic reagents. 

So far, there is no conclusive evidence that a free allyl carbanion is generated from allylsilanes under fluoride ion catalysis. A hypervalent silyl anion, with the silicon still bonded to the allylic moiety, accounts equally well for the results obtained. Based on a variety of experimental results, it is in fact more likely that a nonbasic hypervalent silyl anion is involved rather than the basic free allyl carbanion first postulated14-23. When allylsilanes are treated with fluoride in the presence of enones. 1,4-addition takes place along with some 1,2-addition13. 

Mobility of Silyl Croups and the Importance of Hypervalent Silicon Species... 

It is interesting to note that many of the techniques developed in phosphorus chemistry are npw being routinely applied to hypervalent molecules of other elements. For instance, Martin et al. have studied the pseudorotational (Berry) mechanism for the inversion of 10-Si-5-siliconates (1) by 19F n.m.r. and demonstrated a linear correlation between AG for inversion at silicon and the a values of the variable ligand, Y The energy barriers for... 

Silicon and Phosphinomethanides A Novel Entry to Hypervalent and Low Valent Organosilicon Chemistry... 

Stable hypervalent silicon complexes with chelating diphosphinomethanide ligands similar to II, but with hexacoordinated silicon centers are obtained from 2 and SiCl4 (Eq.(4)). 

It was found that treatment of a mixture of 120 and 121 with tris(diethylamino-sulfonium) trimethyldifluorosilicate [TASF(Et)] resulted in smooth addition-elimination to the naphthoquinone to form the y-alkylation product 125 (85 %). TASF(Et) is a convenient source of soluble, anhydrous fluoride ion [47]. It is believed that exposure of 121 to TASF(Et) results in fluoride transfer to generate a hypervalent silicate anion, as depicted in structure 124. The transfer of fluoride between TASF(Et) and 121 may be driven by stabilization of the anionic species 124 by delocalization of the carbon-silicon bond into the LUMO of the unsaturated ketone. 1,4-Addition-elimination of this species to the naphthoquinone 120 would then form the observed product. 

Open-chain alkanes, alkyl halide reduction, 29-31 Organosilicon hydrides bond strengths, 5-6 hypervalent silicon species, 9-11 ionic hydrogenation, 5 trivalent silicon species, 7-9 Orthoesters, reduction of, 97-99 Oxime reduction, 102... 

It is an intriguing idea to stabilize low-valent silicon species, such as silyl cations, silylenes, silenes, and disilenes using intramolecularly coordinating ligands. Corriu et al. succeeded in the preparation of the first hypervalent silyl cation [(8-Me2NCioH6)2SiH]+l/2[l8]z 782 by the reaction of the hexacoordinated diorganosilane... 

This reaction proceeds with a variety of aryloxy ligands and a spectrum of substituents R and R at the phosphorus centre. Alkalimetal fluorides, tetrabutylammonium fluoride (TBAF), hypervalent silicon Ph3Sip2 and tin Ph3Snp2 difluorides are suitable fluoride anion donors. The exchange of aryloxy ligand proceeds efficiently in solvents such as THF, MeCN and CH2CI2. 

While the last elass of eomplexes considered in this section, the compounds 145, closely resemble the usual silane a-complexes, other multicenter H Si interactions discussed above have spectroscopic and structural features common to both the IHI and a-complexes. This enigmatic situation can be explained well by the structure 132 in terms of a a-coordination of the Si-H bonds of the hypervalent ligand (H +iSiX3)" 1) to metal, which thus includes both the hypervalent interaction of the silicon with the hydride atoms and the a-complexation of the Si-H bonds to metals. The key features of complexes with multicenter H Si interactions are summarized in Table VIII, where a comparison with the IHI and the residual H-Si interactions in silane a-complexes is given. 

It is obvious that studying interligand Si-H interactions has reached a great extent of sophistication. At least three classes of nonclassical Si-H bonding can be identified. These are the electron-deficient residual Si H interactions in silane a-complexes and agostic complexes, electron-rich IHI MH SiX, and the more recent multicenter H Si interactions, which are the subject of current debate and have features common to both IHI and a-complexes. This surprising diversity stems from the special role the substituent at silicon can play in tuning the extent of Si H interaction, and from the propensity of silicon to be hypervalent. 

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