Five-coordinate silicon intermediate

In conclusion, all these observations suggest the presence of a five-coordinate silicon intermediate both in nucleophilic displacements at silicon and in the racemization of halosilanes as previously suggested (5, 59). The ability to form a pentacoordinate silicon adduct is not controlled by the electronegativity of the substituents at silicon, but by the tendency of the Si—X bond to be stretched under the influence of a nucleophile ... 

COFj will react with three equivalents of MegSiCFg In MeCN, in the presence of KF upon work up, followed by acidification with concentrated sulfuric acid, (CF 3)30011 is isolated in ca. 75% yield [1159aaj. The reaction is believed to proceed by the mechanism illustrated in Fig. 13.11, involving a five-coordinate silicon intermediate, [Me3SiF(CF3)]. ... 

Are the nucleophilic substitutions at silicon consistent with the occurrence of the rate-determining formation of a five-coordinate silicon intermediate ... 

Smith [26] studied the hydrolysis of methylmethoxysilanes and determined the reactions to be first order in both the substrate and the acid. He is one of the few to suggest a mechanism other than the formation of a siliconium ion. He suggested that a five-coordinate reaction intermediate in which the entering and the leaving groups attach to the same face of the silicon ... 

Various theoretical studies (2, 3) reveal that nucleophilic substitution on silicon always proceeds through a five-coordinate silicon adduct intermediate. Thus the Sn2 mechanism on Si never resembles that of carbon. 

In associative ligand exchange, the intervention of hexacoordinated intermediates has been demonstrated in various intermolecular ligand exchange processes at five-coordinated silicon compounds (286,355) (eq. [114]). 

Nucleophilic substitution at silicon, as described in previous sections, usually proceeds through five- or possibly six-coordinated silicon intermediates or transition states. For silicon bearing the common alkyl or aryl ligands, the barrier to formation of such extracoordinated intermediates is quite low. In these examples bimolecular substitution is so facile that unimolecular substitution, even in the most favourable cases, is so slow by comparison as to be unobservable. 

Reacting la,b with MeLi in HMPA as active solvent and in the presence of MeOH as trapping agent, the attack of a methyl anion at the silicon atom of the silacyclobutane is the first reaction step and gives a pentacoordinated silicon anion (Scheme 2). Such five-coordinated species are discussed as intermediates during the ring opening polymerization of silacyclobutenes, -butanes, and -pentenes [2]. Furthermore, five-coordinated silicon species are well described to be stable compounds [3]. 

As previously pointed out by Lasaga and Gibbs (1990), there is reason to believe that the hydroxide exchange reaction between water and quartz proceeds by way of a fivefold coordinate silicon intermediate. The existence and nature of this five-fold coordinate silicon atom was further investigated by Kubicki et al. (1993). They determined the gas-phase reaction path of the addition of hydroxide to orthosilicic acid and a subsequent ab stracti on of H2O. 

The crux of organic mechanistic stereochemistry may be the Walden inversion, the inversion of stereochemistry about a four-coordinate carbon atom by nucleophilic attack of, for example, a hydroxide ion on an alkyl halide. Many reactions of inorganic molecules follow the same mechanism. In contrast, the dissociative mechanism of tertiary halides to form tertiary carbocatanion intermediates is essentially unknown among the nonmetallic elements silicon, germanium, phosphorus, etc. The reason for this is the generally lower stability of species with coordination numbers of less than 4, together with an increased stability of five-coordinate intermediates. This difference is attributable to the presence of d orbitals in the heavier elements (Chapter 18). 

In the past few years, numerous experimental results have illustrated the fundamental importance of penta- and hexacoordinate silicon species in reactions at silicon. The implication of pentacoordinate intermediates in substitution reactions at silicon is now well accepted (10, 11) the nucleophilically induced racemization (13,268) and hydrolysis (or alcoholysis) of halosilanes (268, 269), both controlled by entropy factors, take place through expansion of coordination at silicon. Pentacoordinate species with two or three carbon atoms attached to silicon have been isolated (129, 155) and finally, five-coordinate anions with five carbon atoms around silicon have been identified in the gas phase (107). This shows how much the expansion of coordination at silicon is an energetically favorable process. 

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. 

Under basic conditions (pH > 2.5) it is likely that water dissociates to produce nucleophilic hydroxyl anions in a rapid first step. The hydroxyl anion then attacks the silicon atom. Her [124] and Keefer [125] propose a mechanism in which hydroxyl anion displaces OR— with inversion of the silicon tetrahedron (Figure 24.14), while Pohl and Osterholz [126] favor a mechanism involving a stable five-coordinated intermediate which decays through second transition state in which any of the surrounding ligands can acquire a partial negative charge. 

The computation was performed up to the MP2/6-31G(d) level. They gathered evidence suggesting that the five-fold coordinate silicon structure may be a long-lived intermediate in basic solutions and can possibly be observed experimentally (Kinrade et al. 1999). The technique they used in finding the transition state was primarily constrained optimizations followed by Bemy optimization. 

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