The Pentacoordinate Silicon

An early observation in the development of fluorosilane cross-coupling noted the affect of heteroatom substitution at the silicon on the facility of coupling. In the cross-coupling reactions between (l-octenyl)silyl fluorides and 1-iodonaphthalene, it was determined that fluoride substitution on silicon was essential for the reaction 

Despite much speculation over the years and indirect evidence, a dear mechanistic picture, supported by experimental evidence (kinetics, reactive intermediates, calculations), is stiU lacking. More recently, however, a number of investigations including quantitative evaluation of substituent effects, rate equation, and spectroscopic identification of intermediates have provided crudal insights into the mechanistic details of these important reactions. 

---

The study of compounds containing pentacoordinate silicon atoms currently represents one of the main areas of research in silicon chemistry. This is evident from the numerous reviews and proceedings published on this topic in recent years.112 Most of the pentacoordinate silicon compounds described in the literature are either salts with A5.S7-silicate anions or neutral silicon complexes with a 4+1 coordination to silicon. This review deals with a completely different class of pentacoordinate silicon compounds zwitterionic A S /-silicatcs. These molecular compounds contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom. 

Generally, the pentacoordinate silicon compounds described in this chapter are sensitive to water and very easily undergo hydrolytic Si-O cleavage reactions in solution. This has been used for the synthesis of the octa(silases-quioxane) 46, which was obtained in 90% yield by treatment of compound 35 with water in boiling acetonitrile (Scheme 8).34... 

The pentacoordinate silicon compounds 50,43 51,43 and 529 are spirocyclic zwitterionic A557-silicates with an Si04C skeleton. They contain two ethane-l,2-diolato(2-) ligands or two meso-oxolane-3,4-diolato(2-) ligands. 

The pentacoordinate silicon compounds 81, 8,54 82,54 83,54 and 8455 are spirocyclic zwitterionic A5S7-silicates with an Si04C skeleton. The chiral zwitterions contain two diolato(2—) ligands that formally derive from aceto-hydroximic acid and benzohydroximic acid (tautomers of acetohydroxamic acid and benzohydroxamic acid). 

The pentacoordinate silicon compounds 94,23 95,23 96-98,60 99,60,61100,61,62 101-103,60 104,61,62 105,62 106,62 and 10761,63 are monocyclic zwitterionic A5S7-silicates with an Si02FC2 skeleton. The chiral zwitterions each contain one bidentate diolato(2-) ligand that formally derives from 1,2-dihydroxy-benzene, salicylic acid, glycolic acid, oxalic acid, benzohydroximic acid (tautomer of benzohydroxamic acid), 2-methyllactic acid, or (S)-mande-lic acid. 

Nitrogen-coordinated pentacoordinate complexes have been used as stereoselective reducing agents in the preparation of erythro-(meso)- 1,2-diols from diketones and a-hydroxyketones109. The reducing agent was the (l-naphthylamino-8)trihydridosilane 92e. After formation of the dioxo chelate from the diketone (equation 32), the diol was obtained from the pentacoordinate silicon complex by reduction with LiAlILt. 29 Si NMR spectroscopy was used for the product-ratio analysis in this reaction, which was found to yield primarily the erythro diols. 

The rate and pathway of the methanolysis appear to depend strongly on the steric and electronic effects of the equatorial substituents. Because of the steric hindrance at the pentacoordinate silicon induced by N(Y/ substitution in N-silylated triazasilatranes, nucleophiles can attack their four-coordinate silicon atoms (equation 189)414. 

In 1982, Sakurai [7] described a catalytic version of this reaction (Scheme 13.4). The addition of small quantities of fluoride anions to the allylsilane 1 generates the pentacoordinated silicon species 10, probably in equilibrium with the starting materials 1 and 11. This activated species can react with the carbonyl derivative 6 to yield the alkoxide 12 which is trapped by fluorotrimethylsilane. This last step not only furnishes the silylated compounds 13 but also regenerates the fluoride catalyst 11. Acidic work-up then leads to the desired homoallylic alcohol 7. 

Oxidative addition of the silane to the metal is fast and reversible 30 therefore unless the pentacoordinated silane drastically slows down the oxidative addition process, pentacoordination will not alter the rate of the reaction at this stage of the cycle. The increased reactivity of le may be explained by the attack of the alcohol on the pentacoordinated silane that would form after oxidative addition (Figure 9A). The rate of the alcohol addition is increased by the higher reactivity of the pentacoordinated silicon center. This may explain the slower reactivity for those alkoxysilanes that cannot form this intramolecular coordination complex due to the absence of a nearby Lewis basic atom. We had observed during the comparison of aliphatic alcohol to benzyl alcohol that the nucleophilicity of the alcohols has an effect on the rate of the reaction. This is evidence that the alcohol and the silane are involved in the rate-determining step with 10 % Pd/C catalytic system. 

This conclusion was also confirmed by multinuclear (13C, 15N, 29Si) NMR spectroscopy (Table VI). The 29Si resonances of complexes 11, except that for iig, shifted to lower field upon cooling. This has been rationalized by the ionization reaction A E (Scheme 1), which converts the pentacoordinate silicon to tetracoordinate, resulting in the shift of... 

In contrast, two NH3 molecules add to SiH3 the energy of the second addition to H3NSiH3 is smaller than that of the first, but is still very large (ca. 40 kcal mol" ). The pentacoordinated silicon minimum possesses D i, symmetry, i.e. both Si-N distances are equal (Fig. 8). 

This provides a particularly convenient way of measuring reaction rates, simply by running the NMR spectra of a series of mixtures and analysing the results. The total rate of nucleophilic substitution at the pentacoordinate silicon can be measured by using dynamic NMR programs such as DNMR5 (QCPE) to model the resonances from the bound and non-coordinated NMI and their interconversions. At the slow exchange limit sharp, separate resonances are observed for the coordinated and non-coordinated NMI. 

The initial results have produced some interesting data. For the reaction with inversion of configuration there does not appear to be any rate dependence on the concentration of NMI. The inversion reaction, being zero order in NMI cannot take place by an elementary reaction with direct attack of NMI at the pentacoordinate silicon. The rate determining step must involve only 18 and we postulate that the inversion mechanism follows the path shown in Scheme 7, in which the breaking of the 0 "Si bond is the rate limiting step, followed by fast attack of NMI on the open chain four coordinate silicon compoimd. 

Over the past few years, a series of zwitterionic spirocyclic bis[l,2-benzenediolato(2-)]-organosilicates, such as compounds 1 [Ic] and 2 [la] (Fig. 1), and related zwitterions with 2,3-n q)hthalene-diolato(2-) and symmetrically substituted l,2-benzenediolato(2-) ligands have been structurally charac-terized by X-ray diffraction [1]. The coordination polyhedra around the pentacoordinate silicon atoms of such compounds can be described using the idealized geometries of the trigonal bipyramid (TBP) and the square pyramid (SP). In the solid state, almost all transitions between a... 

Based on kinetic and stereochemical considerations, Muller revised this mechanistic proposal in favor of a two-step associative mechanism ". The monomer would be added to the a-carbon of the pentacoordinated siliconate chain (49) followed by migration of the sUyl group to the carbonyl of the monomer (equation 48). It is then essential that the exchange of the catalyst between chain-ends is fast compared to chain propagation. 

In addition, an analogous species (3) containing a silicon-hydrogen IxMid was obtained by treating HSiCU with 1 equiv. of a lithium amidinate ligand (Fig. 3, Table 2). As may be expected, the hydrogen atom adopts an equatorial position at the pentacoordinate silicon atom. 

When oxidation with hydrogen peroxide leads to undesired products, it is noteworthy that the bistrimethylsilylperoxide, for which a very cheap and convenient synthesis was proposed (ref. 44), may be successfully used (ref. 43). The behaviour of bis(trimethylsilyl)peroxide supports the second interpretation. Indeed, the replacement of HO-OH by Me3SiO-OSiMe3 in the first mechanism involves, in the last step, the reaction of Me3SiOSiMe3 on the pentacoordinated silicon moiety which seems very unlikely. 

Depending on the internal energy and the substituents attached to the pentacoordinate silicon adduct anions, not only exchange processes, like reaction 146, or substitutions (reactions 143-145) occur, but alkane elimination is also frequently observed, in particular under ICR conditions. Alkane elimination is favoured if the adduct does not contain a good leaving group (allyl, alkoxide). Three instructive examples are described in reactions 147-149164b. 

The two distinct sets of energies shown in Tables 6 and 7, respectively, probably reflect the operation of two quite different dynamic processes. The first, higher, values may be attributed to the energy required to sever the N -> Si coordinate bond and permit inversion at the nitrogen atom, in order to equilibrate the methyl group environments, whereas the second, lower, set corresponds to pseudorotation at the pentacoordinate silicon atom. 

---