Stereochemistry of silicon

The compounds formed by one Si atom and one to two other non-metallic atoms, such as SiX or SiX2 (X = H, F, O, and Cl), are not stable species. 

A number of silenes (R2Si=CR2) and disilenes (R2Si=SiR2) containing double bonds, where each Si atom is three-coordinated, are known. 

Numerous compounds of octahedral hexa-coordinate Si are known. Some examples are shown below. 

In the salt [C(NH2)3]2[SiF6], the Si-F bond length in octahedral SiFg2 is 168 pm, which is longer than the Si-F bonds in SiFs-. 

The chemistry of silicon oxygen compounds with SiOs and SiC 6 skeletons in aqueous solution is of special interest. It has been speculated that such Si(IV) complexes with ligands derived from organic hydroxy compounds (such as pyrocatechol derivatives, hydroxycarboxylic acids, and carbohydrates) may play a significant role in silicon biochemistry by controlling the transport of silicon. 

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Although the stereochemistry of silicon in these compounds presents no points of special interest, the bond arrangement being the normal tetrahedral one, the siloxanes include some interesting ring systems. 

The elucidation of reaction mechanisms in organic chemistry has involved both kinetic and stereochemical studies. Similarly, the stereochemistry of silicon (1,2) has played a determining role in understanding the reaction mechanism in organosilicon compounds. 

The chemistry and stereochemistry of aminoboranes containing the siLicon—nitrogen—boron linkage have been the subject of numerous studies. Many of these compounds are useful precursors to other B—N systems including diboryl-amines (45) and B—H substituted aminoboranes (46). A series of... 

L. H. Sommer, Stereochemistry, Mechanism and Silicon An Introduction to the Dynamic Stereochemistry and Reaction Mechanisms of Silicon Centers, McGraw-Hill, New York, 1965, p. 126. 

Another strategy to control the regio- and stereochemistry of cycloaddidon is a silicon-tethered reacdon, as discussed in the secdon of nitronates fSecdon 8.2.3 fEq. 8.65. ... 

Vinylsilanes react readily with a range of electrophiles to give products of substitution (1). The overall stereochemistry of such substitution will depend on a number of factors, including the stereochemistry of addition and subsequent elimination when 1,2-adducts are discrete species. However, the regiochemistry of substitution is normally unambiguous, the -effect ensuring that carbonium-ion development on attack by the electrophile will occur at the carbon terminus remote, i.e. /3, to silicon ... 

Although the allylation reaction is formally analogous to the addition of allylic boranes to carbonyl derivatives, it does not normally occur through a cyclic TS. This is because, in contrast to the boranes, the silicon in allylic silanes has little Lewis acid character and does not coordinate at the carbonyl oxygen. The stereochemistry of addition of allylic silanes to carbonyl compounds is consistent with an acyclic TS. The -stereoisomer of 2-butenyl(trimethyl)silane gives nearly exclusively the product in... 

The 29Si resonance is therefore a single narrow line. However for dialkylpolysilanes with two different alkyl groups on each silicon, (RR Si)n, each silicon atom is a chiral center and the resonance for a particular silicon will depend upon the relative stereochemistry of other nearby silicon atoms. For such polymers, a rather symmetrical cluster of peaks is observed (Figure 5). These results are consistent with atactic structures, having a statistical (Bernoullian) distribution of relative configurations.(32,33)... 

Most optically active polysilanes owe their optical activity to induced main-chain chirality, as outlined above. However, backbone silicon atoms with two different side-chain substituents are chiral. Long-chain catenates, however, are effectively internally racemized by the random stereochemistry at silicon, and inherent main-chain chirality is not observed. For oligosilanes, however, inherent main-chain chirality has been demonstrated. A series of 2,3-disubstituted tetrasilanes, H3Si[Si(H)X]2SiH3 (where X = Ph, Cl, or Br), were obtained from octaphenylcyclote-trasilane and contain two chiral main-chain silicon atoms, 6.16 These give rise to four diastereoisomers the optically active S,S and R,R forms, the activity of which is equal but opposite, resulting in a racemic (and consequently optically inactive) mixture and the two meso-forms, S,R and R,S, which are optically inactive by internal compensation. It is reported that the diastereoisomers could be distinguished in NMR and GC/MS experiments. For the case of 2-phenyltetrasilane, a racemic mixture of (R)- and (A)-enantiomers was obtained. 

The ability of silicon to direct the stereochemistry is a technique just beginning to be appreciated. 

Most of the zwitterionic compounds studied so far are chiral, with a chiral A5S/-silicate skeleton. Most of them have been isolated as racemic mixtures and in some cases as enantiomerically pure compounds, some of the optically active compounds being configurationally stable in solution. With these experimental investigations, in combination with computational studies, a new research area concerning the stereochemistry of molecular pentacoordinate silicon compounds has been developed. 

Iminium ion-vinylsilane cyclizations closely related to the one described here have been used to prepare indolizidine alkaloids of the pumiliotoxin A and elaeokanine families, indole alkaloids, amaryllidaceae alkaloids, and the antibiotic (+)-streptazolin. The ability of the silicon substituent to control the position, and in some cases stereochemistry, of the unsaturation in the product heterocycle was a key feature of each of these syntheses. 

The foregoing considerations show that, in order to reproduce the stereochemical trends, it is not necessary to introduce either d orbitals for the silicon atom or pseudo-rotations for the transition state. Conversely, the stereochemistry of substitution reactions on silicon compounds cannot be taken as a proof of d orbitals intervention in silicon chemistry. 

In the following sections we shall examine the factors controlling the stereochemistry of substitution at silicon. 

The stereochemistry at silicon is extremely sensitive to the nature of the nucleophiles (Tables I, II, and VI). As a consequence, the stereoselectivity, i.e., either percentage of RN or percentage of IN, is a quite sensitive and reliable measure of the dependence of the mechanism upon small changes in the structure of the anion, the metal, or the solvent. The analysis of the experimental data can take the following form ... 

Consistent with these observations are the experiments with phenoxide anions (56) and the relationship between the stereochemistry at silicon and the regioselectivity of attack on a-enones (57). 

In concluding this section on the influence of the nature of the nucleophile, it is important to stress the dominant influence of the nucleophile on the stereochemistry at silicon. This effect cannot be interpreted in terms of the stability of the intermediate on the basis of the apicophilic-ity rule as stated in phosphorus chemistry. It fails to explain the retention of configuration as stereochemical outcome. No better explanation can be extracted from the quasicyclic SNi-Si mechanism (/. 2). On the other hand, data obtained with various nucleophiles show clearly that the stereochemistry is controlled by the electronic character of the nucleophile. In other words, this factor at first determines the geometry of attack of the nucleophile at silicon, which leads in a first determinant step to the formation of a pentacoordinate intermediate (55). We proposed the following ... 

Anh and Minot (6) reported a rationalization of the stereochemistry at silicon by an extension of Salem s treatment of the Walden inversion (6 ). The frontier-orbital approximation is assumed, i.e., the major interaction during the reaction occurs between the HOMO of the nucleophile and the LUMO of the substrate o-f j-x. The calculated structure of the latter is shown below, with the big lobes of the hydrid AOs pointing toward each other (Scheme 9). 

In the foregoing discussion (Section II,B), we stressed the dominant influence of the electronic character of the nucleophile on the stereochemistry at silicon. We wish now to propose the following concerning this effect. 

Studies in the stereochemistry of nucleophilic displacements at silicon over the past few years have been numerous. Significant progress has been achieved in establishing the controlling factors of the stereochemistry (i) the lability of the leaving group, (ii) the electronic character of the nucleophile, and (iii) the bond angles at silicon. 

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