Pentacoordinate silicon compounds nucleophilic substitution

Recently, the stereochemistry of nucleophilic substitution at silicon has been reviewed by Holmes2, and the role of pentacoordinate silicon compounds as reaction intermediates has been reviewed by Corriu and coworkers3. 

Summary The rich variety of the coordination chemistry of silicon is discussed and some theoretical issues are raised. In an attempt to understand further the underlying chemistry, some thermodynamic and kinetic parameters for the formation and substitution of pentacoordinate silicon compounds have been measured by NMR methods. Values of -31 3 kJ mol for SHand -100 10 J K mor for A5-were measured for the intramolecular coordination of a pyridine ligand to a chlorosilane moiety. A detailed kinetic analysis of a nucleophilic substitution at pentacoordinate silicon in a chelated complex revealed that substitution both with inversion and retention of configuration at silicon are taking place on the NMR time-scale. The substitution with inversion of configuration is zero order in nucleophile but a retentive route is zero order in nucleophile at low temperature but shows an increasing dependence on nucleophile at higher temperatures. These results are analysed and mechanistic hypotheses are proposed. Some tentative conclusions are drawn about the nature of reactivity in pentacoordinate silicon compounds. 

The expansion of coordination at silicon. Pentacoordinated species seem to be quite intimately involved in many processes taking place at silicon. Expansion of coordination is the fundamental step not only in the nucleophilic induced racemization reviewed some years ago (13), but also in nucleophilic substitution activated by nucleophiles. A part of this review is devoted to the stereochemical and mechanistic aspects of nucleophilic activation. Furthermore, in connection with a possible isomerization of trigonal bipyramidal silicon by Berry pseudorotation, the dynamic stereochemistry of pentacoordinated silicon compounds is discussed. 

Allylsilanes are intrinsically nonnucleophilic. We believe that addition of fluoride ion to an allylsilane forms a silicate anion, which can react at either end of the tc-system. Thus from a mechanistic viewpoint, substitution of an allylsilane under nucleophilic conditions is more appropriately considered an addition-elimination reaction with the terms Se2 and Se2 aptly representing the elimination process. For recent evidence of pentacoordinate silicate intermediates in fluoride ion catalyzed allylation using trimethylallylsilane, see a) M. Kira, M. Kobayashi and H. Sakurai, Tetrahedron Lett.. 28. 4081 (1987) b) M. Kira, K. Sato and H. Sakurai. J. Am. Chem. Soc.. 110, 4599 (1988) c) T. Hayashi, Y. Matsumoto, T. Kiyoi, Y. Ito, S. Kohra, Y. Tominaga and A. Hosomi, Tetrahedron Lett., in press d) H. Sakurai, Lewis Acid Character and Selective Reactions of Pentacoordinate Silicon Compounds, this volume. 

A series of binuclear pentacoordinated silicon complexes 85 of diketopiperazine have been synthesized and substituent (or leaving group) effects on the Si-O coordination have been studied for five analogues with X=F, Cl, OTf, Br, and I [226]. Variable-temperature NMR spectroscopy (supported by X-ray crystallography) shows, for the first time in binuclear pentacoordinated silicon complexes, a complex equilibrium with both nonionic (0-Si) and ionic (Si-X) dissociation of the axial bonds in the silicon-centered trigonal bipyramids. The two dissociation pathways are consistent with a model for nucleophilic substitution at the silicon atom in a binuclear pentacoordinated silicon compound (Scheme 21) [226]. 

Today it is widely accepted that fivefold coordinated silicon plays a key role in the reaction mechanisms of the nucleophilic substitution having a trigonal bipyramidal transition state species which ressemble these transition states can be isolated in some special cases. The structural features fit well to kinetic data and possibly explain the significantly higher reactivity (proved by experimental data) of Si-pentacoordinated compounds compared to their tetracoordinated analoga. 

The main methods for the synthesis of hexacoordinate silicon compounds are similar to those for pentacoordinate complexes and were outlined in a recent review6. These methods include (a) addition of nucleophiles (neutral or anionic) to tetracoordinate silanes (b) intermolecular or intramolecular coordination to an organosilane (c) substitution of a bidentate ligand in a tetrafunctional silane. The following discussion focuses mainly on new complexes, reported since the recent reviews6,7 were published. 

We have reported a new technique for mapping the progress of nucleophilic substitution at silicon, through a pentacoordinate silicon [7]. The method requires the synthesis of a series of model compounds, each of which corresponds to a point on the reaction profile. The first example was the pyridone series represented in Scheme 5. 

It is well recognised that, in some circumstances, the prior coordination of a nucleophile to tetracoordinate silicon can produce a pentacoordinate silicon centre that is activated to nucleophilic attack [1,8]. We have previously published preliminary data on the use of chelated pentacoordinate silylamides as reactivity probes for nucleophilic activation [8]. The work presented here is an extension of those studies and incorporates a stereochemical and kinetic investigation in a single series of experiments. The compound chosen for the experiments was 18 which has several advantages for a kinetic and stereochemical study. The coordinated A -methylimidazole (NMI) group is readily displaced by another NMI molecule in a degenerate substitution reaction. 

Pentacoordinate organosilicon species formed by coordination of a neutral two-electron ligand to tetracoordinate organosilanes constitute an interesting class of compounds. These systems can serve as models for the study of nucleophilic substitution at tetracoordinate silicon. 

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