Pseudorotation at silicon

The possibility of pseudorotation at silicon was first suggested by Klanberg and Muetterties31 in their study of pentacoordinate fluorosilicates SiFsA RSiF4- and R2SiF3-. Recently there have been a number of studies on the pseudorotation of pentacoordinate silicon species. 

In contrast to the high barrier found in 103 for pseudorotation at silicon, the 13C, and 29Si NMR spectra of 105 indicate rapid inversion about silicon already at room temperature the two fluoro ligands are equivalent. 

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... 

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. 

Since the kinetics of isomerization of 195 and 196 are different, severalmech-anisms can be operating. Study of compound 196 when compared with 195 does not adduce any supplementary evidence for a pseudorotation mechanism. However, it confirms the multiplicity of exchange processes possible at silicon. 

The intervention of one mole of nucleophile in the inversion process contrasts with the results obtained for racemization of chlorosilane where two moles of nucleophiles are involved (13). To account for their results, Martin and Stevenson proposed that inversion at tetracoordinate silicon arises from an isomerization of a pentacoordinated intermediate via Berry pseudorotation steps. Several pathways were proposed, the most interesting being a nucleophilic attack at a carbon-oxygen edge of the tetrahedral silicon compound. In the resulting intermediate, a single Berry pseudorotation with the nucleophile as pivot and equatorial departure of the nucleophile causes an inversion at silicon (eq. [110]). A... 

Among the multiplicity of processes taking place at silicon, pseudorotation has not yet been unambiguously established. It represents an exciting area for investigation, and we hope that further work will be stimulated. 

Reaction mechanisms of nucleophilic attack at silicon 4. Pseudorotation in pentacoordinate silicon species... 

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. 

The barrier for inversion of configuration at silicon was, however, reported recently in a different type of complex, 100c, in which the chelate cycle is apparently more flexible than in 91a and allows for the transformation through a 120° angle. The barrier (18.7 kcalmol ) for pseudorotation (epimerization) was determined from the coalescence of signal pairs of the two diastereomers. Indeed, this barrier is substantially higher than those measured for the two-step pseudorotation, as might have been expected. 

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