Silicon pentacoordinate species

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. 

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. 

Corriu and coworkers have proposed50 an alternative mechanism involving extension of the coordination of the silicon atom. The first step involves rapid and reversible attack by a molecule of the nucleophilic catalyst, Nu, to give the pentacoordinate species 23 (equation 11). 

Numerous experimental observations of the increased reactivity of pentacoordinate silicon over tetracoordinate silicon have been reported. Corriu and coworkers have found that the relative reactivity of PbMeSiI 3 and PhMeSiF2 towards t-BuMgBr was >1000 155. The pentacoordinate species 29 reacts with alcohols and acids to give mono- or disubstitution products whereas the corresponding four-coordinate species 30 does not react at all56. 

Several aromatic and heterocyclic acyl trimethylsilanes have been used as acyl anion equivalents by treatment with fluoride ion (Scheme 81, path a)23 133 154b160191192. Provided that the acyl substituent is electron-withdrawing, and that there are no aryl substituents on the silicon atom, acyl anions can be trapped by various electrophiles in moderate to good yields indeed, acyl anions and pentacoordinate silicon anionic species have both been detected in gas-phase reactions of acyl silanes with fluoride ion193. 

The enhanced reactivity of SCB-derived enol ethers is attributed to the combination of ring strain and the potential for silicon to expand its coordination number form penta- to hexacoordinate compounds. Specifically, for SCBs, the reaction with nucleophiles allows for relief of the strain energy via rehybridization of the geometry at silicon from tetrahedral to trigonal bipyramidal (tbp) upon formation of a pentacoordinate species. 

In 1999, Demnark disclosed that silacyclobutane derivatives (34) were very effective coupling partners for the transfer of alkenyl and aryl groups. The original theory was that the release of strain energy upon treatment with fluoride to make the silicon pentacoordinate is what made these species so active. However, later studies showed that under the conditions of the cross-coupling reactions, silacyclobutanes formed silanols (35) and disoloxanes (36), which themselves proved... 

The difficulty in interpreting these results, at the time, arose from the lack of reliable points of comparison for the expected Sn chemical shifts of stannylium ions. There was no doubt that values higher than 300 were deshielded to an unprecedented extent, but were these shifts sufficient to demonstrate tricoordination or trivalency Computation in the early 1990s could not provide a reliable answer. Arshadi et al bypassed the calculational problem in 1996 by publishing a remarkable empirical correlation between structurally analogous silicon and tin compounds ( Si chemical shift vs Sn chemical shift). Since reliable calculations were available for the Si chemical shifts of trialkylsilylium ions, this plot could provide at least an indication of the expected Sn chemical shifts of trialkylstannylium ions, which proved to be 5 ca. 1700. The species observed by Birchall, Lambert, and Sakurai thus were very far from the expected chemical shift and hence from the ideal tricoordinate geometry of the stannylium ion. Since the values are deshielded to some extent, pentacoordination could be ruled out. The best description of the structures observed by all these authors, therefore, is the bond-stretched, solvent-coordinated stannyl cation 8. Lambert and Kuhlmann observed high conductivity, so that the neutral anion-coordinated variant 9 could be eliminated. Such structures (8) also apply to those reported in 1992 by Edlund et al as the tetrahedral part of the equilibrium with pentacoordinate species. 

Of particular interest are the reactions reported by Sullivan, DePuy, and Damrauer (107). In the gas phase, pentavalent silicon anions, including silicon anions with five carbon substituents, have been generated by reaction of anions with substituted silanes. For example, direct addition of F" or of the allyl anion occurs, leading to anions formulated as pentacoordinate species (Scheme 21). 

Nucleophilic substitution on silicon—stable hypercoordinated species Another demonstration of the role of ionic structures is the nucleophilic substitution on Si, which proceeds via pentacoordinated intermediates [81,82], in contrast to the situation in carbon where the pentacoordinated species is a transition state. Recently, Lauvergnat et al. [83], Shurki et al. [84], Sini et al. [85], and Shaik et al. [86] have performed BOVB/6-31G (and a few other basis sets) calculations for a C-X and Si-X bonds (X = H, F, Cl) and made an interesting observation that the minimum of the ionic curve... 

To the best of our knowledge, cationic pentacoordinate silicon(IV) complexes with SiOs skeletons have not yet been described, whereas the chemistry of cationic hexacoordinate silicon(IV) species with SiO(> skeletons is well established. 

Transition metal-free hydrosilylation of carbonyl compounds can be realized with the use of Brpnsted or Lewis acids as well as Lewis bases. Alkali or ammonium fiuorides (CsF, KF, TBAF, and TSAF) are highly effective catalysts for the reduction of aldehydes, ketones, esters, and carboxylic acids with H2SiPh2 or PMHS. Lithium methoxide promotes reduction of esters and ketones with trimethoxysilane. A generally accepted mechanism of Lewis base-catalyzed hydrosilylation of carbonyl compovmds involves the coordination of the nucleophile to the silicon atom to give a more reactive pentacoordinate species that is attacked by the carbonyl compound giving hexacoordinate silicon intermediates (or transition states), in which the hydride transfer takes place (Scheme 30) (235). 

The SCD involves diabatic state curves of mixed character. Sometimes, however, the mixed character of the curves will conceal important information which can be revealed by looking at the VB configurations individually. Figures 5(a) versus 5(b) shows the HL and triple ionic structures for Sn2 reactions on carbon versus silicon. It is seen that the Si case is favored over the C case by having a much more stable triple ionic structure as well as a larger covalent-ionic VB mixing. Consequently, the Sn2(C) is typified by a TS and a barrier, while the Sn2(S1) case by a stable pentacoordinated species, (SiHsXa) . 

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. 

Alternatively, unreactive mixtures of organosilicon hydrides and carbonyl compounds react by hydride transfer from the silicon center to the carbon center when certain nucleophilic species with a high affinity for silicon are added to the mixture.76 94 This outcome likely results from the formation of valence-expanded, pentacoordinate hydrosilanide anion reaction intermediates that have stronger hydride-donating capabilities than their tetravalent precursors (Eq. 6).22,95 101... 

Pentacoordinate silicon complexes can also be prepared via the reaction of diaryldichlorosilanes with carbenes 4 (R = Et, Pr R = Me). Interestingly, a 2-(trimethylsilyl)imidazolium salt (56) was formed when Me3SiI was treated with 4 (39). Unfortunately, no information regarding the crystal structures of these species is available. 

As demonstrated by single-crystal X-ray diffraction, the /-coordination polyhedra of 85-87 are distorted trigonal bipyramids, with each of the axial positions occupied by the oxygen atoms. This is shown for compound 86 in Fig. 11. In all cases, the crystals are formed from pairs of (A)- and (A)-enantiomers. Selected geometric parameters for 85-87 are listed in Table XIII. As can be seen from the Si-O [1.8004(10)-1.829(6) A], Si-N [1.741(7)-1.764(6) A], and Si-C distances [1.867(8)-1.915(2) A], the A/02N2C frameworks of 85-87 are built up by five normal covalent bonds and do not involve a bonding system in the sense of the 4+1 coordination usually observed for pentacoordinate silicon species with Si-N bonds. 

The oxidative cleavage of the Si—C bond requires the presence of at least one heteroatom on silicon. The proposed mechanism involves formation of a pentacoordinate silicon species as the initial key intermediate and a hexacoordinate silicon species in the transition states (equation 75). 

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