Pentacoordinate silicon anions

The reactivity of penta- and hexacoordinated silicon compounds has been described to be very different from the reactivity of the corresponding tetracoordinated derivatives [ 1], An increase in reactivity towards nucleophiles has been observed in the case of neutral and anionic pentacoordinated silicon compounds as exemplified by the following Schemes [2],... 

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. 

Corriu and coworkers37 showed that alkoxysilanes R Si(OR )4-n [in particular RSi(OMe)3] give better yields for the DDB cyclization than do chlorosilanes. The reaction of anionic pentacoordinated silicon complexes [RSi(02CgH4-0)2] Na+ with DDB and subsequent LiAlPLi reduction give 1-R-diphenylsiloles (R = Me, Ph)38. Bis(silacyclopentadien-l-yl)alkanes were formed from DDB and a, a>-bis(dihalomethylsilyl)alkanes39. 

The silicon-halogen, silicon-oxygen, and silicon-sulfur bonds of the haiogenosil-anes, siiyl ethers, and siiyi thioethers are cleaved by reaction with LAH, AIH3, or DIBAH, and the corresponding siiyl hydrides are obtained [CGI, CG2]. Ultrasound activation can be applied [LG2] (Figure 5.9), Anionic pentacoordinated silicon compounds are reduced to hydrogenosilanes by LAH or DIBAH [BC6]. 

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). 

Although anionic, pentacoordinate silicon species have received attention, the isolation of siliconates 194, 103, 195, 196 (Figure 28) by Perozzi, Martin and colleagues (155) has provided an opportunity to study in detail both structure and stereomutation of a pentacoordinate silicon compound (155, 321). This point will be discussed further in Sect. V-C. The crystal structure of 195 shows that the silicon is pentacoordinated with the fluorine and the two carbon atoms forming the equatorial plane of a somewhat distorted trigonal bipyramid (321). It is essentially the same as that proposed for similar siliconates on the basis of NMR data obtained in the solution phase (155). 

Although the nitrogen of the N = S = N group has low nucleophilicity, an intramolecular cyclization can occur in the presence of a catalyst (fluoride ion). Thus, heating of l-pentafluorophenyl-3-trimethylsilyl-l,3-diaza-2-thiaallene 170 in acetonitrile in the presence of cesium fluoride leads to 4,5,6,7-tetrafluoro-2,l,3-thiadiazole 171 (90JFC(50)359) (Scheme 157). It is assumed that 171 is formed via the intermediate pentacoordinated silicon anion, which is further transformed into the N-nucleophile with elimination of trimethylfluorosilane. 

It has been found that the general structural features which stabilize a square pyramidal geometry (or rectangular pyramidal geometry for spirobicyclic derivatives) for phosphoranes23 may be extended to the isoelectronic pentacoordinate silicon anions. These requirements may be summarized as follows ... 

Reacting la,b with MeLi in HMPA as active solvent and in the presence of MeOH as trapping agent, the attack of a methyl anion at the silicon atom of the silacyclobutane is the first reaction step and gives a pentacoordinated silicon anion (Scheme 2). Such five-coordinated species are discussed as intermediates during the ring opening polymerization of silacyclobutenes, -butanes, and -pentenes [2]. Furthermore, five-coordinated silicon species are well described to be stable compounds [3]. 

Whereas further purification of 6 was unsuccessful because of the lability in air and moisture, the crystals gave the satisfactory spectroscopic data to indicate the structure. Although the Si-H coupling constants for anionic pentacoordinate hydridosilicates have never been reported, the value ( Jsi.nl = 253.5 Hz) is quite reasonable in comparison with the value reported for the (tetrafluoroethyl)silane-trimethylamine complex (246 Hz), indicative of the equatorial location of the hydrido ligand which is bonded to the sp hybridized orbital on silicon. 

Acyl silanes are extremely sensitive towards nucleophiles and nucleophilic bases for example, alcoholic solutions of benzoyl triphenylsilane containing a trace of aqueous hydroxide ion rapidly produce triphenylsilanol and benzaldehyde . Three reasonable mechanisms may be conceived for this reaction (Scheme 60) Sat2 displacement at the silicon atom (path A) nucleophilic attack at the carbonyl carbon atom followed by Brook rearrangement, initially to give a hemiacetal (path B) and nucleophilic attack at the silicon atom to form a pentacoordinate silicon anionic intermediate, followed by migration of the nucleophile to the carbonyl group and subsequent Brook rearrangement (path C). 

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