Tetracoordinated silyl

Owing to the high Lewis acidity the group 14 organometallic cations are polymerization catalysts par excellence. so Silanorbonyl cations and triethylsilyl arenium have been shown to be efficient catalysts for metal-free hydrosilylation reactions. Chiral silyl cation complexes with acetonitrile have been applied as cata -lysts in Diels Alder-type cyclization reactions °792 intramolecularly stabilized tetracoordinated silyl cations have been successfully used as efficient catalysts in Mukaiyama-type aldol reactions. 

Of particular interest in the context of this review are the solvent- or anion-stabiUzed tetracoordinated silyl cations 7. In the case of the solvent complexes 11-16, structural and/or NMR spectroscopic data clearly indicate a covalent... 

If there is no other interaction, the reaction proceeds through an acyclic TS and steric factors determine the amount of syn versus anti addition. This is the case with BF3, where the tetracoordinate boron-aldehyde adduct does not offer any free coordination sites for formation of a cyclic TS. Stereoselectivity increases with the steric bulk of the silyl enol ether substituent R1.50... 

The parameters of organophosphorus betaines 5, 6,15,17, 20, 21, 24, 25, and 31 and the corresponding phosphonium cations R3P+ CHR1] 2, as well as cations of silylated phosphonium salts R CHR -SiR2,82 84 are quite similar. The 31P NMR signals of the betaines lie in the region typical of tetracoordinated phosphorus.85,86... 

No conformational dependence (NCD). Groups of this type include monatomic substituents such as hydrogen and the halogens cylindrical substituents such as the ethynyl and cyano groups, and tetracoordinate symmetric top substituents such as the methyl, trifluoromethyl and silyl groups. 

While for silyl and germyl cation complexes tetracoordination of the element prevails, stannyl cations favor pentacoordination. That is, the trigonal-bipyramidal environment of the Sn atom in the diwater complex of the stannylium ion Me3Sn ... 

The tetracoordinate silicon cation is a rather common species in solution. It may be generated by heterolytic cleavage of a bond from silicon to a reactive ligand, as a result of interaction of the silicon center with an uncharged nucleophile like amine, imine, phosphine, phosphine oxide, and amide. Since these nucleophiles are also known to be effective catalysts for many displacements at silicon including important silylation processes (86,89,235-238), the cations of tetracoordinate silicon have received attention as possible intermediates in these reactions according to Eq. (40) (78,235,239-243). 

Tetracoordinate Si+ complexes may undergo transformations in the mobile ligand, since nucleophilic attack of the counterion may be directed toward electrophilic centers located in the ligand group. Some reactions involving these transformations are of importance in synthesis (86,272-274). For example, the silylation of phosphorus alkyl esters has been broadly explored by bioorganic chemists as a convenient method of generation of phosphorus acids [Eq. (57)] (273). The mechanism of the silylation reaction [Eq. (57)] has been well documented for the cases where X is... 

Four crystal structures of lithiated bis(silyl)hydrazines are described.16,32-35 The Li centers exhibit tri- and tetracoordination, but because of Li-C interactions, the Li atoms may also adopt higher coordination numbers. All lithiated bis(silyl)hydrazines show side-on and end-on coordinated lithium. 

While for silyl and germyl cation complexes tetracoordination of the element prevails, stannyl cations favor pentacoordination. That is, the trigonal-bipyramidal environment of the Sn atom in the diwater complex of the stannylium ion Me3Sn + [Me3Sn(H20)2]+ 84178 has its counterpart in the tetrahedral structure of the pro-tonated silanol 85.116 The structure of the bis(acetonitrile)tricyclohexyltin hexa-fluoroantimonate 86 SbF6179 reveals pentacoordination for the tin atom,180 while all... 

The transsilylation reaction described above was studied with 8 and the silylated hydrazides l.33,34 The E,Z stereoisomers of la35 reacted in different ways, shown in Eqs. (6) and (7). Initially both isomers form unstable tetracoordinate silane intermediates (9a-E, 9a-Z), which can be observed at low temperatures in CD2C12 solutions by 29Si NMR spectroscopy. The intermediates 9a-E and 9a-Z could be observed for about 20 min at — 70 °C, before reacting further to form five-membered and six-membered chelate products 10a and 11a, respectively. At ambient temperature the reaction proceeds immediately without observation of intermediates. 

In summary, trialkylsilylium ions R3Si+ are stabilized by forming tetracoordinated Si complexes of type III. Pentacoordination at Si will only occur if there are just weak steric interactions between substituents R and solvent S. Mostly, this is parallel to a low internal stability of the silylium ion in question. Accordingly, R2HSi(S)2+ complexes can be observed experimentally while this is not possible for trialkyl silyl cations. [43]... 

The hypersilyl group allows isolation of stable gallium silyl compounds with tri- and tetracoordinated gallium centres. The GaSi bond seems to be inert to protic reagents. 

In cases in which the high Lewis acidity of the cationic silicon is decreased by an intramolecular Lewis basic group (LB), as in the tetracoordinated cation 9 (and likewise in the siliconium ion 10 Scheme 2), interactions between the positively charged silicon atom and the solvent and/or anion are of minor importance. The structures, spectroscopic properties, and reactivities of these sUyl cations are greatly determined by the electron-donating ability of the LB groups. As a consequence, the structural and spectroscopic features of silyl cations 21 and 22, which are stabilized by intramolecular electron donation from an aryl substituent, closely resemble those of benzenium ions 11 (Scheme 5) [33, 34]. The intramolecular stabilization operative in silylium ions 9 offers the intriguing possibility of... 

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