Silicon chelates effect

An effective route to O Si pentacoordinate silicon chelates consists of the reaction of chloromethyl(dimethyl)chlorosilane (ClCH2SiMe2Cl, 8) with a variety of N- or O-trimethylsilylamides, 5 lactams, ureas, 5... 

Theoretical and experimental studies on penta-coordinate silicon derivatives demonstrate that their existence is determined by a combination of factors electronegativity of the substituents and steric interactions between substituents. One should also emphasize the role and significance of polydentate ligand (chelate effect), the size and number of chelate rings involving the silicon atom, and strain reduction for five-membered ring system, wich can stabilize unusual structures, as well as to the role of medium effect. This is consistent with the results of NMR spectroscopic studies. 

Silatranes 1 are meanwhile classical cage compounds with donor-acceptor interactions and represent examples of hypercoordinated silicon [2], The donor-acceptor contact in 1 is formed by an interaction of the Lewis-basic amino group with the Lewis-acidic silicon center favored by the chelate effect. Numerous examples show that electron-rich transition metal complexes also possess Lewis-basic properties [3, 4]. Isolobal replacement of the NR3-unit in 1 by a d ML4-unit [5] leads to compounds of type 2 [1,6, 7]. These Si/Ni-cages 2 can be regarded as metallosilatranes. Here we report on the s5mthesis, structure and bonding of 2. 

Note that in contrast to the complexes of SiCU with neutral phosphines, as shown above, the complexes with the carbanionic ligands C have at least one silicon-carbon bond. Apparently, this is enough to effectively make an additional P->Si donor-acceptor bond impossible. It is somewhat surprising that this is the case also in solution where an intramolecular P-Si bond should be particularly favored by the chelate effect. 

It is known that silicon with bulky substituents can serve as an efficient stereo-directing functional element. In addition, silicon is a synthetic equivalent for an aUcoxy substituent, which opens a straightforward approach to the stereocontroUed aldol addition of acyclic molecules (Scheme 3.65). The protocol by Fleming ti al. starts with 1,4-addition of a silylcuprate to enoate 325 to give enolate (Z)-326 selectively under kinetic control, probably due to a chelating effect of the metals. On re-protonation and subsequent deprotonation with LDA, the enolate... 

The tautomeric equilibrium between penta- and hexa-coordinated silicon chelates (Scheme 5.60) was studied by Si NMR spectroscopy [3( Si) = —57.6ppm for penta-coordination and 5( Si) = —134.0ppmfor hexa-coordinationj. The study is very detailed involving the effects of temperature, solvent, substituent, and counterion effects [142]. 

Axially chiral bis-isoquinoline N,N -bisoxide (S)-17 has been reported to promote the addition of Me3SiCN (1.5-2.0 equiv.) to imines 78, derived from aromatic aldehydes (Scheme 7.17) here, CH2CI2 was identified as an optimal solvent. The reaction is stoichiometric in 17, and exhibits partial dependence on the imine electronics (62-78% ee), but much less than that observed for the allylation of PhCHO catalyzed by QINOX (24) (vide supra). The o-substitution had a positive effect in the case of Cl (95% ee), but a very negative effect in the instance of MeO (12% ee). Chelation of the silicon by the N-oxide groups was suggested to account for the stereochemical outcome. Analogues of 17 were much less successful [75]. 

Thus, certain structural features, in particular incorporation of the central atom into a five-membered ring, chelate, macrocyclic and cryptate effects may stabilize the geometry of penta- and hexa-coordinate silicon species. Much of the insight as to which structural features might be expected to stabilize this species comes from studies of species involving higher valent states of other nonmetallic elements of the third row of the periodic table os-so )... 

For the cationic pentacoordinate tropolonates there appears to be an effect of the nature of the non-chelating group R on chemical shift. Those tropolone complexes in which the silicon atom is bonded to an sp hybridized carbon in R ([47] and [48]) exhibit a chemical shift of — 141 ppm compared with hexacoordinate T3Si (<3 = — 139 ppm). 

The acyclic compound l-NaphPh(H)SiOCOPh is not significantly converted to benzaldehyde on thermolysis, even at temperatures considerably higher than those (100-160 °C) which are effective with the chelate system. This reaction may proceed via the transient formation of a chelated silanone 82, since the trisiloxane 80 is isolated as the silicon-containing product. Confirmation of this mechanism was obtained in the reaction of 78 with carbon dioxide, from which the silyl ester of formic acid 81 can be isolated and... 

Two chelating agents, ethylenediaminetetraacetic acid (EDTA) and 8-hydroxyquinoline, also have been used for calcium and magnesium determinations. They are effective in controlling a number of interfering ions, including phosphate, sulfate, aluminum, silicon, boron, and selenium. 

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