Silicon Lewis acids

Regio-, enantio-, and diastereo-selective vinylogous aldol additions of silyl dienol ethers to aldehydes use a Lewis base (a chiral bis-BINAP-phosphoramide) to activate a Lewis acid (silicon tetrachloride).139... 

Although adducts between the Lewis acid silicon and sundry Lewis bases are important as intermediates in the reactions of Si—Si bonds, very few stable adducts are known. 

Woerpel and Nevarez postulated a stepwise mechanism for the formation of 185 (Scheme 7.54).127 Coordination of benzaldehyde to the Lewis acidic silicon atom of 169g86 triggers an attack by the pendent A-aryl group to form zwitterion 189. Isomerization of 189 to azomethine ylide 190128 enabled a 4n-electrocyclization to produce aziridine 185. The thermodynamic driving force for this reaction was attributed to the relief of the silaaziridine s ring strain. 

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. 

The same trend is evident for the more Lewis-acidic silicon halides or pseudo-halides, " when the coordination number is increased from 4 to 6 (Scheme 8). Variable temperature Si NMR studies show the equilibrium between four-, five- and six-coordinate silicon atoms in imine and enaminechelate complexes. There are numerous Si NMR spectroscopic studies of hypervalent silicon... 

From these data, we have proposed a mechanism shown below, which involves coordination of carbonyl oxygen to the Lewis acidic silicon center followed by hydride transfer. Combination of the Lewis acidity and the ability of hydride transfer in pentacoordinate hydridosilicates opens a new versatile reducing systems. 

The concept of Lewis base activation of Lewis acids was developed by Denmark, and it takes advantage of the fact that the Lewis acidity of silicon is increased when the coordination sphere is enlarged. If chiral Lewis bases are provided, here chiral phosphoramides and bisphosphoramide, SiCl can be efficiently employed in asymmetric Mukaiyama aldol reactions. The Lewis base activation can be performed on trichlorosilyl enolates (24), which can be generated in situ by mercury-mediated trans-silylalion of a TMS silyl enol ether (18) in the presence of SiCl. Generation of the hypervalent silicon species produces a more Lewis acidic silicon moiety, which acts to coordinate and activate the aldehyde that has to be brought to reaction (Scheme 2.128) [48]. 

Silicon compounds can also act as Lewis acids, whereas carbon compounds typically cannot. Because a silicon atom is bigger than a carbon atom and can expand its valence shell by using its d-orbitals, it can accommodate the lone pair of an attacking Lewis base. A carbon atom is smaller and has no available d-orbitals so in general it cannot act as a Lewis acid. An exception to this behavior is when the carbon atom has multiple bonds, because then a Tt-bond can give... 

Carbon is the only member of Group 14/IV that commonly forms multiple bonds with itself singly bonded silicon atoms can act as Lewis acids because a silicon atom can expand its valence shell. 

Silicon reacts directly with chlorine to form silicon tetrachloride, SiCl4 (this reaction was introduced in Section 14.17, as one step in the purification of silicon). This compound differs strikingly from CC14 in that it reacts readily with water as a Lewis acid, accepting a lone pair of electrons from H20 ... 

Carbon forms ionic carbides with the metals of Groups 1 and 2, covalent carbides with nonmetals, and interstitial carbides with d-block metals. Silicon compounds are more reactive than carbon compounds. They can act as Lewis acids. 

The reactivities of alkenylsilanes in the presence of a Lewis acid vary depending upon the nature of substituents on silicon as shown in Table 1. 

It is well known in the literature that aluminum chloride, a strong Lewis acid, is a very effective catalyst in Friedel-Crafts alkylations with silicon... 

All these silicon reagents which activate metal surfaces are also Lewis acids, which can also contribute as such to the acceleration of any of the subsequently discussed reactions, because of their inherent Lewis acid properties. 

These TMS-carbamate-mediated NCA polymerizations resemble to some extent the group-transfer polymerization (GTP) of acrylic monomers initiated by organo-silicon compounds [40]. Unlike GTPs that typically require Lewis acid activators or nucelophilic catalysts to facilitate the polymerization [41], TMS-carbamate-mediated NCA polymerizations do not appear to require any additional catalysts or activators. However, it is still unclear whether the TMS transfer proceeds through an anionic process as in GTP [41] or through a concerted process as illustrated in Scheme 14. 

A good example is B (0113)3 which uses a vacant 2 p orbital to form an adduct with ammonia. The elements in the p block beyond the second row of the periodic table have empty valence d orbitals that allow them to act as Lewis acids. The silicon atom in Sip4 is an example. 

Although the allylation reaction is formally analogous to the addition of allylic boranes to carbonyl derivatives, it does not normally occur through a cyclic TS. This is because, in contrast to the boranes, the silicon in allylic silanes has little Lewis acid character and does not coordinate at the carbonyl oxygen. The stereochemistry of addition of allylic silanes to carbonyl compounds is consistent with an acyclic TS. The -stereoisomer of 2-butenyl(trimethyl)silane gives nearly exclusively the product in... 

The addition reaction of allylsilane to acetaldehyde with BF3 as the Lewis acid has been modeled computationally.95 The lowest-energy TSs found, which are shown in Figure 9.2, were of the synclinal type, with dihedral angles near 60°. Although the structures are acyclic, there is an apparent electrostatic attraction between the fluorine and the silicon that imparts some cyclic character to the TS. Both anti and syn structures were of comparable energy for the model. However, steric effects that arise by replacement of hydrogen on silicon with methyl are likely to favor the anti TS. 

The electrophilicity of silicon is enhanced in five-membered ring structures. Chloro dioxasilolanes, oxazasilolidines, and diazasilolidines react with aldehydes in the absence of an external Lewis acid catalyst.113... 

In the l3C-NMR spectrum of the iminosilane, the C20 signal of the THF molecule appears at 73.78 ppm—a downfield shift of nearly 6 ppm compared with that for free THF. This reflects the strong Lewis acid character of three-coordinated silicon in the iminosilane. 

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