Nucleophilic addition reactions silicon nucleophiles

Thus it is clear that a wide variety of polar tr-bonded reagents will react with the silicon-carbon double bond. In general, the reactions can be explained on the basis of a two-step addition reaction initiated by nucleophilic attack on silicon. 

Group-transfer polymerizations make use of a silicon-mediated Michael addition reaction. They allow the synthesis of isolatable, well-characterized living polymers whose reactive end groups can be converted into other functional groups. It allows the polymerization of alpha, beta-unsaturated esters, ketones, amides, or nitriles through the use of silyl ketenes in the presence of suitable nucleophilic catalysts such as soluble Lewis acids, fluorides, cyanides, azides, and bifluorides, HF. ... 

The results in Table 3 were explained as shown in Scheme 4. From the fact that no kinetic isotope effect was observed in the reaction of phenyl-substituted disilenes with alcohols (Table 1), it is assumed that the addition reactions of alcohols to phenyltrimethyl-disilene proceed by an initial attack of the alcoholic oxygen on silicon (nucleophilic attack at silicon), followed by fast proton transfer via a four-membered transition state. As shown in Scheme 4, the regioselectivity is explained in terms of the four-membered intermediate, where stabilization of the incipient silyl anion by the phenyl group is the major factor favoring the formation of 26 over 27. It is well known that a silyl anion is stabilized by aryl group(s)443. Thus, the product 26 predominates over 27. However, it should be mentioned that steric effects also favor attack at the less hindered SiMe2 end of the disilene, thus leading to 26. 

Because of the stability of iron tricarbonyl diene complexes, conjugated dienals are protected from polymerization when complexed, while other reactions can be carried out at the aldehyde functionaUty. A number of synthetically attractive nucleophilic transformations of the aldehyde can be performed on these complexes. These include, aldol reactions, Michael additions, reactions with organozinc, -silicon, -boron, and -tin... 

Alkynylsilanes can function as carbon nucleophiles in addition reactions to electrophilic ir-systems. In principle electrophilic addition reactions to alkynylsilanes can occur to produce a- or P-silyl-substituted vinyl cations, as illustrated in Scheme 37. The a-silyl carbocation is not the most stabilized cation, the reason being that the caibon-silicon bond can achieve coplanarity with the vacant orbital on the -carbo-nium ion, making possible 3-stabilization through hyperconjugation. Depending on the configuration of the carbocation, the developing vacant orbital can exist as a p-orbital, as in structure (75a), or an rp -hy-brid, as in structure (75b). 

Silicon compounds with coordination number larger than four are the object of many studies first with respect to their application as catalysts in organic and inorganic syntheses and second as starting materials for the preparation of a broad variety of organosilicon compounds [1]. Additionally, hypervalent silicon hydride compounds can successfully be used as model compounds to study, for instance, the mechanism of nucleophilic substitution reactions, which is of great interest since the silicon atom is able to easily extend its coordination number [1]. Moreover, hypervalent silanes are suitable as starting materials for the synthesis and stabilization of low-valent silanediyl transition metal complexes [2-5]. 

A nucleophile adds, forming a trigonal bipyramidal pentacovaient intermediate that then ejects the leaving group in the microscopic reverse of the nucleophilic addition reaction. The pentacovaient intermediate is often short-lived, and this is shown by enclosing it in brackets. The pentacovaient intermediate bonds that are colinear are called axial (or apical) bonds, whereas the three bonds that lie in the plane perpendicular are called equatorial. Three common elements that can react by this pathway are silicon, phosphorus, and sulfur. Hydrolysis of phosphate esters like RNA occurs by this path. 

More recently, Majetich et al. have proposed pentacoordinate allylsilicate intermediates formed from fluoride ion addition to silicon as an ambient nucleophilic species. However, nothing had been known so far about pentacoordinate allylsilicates, before we have initiated the present study, whereas a number of pentacoordinate organosilicates have been well characterized. It is an interesting problem whether the fluoride-catalyzed reactions actually involve free allyl anions or not. The author will discuss this problem in the next section somewhat in detail. In this section the author will describe preparation of well-defined pentacoordinate allylsilanes and their reactions which have been published recently. ... 

The aldol reaction is one of the most useful carbon-carbon bond forming reactions in which one or two stereogenic centers are constructed simultaneously. Diastereo-and enantioselective aldol reactions have been performed with excellent chemical yield and stereoselectivity using chiral catalysts [142]. Most cases, however, required the preconversion of donor substrates into more reactive species, such as enol silyl ethers or ketene silyl acetals (Scheme 13.45, Mukaiyama-type aldol addition reaction), using no less than stoichiometric amounts of silicon atoms and bases (Scheme 13.45a). From an atom-economic point of view [143], such stoichiometric amounts of reagents, which afford wastes such as salts, should be excluded from the process. Thus, direct catalytic asymmetric aldol reaction is desirable, which utilizes unmodified ketone or ester as a nucleophile (Scheme 13.45b). Many researchers have directed considerable attention to this field, which is reflected in the increasing... 

A sophisticated chemoselective 1,4-addition of silicon nucleophiles to a.P Unsaturated aldehydes (1,2- versus 1,4-addition) was disclosed by the groups of Ibrahem and Cordova, merging copper(I)-catalyzed silicon-boron bond activation with iminium ion catalysis (Scheme 13) [40]. Proline derivative (S)-68 induced good to high levels of enantiocontrol in reactions of enals with either aromatic [22a-c —> (S)-67a-c] or aliphatic substituents [22d —> (/ )-67d] in the p-position. The chemoselectivity was excellent throughout. Moreover, p,p-disubstituted substrates were also converted chemoselectively with acceptable 76% ee [(S)-69],... 

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