Electrophiles silicon

Enolate reactivity depends on the electrophile. Enolates generally form CC bonds with carbon electrophiles, and OSi bonds with silicon electrophiles. 

Silicon presents an attractive option among eledrophilic activating and dehydrating agents of hemiacetals because of the wide commercial availability of eledrophilic silicon sources. The two main classes of silicon electrophiles used, namely silyl halides and silyl sulfonates, have been demonstrated to promote a variety of glycosylations including some examples of oligosaccharide synthesis. 

Related studies have recently been reported by Nakamura et al. In this case, the reaction, named silyldemetalation, required the capture of the vinyl gold intermediate by a silicon electrophile [108]. 

Perhaps the most direct method of synthesizing an acyl silane is by reaction of an acyl lithium, prepared by carbonylation of an alkyl lithium at —110°C, with a silicon electrophile, illustrated in Scheme 28106,107. Although this method is successful for a variety of alkyl acyl silanes in moderate yields, low temperatures must be used, and the method is not suitable for aryl acyl silanes. 

Second only to lithium enolates in usefulness are silyl enol ethers. Silicon is less electropositive than lithium, and silyl enol ethers are more stable, but less reactive, than lithium enolates. They are made by treating an enolate with a silicon electrophile. Silicon electrophiles invariably react with enolates at the oxygen atom firstly because they are hard (see p. 237) and secondly because of the very strong Si-O single bond. The most common silicon electrophile is trimethylsilyl chloride (Me3SiQ), an intermediate made industrially in bulk and used to make the NMR standard tetramethyl silane (Me4Si). 

You have seen reaction at oxygen before, Enolates react on oxygen with silicon electrophiles and we found the products, silyl enol ethers, useful in further reactions. Enol esters also have their uses—as precursors of lithium enolates, for example. You saw one being used like this on p. 683,... 

Cyclopropenylcarbinyl anions have been generated, including the parent species (171) as well as substituted derivatives. Reaction with carbon or silicon electrophiles favored substitution on the ring. ... 

The addition of sulfur nucleophiles to alkynes is a less developed transformation. However, Yamamoto described the attack of the sulfur atom of aryl thioethers to afford benzothiophenes (equation 31). More recently, it has been showed that propargylic thioethers or thioacetals undergo migration to give carbenes that cyclize in hydroarylation processes and thiocarbamates that evolve by propargylic rearrangement. Thiosilanes can perform as both sulfur nucleophiles and silicon electrophiles in intramolecular reactions to afford benzothiophenes (equation 32). ... 

Monolithiation of 2,6-dibromopyridine is best achieved by inverse addition -dibromide to n-butyllithium, or by using dichloromethane as solvent - probably a unique application of this solvent to lithiation. A normal lithiation of 2,5-dibromopyridine, but at -90 °C, produces clean 2-substituted product with a hindered silicon electrophile. ... 

The iron-catalysed reaction of heteroarenes, including indoles, pyrroles, thiophene, and furan, with 3-methyl-2-quinonyl boronic acids allows the formation of alkylated products, such as (68), rather than the more usual alkenylated products. The unusual alkylation, at the 5-position, of oxindoles to give products such as (69) has been reported using the acid-catalysed reaction with benzylic alcohols in nitromethane. Silylation of indole, at the 3-position, to give (70) has been achieved using a cationic ruthenium(II) sulfide complex as a catalyst. A sulfur-stabilized silicon electrophile is formed resulting in a Wheland intermediate which is deprotonated by sulfur atom. ... 

The required silanes can be formed by the reaction of organolithium species or Grignard reagents with silicon electrophiles, but also by palladium- and rhodium-catalysed coupling reactions (Scheme 2.108). 

Deprotonation of -amidobenzene and /3-amidophospho-nates may be achieved using j-BuLi. The resulting species may be alkylated, e.g., with phosphorus or silicon electrophiles (eq 43), in a reaction that is mediated by a complex-induced proximity effect (CIPE). ... 

Classical synthetic routes to arylsilanes consist of the reaction of aryl Grignard or aryllithium compounds with silicon electrophiles and many of these routes are presented in the various reviews. Recently, metal-catalyzed reactions of aryl halides with silanes have proven to be useful routes to functionalized arylsilanes. Alami reported the first platinum-catalyzed selective silylation of aryl iodides and bromides 9 having electron-withdrawing group to give arylsilanes 10 with triethylsilane and sodium... 

Reaction with Nucleophilic Reagents with Silicon Electrophiles... 

Scheme 3-3. The reaction of nucleophilic reagents with silicon electrophiles.

---