Reactions of Alkynylsilanes

A solution of hex-l-yne (0.3 mol) in ether (100 ml) was treated consecutively at — 78 °C with a solution of n-butyl lithium (0.306 mol) in hexane and with TMSC1 (0.306mol). The reaction mixture was brought to ambient temperature, stirred for 2h, and then quenched with ice-water. The layers were separated, and the aqueous layer was re-extracted with pentane. The combined organic extracts were washed with water and brine, and dried. Concentration and distillation gave the alkynylsilane (0.267mol, 89%), b.p. 71-73 °C/36 mmHg. 

Both terminal and internal propargylsiianes are versatile intermediates for the preparation of allylsilanes (Chapter 5). 

To a solution of t-BuLi (21 mmol, 2.3 m in pentane), cooled to — 78°C, were added sequentially with stirring ether (20 ml), TMEDA (20 mmol), and hept-2-yne (20 mmol). The yellow slurry thus produced was allowed to come to 0°C, and was stirred at this temperature for a further hour. The yellow solution was then cooled to — 78°C, and treated dropwise with TMSCI (24mmol). The mixture, on reaching ambient temperature, was poured on to ice-water (20ml), and the layers were separated. The aqueous layer was extracted with ether (3 x 20 ml), and the combined extracts were washed with aqueous HC1 (20 ml, 3 m) and brine, and dried. Concentration and distillation gave the product (16.2mmol, 81%), b.p. 69-70°C/ 10 mmHg. 

To a cooled (2-8°C) solution of A1C13 in carbon disulphide or nitrobenzene was added dropwise an equimolar mixture of the alkynylsilane and the acid chloride (or anhydride), dissolved in a little of the same solvent. After being stirred for 30 min, the reaction mixture was poured onto dilute sulphuric acid/ice. Normal isolation procedures gave the alkynone (50-90%). 

Phenyl trimethylsilylethyne (Chapter 20) undergoes a fluoride-ion-catalysed addition to aldehydes and ketones. This provides a remarkably mild, relatively non-basic method for the generation of an alkynyl anion or 

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An interesting X-ray structure of the highly strained, sp hybridized vinyl cation 8 was recently reported by Muller et al. The synthesis of 8 was accomplished by the reaction of alkynylsilane 7 and triphenylmethyl (trityl) cation. 

Reactions of alkynylsilanes with electrophiles in the presence of a Lewis acid catalyst such as BF3, AICI3, or TiCl4 generally occur regioselectively at the silicon-bearing carbon, due to the [3-effect, and under mild conditions. 

The GaCls-promoted reaction of alkynylsilanes with arenes, fhen treatment wifh MeLi followed by hydrolysis, yields -arylvinylsilanes (Scheme 10.206) [538]. When D2O is added in fhe hydrolysis step, the a-carbon of fhe vinylsilane product is deuterated. This indicates fhe presence of vinylgallium species in fhe reaction mixture. The arylation mechanism probably involves the Friedel-Crafts-type reaction of GaClj-coordinated alkynylsilanes at the -position. 

Alkynyltrimethylsilanes act as effective alkynyl donors in the presence of a fluoride ion source [15]. Recent studies on the Sonogashira-type reaction of alkynylsilanes have revealed that the use of a Cu(I) [554] or Ag(I) [555] salt leads to highly efficient coupling with vinyl and aryl triflates (Scheme 10.213). 

Although the aluminum chloride or tin(IV) chloride-promoted reactions of alkenylsilanes with acid halides or highly reactive alkyl halides such as chloromethyl alkyl ethers or the corresponding sulfides afford the Friedel-Crafts type products, it is noted that these reactions are not always stereospecific (eq (16)) [13]. The similar reactions of alkynylsilanes with electrophiles have been studied (eq (17)) [14]. 

An alternative, less general procedure for the preparation of alkynyl(phenyl)iodonium triflates in moderate yields employs the reaction of alkynylsilanes [493], alkynylstannanes [494], or alkynylboronates [297] with Zefirov s reagent, 2PhI0-Tf20. This method is also applicable to the synthesis of the parent ethynyl(phenyl)iodonium triflate [495]. 

N. Sakai, N. Uchida, T. Konakahara, Synlett 2008, 1515-1519. Facile and selective synthesis of propargylic amines and 1,6-diynes one-pot three-component coupling reactions of alkynylsilanes, aldehydes and amines by a cooperative catalytic system comprised of CuCl and Cu(OTf)j. 

A useful alternative under neutral conditions is the coupling reaction of alkynylsilanes with aryl- (or alkenyl-) triflates using a CuCl/Pd cocatalyst system (eq 18). For the mechanistic pathway the authors presume that a transmetalation from silicon to copper takes place as the keystep (Cul being less effective in the transmetalation), allowing the synthesis of symmetrical and un-symmetrical arylalk)mes. 

Scheme 3-52. Preparation of vinylsilanes by the reaction of alkynylsilanes with...

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