Alkynylsilanes reactions

This section describes Michael-analogous processes in which, mostly under electrophilic conditions, ally - or alkynylsilanes undergo addition to enones or dienones (Sakurai reactions). The intramolecular addition of allylsilanes is an extremely useful reaction especially for the construction of carbocyclic ring systems, which occurs in a diastereoselective manner, in many cases with complete asymmetric induction. 

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 (0.306 mol). 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/36mmHg. 

Terminal alkynes can be converted readily into alkynylsilanes by reaction of the corresponding alkyne anion or its metalloid equivalent with a suitable chlorosilane (/). The reverse reaction, that of liberation of the alkyne, is quite facile, being effected by several reagent combinations, including hydroxide ion, methanolysis, fluoride anion, silver(i) followed by cyanide anion, and methyl lithium-lithium bromide (2). 

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%). 

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. 

As was mentioned above (see Section 12.13.4.2), the 1,1-organoboration of alkynylsilanes, -stannanes, and -germans with trialkylboranes and boracyclanes occurs under very mild (Sn), mild (Ge), or harsh (Si, > 100 °C) reaction conditions <1995CCR125>. Similar reactions with triallylboranes proceed in general under milder conditions in comparison with trialkylboranes and furthermore a competition between 1,1- and 1,2-allylboration often takes place... 

Cyclization reactions of vinyl- and alkynylsilanes have been reviewed100. The course of the reaction of the cyclohexenone derivative 184 depends on the catalyst employed ethylaluminium dichloride gives solely the product 185 of 1,6-addition, whereas tetrabuty-lammonium fluoride yields a mixture containing 69% of the 1,4-adduct 186 and 31% of the bridged compound 187 (equation 89)101. Intramolecular addition reactions of allylic silanes102 may also be catalysed by Lewis acids (equation 90) or fluoride ions, and in this case an allyl anion or a pentavalent silicon intermediate may be involved (equation 91). Such reactions are exemplified by the formation of a 1 5 mixture of the diastereomers 189 and 190 when the cyclohexenone derivative 188 is treated with ethylaluminium dichloride (equation 92). In the presence of fluoride anion the ratio of the isomers is reversed103. 

Another application of [Bu4N][Ph3SiF2] 826 involves the silicon-carbon bond cleavage of allyl-, benzyl-, and alkynylsilane derivatives 827-829.826 Subsequent reactions of the generated carbanions with electrophiles (Scheme 112) and alkyl halides (Scheme 113) provide high yields of carbon-carbon coupled products. 

Electrophilic substitution reactions on alkynylsilanes are also known. In 1978 K. Ultimootd, M. Tanaka and coworkers by an intramolecular reaction carried out the synthesis of a macrocyclic ketone. 

The homoallylic alcohol can be liberated by oxidation followed by base-catalyzed f>-elimination. The alcohols obtained in this way are formed in 70 5% e.e. A similar reaction occurs with alkynylsilanes to give propargylic alcohols in 70-90% e.e.72... 

Reactions of lithium and titanium compounds 2, generated in situ by deprotonation of alkynylsilanes with tm-butyllithium, followed by addition of titanium(IV) isopropoxide and an aldehyde result either in a-hydroxyallenes without axial dissymmetry or in /J-hydroxyalkyn-es90 91. 

The dominance of an alkoxy group over a silyl group is manifested in the generation of amides from reaction of oc-silyl enol ethers with chlorosulfonyl isocyanate, followed by hydrolysis. Simple vinylsilanes and alkynylsilanes undergo carbamidation [34],... 

When the hydrogen of the HA is attached to carbon, this reaction is the same as 2-21. We do not list the many hydrolyses of sodium or potassium enolates, etc. found in Organic Syntheses. The hydrolysis of a Grignard reagent to give an alkane is found at OS II, 478 the reduction of a vinylic tin compound at OS 66, 75 and the reduction of an alkynylsilane at OS 67, 149. 

Dehydrogenative silylation has also been observed for terminal alkyne substrates. Doyle and co-workers reported in 1991 that a small amount (6%) of alkynylsilane was observed in the product mixture that results from reaction of phenylacetylene and Et3SiH catalyzed by Rh2(pfb)4.41 The remaining components of the product mixture resulted from hydrosilylation. Crabtree and co-workers have found that in reaction of terminal alkynes with various tertiary silanes, dehydrogenative silylation can become the predominant route, depending on reaction conditions [Eq. (7)].42... 

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