Under co-catalysis triisopropylsilylacetylene reacted with enones to form β-ethynylketones in high yields. The reaction worked well with ethynylTBS, ethynylTBDPS as well, although ethynylTES gave only 40% yield. Under the reaction conditions the non-silylated terminal acetylenes phenylacetylene and 1-octyne gave alkyne oligomerization. An asymmetric version of the reaction, which gave good yields and acceptable ee values, was also presented.33
Scheme 27. β-Ethynylation of α,β-unsaturated ketones
Carreira and coworkers reacted terminal acetylenes including ethynyltrimethylsilane with aldehydes in the presence of (+)-N-methylephedrine to give the propargyl alcohol in high yield and high enantiomeric excess.34
Scheme 28. Asymmetric ethynylation of an aldehyde
The aldehyde 24 was reacted with ethynyltrimethylsilane under Carreira conditions to give a single diastereomer of 25, which was O-silylated followed by protiodesilylation of the TMS group. This material was carried forth in a synthesis of hyptolide and 6-epi-hyptolide.35
Scheme 29. Diastereoselective ethynylation of 24 in a synthesis of hyptolide
In keeping with the common use of ethynylsilanes as surrogates for the simple ethynyl organometallics, an ‘in situ’ process for the ethynylation of aldehydes was developed. In this chemistry a combination of ZnBr2, TMSOTf, and Hünig’s base was used to generate the ethynylzinc reagent in situ and along with a silylating agent this was reacted with the aldehyde to generate the doubly silylated propargyl alcohol.36
Scheme 30. ‘In situ’ ethynylation of aldehydes
The aminomethylation of terminal alkynes was applied to a variety of acetylene derivatives including one example with ethynyltriethylsilane, which provided the triethylsilylated propargyl amine in good yield. This was subsequently protiodesilylated and the resulting propargyl amine converted to a mixed bis(aminomethyl)alkyne in a 49% yield over the three steps.37
Scheme 31. Aminomethylation of terminal alkynes
Ethynyltriisopropylsilane was employed in a Ni-atalyzed, three-component reaction of the ethynylsilane, an alkyne, and norbornene. A variety of norbornene derivatives were reacted with good success. When ethynyltriisopropylsilane was used as the sole acetylene reactant the bis(triisopropylsilyl)-1,5- enyne was produced. One example with a bicyclo[2,2,2]octane gave only a 12 percent yield when reacted with ethynyltriisopropylsilane.38
Scheme 32. Three-component reaction of norbornene with and ethynylTIPS and an alkyne
Ethynyltrimethylsilane could be directly alkylated to 1-trimethylsilyldodec-1-yne on modest yield. The yield of this sole silicon example was comparable to the direct alkylation of other terminal alkynes.39
Scheme 33. Copper-catalyzed alkylation of ethynyltrimethylsilane