13 - Aminoalkylation of Acetylenic Compounds

Abstract

The formation of the acetylenic tertiary amines involves the reaction of acetylenic compounds with formaldehyde and secondary amines. Aliphatic 1-alkynes, which are less acidic than phenylacetylene, react sluggishly. The formation of the acetylenic tertiary amines is visualized as a nucleophilic attack of the acetylide anion on the imonium ion which is in equilibrium with the dialkylamino methanol. These compounds are easily formed at room temperature or slightly elevated temperatures from secondary amines and polymeric formaldehyde (paraformaldehyde). Acetylenes with a conjugated unsaturated system, RCH═CHC≡CH or RC≡CC≡CH, arylacetylenes, ArC≡CH, and ethynyl sulphides, HC≡CSR, react more easily than do alkylacetylenes. In the latter compounds the ethynyl proton is less “mobile.” The reaction times, which are quite long for alkylacetylenes, can be shortened to one or a few hours by using a small amount of a copper salt (Cu(OAc)2,CuCl, and CuBr). This forms a copper(I) compound with the acetylene. The copper acetylide is visible as a yellow solid during the reaction. This solid disappears or is replaced by a red suspension of copper, when the conversion has finished. The yields of the copper-catalysed Mannich reactions with acetylenic derivatives are usually high.