Copper-, cerium- and dysprosium-catalyzed synthesis of monosubstituted allenes from N,N-diisopropyl substituted propargylamines at room temperature

Allenes have been demonstrated as synthetically versatile starting materials in organic synthesis. Allenation of terminal alkynes (ATA) reaction produces substituted allenes from terminal alkynes, carbonyl compounds, amines in the presence of Cu, Zn, Cd, Au, Ag and Pd salts. All known ATA reactions, catalyzed by the salts of the above metals, take place at an elevated temperature from 40 °C (in the case of Au, Ag) to 100 °C and higher (in the case of Cu, Zn, Cd). It is suggested that the formation of the allenes happens in a multistep process in which, at first, AТА reaction takes place and, the formed propargylamine then reacts further to generate the allene. Although the ATA reaction in some cases can be carried out at room temperature, not a single example of the conversion of propargylamine to allene at room temperature has been known. We have discovered for the first time that the [CuI – TIBAL], [Ce2(SO4)3 – TIBAL] and [Dy2(SO4)3 – TIBAL] catalytic systems makes it possible to prepare monosubstituted allenes from N,N-diisopropyl substituted propargylamines in dichloromethane or dichloroethane in high yield even at room temperature. Typically we used 0.2 equivalent of metal salt and 1-2 equivalents of TIBAL. This study demonstrates a remarkable activating effect of the TIBAL additive. In addition, the catalytic effect of lanthanide salts (Ce, Dy) on the conversion of propargylamines into monosubstituted allenes was discovered for the first time.