Cross dimerization of terminal alkynes catalyzed by [(Et2N)3U][BPh4]

The cationic complex [(Et2N)3U][BPh4] reacts with a mixture of terminal alkynes inducing the synthesis of the cross dimerization products. For equimolar amounts of aliphatic alkynes (iPrCCH, tBuCCH) the head-to-tail geminal dimer of iPrCCH and the geminal cross dimer resulting from the insertion of iPrCCH into the UCCR (R=iPr, tBu) moiety are obtained. When a mixture of PhCCH is reacted with either iPrCCH or tBuCCH, different products are obtained depending on the molar ratio of the alkynes. The dimerization of iPrCCH with an excess of PhCCH produces the geminal head-to-tail cross dimer issued from the insertion of the aliphatic alkyne into the UCCPh moiety, and the geminal dimer of PhCCH. Inverting the molar ratio of the alkynes and using the deuterium labeled aliphatic alkyne iPrCCD, the deuterated geminal head-to-tail cross dimer is obtained preferentially with small amounts of the deuterated head-to-tail dimer of iPrCCD. The mixture of tBuCCH and PhCCH is converted into the geminal head-to-tail cross dimer in good yield if the former alkyne is in large excess. The addition of external EtNH2 in the cross dimerization of iPrCCH with PhCCH induces a different chemoselectivity producing mainly the cis-dimer of PhCCH. The use of a bulky amine, tBuNH2, with tBuCCH causes the decomposition of the catalytic complex, forming the salt [tBuNH3][BPh4]·tBuNH2.