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

The cationic complex [(Et2N)(3)U][BPh4] reacts with a mixture of terminal alkynes inducing the synthesis of the cross dimerization products. For equimolar amounts of aliphatic alkynes ((PrC)-Pr-i=CH, (t)BuCdropCH) the head-to-tail geminal dimer of (i)PrCdropCH and the geminal cross dimer resulting from the insertion of (i)PrCdropCH into the U-CdropCR (R = Pr-i, Bu-t) moiety are obtained. When a mixture of PhCdropCH is reacted with either (i)PrCdropCH or (t)BuCdropCH, different products are obtained depending on the molar ratio of the alkynes. The dimerization of (i)PrCdropCH with an excess of PhCdropCH produces the geminal head-to-tail cross dimer issued from the insertion of the aliphatic alkyne into the U-CdropCPh moiety, and the geminal dimer of PhCdropCH. Inverting the molar ratio of the alkynes and using the deuterium labeled aliphatic alkyne (i)PrCdropCD, the deuterated geminal head-to-tail cross dimer is obtained preferentially with small amounts of the deuterated head-to-tail dimer of (i)PrCdropCD. The mixture of (t)BuCdropCH and PhCdropCH 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 (i)PrCdropCH with PhCdropCH induces a different chemoselectivity producing mainly the cis-dimer of PhCdropCH. The use of a bulky amine, (BuNH2)-Bu-t, with (t)BuCdropCH causes the decomposition of the catalytic complex, forming the salt [(BuNH3)-Bu-t][BPh4].(BuNH2)-Bu-t. (C) 2002 Elsevier Science B.V. All rights reserved.