Oligomerization and Hydroamination of Terminal Alkynes Promoted by the Cationic Organoactinide Compound [(Et_2N)_3U][BPh_4]

摘要

The three ancillary amido moieties in the cationic complex [(Et_2N)_3U][BPh_4] are highly reactive and are easily replaced when the complex is treated with primary amines. The reaction of [(Et_2N)_3U][BPh_4] with excess tBuNH_2 allows the formation of the cationic complex [(tBuNH_2)_3(tBuNH)_3U][BPh_4]. X-ray diffraction studies on the complex indicate that three amido and three amine ligands are arranged around the cationic metal center in a slightly distorted octahedral mere geometry. The cationic complex reacts with primary alkynes in the presence of external primary amines to primarily afford the unexpected cis dimmer and, in some cases, the hydroamination products are obtained concomitantly. The formation of the cis dimer is the result of an envelope isomerization through a metal-cyclopropyl cationic complex. In the reaction of the bulkier alkyne tBuC(ident to)CH with the cationic uranium complex in the presence of various primary amines, the cis dimmer, one trimer, and one tetramer are obtained regioselectively, as confirmed by deuterium labeling experiments. The trimer and the tetramer correspond to consecutive insertions of an alkyne molecule into the vinylic CH bond trans to the bulky tert-butyl group. The reaction of (TMS)C(ident to)CH with the uranium catalyst in the presence of EtNH_2 followed a different course and produced the gem dimmer along with the hydroamination imine as the major product. However, when other bulkier amines were used (iPrNH_2 or tBuNH_2) both hydroamination isomeric imines sand E were obtained. During the catalytic reaction, the E (kinetic) isomer is transformed into the most stable Z (thermodynami) isomer. The unique reactivity of the alkyne (TMS)C(ident to)CH with the secondary amine Et_2NH is remarkable because it afforded the trans dimer and the corresponding hydroamination enamine. The later probably results from the insertion of the alkyne into a secondary metalamide bond, followed by protonolysis.