Synthesis and Reactivity of Phosphine-Quinolinolato Rhodium Complexes: Intermediacy of Vinylidene and (Amino)carbene Complexes in the Catalytic Hydroamination of Terminal Alkynes

Abstract

We report here on the syntheses of rhodium(I) complexes bearing a phosphine-quinolinolate ligand and the isolation of two classes of important intermediates in the anti-Markovnikov hydroamination of terminal alkynes with secondary amines: vinylidene-bridged dirhodium complexes and (amino)carbene complexes. We first prepared various rhodium(I) complexes bearing a phosphine-quinolinolate ligand and found that some of these complexes had catalytic activity for the anti-Markovnikov hydroamination of terminal alkynes with secondary amines. The reaction of stoichiometric amounts of a (PNO)Rh complex with terminal alkynes provided vinylidene-bridged dirhodium complexes. When the resulting vinylidene complexes were reacted with secondary amines, (amino)carbene complexes were generated, which gave the hydroamination product upon heating with an additive. These results suggest that this anti-Markovnikov hydroamination catalyzed by (PNO)Rh complexes proceeds via vinylidene and (amino)carbene complexes.

Carbon–Carbon Bond Formation via Catalytically Generated Aminocarbene Complexes: Rhodium‐Catalyzed Hydroaminative Cyclization of Enynes with Secondary Amines

“…Then the proportion of 4 ab gradually increased and the ratio of products 3 ab / 4 ab reached circa 2:1 after 2 h. This result suggests that there exists a reversible isomerization process between 3 and 4 . When the reaction was monitored, a signal of a phosphorus atom coupled to a rhodium atom was also observed at 52.3 ppm (d, J P‐Rh =218.2 Hz) in 31 P{ 1 H} NMR analysis and is assignable to that of the (PNO)Rh aminocarbene complex . On the basis of this result, it is suggested that the hydroaminative cyclization of enynes proceeds via rhodium aminocarbene intermediates.…”

“…Recently, we found that phosphine‐quinolinolato rhodium [hereinafter, abbreviated as (PNO)Rh] complexes can be used as a catalyst for anti‐Markovnikov hydroamination of terminal alkynes, and vinylidene and aminocarbene complexes were suggested to be involved in the hydroamination as important intermediates . On the basis of the results, we envisioned that the (PNO)Rh aminocarbene complex generated in situ could be used for further C−C bond formations to achieve catalytic processes.…”

Carbon–Carbon Bond Formation via Catalytically Generated Aminocarbene Complexes: Rhodium‐Catalyzed Hydroaminative Cyclization of Enynes with Secondary Amines

“…Recently, we found that phosphine-quinolinolato rhodium [hereinafter, abbreviated as (PNO)Rh] complexes can be used as a catalyst for anti-Markovnikov hydroamination of terminal alkynes, and vinylidene and aminocarbene complexes were suggested to be involved in the hydroamination as important intermediates. [4,5] On the basis of the results, we envisioned that the (PNO)Rh aminocarbene complex generated in situ could be used for further C À C bond formations to achieve catalytic processes. There have been many reports describing the reactivity of aminocarbene complexes.…”

Formation of α-Monosubstituted Propargylamines from Terminal Alkynes and Secondary Amines Using a (PNO)Rh/Cu Tandem Catalyst System

“…9 For example, we found the combination of an 8-quinolinolatorhodium complex bearing a cyclooctadiene ligand (Rh(Q)(cod)) with triarylphosphines to be effective catalysts for the anti-Markovnikov hydroamination of arylacetylenes with secondary amines at room temperature. 9b We recently synthesized novel rhodium(I) complexes bearing a PNO tridentate ligand containing an 8-quinolinolate and a phosphine moieties (Figure 1) 10 and found that the (PNO)Rh complexes can produce the anti-Markovnikov hydroamination product from an aliphatic terminal alkyne and piperidine at 110°C. It was also shown that vinylidene-bridged dirhodium complexes and (amino)carbene complexes can be prepared from (PNO)Rh complex 1a with terminal alkynes and then with amines and are important intermediates in the anti-Markovnikov hydroamination of terminal alkynes.…”

“…As we previously reported for the (PNO)Rh-catalyzed anti-Markovnikov hydroamination of terminal alkynes, 10 enamines were considered to be produced in this reaction as well via formation of a vinylidene intermediate and then conversion to an (amino)-carbene complex, followed by 1,2-β-H shift. In the presence of a copper catalyst, a copper acetylide may be generated by the reaction with a terminal alkyne and a base.…”