Late Transition Metal Catalyzed Intramolecular Hydroamination:  The Effect of Ligand and Substrate Structure

Abstract

A series of cationic rhodium(I) and iridium(I) complexes of the type [M(N-N)(CO) 2 ][BPh 4 ] containing the imidazole-based bidentate nitrogen donor ligands (N-N), including bis(N-methylimidazol-2-yl)methane, and bis(N-methylbenzimidazol-2-yl)methane (bbnzim) (6a M ) Rh, 6b M ) Ir), were synthesized and characterized. Complexes incorporating the pyrazole analogue bis(1-pyrazolyl)methanewere also prepared. The efficiency of each of the complexes as catalysts for the cyclization of 4-pentyn-1-amine to 2-methyl-1-pyrroline is reported. The influence of structural variations of the nitrogen donor ligand on the catalytic efficiency of cationic complexes of the type [M(N-N)(CO) 2 ][BPh 4 ] for the hydroamination reaction was found to be much less than the influence of the nature of the counterion. The scope of the substrates for the intramolecular hydroamination of alkynamines was also investigated using Rh and Ir catalysts with the bim and bpm ligands, using 3-butyn-1-amine (7a), 3-pentyn-1-amine (7b), 4-phenyl-3-butyn-1-amine (7c), 4-pentyn-1-amine (8a), 4-hexyn-1-amine (8b), 5-phenyl-4-pentyn-1-amine (8c), 5-hexyn-1-amine ( 9), and 6-heptyn-1-amine (10) as substrates. The rhodium and iridium catalysts under investigation preferentially catalyze the formation of five-membered rings and do not catalyze the formation of four-or seven-membered rings. The effect of the substituents on the alkyne on the efficiency of hydroamination differentiates Rh(I) and Ir(I) and suggests the nature of the reactive metal-bound alkynyl intermediate.

Intramolecular hydroamination catalysed by Ag complexes stabilised in situ by bidentate ligands

“…Previous results have shown that the cyclisation of alkynylamines to form sixmembered rings is typically harder to achieve than five-membered rings [4,9], which was also shown to be the case here. The combination of AgOTf (2 mol%) and 3 (2 mol%) promoted a conversion of 11 to 12 of only 34% after 21 h, with no significant improvement after this time.…”

“…Synthesis of 1-(2-(diphenylphosphino)phenyl)pyrazole (4) To further investigate the use of P,N-donor ligands in hydroamination catalysis the novel pyrazole-phosphine ligand 1-(2-(diphenylphosphino)phenyl)pyrazole (4) was synthesised (Scheme 1). In this ligand, modelled on the previously reported ligand 3 [14], a phenyl bridge replaces the ethyl backbone.…”

“…Catalysed cyclisation of alkynyl-and alkenylamines via intramolecular hydroamination has shown great potential for the energy efficient synthesis of N-heterocycles [1]. Lanthanide [2], early [3] and late [4][5][6][7][8][9][10] transition metal complexes have been shown to successfully catalyse this reaction. The advantage of late transition metals is their lower oxophilicity (compared to early transition metal or lanthanide metal catalysts), which leads to higher functional group tolerance and less sensitivity to water or oxygen exposure.…”

Synthesis of Saturated Heterocycles via Metal-Catalyzed Alkene Hydroamination or Hydroalkoxylation Reactions

“…This seminal work, along with more recent reports describing Ircatalyzed hydroamination of aminoalkynes [54], laid the foundation for the development of new iridium catalysts for intramolecular hydroamination of unactivated alkenes [40]. Recently iridium complexes have emerged as highly active catalysts for intramolecular hydroamination of alkenes with either primary or secondary amines.…”

Syntheses and characterization of mono and dinuclear complexes of platinum group metals bearing benzene-linked bis(pyrazolyl)methane ligands

“…Arene ruthenium, rhodium and iridium complexes of bis(pyrazolyl)methanes have attracted attention due to their catalytic ability in reactions such as the alcoholysis of ketones and silanes, hydroformylation and hydroaminomethylation of alkenes and hydroamination [14][15][16]. Besides these, nitrogen donor ligands with platinum group metals have been shown to be effective catalysts for oxidation reactions [17] and for ring-opening metathesis polymerization [18] and recent studies of arene ruthenium complexes have shown that they are found to inhibit cancer cell growth [19,20].…”