Synthesis and Application of Chiral Phosphino-Imidazoline Ligands:  Ir-Catalyzed Enantioselective Hydrogenation

Abstract

[reaction: see text] A series of chiral phosphino-imidazolines (PHIM ligands) 1a-j with different substituents at the stereogenic center, the nitrogen atom of the imidazoline ring, and at the phosphorus atom were synthesized. Iridium complexes derived from these ligands have been evaluated as catalysts for the enantioselective hydrogenation of unfunctionalized olefins. In several cases, higher enantiomeric excesses were observed than with analogous phosphino-oxazoline ligands.

Changing the Palladium Coordination to Phosphinoimidazolines with a Remote Triazole Substituent

“…The analogous phosphinoimidazoline (PHIM) ligands (Figure 1, right) could represent an even more convenient alternative: the topology of the imidazoline ligand allows the use of readily available C 2 -symmetric diamine fragments, while the second nitrogen atom represents an additional source of molecular diversity (R 2 ), allowing the programmed modification of the electronic properties of the coordinating nitrogen atom. The additional nitrogen atom could also serve for the heterogenization of the phosphinoimidazoline ligand onto insoluble organic resins, a field which still remains completely unexplored.PHIM ligands have shown to be very efficient in the Ir-catalyzed enantioselective hydrogenation of prochiral olefins [2] and imines, [3] as well as in the Pdcatalyzed asymmetric Heck reaction.[4] However, they have never been used in Pd-catalyzed asymmetric allylic substitution reactions, a domain where PHOX ligands are one of the most important players. [1d,5] We wish to report in the present paper the structural optimization of modular PHIM ligands using the Pd-catalyzed asymmetric allylic substitution reaction as a benchmark.…”

The Development of Bidentate P,N Ligands for Asymmetric Catalysis

“…The group of Pfaltz has applied a number of these ligands in the iridium-catalysed enantioselective hydrogenation of various alkene substrates. [132] Ligands 345 and 346 gave the best results, surpassing those achieved with the oxazoline ligands in some cases. Busacca et al tested ligands such as 347 in the asymmetric intermolecular Heck reaction and discovered several interesting electronic effects, Scheme 26.…”

The Hendrickson ‘POP’ reagent and analogues thereof: synthesis, structure, and application in organic synthesis

“…25 In addition, derivatives of these heterocycles have attracted substantial attention because of their interesting biological activities. 26 Consequently, the design of enantiospecific routes to imidazolines and their derivatives constitutes an important step toward the construction and biological evaluation of macrolactam natural product analogues containing these heterocycles.…”