Enantioselective Hydrogenations of Arylalkenes Mediated by [Ir(cod)(JM-Phos)]+ Complexes

Abstract

Phosphine oxazoline ligands la-j were converted to the corresponding [Ir(cod)(phosphine oxazoline)]+ complexes 2a-j. X-ray diffraction analyses of complexes 2b, 2h, 2i, and 2j were performed. The tert-butyl-, 1,1-diphenylethyl-, and phenyl-oxazoline complexes (2b, 2h, and 2i, respectively) had typical square planar metal environments with chair-like metallocyclic rings. However, the 3,5-di-tert-butylphenyl oxazoline complex 2j was distorted toward a tetrahedral metal geometry. This library of complexes was tested in asymmetric hydrogenations of several arylalkenes. High enantioselectivities and conversions were observed for some substrates. A possible special role for the HPh2C-oxazoline substituent in asymmetric hydrogenations was identified and is discussed. In attempts to rationalize why high enantioselectivities were not observed for some alkenes, a series of deuterium labeling experiments were performed to probe for competing reactions that occurred prior to the hydrogenation step. Double bond migrations were inferred for several substrates, and this is a significant complication in asymmetric hydrogenations of arylalkenes that had not been discussed prior to this study. A mechanistic rationale is proposed involving competing double bond migration for some but not all substrates. Appreciation of this complication will be valuable in further studies aimed at optimization of enantioselection in asymmetric hydrogenations of unfunctionalized alkenes.

Threonine-Derived Phosphinite-Oxazoline Ligands for the Ir-Catalyzed Enantioselective Hydrogenation

“…[4] Although iridium-PHOX catalysts gave excellent enantioselectivities and high turnover numbers in the hydrogenation of certain olefins such as 1,2-diaryl-1-alkyl-substituted alkenes, the substrate scope of these catalysts proved to be limited. Therefore, we and others started a search for new P,Nligands which enhance the application range of iridiumcatalyzed hydrogenation.Burgess and coworkers reported the JM-Phos ligands [5] (Figure 1) and, more recently, analogous oxazolines connected to an imidazolylidene instead of a phosphine group. [6] We prepared PHOX analogues (PyrPHOX; Figure 1) in which the phenyl bridge was replaced by a pyrrole ring.…”

“…Burgess and coworkers reported the JM-Phos ligands [5] (Figure 1) and, more recently, analogous oxazolines connected to an imidazolylidene instead of a phosphine group. [6] We prepared PHOX analogues (PyrPHOX; Figure 1) in which the phenyl bridge was replaced by a pyrrole ring.…”

Hydrogenation of Unfunctionalized Alkenes

“…Extensive variations of the ligand backbone, oxazoline and phosphine groups have proved successful, although common to all these ligands is an sp 2 -hybridized coordinating nitrogen atom. Phosphorus coordination occurs most commonly via a phosphine group ( (4) [6,8] , (5) [9] , (6) [10] , (7) [11] , (11) [12] , (12) [13] , (15) [14] , (17) [15] , (20) [16] , (21) [17] and (22) [18] ), though ligands with phosphinites ( (8) [19] , (13) [20] , (14) [20,21] , (16) [22] , (18) [23] and (19) [24] ) and phosphites ( (9) and (10) [25] ) have also successfully been developed. More recently, phosphorus coordination has been replaced by the use of electron -rich N -heterocyclic carbenes [26] ( (23) [27] , (24) and (25) [28] and (26) [29] ) (Figure 2.4 ).…”

Design of Chiral Ligands for Asymmetric Catalysis: From C2‐Symmetric P,P‐ and N,N‐Ligands to Sterically and Electronically Nonsymmetrical P,N‐Ligands

“…Subsequently, we added phosphinoimidazolines, such as type 14, to our collection of ligands (32,33), as well as pyridine-and quinoline-derived P,N-ligands of type 15 and 16 (34), all of which gave very promising results. Recently, other research groups have become interested in this class of catalysts and have reported additional variants of Ir complexes with P,N-ligands (35)(36)(37)(38)(39)(40) and, as a further modification, oxazoline ligands with a heterocyclic carbene unit instead of a phosphino group (41). Clearly, Ir complexes derived from P,N-ligands represent a new class of catalysts that significantly expands the application range of asymmetric hydrogenation.…”