Highly Enantioselective Iridium‐Catalyzed Hydrogenation of Trisubstituted Olefins, α,β‐Unsaturated Ketones and Imines with Chiral Benzylic Substituted P,N Ligands

Abstract

The benzylic substituted P,N ligands, diphosphinobenzyloxazolines, showed their high catalytic activity as well as asymmetric induction in the iridium-catalyzed asymmetric hydrogenation of unfunctionalized alkenes, a,b-unsaturated esters, allyl alcohols, a,b-unsaturated ketones, and imines, providing the corresponding chiral products in high ee with high conversion.Keywords: asymmetric hydrogenation; imines; iridium; P,N ligands; unfunctionalized olefins; a,b-unsaturated ketonesThe enantioselective hydrogenation as one of the most powerful tools in asymmetric catalysis for the preparation of optically active compounds has attracted great attention. Different transition metals with chiral ligands have been used as catalysts in the hydrogenation reactions of a variety of unsaturated compounds.[1] Among them, chiral iridium complexes have showed their unique asymmetric induction and catalytic activity for some substrates, [2] especially for non-functionalized alkenes, [2d-g] enamines, [2h,i,x] imines, [2j-m,w] quinolines, [2n-t] a,b-unsaturated ketones, [3] and carboxylic acid derivatives. [2u] In spite of the great progress achieved, only a limited number of ligands have been found to be effective in iridium-catalyzed asymmetric hydrogenation, although a wide variety of ligands with different frameworks has been synthesized and used in asymmetric catalysis.[4] Recently, we developed benzylic substituted ligands and showed their catalytic activity as well as asymmetric induction (Scheme 1).[5] It was deduced that the ligands with one more carbon atom have increased tether length between the donor atoms and this makes the transition metal complexes have more conformers to fit different substrates. Herein we report the Ir-catalyzed asymmetric hydrogenation of non-functionalized alkenes as well as imines and a,b-unsaturated ketones using the catalyst derived from benzylic substituted ligand 3, providing the corresponding products in high enantioselectivities.Asymmetric hydrogenation of unfunctionalized olefins had been a challenge for a long time and the breakthrough appeared when Pfaltz used Ir catalysts with the famous PHOX ligands.[3a] Since then, many P,N ligands were used in the Ir-catalyzed asymmetric hydrogenation of this type of olefins, some of which provided products in high enantioselectivity. Thus, the asymmetric hydrogenation of olefin 4a was chosen to test the effectiveness of the ligand. It showed that product 6a in 97% ee with 100% conversion was obtained when 0.3 mol% of catalyst 5a under 50 bar of hydrogen pressure at 25 8C was used. Encouraged by this result, the impact of different aryl groups on P as well as the substituents on the oxazoline ring on the reaction was investigated [Eq. (1), Table 1]. It can be Scheme 1. Structure of benzylic substituted ligands.

Highly Enantioselective Iridium‐Catalyzed Hydrogenation of α,β‐Unsaturated Esters

“…Contrary to (E)-b,b-disubstituted a,b-unsaturated esters, members of the (Z)-configured cohort have proven very hard to hydrogenate with excellent ee by using N,P-chelated iridium catalysts. [20] We found the hydrogenation of substrate 11 by [(A)*IrA C H T U N G T R E N N U N G (cod)] + A C H T U N G T R E N N U N G [BAr F ] À to be no exception, as an Table 1. Asymmetric hydrogenation of (E)-b,b-disubstituted a,b-unsaturated esters.…”

Asymmetric Hydrogenation of Olefins Using Chiral Crabtree-type Catalysts: Scope and Limitations

“…126a Ligand 13a (Figure 4) was used to obtain enantioselectivities over 90% for several β,β-substituted α,β-unsaturated esters. 57 Both the β-methyl (S167) and β-ethyl (S168) derivative could be highly selectively reduced (Table 9, entries 3 and 4), but more significantly, the Z-alkene S169 could be reduced in 92% enantioselectivity (entry 5).…”

A Mild Dihydrobenzooxaphosphole Oxazoline/Iridium Catalytic System for Asymmetric Hydrogenation of Unfunctionalized Dialins

“…A survey of the literature revealed that asymmetric hydrogenation of these exocyclic imines using iridium catalysts generally afforded low enantioselectivities because the conformational strain, upon coordination to iridium, in the transition state caused by the cyclic structures. [24] An encouraging result of iridium-catalyzed enantioselective hydrogenation of ketimines was reported by Ding and co-workers and up to 98 % ee was achieved with a spirophosphine oxazoline ligand. [25] .…”