Iridium‐Catalyzed Enantioselective Hydrogenation of Terminal Alkenes

Abstract

Iridium complexes derived from chiral P,N ligands are efficient catalysts for the enantioselective hydrogenation of 2-aryl-substituted terminal alkenes. Using 0.1 -1 mol % of catalyst at room temperature and ambient hydrogen pressure, high enantioselectivities (88 -94% ee), full conversions after short reaction times and essentially quantitative yields were obtained for a range of differently substituted 2-arylalkenes. Among six iridium complexes that were tested, the most selective catalyst was a complex with a phosphinite-oxazoline ligand derived from threonine (IrThrePHOX). In contrast to the hydrogenation of trisubstituted alkenes, a strong pressure effect was observed for this class of substrates. Lowering the hydrogen pressure from 50 to 1 bar resulted in a strong increase of the ee values.

Synthesis and Application of Chiral N‐Heterocyclic Carbene–Oxazoline Ligands: Iridium‐Catalyzed Enantioselective Hydrogenation

“…Since previous work on substrate 24 showed that low hydrogen pressure increases the asymmetric induction, [5,32] catalyst screening was performed at 1 bar H 2 gas pressure (Table 5).…”

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

“…132 For terminal allylic alcohols, the Ir-catalyzed asymmetric hydrogenation has achieved ee's up to 88% in the hydrogenation of S126 using phosphinite-oxazoline 19a (Figure 4) ligand (Table 7, entry 32). 67 Introducing a phosphite moiety in the ligand design is advantageous, achieving enantioselectivities up to 95% in the reduction of S127 (Table 7, entry 33).…”

Adaptative Biaryl Phosphite–Oxazole and Phosphite–Thiazole Ligands for Asymmetric Ir‐Catalyzed Hydrogenation of Alkenes

“…The complexes were characterized by elemental analysis and 1 H, 13 C, and 31 P NMR spectroscopy. The spectral assignments (see the Experimental Section) were based on information from …”

“…Few known catalytic systems provide high enantioselectivities for these substrates, and those that do are usually limited in substrate scope. [13,14] Pfaltz found that in the hydrogenation of terminal alkenes, the selectivity is highly pressure dependent. Hydrogenation at an atmospheric pressure of H 2 gave significantly higher ee values than at higher pressures.…”

“…These results, which again clearly show the efficiency of using modular scaffolds in ligand design, are among the best that have been reported for this demanding substrate class. [13,14] We then studied the asymmetric hydrogenation of other 1,1-disubstituted aryl-alkyl substrates (S14-S20) and 1,1-disubstituted heteroaryl-alkyl olefins (S21-S23) by using the phosphite-oxazole/thiazole ligand library L1 a-g-L7 a-g. The most noteworthy results are shown in Table 5.…”