catalyzed the reduction of three cyclic β-enamides with promising
enantioselectivities (up to 92% ee; Fig. 28). However, this catalyst showed low
activities (55–81% conversion after 18 h at rt. and at P H2 ¼ 50 bar) [203].
It has not been until very recently that Ir-catalysts have shown their high
efficiency in the reduction of this type of challenging substrates. In 2016, two reports
appeared demonstrating the potential of Ir-catalysts modified with P,N-ligands and
showing that Ir-P,N catalysts can be also efficient in the reduction of alkenes bearing
metal-coordinating groups [25, 26]. Riera and Verdaguer et al. found that bulky
P-stereogenic phosphine-oxazoline ligands L36 (Fig. 18) provided the highest
selectivity ever reported for the reduction of cyclic enamides derived from α- and
β-tetralones (Fig. 29a, ees up to 99% at only 3 bars of H 2 ), surpassing the results
obtained with the widely studied Rh- and Ru-catalysts [25]. In the same year,
Diéguez and co-workers also reported the successful application of PHOX-derived
phosphite ligands L39 (Fig. 29b) to the hydrogenation of cyclic β-enamides [26]. A
range of 2-aminotetralines and 3-aminochromanes were obtained in high yields and
excellent enantioselectivities (ees up to 99%, Fig. 29b). The hydrogenation of cyclic
α-enamides also proceeded in high enantioselectivities (ees up to 96%). In addition,
the reactions could be carried out in environmentally friendly solvents, propylene
carbonate, with no loss of selectivity.
The already mentioned ligands L37 (Fig. 18) that were successful for disubstituted, trisubstituted, and tetrasubstituted unfunctionalized olefins also provided
excellent enantioselectivities in the hydrogenation of various cyclic β-enamides
(6 examples, ees up to 99%) [27]. Moreover, both enantiomers of the reduced
products could be accessed with the correct choice of the phosphite-based ligand.
Fig. 27 Hydrogenation of
tetrasubstituted
α-fluoro-β-enamino esters
catalyzed by Ir-ZhaoPhos
catalyst
Fig. 28 Ir-catalyzed
hydrogenation of β-aryl
cyclic enamides using
SIAPhos ligand
Iridium-Catalyzed Asymmetric Hydrogenation
181
enantioselectivities (up to 92% ee; Fig. 28). However, this catalyst showed low
activities (55–81% conversion after 18 h at rt. and at P H2 ¼ 50 bar) [203].
It has not been until very recently that Ir-catalysts have shown their high
efficiency in the reduction of this type of challenging substrates. In 2016, two reports
appeared demonstrating the potential of Ir-catalysts modified with P,N-ligands and
showing that Ir-P,N catalysts can be also efficient in the reduction of alkenes bearing
metal-coordinating groups [25, 26]. Riera and Verdaguer et al. found that bulky
P-stereogenic phosphine-oxazoline ligands L36 (Fig. 18) provided the highest
selectivity ever reported for the reduction of cyclic enamides derived from α- and
β-tetralones (Fig. 29a, ees up to 99% at only 3 bars of H 2 ), surpassing the results
obtained with the widely studied Rh- and Ru-catalysts [25]. In the same year,
Diéguez and co-workers also reported the successful application of PHOX-derived
phosphite ligands L39 (Fig. 29b) to the hydrogenation of cyclic β-enamides [26]. A
range of 2-aminotetralines and 3-aminochromanes were obtained in high yields and
excellent enantioselectivities (ees up to 99%, Fig. 29b). The hydrogenation of cyclic
α-enamides also proceeded in high enantioselectivities (ees up to 96%). In addition,
the reactions could be carried out in environmentally friendly solvents, propylene
carbonate, with no loss of selectivity.
The already mentioned ligands L37 (Fig. 18) that were successful for disubstituted, trisubstituted, and tetrasubstituted unfunctionalized olefins also provided
excellent enantioselectivities in the hydrogenation of various cyclic β-enamides
(6 examples, ees up to 99%) [27]. Moreover, both enantiomers of the reduced
products could be accessed with the correct choice of the phosphite-based ligand.
Fig. 27 Hydrogenation of
tetrasubstituted
α-fluoro-β-enamino esters
catalyzed by Ir-ZhaoPhos
catalyst
Fig. 28 Ir-catalyzed
hydrogenation of β-aryl
cyclic enamides using
SIAPhos ligand
Iridium-Catalyzed Asymmetric Hydrogenation
181