fluorinated olefins [87, 126, 130], vinyl boronates [88], enol phosphinates [84], and
E- and Z-chiral sulfones [131, 132], allylic alcohols [90, 133], and the
monohydrogenation of 1,4-dienes [134]. Among the hydrogenation of dienes,
many purely alkyl-substituted were successfully hydrogenated (ees up to 99%). In
addition, they are able to selectively hydrogenate only one of the double bonds,
leaving room for further synthetic manipulations. In this respect, Andersson’s group
use this methodology for the total synthesis of (-)-juvabione, a natural sesquiterpene
exhibiting juvenile hormone activity using Ir/L28 catalyst [135].
Another interesting example of ligand design was the phosphite-thiazoline ligand
L30 (Fig. 13), in which the oxazoline group in ligands L20 was replaced by a
thiazoline moiety. The introduction of a thiazoline moiety has not only provided
enantioselectivities up to >99% for a range of α,β-unsaturated ketones, vinylsilane,
and trifluoromethyl olefins but also has increased the enantioselectivities of
Z-trisubstituted olefins while maintaining the excellent enantioselectivities for a
range of E-trisubstituted and 1,1-disubstituted minimally functionalized olefins [136].
2.1.6 Carbene-Nitrogen Ligands
Another type of effective catalysts is the Ir/carbene-nitrogen complexes. An important advantage of N-heterocyclic carbene (NHC) catalysts compared to their phosphine analogues concerns their better tolerance for acid-sensitive substrates. In 2001,
Burgess’ group reported for the first time that NHC-oxazoline-based Ir-catalysts
(ligand L31, Fig. 14, R
1
¼ 2,6i
Pr 2 -Ph and R
2
¼ 1-Ad) can also be applied in the
hydrogenation of unfunctionalized olefins with results comparable to the commonly
used Ir-P,N catalysts [137, 138]. These catalysts afforded high enantioselectivities
(up to 98% ee) in a limited group of unfunctionalized olefins, mainly trisubstituted,
and for the more challenging disubstituted olefins, only one example was reported
with low enantioselectivity. Since then, a few more carbene-N ligands have been
developed but with less success [139–143], except for the family of Ir-NHC-pyridine
catalysts [144] developed by Pfaltz’s group (with ligands L32, Fig. 14, R ¼ 2,6diisopropylbenzene) that showed similar enantioselectivities to the Burgess ones.
So, high enantioselectivities (>90% ee) were observed, even for Z-trisubstituted
(94% ee) and endocyclic substrates (96% ee).
2.1.7 Application of P-O/S Ligands
In contrast to other catalytic processes and to the Rh-/Ru-hydrogenation, for the
reduction of unfunctionalized olefins, the possibility of changing the nature of the
N-donor atom in the ligand design of heterodonor ligands was not contemplated
until recently. In 2011, Pfaltz successfully reported the application of proline-based
P,O ligands L33 in the asymmetric hydrogenation of trisubstituted alkenes (Fig. 15,
R
1
¼ Ph,
t Bu, Cy, o-Tol and R
2
¼
t Bu, 1-Ad, CPh 3 , 1Ad-NH, MesNH, CPh 3 NH)
[145–147]. Phosphines bearing either a bulky amide or urea groups at the pyrrolidine
168
J. Margalef et al.
E- and Z-chiral sulfones [131, 132], allylic alcohols [90, 133], and the
monohydrogenation of 1,4-dienes [134]. Among the hydrogenation of dienes,
many purely alkyl-substituted were successfully hydrogenated (ees up to 99%). In
addition, they are able to selectively hydrogenate only one of the double bonds,
leaving room for further synthetic manipulations. In this respect, Andersson’s group
use this methodology for the total synthesis of (-)-juvabione, a natural sesquiterpene
exhibiting juvenile hormone activity using Ir/L28 catalyst [135].
Another interesting example of ligand design was the phosphite-thiazoline ligand
L30 (Fig. 13), in which the oxazoline group in ligands L20 was replaced by a
thiazoline moiety. The introduction of a thiazoline moiety has not only provided
enantioselectivities up to >99% for a range of α,β-unsaturated ketones, vinylsilane,
and trifluoromethyl olefins but also has increased the enantioselectivities of
Z-trisubstituted olefins while maintaining the excellent enantioselectivities for a
range of E-trisubstituted and 1,1-disubstituted minimally functionalized olefins [136].
2.1.6 Carbene-Nitrogen Ligands
Another type of effective catalysts is the Ir/carbene-nitrogen complexes. An important advantage of N-heterocyclic carbene (NHC) catalysts compared to their phosphine analogues concerns their better tolerance for acid-sensitive substrates. In 2001,
Burgess’ group reported for the first time that NHC-oxazoline-based Ir-catalysts
(ligand L31, Fig. 14, R
1
¼ 2,6i
Pr 2 -Ph and R
2
¼ 1-Ad) can also be applied in the
hydrogenation of unfunctionalized olefins with results comparable to the commonly
used Ir-P,N catalysts [137, 138]. These catalysts afforded high enantioselectivities
(up to 98% ee) in a limited group of unfunctionalized olefins, mainly trisubstituted,
and for the more challenging disubstituted olefins, only one example was reported
with low enantioselectivity. Since then, a few more carbene-N ligands have been
developed but with less success [139–143], except for the family of Ir-NHC-pyridine
catalysts [144] developed by Pfaltz’s group (with ligands L32, Fig. 14, R ¼ 2,6diisopropylbenzene) that showed similar enantioselectivities to the Burgess ones.
So, high enantioselectivities (>90% ee) were observed, even for Z-trisubstituted
(94% ee) and endocyclic substrates (96% ee).
2.1.7 Application of P-O/S Ligands
In contrast to other catalytic processes and to the Rh-/Ru-hydrogenation, for the
reduction of unfunctionalized olefins, the possibility of changing the nature of the
N-donor atom in the ligand design of heterodonor ligands was not contemplated
until recently. In 2011, Pfaltz successfully reported the application of proline-based
P,O ligands L33 in the asymmetric hydrogenation of trisubstituted alkenes (Fig. 15,
R
1
¼ Ph,
t Bu, Cy, o-Tol and R
2
¼
t Bu, 1-Ad, CPh 3 , 1Ad-NH, MesNH, CPh 3 NH)
[145–147]. Phosphines bearing either a bulky amide or urea groups at the pyrrolidine
168
J. Margalef et al.