Highly Enantioselective Hydrogenation of Non- ortho-Substituted 2-Pyridyl Aryl Ketones via Iridium- f-Diaphos Catalysis

Article abstract of DOI:10.1021/acs.orglett.9b01415

This work disclosed a highly enantioselective hydrogenation of non-ortho-substituted 2-pyridyl aryl ketones via Ir/f-diaphos catalysis. This catalytic system allows for full control over the configuration of the stereocenter, affording two enantiomers of the desired products with extremely high enantioselectivity (up to >99% ee in most cases) and excellent reactivity (TON of up to 19600, TOF of 1633 h^-1) under mild conditions. Density functional theory calculations and control experiments revealed that the relay hydrogen bonding among the solvent isopropanol, substrate, and ligand is crucial for high ee's.

Full text of DOI:10.1021/acs.orglett.9b01415

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Highly Enantioselective Hydrogenation of Non-ortho-Substituted
2‑Pyridyl Aryl Ketones via Iridium‑f‑Diaphos Catalysis
Sanfei Nian,†,∥ Fei Ling,†,∥ Jiachen Chen,† Ze Wang,† Haiwei Shen,† Xiao Yi,† Yun-Fang Yang,‡,§
Yuanbin She,‡ and Weihui Zhong*,†
†Key Laboratory for Green Pharmaceutical Technologies and Related Equipment of Ministry of Education, College of
Pharmaceutical Sciences, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China
‡College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China
§Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States
S
* Supporting Information
ABSTRACT: This work disclosed a highly enantioselective
hydrogenation of non-ortho-substituted 2-pyridyl aryl ketones
via Ir/f-diaphos catalysis. This catalytic system allows for full
control over the configuration of the stereocenter, affording
two enantiomers of the desired products with extremely high
enantioselectivity (up to >99% ee in most cases) and excellent
reactivity (TON of up to 19600, TOF of 1633 h−1) under
mild conditions. Density functional theory calculations and
control experiments revealed that the relay hydrogen bonding
among the solvent isopropanol, substrate, and ligand is crucial
for high ee’s.
nantiopure aryl N-heteroaryl methanols make up a class of
valuable structural motifs, widely used in the synthesis of
E
numerous pharmaceuticals, agrochemicals, and biologically
active compounds,3 such as bepotastine besilate,4 carbinox-
amine,5 and (R,S)-mefloquine6 (Figure 1). Thus, the develop-
ment of efficient methods for the catalytic enantioselective
synthesis of these compounds is of substantial interest in the
academic community and industrial sector.7−11 Asymmetric
hydrogenation of prochiral ketones is one of the most
straightforward methods for accessing chiral alcohols, and
numerous fantasic ligands have been successfully developed for
the highly enatioselective hydrogenation of simple ketones in
the past several decades.1,2 In contrast, little attention has been
paid to the direct asymmetric hydrogenation of aryl N-
heteroaryl ketones. Due to the coordination effect of the
pyridine, the hydrogenation of such compounds became more
challenging than that of simple ketones and remained a
challenging task to be solved. In 2001, Noyori disclosed a
pioneering work about the asymmetric hydrogenation of 2-
acetylpridine with the trans-RuCl2[(R)-xylBINAP]-[(R)-
Figure 2. Privileged ligands for asymmetric hydrogenation of 2-
pyridyl aryl ketones.
Scheme 1. Tested f-Diaphos Ligands for the Asymmetric
Hydrogenation of 1a
daipen] complex as a catalyst, and the reaction performed
well to afford chiral 1-(pyridin-2-yl)ethan-1-ol in 96% ee, due
to the addition of isopropyl borate to inhibit the coordination
Figure 1. Representative drugs containing an aryl N-heteroaryl
alcohol moiety.
Received: April 23, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01415
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Organic Letters
Letter
a
Table 1. Optimization of Reaction Conditions
b
c
d
entry
ligand
solvent
base
conversion (%)
ee (%)
conformation
1
L1
L2
L3
L4
L5
L3
L3
L3
L3
L3
L3
L3
L3
L3
L3
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
EtOH
MeOH
CH2Cl2
toluene
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOK
tBuONa
NaOMe
K2CO3
tBuOLi
tBuOLi
95
>99
>99
>99
>99
63
96
99
S
S
S
R
R
S
S
S
S
S
S
S
−
S
S
2
3
>99
>99
>99
92
4
5
6
7
57
11
8
86
87
9
35
43
10
11
12
13
>99
>99
>99
NR
>99
45
56
84
83
−
94
e
14
f
15
94
a
Reaction conditions: 2 mmol scale, 0.2 M substrate, 0.05 mol % [Ir(COD)Cl]2, 0.105 mol % ligand L, 5 mol % base, 10 mL of solvent, room
b
c
d
e
f
temperature (25−30 °C), 12 h. Determined by GC analysis. Determined by HPLC analysis. Compared with the literature. At 60 °C. At 0 °C.
effect of pyridine.12 Inspired by Noyori’s research, several
similar ruthenium complexes have thus been devoted to the
preparation of aryl N-heteroaryl methanols (Figure 2).8
However, all of the results indicated that only the substrates
bearing an ortho substituent on the phenyl ring could yield
excellent ee (99%), while poor ee’s (27−62%) were observed
when using other 2-pyridyl aryl ketones. Recently, Zhang and
co-workers made progress in terms of the asymmetric
hydrogenation of 2-pyridyl aryl ketones using the [Rh-
(COD)-Binapine]BF4 complex as an efficient catalyst,
producing the corresponding alcohols with excellent ee’s
(99%) regardless of whether there was an ortho-substituted
group (Figure 2).13 Despite the great successes described
above, several important but unsolved challenges remain. (1)
The turnover number (TON) was far lower than that needed
to meet the requirements of industrial production, and the
enantioselectivity can be further improved. To date, the best
result of 99% ee with a TON of 4450 was achieved by Zhang’s
group. (2) The catalytic system was limited to a Ru- or Rh-
diphosphine system, and the Ir tridentate ligand complex has
not yet been exploited. (3) Density functional theory (DFT)
calculations for the elucidation of this reaction mechanism
have not been studied, nor has the influence of the substituents
on the pyridine rings. (4) Enantio-divergent synthesis of
enantioenriched 2-pyridyl aryl alcohols with one kind of ligand
remains an unexplored issue, but it is highly desirable, because
the biological activities of these alcohols are closely dependent
on their absolute configurations. Thus, there is an urgent
demand for sufficient and effective ligands to solve these
problems.
divergent entry to (R)- and (S)-aryl N-heteroaryl methanols
without substrate limitations (Figure 2). The Ir/f-diaphos
catalytic systems exhibited outstanding enantioselectivity and
superb reactivity, delivering both (R)- and (S)-2-pyridyl aryl
alcohols in up to >99% ee (in most cases) with a TON of up to
19600 and a TOF of 1633 h−1. In addition, this method was
also compatible with 2-pyridyl alkyl ketones, leading to (S)-1-
(pyridin-2-yl)ethan-1-ol in >99% ee. More importantly,
preliminary DFT calculations and control experiments revealed
that the position of the N atom in the substrate, the solvent,
and two hydrogen atoms on the cyclohexane of the diaphos
ligand are important elements for achieving high ee’s.
We initially selected 1a as a model substrate to evaluate
various chiral Ir/f-diaphos catalysts generated in situ by mixing
[Ir(COD)Cl]2 with f-diaphos ligands L1−L5 (Scheme 1). To
our delight, L1 exhibited good results with respect to
producing (S)-2a with 95% conversion and 96% ee (Table 1,
entry 1). With an increase in steric hindrance on the benzene
ring of the sulfonyl group from L1 to L2 with the switch from
4-methyl to 2,4,6-trimethyl, the enantiomeric selectivity of the
reaction increased from 96% to 99% ee with full conversions
(Table 1, entry 2). Notably, when ligand L3 was used,
excellent enantioselectivity and superb reactivity were
observed, leading to (S)-2a with >99% conversion and >99%
ee. Moreover, L4, an enantiomer of L1, also performed well in
the reduction of 1a with the opposite configuration of the
product [>99% conversion and >99% ee (Table 1, entry 4)].
After a set of (SC,RFC,S,S)-ferrocene-based ligands had been
screened, L5 turned out to be optimal, affording (R)-2a in
>99% ee with full conversion (Table 1, entry 5). In addition, a
i
Very recently, our group has presented an iridium-catalyzed
asymmetric hydrogenation of aryl alkyl ketones and diaryl
ketones with up to >99% ee using newly developed tridentate
ferrocene-based diamine-phosphine sulfonamide ligands (f-
diaphos).14 We envisioned that the Ir/f-diaphos catalytic
system may work well with the hydrogenation of the 2-pyridyl
aryl ketones. Herein, we reported a very efficient route for the
set of solvents were screened for (S)-2a; PrOH was found to
be superior to EtOH, MeOH, CH2Cl2, and toluene (Table 1,
entries 6−9, respectively). Notably, the basicity of the bases
was crucial for this reaction; less basic organic bases offered
ee’s that were better than those of strong ones, while inorganic
base K2CO3 failed to give (S)-2a (Table 1, entries 10−13).
Finally, the reaction temperature was examined; changing the
B
DOI: 10.1021/acs.orglett.9b01415
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Organic Letters
Letter
Scheme 2. Scope of 2-Pyridyl Aryl Ketones 1
Scheme 4. Control Experiments
temperature led to a slightly decreased ee, and the reaction
efficiency was sharply decreased when the reaction was
conducted at 0 °C (Table 1, entries 14 and 15). On the
basis of these results, the optimized reaction conditions were
set as follows: 0.1 mol % Ir−L3 or Ir−L5 complex, 5 mol %
i
tBuOLi, 3 MPa of H2, PrOH, rt, 12 h for (S)-2a or (R)-2a.
With the optimum reaction conditions in hand, we next set
out to determine the versatility of this reaction system in the
asymmetric hydrogenation of 2-pyridyl aryl ketones. Various
chiral ketones were hydrogenated to the corresponding
alcohols in excellent ee’s, exceeding >99% in most cases with
full conversion (Scheme 2). The substituents on the phenyl
rings had little influence on the enantioselectivity; both
electron-donating groups and electron-withdrawing groups
were reduced to the desired products 2b−2f in excellent ee’s.
Notably, a slight decrease in enantioselectivity was observed
when using a substrate with a strong electron-withdrawing
group (−CF3), producing (S)-2d and (R)-2d in 96% and 97%
ee’s, respectively. Moreover, decent results were obtained
regardless of the substituted position on the phenyl ring, giving
the expected products 2g−2j in 91% to >99% ee’s. It is worth
noting that disubstituted substrate 1k was hydrogenated
smoothly to corresponding products (S)-2k and (R)-2k in
high ee’s. In addition, the benzene ring could be effectively
replaced by other aromatic substituents like naphthalene (1l)
and benzo[d][1,3]dioxole (1m) to give (−)-alcohols in >99%
and 99% ee’s, respectively, (+)-alcohols in >99% ee’s. Next, the
effect of the substituent on the pyridine ring was examined.
Pleasingly, both enantiomers were obtained in extremely high
ee’s, except for a relatively lower enantioselectivy obtained
from the asymmetric hydrogenation of bromo-substituted
compound 1r. Finally, we investigated the asymmetric
hydrogenation of substituted dipyridylketones. However, the
catalyst could not recognize dipyridylketones with a para
substituent, affording racemic 2v in moderate yield. For-
tunately, moderate ee’s were obtained upon increasing the
asymmetrical bias in the substrate (1v). The results described
above suggested that the current Ir/f-diaphos catalytic system
tolerates a broad substrate scope of 2-pyridyl aryl ketones in
good efficiency with extremely high enantioselectivity.
a
Condition A: 2 mmol scale, 0.2 M substrate, 0.05 mol %
[Ir(COD)Cl]2, 0.105 mol % L3, 5 mol % tBuOLi, 10 mL of
iPrOH, room temperature (25−30 °C), full conversion in all cases
b
after 12 h. Condition B: 2 mmol scale, 0.2 M substrate, 0.005 mol %
[Ir(COD)Cl]2, 0.0105 mol % L5, 5 mol % tBuOLi, 10 mL of iPrOH,
room temperature (25−30 °C); full conversion in all cases after 12 h.
c
d
L2 was used. S/C = 100.
Scheme 3. Gram-Scale Reactions and Another
Transformation
C
DOI: 10.1021/acs.orglett.9b01415
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Organic Letters
Letter
To further exemplify the utilities of the developed iridium
catalytic system, several gram-scale reactions with lower
catalyst loadings were conducted, as shown in Scheme 3. To
our delight, the Ir−L3 complex exhibited good stability and
excellent reactivity. When the catalyst loading was decreased to
0.005 mol % (S/C = 20000), the asymmetric hydrogenation
still proceeded smoothly with 21.8 g of 1e to afford product
(S)-2e in a 21.5 g isolated yield (98%) and >99% ee within
only 12 h at 30 °C under a hydrogen pressure of 50 atm
(Scheme 3a). Notably, product (S)-2e is a key intermediate of
bepotastine besilate and carbinoxamine.15 More importantly,
the current catalytic system was also compatible with 1-
(pyridin-2-yl)ethan-1-one 3a, leading to (S)-4a in 99% yield
with >99% ee when using L7 as the ligand (Scheme 3b).
Finally, a set of control experiments and DFT studies have
been carried out to gain insight into the reaction mechanism
(see the Supporting Information for details). The impact of the
position of the N atom in the substrates on the asymmetric
hydrogenation reaction was first examined (Scheme 4, 1). As
expected, the 3- or 4-pyridyl ketone exhibited an enantiose-
lectivity much lower than that of the 2-pyridyl substrate,10c
indicating that the N atom adjacent to the carbonyl group
plays a crucial role for high ee’s, perhaps because of the
interaction with the Ir/f-diaphos complex. Next, we synthe-
sized tridentate ligand L10 bearing an ethylenediamine moiety
and applied it in the iridium-catalyzed asymmetric reduction of
1a; however, a lower ee value was obtained compared with that
of L1 (Scheme 4, 2, 73% ee vs 96% ee). This outcome
demonstrated the importance of the cyclohexane ring in the
ligand for this hydrogenation. In addition, DFT calculations
were performed by using ligand L3 and substrate 1a (Figure
S1). The common active catalyst was believed to be an
Ir(III)−dihydride complex A,16 which is derived from the
reaction of the Ir(I) precursor, L3, the base, and two H2
molecules along with dissociation of the COD ligand. The
hydrogenation of 1a with complex A could take place via the
two most possible transition states, TS-1 and TS-2, which
forms major product (S)-2a and its enantioisomer (R)-2a,
respectively. The relative enthalpy of TS-1 is 2.0 kcal/mol
lower than that of TS-2, because of the relayed hydrogen
bonding effect among the N atom in the substrate, two
hydrogen atoms in the ligand, and the O−H goup from the
solvent (Table S2). Therefore, (S)-2a is the major product,
and the theoretically predicted ee is 93%, which is slightly
lower than that of experimental observation.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01415.
■
S
Experimental details, spectral data, copies of 1H and 13
NMR spectra, and HPLC charts (PDF)
C
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: weihuizhong@zju.edu.cn.
ORCID
Yuanbin She: 0000-0002-1007-1852
Weihui Zhong: 0000-0003-0349-2226
Author Contributions
∥S.N. and F.L. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Natural Science Foundation of
China (21676253 and 21706234) and the Natural Science
Foundation of Zhejiang Province of China (LY19B060011) for
financial support.
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