Iridium-Catalyzed Transfer Hydrogenation of 1,10-Phenanthrolines using Formic Acid as the Hydrogen Source

Article abstract of DOI:10.1002/adsc.201500909

The iridium-catalyzed highly regioselective transfer hydrogenation of a variety of 2-substituted and 2,9-disubstituted 1,10-phenanthrolines under mild conditions with formic acid as the hydrogen source is described. In the presence of a catalytic amount of the iridium complex [CpIrCl₂]₂, the transfer hydrogenation proceeded smoothly in 1,4-dioxane under ligand-free conditions, exclusively leading to a range of 1,2,3,4-tetrahydro-1,10-phenanthroline products in high yields. The catalyst generated in situ from [CpIrCl₂]₂ and (R,R)-(CF₃)₂C₆H₃SO₂-dpen [N-(2-amino-1,2-diphenylethyl)-3,5-bis(trifluoromethyl)benzenesulfonamide] could efficiently catalyze the asymmetric transfer hydrogenation of these 1,10-phenanthrolines in isopropyl alcohol (i-PrOH) to afford chiral 1,2,3,4-tetrahydro-1,10-phenanthrolines in high yields with up to >99% ee. The key to the success of the reduction is the choice of solvent and hydrogen source.

Full text of DOI:10.1002/adsc.201500909

COMMUNICATIONS
DOI: 10.1002/adsc.201500909
Very Important Publication
Iridium-Catalyzed Transfer Hydrogenation of 1,10-Phenanthro-
lines using Formic Acid as the Hydrogen Source
a
a
a
a
a
a
Conghui Xu, Lingjuan Zhang, Chaonan Dong, Jianbin Xu, Yixiao Pan, Yali Li,
a
a
a
a,
Hanyu Zhang, Huanrong Li, Zhiyong Yu, and Lijin Xu *
Department of Chemistry, Renmin University of China, Beijing 100872, Peopleꢀs Republic of China
a
Fax: (+86)-10-6251-6444; phone: (+86)-10-6251-1528; e-mail: xulj@chem.ruc.edu.cn
Received: September 29, 2015; Revised: December 6, 2015; Published online: February 11, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500909.
Abstract: The iridium-catalyzed highly regioselec-
tive transfer hydrogenation of a variety of 2-substi-
tuted and 2,9-disubstituted 1,10-phenanthrolines
under mild conditions with formic acid as the hy-
drogen source is described. In the presence of a cat-
alytic amount of the iridium complex [Cp*IrCl ] ,
such as quinolines, quinoxalines, isoquinolines, in-
doles, pyridines and pyrazines have been successfully
reduced with high selectivities by using isopropyl al-
cohol or formate as the hydrogen source in the pres-
[3]
ence of an Ru, Rh or Ir catalyst. It is noteworthy
that Xiao and his co-workers found that the transfer
hydrogenation of N-heteroarenes including asymmet-
ric reduction of quinolines could be smoothly per-
formed in water without calling for organic solvents
2
2
the transfer hydrogenation proceeded smoothly in
,4-dioxane under ligand-free conditions, exclusive-
1
ly leading to a range of 1,2,3,4-tetrahydro-1,10-
phenanthroline products in high yields. The catalyst
generated in situ from [Cp*IrCl2]2 and (R,R)-
[3i,k–m]
and ligand modification.
However, despite these
exciting advances, there is still much room for im-
provement, particularly in terms of N-heteroaryl sub-
strate scope and catalytic efficiency.
(
CF ) C H SO -dpen
[N-(2-amino-1,2-diphenyl-
3
2
6
3
2
ethyl)-3,5-bis(trifluoromethyl)benzenesulfonamide]
could efficiently catalyze the asymmetric transfer
hydrogenation of these 1,10-phenanthrolines in iso-
propyl alcohol (i-PrOH) to afford chiral 1,2,3,4-tet-
rahydro-1,10-phenanthrolines in high yields with up
to >99% ee. The key to the success of the reduc-
tion is the choice of solvent and hydrogen source.
Unlike 1,10-phenanthroline and its derivatives,
which have been extensively studied as one of the
most versatile bidentate ligands in organometallic
[4]
chemistry and catalysis, the partially reduced 1,2,3,4-
tetrahydro- and 1,2,3,4,7,8,9,10-octahydro-1,10-phen-
anthrolines have attracted much less interest despite
the fact that they can serve as important ligands for
[5]
Keywords: iridium; 1,10-phenanthrolines; reduc-
tion; 1,2,3,4-tetrahydro-1,10-phenanthrolines; trans-
fer hydrogenation
olefin polymerization, as valuable synthetic inter-
[6]
mediates to prepare chiral materials, as antagonists
[7]
with potent activity against HIV-1 and as vicinal di-
[8]
amine and N-heterocyclic carbene ligands.
As
a direct consequence, the construction of these com-
pounds is largely underexplored. Traditional methods
for the preparation of 1,2,3,4-tetrahydro- and
Transition metal-catalyzed transfer hydrogenation re- 1,2,3,4,7,8,9,10-octahydro-1,10-phenanthrolines
pri-
actions employing a hydrogen donor other than mo- marily rely on reduction with stoichiometric or excess
[
8b,9]
lecular hydrogen have become a powerful rival to the amounts of reducing agent
and transition metal-
[10]
hydrogenation reaction because of their versatility, catalyzed heterogeneous hydrogenation. The substi-
[
1]
operational simplicity and safety. The past two de- tuted phenanthrolines could also be reduced using
cades have witnessed considerable progress in the a Hantzsch dihydropyridine under the catalysis of
transfer hydrogenation of carbonyl compounds and BINOL-derived phosphoric acid (BINOL: 1,1’-bi-
imines, and the catalytic systems based on ruthenium, naphthol). Based on a cyclometallated Ir(III) cata-
rhodium and iridium complexes are notable for their lyst, Xiao and co-workers reported the partial hydro-
[11]
[
1c–m,2]
remarkable catalytic performance.
Meanwhile, genation of 2,9-dimethyl-1,10-phenanthroline to give
the transition metal-catalyzed transfer hydrogenation the 1,2,3,4-tetrahydro-1,10-phenanthroline product
[12]
of N-heteroarenes has drawn the attention of synthet- under mild conditions. Notably, Fan and co-workers
ic chemists, and a variety of N-heteroaryl compounds, found that 2-substituted and 2,9-disubstituted 1,10-
Adv. Synth. Catal. 2016, 358, 567 – 572
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
567

Conghui Xu et al.
COMMUNICATIONS
phenanthroline derivatives could undergo efficient Table 1. Screening conditions for catalytic transfer hydroge-
[
a]
nation of 1a.
asymmetric hydrogenation catalyzed by a cationic
ruthenium complex, providing 1,2,3,4-tetrahydro- and
1
,2,3,4,7,8,9,10-octahydro-1,10-phenanthroline prod-
[13]
ucts in high yields and selectivities. In consideration
of the limitations of these known protocols and the
appeal of transition metal-catalyzed transfer hydroge-
nation, it appears very attractive to develop efficient
catalytic systems for the transfer hydrogenation of
Entry Catalyst
precursor
Hydrogen
source
Solvent
Yield
[%]
[b]
[
14]
1,10-phenanthrolines. Following our continuing in-
[15]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
[Cp*IrCl2]2 HCO H
H O
87
25
33
50
NR
68
trace
12
trace
39
57
2
2
terest in reducing N-heteroarenes, herein we report
that [Cp*IrCl ] could efficiently catalyze the transfer
[Cp*IrCl2]2 HCO H
MeOH
EtOH
i-PrOH
TFE
2
2
2
[Cp*IrCl2]2 HCO H
2
hydrogenation of 2- and 2,9-substituted 1,10-phenan-
[Cp*IrCl2]2 HCO H
2
throline derivatives with HCO H as the hydrogen
2
[Cp*IrCl2]2 HCO H
2
source in 1,4-dioxane under ligand-free conditions to
give 1,2,3,4-tetrahydro-1,10-phenanthroline products
in high yields, and the combination of [Cp*IrCl2]2
with chiral (CF ) C H SO -dpen constitutes an effi-
[Cp*IrCl2]2 HCO H
THF
2
[Cp*IrCl2]2 HCO H
CH Cl
2
2
2
[Cp*IrCl2]2 HCO H
toluene
DMSO
2
[Cp*IrCl
[Cp*IrCl2]
]
HCO
HCO H
HCO H
H
3
2
6
3
2
2
2
2
2
2
0
1
2
3
4
5
6
7
8
CH CN
cient catalytic system for the asymmetric transfer re-
duction of 1,10-phenanthroline derivatives to give
chiral 1,2,3,4-tetrahydro-1,10-phenanthroline products
in up to >99% ee.
We initiated our investigation with the screening of
the reaction conditions for transfer hydrogenation of
2
3
[Cp*IrCl2]
2
DMF
[Cp*IrCl2]2 HCO H
1,4-dioxane 96
1,4-dioxane NR
1,4-dioxane NR
2
[Cp*IrCl2]2 HCO Na
2
[Cp*IrCl2]2 HCO Li
2
[Cp*IrCl2]2 HCO H/NEt (5/2) 1,4-dioxane 34
2
3
[c]
[Cp*IrCl2]2 HCO H
1,4-dioxane 82
1,4-dioxane NR
1,4-dioxane NR
1,4-dioxane NR
1,4-dioxane NR
2
2,9-dimethyl-1,10-phenanthroline (1a) in the presence
[Cp*RhCl2]2 HCO H
2
[
[
d]
e]
of a transition metal catalyst derived from a half-
[Ru]
[Ru]
none
HCO H
2
sandwich complex (Rh, Ir or Ru). Considering the 19
substantial advantage of using water as the reaction 20
HCO H
2
HCO H
2
[
16]
medium, we first tried the reduction of 1a in water
[
a]
Reaction conditions: 1a (0.2 mmol), catalyst precursor
1 mol%), hydrogen source (1.6 mmol), solvent (2.0 mL),
408C, 24 h. NR=no reaction.
Isolated yield.
with HCO H as the hydrogen source under the cataly-
2
(
sis of [Cp*IrCl ] . After 24 h, the reaction afforded
2
2
[
[
[
[
b]
c]
d]
e]
the partial hydrogenation product, 2,9-dimethyl-
,2,3,4-tetrahydro-1,10-phenanthroline (2a) in 87%
1
Room temperature.
isolated yield, and no double reduced product was de-
tected (Table 1, entry 1). Encouraged by this result,
we then screened other solvents, such as MeOH,
EtOH, i-PrOH, TFE (trifluoroethyl alcohol), THF
[Ru(p-cymene) Cl ] was employed.
2
2
2
[Ru(benzene)
Cl
2
]
2
was employed.
2
(
tetrahydrofuran), CH Cl , toluene, DMSO (dimethyl isolated 2a to 82% (Table 1, entry 16). We also inves-
2 2
sulfoxide), CH CN, DMF (N,N-dimethylformamide) tigated the catalytic performance of other half-sand-
3
and 1,4-dioxane. Among these solvents, 1,4-dioxane wich complexes, but they were inferior to [Cp*IrCl2]2
afforded the best result, and 2a was isolated in 96% (Table 1, entries 17–19). In the absence of the catalyst
yield (Table 1, entries 2–12). Realizing that the choice precursor, no reaction took place (Table 1, entry 20).
of hydrogen source may affect the transformation ef-
With the optimized reaction conditions in hand, we
ficiency, we then examined the performance other hy- then investigated the generality of this reduction reac-
drogen sources. Indeed, no reduction was observed tion. As shown in Table 2, the simple 1,10-phenan-
using HCO Na or HCO Li as the hydrogen source throline (1b) and symmetrical 2,9-dialkylated 1,10-
2
2
(
Table 1, entries 13 and 14). With the frequently em- phenanthrolines (1c–f) proceeded smoothly to gener-
ployed azeotropic HCO H/NEt (5/2) mixture as the ate the partially hydrogenated products (2b–f) in ex-
2
3
hydrogen source, the reaction occurred, but a signifi- cellent yields, suggesting that the reaction was insensi-
cantly diminished yield was obtained (Table 1, tive to the length of the side chain. It was observed
entry 15). Obviously, the reduction favors the strong that the symmetrical 1,10-phenanthrolines with bulky
acidic conditions, and 1a is probably reduced through substituents at the 2- and 9-positions (1g and 1h) par-
an ionic mechanism in its protonated form. A similar ticipated well in this reaction, and the corresponding
substrate activation strategy is well known among products (2g and 2h) were isolated in 92% and 94%
[17]
transition metal-catalyzed reduction reactions. Fur- yields, respectively. The more bulky 2,9-di-tert-butyl-
ther optimization indicated that carrying out the re- 1,10-phenanthroline (1i) was also reduced, although
duction at room temperature decreased the yield of a higher catalyst loading (2 mol%) was required. The
5
68
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Adv. Synth. Catal. 2016, 358, 567 – 572

COMMUNICATIONS Iridium-Catalyzed Transfer Hydrogenation of 1,10-Phenanthrolines
[
a,b]
Table 2. Ir-catalyzed transfer hydrogenation of 1,10-phenanthrolines.
[
[
[
a]
b]
c]
Reaction conditions: 1 (0.2 mmol), [Cp*IrCl ] (1 mol%), HCO H (1.6 mmol), 1,4-dioxane (2.0 mL), 408C, 24 h.
Isolated yield.
2
2
2
[
Cp*IrCl ] (2 mol%) was employed.
2
2
unsymmetrical 2,9-dialkylated 1,10-phenanthroline 1j 4,7-phenanthroline (1w) under the current catalytic
underwent selective reduction, delivering a mixture of conditions. Transfer reduction of 1u and 1w led to the
regioisomeric products 2j and 2j with the former as formation of 2-methyl-1,2,3,4-tetrahydroquinoline
1
2
the major product due to steric hinderance. Likewise, (2u) and 1,2,3,4-tetrahydro-4,7-phenanthroline (2w) in
the 2-alkylated 1,10-phenanthrolines (1k–1n, 1p) gave 80% and 35% yields, respectively, but no reaction was
regioisomeric products (2k , 2k , 2m , 2m , 2n , 2n , observed in the case of 1t or 1v.
1
2
1
2
1
2
2
p , 2p ) with low regioselectivities. Interestingly, 2-
Encouraged by these results, we next turned our at-
isopropyl-1,10-phenanthroline (1o) and 2-(tert-butyl)- tention to the asymmetric transfer hydrogenation of
,10-phenanthroline (1q) could readily engage in the these 1,10-phenanthroline derivatives. We first exam-
reduction, exclusively affording 2o and 2q in 91% and ined the asymmetric reduction of 1a with HCO H in
1
2
1
2
90% yields, respectively, as a result of the strong different solvents in the presence of a chiral iridium
steric hinderance. However, 2,9-diphenyl-1,2,3,4-tetra- catalyst generated in situ from [Cp*IrCl ] and the
2
2
[18]
hydro-1,10-phenanthroline (1r) was less reactive, re- Noyori ligand Ts-dpen (L1) (Table 3, entries 1–11).
quiring a higher catalyst loading (2 mol%) to reach A dramatic solvent effect was observed, and the use
a high isolated yield of the reduction product 2r. In of i-PrOH as the solvent was the most beneficial in
contrast, 2-phenyl-1,10-phenanthroline (1s) showed terms of enantioselectivity (Table 3, entry 4). Al-
good reactivity to provide the reduction product 2s in though 1,4-dioxane delivered the best yield, a very
9
0% yield, and the reduction took place exclusively at low enantioselectivity was obtained (Table 3,
the unsubstituted pyridyl ring. It should be noted that entry 11). With i-PrOH as the solvent, a survey of var-
only one pyridyl ring was reduced in all cases studied, ious chiral diamine ligands was then carried out. It
and full reduction of the two pyridyl rings was not ob- was found that both steric and electronic effects of
served.
these ligands have a profound influence on the reac-
We also examined the transfer hydrogenation of tion efficiency and enantioselectivity (Table 3, en-
quinoline (1t), quinaldine (1u), isoquinoline (1v) and tries 12–18), and (R, R)-(CF ) C H SO -dpen (L5)
3
2
6
3
2
Adv. Synth. Catal. 2016, 358, 567 – 572
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569

Conghui Xu et al.
COMMUNICATIONS
Table 3. Screening conditions for the asymmetric transfer that more than 99% ee values were achieved in the
[a,b]
hydrogenation of 1a.
reduction of compounds 1d and 1e. The size of the
alkyl substituent did not influence the enantioselectiv-
ity of reaction as evidenced by the fact that excellent
ee values were observed in the reduction of bulky
substrates 1g and 1h. However, in the case of the
more sterically demanding substrate 1i, only the race-
mic product was isolated. The unsymmetrical 2,9-di-
alkyl-substituted 1,10-phenanthroline substrate (1j)
could give the two chiral products [(R)-2j and (R)-
1
2
j2] with high enantioselectivities, albeit with low re-
gioselectivities. The current catalytic system did not
work well for the asymmetric reduction of 2-substitut-
ed 1,10-phenanthrolines (1k–1n), and very low enan-
tioselectivities were obtained [(R)-2k –(R)-2n ]. In
1
2
these monosubstituted substrates, the absence of
a second substituent makes the facial discrimination
less effective, thereby leading to lower ee values. The
reduction of 2,9-diphenyl 1,10-phenanthroline (1r)
only resulted in the formation of racemic product. We
also carried out the asymmetric transfer hydrogena-
tion of quinaldine (1u) using HCO H and HCO H/
[
b]
[c]
Entry
Solvent
Ligand
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
9
H O
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L6
L7
L8
L5
90
42
47
65
4
2
MeOH
EtOH
i-PrOH
THF
87
90
92
71
ND
ND
81
52
37
17
97
84
22
98
94
97
48
ND
2
2
77
Et N (5/2) azeotrope as the hydrogen source, respec-
3
CH Cl2
trace
trace
70
58
16
95
81
69
30
94
82
86
19
2
tively. The reaction with HCO H gave the desired
2
DMSO
DMF
product
(R)-2-methyl-1,2,3,4-tetrahydroquinoline
[
(R)-2u] in 86% yield and 68% ee, whilst using the
CH CN
3
azeotrope resulted in a lower yield of 57% and a simi-
lar ee value of 67%. Obviously, the addition of NEt3
10
11
12
13
14
15
16
17
18
19
toluene
1,4-dioxane
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
is detrimental to the reaction.
In conclusion, we have developed mild and efficient
transfer hydrogenation reaction conditions that allow
selective reduction of a wide range of 2-substituted
and 2,9-disubstituted 1,10-phenanthrolines to exclu-
sively give 1,2,3,4-tetrahydro-1,10-phenanthroline
products in high yields and enantioselectivities (up to
[d]
NR
>
99% ee) using HCO H as the hydrogen source. It is
2
[a]
Reaction conditions: 1a (0.2 mmol), [Cp*IrCl2]2 noteworthy that the choice of solvent and hydrogen
(
1 mol%), ligand (2.4 mol%), HCO H (1.6 mmol), sol-
vent (2.0 mL), 408C, 24 h. ND=not detected.
Isolated yield.
2
source is critical to catalysis. The present catalysis is
operationally simple, and provides a valuable alterna-
tive to the known methods for 1,10-phenanthroline
reduction. Further studies toward the full reduction of
the two pyridyl rings of 1,10-phenanthrolines are cur-
rently underway in our laboratory.
[
[
[
b]
c]
Determined by HPLC analysis.
No HCO H was added.
d]
2
proved to be the best choice, and the desired product
was furnished with 94% yield and 98% ee (Table 3,
entry 15). We finally examined the asymmetric reduc-
Experimental Section
tion of 1a in i-PrOH in the absence of HCO H, but
2
no reaction occurred (Table 3, entry 19). Obviously i-
PrOH did not serve as a hydrogen source in this case.
A variety of 2,9-disubstituted and 2-substituted
General Procedure for Transfer Hydrogenation of
1
,10-Phenanthrolines
A mixture of 1,10-phenanthroline 1 (0.2 mmol), HCO H
2
1
,10-phenanthroline derivatives were then subjected
(
75.0 mg, 1.6 mmol), [Cp*IrCl ] (1.6 mg, 1 mol%), 1,4-diox-
2 2
to asymmetric transfer hydrogenation under the opti-
mized reaction conditions (Table 4). As expected, var-
ious symmetrical 2,9-dialkylated 1,10-phenanthroline
derivatives were readily reduced, affording the corre-
ane (2.0 mL) was stirred at 408C for 24 h under an N at-
mosphere. After cooling down to room temperature, water
2
(10.0 mL) was added to the reaction mixture. The reaction
mixture was extracted with CH Cl (3”10.0 mL). The com-
2
2
sponding chiral products in excellent yields and enan- bined organic layer was dried with anhydrous Na SO4 for
2
[
19]
tioselectivities in almost all the cases. It is notable a while and filtered. The filtrate was concentrated by
5
70
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ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2016, 358, 567 – 572

COMMUNICATIONS Iridium-Catalyzed Transfer Hydrogenation of 1,10-Phenanthrolines
[
a,b]
Table 4. Ir-catalyzed asymmetric transfer hydrogenation of 1,10-phenanthrolines.
[a]
Reaction conditions: 1 (0.2 mmol), [Cp*IrCl ] (1 mol%), L5 (2.4 mol%), HCO H (1.6 mmol), i-PrOH (2.0 mL), 408C,
2
2
2
2
4 h.
Isolated yield. Enantiomeric excess was determined by HPLC.
Cp*IrCl ] (2 mol%) and L5 (4.8 mol%) were employed.
[
[
b]
c]
[
2
2
vacuum evaporation and the residue was purified by column References
chromatography on silica gel using a mixture of ethyl ace-
tate and hexane to give the pure 1,2,3,4-tetrahydro-1,10-
phenanthroline product.
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(
CF ) C H SO -dpen (2.3 mg, 2.4 mol%), i-PrOH (2.0 mL)
3
2
6
3
2
was stirred at 408C for 24 h under an N atmosphere. After
2
cooling down to room temperature, water (10.0 mL) was
added to the reaction mixture. The aqueous solution was ex-
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2
layer was dried with anhydrous Na SO for a while and fil-
2
4
tered. The filtrate was concentrated by vacuum evaporation
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Acknowledgements
Financial support from the National Natural Science Founda-
tion of China (21372258, 21072225 and 21172258) is greatly
acknowledged.
Adv. Synth. Catal. 2016, 358, 567 – 572
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of product (R)-2c remained unchanged during the
course of the reaction, indicating that there is no ele-
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5
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Adv. Synth. Catal. 2016, 358, 567 – 572