Ruthenium-Catalyzed Enantioselective Hydrogenation of Hydrazones

Article abstract of DOI:10.1021/acs.orglett.0c02756

Prochiral hydrazones undergo efficient and highly selective hydrogenation in the presence of a chiral diphosphine ruthenium catalyst, yielding enantioenriched hydrazine products (up to 99% ee). The mild reaction conditions and broad functional group tolerance of this method allow access to versatile chiral hydrazine building blocks containing aryl bromide, heteroaryl, alkyl, cycloalkyl, and ester substituents. This method was also demonstrated on >150 g scale, providing a valuable hydrazine intermediate en route to an active pharmaceutical ingredient.

Full text of DOI:10.1021/acs.orglett.0c02756

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Letter
Ruthenium-Catalyzed Enantioselective Hydrogenation of
Hydrazones
Christopher H. Schuster,*,§ James F. Dropinski,§ Michael Shevlin, Hongming Li, and Song Chen
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ABSTRACT: Prochiral hydrazones undergo efficient and highly selective hydrogenation in the presence of a chiral diphosphine
ruthenium catalyst, yielding enantioenriched hydrazine products (up to 99% ee). The mild reaction conditions and broad functional
group tolerance of this method allow access to versatile chiral hydrazine building blocks containing aryl bromide, heteroaryl, alkyl,
cycloalkyl, and ester substituents. This method was also demonstrated on >150 g scale, providing a valuable hydrazine intermediate
en route to an active pharmaceutical ingredient.
molecular architectures and interesting biological activities.3
ince the pioneering work of Emil Fischer in the 1870s,
Many of these structures contain N−N linkages attached at
Shydrazine derivatives have become an indispensable class
applications from dyes, corrosion inhibitors and fuels, to
catalysts, synthetic reagents, pharmaceuticals, and agro
chemicals (Scheme 1A).2 Nature itself has also incorporated
N−N motifs in a variety of natural products resulting in unique
of molecules of great importance to modern life.1 Such N−N
chiral centers, inspiring the development of new methods for
their selective synthesis.4 Several of these approaches are
highlighted in Scheme 1B and include both C−N bond
forming and N−N bond forming transformations.
bond containing compounds can be found in a wide range of
One of the most successful methods that has emerged for
the synthesis of chiral hydrazines is the selective reduction of
prochiral hydrazones. Early examples of this approach relied on
preexisting stereocenters in order to deliver reducing agents in
a diastereoselective manner.5 A breakthrough in the field was
achieved by Burk, who in 1992 reported the first example of
enantioselective reduction of hydrazones, utilizing a rhodium/
DuPhos catalyst.6 Since this initial report, many excellent
examples of enantioselective hydrazone reduction have been
disclosed (Scheme 1B). Following the work of Burk, many of
these reports employ the use of rhodium7 based catalysts, with
fewer examples of palladium,8 nickel,9 iridium,10 and cobalt11
based catalysts. Despite the great success of ruthenium
catalysts in related imine reduction chemistry,12 to the best
of our knowledge there are no examples of utilizing group 8
metals in hydrazone reduction. Herein, we report the first
example of enantioselective ruthenium-catalyzed hydrogena-
tion of hydrazones.
Scheme 1. Synthetic Approaches towards Chiral Hydrazines
As part of a recent campaign toward a pharmaceutical
intermediate, an asymmetric preparation of large amounts of
Received: August 17, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02756
© XXXX American Chemical Society
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A

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hydrazine 2 was required (Table1). Due to the relative ease of
deprotection, initial focus was placed on targeting an
Importantly, the catalyst loading could be lowered to 0.3 mol
% and still maintain efficient hydrazone reduction (entry 11).
With a highly active and enantioselective catalyst system in
hand, the scope of ruthenium-catalyzed hydrazone reduction
was examined (Table 2). Notably, both electron-rich and
Table 1. Selected Screening Data for the Enantioselective
a
Reduction of 1
Table 2. Scope of Enantioselective Hydrogenation of
a
Hydrazones
b
c
c
entry
solvent
metal
ligand
T002-1
J011-1
conv (%)
ee (%)
1
2
3
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MIBK
Rh
Rh
Rh
Rh
Rh
Ru
Ru
Ru
Ru
Ru
Ru
7
96
94
93
22
74
86
88
81
92
97
97
99
98
97
J004-1
d
4
(R)-MP2-SegPhos
(S,S)-iPr-DiPAMP
W003-1
W017-1
W022-1
W022-1
W022-1
W022-1
5
6
7
8
9
91
>98
>98
>98
>98
>98
>98
ef
,
10
11
MIBK
MIBK
eg
,
a
Conditions: metal precursor, ligand, solvent (0.02M), 500 psi H2, 50
b
°C. Rh = (NBD)2RhBF4, Ru = (methallyl)2Ru(cod) + HBF4−OEt2
(2 equiv to Ru). As determined by chiral SFC analysis at 210 nm.
Absolute configuration is (R) unless otherwise noted. (S) product
obtained. 0.2 M in MIBK (methylisobutylketone). 1.0 mol % Ru/
c
d
e
f
g
ligand. 0.3 mol % Ru/ligand.
enantioselective reduction of Cbz protected hydrazone 1,
which was prepared from the commercially available ketone
and subjected to screening conditions (Table 1). Initial
attempts were inspired by the work of Tan and Yoshikawa7e
and centered around the use of rhodium/chiral phosphine
based catalysts (entries 1−5). Unfortunately, these conditions
failed to yield sufficiently high levels of enantioselectivity, with
the best result giving 88% ee when employing (R)-MP2-
SegPhos (Figure 1) in combination with (NBD)2RhBF4 at 50
a
Conditions: (methallyl)2Ru(cod) (1 mol %), W022-2 (1.1 mol %),
HBF4−OEt2 (2 mol %), MIBK (0.2 M), 50 °C, 100 psi H2 unless
otherwise noted, yield refers to assay yield of reaction mixture
b
c
determined by quantitative HPLC analysis. 200 psi H2. 500 psi H2.
electron-poor aryl substituents are well tolerated, fully
converting to the desired hydrazides 4 and 5 with excellent
levels of enantioselectivity. This is in stark contrast to rhodium-
catalyzed hydrazone hydrogenations, which were shown by
Burk to exhibit a strong electronic dependence on the aryl
substituent with deficient groups consistently outperforming
donating groups.6b Sterically challenging ortho-methyl contain-
ing substrate 6 and aryl bromide 7 were reduced in high yield.
Only moderate enantioselectivity for formation of 6 was
achieved, possibly due to a conformational change induced by
having an ortho substituent. Crucially, 7 exhibited no signs of
reductive dehalogenation. Examination of other substitution
patterns revealed both ethyl substitution (8) and cyclohexyl
substitution (9)17 gave efficient reaction, highlighting the
ability of the catalyst to distinguish between aryl groups and
Figure 1. Chiral diphosphine ligands used in Table 1.
°C in methanol (entry 4). It was not until screening
ruthenium/chiral phosphine based catalysts that high levels
of enantioselectivity were obtained, with Walphos13 and
(methallyl)2Ru(cod)14 combinations performing most effec-
tively (entries 6−8). Further optimization of reaction
conditions identified MIBK15 as the ideal solvent together
with Walphos derivative W022 as ligand, affording full
conversion to desired product 2 with 99% ee (entry 9).16
B
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groups larger than methyl. Interestingly, the selectivity of the
reaction decreases significantly when both hydrazone sub-
stituents are alkyl with hydrazine 10 being produced with only
10% ee. Previously, work by Zhou has shown trifluoromethyl-
substituted hydrazones to undergo selective reduction in the
presence of chiral Pd-catalysts under forcing condtions.8b8d
Notably, the formation of trifluoromethyl-substituted hydra-
zine 11 proved difficult for the ruthenium catalyst, achieving
only an 18% conversion and 43% ee.
slightly diminished enantioselectivity. Of note, simple alkyl-
substituted hydrazones do not undergo reduction, with no
conversion to the dimethyl substituted target 20, possibly due
to unproductive catalyst binding by this basic substrate.
The vast majority of enantioselective hydrazone reductions
utilize benzoyl-protected hydrazones as substrates.6−11 Inter-
estingly, this commonly employed protecting group does not
achieve high levels of enantioselectivity under the standard
ruthenium-catalyzed conditions, leading to benzoyl-protected
hydrazine 21 in good yield but with only moderate
enantioselectivity. To further probe this observation, electroni-
cally differentiated benzoyl-protected hydrazones were pre-
pared and subjected to the standard reaction conditions.
Notably, both a more electron-rich benzoyl-group (22) and a
more electron-poor benzoyl group (23) led to similar levels of
enantioselectivity. This is in contrast to rhodium-catalyzed
hydrazone hydrogenations where a strong dependence on
protecting group electronics leads to large changes in
enantioselectivity. Studies by Burk showed drastic changes in
enantioselectivity with more donating protecting groups
leading to increased selectivity, presumably due to more
tightly bound chelates in the enantiodetermining step.6b The
ruthenium-catalyzed conditions reported here appear to be
largely complementary to the majority of other metal catalyzed
hydrazone reductions, with the use of carbonate-based
protecting groups giving the best enantioselectivities.
Additionally, cyclic hydrazones were examined, giving
hydrazine products 12 and 13 in moderate to good yields
and with excellent enantioselectivity. Substrates containing
heteroaryl groups also underwent smooth reduction, affording
enantioenriched hydrazines containing furan (14), thiophene
(15), and pyridine (16). Significantly, the ruthenium catalyst is
tolerant of thiophene which can often poison transition metal
catalysts. The presence of a basic pyridine moiety appears to
adversely effect catalyst efficiency with only moderate yield and
selectivity obtained for 16. It was reasoned that the unhindered
pyridine could participate as an unwanted ligand for the
ruthenium catalyst and thereby inhibit catalysis. Attempts to
quench this interference by treating the substrate with a
stoichiometric equivalent of HBF4 proved unsuccessful, leading
to no conversion to 16. α-Hydrazino acids are interesting
analogs of amino acids which have garnered significant
attention as synthetic targets.18 Reduction of an α-ester
hydrazone proved successful, and α-hydrazino-ester 17 was
prepared in good yield and with excellent enantioselectivity.
In order to examine the scope and effect of the hydrazone
protecting groups in the hydrogenation reaction, analogs of
acetophenone-derived hydrazone were prepared and subjected
to the standard conditions (Table 3). Efficient reduction was
achieved when carbamate protecting groups were employed
with only a slight decrease in enantioselectivity going from Cbz
(3) to Boc (18) protected hydrazines. Acetyl protected
hydrazone was also reduced effectively by the ruthenium
catalyst system, generating hydrazine 19 in high yield but with
Access to large quantities of hydrazine 2 were required, and
therefore the hydrogenation of hydrazone 1 was examined in
scale-up studies (Scheme 2). Importantly, the hydrogenation
Scheme 2. Scale-up Hydrogenation of Hydrazone 1
a
Table 3. Examination of Additional Protecting Groups
proceeded smoothly with ruthenium loadings as low as 0.3 mol
%, achieving full conversion with excellent enantioselectivity.
Subsequent deprotection with Pd/C and conversion to the
crystalline HCl salt allowed isolation of 24 without any
detectable racemization.
In conclusion, the first enantioselective ruthenium-catalyzed
hydrogenation of hydrazones has been demonstrated. Mild
reaction conditions permitted the use of a broad range of
substrates, leading to useful chiral hydrazine building blocks
with high enantioselectivities. Complementary to the majority
of existing methods, easily cleaved carbamate-based protecting
groups provided the best reactivity and enantioselectivity.
Finally, the hydrogenation proved to be readily scalable at low
catalyst loadings, enabling access to large quantities of valuable
enantioenriched hydrazine intermediate 24.
ASSOCIATED CONTENT
* Supporting Information
■
sı
a
Conditions: (methallyl)2Ru(cod) (1 mol %), W022−2 (1.1 mol %),
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02756.
HBF4−OEt2 (2 mol %), MIBK (0.2 M), 50 °C, 100 psi H2 unless
otherwise noted, yield refers to assay yield of reaction mixture
determined by quantitative HPLC analysis. 500 psi H2 and 5 mol %
b
Experimental procedures and characterization of all new
compounds (PDF)
catalyst.
C
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Letter
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AUTHOR INFORMATION
■
Corresponding Author
Christopher H. Schuster − Department of Process Research
and Development, Merck & Co., Inc., Rahway, New Jersey
07065, United States; orcid.org/0000-0003-4110-3551;
Email: christopher.schuster@merck.com
Authors
James F. Dropinski − Department of Process Research and
Development, Merck & Co., Inc., Rahway, New Jersey 07065,
United States
Michael Shevlin − Department of Process Research and
Development, Merck & Co., Inc., Rahway, New Jersey 07065,
United States
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Hongming Li − Department of Process Research and
Development, Merck & Co., Inc., Rahway, New Jersey 07065,
United States
Song Chen − Department of Synthetic Chemistry, Pharmaron
Beijing Co., Beijing 100176, P. R. China
(7) (a) Yamazaki, A.; Achiwa, I.; Horikawa, K.; Tsurubo, M.;
Achiwa, K. Synlett 1997, 1997, 455. (b) Ireland, T.; Tappe, K.;
Grossheimann, G.; Knochel, P. Chem. - Eur. J. 2002, 8, 843.
(c) Tappe, K.; Knochel, P. Tetrahedron: Asymmetry 2004, 15, 91.
(d) Gavryushin, A.; Polborn, K.; Knochel, P. Tetrahedron: Asymmetry
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Qu, B.; Rodriguez, S.; van der Veen, L.; Reeves, D. C.; Gonnella, N.
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02756
Author Contributions
§C.H.S. and J.F.D. contributed equally.
Notes
(8) (a) Wang, Y. Q.; Lu, S. M.; Zhou, Y. G. J. Org. Chem. 2007, 72,
3729. (b) Chen, Z. P.; Hu, S. B.; Zhou, J.; Zhou, Y. G. ACS Catal.
2015, 5, 6086. (c) Chen, Z. P.; Chen, M. W.; Shi, L.; Yu, C. B.; Zhou,
Y. G. Chem. Sci. 2015, 6, 3415. (d) Chen, Z. P.; Hu, S. B.; Chen, M.
W.; Zhou, Y. G. Org. Lett. 2016, 18, 2676.
(9) (a) Xu, H.; Yang, P.; Chuanprasit, P.; Hirao, H.; Zhou, J. Angew.
Chem., Int. Ed. 2015, 54, 5112. (b) Yang, P.; Zhang, C.; Ma, Y.;
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to acknowledge Ryan Cohen (Merck)
for analytical support as well as Alan Hyde, LC Campeau,
Jamie McCabe Dunn and William Morris (all Merck) for
helpful suggestions towards the preparation of this manuscript.
(10) Chang, M.; Liu, S.; Huang, K.; Zhang, X. Org. Lett. 2013, 15,
4354.
(11) Hu, Y.; Zhang, Z.; Zhang, J.; Liu, Y.; Gridnev, I. D.; Zhang, W.
Angew. Chem., Int. Ed. 2019, 58, 15767.
(12) For select reviews, see: (a) Noyori, R.; Hashiguchi, S. Acc.
Chem. Res. 1997, 30, 97. (b) Clapham, S. E.; Hadzovic, A.; Morris, R.
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