(η5-Pentamethylcyclopentadienyl)iridium Complex Catalyzed Imine Reductions Utilizing the Biomimetic 1,4-NAD(P)H Cofactor and N-Benzyl-1,4-dihydronicotinamide as the Hydride-Transfer Agent

Article abstract of DOI:10.1002/cctc.201601307

The interaction between synthetic organometallic complexes and metabolic cofactors has proven to be a newly emerging topic in bioorganometallic chemistry. Thus, the first cationic Cp*Ir-catalyzed (Cp=η⁵-pentamethylcyclopentadienyl) imine reduction in neutral buffered aqueous medium was examined. The reaction was found to proceed through hydride transfer from NADH as the hydride source at room temperature in air. Cationic Cp*Ir complexes proved to be the most efficient catalysts for this transformation. We also highlighted that the choice of the proton source was essential. The method was subsequently applied to cyclic and noncyclic imines. Eventually, the concept was extended to the reductive alkylation of one amine.

Full text of DOI:10.1002/cctc.201601307

Accepted Article
Title: Iridium catalyzed imine reduction by hydride transfer using
coenzyme NADH
Authors: Olivier Riant, mathieu Soetens, and Fleur Drouet
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Cp*Ir catalyzed imine reductions utilizing the biomimetic 1,4-
NAD(P)H cofactor and N-benzyl-1,4-dihydronicotinamide as the
hydride transfer agent
Mathieu Soetens [a], Fleur Drouet [a], Olivier Riant* [a]
Abstract: Interactions between synthetic organometallic complexes
and metabolic cofactors proved to be a newly emerging topic in bio-
organometallic chemistry. Thus, the first cationic Cp*Ir-catalyzed
imine reduction in neutral buffered aqueous medium is reported. The
reaction proceeds via a hydride transfer from NADH as the hydride
source at room temperature and under air. Cationic Cp*Ir complexes
proved to be the most efficient catalysts for this transformation. We
also highlighted that the choice of the proton source was essential.
dihydronicotinamide (Bn-NADH) to [Cp*Rh(bpy)(D2O)](OTf)2 as
the hydride acceptor in D2O/THF-d during deuterium
incorporation studies.[4c] Then, in 2012, Sadler reported the direct
1
observation and characterization by H-NMR of a ruthenium or
iridium hydride obtained directly by treatment of the
corresponding aqua complexes by NADH (Scheme 1b).[6] Studies
about the properties and stability of those hydrides were reported
by Fukuzumi and Miller.[7] Those hydrides could further reduce
The method was subsequently applied on cyclic and non-cyclic imines. biologically relevant substrates, such as pyruvate to lactate. This
Eventually, the concept was extended to one example of reductive
alkylation of an amine.
system was also applied to quinones by Sadler and Fukuzumi.[8]
Those metal hydride are already known as versatile tools for
reduction of carbonyls and imines in water,[9] and more complex
functional groups such as dihydroisoquinolines.[10]
Even if those hydride transfer have already found applications
within biological medium, we noticed that to date, few examples
involving NAD(P)H cofactors and organometallic complexes have
been published so far. Ariga applied the concept of the reduction
of quinones by NADH and a cationic Cp*Ir complex reported by
Sadler to quench the fluorescence of a rhodamine and used it as
a probe of the tricarboxylic acids cycle.[11] More recently, Do
showed that it was possible to use NADH and sub-stoichiometric
amounts of a cationic Cp*Ir complex to reduce cytotoxic
aldehydes in the water medium to the corresponding alcohols in
water, proving that the complex acts as a catalyst.[12] However,
there has been no report of catalytic hydride transfer reduction of
imines involving NAD(P)H and a metal complex to the best of our
knowledge. Imines are more challenging substrates than
aldehydes due to their reduced electrophilic character and their
hydrolytic behavior in water at acidic pH. Herein, we report the
cationic Cp*Ir promoted catalytic transfer reduction of a set of
imines in water using NADH or one of its analogues as the hydride
NAD(P)H is widely used by the metabolism as a cofactor for its
role as an important electron carrier in many biological pathways,
including glycolysis, fermentation and Krebs cycle and is known
to have a key role in the metabolism of every living cell, especially
cancer cells due to lower oxygen tension.[1]
Interactions between organometallics and NAD cofactors in the
context of NADH regeneration has been a key topic in bio
organometallic chemistry for over 3 decades and has been
extensively studied.[2] The rhodium (III) complex [Cp*RhCl(bpy)]+,
in combination with HCOONa as a hydride source, was first
employed by Steckhan in the context of NADH regeneration
(Scheme 1a).[3] Later, Fish explained the 1,4 regioselectivity of the
hydride transfer from the metal center to NAD+ by the coordination
of the NAD+ amide moiety to the metal.[4] The concept was later
applied by Sadler in living cells, where he reported that Noyori
type ruthenium hydride complexes could reduce NAD+ to NADH
in human ovarian cancer cells in the presence of formate anion,
leading to cell death through a novel and original reductive stress
mechanism.[5]
Fish also brought experimental evidences of the reversibility of
that hydride transfer from the NADH analogue N-benzyl-1,4-
source (Scheme 1c).
Scheme 1 a) Steckhan and Fish: electrochemical or reduction of a d6 complex
to the hydride and reduction of NAD+ to 1,4-NADH, b) Sadler: Oxidation of
NADH to NAD+ and formation of the hydride and subsequent reduction of
pyruvate to lactate, c) This work: oxidation of NADH to NAD+ and reduction of
an imine using the in situ generated hydride.
[a]
M. Soetens, Dr. F. Drouet, Prof. Dr. O. Riant
Université catholique de Louvain
Institute of Condensed Matter and Nanosciences (IMCN)
1, Place Louis Pasteur
B-1348 Louvain-la-Neuve, Belgium.
E-mail: olivier.riant@uclouvain.be
In the first phase of our investigation, we decided to optimize the
reduction of a water stable and water soluble model imine using
a stoichiometric amount of NADH (Scheme 2a). Recent studies
Supporting information for this article is given via a link at the end of
the article
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COMMUNICATION
by Pikho, Yu and Vidal drew our attention to dihydroisoquinolines
as good model substrates for this reduction.[10a] We therefore
oriented our screening towards water soluble catalysts able to
react with NADH and form a hydride stable in neutral buffered
conditions and also able to reduce imines in water. We started our
investigations by screening ruthenium (C1 and C2) and cationic
Cp*Ir (C3-C6) catalysts using a 50 mM phosphate buffer at pH 7
as medium (Scheme 2b). Despite the numerous reports by Sadler
on hydride transfers from NADH to Ru complexes, C1 and C2,
even at high catalytic loading, did not show any good reactivity for
the reduction of the imine 1.[6] After switching to cationic Cp*Ir
complexes with lower catalytic loading (2 mol%), we noticed that
an electron rich bipyridine (C6) was vital for an efficient reduction
of our model substrate. We therefore obtained a yield of 43%
using 2 mol% of C6.
of the acid. Preliminary studies rapidly showed a different
behavior for the acids depending on their pKa (Figure 1). Our
screening among acids ranging from pKa of -0.3 to 10 revealed
three different phenomena (See Supporting information Table 2).
Firstly, for the strong acids (pKa <4), we observed a decrease of
the yield after the addition of 1 equivalent and no more conversion
for NaHSO4 and trifluoroacetic acid (TFA) when 3 equivalents are
used. Secondly, the medium acids (pKa between 4 and 8-9)
afforded a maximum of yield using 2 equivalents and only a slight
decrease when a third equivalent is used. Finally, the weak acids
have little to no effect compared to the situation in absence of acid.
We hence suspected that the pKa threshold of 4 corresponds to
the pKa of the protonation of the hydride.[13] When the acid is too
strong, the hydride is reprotonated faster than it can reduce the
imine. To further confirm this hypothesis, a yield of 52% was
obtained when 2.5 equivalents of TFA acid were added over 5
hours, avoiding thus the contact between the acid and the hydride.
Then, the second threshold corresponds to the activation of the
imine in solution by protonation. When the acid is too weak
(PhOH), there is no effect as no activation of the substrate is
possible. Our screening showed that KH2PO4 was the optimal
acid for our purpose. We were pleased to obtain 60% yield (TON
= 30) with the cationic Cp*Ir complex C6 using either 2.5
equivalents of KH2PO4. Alternatively, a constant pH was reached
by working with a 100 mM pH 7 phosphate buffer to obtain the
same yield of 60% (Table 1 Entry a). The pH was found to change
from 7.0 to 7.4 during the reaction. While performing the reaction
in 100 mM buffers with pH ranging from 4 to 10, the same trend
was observed. The reaction was found to exhibit an optimum for
neutral pH buffers (See supporting information Table 4).
Scheme
2 a) Model reaction studied: reduction of 6,7-dimethoxy-1-
Some minor improvements of the yield were noted when an
excess of NADH was used (72%), even though this option was
not further investigated for economic reasons. NADPH exhibited
a similar reactivity, providing 62% of yield (Table 1, Entry a).
Sodium formate also gave us a yield of 99% while 10 equivalents
are used (Table 1, Entry c). The formate turns out to be an efficient
hydride source for our reaction. This result is especially interesting
as it shows that it can be used as a NADH substitute in biological
applications, as living cells can tolerate millimolar concentrations
of formate.[5]
methyltetrahydroisoquinoline, b) Structure of the catalysts C1-C6 used for this
study and yield (%) for the model reaction. Conditions: substrate at 15 mM in 50
mM pH 7 phosphate buffer, C1-C2 10 mol% and C3-C6 2 mol%, NMR yields
versus dimethyl terephthalate as internal standard.
Since the obtained amine 2 is present in its protonated form at pH
7, we expect that at least 2 H+ from the medium should be
consumed in the overall process. The formed imine can also trap
the Cp*Ir aqua complex by coordination to the metal center,
preventing thus the catalyst from performing another catalytic
cycle. We believe that protonation of the formed amine can
impede this inhibition mechanism by preventing the coordination
of the amine to the iridium. Therefore, we investigated the impact
70%
60%
Scheme 3 Study of various hydride donors
-0,3
pKa of the acid:
50%
1,9
Table 1 Yield obtained for the various hydride donors of this study
40%
3,13
30%
Entry
a
Hydride source
Equiv.
1
2
Yield (%)
60a
4,2
NADH
72a
20%
b
c
NADPH
1
10
1
61a
99b
83b
99b
4,75
7,21
9,95
10%
0%
HCOONa
d
Bn-NADH
2
0
1
2
3
Conditions: Substrate 15 mM in 100 mM phosphate buffer under
air. a NMR yield using dimethyl terephthalate, b isolated yield
Equivalent of acid
Figure 1 Impact of the use of acids used at 0, 1, 2 and 3 equivalents. pKa: TFA
(-0.3), NaHSO4 (1.99), citric acid (3.13), PhCOOH (4.2), AcOH (4.75), KH2PO4
(7.21), phenol (9.95)
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After a successful validation of the methodology with NADH, we
decided to apply the concept on a panel of imines (Scheme 4,
Table 2). We hereby chose a NADH analogue bearing the same
redox active dihydropyridine as an economical alternative to
NADH. Indeed, many NAD analogues were previously
reported.[14] Among those, we chose the N-benzylated analogue
of NADH (Bn-NADH) for its acceptable aqueous solubility and
since its interactions with metal complexes were studied in the
literature.[4] It was important to note that 2 equivalents of Bn-
NADH were needed to get full conversion (Table 1, Entry d). We
verified that the N-benzyl-1,4-dideuteronicotinamide (Bn-NADH-
d2, 79% deuterated) led to a deuterium incorporation of 72 %,
indicating thus that the hydride is coming from the Bn-NADH-d2
and that the Ir-D attacks the imine. This catalytic system shows
excellent yields on cyclic imines (Table 2, Entries a–c), good
yields on salicylideneaniline derivatives (Table 2, Entries d–f) and
on N-benzylidene derivatives (Table 2, Entries g–l). Some
selected examples allow to compare the results of Bn-NADH with
NADH (Table 2, Entries a, d, g, n).
j
k
l
81
99
91
60
m
n
88
63a
Conditions: Substrate 15 mM in 100 mM phosphate buffer
under air, 2 equiv. of Bn-NADH, isolated yields after flash
column chromatography, a 1 equiv. NADH, NMR yield using
dimethyl terephthalate as internal standard.
To further demonstrate the potential of this method in more
complex cases, we chose the water soluble model amine 2 to
perform the reductive alkylation by the aldehyde 3 (Scheme 5a).
We studied the influence of the acid (AcOH in our case), the
equivalents of Bn-NADH, the impact of the use of a buffer and the
number of equivalents of aldehyde 3 (See supporting information
Table 5). It turned out that using an excess of 3 provides a better
yield compared to a stoichiometric quantity. The best conditions
were found with 2 equivalents of Bn-NADH, 3 equivalents of
aldehyde 3 and 2 equivalents of AcOH. An excess of aldehyde
ensures a full complete conversion of the amine. The product 4
could even be isolated with a 93% yield.
We eventually extended this concept to a more complex
sequence, involving the one pot reduction of the imine 1 followed
by the reductive alkylation of the in situ generated amine 2 by the
aldehyde 3 and we obtained the desired product 4 with a yield of
56% (Scheme 5b). The selectivity for the reductive alkylation was
explained by the higher reactivity of the iminium compared to the
aldehyde, even if only small amounts of iminium are present in
solution due to equilibrium.
Scheme 4 a) Study of the scope of the reaction on a set of cyclic and acyclic
imines; b) NADH analogue used for the application of the concept
Table 2 Reduction of cyclic and non-cyclic amines using Bn-NADH and NADH
Entry
a
Substrate
Yield (%)
99
60a
87
b
c
58a
99
72
d
e
62a
99
99
f
97
g
40a
h
i
78
82
Scheme 5 a) Reductive alkylation of the amine 2 by the aldehyde 3, b) One pot
imine reduction and reductive alkylation
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[6] S. Betanzos-Lara; Z. Liu; A. Habtemariam; A. M. Pizarro; B. Qamar; P. J.
Sadler. Angew. Chem., Int. Ed. 2012, 51, 3897-3900.
[7] (a) Y. Maenaka; T. Suenobu; S. Fukuzumi. J. Am. Chem. Soc. 2012, 134,
367; (b) C. L. Pitman; K. R. Brereton; A. J. M. Miller. J. Am. Chem. Soc.
2016, 138, 2252-2260.
[8] (a) Z. Liu; R. J. Deeth; J. S. Butler; A. Habtemariam; M. E. Newton; P. J.
Sadler. Angew. Chem., Int. Ed. 2013, 52, 4194-4197; (b) T. Suenobu; S.
Shibata; S. Fukuzumi. Inorg. Chem. 2016, 55, 7747-7754.
[9] (a) X. Wu; J. Xiao, Hydrogenation and Transfer Hydrogenation in Water. In
Metal-Catalyzed Reactions in Water, Wiley-VCH Verlag GmbH & Co.
KGaA: 2013; pp 173-242; (b) D. Wang; D. Astruc. Chem. Rev. 2015, 115,
6621; (c) J. S. Willemsen; J. C. van Hest; F. P. Rutjes. Chem. Commun.
(Cambridge, U. K.) 2013, 49, 3143-3145; (d) J. r. m. Canivet; G. S?ss-Fink.
Green Chem. 2007, 9, 391.
[10] (a) L. Evanno; J. Ormala; P. M. Pihko. Chem. Eur. J. 2009, 15, 12963-
12967; (b) Y. M. Wilson; M. Dürrenberger; E. S. Nogueira; T. R. Ward. J.
Am. Chem. Soc. 2014, 136, 8928; (c) N. Pannetier; J.-B. Sortais; J.-T.
Issenhuth; L. Barloy; C. Sirlin; A. Holuigue; L. Lefort; L. Panella; J. G. de
Vries; M. Pfeffer. Adv. Synth. Catal. 2011, 353, 2844-2852; (d) C. Wang; C.
Li; X. Wu; A. Pettman; J. Xiao. Angew. Chem., Int. Ed. 2009, 48, 6524-
6528; (e) Z. Wu; M. Perez; M. Scalone; T. Ayad; V. Ratovelomanana-Vidal.
Angew. Chem., Int. Ed. 2013, 52, 4925-4928; (f) J. Wu; F. Wang; Y. Ma; X.
Cui; L. Cun; J. Zhu; J. Deng; B. Yu. Chem. Commun. (Cambridge, U. K.)
2006, 16, 1766-1768.
In this study, we report a novel and efficient imine reduction with
good yields using NADH and the NADH mimic Bn-NADH as a
hydride source and catalytic amounts of the cationic Cp*Ir catalyst
C6. HCOONa was shown in one example to be another hydride
source for the reaction we studied. This method was further
applied to the reductive alkylation of a secondary amine. We
finally showed the applicability of the method for a one pot
reduction of an imine and the reductive alkylation of the amine
generated in situ.
We expect that this method would find numerous applications in
homogeneous in cellulo catalysis in the field of bioorganometallic
chemistry.[15] This methodology could be employed as a tool for
protein labelling by reductive alkylation in their native environment
without the use of an external hydride source like HCOONa.[16] It
could also be applied for targeted decaging of exogenous
probes[17] or drugs[18] in cellular medium.
[11] H. Komatsu; Y. Shindo; K. Oka; J. P. Hill; K. Ariga. Angew. Chem., Int. Ed.
2014, 53, 3993-3995.
[12] A. H. Ngo; M. Ibañez; L. H. Do. ACS Catalysis 2016, 6, 2637-2641.
[13] T. Abura; S. Ogo; Y. Watanabe; S. Fukuzumi. J. Am. Chem. Soc. 2003,
125, 4149.
[14] (a) L. Zhang; J. Yuan; Y. Xu; Y. H. P. Zhang; X. H. Qian. Chem. Comm.
2016, 52, 6471-6474; (b) C. E. Paul; I. W. C. E. Arends; F. Hollmann. ACS
Catalysis 2014, 4, 788-797; (c) T. Knaus; C. E. Paul; C. W. Levy; S. de
Vries; F. G. Mutti; F. Hollmann; N. S. Scrutton. J. Am. Chem. Soc. 2016,
138, 1033-1039.
Acknowledgements
F.R.S-FNRS FRIA is gratefully acknowledged for financial funding.
The authors thank Prof. Tom Leyssens and Gabriel Mercier for
providing imines to exemplify the methodology.
[15] (a) P. K. Sasmal; C. N. Streu; E. Meggers. Chem. Comm. 2013, 49, 1581-
1587; (b) J. J. Soldevila-Barreda; P. J. Sadler. Curr. Opin. Chem. Biol. 2015,
25, 172.
Keywords: hydride transfer • bioorthogonal chemistry •
biomimetic • iridium catalysis • reductive alkylation • NADH
[16] J. M. McFarland; M. B. Francis. J. Am. Chem. Soc. 2005, 127, 13490-
13491.
[1] (a) W. Kaelin; C. Thompson. Nat. Cell Biol. 2010, 465, 562-564; (b) A.
Chiarugi; C. Dolle; R. Felici; M. Ziegler. Nat. Rev. Cancer 2012, 12, 741-
752.
[17] (a) C. Streu; E. Meggers. Angew. Chem., Int. Ed. 2006, 45, 5645-5648; (b)
R. M. Yusop; A. Unciti-Broceta; E. M. Johansson; R. M. Sanchez-Martin;
M. Bradley. Nat. Chem. 2011, 3, 239-243; (c) M. I. Sanchez; C. Penas; M.
E. Vazquez; J. L. Mascarenas. Chem. Sci. 2014, 5, 1901-1907; (d) M.
Tomas-Gamasa; M. Martinez-Calvo; J. R. Couceiro; J. L. Mascarenas. Nat.
Commun. 2016, 7, 12538; (e) J. Li; J. Yu; J. Zhao; J. Wang; S. Zheng; S.
Lin; L. Chen; M. Yang; S. Jia; X. Zhang; P. R. Chen. Nat. Chem. 2014, 6,
352.
[18] (a) J. T. Weiss; J. C. Dawson; C. Fraser; W. Rybski; C. Torres-Sanchez; M.
Bradley; E. E. Patton; N. O. Carragher; A. Unciti-Broceta. J. Med. Chem.
2014, 57, 5395-5404; (b) J. T. Weiss; J. C. Dawson; K. G. Macleod; W.
Rybski; C. Fraser; C. Torres-Sánchez; E. E. Patton; M. Bradley; N. O.
Carragher; A. Unciti-Broceta. Nat. Commun. 2014, 5, 3277.
[2] (a) F. Hollmann; I. Arends; K. Buehler. ChemCatChem 2010, 2, 762-782;
(b) T. Quinto; V. Köhler; T. R. Ward. Top. Catal. 2013, 57, 321-331.
[3] R. Ruppert; S. Herrmann; E. Steckhan. J. Chem. Soc., Chem. Commun.
1988, 17, 1150-1151.
[4] (a) H. C. Lo; O. Buriez; J. B. Kerr; R. H. Fish. Angew. Chem., Int. Ed. 1999,
38, 1429-1432; (b) H. C. Lo; R. H. Fish. Angew. Chem., Int. Ed. 2002, 41,
478-481; (c) H. C. Lo; C. Leiva; O. Buriez; J. B. Kerr; M. M. Olmstead; R.
H. Fish. Inorg. Chem. 2001, 40, 6705-6716.
[5] J. J. Soldevila-Barreda; I. Romero-Canelon; A. Habtemariam; P. J. Sadler.
Nat. Commun. 2015, 6, 6582.
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Cp*Ir catalyzed imine reductions
utilizing the biomimetic 1,4-NAD(P)H
cofactor and N-benzyl-1,4-
dihydronicotinamide as the hydride
transfer agent
The first cationic Cp*Ir-catalyzed imine reduction in neutral buffered aqueous medium
under air at room temperature is reported. The reaction proceeds via a hydride
transfer from NADH as the hydride source. Cp*Ir(N,N) complexes proved to be the
most efficient catalysts for this transformation. The method was subsequently applied
to the reduction of cyclic and non-cyclic imines. Eventually, the concept was extended
This article is protected by copyright. All rights reserved.