Hydrogen-Transfer-Mediated α-Functionalization of 1,8-Naphthyridines by a Strategy Overcoming the Over-Hydrogenation Barrier

Article abstract of DOI:10.1002/anie.201707702

A general catalytic hydrogen transfer-mediated α-functionalization of 1,8-naphthyridines is reported for the first time that benefits from a hydrogen transfer-mediated activation mode for non-activated pyridyl cores. The pyridyl α-site selectively couples with the C8-site of various tetrahydroquinolines (THQs) to afford novel α-functionalized tetrahydro 1,8-naphthyridines, a class of synthetically useful building blocks and potential candidates for the discovery of therapeutic and bio-active products. The utilization of THQs as inactive hydrogen donors (HDs) appears to be a key strategy to overcome the over-hydrogenation barrier and address the chemoselectivity issue. The developed chemistry features operational simplicity, readily available catalyst and good functional group tolerance, and offers a significant basis for further development of new protocols to directly transform or functionalize inert N-heterocycles.

Full text of DOI:10.1002/anie.201707702

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Accepted Article
Title: Hydrogen Transfer-Mediated a-Functionalization of 1,8-
Naphthyridines by a Strategy Overcoming the Over-
Hydrogenation Barrier
Authors: Xiuwen Chen, He Zhao, Chunlian Chen, Huanfeng Jiang, and
Min Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201707702
Angew. Chem. 10.1002/ange.201707702
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10.1002/anie.201707702
Angewandte Chemie International Edition
Hydrogen Transfer-Mediated -Functionalization of 1,8-Naphthyridines by
a Strategy Overcoming the Over-Hydrogenation Barrier
Xiu-Wen Chen, He Zhao, Chun-Lian Chen, Huan-Feng Jiang, and Min Zhang*
Abstract: By a hydrogen transfer-mediated activation mode for non-
activated pyridyl nucleus, a general catalytic hydrogen transfer-
mediated -functionalization of 1,8-naphthyridines is reported for
the first time. Its -site selectively couples with the C8-site of various
tetrahydroquinolines (THQs) to afford novel -functionalized
tetrahydro 1,8-naphthyridines, a class of synthetically useful building
blocks and potential candidates for the discovery of therapeutic and
bio-active products. The utilization of THQs as inactive hydrogen
donors (HDs) appears to be a key strategy to overcome the over-
hydrogenation barrier and address the chemo-selectivity issue. The
developed chemistry features operational simplicity, readily available
catalyst and good functional tolerance, and offers a significant basis
for further development of new protocols to directly transform or
functionalize inert N-heterocycles.
N-heteroarenes via a hydrogen transfer-mediated activation
mode for non-activated pyridyl nucleus. As shown in Scheme 1,
in the presence of a suitable HD and catalyst (Cat), the first
transfer hydrogenation (TH) of pyridyl ring of 2 would afford an
allylic amine A and its tautomers (enamine B and imine C). Then,
the nucleophlile 1 (NuH) would undergo -addition to imine C,
thereby releasing the tetrahydro -functionalized product 3.
Scheme 1. Envisioned transfer hydrogenative coupling protocol.
Hydrogen transfer-mediated carbon-carbon and carbon-
heteroatom bond formations have emerged as powerful tools in
synthetic organic chemistry, since they enable key steps in
creation of various functionalized products.[1] For examples,
except for the hydrogen-borrowing methodologies extensively
employed for the N-alkylation[2] and C-alkylation[3], Krische has
demonstrated outstanding contributions on the coupling of
alcohols with C-C double/triple bonds.[4] With the use formic acid
as the hydrogen donor (HD), the Bruneau group has reported
the non-directed C(sp3)-H bond alkylation of saturated cyclic
amines.[5] In addition, Li and the co-workers have illustrated the
conversion of phenol derivatives into amines in the presence of
HCO2Na.[6] Despite these significant achievements, the strategy,
via selective trap of the transient partially hydrogenated N-
heteroarenes, remains a challenge, although it would shed new
light to access functionalized molecules that are inaccessible or
difficult to prepare with the existing methods.
However, particularly worth mentioning is that the presence
of transition metal complexes and hydrogen donors (HDs) can
easily form in-situ the highly reactive metal hydride species,
which would tend to undergo the second TH to generate the
over-hydrogenated by-products 2’ (from C to 2’).[11] Hence, it is
understandable that the above transfer hydrogenative -
functionalization of pyridyl nucleus (Scheme 1) was not practical
in the vast majority of cases when we employed different
combinations of metal catalysts (i.e., Ru, Ir, Co, Rh) with
conventional HDs (such as alcohols,[2-3] NH3BH3,[12] Hantzsch
esters,[13] HCO2H,[5] and HCO2Na [6]). Thus, to achieve the
selective formation of product 3, there should be a compatible
catalyst system to ensure that the trapping rate of imine C by
nucleophile (NuH: 1) is much faster than the second TH process,
thus suppressing the formation of undesired product 2’.
In sharp contrast with the conventional HDs, the metal-
catalyzed dehydrogenation of cyclic amines,[14] such as
tetrahydroquinolines (THQs), is much difficult due to the
thermodynamic stability and kinetic inertness. Consequently, the
utilization of THQs as the hydrogen donors would exhibit
relatively low TH efficiency, and favor the capture of imine (C) by
nucleophile. These features and the extensive functions of -
Pyridyl ring is ubiquitous in bioactive molecules, functional
materials, dyes, agrochemicals, pharmaceuticals, and natural
products. Hence, the direct transformation of pyridyl ring into
functionalized frameworks is of significant importance. However,
such a goal, without the assistance of directing groups[7] or pre-
installation of functionalities[8], remains highly challenging. As
part of our continuous research interest in construction of N-
heterocycles by hydrogen transfer strategy,[9] we have recently
demonstrated a direct reductive quinolyl β-C-H alkylation using
formic acid as the hydrogen donor (HD).[10] Encouraged by this
work, we also wished to realize the direct -functionalization of
functionalized
tetrahydro-1,8-naphthyridines
(THNs)[15-16]
motivated us to study the hydrogen transfer-mediated -
functionalization of 1,8-naphthyridines with THQs. As shown in
Scheme 2, the reaction of THQ 1a (1.5 equiv) and naphthyridine
2a was initially envisioned as follows: the β-site of enamine B-1a,
arising from dehydrogenation of 1a, would undergo nucleophilic
addition to the -site of the partially transfer hydrogenated 1,8-
naphthyridine (C-2a). Then, the second hydrogen transfer to the
coupling adduct 3aa-1 would give rise to the -functionalized
THN 3aa’. However, we observed that, instead of this
anticipated compound 3aa’, the C8-site of THQ 1a selectively
coupled with the -site of C-2a to give product 3aa possessing a
synthetically useful 2-(aminomethyl)aniline motif in 31% yield.
Based on this finding employing THQs as the inactive HDs, we
wish herein to report an iridium-catalyzed hydrogen trasfer-
mediated -functionalization of 1,8-naphthyridines for the first
[a]
X.-W. Chen, H. Zhao, C.-L. Chen, Prof. Dr. H.-F. Jiang, Prof. Dr. M.
Zhang
Key Lab of Functional Molecular Engineering of Guangdong
Province, School of Chemistry and Chemical Engineering, South
China University of Technology, Guangzhou 510641, China
E-mail: minzhang@scut.edu.cn
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
This article is protected by copyright. All rights reserved.

10.1002/anie.201707702
Angewandte Chemie International Edition
time, offering
functionalized frameworks.
a
direct conversion of N-heteroarenes into
the reactions proceeded smoothly and furnished the desired
products (3aa-3am) in moderate to good isolated yields. It was
found that the electronic properties of the substituents on the
aryl ring of 1,8-naphthyridines significantly affected the product
yields. Especially, substrates
2
having strong electron-
withdrawing groups gave much higher yields (3aa-3ae) than
those with electron-donating groups (3ah, 3aj), which can be
rationalized as the electron-withdrawing groups enhance the
electrophilicity of the in-situ formed imine intermediate, thus
favouring the coupling step (Scheme 1, from C to product 3).
Interestingly, the heteroaryl substituted 1,8-naphthyridine (2i)
also underwent efficient formation of the corresponding product
(3ai),
a
potential multidentate ligand to be applied in
organometallic chemistry and catalysis,[17] and its structure was
confirmed by X-ray diffraction [see the Supporting Information
Scheme 2. The unexpected new finding.
(SI), Figure S1]. Substrate 2k,
a nitrogen-modified 1,8-
naphthyridine, effectively coupled with THQ 1a to give
compound 3ak in 62% yield, this example demonstrates the
potential of the methodology to be applied to other heterocyclic
scaffolds. 3-Phenyl-1,8-naphthyridine (2l) possesses two
reactive -sites, whereas the coupling occurs selectively on the
non-substituted pyridyl ring (3am), presumably because of
influence of steric hindrance. Noteworthy, a series of functional
groups such as -CN, -NO2, -CF3, -Me, ester and halogens (-Cl, -
Br) are well tolerated in the reaction, which offers the potential
for molecular complexity via further transformations.
Our initial studies focused on developing a more efficient
catalyst system for the coupling of 1a and 2a as a model
reaction. Gratifyingly, replacing [Ir(COD)Cl]2 (Scheme 2) with
[Cp*IrCl2]2 improved the yield of 3aa to 49% (Table 1, entry 1),
whereas the ruthenium and palladium catalysts, frequently
employed for TH reactions, showed no activity (Entries 2 and 3).
In the absence of catalyst the product was not formed (entry 4),
indicating that the iridium plays a crucial role in affording product
3aa. Then, the addition of base and acid significantly diminished
the reaction efficiency (entries 5-6). The solvent screening
(entries 7-11) showed that toluene and t-amyl alcohol gave
further increased yields (entries 7-8). From the environmental
compatibility viewpoint, we chose t-amyl alcohol as the preferred
solvent. Moreover, 1.5 equivalents of THQ was sufficient for the
reaction (entry 8), and both increase and decrease of reaction
temperatures diminished the product yields (entry 12). The
reaction exposed in air only gave a 19% yield (entry 13). Hence,
the optimal conditions are as indicated in entry 8 of Table 1.
Table 1: Screening of optimal reaction conditions.[a]
Entry
Catalyst
Additive
Solvent
3aa, yield%[b]
1
2
2
4
5
6
7
8
9
[Cp*IrCl2]2
Ru3(CO)12
Pd(OAc)2
-
[Cp*IrCl2]2
[Cp*IrCl2]2 CF3CO2H
[Cp*IrCl2]2
[Cp*IrCl2]2
[Cp*IrCl2]2
-
-
-
-
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
toluene
t-amyl alcohol
p-xylene
DMF
DMSO
t-amyl alcohol
t-amyl alcohol
49
NR
NR
-
Na2CO3
Trace
32
-
-
-
-
-
-
-
70
(72, 72)[c]
63
10 [Cp*IrCl2]2
11 [Cp*IrCl2]2
12 [Cp*IrCl2]2
13 [Cp*IrCl2]2
11
Trace
(60, 68)[d]
19[e]
[a] Reaction conditions: unless otherwise stated, all the reactions were
performed with 1a (0.3 mmol), 2a (0.2 mmol), Cat (1 mol %), solvent (1.0
mL) at 130 oC for 18 h under N2 protection. [b] Isolated yield. [c] Yields
obtained with 0.3 and 0.4 mmol 1a, respectively. [d] Yields obtained at
120 oC and 140 oC, respectively. [e] Under air.
Scheme 3. Variation of 1,8-naphthyridines. Standard conditions: 1a
(0.3 mmol), 2 (0.2 mmol) and [Cp*IrCl2]2 (1 mol %) in t-amyl alcohol
(1.0 mL) was performed at 130 oC for 18 h under N2 protection.
With the availability of the optimal conditions, we next
investigated the generality of synthetic protocol. First, the
reactions of THQ 1a in combination with a variety of 1,8-
naphthyridines (2) were evaluated. As illustrated in scheme 3, all
Subsequently, we turned our attention to the variation of
both coupling partners (Scheme 4). The reactions of 1,8-
naphthyridines
2 with various tetrahydroquinolines 1 was
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10.1002/anie.201707702
Angewandte Chemie International Edition
explored. Similar to the results described in Scheme 4, all the
reactions efficiently afforded the desired products in moderate to
high isolated yields (3ba-3hj). It was found that the substituents
on the tetrahydroquinolyl skeleton significantly influenced the
product formation. The electron-donating group containing THQs
1 gave the products in much higher yields (3ba-3ga) than those
of electron-deficient ones (3ha, 3hj). This phenomenon is likely
due to the electron-rich substituents which enhance the
nucleophlicity of the C8-site of the THQ intermediates, thus
favouring the addition process (Scheme 1, from C to product 3).
nucleophilic addition of the C8-H of THQ (1a) to imine activated
by iridium catalyst also rationalizes the result (SI, Scheme S1).
Scheme 5. The control experiments.
Finally, we were interested in demonstrating the utility of the
developed chemistry. Compounds 3aa and 3ea were treated
with both alkyl (4a) and aryl (4b, 4c) aldehydes in acetic acid at
70 oC for
5 h, which underwent effective intermolecular
condensation to afford the cyclization products (5aaa-5aac,
5eab) in moderate to good isolated yields (Scheme 6).
Noteworthy, the obtained products feature a four N-heterocycle
fused motif (ring-A, B, C and D) that constitutes the core
structure of many products exhibiting interesting bioactivities,
such as intercalation of DNA[19a] and inhibition of
butyrylcholinesterase[19b]
, whereas the preparation of such
structural compounds is quite difficult with conventional methods.
Scheme 4. Both variations of two coupling partners.
To gain insights into the reaction mechanism, several control
experiments were performed. The model reaction was
interrupted after 5 h to analyse the intermediates. Product 3aa,
tetra- and di-hydronaphthyridine 2a’, C-2a, as well as quinoline
1a’ were detected in 15%, 1%, 1% and 10% yields, respectively
(Scheme 5, Eq. 1). Then, the reaction of 2a’ with THQ 1a or
quinoline 1a’ under the standard conditions failed to give the
product 3aa (Eq. 2), supporting that effective suppressing the
formation of over-hydrogenated tetrahydronaphthyridine 2a’ is
the key factor in chemo-selective generation of 3aa. Then, the
comparative experiments for the addition of 1a to imine C-2a
showed that the iridium(III) catalyst plays a crucial role in the
activation of the C8-H site of THQ 1a (Eq.3). Further, the
reaction of deuterated THQ (1e-dn) and 2a gave product 3ea-dn
with retention of the deuterium ratio of the tetrahydroquinolyl
motif, whereas different deuterium value was found on the newly
formed tetrahydronaphthyridyl unit, revealing that the hydrogen
is transferred from the excess part of THQ 1a to the 1,8-
naphthyridine 2a (1a : 2a = 1.5 : 1). All these results are
consistent with the reaction pathway proposed in Scheme 1. The
addition of C8-H of THQ (1a) to imine C-2a (Scheme 2) is
suggested as follows: THQ 1a with iridium catalyst undergoes
C8-H metalation[18] under the assistance of the N-coordination,
then product 3aa is afforded via C8-Ir insertion into the imino unit
of C-2a followed by protodemetallation. Alternatively, a direct
Scheme 6. The synthetic utility.
In summary, via a hydrogen transfer-mediated activation
mode for the non-activated pyridyl nucleus, we have developed
a
general
functionalization of 1,8-naphthyridines for the first time. Its -site
selectively couples with the C8-site of various
catalytic
hydrogen
transfer-mediated
-
tetrahydroquinolines to afford novel -functionalized tetrahydro-
1,8-naphthyridines, a class of synthetically useful building blocks
and potential candidates for the discovery of therapeutic and
bio-active molecules. The strategy utilizing tetrahydroquinolines
as inactive hydrogen donors was applied to overcome the over-
hydrogenation barrier and address the chemo-selectivity issue.
The developed chemistry features operational simplicity, readily
available catalyst and good functional tolerance, and offers a
significant basis for further development new protocols to
directly transform or functionalize inert N-heterocycles.
Acknowledgements
This article is protected by copyright. All rights reserved.

10.1002/anie.201707702
Angewandte Chemie International Edition
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and
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Program
of
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Keywords: Iridium Catalysis • Transfer Hydrogenative Coupling
• Tetrahydroquinolines • 1,8-naphthyridines • Hydrogen Donor
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