Water-promoted dehydrative coupling of 2-aminopyridines in heptane: Via a borrowing hydrogen strategy

Article abstract of DOI:10.1039/d1ra04118e

A synthetic method for dehydrative N-benzylation promoted by water molecules in heptane using a π-benzylpalladium system has been developed. The presence of water significantly accelerates carbon-nitrogen bond formation, which is accomplished in an atom-economical process to afford the corresponding N-monobenzylated products. A crossover experiment afforded H/D scrambled products, which is consistent with a borrowing hydrogen mechanism. Kinetic isotope effect measurements revealed that benzylic carbon-hydrogen bond cleavage was the rate-determining step.

Full text of DOI:10.1039/d1ra04118e

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Water-promoted dehydrative coupling of 2-
aminopyridines in heptane via a borrowing
hydrogen strategy†
Cite this: RSC Adv., 2021, 11, 23144
*
*
Taku Nakayama, Hidemasa Hikawa,
Shoko Kikkawa
and Isao Azumaya
A synthetic method for dehydrative N-benzylation promoted by water molecules in heptane using a p-
benzylpalladium system has been developed. The presence of water significantly accelerates carbon–
nitrogen bond formation, which is accomplished in an atom-economical process to afford the
corresponding N-monobenzylated products. A crossover experiment afforded H/D scrambled products,
which is consistent with a borrowing hydrogen mechanism. Kinetic isotope effect measurements
revealed that benzylic carbon–hydrogen bond cleavage was the rate-determining step.
Received 27th May 2021
Accepted 23rd June 2021
DOI: 10.1039/d1ra04118e
rsc.li/rsc-advances
of HAuCl4 in the hydrophilic core for intramolecular alkyne
carboxylation.9c
Introduction
Inspired by these reports, we became interested in designing
a new catalytic system via water-assisted self-assembly of polar
substrates with a palladium catalyst in non-polar solvents
(Scheme 1). We herein present the dehydrative coupling of 2-
aminopyridines with benzylic alcohols in heptane using a p-
benzylpalladium system, furnishing the corresponding N-
benzyl-2-aminopyridines.10 As the reaction begins, in situ
generated water accelerates the aggregation of polar substrates
with a Pd(0)/TPPMS catalyst in heptane, forming an active p-
benzylpalladium catalyst system. The oxidative addition of
The borrowing hydrogen methodology has emerged as a prom-
ising greener synthetic strategy for straightforward carbon–
nitrogen bond formation utilizing readily available and low-
toxicity benzyl alcohols instead of benzyl halides as coupling
partners.1 Various catalyst systems using iridium(III),2 ruth-
enium(II),3 or other metals4 have been developed. Although this
strategy affords an attractive atom-economical pathway and is
an appealing synthetic shortcut for constructing valuable
products, the reactions generally require high temperatures,
organic solvents and strong bases. Thus, an efficient system
possessing greater catalytic activity that can be performed at
lower temperatures in greener solvents without any additives is
highly desirable for sustainable carbon–nitrogen bond forming
reactions.5 Recently, our group has been exploring a greener
borrowing hydrogen methodology involving p-benzylpalladiu-
m(II) complexes6 in water.7
Recent studies disclosed that hydrophilic interactions play
an important role in controlling self-assembly in biological
processes.8 In 2008, McLain et al. showed that charge-based
interactions play an important role in dipeptide association in
aqueous solution.8b In non-polar organic solvents, reverse
micelles (RMs), with the polar groups concentrated in the
interior of the aggregate, are formed via the self-assembly of
surfactant molecules.9 Notably, the presence of water molecules
is the driving force for reverse micelle formation in nanosized
water pools.9b In 2014, Zhao et al. prepared gold cluster catalysts
within interfacially cross-linked reverse micelles via extraction
Faculty of Pharmaceutical Sciences, Toho University, 2-2-1 Miyama, Funabashi, Chiba
274-8510, Japan. E-mail: hidemasa.hikawa@phar.toho-u.ac.jp; isao.azumaya@phar.
toho-u.ac.jp
† Electronic supplementary information (ESI) available: Copies of 1H and 13C
Scheme 1 Pd-catalyzed dehydrative coupling between aminopyridine
1a and alcohol 2a.
NMR spectra for all compounds. See DOI: 10.1039/d1ra04118e
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Table 1 Reaction optimizationa
using 3 equiv. of alcohol 2, the reaction did not proceed aer
4 h, but was completed in 16 h (entry 2). Screening of other
palladium catalysts found that palladium(II) triuoroacetate
resulted in an almost quantitative yield, whereas palladium(II)
dichloride was not applicable (entries 3 and 4). The reaction
using tris(dibenzylideneacetone)dipalladium(0) at 120 ꢀC for
16 h gave 3a in 77% yield (entry 5). Although the reaction pro-
ceeded to completion using octane instead of heptane (entry 6),
the use of other low-polarity organic solvents resulted in lower
yields (entries 7–11). Furthermore, little reaction occurred
under neat conditions (entry 12). Other water-soluble phos-
phine ligands such as P(C6H4SO3Na)3 L2 (TPPTS) and (diphe-
nylphosphino)benzoic acids L3–4 were less effective than
TPPMS (entries 13–15), and a lower yield was also obtained
without a phosphine ligand (entry 16). Since the reaction was
completed in 4 h under an Ar atmosphere, oxygen was not
essential to the borrowing hydrogen reaction (entry 17).
Entry
Catalyst
Ligand
Solvent
NMR yield (%)
1
2
3
Pd(OAc)2
Pd(OAc)2
Pd(TFA)2
PdCl2
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
None
L1
Heptane
Heptane
Heptane
Heptane
Heptane
Octane
Quant
0b (quant)c
97
0
77
Quant
2
38
4
0
20
8
5
0
6
4
5d
6
Pd2(dba)3$CHCl3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
e
7
8
9
Hexane
MCHf
Toluene
(CHCl2)2
1,4-Dioxane
None
Heptane
Heptane
Heptane
Heptane
Heptane
10
11
12
13
14
15
16
17g
2. Reaction scope
Next, we explored the scope of 2-aminopyridines 1 and benzylic
alcohols 2 capable of undergoing a borrowing hydrogen reac-
tion (Scheme 2). A variety of 2-aminopyridines 1 were well
tolerated, giving the desired products 3b–e in 75–96% yields
(Scheme 2A). The reaction of simple 2-aminopyridine, bearing
no substituent at the 5-position, also proceeded smoothly (3f,
90%). Advantageously, a substrate containing a sensitive
reducible nitro group could be utilized, furnishing the corre-
sponding desired product 3g, albeit in low yield. The reaction of
6-aminonicotinic acid gave an extremely poor result (only 7%
isolated yield of N-benzylated product 3h). To our delight, the
utilization of benzyl alcohol (2a) directly as a coupling partner
could be applied to 3-aminopyridine, 2-aminoquinoline and
nicotinamide (niacinamide) as amine substrates, furnishing
the corresponding pharmaceutically active N-benzylated motifs
in moderate to excellent yields (3i 88%; 3j, 82%; 3k, 66%).12
Unfortunately, no reactions occurred when using 4-amino-
pyridine, 2-aminopyrimidine and 2-(methylamino)pyridine
since the amino groups had poor nucleophilicity. Benzylic
alcohols with electron-donating groups were effective coupling
partners (3j–s, 65–93%) (Scheme 2B). A sterically hindered 2-
methoxybenzyl alcohol (2s) was well tolerated to obtain the N-
benzylated product 3s in 84% yield. Furthermore, the reactions
of electron-decient uorobenzyl alcohols afforded the desired
products 3t–v in 56–88% yields, although in octane, higher
temperatures were required to achieve better yields. In contrast,
no reaction proceeded when using 4-nitrobenzyl alcohol and 2-
phenylethyl alcohol, likely because the corresponding p-benzyl
Pd(II) cation species was not generated.
30
99
a
Optimizations were performed with 1 mmol of 1a and 5 mmol of 2a in
the presence of 5 mol% Pd catalyst and 10 mol% TPPMS (10 mol%) in
solvent (4 mL) at 100 ꢀC for 4 h under air. b 3 equiv. of alcohol 2a. c For
16 h. d Conducted at 120 ꢀC for 16 h. e 2.5 mol%. f Methylcyclohexane.
g
Under Ar.
alcohols to palladium(0) complexes is generally difficult due to
the poor performance of the hydroxyl moiety as a leaving group.
In the present work, the rst example of direct conversion of
non-activated benzyl alcohols to the p-benzylPd(II) system has
been developed in organic solvents, and applied to the dehy-
drative cross-coupling reaction. This water-in-oil reaction
system is substantially different from our previous method (an
oil-in-water type reaction system) concerning the self-assembly
between palladium catalysts and polar substrates (Scheme 1A
vs. B), and shows a signicant advancement over our previous
synthetic protocol for more sustainable chemistry.
Achieving the direct introduction of various functionalities
to aminopyridines, which are found in a wide variety of phar-
maceuticals, is of enormous interest.11 Notably, our p-benzylP-
d(II) system can be applied to a variety of 2-aminopyridine
substrates and benzylic alcohols under mild conditions.
3. Effect of water
Results and discussion
1. Optimization of N-benzylation
To measure the full reaction prole, dehydrative coupling
between 2-aminopyridine 1a and alcohol 2a at 100 ꢀC in
We initially examined the Pd-catalyzed reaction between 5- heptane was monitored by H NMR spectroscopy. The forma-
(triuoromethyl)pyridin-2-amine (1a) and benzyl alcohol (2a) in tion of N-benzylated product 3a along with benzaldehyde (4a)
heptane (Table 1). The reaction was completed in 4 h at 100 ꢀC was observed (Fig. 1A). No imine intermediate was detected,
under air, furnishing the desired product 3a (entry 1). When suggesting its rapid hydrogenation to the corresponding
1
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Scheme 2 Substrate scope of N-benzylation. Yield of isolated products. a3 equiv. of alcohol. bNMR yield. c10 mol% Pd(OAc)2, 20 mol% TPPMS,
150 ꢀC, 24 h in octane.
desired product 3a. The consumption of alcohol 2a followed addition of 0.5 mmol water compared to no water. The addition
pseudo-rst-order kinetics including the presence of two of 1 mmol water resulted in almost the same outcome (60%). In
mechanistic periods (0–2 h: k1 ¼ 0.01 hꢁ1; 2–4 h: k2 ¼ 0.35 hꢁ1
)
contrast, no benzylation was observed in the presence of excess
(Fig. 1B). Based on these results, we hypothesized that in situ water (2 mmol). When changing the additional water ratio, the
generated water molecules might promote the dehydrogenation reaction was accelerated in the following order: 0.5 mmol z
of 2a. Therefore, we investigated the detailed reaction mecha- 1 mmol > no addition of water (in situ generated 0.35 mmol of
nism, especially the role of water molecules in heptane.
water molecules aer 3 h) [ 2 mmol, suggesting that traces of
Thus, we conducted the reaction while adding different water present in situ (#1 mmol) promoted aggregation between
amounts of water (0.5, 1 and 2 mmol) (Scheme 3). As expected, the polar substrates and the Pd catalyst in heptane, forming the
the initial reaction rate in heptane dramatically increased upon active p-benzylpalladium(II) catalyst system. Since the forma-
tion of reverse micelles via aggregation of polar substrates did
not occur by adding excess water, the water-in-oil type reaction
employing the p-benzylpalladium species did not take place.
Indeed, the addition of water (1 mmol) was not effective without
stirring or addition of n-Bu4NBr.
Scheme 3 N-Benzylation in the presence of different amounts of
Fig. 1 Monitoring the reaction.
water.
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3-methylbenzyl alcohol 2r (5 mmol) to the standard reaction
conditions (Scheme 4). If the nucleophilic substitution of
alcohol 2r (dehydrative Tsuji–Trost type reaction) proceeds,
deuterium labeled product 3r-d would not be obtained. In
contrast, a borrowing hydrogen reaction would afford the H/D
scrambled N-benzylated product 3r-d. As expected, a mixture
of crossover product 3r-d and non-crossover product 3r was
obtained in 18% yield. The results obtained from a 13C NMR
analysis showed that the carbon of the methylene group
exhibited a 1 : 1 : 1 triplet for 3r-d. Furthermore, the ratio of the
1
integrated values in the H NMR spectrum showed 18% incor-
Fig. 2 Comparison of pseudo-first-order kinetic plots for reaction of
2a in heptane (A) with addition of H2O vs. D2O, and (B) in H2O vs. D2O.
poration of deuterium at the benzylic position of 3r-d (see ESI†).
This experimental evidence clearly indicates a borrowing
hydrogen reaction and rules out an alternative Pd-catalyzed SN2-
type substitution reaction.
5. Mechanistic studies
First, we examined the Pd-catalyzed disproportionation of
alcohol 2a (Scheme 5A). When the alcohol 2a was converted to
aldehyde 4a, the generation of toluene (5a) was clearly observed by
1H NMR spectroscopy. This result suggested that b-hydride elim-
ination of benzylPd(II)-alkoxide complex B led to the aldehyde 4a,
which then reacted with benzylPd(II) hydride species C via reduc-
tive elimination to form toluene 5a with regenerated Pd(0)Ln.
We next studied the kinetic isotope effect in an intermolecular
competition experiment using benzyl alcohol and its deuterium-
labeled analog. As expected, a kinetic isotope effect (KIE) of 3.1
was observed for the reaction with substrate 1a (Scheme 5B). A KIE
of this magnitude is in agreement with the benzylic C–H bond
cleavage featured in the turnover limiting step.
Scheme 4 Deuterium labeling experiment.
Water-promoted dehydrogenation of 2a in heptane exhibited
pseudo-rst-order plots with a higher primary kinetic solvent
isotope effect (KSIE ¼ 4.6). This value was much larger than the
KSIE value of 1.4 measured in water reactions (Fig. 2A and B),7b
suggesting that hydrogen bonds play a signicant role in the
dehydrative coupling reaction in heptane.
4. Deuterium labeling experiment
We performed a crossover experiment by subjecting an equi-
molar amount of deuterated benzyl alcohol 2a-d2 (5 mmol) and
Scheme 5 Mechanistic studies.
Scheme 6 Proposed catalytic cycle.
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The C–H bond cleavage of alcohol 2a via b-hydride elimination is
the rate-determining step (KIE value of 3.1 in Scheme 5B). Notably,
water molecules promote the aggregation of polar substrates with
palladium catalysts in heptane, furnishing the active p-benzyl-
palladium system smoothly in the rate-determining step.
Furthermore, benzyl alcohol (2a) disproportionates to benzalde-
hyde with toluene (Scheme 5A), which provides support for the
proposed p-benzylpalladium catalyst mechanism.
Steps 3 and 4. Finally, reductive amination between aldehyde
4a and the amine substrate catalyzed by Pd(II) hydride C
proceeds to generate the desired product 3a and regenerates the
benzylPd(II) A.
6. Mechanistic considerations
Considering the results above, a proposed catalytic cycle for the
borrowing hydrogen reaction in heptane is illustrated in
Scheme 6.
Step 1. Initially, oxidative addition of alcohol 2a to the Pd(0)
species13 generates the p-benzylPd(II) intermediate A.14 In this
step, polar substrates with water molecules activate the hydroxy
group of 2a through a hydrogen bonding network to generate
complex A.15,16 This mechanism is consistent with the observed
KSIE (kH2O/kD2O ¼ 4.6).
Step 2. Next, b-hydride elimination affords the key interme-
diate aldehyde 4a.17 Since the alcohol–Pd(II) complex is depro-
tonated by the acetoxy anion,17b,18 strong bases are not required.
To verify the formation of the p-benzylpalladium catalyst
system, we examined the Tsuji–Trost type reaction of aniline
(1w) in heptane (Scheme 7A). An electron-sufficient amine
substrate should react with the p-benzylPd(II) species by
nucleophilic substitution. As expected, the reactivity of 1w was
signicantly different from that of 2-aminopyridine 1a,
furnishing not only N-benzylaniline (3w) but also dibenzylated
product 6w (3w, 20%; 6w, 34%). Furthermore, when using 3w as
a starting material, the corresponding dibenzylated product 6w
was formed via the condensation of an alcohol with a benzylic C–H
bond (Scheme 7B). These results provide convincing evidence
supporting the formation of the p-benzylPd(II) species. This cata-
lyst system enables the dehydrative N-benzylation C–H bond acti-
vation cascade reaction of electron-rich analog 3w. The h1-s-benzyl
nucleophile attacks an electrophilic h3-p-benzyl moiety in the bis-
benzylPd(II) complex D, generating a new benzylic C–C bond. This
proposed mechanism is consistent with the nucleophilic reactivity
of bis-p-allylpalladium complexes reported by Yamamoto et al.19
Indeed, Pd-catalyzed benzylic C–H benzylation of electron-
sufficient N-benzylpyridine substrate 3a did not occur (Scheme
7C). Furthermore, the treatment of 3a with D2O (1 equiv.) as the D-
atom source in heptane showed no deuterium incorporation at the
benzylic position of 3a (The reaction in D2O as a solvent also gave
the same result, see ESI†). These results clearly demonstrated that
Pd-catalyzed benzylic C–H bond cleavage of 3a did not occur, and
therefore rule out D/H exchange between N-benzylated product 3a-
d2 and 3r in our crossover experiment (see Scheme 4).
On the basis of these results, dehydrogenation of benzyl alcohol
(2a) by p-benzylPd(II) complex A proceeds faster than nucleophilic
substitution of species A, and leaves the weak 2-aminopyridine
nucleophile 1a intact (Scheme 7D). Subsequently, reductive ami-
nation of 1a with the resulting aldehyde 4a leads to 3a via the
borrowing hydrogen pathway. This is consistent with the result of
the crossover experiment (see Scheme 4). When adding aniline
(1w), N-benzylated product 3a was not obtained due to the
poisoning effect of aniline (Scheme 7E).
Scheme
7 (A) and (B) Pd-catalyzed benzylation of aniline. (C)
Deuterium labeling study. (D) Proposed reaction pathway. (E) N-
Benzylation of 2-aminopyridine 1a in the presence of aniline.
Scheme 8 Comparison of the catalytic activity.
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Having established a successful dehydrative N-benzyla-
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Notes and references
tion strategy, we compared the catalytic activity of our p-
benzylpalladium catalyst system with other efficient systems
(Scheme 8). While the Pd(0)/TPPMS-catalyzed reaction in
heptane proceeded to completion within 4 h (see Table 1), the
previous borrowing hydrogen protocols10c,20 resulted in no
reaction. Furthermore, the Pd-catalyzed reaction in water
(our previous work)7b was not effective (only 40% yield),
clearly demonstrating the superiority of the present catalytic
strategy using heptane.
To highlight the synthetic utility, we performed the N-ben-
zylation on a gram scale (see ESI†). N-Benzylated product 3a
(1.56 g) was facilely isolated in 88% yield from pyridine
substrate 1a with alcohol 2a. The developed simple operations
also avoided the use of column chromatography for a gram
scale reaction.
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Conclusions
In summary, we report an efficient palladium-catalyzed dehydrative
N-benzylation in heptane via the borrowing hydrogen methodology.
The strategy provides an efficient method for the facile synthesis of
benzylaminopyridines, which are found in a wide variety of phar-
maceuticals. Notably, the use of a Pd(0)/TPPMS catalyst in a non-
polar heptane solvent with a trace amount of water is critical for
our catalytic system. Water molecules assist in the aggregation of
polar substrates followed by the formation of an active p-benzyl-
palladium catalyst system in heptane, which signicantly boosts the
borrowing hydrogen reaction. We expect that this study will aid in
the design of new catalytic systems using non-polar solvents, and
will serve as an entry point for reaction discovery.
Experimental
General procedure
To a sealed tube were added 2-aminopyridines 1 (1 mmol), Pd
catalyst (0.05 mmol), phosphine ligand (0.1 mmol), alcohols 2 (5
mmol) and heptane (4 mL). The resulting reaction mixture was
heated at 120 ꢀC for 17 h under air. Aer completion of the
reaction, the mixture was cooled to room temperature. The
reaction mixture was diluted with water, and then extracted
with EtOAc. The combined organic phases were washed with
brine, dried over MgSO4, and ltered and concentrated to
dryness under reduced pressure. The resulting residue was
puried by ash column chromatography on silica gel (eluent:
n-hexane/EtOAc) to obtain desired products 3.
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Acknowledgements
AcknowledgementsThis work was supported by JSPS KAKENHI
grant number 19K07003.
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