General synthesis of N-Alkylation of amines with secondary alcohols via hydrogen autotransfer

Article abstract of DOI:10.1021/acs.orglett.9b02990

Direct catalytic N-alkylation of amines with secondary alcohols via hydrogen autotransfer (HA) strategy is very challenging and has been scarcely reported, even under precious metal catalysis. Herein, an efficient N-alkylation of amines, including benzyla

Full text of DOI:10.1021/acs.orglett.9b02990

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
General Synthesis of N‑Alkylation of Amines with Secondary
Alcohols via Hydrogen Autotransfer
Murugan Subaramanian, Siba P. Midya, Palmurukan M. Ramar, and Ekambaram Balaraman*
Department of Chemistry, Indian Institute of Science Education and Research (IISER) Tirupati, Tirupati − 517507, India
S
* Supporting Information
ABSTRACT: Direct catalytic N-alkylation of amines with
secondary alcohols via hydrogen autotransfer (HA) strategy is
very challenging and has been scarcely reported, even under
precious metal catalysis. Herein, an efficient N-alkylation of
amines, including benzylamines using secondary alcohols as
alkylating agents, is reported. This reaction is catalyzed by a
molecularly defined NNN-Ni(II) pincer complex, and the
reaction operates under mild, benign conditions. Various
substrates and functional groups were tolerated. Preliminary
mechanistic studies suggest that the N-alkylation reaction proceeds via a hydrogen autotransfer mechanism.
itrogen-containing organic moieties are an essential
building block and play a vital role in biologically active
For the past few decades, acceptorless dehydrogenative
coupling reaction (ADC) and hydrogen autotransfer (HA)
strategy have been the most convenient methods for the
synthesis of amines using alcohols as alkylating agents.4 The
main advantage of the HA approach is that it enables
sustainable production of amines from widespread alcohols,
thereby avoiding the use of hazardous, mutagenic alkylating
agents, and it generates water as the only byproduct.4−6
Notably, the development of nonprecious, Earth-abundant,
base-metal catalysts is a promising alternative for noble-metal
catalysis and is highly appreciated for the C−C and C−N bond
formation reactions.5 To date, the N-alkylation of anilines with
primary alcohols have been well-explored by using various
base-metal (Mn, Fe, Co, and Ni) catalysts.6 On the other hand,
seminal examples based on noble-metal catalyzed homoge-
neous7 and heterogeneous supported catalyst8 systems were
reported for the secondary alcohol amination reactions.
N
natural products, as well as synthetic organic chemistry.
Among them, amines are one of the most significant
functionalities that have a crucial role in day-to-day life.
Amines are widely used in the synthesis of various
pharmaceuticals, agrochemicals, and fine chemicals (see
Scheme 1).1 Typically, N-alkylated amines (sec-amines) are
Scheme 1. Pharmaceutically Important N-Alkylated Amine
Moieties (Selected Examples)
In 2006, the pioneering work by Beller and co-workers
reported the amination of secondary alcohols using a
ruthenium(0) catalyst.9 This was followed by ruthenium(II)-
catalyzed amination of secondary alcohols via HA strategy,
which was independently reported by the research groups of
Williams10a and Takacs.10b Recently, Seayad and co-workers
reported palladium-catalyzed N-alkylation of anilines using
primary and secondary alcohols.11 However, the electron-rich
phosphine ligand system has limitations, including high cost
and limited availability of noble metals. Indeed, the scope of
secondary alcohols for N-alkylation is very limited and less
explored under 3d-transition-metal catalysis.12 The limited
scope of secondary alcohol N-alkylation is mainly due to the
difficultness in the dehydrogenation to the corresponding
ketones, followed by hydrogenation of the iminium or enamine
prepared by the direct nucleophilic substitution reaction.
However, this conventional method of amine synthesis
requires harsh reaction conditions, prefunctionalized starting
materials, harmful reagents, and the generation of stoichio-
metric acid waste, thus limiting their practical applicability.
Alternative catalytic strategies involve the cross-coupling
reaction of amines with the corresponding activated deriva-
tives,2 reductive N-alkylation, hydroamination, and hydro-
amino−methylation reactions.3
Received: August 22, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02990
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
intermediates.4 Thus, the secondary alcohols are challenging to
use as an alkylating agent, compared with primary alcohols.13
To the best of our knowledge, there is no general strategy for
N-alkylation of amines with secondary alcohols under base-
metal catalysts, and its application remains elusive.
a
Table 1. Optimization Studies
In our continuous efforts on base-metal catalysis,14 recently,
we have reported NNN-Ni(II)-pincer complex catalyzed α-
alkylation of unactivated amides and esters with alcohols.14b
Followed by this, herein, we report NNN-Ni(II)-pincer
complex catalyzed selective N-alkylation of amines with
secondary alcohols via HA strategy. Interestingly, more
challenging substrates were achieved using secondary alcohols
as potential alkylating agents with various anilines and amines
under nickel catalysis. This well-defined catalytic system allows
N-alkylation of various secondary alcohols with low catalyst
loading (1 mol %) of nickel using phosphine-free, air-stable
nitrogen-containing ligands. (See Scheme 2.)
b
entry catalyst
base
solvent
temperature (°C) yield (%)
1
2
I
II
KOtBu
KOtBu
KOtBu
KOtBu
NaOtBu
LiOtBu
KOH
KiOPr
KOAc
KOtBu
−
toluene
toluene
110
110
110
110
110
110
110
110
110
110
110
140
140
140
140
140
140
140
38
32
36
47
33
26
28
25
10
trace
trace
75 (69)
73
68
62
31
22
3
I
m-xylene
n-octane
n-octane
n-octane
n-octane
n-octane
n-octane
n-octane
n-octane
n-octane
n-octane
n-octane
n-octane
n-octane
n-octane
n-octane
4
I
5
I
6
I
7
I
8
I
9
I
10
11
12
13
14
15
16
17
18
−
I
I
I
c
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
Scheme 2. Strategies for the N-Alkylation of Amines
d
II
e
III
IV
V
NiCl2
∼5
a
Reaction condi-
tions: aniline 2a (0.5 mmol), cyclopentanol 1a (0.75 mmol), nickel-
catalyst (2 mol %), base (0.5 mmol), 2 mL of n-octane, at 110 or 140
b
Initially, cyclopentanol 1a was chosen as a benchmark
substrate for N-alkylation of aniline 2a as an amine coupling
partner. Thus, in the presence of nickel(II)-NNN catalyst I
and catalyst II and stoichiometric amounts of a base, at 110
°C, with toluene as a solvent, the reaction resulted in product
3a in 38% and 32% yields, respectively (see Table 1, entries 1
and 2). Interestingly, changing the solvent from toluene to n-
octane increased the yield of product 3a (Table 1, entry 4).
Under identical conditions, various bases such as NaOtBu,
LiOtBu, KOiPr, KOH, and KOAc did not improve the yield of
the product formation (Table 1, entries 5−9). Notably, no
reaction occurred in the absence of the base as well as in the
absence of a nickel catalyst (Table 1, entries 10 and 11).
Hence, the necessity of base and the catalyst are essential for
this transformation. Increasing the temperature from 110 °C to
140 °C offered 75% of the product 3a (Table 1, entry 12).
Further reduction of catalyst loading from 2 mol % to 1 mol %
provides the same results (Table 1, entry 13). Besides, the use
of 1:1 ratio of NiCl2 and the PNP ligand (catalyst III) leads to
62% of 3a under similar conditions (Table 1, entry 15). Under
the optimized reaction conditions, other nickel catalytic
systems IV and V were found to be inefficient for this N-
alkylation reaction (Table 1, entries 16 and 17). Notably, a
trace amount of N-alkylated product 3a was observed using a
catalytic amount of NiCl2 as a catalyst, under standard catalytic
conditions (Table 1, entry 18). This result indicates the
importance of a molecularly defined NNN-Ni(II) catalytic
system.
°C for 12 h. Yield determined by GC using 1,4-dibromobutane as an
c
d
e
internal standard. Isolated yield. 1 mol % of catalyst was used. Used
in a 1:1 ratio of NiCl2 and the PNP ligand.
complex (1). Initially, the scope of secondary alcohols with
aniline under optimized reaction conditions was studied (see
Table 2). Importantly, aliphatic cyclic secondary alcohols such
as cyclopentanol (3a in 69%), cyclohexanol (3b in 63%),
cycloheptanol (3c in 58%), and cyclooctanol (3d in 52%)
successfully underwent N-alkylation reaction with aniline 2a
under standard reaction conditions. Similarly, more rigid cyclic
secondary alcohols such as fluorenol 1e gave 73% isolated yield
of N-alkylated product 3e. It is important to note that
diphenylmethanol afforded the corresponding product 3f in
excellent yields (3f in 93%). Consequently, symmetrical
diphenylmethanol bearing −Me, and −OMe groups gave the
N-alkylated products 3g in 50%, and 3h in 63%, respectively.
In addition, unsymmetrical diphenylmethanol having sterically
hindered methyl group gave the expected product 3j in 37%
yield, and 4-chloro diphenylmethanol yielded 52% of the
product 3i under optimized reaction conditions. Notably, 1,2-
diol did not afford the desired secondary amine under present
reaction conditions and gave exclusively the unsaturated
ketimine product 3k′ in 43% yield, wherein interestingly the
primary alcoholic group remains unaffected. The formation of
imine with secondary alcohol over the primary alcohol is due
to the reversibility of the primary alcohol dehydrogenation. On
the other hand, dehydrogenation of secondary alcohol is
irreversible under our standard reaction conditions and
afforded 80% isolated yields of the dehydrogenated product
(see Scheme 3b). Indeed, the more-flexible aliphatic secondary
alcohols such as 2-hexanol, 2-heptanol, and 2-decanol failed to
Having optimized reaction conditions in hand, we have
started investigating the applicability of the present catalytic
system for the N-alkylation of amines using secondary alcohols
as alkylating agent catalyzed by nickel(II)−NNN pincer
B
DOI: 10.1021/acs.orglett.9b02990
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Organic Letters
Letter
Table 2. NNN-Ni(II)-Catalyzed N-Alkylation of Amines
with Secondary Alcohols: Scope of Alcohols
Scheme 3. Control Experiments and Mechanistic Studies
a
a
Reaction conditions: alcohol 1 (0.75 mmol), aniline 2a (0.5 mmol),
catalyst I (1 mol %), and KOtBu (0.5 mmol) at 140 °C (oil-bath
temperature) in 2 mL of n-octane for 24 h. bIsolated yields. n.d = not
detected.
yield the corresponding N-alkylated products under our
reaction conditions.
Encouraged by the excellent catalytic activity of N-alkylation
of anilines with secondary alcohol, next, we were interested in
studying the reactivity of various benzylamines under standard
reaction conditions (see Table 4). It is important to note that
benzyl amines can undergo dehydrogenation reactions and
might lead to a self-dehydrogenative-coupled product (imine)
under similar reaction conditions.15 Also note that ADC of
unreactive amines and alcohols selectively leads to imines and
not the hydrogenated product (i.e., amines). This is due to
challenges in the transfer hydrogenation of the imine
intermediate to amine product.16 Thus, N-alkylation of benzyl
amines using an alcohol as alkylating agent is very challenging
and rarely reported. Under optimized conditions, benzyl
amines yielded the corresponding N-alkylated products in
moderate to excellent yields. Gratifyingly, the electron-
donating materials, as well as halides such as chloro and
fluoro derivatives, underwent N-alkylation with excellent
selectivity and resulted in products 7a−7e in 40%−61% yields,
respectively. Notably, heterocyclic amines containing oxygen
(7f) and sulfur (7g) were tolerated and gave the corresponding
secondary amines 7f and 7g in 32% and 38% isolated yields,
respectively. Unfortunately, a trace amount of product 7h was
formed in case of the aliphatic amine, as well as no product
being observed in the case of sulfonohydrazide derivatives (7i).
To obtain more-detailed insight into the nickel-catalyzed N-
alkylation of amines with secondary alcohols, several control
experiments were performed under the optimal reaction
conditions. Interestingly, the formation of molecular hydrogen
from the reaction medium was detected by gas chromatog-
raphy analysis (see Scheme 3a). The reaction of diphenylme-
Next, we have investigated the variety of substituted aniline
derivatives for the selective N-alkylation reaction using
diphenylmethanol 4a as the benchmark substrate (see Table
3). The N-alkylation reactions proceed smoothly for a variety
of aniline derivatives. Particularly, unsubstituted aniline and
naphthyl amine afforded the corresponding N-alkylated
products 5a (93% isolated yield) and 5b (83% isolated
yield), respectively. Anilines containing electron-donating
(−Me, −iPr, −OMe, −OEt, −OPh, and −SMe) groups were
well-tolerated for the N-alkylation reaction with secondary
alcohols and yielded the corresponding products 5c−5j in
49%−79% isolated yields. Because of the steric nature of the
methyl group in 2-methyl, product 5d was obtained only in
49% yield. Importantly, the halide functionality such as
bromo-, chloro-, and fluoro-containing groups at the ortho,
meta, and para positions yielded the N-alkylated products in
good to excellent yields (products 5m−5s, 48%−95% yield).
In particular, pharmaceutically active 1,3-dioxalone, 1,4-diox-
alone, and trifluoro-substituted anilines underwent N-alkyla-
tion reaction and resulted in the products 5k (42% yield), 5l
(49% yield), 5t (79% yield), and 5u (57% yield), respectively.
Interestingly, heteroaromatic amine, such as 2-aminopyridine
was efficiently alkylated under our reaction conditions, and
gave the product 5w in 92% isolated yield. However, the
aniline containing strong electron-withdrawing group (−NO2)
resulted in a trace amount of product 5v, and highly hindered
1-adamantylamine (5x) did not give the desired product under
standard reaction conditions.
C
DOI: 10.1021/acs.orglett.9b02990
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Organic Letters
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Table 3. NNN-Ni(II)-Catalyzed N-Alkylation of Amines
with Secondary Alcohols: Scope of Anilines
Table 4. Ni-Catalyzed N-Alkylation of Amines with
Secondary Alcohols: Scope of Benzylamines
a
a
a
Reaction conditions: amine 6 (0.5 mmol), diphenylmethanol 4a
(0.75 mmol), catalyst I (1 mol %), and KOtBu (0.5 mmol) at 140 °C
(oil-bath temperature) in 2 mL of n-octane for 24 h. Isolated yields.
b
Scheme 3g), which suggested that the reaction does not follow
the free-radical pathway or single-electron transfer (SET). A
competitive reaction between 3-chloroaniline (2o) and 3-
chlorobenzylamine (6e) with diphenylmethanol (4a) gave
aniline alkylated product (5o) in 79% isolated yield, and a
trace amount of benzylamine alkylated product 7e (see
Scheme 3h) exclusively. This result showed that the reactivity
of anilines is superior to the benzylamines for the N-alkylation
of secondary alcohol under optimized conditions.
In summary, direct N-alkylation of amines (anilines and
benzylamines) with secondary alcohol via hydrogen autotrans-
fer (HA) strategy is reported. This reaction is catalyzed by a
molecularly defined NNN-Ni(II) pincer complex, and the
reaction operates under mild, benign conditions. Various
substrates and functional groups were tolerated. Preliminary
mechanistic studies suggest that the N-alkylation reaction
proceeds via hydrogen autotransfer strategy.
a
Reaction conditions: amine 2 (0.5 mmol), diphenylmethanol 4a
(0.75 mmol), catalyst I (1 mol %), and KOtBu (0.5 mmol) at 140 °C
(oil-bath temperature), in 2 mL n-octane for 12 h. Isolated yields.
b
thanol under standard reaction conditions yields benzophe-
none 4a′ in 80% isolated yields (Scheme 3b). This result
suggested that the formation of benzophenone proceeds via
the dehydrogenative pathway. Furthermore, the preparation of
benzophenone imine 5a′ was successfully achieved by refluxing
benzophenone 4a′ and aniline 2a in the presence of KOtBu.
Indeed, the transformation of imine was promoted/accelerated
by the nickel catalyst (see Scheme 3c).
ASSOCIATED CONTENT
* Supporting Information
■
S
Importantly, the reaction of diphenylmethanol 4a with
aniline 2a under standard conditions in 3 h gave
benzophenone imine intermediate 5a′ (minor) and N-
alkylated product 5a (major) (Scheme 3d). In addition,
treatment of benzophenone imine intermediate 5a′ with
diphenyl methanol 4a under optimized conditions resulted
the desired product 5a in excellent yields (see Scheme 3e).
Moreover, the deuterium labeling experiment was performed
with aniline 2a and diphenyl methanol 4a-D under standard
conditions and afforded 30%-deuterium-incorporated N-
alkylated amine product 5a-D (see Scheme 3f). The lesser
deuterium incorporation may be due to the hydrogen−
deuterium (H−D) exchange by the presence of formed
water molecule during the catalytic process. Next, the presence
of radical scavengers did not affect the product formation and
afford excellent yield of 5a under standard conditions (see
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02990.
Experimental section; mechanistic studies, kinetic
1
studies, H and 13C NMR spectra, and mass spectra
(PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: eb.raman@iisertirupati.ac.in.
ORCID
Ekambaram Balaraman: 0000-0001-9248-2809
Notes
The authors declare no competing financial interest.
D
DOI: 10.1021/acs.orglett.9b02990
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
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ACKNOWLEDGMENTS
■
This research work was supported by IISER Tirupati, and
SERB, India (Grant No. CRG/2018/002480/OC). M.S.
thanks the UGC for the research fellowship.
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DOI: 10.1021/acs.orglett.9b02990
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