Room temperature N-heterocyclic carbene manganese catalyzed selective N-alkylation of anilines with alcohols

Article abstract of DOI:10.1039/c9cc02989c

The first example of room temperature non-noble metal homogeneous system catalyzed selective N-alkylation of anilines with alcohols by a bis-NHC manganese complex is presented. This system was applied to a large range of alcohols and anilines, including biologically relevant motifs and challenging methanol. Experimental and computational studies suggest an outer-sphere mechanism for this NHC-Mn system.

Full text of DOI:10.1039/c9cc02989c

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DOI: 10.1039/C9CC02989C
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Room Temperature N-Heterocyclic Carbene Manganese Catalyzed
Selective N-Alkylation of Anilines with Alcohols
Ming Huang,a Yukui Li,a Yinwu Li,a Jiahao Liu,a Siwei Shu,b Yan Liu,b and Zhuofeng Ke*,a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The first example of room temperature non-noble metal
homogeneous system catalyzed selective N-alkylation of anilines
with alcohols by a bis-NHC manganese complex is presented. This
system was applied to a large range of alcohols and anilines,
including biologically relevant motifs and the challenging
methanol. Experimental and computational studies suggest an
outer-sphere mechanism for this NHC-Mn system.
The selective N-alkylation of amines represents a powerful
method for synthetic application in fine and pharmaceutical
chemicals.1 Traditional syntheses of N-alkylated amines with
alkyl halides often generate side products or waste and suffer
from selectivity.2 Recently, the dehydrogenative coupling of
alcohols with amines by a borrowing hydrogen (BH) or
hydrogen autotransfer (HA) methodology has been developed
for more sustainable N-alkylation (Scheme 1a).3
Scheme 1. Manganese Complexes Catalyzed the N-Alkylation of
Amines with Alcohols.
Various catalytic systems have been developed for this
transformation. The traditional ones are usually based on
precious metals such as Ir,4 Ru,5 and others6 (Scheme 1b).
However, concerns regarding the cost, limited availability, and
toxicity of precious-metal catalysts, numerous efforts have
been devoted to developing non-noble metal catalysts with
Fe,7 Co,8 Ni,9 and Mn10 for the N-alkylation of amines with
alcohols (Scheme 1b). For example, Feringa and Barta et al.
reported Fe(II)-catalyzed direct alkylation of amines with
the PNP ligand for the N-alkylation of amines with alcohols.10a
o
Shortly afterward, the groups of Kirchner (4 mol% Mn, 130 C)
o
and Sortais (5 mol% Mn, 120 C) reported the PN3P complexes
for the dehydrogenative coupling of methanol with amines.10b,
10c
Although significant progress has been made for the non-
noble metal catalysts, challenges still remain in harsh reaction
conditions, high catalyst loading, lower activity, as well as
o
narrower substrate scope. High temperature (> 80 C) and the
o
alcohols at 120-130 C (5 mol% Fe).7a The PN5P-Co(II),8a PNP-
phosphine-containing ligands are usually required. Therefore,
a room temperature environmentally-benign phosphine-free
non-noble metal system is highly desired.
Co(II),8b or PCP-Co(II)8c systems were reported to catalyze N-
o
alkylation with alcohols at 80-130 C. Ni(II)-catalysts were also
To this goal, we envision that Mn(I) should be a good
candidate, as compared with other metal centers above, for its
low electronegativity, stronger reductivity, and higher hydricity
in metal hydride, which should be key important for the BH or
HA approach. As depicted in Scheme 1c, besides the Mn(I)
metal center, we would like to select the versatile N-
heterocyclic carbenes (NHCs) ligands11 to replace the general
developed, which generall catalyzed the related N-alkylation
reactions at 120-130 oC.9 Lately, Mn-complexes offer attractive
alternatives, which have led to promising applications for the
N-alkylation of amines with alcohols.10 In 2016, Beller et al.
o
reported the first Mn(I) (3 mol% Mn, 80 C) catalyst based on
phosphine. We envision that the strong
-donation of NHCs
a.School of Materials Science and Engineering, PCFM Lab, Sun Yat-sen University,
Guangzhou 510275, P. R. China. E-mail: kezhf3@mail.sysu.edu.cn
b.School of Chemical Engineering and Light Industry, Guangdong University of
Technology, Guangzhou 510006, P. R. China.
Electronic Supplementary Information (ESI) available: General experimental
procedures, additional schemes and figures, characterization data and NMR
spectra are available details. See DOI: 10.1039/x0xx00000x
facilitating the BH or HA process. In addition, NHCs ligands
have enjoyed increasing popularity in the design of Ru/Ir
catalysts for improving catalytic activity in N-alkylation of
amines.4b, 4c, 12 Thus, it would be advantageous to explore
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NHC-Mn(I) for this transformation. More importantly, NHC-
Mn(I) complexes have been applied in the electrocatalytic
reduction of CO2 and the hydrosilylation of ketones very
recently,13 implying their reduction ability. We further
performed density functional theory (DFT) to evaluate the
metal hydride reactivity for bis-NHC–Mn(I) complex, as
compared with those of previous phosphine metal hydrides.
Encouragingly, as shown in Scheme S5, DFT predict a higher
orbital energy of metal-hydride for bis-NHC–Mn(I) (-6.03 eV)
than those for other phosphine-Mn complexes (< -6.06 eV),
suggesting a possible prior reactivity of NHC–Mn(I) hydride
towards transfer hydrogenation.14 With these inspirations, we
became keenly interested in exploring an easily accessible and
highly active NHC-based Mn(I) catalyst for sustainable N-
alkylation. Herein, we describe a bis-NHC-Mn(I) complex as the
first example of room temperature non-noble homogeneous
system catalyzed selective N-alkylation of anilines with
alcohols.
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DOI: 10.1039/C9CC02989C
Scheme 3. The scopes of anilines. [a] [a 2 (0.5 mmol), 1a (0.5
mmol), Mn-1 (1.5 mol%), t-BuOK (1 equiv.) and toluene (0.25
mL), RT, 48 h, isolated yields. [b]Mn-1 (3.0 mol%).
Complexes Mn-1-4 (Scheme 2) were synthesized to study
the feasibility of NHC–Mn catalyzed N-alkylation of amines
with alcohols. These complexes were prepared in moderate
yields by the treatment of MnBr(CO)5 with corresponding
imidazolium salts in the present of t-BuOK (see ESI for more
details).13a Gratifyingly, in the presence of 1.5 mol% of Mn-1
and 1 equiv. of t-BuOK, at 50 oC, the yield of 3a was up to 98%
within 24 h. With the replacement of Me group with Ph group
on the wingtip of NHC ligand, Mn-2 led to lower yield (58%).
The catalytic performance of Mn-NHC complex was
suppressed significantly when pyridyl-NHC (Mn-3) or picolyl-
NHC (Mn-4) was used as the ligand. Subsequently, the
influences of reaction parameters were investigated (Table S1).
Deficiently, the low yield was obtained when the reaction was
performed under the air atmosphere (Table S1, entry 10).15
Notably, Mn-1 was also similarly active in non-dried solvents
(98%, Table S1, entry 14). Most significantly, this
transformation also smoothly occurred at room temperature,
85% and 98% yields were obtained in 24 h and 48 h,
respectively (Table S1, entries 15-16). To the best of our
knowledge, this is the first example of the N-alkylation of
anilines with alcohols at room temperature using non-noble
metal homogeneous catalysts.16
(Scheme 3). Substrates bearing electron-withdrawing and
electron-donating substituents at the para-position of aniline
afforded the N-mono-alkylated anilines 3b-g in good yields
(50–93%) at RT. 3-Bromoanilines and 3-methylaniline operated
smoothly to products 3h (64% yield) and 3i (85% yield),
respectively. Similarly, sterically hindered anilines with ortho-
substituents provided corresponding monoalkylated amines
3j
and 2-naphthylamine could be transformed into the desired
products 3m in 82-86% yields. Alternative to the Beller’s
-l in 40–68% yields with 3 mol% Mn-1. The 1-naphthylamine
-
n
catalytic system, our catalyst system is also tolerated to amine
containing C=C bond, which gave 3o in 80% yield.10a Employing
1, 4-benzodioxan-6-amine and 3-aminopyridine led to 3p and
3q in moderate isolated yields (50% and 57%, respectively)
with 3 mol% Mn-1. However, attempts at the alkylation of
aliphatic amines with alcohols were unsuccessful, yielding only
traces of products (Scheme S3).
Next, we explored the variability of alcohols (Scheme 4).
Pleasingly, both electron-rich and electron-deficient benzylic
alcohols could be employed efficiently in our process (4a
86%). A series of functional groups such as halide groups (4a
and 4i), thioether group (4f), and trifluoromethyl group (4j
–j, 58-
To determine the broad applicability of this catalytic
platform, we then subjected the reaction to a wider range of
aromatic amines and alcohols. Firstly, we explored the N-
alkylation of structurally diverse anilines with benzyl alcohol
-
c
)
were compatible. Alcohols with a heteroaromatic group such
as 3-pyridinemethanol and 3-thienylmethanol, also could
convert to the desired products 4k-l in 70-78% yields with 3
mol% Mn-1. We next turned our attention to the more
challenging aliphatic alcohols. Citronellol with an unsaturated
C=C bond and phenylpropanol were initially surveyed, and the
reactions proceeded smoothly to give the desired products 4m
and 4n in 78 and 65% yield, respectively. Similar results were
obtained for both the long-(C8) and short-chain (C2) aliphatic
alcohols, which delivered the corresponding products 4o-s in
good yields (55-75%).
Encouraged by the results of aliphatic alcohols, we further
investigated the N-methylation of amines with methanol,
Scheme 2. N-alkylation of aniline with benzyl alcohol
with different NHC-Mn Complexes.
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9CC02989C
Scheme 6. Proposed reaction mechanism based on
experimental and DFT results.
isolated yields, indicating our catalytic system is tolerated to
the vinyl groups, which is rare in noble-metal catalysis.10a
To gain mechanistic insight into this catalytic system,
experimental and DFT calculations were performed. Firstly, the
control reaction of imine 3aa with benzyl alcohol 1a in the
optimal conditions gave 3a (53%) via transfer hydrogenation of
the C=N bond (Scheme S4a), supporting the hydrogen-transfer
of mechanism.19 Then the low kinetic isotope effect (KIE = 1.1)
was measured by both the intermolecular competition
reaction (Scheme S4b) and parallel reaction (Scheme S4c),
which indicated the reversibility of the ß-hydride elimination
step and the alcohol oxidation wasn’t involved in the rate-
determining step (RDS) of this reaction.9a And the parallel
competition reaction also could be indicative for an off-cycle
equilibrium (e.g. Scheme S6, protonation of the Mn-H
intermediate). Indeed, the production of hydrogen gas was
detected from the head gas of the reaction of 1a with 2a
catalyzed by Mn-1 (Fig S8).20 A single hydrido signal at -6.46
ppm (Fig S4) was observed in the 1H NMR spectroscopy, which
confirmed the participation of Mn−H species in the catalytic
system. Based on these experimental observations and
literatures,21 either an inner-sphere mechanism or an outer-
sphere mechanism may be possible for this non-bifunctional
Mn-NHC catalysis (Scheme S6).
[a]
Scheme 4. The scopes of alcohols.[a] 2a (0.5 mmol), 1 (0.5
mmol), Mn-1 (1.5 mol%), t-BuOK (1 equiv.) and toluene (0.25
mL), RT, 48 h, isolated yields. [b]Mn-1 (3.0 mol%).[c]GC yield.
which is highly challenging due to the high dehydrogenation
activation barrier of methanol (

H = 21 kcal/mol) compared to
those of higher alcohols (ethanol,
△
H = 16 kcal/mol).17 After
the optimization of the reaction conditions (Table S3), we
performed N-methylation with 1.5 mol% Mn-1 at 100 C. As
shown in Scheme 5, a series of anilines afforded the N-
methylated products in good to excellent isolated yields (5b–p:
53–94%). Again, the reducible vinyl-substituted anilines and
heteroaromatic were tolerant, providing 5i and 5p in 82% and
94% yields, respectively.
To highlight the synthetic practicality of the present
protocol, a gram-scale (10 mmol) reaction with 1a and 2a was
performed, to deliver 3a in 80% yield. The synthetic utility was
also demonstrated for the synthesis of resveratrol-derived
amines (Eq.1), which are known to be active for Alzheimer’s
disease.18 Noticeably, with the 4-aminostilbene as a substrate,
o
DFT studies (Scheme 6, Fig S9 and S10) indicate the
preference of the outer-sphere mechanism without the
dissociation of the CO ligand (Scheme 6 and Scheme S6). The
the alkylated derivatives 7a
–
7d were synthesized in 80–90%
outer-sphere hydride elimination step22
(
TS1
kcal/mol) and the hydride insertion step (TS2
kcal/mol) are calculated to be more favored than the
elimination (TSB1
G‡ = 36.6 kcal/mol) and the inner-sphere
hydride insertion (TSB2
G‡ = 46.4 kcal/mol) processes. The
,


G‡ = 20.1
G‡ = 23.7
-H
,

,

,

outer-sphere mechanism is strongly supported by our in-situ IR
spectroscopy study, in which no CO dissociated Mn
intermediate was observed during the catalytic cycle (Fig S5
and Fig S7).15b The set of CO stretches (1999, 1915, 1875 cm-1)
corresponding to bis-NHC-MnBr(CO)3, and the new set of CO
stretches (1973, 1891, 1868 cm-1) observed in the reaction
process, which corresponds probably to the generated bis-
NHC-Mn(OR)(CO)3, suggesting the existence of Mn species
with three coordinated carbonyls. The overall activation free
energy is calculated to be 23.7 kcal/mol. The success of this
system at room temperature may probably be attributed to
the high activity of NHC-Mn in the outer-sphere mechanism, in
which the carbonyl dissociation is unnecessary.
[a]
Scheme 5. N-alkylation with MeOH.[a]
2 (0.5 mmol),
MeOH (0.5 mL), Mn-1 (1.5 mol%), t-BuOK (1 equiv.) and
toluene (1 mL), 100 oC, 24 h, sealed tube, isolated yields.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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E. Pittenauer, G. Allmaier and K. Kirchner, Org. LVeitetw.,A2rti0cl1e6O,n1lin8e,
In summary, we have developed the first room temperature
phosphine free NHC-Mn homogeneous system for the N-
alkylation of anilines with alcohols at a low catalyst loading. A
large variety of aromatic amines and primary alcohols
including biologically relevant motifs and the challenging
methanol were efficiently converted into mono-N-alkylated
amines. Experimental and computational studies suggested an
outer-sphere mechanism for this non-bifunctional NHC-Mn
system. The present results demonstrated the potential of
NHC manganese as new type of highly active catalysts in
(de)hydrogenation and sustainable chemistry.
This work was supported by the NSFC (21673301, 21473261,
and 21502023), the Guangdong Natural Science Funds for
Distinguished Young Scholar (No. 2015A030306027), the Tip-
top Youth Talents of Guangdong special support program
(No.20153100042090537) and the Fundamental Research
Funds for the Central Universities.
3462-3465.
DOI: 10.1039/C9CC02989C
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Conflicts of interest
There are no conflicts to declare.
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