Phosphine-Free Well-Defined Mn(I) Complex-Catalyzed Synthesis of Amine, Imine, and 2,3-Dihydro-1 H-perimidine via Hydrogen Autotransfer or Acceptorless Dehydrogenative Coupling of Amine and Alcohol

Article abstract of DOI:10.1021/acs.organomet.9b00131

The application of nontoxic, earth-abundant transition metals in place of costly noble metals is a paramount goal in catalysis and is especially interesting if the air- and moisture-stable ligand scaffold is used. Herein, we report the synthesis of amines/imines directly from alcohol and amines via hydrogen autotransfer or acceptorless dehydrogenation catalyzed by well-defined phosphine-free Mn complexes. Both imines and amines can be obtained from the same set of alcohols and amines using the same catalyst, only by tuning the reaction conditions. The amount and nature of the base are found to be a highly important aspect for the observed selectivity. Both the primary and secondary amines have been employed as substrates for the N-alkylation reaction. As a highlight, we showed the chemoselective synthesis of resveratrol derivatives. Furthermore, the Mn-catalyzed dehydrogenative synthesis of structurally important 2,3-dihydro-1H-perimidines has also been demonstrated. Density functional theory calculations were also carried out to model the reaction path and to calculate the reaction profile.

Full text of DOI:10.1021/acs.organomet.9b00131

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Phosphine-Free Well-Defined Mn(I) Complex-Catalyzed Synthesis of
Amine, Imine, and 2,3-Dihydro‑1H‑perimidine via Hydrogen
Autotransfer or Acceptorless Dehydrogenative Coupling of Amine
and Alcohol
Kalicharan Das, Avijit Mondal, Debjyoti Pal, Hemant Kumar Srivastava,* and Dipankar Srimani*
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India
S
* Supporting Information
ABSTRACT: The application of nontoxic, earth-abundant transition metals in place of costly noble metals is a paramount goal
in catalysis and is especially interesting if the air- and moisture-stable ligand scaffold is used. Herein, we report the synthesis of
amines/imines directly from alcohol and amines via hydrogen autotransfer or acceptorless dehydrogenation catalyzed by well-
defined phosphine-free Mn complexes. Both imines and amines can be obtained from the same set of alcohols and amines using
the same catalyst, only by tuning the reaction conditions. The amount and nature of the base are found to be a highly important
aspect for the observed selectivity. Both the primary and secondary amines have been employed as substrates for the N-
alkylation reaction. As a highlight, we showed the chemoselective synthesis of resveratrol derivatives. Furthermore, the Mn-
catalyzed dehydrogenative synthesis of structurally important 2,3-dihydro-1H-perimidines has also been demonstrated. Density
functional theory calculations were also carried out to model the reaction path and to calculate the reaction profile.
halides, triflates, tosylates, or mesylates and then reacting with
primary amines to get the N-alkylated product.7 These
multistep strategies use hazardous reagents and lengthy
work-up procedures, which generates a large amount of
waste.8 They also suffer from poor atom economy9 and a low
level of selectivity. To overcome these drawbacks, new
catalytic protocols involving direct N-alkylation of amines by
alcohol using “hydrogen autotransfer” (HA) or “borrowing
hydrogen”(BH) strategies have been developed.10 The
catalytic cycle (Scheme 1) consists of three steps: (i)
dehydrogenation of alcohol to form aldehyde/ketone, (ii)
imine generation, and (iii) hydrogenation of imine to form
amine (hydrogen autotransfer).
INTRODUCTION
■
Nitrogenous compounds such as amine and imines are known
for their valuable application in chemistry.1 They are
ubiquitous in many natural products and widely used as
dyes, pharmaceuticals, agrochemicals, lubricants, and surfac-
tants.2 Thus, the development of an environmentally benign,
atom-economical catalytic approach for construction of the
C−N bond is a paramount aim in catalysis.3 The most
important processes to synthesize amines are the Buchwald−
Hartwig amination4 and Ullmann reactions.5 Although the
high significance and wide application of these processes
proved their efficacy in the organic synthesis, they often suffer
from the generation of substantial amounts of side products or
wastes. The usage of alcohols as starting materials has attracted
significant attention in view of recent sustainable develop-
ments, as the alcohols are easily available either by different
industrial processes or can be obtained renewably from
lignocellulose.6 The conventional approach for the construc-
tion of the C−N bond from alcohols is converting first the
alcohol functionality to a suitable leaving group such as
The process is completely environmentally benign as water
is the only byproduct. Although the pioneering works on the
N-alkylation of amines by alcohols were described independ-
ently by Watanabe11 and Grigg12 at the beginning of 1980s,
Received: February 27, 2019
© XXXX American Chemical Society
A
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condition. In addition, the first dehydrogenative synthesis of
structurally important 2,3-dihydro-1H-perimidine derivatives
using the Mn complex has also been demonstrated under the
phosphine-free condition.
Scheme 1. Synthesis of Imines/Amines via Borrowing
Hydrogen/Dehydrogenative Condensation
RESULTS AND DISCUSSION
■
Initially, we investigated the catalytic applicability of
complexes 1−321 toward the N-alkylation reaction of amine
with alcohol. Aniline and 4-methoxybenzyl alcohol were taken
as a model substrate to optimize the reaction condition. When
a toluene solution containing aniline (1 mmol), 4-methox-
ybenzyl alcohol (1.1 mmol), and potassium tert-butoxide
(tBuOK) (1.2 mmol) was refluxed for 24 h in the presence of
5 mol % catalyst 1, N-(4-methoxybenzyl)aniline was obtained
in a 79% yield (Table 1, entry 1). Keeping the other
conditions unaltered when xylene was used, the yield of the N-
alkylated product was dropped to a 31% yield (Table 1, entry
2). Next, we studied the catalytic activity of the catalyst 2;
under the similar reaction condition using the toluene solvent,
catalyst 2 gave only a 48% desired product yield (Table 1,
entry 3). Gratifyingly, catalyst 3 gave a yield of 85% of the N-
the significant progress has been achieved only after 2000
using precious metals.13−17 The replacement of precious noble
metal catalysts by eco-friendly, inexpensive earth-abundant
metals is a key challenge in the homogeneous catalysis. In
recent years, explosive growth in the catalysis by base metals
has been observed.18 Since 2016, manganese, being the third
most abundant transition metal in the earth’s crust, has
attracted considerable attention toward the de(hydrogenative)
synthesis of complex organic molecules from renewable
starting materials.19 However, the N-alkylation of alcohols
with amine or the dehydrogenative synthesis of imines from
alcohol and amine using environmentally benign, nontoxic Mn
is still in the nascent stage (Figure 1).20 Beller and co-workers
Table 1. Optimization of the Reaction Conditions for the
a
N-Alkylation Reaction of Amine with Alcohols
base
amine:alcohol
(mmol)
yield
b
entry
1
catalyst
solvent
(mmol)
(%)
1
1
2
2
3
3
3
3
3
3
3
3
3
3
toluene tBuOK
(1.2)
tBuOK
(1.2)
toluene tBuOK
(1.2)
tBuOK
(1.2)
toluene tBuOK
(1.2)
tBuOK
(1.2)
toluene tBuOK
(1.2)
toluene tBuOK
(1.2)
toluene tBuOK
(1.0)
toluene tBuOK
(0.75)
toluene tBuOK
(1.5)
toluene tBuOK
(1.2)
toluene tBuOK
(1.2)
toluene tBuOK
(1.2)
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.2
1:1.3
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
79
31
48
14
85
34
98
96
77
49
42
71
80
33
2
xylene
3
4
xylene
5
6
xylene
7
8
Figure 1. Phosphine-free Mn complexes and the molecular structure
of complex 1 with a thermal ellipsoid at the 30% probability level (all
the hydrogens except N2 and the counter ion are not shown for
clarity).
9
10
11
first demonstrated the efficient N-alkylation of alcohol using
the Mn-PNP pincer complex.20a Milstein and co-workers
reported the first imine synthesis through the dehydrogenative
coupling of alcohols and amines catalyzed by the pyridine-
based Mn-PNP pincer complex.20b Although phosphine
ligands are known for their significant applications in the
homogeneous catalysis, still, there are some drawbacks
associated with their air and moisture sensitivities, complex
synthetic procedures, and relatively high cost. Thus, catalysis
using phosphine-free, earth-abundant metal complexes are
highly desirable in terms of sustainability and cost-effective-
ness. Herein, we report first phosphine-free well-defined Mn
complex (Figure 1) catalyzed N-alkylation of amines with
alcohol and the dehydrogenative synthesis of imine. The
selective synthesis of N-alkylated amines or imines is achieved
by using a single precatalyst, only by tuning the reaction
c
12
d
13
e
14
15
16
3
3
toluene KOH (1.2)
toluene K2CO3
(1.2)
toluene tBuOK
(1.2)
1:1.2
1:1.2
50
27
17
Mn(CO)5Br
1:1.2
20
a
Reaction conditions: aniline (1 mmol), 4-methoxy benzyl alcohol
(1.1−1.3 mmol), catalyst (5 mol %), base (0.75−1.5 mmol), 140 °C
(oil bath temperature), 24 h, solvent (2 mL). NMR yield using
b
c
d
e
CH3CN as internal standard. 2.5 mol %. 12 h. 100 °C loading.
B
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a b
,
Table 2. Scope of N-Alkylation Reaction of Primary and Secondary Amines with Different Primary Alcohol
a
Reaction conditions: amine (1 mmol), alcohol (1.2 mmol), catalyst 3 (5 mol %), tBuOK (1.2 mmol), 140 °C (oil bath temperature), 24 h,
b
c
d
toluene (2 mL). Isolated yield. Diaminobenzene (0.5 mmol), alcohol (1.5 mmol), 36 h. 18 h.
alkylated product, which was further improved to 98% when
aniline and 4-methoxybenzyl alcohol were taken in a 1:1.2
ratio yield (Table 1, entries 5 and 7). The amine:alcohol ratio
(1:1.2) used in our protocol is relatively lower than the
previously reported PNP-Mn(I) complex20c or nickel com-
plex.18o The yield of desired N-(4-methoxybenzyl)aniline was
decreased with a lower amount of tBuOK (Table 1, entries 9
and 10) or by using a weaker base such as KOH or K2CO3
(Table 1, entries 15 and 16).
Thus, when 1.2 mmol of a weaker base such as KOH was
used, only a yield of 48% of the desired N-alkylated product
was obtained (Table 1, entry 15) together with a 41% yield of
imine, although decreasing the amount of tBuOK (0.75)
selectivity toward the desired amine was dropped to 49%
(Table 1, entry 10). Thus, the nature and stoichiometry of the
base are crucial for the observed selectivity. The lower
temperature or the shorter reaction time is found to have a
detrimental effect on the yield of the reaction (Table 1, entries
13 and 14). MnBr(CO)5 gave only a yield of 20% of the
desired amine under the optimized reaction condition (Table
1, entry 17).
After establishing the optimal reaction condition, we
investigated a wide range of the substrate scope to
demonstrate the generality of the reaction (Table 2). At the
outset, different substituted aniline and alcohols were tested.
Substrates having the electron-donating or electron-with-
drawing group in the aryl group of aniline or benzyl alcohols
were reacted smoothly under the optimized condition to
afford the N-monoalkylated product in high yields. Even the
sterically hindered anilines such as 2-(ethylthio)aniline and
2,4,6-trimethylaniline underwent N-alkylation smoothly, af-
fording the corresponding N-monoalkylated product in good
yields. Alcohols possessing heteroaromatic moieties or
naphthyl moieties have also worked well. Furthermore, we
investigated the N-alkylation of diaminobenzene. There is a
possibility of formation of the dialkylated (N-alkylation of
C
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a b
,
Table 3. Scope of N-Alkylation Reaction of Heterocyclic Amines
a
Reaction conditions: amine (1 mmol), alcohol (1.2 mmol), catalyst 3 (5 mol %), base (1.2 mmol), 140 °C (oil bath temperature), 24 h, toluene
b
c
d
(2 mL). Isolated yield. Diaminobenzene (0.5 mmol), alcohol (1.5 mmol), 36 h. Melamine (0.5 mmol), alcohol (2.25 mmol), catalyst 3 ( 0.08
mmol), tBuOK (1.6 mmol), 52 h.
both NH2) and mono-alkylated (N-alkylation of one of NH2)
product. Indeed, when 1,3-diaminobenzene and 4-chloroben-
zyl alcohol were taken in a 1:2.4 ratio, the mixtures of mono-
and dialkylated products were formed 30 and 40%,
respectively. Increasing the ratio to 1:3, the dialkylated
product was isolated to an 85% yield after 36 h. In the case
of 1,2-diaminobenzene, only a 46% yield of the desired
dialkylated product was obtained due to the formation of 1,2-
disubstituted benzimidazole.21a Pure aliphatic amine such as
cyclohexyl amine has reacted smoothly under the reaction
condition. To demonstrate the usefulness of our protocol, we
explored the N-alkylation of secondary amine to synthesize
unsymmetrically substituted tertiary amine. The challenging
substrates such as N-methylaniline18o and less basic N-
methylbenzylamine have reacted well with different benzylic
and heteroaryl alcohols, which, to the best of our knowledge,
are not yet reported with any manganese catalyst.
alcohols. Upon refluxing melamine (0.5 mmol) with 4-
chlorobenzyl alcohol (2.25 mmol) in toluene led to the
formation of the corresponding trialkyalted product in a 58%
yield after 36 h, which was further improved to 95%, only by
increasing the reaction time (52 h).
Resveratrol-derived amines are known for their activity
toward the treatment of Alzheimer’s disease.27 Thus, we have
applied our protocol for the chemoselective N-alkylation of 4-
aminostilbene with different alcohols to afford resveratrol
derivatives in a good isolated yield (Scheme 2).
a b
,
Scheme 2. Synthesis of Resveratrol Derivatives
To illustrate the synthetic utility, the scope of the N-
alkylation reaction with regard to different heterocyclic amines
was investigated further (Table 3). 2-(N-alkylamino)-
benzothiazoles are structurally important units in many
bioactive compounds22 and have a broad range of physio-
logical and pharmacological activities.23 Thus, initially, we
explored the N-alkylation of 2-aminobenzothiazole with
different alcohols, gratifyingly N-exosubstituted 2-(N-
alkylamino)benzothiazoles were obtained in a high yield.
The reaction catalyzed by the Mn complex is completely
regioselective toward N-exosubstituted 2-(N-alkylamino)-
benzothiazoles, although N-alkylation of 2-aminobenzothia-
zoles with alkyl halides give N-endosubstituted 3-alkyl-2-
iminobenzothiazolines.24 Next, we investigated the N-
alkylation of aminopyridines as the selective N-alkylation of
aminopyridine is a challenging role as amides can be formed as
byproducts.25 To our delight, the N-alkylation of amino
pyridines proceeds well to afford the desired N-alkylated
product in a good yield. Next, we were interested in the tri-N-
alkylation of melamine as substituted s-triazine derivatives are
known to have different biological activities.26 Thus, we
studied the reaction of melamine with different benzyl
a
Reaction conditions: 4-aminostilbene (1 mmol), alcohol (1.2
mmol), catalyst 3 (0.05 mmol), t-BuOK (1.2 mmol), toluene (2
mL), 140 °C (oil bath temperature), 24 h. Isolated yield.
b
Next, we are interested to synthesize imine directly from the
alcohol and amines using our Mn complexes, only by tuning
the reaction condition (Table 4). In 2016, Milstein and co-
workers first reported the Mn-catalyzed dehydrogenative
synthesis of imines,20b and very recently, Kempe and co-
workers demonstrated sustainable imine formation using the
triazine-based PNP-Mn complex.20c Thus, the dehydrogen-
D
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a b
,
Table 4. Dehydrogenative Condensation Reaction of Alcohol and Amine
a
Reaction conditions: amine (1 mmol), alcohol (1.2 mmol), catalyst 3 (5 mol %), KOH (0.3 mmol), 140 °C (oil bath temperature), 24 h, toluene
b
(2 mL). NMR yield (the yield in the parenthesis is the isolated yield).
a b
,
Table 5. Scope of the Reaction To Synthesize 2,3-Dihydro-1H-Perimidine Derivatives
a
Reaction conditions: catalyst (5 mol %), 1,8-diaminonaphthalene (1 mmol), alcohol (1.2 mmol), base (0.3 mmol), 140 °C (oil bath temperature),
b
c
24 h, toluene (2 mL). Isolated yield. Alcohol (2.0 mmol), base (0.5 mmol), 72 h.
ative condensation of alcohol and amine using phosphine-free
Mn complexes is highly desirable. Hence, when a toluene
solution having aniline (1 mmol) and 4-methoxybenzyl
alcohol (1.2 mmol) was refluxed in the presence of KOH
(0.3 mmol) and 5 mol % catalyst 3 under argon flow, the
desired imine was obtained in an 88% yield.
E
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Scheme 3. Control Experiments
a
Scheme 4. Calculated Energy Profile of ADC Reaction of Benzyl Alcohol with Complex III
a
Energies are obtained at the B3PW91 DFT method (see Tables S2−S4 for details).
It is important to note that the quantity of the base used in
our system is significantly lower compared to that in the
triazine-based Mn-PNP system.20c Encouraged by the
versatility of our current catalytic system, we tried to apply
our protocol to synthesize 2,3-dihydro-1H-perimidines as they
are an important class of compounds having useful biological
activity.28 Thus, when 1,8-diaminonaphthalene is reacted with
different primary alcohol in the presence of catalyst 3,
corresponding 2,3-dihydro-1H-perimidine derivatives were
isolated in good yields (Table 5). To the best of our
knowledge, this is the first example of the dehydrogenative
synthesis of 2,3-dihydro-1H-perimidines using the Mn
complex under the phosphine-free condition.
F
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To gain a mechanistic insight, some control experiments
were performed (Scheme 3). When an equimolar mixture of
(E)-1-(4-methoxyphenyl)-N-phenylmethanimine and 4-me-
thoxybenzyl alcohol was treated with catalyst 3 in the presence
of KOtBu (1.2 mmol), N-(4-methoxybenzyl)aniline was
obtained in an 84% yield, whereas when the reaction was
done under the similar condition in the absence of the alcohol,
no conversion of the imine to the corresponding amine was
observed. Next, we studied the hydrogen autotransfer reaction
in the presence of alkene. Thus, when an equimolar mixture of
(E)-1-(4-methoxyphenyl)-N-phenylmethanimine, trans-stil-
bene, and 4-methoxybenzyl alcohol was treated under the
optimized reaction condition of hydrogen autotransfer, 65% of
corresponding amine was detected, and stilbene was not
hydrogenated. Furthermore, when a mixture of (E)-1-(4-
methoxyphenyl)-N-phenylmethanimine and trans-stilbene in a
1:5 ratio is treated with 4-methoxy benzyl alcohol (1 mmol)
under the similar reaction condition, the yield of the desired
amine dropped to 20%. Thus, the excess amount of alkene
found to retard the hydrogen autotransfer reaction of imine.
On the basis of the previous reports,19k,29 our observations
and with the help of DFT calculations,30 the plausible
mechanism has been proposed and is presented in Scheme
4. Benzyl alcohol was taken as a model substrate to investigate
the manganese (catalyst 3)-catalyzed dehydrogenation reac-
tion. First, the cationic complex in the presence of base and
under heating condition gave the five-coordinated amide
complex I [PyCH2NCH2CH2Mn(CO)2]. The catalysis
commences with the adduct formation III through the
hydrogen bonding between complex I and benzyl alcohol.
We selected complex II as a reference point, which is more
stable than complex III by 6.22 kcal/mol. Next, benzaldehyde
was formed together with the manganese hydride complex IV
via the hydride transfer of the α-methylene group of benzyl
alcohol onto the metal center. This is going through the
transition state TS(III-IV), and the activation barrier for this
reaction is 18.49 kcal/mol. This is the rate-determining step,
and it is in accordance with the previous report.29c Next, the
barrier was calculated for the release of H2 from the complex
IV with the involvement of benzyl alcohol. The first step is the
generation of the adduct complex V via hydrogen bonding
between N−H of the complex IV and O of benzyl alcohol.
Then, it leads to the formation of the manganese-coordinated
η2-H2 complex VI through TS(V-VI). The activation barrier
for this formation is 14.26 kcal/mol. The stabilization for
complex VI is 3.22 kcal/mol compared to the TS(V-VI).
Finally, dissociation of the H2 molecule has an activation
barrier of only ∼1 kcal/mol to regenerate complex III, which
shows a high stabilization of 24.01 kcal/mol from the TS as
presented clearly in Scheme 4. The effect of various DFT
methods, basis sets, dispersion corrections, etc. on the energy
profile of the reaction was tested (Tables S2−S4, Supporting
Information). The level of the calculation does not affect the
overall trend of the results. Changes in geometry of these
complexes along the path of the reaction are summarized in
Table S5 (see Supporting Information).
condition. The nature and stoichiometry of the applied base
are crucial to obtain the maximum selectivity. This effective
protocol has also been applied to synthesize the 2,3-dihydro-
1H-perimidine derivatives. To the best of our knowledge, this
is the first sustainable synthesis of 2,3-dihydro-1H-perimidines
using the phosphine-free Mn complex. Furthermore, a
mechanistic insight based on control experiments and DFT
calculations is also presented. The versatility and the selectivity
of the catalysts having a nontoxic earth-abundant manganese
metal and a non-phosphine ligand system make this protocol
sustainable and attractive.
EXPERIMENTAL SECTION
■
General Considerations. All chemicals were purchased from
common commercial sources and used without any further
purification. All solvents were dried by using standard procedure.
The preparation of catalyst was carried out under an argon
atmosphere with freshly distilled THF. All catalytic reactions were
carried out under an argon atmosphere using dried glassware and
standard syringe/septa techniques.
The DRX-400 Varian spectrometer and Bruker Avance III 600 and
400 spectrometers were used for recording NMR (1H and 13C)
spectra using CDCl3 as the solvent and TMS as an internal standard.
Chemical shifts (δ) and spin−spin coupling constant (J) are reported
in ppm and in Hz, respectively, and other data are reported as
follows: s = singlet, d = doublet, t = triplet, m = multiplet, q = quartet,
and br s = broad singlet. FTIR were collected on a PerkinElmer IR
spectrometer. A Q-Tof ESI-MS instrument (model HAB 273) was
used for recording mass spectra. X-ray crystallographic data were
collected using an Agilent Super Nova (single source at offset, EOS)
diffractometer. Data refinement and cell reduction were carried out
by CrysAlisPro. Structures were solved by direct methods using
SHELXS-97 and refined by full-matrix least squares on F2 using
SHELXL-97. All of the non-H atoms were refined anisotropically.
SQUEEZE was used to reduce the contribution of dichloromethane
to the overall electron density. The purity determination of the
substrates and reaction monitoring was accomplished using TLC on
silica gel 60 F254 plates (from Merck Company), and SRL silica gel
(100−200 mesh) for column chromatography was used.
General Experimental Procedure for the N-Alkylation
Reaction of Amine with Alcohol. A mixture of aniline derivatives
(1.0 mmol), primary alcohol (1.2 mmol), tBuOK (1.2 mmol), and
catalyst 3 (0.05 mmol) was refluxed with 2 mL of toluene at 140 °C
for 24 h under an argon balloon. After cooling, chloroform was added
to dilute the mixture and filtered through Celite. The filtrate was
concentrated under reduced pressure, and the residue was purified
using silica gel column chromatography with 2−20% ethyl acetate in
hexane as an eluent.
General Experimental Procedure for the Synthesis of Imine
Derivatives. A mixture of amine (1.0 mmol), primary alcohol (1.2
mmol), KOH (0.3 mmol), and catalyst 3 (0.05 mmol) was refluxed
with 2 mL of toluene at 140 °C for 24 h in an open system under an
argon balloon. After cooling, chloroform was added to dilute the
mixture and filtered through Celite. The filtrate was concentrated
under reduced pressure, and the residue was purified using silica gel
column chromatography with 2−20% ethyl acetate in hexane as an
eluent to get a pure compound.
General Experimental Procedure for the Synthesis of 2,3-
Dihydro-1H-Perimidines Derivatives. A mixture of naphthalene-
1,8-diamine (1.0 mmol), primary alcohol (1.2 mmol), KOH (0.3
mmol), and catalyst 3 (0.05 mmol) was refluxed with 2 mL of
toluene at 140 °C for 24 h in an open system under an argon balloon.
After cooling, chloroform was added to dilute the mixture and filtered
through Celite. The filtrate was concentrated under reduced pressure,
and the residue was purified by silica gel column chromatography
with 2−20% ethyl acetate in hexane as an eluent to get a pure
compound.
CONCLUSIONS
■
In conclusion, we demonstrated the first phosphine-free Mn-
based protocol to synthesize both amine and imine from the
same set of alcohol and amine, only by tuning the reaction
condition. We have observed a broad range of a substrate
scope for both the reaction under the optimized reaction
Computational Methodology. All the calculations were
performed using the Gaussian-09 (revision D.01) program package.31
G
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Geometries of considered reactants, transition states, and inter-
mediates were fully optimized using the B3PW91 method.32 This
method uses the same exchange functional as the B3LYP method but
employs different and comparatively a newer correlation functional.
The double-ζ quality SDD basis set with an effective core potential
(ECP) was selected for a manganese atom, and the 6-31G(d,p) basis
set was selected for all other atoms. Methods and basis sets were
selected based on previous reports.29b Frequency calculations were
performed at the same level of theory to distinguish the obtained
stationary points as the minima or transition state on the potential
energy surface. The effect of the toluene solvent was included in all
the calculations with the help of the IEF-PCM method available in
the Gaussian-09 program.33 The reaction path was validated by
following the reaction profile with the help of intrinsic reaction
coordinate (IRC) calculations with mass-weighted coordinates.34 The
effect of higher basis set, different DFT methods, and dispersion
corrections on the energy profile of the reaction was tested (Tables
S2−S3). All four tested dispersion corrections show very similar
energetics (Table S3). The overall energetics from dispersion
corrected and uncorrected methods is also similar (Table S3), and
the only difference is that the dispersion correction provides some
extra stabilization to TS(III-IV). The temperature variation has
almost no effect on thermal corrections (Table S4).
Department of Science and Technology (grant DST-SERB
SB/S2/RJN/004-2015). We would like to thank CIF and
Department of Chemistry IITG, DST-FIST program and
Param-Ishan supercomputing facility at IITG for the
instrumental facilities, computational facilities, infrastructural
support, and startup grant. K.D., A.M., and D.P. are thankful
to the institute for their fellowship.
REFERENCES
■
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Experimental procedures, characterization data, theoreti-
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X-ray crystallographic data (CCDC 1881759) for 1
(XYZ)
Accession Codes
CCDC 1881759 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: hemantkrsri@gmail.com (H.K.S.).
*E-mail: dsrimani@iitg.ac.in; Fax: (+91)-361-258-2349;
Phone: (+91)-361-258-3312 (D.S.).
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ORCID
Kalicharan Das: 0000-0003-4347-1390
Hemant Kumar Srivastava: 0000-0001-6589-6854
Dipankar Srimani: 0000-0001-8826-9773
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Author Contributions
D.S. and K.D. designed the experiments. K.D., A.M., and D.P.
carried out the experiments. H.K.S. performed the computa-
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wrote the manuscript with the input of all authors.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We would like to thank SERB, DST (ECR/2016/000108) and
DST-INSPIRE (IFA-14-CH-143) for financial support. H.K.S
is thankful to Science and Engineering Research Board,
H
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