Aluminium complex as an efficient catalyst for the chemo-selective reduction of amides to amines

Article abstract of DOI:10.1039/c9dt01806a

We report an efficient protocol for the catalytic chemo-selective reduction of tert-amides with pinacolborane (HBpin) to afford the corresponding amines in high yields using aluminium complexes [κ²-{Ph₂P(X)NC₉H₆N}Al(Me)₂] [X = S (2a), Se (2b)] as pre-catalysts at room temperature. The aluminium complexes were prepared from the reaction of [Ph₂P(X)NC₉H₆N] [X = S (1a), Se (1b)] and trimethylaluminium in toluene. The solid-state structure of complex 2b is established. Tertiary amides with a wide array of electron-withdrawing and electron-donating functional groups were easily converted to the desired products through the selective cleavage of the amides' CO bond by aluminium hydride as an active species. A kinetic study of the catalytic reaction is also reported.

Full text of DOI:10.1039/c9dt01806a

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DOI: 10.1039/C9DT01806A
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ARTICLE
Aluminium complex as an efficient catalyst for chemo-selective
reduction of amides to amines
Suman Das,a† Himadri Karmakar,a† Jayeeta Bhattacharjee,a† Tarun K. Panda*a
Received 00th January 20xx,
Accepted 00th January 20xx
We report an efficient protocol for the catalytic chemo-selective reduction of tert-amides with pinacolborane
(HBpin) to afford corresponding amines in high yield using aluminium complexes [2-{Ph2P(X)NC9H6N}Al(Me)2]
[X = S (2a), Se (2b)] as pre-catalysts at room temperature. The aluminium complexes were prepared from the
reaction of [Ph2P(X)NC9H6N] [X = S (1a), Se (1b)] and trimethylaluminium in toluene. The solid-state structure
of complex 2b is established. Tertiary amides with a wide array of electron-withdrawing and electron-donating
functional groups were easily converted to the desired products through the selective cleavage of the amides’
C=O bond by aluminium hydride as an active species. A kinetic study of the catalytic reaction is also reported.
DOI: 10.1039/x0xx00000x
www.rsc.org/
to compel the reaction, which makes this process unfavourable.
On the other hand, using stoichiometric quantities of traditional
aluminium hydride23 (LiAlH4) and boron hydrides (B2H6 or
Introduction
There is an ever-increasing demand for efficient catalytic
reduction of amides to amines under mild conditions, since it
represents a highly desired transformation in organic chemistry,
agrochemical and materials chemistry, as well as the
pharmaceutical field.1,2 Due to this, development of numerous
strategies for synthesis of amines continues to attract interest
from chemists.3 Reductive transformations of amides to amines
are among the most straightforward methods of amine synthesis,
since the starting materials can be readily accessed synthetically,
as well as being naturally and predominantly available among
biological molecules.4−6 Therefore, numerous transition-metal
and main-group metal complexes enjoy a pre-eminent place as
homogeneous and heterogeneous catalysts in the reduction of
amides,7 including deoxygenation of amides,8−13 hydrogenation
of amides and imines.14−17 Among the various types of catalytic
hydrogenation systems, ruthenium,[18] rhodium,[19] palladium,[20]
platinum, and[21] iridium22 have shown high efficiency, but these
elements are expensive and susceptible to possible constraints in
terms of supply due to their geological scarcity. Although the
catalytic hydrogenation pathway is highly in demand due to its
atom economy, most amide hydrogenation necessitates
conditions such as elevated pressure and temperature (>150 °C)
BH3·THF)24 to achieve amide reduction is already well reported.
However, clear drawbacks of these reagents are their sensitivity
to air and moisture, attendant formation of environmentally
hazardous waste materials, as well as lack of selectivity in the
presence of various functional groups. Catalytic hydrosilylation of
amides, using a range of precious-metal catalysts such as Pt,25 Ir,26
Rh,27 and Ru,28 is also well reported in the literature. Given that
high selectivity and broad tolerance towards the presence of
other functional groups are important factors for the acceptance
and application of a given catalytic process, a new methodology
with inexpensive and environment-friendly catalysts is still
necessary to address the challenges faced during amide
reduction.
Catalytic hydroboration of amides with pinacolborane (HBpin)
has not been explored much – except recently and only in case of
a reduction of carbonyl moieties29 – and needs considerable
development. As the starting material, HBpin does not react with
secondary and tertiary amides at room temperature, and also
shows good selectivity and functional group tolerance. Thus, it is
preferable to use HBpin for the catalytic reduction of tertiary and
secondary amides to amines.
a.Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi,
Sangareddy 502285, Telangana, India. Fax: + 91(40) 2301 6032; Tel: +
91(40) 2301 6036; E-mail: tpanda@iith.ac.in.
† Equal contribution from the authors.
Electronic Supplementary Information (ESI) available: Text giving
experimental details for the catalytic reactions, 1H, 13C{1H} and spectra of
amines 3a – 3zb, and 4a – 4i and compounds, 1a, 1b, 2a, 2b in Supporting
Information. For crystallographic details in CIF see DOI: 10.1039/b000000x/.
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analysis, and their solid-state structures were established by single-
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DOI: 10.1039/C9DT01806A
crystal X-ray diffraction analysis.
Scheme 1. Synthesis of P–N chalcogenide ligands and Al complexes
1a, 1b, 2a, and 2b.
Figure 1. Previous reports for amide reduction.
In the 1H NMR spectra of ligands 1a and 1b, the signal for the amine
NH hydrogen atom appears as a doublet at H = 7.86 ppm (1a) and
7.81 ppm (1b) due to coupling with the phosphorus atom (2JHP of 3.6
Hz) (Figure FS 1 and 4 in ESI). In the 31P NMR spectra, ligand 1a
exhibits a sharp singlet at P = 51.3 ppm and ligand 1b displays a
sharp singlet at P = 47.5 ppm along with two satellite peaks due to
coupling with the adjacent selenium atom. (Figure FS 3 and 6 in ESI).
The solid-state structures of ligands 1a and 1b were established by
single-crystal X-ray diffraction. Their molecular structures are given
in Figures 2 and 3. The P–S bond distance in 1a is 1.9392(10) Å and
the P–Se bond distance in 1b is 2.0981(9) Å, which are very similar to
those previously reported for [Ph2P(S)NHCPh3] [1.9472(7) Å]34 and
[Ph2P(Se)-NH(2,6-Me2C6H3)] [2.1019(8) Å]35 respectively, and are
hence diagnostic of double bonds (P=S and P=Se). The P–N bond
distances of 1.670(2) Å and 1.672(3) Å correspond to those measured
in other phosphinamines.36-37
Recently, Sadow et al. reported the magnesium-catalysed
(ToMMgMe) hydroboration of amides by deoxygenation to
amines, but the substrate scope of the reaction is very limited.30
Our research group has reported that a number of earth-
abundant materials and metals, such as alkali, alkaline earth
metals,31 titanium,32 and aluminium33 can be used as novel
catalysts for the hydroboration of a number of carbonyl
compounds including aldehydes, ketones, and carboxylic acids,
followed by organic nitriles and unsaturated substrates. Due to
the earth-abundant nature of the above-mentioned metal
complexes, as well as the fact that they are inexpensive, easily
accessible, biocompatible, and have low toxicity, these metal
complexes are attractive options for various types of catalytic
hydroboration of C–X unsaturated bonds. In continuation of our
Al mediated catalytic hydroboration reaction,33 we extended this
protocol to amide moieties. In this paper, we report the use of
aluminium bis-alkyl complexes [2-{Ph2P(X)NC9H6N}Al(Me)2] [X =
S (2a), Se (2b)] supported by functionalised amidophosphine
ligands as an efficient protocol for the catalytic chemo-selective
reduction of tert-amides with HBpin to afford corresponding
amines in high yield at room temperature. To the best of our
knowledge, this paper describes the first example of catalytic
hydroboration of amides using earth-abundant aluminium.
Results and discussion
Synthesis of ligands. Aminophosphine-chalcogenide ligands
[Ph2P(X)NHC9H6N] (X = S; 1a, Se; 1b) were synthesised in good yield
using a protocol similar to that reported in literature, that is, by the
reaction of 8-aminoquinoline with chloro-diphenylphosphine (1:1
equiv.) in the presence of trimethylamine (1.5 equiv.) in a mixture of
toluene and THF (1:1) followed by the addition of elemental sulphur
or selenium (1.2 equiv.) in 90o C (Scheme 1).34–35 Both ligands 1a and
1b were characterised using multinuclear NMR and combustion
Figure 2. Molecular structure of ligand 1a in the solid state. Selected
bond lengths (Å) and angles (deg) are given. P1–N2 1.670(2), P1–S1
1.9392(10), P1–C10 1.808(3), P1–C16 1.809(2), N2–P1–S1 115.91(8),
N2–P1–C16 100.39(11), N2–P1–C10 104.90(12). CCDC No. 1912620.
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Figure 4. The molecular structure of aluminium comVpileewxA2rtibcleinOntlhinee
DOI: 10.1039/C9DT01806A
solid state. Selected bond lengths (Å) and angles (deg) are given. Al1–
N1 1.963(5), Al1–N2 1.909(5), Al1–C10 1.955(7), Al1–C11 1.955(7),
N2–P1 1.656(5), P1–Se1 2.1093(17), N1–Al1–N2 84.88(19), N1–Al1–
C10 109.8(3), N1–Al1–C11 104.0(3), N2–Al1–C10 117.5(3), N2–Al1–
C11 113.3(3), Al1–N2–P1 126.7(2). CCDC No. 1912621.
As mentioned earlier, the molecular structure of complex 2b in the
solid state was confirmed by single-crystal X-ray diffraction analysis.
Complex 2b crystallises in the triclinic space group P-1, with two
molecules in the unit cell. Crystallographic and refinement
parameters are given in Table TS1. The molecular structure of
complex 2b is depicted in Figure 4. The solid-state structure of
complex 2b confirms the ligation of two aminophosphine-selenide
ligands to the Al ion through anionic phosphinamido nitrogen and
the nitrogen atom of the 8-aminoquinoline moiety of ligand 1b. The
selenium atom remains non-coordinated. Additionally, two methyl
groups are attached to the aluminium ion, resulting in a distorted
tetrahedral geometry around the metal centre. Two sets of Al–N
distances, 1.909(5) Å and 1.963(5) Å, are consistent with the
Figure 3. The molecular structure of ligand 1b in the solid state.
Selected bond lengths (Å) and angles (deg) are given. P1–N2 1.672(3),
P1–Se1 2.0981(9), P1–C10 1.807(3), P1–C16 1.798(3), N2–P1–Se1
115.99(10), N2–P1–C16 100.87(14), N2–P1–C10 105.15(14). CCDC
No. 1912619.
Synthesis of Aluminium complexes. Trimethylaluminium was distances of the anionic amido–Al and neutral nitrogen–Al bonds in
reacted with one equivalent of aminophosphinchalcogenides the ligand 1b and are within the range that we previously reported
for complex [2-{2-F-C6H4NP(Se)Ph2}2Al-(Me)] and other reported Al
[Ph2P(X)NHC9H6N] to afford, in good yield (Scheme 1), corresponding
aluminium complexes [2-{Ph2P(X)NC9H6N}Al(Me)2] [X = S (2a), Se complexes.32
(2b)] as colourless solids. Both the Al complexes showed good
solubility in common organic solvents such as THF and toluene. Catalytic reduction of amides. After successful synthesis of the two
1
Complexes 2a and 2b were characterised using H, 13C{1H}, 31P{1H} aluminium complexes 2a and 2b, we tested them as pre-catalysts for
NMR spectral data, and combustion analysis. The solid-state the selective reduction of tertiary amides with HBpin to afford the
structure of complex 2b was established by single-crystal X-ray corresponding amines. The reduction of N,N-dimethyl benzamide
1
crystallography. In the H NMR spectra of complexes 2a and 2b with HBpin in the presence of 5 mol% of complex 2a or complex 2b
measured in C6D6, the absence of a doublet resonance signal at 7.86 at room temperature was chosen as the model reaction (Table 1).
ppm and 7.81 ppm assigned to the –NH proton of ligands 1a and 1b First, we tested the reaction in the absence of the catalyst, which did
respectively confirms the formation of the monoanionic fragment of not afford any amine, thus confirming the necessity of the catalyst
ligand 1a and 1b respectively. Complexes 2a and 2b display two sharp (Table 1, entry 1). However, reactions using complexes 2a and 2b as
catalysts under solvent-free conditions afforded high yields – 84%
and 88% respectively – within 12 hours (Table 1, entries 2–3).
Extending the reaction time to 24 hours when using complex 2b as
the catalyst increased the yield of the product to 94% (Table 1, entry
4). Hence, complex 2b was chosen as the best catalyst. With the
selected catalyst 2b, we examined the effect of solvent on the
selective reduction of N,N-dimethyl benzamide by HBpin, for which
various solvents such as toluene, THF, and hexane, were screened.
However, in all cases, we observed slow reactivity even after lengthy
reaction times (Table 1, entries 5–7). Further, catalyst loading of 2.5%
or 1% substantially lowered the reaction rate, and yields of 70% and
62% respectively were observed after 12 hours of the reaction (Table
1, entries 8–9). We increased the molar ratio of HBpin to four
equivalents with respect to the N,N-dimethylbenzamide; however,
no significant increase in yield was observed (Table 1, entry 10).
Therefore, we set out to examine the scope of various substrates of
tert-amide, with room temperature and solvent-free conditions
taken as optimal, in the presence of 5 mol% of the aluminium
complex 2b.
singlets at H = –0.186 ppm and –0.416 ppm respectively in respect
of resonance signals of the two methyl groups attached to the metal
ions (Figures FS 7 and 10 in ESI). Additionally, in the 31P {1H} NMR
spectra, complexes 2a and 2b exhibit resonance signals at P = 64.6
ppm and 58.2 ppm respectively, which are significantly downfield
shifted compared to the signals of the corresponding ligands, which
are P = 51.3 ppm (1a) and 47.5 ppm (1b) (Figures FS 8 and 11 in ESI).
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the presence of the corresponding halide-substitutedVaiemwiAdreticsle(TOanblinlee
Table 1. Screening of Al complexes as a pre-catalysts for reduction of
tert-amide to yield the corresponding amine.
DOI: 10.1039/C9DT01806A
2, entries 3k–3Zb) (Figures FS 43–95 in ESI).
Table 2. Substrate scope for reduction 3o amide to corresponding
amine synthesis catalysed by complex 2b.
Entry
Catalyst
Solvent Cat (X t (h)
mol)
T (°)
Yielda(%)
1
2
3
4
5
6
7
8b
9c
10d
None
2a
2b
2b
2b
2b
2b
2b
2b
Neat
Neat
Neat
Neat
Toluene
THF
Hexane
Neat
Neat
Neat
-
5
5
5
5
5
5
2.5
1
12
12
12
24
24
24
24
12
12
24
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
0
84
88
94
69
65
58
70
62
92
2b
5
aIsolated yield. b,cReaction was performed using 2.5 mol% (entry 8) and 1
mol% (entry 9) of catalyst 2b, dReaction was performed using molar ratio
of HBpin : N,N-dimethylbenzamid = 4:1.
With these optimised conditions, we first investigated the scope of
the reaction in the presence of aliphatic, aromatic as well as
heterocyclic amides with HBpin. Table 2, which summarises the
results, indicates that all the aliphatic, aromatic, and heterocyclic
amides are reduced smoothly under the optimised reaction
conditions with HBpin (Table 2, entries 3a–3zb). In each case, the
yield of the corresponding amine product was isolated, and then the
yield was calculated (Figures FS 13–95 in ESI). The reduction of N, N-
dimethylacetamide proceeded rapidly and a quantitative yield (96%)
of N, N-dimethyl ethanamine was obtained within five hours (Table
2, entry 3a). Similarly, N, N-dimethyl propionamide, N,N-diethyl
acetamide and N-methyl pyrrolidine-2-one gave excellent yields
(>90%) of the corresponding amines at room temperature (Table 2,
entries 3b, 3c, and 3e) (Figures FS 15–20 and FS25–27 in ESI). When
2,2,2-trifluoro-N,N-dimethylacetamide was used as the substrate,
we were pleased to find that 88% of the product was obtained under
similar conditions, indicating that the electron-withdrawing trifluoro
group has no significant impact on the reaction (Table 2, entry 3d)
(Figures FS 21–24 in ESI). Aryl amides with no substitution in the
benzene ring, such as N,N-dimethyl-benzamide, N,N-diisopropyl
benzamide, N,N-diethyl benzamide, N-methyl-N-phenyl benzamide,
and phenyl-(pyrrolidine-1-yl)methanone reduced smoothly,
producing the corresponding tert-amines in nearly quantitative
yields (Table 2, entries 3f–3j) (Figures FS 28–42 in ESI). To explore the
electronic effect of the amide reduction reaction, we treated a series
of differently substituted benzamides where substitution on the
nitrogen atom was varied from ethyl, isopropyl, and methyl-phenyl
groups and substitution on the para-position of the benzene ring was
changed from electron-donating groups such as methoxy to
electron-withdrawing ones, such as fluoro, chloro, bromo, and iodo
groups. We observed that the presence of electron-donating groups
at the para-position of the benzene ring resulted in higher yields than
All reactions were performed in the neat condition under ambient
temperature for 12 hours using different amides (0.5649 mmol, 1
equiv.), HBpin (1.129 mmol, 2 equiv.), and complex 2b (22 mg,
0.02824 mmol, 5 mol%). Yields were calculated as an isolated yield
basis.
Table 3. Chemo-selective catalytic reduction of 3° amide by catalyst
2b.
All reactions were performed in the neat condition under ambient
temperature for 12 hours using different amides (0.5649 mmol, 1
equiv.), HBpin (1.129 mmol, 2 equiv.), and complex 2b (22 mg,
0.02824 mmol, 5 mol%). Yields were calculated as an isolated yield
basis.
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After demonstrating the general applicability of our procedure, we The proposed mechanism for the catalytic hydroboration of amide
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DOI: 10.1039/C9DT01806A
wanted to investigate the functional group tolerance and chemo- with HBpin is shown in Scheme 2. Initially, the aluminium pre-catalyst
selectivity of the catalytic system. Thus, we studied particularly 2b reacts with HBpin to form the active aluminium hydride species
challenging substrates, which we estimated might undergo (I), which further reacts with one equivalent of the amide, during
additional reductive transformations in the presence of HBpin. To which the nucleophilic hydride ion attacks the carbonyl carbon of the
our delight, in each case, only the amide moieties reduced, while the amide, producing a tetrahedral intermediate (III). Complex (III)
additional ester, ether, nitro, cyano, and non-conjugated double eventually rearranges itself to give the iminium species (IV), which
bonds remained intact, representing the high chemo-selectivity of further reacts with another molecule of HBpin to yield the amine
the aluminium complexes (Table 3, entries 4a–4i) (Figures FS 96–122 product as well as the boryl aluminium species (V). In the final step,
in ESI). All the results are summarised in Table 3.
the active aluminium hydride species is regenerated using another
molecule of HBpin, by eliminating the corresponding free
dioxaborolane product. No decomposition of HBpin was observed
during the reaction.
Kinetic Study. To determine the initial rate of catalytic reduction of
amines, kinetic experiments were performed with respect to the
starting material and catalyst 2b. Reactions were performed with
varied concentrations of catalyst 2b, as well as N,N-dimethyl
benzamide and HBpin while keeping the concentration of other
reagents unchanged. Hence, in situ NMR experiments were carried
out by loading complex 2b (0.03, 0.04, 0.05, 0.06, 0.07 M) from a
stock solution and adding N,N-dimethyl benzamide (0.126 g, 1.0
mmol), HBpin (0.108 g, 1.0 mmol), and C6D6 (1 mL) to it. The
temperature of the solution mixture was set at 60oC. At indicated
time intervals, the solution was analysed by 1H NMR, which revealed
that the rate law of the reactions displays a first-order dependence
on complex 2b (Figures FS123–FS126 in ESI). Increasing the amount
of N,N-dimethyl benzamide and HBpin also displayed first-order
dependence on N,N-dimethyl benzamide (Figure FS127 in ESI), HBpin
(Figure FS 128 in ESI). Hence, the overall rate of the reaction gave rise
to the following kinetic rate equation:
Conclusions
We have demonstrated the synthesis of two aluminium complexes,
2a and 2b, stabilised by the quinoline-based amino-
phosphinechalcogenide ligand. The solid-state structure of complex
2b is established. Aluminium complex 2b was used as a pre-catalyst
for the catalytic chemo-selective reduction of 3° amides with HBpin
to afford corresponding amines in high yields at room temperature.
Catalyst 2b exhibited high conversion, superior selectivity, and broad
functional group tolerance during the reaction.
Experimental
General: All manipulations of air-sensitive materials were performed
with the rigorous exclusion of oxygen and moisture in flame-dried
Schlenk-type glassware either on a dual manifold Schlenk line,
interfaced to a high vacuum (10-4 torr) line or in an argon-filled M.
Braun glove box. Hydrocarbon solvents (toluene and n-pentane)
were distilled under nitrogen from LiAlH4 and stored in the glove box.
1H NMR (400 MHz) and 13C{1H} NMR (100 MHz) spectra were
recorded on a BRUKER AVANCE III-400 spectrometer. BRUKER ALPHA
FT-IR was used for FT-IR measurement. Elemental analyses were
performed on a BRUKER EURO EA at the Indian Institute of
Technology Hyderabad. All the amides were prepared according to
the published procedure.22 The NMR solvent C6D6 was purchased
from Sigma Aldrich. Crystallographic data for the structures reported
in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no. CCDC
1912620 (1a), 1912619 (1b) 1912621 (2b).
dp/dt = kobs [N,N-dimethylbenzamide]1. [HBpin]1. [2b]1
Preparation of [Ph2P(X)NC9H6N] [X = S (1a), Se (1b)]. In a dry 25 mL
Schlenk flask, a mixture of 8-aminoquinoline (500 mg, 3.472 mmol)
and 25 mL of toluene/THF was placed. To this solution,
chlorodiphenylphosphine (765 mg, 3.472 mmol) was added, after
which trimethylamine (527 mg, 5.208 mmol) was also added. The
resulting reaction mixture was stirred continuously for 12 hours at
room temperature. A white precipitate was obtained, which was
filtered, and the filtrate was collected into another dry Schlenk flask.
Elemental sulphur (166 mg, 5.208 mmol for ligand 1a) or selenium
(401 mg, 5.208 mmol for ligand 1b) was added to it and the reaction
mixture was heated at 90oC for 24 hours. Then the solution was
Scheme 2. Proposed mechanism for the selective reduction of
amides to amines.
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filtered and the solvent was evaporated under vacuum, resulting in
an oily compound which was washed with n-hexane. The title
compound was re-crystallised from THF/toluene at ‒35oC and
colourless crystals were obtained after three days.
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General procedure for catalytic aDluOmI: i1n0i.1u0m39/Cc9oDmT0p18le0x6A–
mediated reduction of amides with HBpin.
The required amide precursor (0.5649 mmol, 1 equiv.) was added to
the reaction mixture of HBpin (1.129 mmol, 2 equiv.) and 5 mol%
ligand 1a (22 mg, 0.02824 mmol). This was done in a 25 mL dry
Schlenk flask inside a glovebox. The colourless reaction mixture was
stored at room temperature or heated to 40–60°C, depending on the
nature of nucleophiles. After 12 hours, the reaction mixture was
quenched with a 4N HCl acid workup for four–five hours, and then
washed with dichloromethane two–three times. Subsequently, the
aqueous part of the reaction mixture was evaporated using a rotary
evaporator, and a colourless product was obtained. The products
1a: Yield: 1.20 g, 85%. 1H NMR (400 MHz, CDCl3, 25 ºC): δH 8.73 - 8.71
(dd, 1H 1JHH = 3.6, 1.2 Hz, Ar), 8.11 - 8.06 (m, 4H, Ar), 7.86 (d, 1H NH),
7.54 - 7.45 (m, 5H, Ar), 7.41 - 7.38 (m, 1H, Ar) 7.32 - 7.24 (m, 3H, Ar)
ppm. 13C{1H} NMR (100 MHz, CDCl3 ): C 147.9 (Ar), 139.2 (Ar), 137.3
(Ar), 136.3 (Ar), 133.0 (Ar), 131.6 (Ar), 128.9 (Ar), 126.8 (Ar), 121.7
(Ar) 119.2 (Ar), 114.0 (Ar), ppm. 31P{1H} NMR (161.9 MHz, CDCl3): δP
= 51.3 ppm. Elemental Analysis: C21H17N2PS (360.41): Calcd. C 69.98,
H 4.75, N 7.77. Found C 69.71, H 4.53, N 7.47.
1b: Yield: 1.34 g, 85%. 1H NMR (400 MHz, CDCl3, 25 ºC): δH 8.72 - 8.70
(dd, 1H 1JHH = 4.2, 1.5 Hz, Ar), 8.11 - 8.05 (m, 4H, Ar), 7.81 (d, 1H NH),
7.51 - 7.44 (m, 5H, Ar), 7.40 - 7.37 (m, 1H, Ar) 7.33 - 7.27 (m, 3H, Ar)
ppm. 13C{1H} NMR (100 MHz, CDCl3 ): C 147.9 (Ar), 139.2 (Ar), 137.3
(Ar), 136.3 (Ar), 133.0 (Ar), 131.6 (Ar), 128.9 (Ar), 126.8 (Ar), 121.7
(Ar) 119.2 (Ar), 114.0 (Ar), ppm. 31P{1H} NMR (161.9 MHz, CDCl3): δP
= 47.5 ppm. Elemental Analysis: C21H17N2PSe (407.30): Calcd. C 61.92,
H 4.21, N 6.88. Found C 61.58, H 4.11, N 6.64.
1
were identified according to H, 13C, and DEPT NMR spectroscopy
(where necessary), as well as MS analysis.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Preparation of [2-{Ph2P(X)NC9H6N}Al(Me)2] [X = S (2a), Se (2b)]. In
a dry 25 mL Schlenk flask, ligand 1a (200 mg, 0.555 mmol) or 1b (200
mg, 0.4914 mmol) and 5 mL of toluene were added. To this solution,
trimethylaluminium (40.04 mg, 0.555 mmol) (2a) or (35.4 mg, 0.4914
mmol) (2b) was added, and the resulting reaction mixture was stirred
continuously for 12 hours at room temperature. The solvent was
then evaporated under vacuum to produce a yellow-coloured
residue, which was dissolved in 3 mL of toluene and set aside for re-
crystallisation at ‒35oC. Yellow-coloured crystals were obtained after
three days.
This work was financially supported by the Science and Engineering
Research Board (SERB), Department of Science and Technology
(DST), India, under project no. (EMR/2016/005150). S.D. thanks
MHRD and J.B. thanks UGC for their Ph.D. fellowships. Instrumental
support was provided by the Indian Institute of Technology
Hyderabad.
Notes and references
1. Modern Amination Method; Wiley: New York, 2000. (b)
Modern reduction methods; P. G. Andersson and I. J.
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2a: Yield: 73 mg, 88%. 1H NMR (400 MHz, C6D6, 25 ºC): δH 8.21 - 8.15
1
(m, 4H, Ar), 7.69 - 7.68 (dd, 1H, JHH = 4.6 Hz, Ar), 7.31 - 7.24 (m, 1H
Ar), 7.15 - 7.11 (m, 1H, Ar), 7.06 - 7.00 (m, 7H, Ar) 6.84 - 6.80 (t, 1H,
Ar) 6.58 - 6.56 (d, 1H, 1JHH = 8.2 Hz, Ar), 6.43 - 6.40 (m, 1H, Ar), -0.18 2. a) R. C. Larock, in Comprehensive Organic
(s, 6H, CH3) ppm. 13C{1H} NMR (100 MHz, C6D6 ): C 143.8 (Ar), 139.7
(Ar), 135.3 (Ar), 134.4 (Ar), 132.5 (Ar), 131.3 (Ar), 129.5 (Ar), 128.9
(Ar), 128.4 (Ar) 127.9 (Ar), 125.5 (Ar), 121.3 (Ar), 115.4 (Ar), 67.7
(CH3), 25.6 (CH3), 21.2 (CH3) ppm. 31P {1H} NMR (161.9 MHz, C6D6): δP
= 64.6 ppm. Elemental Analysis: C27H30AlN2OPS (488.56): Calcd. C
66.38, H 6.19, N 5.73. Found C 66.11, H 5.89, N 5.51.
Transformations:
A
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2b. Yield: 84 mg, 88%. 1H NMR (400 MHz, C6D6, 25 ºC): δH 8.24 - 8.19
(m, 4H, Ar), 7.62 - 7.61 (d, 1H 1JHH = 4.7 Hz, Ar), 7.45 - 7.43 (m, 1H Ar),
7.26 - 7.24 (m, 1H, Ar), 7.02 (m, 6H, Ar) 6.84 - 6.80 (t, 1H, Ar) 6.58 -
6.56 (d, 1H 1JHH = 8.2 Hz, Ar) 6.43 - 6.40 (m,1H, Ar), -0.26 (s, 6H, CH3)
ppm. 13C{1H} NMR (100 MHz, C6D6 ): 143.7 (Ar), 139.9 (Ar), 134.7 (Ar),
132.8 (Ar), 131.3 (Ar), 129.5 (Ar), 128.9 (Ar), 128.4 (Ar) 127.9 (Ar),
121.3 (Ar), 118.1 (Ar), 115.6 (Ar), 67.7 (CH3), 25.6 (CH3) ppm. 31P{1H}
NMR (161.9 MHz, C6D6): δP = 58.2 ppm. Elemental Analysis:
C27H30AlN2OPSe (535.46): Calcd. C 60.56, H 5.65, N 5.23. Found C
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View Article Online
DOI: 10.1039/C9DT01806A
Aluminium complex as an efficient catalyst for chemo-selective reduction of
amides to amines
Suman Das,a Himadri Karmakar, a Jayeeta Bhattacharjeea and Tarun K. Panda*a
Graphical Abstract
Catalytic chemo-selective reduction of tert-amides with pinacolborane (HBpin) to furnish
corresponding tert-amines using earth abundant Al complex under solvent free, base free and mild
conditions is reported.
n
i
p
B
H
O
R1
R1
N
R2
N
R2
37 examples
Broad substrate scope
Yield up to 96%
Cheaper earth abundant Al metal
Solvent-free and base free condition
Mild reaction condition,
Chemo-selective reduction