Conversion of racemic alcohols to optically pure amine precursors enabled by catalyst dynamic kinetic resolution: Experiment and computation

Article abstract of DOI:10.1039/d0cc02881a

An unprecedented base metal catalysed asymmetric synthesis of α-chiral amine precursors from racemic alcohols is reported. This redox-neutral reaction utilises a bench-stable manganese complex and Ellman's sulfinamide as a versatile ammonia surrogate. DFT

Full text of DOI:10.1039/d0cc02881a

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DOI: 10.1039/D0CC02881A
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Racemic alcohols to optically pure amine precursors enabled by
catalyst dynamic kinetic resolution: experiment and computation
Luis Miguel Azofra,*a Mai Anh Tran,b Viktoriia Zubar,b Luigi Cavallo,c Magnus Ruepingbc and
Received XXth XXXX 2020,
Accepted XXth XXXX 20XX
Osama El-Sepelgy*b
DOI: 10.1039/X0XX00000X
An unprecedented base metal catalysed asymmetric synthesis of α-
The direct asymmetric amination of alcohols to produce
chiral amine precursors from racemic alcohols is reported. This chiral aniline,3 amino alcohols,4 oxazolidinones,5 and
redox-neutral reaction utilises a bench-stable manganese complex hydrazones6 has been demonstrated using ruthenium and
and Ellman’s sulfinamide as versatile ammonia surrogate. DFT iridium catalysis. In contrast, the direct production of chiral
calculations explain the unusual finding of the highly primary amines from racemic alcohols is significantly more
stereoselective transformation enabled by
undergoes an unusal dynamic kinetic resolution.
a
catalyst that challenging. In this regard, ruthenium catalysed protocol was
disclosed. However, this noble metal method is limited to the
methyl substituted chiral amines.7 Apart from metal catalysis,
Chiral primary amines are of great importance in
pharmaceutical and agrochemical industries. In fact, at least
40% of the optically active drugs are chiral amines. Biocatalytic
kinetic resolution of racemic amines with 50% maximum yield
is probably the most widely practiced method for the
production of enantiopure primary amines. Additionally,
hydrogenation using man-made catalysis (transition-metal
catalysis and organocatalysis), which require hydrogen gas
the use of a dual enzymatic system for the direct amination of
secondary alcohols has been reported.8 Therefore, the
development of a new catalytic system, ideally based on a non-
precious metal catalyst,9 which can transform a broad range of
racemic alcohols to enantiopure primary amines, is an elusive
goal.10
Ellman’s sulfinamide represents as an industrially relevant
reagent, frequently used as an ammonia surrogate in the
synthesis of enantiopure primary amines.11 This transformation
typically requires three chemical steps, i.e., stoichiometric
oxidation, condensation using stoichiometric titanium reagent
and stoichiometric reduction, producing large quantities of
waste.
under higher pressure or stoichiometric amounts of
a
reductant, often involve the use of hard to remove protecting
groups. Besides, methods involving chemoenzymatic dynamic
kinetic resolution, cascade deracemization of racemic amines
have been reported.1
The hydrogen autotransfer (HA) strategy has captured much
attention during the last few years, mainly due to its synthetic
importance as a powerful environmentally benign approach for
the construction of C–C and C–N bonds. In this regard, progress
has been made in transition-metal catalysed C-alkylations and
N-alkylations with non-activated alcohols. The reactions mainly
rely on the production of achiral or racemic products.2
Encouraged by recent studies on metal catalysed hydrogen
autotransfer,2,12,13 we envisioned that an effective base-metal
catalyst might potentially catalyse the oxidative
dehydrogenation of the sec-alcohols (rac-1), to form the
corresponding ketone and shuttle the abstracted hydrogen
required for the stereoselective hydrogenation. The in situ
formed ketone undergoes condensation with an ammonia
equivalent to produce an imine and only water as a by-product.
Finally, the imine will be reduced in a stereoselective fashion to
produce the desired product. The chemoselectivity of the
reaction depends on the potential of the metal catalyst to
return the stored hydrogen to the final product.
a. Instituto de Estudios Ambientales y Recursos Naturales (i-UNAT), Universidad de
Las Palmas de Gran Canaria (ULPGC). Campus de Tafira, 35017, Las Palmas de Gran
Canaria, Spain.
E-mail: luismiguel.azofra@ulpgc.es
Herein, we present a new manganese catalysed synthesis of
optically active amines from racemic alcohols. Interestingly, our
DFT mechanistic study on this stereoselective reaction explains
how a racemic catalyst could lead to optically pure product.
Notably, we found that the high stereocontrol is enabled by
unusual catalyst dynamic kinetic resolution (Scheme 1a). The
b. Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074
Aachen, Germany.
E-mail: Osama.Elsepelgy@rwth-aachen.de
c. KAUST Catalysis Center (KCC), King Abdullah University of Science and Technology
(KAUST), Thuwal 23955-6900, Saudi Arabia.
Electronic Supplementary Information (ESI) available: Full experimental and
computational
details
and
optimised
Cartesian
coordinates.
See
DOI: 10.1039/X0XX00000X
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chiral imine intermediate kinetically discriminates the catalyst the sec-phenethyl alcohol derivatives. Thus, 1-tetralol (1k) was
View Article Online
racemic mixture (Scheme 1b). While only one enantiomer of the converted to the desired product in 84%DyOieI:l1d0..1T0h3i9s/Da0mCiCn0e2i8s8o1Af
catalyst can be involved in the stereoselective hydrogenation relevance as it is used in the synthesis of diverse bio-related
reaction, the ketone intermediate plays the crucial role of the compounds.16 The ethyl substituted alcohol 1l and the more
racemisation of second catalyst enantiomer (Scheme 1a). We challenging alcohol 1m were found to be reactive using the
are not aware of a literature report with such unusual presented catalytic system. With this success, we turned our
observation.
attention to the more demanding non-benzylic alcohols 1n-1q.
The chiral amines bearing a cyclohexyl substituent (R,Rs)-3n and
cyclopropyl substituent (R,Rs)-3p were produced in very good
diastereomeric ratio as well. Interestingly, the amphetamine,
which is used in the treatment of attention deficit hyperactivity
disorder, can be obtained from the rac-1q in 78% yield.
Heterocycles-containing alcohols were also tolerated and
underwent reaction to afford the chiral amines precursors
(R,Rs)-3r to (R,Rs)-3u in very good yields and with high
diastereoselectivity. Interestingly, despite the importance of
the optically pure 3s and 3r in the synthesis of HIV protease
inhibitors,17 the asymmetric synthesis of the corresponding
primary amines is not yet reported. Noteworthy, even pyridine
containing substrates and products, such as 3u are tolerated in
this manganese catalysed reaction.
a) ketone enables, catalyst racemization
CO
PPh2
Mn
Br
P
N
H
catalyst activation
base
Mn
CO
CO
N
N
N
CO
CO
16e specie
alcohol
alcohol
H
H
O
H
H
(R)
P
N
N
N (S)
Mn
CO
CO
Mn
N
OC
OC
racemization
P
C(S)
C(R)
b) stereoselective hydrogenation enabled by catalyst DKR
(RS
)
N
(RS
)
HN
(R)
C(S)
+
C( )
R
C(S)
C(R)
+
inactive
Table 1. Manganese catalysed of asymmetric amination of
sec-alcohols.a
active
-16e specie
N
(SS
)
CO
Br
(SS
)
O
S
HN
H
PPh2
O
S
OH
(S)
+
C(S)
+
N
C(R)
HN
R1
tBu
R2
[Mn], Cs2CO3
R2
-16e specie
active
inactive
Mn
+
H2
N
tBu
R1
+
H2
O
140 ºC, 16 h
CO
N
rac-1
CO
(Rs)-2
(R,R)-3
Scheme 1. Manganese-catalysed diasteroselective hydrogen
O
S
autotransfer.
O
O
O
S
S
S
HN
HN
HN
HN
Cl
Me
Me
Me
Me
We started our study with the selection of the appropriate
base metal catalyst and the optimisation of the suitable
reaction parameters. After careful investigations, we found that
the use of the air stable PNN-Mn complex (5 mol%) in
combination with Cs2CO3 (10 mol %) and t-amyl alcohol (0.5 M)
is the optimal combination for this reaction (see Supporting
Information for details).
Cl
F
(R,Rs)-3a
85%, 99:1 dr
(R,Rs)-3db
(R,Rs)-3b
80%, 98:2 dr
(
R Rs
)-3c
,
65%, 98:2 dr
74%, 97:3 dr
O
S
O
O
S
O
S
HN
S
HN
HN
HN
Me
MeO
O
Me
Me
Me
MeO
tBu
O
(R,Rs)-3e
87%, 97:3 dr
(R,Rs)-3f
85%, 99:1 dr
(R,Rs)-3h
95%, 98:2 dr
(R,Rs)-3g
Next, the variability and the applicability of the asymmetric
hydrogen autotransfer reaction were investigated (Table 1). We
initially explored different racemic benzylic alcohols. The
alcohols 1a-1i bearing different electron donating and electron
withdrawing groups were applicable without significant effect
on the reactivity or the stereochemical outcome and all desired
products 3a-3i were isolated in very good yields and optical
purity. Similarly, the naphthyl substituted sulfinamide (R,Rs)-3j
was isolated in very good yield and selectivity. Importantly,
some of these amines are key intermediates in the synthesis of
pharmaceuticals and bioactive molecules. For example,
Carpropamid,14 an agriculture fungicide, can be prepared from
(R,Rs)-3b, whereas the Alzheimer’s and Parkinson’s drug
Rivastigmine can be produced using the sulfinylamine (S,Ss)-
3g.15 Noteworthy, the classical synthesis of these α-chiral
amines involves the addition of MeLi to N-tert-butylsulfinyl
aldimes. However, this reaction often suffers from
unfavourable diastereocontrol, even at lower temperature. To
our delight, our catalytic system was found not to be limited to
83%, 98:2 dr
O
S
O
S
O
O
S
HN
HN
S
HN
HN
Me
Me
Et
(R,Rs)-3ic
(R,Rs)-3j
76%, 98:2 dr
(R,Rs)-3lb
(R,Rs)-3kc
84%, 94:6 dr
O
70%, 95:5 dr
80%, 94:6 dr
O
O
O
S
S
HN
S
S
HN
HN
HN
3
Me
Me
Me
Me
(R,Rs)-3mb,d
(R,Rs)-3n
55%, 93:7 dr
(R,Rs)-3ob,d
74%, 82:18 dr
(R,Rs)-3pc
86%, 97:3 dr
89%, 93:7 dr
O
O
O
O
S
S
S
HN
S
HN
HN
HN
Ph
Me
Me
Me
S
X
Cl
N
(R,Rs)-3rb,d
86%, 96:4 dr
X = S, (R,Rs)-3sb,d 90%, 92:8 dr
X = O, (R,Rs)-3tb,d, 83%, 80:20 dr
(R,Rs)-3q
78%, 84:16 dr
(R,Rs)-3ub,d
65%, 95:5 dr
aReaction conditions: 1 (0.75 mmol), (R)-2a (0.5 mmol), [Mn] (0.025
mmol) and Cs2CO3 (0.05 mmol) in t-amyl alcohol (1 mL) were stirred at
140 °C (aluminum block), for 16h in a glass tube under argon. Yields
2 | Chem. Commun., 2020, XX, 1-4
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after column chromatography are given. b [Mn] (0.05 mmol), Cs2CO3 (0.1
mmol). c 48 h. d 1 (1 mmol).
View Article Online
DOI: 10.1039/D0CC02881A
(RS
)
(RS
)
NH
H
NH
H
(S)
(R)
Ph
Ph
C(S)-D(S,S)
36.3
—2.9
—1.0
C(R)-D(R,R)
29.1
Ph
Ph
H
H
H
H
(RS
)
(RS
)
(RS
)
C(S)-D(S,R)
39.5
(RS
H
)
C(R)-D(R,S)
33.1
N
Ph
N
Ph
N
N
H
H
H
(S)
N
P
(S)
N
P
N
N
N
N
Mn
Mn
CO
Mn
Mn
CO
CO
CO
CO
CO
(RS
)
CO
P
N
P
N
CO
(RS
)
N
D(R,R)
19.5
D(S,R)
19.3
D(S,S)
15.1
D(R,S)
19.9
N
Ph
Ph
OH
Ph
Ph
H
N
H
N
H
H
H
H2
O
N
P
Ph
P
N
Mn
Mn
Mn
CO
CO
CO
CO
CO
O
P
N
OH
CO
N
(RS
)
NH2
C(R)
5.1
C(S)
5.1
A
0.0
Ph
Ph
H
O
O
Ph
H
Ph
Ph
Ph
H
OH
H
H
O
O
H
N
O
H
Ph
H
O
Ph
Ph
H
B(S,S)-C(S)
P
H
N
A-B
—
A-B
—
P
B(R,R)-C(R)
20.1
N
20.1
(S)
N
Mn
CO
CO
1a
(S)
N
Mn
N
CO
CO
Mn
N
B(S,R)-C(S)
Mn
N
CO
CO
B(R,S)-C(R)
19.9
CO
CO
19.9
P
P
O
B(S,R)
4.1
B(R,S)
4.1
B(S,S)
1.9
B(R,R)
1.9
(RS
)
=
S
(R)
tBu
Scheme 2. Proposed reaction mechanism for the stereoselective manganese catalysed hydrogen autotransfer amination of rac-
alcohols. Free reaction and activation energies (at 140 C) are shown in kcal mol–1 at M06/TZVP level using toluene as solvent.
In order to understand the origin of the stereoselective of (S) stereogenic centre is 33.1 kcal mol–1. On the other hand,
hydrogenation, we have carried out a DFT modelling for the the barriers for the hydride transfer using the catalyst
asymmetric amination of 1-phenylethanol (Scheme 2). The 16e enantiomer C(S) are 39.5 and 36.3 kcal mol–1. Interestingly,
species A, which can be generated by treating the manganese when an imine bearing (S)-sulfinamide group is used as a
pre-catalyst [Mn] with appropriate base, dehydrogenates the substrate, only the (S) enatiomer of the catalyst is involved in
alcohol substrate rac-1a to produce acetophenone and the imine hydrogenation step and creates new stereogenic with
manganese hydride complex C. Based on our DFT results, the (S) configuration. With this process being controlled by kinetics,
alcohol dehydrogenation step takes place in a stepwise fashion it supposes a diasteoisomeric ratio equal to 99:1. That is in fully
via the formation of four different diastereoisomers of agreement with our experimental results.
manganese alkoxide intermediates B, bearing chiral nitrogen
The last step involves the proton transfer to the product
and carbon atoms. However, the proton transfer A-B is a nitrogen atom. The proton could be internally transfer from the
barrier-less process. The calculated free activation energy for catalyst NH group to generate the product and the 16e species
the hydride transfer B-C steps are between 19.9 and 20.1 kcal A. Alternatively, in the presence of excess rac-1a, direct proton
mol–1 at the M06/TZVP level of theory. The β-hydride transfer from the alcohol substrate could take place to produce
elimination will lead to the formation of racemic mixture of the the desired product and the intermediates B without the
hydrogenated catalyst C(S) and C(R).
regeneration of the 16e species A.18 Notably, calculations show
The condensation reaction between the in situ generated that the undesired catalyst enantiomer C(S) can be easily
ketone and (Rs)-2 results in the generation of a C=N bond which racemised with the assist of the acetophenone intermediate. In
can be potentially hydrogenated by the action of racemic other words, the steps C(S)-B(S,R), C(S)-B(S,S), B(S,S)-A and
manganese catalyst C(S) and C(R). Similarly, to the alcohol B(S,R)-A are reversible. We are not aware of previously
substrate dehydrogenation, we found that the hydrogenation reported mechanism with similar observation of the catalyst
of the imine intermediate takes place via
mechanism. Since the hydrogenated catalyst exists as a racemic
a
stepwise racemisation assisted by in situ generated intermediate.
In conclusion, we have developed an unprecedented base-
mixture that establishes the possibility of the hydride transfer metal catalysed stereoselective amination of racemic alcohols
(C-D) to form four different intermediates D. Importantly, we using the hydrogen autotransfer strategy. The produced
found that the hydride transfer step (C-D) is the rate enatiomerically pure sulfinamides could be easily converted to
determining step as well as the stereodetermining step. When the corresponding α-chiral amine upon stirring in methanolic
the manganese catalyst C(R) was used for the hydrogenation of HCl at room temperature.19 Notably, protocol uses inexpensive
an imine bearing (R)-sulfinamide group, the computed barrier earth-abundant manganese complex and readily available
for the hydride transfer is 29.1 kcal mol–1 for the creation of a substrates. Our DFT calculations demonstrate the origin of high
new (R) stereogenic centre, while the barrier for the generation diastereoselectivity which is enabled by an unusual catalyst
This journal is © The Royal Society of Chemistry 2020
Chem. Commun., 2020, XX, 1-4 | 3
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
L.M.A thank Universidad de Las Palmas de Gran Canaria
(ULPGC) for support. Gratitude is also due to the KAUST for
using the supercomputer Shaheen II for providing the
computational resources.
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4 | Chem. Commun., 2020, XX, 1-4
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