Mild dynamic kinetic resolution of amines by coupled visible-light photoredox and enzyme catalysis

Article abstract of DOI:10.1039/c8cc07990k

Herein, we described photoenzymatic dynamic kinetic resolution (DKR) of amines under mild conditions. The racemization of amines via a photoredox-mediated hydrogen atom transfer (HAT) protocol in conjunction with an enzyme catalyst to achieve the DKR of amines allows a variety of primary amines to be converted into a single enantiomer in high yield and with excellent enantioselectivity. Notably, this protocol can also be extended to 1,4-diamine derivatives with high levels of diastereo- and enantioselectivity.

Full text of DOI:10.1039/c8cc07990k

Showcasing research from the group of Professor Shaolin Zhou,
Key Laboratory of Pesticide & Chemical Biology, Ministry of
Education, CCNU-uOttawa Joint Research Centre, College of
Chemistry, Central China Normal University (CCNU), Wuhan,
P.R. China.
As featured in:
Mild dynamic kinetic resolution of amines by coupled
visible-light photoredox and enzyme catalysis
A mild and efficient dynamic kinetic resolution (DKR) of amines
was achieved by combining visible-light-induced photoredox
catalysis and enzyme catalysis. This dual catalytic system was
appropriate for both monoamines and 1,4-diamines.
See Shaolin Zhou et al.,
Chem. Commun., 2018, 54, 14065.
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Mild dynamic kinetic resolution of amines by
coupled visible-light photoredox and enzyme
catalysis†
Cite this: Chem. Commun., 2018,
4, 14065
5
Received 8th October 2018,
Accepted 31st October 2018
Qiong Yang,‡ Fengqian Zhao,‡ Na Zhang, Mingke Liu, Huanhuan Hu,
Jingjie Zhang and Shaolin Zhou
*
DOI: 10.1039/c8cc07990k
rsc.li/chemcomm
Herein, we described photoenzymatic dynamic kinetic resolution use of thiyl radicals was reported by the research groups of Gil
1
1
(
DKR) of amines under mild conditions. The racemization of amines and Bertrand. Notably, this method has been used for the
via a photoredox-mediated hydrogen atom transfer (HAT) protocol DKR of a series of aliphatic amines and resulted in excellent ee
in conjunction with an enzyme catalyst to achieve the DKR of values and good yields under very mild reaction conditions
amines allows a variety of primary amines to be converted into a (38–40 1C). However, the requirement of a large amount of
single enantiomer in high yield and with excellent enantioselec- expensive and explosive AIBN as a radical initiator or/and high-
tivity. Notably, this protocol can also be extended to 1,4-diamine energy ultraviolet irradiation may limit the broad application of
derivatives with high levels of diastereo- and enantioselectivity.
this method. Therefore, a mild, sustainable and efficient DKR
protocol appropriate for a broad spectrum of substrates is of
Chiral amines are an integral part of numerous natural products great interest.
and bioactive molecules, and they are utilized as chiral ligands,
Asymmetric photocatalysis induced by visible light is an
catalysts and kinetic resolution reagents in organic synthesis and emerging strategy for the sustainable production of enantio-
1
12
catalysis. A variety of effective methods, including the asymmetric merically pure compounds. In addition to single chiral photo-
2
13–15
hydrogenation of imines and enamines, the asymmetric alkyla- catalysts,
tion of imines, the asymmetric hydroamination of alkenes, and at least an achiral photoredox catalyst and a chiral cocatalyst,
kinetic resolution, have been well documented in the literature; have found wide applications in this field. To date, chiral organic
however, enzymatic kinetic resolution (KR) of racemic amines small molecules, chiral Lewis acids, and chiral transition-metal
remains the most common approach to accessing enantiopure complexes have been frequently employed as cocatalysts to
amines, especially for larger scale preparations. Unfortunately, improve the stereoselectivity. Compared to chemical catalysts,
dual- and multi-catalyst systems, which consist of
3
4
5
16
17
18
6
this method suffers from a maximum yield of 50%. An attractive biocatalysts often feature higher regioselectivity and stereoselectivity
strategy circumventing this limitation by combining the kinetic and therefore have great potential as cocatalysts in asymmetric
resolution process with in situ racemization is dynamic kinetic photocatalysis. Despite their simplicity, however, such a combi-
resolution (DKR).
nation remains challenging because these catalysts often operate
7
Despite tremendous efforts over the past decade, the DKR under different conditions. Progress in this field was limited until
of amines has been largely hampered by the lack of efficient recently. Earlier this year, the synthesis of enantiomerically pure
amine racemization catalysts. At present, most research has 1,3-mercaptoalkanols was described through a one-pot photo-
8
9
19
focused on using transition metals, such as Ru, Pd, Co and enzymatic cascade reaction. In 2018, Hartwig and coworkers
1
0
Ni, as amine racemization catalysts. However, most of these highlighted the combination of an Ir-based or flavin mononucleotide
reactions require elevated temperatures (above 70 1C), which (FMN) photocatalyst with an ene-reductase for the asymmetric
2
0
are incompatible with several functional groups. An interesting reduction of E/Z mixtures of alkenes. In 2018, Wenger and
radical approach for the racemization of amines through the coworkers demonstrated a water-soluble Ir-based photocatalyst
in combination with the enzyme monoamine oxidase to convert
2
1
Key Laboratory of Pesticide & Chemical Biology, Ministry of Education,
CCNU-uOttawa Joint Research Centre, College of Chemistry,
Central China Normal University, 152 Luoyu Road, Wuhan, Hubei 430079, China.
E-mail: szhou@mail.ccnu.edu.cn
cyclic imines into chiral cyclic amines.
Intrigued by these results and with our interest in chemo-
enzymatic DKR reactions, herein, we present our preliminary
results on the racemization of amines via a photoredox-mediated
HAT protocol in combination with enzymatic resolution to achieve
the DKR. This system was appropriate for both monoamines and
22
†
Electronic supplementary information (ESI) available: Experimental proce-
dures, isolated products’ characterization data. See DOI: 10.1039/c8cc07990k
These two authors contributed equally to this work.
‡
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a
Table 1 Optimization of the racemization conditions
Table 2 Optimization of the DKR
Entry
Variation from the standard conditions
Ir(ppy) (dtb-bpy)PF
ee (%)
1
2
3
4
5
6
7
2
6
4
À1
Entry
N-435 mg mmol
Yield (%)
ee (%)
Ru(bpy) Cl instead of Ir(ppy)
fac-Ir(ppy) instead of Ir(ppy)
Ir(ppy) (bpy)PF instead of Ir(ppy)
Toluene instead of CH CN
3
2
2
(dtb-bpy)PF
(dtb-bpy)PF
6
83
99
99
72
69
30
94
94
94
1
2
3
50
100
200
47
64
98
99
99
99
3
2
6
2
6
2
(dtb-bpy)PF
6
3
Cyclopentyl methyl ether instead of CH
3
CN
a
General reaction conditions: amine rac-1a (0.4 mmol), photocatalyst
Without 4 Å-MS
b
b
2 6
Ir(ppy) (dtb-bpy)PF (2 mol%), n-OctSH (50 mol%), Novozym 435, b-methoxy-
8
9
1
Without Ir(ppy)
2
(dtb-bpy)PF
6
propionate (2 equiv.), 4 Å-MS (400 mg) and MeCN (4 mL) stirred in a Schlenk
tube irradiated by a white LED lamp (32 W) at 38 1C under an Ar atmosphere
for 24 h. The ee was determined by chiral HPLC.
Without n-OctSH
Without light
b
0
a
General reaction conditions: amine (S)-1a (99% ee, 0.4 mmol), photo-
(dtb-bpy)PF (2 mol%), n-OctSH (50 mol%), 4 Å-MS
400 mg) and MeCN (4 mL) stirred in a Schlenk tube irradiated by a
catalyst Ir(ppy)
2
6
(
corresponding chiral amide (R)-3a in 99% ee and 98% yield
(Table 2, entry 3).
white LED lamp (32 W) at 38 1C under an Ar atmosphere for 24 h. The ee
b
was determined by chiral HPLC. (S)-1a with 94% ee was used.
Under the optimized conditions, the generality of this
cooperative catalyst system was studied in more detail. Various
the more challenging 1,4-diamines, and they could be converted racemic secondary aliphatic amines (see the ESI†) were
into a single enantiomer in high yield and with excellent enantio- employed and provided the corresponding chiral amides in
selectivity.
high yields and with excellent enantioselectivities (Table 3).
We commenced our study by evaluating the visible-light Arene-substituted amines (1a to 1m), bearing electron-withdrawing
photoredox-mediated racemization of (S)-4-phenyl-2-butanamine and electron-donating moieties in the para, meta and ortho
((S)-1a) under white LED irradiation (32 W) at 38 1C. A variety of positions, gave the corresponding amides in moderate to good
photocatalysts, HAT catalyst thiols and their respective loadings yields (61–97%) and with excellent enantiomeric excesses
were investigated in acetonitrile. We found that the use of 2 mol% (all 99%) (3a to 3m); however, substrates bearing a p-methoxy
of Ir(ppy)
Å molecular sieves (4 Å-MS) led to complete racemization and 61%, respectively). The DKR of long chain aliphatic amines
Table 1, entry 1). Replacing this photocatalyst with other photo- (1o to 1r) also proceeded very well and gave the corresponding
catalysts, such as Ru(bpy) Cl , fac-Ir(ppy) , and Ir(ppy) (bpy)PF products in moderate to good yields (62–84%) and with excellent
2 6
(dtb-bpy)PF and 50 mol% of n-OctSH in the presence of (1f) and p-Cl (1m) group showed slightly decreased yields (64%
4
(
3
2
3
2
6
,
resulted in very low or no activity (Table 1, entries 2–4). Replacing enantiomeric excesses (all 99%); however, short chain sec-butyl-
acetonitrile with other solvents (such as toluene and cyclopentyl amine 1n afforded a decreased yield (58%) and ee (73%). Further-
methyl ether) showed inferior results (Table 1, entries 5 and 6). more, 2-cyclohexylethylamine (1s) was smoothly converted to the
Notably, the racemization rate decreased in the absence of 4 Å-MS corresponding amide in 84% yield and with 99% ee.
(
Table 1, entry 7). Although the exact role of 4 Å-MS in this reaction
Having demonstrated the high efficiency of our protocol for
23
remains unclear, considering a previous report and our results the synthesis of enantiomerically pure monoamides, we were
together, a plausible explanation for the positive effect of 4 Å-MS is keen to know if our protocol was applicable to the synthesis of
the coordination between the 4 Å-MS (Al in the framework) and diastereo- and enantiomerically pure acyclic diamines, especially
the amines to accelerate the generation of the a-amino radical. 1,4-diamines, which are key structural units in many biologically
Finally, no racemization occurred in the absence of either the active compounds [notable examples include lopinavir (an HIV
photocatalyst (Ir(ppy) (dtb-bpy)PF ), HAT catalyst (n-OctSH), or protease inhibitor) and cobicistat (a potent selective inhibitor of
light (Table 1, entries 8–10). These results demonstrated that human CYP3A) ]. The diastereo- and enantioselective synthesis of
the photocatalyst, thiol and light were essential for the amine 1,4-diamines is very challenging, and there are relatively few
3+
24
2
6
25
racemization.
methods available. The general synthetic route to these structural
26
Having established the optimized racemization conditions, units usually involves tedious multi-step reactions (5 or more).
we tried to combine this system with enzymatic resolution via Thus, the development of an economic and efficient catalytic
acylation using Candida antarctica lipase B (Novozym 435) as protocol for the preparation of these diastereo- and enantiomeri-
the enzyme catalyst and methyl b-methoxypropionate as the cally pure acyclic 1,4-diamines is of great interest. We set out to
acyl donor (Table 2). To our delight, the initial results showed investigate the DKR of hexane-2,5-diamine (4). Gratifyingly, the
good compatibility among the photoredox catalyst, the HAT DKR of hexane-2,5-diamine (4, dl :meso = 1 : 1) occurred smoothly
catalyst and the enzyme catalyst (Table 2, entry 1). After careful to afford corresponding diamide 5 in 64% isolated yield with
evaluation of the loadings of Novozym 435 and the acyl donor, a excellent diastereo- and enantioselectivity. To the best of our
complete conversion of rac-1a was achieved, affording the knowledge, this is the first report of the DKR of 1,4-diamines,
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Table 3 Scope of primary amines
Scheme 1 Proposed catalytic cycles for amine racemization.
photocatalyst, Ir(ppy) (dtb-bpy)PF , would generate a photoexcited
2
6
III/II
complex, which could be reduced (E1/2*
MeCN) by the thiol catalyst n-OctSH to produce Ir (ppy) (dtb-bpy)
2
= +0.66 V vs. SCE in
27
ll
as well as thiyl radical 6. At this stage, owing to the polarity-
matching effect in the HAT, the electrophilic thiyl radical 6 could
easily abstract a hydrogen atom from an electron-rich a-C–H bond
of (S)-1a to afford a-amino radical 7 and n-OctSH [(a comparable
amine, isopropylamine, has an a-C–H bond dissociation energy =
À1
À1 28
3
8
82.8 kJ mol ; a comparable thiol, BuSH, has an S–H BDE = 370 Æ
.4 kJ mol ) ]. Reverse hydrogen transfer from n-OctSH to
a-amino radical 7 affords racemic amine rac-1a. Thiyl radical 6 then
ll
oxidizes the reduced catalyst Ir (ppy)
2
(dtb-bpy) and is protonated to
III/II
29
turn over the photoredox catalytic cycle [(E1/2 = À1.51 V vs. SCE;
a
red
28
comparable thiol, cysteine, has E1/2(thiol) = À0.85 V versus SCE) ].
In conclusion, we have developed a mild photoenzymatic
protocol for the DKR of amines that allows a variety of primary
amines to be converted into a single enantiomer in high yield
and with excellent enantioselectivity. Remarkably, this method
can also be extended to 1,4-diamine derivatives and achieve
high diastereo- and enantioselectivities. Moreover, we have provided
insight into the mechanism of the amine racemization through
deuterium labelling studies. Further studies on the applica-
a
General reaction conditions: amine 1a–1s (0.4 mmol), acyl donor
(dtb-bpy)PF (2 mol%), n-OctSH (50 mol%),
Novozym 435 (80 mg), 4 Å-MS (400 mg) and MeCN (4 mL) stirred in a
(2 equiv.), photocatalyst Ir(ppy)
2
6
Schlenk tube irradiated by a white LED lamp (32 W) at 38 1C under an tions of the photoenzymatic protocol to other synthetically
b
Ar atmosphere for 2 d; isolated yields are given. The ee was determined by
c
d
interesting substrates are ongoing in our laboratory.
chiral HPLC. The ee was determined by chiral GC. Acyl donor (7 equiv.),
e
We are grateful for the financial support from the National
Novozym 435 (120 mg), 2.5 d. Acyl donor (7 equiv.), Novozym 435 (120 mg),
f
g
3
(
d. Acyl donor (7 equiv.), Novozym 435 (200 mg), 6 d. Acyl donor Natural Science Foundation of China (no. 21372091), Research
7 equiv.), Novozym 435 (160 mg), 3 d.
Funds for the Central Universities (no. CCNU18TS2041) and the
11 Project (B17019). We also thank Prof. Howard Alper from
and we believe our protocol provides an efficient route to access the Department of Chemistry and Biomolecular Sciences, Univer-
diastereo- and enantiomerically pure 1,4-diamines. sity of Ottawa, and Profs Wen-Jing Xiao, Liang-Qiu Lu and Jia-Rong
1
A deuterium labelling study was performed to elucidate the Chen all from CCNU, for the helpful discussions.
racemization mechanism. As thiols have recently been identified
as excellent hydrogen atom donors for intercepting carbon radicals
in various radical reactions, we envision that deuterium atom
Conflicts of interest
abstraction from a deuterated thiol by the proposed key a-amino
radical intermediate would yield the corresponding deuterated
There are no conflicts to declare.
product. Therefore, a deuterium labelling study was conducted Notes and references
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1
2
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14068 | Chem. Commun., 2018, 54, 14065--14068
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