Acid-Assisted Ru-Catalyzed Enantioselective Amination of 1,2-Diols through Borrowing Hydrogen

Article abstract of DOI:10.1021/acscatal.6b02959

Here, we present a highly enantioselective synthesis of 1,2-amino alcohols from readily available racemic 1,2-diols through a borrowing hydrogen process. An intriguing acid effect was discovered for this Ru-catalyzed amination reaction, which led to a sig

Full text of DOI:10.1021/acscatal.6b02959

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Letter
Acid-Assisted Ru-Catalyzed Enantioselective
Amination of 1,2-Diols through Borrowing Hydrogen
Li-Cheng Yang, Ya-Nong Wang, Yao Zhang, and Yu Zhao
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02959 • Publication Date (Web): 23 Nov 2016
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Acid-Assisted Ru-Catalyzed Enantioselective Amination of
1,2-Diols through Borrowing Hydrogen
Li-Cheng Yang,† Ya-Nong Wang,† Yao Zhang,*,§ and Yu Zhao*,†
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†Department of Chemistry, National University of Singapore, 3 Science Drive 3, Republic of Singapore, 117543
§College of Chemistry, Liaoning University, Shenyang, 110036, People’s Republic of China
ABSTRACT: We present here a highly enantioselective synthesis of 1,2-amino alcohols from readily available racemic 1,2-
diols through a borrowing hydrogen process. An intriguing acid effect was discovered for this Ru-catalyzed amination
reaction, which led to a significant improvement of the stereoselectivity of the process. Preliminary mechanistic studies
suggest a scenario of Brønsted acid-assisted dynamic kinetic asymmetric amination of alcohols.
KEYWORDS: amination, borrowing hydrogen, acid catalysis, amino alcohol, enantioselectivity
Due to the widespread use of chiral β-amino alcohols in
asymmetric chemical synthesis as well as in fine chemi-
cals and pharmaceutical industry,1 the efficient synthesis
of them in enantiopure form has been an important target
for organic chemists.2 In addition to the subclass that is
easily accessible from reduction of α-amino acids, several
synthetic protocols have also been developed to access
other β-amino alcohols with good stereocontrol, includ-
ing asymmetric aminohydroxylation of alkenes,3 ring-
opening of epoxides by amines4 and resolution of the ra-
cemic starting compounds.5 However, highly efficient and
selective methods are still highly desired for the access of
this important class of compounds.
branched alcohols to prepare diastereo- and enantiopure
α-branched amines.11c It is important to note that essen-
tially all previous catalytic systems for amination of alco-
hols were carried out under basic or neutral conditions. In
our system, on the other hand, the acid co-catalyst played
a key role for promoting effective imine condensation as
well as the reduction of imine, and the chirality of the
acid was crucial in achieving high stereoselectivity of the
process.
Amination of alcohols through borrowing hydrogen
methodology has received much attention as a signifi-
cantly atom-economical and environmentally friendly
synthetic strategy for amine synthesis.6 In this overall
redox-neutral process, the alcohol substrate is converted
directly to the amine product without the need for any
extra stoichiometric reagent, and water is generated as
the sole side product. Great process has been made in this
field of research from the efforts of many research
groups.7-10 The development of asymmetric variants of this
process, however, only emerged in the past few years.11
Our group reported the first enantioselective amination
of secondary alcohols catalyzed by iridium and chiral
phosphoric acid (Scheme 1a),11b and also applied the sys-
tem to a highly stereo-convergent amination of α-
Scheme 1. Initial Attempts at Amination of Diols
In an effort to extend the utility of this catalytic system,
we were particular attracted to the amination of diols to
access diamines which are important motifs in asymmet-
ric synthesis. Initial attempts to obtain the di-amination
of secondary diols such as meso-hydrobenzoin, unfortu-
nately, met with limited success; only mono-amination
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took place to yield the amino alcohol with low efficiency
11
Ru-2
Ru-3
4a
4a
1a
1a
75
81
89:11
89:11
and selectivity (Scheme 1b). The amination of primary,
secondary diols such as 2 (Table 1) was pioneered by the
group of Oe and Ohta using Ru-based catalyst; similarly,
only amination of the primary alcohol took place to yield
the amino alcohol products.11j The enantioselectivity of
the reaction, however, was also far from synthetically use-
ful level. Very recently the Beller group reported an ele-
gant reaction between diols and urea through a sequence
of nucleophilic substitution followed by intramolecular
amination of alcohols to deliver oxazolidin-2-ones in high
enantioselectivity.11g Based on our previous endeavours,
we were curious whether the cooperative metal and acid
catalysis could help deliver a higher enantioselective syn-
thesis of this useful class of amino alcohols. Herein we
report our finding of Brønsted acid-assisted Ru-catalyzed
highly enantioselective amination of 1,2-diols.
12
9
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aGeneral conditions: 2 (0.3 mmol), morpholine (0.1 mmol) and the
catalysts were dissolved in toluene (0.1 M) and heated to 100 °C for
24 h. bNMR yield using 1,3,5-trimethoxybenzene as the internal
standard. cDetermined by HPLC on a chiral stationary phase.
The amination of 2 with morpholine catalyzed by our
previous iridium-based system unfortunately produced 3a
with only moderate yield and er (entry 1, Table 1), and no
further improvement was achieved after much optimiza-
tion (results not shown). After systematic screening of
different metal precursors and chiral ligands, the Ru-
Josiphos system proved to be the optimal choice as re-
ported previously by the group of Oe.11j To our excitement,
while the control reaction using Ru-1 and Josiphos yield-
ed 3a with only moderate er (entry 2), the addition of acid
co-catalyst 1a led to a significant increase of the enanti-
oselectivity of the reaction (93:7 er, entry 3). At this point,
efforts were directed towards the systematic screening of
various chiral phosphoric acids 1b-1d (entries 4-6), differ-
ent chiral ligands 4b-4e (entries 7-10) or metal precursors
Ru-2 and Ru-3 (entries 11-12). All these attempts, unfortu-
nately, led to no further improvement on the enantiose-
lectivity of this reaction.
An interesting discovery was made when we used the
achiral diphenyl phosphate as the acid co-catalyst, which
provided a similar boost on the enantioselectivity (88:12
er, entry 1, Table 2). Realizing the chirality of the acid was
not essential for the selectivity of this catalytic system, we
moved on to screen a range of simple Brønsted acids for
this reaction (entries 2-8). This acid effect turned out to
be general, as a range of Brønsted acids all resulted in
enhanced level of enantioselectivity (entries 1-8). The use
of benzoic acid proved to be the optimal choice for both
yield and selectivity (entry 6). Under these conditions, the
loading of diol could be reduced to 1.5 equiv. with no ero-
sion on the efficiency of the process (entry 9 vs. entry 10).
Finally, extended reaction time of 48 h led to complete
conversion to the desired product (entry 11).
Table 2. Acid Effect in Borrowing Hydrogena
Table 1. Optimization of Amination of 1,2-Diola
entry
acid
diphenyl phosphate
TFA
yield(%)b
erc
yield(%)b
erc
1
2
34
54
76
83
66
79
88
37
79
59
99
88:12
94:6
93:7
90:10
88:12
94:6
93:7
91:9
entry
[M]
[Ir]
ligand
\
acid
1a
\
1
2
54
90
83
41
72:28
72:28
93:7
3
CH3CO2H
TfOH
Ru-1
Ru-1
Ru-1
Ru-1
Ru-1
Ru-1
Ru-1
Ru-1
Ru-1
4a
4a
4a
4a
4a
4b
4c
4
3
1a
1b
1c
5
TsOH
4
5
89:11
91:9
6
PhCO2H
79
87
75
40
29
80
7
m-NO2PhCO2H
2,5-di-NO2PhCO2H
PhCO2H
6
7
1d
1a
1a
1a
1a
80:20
54:56
20:80
89:11
90:10
8
9d
10e
11d,f
94:6
94:6
94:6
PhCO2H
8
9
10
PhCO2H
4d
4e
a-cSee Table 1. dUse of 1.5 equiv. diol. eUse of 1.0 equiv. diol. 48 h
reaction.
f
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With the optimal conditions in hand, we examined
the scope of amines for the amino alcohol synthesis
(Scheme 2). In addition to morpholine 3a, the use of pi-
peridine led to the formation of 3b in excellent yield and
high er of 95:5. The seven-membered amine 3c was ob-
tained in an even higher er of 96:4. The use of tetrahy-
droisoquinoline worked out similarly well (94.5:5.5 er for
3d). When an acyclic secondary amine was used, the en-
antioselectivity dropped significantly and 3e was generat-
ed with a moderate 79:21 er. It is noteworthy, however, a
closely related product was obtained in an essentially ra-
cemic form in the absence of the acid co-catalyst.11j
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aSee Scheme 2.
Scheme 3. Scope of 1,2-diols for 1,2-Amino Alcohol
Synthesisa
While the exact nature of this intriguing acid effect is
currently not clear, we propose the reaction pathway
shown in Scheme 4 as a working hypothesis. The amina-
tion initiated with the oxidation of 2 to 5 (or possibly to
the tautomer 6 as well, which is a tautomer of 5). In the
original report by Oe and Ohta, it was proposed that 7
could be formed and converted to 8 via reductive amina-
tion (pathway A, Scheme 4a).11j The enantio-determining
step was then proposed to be the asymmetric transfer
hydrogenation (ATH) of ketone 8. To shed some light on
the mechanism, we prepared amino ketone 8 and subject-
ed it to the transfer hydrogenation condition using the
same catalytic system in the absence or presence of acid.
For this reaction, however, no improvement on enanti-
oselectivity was observed at all (Scheme 4c), which was
inconsistent with our observation of the acid effect. We
therefore propose an alternative pathway B (Scheme 4b)
in which 5 underwent imine condensation to generate 9,
which should be more energetically favored over oxida-
tion to 7, and the acid co-catalyst should significantly
facilitate such a transformation. It is believed that the
reduction of the most electrophilic iminium 9 should be
highly favored over reduction of 10 and 8 to yield amino
alcohol 3. We propose that this enantioselective process
could be operative through a dynamic kinetic asymmetric
reduction12 of 9: the two enantiomers of iminium 9 are
expected to undergo facile racemization through the achi-
ral enamine 10, while the chiral Ru-catalyst could differ-
entiate the two enantiomers of 9 to promote a stereose-
lective reduction to deliver 3. It is important to note that
while amino ketone 8 is also a tautomer of 10, the isomer-
ization of ketone 8 to enol 10 is expected to be very slow,
which might explain the lack of acid effect for the reduc-
tion of α-aminoketone 8 (Scheme 4c). Overall the benefi-
aGeneral conditions: 2 (0.15 mmol), amine (0.1 mmol) and the cata-
lysts were dissolved in toluene (0.1 M) and heated to 100 °C for 48 h.
bA mixture of amino alcohols were obtained. The isolated yield of the
major isomer is shown.
Scheme 2. Scope of Amines for 1,2-Amino Alcohol
Synthesisa
Gratifyingly, this amination is not limited to the use of
secondary amine substrates. When p-anisidine was used,
product 3f could be obtained in reasonable yield of 70%
with 91:9 er. When aniline was used for the reaction,
however, a mixture of products including 3g and the cor-
responding isomer (with amination of the secondary al-
cohol) was produced in a ratio of 4:1. The major isomer 3g
was obtained in 63% yield with an excellent 96.5:3.5 er.
When electron-deficient aniline was used, amino alcohol
3h was also formed as a mixture with its isomer in a ratio
of 3:1. There seemed to be some correlation of the elec-
tronic property of the anilines with the regioselectivity of
this reaction.
The same set of conditions could be applied to a wide
range of diol substrates (Scheme 3). For aryl-containing
diols, different substituents including electron-donating
or electron-withdrawing ones at para- or meta-positions
were all well-tolerated. Amino alcohols 3h-3n were ob-
tained in uniformly good enantioselectivity and high yield.
Notably, a highly sterically hindered diol was also tolerat-
ed to produce 3o in excellent yield and er. In addition to
aryl-substituted diols, aliphatic diols could also be used to
yield amino alcohols such as 3p and 3q in high yield and
er, which greatly expanded the scope of this catalytic sys-
tem.
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Corresponding Author
cial effect of achiral Brønsted acids on the enantioselectiv-
ity of the reaction may come from an accelerated tautom-
erization/racemization.
zhaoyu@nus.edu.sg; zhangyao@lnu.edu.cn
Notes
.
a) ATH of -amino ketone
The authors declare no competing financial interests.
O
OH
[O]
OH
OH
R
R
R
ASSOCIATED CONTENT
Supporting Information
6
2
Pathway A
[O]
[O]
OH
R1
OH R1
N
O
R1
N
O
HN
R2
[O]
ATH of ketone
Experimental procedures and characterization data for all
the products are provided. This material is available free of
charge via the Internet at http://pubs.acs.org.
9
O
O
R2
R
R2
R
R
reductive
amination
5
7
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3
b) Dynamic kinetic asymmetric reduction
[Ru-H]
[Ru]
OH R1
N
OH R1
N
R2
R2
R2
R
R2
ACKNOWLEDGMENT
R
kfast
9
3
kfast
We are grateful for the generous financial support from Sin-
gapore National Research Foundation (R-143-000-477-281
and R-143-000-606-281), the Ministry of Education (MOE) of
Singapore (R-143-000-613-112), and the National Natural Sci-
ence Foundation of China (21502082).
OH
Pathway B
O
R1
N
OH R1
O
R
facilitated
by acid
N
R
R2
5
R
10
8
R1
kfast
HN
R2
[Ru-H]
[Ru]
OH R1
N
OH R1
N
R
R
R2
kslow
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ent-3
ent-9
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c) ATH of preformed -amino ketone
OH
5 mol % Ru-1, 6 mol % 4a
O
O
OH
O
+
OH
N
N
3a
without PhCO2H: 33%, 85:15 er
Ph
3 equiv.
toluene, 100 °C, 24 h
Ph
Ph
8
with PhCO2H: 43%, 86:14 er
d) use of enantiopure diol
OH
OH
OH
O
standard conditions
OH
or
OH
N
Ph
Ph
Ph
O
(R)-2
3a
(S)-2
N
H
with (R)-2: >95% conv., 96.5:3.5 er
with (S)-2: >95% conv., 94:6 er
with rac 2: >95% conv., 94:6 er
Scheme 4. Mechanistic Studies and Proposed Reac-
tion Pathway
To shed more light on the mechanism, the use of en-
antiopure diol substrates were tested under the standard
conditions (Scheme 4d). Interestingly the use of (S)-2 led
to the same selectivity with the use of racemic 2, while the
use of the matched substrate (R)-2 delivered 3a in higher
er. This observation is consistent with the dynamic kinet-
ic asymmetric reduction pathway, although we cannot
rule out other possible reaction pathway or combination
of different pathways for this transformation.
In summary, we have developed a highly enantioselec-
tive synthesis of β-amino alcohols through mono-
amination of readily available racemic diols. The addition
of a simple achiral acid co-catalyst proved to be the key
factor to achieve high enantioselectivity in this Ru-
catalyzed process. A possible pathway of dynamic kinetic
asymmetric amination of alcohols was proposed. The de-
velopment of efficient methods to prepare other classes of
amino alcohols and diamines are currently under investi-
gation in our laboratories.
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