Dynamic kinetic asymmetric amination of alcohols: From a mixture of four isomers to diastereo- and enantiopure α-branched amines

Article abstract of DOI:10.1021/jacs.5b02212

The first dynamic kinetic asymmetric amination of alcohols via borrowing hydrogen methodology is presented. Under the cooperative catalysis by an iridium complex and a chiral phosphoric acid, α-branched alcohols that exist as a mixture of four isomers undergo racemization by two orthogonal mechanisms and are converted to diastereo- and enantiopure amines bearing adjacent stereocenters. The preparation of diastereo- and enantiopure 1,2-amino alcohols is also realized using this catalytic system.

Full text of DOI:10.1021/jacs.5b02212

Communication
pubs.acs.org/JACS
Dynamic Kinetic Asymmetric Amination of Alcohols: From A Mixture
of Four Isomers to Diastereo- and Enantiopure α‑Branched Amines
Zi-Qiang Rong,§ Yao Zhang,†,§ Raymond Hong Bing Chua, Hui-Jie Pan, and Yu Zhao*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Republic of Singapore, 117543
S
* Supporting Information
Scheme 1. Rationale for Dynamic Kinetic Asymmetric
Amination of Alcohols
ABSTRACT: The first dynamic kinetic asymmetric
amination of alcohols via borrowing hydrogen method-
ology is presented. Under the cooperative catalysis by an
iridium complex and a chiral phosphoric acid, α-branched
alcohols that exist as a mixture of four isomers undergo
racemization by two orthogonal mechanisms and are
converted to diastereo- and enantiopure amines bearing
adjacent stereocenters. The preparation of diastereo- and
enantiopure 1,2-amino alcohols is also realized using this
catalytic system.
ynamic kinetic asymmetric transformation (DYKAT)1 or
2
Ddynamic kinetic resolution (DKR) has proven to be a
highly valuable strategy in asymmetric synthesis, as it converts
both enantiomers of a racemic substrate into enantioenriched
product and overcomes the limitation of 50% theoretical yield
in traditional kinetic resolution.3 Such stereoconvergent
processes require a rapid racemization of the substrate (or an
intermediate) or the conversion of the racemic substrate to an
achiral intermediate prior to the enantio-determining step.4 Out
of various strategies developed for such transformations,
racemization of tertiary stereocenters α to carbonyl or imine
functionality through the achiral enolate5 or enamine6 (as
reported from the groups of Noyori, Zhou, Johnson, List and
others) and metal-catalyzed racemization of alcohols through a
redox process (as reported by the Backvall group, the Fu group
̈
and others)7 have found wide application in asymmetric
catalysis.8 One representative example of reductive amination of
α-branched cyclic ketones catalyzed by chiral phosphoric acid
1a is shown in Scheme 1a.6e In a few rare cases, a “double”
DKR process could be realized in which two isolated
stereocenters in the substrate are racemized using the same
strategy to yield chiral alcohols bearing multiple stereo-
centers.5g,7c,e We report here a unique system in which α-
branched alcohols (easily prepared as a mixture of four
stereoisomers) undergo racemization by two orthogonal
mechanisms to produce diastereo- and enantiopure acyclic
amines and amino alcohols bearing two adjacent stereocenters.
Recently our group reported the first example of catalytic
asymmetric amination of alcohols via borrowing hydrogen
promoted by Ir-complex 2a in combination with TRIP 1a
(Scheme 1b).9a The groups of Dong and Guan also reported
highly efficient Ru-catalyzed diastereoselective amination of
alcohols using Ellman’s chiral t-butanesulfinamide.9b Such a
process using borrowing hydrogen methodology10,11 has long
been recognized as a highly atom economical and green
method for the production of amines, as the alcohol substrate is
utilized as the hydrogen donor and no external reductant is
required for this transformation.12 In our enantioselective
variant, both enantiomers of the racemic alcohol were
converted to enantioenriched amine product through a redox
DYKAT pathway (dehydrogenation of the alcohol to ketone,
condensation of ketone to form imine followed by asymmetric
transfer hydrogenation of imine).
In an effort to extend the synthetic utility of this catalytic
system, we became interested in the asymmetric amination of
alcohol 3 that can be easily prepared as a mixture of four
stereoisomers (Scheme 1c). It was envisioned that the two
Received: March 2, 2015
Published: April 2, 2015
© 2015 American Chemical Society
4944
DOI: 10.1021/jacs.5b02212
J. Am. Chem. Soc. 2015, 137, 4944−4947

Journal of the American Chemical Society
Communication
stereocenters in 3 could both be racemized via first oxidation to
ketone A and then tautomerization of the iminium intermediate
(B and B′) through enamine C. Such a process, however,
challenges the catalyst with an additional dimension of
selectivity in the asymmetric reduction step: it has to
differentiate between B and B′, in addition to the enantiotopic
faces of the imine moiety in order to realize high diastereo- and
enantioselectivity for the synthesis of 4.
Intrigued by this possibility of a highly stereoconvergent
transformation, we initiated our studies by examining the
reaction of 3a (as a 2:1 mixture of diastereomers) with aniline
to generate chiral amine 4a (Table 1). Preliminary studies
high efficiency (entry 3 vs entry 4), while the dr remained
moderate (86:14).
Further efforts to optimize the diastereoselectivity focused on
the screening of catalyst structures. Iridium complexes 2b to 2e
bearing different sulfonamide moiety or chiral diamine
backbone all proved to be much less efficient and less
stereoselective (entries 5−8).13 As for the chiral phosphoric
acid, the bulky tri-isopropylphenyl substituent in 1a proved
uniquely effective, as 1b or 1c turned out to be much less
reactive and selective (entries 9−10). Catalyst 1d that was
reported to be more acidic than the corresponding 1a again
showed reduced reactivity and selectivity (entry 11).14 Finally
different chiral backbones of the phosphoric acid were tested.
To our excitement, the use of spirocyclic 1e15 produced 4a in
excellent dr of 97:3 and er of 98.5:1.5 (entry 12). At this point,
the use of 3a as the limiting reagent proved successful, however,
the yield of 4a reduced to 48% (entry 13). Consistent with our
previous studies,9a the use of ent-2a and 1e proved to be the
mis-matched case that resulted in lower stereoselectivity (entry
14). Gratifyingly, more concentrated conditions and a
prolonged reaction time led to marked improvement (entries
15−16). The yield of 4a was improved to 81% with excellent
stereoselectivity, realizing a formal conversion of four isomers
of 3a to essentially one stereoisomer of 4a.
Table 1. Optimization of DYKAT
a
b
c
entry
[Ir]
2a
acid 3a:aniline yield (%)
dr
er
d
1
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
1e
1e
1e
1e
1.5:1
1.5:1
1.2:1
1.1:1
1.2:1
1.2:1
1.2:1
1.2:1
1.2:1
1.2:1
1.2:1
1.2:1
29
74
72
32
19
62
8
89:11
86:14
86:14
86:14
80:20
80:20
75:25
67:33
33:67
50:50
50:50
97:3
97:3
96:4
2
3
2a
2a
96.5:3.5
97:3
4
2a
5
2b
91.5:8.5
91:9
6
2c
With the optimal conditions in hand, we moved on to
examine the scope of this dynamic kinetic asymmetric
amination system (Scheme 2). Various para-substitutents on
7
2d
62:38
49:51
70:30
74:26
91:9
8
2e
16
24
6
9
2a
10
11
12
13
14
2a
Scheme 2. Scope of Dynamic Kinetic Asymmetric Amination
2a
22
73
48
31
60
81
a
of Alcohols
2a
98.5:1.5
99:1
2a
1:1.2
97:3
ent-2a
2a
1:1.2
1:1.2
1:1.2
86:14
97:3
10:90
99:1
e
15
ef
,
16
2a
97:3
99:1
a
b
Isolated yield. The dr value was determined by NMR spectroscopy
c
of the crude reaction mixture. The er value was determined by HPLC
on a chiral stationary phase. t-Amyl alcohol as the solvent. [0.4 M]
concentration. 60 h reaction time.
d
e
f
showed that the optimal conditions from our previous studies
(t-amyl alcohol as solvent)9a produced 4a in a good dr of 89:11
and excellent enantioselectivity, albeit with a low yield (entry
1). As the mixture of 4a and epi-4a cannot be easily separated,
we set our goal for achieving both high stereoselectivity and
high efficiency of the system. After screening various solvents,
toluene proved to be the preferred choice that produced 4a in
much enhanced efficiency with slightly decreased stereo-
selectivity (entry 2). The use of slight excess of 3a (1.2 equiv
or higher) vs aniline was necessary at this stage to maintain the
a
See SI for the detailed procedure. Some products were converted to
the corresponding amide (2.0 equiv DIPEA, 2.0 equiv AcCl, DCM, 3
h) to determine the er. The reported yields refer to the two-step
combined yield.
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DOI: 10.1021/jacs.5b02212
J. Am. Chem. Soc. 2015, 137, 4944−4947

Journal of the American Chemical Society
Communication
the aryl ring that are electron-donating, electron-withdrawing
and electron-neutral can be well-tolerated to yield 4b−4e in
uniformly good yield and excellent stereoselectivity. The
relative and absolute configuration of 4e was unambiguously
assigned by single crystal X-ray analysis, and those of other
products were assigned by analogy. Chiral amines with a meta-
substituent on the aryl ring (4f, 4g) and also other aryl groups
(4i and 4j) were produced with similar success. Ortho-
substitution on the aryl ring, however, proved to be too
sterically hindered; low yield as well as reduced diastereose-
lectivity was obtained for 4h.16 Extension to alkyl, alkyl-
substitution at the α-position such as 4k also worked out well,
albeit with a reduced dr of 90:10. Gratifyingly, the smaller
substituent on the α-stereocenter can be extended to alkyl
groups bigger than methyl (as in 4l−4n) with excellent results.
Finally, p-anisidine can also be used instead of aniline to
produce 4o with slightly reduced stereoselectivity to that of 4a.
To further extend the synthetic utility of this system, we
examined the reaction using monoprotected diols in an effort to
access diastereo- and enantiopure 1,2-amino alcohols. As shown
in Scheme 3a, we chose the readily available mono TBS-ether
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and characterization data for all the
products. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*zhaoyu@nus.edu.sg
Present Address
†College of Chemistry, Liaoning University, Shenyang, China,
110036
Author Contributions
§These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the generous financial support from
Singapore National Research Foundation (NRF Fellowship)
and National University of Singapore.
Scheme 3. Access to Chiral Amino Alcohols
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