Catalytic enantioselective amination of alcohols by the use of borrowing hydrogen methodology: Cooperative catalysis by iridium and a chiral phosphoric acid

Article abstract of DOI:10.1002/anie.201307789

The catalytic asymmetric reduction of ketimines has been explored extensively for the synthesis of chiral amines, with reductants ranging from Hantzsch esters, silanes, and formic acid to H2 gas. Alternatively, the amination of alcohols by the use of borrowing hydrogen methodology has proven a highly atom economical and green method for the production of amines without an external reductant, as the alcohol substrate serves as the H2 donor. A catalytic enantioselective variant of this process for the synthesis of chiral amines, however, was not known. We have examined various transition- metal complexes supported by chiral ligands known for asymmetriC-Hydrogenation reactions, in combination with chiral Bronsted acids, which proved essential for the formation of the imine intermediate and the transfer-hydrogenation step. Our studies led to an asymmetric amination of alcohols to provide access to a wide range of chiral amines with good to excellent enantioselectivity.

Full text of DOI:10.1002/anie.201307789

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Chemie
DOI: 10.1002/anie.201307789
Asymmetric Catalysis
Catalytic Enantioselective Amination of Alcohols by the Use of
Borrowing Hydrogen Methodology: Cooperative Catalysis by Iridium
and a Chiral Phosphoric Acid**
Yao Zhang, Ching-Si Lim, Derek Sui Boon Sim, Hui-Jie Pan, and Yu Zhao*
Dedicated to Professor Marc L. Snapper on the occasion of his 55th birthday
Abstract: The catalytic asymmetric reduction of ketimines has
been explored extensively for the synthesis of chiral amines,
with reductants ranging from Hantzsch esters, silanes, and
formic acid to H2 gas. Alternatively, the amination of alcohols
by the use of borrowing hydrogen methodology has proven
a highly atom economical and green method for the production
of amines without an external reductant, as the alcohol
substrate serves as the H2 donor. A catalytic enantioselective
variant of this process for the synthesis of chiral amines,
however, was not known. We have examined various tran-
sition-metal complexes supported by chiral ligands known for
asymmetric hydrogenation reactions, in combination with
chiral Brønsted acids, which proved essential for the formation
of the imine intermediate and the transfer-hydrogenation step.
Our studies led to an asymmetric amination of alcohols to
provide access to a wide range of chiral amines with good to
excellent enantioselectivity.
ketone, condensation to give an imine, and subsequent
imine hydrogenation and utilizes the alcohol substrate as
the H2 donor (Scheme 1b). Great progress has been made in
the search for efficient catalytic processes for the reactions of
various amines with primary and even secondary alcohols in
the past decade, with state-of-the-art systems reported by the
research groups of Beller, Williams, Fujita, Kempe, Crabtree,
Yus, Peris, Martꢀn-Matute, and others.[7–9] A catalytic, enan-
tioselective amination of alcohols by the use of borrowing
hydrogen methodology would be a highly efficient way of
preparing chiral amines from simple alcohol and amine
starting materials without the need for an external reductant
and with the generation of H2O as the only side product. Such
a process, to our surprise, has remained elusive until now.[10]
We report herein our recent studies towards the first such
process.
We initiated our studies by examining the reaction of 2-
octanol (1a) and p-anisidine (2a) to afford 3a. As for the
catalyst, we decided to start with metal complexes that have
proven successful for asymmetric transfer-hydrogenation
reactions,[3] such as those supported by a privileged chiral
monotosylated diamine[11,3e–h] or various bisphosphine
ligands. Despite extensive experimentation, however, no
efficient conversion into the desired product 3a was observed
in the presence of iridium-, rhodium-, or ruthenium-based
catalysts, even in refluxing toluene with the addition of 4 ꢁ
Owing to the widespread use of chiral amines in the
pharmaceutical and fine-chemicals industries, the develop-
ment of efficient methods to prepare these compounds in high
enantiomeric purity has been an important goal in organic
synthesis.[1] One of the most explored reaction types for this
purpose is the asymmetric hydrogenation of ketimines (or
reductive amination of ketones; Scheme 1a).[2] Various sys-
tems based on transition-metal catalysis,[3] organocatalysis,[4]
and cooperative catalysis involving both[5] have been docu-
mented for imine hydrogenation, with reductants ranging
from Hantzsch esters, silanes, and formic acid to H2 gas.
In a related area of research, the amination of alcohols by
the use of borrowing hydrogen methodology[6] (also known as
the hydrogen autotransfer process)[6d] has long been recog-
nized as a highly atom economical and green method for the
production of amines. This approach involves the three steps
of dehydrogenation of an alcohol to the corresponding
[*] Dr. Y. Zhang, C.-S. Lim, D. S. B. Sim, H.-J. Pan, Prof. Y. Zhao
Department of Chemistry, National University of Singapore
3 Science Drive 3, Singapore 117543 (Singapore)
E-mail: zhaoyu@nus.edu.sg
Homepage: http://zhaoyu.science.nus.edu.sg
[**] We are grateful for the generous financial support from the
Singapore National Research Foundation (NRF Fellowship) and the
National University of Singapore.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307789.
Scheme 1. Different strategies for the synthesis of chiral amines.
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molecular sieves (MS; see examples in Table 1, entries 1 and
2).
acid additives, even though all known amination reactions of
alcohols on the basis of borrowing hydrogen methodology
were carried out under basic or neutral conditions.[6–9] As
shown by the representative results listed in entries 3–5 of
Table 1, whereas acid additives did not improve the reaction
catalyzed by Ru complex 4 (entry 3), the combination of an
acid (such as p-toluenesulfonic acid or phosphoric acid) and
the Ir complex (R,R)-5 successfully led to the formation of 3a,
although the level of efficiency and selectivity remained
disappointingly low (entries 4,5). Chiral phosphoric acids
were then tested. The identity of the 3,3’-substituents in the
chiral backbone had a dramatic effect on the reaction
outcome (Table 1, entries 6–9), whereby 10e provided the
highest level of enantioselectivity and promising efficiency.
As two chiral sources were used, the combination of 10e with
the enantiomeric Ir complex (S,S)-5 was tested. This pair
provided significantly higher selectivity (87% ee), but with
diminished 32% conversion (Table 1, entry 10). The matched/
mismatched relationship of the two chiral sources is signifi-
cant, whereby the chirality of the Ir complex is the determin-
ing factor.
We then switched our focus to the modification of the Ir
complex. Variation of the substitution on the sulfonamide
moiety as well as the chiral-diamine backbone led to the
identification of complex (S,S)-6 as the catalyst that provided
the optimal combination of reactivity and selectivity (Table 1,
entries 10–14). Mesylate (S,S)-8 and cyclohexanediamine-
derived (S,S)-9 were surprisingly not effective at all. Further
fine-tuning of various reaction parameters showed that
a higher loading of the alcohol substrate (3 equiv or 5 equiv
rather than 1.5 equiv; Table 1, entries 15 and 16 versus
entry 11) led to an increase in enantioselectivity to
> 90% ee. In an effort to identify an alternative to this
higher alcohol loading, which is detrimental to the overall
efficiency of the process, we wondered whether the beneficial
effect comes from a higher concentration of the hydrogen
donor or simply an increase in the polarity of the reaction
medium. tert-Amyl alcohol, which is not a hydrogen donor but
a much more polar solvent, was then tested as the solvent.
Gratifyingly, this change of solvent led to an increase in the
ee value of the product to 89% (Table 1, entry 17 versus
entry 11). Finally, a higher loading of 10e proved beneficial.
Under the optimal conditions, under which the alcohol was
used in slight excess (1.5 equiv), 3a was produced in 90%
yield with 93% ee (Table 2, entry 1).
Careful examination of the crude reaction mixture
indicated the formation of a small amount of the correspond-
ing ketone, which led us to speculate that the imine
condensation was the problematic step. Inspired by previous
studies on reductive amination by the research groups of
MacMillan[4g] and Xiao,[5e] in which chiral phosphoric acids
proved effective in promoting the condensation of ketones
and anilines even at 35–508C, we decided to test the effect of
Table 1: Optimization of the enantioselective amination of 1a.
Entry [M]
Additive
Solvent
Conv. ee
[%][a]
[%][b]
1
2
3
(R,R)-4
(R,R)-5
–
–
toluene
toluene
<5
<5
<5
32
N.D.
N.D.
N.D.
7
(R,R)-4 TfOH (5 mol%) toluene
(R,R)-5 TfOH (5 mol%) toluene
4
5
6
7
8
(R,R)-5 10a (5 mol%)
(R,R)-5 10b (5 mol%)
(R,R)-5 10c (5 mol%)
(R,R)-5 10d (5 mol%)
(R,R)-5 10e (5 mol%)
(S,S)-5 10e (5 mol%)
(S,S)-6 10e (5 mol%)
(S,S)-7 10e (5 mol%)
(S,S)-8 10e (5 mol%)
(S,S)-9 10e (5 mol%)
(S,S)-6 10e (5 mol%)
(S,S)-6 10e (5 mol%)
(S,S)-6 10e (5 mol%)
(S,S)-6 10e (10 mol%)
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
tert-amyl alcohol
tert-amyl alcohol
33
60
<5
83
50
32
>99
96
<5
<5
>99
>99
95
12
9
N.D.
À17
À46
87
81
77
N.D.
N.D.
90
9
10
11
12
13
14
15[c]
16[d]
17
18
92
89
93
>99
[a] Conversion was determined by NMR spectroscopy with 4,4’-di-tert-
butylbiphenyl as an internal standard. [b] The ee value was determined by
HPLC on a chiral stationary phase. [c] The reaction was carried out with
3 equivalents of the alcohol. [d] The reaction was carried out with
5 equivalents of the alcohol. N.D.: not determined; Tf=trifluorometh-
anesulfonyl. See the Supporting Information for details.
The same set of conditions can be applied to a wide range
of alcohol substrates (Table 2). Alcohols bearing linear alkyl
substituents (substrates 1a–c) were transformed into 3 with
high enantioselectivity (Table 2, entries 1–3). The reactions of
substrates bearing a-branched substituents were particularly
successful (up to 97% ee; Table 2, entries 4–6). Functional-
ities such as benzyl ether and silyl ether groups were well-
tolerated (products 3i and 3j), even though the reaction
involved reductive and acidic conditions (Table 2, entries 9
and 10). Aryl–alkyl-substituted products were also generally
obtained with good to excellent enantioselectivity (Table 2,
entries 11–17). The electronic properties of the substrates
seemed to play a role, whereby products bearing electron-
donating substituents were obtained with higher ee values
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Table 2: Scope of the enantioselective amination with respect to the
Table 3: Scope of the enantioselective amination with respect to the
amine substrate.[a]
alcohol substrate.[a]
Entry
R
Product
Yield [%][b]
ee [%]
Entry
R1
R2
Product
Yield [%][b]
ee [%]
1
2
3
4
5
Ph
3u
3v
3w
3x
3y
91
97
81
98
95
88
91
83
89
88
1
2[c]
3[d]
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20[f]
n-hexyl
nBu
nPr
iPr
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
3a
3b
3c
3d
3e
3 f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
90
90
86
88
92
93
75
98
64
95
81
97
72
69
93
92
90
96
97
91
82
83
85
91
91
91
96
83
4-MeC6H4
4-ClC6H4
4-PhC6H4
3-MeOC6H4
cyclohexyl
cyclopropyl
iPrCH2
PhCH2CH2
BnO(CH2)3
TBSO(CH2)3
Ph
4-MeC6H4
4-MeOC6H4
4-BrC6H4
4-CF3C6H4
3-MeC6H4
1-naphthyl
4-MeC6H4
1-naphthyl
iPr
6
3z
84
81
[a,b] See Table 2.
However, despite extensive efforts to extend the scope of the
reaction to other amines, such as benzylamine, tosylamide, or
secondary amines, the current method failed to yield the
product with good efficiency or selectivity. The effective
condensation of the ketone intermediate with different
amines and the asymmetric transfer hydrogenation of
imines are challenges to be addressed.
The intramolecular amination of alcohols has been
demonstrated by Fujita et al. as an efficient way to produce
valuable heterocyclic compounds.[7a] When we applied our
optimal conditions to the reaction (Scheme 2), a good yield
90 (40)[e]
83
80
75
71
80
70 (94)[e]
94
94
73
69
75
Et
Et
3s
3t
[a] Unless stated otherwise, reactions were carried out in tert-amyl
alcohol at reflux for 24 h. All reagents were used as received from the
commercial supplier without purification. The absolute configuration of
the products was determined by comparison of the optical rotation with
reported data or by analogy (see the Supporting Information for details).
[b] Yield of the isolated product. [c] The reaction was carried out with
3 equivalents of the alcohol. [d] The reaction was carried out with
5 equivalents of the alcohol (see the Supporting Information for details).
[e] The values in parentheses refer to the corresponding reaction carried
out at 808C for 48 h. [f] The reaction was carried out in refluxing toluene
for 24 h. Bn=benzyl, PMP=p-methoxyphenyl, TBS=tert-butyldime-
thylsilyl.
Scheme 2. Example of the intramolecular amination of alcohols.
(Table 2, entry 13 versus entries 14 and 15). For certain more
reactive substrates, a lower reaction temperature could
increase the ee value of the product dramatically, albeit at
the cost of conversion (Table 2, entry 15). This catalytic
system is not limited to methyl-substituted alcohols, although
the enantioselectivity dropped to 69–75% ee for ethyl-con-
taining substrates as a result of the diminished size difference
of the two substituents (Table 2, entries 18–20). Use of the
chiral phosphoric acid in excess with respect to the Ir complex
proved essential for alcohols with aryl–alkyl substitution (but
not for alkyl–alkyl-substituted alcohols, as shown in Table 1,
entry 16). For example, for the reaction in Table 2, entry 11, if
5 mol% of 10e was used (the same loading as that of (S,S)-6),
3k was obtained with only 9% conversion.
Different types of amine substrates were also examined
(Table 3). Various anilines, including those bearing substitu-
ents of an electron-donating or electron-withdrawing nature,
were well-tolerated (Table 3, entries 1–5). It is particularly
notable that heterocyclic amines, such as 5-aminoindole, can
be successfully coupled with an alcohol to yield the chiral
amine in good yield and enantioselectivity (Table 3, entry 6).
and promising enantioselectivity (68% ee) were observed for
the quinoline product 12 bearing a simple methyl substituent.
For this reaction, the higher loading of 10e relative to 6 was
not necessary. Thus, the intramolecular imine-condensation
step is arguably efficient enough, even in the absence of the
external acid cocatalyst.
Notably, to the best of our knowledge, our system
represents the first highly enantioselective transfer hydro-
genation of imines with an alcohol as the hydrogen donor,
despite the great success of the use of related Ir and Ru
complexes for the asymmetric transfer hydrogenation of
imines with formic acid–Et3N[3e,f] as well as the asymmetric
hydrogenation of imines with H2.[3g,h,5d–g] To better understand
this cascade process, we examined the transfer hydrogenation
of the corresponding imine 13 with 1k as the hydrogen donor
under similar conditions. As summarized in Table 4, in the
absence of 10e, very low efficiency and no stereoselectivity
were observed with Ir complex 6 (entry 1). Even in the
presence of 10e, the ee value of the product was far from
satisfactory (Table 4, entries 2 and 3). A higher loading of the
alcohol resulted in a higher ee value, which led us to speculate
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to yield the amine with high enantioselectivity,[12] and in this
case, the chiral Ir complex is the determining factor in terms
of the configuration of the chiral amine product.
Table 4: Transfer hydrogenation of a preformed imine.
In conclusion, we have developed the first enantioselec-
tive amination of alcohols by the use of borrowing hydrogen
methodology under the catalysis of a chiral Ir complex in
cooperation with a chiral phosphoric acid. The identification
of new and more efficient catalytic systems to access a wider
range of chiral amines beyond anilines as well as N-containing
heterocyclic compounds with high stereoselectivity is the
focus of current efforts in our laboratories.
Entry
Amount of 1k [equiv]
10e [mol%]
Yield [%]
ee [%]
1
2
3
2.0
2.0
10.0
–
10
10
13
>95
>95
<5
60
71
that the high enantioselectivity observed in our cascade
amination reaction may be partly due to the slow formation of
the imine intermediate (and thus a practically high proportion
of the alcohol present).
Received: September 4, 2013
Revised: November 15, 2013
Published online: December 20, 2013
We propose the catalytic cycle shown in Scheme 3 as
a working hypothesis. Complex I formed between the Ir
complex 6 and the chiral phosphoric acid 10e[5d–g] (as
Keywords: amination · borrowing hydrogen methodology ·
enantioselectivity · hydrogenation · tandem catalysis
.
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À
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Angew. Chem. Int. Ed. 2014, 53, 1399 –1403
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