Iron-catalyzed amination of alcohols assisted by Lewis acid

Article abstract of DOI:10.1039/c5cc03399c

An efficient Lewis acid-assisted, iron-catalyzed amination of alcohols using borrowing hydrogen methodology was developed. In particular, silver fluoride was identified to be a highly effective additive to overcome the low efficiency in the amination of secondary alcohols catalyzed by Kn?lker's complex.

Full text of DOI:10.1039/c5cc03399c

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ARTICLE TYPE
Iron-Catalyzed Amination of Alcohols Assisted by Lewis Acid
Hui-Jie Pan, Teng Wei Ng and Yu Zhao*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
5
An efficient Lewis acid-assisted, iron-catalyzed amination of alcohols using borrowing hydrogen
methodology was developed. In particular, silver fluoride was identified to be a highly effective additive
to overcome the low efficiency in the amination of secondary alcohols catalyzed by the Knӧlker’s
complex.
system to dynamic kinetic amination of α-branched alcohols was
Introduction
35 also achieved.7b In an effort to discover more efficient and
economical catalyst for this process, we were particularly
attracted to the possibility of using the Knӧlker’s complex 18
(Scheme 1b) for amination of alcohols. The use of 1 for
hydrogenation and transfer hydrogenation of carbonyls is well
40 established,9 and the Beller group has reported highly efficient
and enantioselective hydrogenation of imines catalyzed by 1 and
a chiral phosphoric acid.10 Very recently, the Feringa group and
the Wills group reported successful amination of alcohols
catalyzed by iron complex 2 (precursor to Knӧlker’s complex)
45 and the analogous complex 3, respectively (Scheme 1b),11 which
prompted us to report our investigation on related studies as soon
as possible. It is noteworthy that in their reports as well as other
early reports on Fe-catalyzed amination reactions through a
borrowing hydrogen process, only primary alcohols (and selected
50 symmetrical secondary alcohols) can be effectively transformed
to achiral amine products. Secondary alcohols, on the other side,
are in most cases unreactive (consistent with our own findings).
In our studies, we have identified silver fluoride as an effective
additive that enabled highly efficient amination of a wide range
55 of secondary alcohols catalyzed by 1.
10
Owing to the widespread use of amines in pharmaceuticals,
agrochemicals, additive and dyes, the development of efficient
and environmentally benign methods to prepare amines has long
been an important goal in organic synthesis.1 Among the various
strategies for amine synthesis, the amination of alcohols using a
15 borrowing hydrogen methodology2 (or hydrogen autotransfer
process) has been recognized as a highly atom economical choice
(Scheme 1a). In this overall redox-neutral process, the alcohol
substrate serves another role as the H2 donor so no external
reductant is needed, and water is generated as the only side
20 product. Various catalytic systems have been developed for this
transformation, most of which use catalysts based on precious
metals such as Ir,3 Ru4 and others.5 The development of efficient
catalytic
system
using
abundant,
inexpensive
and
environmentally friendly metals (and especially iron) will be
25 highly desired.6
The amination of 2-octanol with p-anisidine was chosen as the
model reaction (Table 1). When the reaction was carried out
using 10 mol% 1, 6a was formed in only 4% yield after refluxing
in toluene for 48 h (entry 1). In the crude reaction mixture, the
60 corresponding ketone could be observed in a substantial amount
together with a small amount of imine (<5%), providing
experimental support for the borrowing hydrogen process.
However, the overall efficiency was disappointing by the use of
only 1 as the catalyst. In our previous system a chiral Brønsted
65 acid proved essential for facilitating the imine condensation as
well as the asymmetric reduction step.7a Inspired by this
observation as well as the report from the Beller group,10 we
screened various Brønsted acids for this reaction (entries 2-4).
Unfortunately no significant improvement was observed in most
70 cases, with only formic acid leading to an improved but still poor
yield of 29% (entry 2). The use of phosphoric acid proved
ineffective (entry 4). Chiral phosphoric acids were also
examined; no product formation was observed to our
disappointment (results not shown).
Scheme 1 Fe-complexes used in amination of alcohols
Our group has been interested in the application of borrowing
30 hydrogen methodology to chiral amine synthesis and we recently
reported the first example of enantioselective amination of
alcohols based on cooperative catalysis of a chiral iridium
complex and a chiral phosphoric acid.7a The extension of this
75
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Table 1 Screening of Brønsted and Lewis acid additives for the iron-catalyzed amination of secondary alcohola
View Article Online
DOI: 10.1039/C5CC03399C
Entry
1
Additive
None
Time (h)
48
Yield (%)b
Entry
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32c
Additive
Time (h) Yield (%)b
4
20 mol% Ni(OTf)2
20 mol% Ni(acac)2
20 mol% Co(acac)2
20 mol% Co(acac)3
20 mol% Ti(OiPr)4
20 mol% Ag2O
20 mol% AgBF4
20 mol% AgOTf
20 mol% AgSbF6
20 mol% Ag2CO3
20 mol% AgF
48
48
48
48
48
48
48
48
48
48
48
24
24
24
24
24
36
29
4
2
20 mol% HCOOH
20 mol% TsOH
20 mol% P-acid
20 mol% Cu(OTf)2
20 mol% CuCl2
20 mol% CuCl
20 mol% CuBr
20 mol% CuI
48
29
4
3
48
4
48
<2
22
<2
<2
<2
<2
<2
<2
<2
<2
30
38
<2
11
55
6
5
48
6
48
7
48
9
8
48
23
39
8
9
48
10
11
12
13
14
15
16
20 mol% FeCl2
20 mol% FeBr2
20 mol% FeCl3
20 mol% FeBr3
20 mol% Fe2O3
20 mol% Fe(acac)3
20 mol% NiCl2
48
48
67
18
46
97
<2
70
48
5 mol% AgF
48
10 mol% AgF
48
40 mol% AgF
48
20 mol% CsF
48
40 mol% AgF
5
aUnless noted otherwise, the reaction was carried out with 5a (0.2 mmol), 4a (5 equiv), 1 (10 mol%) and the additive (20 mol%) in
refluxing toluene. Yield of 6a was determined by GC analysis using n-dodecane as the internal standard. 7 was used as the catalyst
instead of 1.
b
c
10
At this stage, it was decided that a wide range of Lewis acids
should be examined. Various readily available salts of Cu, Fe, Ni,
Co, Ti and Ag were then subjected to the reaction catalyzed by 1.
As summarized in entries 5-27, Table 1, no general trend could
be observed, but the strongly Lewis acidic triflate and antimonate
15 salts all resulted in noticeable improvement (entries 5, 17, 24 and
25). Ti(OiPr)4 and AgF, in particular, proved to be the most
effective additives that yielded 6a in 55% and 67% yield,
respectively (entries 21 and 27). Further optimization with
Ti(OiPr)4 was not successful. To our excitement, the test of
20 loading of AgF proved fruitful (entries 28-30): an excellent 97%
yield of 6a was obtained in 24 h when 40 mol% AgF was used.
When other F- source such as CsF was used instead, no reaction
was obtained (entry 31). Finally, complex 7,12 an air-stable
precursor of 1, was tested. Under otherwise identical conditions, a
25 slightly reduced yield of 70% was obtained (entry 32).
35 Scheme 2 Test of AgF-catalyzed amination of alcohol
With the optimized conditions in hand, we moved on to
explore the substrate scope. As shown in Scheme 3, a wide range
of secondary alcohols underwent reaction to yield the
40 corresponding amines in good to excellent yields. In the case of
aliphatic alcohols, not only the linear ones such as 4a and 4b, but
those with α- and β-substituents could also be converted to the
desired products 6c and 6d in good yields. Benzylic amines 6e
and 6f could also be accessed in good yields. Again, no
45 conversion to these products was obtained in the absence of AgF.
In the Wills’ report using 3 as the catalyst it was also noted that
the reactions of aniline and secondary benzylic alcohols resulted
in only low conversion to impure products. Beside 2-octanol, 3-
As Ag-catalyzed reactions of alcohols and amines via direct
nucleophilic substitution have been documented in the
literature,13 we tested the amination of 4a using 40 mol% AgF in
the absence of 1 (Scheme 2). No product formation was observed
30 under these conditions, further supporting a borrowing hydrogen
process catalyzed by 1 instead of Ag-catalyzed nucleophilic
substitution mechanism.
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octanol bearing an ethyl substituent was also a suitable substrate
yielding 6g in 82% yield, although a longer reaction time of 48 h
a high yield of 80% when 1.2 equiv. of 4o was used (Scheme 4b).
Under similar conditions, alkene-containing substrate underwent
was needed to achieve high conversion. It is noteworthy that the 35 amination smoothly to yield 6p in excellent yield V(iSewchAertmiclee O4ncli)n.e
amination of symmetrical secondary alcohols such as
cyclohexanol and isopropanol using aniline were reported by the
Feringa group; the yield of the corresponding products 6b and 6h
were only 12-14%, further highlighting the beneficiary effect of
The alcohols containing other functionaliDtiOesI: 1s0u.c1h03a9s/Ca5nCCes0t3e3r9o9rC
an ether group, however, failed to yield any product, representing
some limitation of the current catalytic system. The use of diol as
in previous work by Feringa worked out well (Scheme 4d).
5
AgF. Different anilines could also be used for the amination 40 Finally, secondary amine such as piperidine 5g could undergo
reaction (6i-6l). While aniline worked well to yield 6i in excellent
reaction with 4n to yield tertiary amine 8d in a moderate yield of
10 yield, electron deficient p-Cl aniline proved to be much less
efficient (6j). In addition to different anilines, the use of
benzylamine such as 5b for amination reaction was also
examined. Unfortunately, in this case the improvement from AgF
was not as significant. Amine 6m was produced in a 30% yield
15 that will require further optimization.
51% (Scheme 4e).
OMe
OMe
a)
10 mol% 1
OMe
40 mol% AgF
n-octyl
n-octyl
N
N
n-octyl
+
+
H
OH
n-octyl
110 oC in toluene
24 h
H2N
5a
4n
(5 equiv)
6n
8a
27%
72%
b)
OMe
OMe
same as above
R
HN
Ph
+
5a
N
Ph
R
+
10 mol% 1
Ph
OH
OH
40 mol% AgF
Ph
+
6o
4o
8b
HN
R1 R2
H2N
110 oC, 24 h in toluene
with 5 equiv 4o:
65%
80%
21%
<5%
4
5
R1 R2
6
with 1.2 equiv 4o:
(5 equiv)
OMe
OMe
OMe
OMe
c)
OMe
same as above
HN
HN
Me
HN
i-Pr
HN
+
5a
N
H
OH
i-Pr
n-hexyl
Me
Me
Me
Me
6p
4p
(1.2 equiv)
6b
6c
95%
6a
97%
6d
70%
77%c
67%
d)
OMe
(<5% w/o AgF)
same as above
+
5a
N
HO
OH
OMe
OMe
OMe
OMe
8c
4q
(1.2 equiv)
61%
HN
HN
Me
HN
HN
e)
Me
n-pentyl
Et
n-octyl
same as above
HN
N
MeO
n-octyl
+
OH
6h
79%
6e
6f
64%d
6g
4n
(5 equiv)
5g
8d
51%
82%d
76%
(<5% w/o AgF)
OMe
Cl
MeO
HN
45 Scheme 4 Amination of primary alcohols
HN
HN
HN
n-hexyl
Me
n-hexyl
Me
n-hexyl
Me
n-hexyl
Me
6j
6k
6i
6l
79%
36%
97%
26%
10 mol% 1
w or w/o 40 mol% AgF
OH
Me
Ph
HN
+
Ph
NH2
n-hexyl
n-hexyl
Me
6m
110 oC, 24 h in toluene
4a
(5 equiv)
5f
No AgF: 11%
with AgF: 30%
aSee Table 1 for general conditions. bAll the yields refer to
isolated yields of 6. cIsopropyl alcohol as the solvent. d48 h
20 reaction.
Scheme 3 Scope of amination of secondary alcoholsa,b
Scheme 5 Proposed catalytic cycle
The amination of selected primary alcohols that were used in
25 Feringa’s and Wills’ reports were also tested (Scheme 4). Under
the optimized conditions as before, 1-octanol and benzyl alcohol
underwent reaction with p-anisidine to yield a mixture of
secondary amine and tertiary amine products. In this case, the
higher reactivity of this system resulted in a partial double
30 amination reaction to yield the tertiary amine side product. A
simple solution was to use close-to-stoichiometric loading of the
alcohol. As an example, secondary amine 6o could be obtained in
50
This reaction is believed to proceed through a borrowing
hydrogen mechanism, consistent with previous reports. A control
reaction using enantiopure alcohol yielded the amine product in a
racemic form. The exact nature of activation by AgF is not clear
55 at this point. Considering the general improvement of the product
yield from various Lewis acids examined in Table 1, we believe
that AgF serves as the Lewis acid to facilitate imine condensation
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and activate the imine intermediate towards reduction by the iron
hydride species 1 in the catalytic cycle shown in Scheme 5.
Computational studies will be carried out to help elucidate the
details of this activation by AgF.
In conclusion, we report herein a highly efficient iron-catalyzed
amination of primary as well as secondary alcohols assisted by
Lewis acid. The development of an enantioselective variant of
this reaction by examining chiral Lewis acids and chiral analogs
of Knӧlker’s complex is currently under investigation in our
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Department of Chemistry, National University of Singapore, 3 Science
Drive 3, 117543, Republic of Singapore. E-mail: zhaoyu@nus.edu.sg
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