New Iridium Catalysts for the Selective Alkylation of Amines by Alcohols under Mild Conditions and for the Synthesis of Quinolines by Acceptor-less Dehydrogenative Condensation

Article abstract of DOI:10.1002/chem.201402952

A novel family of iridium catalysts stabilised by P,N-ligands have been introduced. The ligands are based on imidazo[1,5-b]pyridazin-7-amines and can be synthesised with a broad variety of substitution patterns. The catalysts were synthesised quantitatively from the protonated ligands and a commercially available iridium precursor. The catalysts mediate the alkylation of amines by alcohols under mild conditions (70 °C). In addition, the synthesis of quinolines from secondary or primary alcohols and amino alcohols is reported. This sustainable synthesis proceeds through the liberation of two equivalents of water and two equivalents of dihydrogen. The investigations indicate that catalysts suitable for hydrogen autotransfer or borrowing hydrogen chemistry might also be suitable for acceptor-less dehydrogenative condensation reactions. Your H or mine? A family of catalysts that mediate the alkylation of amines by alcohols under mild conditions are introduced (see scheme; HA=hydrogen autotransfer, BH=borrowing hydrogen, ADC=acceptor-less dehydrogenative condensation). The efficient synthesis of quinolines from alcohols and amino alcohols through the liberation of two equivalents of dihydrogen is also mediated by this catalyst family.

Full text of DOI:10.1002/chem.201402952

DOI: 10.1002/chem.201402952
Full Paper
&
Sustainable Chemistry
New Iridium Catalysts for the Selective Alkylation of Amines by
Alcohols under Mild Conditions and for the Synthesis of
Quinolines by Acceptor-less Dehydrogenative Condensation
Susanne Ruch, Torsten Irrgang, and Rhett Kempe*[a]
Abstract: A novel family of iridium catalysts stabilised by
P,N-ligands have been introduced. The ligands are based on
imidazo[1,5-b]pyridazin-7-amines and can be synthesised
with a broad variety of substitution patterns. The catalysts
were synthesised quantitatively from the protonated ligands
and a commercially available iridium precursor. The catalysts
mediate the alkylation of amines by alcohols under mild
conditions (708C). In addition, the synthesis of quinolines
from secondary or primary alcohols and amino alcohols is re-
ported. This sustainable synthesis proceeds through the lib-
eration of two equivalents of water and two equivalents of
dihydrogen. The investigations indicate that catalysts suita-
ble for hydrogen autotransfer or borrowing hydrogen
chemistry might also be suitable for acceptor-less dehydro-
genative condensation reactions.
Introduction
developed into a practical synthetic method by (for instance)
the groups of Beller,[7] Grigg,[5,8] Fujita,[9] Williams,[7a,10] Yus[11]
and us.[12] The reaction can also be used to alkylate ammo-
nia[7b,13] and is currently being investigated diversely with
regard to catalyst development and substrate scope broaden-
ing.[14] Mechanistically related to the HA/BH reaction are ADC
reactions (Scheme 1). Here, the reduction step is suppressed
and dihydrogen is liberated, which leads to imines[15] or ole-
fins.[16] The ADC concept becomes synthetically useful especial-
ly if imine and olefin formation are combined.[17] In this regard,
an environmentally benign access to unsymmetrically substi-
tuted N-heteroaromatic compounds[18] has been reported by
the groups of Beller,[19] Milstein,[20] Saito[21] and us.[22]
The hydrogen autotransfer (HA),[1] or borrowing hydrogen
(BH),[2] reaction has received much attention in recent years.[3]
It is an efficient protocol for selective CÀN and CÀC bond-for-
mation reactions.[4] The state of the art mechanistic proposal
for the HA/BH reaction is shown in Scheme 1 (oxidation, con-
Herein, we report on a new family of catalysts that are able
to mediate both HA/BH and ADC reactions. The alkylation of
amines was accomplished under very mild conditions (HA/BH)
and the ADC concept was used to synthesise quinolones from
alcohols and amino alcohols. The results indicate that catalysts
efficient for HA/BH reactions might also be very useful for ADC
chemistry.
Scheme 1. The mechanism of the HA/BH reaction and the mechanism of the
related acceptor-less dehydrogenative condensation (ADC). The suppression
of the reduction step of the HA/BH reaction gives rise to imines or olefins or
their combinations. If this suppression takes place, catalyst recycling has to
occur via hydrogen liberation. X=N, CÀH; [M]=transition-metal catalyst
based on Ir or Ru, for instance.
Results and Discussion
densation, and reduction but no liberation). Amine alkylation
by means of the HA/BH concept was discovered by the groups
of Grigg[5] and Watanabe.[6] In the beginning, it was a rather
unselective reaction that required harsh conditions and was
Recently, we introduced a series of imidazo[1,5-b]pyridazine-
substituted amido ligands, which stabilised early- and late-tran-
sition-metal complexes.[23] They can be synthesised through
the nucleophilic ring transformation of oxadiazolium halides
1 with amines followed by a cyclocondensation reaction with
1,3-diketones. By using amino alcohols as nucleophilic re-
agents, chiral imidazo[1,5-b]pyridazine-substituted amino alco-
hols can be synthesised. The resulting Group 9 metal com-
plexes exhibit good activities and excellent enantioselectivities
in the asymmetric hydrogenation of ketones[24] and imines.[25]
Herein, the synthesis of novel N-phosphino-substituted
[a] S. Ruch, Dr. T. Irrgang, Prof. Dr. R. Kempe
Lehrstuhl Anorganische Chemie II (Catalyst Design), Universitꢀt Bayreuth
Universitꢀtsstrasse 30, NW I, 95440 Bayreuth (Germany)
Fax: (+49)921552157
E-mail: kempe@uni-bayreuth.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402952.
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Scheme 4. Test reaction for the optimisation of the reaction conditions.
Table 1. Results of studies to screen the solvent, base, aniline/base ratio
and aniline/alcohol ratio.[a]
Entry Solvent
Base
Aniline/base Aniline/alcohol Yield [%][b]
1
2
3
4
5
6
7
8
dioxane
THF
toluene
DME
diglyme KOtBu
diglyme K3PO4
diglyme NaOH
diglyme KOH
KOtBu
KOtBu
KOtBu
KOtBu
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.6
1:1.4
1:1.1
1:1
10
20
13
12
58
12
19
8
16
63
66
72
68
28
12
47
50
75
70
Scheme 2. Synthesis of ligands 5a–d.
imidazo[1,5-b]pyridazine amines 5 is reported. They can be
synthesised by P functionalisation of 4a,b[26] and 4c in good to
very good yields and high purities (Scheme 2). The modular
nature of this multi-component procedure ensures the intro-
duction of a broad variety of substitution patterns.
9
diglyme NaOtBu 1:1.2
10
11
12
13
14
15
16
17
18
19
diglyme KOtBu
diglyme KOtBu
diglyme KOtBu
diglyme KOtBu
diglyme KOtBu
diglyme KOtBu
diglyme KOtBu
diglyme KOtBu
diglyme KOtBu
diglyme KOtBu
1:1.1
1:1
The corresponding iridium complexes 6 were prepared in
quantitative yields from 5 and [{Ir(cod)MeO}2][27] (cod=1,5-cy-
clooctadiene) by methanol elimination (Scheme 2) in THF at
room temperature. X-ray crystal-structure analysis of 6a[28] was
performed to determine the molecular structure (Scheme 3) of
one of the pre-catalysts.
1:0.9
1:0.7
1:0.5
1:0.3
1:0.9
1:0.9
1:0.9
1:0.9
[a] Reaction conditions: 708C, 24 h, aniline (2.0 mmol), 0.1 mol% Ir load-
ing. DME=dimethoxyethane. [b] Determined by GC analysis with n-do-
decane as an internal standard.
Five bases were combined with five different solvents
(Table 1, entries 1–9). Diglyme and KOtBu were identified to be
the best combination of solvent and base (yield 58%; Table 1,
entry 5).
For further optimisation of the reaction parameters, both
the aniline/base and aniline/alcohol ratios were investigated
(Table 1, entries 10–19). Varying the base loading between 0.7
and 1.1 equivalents had a minor effect. For 0.9 equivalents of
base in relation to the amount of aniline, the yield is maximal
(Table 1, entry 12). The ratio of aniline to alcohol was also ex-
amined. The optimum aniline to benzyl alcohol ratio was ob-
served at 1:1.1 (Table 1, entry 18).
Scheme 3. Synthesis and molecular structure of 6a. Selected bond lengths
[ꢁ] and angles [8]: Ir1ÀN1 2.086(8), Ir1ÀP1 2.308 (2), P1ÀN4 1.665(9), N1ÀC9
1.407(12), N4ÀC9 1.311(12), P1ÀC16 1.847(11), P1ÀC19 1.848(12); N1-Ir1-P1
80.2(2), N1-Ir1-C9 114.0, 114.0, N4-P1-Ir1 105.6(3), N4-P1-C9 111.8(3), N4-C9-
N1 128.2(8), C16-P1-C19 104.0.
A planar five-membered (Ir1-N1-C9-N4-P1) ring with a P1-Ir1-
N1 angle of 80.28 is formed. Additionally, the coordination
sphere of iridium is saturated due to coordination of COD. N4
is deprotonated and the anionic charge is delocalised over the
P1-N4-C9-N1 backbone. The plane of the imidazo[1,5-b]pyrida-
zine framework deviates by 3.88 from the five-membered che-
late ring. The isopropyl substituents form a C16-P1-C19 angle
of 104.08.
The formation of N-benzylaniline (7a) from aniline and
benzyl alcohol (Scheme 4) was chosen as a model reaction for
the optimisation of the amine alkylation conditions. The cata-
lyst was rapidly formed in situ from a stock solution of 5a and
[{Ir(cod)MeO}2] (0.5 equiv) in THF.
Table 2. Results for catalyst screening studies.[a]
Entry
Catalyst
R1
R2
Yield [%][a]
1
2
3
4
5
6a
6b
6c
6d
–
Ph
Ph
iPr
Ph
iPr
iPr
–
75
95
69
20
2
Ph-p-OCH3
tBu
–
[a] Reaction conditions: aniline (2.0 mmol), benzyl alcohol (2.2 mmol),
0.1 mol% catalyst loading, KOtBu (1.8 mmol), diglyme (2 mL), 708C, 24 h.
[b] Determined by GC analysis with n-dodecane as an internal standard.
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By using the optimised reaction parameters, a screening of
complexes 6a–d was carried out (Table 2). Both complexes
with a phenyl substituent (6a+b), and complex 6c with
a para-methoxyphenyl substituent, at the imidazo[1,5-b]pyrida-
zine backbone show distinctly higher activities than that with
the electron-donating tert-butyl substituent (6d). In contrast to
the isopropyl substituents on phosphorus (6a), the phenyl
substituents (6b) lead to a further increase in yield of 20%
(Table 2, entry 2).
in very good to excellent yields. Interestingly, alkylations of ani-
line derivatives with electron-withdrawing substituents, such
as bromide and chloride, reach a maximum yield at very low
catalyst loadings of 0.025 (Table 3, entries 7 and 10) and
0.05 mol% (Table 3, entries 8 and 9).
Compounds containing
a quinoline framework exhibit
a wide variety of pharmacological and biological activities.
They are, for example, applied as anti-malarials,[29] anti-asthma-
tics,[40a,30] anti-inflammatories,[31] anti-bacterial,[32] anti-hyperten-
sitive,[33] anti-cancer,[34] tyrosine kinase inhibitory agents[35] and
they are known for tumour-initiating activities.[36] In addition,
quinolines are valuable synthons for the preparation of nano-
and mesostructures and polymers with enhanced electronic
and photonic functions.[37] Based on the many applications of
quinolines, their synthesis[38] by transition-metal catalysis is
a very vital field of chemistry.[11f,39,40]
To demonstrate the applicability of the optimised reaction
conditions, different aniline derivatives were alkylated with
benzyl alcohol by using catalyst 6b. Table 3 presents the re-
sults (7a–j) of the substrate scope screening. Both, electron
donating (Table 3, entries 2–6) and -withdrawing substituents
(Table 3, entries 7–10) in the ortho, meta or para position to
aniline are tolerated and provide alkylated aniline derivatives
As a useful test reaction to find the optimal reac-
tion conditions, we first investigated the formation of
Table 3. Results for substrate screening (7a–j) studies.[a]
7,8,9,10-tetrahydro-6H-cyclohepta[b]quinoline
(8a)
from 2-aminobenzyl alcohol and cycloheptanol in
a closed reaction system (Scheme 5).
Entry
Amine
Product
Ir loading
[%]
Yield
[%][b]
Two solvents were investigated with five different
bases (Table 4, entries 1–6). The yield was 14% higher
with THF than that with diglyme, and KOtBu seemed
to be the best base. The screening of the 2-amino-
benzyl alcohol to base ratio (Table 4, entries 7–16) re-
sulted in a maximum yield of 78% when using one
equivalent of base (Table 4, entry 11). However, even
0.5 equivalents of base (Table 4, entry 13) resulted in
a yield of 68%. Because catalytic amounts of base
worked well, the screenings were continued with
0.5 equivalents of base. Increasing the ratio of cyclo-
heptanol and 2-aminobenzyl alcohol to 5 enhanced
the yield of the desired quinoline to 92% (Table 4,
entry 17).
1
2
7a
7b
0.1
95
89
0.15
3
4
7c
0.5
84
85
7d
0.15
5
6
7e
7 f
0.1
91
95
After identifying the ideal reaction conditions, the
catalyst (6a) loading was varied (Table 5). At
0.1 mol%, the yield was almost quantitative after
24 h.
0.13
Screening of catalysts 6a–d did not result in
a better performing catalyst than 6a. Using the fa-
vourable reaction conditions, we set out to test the
reactivity of two amino alcohols with primary and
secondary alcohols. The results are listed in Table 6.
The reaction of 2-aminobenzyl alcohol with cycloal-
cohols gave rise to 2,3-substituted quinolines. Cyclo-
heptanol gave excellent yields (97%; Table 6, entry 1).
If cycloheptanol is used, a significantly lower yield is
observed due to ring constraints. If (2-amino-5-chlor-
ophenyl)methanol is used, a slightly reduced yield is
observed (Table 6, entry 9). The reactions of 2-amino-
benzyl alcohol with 1-phenylethanol (and its deriva-
tives) lead to 2-substituted quinolines and require
a higher catalyst loading (Table 6, entries 3 and 4).
Bromo substituents are tolerated, but lower yields
are observed due to dehalogenation. In addition to
aryl substituents, alkyl substituents can be introduced
7
7g
7h
7i
0.025
0.05
99
96
89
97
8
9
0.05
10
7j
0.025
[a] Reaction conditions: aniline derivative (2.0 mmol), benzyl alcohol (2.2 mmol), cata-
lyst 6b, KOtBu (1.8 mmol), diglyme (2 mL), 708C, 24 h. [b] Determined by GC analysis
with n-dodecane as an internal standard.
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Conclusion
We introduced a novel iridium catalyst system. The ligand syn-
thesis protocol allowed an easy variation of the ligand substi-
tution patterns and the formation of the iridium pre-catalyst
complex was quantitative (isolated in 96% yield). The catalyst
allowed the efficient alkylation of anilines by alcohols (HA/BH)
under mild conditions (708C). Furthermore, the synthesis of
quinolines through the ADC reaction, starting from the 2-ami-
nobenzyl alcohol and primary as well as secondary alcohols,
with a low catalyst loading and a catalytic amount of base was
accomplished. The results indicated that catalysts which medi-
ate HA/BH reactions efficiently might also be competitive in
ADC.
Scheme 5. Test reaction for the optimisation of the reaction conditions for
the quinoline synthesis.
Table 4. Results for screening of solvent, base, substrate/base ratio and
substrate/alcohol ratio.[a]
Entry Solvent Base
Amino alcohol/
base
Amino alcohol/al- Yield
cohol
[%][b]
1
2
3
4
5
6
7
8
diglyme KOtBu 1:1.2
KOtBu 1:1.2
K3PO4 1:1.2
1:2.2
1:2.2
1:2.2
1:2.2
1:2.2
1:2.2
1:2.2
1:2.2
1:2.2
1:2.2
1:2.2
1:2.2
1:2.2
1:2.2
1:2.2
1:2.2
1:5
1:4
1:3.5
1:3
1:2.5
1:2
1:1.5
1:1
60
74
2
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
KH
KOH
1:1.2
1:1.2
66
43
59
55
69
69
73
78
73
68
66
21
0
92
80
80
79
81
68
59
52
NaOtBu 1:1.2
KOtBu 1:3
KOtBu 1:2
Experimental Section
General considerations
9
KOtBu 1:1.8
KOtBu 1:1.4
KOtBu 1:1
KOtBu 1:0.8
KOtBu 1:0.5
KOtBu 1:0.4
KOtBu 1:0.2
KOtBu 1:0
KOtBu 1:0.5
KOtBu 1:0.5
KOtBu 1:0.5
KOtBu 1:0.5
KOtBu 1:0.5
KOtBu 1:0.5
KOtBu 1:0.5
KOtBu 1:0.5
All reactions with air-sensitive compounds were performed under
dry argon or nitrogen atmospheres by using Schlenk and glove
box techniques. Non-halogenated solvents were dried over sodium
benzophenone ketyl and halogenated solvents over P2O5. Deuter-
ated solvents were vented, stored over molecular sieves and dis-
tilled. All chemicals were purchased from commercial vendors.
NMR spectroscopy measurements were carried out by using
a Varian INOVA 300 MHz spectrometer at 298 K. Relative to the
deuterated solvent, chemical shifts are reported in ppm. For ele-
mental analyses, a Vario elementar EL III instrument was used. GC-
MS measurements were obtained with an Agilent Technologies
7890A/5975C system equipped with a HP-5MS column (30 mꢂ
0.32 mmꢂ0.25 mm). GC analyses were carried out with an Agilent
6890N network system equipped with a HP-5 column (30 mꢂ
0.32 mmꢂ0.25 mm). X-ray crystal-structure analysis was performed
by using a STOE-IPDS II diffractometer (l(MoKa)=0.71073 ꢁ), in-
cluding an Oxford Cryostream low-temperature unit. Structure so-
lution and refinement were accomplished by using the SIR97,[41]
SHELXL-97[42] and WinGX[43] programs.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
[a] Reaction conditions: 0.05 mol% 6a, 1108C, 24 h. [b] Determined by
GC analysis with n-dodecane as an internal standard.
Table 5. Catalyst 6a loading.[a]
Entry
Ir loading [%]
Yield [%][b]
General procedure for ligand synthesis (5a–d)
1
2
3
4
5
6
0.2
0.1
0.05
0.025
0.013
–
98
97
92
57
29
2
Compound 4 (1 equiv) was added to THF (30 mL). NEt3 was added
in excess dropwise at room temperature. After stirring the reaction
mixture for 1 h, the dilution was cooled to 08C and chlorodiisopro-
pylphosphine (1 equiv) was added. The mixture was stirred over-
night at room temperature and afterwards at 508C for 1 day. The
white precipitate was filtered off and the solvent was removed.
Then the remaining solid was dissolved in Et2O and filtered again.
The solvent was removed and the remains were evacuated at 608C
overnight. The solid was recrystallised in Et2O. Yields between 68
and 91% were obtained.
[a] Reaction conditions: 2-aminobenzyl alcohol (1.0 mmol), cyclohexanol
(5.0 mmol), KOtBu (0.5 mmol), THF (2 mL), 1108C, 24 h. [b] Determined by
GC analysis with n-dodecane as an internal standard.
at the 2-position. Steric crowding leads to lower yields (Table 6,
entry 5). If primary alcohols are used, 3-substituted quinolines
can be synthesised (Table 6, entries 6–8).
General procedure for complex synthesis
A comparison with the catalyst introduced recently by the
Milstein group (0.5 mol% of a bipyridyl-based ruthenium
pincer complex, stoichiometric amounts of a base, four exam-
ples)[20b] indicates more efficiency, larger substrate scope and
better functional group tolerance of the iridium catalyst dis-
cussed herein.
Compound 5 (1 equiv) was dissolved in THF (10 mL) and then [{Ir-
(MeO)(cod)}2] (0.5 equiv) was added. The solution changed in
colour from orange to red. The mixture was stirred for 3 h at 508C
and overnight at room temperature. After removing the solvent,
the red solid was recrystallised in hexane/THF=2/1. The yield was
almost quantitative.
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Acknowledgements
Table 6. Results for substrate screening (8a–j) studies.[a]
Entry Amino alcohol Alcohol
Product
Ir
Yield
Financial support by the Deutsche Forschungsge-
meinschaft (DFG KE 756/23-1) is gratefully acknowl-
edged. We also thank Dr. I. Haas for solving of the
structure of catalyst 6a.
loading [%][b,c]
[mol%]
1
2
3
0.1
0.1
0.2
97 (94)
53 (50)
90 (86)
Keywords: alkylation · dehydrogenation · hydrogen ·
iridium · N,P ligands
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[a] Reaction conditions: 2-aminobenzyl alcohol derivative (1.0 mmol), alcohol
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was cooled to room temperature and quenched by adding water.
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Chem. Eur. J. 2014, 20, 13279 – 13285
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Received: April 4, 2014
Published online on September 3, 2014
Chem. Eur. J. 2014, 20, 13279 – 13285
www.chemeurj.org
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