Iridium-catalyzed alkylation of methylquinolines with alcohols

Article abstract of DOI:10.1021/jo3019347

Iridium-catalyzed alkylation of methylquinolines at the methyl substituent was achieved using alcohols as alkylating agents. The reaction proceeded through a transfer hydrogenation pathway from the alcohol to the Ir complex, affording an aldehyde and Ir-H species, followed by base-assisted aldol condensation and hydrogenation. This method provides an atom-economical and convenient route to alkylquinolines from easily accessible methylquinolines.

Full text of DOI:10.1021/jo3019347

Note
pubs.acs.org/joc
Iridium-Catalyzed Alkylation of Methylquinolines with Alcohols
Yasushi Obora,* Shinji Ogawa, and Nobuyuki Yamamoto
Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Suita,
Osaka 564-8680, Japan
S
* Supporting Information
ABSTRACT: Iridium-catalyzed alkylation of methylquinolines
at the methyl substituent was achieved using alcohols as
alkylating agents. The reaction proceeded through a transfer
hydrogenation pathway from the alcohol to the Ir complex,
affording an aldehyde and Ir−H species, followed by base-
assisted aldol condensation and hydrogenation. This method provides an atom-economical and convenient route to
alkylquinolines from easily accessible methylquinolines.
uinolines and their derivatives are important motifs in
ligands such as py2NP(iPr)2.11 Reflecting on these related
studies, we realized the importance of the necessity of
investigating efficient synthetic methods for obtaining alkylqui-
nolines from easily accessible methylquinolines using simple
catalytic systems. Here we report a simple and versatile method
for the preparation of various alkylquinolines from methyl-
quinolines and alcohols in the presence of an [Ir(OH)(cod)]2
catalyst combined with a PPh3 ligand and a base.
Q pharmaceutical and agricultural chemistry and are used
as building blocks in total syntheses of natural products.1
Various methods for synthesizing quinolines and quinoline-
based natural products have therefore been developed.2 In
particular, the introduction of alkyl-chain moieties into
benzoquinones is an important chemical transformation.3
Conventionally, methylquinolines have been chosen as the
starting materials for the preparation of alkylated quinoline
derivatives because they are easily accessible.3 The reaction of a
methylquinoline with n-BuLi followed by reaction with an alkyl
halide is the classic method of synthesizing alkylquinolines.3
However, since a prerequisite of the conventional reaction is
the use of aryl halides as substrates, this method suffers from
formation of undesired stoichiometric amounts of waste salts
generated from alkyl halides and bases. The development of an
improved reaction that is environmentally benign and uses
easily accessible starting materials for the introduction of alkyl
groups on methylquinolines is therefore highly desirable.
It is well-known that Ir and Ru complexes serve as efficient
catalysts for transfer hydrogenation (also known as hydrogen
borrowing) from alcohols to aldehydes and ketones.4 Much
research has focused on both α-alkylation and β-alkylation
reactions using alcohols as alkylating agents.5 Our group has
focused on Ir-catalyzed α-alkylations of methyl ketones, methyl
esters, and active methylene compounds, and Guerbet-type
dimerizations of alcohols.6 We achieved Ir-catalyzed alcohol
transformations to esters,7 and reactions of alcohols with
alkynes or enones led to homoallylic alcohols,8 enones,9 and
1,3-diketones.10
To determine the optimum reaction conditions, the reaction
of 2-methylquinoline (1a) with benzyl alcohol (2a) was chosen
as a model reaction and carried out under various reaction
conditions. The results for the alkylation reactions are shown in
Table 1. For example, the reaction of 1a (3 mmol) with 2a (1
mmol) was performed in the presence of [Ir(OH)(cod)]2 (0.05
mmol, 5 mol %) combined with PPh3 (0.20 mmol, 20 mol %)
and t-BuOK (0.5 mmol, 50 mol %) in 1,4-dioxane (1 mL) at
130 °C for 24 h, giving 3a in quantitative yield (Table 1, entry
1). With regard to the Ir complex, [Ir(OMe)(cod)]2,
[IrCl(cod)]2, and [Cp*IrCl2]2 gave comparable good yields
(Table 1, entries 2−4). However, when IrCl3·3H2O and an Rh
analogue, [RhCl(cod)]2, were used, the reactions were sluggish
(Table 1, entries 5 and 6). As the base added in the reaction, t-
BuOK gave the best result, and strong bases such as KOH gave
the product in good yield, but weak bases such as Na2CO3
resulted in total inactivity under these conditions (Table 1,
entries 1, 7, and 8). It was found that the addition of a
phosphine ligand was necessary to obtain higher yields from the
reaction; the reaction was sluggish if a diphosphine was used, or
if the reaction was performed in the absence of a phosphine
ligand (Table 1, entries 9−12). In this reaction, high yield
formation of 3a was achieved by using excess (3 equiv to 2a)
amount of 1a (Table 1, entry 1). However, the conversion of 1a
was 35% and most of unreacted 1a was recovered (59%) by
distillation after the reaction. On the other hand, the equimolar
reaction of 1a with 2a resulted in decrease in the yield of 3a
(Table 1, entry 13) because 1a possesses a less acidic methyl
During the course of our investigations, we became aware of
the development of methyl alkylation of methylquinolines by a
transfer hydrogenation method using alcohols as alkylating
agents. This is a convenient method for obtaining synthetically
useful quinoline derivatives and also for achieving high-yield
direct alkylation of less acidic methyl substituents on the
aromatic compound using a simple Ir catalyst system. Recently,
Kempe and co-workers reported the alkylations of methylpyr-
imidines and picolines using Ir complexes bearing P,N-type
Received: September 6, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/jo3019347 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
Table 1. Ir-Catalyzed Reactions of 2-Methylquinoline (1a)
with Benzyl Alcohol (2a) under Various Conditions
Table 2. Ir-Catalyzed Reactions of 2-Methylquinoline (1a)
with Primary Alcohols 2
a
a
b
entry
Ir catalyst
base
ligand
PPh3
yield 3a (%)
1
2
3
4
5
6
7
8
9
10
[Ir(OH)(cod)]2
[Ir(OMe)(cod)]2
[IrCl(cod)]2
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
KOH
quant (92)
PPh3
86
81
64
PPh3
[Cp*IrCl2]2
IrCl3·3H2O
PPh3
c
PPh3
1
[RhCl(cod)]2
[Ir(OH)(cod)]2
[Ir(OH)(cod)]2
[Ir(OH)(cod)]2
[Ir(OH)(cod)]2
[Ir(OH)(cod)]2
[Ir(OH)(cod)]2
[Ir(OH)(cod)]2
PPh3
7
PPh3
58
d
Na2CO3
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
PPh3
n.d.
PCy3
P(n-Oct)3
dppp
None
PPh3
59
40
12
10
e
11
12
f
g
13
41
a
Conditions: 1a (3 mmol) was allowed to react with 2a (1 mmol) in
the presence of Ir catalyst (0.05 mmol), ligand (0.20 mmol), and base
b
(0.50 mmol) in 1,4-dioxane (1 mL) at 130 °C for 24 h. GC yields
c
based on 2a except the value in the parentheses. 28% of 2-
d
e
stylquinoline was observed. Not detected by GC. dppp (0.10 mmol)
f
g
was used. 2-Methylquinoline (1 mmol) was used. Conversions of 1a
and 2a were 14% and 46%.
group and shows low reactivity in the reaction. No reaction
took place in the absence of an Ir complex.
1,4-Dioxane is the most suitable solvent for the reactions
(Table 1, entry 1). The use of other selected solvents under
these conditions gave the following results: p-xylene (43%),
diglyme (85%), 1,2-dichlorobenzene (34%), and octane (50%).
The methyl alkylation of 2-methylquinoline proceeded
smoothly using various substituted benzyl alcohols bearing
electron-donating/-withdrawing groups (Table 2, entries 1−7),
as well as using aliphatic alcohols (Table 2, entries 8−11),
giving the corresponding products (3b−3l) in good to excellent
yields under the same condition as for entry 1 in Table 1; the
results are shown in Table 2. Although this reaction tolerated
various primary alcohols as alkylating agents, unfortunately, the
use of secondary alcohols such as benzhydrol was sluggish
under these conditions.
The present catalytic system was successfully extended to
reactions of various heteroaromatic compounds, and the results
are shown in Table 3. 4-Methylquinoline gave the correspond-
ing product in good yield (Table 3, entry 1), but 3- and 6-
methylquinoline were totally inactive (Table 3, entries 2 and 3).
The use of 2-methylquinoxaline and 2-methylbenzoxazole gave
the desired methyl-alkylated products (3n and 3o) in high to
excellent yields (Table 3, entries 4 and 5). In contrast, the use
of 2-methylpyridine gave the corresponding alkylated pyridine
(3p) in only 21% yield (Table 3, entry 6).
a
Conditions: 1a (3 mmol) was allowed to react with 2 (1 mmol) in
the presence of Ir catalyst (0.05 mmol), ligand (0.20 mmol), and base
(0.50 mmol) in 1,4-dioxane (1 mL) at 130 °C for 24 h. t-BuOK (0.70
mmol) was used. 2-Methylquinoline (10 mmol) was used.
b
c
methylquinoline, 32 for 3-methylquinoline, and 29.5 for 2-
methylpyridine).12
These results suggest that the use of methylquinolines with
benzo substituents on the methylpyridine scaffold is a
prerequisite for the reaction to proceed efficiently and in
good yield. Furthermore, the use of 2- or 4-methylquinoline
efficiently increases the acidity of the methyl protons for the
deprotonation step on addition of a base in the catalytic system
(pKa values are 25 for 2-methylquinoline, 25 for 4-
A similar enhancement of the reaction by a benzo-substituent
effect was observed in our study of Ir-catalyzed reactions of 1-
naphthylamine and diols, giving benzo[h]quinolines and
benzoindoles.13
To obtain further insights into the present reaction, the time
course of the reaction of 1a and 2a was monitored (Figure 1).
In the early stage of the reaction, the formation of 2-
B
dx.doi.org/10.1021/jo3019347 | J. Org. Chem. XXXX, XXX, XXX−XXX

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Note
The above alkylation reaction is therefore thought to proceed
through the following sequential reactions, involving three key
steps, as proposed in previous reviews (Figure 2):4 (i)
hydrogen transfer from alcohol 2 to an Ir complex, giving
aldehyde A and an Ir hydride intermediate B; (ii) base-
catalyzed aldol condensation by a coupling reaction of A and 1a
to form alkenylquinoline 4;1c,14 and (iii) selective hydro-
genation of 4 by the Ir hydride complex generated during the
course of the reaction, leading to alkylated quinoline 3.
In conclusion, an efficient and selective alkylation of
methylquinolines was successfully developed using an Ir
complex combined with a phosphine ligand and a base. The
reaction provides a simple and atom-economical direct route to
alkylquinolines, as the exclusive products, in good to excellent
yields.
Table 3. Ir-Catalyzed Reactions of Heteroaromatic
Compounds 1 with Benzyl Alcohol (2a)
a
EXPERIMENTAL SECTION
■
General. GC analysis was performed with a flame ionization
detector using a 0.22 mm × 25 m capillary column (BP-5). 1H and 13
C
NMR were measured at 400 and 100 MHz, respectively, in CDCl3
with Me4Si as the internal standard. The products were characterized
1
by H NMR, 13C NMR, and GC−MS. The yields of products were
estimated from the peak areas based on the internal standard
technique using GC.
Compounds 3a,15a 3b,15b 3c,15c 3e,15d 3i,15e 3j,15e 3k,15e 3m,15f
3n,15g 3o,15h 3p,15i and 4a1c were reported previously.
Typical Reaction (Table 1, entry 1). A mixture of [Ir(OH)-
(cod)]2 (32 mg, 0.05 mmol), PPh3 (52 mg, 0.20 mmol), t-BuOK (56
mg, 0.5 mmol), 1a (0.43 g, 3 mmol), and 2a (0.1 g, 1 mmol) in 1,4-
dioxane (1 mL) was stirred at 130 °C for 24 h under Ar. The
conversions and yields of products were estimated from peak areas
based on an internal standard using GC, and the product 3a was
obtained in quantitative yield. The product 3a was isolated by column
chromatography (230−400 mesh silica gel, n-hexane/ethyl acetate =
10/1) and Kugelrohr distillation (100 °C (pot)/0.3 mmHg, 1 h) in
92% yield (215 mg) as yellow liquid. After the reaction, unreacted 1a
(1.77 mmol, 253 mg, 59%) was recovered by Kugelrohr distillation (95
°C (pot)/0.3 mmHg, 1 h).
a
Conditions: 1 (3 mmol) was allowed to react 2a with (1 mmol) in
the presence of Ir catalyst (0.05 mmol), ligand (0.20 mmol), and base
(0.50 mmol) in 1,4-dioxane (1 mL) at 130 °C for 24 h. t-BuOK (0.70
mmol) was used. 1 (5 mmol) was used. Not detected by GC. t-
b
c
d
e
Compound 3b. Yield 88% (232 mg), yellow solid, mp 56−57 °C
BuOK (1 mmol) was used.
(lit.15b mp 58−59 °C)
Compound 3m. Yield 56% (131 mg), yellow solid, mp 99−100 °C
(lit.15f mp 96−97 °C)
Compound 3o. Yield 72% (161 mg), white solid, mp 53−54 °C
(lit.15h mp 53−54 °C)
1
Compound 3d. Yield 76% (188 mg), yellow liquid; H NMR δ
8.09−7.00 (m, 10H), 3.29−3.25 (m, 2H), 3.13−3.09 (m, 2H), 2.312
(s, 3H); 13C NMR δ 161.9 (C), 148.0 (C), 139.7 (C), 136.2 (CH),
136.0 (C), 130.2 (CH), 129.4 (CH), 128.9 (CH), 128.8 (CH), 127.5
(CH), 126.8 (C), 126.1 (CH), 126.0 (CH), 125.8 (CH), 121.5 (CH),
39.7 (CH2), 33.2 (CH2), 19.3 (CH3); IR (neat, cm−1) 3053, 2920,
1599, 1504, 1427, 827, 785, 745; GC−MS (EI) m/z (relative
intensity) 247 (89) [M]+, 246 (100), 156 (56), 142 (2), 128 (14), 105
(11), 91 (3), 77(14); HRMS (EI-TOF) m/z calcd for C18H17N [M]+
247.1361, found 247.1357.
Compound 3f. Yield 89% (258 mg), yellow solid, mp 66−67 °C;
1H NMR δ 8.09−7.20 (m, 10H), 3.31−3.27 (m, 2H), 3.14−3.10 (m,
2H), 1.31 (s, 9H); 13C NMR δ 162.0 (C), 148.8 (C), 148.0 (C), 138.5
(C), 136.2 (CH), 129.3 (CH), 128.9 (CH), 128.1 (CH), 127.5 (CH),
126.8 (C), 125.7 (CH), 125.3 (CH), 121.5 (CH), 41.0 (CH2), 35.4
(CH2), 34.4 (C), 31.4 (CH3); IR (neat, cm−1) 2959, 1599, 1500, 1425,
827, 752; GC−MS (EI) m/z (relative intensity) 289 (94) [M]+, 288
(100), 274 (27), 258 (2), 244 (1), 232 (15), 156 (34), 147 (8), 142
(4), 132 (6), 128 (11), 119 (3), 118 (2), 117 (18), 105 (5), 104 (2),
103 (3), 91 (10), 89 (3), 77 (4), 57 (3); HRMS (EI-TOF) m/z calcd
for C21H23N [M]+ 289.1830, found 289.1840.
Figure 1. Time-course monitoring of reaction of 1a with 2a.
phenylethenylquinoline (4a) was detected, which showed that
4a was an intermediate in the reaction. We interrupted the
reaction course at 1 h, and the conversions of 1a and 2a were
10% and 41%, respectively. Products 3a and 4a were isolated
during the course of the reaction. The results, shown in Figure
1 and Scheme 1, clearly indicate that 4a was an intermediate in
the reaction.
Compound 3g. Yield 82% (232 mg), yellow solid, mp 125−126
1
°C; H NMR δ 8.10−7.17 (m, 13H), 3.39−3.28 (m, 4H), 3.13−3.09
(m, 2H); 13C NMR δ 161.7 (C), 148.0 (C), 139.0 (C), 136.2 (CH),
C
dx.doi.org/10.1021/jo3019347 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
Scheme 1. Observed Reaction Intermediate 4a in the Reaction of 1a with 2a
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the MEXT-Supported Program for
the Strategic Research Foundation at Private Universities
(2010-2014), ALCA from Japan Science and Technology
Agency (JST), and the Kansai University Research Grants:
Grant-in Aid for Encouragement of Scientists, 2012.
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Figure 2. Plausible reaction pathway.
133.6 (C), 132.0 (C), 129.4 (CH), 128.9 (CH), 127.9 (CH), 127.6
(CH), 127.5 (CH), 127.5 (CH), 127.3 (CH), 126.8 (C), 126.6 (CH),
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1H NMR δ 8.07−7.12 (m, 10H), 3.26−3.22 (m, 2H), 3.14−3.10 (m,
2H); 13C NMR δ 161.2 (C), 147.9 (C), 139.9 (C), 136.2 (CH), 131.6
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126.7 (C), 125.8 (CH), 121.4 (CH), 40.6 (CH2), 35.0 (CH2); IR
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Compound 3l. Yield 78% (199 mg), yellow liquid; H NMR δ
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(CH), 36.8 (CH2), 33.5 (CH2), 32.7 (CH2), 28.9 (CH2), 25.7 (CH2),
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* Supporting Information
1H, and 13C NMR spectra for products. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: obora@kansai-u.ac.jp.
■
D
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