FeCl3-Mediated synthesis of polysubstituted tetrahydroquinolines via domino Mannich/Friedel-Crafts reactions of aldehydes and amines

Article abstract of DOI:10.1039/c1ob05646h

A useful method to construct highly substituted tetrahydroquinolines has been developed through an iron(iii) chloride-mediated domino Mannich and intramolecular Friedel-Crafts alkylation followed by intermolecular Friedel-Crafts alkylation reactions of al

Full text of DOI:10.1039/c1ob05646h

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FeCl3-Mediated synthesis of polysubstituted tetrahydroquinolines via domino
Mannich/Friedel–Crafts reactions of aldehydes and amines†
Yan-Fang Yang,a Xing-Zhong Shu,a Hai-Long Wei,a Jian-Yi Luo,a Shaukat Ali,a Xue-Yuan Liua and
Yong-Min Liang*a,b
Received 25th April 2011, Accepted 17th May 2011
DOI: 10.1039/c1ob05646h
A useful method to construct highly substituted tetrahydro-
quinolines has been developed through an iron(III) chloride-
mediated domino Mannich and intramolecular Friedel–
Crafts alkylation followed by intermolecular Friedel–Crafts
alkylation reactions of aliphatic aldehydes with aromatic
amines.
Tetrahydroquinoline derivatives are an important class of bio-
logically active compounds, which are widely used in organic
synthesis and pharmaceutical chemistry.1 Currently, much effort
Scheme
1
Proposed domino Mannich/intramoleculer FC alkyla-
in this area is focused on constructing polysubstituted tetrahydro-
quinolines, for instance the aza Diels–Alder reactions,2 hydrogena-
tions of quinolines,3 benzotriazole-mediated indirect electrophilic
substitution,4 electrophilic cyclization,5 ring expansion reactions,6
palladium-catalyzed cross-coupling reactions,7 organocatalytic
hydroarylations of enals,8 hydroaminations of aniline alkynes,9
and intramolecular redox reactions.10 However, to the best of
our knowledge, intermolecular Friedel–Crafts (FC) alkylation to
construct polysubstituted tetrahydroquinolines has been rarely
reported. Crabb et al. reported a protonic acid-catalyzed conden-
sation of anilines with two molecules of an aldehyde affording
a mixture of the cis- and trans- isomers of 2,6-dimethyl-4-
hydroxy-1,2,3,4-tetrahydroquinoline in an approximate ratio of
1 : 2 [Scheme 1, eqn (1)].11 In recent years, significant efforts have
been focused on benzylic arylation chemistry utilising Friedel–
Crafts alkylations.12 Friedel–Crafts reactions of benzylic alcohols
have been studied with traditional Lewis and Brønsted acids.13
Beller et al. demonstrated that late transition metal salts such as
HAuCl4, IrCl3, [MesW(CO)3], RhCl3, H2PdCl4, H2PtCl6 and FeCl3
effectively catalyze the addition of benzyl acetates and benzyl
alcohol to arenes.14 We envisioned that FeCl3 is an attractive alter-
native to rare-earth triflates since it is non-toxic, cheap and readily
available.15 With these thoughts in mind, we decided to test a new
domino reaction involving a cascade Mannich/intramolecular
tion/intermolecular FC alkylation reactions.
Friedel–Crafts alkylation/intermolecular Friedel–Crafts alkyla-
tion sequence.
Domino reactions are attractive to industrial and laboratory
chemists because of their potential to save solvents, reagents,
time and energy.16 Our group is persistently interested in
domino reactions to synthesize various functionalized heterocyclic
compounds.17 Herein, we report our results of the cascade
reactions of aliphatic aldehydes with aromatic amines in the
presence of FeCl3. This strategy provides a pathway in one-pot
manner to the synthetically useful tetrahydroquinolines.
We conducted many trials to approach our goal by treating a
threefold excess of N-methylaniline 1a (1.2 mmol) with phenylac-
etaldehyde 2a (0.4 mmol) in the presence of 0.3 equivalents of iron
˚
catalyst and 50 mg of 4 A molecular sieves (MS) in CH3NO2 (3 mL)
under argon. Gratifyingly, the desired product 3a was formed in
33% and 38% yield by using of FeCl3 and FeBr2 respectively, after
8 h at 60 ◦C (Table 1, entries 1 and 4). No reaction was observed
in anisole when FeBr2 was used as catalyst, whereas FeCl3 gave
a 78% yield (entries 5 and 6). Using anisole as solvent, the yield
was improved greatly. The reaction performed in other solvents
afforded inferior yields (entries 7–10). Lewis acid such as Sc(OTf)3
and InCl3 showed comparable catalytic activity to give 3a in 61%
and 71% yield, respectively (entries 13 and 14). Brønsted acids were
also examined, and only HSbF6·6H2O worked and gave 62% yield
(entry 16), whereas TFA and TsOH did not catalyze this reaction.
10 mol% and 20 mol% of FeCl3 gave lower yields (entries 19 and
20). When the reaction was conducted at ambient temperature, it
proceeded with a lower reaction yield (entry 21, 45%). Surprisingly,
a higher reaction temperature did not increase the yield (entry 22,
51%). As the reaction proceeds with loss of two equivalents of
aState Key Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou, 730000, People’s Republic of China
bState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical
Physics, Chinese Academy of Science, Lanzhou, 730000, People’s Republic
of China. E-mail: liangym@lzu.edu.cn; Fax: (+86)-931-891-2582
† Electronic supplementary information (ESI) available: 1H and 13C NMR
spectra for compounds 3a–3k, 4a–4f. CCDC reference number 807037.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c1ob05646h
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Table 1 Optimization of reaction conditionsa
˚
water, the addition of 4 A MS was essential. After the systematic
˚
screening, the◦use of 0.3 equivalents of FeCl3 with 4 A MS in dry
anisole at 60 C under argon was considered to be the optimum
and selected as the standard conditions.
With the optimized conditions in hand, we first explored the
scope of amines for this reaction, as summarized in Table 2. The
reaction proceed well with substituents on the meta positions of
N-methylanilines. Aromatic amines with electron-donating groups
on the benzene rings gave higher yields than those with electron-
withdrawing groups except the 3-methoxy group (entries 2–7).
This shows that steric effects had a strong influence on the
reaction. With a 3,5-dimethyl group on the aniline, however,
no desired product was obtained (entry 8). The methodology
also tolerated well the R2 position being occupied by a phenyl
group, affording the desired product 3h in 58% yield (entry 9). To
investigate the steric effects, we found that 3-methyldiphenylamine
1i reacted with 2a to produce the major product 3i in 47%
yield, showing little steric hindrance (entry 10). Interestingly, the
use of a highly hindered secondary amine 1j was also allowed,
which gave an approximate 1 : 1 mixture of two isomers of 5,6,7-
trisubstituted julolidine 3j (entry 11).18 Furthermore, N-methyl-1-
naphthylamine gave the by-product 5-methyl-1-naphthylamine in
50% yield, but the desired product 3k was obtained in 10% yield
(entry 12). Finally, it is worthwhile to note that N-methylanilines
with ortho and para substituents didn’t work at all. According to
Entry Catalyst
Loading (mol%) Solvent
Yield (%)b
1
2
3
4
FeCl3
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
CH3NO2
CH3NO2
CH3NO2
CH3NO2
Anisole
Anisole
CH3CN
DCE
33
Fe(NO3)3
Fe(acac)3
FeBr2
NR
NR
38
NR
78
48
39
5
6
FeBr2
FeCl3
7
8
FeCl3
FeCl3
9
FeCl3
FeCl3
AlCl3
Cu(OTf)2
Sc(OTf)3
InCl3
Toluene
Chlorobenzene 22
38
10
11
12
13
14
15
16
17
18
19
20
21c
22d
Anisole
Anisole
Anisole
Anisole
Anisole
Anisole
Anisole
Anisole
Anisole
Anisole
Anisole
Anisole
NR
32
61
71
47
62
NR
NR
15
BF3·Et2O
HSbF6·6H2O 30
TFA
30
30
10
20
30
30
TsOH
FeCl3
FeCl3
FeCl3
FeCl3
1
the H NMR and X-ray diffraction analysis of 3e (Fig. 1), the
31
45
51
2,3-trans-2,4-trans isomers were proved to be the major products;
in most cases, the minor isomers exhibited the 2,3-trans-2,4-cis
configuration.
To expand further the scope of the reaction, we also investi-
gated other aldehydes. Aldehydes unbranched at the a-position
react similarly producing 1,2,3,4-tetrahydroquinoline derivatives.
Table 3 demonstrates the generality and scope of the reaction
a
˚
Conditions: 1.2 mmol 1a, 0.4 mmol 2a and 50 mg of 4 A MS with 0.3
equivalents of catalyst in solvent (3.0 mL) at 60 ◦C under Ar after 8 h.
b Isolated yield. c Performed at room temperature after 24 h. d Performed
at 80 ◦C.
Table 2 FeCl3-Mediated synthesis of tetrahydroquinolines with aromatic aminesa
1
Entry
R1
R2
Time (h)
Yield (%)b
Ratio of isomersc
1
2
3
4
5
6
7
8
H
3-Me
3-OMe
3-F
3-Cl
3-Br
3-CO2Me
3,5-Dimethyl
Me
Me
Me
Me
Me
Me
Me
Me
8
78 (3a)
72 (3b)
42 (3c)
69 (3d)
65 (3e)
56 (3f)
47 (3g)
NR
58 (3h)
47 (3i)
38 (3j)
10 (3k)
88 : 12
72 : 28
75 : 25
90 : 10
86 : 14
89 : 11
87 : 13
—
86 : 14
79 : 21
51 : 49
67 : 33
10
12
10
10
14
12
10
10
10
12
12
9
H
H
Ph
3-Me-Ph
10
11
12
1,2,3,4-Tetra-hydroquinoline
N-Methyl-1-naphthylamine
Reactions were conducted with 1 (1.2 mmol), 2a (0.4 mmol) and 50 mg of 4 A MS with 0.3 equivalents of FeCl3 in anisole (3 mL) at 60 ◦C under Ar.
b Isolated yield. c The ratio of (2,3-trans-2,4-trans) isomer to (2,3-trans-2,4-cis) isomer was determined by 1H NMR analysis.
a
˚
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Table 3 FeCl3-Mediated synthesis of tetrahydroquinolines with aliphatic aldehydesa
Entry
1
Aldehyde
Product
Yield (%)b
38
Ratio of isomersc
90 : 10
2
48
94 : 6
3
40
88 : 12
4
38
>99 : 1
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Table 3 (Contd.)
Entry
5
Aldehyde
Product
Yield (%)b
70
Ratio of isomersc
48 : 52
6
42
<1 : 99d
Reactions were conducted with 1a (1.2 mmol), 0.4 mmol of 2, 50 mg of 4 A MS and 0.3 equivalents of FeCl3 in anisole (3 mL) at 60 ◦C under Ar after
10 h. b Isolated yield. c Determined by the 1H NMR analysis. d dr = 3 : 1.
a
˚
of N-methylaniline 1a with aliphatic aldehydes 2 to form the
corresponding products under optimized conditions. However,
acetaldehyde was less efficient in this transformation (entry 1).
The increase in the length of the aldehyde chain caused a decrease
in yield of the reaction (entries 2-4). When octanal was added to
the reaction system, no desired product was observed. Enolization
of the aldehyde becomes difficult with the increase in chain length.
In addition, reactions of 3-phenylpropanal with N-methylaniline
can also be carried out, affording a mixture of 1 : 1 isomers, which
The mechanism for this transformation is proposed to be as de-
picted in Scheme 2.20 Iminium ion A is formed by the condensation
of aniline 1 with aldehyde 2, which adds to a molecule of iron(III)
enolate21 2¢ to produce aldehyde B by a Mannich reaction.22 Then,
aldehyde B undergoes an FeCl3-catalyzed intramolecular Friedel–
Crafts type ring closure to furnish a benzyl alcohol intermediate C,
which in the presence of a proton loses water and releases FeCl3 to
produce carbocation intermediate D. Finally, excess N-protected
aniline23 as the aromatic nucleophile attacks the carbocation
intermediate D via an intermolecular Friedel–Crafts reaction,
which loses a proton to afford tetrahydroquinoline skeleton 3
or 4. In this transformation, carbocation intermediate D is the
key intermediate leading to compound 3 or 4. If we decrease the
temperature or reduce the catalyst loading, there will be some
iminium ion A left unreacted. Thus, the SN1 step to generate
carbocation intermediate D is the rate determining step.
1
was determined from the H NMR spectrum, in 70% combined
yield (entry 5). When enantiomerically pure methyl-(S)-2-N,N-
di-tert-butoxycarbonyl-5-oxopentanoate 2f, which can be readily
synthesized in four conventional steps from L-glutamic acid,19
was subjected to the standard reaction conditions, the product
4f was obtained as a mixture of 3 : 1 diastereoisomers (entry 6).
The enantiomeric excess of each isomer was determined to be
higher than 99% by chiral HPLC analysis. It appears that 2f did
not racemize during this process. The relative configurations of the
major isomers 4a, 4b, and 4f were assigned by Nuclear Overhauser
Effect (NOE) spectroscopy (cf. ESI†) and additionally confirmed
by an X-ray crystal structure analysis of 3e (Fig. 1).
In summary, we have developed a simple method for a
one-pot domino Mannich/intramolecular Friedel–Crafts alkyla-
tion/intermolecular Friedel–Crafts alkylation reactions by using
iron(III) chloride as catalyst, which leads to the synthesis of 1,2,3,4-
tetrahydroquinoline derivatives. The advantages of this method
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residue was purified by flash chromatography using petroleum
ether/ethyl acetate as eluent on alkalescent silica gel to give the
desired product.
Acknowledgements
We thank the NSF (NSF-20872052, NSF-20090443, NSF-
21072080), and the Fundamental Research Funds for the Central
Universities (lzujbky-2010-k09) for financial support.
Notes and references
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3a–3k, 4a–4f
˚
To a solution of 1 (1.2 mmol), 2 (0.4 mmol) and 4 A MS 50 mg
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The mixture was stirred under argon at 60 ◦C. On completion
of the reaction as shown by TLC analysis, the reaction mixture
was filtered and partitioned between sat. aq. NaHCO3 and ethyl
acetate. The aqueous layer was further extracted with ethyl
acetate (3 ¥ 20 mL). The combined organic extracts was dried
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