Highly efficient and regioselective platinum(ii)-Catalyzed tandem synthesis of multiply substituted indolines and tetrahydro-Quinolines

Full text of DOI:10.1002/anie.200805383

Angewandte
Chemie
DOI: 10.1002/anie.200805383
Synthetic Methods
Highly Efficient and Regioselective Platinum(II)-Catalyzed Tandem
Synthesis of Multiply Substituted Indolines and Tetrahydro-
quinolines**
Xin-Yuan Liu and Chi-Ming Che*
Transition-metal-catalyzed tandem cyclization reactions can
provide an efficient way to construct polycylic structures from
readily accessible organic compounds.[1] In particular, the use
of platinum and gold complexes as catalysts has recently
generated a variety of methods for the synthesis of organic
compounds. These syntheses exhibit good selectivity, have
high atom-economy, and are carried out under mild reaction
Scheme 1. The highly selective formation of indolines or tetrahydroqui-
nolines from metal-catalyzed tandem reactions.
conditions and at low catalyst loadings.[2]
Following our recent work on the synthesis of substituted
1,2-dihydroquinolines and pyrrolo[1,2-a]quinolines from
reactions of alkynes with amines or aminoalkynes through
gold(I)-catalyzed tandem cyclization,[3] we turned to a more
complicated system, that is, the metal-catalyzed reaction of
aminoalkynes (1) with 1,3-diketones (2; see Scheme 1). A
wide variety of products could be expected from this reaction
based on well-documented metal-catalyzed reactions such as
1) intermolecular addition of 2 to alkynes,[2b,4] 2) condensa-
tion of 2 with amines to give b-enaminones,[2a,4c,5] and
3) hydroamination of 1 to generate an enamine intermedia-
te[2a,b,4c] and possible reaction of the enamine with 2 to give a
mixture of products.[6] Unexpectedly, we detected neither a b-
enaminone nor the product resulting from the intermolecular
addition of 2 to the alkynyl moiety of 1, when 1 and 2 were
with the platinum(II) compound as the catalyst. The reaction
was found to result in the isolation of indoline or tetrahy-
droquinoline derivative 3 in up to 99% yield by a highly
selective tandem cyclization. This new method for the syn-
thesis of indoline or tetrahydroquinoline derivatives is
reported herein (Scheme 1).
for the construction of indolines use precursors that already
contain the six-membered ring of the indoline bicyclic core.[9]
Few methods are known for the assembly of both the six- and
five-membered rings (of the indoline core) from acyclic
precursors. All of these methods involve an intramolecular
[4+2] cycloaddition step.[10] A notable example is the stoi-
chiometric intramolecular [4+2] cycloaddition of ynamides
and conjugated enynes—a method that is particularly useful
for the construction of indolines bearing multiple substituents
on the six-membered ring.[10d] The method reported herein
(Scheme 1) is useful for constructing indoline compounds
bearing multiple substituents on both the six- and five-
membered rings. Also, the method has a very broad substrate
scope and can be used to synthesize tetrahydroquinoline
derivatives. The indoline or tetrahydroquinoline compounds
can be obtained in good to excellent yields and with high
regioselectivity and with almost complete chemoselectivity
under mild reaction conditions.
To optimize the reaction conditions, we treated 4-
methoxy-N-(pent-4-ynyl)aniline (1A) with 2,4-pentanedione
(2a) in different solvents and screened a variety of metal
catalysts. These catalysts were based on Cu, Co, Ni, Ag, Au,
Pd, or Pt compounds and the results are shown in Table S1 of
the Supporting Information. These studies revealed that
methanol was the solvent of choice, and catalyst K2PtCl4 gave
the product 3Aa in the best yield. The yield could be
improved either by lowering the reaction temperature to
408C or by adding activated, powdered molecular sieves (4 ꢀ
M.S.). The catalyst loading could be reduced from 5 to
1 mol% without affecting the yield of 3Aa. However, a
decrease in the yield of 3Aa was observed when the amount
of 2a was reduced from 4 to 1.2 equivalents. The optimal
reaction conditions were found to be 1 mol% of K2PtCl4 in
the presence of molecular sieves (4 ꢀ) with methanol as the
solvent at 408C for 22 hours, and gave the product 3Aa in
88% yield (Table 1, entry 1). Trifluoromethanesulfonic acid
did not catalyze this reaction (Table S1, entry 28, in the
Supporting Information).
Indoline frameworks are ubiquitous structural motifs in a
myriad of biologically active alkaloid natural products[7] and
pharmaceutically active compounds.[8] Consequently, there
has been continued interest in the development of efficient
methods for the syntheses of indolines bearing multiple and
diverse substituent patterns. Most of the methods available
[*] X.-Y. Liu, Prof. Dr. C.-M. Che
Department of Chemistry and Open Laboratory of Chemical Biology,
Institute of Molecular Technology for Drug Discovery and Synthesis,
The University of Hong Kong
Pokfulam Road, Hong Kong (P.R. China)
Fax: (+852)2857-1586
E-mail: cmche@hku.hk
[**] This study was supported by The University of Hong Kong
(University Development Fund), The Hong Kong Research Grants
Council, and The University Grants Committee of the Hong Kong
SAR of China (Area of Excellence Scheme, AoE/P-10/01).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200805383.
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Table 1: Synthesis of indolines and tetrahydroquinolines from 2a and various aminoalkynes 1A–W through platinum(II)-catalyzed tandem reaction.[a]
Entry
X
Aminoalkyne
t
[h]
Product
Yield Entry
[%][b]
X
Aminoalkyne
t
[h]
Product
Yield
[%][b]
1
4-OMe
1A 22
3Aa 88
14
1N 23
3Na 89
2
3
2-OMe
4-OPh
1B 22
1C 22
3Ba 96
3Ca 91[c] 16[d] Me
15[d] Ph
1O 72
1P 24
3Oa 51
3 Pa 65[e]
4
5
4-Me
1D 32
3Da 76
17[d]
1Q 72
3Qa 76
3Ra 77
1E 22
3Ea 95
18
4-OMe
1R 18
6[d]
2-F
1F 24
1G 36
3Fa 83
3Ga 79
19[d] 2-OMe
1S 58
1T 58
3Sa 79
3Ta 78
7[d]
4-Cl
20[d]
8
9
4-Cl
H
1H 18
1I 18
3Ha 99
3Ia 99
21
22
H
Cl
1U 32
1V 32
3Ua 88
3Va 91
10
11
1J 22
3Ja 95
23
1W 30
3Wa 84
1K 20
3Ka 94
12
13
OMe
OPh
1L 20
1M 20
3La 95
3Ma 99
[a] Reaction conditions: 1 (0.2 mmol), 2,4-pentanedione (0.8 mmol), K2PtCl4 (1 mol%), 4 ꢀ M.S. (20 mg), MeOH (1 mL), 408C. [b] Yield of isolated product
based on 1. [c] Determined by 1H NMR spectroscopy. [d] Reaction conditions: 1 (0.2 mmol), 2,4-pentanedione (0.8 mmol), K2PtCl4 (5 mol%), 4 ꢀ M.S. (20 mg),
MeOH (1 mL), 658C. [e] The reaction also afforded 3Ra in 24% yield.
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Table 2: Synthesis of highly substituted indolines from 1I and 1,3-diketones
2b–k.[a]
Under the optimized reaction conditions, we treated 2a
with a variety of 1,4-aminoalkynes (Table 1) bearing aryl
groups (with electron-donating substituents: 1A–D; Table 1,
entries 1–4 or with electron-withdrawing substituents: 1F–G;
Table 1, entries 6 and 7), a naphthalene group (1E; Table 1,
entry 5), benzyl groups (1H–I; Table 1, entries 8 and 9), and
alkyl groups (1J–K; Table 1, entries 10 and 11), and we
isolated the indoline derivatives 3Aa–Ka in 76–99% yield.
For the aryl groups with electron-withdrawing substituents, a
higher catalyst loading (5 mol%) and temperature (658C)
were required (Table 1, entries 6 and 7). Notably, the 1,4-
aminoalkynes with R3 as a cyclohexyl group underwent the
platinum(II)-catalyzed tandem cyclization and furnished
spirocyclic indolines 3La–Na in 89–99% yield (Table 1,
entries 12–14). The 1,4-aminoalkynes having an internal
alkyne group are also applicable to this tandem cyclization
reaction, and the reaction afforded highly substituted indo-
lines 3Oa–Qa in 51–76% yield, although a longer reaction
time (for 1O,Q), a higher temperature, and an increased
catalyst loading (5 mol%) were required for complete sub-
strate conversion (Table 1, entries 15–17).
Entry
R
or
X
1,3-Diketone
t [h] Product
Yield [%][b]
(selectivity)[c]
1
2
Et
Ph
2b 24
3Ib 98
3Ic 95
2c 22
2d 24
2e 72
3
3Id 92
We then extended the reaction to 1,5-aminoalkynes.
Under the optimized reaction conditions, treatment of 2a
with 1R–W containing aryl and alkyl substituents gave
1,2,3,4-tetrahydroquinolines 3Ra–Wa in 77–91% yield
(Table 1, entries 18–23). 1,2,3,4-Tetrahydroquinolines are of
importance in medicinal chemistry.[11]
4[d]
5[e]
CF3
Me
3Ie 61
3If 98 (>20:1)
2 f
5
Besides 2a, a series of symmetric 1,3-diketones with alkyl
and phenyl substituents were similarly treated with 1I, and
the corresponding indolines 3Ib–Id were obtained in 92–98%
yield (Table 2, entries 1–3). For hexafluoroacetylacetone
(2e), the indoline 3Ie was obtained in a lower yield (61%;
Table 2, entry 4)—the low yield could be attributed to the
electronic effect of the trifluoromethyl groups. Interestingly,
unsymmetrical 1,3-diketones could also undergo the platin-
um(II)-catalyzed tandem cyclization to afford highly substi-
tuted indolines with high regioselectivity. The regioselectivity
depends on both steric and electronic properties of the
substituents on the 1,3-diketone substrates. For example,
reactions of 2 f–i bearing an electron-withdrawing group
(CF3) gave 3If–Ii[12] in 94–99% yield with high regioselectivity
(Table 2, entries 5–8). Compounds 2j and 2k also underwent
the tandem cyclization reaction, and gave 3Ij[12] and 3Ik[12] in
93% and 90% yield, respectively—albeit with diminished
regioselectivity (Table 2, entries 9 and 10).
A proposed mechanism for this tandem cyclization
reaction is depicted in Scheme 2, which involves platinum-
catalyzed intramolecular hydroamination[3b,13] of aminoal-
kynes 1 to generate enamine II, and subsequent nucleophilic
attack of the enol form (2’)[14] of 2 via 1,4- or 1,2-addition,[15]
with subsequent cyclization to give intermediate III, which
eliminates two water molecules to give 3. The enamine II
could also be formed through intramolecular cyclization of an
aminoketone intermediate I, which might be generated by the
platinum-catalyzed hydration of 1.[16] In the literature, a
similar cyclization reaction of enamines with 1,3-diketones
has been reported[6]—however, the substrates were confined
to push–pull enamines (which bear an additional electron-
acceptor group at the b-position and a methyl group at the a-
6[e]
7[e]
8[e]
H
Me
Cl
2g 12
2h 18
2i 18
3Ig 99 (15:1)
3Ih 97 (15:1)
3Ii 94 (13:1)
9
2j 20
3Ij 93 (5:1)
10
2k 24
3Ik 90 (6.3:1)
[a] Reaction conditions: 1I (0.2 mmol), 1,3-diketone (0.8 mmol), K2PtCl4
(1 mol%), 4 ꢀ M.S. (20 mg), MeOH (1 mL), 408C. [b] Yield of isolated
product based on 1I. [c] Selectivity determined by 1H NMR spectroscopy.
[d] Reaction conditions: 1I (0.2 mmol), 2e (0.8 mmol), K2PtCl4 (5 mol%),
4 ꢀ M.S. (20 mg), MeOH (1 mL), 658C. [e] Reaction conditions: 1I
(0.2 mmol), 2 f–i (0.4 mmol), K2PtCl4 (1 mol%), 4 ꢀ M.S. (20 mg), MeOH
(1 mL), 408C.
position) and hexafluoroacetylacetone or 1,1,1-trifluoroace-
tylacetone (both with a highly electrophilic carbonyl group)—
but the reaction occurred under harsh conditions and gave a
mixture of products in low yields and with poor selectivities.[6]
Electrospray ionization mass spectroscopic analysis of a
solution of 1A and K2PtCl4 (20 mol%) in MeOH that was
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the reaction was subjected to microwave
irradiation, the reaction time was shortened
from 24–72 hours to 1.3–2.5 hours and the
corresponding products were obtained in up
to 99% yield (Table 3, entries 5–9).
In summary, we have described a new,
efficient platinum(II)-catalyzed tandem cyc-
lization reaction from simple, readily avail-
able starting materials to furnish multiply
substituted indolines in excellent yields with
high regioselectivity under mild reaction
conditions. The procedure is easy to per-
form and allows for a straightforward,
diversity-oriented and regioselective syn-
thesis of indoline derivatives with a broad
substrate scope, thus rendering the method
a valuable addition to alternative indoline
syntheses.
Scheme 2. A proposed mechanism.
Table 3: Synthesis of indolines and tetrahydroquinolines from 2a and
various aminoalkynes under different reaction conditions.
Received: November 4, 2008
Published online: February 11, 2009
Keywords: 1,3-diketones · aminoalkynes · cyclization · platinum ·
.
synthetic methods
Entry
Aminoalkyne
t [h]
Product
Yield [%][a]
1[b]
2[d]
3[d]
4[d]
5[e]
6[e]
7[e]
8[e]
9[e]
1I
19
22
18
18
2
2.2
2.5
2.5
1.3
3Ia
97[c]
74
67
61
86
90
57
76
99
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1A
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1I
1F
1G
1O
1T
1U
3Aa
3Ha
3Ia
3Fa
3Ga
3Oa
3Ta
3Ua
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(10 mmol), 2,4-pentanedione (15 mmol), K2PtCl4 (1 mol%), 4 ꢀ M.S.
(2 g), MeOH (20 mL), 408C. [c] Yield of isolated product based on 1I.
[d] Reaction conditions: 1 (0.2 mmol), 2a (0.8 mmol), K2PtCl4 (1 mol%),
H2O (1 mL), 408C. [e] Reactions were conducted with substrate
(0.1 mmol), K2PtCl4 (1–5 mol%) in MeOH (1 mL) at 1108C under
microwave irradiation.
stirred for 2 hours at 408C revealed a peak at m/z 190, which
is attributable to the protonated molecular ion of enamine II
(R1 = p-MeOC6H5). The K2PtCl4-catalyzed reaction of 1A at
408C for 2 hours and subsequent treatment with water gave
N-(4-methoxyphenyl)-5-aminopentan-2-one in 95% yield
[Eq. (1) of Scheme S1 in the Supporting Information]. Treat-
ment of the aminoketone with 2a for 20 hours with or without
the catalyst gave 3Aa in similar yields [Eq. (2) Scheme S1 in
the Supporting Information]. This outcome suggests that the
Pt species may not provide significant activation for the last
stage of the cyclization cascade, but only activate the
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a gram scale in the presence of K2PtCl4 (1 mol%) for 19 hours
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