Palladium-Catalyzed Oxidative Cyclization for the Synthesis of Indolyl/Pyrrolyl 3-Phosphonates

Article abstract of DOI:10.1002/adsc.201600321

Imino-/enaminophosphonates derived from amines and diethyl phenacyl phosphonates undergo oxidative cyclization via C H bond activation catalyzed by palladium chloride to provide a convenient route for the synthesis of substituted indol-3-yl and pyrrol-3-yl phosphonates. (Figure presented.) .

Full text of DOI:10.1002/adsc.201600321

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DOI: 10.1002/adsc.201600321
Palladium-Catalyzed Oxidative Cyclization for the Synthesis of
Indolyl/Pyrrolyl 3-Phosphonates
Aniket Mishraa and Indubhusan Deba,*
a
Organic & Medicinal Chemistry Division, Indian Institute of Chemical Biology, 4-Raja S. C. Mullick Road, Jadavpur,
Kolkata – 700032, India
Fax : (+91)-33-2473-5197; phone: (+91)-33-2499-5938; e-mail: indubhusandeb@iicb.res.in or indubhusandeb@gmail.com
Received: March 23, 2016; Revised: April 18, 2016; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600321.
Abstract: Imino-/enaminophosphonates derived
phonate-containing heterocycles, we sought to find
a catalytic method for indolyl 3-phosphonates via oxi-
from amines and diethyl phenacyl phosphonates un-
À
dergo oxidative cyclization via C H bond activation
catalyzed by palladium chloride to provide a con-
venient route for the synthesis of substituted indol-
3-yl and pyrrol-3-yl phosphonates.
À
Keywords: C H bond activation; imine-enamines;
indoles; ketophosphonates; phosphonates; pyrroles
The ubiquitous presence of indole and pyrrole skele-
tons in various natural products, pharmaceuticals and
synthetic materials make them immensely valuable
heterocycles.[1] Hence development of new and effi-
cient methods for their synthesis and derivatization
assumes high significance.[2] Ketones, hydrazines, aryl-
amines and their derivatives are the most common
and easily accessible starting materials for the synthe-
sis of various azaheterocycles.[3] Over the past de-
cades, a plethora of methods through C H/C X func-
tionalization have been developed for the synthesis of
indoles, pyrroles and carbazoles.[4] In this context, the
Glorius and Yoshikai groups have developed a con-
cept of palladium-catalyzed oxidative cyclization of a-
arylenamine/imines independently.[5] On the other
hand, significant attention has also been devoted to
synthesize biologically active[6] (Figure 1) and synthet-
ically important organophosphorus molecules,[7] in-
Figure 1. Bioactive phosphonates.
À
À
À
cluding conventional methods and direct C P bond
formation reactions.[8]
Recently, 3-phosphinoylindoles have been synthe-
À
À
sized employing direct C H/C P bond formation ap-
proaches [Scheme 1, Eq. (1) and Eq. (2)].[9]
However, the synthesis of indolyl 3-phosphonates
using transition metal-catalyzed oxidative cyclization
of imines or enamines remains untouched. Encour-
aged by the recent reports on the synthesis of phos- Scheme 1. Strategy for indolyl-phosphine derivatives.
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dative cyclization of enamines/imines derived from b- DBU, DIPEA, etc. as additives remained ineffective.
ketophosphonates and amines [Eq. (3)].[10] Herein, we The yield could not be improved either by extending
disclose a PdCl2/Cu(OAc)2-catalyzed oxidative cycli- the reaction time or increasing the reaction tempera-
zation approach for the synthesis of indolyl 3-phos- ture. However, to make this method general, we have
phonates starting from N-arylimines/enamines, pre- chosen 1008C as the optimized reaction temperature.
pared from arylamines and b-ketophosphonates [Eq. Using the optimized reaction conditions (Table 1,
(3)].
entry 8), we expanded the scope of indole formation
We commenced our study using the freshly pre- from the corresponding enamines.
pared mixture of imine-enamine 1a/2a,[10] obtained
Employing the PdCl2/Cu(OAc)2 catalytic combina-
from aniline and diethyl phenacylphosphonate, as tion, an array of 2-substituted indolyl 3-phosphonates
substrate. Screening of various palladium catalysts led was synthesized in good to excellent yields from the
us to find that the combination of PdCl2 (10 mol%) corresponding imine/enamine mixtures, derived from
and Cu(OAc)2 (1 equiv.) in DMSO serves as an effi- substituted anilines and b-ketophosphonates with tol-
cient catalytic system to obtain indolyl 3-phosphonate erance of a broad range of functional groups as illus-
3a in 82% yield at 1008C or 608C (Table 1, entries 8 trated in Table 2. The effect of substituents on the ar-
and 12). The use of several other palladium sources omatic ring undergoing indole formation was tested
resulted in lower yields (Table 1). Replacement of using imines/enamines 1/2 derived from anilines and
Cu(OAc)2 with other oxidants also proved less effec- b-ketophosphonates (Table 2).
tive or not effective at all. Even the use of various sol-
vents (DMF, DMA and MeCN) other than DMSO re- methyl) or electron-withdrawing groups on the para
sulted in poor conversion. position of the arene underwent cyclization and readi-
Substrates bearing electron-donating (methoxy, S-
Increasing the amount of Cu(OAc)2 to 2 equiv. did ly afforded the products in moderate to good yields
not improve the yield of the product, while 0.5 equiv. (3a–3g). Interestingly, substrates containing a halogen
of Cu(OAc)2 proved less efficient (65%). Lowering (F, Cl or Br) at ortho, meta and para positions provid-
the catalyst loading (PdCl2, 5 mol%) also proved less ed the indoles in good to excellent yields (3d, 3g-3l).
satisfactory (Table 1, entries 3 and 14). Complete hy- On the other hand for iodo-containing substrate 2m,
drolysis of the starting material was observed in the the desired product 3m was obtained in 32% yield
presence of 1 equiv. of PdCl2 (Table 1, entry 19). Ad- along with deiodinated parent indole 3a (39% yield).
dition of different bases such as Ag2CO3, K2CO3, We observed that the substrates 2f, 2k, 2p and 2r
Table 1. Optimization of the reaction conditions for the cyclization of imine-enamines.[a]
Entry
Catalyst (mol%)
Additive
Solvent
Temp. [8C]
Yield [%][b]
1
2
3
4
5
6
7
8
Pd(OAc)2 (10)
Pd(OAc)2 (5)
PdCl2 (5)
Pd(MeCN)2Cl2 (5)
Pd(xanthphos)Cl2 (5)
Pd(dppf)Cl2 (5)
Pd[P(O-tol)3]2Cl2 (5)
PdCl2 (10)
PdCl2 (10)
PdCl2 (10)
PdCl2 (10)
PdCl2 (10)
PdCl2 (10)
PdCl2 (5)
PdCl2 (10)
PdCl2 (10)
PdCl2 (10)
–
PdCl2 (100)
O2/TBAB
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
O2/TBAB
CuCl2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
–
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
60
62[c]
52
58
52
54
56
28
82
100
100
100
100
100
100
100
100
100
120
60
60
60
60
60
60
60
60
[d]
9
–
10
11
12
13
14
15
16
17
18
19
58
68
82
65[e]
55
38
39
73
nr
DMA
CH3CN
DMSO
DMSO
[d]
–
[a]
Reaction conditions: unless otherwise mentioned all reactions performed with mixture of 1a and 2a (0.2 mmol), PdCl2
(10 mol%), Cu(OAc)2 (1 equiv.), DMSO (0.07M), 60–1008C, N2, 16 h.
Isolated yield; nr=no reaction.
2.0 equiv of tetrabutylammonium bromide (TBAB).
Hydrolysis of 1a/2a was observed.
[b]
[c]
[d]
[e]
0.5 equiv. of Cu(OAc)2.
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Table 2. Scope of palladium-catalyzed oxidative cyclization of imine-enamines.[a]
[a]
Reaction conditions: mixture of 1 and 2 (0.2 mmol), PdCl2 (10 mol%), Cu(OAc)2 (1 equiv.), DMSO (2.8 mL), 1008C, 16 h,
N2. Yields refer to isolated products.
At 608C.
[b]
bearing electron-withdrawing group(s) on para and diethyl acetal and aniline did not afford the desired
meta positions of the amine provided relatively lower indole 3ad. The structures of all the products were de-
yields, which may be due to the rapid decomposition duced by spectroscopic studies (see the Supporting In-
of starting material 1/2. However, the presence of an formation). A single-crystal X-ray study of 3e and 3i
electron-withdrawing group at an ortho position of further confirmed the structures (see the Supporting
the amine provided the products 3l and 3n in excel- Information).[11] To determine the scalability of the
lent yields. Next, we turned our attention of using the cyclization process, we conducted a gram-scale experi-
enamines, derived from substituted diethyl phenacyl- ment using the mixture of imine and enamine (1a+
phosphonates for cyclization. Under the optimized 2a) (1 g, 3 mmol) at 608C and obtained the product in
conditions employed, the enamines 2s–2ac derived 79% yield (0.78 g) (Scheme 2a). In a one-pot varia-
from substituted ketophosphonates and aniline as tion, where the crude imine/enamine, prepared from
well as their p-substituted derivatives were successful- the corresponding amine and ketone was subjected to
ly converted to indoles 3s–3x and 3aa–3ac in good to cyclization without further purification, the indole 3a
excellent yields (Table 3).
was obtained in 60% yield (Scheme 2b).
It should be noted that the nature of the substitu-
This result encouraged us to extend the methodolo-
ents on the second aryl rings of phosphonates 2s–2ac gy and explore whether phosphonate-functionalized
has a minor effect on the yields. Electron-withdrawing pyrroles could also be synthesized from enamines de-
groups (F, CF3, CN, NO2) on phosphonates worked rived from ketophosphonates and allylamine via pal-
well to provide the desired products 3t, 3u, 3v and ladium-catalyzed dehydrogenative cyclization,[12] in
3aa in good to excellent yields. Unfortunately, the re- order to broaden the synthetic utility of this process.
action remained unsuccessful when pinacolonyl phos- After thorough optimization, we found that under
phonate and acetonyl phosphonate were used for the modified conditions employing PdCl2 (20 mol%) in
preparation of the corresponding enamines. However, the presence of Cu(OAc)2, enamine 4a also participat-
the imine-enamine derived from bromoacetaldehyde ed in the reaction and afforded the pyrrolyl 3-phos-
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Table 3. Scope of palladium-catalyzed oxidative cyclization of imine-enamines.[a]
[a]
Reaction conditions: mixture of 1 and 2 (0.2 mmol), PdCl2 (10 mol%), Cu(OAc)2 (1 equiv.), DMSO (2.8 mL), 1008C, 16 h,
N2. Yields refer to isolated products.
At 608C.
[b]
phonate 5a in 60% yield (Scheme 3). b-Ketophospho- ents underwent smooth cyclization to provide the de-
nates bearing 4-phenylaryl and 4-bromoaryl substitu- sired products 5b and 5c in 56% and 52% yields, re-
spectively (Scheme 3).
Scheme 3. Scope for synthesis of pyrroles via dehydrogena-
tive cyclization.
Scheme 2. Gram-scale and one-pot reactions.
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product was purified by column chromatography on neutral
alumina using petroleum ether/ethyl acetate as eluent to
obtain the desired product.
Full characterization data and copies of relevant spectra
of all new products are provided in the Supporting Informa-
tion.
We believe that the indole formation reaction is
likely to involve oxidative addition of the enamine
C H bond to Pd(II), followed by second C H bond
palladation and reductive elimination of the resulting
indole (Scheme 4).[5b,d,4f,13]
À
À
General Procedure for Synthesis of Pyrrolyl 3-
Phosphonates
In a 10-mL, oven-dried, two-necked round bottom flask
a
mixture of 0.2 mmol of imine-enamine, Cu(OAc)2
(1 equiv.) and PdCl2 (20 mol%) were charged. To that
2.8 mL of anhydrous DMSO were added. The reaction flask
was quickly evacuated and backfilled with nitrogen (this
process was repeated three times). Then it was slowly
heated to 408C (bath temperature) while stirring in an oil
bath. After the mentioned time the reaction was stopped
and allowed to cool to room temperature. Then the reaction
mixture was passed through a short pad of celite and
washed with 5 mL of ethyl acetate. To that 20 mL of water
were added and the organic layer was separated. The aque-
ous layer was extracted with ethyl acetate (3ꢁ) and the com-
bined organic layer was washed with brine, dried over anhy-
drous Na2SO4 and concentrated under vacuum. The crude
product was purified by column chromatography on neutral
alumina using petroleum ether/ethyl acetate as eluent to
obtain the desired product.
Scheme 4. A plausible mechanism for indole formation.
Full characterization data and copies of relevant spectra
of all new products are provided in the Supporting Informa-
tion.
Conclusions
In summary, we have demonstrated that a PdCl2/
Cu(OAc)2 catalytic system allows the formation of in-
dolyl 3-phosphonates and pyrrolyl 3-phosphonates in
good to excellent yields via oxidative cyclization of
the corresponding imino/enaminophosphonates with
a broad range of substrates.
Acknowledgements
AM thanks CSIR for his fellowship and ID thanks IICB-
CSIR (MLP-116, BSC-0115 and BSC-0206), DST-India (CS-
03/2013) and BMS-USA for research funds. We thank Dr. R.
Natarajan for X-ray analyses as well as Dr. Prasant C. Singh
and Dr. B. Achari for valuable suggestions.
Experimental Section
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Palladium-Catalyzed Oxidative Cyclization for the Synthesis
of Indolyl/Pyrrolyl 3-Phosphonates
7
Adv. Synth. Catal. 2016, 358, 1 – 7
Aniket Mishra, Indubhusan Deb*
Adv. Synth. Catal. 0000, 000, 0 – 0
7
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÞÞ
These are not the final page numbers!