Regioselective Direct C3-Phosphorylation of N-Sulfonylindoles under Mild Oxidative Conditions

Article abstract of DOI:10.1002/adsc.201601204

The reactions of N-sulfonylindoles with H-phosphine oxides under oxidative conditions give a wide range of C-3 phosphorylated free (NH)-indoles. Several mild oxidants, such as AgNO₃, di-tert-butyl peroxide (DTBP), and K₂S₂O₈, can be used to promote this transformation. (Figure presented.).

Full text of DOI:10.1002/adsc.201601204

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DOI: 10.1002/adsc.201601204
Regioselective Direct C3-Phosphorylation of N-Sulfonylindoles
under Mild Oxidative Conditions
Feng Su,a Weidong Lin,a Pengfei Zhu,a Dezhi He,a Jianbin Lin,a Hui-Jun Zhang,a,*
and Ting-Bin Wena
a
Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 Fujian,
Peopleꢀs Republic of China
Fax : (+86)-59-2218-6295; phone: (+86)-59-2218-2635; e-mail: meghjzhang@xmu.edu.cn
Received: October 29, 2016; Revised: January 12, 2017; Published online: && &&, 0000
Supporting information for this article can be found under: http://dx.doi.org/10.1002/adsc.201601204.
Abstract: The reactions of N-sulfonylindoles with
H-phosphine oxides under oxidative conditions give
a wide range of C-3 phosphorylated free (NH)-in-
doles. Several mild oxidants, such as AgNO3, di-tert-
butyl peroxide (DTBP), and K2S2O8, can be used to
promote this transformation.
and 3-positions of indoles can be attacked by the
phosphorus radical.[8,10] Therefore, it is necessary to
develop a general and efficient method for regioselec-
tive C-3 phosphorylation of indoles. Moreover, by re-
duction with silanes, the phosphine oxides are also
the key precursors of phosphine derivatives.[11]
Although indoles are known to be more susceptible
to electrophilic attack at the b-position, the regiose-
lective indole b-nucleophilic substitution was also
made possible by the departure of the N-substituent
as a leaving group from indolic nitrogen.[12] In addi-
tion, Zard et al. have recently discovered a desulfony-
Keywords: heterocycles; phosphorylation; radicals;
regioselectivity; synthetic methods
Aromatic organophosphorus compounds represent an
important class of chemicals in organic and organo-
metallic chemistry,[1] materials science,[2] and medici-
nal chemistry.[3,4] Recently, the construction of 3-phos-
phoindole skeletons has received much attention due
to their significant applications in pharmaceutical
chemistry and transition metal catalysis.[5,6] Among all
the successful strategies towards 3-phosphoin-
ACHTUNGTRENNUNG
doles,[6c,7–9] transition metal-catalyzed direct C(sp2)–H
phosphorylation of indoles seems to be the most
straightforward pathway. In 2014, Yang et al. reported
the first Cu(I)-catalyzed cross-coupling reactions of
C-2 substituted indoles with diphenylphosphine oxide
and chiral (1S,2S,5R)-(À)-menthoxyphenylphosphi-
nate for the synthesis of 3-phosphoindoles
(Scheme 1a).[7] However, the reaction of C-2 unsubsti-
tuted N-methylindole with Ph2P(O)H gave only
a trace amount of the desired product.[7] Very recent-
ly, Zeng and Zou et al. also developed a silver-cata-
lyzed direct C(sp2)–H phosphorylation of C-2 and C-3
substituted indoles with Mg(NO3)2 as additive
(Scheme 1b).[8] It is notable that the reaction of un-
substituted indole with diphenylphosphine oxide took
place in the presence of 2.2 equiv. of Ag2CO3, albeit
giving a mixture of 2-phosphorylated indole and 2,3-
diphosphorylated indole, which suggested that both 2- Scheme 1. C-3 Phosphorylation of indoles.
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lative radical ring closure process for the formation of AgNO3 gave 3aa in a higher yield (73%, entry 3). In-
benzazepin-2-ones.[13] Notably, the fragmentation of creasing the amount of 2a to 3 equiv. led to the for-
the N–SO2R bond next to a carbon radical was in- mation of 3aa in 96% yield (entry 4). When the reac-
volved in the reaction by virtue of the correct align- tion was carried out at higher or lower reaction tem-
ment of the orbital containing the free electron with peratures, the yield of 3aa was reduced obviously
the antibonding orbital of the N–S bond.[13] Encour- (entry 5 and 6). In the absence of any silver salts, no
aged by these results, we envisaged the C-3 phosphor- phosphorylation product was observed (entry 7). The
ylation of N-sulfonylindoles via addition of the phos- reaction also worked well without the addition of 4ꢂ
phoryl radical at the b-position driven by sequential molecular sieves, albeit affording 3aa in diminished
cleavage of the N–SO2R bond (Scheme 1c). Herein, yield (85%, entry 8). Notably, when simple indole was
we wish to disclose our findings on the regioselective treated with diphenylphosphine oxide 2a in the pres-
formation of C-3 phosphorylated indoles through the ence of AgNO3, the desired product 3aa was not ob-
reactions of N-sulfonylindoles with H-phosphine served at all (entry 9). Furthermore, both N-tosylin-
oxides under mild oxidative conditions.
dole and N-mesylindole reacted with 2a giving the de-
According to our experience in the silver-promoted sired product 3aa in good yields (entry 10 and 11).
direct phosphorylation of (benzo)thiazoles,[14,15] we in- Thereafter, several other oxidants, such as DTBP,[17]
itially conducted the reaction of N-phenylsulfonylin- (NH4)2S2O8, Na2S2O8, and K2S2O8,[18] were also exam-
dole 1a and diphenylphosphine oxide 2a in the pres- ined (see the Supporting Information). It was found
ence of 1 equiv. of AgOAc in acetonitrile at 1008C that DTBP and K2S2O8 could also promote the C-3
for 25 h. To our delight, the desired C-3 phosphoryla- phosphorylation of 1a with 2a and afforded the prod-
tion product 3aa was produced in 46% yield (Table 1, uct 3aa in good yields (entries 12–16).
entry 1).[10b,16] The phenylsulfonyl group was indeed
With the optimized reaction conditions in hand, the
removed during the reaction as hypothesized. When substrate scope was examined under both AgNO3-
Ag2CO3 was used instead of AgOAc, the desired and DTBP-mediated conditions, respectively
product 3aa was isolated in 56% yield (entry 2). The (Table 2). A series of N-sulfonylindoles bearing elec-
reaction performed in the presence of 1 equiv. of tron-donating and electron-withdrawing groups react-
ed with diphenylphosphine oxide affording the de-
Table 1. Optimization of the reaction conditions.[a]
sired desulfonylated phosphorylation products in
good yields (3ba–3ia). Surprisingly, the reaction of 5-
chloro-1-(phenylsulfonyl)-1H-indole 1e with 2a in the
presence of AgNO3 gave the desired product 3ea in
only 33% yield. Both the sterically hindered 2-
methyl-1-(phenylsulfonyl)-1H-indole 1h and methyl 1-
(phenylsulfonyl)-1H-indole-2-carboxylate 1i reacted
with 2a in the presence of AgNO3 or DTBP giving
the corresponding products 3ha and 3ia in high yields.
Conducting the reaction of 1-(phenylsulfonyl)-1H-pyr-
rolo[2,3-b]pyridine 1j and 2a in the presence of
AgNO3 did not afford any phosphorylation products
owing to the coordinating properties of the pyridine
nitrogen atom. To our delight, the reaction between
1j and 2a worked smoothly in the presence of 3 equiv.
DTBP providing the desired product 3ja in 62%
yield. Furthermore, treatment of pyrroles 1k and 1l,
respectively, with 2a led to the corresponding a-phos-
phorylation products 3ka and 3la in high yields.[19] No
b-phosphorylation product was observed at all. There-
after, a range of H-phosphine oxides was examined.
Under AgNO3-promoted conditions, the introduction
of both electron-rich and electron-deficient substitu-
ents on the phenyl ring of diarylphosphine oxides led
to the formation of the desired products 3ab–3af in
varying yields (3ab, 60%; 3ac, 31%, 3ad, 27%; 3ae,
35%; 3af, 41%). However, the reactions performed in
the presence of DTBP gave much better results (63–
82%). Subsequently, the reactions of ethyl phenyl-
phosphinate 2g and diethyl phosphonate 2h with N-
Entry
R
[O] (equiv.) T [oC] T [h] Yield [%][b]
1[c]
2[c]
3[c]
4
5
6
SO2Ph (1a) AgOAc (1) 100
SO2Ph (1a) Ag2CO3 (1) 100
SO2Ph (1a) AgNO3 (1) 100
SO2Ph (1a) AgNO3 (1) 100
SO2Ph (1a) AgNO3 (1) 80
SO2Ph (1a) AgNO3 (1) 120
25
15
15
20
20
20
20
20
20
20
20
21
21
21
24
24
46
56
73
96
59
81
0
7
SO2Ph (1a)
–
100
8[d]
9
SO2Ph (1a) AgNO3 (1) 100
85
0
H (1’)
Ts (1a’)
Ms (1a’’)
AgNO3 (1) 100
AgNO3 (1) 100
AgNO3 (1) 100
10
11
12[d]
13[d]
94
61
58
73
84
76
93
SO2Ph (1a) DTBP (3)
SO2Ph (1a) DTBP (3)
120
140
140
14[d,e] SO2Ph (1a) DTBP (3)
15
16
SO2Ph (1a) K2S2O8 (1) 100
SO2Ph (1a) K2S2O8 (2) 100
[a]
Reaction conditions: 1 (0.2 mmol), 2a (3 equiv.), [O], 4ꢂ
molecular sieves (100 mg), and MeCN (3 mL) under an
argon atmosphere in a sealed tube.
Isolated yields.
Using 2a (2 equiv.).
Without molecular sieves.
[b]
[c]
[d]
[e]
Using EtOAc (3 mL).
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Table 2. Synthesis of C-3 phosphorylated indoles with
uct 3ag and 3ah in 45% and 21% yields, respective-
ly.[20] Later, the butyl(phenyl)phosphine oxide 2i could
also react with 1a under oxidative conditions to give
the corresponding 3ai in moderate yield. The reaction
between dicyclohexylphosphine oxide 2j and N-phe-
nylsulfonylindole 1a in the presence of AgNO3 was
performed leading to the formation of 3aj in 76%
yield. However, the DTBP-promoted reaction of 2j
gave only 28% yield of 3aj. Notably, it was also found
that the phosphorylation of 1a with Ph2P(O)H in the
presence of AgNO3 was easily scaled up to 4 mmol
scale, affording 1.06 g of 3aa in 84% yield [Eq. (1)].
AgNO3 or DTBP.[a,b]
Furthermore, several reactions between different
N-sulfonylindoles and H-phosphine oxides in the
presence of K2S2O8 were conducted. Compared with
DTBP-mediated transformations, the C-3 phosphory-
lated indoles were obtained in similar yields
(Table 3).
To investigate the mechanism of these oxidant-pro-
moted C-3 phosphorylations of N-sulfonylindoles, sev-
eral control experiments were performed. For the re-
action of 1a with 2a in the presence of AgNO3, the
addition of BHT and TEMPO as radical scavengers
both suppressed the formation of the phosphorylation
product [Eq. (2)]. Moreover, for the reaction between
1a and 2a in the presence of DTBP in THF, besides
the formation of 3aa, S-phenyl diphenylphosphino-
thioate 4 was also isolated in 41% yield [Eq. (3), see
also the Supporting Information].[21] These prelimina-
ry results and the good regioselectivity of the phos-
phorylation reactions are all consistent with the radi-
cal mechanism as we hypothesized. However, a mech-
anism involving the oxidation of N-sulfonylindole to
a cation radical followed by nucleophilic addition of
phosphinous acid P(OH)R’2 at the 3-position cannot
be excluded (see the Supporting Information).[22]
[a]
Conditions A:
1 (0.2 mmol), 2 (0.6 mmol), AgNO3
(0.2 mmol), 4ꢂ MS (100 mg), MeCN (3 mL), 1008C,
20 h. Conditions B: 1 (0.2 mmol), 2 (0.6 mmol), DTBP
(0.6 mmol), EtOAc (3 mL), 1408C, 21 h.
Isolated yields; the yields of reactions under conditions B
are shown in parenthesis.
For 40 h.
In MeCN.
For 30 h.
[b]
[c]
[d]
[e]
phenylsulfonylindole 1a were conducted. Although no
desired products were observed in the presence of
AgNO3, the corresponding DTBP-promoted reactions
worked smoothly affording the phosphorylation prod-
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Table 3. K2S2O8-promoted phosphorylation of N-sulfonylin-
1008C for 20 or 24 hours. After cooling the reaction mixture
to room temperature, it was quickly filtered through a short
column (2 cm) of silica gel to remove the 4ꢂ MS and salt.
Subsequently, the filtrate was washed with saturated
NaHCO3 and extracted with dichloromethane. The com-
bined extracts were dried over Na2SO4, filtered and concen-
trated under reduced pressure. The residue was purified by
column chromatography on silica gel (petroleum ether/ethyl
acetate=1:1 to 1:3; acetone) to afford the product 3.
doles with H-phosphine oxides.[a,b]
General Procedure for DTBP-Meditated C-3
Phosphorylation of N-Sulfonylindoles
To a 25-mL tube, N-sulfonylindole 1 (0.2 mmol), ethyl ace-
tate (3 mL), H-phosphine oxide 2 (0.6 mmol), and DTBP
(0.6 mmol) were added under an argon atmosphere. Then
the tube was sealed with a Teflon-lined cap, and the reaction
mixture was stirred at 1408C for 21 hours. Subsequently, the
reaction was quenched with saturated NaHCO3 and extract-
ed with dichloromethane. The combined extracts were dried
over Na2SO4, filtered and concentrated under reduced pres-
sure. The residue was purified by column chromatography
on silica gel (petroleum ether/ethyl acetate=1:1 to 1:3; ace-
tone) to afford the product 3.
Acknowledgements
The authors gratefully acknowledge the financial support
from Natural Science Foundation of China (No. 21572188,
21302157, 21302159) and Fundamental Research Funds for
the Central Universities (No. 20720160049). H.-J. Zhang also
thanks Prof. Pierre H. Dixneuf for useful discussions.
[a]
Reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol), K2S2O8
(0.4 mmol), 4ꢂ MS (100 mg), MeCN (3 mL), 1008C,
24 h.
Isolated yields.
[b]
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6 Regioselective Direct C3-Phosphorylation of N-
Sulfonylindoles under Mild Oxidative Conditions
Adv. Synth. Catal. 2017, 359, 1 – 6
Feng Su, Weidong Lin, Pengfei Zhu, Dezhi He, Jianbin Lin,
Hui-Jun Zhang,* Ting-Bin Wen
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