Copper-catalyzed radical cascade cyclization for the synthesis of phosphorated indolines

Article abstract of DOI:10.1039/c4cc10267c

A novel and convenient approach to the synthesis of various phosphorated indolines via a copper-catalyzed radical cascade cyclization reaction has been developed. The reaction employs cheap copper as the catalyst and K₂S₂O₈ as the oxidant under mild conditions. Various alkenes and P-radical precursors are compatible with this transformation. Preliminary mechanistic studies reveal that the addition of the P-radical may initiate the reaction, and then oxidative cyclization may be achieved to afford the desired product. This journal is

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Copper-Catalyzed Radical Cascade Cyclization
for the Synthesis of Phosphorated Indolines
Cite this: DOI: 10.1039/x0xx00000x
Hong-Yu Zhang,a Liu-Liang Mao,a Bin Yanga and Shang-Dong Yang*a.b
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org
A novel and convenient approach to the synthesis of various arylphosphination of alkenes.15 On the basis of these works, we
phosphorated indolines via copper-catalyzed radical cascade hypothesize that the addition of the Pꢀradical to 2ꢀallylaniline
cyclization reaction has been developed. The reaction could trigger the oxidative cyclization to afford the
employs cheap copper as the catalyst and K2S2O8 as the corresponding phosphorated indolines. Herein, we describe the
oxidant under mild conditions. Various alkenes and P-radical first example of copperꢀcatalyzed aminoꢀphosphorus
precursors are compatible with this transformation. difunctionalization via cascade cyclization that exhibits several
Preliminary mechanistic studies reveal that the addition of P- features. 1) cheap copper catalyst and K2S2O8 oxidant are
radical may initiate the reaction, and then oxidative employed without any base under mild conditions; 2) compared
cyclization may be achieved to afford the desired product.
with previous cases, 3c, 4b, 5d, 6g, 7i this reaction may be initiated
from the addition of Pꢀradical, and then oxidative cyclization
may be achieved to access the desired product (Scheme 1).
The nitrogen heterocyclic components have found abundant
applications as structural motifs in pharmaceutically active
compounds and natural products.1 Consequently, many
endeavors have been focused on the development of efficient
approaches to afford these heterocycles.2 Transitionꢀmetal
catalyzed/promoted difunctionalization of derivative 2ꢀ
allylaniline represents one of the most direct and atomꢀ
economical approaches,3 including aminooxygenation,4
aminohalogenation,5 diamination6 and carboaminations.7
However, its application in the synthesis of phosphorated
indolines by aminophosphination via radical cascade
cyclization has never been demonstrated. As expected products
contain the vicinal amine and phosphorus segment,
phosphorated indolines are ubiquitous in pharmaceuticals,8
Scheme 1. CopperꢀCatalyzed Radical Cascade Cyclization
At the outset of this investigation, we chose the
NꢀTsꢀ2ꢀ
common catalysts9 and popular ligands.10 Therefore, allylaniline (1a) and HP(O)Ph2 (2a) as the model substrates and
successfully achieve the aminoꢀphosphorus difunctionalization various silver and copper catalysts in different solvents at
of alkenes to synthesize phosphorated indolines in one different temperatures were tested. Regrettably, only the
operation is highly desirable in organic synthesis, because addition product (4a) was obtained. In light of these factors, we
potentially difficult workꢀup and isolation steps can be avoided hypothesized that silver or copper salts might promote the
and the generation of chemical waste is minimized.
addition of Pꢀradical to alkene, and then maybe need the
In the past decades, there are a number of powerful methods appropriate oxidant to enhance the eletrophilicity of copper
that address C–P bond construction.11ꢀ12 Thereinto, adding the species which will activate the NꢀH bond leading to complete
Pꢀcentered radicals to alkenes or alkynes is well precedented.13 the cyclization. So we used AgNO3 as an oxidant to screen
However, examples that involve in the aminophosphination of different copper catalysts (50 mol %). To our delight, when
alkenes or alkynes in one pot via a cascade sequence remain Cu(ClO4)2·6H2O was used as the catalyst, the desired product
elusive and rudimentary.14 Very recently, our group has
(3a) was obtained in 16% yield (Table 1, entry 1; SI Table S1).
disclosed
the
novel
silverꢀcatalyzed
oxidative Motivated by this result, we further optimized the reaction
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DOI: 10.1039/C4CC10267C
conditions. Various silver oxidants, the loading of atom or the
Nꢀprotecting group, but expected products were
Cu(ClO4)2·6H2O, different catalysts, temperatures and solvents obtained only in moderate yields with low d.r. values (3z-3aa).
were investigated respectively (Table 1, entries 2ꢀ8; SI Tables We also have examined disubstituted alkenes. Disappointingly,
S2ꢀ6). Screening of different radical initiators showed that Pꢀ only addition products were obtained.
centered radicals were also generated from cheaper peroxides
Table 2. Substrates Scopea, b
t-
such as K2S2O8, Na2S2O8 and BuOOH (Table 1, entries 9ꢀ11;
SI Table S7).16 After several trials, the most appropriate oxidant
R4
O
NHR4
PR1R2
N
O
•
20 mol % Cu(ClO4)2 6H2O
was identified as K2S2O8 (Table 1, entry 12; SI Table S8). After
a large number of attempts, we decreased the amount of
Cu(ClO4)2·6H2O to 20 mol % by adding HP(O)Ph2 slowly with
a syringe pump (Table 1, entry 13; SI Table S9). Finally, the
optimized reaction conditions are as follows: Cu(ClO4)2·6H2O
(20 mol %) as catalyst, K2S2O8 (3.0 equiv) as oxidants in
CH3CN at 35 oC for 6 h.
R3
H
PR1R2
R3
3 equiv K2S2O8 , CH3CN, 35 °C
2
1
3
Cl
O
Ts
Ts
O
Ts
O
PPh2
N
P
N
P
N
Cl
3a: 74 %
3b: 83
%
3c: 73 %
OMe
O
O
Ts
O
Ts
N
Ts
P(tBu)Ph
P(OiPr)Ph
Table 1. Selected Reaction Conditions Optimizationa
N
Ts
P(OEt)Ph
O
N
P
N
OMe
O
NHTs
Ts
c
3e
3g: 77 %
: 70 %
Ts
3d
:53 %
3f: 51 %
NHTs
O
O
HPPh2
PPh2
N
cat. / oxidant
solvent, T
: R1 = R2 = OMe: 42 %
3i
3j
3k
O
PPh2
O
Ts
: R1 = R2 = OEt: 53 %
P(OnBu)Ph
PR1R2
N
N
: R1 = R2 = OiPr: 61 %
4a
2a
3a
3l: R1 = R2 = OnBu: 53 %
3m: R1 = R2 = OBn: 45 %
1a
3h: 72 %
b
PhO2S
N
entry
oxidant (equiv)
cat. (mol %)
3a
(%)
yield of
O
O
T (°C)
O
O
Ts
Ts
N
Ms
P(OPh)2
PiPr2
N
PPh2
PPh2
N
AgNO3 (1.0)
AgNO3 (1.0)
1
2
Cu(ClO4)2 6H2O (50)
Cu(OAc)2 (50)
80
80
16
trace
3p
3n
3o: 0 %
: 65 %
3q: 61 %
: 31 %
AgNO3 (1.0)
AgOTf (1.0)
R3
3
4
5
Cu(OTf)2 (50)
Cu(ClO4)2 6H2O (50)
80
80
80
trace
10
: R3 = Me: 45 %
O
3r
O
3v : R3 = Me: 73 %
3w : R3 = OMe: 60 %
3x
Ts
Ts
N
3s : R3 = OMe: 55 %c
3t : R3 = F: 75 %
PPh2
PPh2
N
PPh2
: R3 = F: 66 %
Ag3PO4 (1.0)
Ag3PO4 (1.0)
Ag3PO4 (1.0)
Ag3PO4 (1.0)
K2S2O8 (1.0)
Na2S2O8 (1.0)
tBuOOH (1.0)
K2S2O8 (3.0)
K2S2O8 (3.0)
Cu(ClO4)2 6H2O (50)
Cu(ClO4)2 6H2O (200)
Cu(OTf)2 (200)
Cu(ClO4)2 6H2O (200)
Cu(ClO4)2 6H2O (200)
Cu(ClO4)2 6H2O (200)
Cu(ClO4)2 6H2O (200)
Cu(ClO4)2 6H2O (200)
Cu(ClO4)2 6H2O (20)
Cu(ClO4)2 6H2O (15)
37
65
60
68
48
41
43
73c
74d
60d
R3
3u : R3 = Cl: 70 %
6
7
8
80
80
65
65
65
65
65
35
35
O
SO2
Ts
R5
N
O
O
Ts
N
PPh2
N
PPh2
O
=
R5
9
3y: 64 %
3aa: 49 % (d.r. = 1:1)
H
3z
: 63 % (d.r. = 1:1)
10
11
12
13
14
a2 (0.8 mmol) in CH3CN (2 mL) was added dropwise via an automatic
syringe to the mixture of 1 (0.2 mmol), Cu(ClO4)2·6H2O (0.04 mmol),
K2S2O8 (0.6 mmol) in CH3CN (2 mL) in 6 hour at 35 °C. Yield of isolated
b
K2S2O8 (3.0)
c
product. Reaction was carried out in the presence of 1 (0.2 mmol), 2 (0.8
mmol), Cu(ClO4)2·6H2O (0.4 mmol), K2S2O8 (0.6 mmol) in CH3CN (2 mL) in
aAll reactions were carried out in the presence of 0.2 mmol of 1a and 0.3 6 hour at 65 °C.
mmol of 2a in 2.0 mL different solvents under Ar atmosphere; bYield of
isolated product. c2a (0.8 mmol) was used. d2a (0.8 mmol) in solvent (2 mL)
In order to gain insight into the catalytic procedure, we
was added dropwise via an automatic syringe to the mixture of 1a (0.2
mmol), Cu(ClO4)2·6H2O, K2S2O8 (0.6 mmol) in solvent (2 mL) in 6 hours at
35 °C.
carried out a series of control experiments. As illustrated in
Scheme 2, the process was partially suppressed in the presence
of 1.0 equiv of 1, 1ꢀdiphenylethylene as a radical scavenger and
a trapped compound 6a was isolated, thus suggesting that the
formation of the phosphinoyl radical (2A) was the key step in
this transformation and 1, 1ꢀdiphenylethylene was superior to
1a in the addition of the phosphinoyl radical (2A) (Scheme 2,
equations 1 vs. 2). When 1a was investigated under standard
conditions without 2a, the cyclization product 7a was not
detected. This result illustrates that the addition of the
phosphinoyl radical triggered cyclization (Scheme 2, equation
3). Furthermore, when 1a was added dropwise via an automatic
syringe to the mixture of 2a, Cu(ClO4)2·6H2O, K2S2O8 in
solvent, 4a was the major product and the desired product 3a
was obtained in 9 % yield. Thus, we concluded that the crucial
step was the oxidative addition in the presence of excess Cu
catalyst to obtain complex 4A (Scheme 2, equation 4).
Additionally, the reaction could also proceed through a
stoichiometric [Ph2P(O)Ag] complex in order to afford the
With the optimized conditions in hand, we investigated the
substrate scope as shown in Table 2. Firstly, we evaluated a
series of phosphinoyl radicals. Diverse diphenylphosphine
oxides,
tertꢀbutylphenylphosphine
oxide
and
alkyl
phenylphosphinates served as suitable Pꢀradical precursors for
this transformation, and the desired products were isolated in
good yields (3a-3h). Moreover, reactions could also proceed
well by using dialkyl
Hꢀphosphates and afford the
corresponding products in moderate yields (3i-3n). However,
under the same conditions, diisopropylphosphine oxide failed to
provide the desired product (3o). Subsequently, an investigation
into different
provided corresponding products in good yields (3p
Boc, Ac, and Bz protecting groups did not work at all. Finally,
variously substituted ꢀTsꢀ2ꢀallylanilines bearing different
Nꢀprotecting groups showed that sulfonyl groups
ꢀ
3q), but
N
functional groups on different positions could still afford
desired products in good yields (3r-3y). In addition, we
introduced stereogenic centers at the αꢀposition of the nitrogen
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corresponding product as well as the byproduct 8a14j (Scheme Acknowledgement
2, equation 5).
We are grateful for the NSFC (Nos. 21272100 and 21472076) and,
FRFCU (lzujbkyꢀ2013ꢀct02, PCSIRT (IRT1138), and Program for
New Century Excellent Talents in University (NCETꢀ11ꢀ0215 and
lzujbkyꢀ2013ꢀk07) financial support. S.F. Reichard, MA contributed
editing.
aState Key Laboratory of Applied Organic Chemistry, Lanzhou
University, Lanzhou 730000, P. R. China. Eꢀmail: yangshd@lzu.edu.cn;
Fax: +86ꢀ931ꢀ8912859; Tel: +86ꢀ931ꢀ8912859
bState Key Laboratory for Oxo Synthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics, Lanzhou 730000, P. R. China
†
Electronic supplementary information (ESI) available. See DOI:
10.1039/c3cc
Notes and references
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a1a (0.2 mmol) in CH3CN (2 mL) was added dropwise via an automatic
syringe to the mixture of 2a (0.8 mmol), Cu(ClO4)2·6H2O (0.04 mmol),
K2S2O8 (0.6 mmol) in CH3CN (2 mL) in 6 hour at 35 °C. b1a (0.2 mmol) was
stirred in the presence of Cu(ClO4)2·6H2O (0.2 mmol) and Ph2(O)PAg (0.2
mmol) was added after 1 hour.
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we proposed a tentative mechanism. First, a phosphinoyl
17,
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.
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Scheme 3. Plausible Mechanistic Pathway
In summary, we have demonstrated a novel and convenient
approach to the synthesis of various phosphorated indolines by
copperꢀcatalyzed aminoꢀphosphorus difunctionalization via
cascade cyclization. The reaction employs cheap copper as
catalyst and K2S2O8 as an oxidant under mild condition.
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DOI: 10.1039/C4CC10267C
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