Silver-mediated oxidative C-H/P-H functionalization: An efficient route for the synthesis of benzo[b]phosphole oxides

Article abstract of DOI:10.1021/ja407373g

A Ag-mediated C-H/P-H functionalization reaction of arylphosphine oxides with internal alkynes was described for the direct preparation of benzo[b]phosphole oxides with a high yield. An unusual aryl migration on the P-atom derived from a C-P bond cleavage

Full text of DOI:10.1021/ja407373g

Communication
pubs.acs.org/JACS
Silver-Mediated Oxidative C−H/P−H Functionalization: An Efficient
Route for the Synthesis of Benzo[b]phosphole Oxides
Yun-Rong Chen and Wei-Liang Duan*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Road, Shanghai 200032, China
S
* Supporting Information
activation.9 The use of Rh or Pd catalysts in the presence of
ABSTRACT: A Ag-mediated C−H/P−H functionaliza-
tion reaction of arylphosphine oxides with internal alkynes
was described for the direct preparation of benzo[b]-
phosphole oxides with a high yield. An unusual aryl
migration on the P-atom derived from a C−P bond
cleavage and a new C−P bond formation was also
observed and demonstrated to proceed via the radical
process.
Ag2CO3 as the oxidant facilitates the reaction smoothly,
producing the desired phosphorus heterocycle 3a in good yields
(Table 1, entries 1 and 2). Furthermore, checking the
background reaction revealed that this reaction is prompted
merely by Ag2CO3 (entry 3).10 For this process, examination of
various silver salts indicated that Ag2O is optimal (entries 3−7).
Other metal oxidants, such as copper acetate or iron chloride, did
not show activity (entries 8 and 9). Solvent screening disclosed
that dimethylformamide (DMF) is optimal (entries 10−16).
hosphorus-containing heterocycles have wide applications
Pin organic synthesis, medicinal chemistry, and material
industries.1 Among these compounds, benzo[b]phospholes have
recently received considerable attention because of their inherent
physical and optical properties, which allow the development of
new classes of optoelectrochemical materials.2 Previous reports
have revealed methods for the synthesis of indoles, benzofurans,
and benzothiophene heterocycles. Yet, methods for the synthesis
of benzo[b]phospholes are lacking. Most reported routes start
from o-halide-substituted aryl alkynes, followed by phosphina-
tion and cyclization reactions via multiple steps.3 Therefore, the
development of new, efficient methods for the synthesis of
benzo[b]phosphole oxides would be highly appealing and
essential.
During the past decade, C−H bond functionalization reactions
have been extensively investigated to build diverse complex
molecules, including various heterocyclic compounds in more
efficient ways compared with conventional synthetic methods
based on functional group transformation.4 Typical examples
include the synthesis of indoles from the reaction of aniline
derivatives with alkynes catalyzed by Pd or Rh,5 the construction
of pyrroles from the reactions of enamines with ketones catalyzed
by Rh,6 and the synthesis of hexamember N- or O-containing
heterocycles from the reaction of carboxylic amides or acids with
alkynes and alkenes catalyzed by transition metals.7 However,
only a few studies are reported on the synthesis of P-containing
heterocycles through the C−H functionalization pathway.8 In
this study, we describe a Ag-mediated C−H/P−H functionaliza-
tion reaction of disubstituted phosphine oxides with alkynes to
synthesize conveniently and directly high yields of benzo[b]-
phosphole oxides.
Table 1. Silver-Mediated Oxidative Cyclization of
Diphenylphosphine Oxide with Diphenylacetylene11
catalyst
oxidant
(equiv)
temp (°C)/
yield
a
entry
1
(mol %)
solvent
DMF
time (h)
(%)
Cp*RhCl2
(2.5)
Ag2CO3
(3.0)
120/1
120/1
120/1
85
77
70
2
3
Pd(OAc)2
(5.0)
Ag2CO3
(3.0)
DMF
DMF
DMF
−
Ag2CO3
(3.0)
4
5
6
7
−
−
−
−
−
Ag2O (2.0)
120/1
120/1
120/1
120/1
120/1
85
62
85
0
AgNO3 (4.0) DMF
AgOAc (6.0) DMF
AgOTf (6.0) DMF
b
8
Cu(OAc)2
(2.0)
DMF
0
b
9
−
−
−
−
−
−
−
−
FeCl3 (4.0)
Ag2O (1.5)
Ag2O (1.5)
Ag2O (1.5)
Ag2O (1.5)
Ag2O (1.5)
Ag2O (1.5)
Ag2O (1.5)
DMF
120/1
100/8
100/8
100/8
100/8
100/8
100/8
100/8
100/8
0
94
89
23
21
22
32
74
22
c
10
DMF
c
11
CH3CN
dioxane
toluene
DCE
c
12
c
13
c
14
c
15
t-AmOH
AcOH
DMF
c
16
c
17
Ag2O (20)
NaBrO3
(1.5)
c
18
Ag2O (20)
CAN (2.0)
1 atm of O2
DMF
DMF
100/8
100/8
23
0
c
19
Ag2O (20)
a
b
c
Isolated yield. 2.0 equiv of diphenylphosphine oxide used. 1.5 equiv
Diphenylphosphine oxide was first reacted with diphenylace-
tylene under the catalysis of transition metals. The initial
assumption is to utilize the coordination ability of the P-atom
toward transition metals for the formation and functionalization
of possible metallcycle intermediates via ortho C−H bond
of diphenylphosphine oxide used.
Received: July 18, 2013
Published: October 28, 2013
© 2013 American Chemical Society
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Finally, attempts to use catalytic amounts of Ag2O in the
presence of stoichiometric amounts of oxidants, such as sodium
bromate, CAN, and O2, have not been successful (entries 17−
19).
Table 3. Silver-Mediated Oxidative Cyclization of
Diphenylphosphine Oxide with Disubstituted Acetylenes15
With the optimum conditions in hand, the current method was
applied for various substrates, and the results are shown in Table
2. Monophenylphosphine oxides bearing various substituted
Table 2. Silver-Mediated Oxidative Cyclization of Phosphine
Oxides with Diphenylacetylene
a
The number outside of parentheses is isolated yield for the reaction
carried out through Method A, and the number in parentheses is
b
isolated yield for the reaction carried out through Method B. The
c
reaction was conducted at 110 °C. The structure of the product was
confirmed by X-ray crystal diffraction analysis.12
a
b
apply this strategy in the current reaction. The investigation of
several metal nitrates revealed that the use of 1 equiv of zinc
nitrate in the presence of 5 mol % Ag2O promoted the coupling
reaction of phosphine oxides with alkynes, furnishing the
phosphorus heterocycles in moderate to good yields (Table 3).15
To obtain more insights about the mechanism of the current
reaction, a series of experiments were conducted. First,
competition reactions between diphenylacetylene 2a and
dipropylacetylene 2b with diphenylphosphine oxide were carried
out. The results indicated that 2a reacts with diphenylphosphine
oxide faster than 2b (Scheme 1). This finding can be attributed to
Isolated yield. 2.5 equiv of diarylphosphine oxide and Ag2O used.
groups, such as tert-butyl, cyclohexyl, and ethyloxy, can be
coupled with diphenylacetylene to afford the corresponding
products in moderate to good yields (Table 2, entries 1−4). An
optically pure phenylphosphinate having a (−)-menthol group
can also be converted to the phosphorus heterocycle, albeit with
obvious loss of enantiomeric excess (entry 5). In addition,
phosphine oxides having various electron-rich or -poor
substituted aryl groups were examined with diphenylacetylene
2a. In particular, aside from the desired products being
generated, a series of compounds with aryl migration on the P-
atom was also observed (entries 6−10), and the structures of the
phosphorus heterocycles in entry 6 were confirmed by X-ray
crystal diffraction analysis.12 This aryl migration is derived from a
C−P bond cleavage and a new C−P bond formation process,
which have not been reported to date.13 Table 3 summarizes the
reaction of diphenylphosphine oxide with various disubstituted
alkynes. Both diaryl- and dialkyl-substituted alkynes reacted with
diphenylphosphine oxide smoothly with good yields (Table 3,
entries 1−2). For the unsymmetrically disubstituted alkynes, a
phosphorus atom was installed at the less hindered position of
alkynes preferentially (entries 3−5).14 In addition, ethyl 3-
phenylpropiolate and 3-phenylpropynenitrile are also suitable
acceptors to extend the applicability of the current method
(entries 6−7). During our studies, Yang et al. described that
metal nitrates can be used as the oxidant to regenerate the active
Ag(I) catalyst from Ag(0).8e Their impressive work inspired us to
Scheme 1. Intermolecular Competition Experiments with
Diphenylphosphine Oxide
the fact that 2a is more electron deficient than 2b. With regard to
aryl migration, phosphine oxides with two aryl moieties were
examined, and the phenyl group on the phosphorus atom was
functionalized at a rate similar to the 4-fluorophenyl ring case but
slightly faster than the rate for the 4-methoxylphenyl group
(Scheme 2). The kinetic isotope effect was examined through the
reaction of deuterated diphenylphosphine oxide with 2a, and
KH/KD = 1.05 was observed (Scheme 3). This result indicated
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Communication
Scheme 2. Intramolecular Competition Experiments with
Unsymmetric Diarylphosphine Oxides
Scheme 5. Intramolecular Cyclization Reactions of 4a and 4b
and 3a or from intermediate IV to 3a are feasible via the radical
process. Moreover, the P-centered radicals can be generated
from the reaction of disubstituted phosphine oxides with
Mn(OAc)3.19 Thus, the current reaction was carried out with
Mn(OAc)3 as the oxidant. Two types of benzo[b]phosphole
oxides, along with the involvement of aryl migration, were
isolated in good yields (Scheme 6), which further support the
proposed mechanism in Scheme 4.
Scheme 6. Oxidative Cyclization of Diarylphosphine Oxides
Mediated by Mn(OAc)3
that the C−H bond cleavage on the aryl ring of the phosphine
oxide is not involved in the rate-limiting step.16
Scheme 3. Experiments with Isotopically Labeled
Compounds
Given that aryl migration is often observed in radical
reactions,13 we proposed a tentative mechanism for the current
system (Scheme 4). First, a phosphorus radical is generated from
To demonstrate the utility of the current protocol,20 the
reactions of diynes with diphenylphosphine oxide were
conducted for the one-step synthesis of the bis(benzo[b]-
phosphole oxides) (Scheme 7),21 which has been suggested to
Scheme 4. Proposed Reaction Pathways
Scheme 7. Applications for the Synthesis of
Bis(benzo[b]phosphole oxides) 6a and 6b
have potential applications in organic electronics.3a In addition,
the absorption/emission spectra of the obtained benzo[b]-
phosphole oxides were measured in CH2Cl2, and all the
examined compounds showed fluorescence with 0.6−14.5%
quantum yields (see the Supporting Information (SI)).
In summary, we have described a Ag-mediated oxidative C−
H/P−H functionalization reaction of disubstituted phosphine
oxides with internal alkynes, furnishing the benzo[b]phosphole
oxides with high yields in a simple, efficient way. An unusual aryl
migration/cyclization on the P-atom has also been observed and
proposed via the radical pathway. Further extension of the
current method and application of the obtained P-compounds as
the ligand in catalysis will be pursued.
silver diphenylphosphine oxide,17 and then the radical addition
to the alkyne affords an alkenyl radical I. Alternatively, I also
might be generated through the addition of silver diphenylphos-
phine oxide to alkyne, followed by a silver-induced formation of
an alkenyl radical step.8e,18 Next, the intramolecular addition of
the alkenyl radical I to the aryl ring at the ortho position of the
phosphorus atom, followed by the oxidation with Ag2O and the
removal of a proton, affords the phosphorus heterocycle 3a. For
the formation of the aryl migration product, the alkenyl radical I
attacks the aryl ring at the carbon atom directly attached to the
phosphorus atom,13c,d followed by the C−P bond cleavage,
furnishing a phosphorus radical IV. Subsequently, radical
phosphination on the aryl group that lies at the cis-position of
the phosphorus atom, in the presence of Ag2O as the oxidant,
affords the phosphorus heterocycle with the aryl migration.
To further prove this mechanism, compounds 4a and 4b were
prepared and examined under the radical cyclization conditions
or the current reaction conditions, respectively. The same
phosphorus heterocycle 3a was obtained in both cases (Scheme
5). These results indicated that the steps from intermediate I to II
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and characterization of the products.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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Journal of the American Chemical Society
Communication
F.; Wang, J.-Y.; Sheng, J.; Vora, H. U.; Wang, X.-S.; Yu, J.-Q. J. Am. Chem.
Soc. 2013, 135, 1236.
AUTHOR INFORMATION
■
Corresponding Author
wlduan@mail.sioc.ac.cn
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Basic
Research Program of China (973 Program 2010CB833300),
NSFC (20902099, 21172238), and SIOC.
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