Direct annulations toward phosphorylated oxindoles: Silver-catalyzed carbon-phosphorus functionalization of alkenes

Article abstract of DOI:10.1002/anie.201209475

Silver screen: The AgNO₃-catalyzed carbon phosphorylation of alkenes occurs by an alkene addition/cyclization cascade. Ag⁺ reacts with Ph₂P(O)H to form the crucial active intermediate 1, which promotes the reaction. This method requires a cheap, nontoxic silver salt as the catalyst and substrates for the transformation are simple and readily accessible.

Full text of DOI:10.1002/anie.201209475

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Angewandte
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DOI: 10.1002/anie.201209475
CÀH Functionalization
Direct Annulations toward Phosphorylated Oxindoles: Silver-
Catalyzed Carbon-Phosphorus Functionalization of Alkenes**
Ya-Min Li, Meng Sun, Hong-Li Wang, Qiu-Ping Tian, and Shang-Dong Yang*
Aromatic organophosphorus compounds play very important
roles and are ubiquitous. They can be found in a wide range of
nucleotides, pharmaceuticals, and phosphine-containing
for the AgNO -catalyzed carbon phosphorus functionaliza-
tion of alkenes has been developed and exhibits several
unique features (Scheme 1): 1) unlike palladium, silver first
3
[
1]
ligands. Therefore, the development of a more concise and
reacts with Ph P(O)H and forms the crucial active inter-
2
efficient method for CÀP bond formation is highly desirable
mediate 2A, which promotes the reactions; 2) a cheap,
[
2]
and presents a considerable challenge. Recent, transition
metal catalyzed difunctionalization of alkenes provide a pow-
erful strategy for the synthesis of various organic com-
[
3]
[4]
[5]
pounds,
including diamination,
aminooxygenation,
and aminohalogena-
In particular, the palladium-catalyzed oxidative
[
6]
[7]
dioxygenation,
tion.
fluoroamination,
[
8]
carbo- and heterofunctionalization of alkenes with different
nucleophiles by direct CÀH functionalization of arenes offers
[9]
a new way to synthesize various functionalized oxindoles.
However, application of this method to the synthesis of
phosphorylated oxindoles with phosphorus-based nucleo-
philes has not been achieved, and may be due to the strong
coordinating abilities of both the substrate and the product. In
addition, these related reactions generally used PhI(OAc) as
2
an oxidant, which causes the phosphorus nucleophiles to
decompose under strong oxidative conditions. In the past
several years, we have focused our efforts on the development
of new and efficient protocols for transition metal catalyzed
Scheme 1. Silver-catalyzed difunctionalization of alkenes.
[
10]
CÀP bond formation. It has been found that relatively mild
copper and silver salts can work with Ph P(O)H to form the
2
corresponding active [Ph P(O)Cu] or [Ph P(O)Ag] com-
plexes, which may be applied to the addition of alkenes.
nontoxic silver salt is employed in catalyzing the hydro-
phosphorylation of alkenes for the first time; 3) the substrates
for the transformation are simple and readily accessible, and
no additional ligand or oxidant is needed.
2
2
[11]
In light of these factors, we hypothesized that silver or copper
salts might catalyze carbon phosphorus functionalization of
alkenes and conduct the direct cyclization to afford the
corresponding phosphorylated oxindoles which have the
In an initial study, we chose the N-methyl-N-arylacryl-
amide 1a and Ph P(O)H as the model substrates, and various
2
[
12]
potential to be novel insecticides or herbicides. Moreover,
it also could be used as a P,O-ligand for transition metal
catalyzed reactions. Through our endeavors, a new method
copper and silver catalysts were tested in different solvents at
different temperatures. To our delight, this reaction occurred
in the presence of 50 mol% of either copper or silver salts
such as Cu O, CuCl, Cu(OAc) , Ag O, and AgNO in CH CN
2
2
2
3
3
at 1008C, with AgNO affording the desired product 2a in the
3
[
*] Y. Li, M. Sun, H. Wang, Q. Tian, Prof. S. Yang
State Key Laboratory of Applied Organic Chemistry
Lanzhou University
Lanzhou 730000 (P. R. China)
E-mail: yangshd@lzu.edu.cn
highest yield of 79% (Table 1, entries 1–5). Encouraged by
this result, we further optimized the reaction conditions. Our
focus was to decrease the loading of the silver catalyst,
however, when the quantity of catalyst was reduced to
Prof. S. Yang
20 mol%, only AgNO could promote the reaction to work
3
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics, Lanzhou 730000 (P. R.
China)
smoothly and 2a was obtained in 21% yield (Table 1, entry 6).
Attempts using nitrogen-containing ligands were similarly
unsuccessful. These results prompted us to consider that the
anion may be critical in this reaction. A variety of nitrates
having potential in this transformation were evaluated in the
[
**] We are grateful to the NSFC (Nos. 21272100), Program for
Changjiang Scholars and Innovative Research Team in University
(
PCSIRT: IRT1138), and Research Fund for the Doctoral Program of
presence of 10 mol% AgNO in CH CN at 1008C. The results
Higher Education of China (Nos. 20100211120014) for financial
support. We thank S. F. Reichard, MA for editing the manuscript.
3
3
demonstrate that our hypothesis was reasonable. Using
.5 equivalents of Mg(NO ) ·6H O as an additive gave the
0
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209475.
3
2
2
best result (Table 1, entries 7–9). Excitingly, when the loading
3
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[
a]
[a,b]
Table 1: Reaction conditions screening.
Table 2: Silver-catalyzed carbon phosphorylation of alkenes.
[
b]
Entry
Cat. (mol %)
Cu O (50)
Additive (equiv)
Yield [%]
1
2
3
4
5
6
7
8
9
2
–
–
–
–
–
–
43
51
39
60
79
21
76
78
71
82
38
76
60
56
75
0
68
59
65
61
59
<5
<5
<5
0
CuCl (50)
Cu(OAc) (50)
2
Ag O (50)
2
AgNO (50)
3
AgNO (20)
3
AgNO (10)
Mg(NO ) 6H O (1.0)
3
3
2·
2
AgNO (10)
Mg(NO ) ·6H O (0.5)
3 2 2
Mg(NO ) ·6H O (0.3)
3 2 2
Mg(NO ) ·6H O (0.5)
3 2 2
Cu(NO ) ·3H O (0.5)
3 2 2
La(NO ) ·6H O (0.5)
3 3 2
Yb(NO ) ·6H O (0.5)
3 3 2
Co(NO ) ·6H O (0.5)
3 2 2
Ce(NO ) ·6H O (0.5)
3 2 2
3
AgNO (10)
3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
AgNO (5)
3
AgNO (5)
3
AgNO (5)
3
AgNO (5)
3
AgNO (5)
3
AgNO (5)
3
AgNO (5)
NaNO + H O (0.5)
3 2
3
Ag O (5)
Mg(NO ) ·6H O (0.5)
2
3
2
2
AgOTf (5)
Mg(NO ) ·6H O (0.5)
3 2 2
Mg(NO ) ·6H O (0.5)
3 2 2
Mg(NO ) ·6H O (0.5)
3 2 2
Mg(NO ) ·6H O (0.5)
3 2 2
AgSbF (5)
6
AgBF (5)
4
AgOAc (5)
AgNO (5)
MgCl (0.5)
3
2
AgNO (5)
MgSO (0.5)
3
4
AgNO (5)
Mg(OAc) ·4H O (0.5)
3
2
2
AgNO (5)
MgClO (0.5)
[a] All the reactions were carried out in the presence of 0.3 mmol of 1a–
1z, Mg(NO ) ·6H O (0.5 equiv) in 3 mL CH CN at 1008C. Conditions A:
3
4
AgNO (5)
Mg(NO ) ·6H O (0.5)
75
0
3
3
2
2
3
2
2
3
4
5
–
Mg(NO ) ·6H O (0.5)
5 mol% AgNO , 2.0 equiv HP(O)R R ; Conditions B: 20 mol% AgNO ,
.0 equiv HP(O)R R ; [b] Yield of the isolated product.
3
2
2
3 3
4
5
2
[
a] Reaction conditions: 1a (0.3 mmol), Ph P(O)H (0.6 mmol), catalyst,
2
and additive in dry CH CN (3 mL) with stirring at 1008C under argon for
3
1
2 h. [b] Yield of the isolated product. [c] Reaction temperature: 808C.
para-position of the arylacrylamide reacted well and the
majority of products were obtained with high yields (2e–2l).
With substituent groups at the ortho position of the arylacryl-
amide, the effect of the steric bulk was very distinct and lower
yields were observed as a result (2m–2r). For some substrates
the loading of the catalyst had to be increased to 20 mol% to
furnish the products (2m–2r). Substrates bearing meta sub-
stituents exhibited good reactivity but poor regioselectivity
(2s). When the benzene ring of the substrates was changed to
naphthalene, the reaction successfully provided the product
2t. In addition, cyclization of the tetrahydroquinoline deriv-
ative furnished the tricyclic oxindole 2u in 83% yield.
However, using heterocyclic substrates such as pyridine,
quinolone, and pyrimidine led to no reaction as a result of
lower reactivity. Next, variously substituted alkenes were
evaluated. No reaction occurred in the case of a monosub-
of AgNO was reduced to 5 mol%, the yield of 2a improved
to 82% yield (Table 1, entry 7). Other nitrates such as
Cu(NO ) ·3H O, La(NO ) ·6H O, Y(NO ) ·6H O, Co-
3
3
2
2
3
3
2
3
3
2
(
NO ) ·6H O, and Ce(NO ) ·6H O also performed well in
3 2 2 3 3 2
this reaction (Table 1, entries 10–15). If the additive was an
aqueous solution of NaNO , the desired product 2a was not
produced (Table 1, entry 16). By using 0.5 equivalents of
Mg(NO ) ·6H O as an additive, we revaluated different silver
3
3
2
2
catalysts but lower yields were obtained (Table 1, entries 17–
1). Different magnesium salts were also examined and the
results show that Mg(NO ) ·6H O is still the best choice
2
3
2
2
(
Table 1, entries 22–25). In addition, decreasing the temper-
ature caused the yield descend (Table 1, entry 26). A control
experiment showed that Mg(NO ) ·6H O alone could not
2
stituented olefin (R = H). However, a series of a-substituted
3
2
2
promote this reaction (Table 1, entry 27).
olefins bearing different functional groups, such as phenyl
(2v), benzyl (2w), ester (2x), and phthalimide (2y) groups
can still provide the desired products in moderate to good
yields. Notably, the product of 2y can be easily to converted
With the optimized reaction conditions in hand (Table 1,
entry 7), we turned our attention to the scope with respect to
the substrates. As shown in Table 2, an investigation into
different N-protecting groups showed that methyl and benzyl
are much better than tosyl (2a–2c) and that N-free arylacryl-
amide does not work at all (2d). Substrates with both
electron-donating and electron-withdrawing groups on the
[
13]
into spiropyrrolidinyloxoindoles such as horsfiline.
a,b-
disubstituted arylacrylamide was also subjected to the pro-
cess, thus affording the spirooxindole 2z with 82% yield and
moderate diastereoselectivity. Finally, both diethyl phosphite
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and ethyl phenylphosphinate could be used as substrates, thus
generating the corresponding products in 83% and 43%
yields (2aa, 2bb), both of which can undergo various func-
tional groups conversions through cross-coupling reactions.
The reaction can also be effectively scaled up with similar
efficiency (see the Supporting Information).
To obtain larger rings and to determine whether the
Michael addition occurred during the phosphorylation of the
alkene, we selected 3d as a substrate for the reaction and the
six-membered-rings product 2dd was obtained in 63% yield.
This result also excluded the Michael addition in this trans-
formation [Scheme 2, Eq. (1)]. In addition, we introduced
evidence in this regard. As a unique substrate, 1a worked in
the presence of 5 mol% AgNO and 0.5 equivalents Mg-
3
(NO ) ·6H O in CH CN at 1008C. CÀH functionalization did
3
2
2
3
[14]
not occur and the cyclization product 3c was not detected
(Scheme 3). However, 2A catalyzed a carbon phosphoryla-
tion reaction of 1a under the optimized reaction conditions to
give 2a in 72% yield (Scheme 3). Thus, we conclude that the
first step is phosphorylation with AgNO and the formation of
3
the Ph (O)PAg is the key step in this transformation.
2
A detailed mechanism for this novel transformation is
unknown. To collect data on the catalytic procedure, the
intramolecular and intermolecular kinetic isotope effect
(KIE) experiments were carried out with the deuterium-
labeled substrates [D ]-1a and [D ]-1a (Scheme 4a and b).
1
5
Scheme 2. AgNO -catalyzed carbon phosphorylation of alkenes.
3
a stereogenic center at the a-position of the carbonyl group
and set out to obtain the optically pure product 2ee, but a low
d.r. value was observed [Scheme 2, Eq. (2)].
While investigating the scope with respect to the sub-
strates, the arylacrylamide 4a reacted with Ph P(O)H under
2
the optimal reaction conditions and only hydrophosphoryla-
tion of the alkene occurred to give the uncyclizated product
4
ab in 29% yield (Scheme 3). This result suggests that
Scheme 4. Deuterium-labeled carbon phosphorylation of alkenes.
AgNO coordinates to 4a and leads to alkene addition. In
3
contrast, a pioneering report shows that AgNO can react
3
[15]
with Ph P(O)H to form the complex of Ph (O)PAg (2A).
Two secondary kinetic isotope effects were observed (the
2
2
Two experiments were performed to collect additional
intramolecular k /k = 1.2, and intermolecular k /k = 0.9).
H
D
H
D
First, the intermolecular one indicates that the CÀH bond is
broken after the turnover-limiting step. In addition, the
observation of the intramolecular kinetic isotope effect of 1.2
indicates that CÀC bond formation occurs before CÀH
[
16]
cleavage. [D ]Diphenylphosphine oxide was also examined
1
in the presence of 100 mol% AgNO and 2a was obtained in
3
7
5% yield and no other deuterium-labeled product was
observed. This result illustrates that Ph P(O)H first reacts
2
with AgNO and forms 2A. Simultaneously, the pH value of
3
the reaction solution changes and also indicates that the
HNO is produced (see the Supporting Information).
3
Some metal-catalyzed carbon phosphorylation reactions
of alkenes with diphenylphosphine oxide Ph P(O)H and H-
2
phosphonates (RO) P(O)H are well known to proceed by
2
[
17]
a radical process, so our carbon phosphorylation reaction of
alkenes may proceed by a radical pathway. The preliminary
mechanistic studies support this assumption. Chemical trap-
ping of radicals using TEMPO (2,2,6,6-tetramethyl-1-piper-
Scheme 3. Investigation of phosphorylation and cyclization.
3
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idinyloxy,
a
well-known radical-trapping reagent) was
intermediate 2A under the reaction conditions. It may also
recently also shown to be invaluable in elucidating the
proceed by a pathway involving a silver-promoted generation
[17d,e]
[19]
mechanism of the phosponyl radical.
As illustrated in
of the phosphoryl radical 2B (Scheme 6, path A), which
[17d]
Scheme 5, the addition of 1.0 equivalents of TEMPO sup-
pressed the carbon phosphorylation process, thus suggesting
that this transformation might involve a radical process.
then adds to 1a to give the alkyl radical 2D.
Another
possibility for the formation of the alkyl radical 2D is the
addition of 2A to 1a to form the silver(I) species 2C, which
was oxidized to the alkyl radical 2D (Scheme 6, path B). The
resulting alkyl radical 2D participates in an intramolecular
radical substitution reaction. Addition of the radical to the
aromatic ring to generate the intermediate 2E with a subse-
quent single-electron transfer (SET) from 2E to silver(I)
[19]
would release the product along with HNO and silver(0).
3
In the presence of HNO , the silver(0) was oxidized to
silver(I).
3
Scheme 5. TEMPO-trapped carbon phosphorylation of alkene.
In summary, we have developed a highly efficient protocol
for the preparation of various diphenylphosphoryl oxindoles
by silver-catalyzed difunctionalization of alkenes through
a carbon phosphorylation and CÀH functionalization cascade
To further understand the catalytic cycle of the carbon
phosphorylation, the model reaction was monitored by mass
spectrometry experiment. First, the ESI/MS showed a peak at
process. A cheap, nontoxic silver salt is employed in catalyz-
ing the hydrophosphorylation of alkenes for the first time.
Further studies on the clarification of the reaction mechanism
and application to other substrates are
+
m/z 308.9602, which corresponds to [2A + H] (Scheme 6).
The signal might mean that AgNO can easily react with
3
underway.
Experimental Section
General Procedures for silver-catalyzed
carbon phosphorylation of substrates: In
a Schlenk tube, 1a (0.30 mmol), HP(O)Ph2
(
(
0.6 mmol), AgNO (0.015 mmol), and Mg-
3
NO ) ·6H O (0.15 mmol) were added and
3
2
2
charged with Ar three times. Anhydrous
CH CN (3 mL) was then added. The mix-
3
ture was allowed to stir at 1008C for 12 h
(monitored by TLC). After the substrate
was consumed, the reaction was cooled to
room temperature and tBuONa (1.2 mmol)
was added. The mixture was stirred for an
additional 10 min at room temperature. The
solvent was then removed under vacuum
and the resulting residue was purified by
column chromatography on silica gel
(eluent: hexanes/ethylacetate 1:1) to give
the product 2a.
Received: November 27, 2012
Revised: January 14, 2013
Scheme 6. Proposed mechanisms of AgNO -catalyzed carbon phosphorylation of arylacrylamide.
Published online: February 25, 2013
3
Keywords: alkenes · heterocycles ·
phosphorous · silver · synthetic methods
.
diphenylphosphine oxide to form the intermediate 2A. In
contrast, a signal at m/z 483.1087 may be the silver(I) species
2
C which results from the addition of 1a with 2A. In the
standard catalytic reaction, we were delighted to see two
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(
Scheme 6).
On the basis of the above experiments, we propose
a tentative pathway for this transformation (Scheme 6). First,
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