Pd(II)-catalyzed phosphorylation of aryl C-H bonds

Article abstract of DOI:10.1021/ja404526x

A Pd(II)-catalyzed C-H phosphorylation reaction has been developed using heterocycle-directed ortho-palladation. Both H-phosphonates and diaryl phosphine oxides are suitable coupling partners for this reaction.

Full text of DOI:10.1021/ja404526x

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“Pd(II)-Catalyzed Phosphorylation of Aryl C–H Bonds”
Chen-Guo Feng, Mengchun Ye, Kai-Jiong Xiao, Suhua Li, and Jin-Quan Yu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja404526x • Publication Date (Web): 11 Jun 2013
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Pd(II)-Catalyzed Phosphorylation of Aryl C–H Bonds
Chen-Guo Feng, Mengchun Ye, Kai-Jiong Xiao, Suhua Li, Jin-Quan Yu*
Department of Chemistry, The Scripps Research Institute, 10550 N. Torrey Pines Road, La Jolla, California 92037
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Abstract: A Pd(II)-catalyzed C–H phosphorylation reaction
has been developed using heterocycle-directed ortho-
palladation. Both H-phosphonates and diaryl phosphine
oxides are suitable coupling partners for this reaction.
Aryl phosphonates and derivatives are an important class of
molecules because of their broad application in medicinal
chemistry,1 material chemistry,2 and catalysis.3 Since the
pioneering work reported by Hirao and co-workers in 1981,4
palladium catalyzed cross-coupling of aryl halides with H-
phosphonates has become a practical method to construct C(sp2)-
P bonds.5 During the past decade, the scope of the Hirao reaction
has been significantly expanded to include aryl triflates, tosylates,
diazonium salts and boronic acids as coupling partners.6 Copper
and nickel complexes were also shown to be effective catalysts
for this reaction.7,8 Encouraged by recent progress towards
developing Pd-catalyzed diverse carbon-carbon and carbon
heteroatom bond forming reactions via directed C–H activation,9-
13
we embarked on the development of phosphorylation of C–H
Table 1. Reaction Conditions Optimizationa
bonds as a complementary method for making carbon-phosphorus
bonds, which remains an unsolved problem due to the strong
Pd(OAc)2 (10 mol%)
N
N
coordinating property of the phosphorus coupling partners. Takai
and co-workers used a tethered phosphite as a directing group as
well as the coupling partner successfully avoided this problem and
established the first example of Pd(0)-catalyzed C–H
phosphorylation reaction in an intramolecular fashion (eq 1).14-16
Herein we report an intermolecular C–H phosphorylation of C–H
bonds with a variety of heterocycles (eq 2). The pyridine and
oxazoline containing phophonate products are potentially useful
precursors for medicinal chemistry1 or N, P-bisdendate ligand
preparation.3h
O
O
base or acid
HP(OiPr)2
P(OiPr)2
+
oxidant, BQ
T °C, 13 h
(1.2 equiv)
1a
entry
1
2a
3a
base/acid
oxidant
solvent
DCE
yield (%)b
T (°C)
100
19
22
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag3PO4
AgO
Na2CO3
Na2CO3
2
100
MeCN
3
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
Na2CO3
Na2CO3
Na2CO3
-
1,4-dioxane 17
4
toluene
34
58
29
52
52
54
0
5
t-AmylOH
t-AmylOH
t-AmylOH
t-AmylOH
t-AmylOH
t-AmylOH
6
7
PivOH
AcOH
8
9
NaHCO3
K3PO4
NaTFA
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
10
11
12
13
14
15
16
17
t-AmylOH 47
69
73
34
79
50
44
t-AmylOH
t-AmylOH
t-AmylOH
t-AmylOH
AgOAc
Cu(OAc)2 t-AmylOH
To establish the feasibility of the C-P bond formation from
cyclopalladated complexes and H-phosphonates,17 we treated
complexes I and II with H-phosphonate 2a under various
conditions. We found that stirring I or II with H-phosphonate 2a
in the presence of 1 equiv 1,4-benzoquinone (BQ) in a range of
solvents gave the desired phosphorylation product 3a and 4 in
moderate to excellent yields (eq 3-4). The use of BQ was found to
be essential for the formation of the products. Presumably, BQ
NaOAc
NaOAc
NaOAc
K2S2O8
AgOAc
AgOAc
t-AmylOH
t-AmylOH
t-AmylOH
18
19
120
140
84
72
a Reaction conditions: Diisopropyl H-phosphonate 2a (0.24 mmol) in solvent (2 mL)
was added dropwise to a mixture of 2-phenylpyridine 1a (0.2 mmol), Pd(OAc)2
(0.02 mmol), base or acid (0.4 mmol) and oxidant (0.4 mmol) in solvent (2 mL) in 13
h. b Yields were determined by GC-MS.
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However, Cl on the meta-position (3i), and strongly electron-
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8
promotes the reductive elimination in a similar manner to that
observed in the coupling of C–H bonds with organometallic
reagents.18
withdrawing F (3j), CF3 (3k), CN (3l) and CO2Me (3n) groups at
the para position decreased the yields to 58%, 45%, 42%, 15%
and 58% respectively. The reaction of 2-naphthalene also
proceeded smoothly and gave highly selective β- phosphorylation
product in 79% yield (3o). Moderate to good yields (66-79%)
were obtained when the pyridine rings were substituted by methyl
or MeO groups at various positions (3p-3s).
Based on this reactivity, we proceeded to develop catalytic
conditions for this transformation using 2-phenylpyridine 1a as
the model substrate. Not surprisingly, reacting 2-phenylpyridine
1a with H-phosphonate 2a in the presence of Pd catalyst in one
pot did not give any desired product. Presumably, coordination of
the H-phosphonate reagent with Pd(II) catalyst will inhibit the C–
H activation step. The tautomeric equilibria of H-phosphonates is
well-known and the tri-coordinated phosphite can bind strongly to
Pd(II) center with its lone electron pair.19 To avoid this problem,
we added the H-phosphonate 2a to the reaction dropwise so that
the concentration of it is minimized during the reaction course.
With 10 mol% Pd(OAc)2 as catalyst, Ag2CO3 as oxidant and
Na2CO3 as base, H-phosphonate 2a was added dropwise at 100 oC
in different solvents (Table 1). To our delight, the desired product
was obtained under these reaction conditions; and t-AmylOH
proved to be the best solvent (entry 5). Both a suitable base and
acid promoted the reaction (entries 6-9). While stronger base
K3PO4 completely inhibited the reaction (entry 10), the use of
NaOAc gave product 3a in 69% yield (entry 12). Several other
silver salts were also examined, and AgOAc was found to be the
best choice, improving the yield to 79% (entry 15). Cu(OAc)2 and
K2S2O8 can also been used as oxidant albeit less effective
compared to silver salt oxidants (entries 16 and 17). The reaction
yield was further improved to 84% when reaction temperature
Table 3. C–H Phosphorylation of Pyridine Derivatives a,b
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o
was raised from 100 to 120 C (entry 18). The use of other diaryl
H-phosphonates and cyclic H-phosphonates did not improve the
reaction yields (Table 2).
Table 2. Evaluation of Different Phosphorylation Reagentsa,b
With these optimized reaction conditions in hand, we examined
the scope of arenes using coupling partner 2a and obtained the
isolated yields with substrates 1a-s (Table 3). Arenes with
electron-donating p- and m-methyl substitution gave yields of 73%
and 80% respectively (3b and 3c), while the o-methyl substituted
arene afforded a lower yield of 61% (3d) due to the buttressing
effect of the biphenyl. Similar trends in yields were observed with
MeO substituted arenes (3e-3f). Introduction of moderately
electron-withdrawing Cl on the para-position of arene was well
tolerated and the product was obtained in 67% yield (3h).
To expand the utility of this methodology, several other
nitrogen-based heterocycle scaffolds were examined (Table 4).
Both quinoline- and isoquinoline-directed phosphorylation of 1t
and 1u occurred to give the corresponding products 3t and 3u in
62% and 74% yields respectively. Phosphorylation of
isoquinoline 1v gave the desired product 3v in only 28% yield due
to steric hindrance. We were delighted that 7,8-benzoquinoline
was phosphorylated in 65% yield to give a potentially useful
ligand scaffold 3w. Phosphorylation of 2-phenylpyrimidines gave
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corresponding products in 61-76% yields (3x-3z). We also
In light of the previous observation that Ag(I)-mediated
attempted to use this reaction to prepare the PHOX type ligands,3a
but only in 40% yield (4). Pyrrazole substrate was also
phosphonated to give 5 in 51% yield.
phosphorylation of indoles with H-phosphonates proceeds via a
radical pathway,15d we performed a control experiment in the
absence of Pd catalyst (eq 5). We found that this reaction did not
proceed without palladium catalyst. Since the palladocycles I and
II were shown to react with H-phosphonate 2a to give the
phosphorylation products (eq 1), we believe that our reaction
proceeds through directed palladation and subsequent coupling
with phosphate coupling partners.20 C–H activation of 2-
phenylpyridine 1a generates cyclopalladate species A, which
undergoes anionic ligand exchange with H-phosphonate 2a to
provide complex B.20 The reductive elimination of complex B
facilitated by BQ affords the desired phosphorylation product.
The Ag(I) oxidant reoxidizes Pd(0) to Pd(II) to close the catalytic
cycle. In terms of redox chemistry, this reaction differs from the
Takai’s intramolecular reaction in which Pd(0) inserts into the P-
H bonds to form the P-Pd-H species that cleaves C–H bonds.14
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Table 4. C–H Phosphorylation With Diverse Heterocyclesa,b,c
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Scheme 1. Proposed Reaction Mechanism
Reactions of these phophonates with ArMgX readily afford
triarylphosphine oxides which can be reduced to give
triarylphophine ligands.3h-3i Alternatively, we also demonstrated
the feasibility of preparing diarylphosphine oxide precursors
directly by coupling C–H bonds with various diaryl phosphine
oxides (Table 5), albeit giving moderate yields under current
conditions.
In summary, a Pd(II)-catalyzed intermolecular C–H activation
/phosphorylation reaction has been developed for the first time. A
variety of heterocyclic substrates were phosphorylated to give N-
P bisdentate compounds that are potentially useful in medicinal
chemistry and catalysis.
Table 5. Coupling With Several Diarylphosphine Oxidesa,b
Acknowledgement. We gratefully acknowledge The Scripps
Research Institute, the U. S. NSF (CHE-1011898) for financial
support. We thank Shanghai Institute of Organic Chemistry
(SIOC) and Zhejiang Medicine for a joint postdoctoral fellowship
(C.-G. F.).
Supporting Information Available: Experimental procedures
and spectral data for all new compounds (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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