Palladium-Catalyzed Phosphorylation of Aryl Mesylates and Tosylates

Article abstract of DOI:10.1021/acs.orglett.5b03104

The first general palladium catalyst for the phosphorylation of aryl mesylates and tosylates is reported. The newly developed system exhibits excellent functional group compatibility. For instance, free amino, keto, ester, and amido groups, as well as het

Full text of DOI:10.1021/acs.orglett.5b03104

Letter
pubs.acs.org/OrgLett
Palladium-Catalyzed Phosphorylation of Aryl Mesylates and
Tosylates
Wai Chung Fu, Chau Ming So, and Fuk Yee Kwong*
State Key Laboratory of Chirosciences and Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic
University, Hung Hom, Kowloon, Hong Kong
S
* Supporting Information
ABSTRACT: The first general palladium catalyst for the
phosphorylation of aryl mesylates and tosylates is reported.
The newly developed system exhibits excellent functional
group compatibility. For instance, free amino, keto, ester, and
amido groups, as well as heterocycles, remain intact during the
course of reaction. The mesylated derivatives of biologically
active compounds such as 17β-estradiol and 6-hydroxyflavone
are also shown to be applicable substrates. A one-pot phosphorylation−amination sequence is described for the facile synthesis of
potential pharmacophores.
rganophosphorus compounds such as aryl phosphonates
constitute an important class of compound due to their
compounds to electrophiles, their high cost and the low
hydrolytic stability of aryl triflates have generated application
concerns. Indeed, the use of aryl mesylates and tosylates are
much more desirable due to their cost-effectiveness13 and high
stability,14 as well as the environmental friendliness which can
be realized by the natural biodegradation of methanesulfonic
acid byproducts.15 However, the chemical inertness of these
electrophilic partners makes them prone to the oxidative
addition of a Pd(0) species, resulting in lost activity in catalysis.
Therefore, the identification of a suitable catalyst to facilitate
the aryl mesylate C−P couplings has been proven to be a
persistent challenge. Although a nickel catalyst has been
reported to use aryl mesylates in the C−P bond forming
process,16 the scope was found not to be general and only
moderate product yields were shown. Thus, there is a demand
to develop a system for achieving general and versatile C−P
bond couplings. Herein, we report the first general Pd catalyst
for the phosphorylation of aryl mesylates and tosylates. This
new system is applicable to a wide range of functional groups
with modest catalyst loadings.
In our initial investigation, nonactivated 4-tert-butylphenyl
tosylate and diisopropyl phosphite were selected as the
benchmarking substrates (Scheme 1). A series of remarkable
ancillary ligands were then evaluated for their efficacy in this
C−P bond coupling reaction (Scheme 1). CM-Phos (L1)17 was
found to be the best candidate while Mor-DalPhos18 (L2) and
XPhos19 (L3) were also suitable ligands albeit with lower yields.
Other state-of-the-art ligand skeletons, including mono-
dentate (L4−L6, L11)20 and bidentate phosphines21 (L7−
L10) as well as a N-heterocyclic carbene22 (L12), were also
examined. Nevertheless, they were not effective in promoting
the catalysis, presumably due to the demanding oxidative
O
versatile applications in medicinal chemistry,1 organic syn-
thesis,2 and material chemistry3 (Figure 1). Apart from the
Figure 1. Examples of organophosphorus compounds and their
potential applications.
conventional Grignard or organolithium protocols, the most
straightforward method to prepare such compounds is the Pd-
catalyzed C−P bond formation pathway between a R2P(O)H
compound and a coupling partner.4 The pioneering work of
Hirao in 1981 has prompted the scientific community’s interest
in extending the scope of the process.5 The use of aryl halides,6
boronic acids,7 triflates,8 hydrazines,9 and triarylbismuths10 in
the Pd-catalyzed C−P bond formation has been well
documented. A number of copper11 and nickel12 catalysts
were also shown to promote the C−P bond-forming reactions.
Despite the availability of aryl halides, some unique arenes
exist only in phenolic form. In fact, the regioselective
halogenation for accessing these particular arene patterns may
not be straightforward. For example, biologically active
compounds such as estrogenic hormones contain a phenolic
moiety, but not the halogenated analog. Although triflating
agents could offer a route for transforming phenolic
Received: October 27, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03104
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Scheme 1. Evaluation of Ligand Efficacy
Table 1. Selected Entries of Reaction Optimization
entry
base
solvent
% yield
1
K3PO4
K2HPO4
Na3PO4
K2CO3
DABCO
NEt3
t-BuOH (0.2 M)
10
2
t-BuOH (0.2 M)
34
3
t-BuOH (0.2 M)
37
4
t-BuOH (0.2 M)
15
5
t-BuOH (0.2 M)
21
6
t-BuOH (0.2 M)
49
7
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
t-BuOH (0.2 M)
52
8
DMF (0.2 M)
trace
trace
trace
63
9
dioxane (0.2 M)
10
11
12
13
14
15
16
toluene (0.2 M)
i-PrOH (0.2 M)
t-BuOH/i-PrOH (1:1; 0.2 M)
t-BuOH/i-PrOH (1:1; 0.3 M)
t-BuOH/i-PrOH (1:1; 0.5 M)
t-BuOH/i-PrOH (1:1; 0.3 M)
t-BuOH/i-PrOH (1:1; 0.3 M)
68
78
71
b
88
c
a
94
Reaction conditions: 4-tert-Butylphenyl tosylate (0.3 mmol), DIPEA
(0.9 mmol), diisopropyl phosphite (0.45 mmol), t-BuOH/i-PrOH
(1:1, 1.0 mL, 0.3 M), Pd(OAc)2 (1.0 mol %), Ligand (4.0 mol %)
under nitrogen at 110 °C for 18 h under N2; calibrated GC yields were
reported.
a
Reaction conditions: 4-tert-Butylphenyl tosylate (0.3 mmol), base
(0.6 mmol), diisopropyl phosphite (0.36 mmol), solvent, Pd(OAc)2/
L1 = 1:3 under nitrogen at 110 °C for 18 h under N2; calibrated GC-
FID yields were reported. Pd(OAc)2/L1 = 1:4. Pd(OAc)2/L1 = 1:4,
b
c
DIPEA (0.9 mmol), diisopropyl phosphite (0.45 mmol).
addition of aryl tosylates as suggested by the unreacted
remaining starting materials.
aryl mesylates (Scheme 3). Functionalized aryl mesylates
containing an ester (Scheme 3, 2e), an enolizable ketone
(Scheme 3, 2d), a free amine (Scheme 3, 2h), heterocycles
(Scheme 3, 2i−2l), and an ortho-substituted arene (Scheme 3,
2d) were phorsphorylated successfully with good-to-excellent
yields. Apart from the H-phosphinates, the less electron-
deficient ethyl phenylphosphinate was an applicable phospho-
rus coupling partner as demonstrated by the examples 2l and
2m. Organophosphorus compounds have been documented as
important functionalities found in many pharmacophores. It
would be versatile if our system could directly phosphorylate
natural products or biogically active compounds. The mesylated
17β-estradiol and 6-hydroxyflavone were subjected to the
reaction. Under the standard conditions, the corresponding
products were afforded in moderate (Scheme 3, 2n) and
excellent yields (Scheme 3, 2o). It is worthy to show that the
aliphatic meslyate group in 2n remained intact during the
course of the reaction and can be further functionalized after
deprotection strategies. In fact, 2n can only be prepared from
the phenolic derivatives of 17β-estradiol while no correspond-
ing aryl halides were available. The structure of 2n could also be
derived to a bis-steroidal phosphine ligand,23 which possesses
chirality owning to its distinctive carbon framework, with
reported synthetic routes.24
Having identified the effective ligand, we next surveyed the
reaction conditions for this catalysis (Table 1). i-Pr2NEt
(DIPEA) was found to be the best base, whereas inorganic
bases such as K3PO4 and carbonated bases were inferior (Table
1, entries 1−7). Regarding the solvent screening, i-PrOH gave a
better result than t-BuOH and other solvents (Table 1, entries
8−11). Further solvent investigation showed that i-PrOH/t-
BuOH (1:1) mixtures provided a better product yield (78%) at
a concentration of 0.3 M (Table 1, entries 12−14). The best
metal-to-ligand ratio was found to be 1:4 (Table 1, entry 15 vs
13). Increasing the amount of diisopropyl phosphite and
DIPEA gave a slightly better yield (Table 1, entry 16 vs 15).
Encouraged by the promising Pd/CM-Phos system, we then
turned our attention toward exploring the substrate scope. We
first examined a spectrum of aryl tosylates (Scheme 2). The
reaction proceeded smoothly with a broad range of substituted
aryl tosylates, and mostly 0.5−1.0 mol % of catalyst enabled
catalyzing the coupling with complete substrate conversion.
Electronically neutral (Scheme 2, 1a−1d), rich (Scheme 2, 1e−
1g), and deficient (Scheme 2, 1h) arenes were well-tolerated. A
wide range of heterocycles such as pyridine, thiazole,
isoquinolone, thiophene, and pyrrole furnished the desired
products in good-to-excellent yields (Scheme 2, 1i−1n). It is
worthy to note that particular functional groups including
ketone (Scheme 2, 1h), NH-amide (Scheme 2, 1o), and an
unprotected amino group (Scheme 2, 1p) was compatible in
the system. Dual phosphorylation was also achieved by
employing twofold phosphites and DIPEA to give 1q in 98%
yield.
To realize the capability of the proposed system in the direct
synthesis of biologically active compounds, we have designed a
one-pot sequential reaction including both C−P and C−N
bond formations (Scheme 4). After the first phosphorylation of
3-aminophenyl tosylate, the second electrophile and K2CO3
were directly added to the reaction mixture under nitrogen
without any additional catalyst or solvent. The mixtures were
subjected for an additional 24 h, and 3a was obtained in
In addition to aryl tosylate, the newly developed catalyst
system was capable of facilitating the C−P bond formation of
B
DOI: 10.1021/acs.orglett.5b03104
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Scheme 2. Pd-Catalyzed Phosphorylation of Aryl Tosylates
Scheme 3. Pd-Catalyzed Phosphorylation of Aryl Mesylates
a
Reaction conditions: ArOMs (0.3 mmol), DIPEA (0.9 mmol), H-
phosphonate or H-phosphinate ester (0.45 mmol), t-BuOH/EtOH
(1:1, 1.0 mL, 0.3 M), Pd(OAc)2/L1 = 1:4 under nitrogen at 110 °C
for 18 h under N2, isolated yields were reported. Catalyst loading was
reported in parentheses as mol % of Pd with respect to ArOMs.
a
b
Reaction conditions: ArOTs (0.3 mmol), DIPEA (0.9 mmol), dialkyl
phosphite (0.45 mmol), t-BuOH/i-PrOH (1:1, 1.0 mL, 0.3 M),
Pd(OAc)2/L1 = 1:4 under nitrogen at 110 °C for 18 h under N2;
isolated yields were reported. Catalyst loading was reported in
parentheses as mol % of Pd with respect to ArOTs. Reaction times
Reaction times were not optimized for each substrate. t-BuOH/i-
c
PrOH was used as solvent. DIPEA (1.8 mmol), ethyl phenyl-
phosphinate (0.9 mmol).
Scheme 4. One-Pot Sequential Integration of Aryl
b
c
a
were not optimized for each substrate. Reaction time = 36 h. t-
BuOH was used as solvent. t-BuOH/n-BuOH (1:1) was used as
solvent. t-BuOH/EtOH (1:1) was used as solvent. DIPEA (1.8
Phosphonate
d
e
f
mmol), diisoproyl phosphite (0.9 mmol).
moderate yield. Particularly noteworthy is that the 3-(hetero-
arylamino)phenylphosphonate is a key functionality of the
potential CDK9/CycT1 inhibitors.25
a
Reaction conditions: (i) 3-Aminophenyl tosylate (0.3 mmol), DIPEA
(0.9 mmol), diethyl phosphite (0.45 mmol), t-BuOH/EtOH (1:1, 1.0
mL, 0.3 M), Pd(OAc)2 (2 mol %), L1 (8 mol %) under nitrogen at
110 °C for 18 h under N2. (ii) 6-Quinolinyl mesylate (0.45 mmol),
K2CO3 (0.75 mmol) were added under N2, and the reaction mixtures
were stirred at 110 °C for an additional 24 h.
In summary, the transformation of naturally abundant
phenolic derivatives to their corresponding phosphorus-
containing compounds via an electrophilic pathway has been
a challenge. Here, we show the first general palladium catalyst
for the phosphorylation of aryl mesylates and tosylates with H-
phosphonates and H-phosphinate esters. The chemoselectivity
of the Pd/L1 system was demonstrated by a wide spectrum of
compatible functional groups including an enolizable ketone,
ester, amide, unprotected amine, and heterocycles. Phenolic
derivatives from biologically active compounds also underwent
the reaction smoothly. The one-pot two-step phosphorylation−
amination sequence provides a facile and direct synthesis of
attractive phosphorus-containing pharmcophores. Due to the
attractiveness of the aryl mesylates/tosylates and the above-
mentioned amenability, we believe this new Pd catalyst will
provide efficient access to a variety of synthetically useful and
pharmaceutically attractive organophosphorus compounds.
Future efforts will focus on the synthesis of novel phosphine
ligands using the proposed system.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b03104.
1
Detailed experimental procedures; H, 13C, 31P NMR
spectra; and characterization data of all new compounds
(PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: fuk-yee.kwong@polyu.edu.hk.
C
DOI: 10.1021/acs.orglett.5b03104
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
L.; Wu, G.-J.; Li, Y.; Gao, L.-X.; Han, F.-S. Chem. - Eur. J. 2012, 18,
9622. (d) Zhang, H.-Y.; Sun, M.; Ma, Y.-N.; Tian, Q.-P.; Yang, S.-D.
Org. Biomol. Chem. 2012, 10, 9627. (e) See ref 16.
Notes
The authors declare no competing financial interest.
(13) According to 2014 Aldrich catalog: mesyl chloride ($168/1 L),
tosyl chloride ($91.1/1 kg), triflic anhydride ($1690/1 kg).
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(b) Kozhushkov, S. I.; Potukuchi, H. K.; Ackermann, L. Catal. Sci.
Technol. 2013, 3, 562.
ACKNOWLEDGMENTS
■
We thank the Research Grants Council of Hong Kong (CRF:
C5023-14G), General Research Fund (PolyU 153008/14P),
and State Key Laboratory of Chirosciences for financial
support.
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G. A.; Shin, I. Org. Lett. 2012, 14, 3138.
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DOI: 10.1021/acs.orglett.5b03104
Org. Lett. XXXX, XXX, XXX−XXX