Pd(II)-catalyzed C(sp2)-H hydroxylation with R 2(O)P-coordinating group

Article abstract of DOI:10.1021/ol403028a

A novel R₂(O)P-directed Pd(II)-catalyzed C-H hydroxylation to synthesize various substituted 2′-phosphorylbiphenyl-2-ol compounds is described. Notably, the reaction operates under mild conditions and shows good functional group tolerance, high selectivity, and yield.

Full text of DOI:10.1021/ol403028a

ORGANIC
LETTERS
Pd(II)-Catalyzed C(sp2)ꢀH Hydroxylation
2013
Vol. 15, No. 24
6186–6189
with R2(O)P-Coordinating Group
Hong-Yu Zhang,† Hong-Ming Yi,† Gang-Wei Wang,† Bin Yang,† and
Shang-Dong Yang*,†,‡
State Key Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou 730000, P. R. China, and State Key Laboratory for Oxo Synthesis and Selective
Oxidation, Lanzhou Institute of Chemical Physics, Lanzhou 730000, P. R. China
yangshd@lzu.edu.cn
Received October 22, 2013
ABSTRACT
A novel R2(O)P-directed Pd(II)-catalyzed CꢀH hydroxylation to synthesize various substituted 20-phosphorylbiphenyl-2-ol compounds is
described. Notably, the reaction operates under mild conditions and shows good functional group tolerance, high selectivity, and yield.
Phenols bearing functional groups are of great practical
value to chemists.1 Phenols functioning as important
building blocks are commonly found subunits in a number
of biomolecules and drug molecules.2 Furthermore, phe-
nols or protected phenols with activating groups such as
triflate, nonaflate, mesylate, sulfamate, tosylate, carbo-
nate, and even methyl are attractive for cross-coupling
reactions to introduce other functional groups.3 Various
binaphthol derivatives represent ligands or catalysts of
particular importance that have seen extensive application
in asymmetric catalysis.4 Recently, as one of several new
tactics in practical chemistry, the hydroxyl group has also
been used as a directing group to guide the transition-
metal-catalyzed ortho-position CꢀH bond activation.5
Therefore, the development of a novel and simple method
of catalytically hydroxylating arenes to produce phenols
represents a considerable task and an important challenge.
Indeed, CꢀH activation is the most straightforward and
efficient method for constructing CꢀO bonds. The method
has garnered significant attention over the past several
years.6,7 Versatile ruthenium and palladium catalysts have
proved too powerful for hydroxylation as a result of
† Lanzhou University.
‡ Lanzhou Institute of Chemical Physics.
(1) (a) Rappoport, Z. The Chemistry of Phenols; Wiley-VCH: Wein-
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D. R.; Myers, A. G. Science 2005, 308, 395.
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D. A. Cancer 1967, 20, 333. (b) Tong, X.-G.; Wu, G.-S.; Huang, C.-G.;
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r
10.1021/ol403028a
Published on Web 11/11/2013
2013 American Chemical Society

weakly coordinating directing groups (such as ketones,
amides, esters, and carboxylic acids).8,9 Although these
present studies exhibit notable advantages, they demon-
strate a particular simularity: the directing groups and the
new CꢀOH bond are located on the same aromatic ring.
Very recently, Jiao and co-workers have established a
method to synthesize various substituted 2-(pyrdin-2-yl)-
phenols from 2-phenylpyridines.10 Herein, we report the
firstR2(O)P-directedPd(II)-catalyzedCꢀHhydroxylation
to synthesize various substituted 20-phosphorylbiphenyl-
2-ol compounds (Scheme 1). In contrast to the former
directing groups (pyridine, ketone, amides, esters, and
carboxylic acids), using the R2(O)P as a directing group
has some unique features: first, organophosphorus mole-
cules play an important part in life sciences and pharma-
ceuticals.11 Moreover, the R2(O)P group not only acts as the
directing group but after reduction is also retained as a
chelating group in the desired 20-phosphorylbiphenyl-2-ol
products, which are one type of very important P,O-ligands.
In particular, all procedures are easy to handle, the reaction
condition is simple to assemble, and the reaction tempera-
ture requires 60 °C, which is easily aquired. Furthermore,
this reaction provides a pathway for the preparation of
substituted 20-phosphorylbiphenyl-2-ol compounds that is
far more concise and effective than previous reports.12
Scheme 1. Different Directing Groups Guiding
Transition-Metal-Catalyzed CꢀH Hydroxylation
In the past year, various research groups have reported a
series of transition-metal-catalyzed phosphorous acid or
phosphate ester directed CꢀH functionalization.13 Apart
from these developments, our group explored R2(O)P-
directed Pd(II)-catalyzed CꢀH activation that may under-
go a seven-membered cyclopalladium pretransition state,
followed by successful CꢀH olefination to synthesize
alkeneꢀphosphine compounds.14 Inspired by this research,
we first used 2-(diisopropylphosphoryl)biphenyl (1a)
as the model substrate in the presence of Pd(OAc)2
(10 mol %) as catalyst in 1,2-dichloroethane (DCE) at
80 °C. The reaction was attempted with various oxidants;
ultimately, the desired product 2a was obtained in 64%
yield by using of [bis(trifluoroacetoxy)iodobenzene] (PhI-
(OTFA)2) (Table1, entries 1ꢀ4). Theresults wereencoura-
ging. We further optimized the reaction conditions.
Although the substrate had completely converted in the
former reaction condition, we observed a lower yield of 2a.
Considering the decomposition of the substrate or the
target molecule, we screened the reaction temperatures
and found that depression of the reaction temperature was
in fact beneficial for the reaction, and the best yield of 2a
improved to 69% at 60 °C (Table 1, entries 5ꢀ6). The
loading of the screening showed that the most appropriate
oxidant was 1.5 equiv of PhI(OTFA)2, and product 2a was
generated in 73% yield (Table 1, entries 7 and 8). Further
screening of different palladium catalysts showed that
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Table 1. Reaction Conditions Screeninga
entry
cat. (mol %)
Pd(OAc)2 (10)
oxidant (equiv)
K2S208 (2.0)
1,4-benzoquinone (2.0)
PhI(OAc)2 (2.0)
temp (°C)
solvent
yieldb (%)
1
2
3
4
5
6
7
8
80
80
80
80
60
50
60
60
60
60
60
60
60
60
60
60
60
60
60
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2CI
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
CH3NO2
CF3COOH
CHCl3
CH3COOH
CH3NO2
CH3NO2
CH3NO2
n.r.
n.r.
50c
64
69
56
73
66
81
75
Pd(OAc)2(10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2(10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(TFA)2 (10)
PhI(OTFA)2 (2.0)
PhI(OTFA)2 (2.0)
PhI(OTFA)2 (2.0)
PhI(OTFA)2(1.5)
PhI(OTFA)2(1.2)
PhI(OTFA)2(1.5)
PhI(OTFA)2(1.5)
Phl(OTFA)2(1.5)
PhI(OTFA)2(1.5)
PhI(OTFA)2(1.5)
PhI(OTFA)2(1.5)
PhI(OTFA)2(1.5)
PhI(OTFA)2(1.5)
PhI(OTFA)2(1.5)
PhI(OTFA)2(1.5)
Phl(OTFA)2(1.5)
9
10
11
12
13
14
15
16
17
18
19
Pd(NO3)2 2H2O(10)
3
Pd(CH3CN)2(BF4)2 (10)
PdCI2(10)
58
n.r.
86
Pd(TFA)2 (10)
Pd(TFA)2 (10)
Pd(TFA)2 (10)
Pd(TFA)2 (10)
Pd(TFA)2 (5)
73
59
53c
85
Pd(TFA)2 (3)
Pd(TFA)2 (0)
79
n.r.
a All reactions were carried out in the presence of 0.3 mmol of 1a in 2.0 mL of different solvents under air atmosphere. b Yield of isolated product.
c 20-(Diisopropylphosphoryl)biphenyl-2-yl acetate (2aa) was obtained.
Pd(TFA)2 was a better catalyst than Pd(OAc)2, and the yield
of 2a was significantly improved to 81% (Table 1, entry 9).
Other Pd(II) catalysts, such as Pd(NO3)2 2H2O and Pd-
Table 2. Evaluation of Different Directing Groupsa
3
(CH3CN)2(BF4)2, could prompt the reaction but were not
as efficient as Pd(TFA)2 (Table 1, entries 10 and 11). This was
not the case for PdCl2 (Table 1, entry 12), however, likely
because chloride ion exchange with trifluoroacetate anions is
difficult due to the stable PdꢀCl bonds.10 Investigation of
different solvents indicated that CH3NO2 was the best solvent
suitable for this reaction, and the product (2a) was afforded in
86% yield (Table 1, entries 13ꢀ16). Not surprisingly, when
PhI(OAc)2 was used as oxidant or CH3COOH as solvent, the
acetoxylated product (2aa) was obtained, respectively
(Table 1, entries 3 and 16). Finally, when we evaluated the
loading of Pd(TFA)2, 5 mol % of Pd(TFA)2 was found to be
sufficient to prompt the reaction without an apparent decrease
in reaction yield (Table 1, entries 17 and 18). The control
experiment thus illustrates that Pd(TFA)2 is indispensable to
the reaction (Table 1, entry 19). As our overall experimental
results demonstrate, the optimal reaction conditions involve
the use of 5 mol % of Pd(TFA)2 and 1.5 equiv of PhI(OTFA)2
as the oxidant in 2.0 mL of CH3NO2 for 0.3 mmol of 1a at
60 °C under an air atmosphere (Table 1, entry 17).
a All reactions were carried out under the optimal conditions re-
ported in the text. b Isolated yields.
With the optimized conditions in hand, we first exam-
ined the scope of substrates by varying the phosphate
directing group (Table 2). In addition to 2-(diisopropyl-
phosphoryl)biphenyl, 2-(diethylphosphoryl)biphenyl and
2-(dicyclohexylphosphoryl)biphenyl are also compatible
with this reaction, affording their corresponding products
in good yields (Table 2, 2b,c). After electron effects are
taken into consideration, 2-(diphenylphosphoryl)biphenyl
is converted into the desired product of 2d in a 66%
lower yield. Furthermore, 2-(tert-butyl(phenyl)phosphoryl)-
biphenyl worked smoothly in the reaction; 2e was obtained
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Org. Lett., Vol. 15, No. 24, 2013

Table 3. Scope of Different Substituted Substratea
Scheme 2. Pd(II)-Catalyzed CꢀH Oxidation and Rearrangement
must be decreased to 1.4 equiv; otherwise, a lower yield of
2m will be observed. However, when the methoxyl group
lies on the meta-position of the diisopropylphosphoryl
group, the reaction progressed very well and the products
2n and 2o were obtained in 83% and 81% yields, respec-
tively. We found the electronic effect to be very distinct in
these transformations. Whenelectron-withdrawinggroups
such as F, Cl, and CF3 lay on the 40-position, the corre-
sponding products were afforded in excellent yields in the
presence of 10 mol % of Pd(TFA)2 at 80 °C (2pꢀr). On the
other hand, when 2-diphenylphosphino oxide was selected
as the directing group, relatively lower yields were observed
no matter the electron-donating substituent group or elec-
tron-withdrawing group on both aromatic rings (2sꢀv).
While in the process of investigating substrates, we dis-
covered that 3-(2-(diphenylphosphoryl)phenyl)thiophene
was also compatible with CꢀH hydroxylation, but an un-
expected rearrangement product, 3-(2-(diphenylphosphoryl)-
phenyl)thiophen-2(5H)-one, was obtained in 51% yield
(Scheme 2). As a useful skeleton in the field of medicinal
chemistry,15 this product will provide an efficient and novel
pathway for the synthesis of thiophen-2(5H)-one derivatives.
In summary, we have developed a novel R2(O)P-
directed Pd(II)-catalyzed CꢀH hydroxylation, which is
an effective method to synthesize various substituted
20-phosphorylbiphenyl-2-ol compounds. Notably, this re-
action iseasytohandle, can operate under mild conditions,
and displays good functional group tolerance. Given its
wide utilization in coordination chemistry and the potential
value for medicinal chemistry and biochemistry, we antici-
pate the process described in this paper to be important in
practical applications. Further studies into the mechanism
and further application of this method into the synthesis of
axially chiral phosphines are ongoing in our laboratory.
a All reactions were carried out under optimal conditions as reported
in the text. b Isolated yield. c 1.4 equiv of PhI(OTFA)2 was used; d 10 mol %
of Pd(TFA)2 was used at 80 °C.
in the mixture of monohydroxylation and dihydroxylation
in a ratio of 7.3:1. Interestingly, when diethylbiphenyl-2-
ylphosphonate was used, only phosphoryl lactone 2f was
obtained in 37% yield. Certain phosphates, such as tri-
phenylphosphine oxide and R-diethyl naphthalenyl phos-
phonate, did not work at all. These results imply, however,
that the seven-membered cyclopalladium pretransition
state may play a critical role in this transformation.
Acknowledgment. We are grateful for the financial
support of NSFC (No. 21272100), FRFCU (lzujbky-
2013-ct02), PCSIRT (IRT1138), and NCET-11-0215. We
thank S. F. Reichard at the University of Chicago for
editing the manuscript.
We next investigated the scope of different substituted
2-(diisopropylphosphoryl)biphenyl derivatives (Table 3).
When 2-(diisopropylphosphoryl)-20-methylbiphenyl was
tested, the product of 2i was obtained in a 61% lower yield
as a result of steric hindrance. If the methyl and dimethyl
group lie on the meta- or para-position of phenyl phos-
phate, the corresponding products can be afforded in good
yields (2jꢀl). Interestingly, ifthe methoxyl grouplieson the
para-position of phenylphosphate, the loading of oxidant
Supporting Information Available. Experimental pro-
cedures and full spectroscopic data for all new com-
pounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
(15) Li, X.-H.; Sheng, X.-N. 4Aryl-5H-thiophene-2-one derivatives
and their preparation and use. WO 2004/106321 A1, Dec 9, 2004.
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 24, 2013
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