Copper-catalyzed synthesis of mixed alkyl aryl phosphonates

Article abstract of DOI:10.1021/ja505281v

Copper-catalysis allows the direct oxygen-arylation of dialkyl phosphonates with diaryliodonium salts. This novel methodology proceeds with a wide range of phosphonates and phosphoramidates under mild conditions and gives straightforward access to valuable mixed alkyl aryl phosphonates in very good yields and near perfect selectivity.

Full text of DOI:10.1021/ja505281v

Communication
pubs.acs.org/JACS
Copper-Catalyzed Synthesis of Mixed Alkyl Aryl Phosphonates
Martín Fananas-Mastral* and Ben L. Feringa*
́
̃
Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
S
* Supporting Information
alternative methods for the synthesis of mixed alkyl aryl
phosphonate esters is highly desirable.
ABSTRACT: Copper-catalysis allows the direct oxygen-
arylation of dialkyl phosphonates with diaryliodonium
salts. This novel methodology proceeds with a wide range
of phosphonates and phosphoramidates under mild
conditions and gives straightforward access to valuable
mixed alkyl aryl phosphonates in very good yields and near
perfect selectivity.
Since pioneering work by Stang and co-workers,14 diary-
liodonium salts, which are air- and moisture-stable, nontoxic, and
easy to prepare compounds, have recently gained considerable
attention as mild and selective arylating reagents in organic
synthesis.15−20 It is known that a Cu(I) catalyst can be oxidized
in the presence of a diaryl iodonium salt to form a highly
electrophilic aryl-Cu(III) intermediate.17,21 We hypothesized
that the Lewis-basic phosphoryl oxygen of a dialkyl phospho-
nate22 could react under mild conditions with this aryl-Cu(III)
species giving rise to a phosphonium-like intermediate,23 which
might evolve by substitution of one of the alkyl groups (Scheme
1B). Reductive elimination would form the new C(sp2)−O
bond, regenerate the Cu(I) catalyst, and lead to the formation of
an alkyl aryl phosphonate ester. We report here the development
of catalytic methodology for the synthesis of mixed alkyl aryl
phosphonates from readily available dialkyl phosphonates and, to
the best of our knowledge, the first catalytic oxygen-arylation of
phosphonates with diaryliodonium salts.24,25 The catalytic
transformation operates under mild conditions and proceeds
both with phosphonates and phosphoramidates and a variety of
diaryliodonium salts.
rganic phosphonates represent a highly important class of
1
Ocompounds with a wide range of applications in biology,
agriculture,2 and synthetic organic chemistry.3 In particular,
mixed alkyl aryl phosphonate esters play a key role in nucleotide
chemistry4 and have been used as transition-state analogues in
the study of catalytic antibodies.5 Unsymmetrical phosphonate
esters specifically bind to proteinase amino acid residues,6 show a
variety of important biological activities,7 and are used in
selective covalent immobilization of proteins to surfaces.8
While many methods are available for the synthesis of
symmetrical phosphonate diesters,9 Arbuzov10 and Michaelis−
Becker11 reactions being the most common ones, the synthesis of
mixed alkyl aryl phosphonates mainly relies on multistep
processes based on the reaction of a phenol with an in situ
formed alkyl phosphonochloridate12 or involve the substitution
of a diaryl phosphonate by a metal alkoxide13 (Scheme 1A). In
both cases the reactions are subject to selectivity problems due to
the formation of varying amounts of phosphonic dichloride or
dialkyl phosphonate esters or to instability of the intermediate
phosphonochloridate. More importantly, these methods are
associated with the use of phosphorus chlorides, which are
hazardous and toxic reagents. Therefore, the development of
We started our studies by investigating the reaction between
diethyl 1-butylphosphonate 1a and diphenyliodonium triflate 2a
(Table 1). The initial experiments were carried out using
Cu(OTf)2 as a catalyst26 in dichloroethane at 70 °C. Although
the conversion was moderate, the reaction led to the formation of
the mixed phosphonate 3a and a small amount (4%) of diarylated
product 4a (entry 1). The use of a hindered non-nucleophilic
base such as 2,6-di-tert-butylpyridine (dtbpy)27 had a beneficial
effect on both conversion and selectivity, exclusively leading to
the formation of the monoarylated phosphonate (entry 2).28
A
screening of copper complexes (see Supporting Information,
Table S1) revealed that Cu(OTf)2 and CuCl are the most
efficient catalysts for this transformation, the latter giving slightly
higher conversion (entries 2 and 3). The use of 1.5 equiv of 2a
gave almost full conversion, but a small amount of diphenyl
phosphonate 4a was found (entry 4). Total selectivity could be
restored by carrying out the reaction at a lower temperature
(entry 5). The nature of the solvent has also an important role in
the reaction outcome (entries 5−7). By using dichloromethane
as solvent, full conversion was achieved, and ethyl phenyl
phosphonate 3a was isolated in 90% yield with >99% selectivity29
(entry 7). The reaction also proceeds at room temperature, but
incomplete conversion is observed, probably due to competing
decomposition21a,b of 2a (entry 8). Finally, no reaction was
Scheme 1. Synthesis of Mixed Alkyl Aryl Phosphonate Esters
Received: May 27, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja505281v | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
Table 1. Screening of Reaction Conditions
Table 2. Scope of Dialkyl Phosphonates
a
bc
,
entry
[Cu]
2a equiv
base
solvent T (°C) conv (%)
d
1
2
Cu(OTf)2
Cu(OTf)2
CuCl
1
DCE
70
70
70
70
40
40
40
rt
66
76
80
98
96
85
1
dtbpy
dtbpy
dtbpy
dtbpy
dtbpy
dtbpy
dtbpy
dtbpy
DCE
3
1
DCE
e
4
CuCl
1.5
1.5
1.5
1.5
1.5
1.5
1.5
DCE
5
CuCl
DCE
6
CuCl
toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
7
CuCl
full (90)
8
CuCl
76
0
9
40
40
10
CuCl
83
a
Reactions performed on a 0.1 mmol scale. 3a:4a ratio >99:1 unless
b
c
otherwise noted. Determined by GC analysis. Yield of isolated
product showed in parentheses. 4% of 4a formed. 5% of 4a formed.
d
e
DCE = 1,2-dichloroethane.
observed in the absence of copper complex at 40 °C (entry 9),
while the absence of dtbpy leads to a lower conversion (entry
10). The use of diphenyliodonium salts bearing halide
counteranions did not result in any arylated phosphonate,
while the use of salts displaying other noncoordinating anions led
to lower conversions (see Supporting Information, Table S2).
Having established the optimized conditions (Table 1, entry
7), we set out to investigate the scope of the reaction. First, we
explored this new transformation by using different dialkyl
phosphonates 1 (Table 2). The reaction proved to be very
efficient, with both diethyl alkyl (1a−c) and diethyl aryl
phosphonates (1d) providing in all cases very good yields.
Functionalized benzyl (1e) and alkyl (1f) diethyl phosphonates
and acetal-protected aldehyde 1g also worked well and led to the
corresponding ethyl phenyl phosphonates 3e, 3f, and 3g in high
yields without any traces of side products. Substrates bearing a
pyridyl group (1h) and a phthalimide (1i) did not give any
conversion. We propose that in these cases a stronger
coordination from the pyridine or phthalimide to the aryl-
Cu(III) intermediate might block the reaction. On the other
hand triethyl phosphonoacetate (1j) reacted under these
catalytic conditions and led to the mixed phosphonate 3j,
although no full conversion was achieved This new methodology
is also applicable to dimethyl phosphonates as illustrated by the
synthesis of products 3k and 3l. Interestingly, di-tert-butyl
phosphonates, albeit more reactive,30 are also suitable substrates
for this transformation. High selectivity and full conversion was
reached for the arylation of 1m at room temperature, providing
product 3m in 66% yield.31 Phosphoramidates (1n and 1o) also
undergo this copper-catalyzed arylation with a similar trend in
reactivity. When the reaction with di-isopropyl phosphoramidate
1n was carried out at 40 °C a mixture of mono- and diarylated
products was found. Again, total selectivity toward the
monoarylation could be restored by performing the reaction at
room temperature while using a lower amount of phenyl-
iodonium triflate. Using these conditions isopropyl phenyl
phosphoramidate 3n was obtained in 77% yield as a 1:1 mixture
of diastereoisomers. In contrast, diethyl phosphoramidate 1o led
exclusively to monoarylated product 3o at 40 °C, although the
reaction stopped at 50% conversion when it was carried out at
room temperature.
a
Reactions performed on a 0.2 mmol scale. 3:4 ratio >99:1 unless
b
otherwise noted. Yields refer to isolated pure products. 1 equiv of 2a
was used. 4m was obtained in 32% yield. 4n was obtained in 21%
yield.
c
d
Different diaryliodonium salts were also used for this arylation
of phosphonates. As previously shown by Gaunt, Sanford, and
others,16,17 unsymmetrical diaryliodonium salts bearing one
bulky mesityl ligand allow the selective transfer of the other aryl
group. This approach is attractive from a practical point of view as
only 1 equiv of the desired transferring aryl group is required.
Furthermore, these diaryliodonium salts are easily prepared from
commercially available reagents in one-pot operations.32 To our
delight, diaryliodonium salts 2b−g bearing ortho-, meta- and
para-substituents all worked well in this copper-catalyzed
transformation independently of the electronic properties of
the substituent (Table 3). The corresponding alkyl aryl
phosphonates 3p−w were obtained with perfect selectivity and
good to excellent yields in all cases. Noteworthy is the reaction
with 4-nitrophenyl(mesityl)iodonium triflate 2d, which gives
access to alkyl 4-nitrophenyl phosphonates in a very straightfor-
ward manner. Phosphonates of this type react selectively with a
range of esterase enzymes and are commonly used in the study of
biological processes.5,8,12 The synthetic utility of the new method
was demonstrated by the reaction between 1a and 2c on a gram
scale (4 mmol), which led to 3r (1.22 g) in an excellent 97% yield
using only 5 mol % of CuCl. It is also important to note that
mesityl iodide was obtained quantitatively and 2,6-di-tert-
butylpyridine could be recovered as a triflic salt (see Supporting
Information for further details), adding to the practicality of the
process.
On the basis of our experimental observations and some
preliminary mechanistic investigations (Scheme 2 and Support-
ing Information, Schemes S1−S3), we propose the following
mechanism for the copper-catalyzed oxygen-arylation of
B
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Journal of the American Chemical Society
Communication
anion as the nucleophile in this substitution.35 Taking into
account its low nucleophilicity and the observed reactivity trend
for the different alkoxy groups (Table 1), we propose that this
substitution might follow an SN1-type mechanism.36 The result
obtained with phosphonate 1q (Scheme 2b), in which only the
isopropyl group was substituted, supports such pathway,
although we cannot discard different alternatives.37 Although
the exact role of dtbpy is not entirely clear,38 it could facilitate the
substitution step by scavenging protic impurities and/or
stabilization of the emerging carbocation31 as reported for living
carbocationic polymerizations.39,40 Finally, a reductive elimi-
nation would regenerate the Cu(I) catalyst and afford alkyl aryl
phosphonate 3.
a
Table 3. Scope of the Diaryliodonium Salts
In summary, we have developed an efficient copper-catalyzed
oxygen-arylation of dialkyl phosphonates and phosphoramidates
with diaryliodonium salts based on the formation of high valent
aryl-copper(III) species. The reaction proceeds under mild
conditions, avoids the use of toxic reagents, and affords the
monoarylated alkyl aryl phosphonates in high yields with near
perfect selectivity.
a
Reactions performed on a 0.2 mmol scale. 3:4 ratio >99:1 in all cases.
b
Yields refer to isolated pure products. Yield in brackets refers to the
reaction run on a 4 mmol scale using 5 mol % of CuCl.
ASSOCIATED CONTENT
* Supporting Information
■
S
Scheme 2. Mechanistic Investigations
Optimization tables, mechanistic investigations, experimental
procedures, and characterization data. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
m.fananas.mastral@rug.nl
b.l.feringa@rug.nl
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
phosphonates with diaryliodonium salts (Scheme 3). Initially,
the diaryliodonium salt 2 oxidizes the Cu(I) catalyst giving rise to
The Netherlands Organization for Scientific Research (NWO-
CW), the National Research School Catalysis (NRSC-C), the
European Research Council (ERC advanced grant 227897 to
B.L.F.), the Royal Netherland Academy of Arts and Sciences
(KNAW), and the Ministry of Education Culture and Science
(Gravity program 024.601035) are acknowledged for financial
support.
Scheme 3. Proposed Mechanism for the Copper-Catalyzed
Oxygen-Arylation of Phosphonates
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