TiO2/Cu2O nanoparticle-catalyzed direct C(sp)-P bond formation: Via aerobic oxidative coupling in air and visible light

Article abstract of DOI:10.1039/c9dt04757c

The synthesis of organophosphorus compounds is one of the important goals in organic chemistry. Among these compounds, alkynylphosphonates are significantly utilized as the main precursors for the synthesis of biologically active molecules in medicinal chemistry and have attracted extensive interest in the past few decades. Although few efforts have been made towards the direct and atom-economical synthesis of alkynylphosphonates, efforts towards the utilization of visible light as a green and renewable energy source have not been made to date. Here, we have promoted a strategy to construct a type of nano metal oxide composite photocatalyst (Cu₂O decorated on TiO₂) for the synthesis of alkynylphosphonates via direct C-P bond formation between terminal alkyne and H-phosphonate under visible light irradiation. In this p-n heterojunction photocatalyst, Cu₂O acted as a visible-light absorber; moreover, the CB (conduction band) of TiO₂ was favorable for accepting a photogenerated electron, and the generated electron hole (e^-/h⁺) pair could initiate the reaction. The present study can provide a new way for the synthesis of this important class of phosphorus organic compounds.

Full text of DOI:10.1039/c9dt04757c

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DOI: 10.1039/C9DT04757C
ARTICLE
TiO2/Cu2O nanoparticles – catalyzed direct C(sp)-P bond formation
via aerobic oxidative coupling under air and visible light
Mona Hosseini-Sarvari,*a and Fattaneh Jafaria
Received 00th January 20xx,
Accepted 00th January 20xx
Synthesis of organophosphorus compounds is one of the important goal in organic chemistry. Among them,
alkynylphosphonates are greatly utilized as the main precursors for the synthesis of biologically active molecules in medicinal
chemistry and they have extremely interested in past decades. Among a few efforts for direct and atom economy synthesis
of alkynylphosphonates, there is no report using visible light as a green and renewable energy. Here, we promote a strategy
to construct a type of nano metal oxide composite photocatalyst (Cu2O decorated on TiO2) for the synthesis of
alkynylphosphonates via direct C-P bond formation between terminal alkyne and H-phosphonate, under visible light
irradiation. In this p-n heterojunction photocatalyst, Cu2O assisted as visible light absorbers, and at the same time CB
(conduction bond) of TiO2 is favorable for accepting photogenerated electron and electrony hole (e-/h+) generated could
initiate the reaction. This study could develop a new way for the synthesis of this important class of phosphorus organic
compounds.
DOI: 10.1039/x0xx00000x
common photocatalysts are inexpensive, not toxic and
environmentally friendly.21, 23
In modern chemistry, direct oxidative C-H/heteroatom-H
coupling reactions in term of atom and steps economy are
Introduction
Statement in green chemistry: water, visible light, and air are
clean, available, and not ended natural sources. Photochemistry
has been an important subject in chemistry for a long time.
However, the use of photochemical methods due to its
limitations have made them less useful in the synthesis of
organic compounds. Since 2008, using visible light as a green
energy source for number of organic transformations received
more attention.1-10 At this time new photocatalysts were
provided by research groups. The most photocatalysts which
were used are Ru, Ir complexes and organic dyes, and also in a
few one nanoparticle semiconductors.11-13 Despite being very
worthwhile and many suitable reports using transition metal
complexes (Ir, Ru….) and organic dyes, but they are expensive,
toxic, and homogeneous photocatalysts. So, nano
semiconductors are favorable class of photocatalysts that
among them, p-type Cu2O is considerable attention because of
its narrow band gap (2.0ev).14-19 Serious limitation of Cu2O is its
rapid recombination of photo induced e-/h+ pairs, so this, limit
its photocatalytic activity.20-22 By decorating Cu2O with another
semiconductors such as TiO2 the recombination of e-, h+ pairs
decreased (Cu2O has higher conduction band than TiO2 , so the
transfer of excited electrons and holes between them are
eligible besides both Cu2O and TiO2 in comparison with other
intense tools in organic synthesis.24-26 Among them direct and
selective oxidative coupling of terminal alkynes are still a
challenge, due to the importance of this category of compounds,
it is still important to develop new methods.27-34
Alkynylphosphonates have extremely interested in past
decades, they are one of the important class of triple bond
containing compounds and valuable chemicals in modern
synthesis chemistry. In addition, they are widely used as key
precursors for biologically active molecules in medicinal
chemistry such as a diphosphonate ester, SR12813, by acting as
36
a
strong anti-hypocholesterolemic,35,
and phosphorus-
containing 17β-side chain mifepristone analogues as
progesterone receptor antagonists,37 (Scheme 1, A). They also
could be used for synthesizing diverse phosphorus-containing
molecules through hydrations (produced β-ketophosphonates,
which are used in the HWE reaction38-40), metallacycle
formation, reductions, conjugate-addition reactions, and
cycloaddition reactions41-48 (produced
a wide range of
important phosphorus cyclic compounds, including amino acids
analogues for studies in chemical biology41 or versatile chiral
ligands for metal catalysis46-49) (Scheme 1, B).50, 51
All the reported methods for the direct synthesis of
alkynylphosphonat from terminal alkynes are under thermally
conditions at elevated temperature (Scheme 2).52-59 The major
drawbacks in these reported methods are using large amount
of bases, phosphorus ligands, using large amount of Ag, and
limited to work only with aromatic and electron rich aromatic
alkynes. To the best of our knowledge, there is not any report
a. Department of Chemistry, Faculty of Science, Shiraz University, Shiraz,
7194684795 I.R. Iran. E-mail: hossaini@shirazu.ac.ir
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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for the direct synthesis of alkynylphosphonates at room single electron transfer (SET) to the hole which wVaieswgAertnicelerOantleinde
DOI: 10.1039/C9DT04757C
temperature by using visible light as a green source of energy.
in Cu2O. On the other hand, TiO2 which was decorated on Cu2O
favored the transfer of photoexcited electron and prevent the
recombination of e-/h+ pairs. On the basis of this we became
interested in the direct oxidative coupling of terminal alkyne
and phosphonates.
O
EtO
P
OEt
O
P
O
OEt
X
P
R
EtO
H2O
R
R
-ketophosphonates
OH
With this in mind, we started this project by subjected
phenylacetylene and diethylphosphate to CFL visible light in the
presence of TiO2/Cu2O nanoparticlesin acetonitrile as solvent,
which after 24 h alkynylphosphonate 3a was afforded in 87%
yield (Scheme 3).
R
SR12813
anti-hypocholesterolemic activity
R
R
N
R
3+2
N
P
X
R
N
P
X
O
R
R
R
R
P
Me2N
HO
R
X
4+2
P
R
2+2+2
R
R
O
EtO
OEt
aromatic compounds
B.
Progesterone receptor antagonists
A.
O
P
Nano TiO2/Cu2O nanoparticles
O
EtO
P
H
OEt
Scheme 1. Some phosphorus-containing bioactive molecule (A),
alkynyl phosphorus derivatives as important building blocks (B).
1a
2a
3a
Scheme 3. Synthesized of 3a.
Pd catalyst, 60-700C
Thermal methods
After successful obtain product 3a, we tried to obtain the best
reaction conditions. We next considered a range of solvents,
including CH3CN, DMF, DMSO, EtOAc, THF, MeOH and H2O
under visible light (Table 1). From Table 1, DMF was resulted the
best yield so it was chosen for more reaction conditions (entry
1). In CH3CN, THF, EtOAc, MeOH and DMSO lower yield was
afforded (entries 2-6) and poor result was observed when the
reaction was carried out in H2O (entry 7). In the presence of
water, a large amount of homocoupling by-product of
phenylacetylene was obtained (1,4-diphenylbuta-1,3-diyne). It
is interesting to note that under solvent free condition, the
reaction also underwent smoothly to generate the diethyl
(phenylethynyl) phosphonate in excellent yields but for some
required excess base
Cu catalyst
500C
required large amount of catalyst
reaction at 50-1200C
2 eq Ag2CO3
1200C
required ligands
O
O
P
OR'
Previous works
R
H
R'O P H
OR'
OR
R
'
R.T.
TiO2/Cu2O
Photocatalysis method
no ligand and base
reaction at R.T.
This work
metal free
Scheme 2. Synthesis of alkynylphosphonate.
Recently, we reported TiO2/Cu2O nanoparticles as active terminal alkyne did not afford any product, so for further
photocatalyst for the synthesis of arylphosphonates.60 Based on investigation we chose DMF as solvent (entry 1).
our interest in developing photocatalyst applications of Then the amount and some other catalysts were examined. As
TiO2/Cu2O nanoparticlesherein, we report the first successful shown in Table 1, TiO2/Cu2O nanoparticleswas found to be the
and efficient, direct oxidative coupling of terminal alkynes, and most effective photocatalyst in terms of reaction rate, and yield
H-phosphonate for C(sp)-P bond formatin photocatalyzed by (entry 1). As expected by using Cu2O nanoparticles alone, the
reusable TiO2/Cu2O nanoparticles in mild reaction conditions desired product was generated in 75 % because Cu2O has a good
without use of any additive, base or ligand under visible light visible light absorbant but the recombination of e-/h+ occurs so
irradiation, at room temprature.
fast, so the yield in comparing with the TiO2/Cu2O
nanoparticleswas decreased. Using, TiO2 and p25 alone were
not produced any products, respectively (entries 11, 12). We
also investigated the reaction with different ratio of Cu2O and
TiO2; (5/1, 10/1, 15/1, 34/1). We found that 34/1 of TiO2/Cu2O
nanoparticles (6% w/w) is the best photocatalyst (entries 13-15).
By changing the amount of photocatalyst, 0.01 g of TiO2/Cu2O
nanoparticles was found to be optimal. When 0.005 g of
TiO2/Cu2O nanoparticles was used, the reaction did not go to
completion (entry 16). However, no significant improvement
was observed with 0.02 g of TiO2/Cu2O nanoparticles and by-
product (coupling of phenylacetylene) were observed (entry 17).
Obviously, there was no reaction without use of any
photocatalyst (entry 18). Next we examined the effect of light.
The process was carried out under white, blue, green, red, no
light and also in dark condition (entries 19-23, also see Figure
Results and discussion
TiO2/Cu2O nanoparticles were synthesized as reported in our
previous work60 and fully characterized by different methods.
Some of these data were again subjected in SI (Table S1-S2 and
Figures S1-S4).
As we previously reported,60 in this nanocomposite (TiO2/Cu2O)
Cu2O is the main component for absorbing visible light due to
its narrow band gap. So, after absorbing the visible light
irradiation, e-/h+ pairs generated in Cu2O. The generated hole
(h+) has the ability to generate phosphorus radical. Bond
dissociation energy of P-H bond in phosphites are weak61 so it
could homolytic cleavage to form a phosphorous base radical by
2 | J. Name., 2012, 00, 1-3
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S49). Blue LED and CFL white lights were shown the same results, CFL 15W white in air. To the best of our knowledge, this is the
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DOI: 10.1039/C9DT04757C
so according to availability we were chosen the white light. As first example of the use of TiO2/Cu2O nanoparticles as
can be seen from Table 1, entry 23, in dark reaction condition, photocatalyst for direct oxidative coupling of terminal alkyne
58% yield of the product 3a was obtained. This could be and H-phosphonate under visible light without use of any bases
explained by a non-photocatalytic pathway. According to the and ligands.
previously reported by Han62 and moglie,53 Cu2O (Cu(I)) could One of the goles for a heterogeneous photocatalyst in
activated the terminal alkyne, then the coordination of comparing with the presented homogeeneous photocatalyst is
diethylphosphonate to this and subsequent reductive their ability to recycled and reused. Accordingly, TiO2/Cu2O
elimination to produce the product.
nanoparticles photocatalyst could be separated and recovered
We also investigated atmosphere of the reaction. The process by simple centrifugation from reaction mixture, washing by
was carried out under air, Ar and O2 condition. The results are adding the appropriate solvent, and then set up in a new run.
shown in Table 1 (entries 1, 24, 25).
Following this procedure, the catalyst was recycled effectively
for ten times without any significant decrease in yield or
photocatalytic activity (Figure 1). Leaching of the nano
photocatalyst was examined by ICP. The result was shown that
the content of Cu was 24.30% (w/w). In addition, after 10 runs
of the reaction, we checked the SEM and XRD pattern of the
powder and compared with original one (compare Figure 1b,c
with Figures S1a and S4c). As can be seen from these Figures no
significant changes were observed.
Table 1. Optimizationa
Yield
(%)b
94
87d
80d
82
83
84
40
20
94
75
0
Run
Catalyst (g)
Solvent
Light
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
TiO2/Cu2O(0.01)c
TiO2/Cu2O(0.01)c
TiO2/Cu2O(0.01)c
TiO2/Cu2O(0.01)c
TiO2/Cu2O(0.01)c
TiO2/Cu2O(0.01)c
TiO2/Cu2O(0.01)c
TiO2/Cu2O(0.01)c
TiO2/Cu2O(0.01)c
Cu2O (0.01)e
DMF
CH3CN
DMSO
MeOH
AcOEt
THF
CH3CN/H2O
H2O
No solvent
DMF
CFL
CFL
CFL
CFL
CFL
CFL
CFL
CFL
CFL
CFL
CFL
CFL
CFL
CFL
CFL
a
100
)
80
60
40
20
0
96 96
95 95
94
5
93
6
92
7
91
8
90
9
89
10
TiO2 (0.01)e
DMF
DMF
DMF
DMF
P25 (0.01)c
0
1
2
3
4
TiO2/Cu2O(0.01)f
TiO2/Cu2O(0.01)g
TiO2/Cu2O(0.01)h
47
56
80
Runs
b
)
c
)
DMF
TiO2/Cu2O(0.005)
16
DMF
CFL
68
c
17
18
19
20
21
22
23
24
25
TiO2/Cu2O (0.02)c
No catalyst
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
CFL
CFL
Blue LED
Green LED
Red LED
Neat
80
0
TiO2/Cu2O (0.01)c
TiO2/Cu2O (0.01)c
TiO2/Cu2O (0.01)c
TiO2/Cu2O (0.01)c
TiO2/Cu2O (0.01)c
TiO2/Cu2O (0.01)c
TiO2/Cu2O (0.01)c
93
87
80
78
58
44i
96j
Dark
CFL
CFL
Fig.
1 a) Recyclability of TiO2/Cu2O nanoparticles for
alkynylphosphonates synthesis, b) SEM,and c) XRD after 10 runs.
aReaction conditions: Phenylacetylene (0.5 mmol), diethylphosphite
(0.5 mmol), solvent (0.5 mL), 3 h under air condition and all the lamps
are 15W. bIsolated yield. cTiO2/Cu2O, 6% w/w (molar ratio of Cu2O and
To show the scope and generality, some structurally diverse of
terminal alkyne and H-phosphonate were engaged as the
reaction substrates. The results are summarized in Table 2. The
results demonstrated that all terminal alkyne with
diethyphosphite and dimethylphosphite were transformed to
the corresponding alkynylphosphonates in good to excellent
yields but diphenyphosphite did not yield the coupled product
due to the strically effect even after a long reaction times.
Furthermore, by using various substituted phenylacetylenes
with different electronic properties, different yields in the final
d
e
TiO2 is 34/1). Yield after 24h. Nano Cu2O and TiO2 were prepared
f
according to the experimental section. The molar ratio of TiO2/Cu2O
g
nanoparticlesis 5/1. The molar ratio of TiO2/Cu2O nanoparticlesis is
10/1. hThe molar ratio of TiO2/Cu2O nanoparticlesis 15/1. iThe reaction
is under Ar. jThe reaction is under O2.
products
were
achieved.
4-Methyl-substituted
Finally, from Tables 1 it can be seen that the best result was
achived with TiO2/Cu2O nanoparticles (0.01 g), in DMF under
phenylacetylenes was more reactive compared by
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phenylacetylene (entries 3c, 3d). In addition, heteroaromatic (TEMPO), the desired alkynylphosphonate 3a was obtained only
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DOI: 10.1039/C9DT04757C
terminal alkyne also produced the corresponding product (3k). in a trace amount. Also in another experiment, by adding
triethanolamine (TEA), as a hole scavenger, again the desired
product was obtained in a trace amount. These results
Table 2. One-pot synthesis of alkynylphosphonates derivatives confirmed that the reaction is likely to involve a radical process.
3 from terminal alkyne and H-phosphonate, by using TiO2/Cu2O
nanoparticles as photocatalysta
O
OEt
TiO2/Cu2O nanoparticles
O
P
TEMPO (1 eq)
OEt
EtO
P
H
DMF (0.5 mL)
CFL 15 W, air
OEt
MeO
OMe
P
1a
2a
O
3a (trace)
O
P
EtO
O
OEt
OEt
TiO2/Cu2O nanoparticles
TEA (1.5 eq)
O
P
P
OEt
EtO
H
DMF (0.5 mL)
CFL 15 W, air
OEt
3a, 3h, 94%b
3b, 3h, 87%
1a
2a
MeO
EtO
OEt
3a (trace)
OMe
Scheme 4. Control experiments
P
P
O
O
A proposed mechanism for the visible light induced TiO2/Cu2O
nanoparticles photocatalyzed for the synthesis of
alkynylphosphonates is outlined in Figure 2. Upon visible light
irradiation photoexcitation TiO2/Cu2O nanoparticles generates
electron and hole (e-/h+). Firstly, the hole is able to abstract an
electron from the H-phosphonate via a single electron transfer
process (Oxidation) to generate phosphonate radical cation I. In
continue, phosphonate radical cations were lost a proton (H+)
which is captured by O2 radical anion to generate HO2 radical,
and phosphonate radical II was produced. Then it was attacked
to terminal alkyne as a nucleophile to give intermediate III.
Finally, it was lost an H radical to produce product. H2O2 was
created by reaction of O2, electron of excited catalyst, H radical
and H cation, as a by-product in the reaction mixture. The
generation of H2O2 was proven by NMR and H2O2 paper test
(see SI).
3c, 5h, 96%
3d, 5h, 92%
N
N
OEt
P
OEt
O
OMe
P
OMe
O
3e, 18h, 86%
3f, 18h, 78%
O
O
P
OMe
OEt
OEt
P
OMe
O
O
3g, 18h, 82%
3h, 18h, 82%
OEt
OMe
EtO
MeO
P
P
O
O
O
O
Br
Br
O2
3i, 18h, 92%
3j, 18h, 90%
e-
e-
O
e-
2
N
red
OH
P
R'
O
O2
O2
R
OH
P
R2
O
P
R
PH
R'
O
P
EtO
OEt
Ox
h+ h+
R1
I
h+
TiO2
2
Cu2O
HO2
OH
P
R2
R1
3l, 18h, 90%
3k, 18h,83%
I
O
aReaction conditions: terminal Alkyne (0.5 mmol), H-phosphonate
R1
P
R2
R'
(0.5 mmol), TiO2/Cu2O nanoparticles (0.01 g) under visible light (CFL)
O
O
P
R'
II
R'
- H
P
bIsolated
yield.
(Ar) R
in
DMF
(0.5
mL).
H
R'
(Ar) R
III
HO2
H2O2
(Ar) R
H
Fig. 2 Proposed mechanism for C-P bond formation from
terminal alkyne and H-phosphonate with TiO2/Cu2O
nanoparticles under visible light irradiation.
In relation to the proposed mechanism:
In order to propose the proper mechanism some control
experiments were investigated as shown in Scheme 4. Under
standard reaction condition, by adding a radical scavenger
4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9DT04757C
Conclusion
In summary, we have developed a simple and efficient method
for the synthesis of alkynylphosphonates via C-P bond
formation from terminal alkyne and H-phosphonate, by using
TiO2/Cu2O nanoparticles as photocatalyst under visible light at
room temperature. This procedure suffer from many
advantages such as: i) no additives, ii) no bases and/or toxic
reagent(s) iii) no laborious purifications or setup, iv) very simple
procedure for photocatalyst preparation, v) easy recovery and
reusability of the catalyst vi) cost effective, vii) environmentally
friendly, and more important viii) using visible light as a green
energy source. All these points, make our methodology suitable
for industrialization as a valid contribution to the existing
processes in the field of alkynylphosphonates synthesis, an
important building block for organic synthesis.
Acknowledgements
This work was supported by the Shiraz University.
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