Chlorosilane-Catalyzed Coupling of Hydrogen Phosphine Oxides with Acyl Chlorides Generating Acylphosphine Oxides

Article abstract of DOI:10.1021/acs.orglett.0c01384

We report a new method for the synthesis of acylphosphine oxides by the direct coupling of hydrogen phosphine oxides and acyl chlorides mediated by chlorosilanes. This new protocol is greener and safer, because it precludes the generation of volatile haloalkanes and the use of oxidants employed in the conventional methods. Moreover, moisture-unstable acylphosphine oxides that are difficult to prepare via the conventional methods can be generated using this new method.

Full text of DOI:10.1021/acs.orglett.0c01384

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Letter
Chlorosilane-Catalyzed Coupling of Hydrogen Phosphine Oxides
with Acyl Chlorides Generating Acylphosphine Oxides
Jian-Qiu Zhang and Li-Biao Han*
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ABSTRACT: We report a new method for the synthesis of acylphosphine oxides
by the direct coupling of hydrogen phosphine oxides and acyl chlorides mediated by
chlorosilanes. This new protocol is greener and safer, because it precludes the
generation of volatile haloalkanes and the use of oxidants employed in the
conventional methods. Moreover, moisture-unstable acylphosphine oxides that are difficult to prepare via the conventional methods
can be generated using this new method.
cylphosphine oxides R2P(O)C(O)R′ exhibit broad light
Scheme 2. Preparations of Acylphosphine Oxides
A
absorption range and can decompose on exposing to
light. Therefore, these compounds have broad applications as
efficient radical photoinitiators for photopolymerization, such
as surface coatings, finishes, and printing inks.1 Recently, they
are also found unique applications in organic synthesis.2
Among them, diphenyl(2,4,6-trimethylbenzoyl)phosphine
oxide (TPO) is a successfully commercialized photoinitiator
in the industry. Photolysis of TPO yields phosphinoyl and acyl
radicals (Scheme 1).3 Because both of them are able to initiate
Scheme 1. Photolysis and Application of TPO
To find more efficient alternative ways for the preparation of
R2P(O)C(O)R′, the oxidation of α-hydroxyphosphine oxide
prepared via the addition of secondary phosphine oxides to
aldehydes was extensively studied in recent years (Scheme
2b).8 However, the oxidation efficiency was low and usually a
large amount of oxidants must be used in order to obtain good
yields of the desired products. For example, up to 20 equiv of
MnO2 were required in the oxidizing process.1e The direct
coupling of hydrophosphoryl compounds R1R2P(O)H with
acyl chlorides R3COCl is a straightforward strategy (Schemes
the polymerization, fast polymerization rates, and high curing
efficacy can be achieved. The high degree of whiteness is
another outstanding feature of TPO, since the current
aromatic−ketones-type photoinitiators usually suffer from
undesirable yellowing.4
Currently, acylphosphine oxides were synthesized by the
Arbuzov-type reactions from acyl chlorides (RCOCl) with the
air and moisture-sensitive alkoxylphosphines (R2POR) pre-
pared by chlorophosphines (R2PCl) with low-boiling point-
alcohols (Scheme 2a).5 However, R2PCl is not a readily
available compound and the industrial preparation of R2PCl
uses a heavy-pollution process.6 In addition, during the
preparation of TPO, 1 equiv of the toxic low-boiling-point
chloroalkanes (RCl) such as EtCl are inevitably released as
volatile organic compounds (VOCs) that are hard to handle
and environmentally harmful.7
Received: April 22, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01384
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
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2c and 2d) that can solve the problems for the preparation of
TPO, etc., as mentioned above. However, previous literature
has indicated that it was difficult to efficiently obtain such
acylphosphorus compounds directly from R3COCl and
R1R2P(O)H (Scheme 2c),2b since a large amount of an
undesired byproducts were generated.
Scheme 4. Synthesis of Acylphosphine Oxides Using
Me3SiCl
a
Herein, in our study, we report that, by using chlorosilanes,
we can overcome the obstacles and a variety of acylphosphine
oxide compounds R2P(O)C(O)R′ can be efficiently generated
by the coupling of R1R2P(O)H with acyl chlorides R3COCl.
Furthermore, we successfully found that chlorosilanes could be
used as catalysts for these coupling reactions (Scheme 2d). To
the best of our knowledge, this is the first chlorosilane-
catalyzed coupling reactions of R1R2P(O)H with acyl chlorides
R3COCl generating R1R2P(O)C(O)R3.
Preliminary studies were commenced by mixing 0.5 mmol of
diphenylphosphine oxide (Ph2P(O)H) (1a) with 0.6 mmol of
chlorotrimethylsilane (Me3SiCl) in the presence of 0.6 mmol
of triethylamine (Et3N) in 2 mL of THF at room temperature
(Scheme 3a). After an overnight stirring, 31P NMR showed
Scheme 3. Preparation of TPO Using Me3SiCl
a
Reaction conditions: Step 1: R1R2P(O)H (0.5 mmol), THF (2.0
mL), Me3SiCl (0.6 mmol), Et3N (0.6 mmol), 25 °C, overnight. Step
2: Acyl chloride RCOCl (0.6 mmol), 25 °C, overnight. Isolated yield.
b60 °C for step 1. 2 h for step 1. 70 °C, 24 h for step 2. Estimated
c
d
e
31P NMR yield based on 1a used. 0 °C, 2 h for step 2. 100 °C,
f
g
h
overnight for step 2. 2 h for step 2.
With regard to acyl chlorides, other substrates such as
PhCOCl, i-PrCOCl, and MeCOCl could also be used as the
substrates. High yields of the products from these substrates
were also formed, as confirmed by 31P NMR spectroscopy.
These compounds (3e−3g), which readily decompose in a
moist atmosphere, are difficult to generate via old conventional
methods. To date, bulky acylphosphine oxides are isolable and
easily handled compounds, because the CO group of the
acylphosphine oxides is easily attacked by nucleophiles.10,11 By
using the sterically hindered t-BuCOCl, the stable aliphatic
acylphosphine oxide 3h could be obtained in 98% isolated
yield.
To further demonstrate the usefulness of the current method
for the preparation of acylphosphine oxides, a gram-scale
reaction was conducted. As shown in Scheme 5, 2.02 g of
Ph2P(O)H was treated with 1.2 equiv of Me3SiCl in the
presence of Et3N, followed by the addition of MesCOCl at
room temperature. After removal of the amine salt Et3NHCl
via filtration, followed by removal of the volatiles under a
reduced pressure, the residue was purified by chromatography
on silica gel using ethyl acetate/n-hexane as eluents. TPO
that Ph2P−O−SiMe3 (2a), which appeared at 96.3 ppm, was
generated quantitatively. To our delight, a subsequent addition
of 0.6 mmol of mesitoyl chloride (MesCOCl) yielded the
expected product TPO 3a in a quantitative yield. More
conveniently, as shown in Scheme 3b, the reaction could be
simply performed in one pot, affording TPO in 87% yield.9
This method is applicable to the preparation of a variety of
R1R2P(O)C(O)R3. As shown in Scheme 4, three types of
P(O)−H compoundsi.e., secondary phosphine oxides, H-
phosphonate, and H-phosphinatewere all compatible for this
transformation. For example, diarylphosphine oxides with
methyl, trifluoromethyl, as well as chloro groups on the phenyl
rings all could produce the corresponding acyl phosphine
oxides with high yields (3b−3d). It seems that phosphine
oxides having an electron-withdrawing group such as (p-
CF3C6H4)2P(O)H (1c) and (p-ClC6H4)2P(O)H (1d) reacted
with Me3SiCl faster than Ph2P(O)H (1a). However, the
consequent second step reactions with MesC(O)Cl to give
acylphosphine oxides proceeded slower because a long-time
heating (70 °C, 24 h) were required. On the other hand, the
reaction of (p-MeC6H4)2P(O)H (1b) affording the P−O−Si
intermediate required heating at 60 °C. However, the
subsequent reaction with MesC(O)Cl occurred smoothly at
room temperature. As shown in the table, ethyl phenyl-
phosphinate Ph(EtO)P(O)H (1i) and diethyl phosphonate
(EtO)2P(O)H (1j), as well as phenyl(tert-butyl)phosphine
oxide Pht-BuP(O)H (1k) and dibutylphosphine oxide n-
Bu2P(O)H (1l) all served well to afford the desired products in
satisfactory yields under similar reaction conditions (3i−3l).
Scheme 5. Gram-Scale Preparation of TPO
B
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(2.61 g (75% yield)) was obtained as a pale yellow solid
product.
As shown in Scheme 3a, since Me3SiCl was regenerated
during the synthesis of acylphosphine oxides, we considered
that a catalytic synthesis of R1R2P(O)C(O)R3 should work in
the presence of a catalytic amount of chlorosilanes. Thus, it is
expected that, as illustrated in Scheme 6, Me3SiCl first reacts
(Table 1, entry 2). In order to find a better catalyst, the
reactivity of other silylating agents were screened. Unfortu-
nately, the results showed that the expected more active
trimethylsilyl triflate or iodide hardly worked for this
transformation (Table 1, entries 3 and 4). However, among
other silyl chlorides reagents, we were pleased to find that
trichloro(methyl)silane (MeSiCl3) was proved to be the best
catalyst that can produces TPO in 62% yield (Table 1, entries
5−9). Blank experiments showed that the target product could
be scarcely formed in the absence of a silyl chloride (Table 1,
entry 10). In addition, conducting the reaction at an elevated
or decreased temperature could not improve the reaction
efficiency (Table 1, entries 11−13). Finally, the effect of
solvents was investigated (Table 1, entries 14−18). 1,4-
Dioxane, benzene, and toluene could also be used as the
solvents. However, the reaction hardly proceeded in DMF and
MeCN.
Scheme 6. Generation of TPO Catalyzed by TMSCl
This catalytic reaction is applicable to the synthesis of other
TPO analogues, as illustrated in Scheme 7. For example, (p-
with R2P(O)H to form the P−O−Si intermediate in the
presence of Et3N.12 Subsequent reaction of the P−O−Si
species with MesC(O)Cl produces the target TPO and
regenerates Me3SiCl that reacts with R2P(O)H again to start
another cycle of the reaction.
Scheme 7. MeSiCl3-Catalyzed Synthesis of TPO and Its
a
Analogues
Indeed, it was the case. As shown in Table 1, we investigated
the catalytic synthesis of TPO by examining the reactivity of
a
Table 1. Reaction Condition Optimization
b
entry
RnSiX4−n
solvent
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
temperature (°C)
yield (%)
1
2
3
4
5
6
7
Me3SiCl
Me3SiCl
Me3SiOTf
Me3SiI
Me2SiCl2
MeSiCl3
SiCl4
PhSiCl3
Ph2SiHCl
−
MeSiCl3
MeSiCl3
MeSiCl3
MeSiCl3
MeSiCl3
MeSiCl3
MeSiCl3
MeSiCl3
60
60
60
60
60
60
60
60
60
60
70
90
50
60
60
60
60
60
37
trace
n.d.
4
50
62
13
30
19
trace
64
64
60
65
70
5
c
8
9
10
11
12
13
14
15
16
17
18
THF
a
Reaction conditions: under argon atmosphere, R1R2P(O)H (0.6
toluene
1,4-dioxane
MeCN
DMF
benzene
mmol), MesCOCl (0.9 mmol), MeSiCl3 (0.06 mmol), Et3N (0.6
mmol) and 1,4-dioxane (1.5 mL), 60 °C, overnight. Isolated yield.
c
bYield determined by 31P NMR based on 1c used. 20 mol % of
n.d.
72
d
MeSiCl3 and Et3N (1.2 equiv) were used, 120 °C. 20 mol % of
MeSiCl3, Et3N (2 equiv), and MesCOCl (3 equiv) were used.
a
Reaction conditions: under argon atmosphere, Ph2P(O)H 1a (0.6
mmol), RnSiX4−n (10 mol %), Et3N (0.6 mmol), MesCOCl (0.9
b
mmol), solvent (1.5 mL), 60 °C, overnight. Yield determined by 31
P
MeOC6H4)2P(O)H could couple with MesC(O)Cl, giving a
71% yield of the product 3m. In addition, 1,4-phenylene-
bis(phenylphosphine oxide) (1o), equipped with two P(O)−
H moieties, was smoothly acylated to furnish diacylphosphoryl
product 3o in 74% yield. Furthermore, a more conjugated
di([1,1′-bi(p-amylphenyl]-4-yl)phosphine oxide 1p could also
underwent the catalytic reaction with MesCOCl efficiently,
affording a good yield of product 3p. Last, five-membered
cyclic hydrogen phosphonate 1n was successfully introduced
into the acylphosphoryl skeleton 3n in a high yield.
c
NMR based on 1a used. Without Et3N.
Ph2P(O)H (1a) with 1.5 equiv of MesCOCl in the presence of
10 mol % of Me3SiCl and 1 equiv of Et3N at 60 °C. The
desired product TPO 3a could be obtained in 37% yield
(Table 1, entry 1). As expected, the use of triethylamine was
crucial to this transformation, because only a trace amount of
the desired product was obtained in the absence of Et3N
C
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Danilevicius, P.; Gray, D.; Farsari, M.; Gryko, D. T. Push−pull
Acylo-Phosphine Oxides for Two-Photon-Induced Polymerization.
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BAPO as An Alternative Photoinitiator for the Radical Polymerization
of Dental Resins. Dent. Mater. 2014, 30, 945−953.
In summary, we have disclosed a novel protocol for the
synthesis of acylphosphine oxides from hydrophosphoryl
compounds and acyl chlorides by using the readily available
chlorosilanes. Three types of P(O)H compounds, including
secondary phosphine oxides R1R2P(O)H, H-phosphinate
R1(R2O)P(O)H, and H-phosphonate (RO)2P(O)H, were all
efficiently transformed to the corresponding acylphosphine
oxides in good yields. More importantly, a catalytic amount of
chlorosilane could also work well for the couplings. Compared
to the conventional routes, chemicals used in this new method
are readily available. Since the new method prohibits the
emission of the volatile chloroalkanes and the use of oxidants,
it provides a greener and safer approach to a wide range of
acylphosphine oxides that are useful as photoinitiators.
(2) (a) Sato, Y.; Kawaguchi, S. I.; Ogawa, A. Photoinduced
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Perfluoroalkylated Phosphines Using TMDPO and Perfluoroalkyl
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̅
ASSOCIATED CONTENT
■
Acylphosphine Oxide Promoted Cationic Polymerization. Eur. Polym.
J. 1989, 25, 129−131.
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* Supporting Information
(3) (a) Spichty, M.; Turro, N. J.; Rist, G.; Birbaum, J. L.; Dietliker,
K.; Wolf, J. P.; Gescheidt, G. Bond Cleavage In the Excited State of
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01384.
Experimental procedures, full spectroscopic data, and
1
copies of H, 13C, and 31P spectroscopies (PDF)
̈
̈
Stein, D.; Brauer, J.; Kissner, R.; Krumeich, F.; Schonberg, H.;
Levalois-Grutzmacher, J.; Grutzmacher, H. Phosphorous-Function-
̈
̈
AUTHOR INFORMATION
Corresponding Author
■
alized Bis (acyl) Phosphane Oxides for Surface Modification. Angew.
Chem. 2012, 124, 4726−4730. (c) Sluggett, G. W.; Turro, C.; George,
M. W.; Koptyug, I. V.; Turro, N. J. (2,4,6-Trimethylbenzoyl)
diphenylphosphine Oxide Photochemistry. A Direct Time-Resolved
Spectroscopic Study of Both Radical Fragments. J. Am. Chem. Soc.
1995, 117, 5148−5153. (d) Ikemura, K.; Ichizawa, K.; Yoshida, M.;
Ito, S.; Endo, T. UV-VIS Spectra and Photoinitiation Behaviors of
Acylphosphine Oxide and Bisacylphosphine Oxide Derivatives in
Unfilled, Light-Cured Dental Resins. Dent. Mater. J. 2008, 27, 765−
Li-Biao Han − National Institute of Advanced Industrial Science
and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan;
Division of Chemistry, Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan;
orcid.org/0000-0001-5566-9017; Email: libiao-han@
aist.go.jp
́
́
774. (e) Zalibera, M.; Stebe, P. N.; Dietliker, K.; Grutzmacher, H.;
̈
Author
Spichty, M.; Gescheidt, G. The Redox Chemistry of Mono- And Bis
(Acyl) Phosphane Oxides. Eur. J. Org. Chem. 2014, 2014, 331−337.
(f) Kajiwara, A.; Konishi, Y.; Morishima, Y.; Schnabel, W.; Kuwata,
K.; Kamachi, M. Time-resolved Electron Spin Resonance Study on
Radical Polymerization with (2,4,6-Trimethylbenzoyl) Diphenylphos-
phine Oxide. Direct Estimation of Rate Constants for Addition
Reactions of Diphenylphosphonyl Radicals to Vinyl Monomers.
Macromolecules 1993, 26, 1656−1658. (g) Dursun, C.; Degirmenci,
M.; Yagci, Y.; Jockusch, S.; Turro, N. J. Free Radical Promoted
Cationic Polymerization by Using Bisacylphosphine Oxide Photo-
initiators: Substituent Effect on the Reactivity of Phosphinoyl
Radicals. Polymer 2003, 44, 7389−7396.
Jian-Qiu Zhang − National Institute of Advanced Industrial
Science and Technology (AIST), Tsukuba, Ibaraki 305-8565,
Japan; Division of Chemistry, Faculty of Pure and Applied
Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571,
Japan; orcid.org/0000-0002-5220-3200
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01384
Notes
The authors declare no competing financial interest.
(4) For aromatic-ketones-type photoinitiators, see: (a) Yang, N. C.;
McClure, D. S.; Murov, S.; Houser, J. J.; Dusenbery, R. Photo-
reduction of Acetophenone and Substituted Acetophenones. J. Am.
Chem. Soc. 1967, 89, 5466−5468. (b) Hong, K. H.; Liu, N.; Sun, G.
UV-Induced Graft Polymerization of Acrylamide on Cellulose by
Using Immobilized Benzophenone As A Photo-Initiator. Eur. Polym. J.
2009, 45, 2443−2449. (c) Yilmaz, G.; Aydogan, B.; Temel, G.; Arsu,
N.; Moszner, N.; Yagci, Y. Thioxanthone− fluorenes as Visible Light
Photoinitiators for Free Radical Polymerization. Macromolecules 2010,
43, 4520−4526. (d) Esen, D. S.; Arsu, N.; Da Silva, J. P.; Jockusch, S.;
Turro, N. J. Benzoin Type Photoinitiator for Free Radical
Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2013, 51,
1865−1871. (e) Segurola, J.; Allen, N. S.; Edge, M.; McMahon, A.;
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Clear Coatings Systems: Influence of Photoinitiator Type. Polym.
Degrad. Stab. 1999, 64, 39−48. (f) Rutsch, W.; Berner, G.; Kirchmayr,
ACKNOWLEDGMENTS
■
This study was partially supported by a joint-research of
National Institute of Advanced Industrial Science and
Technology (AIST) with Katayama Chemical Industries Co.,
Ltd. Thanks are due to Miss Jingzhuo Zhao (AIST &
University of Tsukuba) for her help collecting NMR data.
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E
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