Palladium-catalyzed solvent-free preparation of arylphosphonates Arp(O)(OAr)2 from (Aro)3p via the Michaelis-Arbuzov rearrangement

Article abstract of DOI:10.1021/acs.organomet.0c00573

The Pd-catalyzed Michaelis-Arbuzov rearrangement of triaryl phosphites to produce the corresponding arylphosphonates in good to excellent yields is disclosed. In comparison to the traditional methods, this new method is highly atom efficient and is general, as it can be readily extended to aryl phosphonites and phosphinites. A gram-scale reaction of (PhO)₃P to PhP(O)(OPh)₂ with low loading of the catalyst was also demonstrated to show its potentially practical usefulness. A plausible mechanism is proposed.

Full text of DOI:10.1021/acs.organomet.0c00573

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Palladium-Catalyzed Solvent-Free Preparation of Arylphosphonates
ArP(O)(OAr)2 from (ArO)3P via the Michaelis−Arbuzov
Rearrangement
Chunya Li and Li-Biao Han*
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ABSTRACT: The Pd-catalyzed Michaelis−Arbuzov rearrange-
ment of triaryl phosphites to produce the corresponding
arylphosphonates in good to excellent yields is disclosed. In
comparison to the traditional methods, this new method is highly
atom efficient and is general, as it can be readily extended to aryl phosphonites and phosphinites. A gram-scale reaction of (PhO)3P
to PhP(O)(OPh)2 with low loading of the catalyst was also demonstrated to show its potentially practical usefulness. A plausible
mechanism is proposed.
nates RP(O)(OR)2, which have important applications in
organic synthesis, flame retardancy, materials chemistry, and
biological chemistry.3 However, the classic Michaelis−Arbuzov
rearrangement is essentially restricted to the preparation of
aliphatic phosphonates RP(O)(OR)2, as can be seen from its
two sequential SN2 nucleophilic attack mechanisms (Scheme
1(1)): i.e., the three phosphonates ArP(O)(OR)2, RP(O)-
(OAr)2, and Ar′P(O)(OAr)2, which have different properties
and applications that RP(O)(OR)2 does not have,4−8 could
not be similarly prepared. In 1970, Tavs reported an important
modification to the classic Michaelis−Arbuzov reaction.7 By
using nickel catalysts, the dialkyl arylphosphonate ArP(O)-
(OR)2 could be successfully prepared from the combination
(RO)3P/ArX (Scheme 1(2)). Recent extensive reinvestigations
of the Michaelis−Arbuzov reaction also have led to certain
progress in the preparation of RP(O)(OAr)2 from (ArO)3P/
RX (Scheme 1(3)).8 However, the most challenging
preparation of Ar′P(O)(OAr)2 from (ArO)3P/Ar′X still
remains unsolved, and these compounds have to be
synthesized using a very tedious and heavily polluting
stoichiometric Friedel−Crafts process using PCl3 (Scheme
1(4)).9 The generation of the corresponding phosphonium
intermediate is difficult because of the low nucleophilicity of
(ArO)3P in comparison to (RO)3P.10 Moreover, the
subsequent rearrangement of the phosphonium to Ar′P(O)-
(OAr)2 is also difficult, since it requires a C−O bond cleavage
by the catalyst.11 Unlike the case for the Tavs system, the
catalyst must play dual roles in this transformation: i.e., it must
he Michaelis−Arbuzov reaction, the formation of the
T
P(V) phosphonates RP(O)(OR)2 by the rearrangement
of the P(III) phosphites via a combination of (RO)3P/RX, was
discovered more than 120 years ago (Scheme 1).1,2 Because
the starting materials used in the reaction are cheap and readily
available, and also because the reaction process is simple, this
reaction has been, ever since, the most famous and frequently
used premier synthetic tool for the preparation of phospho-
Scheme 1. Michaelis−Arbuzov Reaction for the Preparation
of Phosphonates from Cheap Phosphites and Halides
Received: August 28, 2020
https://dx.doi.org/10.1021/acs.organomet.0c00573
© XXXX American Chemical Society
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A

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Communication
be able to activate both the C−X bond and the C−O bond
efficiently.
Herein, we disclose that a simple palladium−ArOX (X =
OTf, I) catalyst system can efficiently catalyze the trans-
formation of (ArO)3P to ArP(O)(OAr)2 (eq 1). We expect
TfOH after heating at 160 °C for 16 h (run 1). In addition, no
rearrangement took place in the presence of Pd(OAc)2, PdCl2,
Pd2(dba)3, and Pd(PPh3)4 (runs 2−5). Very surprisingly, the
combination Pd(OAc)2/HOTf (run 6) could give a 28% yield
of PhP(O)(OPh)2 (2a) under similar conditions. However,
disappointingly, the yields of PhP(O)(OPh)2 could not be
further improved despite a thorough search of the experimental
conditions. In addition, changing Pd(OAc)2 to other palladium
complexes or other metals such as PdCl2, Pd(PPh3)4,
Ni(OAc)2, NiCl2, Ni(cod)2, IrCl3, Rh(NO3)3, Rh(acac)2,
RhCl(PPh3)3, Cu(OAc)2, Fe(acac)2, Zn(OTf)2, Bi(OTf)3
etc. also did not give positive results. Very excitingly, excellent
yields of the product 2a were obtained by replacing HOTf with
MeOTf (run 7) and PhOTf (run 8). Contrary to our
expectation, a phosphine ligand dramatically retarded the
reaction (runs 9−12), and no product 2a was obtained using
PdCl2 (run 13), although other Pd(II) metals such as
Pd(NO3)2 (run 14) and Pd(CF3CO2)2 (run 15) could catalyze
the reaction. The Pd(0) complex Pd2(dba)3 also efficiently
catalyzed the reaction to give a 97% yield of 2a (run 16). For
ROTf, in addition to PhOTf, other compounds, such as 1-
naphthyl trifluoromethanesulfonate and 4-acetophenyl trifluor-
omethanesulfonate and even TMSOTf (run 17), all could
initiate the rearrangement and give high yields of 2a. In
addition to ROTf, PhI could also be used for this catalytic
reaction (run 20). In contrast, PhBr (run 19) only sluggishly
initiated the reaction and PhCl (run 18) could not initiate the
reaction at all. The high temperature of 160 °C is necessary
under the current conditions, otherwise the yield of 2a
decreased (run 21). Interestingly, when the reaction was
carried out in DMF, the reaction could efficiently take place
even at 120 °C (run 24). DMF was the best solvent among
other solvents such as toluene, N,N-diisorpopylformamide, and
N-methyl-2-pyrrolidone, which only produced low yields of 2a.
Although it was sluggish, the reaction even occurred at 80 °C
(run 22), and a 42% yield of 2a could be obtained at 100 °C
(run 23). Intriguingly, PhI seems superior to PhOTf under the
current conditions, since a trace of 2a was obtained at 120 °C
(run 25), though a 92% yield of 2a was generated at 140 °C
(run 26). Finally, unlike phosphines that retarded the reaction,
the addition of a catalytic amount of R3N could accelerate the
reaction (runs 27 and 28). Therefore, excellent yields of
PhP(O)(OPh)2 (2a) could be obtained by a rearrangement of
the cheap (PhO)3P 1a by using the catalyst Pd2(dba)3/PhI/
Et3N (run 27) or Pd2(dba)3/PhI/Cy2NMe (run 28).
This new palladium-catalyzed Michaelis−Arbuzov type
rearrangement is a rather general reaction. As shown in
Table 2, in addition to arylphosphonates, phosphinates and
phosphine oxides could all be prepared under similar
conditions. Thus, the arylphosphonates 2 were obtained
from the corresponding triaryl phosphites (runs 2−7). All of
the reactions with an electron-donating group (Me, t-Bu,
MeO) took place efficiently to give the phosphonates in high
yields. The bulky chemical 1g bearing two methyl groups on
the benzene ring could also be converted to the corresponding
phosphonate 2g in 87% yield (run 7). However, a substrate
with an electron-withdrawing group, tris(4-fluorophenyl)
phosphite (1f), for example, only gave a 31% yield of 2f
under similar conditions (run 6). The reason for the low yield
of 2f could be that the electron-withdrawing group F decreased
the nucleophilicity of 1f, and thus it has difficulty in forming
the key palladium complex 7 (see eq 4) from the coordination
of 1f and Pd. This transformation can also be readily extended
that this an unprecedented palladium-catalyzed rearrangement
should settle the long-unresolved last challenge for the
Michaelis−Arbuzov reaction and therefore finally complete it.
Recently we reported a new TfOH-catalyzed Michaelis−
Arbuzov rearrangement of (RO)3P to give the phosphonate
RP(O)(OR)2 in high yields.12 Unfortunately, like the classic
Michaelis−Arbuzov reaction, this new reaction is also only
applicable to alkyl phosphites, and no rearrangement took
place at all with (ArO)3P. As shown in Table 1, the
rearrangement product PhP(O)(OPh)2 (2a) was not detected
from a mixture of (PhO)3P (1a) and a catalytic amount of
Table 1. Palladium-Catalyzed Rearrangement of (PhO)3P to
a
PhP(O)(OPh)2
temp (°C), time
yield
b
run
cat.
solvent
(h)
(%)
1
2
3
4
5
6
7
8
9
HOTf
Pd(OAc)2
PdCl2
Pd2(dba)3
Pd(PPh3)4
Pd(OAc)2/HOTf
Pd(OAc)2/MeOTf
Pd(OAc)2/PhOTf
Pd(OAc)2/PhOTf/PPh3
none
none
none
none
none
none
none
none
none
none
none
none
none
none
none
none
none
none
none
none
none
DMF
DMF
DMF
DMF
DMF
DMF
160, 16
160, 16
160, 16
160, 16
160, 16
160, 16
160, 16
160, 16
160, 16
160, 16
160, 16
160, 16
160, 16
160, 16
160, 16
160, 16
160, 16
160, 16
160, 16
160, 16
140, 16
80, 21
none
none
none
none
none
28
92
90
trace
trace
trace
trace
none
20
94
97
85
none
19
62
26
trace
42
90
trace
92
90
10 Pd(OAc)2/PhOTf/dppb
11 Pd(OAc)2/PhOTf/dppp
12 Pd(OAc)2/PhOTf/dppf
13 PdCl2/PhOTf
14 Pd(NO3)2/PhOTf
15 Pd(CF3CO2)2/PhOTf
16 Pd2(dba)3/PhOTf
17 Pd2(dba)3/TMSOTf
18 Pd2(dba)3/PhCl
19 Pd2(dba)3/PhBr
20 Pd2(dba)3/PhI
21 Pd2(dba)3/PhI
22 Pd2(dba)3/PhI
23 Pd2(dba)3/PhI
24 Pd2(dba)3/PhI
25 Pd2(dba)3/PhOTf
26 Pd2(dba)3/PhOTf
27 Pd2(dba)3/PhI/Et3N
100, 21
120, 16
120, 16
140, 16
120, 5
28 Pd2(dba)3/PhI/Cy2NMe DMF
120, 5
97
a
Without solvent: a mixture of P(OPh)3 (1a, 1.14 mmol), 5 mol % Pd
(0.057 mmol), 5 mol % A (0.057 mmol), and the phosphine ligand
(Pd/P = 1/2) were heated at 160 °C for 16 h under N2 in an NMR
tube. Using DMF as a solvent: a mixture of P(OPh)3 (1a, 0.2 mmol),
5 mol % Pd (0.057 mmol), 5 mol % A (0.057 mmol) and 5 mol %
b
R3N in DMF (0.5 mL) were heated in an NMR tube. GC yields
using dodecane as an internal standard.
B
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Communication
Table 2. Scope of the Pd-Catalyzed Rearrangement of
a
(ArO)PZ2 to ArP(O)Z2
to aryl phosphonites to synthesize the corresponding aryl
phosphinates (runs 8−12). Thus, substrates with methyl,
methoxyl, and tert-butyl groups all give the corresponding
products in good yields (runs 8−11). The halogen atom F was
tolerated under the reaction conditions, although the yield
from 1l to 2l was low (run 12). Similarly, arylphosphine oxides
can be prepared by the isomerization of phosphinites in
moderate yields (runs 13−16). Thus, a series of methyl-,
methoxyl-, and tert-butyl-substituted phenyl diphenylphosphin-
ites underwent isomerization to give the corresponding
phosphine oxides in moderate yields (runs 14−16). The
aliphatic phenyl dibutylphosphinite 1q and the bulky phenyl
dicyclohexylphosphinite 1r also served well as substrates to
produce the corresponding phosphine oxides 2q and 2r in 82%
and 66% yields, respectively (runs 17 and 18). The reaction
was not restricted to phenyl phosphites, as other aryl
phosphites also underwent similar reactions to produce the
corresponding phosphonates in excellent yields (runs 19−22).
The potentially synthetic usefulness13 of this new reaction
was preliminarily demonstrated in Scheme 2. The expensive
14
PhP(O)(OPh)2 could be easily obtained in a high yield by
a
Reaction conditions unless specified otherwise: 1.14 mmol of 1, 2.5
simply heating the cheap (PhO)3P14 in the presence of a trace
amount of Pd2(pda)3 (0.5 mol %) and PhOTf (1 mol %).
Nearly 3 g of pure PhP(O)(OPh)2 could be obtained from 10
mmol of (PhO)3P after the reaction mixture was passed
through a short silica gel column.
mol % of Pd2(dba)3 and 5 mol % of PhOTf were charged into a glass
tube and heated at 160 °C for 16 h under N2. 5 mol % of Pd2(dba)3
and 10 mol % of PhOTf, 38 h. 5 mol % of Pd2(dba)3 and 10 mol %
of PhOTf. Yield estimated on the basis of GC analysis. 38 h. 5 mol
% of Pd2(dba)3 and 10 mol % of PhOTf, 4 days. 1i (0.2 mmol), 5
mol % of PhI (0.01 mmol), 5 mol % of Cy2NMe (0.01 mmol), and
DMF (0.5 mL), 140 °C.
b
c
d
e
f
g
To probe the reaction mechanism, control reactions were
conducted.15 First, treatment of Pd2(dba)3 with P(OPh)3 1a
C
https://dx.doi.org/10.1021/acs.organomet.0c00573
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Communication
Scheme 2. Gram-Scale Preparation of PhP(O)(OPh)2 from
(PhO)3P
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00573.
■
sı
Experimental procedures, control experiments, full
1
spectroscopic data, and H, 31P and 13C NMR spectra
(PDF)
quickly generated Pd[P(OPh)3]3 (6a) at room temperature
(eq 2).16 Then, the oxidative addition of 6a to iodobenzene
produced the arylpalladium complex 7a quantitatively at room
temperature (eq 3).17 This reaction can be achieved in one
pot. Thus, Pd2(dba)3 reacted with a mixture of PhI and 1a
smoothly to produce 7a in 68% yield (eq 4). Complex 7a
decomposed to phosphonium salt PhP+(OPh)3·I− (8a), 2a, 1a,
and 6a in DMF at 160 °C (eq 5). Moreover, the phosphonium
salt 8a could also quickly react with Pd2(dba)3 to give a high
yield of 7a (eq 6), showing that there is an equilibrium among
7a, 8a, and Pd(0). In addition, the reaction of 1a with PhI
catalyzed by Pd2(dba)3 in Et2O could produce the
phosphonium salt 8a in 80% isolated yield (eq 7).18 The
rearrangement of the phosphonium salt 8a took place in the
presence of the palladium catalyst to give 2a (eq 8). As shown
by eq 9, phosphonium salt 8a also decomposed but slowly in
the absence of the palladium catalyst.
AUTHOR INFORMATION
Corresponding Author
■
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
Author
Chunya Li − 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/0000-0002-4252-1002
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00573
The exact mechanism for the rearrangement of (PhO)3P to
PhP(O)(OPh)2 is not fully understood. On the basis of these
experimental results, a possible mechanism for the Pd-
catalyzed rearrangement of (PhO)3P to PhP(O)(OPh)2
(Scheme 3) is proposed. Thus, Pd2(dba)3 may first react
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Mr. Daoqing Han for carrying out part of the early
screening experiments for Table .
Scheme 3. Proposed Mechanism for the Pd-Catalyzed
■
C(sp2)−O Cleavage Reactions of P(III) Esters
1,
REFERENCES
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D
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EtONa, EtOH
PhP(O)(OPh)2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→⎯ PhP(O)(OEt)2
rt, overnight
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̈
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(7) (a) Tavs, P. Reaktion von Arylhalogeniden mit Trialkylphos-
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phiten und Benzolphosphonigsaure-dialkylestern zu aromatischen
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Phosphonsaureestern und Phosphinsaureestern unter Nickelsalzkata-
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Herstellung und KMR-spektren einiger α, β-ungesattigter phosphon-
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̈
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(8) Ma, X.; Xu, Q.; Li, H.; Su, C.; Yu, L.; Zhang, X.; Cao, H.; Han,
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(9) (a) Buchner, B.; Lockhart, L. B., Jr An improved method of
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(11) For selected examples of metal-catalyzed C−O bond cleavage,
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E
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Organometallics XXXX, XXX, XXX−XXX