Organic Letters
Letter
a
phosphites from the reaction mixtures containing crown ether
is very difficult.8 In 1992, Budnikova investigated the
phosphorylation of ArOH with P4 by an electrochemical
method to give triphenyl phosphate in 82% yield (Scheme
1B).9 In 2005, the Abdreimova group reported the FeCl3−I2-
catalyzed reaction of phenol with P4 to produce (PhO)3PO
with some interesting results (Scheme 1C).10 In 2011, an
efficient Fe-catalyzed method was found by the Kilian group.
With the use of Fe(acac)3−I2 as the catalysts, the air (30−45
mL/min) and white phosphorus solutions (0.34−0.78 mmol of
P4/h) were introduced into the reaction mixture through
needles, making certain the 100% conversion of white
phosphorus and synthesis of (ArO)3PO with high selectivity
(Scheme 1D).11 To the best of our knowledge, the separation
of OPCs from the mixtures containing a transition metal
proved to be difficult because of their strong coordination
ability. On the contrary, triphenyl phosphite [(PhO)3P] could
not be obtained by using this method. Thus, there is a pursuit
for the synthesis of such organophosphorus compounds in
organic chemistry and industrial process, i.e., directly from
white phosphorus and without the assistance of a transition
metal.
Our continued interest in the green synthesis of phosphor-
othioates from P4 prompted us to explore the possibility of
using organic sulfur/selenium compounds as the catalysts for
the direct catalytic synthesis of triaryl phosphites/phosphates
involving white phosphorus and ArOH.12 We envisioned that
the catalytic activation of P4 with (RSe)2 might produce P−
SeR species, which would undergo a further nucleophilic
substitution reaction with ArOH, eventually leading to P−OAr
bonds.
Table 1. Optimization of Reaction Conditions
b
b
entry
base
catalyst
solvent
T (°C)
yield (%)
1
2
3
4
5
6
7
8
K3PO4
A
B
C
D
E
F
A
A
A
A
A
A
A
A
SS
SS
SS
SS
SS
SS
PhMe
DMSO
solvent
SS
SS
SS
SS
SS
SS
SS
SS
60
60
60
60
60
60
60
60
60
40
80
60
60
60
60
60
60
85
45
0
43
43
27
trace
20
trace
52
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
inorganic
organic
K3PO4 (x %)
K3PO4
K3PO4
c
9
10
11
12
13
14
15
67
d
67−80
5−64
0−85
0−85
<5
e
f
g
A (y%)
A
A
h
16
17
i
i
j
K3PO4
>95, 42
a
Conditions: p-MeO-C6H4OH (5 equiv), P4 (5.6 mg, 0.045 mmol of
P4, 0.18 mmol of P atom, a 0.09 M solution of P4 in toluene, 0.5 mL),
a catalyst, and a base in solvent (1 mL) were stirred for 4 h under an
Ar atmosphere. The yield of 2a was determined by 31P NMR analysis
of the crude reaction mixture using Ph3P(O) as an internal standard.
b
c
In mole percent per P atom. PhMe (0.5 mL), solvent (0.5 mL), and
Initial investigations into the synthesis of triaryphosphites
were carried out with P4 and p-methoxyphenol (1a) as reaction
partners (Table 1). A mixture of both compounds, K3PO4, and
a catalyst was heated at 60 °C under argon. In the presence of
diphenyl diselenide (A) as a catalyst, the reaction in a mixed
solvent of toluene and DMSO afforded the desired product (p-
CH3OC6H4O)3P 2a in 85% yield (entry 1). Inspired by this
result, we next examined other readily available diaryl disulfides
such as diphenyl disulfide (B), 4,4′-dinitrodiphenyl disulfide
(C), 4,4′-dimethyldiphenyl disulfide (D), 4,4′-dimethoxydi-
phenyl disulfide (E), and 4,4′-dichlorodiphenyl disulfide (F)
(entries 2−6), and diphenyl diselenide (A) was the best choice
(entry 1). Toluene or DMSO alone was used as the solvent,
affording product 2a in very low yield (entries 7 and 8). Other
mixed solvents such as toluene with DMF, EtOAc, acetone,
1,4-dioxane, CH3CN, or THF were less effective (entry 9).
Screening the reaction temperature revealed that 60 °C
provided the best result (entries 1, 10, and 11). Other alkali
metal salts (KOH, NaOH, t-BuOK, Cs2CO3, K2CO3, and
Na2CO3) were also suitable bases, affording product 2a in
slightly lower yields (entry 12). With the use of DBU as a base,
product 2a was obtained in 64% yield (entry 13). Pyridine and
NEt3 could not promote this process (entry 13). Further
increasing the amount of K3PO4 to 100 mol %, however,
resulted in 2a with a yield of 35% (entry 14). When the
loading of K3PO4 or diphenyl diselenide (A) was decreased,
the yield of 2a also decreased. The catalyst and base are both
indispensable for this phosphorylation of p-methoxyphenol
(1a) (entries 14 and 15). The reaction under oxygen or air
gave neither 2a nor (p-CH3OC6H4O)3PO 3a, and only some
unknown complicated byproducts were observed by 31P NMR
a solvent (DMF, EtOAc, acetone, 1,4-dioxane, CH3CN, and THF).
d
Inorganic base, yield: KOH, 76%; NaOH, 80%; t-BuOK, 79%;
e
Cs2CO3, 67%; K2CO3, 69%; Na2CO3, 77%. Organic base, yield:
f
DBU, 64%; pyridine or (Et)3N, <10%. K3PO4 (mol %, yield): 0 mol
%, 0%; 10 mol %, 75%; 25 mol %, 82%; 50 mol %, 78%; 100 mol %,
g
35%. A (y mol %, yield): 0 mol %, 0%; 10 mol %, 56%; 20 mol %,
h
82%; 25 mol %, 85%. O2 balloon or open to air (2a, <5%; 3g, <5%).
i
j
1a (6 equiv). 1a (3 equiv).
analysis (entry 16). Furthermore, an almost quantitative yield
was achieved with 6 equiv of 1a (entry 17).
We next evaluated the scope of this diphenyl diselenide (A)-
catalyzed coupling reaction of white phosphorus and phenols
for the synthesis of triaryl phosphites [P(III)] (Scheme 2). It
was found that phenols bearing an electron-donating group
(MeO, EtO, BnO, or MeS) at its para position reacted
efficiently with P4, and the corresponding products 2a−2d
were obtained in 60−95% yields. It is noteworthy that
triphenyl phosphite (TPPI, 2e), a famous antioxidant
stabilizer, was obtained in 88% yield. Phenols bearing Me or
tert-butyl groups furnished the desired products in high yields
(2f−2h). In addition, 1-naphthalenol, 2-naphthalenol, and 4-
phenyl phenol also proceeded with good efficiency (2i−2k).
Phenol with an acetal group afforded the corresponding
product 2l in a yield of 78%.
Inspired by these results, we decided to further explore the
diphenyl diselenide-catalyzed reaction for the synthesis of
triaryl phosphates [P(V) (Scheme 3)]. Triphenyl phosphate
(TPP, 3a), a famous fire retardant, was obtained in 90% yield.
Phenols bearing an alkyl group (Me or Et) at the para position
of the phenyl ring gave the corresponding products 3c and 3d
5159
Org. Lett. 2021, 23, 5158−5163