Air-induced double addition of P(O)-H bonds to alkynes: A clean and practical method for the preparation of 1,2-bisphosphorylethanes

Article abstract of DOI:10.1039/c6gc03240k

An air-induced double addition of diphenyl phosphine oxide to various alkynes is reported. This reaction can proceed efficiently under metal- and solvent-free conditions, and is a clean and practically useful method for the preparation of the valuable 1,2

Full text of DOI:10.1039/c6gc03240k

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Air-induced Double Addition of P(O)-H Bond to Alkynes: A Clean
and Practical Method for the Preparation of 1,2-
Bisphosphorylethanes
Received 00th January 20xx,
Accepted 00th January 20xx
Haiqing Guo,a Aya Yoshimura,a* Tieqiao Chen,a Yuta Sagab and Li-Biao Hana*
DOI: 10.1039/x0xx00000x
www.rsc.org/
An air-induced double addition of diphenyl phosphine oxide to
Bisphosphoryl compound 1 can be easily reduced to the
various alkynes was reported. This reaction can proceed efficeintly corresponding trivalent bisphosphinoethanes which are highly
under metal- and solvent-free conditions, and is a clean and useful phosphine ligands for transition metal complexes.4
practically useful method for the preparation of the valuable Compound
1
is also a useful extractant for metal extractions5
and a potentially effective flame retardant because of its high
phosphorus-content.6 In the literatures, compound
was
bisphosphinylethanes.
1
Organophosphorous compounds are invaluable chemicals in
organic synthesis, catalysis, medicinal chemistry, coordination
chemistry, and material science.1 Despite their importance,
efficient methods for their preparation are rather limited.
Traditional methods for the preparation of organophosphorus
compounds are based on the transformation of
organohalides.1,2 For example, the orthodoxy nucleophilic
substitutions of the toxic phosphorus halides with
organolithiums or Grignard reagents produce the
corresponding organophosphorus compounds. The reaction of
an excess amount of a halide with a phosphite (Michaelis-
Arbusov reaction) is also extensively used. However,
drawbacks of these methods, i.e. toxic starting materials,
harsh conditions, low efficiency and low functional tolerance,
are obvious. To solve these problems, the exploration for a
cleaner and more efficient preparation of organophosphorus
compounds to replace these old ones, is of current concerns.3
prepared by the addition of Ph2P(O)H to a vinylphosphine
oxide,7a,7b substitution reaction with 1,2-dichloroethane,7c,7d
nucleophilic addition to ethylene oxide7e etc7f-7h (Scheme 1).
However, shortcomings are clear, i.e. the difficult accessibility
to vinylphosphine oxides, and dangerous starting materials of
dichloroethane and ethylene oxide. A direct double addition
of the easily accessible alkynes with Ph2P(O)-H compounds is
no doubt one of the most atom-economic ways for the
synthesis of 1 8
. To date, double addition of P(O)-H bonds to
alkynes catalysed by palladium or nickel could produce the
corresponding bisphosphoryl compounds.8a,8b However,
purification of the products was a problem because these
products are solids which cannot be purified by distillation,
and therefore, a complete removal of the metal catalysts from
the products is almost impossible because of the strong
coordination of the products with the metals. A double-
addition was also achieved in “super basic media” KOH-
DMSO8c,8d or in the presence of a catalytic amount of t-
BuOLi.8e However, starting materials with functional groups, a
halogen atom, for example, can be severely damaged under
KOH/DMSO
Ph2P(O)H
P(O)Ph2
less accessible
low functinal tolerance
P(O)Ph2
Ph2P(O)H
Ph2P(O)H
5 mol% Pd(PPh3)4
metal pollution
Cl
+
Cl
low AE
Ph2P(O)H
such a basic condition.
toxic
O
P(O)Ph2
R
P(O)Ph2
cat. t-BuOLi
R
a trace amount of air
1
low AE
(1)
P(O)Ph2
low functional tolerance
toxic & explosive
R
with or without solvent
AE = Atom Efficiency
Ph2P(O)H
1
Scheme 1. Known methods for the preparation of 1.
Air (dioxygen) is harmless and abundant. In addition to the
economic and environmental advantages, another big
advantage using air as a radical initiator is that, being different
from the general used radical initiators such as AIBN that
generates considerable amounts of by-products (impurity) in
the reactions and makes the purification of the desired
products difficult, by-products from air is negligible. Radical
reactions employing dioxygen as a radical could be found in
a.National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1,
Higashi, Tsukuba, Ibaraki, 305-8565, Japan.
b.Katayama Chemical Industries Co., Ltd., 3-Chome, Higasinaniwa-cho, Amagasaki,
Hyogo 660-0892, Japan.
*E-mail: libiao-han@aist.go.jp
Electronic Supplementary Information (ESI) available: General information,
detailed experimental procedures, characterization data and copies NMR
spectroscopies of 1H, 13C and 31P NMR. See DOI: 10.1039/x0xx00000x
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the early literatures, and recently this method is re- phosphorylated by diphenylphosphine oxide, producing the
booming.7a,9
corresponding bisphosphoryl compounds in good yields.
Herein, we report a very clean and practical method for the Substrates with functional groups such as chloro, hydroxyl,
preparation of compounds . Thus, in the presence of a trace nitrile, and ester group all served as good substrates (entries 2-
amount of air, Ph2P(O)H can adds efficiently to a variety of 7). It was reminded that a substrate with a chloro group
alkynes to produce the corresponding 1,2- cannot be used as a substrate in the KOH-DMSO or metal-
1
1
bisphosphinylethanes in high yields (eqn 1). As shown below, mediated double phosphorylations of alkynes. The bulky 1-
this reaction well solves the problems mentioned above ethynylcyclopentanol could also give 48% yield of the product
because it can be carried out even in the absence of a solvent, (entry 8). In addition to these aliphatic alkynes, aromatic
and the pure adducts can be obtained by simply washing the alkynes with an electron-donating or an electron-withdrawing
crude products with a small amount of solvent.
group were also double phosphorylated readily by diphenyl
phosphine oxide under similar conditions. Thus,
phenylacetylene reacted with diphenylphosphine oxide to give
the expected adducts in 70% yields (entry 9). 1-Ethynyl-2,4,5-
trimethylbenzene also served as a good substrate (entry 10).
Aromatic acetylene derivatives with an electron-donating
group such as methoxy and amino group reacted with
diphenylphosphine oxide to give the corresponding adducts in
80% and 88% yields, respectively
Table 1 Air-induced double addition of diphenylphosphine oxide to 1-
octyne.a
a
Reaction conditions:
a
mixture of 0.1 mmol
1-octyne, 0.2 mmol
diphenylphosphine oxide, 25 µL solvent and was heated in a 1 mL closed vial
filled with air at the indicated temperature for 22 h. b 31P NMR yield.
In 2007, we disclosed the first air-induced radical addition of
P(O)H compounds to olefins.7a,10 The resulting phosphoryl
radical could be captured by olefins, leading to the regio-
selective production of anti-Makovnicov phosphoryl
compounds. We envisaged that by replacing the olefins with
alkynes in the reaction, a similar addition might also take place
to produce the valuable alkenylphsphorus compounds.
However, to our surprise, instead of an alkenylphosphoryl
compound (mono addition of P(O)H), 1,2-bisphosphoryl
compound
1 via a double-P(O)H addition to the alkyne was
obtained selectively.
As shown in Table 1, a solution of 1-octyne (0.1 mmol) and
Ph2P(O)H (0.2 mmol) in dioxane which was heated at 100 oC
for 22 h in pure argon atmosphere (O2 < 3 ppm) hardly
produced 1,2-bisphosphine oxide 1a (<5% yield, entry 1). In
sharp contrast, by injecting air (1 mL) into the vial, the yield of
1a was dramatically increased to 80% under similar reaction
conditions (entry 2). Decreasing the reaction temperature led
to a low yield of the product (entry 3). This reaction also
proceeded readily in other solvents such as DMF, anisole,
toluene, chlorobenzene and DMSO (entries 4-8). It was noted
that the double addition product was also obtained selectivity
with a stoichiometric amount of Ph2P(O)H under similar
conditions.
This air-induced double addition was applicable to other
alkynes. As shown in Table 2, a variety of alkynes were double-
2 | J. Name., 2012, 00, 1-3
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Table 2 Air-induced double addition of P(O)-H bond to alkynes.a
(entries 11 and 12). An aromatic acetylene with an electron-
withdrawing CF3 group also produced the corresponding
product (entry 13). Noteworthy is that other valuable
functional groups such as carbonyl and chloro groups were
well tolerated which could not be incorporated in the
substrates in the base-catalysed double additions (entries 14-
16). Heteroaromatic alkyne as exemplified by 2-ethynylpiridine
could also react with diphenylphosphine oxide to afford the
corresponding adduct in 72% yield (entry 17). In addition to
Ph2P(O)H, substrates having an electron-donating group (p-
MeOC6H4)2P(O)H (run 18), and an electron-withdrawing group
(p-CF3C6H4)2P(O)H (run 19) also reacted with 1-octyne to
produce the corresponding products. However, an internal
alkyne 4-octyne did not give satisfactory results under similar
conditions.
As a practically useful synthetic method, more importantly, the
reaction takes place efficiently in the absence of a solvent
which significantly simplifies the preparation of 1 11
As
.
demonstrated in Figure 1, charging 1-octyne (1.5 mmol) and
diphenyl phosphine oxide (1 mmol) into a 10 mL glass tube
with 1 mL air, gave a white solid/liquid mixture (Figure 1 (a)).
Upon heating (150 oC), a transparent homogeneous solution
was obtained (Figure 1 (b)). As the reaction progressed, the
solution solidified, and an almost quantitative yield of the
product was obtained after 43 h as confirmed by 31P NMR
spectroscopy ((Figure 1 (c)). A pure white solid 1a was
obtained by simply washing the crude product with 8 mL
hexane to remove the remained starting materilas (1a is not
soluble in hexane; this reaction has been conducted several
times and the yields were in a range of 91-96%). This process
was also applicable to other alkynes as represented in Scheme
2. Including an aliphatic phosphine oxide [Ph(CH2)4]2P(O)H, all
could produce the corresponding bisphosphorylethnanes via
this simple process, which clearly demonstrated that the
a Reaction conditions: 0.1 mmol alkynes, 0.2 mmol diphenyl phosphine oxide, current reaction is a promisingly practically useful simple and
25 µL dioxane (for aromatic, DMF was used as solvent), 1 mL air, 100 oC
green way for the preparation of these 1,2-bisphosphoryl
b
(for aromatic alkynes, the temperature was 150 oC), 22 h, 1 mL vial. 31P
compounds.
NMR yield. Data in parentheses are isolated yields. c 150 oC, DMF was used
as solvent.
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Figure 1. Profile of the air-induced double addition of diphenylphosphine oxide to 1-octyne conducted under a solvent-free reaction condition.
conditions was noted before7a and was further confirmed by a
P(O)R2
P(O)R2
air (1 mL)
separate reaction under similar conditions (eqn 2).
trace air
R'
R2P(O)H
1 mmol
150 oC, 43 h R'
P(O)Ph2
P(O)Ph2
1.5 mmol
Ph2P(O)H
0.1 mmol
1
n-C6H13
P(O)Ph2
95% yield
(2)
25 uL dioxane n-C6H13
100 oC, 26 h
0.1 mmol
1a
,
1i
, 91%; R = Ph, R' = Ph, , 85%; R = Ph,
R = Ph, R' = n-C6H13
R' = p-NH2C6H4, 1l, 80%; R = Ph, R' = p-ClC6H4, 1o, 75%; R = Ph,
1m 1t
P(O)Ph2
R' = p-CF3C6H4,
, 89%; R = Ph(CH2)4, R' = n-C14H29
,
.
P(O)Ph2
Scheme 2. Air-induced double additions conducted under a solvent-free reaction
R
1
R
condition. Reactions were conducted in a 10 mL glass tube.
Ph2P(O)H
Ph2P(O)H
A possible reaction path for this air-induced double addition
of Ph2P(O)H to alkynes were shown in Scheme 3.7a,10 It is
O2
P(O)Ph2
P(O)Ph2
P(O)Ph2
Ph2(O)P
assumed that first, a phosphoryl radical
reaction of Ph2P(O)H with air. then adds to a terminal alkyne
to eventually produce an alkenylphosphoryl compound
(mono P(O)H addition). Further radical addition of to
A is generated via the
R
A
R
B
A
D
C
C
A
P(O)Ph2
would produce the corresponding product 1,2-bisphosphoryl
compounds . The addition of with Ph2P(O)H under radical
Ph2P(O)H
R
1
C
C
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Scheme 3.
A
plausible mechanism for the air-induced double addition of P(O)-H
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5
6
In conclusion, we have disclosed
a practically clean
approach to the valuable bisphosphinylethanes. This reaction
was triggered by a trace amount of air and took place
efficiently to produce various bisphosphinylethanes in good
yields. The generated adducts could be easily isolated in pure
form via simple work-up.
7
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