Highly Selective Phosphinylphosphination of Alkenes with Tetraphenyldiphosphine Monoxide

Article abstract of DOI:10.1002/anie.201603860

In sharp contrast to tetraphenyldiphosphine, which does not add to carbon–carbon double bonds efficiently, its monoxide, [Ph₂P(O)PPh₂] can engage in a radical addition to various alkenes, thus affording the corresponding 1-phosphinyl-2-phosphinoalkanes regioselectively, and they can be converted into their sulfides by treatment with elemental sulfur. The phosphinylphosphination proceeds by the homolytic cleavage of the P^V(O)?P^IIIsingle bond of Ph₂P(O)PPh₂, followed by selective attack of the phosphinyl radical at the terminal position of the alkenes, and selective trapping of the resulting carbon radical by the phosphino group. Furthermore, the phosphinylphosphination product could be converted directly into its platinum complex with a hemilabile P,O chelation.

Full text of DOI:10.1002/anie.201603860

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Communications
Chemie
International Edition: DOI: 10.1002/anie.201603860
German Edition: DOI: 10.1002/ange.201603860
Phosphanes
Highly Selective Phosphinylphosphination of Alkenes with
Tetraphenyldiphosphine Monoxide
Yuki Sato, Shin-ichi Kawaguchi,* Akihiro Nomoto, and Akiya Ogawa*
Abstract: In sharp contrast to tetraphenyldiphosphine, which
does not add to carbon–carbon double bonds efficiently, its
monoxide, [Ph2P(O)PPh2] can engage in a radical addition to
various alkenes, thus affording the corresponding 1-phosphin-
yl-2-phosphinoalkanes regioselectively, and they can be con-
verted into their sulfides by treatment with elemental sulfur.
The phosphinylphosphination proceeds by the homolytic
presence of a radical initiator [AIBN or V-40 (1,1’-azobis-
(cyclohexane-1-carbonitrile)] or under photoirradiation
(Scheme 1a). The addition reaction did not proceed at all,
V
III
À
cleavage of the P (O) P single bond of Ph2P(O)PPh2,
followed by selective attack of the phosphinyl radical at the
terminal position of the alkenes, and selective trapping of the
resulting carbon radical by the phosphino group. Furthermore,
the phosphinylphosphination product could be converted
directly into its platinum complex with a hemilabile P,O che-
lation.
O
rganophosphorus compounds play a vital role in organic
synthesis, catalysis, materials chemistry, medicinal chemistry,
and coordination chemistry.[1] Therefore, the development of
new synthetic methods to create organophosphorus species is
of great importance. For the selective introduction of
phosphorus moieties onto organic molecules, the addition
reaction to carbon–carbon multiple bonds is one of the most
useful and atom-economical methods.[2] In particular, the 1,2-
III
Scheme 1. Radical addition of the diphosphine 1 (PIII P ) and diphos-
À
V
III
À
phine monoxide 3 [P (O) P ] to alkenes.
À
addition of phosphorus compounds, bearing a P P single
bond, to carbon–carbon multiple bonds is the most straight-
forward method for the preparation of vicinal bis(phos-
phine)s,[3] which are useful bidentate ligands in transition
metal catalyzed reactions.[4] Although several attractive
examples of the addition of phosphorus compounds to
alkynes have been reported,[5] to the best of our knowledge,
the corresponding addition to alkenes has not been achieved,
with the exception of the addition of specified diphosphines,
such as tetramethyldiphosphine [(Me2P)2],[6] tetrachlorodi-
phosphine [(Cl2P)2],[7] tetrafluorodiphosphine [(F2P)2],[8] and
1,1-diaminodiphosphine,[9] to only very a few alkenes.
and 1 was recovered quantitatively. We previously reported
that the regioselective thioselenation of alkenes proceeds
smoothly using
a
disulfide–diselenide mixed system,[10]
although the addition to alkenes did not proceed efficiently
using either a disulfide or diselenide species alone. In such
heteroatom-mixed systems, two different heteroatom-cen-
tered radicals (YC, ZC) can be generated and two heteroatom-
containing compounds (Y-Y, Z-Z) with different radical
capturing abilities exist. The more reactive radical attacks
carbon–carbon multiple bonds and the generated carbon
radical is captured by the heteroatom-containing compound
with the better capturing ability (Scheme 1b).
We initiated our study by investigating the addition of
tetraphenyldiphosphine (1) to 1-dodecene (2a) in either the
With this information in mind, we selected tetraphenyldi-
V
phosphine monoxide (3), bearing a P (O) PIII single bond, as
À
an initial phosphorus compound for the radical addition to
alkenes. This choice was because the homolytic cleavage of
[*] Y. Sato, Dr. A. Nomoto, Prof. Dr. A. Ogawa
Department of Applied Chemistry, Graduate School of Engineering
Osaka Prefecture University
V
III
À
the P (O) P bond can afford two different phosphorus-
centered radicals and 3 has pentavalent and trivalent phos-
phorus atoms, which might have different radical capturing
abilities. The 1,2-addition of 3 to alkenes proceeded regio-
selectively to afford the 1-phosphinyl-2-phosphinoalkanes 4
(Scheme 1c). Recently, organophosphines with vicinal soft
(PIII) and hard [PV(O)] Lewis-base centers[11] were used as
hemilabile ligands in transition metal catalyzed reactions.[12]
Furthermore, the reduction of 4 readily affords bidentate
bis(phosphine) ligands. Therefore, we set out to investigate
1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531 (Japan)
E-mail: ogawa@chem.osakafu-u.ac.jp
Dr. S-i. Kawaguchi
Center for Education and Research in Agricultural Innovation
Faculty of Agriculture, Saga University
152-1 Shonan-cho Karatsu, Saga 847-0021 (Japan)
E-mail: skawa@cc.saga-u.ac.jp
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201603860.
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the novel phosphinylphosphination of alkenes with diphos-
phine monoxides.
NMR tube afforded 6a in a good yield, although a prolonged
reaction time was required (entry 6).
When a mixture of 3 (0.3 mmol), 2a (0.3 mmol), and V-40
(0.03 mmol, 10 mol%) was dissolved in benzene and refluxed
for 20 hours, the complete consumption of 2a and the
formation of the phosphinylphosphination product 4a,
which has a diphenylphosphinyl group at the terminal
The phosphinylphosphination of various alkenes with 3
was conducted in the presence of either a catalytic amount of
V-40 (Conditions A) or under photoirradiation (Condi-
tions B; Table 2). The vinyl ether 2b reacted with 3 efficiently.
After the mixture was treated with S8 (0.9 mmol) at 608C for
5 hours, the BPOS 6b was obtained in a good yield. Allyl
ethers afforded the corresponding 1,2-adducts 6c–h in
excellent yields under photoirradiation. The reaction was
tolerant of various functional groups such as fluoro (2e),
methoxy (2 f), ester (2g), and nitrile (2h) groups. The use of
the allyl ester 2i also gave the corresponding 1,2-adduct 6i in
a good yield. Cyclohexane-containing species (2j and 2k)
gave the corresponding BPOSs in good yields. Styrene (2l)
did not undergo the phosphinylphosphination at all, because
1
carbon atom, were confirmed by H and 31P NMR spectros-
copy (Scheme 2). The regioisomer 5a was not detected, and
Table 2: Phosphinylphosphination of several alkenes 2 with diphosphine
monoxide 3.[a]
Scheme 2. Phosphinylphosphination of 2a with 3.
4a was converted into the bis(phosphino)alkane 1-P-oxide 2-
P-sulfide (BPOS) 6a upon treatment with elemental sulfur.
Since a small amount of the self-polymerization product
of 2a was observed, the ratio of 3 to 2a, initiator, and
temperature were screened (Table 1) to identify the optimal
reaction conditions. When the reaction was conducted at low
temperatures, such as 30 and 608C, the self-polymerization of
2a preferentially occurred (entries 2 and 3). In contrast, at
high temperature (808C) and with the use of excess 3
suppressed the polymerization effectively and thus improved
the yield of 6a (entry 4). The phosphinylphosphination did
not take place without a radical initiator (entry 5). Photo-
irradiation with a xenon lamp (500 W) through a sealed Pyrex
Table 1: Optimization of the phosphinylphosphination of alkene 2a with
3.[a]
Entry
3 (mmol)
Initiator
T [8C]
Yield [%][b]
1
2
0.30
0.30
0.30
0.45
0.45
0.90
V-40
AIBN
V-70
V-40
–
80
60
30
80
80
20
50
23
14
79 (72)
0
3[c]
4
5
6[c,d]
xenon lamp
78 (72)
[a] Reaction conditions: 2a (0.3 mmol), benzene (0.6 mL). [b] Deter-
mined by 1H NMR spectroscopy. Isolated yields are shown within
parentheses. [c] The substrates were dissolved in CH2Cl2. [d] The mixture
was irradiated with a xenon lamp (500 W) through a sealed Pyrex NMR
tube for 55 h at room temperature. V-40=1,1’-azobis(cyclohexane-1-
carbonitrile), V-70=2,2’-azobis(4-methoxy-2,4-dimethylvaleronitrile),
AIBN=2,2’-azobis(isobutyronitrile).
[a] Reaction conditions A: 3 (0.45 mmol), 2 (0.3 mmol), V-40
(0.03 mmol), benzene (0.6 mL), 808C, and 20 h. After the reaction, the
resulting mixture was treated with S8 (0.9 mmol) at 608C for 5 h.
Reaction conditions B: 3 (0.6 mmol), 2 (0.2 mmol), CD2Cl2 (0.4 mL),
xenon lamp (500 W), 40 h, room temperature, and subsequent treatment
with S8. [b] The mixture was irradiated for 55 h. [c] The mixture was
irradiated for 20 h. [d] Phenylacetylene (0.3 mmol) was added.
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of preference for the self-polymerization. In contrast, the
reaction of allylbenzene (2m) proceeded to afford BPOS 6m
in a moderate yield. Furthermore, the terminal alkyne 2n also
served as a viable substrate to give the 1,2-adduct 6n in a good
yield with excellent stereoselectivity.
The present reaction is a novel and straightforward
method to synthesize 1-phosphinyl-2-phosphinoalkanes,
which can be used as ligands for transition metal catalyzed
reactions.[12] Therefore, we set out to create a 4d/transition
metal complex using the present method (Scheme 3). When
a mixture of 3 (0.6 mmol) and 2d (0.2 mmol) was photo-
irradiated, the 1-phosphinyl-2-phosphinoalkane 4d was
Scheme 5. Photoinduced homolytic substitution of 1 and 3 with
isopropyl radical.
contrast to 1, which did not react with the generated isopropyl
radical, 3 caused homolytic substitution at the trivalent
obtained in
a
high yield. Subsequently, PtCl2(PhCN)2
phosphorus
atom[16]
to
generate
isopropyl-
(0.2 mmol) was added to the resulting reaction mixture, and
the platinum complex 7d was successfully isolated in 66%
yield.
(diphenyl)phosphine in 40% yield. This result strongly
indicates that the p-type alkyl radical capturing ability of 3,
such as isopropyl radicals, is much higher than that of 1. So the
phosphinylphosphination of various alkenes with 3 success-
fully proceeded.
A plausible reaction pathway for the present reaction is
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À
illustrated in Figure 1. The P (O) P bond cleavage of 3 is
induced by the radical initiator or near-UV light to generate
the diphenylphosphinyl radical [Ph2P(O)C].[17] The generated
radical reacts with 2 at the terminal carbon atom to generate
the alkyl radical 10. The alkyl radical 10 is captured by 3 to
give 4 by an SH2-type mechanism.
Scheme 3. The facile preparation of platinum(II)/bis(phosphine) mon-
oxide complex 7d.
Preliminary density functional theory (DFT) calculations
(B3LYP/6-31G(d)) were performed to explain the observed
regioselectivity of the reaction. Regarding the addition of 3 to
2o (R = CH3 in Figure 1a), the single occupied molecular
orbital (SOMO) level of 10o (À4.85 eV) was similar to the
highest occupied molecular orbital (HOMO) level of 3
(À5.74 eV), which is localized on the trivalent phosphorus
To shed light on the reaction pathway, the diene 8 was
mixed with 3 in the presence of a catalytic amount of V-40
(Scheme 4). As a result, the cyclized product 9 (anti/syn = 5/1)
Scheme 4. Phosphinylphosphination of diene 8 with 3.
was obtained and the noncyclized 1,2-adducts were not
detected at all. This result strongly indicates that the
phosphinylphosphination proceeds by a radical pathway. In
addition, the rate of carbon radical capture by 3 was estimated
to be 5 ꢀ 103 sÀ1mÀ1 at most, because the rate for the 5-exo
cyclization of 5-hexenyl radical is 4 ꢀ 105 sÀ1.[13] The carbon
radical capture rate of 3 is very slow considering the rate
constant for the radical attack of diphenylphosphinyl radical
[14]
on isoprene [k = (1.4 Æ 0.3) ꢀ 107 sÀ1mÀ1
]
and phenyl vinyl
ether [k = (2.6 Æ 0.2) ꢀ 106 sÀ1mÀ1].[15] Therefore, the key step
of the phosphinylphosphination is the carbon radical capture
by 3.
To clarify the difference in reactivity between 1 and 3, we
conducted the reaction between 2-bromopropane (0.2 mmol)
and either 1 or 3 (0.6 mmol) with hexabutylditin (0.2 mmol)
under photoirradiation (Scheme 5). In this system, an isopro-
pyl radical was generated in situ by the abstraction of bromine
Figure 1. a) A plausible reaction pathway for the highly regioselective
phosphinylphosphination of 3 to alkenes. b) Kohn–Sham molecular
orbitals (HOMO and LUMO) of 3 calculated at the B3LYP/6-31G(d)
level of theory.
À
from 2-bromopropane, and it was caused by the Sn Sn bond
cleavage of hexabutylditin upon near-UV irradiation. In sharp
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level of 3 was À1.10 eV. This result indicates that the reactive
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In summary, we have developed a protocol for the highly
regioselective addition of tetraphenyldiphosphine monoxides
to a variety of alkenes. This approach enables the regiose-
lective introduction of diphenylphosphino and diphenylphos-
phinyl groups on various aliphatic alkenes and phenylacety-
lene. A plausible reaction pathway involving a regioselective
radical capturing process was proposed. We believe this
method will facilitate significant opportunities in the synthesis
of bis(phosphine) monoxide ligands.
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Acknowledgments
This research was supported by a Grant-in-Aid for Explor-
atory Research (26620149, A.O., 26860168, S.K.) from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan.
Keywords: phosphanes · phosphorus · radicals · regioselectivity
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[17] The diphenylphosphino radical (Ph2P·) is also generated by near-
UV light. However, Ph2P(O)C is more reactive with alkenes than
Ph2PC and initiates the radical-chain reaction. In this reaction, the
P (O) P bond cleavage, induced by near-UV light, is much
slower than the radical chain reaction.
[18] 1,2-Bisphosphinoalkane was obtained in < 1% yield.
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Received: April 21, 2016
Published online: && &&, &&&&
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Communications
Phosphanes
Y. Sato, S-i. Kawaguchi,* A. Nomoto,
A. Ogawa*
&&&& — &&&&
Highly Selective
Phosphinylphosphination of Alkenes with
Tetraphenyldiphosphine Monoxide
P(O)P by: Ph2P(O)PPh2 can engage in
radical addition to various alkenes, thus
affording the corresponding 1-phosphin-
yl-2-phosphinoalkanes regioselectively,
and they can be converted into their
sulfides by treatment with S8. The reac-
tion proceeds by homolytic cleavage of
V
III
À
the P (O) P bond of Ph2P(O)PPh2,
selective attack of the phosphinyl radical
at the terminal position of the alkene, and
selective trapping of the resulting carbon
radical by the phosphino group.
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