Cyclization of Bisphosphines to Phosphacycles via the Cleavage of Two Carbon-Phosphorus Bonds by Nickel Catalysis

Article abstract of DOI:10.1021/acs.orglett.9b01355

The nickel-catalyzed cyclization of bisphosphine derivatives to form various phosphacycles is reported. The reaction proceeds via the cleavage of two carbon-phosphorus bonds of the bisphosphine. Unlike the previously reported palladium catalysts, the use

Full text of DOI:10.1021/acs.orglett.9b01355

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cyclization of Bisphosphines to Phosphacycles via the Cleavage of
Two Carbon−Phosphorus Bonds by Nickel Catalysis
Hayato Fujimoto, Momoka Kusano, Takuya Kodama, and Mamoru Tobisu*
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
S
* Supporting Information
ABSTRACT: The nickel-catalyzed cyclization of bisphos-
phine derivatives to form various phosphacycles is reported.
The reaction proceeds via the cleavage of two carbon−
phosphorus bonds of the bisphosphine. Unlike the previously
reported palladium catalysts, the use of nickel as a catalyst
allows for the cyclization that requires C(alkyl)−P bond
cleavage. A phospha-nickelacycle intermediate was successfully
isolated and characterized by X-ray crystallography.
ertiary phosphines are the essential components of
transition metal catalysts in regulating the reactivity and
Scheme 1. Metal-Mediated C−P Bond Exchange and Its
T
Application to the Catalytic Synthesis of Phospholes
selectivity of the catalytic reactions. In phosphine-ligated
metal-catalyzed processes, carbon−phosphorus (C−P) bond
cleavage is frequently encountered as an undesired side
reaction.1 For example, in palladium-catalyzed cross-coupling
reactions of aryl halides, the aryl group of a triarylphosphine
ligand has been known to be incorporated to the product in
place of the aryl group of the aryl halide substrate.2 Such an
undesired reaction proceeds via a C−P bond exchange
pathway (Scheme 1a), which can be mediated by several
metal species with palladium being most active.3 Although this
intermolecular aryl group exchange has limited synthetic utility
due to the reversibility of the reaction,4 Morandi et al.5 and our
group6 independently reported an intramolecular variant,
which enables the palladium-catalyzed selective synthesis of
dibenzophosphole derivatives from readily available bisphos-
phines (Scheme 1b). We report herein that this intramolecular
C−P bond exchange reaction can be catalyzed by nickel.
Nickel complexes are much less active in mediating C−P bond
exchange reactions,3b and there are no general catalytic
reactions7 involving the cleavage of C−P bonds of tertiary
phosphines, except for one isolated example using highly
strained methylenecyclopropa[b]naphthalenes.8 The C−P
bond exchange reaction is mediated most efficiently by
ArPdX species, through reductive elimination to form
phosphonium salt A and Pd(0), followed by oxidative addition
of a C−P bond of A9 to provide an exchanged product
(Scheme 1c, M = Pd). It is generally accepted that nickel can
mediate the oxidative addition process more efficiently than
palladium, whereas reductive elimination by nickel would
require a higher barrier than palladium.10 Therefore, nickel
would be expected to exhibit a complementary reactivity to
palladium in some of the elementary steps of the catalytic
cycle, thus potentially allowing for a transformation that would
be unique to nickel.
using Ni(cod)2 as a nickel(0) catalyst (Table 1). To our
delight, the desired phosphole was formed in 8% without the
need for any additives (entry 1). Because phosphole derivatives
are sensitive to oxygen, the product was quantified as the air-
stable phosphole oxide 2a after an oxidative workup with
H2O2. Inspired by the seminal work by Morandi et al.,5 in
which added PhI serves as a cocatalyst to facilitate the
We initially investigated the nickel-catalyzed cyclization of
2,2′-bis(diphenylphosphino)-1,1′-binaphthyl (BINAP, 1a)
Received: April 17, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01355
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Nickel-Catalyzed Cyclization of 1a to 2a
Scheme 2. Reaction Scope
entry
catalyst
additive
solvent
toluene
toluene
toluene
toluene
1,4-dioxane
DMF
NMR yield of 2a/%
1
2
3
4
5
6
7
8
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(OAc)2
Ni(OTf)2
none
Phl
PhBr
PhOTf
PhOTf
PhOTf
PhOTf
PhOTf
8
19
23
79
61
b
99 (84)
toluene
toluene
trace
trace
a
Reaction conditions: 1a (0.20 mmol), Ni cat. (0.020 mmol), and
additive (0.020 mmol) in solvent (1.0 mL) at 140 °C for 20 h. The
NMR yield was determined using 1,1,2,2,-tetrachloroethane as an
b
internal standard. Isolated yield.
formation of a phosphonium salt, a key intermediate, we
examined a nickel catalyst in conjunction with an aryl halide.
The addition of a catalytic amount of PhI (entry 2) and PhBr
(entry 3) increased the yield of 2a to ca. 20%. We were then
able to increase the yield to 79% when PhOTf was used as a
cocatalyst (entry 4). The use of DMF (entry 6) allowed the
reaction to proceed quantitatively to give 2a in 84% isolated
yield, along with a stoichiometric amount of triphenylphos-
phine oxide, thus confirming the fate of the cleaved
phosphorus residue. The use of Ni(OAc)2 (entry 7) and
Ni(OTf)2 (entry 8) as nickel(II) catalysts failed to promote
the reaction.
The scope of this nickel-catalyzed cyclization reaction was
examined using a series of commercially available bisphosphine
derivatives (Scheme 2). The scope of the substituent on the
phosphorus atom was initially examined. Aryl groups, such as
p-tolyl (1b) and 3,5-xylyl (1c), were found to participate in
this cyclization to provide the corresponding phosphole
derivatives 2b and 2c in excellent yields. The cyclization
reaction is not limited to a binaphthyl-based skeleton, but a
simpler biphenyl system, including 1d−1h, also participated
successfully in the reaction. Although these bisphosphines have
a diverse range of bite angles,11 which could potentially affect
the efficiency of the reaction, all provided the corresponding
phospholes 2d−2h in >90% yields. It was also possible to
synthesize heterocyclic (2i) and six-membered (2j) phospha-
cycles using the corresponding bisphosphines 1i and 1j under
these nickel-catalyzed conditions.
Apart from the earth-abundant nature, the use of nickel in
place of palladium provides an opportunity to realize some
unique transformations. Since oxidative addition occurs more
readily in the presence of nickel than palladium,10 a nickel
catalyst shows superior reactivity to a palladium catalyst in
cyclization reactions of electron-rich substrates (Scheme 3).
For example, a bulky and electron-rich bisphosphine 1k
(DTBM-SEGPHOS) can be cyclized more efficiently using a
nickel-catalyzed method (method A) than the previously
reported palladium-catalyzed methods (methods B and C) to
form the elaborate phosphole 2k. Another example involves
the cleavage of more challenging C(sp3)−P bonds. The
cyclization of an alkylene linked bisphosphine such as 1,4-
a
Reaction conditions: bisphosphine (0.20 mmol), Ni(cod)2 (0.020
mmol), PhOTf (0.020 mmol) in DMF (1.0 mL) for 20 h at 140 °C.
Yields of isolated products are given. 0.10 mmol scale. Run on a 3.0
mmol scale using a NiCl(2-Np)(PCy3)2 catalyst. 1.05 g of 2f was
obtained. Run using 0.040 mmol of Ni(cod)2 and PhOTf. Isolated
as a mixture of triphenylphosphine oxides (1:1).
b
c
d
e
bis(diphenylphosphino)butane (1l, DPPB) failed to proceed
under palladium-catalyzed conditions.6,12 However, to our
delight, when the Ni(cod)2/PhOTf was used, 1l underwent
cyclization to form the aliphatic phosphacycle 2l in 50% yield.
The combination of Pd2(dba)3/PhOTf was also studied, but
cyclization of 1l did not proceed. The phosphacyle 2l can be
used as a framework of the phosphine organocatalyst of
catalytic Appel,13 Staudinger,14 Wittig15 and Mitsunobu16
reactions. Although conventional synthetic methods of 2l
involve the use of reactive phosphorus reagents, this nickel-
catalyzed method gives 2l from commercially available and
stable bisphosphine derivatives. It should be noted that
selectivity between C(sp2)−P and C(sp3)−P bonds of the
phosphonium salt intermediates would not be expected to
significantly affect the product selectivity, since the reverse
selectivity simply results in the reverse reaction to form the
previous intermediate and the compound would still
participate in the productive catalytic cycle.
The reaction is presumably initiated by the oxidative
addition of PhOTf to Ni(0) to form a Ni(OTf)Ph-
(bisphosphine), which then serves as a catalytically active
species. To confirm this possibility, NiCl(2-Np)(PCy3)2 (2-Np
= 2-naphthalenyl)17 was synthesized as a model of Ni(OTf)-
Ph(bisphosphine), and its catalytic activity was examined
(Scheme 4a). The BINAP 1a provided the corresponding
B
DOI: 10.1021/acs.orglett.9b01355
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
22.8 ppm with JPP = 318 Hz) were observed (Figure S2).
Single-crystal X-ray diffraction of the crystallized material
revealed that the six-membered phospha-nickelacycle complex
3 was formed via the cleavage of a C−P bond of 1a (Scheme
4c). Hartwig et al.18 reported on a similar phospha-nickelacycle
by the reaction of a BINAP-ligated Ni(0) complex with an
electron-rich aryl chloride. During the course of the formation
of the phospha-nickelacycle 3, the 2-naphthyldiphenylphos-
phine [PPh2(2-Np)], the cleaved phosphorus residue, was also
observed by FAB-MS. In addition, a BINAP-ligated Ni(I)
complex 4, which is formed by the bimolecular reductive
elimination of 2,2′-binaphthalene from the BINAP-ligated
Ni(II) complex, was also formed (Figure S3). The phospha-
nickelacycle 3 could be used to catalyze the cyclization of 1a to
form 2a in 98% yield, even in the absence of PhOTf, whereas
the Ni(I) complex 4 showed no catalytic activity. Heating the
phospha-nickelacycle 3 in DMF at 100 °C for 10 h afforded 2a
in 73% yield. In this process, the addition of KOTf accelerated
the formation of 2a by approximately 2-fold (Figure S4),
probably by decreasing the electron density of the nickel center
through exchanging the ligand from Cl to OTf.19 These results
indicate that reductive elimination to form the cyclic
phosphonium salt20 is the turnover limiting step of this
reaction, and the role of a PhOTf cocatalyst is best rationalized
to facilitate this C−P bond forming reductive elimination.
In conclusion, we report on the nickel-catalyzed cyclization
of bisphosphines to diverse phosphacycles via the cleavage of
two C−P bonds. The method features nickel-catalyzed C−P
bond cleavage, which allows for the transformation of bulky
and C(sp3)−P bonds. Detailed studies related to the
mechanism of this reaction revealed that the phospha-
nickelacycle 3 is a key intermediate. The formation of the
C−P cleavage complex 3, even at 60 °C, indicates that
precautions are needed when evaluating nickel-catalyzed
reactions using BINAP-related bisphosphine ligands, since
undesired ligand decomposition via C−P bond cleavage may
be involved. Applications of this method to the synthesis of
other heterocycle through carbon−heteroatom bond cleavage
are currently being investigated in our laboratory.
Scheme 3. Comparison with Palladium Catalyst
Reaction conditions: method A, Ni(cod)2/PhOTf; method B,6
a
[(allyl)PdCl]2; method C,5 Pd2(dba)3/PhI. Isolated yield. Isolated as
b
a mixture with triphenylphosphine oxide after second reduction/
oxidation (see the SI for details).
Scheme 4. Mechanistic Studies
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01355.
Experimental details and characterization data (PDF)
Accession Codes
CCDC 1907794 and 1907915 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
a
Molecular structure of the nickelacycle 3 with thermal ellipsoids at
50% probability (except for the phenyl and cyclohexyl groups) and all
the hydrogen atoms are omitted for clarity.
AUTHOR INFORMATION
■
phosphole 2a in 85% yield, even in the absence of PhOTf.
Stoichiometric experiments were performed to gain additional
insights into the reaction mechanism (Scheme 4b). When
NiCl(2-Np)(PCy3)2 was reacted with 1.0 equiv of BINAP 1a
at 60 °C in THF-d8, 31P NMR signals corresponding to two
mutually trans phosphine resonances (doublets at 16.6 and
Corresponding Author
*E-mail: tobisu@chem.eng.osaka-u.ac.jp.
ORCID
Mamoru Tobisu: 0000-0002-8415-2225
C
DOI: 10.1021/acs.orglett.9b01355
Org. Lett. XXXX, XXX, XXX−XXX

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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI (18H01978)
and Scientific Research on Innovative Area “Hybrid Catalysis”
(18H04649)from MEXT, Japan. The authors thank Dr.
Hiroyasu Sato (Rigaku Corporation) for the single crystal X-
ray crystallographic analysis. We also wish to thank the
Instrumental Analysis Center, Faculty of Engineering, Osaka
University, for assistance with HRMS and Elemental Analyses.
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