Direct Synthesis of Dibenzophospholes from Biaryls by Double C-P Bond Formation via Phosphenium Dication Equivalents

Article abstract of DOI:10.1021/acs.orglett.0c00944

We have developed a new strategy for the generation of phosphenium dication equivalents from readily available phosphinic acids and Tf₂O. The in situ-generated dication equivalents can be readily coupled with simple (hetero)biaryls to form the corresponding dibenzophospholes directly. This protocol can also be applied to the concise synthesis of six- A nd seven-membered phosphacycles as well as the largely ?-extended heteroacene derivatives, which are of great interest in the field of organic functional materials.

Full text of DOI:10.1021/acs.orglett.0c00944

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Letter
Direct Synthesis of Dibenzophospholes from Biaryls by Double C−P
Bond Formation via Phosphenium Dication Equivalents
Kazutoshi Nishimura, Koji Hirano,* and Masahiro Miura*
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ABSTRACT: We have developed a new strategy for the generation of phosphenium dication equivalents from readily available
phosphinic acids and Tf2O. The in situ-generated dication equivalents can be readily coupled with simple (hetero)biaryls to form the
corresponding dibenzophospholes directly. This protocol can also be applied to the concise synthesis of six- and seven-membered
phosphacycles as well as the largely π-extended heteroacene derivatives, which are of great interest in the field of organic functional
materials.
Scheme 2. Working Hypothesis (L = neutral Lewis base)
dibenzophosphole is a key motif in the design of
Aphosphorus-containing functional organic materials such
as light-emitting diodes and photovoltaic devices.1 Addition-
ally, its six-2 and seven-membered3 analogues also show
characteristic optoelectronic and physical properties. Accord-
ingly, synthetic chemists have focused considerable attention
on the concise construction of the phosphacyclic framework by
the development of efficient C−P bond-forming strategies.
The most classical and most reliable protocols are the
substitution reactions of the dilithiated biaryl compounds
with dichlorophosphines RPCl2 (Scheme 1, path a)4 and
radical substitution reactions of dibromobiaryls with the
specially designed RP(SnMe3)2 reagents (path b).5 Recently,
the cyclization of biarylphosphine derivatives via C−H,6 C−
X,7 or C−P8 bond cleavage has also been developed as the
simpler and relatively functional group-tolerated alternative
strategy (path c). However, the procedures mentioned above
still suffer from the limited functional group compatibility and
somewhat tedious preparation of prefunctionalized starting
substrates and/or reagents.
Scheme 1. Approaches to Dibenzophosphole Derivatives via
C−P Bond Formation
Meanwhile, our research group recently focused on the
synthetic potential of highly electrophilic and coordinately
unsaturated phosphenium cations9 and developed a metal-free,
Received: March 16, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00944
© XXXX American Chemical Society
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A

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a
Scheme 3. Direct Synthesis of Dibenzophospholes 3 by Double C−P Bond Formation of Biaryls 2 with Phosphinic Acids 1
a
Reaction conditions: 1 (2.0 equiv), 2 (0.10 mmol), Tf2O (4.8 equiv), DMAP (4.8 equiv), DCE (1.5−3.0 mL), N2. See the Supporting
b
Information for detailed conditions for each substrate. Isolated yields are shown. The formed C−P bonds are illustrated with a bold line. On a 1.0
mmol scale for 168 h. At 70 °C for 168 h. With 1a (10 equiv) and Tf2O/DMAP (24 equiv).
c
d
two OH groups could be activated with 2 equiv of Tf2O, the
corresponding RP(OTf)2 would form. Two OTf ligands are
then kicked out by the external neutral Lewis base (L) to
generate highly electrophilic, coordinately more unsaturated
phosphenium P(III) dication equivalent. Subsequent inter- and
intramolecular phospha-Friedel−Crafts (PFC)-type reaction-
s2a,b,14 with the biaryl successfully deliver the targeted
dibenzophosphole derivative.
Figure 1. Six- and seven-membered phosphacycles prepared by
double C−P bond formation.
On the basis of the aforementioned scenario, we began
optimization studies with phenylphosphinic acid (1a; 0.20
mmol) and N-methyl-2-phenylindole (2a; 0.10 mmol) as the
phosphenium dication precursor and model (hetero)biaryl
substrate, respectively (Scheme 3). After extensive screening of
various reaction parameters, we pleasingly found that the
desired reaction proceeded in the presence of Tf2O and N,N-
dimethyl-4-aminopyridine (DMAP) in 1,2-dichloroethane
(DCE) solvent at 120 °C to form the corresponding
indolobenzophosphole oxide 3aa in 85% isolated yield. Some
observations should be noted. The effect of the base was
critical, and almost no reaction occurred without any bases
(<5%). The other potential P dication precursors such as
PhPCl2 and PhP(O)(OH)2 resulted in no formation of the
dibenzophosphole structure.15 The initially formed P(III)
benzophosphole product was spontaneously oxidized with
residual Tf2O and/or its derivatives, and oxide form 3aa was
detected exclusively even without special oxidative workup,
which is similar to our previous observation.6f,10 Additionally,
the reaction was also conducted on a 1.0 mmol scale. We then
examined the scope of double C−P bond-forming reaction.
The phenyl ring of model 2a could be replaced with several
electron-donating and -withdrawing aryl groups, and the
corresponding indolobenzophospholes 3ab−3ae were ob-
tained in good yields. The 2-naphthalene-substituted 2f
furnished 3af as the single isomer through C−P bond
formation at the more congested C1 position of naphthalene
[unambiguously confirmed by X-ray analysis (CCDC
1987698)], which is reflected by the electronically controlled
Tf2O-mediated C−P bond formation of alkynes with the
readily available secondary phosphine oxides R2P(O)H to
form the corresponding phosphinative cyclization products and
benzophospholes.10 This strategy can also be applied to the
cyclization of biarylphosphine oxides, delivering the corre-
sponding dibenzophospholes.6f Because of the continuing
interest in this chemistry, we envisioned that, if the more
coordinately unsaturated phosphenium dications can be
generated, the double C−P bond-forming reaction of simple
biaryls can form the corresponding dibenzophospholes
directly, which is highly attractive because the starting simple
biaryls are readily available through the well-established biaryl
cross-coupling chemistry. Herein, we report a Tf2O-promoted
inter- and intramolecular C−P bond formation sequence of
simple (hetero)biaryls with the easily accessible and stable
phosphinic acids RP(O)(OH)H (path d). This newly
developed protocol allows the rapid synthesis of not only
dibenzophospholes but also six- and seven-membered
phosphacycles and largely π-extended P-containing hetero-
acene derivatives. A related reaction of more reactive 1,3-
dienes with dichlorophosphines RPCl2 to provide the
monocyclic phospholes is known as the McCormack
reaction,11 but the extension to the biaryl system remains
largely elusive.12
Our initial working hypothesis is shown in Scheme 2. The
phosphinic acid RP(O)(OH)H undergoes tautomerization to
the corresponding dihydroxylphosphine RP(OH)2.13 If the
B
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Scheme 4. Direct Synthesis of S,P- (3ar−3at) and O,P-Acenes (3au) via Four-Fold C−P Bond Formation
Scheme 5. 31P{1H} NMR Studies
PFC-type reaction mechanism: the more electron-rich C1
position selectively reacted with 1a over the C3 position. In
addition to the indoles, benzothiophenes 2g and 2h and
benzofuran 2i could be converted to benzothienobenzophosp-
holes 3ag and 3ah and benzofuranobenzophosphole 3ai,
respectively. Notably, in the case of benzothiophene substrates,
both ladder-type (3ag) and bent-type (3ah) products were
readily obtained. Bis(heteroaryl)s such as bisindole 2j and
bisbenzothiophene 2k also underwent the double PFC
reaction to afford N,P- and S,P-acenes 3aj and 3ak,
respectively, in acceptable yields. Particularly notable is the
successful application to simple biaryls: 4,4′-di(tert-butyl)-
biphenyl (2l) and 3-methoxybiphenyl (2m) reacted with the
phosphinic acid 1a to regioselectively produce dibenzophop-
holes 3al and 3am, respectively, in synthetically useful yields.
In the case of 3al, the steric bulkiness of tert-Bu groups is an
important key for controlling the regioselectivity. Actually, less
hindered 4,4′-dimethylbiphenyl provided a complicated
mixture (data not shown). Also in the reaction of 1l, a small
amount of simply ortho-phosphinated byproducts was detected
but not fully identified. The simple 1,1-diphenylethylene (2n)
instead of the biaryl was also viable, and 3-phenylbenzophosp-
hole 3an was isolated in 69% yield, in which the free C2−H
can be easily modified via a halogenation and metal-catalyzed
cross-coupling.16 We also investigated the substitution effect
on phosphinic acid 1. The electron-deficient CF3-substituted
1b showed an efficiency similar to that of parent 1a, whereas
the electron-rich MeO-substituted 1c decreased the yield of
the product (3ba, 3ca, and 3bl).17
This strategy was also extended to six- and seven-membered
phosphacycle synthesis (Figure 1). The diaryl ether and
triarylamine were directly converted to the corresponding six-
membered phenoxaphosphine 3ao and phenophosphazine 3ap
in one shot. The doubly benzothiophene-fused seven-
membered phosphepine 3aq was also readily constructed.
These results demonstrate the high potential of this
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phosphenium dication protocol for the synthesis of configura-
tionally flexible medium-sized phospha macrocycles.
containing planar acenes can also be easily constructed.
Thus, the phosphenium dication-based strategy will find wide
applications in the design and synthesis of new functional
organic materials based on the phosphole molecules.21 Further
development of related C−P bond-forming reactions with
coordinately unsaturated and highly reactive P(III) species and
applications of this concept to other heteroatom species are
currently underway and will be reported in due course.
The most salient advantage of this strategy is the direct use
of relatively simple aromatic substrates in the synthesis of
largely π-extended dibenzophosphole derivatives (Scheme 4).
2,6-Bis(2-benzothienyl)naphthalene 2r, which can be readily
prepared by the Suzuki−Miyaura coupling, was transformed
via 4-fold C−P bond formation to the highly condensed
ladder-type S,P-acenes 3ar in 57% yield. Its bent-type isomer
3as was also accessible under the same conditions from 2,6-
bis(3-benzothienyl)naphthalene 2s. Moreover, the organic
semiconductor scaffold 2t was easily modified with the two
phosphole rings (3at). Furthermore, this double cyclization
could be directly applied to the construction of highly fused
O,P-acene 3au based on the six-membered ring system. Similar
to the case of 3af, the regioselectivity was perfectly controlled
in all cases, which was unambiguously confirmed by X-ray
crystallographic analysis (CCDC 1987699, 1987700, 1987701,
1989209, 1989210, and 1989211).
Although we initially hypothesized the formation of the
highly coordinately unsaturated phosphenium P(III) dication
(Scheme 2), an alternative process via phosphonium P(V) or
phosphenium P(III) monocations is also plausible. To gain
mechanistic insight, we finally performed 31P{1H} NMR
studies in CDCl3 solutions. Upon treatment of PhP(O)-
(OH)H (1a, δ 23 ppm) with 2.0 equiv of Tf2O at 60 °C, some
unidentified signals were observed (Figure S14). However, the
addition of 2.0 equiv of DMAP formed two new signals at δ
185 ppm and δ 110 ppm, which are assigned to PhP(OTf)2
and PhP(DMAP)2(OTf)2, respectively (Figure S15). Actually,
the almost same signal (δ 180 ppm) appeared by simple mixing
of PhPCl2 (δ 161 ppm) and 2.0 equiv of AgOTf (Figure S16),
thus suggesting that the former is PhP(OTf)2. However, the
reported 31P{1H} chemical shift of the latter PhP-
(DMAP)2(OTf)2 in CD2Cl2 is δ 121 ppm,18 which is not
fully consistent with the observed value. Given the labile and
reversible coordinating nature of monodentate DMAP under
Tf2O-mediated conditions, we then tested the more rigidly
coordinating tridentate terpyridine derivative t-Bu-terpy
instead of DMAP (Scheme 5). Gratifyingly, we observed the
distinct signal at δ 28 ppm, which is typical to the
tetracoordinated P(III) center (Figure S17). The control
experiment of PhPCl2 and 2.0 equiv of TMSOTf in the
presence of t-Bu-terpy also provided the same signal at δ 28
ppm (Figure S18), which can support the formation of
tetracoordinated phosphenium dication PhP(t-Bu-terpy)-
(OTf)2.19 The outcomes described above indicate the
intermediacy of the phosphenium dication, rather than P(V)
phosphonium and P(III) phosphenium monocations, in the
present double C−P bond formation with the phosphinic acids
and Tf2O.20 However, the details of the C−P bond-forming
step (stepwise or concerted) remain unclear, and further
studies are essential.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00944.
1H, 13C{1H}, and 19F{1H} NMR spectra, ORTEP
drawing, and detailed optimization studies (PDF)
Accession Codes
CCDC 1987698−1987701 and 1989209−1989211 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.
AUTHOR INFORMATION
Corresponding Authors
■
Koji Hirano − Department of Applied Chemistry, Graduate
School of Engineering, Osaka University, Suita, Osaka 565-
0871, Japan; orcid.org/0000-0001-9752-1985;
Email: k_hirano@chem.eng.osaka-u.ac.jp
Masahiro Miura − Department of Applied Chemistry, Graduate
School of Engineering, Osaka University, Suita, Osaka 565-
0871, Japan; orcid.org/0000-0001-8288-6439;
Email: miura@chem.eng.osaka-u.ac.jp
Author
Kazutoshi Nishimura − Department of Applied Chemistry,
Graduate School of Engineering, Osaka University, Suita,
Osaka 565-0871, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00944
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grants JP
18K19078 [Grant-in-Aid for Challenging Research (Explor-
atory)] to K.H. and JP 17H06092 (Grant-in-Aid for Specially
Promoted Research) to M.M. The authors thank Dr. Yuji
Nishii (Osaka University) for his assistance with X-ray analysis.
In conclusion, we have developed a new protocol for the
generation of highly coordinately unsaturated phosphenium
dication equivalents from the readily available phosphinic acids
and Tf2O and succeeded in the inter- and intramolecular
phospha-Friedel−Crafts reaction sequence under metal-free
conditions. This strategy allows the relatively simple (hetero)-
biaryls to undergo double C−P bond formation to form the
corresponding dibenzophospholes in one shot. Moreover, the
more configurationally flexible six- and seven-membered
phosphacycles as well as highly π-extended phosphorus-
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