Regioselective Synthesis of Benzo[b]phosphole Derivatives via Direct ortho-Alkenylation and Cyclization of Arylthiophosphinamides

Article abstract of DOI:10.1021/acs.orglett.6b02868

A new regioselective synthetic methodology for benzo[b]phosphole derivatives has been developed. Thus, a range of functionalized benzo[b]phosphole oxides could be synthesized via Rh(III)-catalyzed C-H alkenylation of arylthiophosphinamides with alkynes fo

Full text of DOI:10.1021/acs.orglett.6b02868

Letter
pubs.acs.org/OrgLett
Regioselective Synthesis of Benzo[b]phosphole Derivatives via Direct
ortho-Alkenylation and Cyclization of Arylthiophosphinamides
Yuto Unoh,† Yuki Yokoyama,† Tetsuya Satoh,*,†,‡ Koji Hirano,† and Masahiro Miura*,†
†Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
‡Department of Chemistry, Graduate School of Science, Osaka City University, Osaka 558-8585, Japan
S
* Supporting Information
ABSTRACT: A new regioselective synthetic methodology for
benzo[b]phosphole derivatives has been developed. Thus, a
range of functionalized benzo[b]phosphole oxides could be
synthesized via Rh(III)-catalyzed C−H alkenylation of
arylthiophosphinamides with alkynes followed by formal
phospha-Friedel−Crafts cyclization.
enzo[b]phospholes are unique and attractive building
blocks among a series of benzoheterole scaffolds, which
Meanwhile, transition-metal-catalyzed directed C−H bond
functionalization reactions have been regarded as powerful
synthetic tools from atom- and step-economical points of view.7,8
In the context of our studies on the C−H bond functionalization
of aromatic phosphorus compounds,9,10 we recently developed
the Cp*Rh(III)-catalyzed ortho-naphthylation reaction of
arylthiophosphinamides with oxabicyclic alkenes (Scheme 2).9d
B
show intriguing electronic and optical properties caused by
distinctive orbital interaction between butadiene π* and σ*(P-
R).1 While they have attracted considerable interest in the area of
materials chemistry, their synthetic methods have been less
explored than those of the isosteric indoles.2 Conventionally,
benzo[b]phosphole scaffolds have been constructed through the
cyclization of ortho-alkynylarylphosphines,3a−c H-phosphine
oxides,3d and aminophosphines.3e,f These methods often require
tedious multistep preparations of the corresponding cyclization
precursors, involving air- and moisture-sensitive intermediates.
In 2013, we4a and the Duan group4b independently reported the
Ag(I)- or Mn(III)-mediated synthesis of benzo[b]phosphole
derivatives via the direct annulation reaction of secondary
phosphine oxides with internal alkynes (Scheme 1).4 While this
Scheme 2. Rh(III)-Catalyzed C−H Bond Functionalization
Approaches to Benzo[b]phosphole Scaffolds
Scheme 1. Synthesis of Benzo[b]phosphole Derivatives by
Radical Cyclization
The naphthylated products were readily converted to fused
dibenzo[b]phosphole derivatives by intramolecular phospha-
Friedel−Crafts reaction11 in a one-pot procedure. Subsequently,
we envisioned that a regioselective synthesis of benzo[b]-
phospholes could be achieved through the ortho-alkenylation of
arylthiophosphinamides and subsequent intramolecular phos-
pha-Friedel−Crafts reaction. Herein, we report a new method for
constructing benzo[b]phosphole oxides by the C−H alkenyla-
tion/cyclization protocol.
First, we carried out optimization studies on the C−H
alkenylation using N,N-diethyl-P,P-diphenylthiophosphinamide
(1a) and diphenylacetylene (2a) as model substrates (Table S1).
Pleasingly, we found that the desired coupling reaction
proceeded smoothly in the presence of [Cp*Rh-
(MeCN)3(SbF6)2] (4 mol %) and AcOH (4 equiv) in PhCl at
protocol provides a short access to benzo[b]phosphole
derivatives, a regioselectivity problem arises when substituted
phenylphosphine oxides are employed due to the radical
intermediate involving the rearrangement of a phosphorus
moiety. After these reports, several improved methods based on a
similar radical mechanism under metal-catalyzed,5a photo-
catalyzed,5b and metal-free conditions5c,d have also been
disclosed. However, the regioselectivity issue has not been
overcome. Recently, Yoshikai and co-workers reported an
elegant multicomponent coupling for the synthesis of benzo[b]-
phosphole derivatives via carbometalation of alkynes with
arylzinc and magnesium reagents.6 Yet, development of more
efficient synthetic routes to benzo[b]phospholes is desired.
Received: September 23, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02868
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Organic Letters
Letter
120 °C to give alkenylated product 3aa in 90% isolated yield
(Scheme 3). The geometry of the alkenyl moiety was determined
to be E by X-ray crystal structure analysis (see the Supporting
Information for details).
Scheme 5. Synthesis of Benzo[b]phosphole Oxides 4 by Semi-
One-Pot Protocol
a
Scheme 3. Rh(III)-Catalyzed Regioselective C−H
Alkenylation of 1a with 2a
We next examined the second step, that is, cyclization. In our
previous report for the intramolecular phospha-Friedel−Crafts
reaction of arylthiophosphinamides, an excess amount of TfOH
was used.9d Thus, the crude product formed by the Rh-catalyzed
alkenylation in Scheme 3 was treated with TfOH under similar
conditions (Scheme 4). Unfortunately, the desired benzo[b]-
Scheme 4. Attempt at Phospha-Friedel−Crafts Reaction via
Intermediate 3aa
phosphole 4aa-S was observed only in a trace amount, and
instead, phosphonium salt 5 was formed in 73% yield as a
diastereomeric mixture. Obviously, 5 seems to be formed via
protonation of the alkenyl moiety of 3aa with TfOH and
electrophilic attack of a resulting carbocation to the PS bond.
We also examined other activating reagents including HCl, AlCl3,
BF3·OEt2, AgOTf, and Me3OBF4, but the attempts were
unsuccessful (Table S2).
Next, we examined another cyclization route via a P(III)
intermediate. Thus, 3aa was reduced to aminophosphine by
treatment with MeOTf and P(NMe2)3.12 The resulting P(III)
compound 6 was treated with TfOH. After workup with H2O2,
benzo[b]phosphole oxide 4aa was successfully obtained in 72%
yield (Scheme 5).
a
Reaction conditions: (1) 1 (0.5 mmol), 2 (0.25 mmol), [Cp*Rh-
(MeCN)3][SbF6]2 (0.01 mmol), AcOH (1.0 mmol) in PhCl (2 mL)
at 120 °C under N2 for 12 h; (2) MeOTf (1.0 mmol) in CH2Cl2 (3
mL), rt, 6 h; (3) P(NMe2)3 (1.0 mmol), rt, 1 h; (4) TfOH (0.5 mL),
rt, 12 h; (5) H2O2 workup. See the Supporting Information for details.
Isolated yields are shown based on the amount of 2. Workup was
carried out using S8 powder instead of H2O2.
The results for the synthesis of a series of benzo[b]phosphole
oxides by the semi-one-pot protocol are summarized in Scheme
5. A variety of diarylacetylenes 2a−2h smoothly coupled with 1a
to afford the corresponding benzo[b]phosphole oxides 4aa−4ah
in moderate to good yields. The reaction of a more electron-
deficient alkyne, bis(4-ethoxycarbonylphenyl)acetylene, did not
give any expected product at all. In the cases with asymmetrical
alkylarylacetylenes, the Rh-catalyzed hydroarylation reaction
proceeded regioselectively to lead to 2-aryl-3-alkylbenzo[b]-
phosphole derivatives 4ai−4aj exclusively. It is worth noting that
this regioselectivity is complementary to that of Ag(I)- or
b
out. The para-substituted 1b−1f reacted with 2c or 2d
regioselectively to give 4bc−4fc and 4dd in good yields. 3-
Methylphenyl and 2-naphtylthiophosphinamides, 1g and 1h,
coupled with 2c at the less hindered positions to form 4gc and
4hc. By post-treatment using sulfur powder in place of H2O2 in
the reaction of 1a with 2a, benzo[b]phosphole sulfide 4aa-S was
selectively obtained in 53% yield.
Mn(III)-mediated annulation reaction reported previously.4,5
A
dialkylacetylene, 4-octyne, did not couple with 1a at all. Next, the
reactions of substituted diarylthiophosphinamides were carried
B
DOI: 10.1021/acs.orglett.6b02868
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Organic Letters
Letter
Under our new conditions, silyl acetylenes did not undergo the
Rh-catalyzed hydroarylation with 1. Utilizing the different
reactivity of alkynes, we attempted the chemoselective coupling
reaction of 1a with diyne 2m (Scheme 6). Under standard
Scheme 7. Plausible Reaction Mechanism of C−P Bond-
Forming Step
Scheme 6. Chemoselective Coupling Reaction of 1a with
Diyne 2m
conditions, the alkyl acetylene unit of 2m over the silyl acetylene
moiety selectively coupled with 1a to form 4am in 45% yield. The
TIPS group was removed upon treatment with TfOH.
To gain some mechanistic information about the C−P bond-
forming ring-closure step, we investigated the difference of
reactivities between P(III) and P(V) intermediates using an aryl
model substrate 1j. Under our previous conditions (TfOH, 100
°C),9d 1j was smoothly converted to dibenzophosphole 7 in 75%
yield (eq 1). In sharp contrast, the present ring-closing procedure
relatively strong fluorescence in a range of 446 to 519 nm.
Remarkably, 3-methyl 4ai showed a blue-shifted emission in a
high quantum yield (420 nm, ΦF = 0.9).
In summary, we have developed a new regioselective synthetic
sequence leading to benzo[b]phosphole derivatives via rhodium-
(III)-catalyzed C−H alkenylation followed by intramolecular
cyclization in a semi-one-pot manner. Thus, a range of
functionalized benzo[b]phosphole oxides can be synthesized
from readily available arylthiophosphinamides. This new
methodology may contribute to expand the utility of
benzophospholes in materials chemistry.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02868.
Experimental procedures, additional results, and charac-
terization data of products (PDF)
Crystallographic data for 3aa (CIF)
involving P(III) species did not give 7 at all (eq 2). These results
indicate that phospha-Friedel−Crafts-type aromatic electrophilic
substitution in P(III) intermediate 1j′ does not occur on its
benzene ring under the present conditions. The lack of such a
reactivity was also observed in related Lewis-acid-promoted
phospha-Friedel−Crafts reactions.11a,c,e
Based on the experimental results, plausible reaction pathways
are depicted in Scheme 7. In route (a), TfOH protonates the
alkenyl moiety of amino phosphine I (= 6) to generate
carbocations II and III, which may exist in equilibrium with
the major contribution of tertiary carbocation II. The
phosphonium salt 5 in Scheme 4 may be generated from 3aa
via a similar tertiary carbocation. In the case of aminophosphine
I, five-membered phosphonium salt IV selectively formed from
the secondary carbocation III with “nucleophilic P center”, after
which diethylamine is eliminated to generate benzo[b]-
phosphole V.3e Although the other general phospha-Friedel−
Crafts route from “electrophilic P center” in route (b)6 cannot be
completely excluded at this stage, route (a) seems to be the major
route based on the control experiment shown in eq 2.
Crystallographic data for 5 (CIF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: satoh@sci.osaka-cu.ac.jp.
*E-mail: miura@chem.eng.osaka-u.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partly supported by a Grant-in-Aid for JSPS
Research Fellow 14J00760 to Y.U., Grants-in-Aid from MEXT,
JSPS KAKENHI JP16H01037 (in Precisely Designed Catalysts
with Customized Scaffolding) and JST (ACT-C), Japan, to T.S.,
JSPS KAKENHI 15H05485 (Grant-in-Aid for Young Scientists
(A)) to K.H., and JSPS KAKENHI JP 24225002 (Grant-in-Aid
for Scientific Research (S)) to M.M.
As expected, most of benzo[b]phosphole oxides obtained in
this study showed remarkable fluorescence in their solid state.13
Optical properties including absolute fluorescent quantum yields
determined by using an integrating sphere system are
summarized in Table S4. 2,3-Diaryl products 4aa−4ie exhibit
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