FeCl2Catalyzed Three-Component Reactions of Phospholes, Pyrrolidine, and Ketones (Aldehydes): Chemoselective Synthesis of 1-Phosphafulvenes

Article abstract of DOI:10.1021/acs.orglett.1c00602

We have developed an unprecedented approach for the synthesis of transient 1-phosphafulvenes through three component reactions of phospholes. The generation of 1-phosphafulvenes was demonstrated by in situ [6 + 4] cycloaddition with 2H-phospholes and [6 + 6] self-dimerization. The [6 + 4] and [6 + 6] reaction pathway could be modulated by the starting ketones and aldehydes. The construction of 1-phosphafulvenes is illustrated by a proposed mechanism combining nucleophilic addition of phospholide to the iminium or isomerized azomethine ylide and a [1,5]-shift of phosphole.

Full text of DOI:10.1021/acs.orglett.1c00602

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Letter
FeCl2 Catalyzed Three-Component Reactions of Phospholes,
Pyrrolidine, and Ketones (Aldehydes): Chemoselective Synthesis of
1‑Phosphafulvenes
Yanjie Liu, Xinran Fan, Rongqiang Tian,* and Zheng Duan*
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ABSTRACT: We have developed an unprecedented approach for the synthesis of transient 1-phosphafulvenes through three
component reactions of phospholes. The generation of 1-phosphafulvenes was demonstrated by in situ [6 + 4] cycloaddition with
2H-phospholes and [6 + 6] self-dimerization. The [6 + 4] and [6 + 6] reaction pathway could be modulated by the starting ketones
and aldehydes. The construction of 1-phosphafulvenes is illustrated by a proposed mechanism combining nucleophilic addition of
phospholide to the iminium or isomerized azomethine ylide and a [1,5]-shift of phosphole.
iminium ion by H transfer, have been identified as reactive
intermediates (Scheme 1b).
he widely utilized three component coupling reactions
1
2
3
3
Tsuch as Mannich, Strecker, A -coupling, and Kabach-
nik−Fields4 reactions have been successfully applied in plenty
of synthetic procedures including heterocyclic frameworks,
On the other hand, pentafulvenes are a class of highly
reactive compounds with unique electronic properties and
have been recognized as the eminent building block of many
polycyclic frameworks and natural products.9 One standard
protocol for the preparation of pentafulvene is the con-
densation of cyclopentadiene with aldehydes or ketones in the
presence of secondary amines, particularly pyrrolidine, which
was developed by Stone and Little (Scheme 1c).10 Pyrrolidine
acts not only as a base but also as a precursor for the iminium
ion to render the carbonyl carbon more electrophilic. Due to
the high reactivity of the CP bond,11 the chemistry of
phosphafulvenes (the phospha-analogues of pentafulvenes) has
been virtually unexplored. The first isolable 1,3,6-triphospha-
fulvene was synthesized by kinetic stabilization of a bulky
group.12 We have revealed the versatile cycloaddition
chemistry and nonbenzenoid aromaticity of 1-phosphafulvenes
by dissociation of α-C2-bridged biphospholes recently.13
However, the chemistry of 1-phosphafulvenes was significantly
hampered because of the lack of an efficient synthetic route.
We hypothesized that incorporating a phosphorus atom into a
medicinal chemistry, and natural product synthesis.5
A
common feature of these reactions is that they involve an
intermediate iminium ion, which is formed by condensation of
a secondary amine with an aldehyde (or ketone) (Scheme 1a).
Seidel et al. recently developed a new strategy for the redox-
neutral α-C−H functionalization of simple secondary amines
(e.g., pyrrolidine).6 These redox-neutral transformations7
combine a reductive N-alkylation with an oxidative α-
functionalization. Azomethine ylides,8 isomers of classic
Scheme 1. (a) Reactions Involving Intermediate Iminium
Ions; (b) Redox-Neutral α-C−H Functionalization of
Secondary Amines; (c) Stone and Little Route to
Pentafulvenes
Received: February 19, 2021
Published: March 29, 2021
https://doi.org/10.1021/acs.orglett.1c00602
© 2021 American Chemical Society
Org. Lett. 2021, 23, 2943−2947
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cyclopentadiene (2H-phosphole)14 in the reaction with an
intermediate iminium ion will afford 1-phosphafulvene, a novel
low coordinate phosphorus species.
a b
,
Scheme 2. Scope of the Three-Component Reaction
It was known that 3,4-dimethyl-1-phenylphosphole can be
converted into the corresponding 2H-phosphole through a
[1,5]-shift at high temperature.15 We selected 3,4-dimethyl-1-
phenylphosphole as the reaction partner in the proposed three-
component reaction with benzophenone and pyrrolidine
(Table 1). The three-component reaction was heated at 170
a
Table 1. Optimization of the Reactions Conditions
entry
cat. (5 mol %)
3a yield (%)
1
2
3
4
5
6
7
8
9
none
31
26
43
58
62
80
91
53
59
PhCO2H
CH3CO2H
ZnCl2
AlCl3
FeCl3
FeCl2
FeCl2
FeCl2
b
c
a
Reaction conditions: 1a (1.0 mmol), 2a (0.55 mmol), pyrrolidine
b
(1.0 mmol), toluene (5 mL), 170 °C, N2. With 2.5 mol % FeCl2.
c
With 10 mol % FeCl2.
°C in a sealed heavy wall pressure tube and provided a tricyclic
compound 3a featuring a diphospha seven-membered ring
(entry 1). The 31P NMR of 3a shows characteristic AB system
1
a
peaks at −30.4 ppm and −11.5 ppm with a JPP coupling
Reaction conditions: 1 (2.0 mmol), 2 (1.1 mmol), pyrrolidine (168
μL, 2.0 mmol), FeCl2 (13 mg, 0.1 mmol), toluene (5 mL), 170 °C, 10
constant at 260 Hz, which stems from the spin of two
nonidentical phosphorus nuclei. One C−N bond in the
pyrrolidine ring was cleaved during the reaction. Using
carboxylic acids as catalysts16 did not promote the yield of
reaction (entries 2 and 3). Different Lewis acids were then
evaluated as catalysts to facilitate the three-component
reaction. Both ZnCl2 and AlCl3 promoted the yield to 58%
and 62%, respectively (entries 4 and 5). Gratifyingly, with 5
mol % ferrous chloride as catalyst, 3a was obtained in 91%
yield (entry 7). Reducing the amount of catalyst to 2.5 mol %
led to a decrease in yield (entry 8). The reaction with 0.5 equiv
pyrrolidine provides 3a in much lower yield (34%). Other
secondary amines such as piperidine and diethylamine did not
afford 3a under the same reaction conditions.
Optimized reaction conditions were employed to evaluate
the scope of the reaction with a number of different
phospholes and ketones (Scheme 2). A variety of phospholes
with different substituents on the phenyl rings bearing
electron-rich and electron-deficient groups afforded the
corresponding products in good to excellent yields (3a−3e).
Electron-donating groups (3b, 3c) increase the reaction yields
while an electron-withdrawing group decreases the yield (3d).
The heteroaromatic substituent such as thienyl was tolerated in
this reaction (3f). Besides, the three-component reaction
works smoothly with different diarylketones (3g−3j). Aliphatic
ketones without α-H provided the corresponding products
with slightly decreased yields (3k and 3l). However, no
reaction was observed with aliphatic ketones has α-H. It is
b
h, N2. Isolated yield.
noteworthy that a similar structure of 3 has been synthesized in
quite low yield (10%).13b
Considering 2H-phospholes are highly reactive in Diels−
Alder reaction,14 the [6 + 4] reaction of 2-phenyl-2H-
phosphole with 1-phosphafulvene would result in the
formation of 3. To obtain further insights into the course of
the reaction, 3a was subjected to the reaction with N-phenyl
maleimide or 3,4-dimethylbutadiene at high temperature. The
retro-[6 + 4] cycloaddition reaction of 3a afforded transient
2H-phosphole and 1-phosphafulvene, which were trapped in
situ by Diels−Alder reaction with N-phenyl maleimide and 3,4-
dimethylbutadiene, respectively (Scheme 3a). The [4 + 2]
adducts 4, 5, 6, 7 were isolated in good yields, providing
additional support for the [6 + 4] pathway. More importantly,
the easy dissociation of 3a suggests that 3a is a useful precursor
to reveal phosphafulvene chemistry. Furthermore, the three-
component reaction also provided α-C2-bridged biphosphole 8
via [6 + 6] cycloaddition of 1-phosphafulvenes but required a
higher temperature of 210 °C (Scheme 3b). 8 was partially
oxidized when growing single crystals for an X-ray diffraction
study.
Interestingly, when benzaldehyde was subjected to the
reaction conditions, the desired [6 + 4] adduct was not
obtained. Instead, the α-C2-bridged biphosphole 9a was
obtained in good yield in the presence of 20 mol % FeCl2
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Scheme 3. (a) Retro-[6 + 4] Cycloaddition of 3a and in Situ
Trapping Reaction; (b) Synthesis of α-C2-Bridged
Biphosphole 8
Scheme 5. (a) Conversion of Dimer 10 to 3a; (b) Proposed
Reaction Pathway
(Scheme 4). The reaction of pyrrolidine and phospholes with
various aromatic or aliphatic aldehydes consistently led to the
Scheme 4. Substrate Scope of the Reaction of Phospholes
a b
,
with Aldehydes
mediate azomethine ylides through internal redox isomer-
ization of iminium ion,6 we proposed a possible mechanism for
the chemoselective formation of [6 + 4] products 3 and
biphospholes 9 (Scheme 5b).
The [1,5]-sigmatropic shift of the exocyclic aryl group of
phosphole 1 generates 2H-phosphole.14 The pyramidal
geometry of phosphorus in 1 favors this process.19 Notably,
the treatment of 2H-phosphole dimer 10 with a base could
yield the corresponding phospholide ion A.14,18 The X-ray
crystal structure analysis of phospholide anions20 shows that
the lithium ion is displaced toward the phosphorus atom,
suggesting a high electron density at P. Moreover, all the
known reactions of phospholide anions take place at
phosphorus.21
Meanwhile, the condensation of pyrrolidine with a ketone
leads to the formation of an intermediate iminium ion B1 at
high temperature. The subsequent redox isomerization
provides azomethine ylide B2. The formation of iminium ion
increases the electrophilicity of carbonyl carbon. The
nucleophilic addition of phospholide A to azomethine ylide
B2 provides C2 exclusively. The migration of pyrrolidine
moiety from P to α-C affords D2. We have demonstrated that
amino substituent favors the migration of the sp3 carbon group
from P to α-C by DFT calculations.13a The amine elimination
cleaves a C−N bond of the pyrrolidine ring and generates 1-
phosphafulvene E2. The further [6 + 4] cycloaddition of E2
with 2H-phosphole produces the isolated product 3. The
electron-withdrawing group reduces the nucleophilicity of
phospholide A and produces 3d in low yield.
a
Reaction conditions: 1 (2.0 mmol), aldehyde (2.15 mmol),
pyrrolidine (168 μL, 2.0 mmol), FeCl2 (51 mg, 0.4 mmol), toluene
b
(5 mL), 170 °C, 16 h, N2. Isolated yield.
formation of α-C2-bridged biphospholes 9a−9f in good yields.
Electron-poor aldehyde led to drastic decrease of the yield
(9c). We have demonstrated that the [6 + 6] dimerization of
1-phosphafulvenes afforded α-C2-bridged biphospholes.13b
The chemoselective 1,1-addition of α-C2-bridged biphospholes
to terminal alkynes provides simple access to the 1,3-
diphosphepines.13d
In order to gain mechanistic insight, the reaction process was
monitored by 31P NMR spectroscopy (Figure S1). The AB
system peaks at 34.9 ppm and −31.8 ppm (1JPP = 199 Hz)
correspond to the [4 + 2] dimer 10 of transient 2H-
phosphole,17 which is consistent with literature report.18
Subsequently, the dimer 10 was independently synthesized
from phosphole 1a and was converted into the [6 + 4] product
3a in 81% yield (Scheme 5a).
For the iminium ion derived from an aldehyde, the addition
of the phospholide A to the exocyclic iminium ion B1
generates phosphole C1 selectively. The [1,5]-shift and
elimination of pyrrolidine moiety ultimately lead to the
formation of 1-phosphafulvene E1. The two routes could be
On the basis of the above experiments and reported
mechanisms involving intermediate iminium ions and inter-
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Letter
modulated by starting aldehyde and ketone. This proposed
mechanism readily illustrates the formation of the 1-
phosphafulvenes E1 and E2.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21672193), the Ministry of Industry and
Information Technology of the People’s Republic of China
(Z135060009002), the 111 Project (D20003), Key Scientific R
esearch Projects of Colleges and Universities in Henan
Province (20A150040) and Zhengzhou University of China
(JC2020530190).
In summary, we have described redox-neutral FeCl2
catalyzed three-component reactions of phospholes, pyrroli-
dine, and ketones (or aldehydes). The extension of iminium
ions and azomethine ylides chemistry to phosphacycles let us
disclose a facile access to transient 1-phosphafuvlenes. Given
their unique π-conjugated structure, phosphafulvenes will
attract considerable research interest. Therefore, this simple
approach will bring a bright prospect for phosphafulvene
chemistry, which represents a new frontier of low coordinate
phosphorus chemistry.
DEDICATION
■
We dedicate this work in memory of Professor Franco
Mathey, a distinguished phosphorus chemist.
̧
is
REFERENCES
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
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AUTHOR INFORMATION
Corresponding Authors
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Rongqiang Tian − College of Chemistry, Green Catalysis
Center, International Phosphorus Laboratory, International
Joint Research Laboratory for Functional Organophosphorus
Materials of Henan Province, Zhengzhou University,
Zhengzhou 450001, PR China; orcid.org/0000-0002-
5460-1691; Email: tianrq@zzu.edu.cn
Zheng Duan − College of Chemistry, Green Catalysis Center,
International Phosphorus Laboratory, International Joint
Research Laboratory for Functional Organophosphorus
Materials of Henan Province, Zhengzhou University,
Zhengzhou 450001, PR China; orcid.org/0000-0002-
0173-663X; Email: duanzheng@zzu.edu.cn
Authors
Yanjie Liu − College of Chemistry, Green Catalysis Center,
International Phosphorus Laboratory, International Joint
Research Laboratory for Functional Organophosphorus
Materials of Henan Province, Zhengzhou University,
Zhengzhou 450001, PR China
Xinran Fan − College of Chemistry, Green Catalysis Center,
International Phosphorus Laboratory, International Joint
Research Laboratory for Functional Organophosphorus
Materials of Henan Province, Zhengzhou University,
Zhengzhou 450001, PR China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00602
Notes
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The authors declare no competing financial interest.
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