Visible-Light-Catalyzed Phosphonation-Annulation: an Efficient Strategy to Synthesize β-Phosphonopyrrolidines and β-Phosphonolactones

Article abstract of DOI:10.1002/cjoc.201800360

A new type of phosphine radical precursor for C—P bond formation has been developed and successfully used in visible-light-catalyzed phosphonation-annulation of unsaturated sulfonamides and carboxylic acids. The catalytic process is simple and green acces

Full text of DOI:10.1002/cjoc.201800360

Visible-Light-Catalyzed Phosphonation–Annulation: An Efficient Strategy
to Synthesize β-Phosphonopyrrolidines and β-Phosphonolactones†
Chong Li,a Zhi-Chao Qi,a Qiang Yang,a Xiao-Yue Qiang,a and Shang-Dong Yang*,a,b
ABSTRACT A new type of phosphine radical precursor for C-P bond formation has been developed and successfully used in visible-light-catalyzed
phosphonation-annulation of unsaturated sulfonamides and carboxylic acids. The catalytic process is simple and green accessible, and no additional
oxidant or base is needed. Mechanistic studies suggest that the reaction proceeds via a single electron transfer pathway.
KEYWORDS Phosphine radical, Photocatalysis, Pyrrolidines, Lactones, Phosphonation-annulation
challenge in current C-P bond formation via phosphine radicals (A,
13f-i]
Scheme 1)[
. Exploring the new strategy to synthesize
Introduction
organophosphorus compounds has become an important topic in
this field[14]. Visible light as a sustainable energy has played an
important role in chemistry since the pioneering researches
Pyrrolidines and lactones are very important organic compound
frameworks that are widely found in natural products,
pharmaceuticals, organic materials, and concerned by chemists
increasingly [1]. Therefore, methods of synthesizing pyrrolidines
and lactones have been extensively studied, including Lewis acid
or Lewis base catalysis [2], BrØnsted acid catalysis[3], transition
metal catalysis[4], and electrophilic halides reaction[5]. With the
rise of radical chemistry, radical reaction has also become a crucial
reported by MacMillan, Yoon, and Stephenson[15]
.
The
accomplishment on synthesis of organophosphorus compounds
via photo-redox catalysis has been documented since 2011[16]
,
examples for the synthesis of phosphorylated pyrrolidines and
lactones are still very rare to date[13i, 17]. In the past several years,
our group has been working on exploring concise and efficient
methodologies of phosphine radicals and made significant
progress[13d-g]. Herein, we wish to report our latest advance in the
synthesis of phosphorylated pyrrolidines and lactones (B, Scheme
1). In this transformation, a new type of phosphine radical
precursor had been synthesized and used in the visible light
catalyzed phosphonation-annulation reaction firstly. Meanwhile,
this catalytic process is simple and green accessible, and no
additional oxidant or base is needed.
method for the synthesis of such compounds[4a, 6]
.
Scheme 1. Phosphonation-annulation via phosphine radicals.
Results and Discussion
Our investigation began with unsaturated sulfonamide 1a and
phosphine radical precursor 2a in the presence of 1.0 mol % PC
(photocatalyst) in DCM under argon atmosphere at room
temperature (Table 1, entry 1). Fortunately, diethyl-
((2-phenyl-1-tosylpyrrolidin-2-yl)-methyl)-phosphonate (3a) was
isolated in 73 % yield when fac-Ir(ppy)3 (ppy = 2-phenylpyridine)
as photocatalyst. Which might mean that phosphine radicals can
be produced smoothly under this condition. Then, the amount of
phosphine radical precursor was changed, to our delight, when 2a
loading to 1.2 to 1.5 equivalent increased the yield to 82 % and
95 % (Table 1, entry 2-3). Different protecting groups for
phosphine radical precursors have been investigated, including
Me, Ac, p-NBz (Table 1, entry 4-6), only Bz as phosphine radical
precursor’s protecting group could give an excellent yield, and Ac
protecting group also get product 3a in 32 % yield. When Me and
p-NBz served as protecting group, no product was detected under
existing conditions. Following other solvents such as DCE, DME,
toluene and CH3CN were also screened, DCM was found to be the
best solvent (Table 1, entry 7-10). After that, a further exploring
the reaction condition by changing the photocatalyst, such as
Eosin Y, Rhodamine B, Rose Bengal, and Ru(bpy)3Cl2·6H2O was
found ineffective on catalysis, with fac-Ir(ppy)3 serving as the best
choice (Table 1, entry 3, entry 11-14). Furthermore, a control
experiments revealed that photocatalyst and light were all
necessary for phosphonation-annulation (Table 1, entry 15-16).
Organophosphorus compounds are used in agriculture[7]
,
biochemistry[8], medicinal chemistry[9], materials[10], and organic
synthesis[11]
. Taking their multifunctional applications into
consideration, design and synthesis of novel organophosphorus
compounds have been extensively received attention in recent
years[12]
. Recently, there are heightened concerns about
phosphine radicals for synthesizing organophosphorus
compounds[13]. However, there is no denying that the production
of phosphine radicals is often accompanied by stoichiometric
amounts of a radical initiator, external oxidants and higher
equivalent phosphine radical precursor[13], all of which remain as a
a State Key Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou 730000, P. R. China.
E-mail: yangshd@lzu.edu.cn.
†
Dedicated to Professor Xiyan Lu on the occasion of his 90th birthday.
b State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou
Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000,
P. R. China.
This article has been accepted for publication and undergone full peer review but has not
been through the copyediting, typesetting, pagination and proofreading process, which
may lead to differences between this version and the Version of Record. Please cite this
article as doi: 10.1002/cjoc.201800360
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Next, we tested the influence of reaction atmosphere (Table 1,
entry 17). It indicated that there were only trace products in air
atmosphere. Finally, no desired products were formed when other
phosphine radical precursors were used, such as HP(O)Ph2 and
HP(O)(OEt)2 (Table 1, entry 18).
Scheme 2. Scope of the Phosphonation-Annulation Reaction of
Unsaturated Sulfonamides a,b
With the optimized reaction conditions in hand, we next shifted
our attention to investigate the scope of various unsaturated
Table 1. Optimization of Reaction Conditions a.b
a Reaction conditions: 1 (0.2 mmol), 2a (0.3 mmol), fac-Ir(ppy)3 (1.0 mol %)
in 2 mL DCM was irradiated by 5 W blue LED for 6 h at room temperature
under Ar. b Isolated yield based on 1. c The dr value was determined by 31P
NMR without column chromatography. d1.5 equiv. K3PO4 was added under
standard reaction conditions.
Scheme 3. Scope of the Phosphonation-Annulation Reaction of
Unsaturated Carboxylic Acids a,b
a Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), PC (1.0 mol %) in 2
mL solvent was irradiated by 5 W blue LED for 6 h at room temperature
e
under Ar. b Isolated yield based on 1a. c 2a (0.2 mmol). d2a (0.24 mmol).
N.D. = none detected. f n.r.= no reaction. g No light. h Under air atmosphere.
I 2a=HP(O)Ph2 or HP(O)(OEt)2.
sulfonamides (1b-1q) with 2a (Scheme 2). To our delight,
substrates bearing various substituents led to the desired
β-phosphonopyrrolidines in moderate to good yields (30%−96%),
when unsaturated sulfonamides were explored in the presence
1.0 mol % fac-Ir(ppy)3. In general, a series of unsaturated
sulfonamides, with various substituents on the aromatic ring were
proven to adapt to this reaction condition and provided the
corresponding product in moderate to excellent yield (30%-96%,
3b-3k). The efficient formation of 3b-3f illustrated that not only
electron-donating Me, OMe but also electron-withdrawing F, Cl,
Br substituents were tolerated on the aryl ring. The steric bulk on
the aromatic ring could affect the reaction. The lower yields were
obtained with the larger steric bulk (3j, 3k). Moreover, for
meta-substituted
sulfonamides
1g
and
1h,
phosphonation-annulation reaction was occurred with moderate
yield (75%, 74%), these lower yields might be affected by steric
hindrance slightly. When the N-protecting group was a chiral
group, we could get a chiral atropoisomeric product (3n) in 36%
yield and low diastereomeric ratios (dr =1.21:1). What’s more,
non-terminal olefins 1l could also achieve good yield (79%) and
high diastereomeric ratios (dr >20:1). β-phosphonopiperidine (3m)
were accessible obtained under the standard reaction conditions
a Reaction conditions: 4 (0.2 mmol), 2a (0.3 mmol), fac-Ir(ppy)3 (1.0 mol %)
in 2 mL DCM was irradiated by 5 W blue LED for 6 h at room temperature
under Ar. b Isolated yield based on 4.c The dr value was determined by 31P
NMR without column chromatography.
endo-products (3o, 3p) in good yields (84%, 53%) and high
diastereomeric ratios (dr >20:1). Unfortunately, unsaturated
sulfonamides 1q could not react in our condition, which might be
due to the fact that the tertiary carbon cation is not as stable as
the benzyl carbon cation.
in
moderate
yield
(46%).
Notably,
endo-selective
phosphonocycloamination for internal β- or γ-aminoxyalkenes (1o,
1p) with 2a gave the corresponding
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To further explore the scope of application of 2a, various
unsaturated carboxylic acids were tested. As shown in scheme 3,
aromatic-substituted lactones 5b and 5c were generated in good
yields with 73% and 90%. In addition, substrates with alkyl chain
substituted could also obtain better yields (5d, 5e). Non-terminal
olefins 4f achieved good yield in 86% and high diastereomeric
Scheme 6. A Plausible Reaction Mechanism.
ratios
(dr
>20:1).
Moreover,
endo-selective
phosphonolactonization for internal carboxy alkenes gave the
corresponding endo-product (5g-5i) in moderate to good yields
and high diastereomeric ratios (dr >20:1). However, the longer
the alkyl chain of the substrate is （4g-4i）, the lower the yield is.
More importantly, a gram-scale reaction was performed to
explore the utility of our strategy in organic synthesis.
Diethyl-((2-phenyl-1-tosylpyrrolidin-2-yl)-methyl)-phosphonate
(3a) 1.16g was obtained when decreasing the load of fac-Ir(ppy)3
(0.1
mol
%)
(A,
Scheme
4).
Furthermore,
phosphonocycloetherfication product 7 was isolated with 91%
yield under the optimal reaction condition (B, Scheme 4).
Scheme 4. Gram-Scale Reaction and Phosphonocycloetherfication
Reaction.
Base on the analysis of mechanism experiments and published
literatures[18]
,
a
mechanism was proposed for the
phosphonation-annulation reaction (Scheme 6). Upon blue light
photoexcitation, IrIII is excited to a photoexcited state IrIII*. IrIII*
IV/III*
oxidatively quenched by 2a to generate IrIV (E1/2
= -1.73 V vs.
SCE)[19] and P-radical A. Then P-radical A is rapidly trapped by
unsaturated substrates 1a to form a radical intermediate B, which
undergoes a single electron transfer (SET) oxidation to generate
benzyl cation intermediate C and regenerate IrIII photocatalyst
simultaneously. Subsequently, benzyl cation is nucleophilic
attacked to form desired products.
Conclusions
In summary, a new type phosphine radical precursor has been
developed to generate phosphine radical through photo-redox
catalysis without additional oxidant and base. This phosphine
radical precursor offers a straightforward and efficient strategy to
C-P bond formation via visible light photocatalysis. The significant
transformation exhibits a great value in organic synthesis and
medicine formation which is oxidant sensitive. As well as sundry
β-Phosphonopyrrolidines and β-Phosphonolactones were formed
smoothly.
Scheme 5. Mechanism Experiments
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2018xxxxx.
Acknowledgement
We are grateful to the financial of the NSFC (Nos. 21472076
and 21532001) and International Joint Research Centre for Green
Catalysis and Synthesis, Gansu Provincial Sci. & Tech. Department
(No.2016B01017).
To gain insight into the mechanism, the typical radical inhibitor
TEMPO (2,2,6,6-tetramethyl-1-piper-idinyloxy) inhibited the
reaction completely (A, Scheme 5). When 1a was replaced by
radical scavenger 1,1-diphenylethylene (8) under standard
conditions, coupling-product: phosphondifunctionalized-product
(9a:9a’=4.55:1) was obtained 53% yield totally (C, Scheme 5). This
indicated that P(O)(OEt)2 and OBz were generated in the system.
Trace product was detected when O2 was instead of Ar (B, Scheme
5). The above experiments showed that the reaction might have
experienced a free radical pathway.
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[1] Entry for the Table of Contents
Page No.
Visible-Light-Catalyzed
Phosphonation–Annulation:
An
Efficient Strategy to Synthesize
β-Phosphonopyrrolidines
β-Phosphonolactones
and
Chong Li,a Zhi-Chao Qi,a Qiang Yang,a
Xiao-Yue Qiang,a and Shang-Dong
Yang*,a,b
A new type of phosphine radical precursor for C-P bond formation has been developed
and successfully used in visible-light-catalyzed phosphonation-annulation of
unsaturated sulfonamides and carboxylic acids. The catalytic process is simple and
green accessible, and no additional oxidant or base is needed. Mechanistic studies
suggest that the reaction proceeds via a single electron transfer pathway.
This article is protected by copyright. All rights reserved.