Cobaloxime Catalysis for Enamine Phosphorylation with Hydrogen Evolution

Article abstract of DOI:10.1021/acs.orglett.0c01709

Direct phosphorylation of enamine and enamide with hydrogen evolution was realized via cobaloxime catalysis under visible-light irradiation. Control experiments and spectroscopic studies demonstrated a reductive quenching pathway of cobaloxime catalyst to produce phosphinoyl radical, which underwent cross-coupling with various enamines (and enamides) to give diverse β-phosphinoyl products in good to excellent yields. More interestingly, Z/E mixture of acyclic enamines could convert into single Z-products with good reactivity.

Full text of DOI:10.1021/acs.orglett.0c01709

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Letter
Cobaloxime Catalysis for Enamine Phosphorylation with Hydrogen
Evolution
Tao Lei, Ge Liang, Yuan-Yuan Cheng, Bin Chen, Chen-Ho Tung, and Li-Zhu Wu*
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ABSTRACT: Direct phosphorylation of enamine and enamide
with hydrogen evolution was realized via cobaloxime catalysis
under visible-light irradiation. Control experiments and spectro-
scopic studies demonstrated a reductive quenching pathway of
cobaloxime catalyst to produce phosphinoyl radical, which underwent cross-coupling with various enamines (and enamides) to give
diverse β-phosphinoyl products in good to excellent yields. More interestingly, Z/E mixture of acyclic enamines could convert into
single Z-products with good reactivity.
Scheme 1. Phosphorylation of Enamine and Enamides
hosphorus-containing complexes are widely applied in
P
biological, pharmaceutical, and material fields.1 The great
demands on its diversity and availability have inspired
increasing research efforts of chemists toward efficient and
general synthetic routes to its derivatives.2 Among them, direct
cross coupling of β-C (sp3)−H in enamine with P(O)−H
represents a promising and straightforward approach to β-
phosphinoyl enamine, which can easily produce diverse
downstream phosphorus-containing compounds such as
relative ketones and β-amino phosphonic acids.3 To the best
of our knowledge, no enamine substrate has been applied in
the above-mentioned transformation, although successful
reports based on stoichiometric Mn3+,4 hypervalent iodonium
salt,5 and transition-metal catalysis6 have realized the β-
phosphorylation of enamides in which the nitrogen atom must
be protected by one acyl group (or sulfonly group) in the
strong oxidative atmosphere. Recently, cobaloxime was found
to activate H-phosphine oxide into phosphinoyl radical species
under mild conditions,7 which inspired us to examine the
direct phosphorylation of enamine via radical addition by
cobaloxime catalysis.8,9
To our delight, under visible-light irradiation of catalytic
Co(dmgH)2pyCl, different enamines with Z/E mixture
coupled with diverse diarylphosphine oxides at room temper-
ature to produce specific Z-β-phosphinoyl enamines irrespec-
tive of the electron property and steric hindrance, which was
unlocked in previous reports (Scheme 1). Moreover, various
enamides were also suitable reactants and exhibited high
reactivity. Different from our previous phosphorylation of
terminal olefins via intermolecular hydrogen atom transfer
process,7 a cross-coupling hydrogen evolution10−12 pathway
smoothly proceeded without any external oxidative agents or
hydrogen acceptors, establishing a general and highly atom-
economic method for β-phosphorylation of enamine and
enamide.
substrates to explore the feasibility of cobaloxime catalysis.
With 10 mol % Co(dmgH)2pyCl as catalyst and 1 equiv of
pyridine as base in DMF, blue light irradiation for 14 h gave
the target product 3 in 7% yield (Table 1, entry 1). Further
screen of other solvents including MeOH, MeCN, DCE, DCM
showed that DCE and DCM were the highest to afford 3 with
76% and 74% yields relatively (Table 1, entries 2−5).
Considering the easy workup, DCM was chosen for the next
optimization on photocatalysts. As shown in Table 1, CoIII
salts (Co(dmgH)2pyCl, Co(dmgH)(dmgH2)Cl2), Co-
(dmgH)2(4-CO2MePy)Cl) were higher in catalytic ability
than CoII (Co(dmgBF2)2(L)2, L = MeCN, H2O) (Table 1,
entries 6−9). As for the base, omitting pyridine or replacement
by TEA, DABCO, Na2HPO4, or Na2CO3 gave lower
conversion yields (Table 1, entry 10; Table S1). Prolonging
the reaction time to 48 h provided the highest 93% yield of
product 3 (Table 1, entry 11). Control experiments confirmed
Received: May 20, 2020
Initially, 3-phenyl-3-(phenylamino)acrylonitrile (Z/E = 1:1)
1 and diphenylphosphine oxide 2 were selected as model
https://dx.doi.org/10.1021/acs.orglett.0c01709
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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Letter
a
Table 1. Optimization of Reaction Conditions
Scheme 2. Mechanistic Experiments
b
entry
photocatalyst
Co(dmgH)2pyCl
Co(dmgH)2pyCl
Co(dmgH)2pyCl
Co(dmgH)2pyCl
solvent
yield (%)
1
2
3
4
5
6
7
8
9
DMF
MeOH
MeCN
DCE
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
7
9
51
76
74
62
68
18
Co(dmgH)2pyCl
Co(dmgH)2(dmgH2)Cl2
Co(dmgH)2(4-CO2MePy)Cl
Co(dmgBF2)2(H2O)2
Co(dmgBF2)2(MeCN)2
Co(dmgH)2pyCl
36
50
C
10
11
d
Co(dmgH)2pyCl
93 (86)
12
13
0
0
e
Co(dmgH)2pyCl
a
Reaction conditions (unless otherwise specified): enamine 1 (0.1
mmol), diphenylphosphine oxide 2 (1.5 equiv, 0.15 mmol),
cobaloxime (10 mol %) and pyridine (1.0 equiv, 0.1 mmol) in
solvent (2 mL) was irradiated with blue LEDs for 14 h under argon
b
atmosphere at room temperature. 31P NMR yield with triphenyl-
C
phosphane as internal standard. Isolated yield in parentheses. No
pyridine. 48 h. No light.
d
e
Scheme 3. Proposed Mechanism
no reaction at all occurred without catalyst or light (Table 1,
entries 12 and 13), indicating the nature of photocatalysis.
As only cobaloxime catalyst responds to visible light and
other two substrates have no absorption at 450 nm (Figure
S1), a new absorption peak between 400 and 500 nm was
assigned to Co(II) species after irradiating the system of
cobaloxime and diphenylphosphine oxide 2 for 10 min (Figure
S2a). Significantly, further addition of enamine into the above-
mentioned reaction mixture, in addition to the observable
Co(I) signal (Figure S2b),13 a characteristic signal of
phosphinoyl radical14 was directly obtained by electron-spin
resonance (ESR) with 5,5-dimethyl-1-pyrroline N-oxide
(DMPO) as radical capture. These results demonstrated that
excited cobaloxime catalyst could interact with diphenylphos-
phine oxide 2 to provide low-valent Co species and
phosphinoyl radical. As supported by spectroscopic results,
2-methyl-2-nitrosopropane (MNP) and 2,2,6,6-tetramethylpi-
peridin-1-oxyl (TEMPO) as radical-trapping agents seriously
suppressed the conversion efficiency, and the radical clock
reaction with 2 equiv of (1-cyclopropylvinyl)benzene led to the
cyclopropane-opening products (Scheme 2a). Furthermore, no
external oxidant was needed in our system, and hydrogen gas
was detected by gas chromatography rather than hydro-
genation of substrate or product (Scheme 2b). As for the Z-
selectivity of product from Z/E mixture of acyclic enamine 1,
ethyl (Z)-3-phenyl-3-(phenylamino)acrylate 1′ with an ester
group was employed to react with diphenylphosphine oxide 2,
which gave a Z/E (2:1) mixture of product 3′ with 99% yield
(Scheme 2c). This result revealed the high selectivity probably
arised from the intramolecular hydrogen-bond effect between
N−H and phosphinoyl group in the product.
the formation of Co(II) species. Generated electrophilic
intermediate A was then added to the CC bond of enamine
to form carbon radical B adjacent to nitrogen atom, which
direct coupled with Co (II) to form C−Co(III) intermediate
(C). Via β-H elimination, the product 3 was obtained
accompanyed by the formation of Co(III)−H. Co(III)−H
reacted with another proton to release hydrogen and
regenerate cobalt(III), completing the reaction circle. In this
process, the possibility of either single electron transfer
between Co(II) and intermediate B to final product 3 and
Co(I) which captured one proton to generate Co(III)−H, or
the direct hydrogen abstract by Co(II) from intermediate B to
produce 3 and Co(III)−H could not be completely
excluded.10c
On the basis of the above experimental results, a rational
mechanism was proposed as listed in Scheme 3. Under visible-
light irradiation, photoexcited cobalt(III) oxime oxidized
diphenylphosphine oxide 2 into phosphinoyl radical A with
With the understanding of mechanism in mind, more efforts
were focused on the generality of this cobaloxime catalysis for
enamine phosphorylation. Various enamines even enamides
B
https://dx.doi.org/10.1021/acs.orglett.0c01709
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Letter
a
Scheme 4. Scope of Enamine and Enamide
a
Reaction condition (unless otherwise specified): enamine (Z/E = 2:1−1:2) or enamide (0.1 mmol), diarylphosphine oxide (1.5 equiv, 0.15
mmol), Co(dmgH)2pyCl (10 mol %) and pyridine (0.1 mmol) in DCM (2 mL) was irradiated with blue LEDs for 48 h under argon atmosphere at
b
c
room temperature. 10, 13, 3′, and 19−20 came from Z-enamine substrates. Z/E value was detected via 31P NMR spectra. 1 mmol reaction.
d
Reaction time was 22 h.
were examined to react with diarylphosphine oxides. As shown
in Scheme 4, analysis of the results gave the following
conclusions: (1) Both electron-donating and -withdrawing
substitution on the N-aryl group showed good tolerance (3−
11). Good to excellent yields were obtained in which a strong
electron-deficient ester substrate could afford 96% yield of
products (10). P- and m-methyl enamines gave relative
products in 79% and 84%, showing little steric effect (4, 5).
In addition, larger conjugated 4-phenyl substitution (12) and
simple NH2 (13) all were competent to react smoothly and
exhibited good reactivity. As for the α position of enamine,
variations on the arene moiety including 3-methyl (14), 4-
methoxyl (15), 4-fluoro (16), 4-chloro (17), and 4-bromo
(18) substitutions were facile to produce β-phosphinoyl
enamines with 69−98% yields. It should be highlighted that
all these products were single Z configurations from enamines
with a Z/E mixture (Z/E = 2:1−1:2), except 10 and 13 from
Z-substrates. (2) In terms of the β-substitution, besides CN,
other groups like esters gave excellent reaction tolerance. Ethyl
(10, 3′), butyl (19), benzyl (20),and even cyclohexyl (21)
esters all gave ≥95% yield of products albeit with a low Z/E
ratio, due to the competition of intramolecur hydrogen bond
between N−H···(O)P and N−H···(O)C. (3) In order to
extend the synthetic application of this method, linear and
cyclic enamides were further tested (22−27). N-(1-Phenyl-
prop-1-en-2-yl)acetamide with a methyl group in the α-
position gave moderate yield (22). Enamides derived from
various benzocyclic ketones such as 2,3-dihydro-1H-inden-1-
one (five-membered, 23), 3,4-dihydronaphthalen-1(2H)-one
(six membered, 24, 25), chroman-4-one (six membered, 26),
and 6,7,8,9-tetrahydro-5H-benzo[7]annulen-5-one (seven
membered, 27) were suitable reactants to produce the
corresponding products in 65%−95% yields. (4) In terms of
reaction partner, diarylphosphine oxide with alkyl, alkoxyl, and
halogen substitutions all exhibited good reactivity (28−31).
Di(naphthalen-2-yl)phosphine oxide bearing a large hindrance
could also react with 3-phenyl-3-(phenylamino)acrylonitrile
(Z/E = 1:1) and afforded 52% yield with single Z isomer (32).
(5) In order to further explore the practicality of this method,
scale-up reaction of 3-amino-3-phenylacrylonitrile, ethyl (Z)-3-
phenyl-3-(phenylamino)acrylate, and N-(3,4-dihydronaphtha-
len-1-yl)acetamide in 1 mmol were carried out to afford the
C
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Letter
target products 13, 3′, and 25 in 73%, 99%, and 60% yields
with almost no loss in yield. The complex cholesterol
derivative was also feasible to give the target product with
90% yield (33). All of the results revealed the great potential of
cobaloxime catalysis for synthetic application.
Academy of Sciences, Beijing 100049, P.R. China; orcid.org/
0000-0003-0437-1442
Chen-Ho Tung − Key Laboratory of Photochemical Conversion
and Optoelectronic Materials, Technical Institute of Physics and
Chemistry, The Chinese Academy of Sciences, Beijing 100190,
P.R. China; School of Future Technology, University of Chinese
Academy of Sciences, Beijing 100049, P.R. China; orcid.org/
0000-0001-9999-9755
In conclusion, we have successfully realized the phosphor-
ylation of enamine and enamide via cobaloxime catalysis.
Under visible-light irradiation, different functionalized enam-
ines and enamides showed good reactivity to reaction with
diverse H-phosphine oxides, providing valuable added β-
phosphinoyl products with no external oxidant or hydrogen
atom acceptors. The proton and electron were confirmed to
couple into hydrogen, revealing the great advantages on
reaction simplicity and atom-economy. Furthermore, the
selective specificity for Z-isomer product in some enamines
conversion, highlighting the accurate application of intra-
molecular hydrogen bond in chemical transformation. The
features of mild reaction conditions, good functional group
tolerance, and conversion yields make this strategy a promising
alternative for the construction of β-phosphinoyl enamines and
enamides, which could be further employed into the last-stage
decoration of complex molecules.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01709
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Ministry of Science and
Technology of China (2017YFA0206903), the National
Natural Science Foundation of China (21861132004), the
Strategic Priority Research Program of the Chinese Academy
of Science (XDB17000000), the Key Research Program of
Frontier Sciences of the Chinese Academy of Science
(QYZDY-SSW-JSC029), and the K. C. Wong Education
Foundation.
ASSOCIATED CONTENT
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REFERENCES
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01709.
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AUTHOR INFORMATION
Corresponding Author
■
Li-Zhu Wu − Key Laboratory of Photochemical Conversion and
Optoelectronic Materials, Technical Institute of Physics and
Chemistry, The Chinese Academy of Sciences, Beijing 100190,
P.R. China; School of Future Technology, University of Chinese
Academy of Sciences, Beijing 100049, P.R. China; orcid.org/
0000-0002-5561-9922; Email: lzwu@mail.ipc.ac.cn
Authors
Tao Lei − Key Laboratory of Photochemical Conversion and
Optoelectronic Materials, Technical Institute of Physics and
Chemistry, The Chinese Academy of Sciences, Beijing 100190,
P.R. China; School of Future Technology, University of Chinese
Academy of Sciences, Beijing 100049, P.R. China
Ge Liang − Key Laboratory of Photochemical Conversion and
Optoelectronic Materials, Technical Institute of Physics and
Chemistry, The Chinese Academy of Sciences, Beijing 100190,
P.R. China; School of Future Technology, University of Chinese
Academy of Sciences, Beijing 100049, P.R. China
Yuan-Yuan Cheng − Key Laboratory of Photochemical
Conversion and Optoelectronic Materials, Technical Institute of
Physics and Chemistry, The Chinese Academy of Sciences,
Beijing 100190, P.R. China; School of Future Technology,
University of Chinese Academy of Sciences, Beijing 100049, P.R.
China
Bin Chen − Key Laboratory of Photochemical Conversion and
Optoelectronic Materials, Technical Institute of Physics and
Chemistry, The Chinese Academy of Sciences, Beijing 100190,
P.R. China; School of Future Technology, University of Chinese
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