Visible light-mediated photocatalytic phosphorylation of vinyl azides: A mild synthesis of β-ketophosphine oxides

Article abstract of DOI:10.1080/00397911.2019.1696364

The photoredox-catalyzed phosphorylation of vinyl azides is described in this paper. The reaction proceeded smoothly using an inexpensive and easily-accessible Eosin Y as a photocatalyst under mild reaction conditions without any other oxidant, metal, or

Full text of DOI:10.1080/00397911.2019.1696364

Synthetic Communications
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Visible light-mediated photocatalytic
phosphorylation of vinyl azides: A mild synthesis
of β-ketophosphine oxides
Hye Im Jung & Dae Young Kim
To cite this article: Hye Im Jung & Dae Young Kim (2019): Visible light-mediated photocatalytic
phosphorylation of vinyl azides: A mild synthesis of β-ketophosphine oxides, Synthetic
Communications, DOI: 10.1080/00397911.2019.1696364
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https://doi.org/10.1080/00397911.2019.1696364
Visible light-mediated photocatalytic phosphorylation of
vinyl azides: A mild synthesis of b-ketophosphine oxides
Hye Im Jung and Dae Young Kim
Department of Chemistry, Soonchunhyang University, Asan, Chungnam, Republic of Korea
ABSTRACT
ARTICLE HISTORY
Received 11 September 2019
The photoredox-catalyzed phosphorylation of vinyl azides is
described in this paper. The reaction proceeded smoothly using an
inexpensive and easily-accessible Eosin Y as a photocatalyst under
mild reaction conditions without any other oxidant, metal, or addi-
tive. This synthetic method affords a convenient way to prepare
b-ketophosphine oxides.
KEYWORDS
b-ketophosphine oxides;
photoredox reaction; radical
process; vinyl azides
GRAPHICAL ABSTRACT
Introduction
Organophosphorus compounds are attracting considerable attention in medicinal chem-
istry and material science due to their chemical and biological properties.[1] They can
also be used as synthetic intermediates in organic chemistry.[2] Among the various
organophosphorus compounds available, b-ketophosphine oxides are highly valuable
organic compounds that serve as versatile building blocks in organic synthesis[3] and
potential ligands in various complexes owing to their coordination property.[4]
Therefore, the development of practical and novel methods for their synthesis has been
a subject of intensive research. Generally, b-ketophosphine oxides are synthesized by
the Arbuzov-type reactions of a-halogenated ketones[5] or the a-acylation of alkylphos-
phine oxides.[6] A number of methods have been reported based on the transition
metal-catalyzed phosphorylation of alkenes[7] and their derivatives such as cinnamic
acids,[8] allyl alcohols,[9] cinnamyl/alkynyl carboxylates,[10] carbonyl compounds,[11] and
alkynes,[12] Recently, Yu et al. reported on the Mn-catalyzed phosphonylation of vinyl
azide with phosphine oxides to afford b-ketophosphine oxides (Scheme 1a).[13] Despite
the above achievements, more practical, novel, and environmentally benign protocols
for the synthesis of b-ketophosphine oxides are still highly desired.
CONTACT Dae Young Kim
dyoung@sch.ac.kr
Department of Chemistry, Soonchunhyang University, Asan,
Chungnam, 31538, Republic of Korea
Supplemental data for this article can be accessed on the publisher’s website
ß 2019 Taylor & Francis Group, LLC

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H. I. JUNG AND D. Y. KIM
(a)
(b)
Scheme 1. Strategy for photoredox-catalyzed phosphorylation of vinyl azides.
The visible light-mediated photocatalysis has become an efficient protocol for organic
chemists to introduce the functional group to organic molecules due to its environmen-
tal friendliness and potential application in industry.[14] To the best of our knowledge,
photocatalytic phosphorylation of vinyl azides with phosphine oxides has not yet been
reported. Thus, we envisioned the transformation of vinyl azides into b-ketophosphine
oxides by the visible light-mediated photocatalysis with diphenylphosphine oxide as the
phosphorus radical precursor (Scheme 1b).
Results and discussion
In connection with our ongoing research program on investigating the C–H bond func-
tionalization via redox reactions,[15] we recently represented the visible light-mediated
photoredox functionalization of alkenes and aromatics.[16] Herein, we report visible
light-mediated photocatalytic phosphorylation of vinyl azides with phosphine oxides.
To determine the optimized reaction conditions for the photocatalytic phosphoryl-
ation of vinyl azides, we examined the reaction of (1-azidovinyl)benzene (1a) and
diphenylphosphine oxide (2) in dichloromethane in the presence of 3 mol% of photoca-
talyst with 10 W white LED. We conducted this reaction with various metal photocata-
lysts and organic dyes (Table 1, entries 1–8). By screening photocatalysts, Eosin Y was
found to be the optimal photocatalyst for this phosphorylation. (82% yield, Table 1,
entry 4). Various commonly used solvents such as dichloromethane, chloroform, THF,
ethyl acetate, 1,4-dioxane, acetonitrile, DMF, DMSO, methanol, and water were exam-
ined (Table 1, entries 4 and 9–18). The results showed that the reaction gave the highest
yield in dichloromethane (entry 4). Reducing the loading of photocatalyst (Eosin Y)
leads to 2 mol% slightly reduced the yield of product 3a (Table 1, entry 19).
Furthermore, when the visible light sources were switched to green or blue LEDs, a
lower yield was obtained (Table 1, entries 20–21). The controlled experiments demon-
strated the critical roles of the photocatalyst and visible light in this phosphorylation
(Table 1, entries 22–23).
With the optimal reaction conditions determined, we turned our attention to study
the substrate scope of vinyl azides 1 with diphenylphosphine oxide (2) (Table 2). The
reactions of vinyl azides 1a–1g showed that a variety of functional groups were well tol-
erated, including methyl, fluoro, chloro, and bromo substituents. The corresponding
b-ketophosphine oxides 3a–3g were obtained with 58–89% yields (Table 2).

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3
Table 1. Optimization of the reaction conditionsa.
Entry
Photocatalyst
Solvent
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
Ru(bpy)3(PF6)2
fac-Ir(ppy)3
Na2Eosin Y
Eosin Y
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
23
23
25
20
22
22
22
22
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
46
48
73
82
61
58
68
61
24
28
29
50
38
17
15
45
26
24
63
45
62
Trace
Trace
Trace
Eosin B
Rose Bengal
Rhodamine B
Rhodamine 6G
Eosin Y
9
10
11
12
13
14
15
16
17
18
19c
19d
20e
21f
22g
23h
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
–
Eosin Y
DCE
THF
EtOAc
1,4-dioxane
MeCN
DMF
DMSO
MeOH
H2O
H2O
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
aReaction conditions: (1-azidovinyl)benzene (1a, 0.1 mmol), diphenylphosphine oxide 2 (0.3 mmol), photocatalyst
(0.003 mmol), solvent (1 mL), white LEDs, room temperature under air.
bIsolated yield.
c2 mol% Eosin Y loading.
dGreen LEDs instead of white LEDs.
eBlue LEDs instead of white LEDs.
fThe reaction was performed in the dark.
gThe reaction was performed without Eosin Y.
hThe reaction was performed in the presence of TEMPO (5 equiv).
The naphthyl- and heteroaryl-substituted vinyl azides 1h and 1i provided the products
with moderate yields (70–71%, Table 2, for 3h–3i).
Then 2-azidohex-1-ene was used as the substrate, corresponding b-ketophosphine
oxide 5 was obtained in 44% yield (Scheme 2).
The reaction of (1-azidovinyl)benzene 1a with di-p-tolylphosphine oxide 6 was
also compatible under the optimal reaction conditions to afford the corresponding
b-ketophosphine oxide 7 in 71% yield (Scheme 3).
To demonstrate the practicality of this photocatalytic phosphorylation of vinyl
azides, the gram-scale reaction was conducted. As shown in Scheme 4, the reaction of
(1-azidovinyl)benzene (1a) with diphenylphosphine oxide (2) under the optimized reac-
tion conditions afforded the corresponding b-ketophosphine oxide 3a with 81% yield.
To further elucidate the reaction mechanism, some controlled experiments were per-
formed. The results showed that the absence of either the visible light or photocatalyst
completely shut down the reactivity (Table 1, entries 22–23). These reaction factors
play an important role in this phosphorylation reaction. The addition of 5 equiv of
2,2,6,6-tetramethylpiperidine N-oxide (TEMPO) per vinyl azide 1a under the standard

4
H. I. JUNG AND D. Y. KIM
Table 2. Substrate scope.a,b
.
aReaction conditions: vinyl azide 1 (0.1 mmol), diphenylphosphine oxide (2, 0.3 mmol), Eosin Y (0.003 mmol), CH2Cl2
(1 mL), white LEDs, room temperature under air.
bIsolated yield.
Scheme 2. Photocatalytic phosphorylation of alkyl-substituted vinyl azide 4.
Scheme 3. Photocatalytic phosphorylation with di-p-tolylphosphine oxide 6.

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5
Scheme 4. Gram-scale synthesis of 3a.
Figure 1. Plausible reaction pathway.
reaction conditions resulted in a trace amount of the desired b-ketophosphine oxide 3a
(Table 1, entry 24). This result is likely supporting a radical pathway to be operative in
the present reaction. Based on our preliminary results as well as related literature,11 we
propose the plausible reaction pathway shown in Figure 1.
Irradiation of the photocatalyst Eosin Y with visible light leads to the excited-state
0
ꢀ
ꢀ
Eosin Y . Initial oxidation of phosphinous acid 2 by Eosin Y , via single-electron
transfer process, should generate radical cation 200. Upon deprotonation, phosphinoyl
radical I is formed and undergoes an addition with vinyl azides 1 to furnish iminyl rad-
ical intermediates II. Radicals II is reduced by Eosin Y.to afford the anions III.
Protonation and hydrolysis lead to yield the b-ketophosphine oxides 3.
Conclusion
In conclusion, we have developed a practical synthetic method for preparing b-keto-
phosphine oxide derivatives from the reaction between vinyl azide derivatives and
diphenylphosphine oxides, accelerated by the inexpensive and easily-accessible Eosin Y
as a photocatalyst under air. This protocol is practical and environmentally friendly by
using shelf-stable diphenylphosphine oxide as phosphorus radical source and visible

6
H. I. JUNG AND D. Y. KIM
light as a source of energy. This reaction represents a convenient and mild method of
preparation of b-ketophosphine oxides.
Experimental
General information
All commercial reagents and solvents were used without purification. TLC analyses were
carried out on pre-coated silica gel plates with F254 indicator. Visualization was accom-
plished by UV light (254 nm). Purification of reaction products was carried out by flash
chromatography using E. Merck silica gel 60 (230–400 mesh). 1H NMR,13C NMR, and 19
F
NMR spectra were recorded at 400 MHz, 100 MHz, 376 MHz, respectively, on a Jeol ECS
400 MHz NMR spectrometer. Chemical shift values (d) are reported in ppm relative to
Me4Si as the internal references and PhCF3 as the external references. Mass spectra (MS-
EI, 70 eV) were conducted on GC-MS Shimadzu QP2010. High-resolution mass spectra
were measured on Jeol HX110/110A using electrospray ionization technique.
General procedure for the synthesis of b-ketophosphine oxide derivatives
An oven-dried flask was equipped with a magnetic stir bar, (1-azidovinyl)arene 1
(0.1 mmol) diphenylphosphine oxide 2 (0.3 mmol), Eosin Y (1.9 mg, 0.003 mmol), and
methylene chloride (1 mL) under air atmosphere. The reaction mixture was allowed to
stir for 20–24 h under irradiation using 10 W white LEDs. After the reaction was fin-
ished, the mixture was concentrated in vacuum and purified by chromatography on sil-
ica gel (ethyl acetate: n-hexane ¼ 2:1) to afford the b-ketophosphine oxides 3.
2-(Diphenylphosphoryl)-1-phenylethanone (3a): Yield: 82%; White solid; m.p.
1
136–138 ꢁC; H NMR (CDCl3, 400MHz): d 7.99 (d, J ¼ 7.2 Hz, 2H), 7.83–7.78 (m, 4H),
7.54–7.42 (m, 9H), 4.15 (d, J ¼ 15.2 Hz, 2H); 13C NMR (CDCl3, 100 MHz): d 192.8
(d, J ¼ 5.7 Hz), 137.0, 133.6, 132.2 (d, J ¼ 2.8 Hz), 131.9 (d, J ¼ 103 Hz), 131.1 (d,
J ¼ 9.5 Hz), 129.3, 128.6 (d, J ¼ 12.4 Hz), 128.5, 43.4 (d, J ¼ 57.2 Hz); 31P NMR (CDCl3,
162 MHz): d 27.5; HRMS (ESI) calcd for C20H18O2P [M þ H]þ 321.1039; found 321.1041.
1
Supplementary data (copies of H, 13C, and 19F NMR spectra of all the synthesized
compounds) associated with this article can be found through the
“Supplementary Content”.
Funding
This research was supported by Soonchunhyang University Research Fund and Basic Science
Research Program through the National Research Foundation of Korea [NRF-
2019R1F1A1059821].
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