Transition Metal-free Phosphorylation of Vinyl Azides: A Convenient Synthesis of β-Ketophosphine Oxides

Full text of DOI:10.1002/bkcs.11964

Note
DOI: 10.1002/bkcs.11964
BULLETIN OF THE
J. Jang and D. Y. Kim
KOREAN CHEMICAL SOCIETY
Transition Metal-free Phosphorylation of Vinyl Azides: A Convenient
Synthesis of β-Ketophosphine Oxides
*
Jihoon Jang and Dae Young Kim
Department of Chemistry, Soonchunhyang University, Chungnam 31538, South Korea.
*E-mail: dyoung@sch.ac.kr
Received September 26, 2019, Accepted December 18, 2019
Keywords: Phosphorylation, Vinyl azides, β-Ketophophine oxides, Metal-free coupling, Radical process
In recent years, organophosphorus compounds have attracted
considerable attention in medicinal chemistry and material sci-
ence due to their chemical and biological properties.1 They can
be also used as synthetic intermediates in organic chemistry.2
Among the various organophosphorus compounds available,
β-ketophosphine oxides are highly valuable organic com-
pounds that serve as versatile building blocks in organic syn-
thesis3 as well as potential ligands in various complexes due to
their coordination property.4 Therefore, the development of
practical and efficient methods for their synthesis has been a
subject of intensive research. In general, β-ketophosphine
oxides are synthesized by the Arbuzov-type reactions of
α-halogenated ketones5 and the α-acylation of alkylphosphine
oxides.6 Various studies have reported on a number of
methods based on the transition metal-catalyzed phosphoryla-
tion of alkenes7 and their derivatives, such as cinnamic acids,8
allyl alcohols,9 cinnamyl/alkynylcarboxylates,10 carbonyl
compounds,11 and alkynes.12 Recently, Yu et al. reported on
the Mn-catalyzed phosphonylation of vinyl azide with phos-
phine oxides to afford β-ketophosphine oxides (Scheme 1
(a)).13 Despite the advancements described above, more mild
and environmentally benign approaches for the synthesis of
β-ketophosphine oxides are still highly desired. To the best of
our knowledge, no study has investigated the transition metal-
free oxidative coupling of vinyl azides with phosphine oxides.
We thus envisioned the transformation of vinyl azides into
β-ketophosphine oxides by the K2S2O8-mediated phosphoryla-
tion with diphenylphosphine oxides as the phosphorus radical
precursor (Scheme 1(b)).
As part of our ongoing research program on the C–H
bond activation,14 we recently presented the radical-
mediated functionalization of alkenes and aromatics.15
Herein, we report on the transition metal-free oxidative
phosphorylation of vinyl azides with phosphine oxides.
We began by investigating the transition metal-free phos-
phorylation of (1-azidovinyl)benzene (1a) and dip-
henylphosphine oxide (2) in the presence of K2S2O8 in
acetonitrile at 60 ꢀC. Fortunately, the desired β-ketophosphine
oxide 3a was obtained in 82% yield (Table 1, entry 1). The
results showed that several other oxidants could also be used,
albeit with relatively low yields compared to K2S2O8, such as
Na2S2O8, (NH4)2S2O8, tert-butyl hydrogen peroxide (TBHP),
and PhI(OAc)2. The reaction was further investigated in vari-
ous solvents (Table 1, entries 1 and 6–11). Of these, acetoni-
trile was found to be the optimal solvent in this reaction
(Table 1, entry 1). Next, we lowered the amount of oxidant to
1.5 equiv, and this led to an increased yield of
β-ketophosphine oxide 3a (Table 1, entry 13). However, a fur-
ther decrease in the amount of oxidant to 1.0 equiv led to a
decreased yield (Table 1, entry 14). No conversion was
observed in the absence of K2S2O8 as the oxidant (Table 1,
entry 15). Finally, the reduction of the amount of 2 to 2.0
equiv resulted in a decrease in yield even at the extended reac-
tion time (Table 1, entry 16).
After determining the optimized reaction conditions, we
examined the substrate scope of vinyl azides 1 with dip-
henylphosphine oxide (2) (Table 2). The reactions of the
(1-azidovinyl)benzene derivatives 1a–1g with various sub-
stituents in the aryl group furnished the corresponding
β-ketophosphine oxides 3a–3g with 61–91% yields
(Table 2). The heteroaryl-substituted vinyl azide 1h provided
the products with moderate yields (70%, Table 2, for 3h).
Alkyl-substituted vinyl azide was also examined. When
2-azidohex-1-ene was used as the substrate, corresponding
β-ketophosphine oxide 3i was obtained in 61% yield.
To demonstrate the practicality of the transition metal-
free phosphorylation of vinyl azides, the gram-scale reac-
tion was conducted under optimized reaction conditions.
As shown in Scheme 2, the reaction of (1-azidovinyl)ben-
zene (1a) with diphenylphosphine oxide under the opti-
mized reaction conditions (2) afforded the desired
β-ketophosphine oxide 3a with 82% yield.
To further elucidate the reaction mechanism, some con-
trolled experiments were conducted. The results showed that
the absence of K2S2O8 completely shut down the reactivity
(Table 1, entry 15). A trace of the product was detected in the
presence of 2,2,6,6-tetramethylpiperidine N-oxide (TEMPO)
as a radical scavenger (Table 1, entry 17). This indicates that
radical intermediates may be involved in this pathway. Based
on our preliminary results as well as previous literature, we
propose the plausible reaction pathway shown in Figure 1.16
Initially, phosphinous acid A reacted with K2S2O8 to generate
radical cation B via a single-electron transfer process. Follow-
ing deprotonation, phosphinoyl radical C is formed, which
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Table 1. Optimization of the reaction conditions.a
Entry
Oxidant
Solvent
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
K2S2O8
Na2S2O8
(NH4)2S2O8
TBHP
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DMSO
DMF
EtOH
CH2Cl2
DCE
8
8
8
8
8
12
12
12
12
12
12
8
8
8
8
15
8
82
74
78
5
Scheme 1. Strategy for phosphorylation of vinyl azides.
PhI(OAc)2
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
—
8
undergoes an addition with vinyl azides 1 to furnish iminyl
radical intermediate D. Radical D is reduced to afford anions
E. Protonation and hydrolysis leads to the yield of the
β-ketophosphine oxides 3.
In conclusion, we have developed a practical synthetic
method for the preparation of β-ketophosphine oxide deriva-
tives from the reaction between vinyl azide derivatives and dip-
henylphosphine oxides under transition metal-free conditions.
The results presented here show that this protocol is a practical
and convenient method for the preparation of β-ketophosphine
oxide derivatives using K2S2O8 as the sole oxidant.
73
73
68
35
61
49
83
85
71
0
9c
10
11
12d
13e
14f
15
16g
17h
PhMe
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
K2S2O8
K2S2O8
70
trace
a Reaction conditions: (1-azidovinyl)benzene (1a, 0.1 mmol),
diphenylphosphine oxide 2 (0.3 mmol), oxidant (0.3 mmol), solvent
(1 mL), room temperature.
Experimental
General
Procedure
for
the
Synthesis
of
b Isolated yield.
β-Ketophosphine Oxide Derivatives. An oven-dried flask
was equipped with a magnetic stir bar, (1-azidovinyl)arene 117
(0.1 mmol) diphenylphosphine oxide 2 (0.3 mmol), K2S2O8
(1.9 mg, 0.15 mmol), and acetonitrile (1 mL). The reaction
mixture was then stirred for 8–20 h at 60 ꢀC. Upon comple-
tion of the reaction, the mixture was concentrated in vacuum
and purified by chromatography on silica gel (ethyl acetate: n-
hexane = 2:1) to afford the β-ketophosphine oxides 3.10–13
2-(Diphenylphosphoryl)-1-phenylethanone (3a). Yield:
85%; white solid; m.p. 136–138 ꢀC; 1H NMR (CDCl3,
400 MHz): δ 7.99 (d, J = 7.2 Hz, 2H), 7.83–7.78 (m, 4H),
7.54–7.42 (m, 9H), 4.15 (d, J = 15.2 Hz, 2H); 13C NMR
(CDCl3, 100 MHz): δ 192.8 (d, J = 5.7 Hz), 137.0, 133.6,
132.2 (d, J = 2.8 Hz), 131.9 (d, J = 103 Hz), 131.1 (d,
J = 9.5 Hz), 129.3, 128.6 (d, J = 12.4 Hz), 128.5, 43.4 (d,
J = 57.2 Hz); 31P NMR (CDCl3, 162 MHz): δ 27.5; EI-
MS: m/z = 320.1 [M+].
2-(Diphenylphosphoryl)-1-(p-tolyl)ethenone (3b). Yield:
91%; white solid; m.p. 140–142 ꢀC; 1H NMR (400 MHz,
CDCl3): δ 7.88 (d, J = 8.4 Hz, 2H), 7.83–7.78 (m, 4H),
7.56–7.44 (m, 6H), 7.21 (d, J = 7.6 Hz, 2H), 4.13 (d,
J = 14.8 Hz, 2H), 2.38 (s, 3H); 13C NMR (100 MHz, CDCl3):
δ 192.3 (d, J = 5.8 Hz), 144.6134.5, 132.1 (d, J = 2.9 Hz),
131.9 (d, J = 103 Hz), 131.1 (d, J = 9.6 Hz), 129.4, 129.2,
128.6 (d, J = 11.4 Hz), 43.2 (d, J = 58.2 Hz), 21.7; 31P NMR
(162 MHz, CDCl3): δ 28.0; EI-MS: m/z = 334.1 [M+].
c At 40 ꢀC.
d 2.0 equiv of K2S2O8 was added.
e 1.5 equiv of K2S2O8 was added.
f 1.0 equiv of K2S2O8 was added.
g 2.0 equiv of 2 was loaded.
h Reaction performed in the presence of TEMPO (5 equiv).
4H), 7.56–7.45 (m, 6H), 7.11–7.07 (m, 2H), 4.11 (d,
J = 15.6 Hz, 2H); 13C NMR (CDCl3, 100 MHz): δ 191.2
(d, J = 5.7 Hz), 166.1 (d, J = 254.5 Hz), 133.4, 132.3
(d, J = 2.8 Hz), 132.2 (d, J = 10.5 Hz), 131.7 (d,
J = 102.1 Hz), 131.1 (d, J = 9.6 Hz), 128.7 (d,
J = 12.4 Hz), 115.7 (d, J = 21.9 Hz), 43.6 (d, J = 56.3 Hz);
31P NMR (CDCl3, 162 MHz): δ 27.3; 19F NMR (376 MHz,
CDCl): δ −104.0; EI-MS: m/z = 338.1 [M+].
1-(4-Chlorophenyl)-2-(diphenylphosphoryl)ethenone (3d).
Yield: 83%; white solid; m.p. 160–163 ꢀC; 1H NMR
(CDCl3, 400 MHz): δ 7.96 (d, J = 8.4 Hz, 2H), 7.82–7.77
(m, 4H), 7.56–7.47 (m, 6H), 7.40 (d, J = 8.8 Hz, 2H), 4.11
(d, J = 14.8 Hz, 2H); 13C NMR (CDCl3, 100 MHz): δ
191.7 (d, J = 5.7 Hz), 140.3, 135.2, 132.3 (d, J = 2.9 Hz),
131.6 (d, J = 103 Hz), 131.0 (d, J = 9.6 Hz), 130.8, 128.9,
128.7 (d, J = 11.4 Hz), 43.6 (d, J = 56.3 Hz); 31P NMR
(CDCl3, 162 MHz): δ 27.3; EI-MS: m/z = 354.1 [M+].
1-(4-Bromophenyl)-2-(diphenylphosphoryl)ethenone (3e).
Yield: 80%; white solid; m.p. 149–152 ꢀC; 1H NMR
(CDCl3, 400 MHz): δ 7.87 (d, J = 8.8 Hz, 2H), 7.82–7.77
(m, 4H), 7.58–7.42 (m, 8H), 4.11 (d, J = 15.2 Hz, 2H); 13C
2-(Diphenylphosphoryl)-1-(4-fluorophenyl)ethenone (3c).
Yield: 82%; white solid; m.p. 156–158 ꢀC; 1H NMR
(CDCl3, 400 MHz): δ 8.07–8.02 (m, 2H), 7.82–7.77 (m,
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Table 2. Substrate scope.a,b
Figure 1. Plausible reaction mechanism.
133.4, 132.2 (d, J = 2.9 Hz), 132.1, 131.8 (d, J = 103 Hz),
131.1 (d, J = 9.5 Hz), 129.9, 128.7 (d, J = 12.4 Hz), 127.5,
118.9, 46.5 (d, J = 57.2 Hz); 31P NMR (CDCl3, 162 MHz):
δ 27.7; EI-MS: m/z = 398.0 [M+].
2-(Diphenylphosphoryl)-1-(thiophen-2-yl)ethenone (3h).
Yield: 70%; colorless oil; H NMR (400 MHz, CDCl3): δ
a Reaction conditions: vinyl azides 1 (0.1 mmol), diphenylphosphine
1
oxide 2 (0.3 mmol), K2S2O8 (0.15 mmol), acetonitrile (1 mL)
7.88 (dd, J = 4.2, 1.0 Hz, 1H), 7.83–7.78 (m, 4H), 7.63 (d,
J = 4.2 Hz, 1H), 7.60–7.44 (m, 6H), 7.09 (dd, J = 4.8,
4 Hz, 1H), 4.07 (d, J = 15.6 Hz, 2H); 13C NMR (100 MHz,
CDCl3): δ 185.1 (d, J = 4.8 Hz), 144.6, 135.4, 135.2, 132.4
(d, J = 1.9 Hz), 131.8 (d, J = 103.9 Hz), 128.8 (d,
J = 12.4 Hz), 128.6, 44.5 (d, J = 57.2 Hz); 31P NMR
(162 MHz, CDCl3): δ 27.5; EI-MS: m/z = 326.1 [M+].
at 60 ꢀC.
b Isolated yield.
NMR (CDCl3, 100 MHz): δ 191.9 (d, J = 5.7 Hz), 135.6,
132.3 (d, J = 1.9 Hz), 131.8, 131.6 (d, J = 105.9 Hz),
131.1 (d, J = 9.5 Hz), 130.8, 129.1, 128.7 (d, J = 12.4 Hz),
43.6 (d, J = 56.2 Hz); 31P NMR (CDCl3, 162 MHz): δ
27.4; EI-MS: m/z = 398.0 [M+].
1-(Diphenylphosphoryl)hexan-2-one (3i). Yield: 61%;
1
colorless oil; H NMR (400 MHz, CDCl3): δ 7.78–7.74 (m,
4H), 7.55–7.48 (m, 6H), 3.59 (d, J = 15.2 Hz, 2H), 2.64 (t,
J = 7.6 Hz 2H), 1.50–1.44 (m, 2H), 1.22–1.18 (m, 2H),
0.83 (t, J = 7.6 Hz 3H); 13C NMR (100 MHz, CDCl3): δ
203.2 (d, J = 5.7 Hz), 132.3, 131.7 (d, J = 105.9 Hz),
130.9 (d, J = 9.6 Hz), 128.8 (d, J = 12.4 Hz), 47.0 (d,
J = 56.2 Hz), 45.1, 25.4, 21.9, 13.8; 31P NMR (162 MHz,
CDCl3): δ 27.2; EI-MS: m/z = 300.1 [M+].
1-(3-Bromophenyl)-2-(diphenylphosphoryl)ethenone (3f).
Yield: 73%; white solid; m.p. 144–147 ꢀC; 1H NMR (CDCl3,
400 MHz): δ 8.04 (t, J = 1.6 Hz, 1H), 7.97 (d, J = 7.6 Hz,
1H), 7.82–7.77 (m, 4H), 7.67–7.64 (m, 1H), 7.57–7.45 (m,
6H), 7.30 (t, J = 8.0 Hz, 1H), 4.12 (d, J = 15.6 Hz, 2H); 13C
NMR (CDCl3, 100 MHz): δ 191.6 (d, J = 5.8 Hz), 138.6,
136.4, 132.3 (d, J = 1.9 Hz), 131.9, 131.7 (d, J = 103.9 Hz),
131.1 (d, J = 10.5 Hz), 130.1, 128.7 (d, J = 12.4 Hz), 128.1,
122.9, 43.5 (d, J = 57.2 Hz); 31P NMR (CDCl3, 162 MHz): δ
27.2; EI-MS: m/z = 398.0 [M+].
Acknowledgment.
This research was supported by
Soonchunhyang University Research Fund.
1-(2-Bromophenyl)-2-(diphenylphosphoryl)ethenone (3g).
Yield: 61%; white solid; m.p. 149–151 ꢀC; 1H NMR
(CDCl3, 400 MHz): δ 7.81–7.75 (m, 4H), 7.56–7.44 (m,
8H), 7.31–7.21 (m, 2H), 4.24 (d, J = 14.4 Hz, 2H); 13C
NMR (CDCl3, 100 MHz): δ 195.6 (d, J = 5.7 Hz), 141.0,
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