Metal-free synthesis of phosphinoylchroman-4-ones via a radical phosphinoylation–cyclization cascade mediated by K2S2O8

Article abstract of DOI:10.3762/BJOC.16.164

A variety of chroman-4-ones bearing phosphine oxide motifs were conveniently synthesized from readily available diphenylphosphine oxides and alkenyl aldehydes via a metal-free tandem phosphinoylation/cyclization protocol. The reaction utilizes K₂S₂O₈ as oxidant and proceeds in DMSO/H₂O at environmentally benign conditions with a broad substrate scope and afforded the title compounds in moderate yields.

Full text of DOI:10.3762/BJOC.16.164

Metal-free synthesis of phosphinoylchroman-4-ones via
a radical phosphinoylation–cyclization cascade
mediated by K2S2O8
Qiang Liu1,2, Weibang Lu1, Guanqun Xie*1 and Xiaoxia Wang1
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2020, 16, 1974–1982.
1Dongguan University of Technology, Dongguan 523808, P. R. China
and 2Department of Applied Chemistry, School of Science, Xi’an
Jiaotong University, Xi’an 710049, P. R. China
doi:10.3762/bjoc.16.164
Received: 09 June 2020
Accepted: 31 July 2020
Published: 12 August 2020
Email:
Guanqun Xie* - 2018008@dgut.edu.cn
Associate Editor: B. Nay
* Corresponding author
© 2020 Liu et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
chroman-4-ones; diphenylphosphine oxide; metal-free; potassium
persulfate; radical cyclization
Abstract
A variety of chroman-4-ones bearing phosphine oxide motifs were conveniently synthesized from readily available diphenylphos-
phine oxides and alkenyl aldehydes via a metal-free tandem phosphinoylation/cyclization protocol. The reaction utilizes K2S2O8 as
oxidant and proceeds in DMSO/H2O at environmentally benign conditions with a broad substrate scope and afforded the title com-
pounds in moderate yields.
Introduction
The chroman-4-one framework is a privileged structural motif through condensation cyclization reactions of o-hydroxyace-
found in numerous natural products and pharmaceuticals with tophenones with ketones/aldehydes [11,12], in addition to other
extraordinary biological and pharmaceutical activities such as alternative transformations [13,14]. Moreover, radical cascade
anticancer activities and anti-HIV activity among others cyclizations of o-allyloxybenzaldehydes by employing appro-
(Figure 1) [1-3]. Therefore, the preparation of functionalized priate radical precursors through visible-light promoted systems
chroman-4-one derivatives has attracted great attention of [15,16], transition-metal-catalyzed systems [17,18], or transi-
experts and scientists in the field of organic synthesis and phar- tion-metal-free systems [19,20], have emerged as a powerful
maceutical sciences over the last few years [1,4,5]. In general, strategy for the synthesis of diversely functionalized chroman-
chroman-4-one derivatives could be obtained via a polarity 4-one derivatives.
reversal strategy enabled by the N-heterocyclic carbene (NHC)-
catalyzed intramolecular Stetter reaction [6-8]. Besides, Organophosphorus compounds are well-known for their medici-
chroman-4-one derivatives were also constructed via intramo- nal, biological, or specific material-related properties and have
lecular oxa-Michael additions of 2’-hydroxychalcones [9,10], or found wide applications in pharmaceutical chemistry, biochem-
1974

Beilstein J. Org. Chem. 2020, 16, 1974–1982.
Figure 1: Biologically active compounds featuring the chroman-4-one framework.
istry, and materials science [21-26]. They represent also excel- available, and versatile oxidant. On the basis of literature
lent ligands for many metals and have been used in catalytic reports [29,30] and our continuing interest in green chemistry
systems for a huge number of organic reactions [21-26]. Due to [31,32], we set the temperature at 70 °C based on the fact that
the importance of the chroman-4-one scaffold on one hand and K2S2O8 thermally decomposes forming sulfate radicals (SO4·−)
that of organophosphorus compounds on the other, the develop- [29,30], which may react with the substrates to furnish such a
ment of concise and efficient approaches for the synthesis of cascade cyclization. To our delight, the anticipated product 3aa
chroman-4-one derivatives decorated with phosphorus function- was obtained in 42% yield in DMSO/H2O (4:1) as reaction me-
alities [27,28], thus combining the characteristics of both com- dium in one pot (Table 1, entry 1). The structure of 3aa was un-
ponents in one molecule may find useful applications. However, ambiguously confirmed by X-ray diffraction analysis of a single
there are only few ways to prepare such compounds. For exam- crystal (Figure 2) and by NMR spectroscopy (see Supporting
ple, in 2008 Rovis et al. [27] reported an intramolecular Stetter Information File 1) [33]. The increase of the amount of K2S2O8
reaction of alkenyl aldehydes to synthesize a series of phos- to 3 equiv resulted in the improvement of the yield of 3aa to
phine oxide and phosphonate-functionalized chroman-4-ones. 52% (Table 1, entry 2). However, adjusting the amount of the
Unfortunately, the preparation of the substrates involved a oxidant K2S2O8 to 4 equiv (Table 1, entry 3) did not further
Rh-catalyzed hydrophosphinylation of a protected functional improve the yield. By further screening of a few solvents, such
alkyne, and the subsequent deprotection with Hg(O2CCF3)2, as MeCN/H2O, DMF/H2O, DMA/H2O, dioxane/H2O, THF/
which is not environmentally benign (Scheme 1a). Besides, in H2O, EtOH/H2O, DCE/H2O, and NMP/H2O, it turned out that
2016 Li’s group [28] reported a silver-catalyzed straightfor- the highest yield was achieved in the DMSO/H2O (4:1) system
ward approach for the synthesis of phosphonate-functionalized (Table 1, entries 4–13). It is notable that product 3aa was not
chroman-4-ones via a phosphoryl radical-initiated cascade observed at room temperature or in the absence of K2S2O8, in-
cyclization of 2-(allyloxy)arylaldehydes using K2S2O8 as an dicating that the reaction was mainly mediated by K2S2O8
oxidant, however, diphenylphosphine oxide (DPPO) was not (Table 1, entries 14 and 15). Increasing the reaction tempera-
suitable for the transformation (Scheme 1b). So the develop- ture to 80 °C afforded better product yields as compared with
ment of metal-free and greener methods to approach chroman- the reactions performed at either 70 °C or 90 °C (Table 1,
4-ones bearing a phosphine oxide moiety is still highly desir- entries 2, 16, and 17). Then, various oxidants such as
able. Herein, we present a transition-metal-free radical cascade (NH4)2S2O8, Na2S2O8, TBHP (tert-butyl hydroperoxide),
cyclization to access the desired chroman-4-one derivatives in DTBP (di-tert-butyl peroxide), and dioxygen were tested and
one pot under environmentally benign conditions (Scheme 1c). the results showed that K2S2O8 exhibited the best efficiency
(Table 1, entries 18–22).
Results and Discussion
Motivated by the desire to develop a metal-free and environ- With the optimal reaction conditions in hand (Table 1, entry
mentally benign protocol for the construction of phosphine 16), we next explored the scope and generality of this protocol
oxide-functionalized chroman-4-ones, we focused on the using various 2-(allyloxy)arylaldehydes 1 for the reaction with
cascade cyclization employing 2-(allyloxy)benzaldehyde (1a) 2a. As shown in Scheme 2, substrates 1 with a range of func-
and diphenylphosphine oxide (DPPO, 2a) as the model sub- tional groups, such as electron-donating groups Me- (1b and
strates with K2S2O8 as the oxidant, which is a cheap, readily 1c), t-Bu- (1d and 1e), and electron-withdrawing groups Cl-
1975

Beilstein J. Org. Chem. 2020, 16, 1974–1982.
Scheme 1: Methods to produce phosphonate-substituted chroman-4-ones.
Table 1: Optimization of the reaction conditions.a
Entry
Oxidant
Solvent
Temp. (°C)
Yield (%)b
1
2
3
4
5
6
7
K2S2O8 (2.0 equiv)
K2S2O8 (3.0 equiv)
K2S2O8 (4.0 equiv)
K2S2O8 (3.0 equiv)
K2S2O8 (3.0 equiv)
K2S2O8 (3.0 equiv)
K2S2O8 (3.0 equiv)
DMSO/H2O (4:1 v/v)
DMSO/H2O (4:1v/v)
DMSO/H2O (4:1v/v)
MeCN/H2O (4:1 v/v)
DMF/H2O (4:1 v/v)
DMA/H2O (4:1 v/v)
dioxane/H2O (4:1 v/v)
70
70
70
70
70
70
70
42
52
50
32
24
21
trace
1976

Beilstein J. Org. Chem. 2020, 16, 1974–1982.
Table 1: Optimization of the reaction conditions.a (continued)
8
K2S2O8 (3.0 equiv)
K2S2O8 (3.0 equiv)
K2S2O8 (3.0 equiv)
K2S2O8 (3.0 equiv)
K2S2O8 (3.0 equiv)
K2S2O8 (3.0 equiv)
–
THF/H2O (4:1 v/v)
EtOH/H2O (4:1 v/v)
DCE/H2O (4:1 v/v)
NMP/H2O (4:1 v/v)
DMSO/H2O (1:1 v/v)
DMSO/H2O (8:1 v/v)
DMSO/H2O (4:1 v/v)
DMSO/H2O (4:1 v/v)
DMSO/H2O (4:1v/v)
DMSO/H2O (4:1v/v)
DMSO/H2O (4:1 v/v)
DMSO/H2O (4:1v/v)
DMSO/H2O (4:1 v/v)
DMSO/H2O (4:1 v/v)
DMSO/H2O (4:1 v/v)
70
70
70
70
70
70
70
rtc
80
90
80
80
80
80
80
trace
trace
trace
18
32
44
0
9
10
11
12
13
14
15
16
17
18
19
20
21
22
K2S2O8 (3.0 equiv)
K2S2O8 (3.0 equiv)
K2S2O8 (3.0 equiv)
(NH4)2S2O8 (3.0 equiv)
Na2S2O8 (3.0 equiv)
DTBP (3.0 equiv)
TBHP (3.0 equiv)
O2
0
58
54
40
50
0
0
0
aReaction conditions: 1a (0.3 mmol, 1 equiv), 2a (1.5 equiv), solvent (v/v, 5 mL), N2 atmosphere, 18 h. bIsolated yields. cRoom temperature.
Figure 2: X-ray structure of compound 3aa (CCDC 2002878).
(1f), Br- (1g), and F- (1h) were well tolerated in this transfor- N-allyl-N-(2-formylphenyl)-4-methylbenzenesulfonamide (1l)
mation, providing the desired products 3aa–ha in 48–62% was examined in the cascade cyclization and the desired prod-
yields. Furthermore, the transformation also proceeded with ucts 3la, 3lf were obtained in 44% and 40% yield.
naphthyl substrate 1i giving the desired product 3ia in 45%
yield. Notably, when the substrate was 2-allylbenzaldehyde Next, to further demonstrate the generality of this strategy, the
(1j), the protocol was also compatible affording the indanone scope of different diphenylphosphine oxide 2 was examined as
derivative 3ja with comparable yield. This outcome is of special shown in Scheme 3. Simple diphenylphosphine oxides, such as
interest, because indanone derivatives are also privileged struc- 2-Me-DPPO (2b), 4-Me-DPPO (2c), and 4-t-Bu-DPPO (2d)
tural motifs found in numerous natural products and pharma- furnished the corresponding products in good yields. Also
ceuticals with extraordinary biological and pharmaceutical ac- multisubstituted diarylphosphine oxides 2e and 2f were well
tivities [34,35]. However, no desired product (3ka) was ob- tolerated under these reaction conditions. Gratifyingly,
tained when there was a nitro group in the substrate (1k). Then, diphenylphosphine oxides bearing fluoro-substituents (2g)
1977

Beilstein J. Org. Chem. 2020, 16, 1974–1982.
Scheme 2: Scope of 2-(allyloxy)arylaldehydes. Reaction conditions: 1 (0.3 mmol, 1 equiv), 2a (1.5 equiv) [2f for product 3lf], DMSO/H2O (4:1 v/v,
5 mL), K2S2O8 (3.0 equiv), N2 atmosphere, 18 h. Yields are isolated yields.
reacted smoothly furnishing the anticipated product 3ag in 58% substrates under the optimized conditions (Scheme 4). The reac-
yield. Furthermore, 1-naphthyl-DPPO (2h) was also suitable for tion afforded the desired product 3ba in a good yield of 56%,
this transformation, and afforded the expected product 3ah in and the structure was also confirmed by X-ray diffraction (see
50% yield. The reaction between diethyl phosphonate (2j) Supporting Information File 1) [33].
and 1a proceeded less efficiently under the conditions and
a low yield of 3aj was obtained. Dimethylphosphine oxide (2k) To gain an insight into the reaction mechanism, we carried out
did not participate in the reaction, likely due to its high some control experiments (Scheme 5). When the reaction was
oxidation potential and poor ability to undergo tautomerization conducted in the presence of radical scavengers such as 2,2,6,6-
[36].
tetramethyl-1-piperidinyloxyl (TEMPO) and butylated hydroxy-
toluene (BHT), the reactions were completely shut down, which
To demonstrate the practicability of this methodology, a gram- indicated that the reaction proceeds through a radical pathway
scale experiment was next performed, employing 1b and 2a as [37-41]. Also, we successfully separated a small amount of by-
1978

Beilstein J. Org. Chem. 2020, 16, 1974–1982.
Scheme 3: Scope of diphenylphosphine oxides. Reaction conditions: 1a (0.3 mmol, 1 equiv), 2 (1.5 equiv), DMSO/H2O (4:1 v/v, 5 mL), K2S2O8
(3.0 equiv), N2 atmosphere, 18 h. Yields are isolated yields.
Scheme 4: Gram-scale reaction.
1979

Beilstein J. Org. Chem. 2020, 16, 1974–1982.
Scheme 5: Control experiments and proposed mechanism.
product 4 which was identified by NMR spectroscopy. These attack on the aldehyde group. The oxygen radical III thus
experiments clearly support a phosphorus-centered radical reac- formed undergoes a formal 1,2-H shift to generate the benzyl
tion pathway. It has been reported that phosphorus-centered radical IV [45,46]. Finally, hydrogen abstraction by the sulfate
radicals could be generated from phosphine oxides in the pres- radical anion (SO4•−) from the benzyl radical IV affords the
ence of potassium persulfate [42-44]. Based on literature prece- final products 3 [45,46].
dent [29,30,42-46] and preliminary mechanistic experiments, a
plausible mechanism was proposed in Scheme 5 which was dif- Conclusion
ferent from the predominant mechanism observed in the Ag-cat- In summary, an environmentally benign and practical radical
alyzed radical cascade for the preparation of phosphonate-func- cascade cyclization was developed to synthesize a series of
tionalized chroman-4-ones [28]. Initially, K2S2O8 thermally phosphonate-functionalized chroman-4-ones from 2-(allyl-
decomposes to form sulfate radical anions (SO4•−) [29,30], oxy)benzaldehydes and diphenylphosphine oxides. This
which react with diphenylphosphine oxide (DPPO, 2) to give protocol proceeds under metal-free conditions and uses cheap
the phosphorus-centered radical I [42-44]. Then, the phos- K2S2O8 as oxidant with easy handling and a broad substrate
phorus centered radical I added to the C–C double bond of 1 to scope. The reaction proceeds through a radical phosphinoyla-
generate a new carbon-centered radical II, with sequential tion–cyclization via a tandem C–P and C–C-bond formation.
1980

Beilstein J. Org. Chem. 2020, 16, 1974–1982.
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Supporting Information File 1
Experimental procedures, spectroscopic and X-ray data and
copies of NMR spectra.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-16-164-S1.pdf]
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Funding
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Org. Chem. Front. 2020, 7, 487–491. doi:10.1039/c9qo01283d
20.Tang, L.; Yang, Z.; Chang, X.; Jiao, J.; Ma, X.; Rao, W.; Zhou, Q.;
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We are grateful for the financial supports from the Science
Foundation for Distinguished Scholars of Dongguan University
of Technology (No. GC200906-10) and the Postdoctoral
Foundation of Dongguan University of Technology (No.
196100040006).
doi:10.1021/acs.orglett.8b02846
21.Chen, L.; Liu, X.-Y.; Zou, Y.-X. Adv. Synth. Catal. 2020, 362,
1724–1818. doi:10.1002/adsc.201901540
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