Silver-mediated radical 5-exo-dig cyclization of 2-alkynylbenzonitriles: Synthesis of phosphinylated 1-indenones

Article abstract of DOI:10.1039/c7cc01666b

A new silver-mediated 5-exo-dig cyclization of 2-alkynylbenzonitriles with disubstituted phosphine oxide and H₂O has been developed. The reaction enables multiple bond-forming events including C-P, C-C and C-O bonds under atmospheric conditions

Full text of DOI:10.1039/c7cc01666b

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Silver-mediated radical 5-exo-dig cyclization of 2-
alkynylbenzonitriles: Synthesis of phosphinylated 1-indenones
Xiao-Tong Zhu,a,b,c Qi Zhao,a Feng Liu,a Ai-Fang Wang,a Pei-Jun Cai,*,b Wen-Juan Hao,a Shu-Jiang
Tu,*,a and Bo Jiang*,a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
O
O
A
new silver-mediated 5-exo-dig cyclization of 2-
OH
O
MeO
MeO
O
N
alkynylbenzonitriles with disubstituted phosphine oxide and H2O
has been developed. The reaction enables multiple bond-forming
events including C–P, C–C and C–O bonds under air conditions,
leading to the concise and direct formation of 28 examples of
phosphorus-containing 1-indenones with generally good yields.
O
Me
OH
HO
O
MeO2S
Br
I
II
ERβ receptor
III
O
Cl
neo-lignin
COX-2 inhibitor
Figure 1. Some bioactive 1-indenone derivatives
Functional 1-indenones are ubiquitous substructure in natural
products (Figure 1, type I)1 and synthetically bioactive
molecules (Figure 1, type II and III),2 which display a broad
spectrum of extraordinary biological and pharmaceutical
properties3 and also behave as key intermediates in the
preparation of natural products4 and pharmaceuticals.5
Consequently, substantial efforts have been made to develop
powerful and reliable methods for the construction of
indenone frameworks. Known procedures for indenone
formation include traditional intramolecular Friedel–Crafts
acylations,6 Grignard reactions,7 and Heck–Larock annulation8
and related process9 as well as recently well-developed metal-
catalyzed C(sp2)–H activation of arenes.10 Besides, Zhang and
Zhou et al. reported manganese(III)-mediated phosphonation-
cyclization of ynones for 1-indenone preparation (Scheme
1a).11a,b Despite these significant advances achieved in this
field, new protocols for the assemble of diverse functionalized
indenones with readily available substrates under mild
conditions are still highly desirable.11c,d
Ar
Ar
+
O
O
Mn(OAc)3
OR
H
P
OR
P
(a)
OR
OR
O
O
O
Ar1
+
Ar
O
P
P
[Ag]
Ar1
(b)
H
Ar
R
Ar1
Ar
Ar1
R1
This work
R2
O
Ar
P
O
P
R3
AgNO3
R
(c)
(d)
H
R3
R1
+
R2 = Ar
CN
2 NC
R
O
R3
4
2
1
P
R3
AgNO3
R2
R1
H2O
O
3
Scheme 1. Profiles for P-centered radical-triggered cyclization
Meanwhile, organophosphorus compounds have been
extensively utilized in organic synthesis, medicinal chemistry,
and materials science.12 Their synthesis has been attracted
considerable attention. Specifically, P-centered radical-
triggered addition to unsaturated systems provides direct and
atom economic approaches toward organophosphorus
compounds with good functional group compatibility.13 Among
them, di-substituted phosphine oxide radicals, in situ
generated from HP(=O)R1R2 precursor, have exhibits high
reactivity with unsaturated bonds and gradually become an
ideal radical partner for forming organophosphorus
compounds.14 For instance, Miura14a and Duan14b et al.
independently reported the Ag-mediated arylphosphine oxide
radical cyclization of symmetrical diarylalkynes for the direct
preparation of benzo[b]phosphole oxides (Scheme 1b). On the
basis of these studies, we questioned whether the reaction
selectivity could be harnessed to establish a regioselective
[3+2] cyclization by using unsymmetrical diarylalkynes
(Scheme 1c). Interestingly, we found the expected
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o
benzo[b]phosphole oxides cannot be generated as we at 80 C generated the unexpected 1-indenone product 3a in
originally planned. Instead, unexpected Ag-mediated 5-exo-dig 42% yield (Table 1, entry 1). Obvious improvement in the yield
cyclization occurred under oxidative conditions in the presence of 3a was achieved (67%) when the co-solvent of CH3CN and
of water, leading to the direct formation of 3-phosphoryl 1- H2O was used as reaction media (entry 2), indicating water
indenones in a functional-group compatible manner (Scheme could facilitate this radical transformation. Other attempted
1d). To the best of our knowledge, this arylphosphine oxide silver salts like silver trifluoroacetate (AgTFA), silver carbonate
radical-induced carbocyclization of 2-alkynylbenzonitriles for (Ag2CO3), silver acetate (AgOAc) and silver oxide (Ag2O) all
the construction of 1-indenones is virtually unexplored so far. showed a lower catalytic capability and gave unsatisfactory
Herein, we would like to elaborate this interesting results (entries 3-6). A decrease of the loading of silver nitrate
transformation. This protocol represents the first domino resulted in a remarkably dropped yield of 3a and the similar
procedure for the direct synthesis of these new 3- outcome was observed with increase of the amount of silver
phosphorylinden-1-ones through
a
mild Ag-mediated nitrate (entries 7-8). Next, the investigation of the solvent
effect revealed that other aprotic solvents, such as 1,2-
carbonylphosphinylation of internal alkynes.
dichloroethane (DCE), dichloromethane (DCM), 1,4-dioxane,
Table 1. Optimization of Reaction Condition for Product 3aa
THF, MeOH, and ethanol (EtOH) proved to be far less effective
than CH3CN. Besides, the reaction efficiency shows an
important dependency on temperature. A lower conversion
was observed with the reaction temperature at either 70 °C or
100 °C (entry 2 vs. entries 15 and 16). Changing the substrate
ratio to 1:3 or 1:5 did not improve the reaction process
(entries 17 and 18). Next, we attempted to combine the
catalytic amount of AgNO3 and additional oxidants such as
tert-butyl hydroperoxide (TBHP), di-tert-butyl peroxide (DTBP)
and O215 to improve the reaction efficiency (entries 19-21). The
results revealed that all these attempts did not show any
improvements with respect to the reaction yield.
entry
Ag salts (equiv)
solvent
t /(oC)
yield (%)b
1
AgNO3 (2.0)
AgNO3 (2.0)
AgTFA (2.0)
Ag2CO3 (2.0)
AgOAc (2.0)
Ag2O (2.0)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DCE
80
80
80
80
80
80
80
80
80
80
80
80
80
80
100
70
80
80
80
80
80
42c
67
2
3
20
4
NRd
trace
NR
48
Having the optimized conditions in hand (Table 1, entry 2), we
next investigated Ag-mediated 5-exo-dig cyclization of various
alkynes with disubstituted phosphine oxide to evaluate the
generality of the methodology. Upon repeating the reaction
5
6
7
AgNO3 (1.5)
AgNO3 (2.5)
AgNO3 (2.0)
AgNO3 (2.0)
AgNO3 (2.0)
AgNO3 (2.0)
AgNO3 (2.0)
AgNO3 (2.0)
AgNO3 (2.0)
AgNO3 (2.0)
AgNO3 (2.0)
AgNO3 (2.0)
AgNO3 (0.1)
AgNO3 (0.1)
AgNO3 (0.1)
8
51
with diphenylphosphine oxide (2a), substrates
1 with different
functional groups on the arylalkynyl moiety all work well,
efficiently delivering the corresponding functionalized inden-1-
9
40
10
11
12
13
14
15
16
17
18
19
20
21
DCM
33
ones
3 with yields ranging from 54% to 76%. A variety of
1,4-dioxane
THF
52
substituents on the aryl ring of the arylalkynyl moiety, such as
fluoro (1b), chloro (1c-d), bromo (1e), methyl (1f-g), ethyl (1h),
t-butyl (1i), and methoxy (1j-1k, PMP = p-methoxyphenyl)
groups, can tolerate the oxidative conditions well. Generally,
substrates bearing electron-poor groups showed better
reactivity and higher yields than electron-donating ones.
Alternatively, a sterically encumbered 4-chloronaphthalen-1-yl
(4-ClNp) counterpart was proven to be a suitable reaction
component, which underwent a similar diarylphosphine oxide-
enabled cyclization process toward 1-indenone 3l in 61% yield.
However, an n-butyl substrate 1m proved to be ineffective for
this reaction, which may be caused by its relative instability of
the vinyl radical intermediate. Next, we evaluated the
electronic nature of the internal arene ring of 2-
alkynylbenzonitriles. Different substituents like methyl, fluoro
and chloro located at the 4- or 5-position of benzonitrile ring
were compatible in these radical cyclization reactions, and the
corresponding inden-1-ones 3n-x were afforded in 50%-73%
yields. Regarding the scope of diaryl phosphine oxides, besides
trace
40
MeOH
EtOH
38
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
43
31
36e
44 f
37g
26h
35i
aReaction conditions: 1a (0.2 mmol), 2a (0.8 mmol), Ag salt (X equiv), solvent
(1.5 mL) and water (0.5 mL) for 4.0 hours, air conditions. bIsolated yield of
product 3a based on 1a. cWithout additional water. dNR= No reaction. eThe
ratio of 1a:2a in 1:3. The ratio of 1a:2a in 1:5. gUse of TBHP (2.0 equiv). hUse
of DTBP (2.0 equiv).iUse of O2 (1.0 atm).
f
Initially,
2-(phenylethynyl)benzonitrile
(
1a
)
and
diphenylphosphine oxide (DPPO, 2a) were selected as the
model substrates to identify the optimal reaction conditions,
with different silver salts as radical promoters and acetonitrile
(CH3CN) as the solvent. The results are summarized in Table 1.
In the presence of 2.0 equivalents of silver nitrate, the reaction
phenyl substrate, 4-methyl (2b) and 3,5-dimethylphenyl (2c
)
analogue could also be accommodated, thus confirming the
reaction efficiency, as 1-indenones 3y-3bb were generated in
good yields. Additionally, the reaction is also applicable to
2 | J. Name., 2012, 00, 1-3
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diethyl phosphite, delivering the corresponding product 3cc in condition (Scheme 3a), but no expected product 3a was
40% yield. Obviously, the current radical 5-exo-dig cyclization observed. Among them, a BHT-P adduct was detected by LC-
can tolerate structurally distinct substrates with steric bulk and MS and 31P NMR analysis,16 indicating the reaction involves a
a
different electronic property, which provides
a
radical mechanism. Subsequently, the reaction was carried out
straightforward and practical pathway for forming richly under Ar conditions to access a 62% yield of 3a (Scheme 3b),
decorated 1-indenones with generally good yields
suggesting that oxygen atom at the carbonyl unit comes from
water. The reaction of 1a did not work without DPPO (2a
)
under the standard conditions, showing that phosphine oxide
radicals triggered 5-exo-dig cyclization (Scheme 3c).
On the basis of above observations and the literature survey,14
a tentative mechanism for this transformation was proposed in
Scheme 4. First, diphenylphosphine oxide (2a) reacts with
AgNO3 to form diphenylphosphine oxide radical
single electron transfer (SET).14 After that, intermolecular
addition of into 1a gives the alkenyl radical , which
undergoes 5-exo-dig cyclization to give intermediate (route
i) rather than 5-endo-trig cyclization to form intermediate
A through a
A
B
D
C
(route ii). The reason may be that the polarized triple bond
(cyano group) favors as radical acceptor more than phenyl ring
because of its easy
protonation of occurs to yield intermediate
π
-band breaking.17 Subsequently, the
E, which is
D
converted into the target product 3a via silver(I)-catalyzed
hydrolysis in the presence of H2O.18
Ph
O
P
O
B
P
Ph
Ag(I)
O
Ph
Ph
5-endo-trig
P
O
Ph PH
cyclization
route ii
Ph
Ph
2a
Ag(0)
CN
A
NC
Ph
C
Ph
Ph
Ph
O
O
P
Ph
Ph
P
Ph
Ph
A
5-
exo-dig
cyclization
route i
CN
1a
N
N
B
D
O
Scheme 2. Synthesis of 3-phosphoryl 1-Indenones (i) Reaction
conditions: 1 (0.2 mmol), 2 (0.8 mmol), AgNO3 (0.4 mmol), H2O (0.5 mL), and
CH3CN (1.5 mL) in the seal reaction tube under air conditions at 80 oC for 4.0
hours. (ii) Isolated yields based on 1.
protonation
Ph
Ph
P
Ph
O
O
P
P
Ph
Ph
Ph
Ph
Ph
Ph
Ag(I)
The structures of the resulting 1-indenones 3 were determined
-Ag(I)
-NH3
by carrying out their NMR and HRMS analysis. Furthermore, in
the case of 3f, its structure was further confirmed by X-ray
diffraction (see Supporting Information).
O
3a
NH
NH
E
H2O
(I)Ag
F
Scheme 4. Plausible Reaction Pathway
OH
O
tBu
tBu
Ph
Ph
P
AgNO3, H2O
Ph
Ph
In summary, we have developed a new P-centered radical-
triggered addition-cyclization that offers a direct protocol
CH3CN, 80 oC, air
(a)
+
2a
O
TEMPO (2.0 equiv)
or BHT (2.0 equiv)
CN
1a
P
O
toward
phosphorus-containing
1-indenones
via
Ph
(LCMS and 31P NMR)
Ph
3a (not observed)
carbonylphosphinylation of 2-alkynylbenzonitriles. This
transformation consists of P-centered radical addition, 5-exo-
dig cyclization and hydrolysis process, resulting in the
successive multiple bond-forming events including C–P, C–C
and C–O bonds in a one-pot manner. This reaction features
flexible structural modification, broad substrate scope and
high functional group tolerance as well as mild reaction
AgNO3, H2O
1a
2a
+
(b)
(c)
3a
(62%)
CH3CN, 80 oC, Ar
AgNO3, H2O
1a
no reaction
CH3CN, 80 oC, air
Scheme 3. Control experiments
To gain mechanistic insight of this transformation, AgNO3- conditions. Currently, experiments toward further biological
mediated reaction of 1a with 2a in the presence of, 2,2,6,6- application are underway in our laboratory.
tetramethyl-1-piperidinyloxy (TEMPO) or butylhydroxytoluene
We are grateful for financial support from the NSFC (No. 21232004,
(BHT), a radical scavenger, were performed under the standard
21472071 and 21602087), PAPD of Jiangsu Higher Education
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DOI: 10.1039/C7CC01666B
Silver-mediated radical 5-exo-dig cyclization of
2-alkynylbenzonitriles: Synthesis of phosphinylated 1-indenones
Xiao-Tong Zhu, Qi Zhao, Feng Liu, Ai-Fang Wang, Pei-Jun Cai, Wen-Juan Hao, Shu-Jiang Tu,
and Bo Jiang
A new silver-mediated 5-exo-dig cyclization of 2-alkynylbenzonitriles with di-substituted
phosphine oxide and H2O has been developed.