Photocatalyzed cascade oxidative annulation of propargylamines and phosphine oxides

Article abstract of DOI:10.1039/c7cc02433a

On account of the broad utilities of organophosphorus compounds, the development of highly efficient and concise phosphination methods is significantly important and urgent. Herein, we disclose a novel method for the synthesis of phosphorylated heterocycles: versatile intermediate propargylamines serving as a new type of radical acceptors incorporated in P-radicals via a photocatalytic strategy. This reaction proceeds through a cascade phosphinoylation/cyclization/oxidation/aromatization pathway using readily available starting materials under mild conditions of light with excellent atom economy, catalyzed by AgOAc or fac-Ir(ppy)₃. One of the phosphorylated quinolines was selected, as an example, as an electron-transporting material for fabricating phosphorescence organic light-emitting diodes displaying excellent electroluminescence performances with a maximum external quantum efficiency of 21.9% with negligible efficiency roll-off ratios.

Full text of DOI:10.1039/c7cc02433a

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Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Photocatalyzed Cascade Oxidative Annulation of Propargylamines
and Phosphine Oxides
Received 00th January 20xx,
Accepted 00th January 20xx
Zheng-Guang Wua, Xiao Liangb, Jie Zhoua, Lei Yua, Yi Wanga,*, You-Xuan Zhenga,*, Yu-Feng Lib,*, Jing-
Lin Zuoa, and Yi Pana
DOI: 10.1039/x0xx00000x
www.rsc.org/
On account of broad utilities of organophosphorous compounds,
development of highly efficient and concise phosphination
In fact, the P−H bond of phosphine oxides tends to undergo
hemolytic cleavage process via the mediation of Ag(I), Mn(III), or
methods is significantly important and urgent. Herein, we disclose peroxides to generate P-centered radicals. Inspired by this elegant
reactivity, various methods to incorporate a phosphoryl group with
radical acceptors such as internal alkynes, alkynoates, isocyanides,
and N-arylacrylamides to synthesize phosphorus-containing
compounds have been established.3
a novel method for the synthesis of phosphorylated heterocycles:
versatile intermediate propargylamine serving as a new type of
radical acceptors incorporating with P-radical via a photocatalytic
strategy. This reaction proceeds through
a
cascade
Propargylamines are synthetically versatile intermediates for
the construction of biologically active heterocycles, which can be
easily prepared via 3A-coupling of amines, aldehydes and alkynes,
like “bread and butters” serving as the most basic components for
phosphinoylation/cyclization/oxidation/aromatization pathway
using readily available starting materials under mild conditions of
light with excellent atom economy, catalyzed by AgOAc or fac-
Ir(ppy)3. One of the phosphorylated quinolines was selected as an
example as an electron-transporting material for fabricating
phosphorescence organic light-emitting diodes displaying
excellent electroluminescence performances with a maximum
external quantum efficiency of 21.9% with negligible efficiency
roll-off ratios.
achieving
a series of cascade reactions with diversity and
complexity.4 We presumed that propargylamine could be employed
as a novel radical acceptor to incorporate with P-centered radicals
for phosphorylated quinolone synthesis (Scheme 1b). The
electronic nature of such intermediate defines the chemical
selectivity of radical-induced cyclization. The active vinyl phosphoryl
species would only attack the ortho-positon of the arylamines,
forming the kinetically favored six-membered ring. This strategy
would benefit from simple raw materials, diverse structures and
excellent atom economy compared with the traditional metal-
catalyzed coupling methods. Furthermore, the phosphorylated
quinoline derivatives could be applied as novel electron-
transporting material (ETM) for fabricating organic light-emitting
diodes (OLEDs) which might expand the utility of this
phosphoylation methodology.
Development of phosphorus chemistry is a subject of continuous
interest and great importance because organophosphorous
compounds have a wide range of applications in organic chemistry
as ligands for transition metal catalysts, in medicinal science as
biologically active molecules, as well as in material chemistry as π-
conjugated materials.1 Due to the introduction of phosphorus atom
into organic molecules can induce marked changes in their physical
and chemical properties, phosphination has become a prevalent
and powerful strategy in the design of new molecules. In light of
this, numerous efforts have been devoted to developing effective
phosphination methods. The transition metal-catalyzed cross-
coupling reactions are the traditional strategies for C−P bond
formation (Scheme 1a).2 Recently, with the rapid progress of radical
chemistry, alternative methods utilizing the active phosphoryl
radical [R1R2P-(O)·] to construct the C−P bond have been explored.
O
N
X
O
P
N
[Ni] or [Pd]
HP R4
R5
R4
+
(X= Br, OTf, B(OH)2 etc.)
(a)
R5
traditional metal-catalyzed cross-coupling reactions
NH2
O
R1
R4
HP
R5
R2
N
R2
O
R1
R1
[Cu]
+
R4
radical path
N
H
(b)
CHO
P
R5
R3
3A-Coupling
R2
+
R3
R3
Cascade Phosphinoylation/Cyclization
/Oxidation/Aromatization Process
versatile intermediate
Scheme 1 Radical Cascade Reaction for the Synthesis of Phosphorylated
Heterocycles.
Firstly, a variety of silver salts were explored for this reaction
of propargylamine 1a and diphenylphosphine oxide 2a (Table
S1). To our delight, the phosphorylated quinoline could be
obtained smoothly in 76% yield with AgOAc (3.0 eqiv) as the
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amount of ruthenium bipyridine complex and 2.0 equivalent of
PIDA gave trace product after stirring in DMF under ambient
promoter through a cascade radical process. Nevertheless, this
transformation required an excess amount of AgOAc and high
temperature. Therefore, an effective, mild and ecofriendly atmosphere for 36 h (Table 1, entry 1). When using iridium
phosphorylation strategy is highly desired.
complexes or Eosin Y, the desired phosphorylated product was
formed in low to moderate yields (entries 2-6). Among them,
Ir(ppy)3 exhibited the best catalytic effects with the yield over 50%
(entry 4). Fortunately, after screening the various bases (entries
7-11), NaHCO3 could promote the reaction yield up to 62%. An
investigation of solvents demonstrated that DMF was the
optimal choice with the yield of 62%, which was better than
other solvents, such as DMSO (40%), NMP (45%), CH3CN (15%),
acetone (trace). The reaction efficiency was further enhanced up
to 78% when performed under argon atmosphere (entry 22). No
product was observed under oxygen or in darkness (entries 21,
23). Furthermore, a small proportion of propargylamine were
oxidized to propargylimine, and the side reaction leads to the
insufficient yield of phosphoryl radical cyclization.
Table 1 Optimisation of the reaction conditionsa
entry
catalyst
oxidant
base solvent yieldb
.
1
2
Ru(bpy)3 6H2O
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
PIFA
-
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
NMP
CH3CN
trace
13%
44%
53%
37%
15%
50%
62%
42%
31%
trace
trace
32%
trace
NR
(dfppy)2Ir(dtbpy)PF6
Ir(mppy)3
Ir(ppy)3
FIrpic
-
3
-
4
-
5
-
6
EosinY
-
7
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Na2HPO4
NaHCO3
LiOAc
K2CO3
DABCO
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
With the optimized reaction conditions, the generality of the
method was investigated and the products were summarized in
Scheme 2. First, the scope of the reaction was explored with
respect to the propargylamine component. As shown in Scheme
8
9
10
11
12
13
14
15
16
17
18
19
20
21c
22d
23e
2
, various functional groups on the aromatic rings of Ar1, Ar2
and Ar3 displayed good tolerance under the optimized
conditions. Propargylamines with whether electron-donating
(methyl, methoxyl and tert-butyl) or electron-withdrawing
(fluorine, chlorine and trifluoromethyl) groups can react
smoothly with diphenylphosphine oxides to give the
corresponding 3-phosphorylated quinolines in moderate to good
yields. In addition, the propargylamines with bulk naphthyl and
biphenyl group at the aromatic rings of Ar1 and Ar2 could also
be transferred to give 3e and 3g with 60% yield. Furthermore,
substrate with other heteroarene such as thienyl could also
react efficiently with 2a to afford the expected product 3o in 33%
yield. For the scope of phosphine oxide derivatives, various
P(O)−H compounds were employed to react with 1a, giving the
Ph2I+OTf-
TBHP
BQ
CBr4
NR
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
40%
45%
15%
acetone trace
DMF
DMF
DMF
NR
78%
0
aReaction condition: propargylamine (0.3 mmol), diphenylphosphine
oxide (0.6 mmol), photocatalyst (3 mol%), oxidant (0.6 mmol), base
(0.45 mmol), solvent (3.0 mL), under an ambient atmosphere for 36 h.
corresponding products 3p−3r in 47
−60% yields. Notably, the
phosphaphenanthrene derivative could also generate the P-
radical to furnish the 3-cyclophosphate quinoline 3q, which was
reported first time. Furthermore, the above mentioned
reactions can also be performed in one-pot four-component
process with corresponding amines, aldehydes, alkynes and
phosphine oxides (Scheme S1).
bIsolated yield. cIn dark. dUnder argon. eUnder oxygen. PIDA
=
phenyliodine diacetate. BQ = 1,4-benzoquinone.
In recent years, visible-light-induced photoredox catalysis has
drawn great attention due to the inherent green character of
light, high reactivity, and good functional group tolerance.
Several recent studies have revealed that the diarylphosphine
oxides could undergo single electron transfer (SET) process to
afford P-centered radical by a reductive quenching cycle of the
excited state of photocatalyst.5 Inspired by this, a new pathway
employing visible-light-induced reaction of propargylamines and
phosphine oxides for organophosphorous compounds was
investigated.
Based on the above experimental results in combination with
literatures,5b,5d a plausible mechanism was depicted in Scheme 3
The photocatalyst Ir(III) is activated by visible light to the
excited-state Ir(III)*, which was oxidized by PIDA to Ir(IV). The
high valence of Ir(IV) would oxidize diphenylphosphine oxide to
produce Ir(III) and P-radical cation. Thereafter, the key
intermediate P-centered radical would be generated through a
hydrogen-atom-transfer (HAT) process, which then undergoes
intermolecular addition with radical acceptor 1a to form the
.
Our study started with a model reaction of propargylamine 1a
with diphenylphosphine oxide 2a in the presence of various
photocatalysts and oxidants (Table 1, and Table S2). A catalytic
alkenyl radical
4
. Subsequently, intramolecular cyclization of
4
produces a cyclohexadienyl-type radical intermediate
5
, which
goes through a single electron oxidation transfer process to
2 | J. Name., 2012, 00, 1-3
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Scheme 3 Plausible Mechanism for Photocatalytic Phosphorylation.
To assess the performances of QDPO as the ETM, a typical
OLED device was fabricated using widely reported materials with
the configuration of indium tin oxide (ITO)/MoO3 (5 nm)/TAPC
(di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane)
(30
nm)/
Ir(tfmppy)2tpip (8 wt%) : mCP [1,3-bis(N-carbazolyl)benzene] (10
nm)/QDPO (30 nm)/LiF (1 nm)/Al (100 nm). In this device, MoO3
and LiF served as hole- and electron-injecting interface modified
materials, respectively. TAPC and mCP acted as hole-
transporting and host material, respectively. The Ir(tfmppy)2tpip
was also developed by our group as a green emitter containing
P=O bonds in ancillary ligand.6a The chemical structures of the
used materials as well as the energy level diagram of the device
architecture were illustrated in Figure S4. In order to compare
the device performances with conventional materials, the other
device using typical electron-transporting material TPBi (2,2’,2’’-
(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole))
with
similar configuration was also fabricated.
The current density – voltage – luminance, current efficiency
luminance, power efficiency luminance and external
–
-
quantum efficiency –luminance curves of the devices are
presented in Figure 1 and Figure S5, and the performance data
are summarized in Table 2. Device based on QDPO exhibits a
lower turn-on voltage of 2.9 V than that of TPBi-based device
(3.1 V), possibly due to the lower LUMO level and high electron
mobility of QDPO. Furthermore, excellent EL performances with
the maximum luminance close to 40000 cd/m2, the peak current
efficiency (ηc,max) of 83.53 cd/A, a maximum external quantum
generate cation intermediate 6. Finally, tandem deprotonation
of 6 and aromatization of 7 afford the target product 3a.
efficiency (EQEmax) of 21.9% and a peak power efficiency (ηp,max
)
Scheme 2. Scope of the Reaction of Propargylamines with Phosphine
Oxides.
of 67.27 lm/W were achieved by device using QDPO. These
performances are higher than those of the TPBi-based device,
which showed a ηc,max of 64.00 cd/A, EQEmax of 16.7% and a
ηp,max of 49.07 lm/W (Table 2). In general, because of the relative
long excited state life-time of the Ir(III) complex, phosphorescent
OLEDs exhibit severe roll-off at a higher current density due to
triplet−triplet annihilaꢀon and triplet-polaron annihilation.
Noticeably, with increase of the current density, the device
exhibits small efficiency roll-off ratios with the ηc of 79.36 and
82.42 cd/A at practical brightness of 100 cd/m2 and 1000 cd/m2,
respectively, as well as EQE of 20.8% and 21.6% without any
light-outcoupling technic. The electroluminescence spectra of
both devices reveals the emission maxima at 523 nm with the
corresponding Commission Internationale de l’Éclairage (CIE)
color coordinates of (0.32, 0.64) and no emission from host and
adjacent materials (Figure 1a). This results indicate that the
recombination of the carriers (hole and electron) occurs only in
In recent decades, phosphorus-containing heterocycles have
been widely used as electron-accepting building blocks in
materials for organic light-emitting diodes (OLEDs). In virtue of
excellent electron transport property, the phosphoryl (P=O)
moiety has been extensively introduced in phosphorescent
metal complexes,6 bipolar host and electron-transporting
materials (ETM) .7 In this case, we selected compound 3a ((2,4-
diphenylquinolin-3-yl)diphenylphosphine oxide: QDPO) as an
example to be employed as an efficient ETM for OLEDs. First, the
physical properties of QDPO, including UV−vis absorpꢀon,
photoluminescence spectra and thermal stability, were
measured, and the key spectral are presented in Figure S1 and
the data are summarized in Table S3. The LUMO/HOMO energy
levels of QDPO were estimated as 2.80 and 6.65 eV, respectively
(
Figure S2 and Table S3).
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Table 2. Key EL Data of the OLEDs.
a)
c)
d)
e)
f)
Vturn-on
Device
[V]
Lmax(voltage)b)
[cd m-2(V)]
ηc.max
[cd A-1]
ηc,100/1000
[cd A-1]
ηp.max
[lm W-1]
EQEmax,100,1000
[%]
QDPO
TPBi
2.9
3.1
38929(13.5)
40422(13.3)
83.53
79.36/82.42
67.27
21.9/20.8/21.6
16.7/15.7/16.3
64.00
b)
60.13/62.70
c)
49.07
a)
V
: turn-on voltage recorded at a brightness of 1 cd m-2;
L
max
: maximum luminance;
η
c,max
: maximum current efficiency; d)current efficiencies
turn-on
measured at the brightness of 100 cd m-2 and 1000 cd m-2; e)ηp,max: maximum power efficiency; f)EQEmax, 100, 1000: maximum external quantum efficiency,
external quantum efficiency at 100 cd m-2 and 1000 cd m-2.
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(b)
QDPO
TPBi
(a)
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TPBi
104
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100
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400
500
600
700
3
4
5
6
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8
9
10 11 12 13
Wavelength (nm)
Voltage (V)
100
100
10
1
(d)
(c)
10
1
QDPO
TPBi
QDPO
TPBi
4
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2000
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Figure 1. Device characteristics: (
density and luminance versus voltage. (
a
) EL spectra at 50 mA. (
c
b) current
) current efficiency versus
luminance. (d) external quantum efficiency versus luminance.
In conclusion, we have developed a simple and effective
method for the synthesis of functionalized phosphorylated
hereocycles through a cascade phosphorylation/cyclization/
oxidation/aromatization process of propargylamines and
phosphine oxides. The novel compound was used as ETM for
fabricating OLEDs and excellent EL performances were
achieved. We anticipate that this methodology will be
particularly useful for the synthesis of organic functional
materials with P=O moiety.
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We gratefully acknowledge the financial support from the
National Natural Science Foundation of China (21402088,
21371093, 91433113, 21301095) and the Major State Basic
Research Development Program (2013CB922101).
Notes and references
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