Preparation of 6-Difluoromethylphosphonated Phenanthridines by Visible-Light-Driven Radical Cyclization of 2-Isocyanobiphenyls

Article abstract of DOI:10.1002/ejoc.201500988

A protocol to obtain a variety of 6-difluoromethylenephosphonated phenanthridines through a radical cyclization process was explored. These reactions, performed with the use of diethyl bromodifluoromethylphosphonate as a radical resource and 2-isocyanobip

Full text of DOI:10.1002/ejoc.201500988

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201500988
Preparation of 6-Difluoromethylphosphonated Phenanthridines by Visible-
Light-Driven Radical Cyclization of 2-Isocyanobiphenyls
[a]
[a]
[a]
[a]
Shuang Wang, Wen-Liang Jia, Lin Wang, and Qiang Liu*
Keywords: Radicals / Cyclization / Fused-ring systems / Biaryls / Nitrogen heterocycles
A protocol to obtain a variety of 6-difluoromethylenephos-
phonated phenanthridines through a radical cyclization pro- withdrawing and electron-donating groups were well toler-
cess was explored. These reactions, performed with the use ated, and the target molecules were obtained in excellent to
smoothly triggered by visible-light photocatalysis. Electron-
of diethyl bromodifluoromethylphosphonate as a radical re- moderate yields.
source and 2-isocyanobiphenyls as radical acceptors, were
prepare the phenanthridine core,[8] and the isocyanide inser-
tion reaction is always a simple and direct route. The iso-
cyanide insertion reaction usually proceeds through a radi-
Introduction
[
9]
Fluorinated molecules are privileged and important, and
they have attracted considerable attention because of their
wide applications in our modern life involving materials,
medicines, and agrochemicals.[ Over recent decades, sub-
stantial efforts have been devoted to developing new syn-
thetic methods for compounds bearing a difluoromethylene
[10]
[11–14]
cal procedure.
Both transition-metal catalysis
and
[15]
visible-light photocatalysis can induce the reaction effec-
tively. For example, Studer and co-workers used FeCl ,
1]
[14a]
3
[12a]
[16]
AgOAc,
-phosphorylated, and 6-trifluoromethylphenanthridines,
respectively. The Yu group achieved the synthesis of 6-tri-
and NiCl2 as initiators to synthesize 6-aroyl-,
6
[
2]
group. In particular, the synthesis of difluoromethyl-
enephosphonated compounds (DFMPCs) has drawn the
[17]
[15a]
fluoromethyl-,
methylated
6-alkylated,
and mono- and difluoro-
[3]
interest of many chemists in very recent years, because
DFMPCs are found in a number of different types of drugs
and are associated with a diverse range of biological activi-
ties.[
[15b]
phenanthridines by using visible light. In
spite of the promising progress achieved in recent years, few
studies have paid attention to the synthesis of difluorometh-
ylenephosphonated (DFMP) phenanthridines.
4,5]
Visible light is a clean, inexpensive, and “almost infi-
nitely” available source of energy.[ In this regard, driving
chemical reactions by visible light highly fulfills the increas-
ing demand for energy conservation. Recently, we reported
a room-temperature visible-light-driven protocol for the
C–H difluoromethylenephosphonation of arenes and het-
6]
[
3e]
eroarenes. In continuation of our interest in utilizing vis-
ible light to drive useful organic reactions, we focused our
attention on the difluoromethylenephosphonation of phen-
anthridines.
The phenanthridine core has caught the attention of
many pharmaceutical and synthetic chemists because it is
associated with bioactivity and applications in natural
[
7]
products synthesis. Although there are various ways to
[
a] State Key Laboratory of Applied Organic Chemistry,
College of Chemistry and Chemical Engineering,
Lanzhou University,
Lanzhou 730000, P. R. China
E-mail: liuqiang@lzu.edu.cn
http://sklaoc.lzu.edu.cn/
Supporting Information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500988.
Scheme 1. Preparation of 6-substituted phenanthridines by iso-
cyanide insertion reactions.
Eur. J. Org. Chem. 2015, 6817–6821
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6817

S. Wang, W.-L. Jia, L. Wang, Q. Liu
SHORT COMMUNICATION
We demonstrated that the phosphonodifluoromethyl Table 1. Optimization of the reaction conditions.[a]
PDFM) radical can be generated from commercially avail-
(
able diethyl bromodifluoromethylphosphonate under vis-
ible-light photocatalytic conditions.[3e] However, attempts
to direct the addition of the PDFM radical to phenanthrid-
ine produced DFMP phenanthridines in low yields (Ͻ5%).
The synthesis strategy was therefore modified to an iso-
cyanide insertion reaction by using bromodifluoromethyl-
phosphonate as a radical precursor. Herein, we report the
synthesis of DFMP phenanthridines by reaction of 2-iso-
cyanobiphenyls with bromodifluoromethylphosphonate un-
der visible-light irradiation (Scheme 1).
Entry
Catalyst
Base
Solvent
Yield [%]
1
2
3
4
5
6
7
8
9
fac-Ir(ppy)
fac-Ir(ppy)
fac-Ir(ppy)
fac-Ir(ppy)
fac-Ir(ppy)
fac-Ir(ppy)
fac-Ir(ppy)
fac-Ir(ppy)
fac-Ir(ppy)
fac-Ir(ppy)
eosin Y
3
3
3
3
3
3
3
3
3
3
K
K
K
K
K
K
2
2
2
2
2
2
HPO
HPO
HPO
HPO
HPO
4
4
4
4
4
CH
CH
2
Cl
CN
2
27
trace
52
59
61
40
15
84
13
3
DMF
THF
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
CO
CO
3
Cs
2
3
KOAc
DBU
1
1
1
1
1
1
1
0
1
2
Et
3
N
20
Results and Discussion
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
trace
trace
83
3 2 2
Ru(bpy) Cl ·6H O
[
b]
c]
3
fac-Ir(ppy)
fac-Ir(ppy)
fac-Ir(ppy)
fac-Ir(ppy)
–
3
3
3
3
Initial studies focused on the difluoromethylenephos-
phonation of 2-isocyano-1,1Ј-biphenyl (1a) with diethyl
bromodifluoromethylphosphonate (2) under visible-light ir-
[
4
91
[
d]
5
6
65
[e]
[f]
n.r.
n.r.[
f]
radiation [blue light-emitting diodes (LEDs), λ = 450 nm]. 17
It was encouraging to see that the reaction of isocyanide 1a
[
a] Reaction conditions: A mixture of 1a (0.2 mmol, 1.0 equiv.), 2
with 2 in a dry CH Cl solution containing tris[2-phenyl-
2
2
(0.4 mmol, 2.0 equiv.), base (0.24 mmol, 1.2 equiv.), and photocata-
2
pyridinato-C ,N]iridium(III) [fac-Ir(ppy) ] (1 mmol-%) as a lyst (0.002 mmol, 1.0 mol-%) in dry solvent (2.0 mL) was irradiated
3
with a 3 W blue LED lamp for 48 h. The reaction was conducted
in a sealed tube full of Ar gas. [b] KOAc (1.6 equiv.). [c] KOAc
(
photocatalyst and K HPO (1.2 equiv.) as a base provided
2
4
aimed product 3a in 27% yield after 48 h of irradiation at
room temperature (Table 1, entry 1). Several solvents such
2.0 equiv.). [d] KOAc (2.4 equiv.). [e] No irradiation. [f] n.r.: no re-
action.
as CH CN, DMF, THF, and toluene were used to replace
3
CH Cl (Table 1, entries 2–5), and toluene was chosen as
2
2
the ideal organic solvent for the reaction. With the best sol-
vent in hand, different bases including K CO , Cs CO ,
2
3
2
3
KOAc, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and electron-donating groups afforded better yields than those
Et N were then tested (Table 1, entries 6–10). We were with electron-withdrawing substituents. For arylisonitriles
3
happy to see that the yield of 3a improved to 84% in the bearing different substituents at the arene moiety contain-
presence of KOAc. Attempts to use other photocatalysts, ing the isocyanide, the yields were also good (Table 2, en-
such as Ru(bpy) Cl ·6H O (bpy = 2,2Ј-bipyridyl) and eo- tries 7–9). The process appears to be general with the posi-
3
2
2
sin Y, were unsuccessful and resulted in almost no product tion of the methyl group on the arene ring bearing the iso-
Table 1, entries 11 and 12). Diethyl bromodifluoromethyl- cyano group, as demonstrated by the efficient conversion of
phosphonate (2) shows an irreversible reduction event at biarylisonitriles to the corresponding DFMP phenanthrid-
0.84 V [vs. saturated calomel electrode (SCE)].[3e] Because ines (Table 2, entries 1, 11, and 12). In addition, multisub-
the potentials of these photocatalysts in the excited state stituted biarylisonitriles also worked well under the stan-
(
–
3
+
2+ [6c]
[
e.g., –0.81 V for Ru(bpy)3 ǞRu(bpy) *]
are more dard reaction conditions (Table 2, entries 1, 13–16), which
3
+
positive than that of the fac-Ir(ppy) ·Ǟfac-Ir(ppy) * cou- highlights the potential of the method in organic synthesis.
3
3
[
3e]
ple (–1.73 V),
the photoredox cycles are unfavorable in
On the basis of the experimental results, our previous
[
3e]
these cases. In addition, the amount of KOAc was carefully work, and the reports of others on the mechanism of the
screened, and 2.0 equivalents KOAc provided the best yield isocyanide insertion reaction,[
9e,14a,16]
we speculate that the
of the target product (91%; Table 1, entries 8, 13–15). Con- reaction takes place through electron transfer (ET) from the
trol experiments indicated that both visible light and the excited photoredox fac-Ir(ppy) catalyst to bromodifluoro-
3
photocatalyst were essential for the transformation.
methylphosphonate, which leads to the generation of the
With the optimized conditions in hand, we examined the PDFM radical. Addition of the PDFM radical to the iso-
substrate scope of this visible-light-driven PDFM radical cyanide functionality in the 2-isocyanobiphenyl provides
isocyanide insertion reaction by employing various biaryl- imidoyl radical A. Subsequently, intramolecular radical
isonitriles. As shown in Table 2, biarylisonitriles with cy- cyclization affords intermediate B, which goes on to gener-
clized aromatic rings substituted with either electron-with- ate deprotonated radical anion C with the aid of a base.
drawing or electron-donating groups afforded the corre- Finally, electron transfer from radical anion C to fac-
IV
sponding products (Table 2, entries 2–6 and 10) in good to Ir (ppy) affords the desired DFMP product and regener-
3
moderate yields (51–96%). Nevertheless, substrates bearing ates the photocatalyst (Scheme 2).
6818
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© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2015, 6817–6821

Preparation of 6-Difluoromethylphosphonated Phenanthridines
Table 2. Scope of the arylisonitriles.[a]
[
(
a] Reaction conditions: A mixture of 1 (0.2 mmol, 1.0 equiv.), 2 (0.4 mmol, 2.0 equiv.), KOAc (0.4 mmol, 2.0 equiv.) and fac-Ir(ppy)
3
0.002 mmol, 1.0 mol-%) in dry toluene (2.0 mL) was irradiated with a blue LED lamp (3 W) for 60 h.

S. Wang, W.-L. Jia, L. Wang, Q. Liu
SHORT COMMUNICATION
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way to prepare 6-difluoromethylenephosphonated (DFMP)
phenanthridines through radical cyclization of 2-iso-
cyanobiphenyls by using inexpensive and commercially
available diethyl bromodifluoromethylphosphonate under
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Received: July 27, 2015
Published Online: September 30, 2015
Eur. J. Org. Chem. 2015, 6817–6821
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