Silver(I)-mediated reaction of 2-isocyanobiaryl with alkyl trifluoroborates: Efficient synthesis of 6-Alkylated phenanthridines

Article abstract of DOI:10.1246/cl.180009

A silver-mediated free-radical cascade cyclization of isocyanides with potassium alkyl trifluoroborates was developed. The procedure tolerates a series of functional groups, including chloro, bromo, ester, ketone, and amide. Additionally, it is not sensitive to steric hindrance. Thus, this reaction represents a very useful strategy for the production of 6-Alkylated phenanthridine derivatives. The addition of alkyl radical to the isonitrile, followed by a radical aromatic cyclization, is involved in this transformation.

Full text of DOI:10.1246/cl.180009

CL-180009
Received: January 5, 2018 | Accepted: February 13, 2018 | Web Released: March 30, 2018
Silver(I)-mediated Reaction of 2-Isocyanobiaryl with Alkyl Trifluoroborates:
Efficient Synthesis of 6-Alkylated Phenanthridines
Siyi Ding, Yuzhen Zhao, Qiang Ma, Shaopeng Tian, Huaping Ren, Min Zhu, Kexuan Li, and Zongcheng Miao*
Key Laboratory of Organic Polymer Photoelectric Materials, School of Science, Xijing University, Xi’an Shaanxi 710123, P. R. China
E-mail: miaozongceng@xijing.edu.cn
R
HAS
A silver-mediated free-radical cascade cyclization of iso-
cyanides with potassium alkyl trifluoroborates was developed.
The procedure tolerates a series of functional groups, including
chloro, bromo, ester, ketone, and amide. Additionally, it is not
sensitive to steric hindrance. Thus, this reaction represents a very
useful strategy for the production of 6-alkylated phenanthridine
derivatives. The addition of alkyl radical to the isonitrile,
followed by a radical aromatic cyclization, is involved in this
transformation.
(a)
Radical Addition
N
N C
N
C
R
R
(X =Br, I)
RX
Mn(acac)3-mediated
Radical initiator
photocatalyst
C(sp3)-H activation
Ag2O-mediated
R-NHNH2
RB(OH)2
R-CF2X (X = Br, SO2Cl
Ether
Alkane
Alcohol
Peroxide
AIBN
R
R-COOH
Metal-free
(b)
R∙BF3K
Ag-catalyzed
Decarboxylation
R-CH2COOK
Keywords: Alkyl radical
| Silver(I)-mediated | Phenanthridines
Figure 1. (a) Previous approaches to form the 6-alkylated
phenanthridine derivatives; (b) Our work using potassium alkyl
trifluoroborates as the alkyl precursors.
Radical chemistry has attracted special research interest
because of its intriguing synthetic opportunities, and radical
chemistry has found widespread applications in medicinal
chemistry, agrochemicals and materials science.1 Over the past
few years, radical reactions have gradually become a versatile
and useful synthetic method for the carbon-carbon bond
formation.2 With the advent of tandem radical reactions, such
as radical addition/cyclization of functionalized alkenes,3 it has
gradually evolved into a valuable approach to the synthesis of
various heterocycles. Phenanthridines belong to an important
class of biological compounds and exist widely in natural
products.4 They are useful drug candidates,5 showing antibacte-
rial, antitumoral, and antilenkemic activities.6 Derivatization
at the 6-position of phenanthridine is a successful strategy to
improve its performance. Several studies have shown that
6-alkyl-substituted phenanthridines have good biological
activities.7 Conceptually, by introducing the alkyl-substituted
groups into the main structure, it is possible to find novel and
interesting bioactive substances.
From an academic standpoint, using 2-isocyanobiphenyls
and alkyl radicals to construct 6-alkylated phenanthridines is
obviously feasible. Adding a carbon-centered radical to the
terminal carbon of 2-isocyanobiphenyls generates a germinal
carbon radical intermediate. Then, the newly generated imidoyl
radical can attach to the double C-C bond. When using
intramolecular aromatic rings to construct the skeletal structure
of phenanthridine, the nitrogen-containing heterocycle and
two chemical bonds are formed sequentially in this process.
Recently, a series of 6-substituted phenthridines were easily
prepared following this synthetic strategy (Figure 1). In 2012,
Chatani and colleagues reported the first synthetic route of
6-alkyl/aryl phenanthridines by Manganese(III) acetylacetonate
[Mn(acac)3]-mediated oxidative cyclization of 2-isocyanobi-
phenyls with alkyl or aryl boronic acids as radical precursors.8
Since then, several groups have successfully reported their
efforts to construct 6-alkylated phenanthridine derivatives
through the reaction of 2-isocyanobiphenyls with the corre-
sponding radical precursors, such as simple ethers,9 alcohols,10
alkanes,10a,11 alkyl halides,12 alkyl hydrazines,13 alkyl carboxylic
acids (or their salts),14 dicumyl peroxide,11b,15 trifluoromethyl-
ated reagents,16 and AIBN.17 Although these methods have their
own specific applications, they still face several drawbacks,
including harsh reaction conditions and limited reaction scope.
Thus, further developments for more practical and general
alkylation methodologies are highly desired. Herein, we present
our recent results on a cascade reaction of 2-isocyanobiaryls
with simple potassium alkyl trifluoroborates for production of
6-alkylated phenanthridine derivatives.
Based on our earlier research, we devised and developed a
rapid and highly efficient method for the radical formation under
relatively mild conditions, using potassium alkyl trifluoroborates
as radical precursor, and using silver(I) as the oxidant. The
corresponding alkyl radicals were immediately trapped with
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), which gave us a
crucial evidence for the existence of the radical intermediate.
Thus far, the successful application of this new radical formation
strategy has been used for the boron-selective oxidative cross-
coupling reaction,18 using arylboronic acids and alkyl trifluoro-
borates as the coupling partners. Encouraged by the preliminary
results, we explored more meaningful applications with the
regard to radical formation. Aryl-isonitrile is another well-
established radical acceptor.19 It is isoelectronic with carbon
monoxide, and they can undergo an insertion reaction to produce
nitrogen-containing heterocycles, which are an important sub-
strate found in many natural products with different biological
activities.
In an initial study, we first carried out the reaction with
phenylpropyl trifluoroborate (1a) and 1.0 equivalent of 2-
isocyano-1,1¤-biphenyl to easily monitor the reaction by gas
chromatography-mass spectrometry (GC-MS) and thin layer
chromatography (TLC) (Table 1). It is worth noting that a
moderate yield of 2a was detected in toluene with trace amounts
of water. It is possible the water kept the alkylboron species in
the reactive boronic acid form. After examining different kinds
562 | Chem. Lett. 2018, 47, 562–565 | doi:10.1246/cl.180009
© 2018 The Chemical Society of Japan

Table 1. Optimization of reaction conditionsa
Ag2O
R
BF3K
R
DCM+ H2O
N
C
N
oxidant
solvent
BF3K
C
N
N
1a
2a
Entry
Oxidant
Solvent
2ad,g (yield [%])
N
N
N
Cl
1
2
3
4
5
6
7
8
9
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
MnO2
Mn(OAc)2
AgNO3
Ag2O
DCMb
DCMc
Toluenec
EtOAcc
MeOHc
DMFc
ND
97 (94)
70
20
ND
14
2a 94%
2b 95%
2c 94%
N
N
N
Br
2e 94%
2f 94%
2d 92%
MeCNc
DCMc
28
O
N
N
N
O
O
2h 97%
ND
ND
ND
trace
ND
2i 96%
2g 95%
DCMc
10
11
12
DCMc
H
N
N
O
O
N
N
N
DCMc,e
DCMc,f
O
O
2l 91%
2k 94%
2j 97%
Ag2O
aReaction conditions: RBF3K (0.2 mmol, 1.0 eq.), 2-isocyano-
1,1¤-biphenyl (0.2 mmol, 1.0 eq.), Ag2O (0.2 mmol, 1.0 eq.),
room temperature, 10 min; bThe reaction was conducted
without the addition of distilled water; cThe reaction was
conducted with addition of distilled water (40 ¯l); dNMR yield,
1,3,5-trimethoxyl benzene as the internal standard reagent.
O
S
O
N
N
N
N
O
O
2n 95%
2o 93%
2m 93%
N
N
N
f
eThe reaction was run under an air atmosphere. The reaction
2r 82%
2p 88%
2q 85%
was conducted under oxygen atmosphere. gYield of the
isolated product, purified by column chromatography. ND:
not determined.
Scheme 1. The reaction of 2-isocyano-1,1¤-biphenyl with alkyl
trifluoroborates with different functional groups. Yield of the
isolated product under the reaction condition: RBF3K (0.2 mmol,
1.0 eq.), 2-isocyano-1,1¤-biphenyl (0.2 mmol, 1.0 eq.), Ag2O
(0.2 mmol, 1.0 eq.), 2 mL DCM and 40 ¯L distilled water as
the solvent, room temperature, 10 min.
of solvents, including protic solvents, polar solvents and
nonpolar solvent, dichloromethane (DCM) with trace amounts
of distilled water provided excellent results. During the reaction,
silver was deposited on the surface of the reactor, which is
difficult to remove and requires nitric acid to get rid of the silver
(0). Attempts to carry out the reaction in air or oxygen
atmosphere resulted in very low yields. Moreover, increasing
or lowering the reaction temperature provided no additional
advantage to the reaction.
can be quantitatively oxidized to produce the relevant radicals.
This reaction was successfully extended to various 2-isocyano-
1,1¤-biphenyl with different functional groups, and the result
are listed in Scheme 2. As indicated, reactions required 10 min
to fully consume the substrates and worked well.
After optimizing the reaction conditions, it can be con-
cluded that the optimized reaction should be performed using the
same molar equivalent of alkyl trifluoroborates and 2-isocyano-
1,1¤-biphenyl at room temperature in the presence of 1.0 equiv-
alent of Ag2O. Under the optimized reaction conditions, the
scope of alkyl trifluoroborates with substituent was studied, as
shown in Scheme 1. As expected, all substrates ran smoothly
under the standard procedure. Primary alkyl trifluoroborates
bearing functional groups, such as ester (1i, 1j and 1m), ketone
(1h), or amide (1k and 1l) were all suitable substrates.
Additionally, alkyl trifluoroborates with a heteroarene group
were compatible (1n, 1o). It is worthy of note that the alkyl
moiety, with bromo- or chloro-group (1c and 1d), remained
intact under these conditions, demonstrating the mild nature
of the reaction conditions. Importantly, the reaction was not
sensitive to steric hindrance, and the branched alkyl trifluoro-
borates (1g, 1m) worked well to deliver the phenanthridine
products in good yields. Furthermore, secondary alkyl-trifluoro-
borates (1p, 1q) and tertiary alkyl-trifluoroborate (1r) were also
viable substrates under the current conditions. Additionally, they
To gain insight into this reaction, a controlled reaction of 1a
and isocyano-1,1¤-biphenyl, under the standard conditions, with
the addition of 2,2,6,6-tetramethyl-1-piperdinoxyl (TEMPO), a
radical scavenger, was carried out. As expected, the cascade
reaction was completely suppressed (Scheme 3a). This result
clearly indicates that the mechanism may undergo a radical
pathway. Taking into account the above result and theoretical
knowledge, we put forward a plausible mechanism (Scheme 3b).
A single electron, which is transferred from the oxidant Ag2O,
can generate alkyl radicals accompanied by the formation of
boron trifluoride (BF3). Then, intramolecular homolytic aromatic
substitution (HAS) of the imidoyl radical on the pending phenyl
ring leads to the formation of the cyclohexadienyl radical
intermediate, and this was proposed as a common step in the
reaction. Subsequently, the single electron transfer (SET) step
occurred to afford the cyclohexadienyl cation, followed by
deprotonative aromatization.
In summary, this method not only describes a rapid and
highly efficient method for the radical formation using potassium
alkylfluoroborates as radical precursor, but also provides a
Chem. Lett. 2018, 47, 562–565 | doi:10.1246/cl.180009
© 2018 The Chemical Society of Japan | 563

its useful features. The application prospects of this reaction in
organic chemistry and functional materials chemistry are very
broad and quite optimistic. Research is ongoing in this area.
R1
R2
O
Ag2O
O
BF3K
R
DCM+ H2O
N
C
N
This work is supported by the Special Fund for Talent
Research of Xijing University (NO. XJ17B01 and NO.
XJ16T05). The authors thank Pro Pengfei Li (Xi’an Jiaotong
University) and Dr Liang Xu (Shihezi university) very much
for their guidance and advice during this work. We would like
to thank LetPub (www.letpub.com) for providing linguistic
assistance during the preparation of this manuscript.
Cl
F
OCH3
O
O
O
N
N
N
O
O
O
3b 94%
3a 92%
3c 91%
Cl
F3
C
F3
C
O
O
O
N
N
O
Supporting Information is available on http://dx.doi.org/
10.1246/cl.180009.
3d 92%
3e 94%
Scheme 2. The reaction of potassium methyl 3-trifluoro-
borato)propanoate with 2-isocyano-1,1¤-biphenyl with different
functional groups. Yield of the isolated product under the
reaction condition: RBF3K (0.2 mmol, 1.0 eq.), 2-isocyano-1,1¤-
biphenyl (0.2 mmol, 1.0 eq.), Ag2O (0.2 mmol, 1.0 eq.), 2 mL
DCM and 40 ¯L distilled water as the solvent, room temperature,
10 min.
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C
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Scheme 3. a) Controlled reaction; b) Proposed mechanism.
new route to construct 6-alkylated phenanthridine motifs. This
reaction is interesting and potentially advantageous. For radical
precursors, in comparison with other organometallic reagents,
organoboronic acid derivatives are more readily available, are
stable in the presence of air and moisture, and compatible
with many functional groups. Moreover, this new strategy
features excellent product selectivity, short reaction time, mild
reaction conditions, simple operation and broad functional group
tolerance. In comparison with conventional strategies, the current
reaction needs one more step to obtain the organoboron reactant
using the corresponding halide as the starting materials. Never-
theless, taking into consideration that many shelf-stable alkyl-
organoboron derivatives are already commercially available, this
reaction should facilitate small-molecule array synthesis due to
4
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564 | Chem. Lett. 2018, 47, 562–565 | doi:10.1246/cl.180009
© 2018 The Chemical Society of Japan

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Chem. Lett. 2018, 47, 562–565 | doi:10.1246/cl.180009
© 2018 The Chemical Society of Japan | 565