Denitrogenative Imidoyl Radical Cyclization: Synthesis of 2-Substituted Benzoimidazoles from 1-Azido-2-isocyanoarenes

Article abstract of DOI:10.1021/acs.orglett.7b01339

A novel access to 2-substituted benzoimidazoles, through unprecedented denitrogenative imidoyl radical cyclization of 1-azido-2-isocyanoarenes, has been developed. This tandem radical process was initiated by adding a C- or P-centered radical to isocyanide, followed by cycloaddition of the imidoyl radical to the azido group. Then, nitrogen loss and hydrogen abstraction of the resulting aminyl radical from surroundings delivered 2-substituted benzoimidazoles. Carbon radicals generated from another annulation process could also be applied, furnishing various heterocycle linked benzoimidazole derivatives.

Full text of DOI:10.1021/acs.orglett.7b01339

Letter
pubs.acs.org/OrgLett
Denitrogenative Imidoyl Radical Cyclization: Synthesis of
2‑Substituted Benzoimidazoles from 1‑Azido-2-isocyanoarenes
Dengke Li, Tingting Mao, Jinbo Huang, and Qiang Zhu*
State Key Laboratory of Respiratory Disease, Guangzhou Institutes of Biomedicine and Health, University of Chinese Academy of
Sciences, Chinese Academy of Sciences, 190 Kaiyuan Avenue, Guangzhou 510530, China
S
* Supporting Information
ABSTRACT: A novel access to 2-substituted benzoimidazoles,
through unprecedented denitrogenative imidoyl radical cycliza-
tion of 1-azido-2-isocyanoarenes, has been developed. This
tandem radical process was initiated by adding a C- or P-centered
radical to isocyanide, followed by cycloaddition of the imidoyl
radical to the azido group. Then, nitrogen loss and hydrogen
abstraction of the resulting aminyl radical from surroundings delivered 2-substituted benzoimidazoles. Carbon radicals generated
from another annulation process could also be applied, furnishing various heterocycle linked benzoimidazole derivatives.
radical intermediate.6 Recently, Studer and Yu independently
socyanide has been extensively studied in acid-promoted
1
Imultiple component reactions (MCRs) and transition-metal- demonstrated a cascade radical cycloaddition of ortho-
diisocyanoarenes to give iodoquinoxalines through an atom
transfer radical addition (ATRA) process, in which one of the
isocyano carbons acted as the imidoyl radical acceptor (Scheme
1b).7 However, cycloaddition of an imidoyl radical to a
heteroatom by forming a C−heteroatom bond is unprecedented.
In continuation of our interest in heterocycle synthesis using
isocyanides,8 we recently demonstrated that 1-azido-2-isocya-
noarenes9 could be used as a new class of functionalized
isocyanide for the synthesis 1,2,3-triazolo[1,5-a]quinoxalines
through copper-catalyzed azide−alkyne [3 + 2] cycloaddition
(CuAAC) followed by isocyanide insertion.10 Meanwhile,
organo azide is widely used in C−N bond formation through
transition-metal-catalyzed or photoinduced azide decomposition
and active nitrenoid or nitrene formation.11 However, reactions
through addition of carbon radical to azide, forming an aminyl
radical after nitrogen loss, are less common.12 An early work by
Montevecchi demonstrated that cyclization of a vinyl radical,
generated by addition of benzenesulfanyl radical to the triple
bond of 2-azidodiphenylacetylene, onto an aromatic azide
followed by denitrogenation and hydrogen abstraction could
give the corresponding indole derivatives.12a Inspired by these
studies,12 we speculate that denitrogenative imidoyl radical
cyclization could be realized using 1-azido-2-isocyanoarenes as
both imidoyl radical precursor and acceptor, to provide an
alternative pathway to construct the benzoimidazole scaffold
(Scheme 1c). It is reasonable to expect that heteroatom-centered
radicals as well as carbon radicals which are generated from
another annulation process could also be applied to this strategy.
Thus, substituents at the C2 position of benzoimidazoles could
be greatly diversified starting from the common 1-azido-2-
isocyanoarene substrates.
catalyzed insertion processes due to its unique structure and
reactivity.2 It can also react with various carbon- or heteroatom-
centered radical species to generate imidoyl radical intermediates
for further transformations.3 Synthetic application of an imidoyl
radical mainly focuses on N-heterocycle synthesis through its
addition to intramolecular alkyne,4 alkene,5 or arene6 function-
alities, generating various substituted indole, quinoline, isoquino-
line, and phenanthridine derivatives (Scheme 1a). For example,
2-isocyanobiphenyls were studied in depth with various radical
sources to give C6 diversified phenanthridines through intra-
molecular homolytic aromatic substitution (HAS) of an imidoyl
Scheme 1. Imidoyl Radical Cyclization Reactions
Received: May 3, 2017
Published: June 5, 2017
© 2017 American Chemical Society
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DOI: 10.1021/acs.orglett.7b01339
Org. Lett. 2017, 19, 3223−3226

Organic Letters
Letter
a
Phosphorus-containing compounds are applied widely in
organic synthesis, pharmaceuticals, and material science.13
Therefore, continuous interests have been paid in developing
new C−P bond-forming methods,14 including phosphinoyl
radical-initiated imidoyl radical formation in the synthesis of 6-
phosphorylated phenanthridines.6q−v We initiated the study with
diphenylphosphine oxide as the radical precursor in a reaction
with 1-azido-2-isocyano-3,5-dimethylbenzene 1a to optimize the
reaction conditions. It was found that Mn(OAc)3·2H2O could
promote the reaction very well in toluene at room temperature
for 3 h, and the products were isolated as a mixture of isomers 3a
and 3a, in a ratio of 1:0.6 in 78% yield (Scheme 2, and see
Scheme 3. Organoboron Reagents Initiated Cyclization
a
Scheme 2. Phosphinoyl Radical Initiated Cyclization
a
Reaction conditions: 1 (0.10 mmol, 1.0 equiv), 2 (0.15 mmol, 1.5
equiv), Mn(acac)3 (0.20 mmol, 2.0 equiv) in toluene (1.5 mL), 80 °C,
b
3−8 h, in Ar. Mn(acac)3 (0.30 mmol, 3.0 equiv), boronic acids 2
(0.30 mmol, 3.0 equiv).
(2.0 equiv) in toluene at 80 °C for 3 h (see SI for details). A
variety of substituted phenyl boronic acids were also tolerable
under the optimal reaction conditions, furnishing the corre-
sponding C2 arylated benzoimidazoles 4b−4j in 68−90% yields
(Scheme 3). Importantly, heteroaryl as well as alkyl boronic acids
could also be applied to the reaction (4k−4n), which greatly
expanded the practicability of the method. 1-Azido-2-isocyanoar-
enes bearing various substituents, such as Me, OMe, Cl and Br,
also underwent the reactions smoothly to give the corresponding
products 4o−4v in moderate to good yields.
Diazonium salts, preformed or generated in situ from anilines,
have also been studied frequently as precursors of aryl
radicals.8e,15 When anilines bearing an appropriate alkenyl or
alkynyl moiety at the ortho position are used, intramolecular aryl
radical cyclization to the C−C unsaturated bond may occur prior
to its addition to isocyanide. If alkyl or alkenyl radicals thus
formed can participate in the current imidoyl radical cyclization,
formation of more complicated benzoimidazoles derivatives is
certainly expected. To test the hypothesis, 2-styrylaniline 5a was
initially tested (entry 1, Table 1). The direct arylation product 6a
was isolated in 65% yield, in which the alkenyl group remained
unchanged due to its unfavorable position for cyclization. To our
delight, the desired radical clock triggered cyclization indeed
took place when relocating the C−C double bond two more
atoms away from the anilinic ring. In the case of 2-(allyloxy)-
aniline 5b, sequential aryl radical cyclization to alkene,
intermolecular imidoyl radical formation, and denitrogenative
cyclization took place to deliver a novel bisheterocyclic system
containing both dihydrobenzofurane and benzoimidazole
scaffolds in one step in 57% yield (entry 2). By changing the
linkage from oxygen to sulfur, and protected nitrogen in alkenyl
anilines 5c−5d, the corresponding dihyrobenzothiophene and
indoline containing benzoimidazole derivatives 6c and 6d were
obtained in 51% and 56% yield, respectively. 6-Exo-trig and 5-
a
Reaction conditions: 1 (0.10 mmol, 1.0 equiv), diphenylphosphine
oxide (0.15 mmol, 1.5 equiv), Mn(OAc)3·2H2O (0.12 mmol, 1.2
equiv) in toluene (1.5 mL), rt for 3 h, in Ar. 1.0 mmol of 1a, 5 h.
b
Supporting Information (SI) for details). Then, the generality of
the current process was studied with a range of 1-azido-2-
isocyanoarenes 1 with diphenylphosphine oxide. As shown in
Scheme 2, 1-azido-2-isocyanoarenes bearing various substitu-
ents, such as Me, OMe, Cl, and Br at different positions,
underwent the annulation smoothly to give the corresponding
products 3a−3j in 52−95% yields. In some cases, the products
existed in equilibrium between the isomeric forms ranging from
1:1 to 1:0.4 as determined by 1H NMR. While in the case of 3c,
3g, and 3h, only one set of peaks for one of the isomers were seen.
When 3-azido-2-isocyano-1,1′-biphenyl 1k, which contained a
competing benzene ring on the other side of the isocyano group,
was tested under slightly modified conditions, benzoimidazole
products 3k and a phenanthridine derivative 3l were isolated in
21% and 20% yield, respectively. The result suggested that the
imidoyl radical intermediate cyclized almost equally fast on the
azido group as compared with the benzene ring.
Next, the feasibility of using carbon radicals in the current
denitrogenative cyclization reaction was investigated. A seminal
work by Tobisu and Chatani in 2012 demonstrated that
organoboron reagents could serve as radical precursors in
reactions with 2-isocyanobiphenyls to give C6 aryl, heteroaryl,
and alkyl substituted phenanthridines.6a Therefore, similar
conditions were applied to 1-azido-2-isocyanoarene substrates
1 (Scheme 3). To our delight, 4,6-dimethyl-2-phenyl-1H-
benzo[d]imidazole 4a was obtained in excellent yield (90%)
from 1a and phenyl boronic acid in the presence of Mn(acac)3
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DOI: 10.1021/acs.orglett.7b01339
Org. Lett. 2017, 19, 3223−3226

Organic Letters
Letter
a
To further confirm the radical mechanism of the reaction,
control experiments in the presence of radical scavengers
including 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) or
butylated hydroxytoluene (BHT) were performed (Scheme 4).
The phosphorylation reactions were completely inhibited with
most of 1a recovered, indicating that radical intermediates were
involved in the reaction.6q−v
Table 1. Aniline Initiated Tandem Cyclization
Scheme 4. Radical Inhibiting Experiments
Based on the observations, a plausible mechanism for the
formation of 3a is proposed in Scheme 5. Initially, phosphorus
Scheme 5. Proposed Mechanism
radical A is formed in the presence of Mn(III) salts, which is a
well-documented process.6r,s Then, addition of intermediate A to
the terminal carbon of isocyanide 1a generates the corresponding
imidoyl radical B.3 Subsequently, cycloaddition of the imidoyl
radical to the azido group generated cyclized aminyl radical D
through intermediate C by nitrogen loss. Finally, hydrogen
abstraction transfer (HAT) of the aminyl radical from
surroundings gives the desired product 3a. For carbon radical
initiated cyclization, a similar sequential radical pathway is taken.
In conclusion, we have developed a novel and practical method
for the synthesis of C2 diversified benzoimidazole derivatives
from 1-azido-2-isocyanoarenes. This tandem process was
triggered by adding various radical species to isocyanide,
followed by cycloaddition of the resulting imidoyl radical
intermediate to the azido group. Subsequent nitrogen loss and
hydrogen abstraction transfer of the aminyl radical intermediate
gave the final product. Carbon radicals generated from another
annulation process could also be applied to this reaction,
furnishing a structurally unique bisheterocyclic system contain-
ing both benzoimidazole and other heterocycles, such as
dihydrobenzofurane, dihydrobenzothiophene, and indoline.
Three chemical bonds were formed in order under mild
conditions. Such kind of cycloaddition of an imidoyl radical to
a heteroatom by forming a C−heteroatom bond is unprece-
dented. This methodology not only provides a practical approach
a
Reaction conditions: 1a (0.10 mmol, 1.0 equiv), 5 (0.30 mmol, 3.0
equiv) in PhCF3 (1.5 mL), rt for 30 min, then 70 °C for 11 h, in Ar.
Isolated yield. BPO = benzoic peroxyanhydride.
b
exo-dig radical cyclization were also feasible for substrates 5e and
5f, giving unique structures of 6e and 6f in moderate yields. In
these processes, multiple chemical bonds including two C−C
and one C−N bonds were formed consecutively in one step. A
one-pot method for the synthesis of these bisheterocyclic
products is not available in the literature. Moreover, these results
provided solid evidence that radical intermediates were involved
in the transformation.8e,15c
It is valuable that other radical species which is not accessible
by the aforementioned approaches could also be applied to this
method. For example, a 1,4-dioxanyl radical generated by heating
BPO in 1,4-dioxane could also react with 1a to afford the
corresponding product 7 in 52% yield.6c
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DOI: 10.1021/acs.orglett.7b01339
Org. Lett. 2017, 19, 3223−3226

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b01339.
Experimental procedure; copies of 1H, 13C, or 31P spectra
for compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zhu_qiang@gibh.ac.cn.
ORCID
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L.; Lagadic, E.; Morlet-Savary, F.; Lohier, J.-F.; Chataigner, I.; Breugst,
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Qiang Zhu: 0000-0002-1243-2391
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (21472190, 21532009, and 21672215) for financial
support.
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