Mn(III)-Mediated Regioselective 6-endo-trig Radical Cyclization of o-Vinylaryl Isocyanides to Access 2-Functionalized Quinolines

Article abstract of DOI:10.1002/adsc.201901300

A Mn(III)-mediated radical cyclization reaction of o-vinylaryl isocyanides and arylboronic acids or diphenylphosphine oxides to access various 2-functionalized quinolines under mild conditions was developed. With the introduction of radical stabilizing substituents (e. g. aryl and methyl group) on vinyl group, this reaction provides a regiospecific 6-endo-trig radical cyclization of o-vinylaryl isocyanides, giving a number of structurally unique and biologically potential 2-functionalized quinoline derivatives.

Full text of DOI:10.1002/adsc.201901300

Title: Mn(III)-Mediated Regioselective 6-endo-trig Radical Cyclization of
o-Vinyl Arylisocyanides to Access 2-Funtionalized Quinolines
Authors: Yan Liu, Shi-Jun Li, Xiaolan Chen, Lu-Lu Fan, Xiao-Yun Li,
Shan-Shan Zhu, Ling-Bo Qu, and Bing Yu
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Adv. Synth. Catal. 10.1002/adsc.201901300
Link to VoR: http://dx.doi.org/10.1002/adsc.201901300

10.1002/adsc.201901300
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Mn(III)-Mediated Regioselective 6-endo-trig Radical Cyclization
of o-Vinyl Arylisocyanides to Access 2-Funtionalized Quinolines
Yan Liu,a,b,# Shi-Jun Li,a,# Xiao-Lan Chen,a,* Lu-Lu Fan,c,* Xiao-Yun Li,a Shan-Shan
Zhu,a Ling-Bo Qu,a and Bing Yua,*
a
College of Chemistry, Zhengzhou University, Zhengzhou 450001, China. E-mail: chenxl@zzu.edu.cn;
bingyu@zzu.edu.cn
b
College of Biological and Pharmaceutical Engineering, Xinyang Agriculture & Forestry University, Xinyang, 464000,
China
c
School of Chemistry & Chemical Engineering, Henan University of Technology, Zhengzhou 450001, China. E-mail:
fanlulu@haut.edu.cn
Yan Liu and Shi-Jun Li contributed equally
#
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract.
A
Mn(III)-mediated radical cyclization
scaffolds, due to their capability to incorporate
various substituents simultaneously with the
construction of ring systems, showing remarkable
operational simplicity and atom-/step-economy.[5]
For instance, a number of ortho-functionalized
aromatic isocyanides have been employed as
radical acceptors reacting with heteroatom (P, S,
Si, etc.)- or carbon (aryl or alkyl)-centered
radicals to generate corresponding imidoyl
reaction of o-vinyl arylisocyanides and arylboronic acids
or diphenylphosphine oxides to access various 2-
funtionalized quinolines under mild conditions was
developed. With the introduction of radical stabilizing
substituents (e.g. aryl and methyl group) on vinyl group,
this reaction provides a regiospecific 6-endo-trig radical
cyclization of o-vinyl arylisocyanides, giving a number of
structurally unique and biologically potential 2-
functionalized quinoline derivatives.
Keywords: Mn(III)-mediated; regioselective; 6-endo-
trig radical cyclization; 2-funtionalized quinolines; o-
vinyl arylisocyanides
radicals,
which
subsequently
undergo
intramolecular cyclization after addition to their
adjacent functionalities to eventually afford
various
N-heterocycles,
including
phenanthridines,[6] quinolines,[7] quinoxalines,[8]
benzimidazoles,[9]
benzothiazoles,[10]
benzoselenazoles,[11] etc (Figure 1).
Isocyanides, owing to their unique structures and
reactivities, have been applied as versatile
building blocks for the construction of nitrogen-
containing compounds, especially amides and
heterocycles, which are of great significance in
biological and pharmaceutical chemistry.[1] For
instance, isocyanide-involved multicomponent
reactions,[2] such as Passerini and Ugi reactions,
provide rapid access to a large array of druglike
molecules.[3] Over the past decades, transition-
metal-catalyzed isocyanide insertion chemistry
have been well developed.[4] Notably, ortho-
functionalized aromatic isocyanides have been
applied as valuable synthons in radical cascade
reactions for the synthesis of N-heterocycle
Figure 1. Radical Cascade Cyclization Involving ortho-
Functionalized Aromatic Isocyanides.
1
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10.1002/adsc.201901300
Advanced Synthesis & Catalysis
Unlike the above-listed radical cycloadditions, stereoelectronic and kinetic control. While, the
as for o-alkenyl arylisocyanides, there are two indirect 6-endo-trig radical cyclization was
possible cyclization pathways according to reported by Alabugin’s group[17] and Shi’s
Baldwin’s rules, i.e., 5-exo-trig and 6-endo-trig, to group[18]
,
respectively, through
a
domino
deliver corresponding N-heterocyclic scaffolds sequence consisting of 5-exo-trig addition/3-exo-
(i.e., indoles and quinolines), respectively. trig cyclization/ring expansion (Scheme 1b).
However, on the basis of the previous reports,[12] Despite those significant progress, the direct 6-
5-exo-trig radical cyclization seems more endo-trig radical cyclization strategy of o-alkenyl
stereoelectronically favored and kinetically arylisocyanides for regiospecific synthesis of
preferred comparing with 6-endo-trig cyclization. quinolines remains in demand. We hypothesized
As a result, the formation of five-membered that the introduction of a radical stabilizing
indole rings is often observed in radical substitution (e.g. phenyl) on C1 position of o-
cyclization reactions of o-alkenyl arylisocyanides vinyl group in arylisocyanide might stabilize the
(Scheme 1a). For example, Ogawa and coworkers key radical intermediate rendering the direct 6-
synthesized bisthiolated indoles via 5-exo-trig endo-trig cycloaddition of imidoyl radical to the
radical cyclization from o-ethenyl arylisocyanides terminal C2 of o-vinyl group (Scheme 1c). Herein,
with disulfide reagents.[13] Afterwards, Studer et we reported a Mn(III)-mediated regioselective 6-
al constructed mono- and bis-trifluoromethylated endo-trig radical cyclization of C1 aryl/methyl-
indoles by adjusting the substituents on o-alkenyl modified
o-vinyl
arylisocyanides
with
groups from o-alkenyl arylisocyanides and arylboronic acids or diphenylphosphine oxides to
Togni’s reagent.[14] Furthermore, the photoredox access 2-funtionalized quinolines under mild
synthesis of 2-phosphinoylindoles though 5-exo- conditions.
trig radical annulation/hydrogen atom transfer
(HAT) process was disclosed by Yang’s group.[15]
Very recently, Reiser’s group utilized Br-/OMe-
Results and Discussion
group promoted umpolung electron transfer to
Diverse methodologies have been established
synthesize a series of 2-substituted indole-3-
toward the synthesis of 2-aryl substituted
glyoxylates.[16]
quinoline derivatives since 1800s, among which,
the representative methods approximately include
classical name reactions,[19] A3 coupling
reactions,[20] dehydrogenative coupling of
aminobenzyl alcohols and amines,[21] and
transition-metal-catalyzed cross-dehydrogenative
coupling (CDC).[22] Although they can provide
easy accesses to targets, the development of
remarkably efficient and practical strategies
though radical cascade cyclization remains highly
desirable. In 2012, Chatani and coworkers
disclosed that with the oxidation of Mn(III) salts
arylboronic acids could generate aryl radical for
the synthesis of phenanthridines.[23] Afterward,
the generation of aryl radical form arylboronic
acids to initiate cascade cyclization of isocyanides
for the synthesis of functionalized heterocycles
has also been reported.[9-10, 24] We initiated this
study by establishing optimal experiment
conditions using the model reaction of 1-
Scheme 1. Radical Cycloaddition of o-Alkenyl
isocyano-2-(1-phenylvinyl)benzene 1a with
Arylisocyanides.
phenylboronic acid 2a in the presence of
manganese (III) salts. After extensive screening of
various reaction parameters including catalyst
In comparison of 5-exo cyclization, 6-endo ring
loadings, solvents, temperature, etc., the
closure seems inaccessible from the aspect of
2
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10.1002/adsc.201901300
Advanced Synthesis & Catalysis
optimized reaction conditions were established as were present on Ar1 rings. Additionally, 1-
follows: 1a (0.2 mmol), 2a (2.0 equiv), and isocyano-2-(1-phenylvinyl)naphthalene (1k) was
Mn(acac)3 (2.0 equiv) were mixed in MeCN (2 also a well-suitable substrate resulting in the
mL) at 80 °C for 1 h (see the Supporting successful synthesis of fused framework
Information for more details).
benzo[h]quinoline 3ka in high yield (88%).
Furthermore, 1l and 1m, with −Me and −F groups
on 4-position of Ar2 rings, could also react with
2a smoothly to provide the corresponding
products 3la and 3ma in good yields (76% and
87%). Among the target products, the structure of
3ka was further confirmed by X-ray
diffraction.[25]
a)
Scheme 2. Substrate Scope of Isocyanides.a) Reaction
conditions: 1 (0.2 mmol), 2a (0.4 mmol), and Mn(acac)3
(0.4 mmol) in MeCN (2 mL) at 80 °C for 1 h. Isolated yields
were given based on substrate 1.
With the optimized reaction conditions
established, we firstly explored the substrate
scope of arylisocyanides 1, which were
synthesized though chemoselective ortho-
alkenylation of anilines (see Supporting
Information for synthetic procedure), and the
results were illustrated in Scheme 2. As it can be
seen,
a
broad
range
of
o-(1-
phenylvinyl)arylisocyanides 1 bearing different
electron-donating groups (4-Me, 6-Me, 3,5-
dimethyl, 5,6-dimethyl, 4-Ph, and 4-MeO) on Ar1
rings could react smoothly with 2a to afford the
corresponding 2-phenyl substituted quinolines
3ba-3ga in 57-79% yields. Meanwhile, 1 bearing
electron-withdrawing groups (−F, −Cl, and −Br)
on 4-position of Ar1 rings also performed well
giving products 3ha-3ja in yields of 97%, 67%,
and 85%, respectively. No obvious electronic
effect was observed when different substituents
Scheme 3. Substrate Scope of Boronic Acids.a) a) Reaction
conditions: 1a (0.2 mmol), 2 (0.4 mmol), and Mn(acac)3
(0.4 mmol) in MeCN (2 mL) at 80 °C for 1 h. Isolated yields
were given based on substrate 1a.
We then continued to evaluate the substrate
scope of boronic acids 2. As it can be seen in
Scheme 3, phenylboronic acids bearing electron-
3
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10.1002/adsc.201901300
Advanced Synthesis & Catalysis
donating and electron-withdrawing groups on Ethyl phenylphosphinate (4d) and dimethyl H-
para-position were well tolerated affording the phosphonate (4e) were also employed was
corresponding products 3ab-3ah in moderate to substrate, however, they failed to afford desired
excellent yields (62-96%). Boronic acids with products (5ad and 5ae). The poor reactivities of
fused arenes, including 2-naphthyl, 1-naphthyl, 9- alkyl phenylphosphinate and dialkyl H-
anthracenyl, and 9-phenanthrenyl, were also phosphonates might be due to their higher
compatible under the standard conditions (3ai- theoretical bond dissociation energies (BDE) of
3al). Heteroarenes substituted boronic acids, such the P-H bonds.[29] Notably, all the synthesized 2-
as 2-furanyl-, 3-furanyl-, 2-thienyl-, 3-thienyl-, 3- phosphoryl quinolines 5 are new compounds.
pyridyl-, 8-quinolinyl-, 5-indolyl-substituted Among them, the structure of 5aa was confirmed
boronic acids 2m-2s, were converted into the by X-ray diffraction.[25]
corresponding quinoline derivatives 3am-3as in
comparable yields as well. However, cyclohexyl
and n-butyl substituted boronic acids (2t and 2u)
only gave trace amount of desired products 3at
and 3au, demonstrating that alkylboronic acids
have relatively poor reactivities in this 6-endo-trig
radical annulation.[24b] Besides, two alkenyl
bronic acids were further evaluated under the
standard conditions to afford the corresponding 2-
alkenylquinolines 3av and 3aw in yields of 79%
and 62%.
It is well known that organophosphorus
compounds occupy an important position in
biochemistry, pharmaceutical chemistry, material
science, and organic synthesis, where phosphorus
substituents modify biological responses,
medicinal properties, and material functions or act
as ligands of transition metals.[26] In light of the
significance
of
these
compounds,
the
development of efficient methods for C-P bond
construction attracts continuous interests.[27] It has
been reported that phosphorus centered radicals
can be generated from phosphine oxides via
homolytic cleavage of H-P bond in the presence
of Mn(III) salts.[9-10, 28] To further expand the
application of this protocol, the reactions of o-
vinyl arylisocyanides 1 with diaryl phosphine
oxides 4 in the presence of Mn(III)-salt were
conducted. To our delight, a number of 2-
phosphoryl quinolines 5 were successfully
synthesized by using Mn(OAc)2·2H2O as oxidant
at 80 °C for 3 h (Scheme 4, also see Supporting
Information for details). A diversity of o-vinyl
arylisocyanides 1 bearing electron-donating or
electron-withdrawing groups on Ar1 or Ar2 ring
reacted efficiently with diphenyl phosphine oxide
4a to furnish the corresponding products (5aa-
5na) in moderate to good yields. Additionally, 4-
methyl, 3,5-dimethyl substituted diarylphosphine
oxides (5b and 5c) were also well tolerated, giving
products 5ab and 5ac with satisfactory yields.
Scheme 4. Substrate Scope of 2‑ Phosphoryl-Substituted
Quinolines. a) a) Reaction conditions: 1a (0.2 mmol), 4 (0.4
mmol), and Mn(OAc)3·2H2O (0.4 mmol) in toluene (2 mL)
at 80 °C for 1 h. Isolated yields were given based on
substrate 1. b) n.d. = not detected.
To demonstrate the synthetic utility of our
strategy, a gram-scale experiment toward 2,4-
diphenylquinoline (3aa) was carried out. We were
delighted to isolate the product 3aa in 70% yield
(Scheme 5a). The practical applicability of this
methodology was further extended to the
synthesis of biologically active quinoxaline
derivatives. As shown in Scheme 5b, the
antifungal reagent (3na), anticancer reagent (3ke),
and a DPPH (2,2-diphenyl-1-picrylhydrazyl)
radical scavenger (3kb)[30] were synthesized from
4
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10.1002/adsc.201901300
Advanced Synthesis & Catalysis
the corresponding o-vinyl arylisocyanides and
boronic acids in ideal yields (66-84%).
Scheme 5. Synthetic Utility and Applicability.
To gain a deeper mechanistic insight into the
reaction process, a series of control experiments
were carried out (Scheme 6). When radical
Scheme 6. Control Experiments.
scavengers,
tetramethylpiperidin-1-yl)-oxidanyl, BHT (2,6-
di-tert-butyl-4-methylphenol), and 1,1-
diphenylethylene, were added to the model
reaction under standard conditions, a significant
loss in yield of 3aa was observed in each case
TEMPO
(2,2,6,6-
The plausible mechanism for this reaction was
further investigated by DFT calculations. As
shown in Figure 2, two pathways (i.e. 5-exo-trig
and 6-endo-trig) are possible for the radical
addition step. Firstly, the phenyl radical attacks
the terminal carbon of isocyano group in 1a
affording the imidoyl radical intermediate A via
the transition state TS1. Then, imidoyl radical A
could add to C1 or C2 of o-vinyl group rendering
corresponding cyclic radical intermediate B’ or B.
For the 5-exo-trig pathway, the carbon could
attack the C1 to form the five-membered ring
radical intermediate B’ via TS2’. As for the 6-
endo-trig pathway, the radical attacked the C2 to
generate six-membered ring radical intermediate
B via TS2. After calculation, it is obvious that the
relative Gibbs free energy of B (-66.1 kJ/mol) is
much lower than that of B’ (-45.3 kJ/mol). Hence,
the radical 6-endo-trig pathway would be more
energetically favorable in thermodynamics, which
is consistent with our experimental results.
However, based on the computational results, the
5-exo-trig pathway could not be completely
excluded and might contribute to the observed
selectivity as well as a minor “invisible” pathway.
In order to verify the significant role of phenyl
substituent in the 6-endo-trig cyclization process,
the calculations of the analogue with H substituent
have been performed as a contrast. The results
show the barriers for the generation of 6-endo-trig
(Scheme
6a-c),
suggesting
that
this
transformation might involve a radical process.
Notably, an adduct 6 was detected in the control
experiment with 1,1-diphenylethylene, indicating
that phenyl radical was generated under the
standard conditions (Scheme 6c). In addition,
when the radical stabilizing substituent on o-vinyl
group was changed from phenyl to methyl, the
corresponding quinoline product 3a’a was
obtained in lower yield (62%, Scheme 6d); when
it was altered to H, the reaction of 1-isocyano-2-
vinylbenzene (1a’’) with phenylboronic acid 2a
unexpectedly gave 3aa rather than desired product
3a”a (Scheme 6e). In this case, 3aa might be
generated via homolytic aromatic substitution
(HAS) of 3a”a by phenyl radical.[31] These results
reflected that the radical stabilizing substituents
on o-vinyl group played a vital role in this reaction.
5
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10.1002/adsc.201901300
Advanced Synthesis & Catalysis
cyclization becomes higher than that of 5-exo-trig
cyclization respectively (see the Supporting
Information for more details), indicating the
phenyl substituent favors formation of radical
center for the 6-endo-trig cyclization product.[12c,
32]
Scheme 7. Proposed Mechanism.
In conclusion, we have developed a Mn(III)-
mediated regioselective 6-endo-trig radical
cyclization reaction of o-vinyl arylisocyanides
and arylboronic acids or diarylphosphine oxides
to synthesize a variety of 2-funtionalized
quinolines. This method not only provides a
practical and efficient approach to structurally
unique and biologically potential 2-functionalized
quinoline derivatives, but also discloses an
unprecedented insight into the direct 6-endo-trig
radical cyclization mechanism which is
complementary to the radical cascade cyclization
of o-vinyl arylisocyanides.
a)
Figure 2. The Free Energy Profiles (Unit: Å).a) All the
Experimental Section
calculations in the manuscript were calculated at the M06-
2X/6-31G(d,p) level. And the solvent effect has been also
considered using self-consistent reaction field (SCRF)
polarizable continuum model, in which acetonitrile is
employed as solvent.
General Procedure: In a 5 mL flask, to the solution of o-
vinyl 1-isocyano-2-(1-phenylvinyl)benzene 1a (41 mg, 0.2
mmol) in MeCN (2 mL, 0.1 M) was added phenylboronic
acid 2a (49 mg, 0.4 mmol) and Mn(acac)3 (141 mg 0.4
mmol). The mixture was allowed to stir at 80 °C for 1 h and
monitored by TLC. After 1a was completely consumed, the
reaction mixture was cooled to room temperature and then
filtered through Celite. The residue was washed with
CH2Cl2 (5 mL × 2). The filtrates were combined, and the
solvents were removed under reduced pressure. The crude
product was finally purified by column chromatography
with petroleum ether/ethyl acetate as an eluent to afford the
desired product 3aa.
Based on the experimental results and previous
reports,[23-24] a plausible mechanism was proposed
(Scheme 7). Initially, the radical R· was in situ
generated from radical precursors (arylboronic
acids or diarylphosphine oxides) in the presence
of Mn(III) salts. Then R· underwent a
chemoselective addition to isonitrile rather than
vinyl group giving the corresponding imidoyl
radical intermediate A.[33] Subsequently, radical A
underwent direct 6-endo-trig cycloaddition to C2
of o-vinyl group to render the cyclic radical
intermediate B. Single electron oxidation of B by
Mn(III) salts led to the formation of cationic
intermediate C. Finally, 2-functionalized
Acknowledgements
We acknowledge the financial support from National Natural
Science Foundation of China (21501010, 21971224), China
Postdoctoral Science Foundation (2019M652561), Key Research
Projects of Universities in Henan Province (20A150006,
20B150021), Henan Province Young Talents Elevation Program
(2019HYTP023) and Hunan Provincial Key Laboratory of
Materials Protection for Electric Power and Transportation
(2019CL03).
quinolines
3
or
5
were obtained after
deprotonation, where the anions in manganese
salts might act as Lewis bases to abstract protons.
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10.1002/adsc.201901300
Advanced Synthesis & Catalysis
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10.1002/adsc.201901300
Advanced Synthesis & Catalysis
UPDATE
Mn(III)-Mediated
Regioselective
6-endo-trig
Radical Cyclization of o-Vinyl Arylisocyanides to
Access 2-Funtionalized Quinolines
Adv. Synth. Catal. Year, Volume, Page – Page
Yan Liu,a,b,# Shi-Jun Li,a,# Xiao-Lan Chen,a,* Lu-Lu
Fan,c,* Xiao-Yun Li,a Shan-Shan Zhu,a Ling-Bo
Qu,a and Bing Yua,*
8
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