Synthesis of Functionalized Benzo[g]indoles and 1-Naphthols via Carbon-Carbon Triple Bond Breaking/Rearranging

Article abstract of DOI:10.1021/acs.orglett.7b03410

Novel carbon-carbon triple bond breaking and rearranging reactions of benzene-linked allene-ynes have been established. The reactions can be selectively controlled toward the formation of two families of skeletally diverse benzo[g]indoles and 1-naphthols

Full text of DOI:10.1021/acs.orglett.7b03410

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis of Functionalized Benzo[g]indoles and 1‑Naphthols via
Carbon−Carbon Triple Bond Breaking/Rearranging
Jia-Yin Wang,†,∥ Peng Zhou,†,‡,∥ Guigen Li,*,‡,§ Wen-Juan Hao,† Shu-Jiang Tu,*,† and Bo Jiang*,†
†School of Chemistry & Materials Science, Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials, Jiangsu
Normal University, Xuzhou 221116, P. R. China
‡Institute of Chemistry & BioMedical Sciences, Nanjing University, Nanjing 210093, P. R. China
§Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States
S
* Supporting Information
ABSTRACT: Novel carbon−carbon triple bond breaking and rearranging
reactions of benzene-linked allene−ynes have been established. The reactions
can be selectively controlled toward the formation of two families of skeletally
diverse benzo[g]indoles and 1-naphthols under mild conditions. Silver salt was
found to efficiently promote indole annulation to give multifunctional
benzo[g]indoles with the installation of two sulfonyl groups into the indole
ring via N−S and N−F bond cleavage of NFSI, whereas NBS and NCS-mediated
benzannulations occurred with the formation of dihalogenated 1-naphthols.
Scheme 1. Allene−Yne-Based Cycloadditions
he study of carbon−carbon triple bond breaking,
T
rearrangement, and metathesis has been an extremely
challenging topic in chemical and material sciences and has
stimulated widespread interest in organic and organometallic
disciplines.1 Differentiated from traditional strategies, this
methodology can enable molecular skeletons to be reorganized
through more atom-economical retrosynthetic analysis so as to
assemble challenging structural units in straightforward and
practical fashions.2 Besides alkyne metathesis,3 previous work
on the cleavage of the carbon−carbon triple bond has been
heavily dependent on the use of stoichiometric organometallic
reagents, such as alkyne−ligand scission on metal complexes4
and oxidative cleavage,5 thereby making their applications less
desirable. Lately, much effort has been made toward transition-
metal catalysis involving Au,6 Pd,7 Ru,8 Rh,9 Cu,10 and Ag11
species in catalytic systems. In contrast, metal-free cleavage of
the unactivated carbon−carbon triple bond has been much less
investigated and still remains an extremely difficult task.12
In recent years, our laboratories have been investigating a
series of domino cyclization reactions for the formation of
multiple functional rings.13 During this study, we prepared
allene−yne-anchored starting materials for various tandem [2 +
2] cycloadditions and functionalization (Scheme 1a).14
Surprisingly, we found the cyclobuta[a]naphthalen-4-ols cannot
be generated as we originally planned.15 Instead, we found
unexpected cleavage of the carbon−carbon triple bond
occurring under mild conditions, leading to the formation of
benzo[g]indoles and 1-naphthols. Further investigation re-
vealed two types of domino reactions involving alkyne breaking
and rearranging steps: (i) Ag-mediated indole annulation led to
functionalized benzo[g]indoles 2 using N-fluorobenzenesulfo-
nimide (NFSI) as both a nitrogen source of indole synthesis
and a sulfonyl anchor for sulfonate formation (Scheme 1b); (ii)
the use of N-bromosuccinimide (NBS) and N-chlorosuccini-
mide (NCS) as halogen sources resulted in dibrominated and
dichlorinated 1-naphthols 3 (Scheme 1c) chemoselectively. To
the best of our knowledge, this is the first breaking/rearranging
of unactivated carbon−carbon triple bonds for the formation of
the above functionalized benzo[g]indoles and 1-naphthols. In
the former process, Ag-mediated the cleavage of N−F and N−S
bonds of NFSI to facilitate the ring-opening of cyclobutenes
and formation of pyrrole ring under oxidative conditions. In the
latter, NBS and NCS played dual roles as both promoters and
electrophiles. Here, we report these novel transformations, as
shown in Scheme 1b,c.
Our initial investigation started with the treatment of
benzene-tethered allene−yne 1a with NFSI in the presence of
AgNO3 and Cs2CO3. The reaction was performed in
Received: November 1, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03410
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
acetonitrile solvent at 50 °C under air atmosphere, delivering
the unexpected benzo[g]indole 2a through indole annulation,
albeit with a low yield of 19% (Table S1, entry S1). The result
is very interesting since it is the first finding that NFSI can act
as both nitrogen and sulfonyl sources for the synthesis of
benzo[g]indol-5-yl sulfonates 2, rather than as the fluorination
reagent often reported in the literature. This discovery
prompted us to further search for the optimal conditions for
this indole annulation. Several others silver salts including
AgOTf, AgTFA, Ag2O, and Ag2CO3, that are often employed in
the catalytic transformations, were screened for this trans-
formation by using 2.0 equiv of NFSI (entries S2−S5). Silver
salts such as AgOTf, AgTFA, or Ag2O gave lower concersions
as compared with AgNO3 (entries S2−S4 vs S1), whereas the
use of Ag2CO3 led to a slightly higher yield of 2a (25%, entry
S5). Increasing Ag2CO3 loading is beneficial to the trans-
formation (entries S6−S8), and its 0.8 equiv gave the best
outcome (49% entry S7). Adjusting the ratio of 1a with NFSI
to 1:3 improved the yield of 2a to 62% (entry S9). Further
increase of the dosage of NFSI showed no positive impact on
the reaction efficiency (entry S10). We next optimized
conditions by using different bases such as K2CO3, Na2CO3,
K3PO4, AcONa, EtONa, and t-BuOK and found all these
attempts did not show any improvements with respect to the
reaction yield (entries S11−S17). Lower conversion into 2a
was obtained with the decrease or increase of Cs2CO3 loading
(entries S18 and S19). Changing the solvents, such as N,N-
dimethylformamide (DMF), 1,4-dioxane, 1,2-dichloroethane
(DCE), trifluorotoluene and tetrahydrofuran (THF), revealed
that all these media completely suppressed the reaction process
(entries S20−S24). The reaction gave a relatively lower
conversion when temperature was changed to either 30 or 70
°C (entries S25−S26). The reaction under argon conditions
gave a very inferior outcome as compared with air conditions
(entry S27).
Scheme 2. Substrate Scope for Forming Products 2
a
Reaction conditions: benzene-tethered allene−ynes 1 (0.2 mmol),
NFSI (0.6 mmol), Ag2CO3 (0.16 mmol), Cs2CO3 (0.2 mmol),
CH3CN (3.0 mL), 50 °C, 5 h. Isolated yields in parentheses are based
on 1.
unexpected dibrominated 1-naphthol 3a was obtained in 39%
yield through carbon−carbon triple bond cleavage (Scheme 3).
Scheme 3. Synthesis of Dibrominated 1-Naphthol 3a
Under the above optimal conditions, we explored the
reaction scope by using a variety of the preformed benzene-
tethered allene−ynes 1 (Scheme 2). A wide range of
substituted benzo[g]indoles 2a−o were isolated in modest to
good yields (46%−67%). Both electron-donating (methyl 1b
and 1c, ethyl 1d) and electron-withdrawing (chloro 1e and 1f,
bromo 1g) groups at different positions of arylalkynyl motifs
can all tolerate this oxidative system. Among them, a sterically
encumbered o-chlorophenyl analogue 1f turned out to be a
suitable reaction partner. Similarly, allene−yne 1h carrying a 1-
naphthyl (1-Np) group still showed high reactivity. However,
exchanging a 1-naphthyl group with an n-butyl counterpart 1i
failed to afford product 2i. Alternatively, substituents with
electronically poor and rich nature on 4- or 5-positions of the
internal arene rings of allene-ynes 1 did not hamper indole
annulation. Various functional groups, such as fluoro (1j and
1k), chloro (1l and 1m), and methyl (1n and 1o), were
adaptable to this reaction, enabling Ag-mediated carbon−
carbon triple bond breaking/rearranging to access the
corresponding benzo[g]indoles 2j−o in yields ranging from
54% to 67%. Notably, pyridine-linked allene−yne 1p was
successfully engaged into the current transformation, providing
the corresponding pyrrolo[3,2-h]quinolin-5-yl sulfonate 2p
albeit with 45% yield.
After a careful screening of conditions was conducted, the
chemical yield can be increased to 75% by adding 1.5 equiv of
water into the system in the absence of Ag salts and bases. We
then continued examining the scope and limitation of the
dibromobenzannulation by changing allene−yne substrates.
As shown in Scheme 4, allene−ynes 1 carrying electron-
neutral, -donating, and -withdrawing substituents on the
arylalkynyl moiety were all compatible for this system. The
presence of electron-rich substituents (Me, t-Bu, and MeO) at
the para-position of the arylalkynyl moiety resulted in the
corresponding products 3b,c, 3h−3j, and 3m in slightly lower
isolated yields than those with electron-withdrawing groups
(3d,e, 3k, and 3n). Next, NCS was utilized to replace NBS in
an attempt to expand the scope of allene-ynes 1. As expected,
the bis-chlorobenzannulation reaction occurred readily under
the above conditions with a variety of functional groups on
both internal arene rings and arylalkynyl moieties of substrates
1. Typical functional groups, such as chloro, fluoro, methyl, tert-
butyl, and ethyl, attached on the arylalkynyl moiety were all
proven to be compatible with the present reaction system. It
was found that electron-withdrawing groups at the C5-position
relative to the internal arene ring showed much better yields
than those with electron-donating ones (3v vs 3aa, 3z vs 3cc),
indicating that the reactivity of the reaction would be
dominated by the electronic nature of substituents. Among
them, a much lower yield was observed for the substrate with a
After the successful formation of benzo[g]indoles 2, we then
employed NBS to replace NFSI for this transformation.
Allene−yne 1a was first subjected to the reaction with 2.0
equiv of NBS under the standard conditions, and the
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a
Scheme 4. Substrate Scope for Forming Products 3
Scheme 5. Control Experiments
Based on the above observations, LC−MS analysis, and
literature survey,14,16 the mechanisms for forming products 2
and 3 were proposed as shown in Scheme 6. First, the
Scheme 6. Plausible Reaction Pathways
a
Reaction conditions: allene−ynes 1 (0.2 mmol), H2O (0.3 mmol),
CH3CN (5.0 mL), 50 °C, 4−8 h. For products 3a−3n: NBS (0.4
mmol). Isolated yields in parentheses are based on 1. For products
3o−dd: NCS (0.3 mmol). Isolated yields in parentheses are based on
NCS.
chloro group linked to the arylalkynyl moiety and a methoxy
group located at C5-position of the internal arene ring for the
formation of the dichlorinated product 3cc. It should be noted
that this is the first method for the multicomponent assembly
of these multifunctionalized 1-naphthols through a sequential
[2 + 2] cycloaddition/C−C cleavage/bis-halogenation pathway
in a one-pot manner. The structures of the products 2a and 3b
were determined by X-ray diffraction analysis (Figure 1 and see
Supporting Information).
intramolecular [2 + 2] cycloaddition of allene−ynes 1
spontaneously occurs to afford cyclobutene intermediate A,
which undergoes oxidation−coordination with silver salt and
NFSI to afford AgIII complex intermediate B,16 followed by
migratory insertion17 and base-promoted ring-opening of
cyclobutane intermediate to access vinyl−AgIII fluoride D.
Next, the fluoride substitutes the sulfonyl group to liberate
benzenesulfonyl fluoride (detected by LC−MS)18 and
intermediate E via N−S bond cleavage, and the following
reductive elimination gives benzo[g]indole intermediate F.
Finally, base-promoted nucleophilic substitution between F and
benzenesulfonyl fluoride yields polysubstituted benzo[g]indole
products 2 (Scheme 6a). Different from the above, the latter
two processes undergo spontaneous [2 + 2] cycloaddition and
the following halohydroxylation of cyclobutene intermediates in
the presence of X−Y (NBS and NCS) and H2O to give G.
Subsequently, the ring-opening of cyclobutene and hydrogen
transfer proceed to form intermediates H followed by
electrophilic substitution with NBS and NCS to yield the
products 3 (Scheme 6b).
In summary, we have discovered unprecedented CC bond
breaking and rearranging reactions of benzene-linked allene−
ynes under mild conditions. These transformations provide
general and practical protocols toward the formation of a range
of richly decorated benzo[g]indoles and 1-naphthols of
chemical and biomedical importance. The original alkyne
motif was split into two parts and was enabled them to remerge
into one molecular framework, thereby maximizing atom
Figure 1. ORTEP drawing of 2a.
Considering NBS and NCS can readily trigger the radical
species, the substrate 1r was first subjected to the reaction with
1.5 equiv of TEMPO or BHT, and the expected product 3d was
afforded in 80% yield; this observation would confirm that the
mechanism does not involve a radical process (Scheme 5a).
Next, substrate 1r was treated with 1.0 equiv of NBS under
standard conditions, delivering the expected product 3d in 40%
yield along with monobrominated product 4 in 10% yield,
which indicates bromination occurred at C4 position prior to
C2 position during the reaction process (Scheme 5b).
C
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* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b03410.
Experimental procedures and spectroscopic data for all
new compounds 2a−p and 3a−cc (PDF)
Accession Codes
CCDC 1517855 and 1577815 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: jiangchem@jsnu.edu.cn.
*E-mail: laotu@jsnu.edu.cn.
*E-mail: guigen.li@ttu.edu.
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Soc. 2001, 123, 8600. (b) Cha, K.-M.; Jo, E.-A.; Jun, C.-H. Synlett
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(10) Sun, J.; Wang, F.; Hu, H.; Wang, X.; Wu, H.; Liu, Y. J. Org.
Chem. 2014, 79, 3992.
ORCID
(11) Shen, T.; Wang, T.; Qin, C.; Jiao, N. Angew. Chem., Int. Ed.
2013, 52, 6677.
(12) Shen, T.; Zhang, Y.; Liang, Y.-F.; Jiao, N. J. Am. Chem. Soc.
Guigen Li: 0000-0002-9312-412X
Bo Jiang: 0000-0003-3878-515X
Author Contributions
∥J.-Y.W. and P.Z. contributed equally.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the NSFC (No.
21332005, and 21472071), PAPD of Jiangsu Higher Education
Institutions, the Outstanding Youth Fund of JSNU
(YQ2015003), NSF of Jiangsu Province (BK20151163),
Robert A. Welch Foundation (D-1361, USA), and the Graduate
Education Innovation Project of Jiangsu Province (No.
KYCX17_1579).
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