Gold(I)-Catalyzed Reactions between N-(o-Alkynylphenyl)imines and Vinyldiazo Ketones to Form 3-(Furan-2-ylmethyl)-1 H-indoles via Postulated Azallyl Gold and Allylic Cation Intermediates

Article abstract of DOI:10.1021/acs.orglett.1c00038

This work describes gold-catalyzed additions of vinyldiazo ketones to N-(o-alkynylphenyl)imines to yield 3-(furan-2-ylmethyl)-1H-indoles involving skeletal rearrangement; these new catalytic reactions are applicable to a wide range of substrates. We postu

Full text of DOI:10.1021/acs.orglett.1c00038

pubs.acs.org/OrgLett
Letter
Gold(I)-Catalyzed Reactions between N‑(o‑Alkynylphenyl)imines
and Vinyldiazo Ketones to Form 3‑(Furan-2-ylmethyl)‑1H‑indoles via
Postulated Azallyl Gold and Allylic Cation Intermediates
Antony Sekar Kulandai Raj,□ Akshay Subhash Narode,□ and Rai-Shung Liu*
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ABSTRACT: This work describes gold-catalyzed additions of
vinyldiazo ketones to N-(o-alkynylphenyl)imines to yield 3-(furan-
2-ylmethyl)-1H-indoles involving skeletal rearrangement; these
new catalytic reactions are applicable to a wide range of substrates.
We postulate a new mechanism involving an initial addition of
diazo ketones to azomethine ylide intermediates to yield gold-
containing N-alkylated indole intermediates that undergo proton-
induced 1,3-group migrations, generating azallyl gold and allylic
cation pairs.
formations involve initial formation of alkenylmetal (Pd, Pt,
Rh, Ir, Au, Cu, Co, In) intermediates that subsequently
undergo consecutive 1,2-migrations over a broad range of
functionalities, including α-alkoxyalkyl,4 allyl,5 benzyl,6 acyl,7
methyl,8 sulfonyl,9 and boryl groups,10 before proceeding to
2,3-disubtituted indole derivatives (I). In contrast with a
consecutive 1,2-migration in eq 1, Oh reported ion pair
intermediates Int-4 in the Ag(I)-catalyzed cycloisomerization
of N-(o-alkynylphenyl)imines to enable 1,3-vinyl migrations. In
this mechanism, vinyl anions Int-4 were greatly stabilized with
two electron-withdrawing groups (eq 2).11 With other N-(o-
alkynylphenyl)imines, Iwasawa recently reported the formation
of azomethine ylides lnt-3′ from N-(o-alkynylphenyl)imines,
which reacted well with alkene in catalytic [3 + 2]-
cycloadditions (eq 3).12 We sought new reactivity of these
azomethine ylides Int-3′. Herein, we report gold-catalyzed
synthesis of 3-(furan-2- ylmethyl)-1H-indoles from the
reactions of azomethine ylides (Int-3′) with vinyldiazo
ketones. Instead of an expected [3 + 2]-cycloaddition, we
observed an initial addition of vinyldiazo ketones to azo
methine ylides (lnt-3′) to yield addition intermediates (Int-5)
and a proton. We note that a consecutive 1,2-migration in eq 1
requires a positive charge on the heteroatoms Y (Y = O, NR)
to enable 1,2-migrations, thus inapplicable to our new system.
The ion pair intermediates in eq 2 fail to work with our
catalysis because the expected allylic anions lnt-6 are unlikely
ubstituted indoles form the core skeletons of a wide range
1
Sof pharmaceuticals and naturally occurring alkaloids. To
date, numerous synthetic approaches have been developed to
generate this ring system via transition-metal catalysis.2 In
Scheme 1, transition-metal-catalyzed 1,3-migratory cyclo-
isomerization of o-alkynylaryl ethers and N,N-disubstituted
anilines provides a powerful approach to the synthesis of 2,3-
disubstituted benzofurans and indoles (eq 1).3 The trans-
Scheme 1. Reactions on Metal-Containing Indole
Intermediates
Received: January 6, 2021
Published: February 9, 2021
https://dx.doi.org/10.1021/acs.orglett.1c00038
© 2021 American Chemical Society
Org. Lett. 2021, 23, 1378−1382
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Organic Letters
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Letter
to survive in the presence of H+. As a concerted 1,3-migration
is thermally forbidden, in this work, we propose formation of
new intermediate pairs involving allylic cations Int-7 and
azallyl gold species Int-8 after a protonation of key
intermediates Int-5.
The significance of this catalysis is that indole-containing
triarylmethane derivatives are easily available (IV-1 to IV-6,
Figure 1).13 Indole derivatives IV-1 and IV-2 are demonstrated
(10 mol %), the yield of product 3a was increased to 84% yield
(entry 2). Other gold phosphine catalysts such as L′AuCl/
AgOTf (L′ = (PhO)3P and PPh3) produced compound 3a in
75 and 83% yields, respectively (entries 3 and 4). Alternative
halide scavengers such as LAuCl/X (X = AgNTf2, AgSbF6, and
NaBARF) gave compounds 3a in 63, 0, and 0% yields,
respectively, with AgSbF6 and NaBARF giving a complicated
mixture of products (entries 5−7). The ligating ability of
−NTf2 seems to be crucial. For LAuCl/AgOTf (L = P(t-
Bu)2(o-biphenyl)), the desired product 3a yields were as
follows (entries 8−10): THF (29%), toluene (76%), and
CH3CN (0%). AgOTf alone was completely inactive (entry
11). AuCl3 and PtCl2 gave a complicated mixture of products
(entries 12 and 13). The structure of compound 3a was
inferred from X-ray diffraction of its relative structure 4c (see
Table 3, entry 3). An overall review of Table 1 suggests that
gold catalysts containing electron-rich ligand and basic
counteranions are more appropriate for this new catalysis,
probably due to an enhanced stabilization of gold carbenes.
We assessed the scope of these catalytic reactions of various
N-(o-alkynylphenyl)imines 1 with vinyldiazo ketone 2a; the
findings are summarized in Table 2. For certain alkyne
Figure 1. Representative bioactive molecules.
Table 2. Scope of the Reaction with Various N-(o-
Alkynylphenyl)imines
to be an anticancer scaffold13a and an antibacterial agent,
respectively.13b Compounds IV-3 and IV-4 are aromatase an
inhibitor13c and an anti-inflammatory agent, respectively.13d
Species IV-5 has been used as an antitubercular agent.13e
Compound IV-6 has been tested as a antiobesity drug.13f
We optimized the reactions of (E)-1-phenyl-N-(2-(phenyl-
ethynyl)phenyl)methanimine 1a with vinyldiazo ketone 2a
using different gold catalysts, and the findings are summarized
in Table 1. Our initial IPrAuCl/AgOTf (10 mol %) evaluation
in DCE at 70 °C (3 h) afforded an indole derivative 3a in 81%
(entry 1). With LAuCl/AgOTf (L = P(t-Bu)2(o-biphenyl))
Table 1. Optimization of the Reaction Conditions
b
entry
catalyst
solvent
t (h)
yield (3a)
c
1
2
3
4
5
6
7
8
IPrAuCl/AgOTf
LAuCl/AgOTf
DCE
DCE
DCE
DCE
DCE
DCE
DCE
THF
toluene
CH3CN
DCE
DCE
DCE
3
2
5
2
4
6
5
6
3
6
6
5
5
81
84
75
83
63
d
a
b
c
(PhO)3PAuCl/AgOTf
PPh3AuCl/AgOTf
LAuCl/AgNTf2
LAuCl/AgSbF6
LAuCl/NaBARF
LAuCl/AgOTf
LAuCl/AgOTf
LAuCl/AgOTf
AgOTf
[1] = 0.1 M. L = P(t-Bu)2(o-biphenyl). Product yields are
reported after separation from a silica column, Tf = trifluorometha-
nesulfonyl.
substrates 1b−1e with 4-substituted species of phenylalkyne
(R = 4-XC6H4; X = CH3, t-butyl, Cl, and Br), their subsequent
products 3b−3e were obtained in 76−82% yields (entries 1−
4). For their 2-thienyl substituted analogue 1f, its correspond-
ing product 3f was produced in 70% yield (entry 5). We also
prepared alkylalkyne substrates 1g−1i (R = n-butyl, isopropyl,
and cyclopropyl), rendering the desired products 3g−3i in
43−52% yields (entries 6−8). We tested the reactions on
substrates 1j−1m, bearing 4-substituted phenyl imines (R1 =
4-XC6H4; X = CH3, t-butyl, Cl, and Br), further providing the
desired products 3j−3m in 72−80% yields (entries 9−12). 2-
29
76
9
10
11
12
13
AuCl3
PtCl2
a
b
[1a] = 0.1 M. Product yields are reported after separation from a
c
silica column. IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine,
d
L = P(t-Bu)2(o-biphenyl), DCE = 1,2-dichloroethane, THF =
tetrahydofuran.
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Letter
Thienylalkyne analogue 1n afforded its resulting product 3n in
70% yield (entry 13). For starting substrates 1o−1q, variations
of the R2 substituents at their bridged benzenes still maintained
high catalytic efficiency, yielding the desired products in 79−
83% yields (entries 14−16).
We assessed the substrate scope using various vinyldiazo
ketones 2 in this catalytic sequence; the yields of products are
summarized in Table 3. We prepared the species of vinyldiazo
Scheme 2. Mechanistic Investigations and Chemical
Functionalization of 4d
Table 3. Scope of the Reaction with Various Vinyldiazo
Ketones
We postulated a mechanism in Scheme 3. The gold catalyst
is expected to form azomethine ylides A via an initial
Scheme 3. A Plausible Reaction Mechanism
a
b
c
[1a] = 0.1 M. L = P(t-Bu)2(o-biphenyl). Product yields are
reported after separation from a silica column, Tf = trifluorometha-
nesulfonyl.
2b−2f bearing distinct 4-phenyl ketones (R3 = 4-XC6H4; X =
CH3, Cl, Br, t-butyl, and OMe), and their resulting products
4a−4e were obtained in 74−84% yields (entries 1−5). X-ray
diffraction of compound 4c confirms our proposed structure.
Again, 3-thienyl ketone species 2g produced the expected
product 4f in 80% yield (entry 6). This new catalysis was
further compatible with α-diazo alkylketones 2h−2j (R3 = n-
butyl, cyclopropyl, and cyclohexyl), rendering the desired
products 4g−4i in 78−84% (entries 7−9). We also checked
the reactions on 2-substituted vinyldiazo ketones 2k−2l (R4 =
Me and n-butyl), producing compounds 4j and 4k in 82 and
68% yields, respectively (entries 10 and 11).
Scheme 2 shows mechanistic investigations and chemical
functionalizations of one representative compound 4d. We
performed an additional experiment between (E)-1-phenyl-N-
(2-(phenylethynyl)phenyl)methanimine 1a and vinyldiazo
ester 2m using optimized conditions, which give a complicated
mixture of product (eq 5); this reaction requires an α-diazo
ketone to activate the reaction. Methylation and tosylation of
species 4d with MeI/KOH and TsCl/KOH afforded N-
methylated or N-tosylated products 5−6 in 95% and 91%
yields respectively (eq 6). We performed a deuterium-labeling
experiment to elucidate the mechanisms of formation of indole
derivatives 3. We prepared a sample d1-1a containing X = 0.82
deuterium content at the imine proton, which afforded
compound d1-3a comprising X = 0.79 D at the methane
carbon, indicating a formal 1,3-benzyl migration (eq 7).
cyclization of N-(o-alkynylphenyl)imine reagents. A nucleo-
philic attack of diazo ketone 2a to azomethine ylides A will
form intermediate B, which is prone to loss of a proton, further
leading to formation of gold containing N-substituted indoles
C. Protonations of indoles are well-known at the C(3)-
carbon14 rather than on the nitrogen atom. Accordingly,
species C undergo protonation to form C(2)-benzyl
carbocations D. As the vinyldiazo moiety of species D is
electron-rich and highly nucleophilic, this electronic effect
leads to a cleavage of the C−N bond of species D, yielding
azallylgold F and bezylic cation E that further stabilized its
resonance form E′. Although an elimination of LAu+ from
intermediates D to yield indole species G is also likely to occur,
we believe that this process is reversible to enable a complete
transformation of species D into E. A recombination of
reactive species E and F produces indole species H, of which
the vinyldiazo ketone can be transformed into a furan ring
according to our previous work.15
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Letter
Molecules 2013, 18, 6620−6662. (c) Zhang, M.-Z.; Chen, Q.; Yang,
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In summary, this work reports gold-catalyzed reactions of N-
(o-alkynylphenyl)imines and vinyldiazo ketones16 to form 3-
(furan-2-ylmethyl)-1H-indole products over a wide scope of
substrates. We postulate an initial gold-catalyzed cyclization of
N-(o-alkynylphenyl)imines to form azomethine ylides that are
subsequently attacked by vinyldiazo ketones to form gold-
containing N-substituted indoles and a proton. Further
protonation of these key indole intermediates likely forms a
pair of intermediates such as azallyl gold and allylic cations to
enable 1,3-formal group migrations.
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Experimental procedures, characterization data, crystal-
1
lography data, and H NMR and 13C NMR spectra for
representative compounds (PDF)
Accession Codes
̈
(e) Pflasterer, D.; Hashmi, A. S. K. Chem. Soc. Rev. 2016, 45, 1331−
CCDC 2049435 contains 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 Cam-
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Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
■
Rai-Shung Liu − Frontier Research Center of Matter Science
and Technology, Department of Chemistry, National Tsing-
Hua University, Hsinchu, Taiwan 30013, Republic of China;
orcid.org/0000-0002-2011-8124; Email: rsliu@
mx.nthu.edu.tw
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Bucher, J.; Rappold, M.; Graf, K.; Rominger, F.; Hashmi, A. S. K.
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Authors
Antony Sekar Kulandai Raj − Frontier Research Center of
Matter Science and Technology, Department of Chemistry,
National Tsing-Hua University, Hsinchu, Taiwan 30013,
Republic of China
Akshay Subhash Narode − Frontier Research Center of
Matter Science and Technology, Department of Chemistry,
National Tsing-Hua University, Hsinchu, Taiwan 30013,
Republic of China
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00038
Author Contributions
□A.S.K.R. and A.S.N. contributed equally.
Notes
(8) Zeng, X.; Kinjo, R.; Donnadieu, B.; Bertrand, G. Angew. Chem.,
Int. Ed. 2010, 49, 942−945.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Ministry of Science and Technology
(MOST 107-3017-F-007-002) and the Ministry of Education
(MOE 106N506CE1), Taiwan, for supporting this work.
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