Diaryliodonium Salt-Mediated Intramolecular C-N Bond Formation Using Boron-Masking N-Hydroxyamides

Article abstract of DOI:10.1021/acs.orglett.9b04076

Intramolecular aromatic C-N bond formation reactions using electron-rich aromatic tethered boron-masking N-hydroxyamide as substrate were realized. These new C-N bond formation reactions involve the in situ generation of a diaryliodonium salt by treatment with hypervalent iodine, deborylation by base treatment, spontaneous N → O acyl migration, cyclization, reductive elimination, elimination of benzoic acid, and tautomerization to indole formation. Hereby, we obtained highly functionalized electron-rich indoles and quinoline in practical yields.

Full text of DOI:10.1021/acs.orglett.9b04076

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Diaryliodonium Salt-Mediated Intramolecular C−N Bond Formation
Using Boron-Masking N‑Hydroxyamides
Makoto Matsumoto,† Kohei Wada,† Kazuki Urakawa,† and Hayato Ishikawa*,†,‡
†Department of Chemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1, Kurokami, Chuo-ku,
Kumamoto 860-8555, Japan
‡Faculty of Advanced Science and Technology, Kumamoto University, 2-39-1, Kurokami, Chuo-ku, Kumamoto 860-8555, Japan
S
* Supporting Information
ABSTRACT: Intramolecular aromatic C−N bond formation reactions
using electron-rich aromatic tethered boron-masking N-hydroxyamide as
substrate were realized. These new C−N bond formation reactions
involve the in situ generation of a diaryliodonium salt by treatment with
hypervalent iodine, deborylation by base treatment, spontaneous N → O
acyl migration, cyclization, reductive elimination, elimination of benzoic
acid, and tautomerization to indole formation. Hereby, we obtained
highly functionalized electron-rich indoles and quinoline in practical
yields.
workers reported direct C−N bond formation, using an in situ
generated diaryliodonium salt (Scheme 1a, eq 2).4 However,
when applying the reported methodology, the synthesis of
election-rich anilines or their equivalents is not easily
accomplished. This is because of an overoxidation reaction
by hypervalent iodine of in situ generated electron-rich
anilines.
Herein, we describe intramolecular aromatic C−N bond
formation by a stepwise protocol using boron-masking N-
hydroxyamide as the substrate to form electron-rich indoles
and quinoline in a one-pot operation (Scheme 1b, eq 3). In the
reaction, an in situ generated, highly reactive diaryliodonium
salt was detected as a key intermediate. This is the first
example of a reaction in which an acyl migration pathway of N-
hydroxyamide is employed in an oxidative C−N bond forming
reaction.
he discovery of new C−N bond formation reactions is an
Timportant topic in current organic chemistry, particularly
because these methods provide a new synthetic strategy for the
development of new organic materials, drugs, and agricultural
chemicals. For example, Buchwald−Hartwig C−N bond
formation reactions were a breakthrough in the evolution of
retrosynthetic analysis.1 To date, aromatic C−N bond
formation using hypervalent iodine as the oxidant has been
reported by several groups.2,3 The unique amino cation or its
equivalents via hypervalent iodine oxidation of amide nitrogen
are reported to afford nonelectron-rich anilines, lactams, or
carbazoles (Scheme 1a, eq 1). Recently, Olofsson and co-
Scheme 1. Oxidative C−N Bond Formation Using
Hypervalent Iodine
It is difficult to employ primary or secondary nitrosoalkanes
as electrophiles because they quickly convert to stabilized cis
and trans dimers or easily tautomerize to thermodynamically
stable oximes.5 After recognizing the challenge presented when
using such an unstable species in nucleophilic C−N bond
formation, we then considered that the generation of a primary
nitrosoalkane from N-hydroxyamide via hypervalent iodine
oxidation, followed by Friedel−Crafts-type cyclization, could
be used to prepare electron-rich indoles (Scheme 2). Thus, the
hypervalent iodine complex 2, prepared from N-hydroxyamide
1 and [bis(trifluoroacetoxy)iodo]benzene (PIFA), should
undergo oxidative cleavage to afford primary nitrosoalkane 4.
Received: November 14, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b04076
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
we discovered is the first example that includes the use of
DMT-MM in N-hydroxyamide synthesis.
Scheme 2. Our Initial Hypothesis: In Situ Generation of
Nitrosoalkane Followed by the Friedel−Crafts-Type C−N
Bond Formation Reaction
With a suitable precursor now in hand, we next examined
the proposed indole synthesis. When 1 equiv of PIFA was
added to a solution of 8 in 1,1,1,3,3,3-hexafluoroisopropanol
(HFIP) at 0 °C (5 min), followed by the addition of aqueous
saturated NaHCO3, the compound 5,7-dimethoxyindole (9)
was obtained in 7% yield, along with 37% of hydrolyzed p-
methoxybenzoic acid.
After several attempts to achieve optimization, we noticed
that the N-hydroxyamide moiety was very unstable; it formed a
complex mixture under the reaction conditions. On the other
hand, we considered that indole formation took place after the
formation of a diaryliodonium salt on an electron-rich aromatic
group via nucleophilic addition to hypervalent iodine.
Therefore, the N-hydroxyamide moiety was masked as a
boron salt9 before PIFA treatment. Thus, N-hydroxyamide 8
was treated with 2 equiv of boron trifluoride−ether complex in
CH2Cl2 to afford the aromatic tethered boron-masking N-
hydroxyamide 12 in quantitative yield (23 °C, 1 h; Table 1). A
single crystal of 12 was obtained and used to confirm the
structure by X-ray analysis (CCDC 1963790).
With the boron-masking N-hydroxyamide 12 now in hand,
we re-examined the indole synthesis. When 12 was treated
with 1 equiv of PIFA in HFIP at 0 °C, the starting material was
immediately consumed, according to TLC analysis. After the
addition of aqueous saturated NaHCO3 solution to the
reaction mixture, our desired product 5,7-dimethoxyindole
The generated primary nitroso moiety would be immediately
trapped by an electron-rich aromatic group tethered to
nitrogen via a Friedel−Crafts-type C−N bond formation to
afford cyclic product 5 along with hydrolyzed p-methox-
ybenzoic acid. Finally, 5 could be easily converted to the
corresponding indoles via dehydration and tautomerization.
In our preliminary investigations (first attempt), we selected
N-hydroxyamide 8, which contains a 1,3-dimethoxybenzene
moiety, as the substrate (Scheme 3). The coupling precursor 7
Scheme 3. Indole Synthesis: First Attempt
a
Table 1. Optimization of Indole Synthesis
was prepared by the reduction of known nitro compound 6
with Al(Hg)6,7 (excess Al(Hg), THF, 23 °C). To prepare N-
hydroxyamide 8, the amino group on a hydroxylamine moiety
had to condense with p-methoxybenzoic acid, selectively.
Initially, p-methoxybenzoyl chloride was employed in a
condensation reaction with hydroxylamine to obtain 8.
However, we were unable to achieve reproducibility; the
amine and hydroxyl groups often reacted to afford an amide/
ester mixture. Thereafter, 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-
4-methylmorpholinium chloride (DMT-MM), developed by
Kunishima et al., was selected as the coupling reagent to
prepare N-hydroxyamide. DMT-MM is reported to be an
amide-selective coupling reagent in the presence of an hydroxyl
group.8 Thus, the treatment of hydroxylamine 7 and p-
methoxybenzoic acid (1 equiv) with DMT-MM (1.2 equiv) in
the presence of 10 mol % N-methyl morpholine afforded the
desired N-hydroxyamide 8 in excellent yield and with good
selectively (CH3CN, −20 °C, 15 h, 93%). This protocol that
b
yield 2
9
b
entry
1
R
yield 1
base (equiv)
conditions
(%)
acid
OMe
quant aqueous sat
NaHCO3
quant Et3N (10)
0 °C, 15 min
77
quant
2
3
4
OMe
H
THF, 0 °C,
2 h
THF, 0 °C,
1.5 h
THF, 0 °C,
10 min
98
97
87
quant
quant
quant
97%
Et3N (10)
NO2
quant Et3N (10)
a
Reaction conditions. Step 1: a reaction mixture of 12−14 (0.0528
mmol) and PIFA (0.0528 mmol) in HFIP (3.6 mL) was stirred for 5
min at 0 °C, followed by evaporation to remove all HFIP. Step 2: base
was added to the crude mixture of products of step 1 in solvent (3.6
b
mL) at 0 °C. Isolated yield.
B
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Letter
(9) was obtained in 77% yield, along with p-methoxybenzoic
acid in quantitative yield (Table 1, entry 1). To understand the
reaction mechanism (the first PIFA treatment), NMR analysis
in deuterated HFIP was carried out. Hypervalent iodine
reacted on the 1,3-dimethoxybenzene moiety in almost full
conversion.3j,10 The reaction site and the structure of iodonium
salt 15 were confirmed by 2D NMR, NOE, and MS (Figure
S1). Thus, a single diaryliodonium salt was generated in situ
via regioselective nucleophilic addition in the first step.
Further improvement in the reaction yield was achieved
when Et3N was used as the base instead of NaHCO3. Thus,
after evaporation of the diaryliodonium salt mixture, anhydrous
Et3N in THF solution was added in a one-pot operation (10
equiv of Et3N, 0 °C, 2 h; Table 1, entry 2). The product 9 was
obtained in almost quantitative yield (98%). It emerged that
the p-methoxy group on the benzoic acid moiety was not
required, although it was an essential functional group in our
initial hypothesis. Thus, when the simple benzoyl-substituted
substrate 13 was employed, the reaction proceeded smoothly
and the desired indole 9 was obtained in excellent yield (1.5 h;
Table 1, entry 3). Furthermore, a p-nitrobenzoyl-substituted
substrate 14 was also a suitable substrate; it promoted indole
formation. Of importance is that the reaction time in the
second step was only 10 min, although here, the reaction yield
was slightly decreased (Table 1, entry 4). We recognized that
the electron-withdrawing group at the benzoyl group would
strongly accelerate C−N bond formation.
Next, our interest moved to the origin of the oxygen of the
recovered benzoic acid. The 18O isotope derivative 16, labeled
on the N-hydroxy amine moiety, was prepared from NaN18O2
(eq 5, below). When the 18O-labeled cyclic boron compound
16 was employed under optimized reaction conditions, 18O-
labeled p-methoxybenzoic acid was obtained in 87% yield
along with indole 9 (see the details in the Supporting
Information). In 1984, Nikishin et al. reported an N → O acyl
migration reaction from N-hydroxy amide to O-acylhydroxyl-
amine.11 We propose that the recovered benzoic acid was
generated via an acyl migration pathway.
Scheme 4. Proposed Reaction Mechanism
study. It is further supported by the effect of an electron-
withdrawing group on benzoic acid (Table 1, entries 2 and 4).
Next, an N → O acyl migration reaction takes place to afford
an O-acyl intermediate 18. It is conceivable that an equilibrium
state exists between compounds 17 and 18. We speculated that
the intramolecular cyclization of 18 affords the six-membered
heterocyclic compound 19. Following a reductive elimination
process4 to form a C−N bond, the indole precursor 20 is
generated. Finally, desorption of p-methoxybenzoic acid
followed by tautomerization affords 5,7-dimethoxyindole (9).
After consideration of the reaction mechanism, O-
benzoylhydroxylamine 21,12 an acyl migration intermediate
in our proposed mechanism, was exposed to PIFA oxidation
(Scheme 5). However, when 21 was treated with 1 equiv of
Scheme 5. Indole Synthesis Using O-Benzoylhydroxylamine
21
Next, a kinetic study of the second step was performed.
Thus, the in situ generated diaryliodonium salt 15, in the
presence of Et3N in THF-d8, was monitored by NMR (Figure
S2). The diaryliodonium salt 15 reacted to form three
products, specifically, indole 9, p-methoxybenzoic, and
iodobenzene, with no reaction intermediates. (15 was fully
consumed.) In addition, when the reaction was carried out in
the presence of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)
as a radical scavenger, our desired indole was produced
smoothly without any side products. We therefore excluded
the radical mechanism via single electron transfer (SET).
Our proposed reaction mechanism derived from the above
experiments is shown in Scheme 4. The cyclic boron
compound 12 is converted to diaryliodonium salt 15 by
PIFA treatment in non-nucleophilic HFIP. The treatment of
the iodonium salt 15 with Et3N results in nucleophilic
deborylation to afford the oxoanion 17. This deborylation
step is the rate-limiting step estimated by the above kinetic
PIFA in HFIP followed by Et3N treatment, our desired indole
9 was obtained in only 5% yield, along with an inseparable
complex mixture. We subsequently determined that the
electron-rich indole that we obtained, 9, was highly reactive
toward PIFA oxidation. When indole 9 was treated with 1
equiv of PIFA in HFIP at 0 °C, 9 was immediately oxidized
and a complex mixture was obtained. Thus, the stepwise C−N
bond formation process that we discovered here is essential for
the synthesis of electron-rich indoles. In particular, the usage of
boron-masking N-hydroxyamide as the substrate and acyl
migration after the preparation of diaryliodonium salts are the
keys to success.
C
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Having established fully optimized conditions, we next
examined the substrate scope of the reaction we developed.
When brominated 12 was employed, 4-bromo-5,7-dimethoxy
indole (22) was obtained in 78% yield (Table 2, entry 2). In
the case of the substrate having a monosiloxyphenyl group, the
indole synthesis proceeded in good conversion to afford the
corresponding indoles; the silyl group was mainly eliminated
[Table 2, entry 3; total indole 66% (23, 15%; 24, 51%)]. From
this result, we recognized that one hydroxy moiety on an
aromatic group was sufficient to promote the newly discovered
C−N bond formation reaction. Furthermore, dialkyl-substi-
tuted monomethoxy substrates are also suitable substrates to
promote the indole formation. The three-membered ring
indole 25 was obtained in 48% yield (Table 2, entry 4). The
unique biphenyl indole 26 was also successfully prepared
through our newly discovered methodology (70%; Table 2,
entry 5). C3-substituted indoles were also obtained. When
substrates with methyl or phenyl group substituents on tether
were employed in the reaction, the desired C3-substituted
indoles 27 and 28 were obtained in high yields (27, 76%; 28,
99%; Table 2, entries 6 and 7). A C2-methyl-substituted indole
29 was also obtained in excellent yield (82%; Table 2, entry 8).
Next, the one-carbon-elongated substrate 30 was employed
to prepare hydroquinoline 31 or its double bond isomers
(Scheme 6). When 30 was treated with PIFA followed by Et3N
a
Table 2. Substrate Scope
Scheme 6. Synthesis of 6,8-Dimethoxyquinoline
treatment, the unexpected 6,8-dimethoxyquinoline (32) and
6,8-dimethoxy-1,2,3,4-tetrahydroquinoline (33) were obtained
in 43% and 31% yields, respectively. These results indicated
that a redox disproportionation reaction of the generated 31
took place after the desired intramolecular C−N bond
formation reaction. Therefore, additional PIFA oxidation to
quinoline 32 from tetrahydroquinoline 33 was carried out in a
one-pot operation. After removal of the Et3N and solvent from
the reaction mixture, PIFA in HFIP was directly added to the
reaction vessel, once again. As a result, quinoline 32 was
obtained as the sole product in 71% yield. Thus, our
methodology was expanded to include the synthesis of
quinoline.
In conclusion, we have discovered a new stepwise and
oxidative intramolecular C−N bond formation reaction, which
uses a boron-masking N-hydroxyamide. The reaction com-
menced with the generation of a diaryliodonium salt by
treatment with hypervalent iodine, deborylation by base
treatment, spontaneous N → O acyl migration, cyclization,
reductive elimination, elimination of benzoic acid, and
tautomerization to afford multisubstituted indoles or quinoline.
The six-step transformation proceeded in one pot, following a
simple protocol. The methods described here are useful for the
efficient synthesis of electron-rich indoles, which are usually
difficult to synthesis by oxidation. Unprotected electron-rich
a
Reaction conditions. Step 1: a reaction mixture of the boron complex
(0.0528 mmol) and PIFA (0.0528 mmol) in HFIP (3.6 mL) was
stirred at 0 °C, followed by evaporation to remove all HFIP. Step 2:
Et3N (0.528 mmol) was added to the crude mixture of the products of
step 1 in THF (3.6 mL) at 0 °C. Isolated yield. Excess aqueous
saturated NaHCO3 was employed in step 2 (see the details in the
Supporting Information).
b
c
D
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Organic Letters
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indoles are usually highly sensitive toward oxidants. Thus, in
this stepwise protocol, the usage of boron-masking N-
hydroxyamide as the substrate and acyl migration after the
preparation of diaryliodonium salts are the keys to success.
This method may also be referred to as aromatic C−H
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currently underway in our group.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04076.
1H-NMR data; kinetic study; experimental details and
characterization data for all new compounds (PDF)
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55, 8928−8932. (b) Purkait, N.; Kervefors, G.; Linde, E.; Olofsson, B.
Angew. Chem., Int. Ed. 2018, 57, 11427−11431.
Accession Codes
CCDC 1963790 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-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: h_ishikawa@kumamoto-u.ac.jp.
ORCID
(7) Paul, B.; Shee, S.; Chakrabarti, K.; Kundu, S. ChemSusChem
2017, 10, 2370−2374.
(8) (a) Kunishima, M.; Kawachi, C.; Iwasaki, F.; Terao, K.; Tani, S.
Tetrahedron Lett. 1999, 40, 5327−5330. (b) Kunishima, M.; Kawachi,
C.; Monta, J.; Terao, K.; Iwasaki, F.; Tani, S. Tetrahedron 1999, 55,
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Hayato Ishikawa: 0000-0002-3884-2583
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We would like to thank Prof. Tohru Fukuyama, Emeritus
Professor (University of Tokyo), for kind advice about the
reaction mechanisms. We gratefully acknowledge the financial
support of a Grant-in-Aid for Scientific Research (B)
(17H03059) from JSPS and Astellas Foundation for Research
on Metabolic Disorders.
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