Cascade Approach to Highly Functionalized Biaryls by a Nucleophilic Aromatic Substitution with Arylhydroxylamines

Article abstract of DOI:10.1021/acs.orglett.9b00927

A transition-metal free synthesis of highly functionalized 2-hydroxy-2′-amino-1,1′-biaryls from N-arylhydroxylamines has been developed. This operationally simple and readily scalable approach relies on a cascade of reactions that initially generates transient N,O-diarylhydroxylamines, via direct O-arylation, which then undergo rapid [3,3]-sigmatropic rearrangement and subsequent rearomatization to form NOBIN-type products. These structurally diverse functionalized biaryls are obtained under mild conditions in good to excellent isolated yields.

Full text of DOI:10.1021/acs.orglett.9b00927

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cascade Approach to Highly Functionalized Biaryls by a
Nucleophilic Aromatic Substitution with Arylhydroxylamines
†
†
†
‡
Lirong Guo,†,§ Fengting Liu,†,§ Liying Wang, Hairui Yuan, Lei Feng, Laszlo Kurti,
́
́
̈
and Hongyin Gao*,†
†School of Chemistry and Chemical Engineering, Key Laboratory of Colloid and Interface Chemistry, Ministry of Education,
Shandong University, 27 South Shanda Road, Ji’nan, Shandong 250100, China
‡Department of Chemistry, Rice University, BioScience Research Collaborative, Houston, Texas 77005, United States
S
* Supporting Information
ABSTRACT: A transition-metal free synthesis of highly functionalized 2-hydroxy-2′-amino-1,1′-biaryls from N-arylhydroxyl-
amines has been developed. This operationally simple and readily scalable approach relies on a cascade of reactions that initially
generates transient N,O-diarylhydroxylamines, via direct O-arylation, which then undergo rapid [3,3]-sigmatropic
rearrangement and subsequent rearomatization to form NOBIN-type products. These structurally diverse functionalized
biaryls are obtained under mild conditions in good to excellent isolated yields.
he biaryl motif is a privileged structure because of its
presence in a large number of bioactive natural products,
be an efficient catalyst to promote the enantioselective [2 + 2]
photocycloaddition of 4-alkenyl-substituted coumarins (Figure
1. Compound B).5 In view of the importance of the biaryl
motif, it is not surprising that much attention has been devoted
to develop novel strategies for the construction of biaryl unit
and numerous methods have been reported.6 Among these
methods, transition-metal-catalyzed cross-coupling of aryl
(pseudo)halides with organometallic reagents has been well
developed and widely applied in biaryl synthesis,7 for instance,
Suzuki,8 Kumada, Negishi, and Stille reactions (Scheme 1a).
During the past two decades, transition-metal-catalyzed direct
dehydrogenative C−H/C−H cross-coupling9 and oxidative
homocoupling6a,d,l of two aromatic compounds were found to
be efficient alternative approaches to biaryls from the
viewpoint of atom and step economy (Scheme 1a).10
However, the generally harsh conditions, employment of
expensive metal catalysts/ligands, and scarcity of specific
starting materials often limit the versatility and utility of these
methods for biaryl synthesis. In addition, the heavy metal
residues in the products pose a serious challenge in both the
development of pharmaceuticals and novel functional materi-
als. Thus, there is need for the continued development of
alternative and more efficient synthetic strategies that allow the
expedient and greener construction of structurally diverse
biaryls in the absence of transition metals.
T
pharmaceuticals, catalysts, as well as organic functional
materials (Figure 1).1 In particular, axially chiral biaryl
Figure 1. Representive examples of important biaryl compounds.
molecules, as exemplified by BINOL, BINAP, NOBIN,
BINAM, and their derivatives, are of increasing importance
due to their extensive applications as chiral catalysts and
ligands.1a,c,2 For instance, chiral BINAP, MAP (monodentate
aminophosphine ligand), and phosphoric acids, which were
derived from BINOL, have been intensively evaluated in
asymmetric catalysis;2d,3 the NOBIN-derived ligand (Figure 1,
Compound A) has been successfully employed in a series of
asymmetric aldol reactions in the presence of titanium
catalysts.4 A NOBIN derived organocatalyst B was found to
In the recent past, several transition-metal free approaches to
biaryls have been reported10,11 including: (1) SET-type (single
Received: March 15, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00927
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
rearrangement/rearomatization cascade to afford the corre-
sponding biaryl products 7 (Scheme 1c). This new approach
utilizes N-protected arylhydroxylamines, which are readily
prepared from the corresponding nitroarenes using well-
developed protocols.25
Scheme 1. Synthetic Strategies to Biaryl Compounds
Initially, we chose N-hydroxy-N-(naphthalen-2-yl)-
benzamide 4a and 4-fluoronitrobenzene 5a as model substrates
to optimize the reaction conditions. The screening of
frequently used organic bases revealed that the desired biaryl
product 7a can be afforded in moderate to good yields (Table
1, entries 1−6). Various inorganic bases (NaOH, KOH,
a
Table 1. Optimization of Reaction Conditions
b
entry
base
Et3N
DIPA
DBU
LDA
tBuONa
tBuOK
NaOH
KOH
Na2CO3
K2CO3
Cs2CO3
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
solvent
time
yield (%)
1
2
3
4
5
6
7
8
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
CH2Cl2
THF
24 h
24 h
2 h
2 h
2 h
2 h
2 h
2 h
2 h
34
60
58
73
58
68
78
92
79
75
48
97
83
77
21
34
29
44
0
electron transfer) cross-coupling between aryl halides and
arenes12 or aryl metal reagents,13 (2) Brønsted acid-catalyzed
atropselective BINAM synthesis through benzidine rearrange-
ment,14 (3) Brønsted acid-catalyzed arylation of 2-naphthols
with quinone derivatives for the synthesis of axially chiral
biaryldiols,15 (4) a similar strategy to NOBIN analogues using
2-naphthylamines as nucleophiles instead of 2-naphthols with
quinone derivatives,16 (5) Brønsted acid-catalyzed arylation of
indole derivatives for the synthesis of axially biaryls,17 (6)
regioselective strategies to biaryls from aryl sulfoxides18 or
aryliodanes19 with phenols via a [3,3]-sigmatropic rearrange-
ment, (7) and others,20 a restricted range of substrates could
engage among these transformations.
9
10
11
12
2 h
12 h
24 h
24 h
24 h
48 h
48 h
48 h
48 h
48 h
48 h
c
13
14
d
15
16
17
18
19
In 2013, Kurti and Ess and co-workers described the
reaction of 2-halonitroarenes 1 with excess (3 equiv)
arylmagnesium reagents at low temperature to generate
halogenated 2-amino-2′-hydroxy-1,1′-biaryls 3 upon aqueous
workup conditions.21 The key intermediate was presumed to
be an N,O-biarylhydroxylamine 2 that could undergo a facile
[3,3]-sigmatropic rearrangement, followed by an intramolecu-
lar proton-transfer/rearomatization to afford the final biaryl
product 3 (Scheme 1 b). The mechanistic pathway for the
generation of 2 is analogous to the Bartoli indole synthesis.22
This approach allows the rapid preparation of multihalogen-
substituted and axially chiral NOBIN-type biaryls. While
powerful, there are two main limitations of this method: (a)
the best results are obtained with ortho-halogen-substituted
nitroarene substrates, in the absence of this structural motif,
complex reaction mixtures are usually obtained; (b) three
equivalents of the same aryl Grignard reagent is required,
which limits the scope of potential substrates, especially those
with base- and Grignard-sensitive functional groups such as
nitriles, esters, ketones, and aldehydes. Given this background,
it became clear to us that if the key intermediate N,O-
biarylhydroxylamines 2 could be obtained via a different
process, the structural diversity of the NOBIN-type biaryl
products would be dramatically improved.
acetone
CH3CN
toluene
DMSO
e
20
21
a
Unless otherwise noted, all reactions were carried out under the
following conditions: 4a (0.2 mmol), 5a (2.0 equiv), base (2.0 equiv),
b
c
solvent (2 mL) at 30 °C. Yields of isolated products. 1.5 equiv of 5a
d
e
was employed. 1.2 equiv of 5a was employed. 2.0 equiv of 4-chloro-
nitrobenzene was employed instead of 5a. DIPA = diisopropylamine;
DBU = 1,8-diazabicyclo-[5.4.0]undec-7-ene; LDA = lithium diiso-
propylamide.
Na2CO3, K2CO3, Cs2CO3, K3PO4) were then tested (Table 1,
entries 7−12), and we found that the use of KOH and K3PO4
resulted in excellent yields (Table 1, entries 8 and 12). The
yields were slightly decreased when reducing the amount of
base (Table 1, entries 13 and 14). Next, a solvent-screen was
carried out and we observed that less polar solvents were not as
efficient as DMSO (Table 1, entries 15−19). When 4-
chloronitrobenzene was used instead of 5a as the arylating
agent, the same biaryl product 7a was isolated in a much lower
yield, presumably because of the significantly lower reactivity
of the chloronitroarene compared to the corresponding
fluoronitroarene in SNAr reactions (Table 1, entry 20). The
optimization studies revealed that the combination of 2.0 equiv
of 5a, 2.0 equiv of K3PO4, in DMSO at 30 °C were an optimal
choice for this transformation (Table 1, entry 12).
Thus, we envisioned that the direct O-arylation of
arylhydroxylamines 4 with electron-deficient haloarenes
would occur via a nucleophilic aromatic substitution
(SNAr)23 pathway to form transient intermediate N,O-diary-
lhydroxylamines24 6, which are expected to undergo the [3,3]-
B
DOI: 10.1021/acs.orglett.9b00927
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
With the optimized conditions in hand, we next turned our
attention to exploring the scope and limitations of this
transformation. We were pleased to find that this cascade
nucleophilic aromatic substitution followed by [3,3]-rearrange-
ment and rearomatization protocol is applicable to a wide
range of arylhydroxylamines and electron-deficient haloarenes,
to afford the highly functionalized NOBIN-type biaryls in good
to excellent yields under standard conditions (Scheme 2).
Treatment of N-hydroxy-N-(naphthalen-2-yl)benzamide 4a
and fluoronitroarenes with different electron-deficient groups,
such as Cl, Br, CF3, NO2, CN, Ac, CO2Me, etc., gave the
corresponding biaryl products in good to excellent yields
(Scheme 2, entries 1−16). However, the reactions of 4a and
fluoronitroarenes with electron-donating groups, such as Me,
OMe, usually resulted in complex reaction mixtures under
standard reaction conditions. Notably, the reactions of N-
hydroxy-N-(naphthalen-2-yl)benzamide 4a and 4-fluorobenzo-
nitrile 5q or 6-fluoropicolinonitrile 5r also proceeded smoothly
to give the desired biaryl products 7q and 7r in moderate
yields, respectively (Scheme 2, entries 17 and 18).26
Scheme 2. Substrate Scope
We next investigated the substrate scope with respect to
arylhydroxylamines in this reaction using commercially
available 1-fluoro-4-nitrobenzene 5a and 2-fluoro-5-nitro-
benzonitrile 5b as representative electrophiles (Scheme 2,
entries 19−33). Naphthalenyl or phenylhydroxylamines with
electron-donating or electron-withdrawing groups were
tolerated well and engaged in reactions with fluoronitroben-
zene 5a or 5b to generate the corresponding biaryl products in
moderate to good yields. In general, arylhydroxylamines with
electron-donating groups resulted in higher yields of the biaryl
products than those arylhydroxylamines with electron-with-
drawing groups under the optimal reaction conditions
(Scheme 2, entries 19 vs 20, 21 vs 22). An arylhydroxylamine
with a strong electron-withdrawing group such as nitro group
was also amenable to the optimized reaction conditions once
the protecting group of the nitrogen atom was switched to an
electron-donating group such as the methyl group instead of a
benzoyl group (Scheme 2, entry 32). To our delight, this
cascade approach could also be applied to the reactions of
heteroarylhydroxylamines such as 6-fuloro-N-methylpyridyl-
hydroxylamine with 4-fluoronitrobenzene 5a to furnish the
corresponding biaryl product in moderate yield (Scheme 2,
entry 33). In addition, the combination of pyridyl-
hydroxylamines with electron-deficient fluoropyridines gave
the bipyridyl products in moderate yields under the optimal
reaction conditions (Scheme 2, entries 34−36). The structure
of the product was unambiguously confirmed by the single
crystal X-ray diffraction of compound 7d (Scheme 2, entry 4).
It is noteworthy that this transformation can be readily scaled
up and grams of biaryl products can be synthesized under
standard conditions (Scheme 3a).
a
Reaction conditions: 4 (0.3 mmol), 5 (0.6 mmol), K3PO4 (0.6
b
mmol), DMSO (3 mL) at 30 °C for 24 h. Yields of isolated products.
c
d
2.0 equiv of tBuOK was employed as base. Reaction was run at 50
To gain some mechanistic insight, we conducted several
control experiments. The reactions proceeded smoothly to
generate the title products in the presence of radical-trapping
reagents, for instance, TEMPO and BHT (Scheme 3b). These
results revealed that a radical route can be ruled out and a SNAr
pathway is more likely.
To demonstrate the synthetic utility of this protocol, biaryl
product 7f was further elaborated as shown in Scheme 4. The
nitro group of 7f can be selectively reduced by zinc to furnish
the corresponding amine 8 (Scheme 4a).27 The deprotection
of amine and the reduction of nitro group can be achieved in
one-pot in the presence of hydrazine hydrate to afford biaryl
e
f
°C for 12 h. 2.0 equiv of Cs2CO3 was employed as base. Reaction
was run at 50 °C for 3 h. Bz = benzoyl, Bn = benzyl.
diamine 9 in 61% yield (Scheme 4b).28 The biaryl triflate 10,
which is derived from 7f in good yield,29 can be converted into
carbazole 12 through an intramolecular amination (Scheme
4c,e).30 The nitro group of biaryl compound 11, which was
methylated from 7f in moderate yield,31 could be easily
converted into indole 13 through Bartoli indole synthesis
(Scheme 4d,f).32
C
DOI: 10.1021/acs.orglett.9b00927
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
via www.ccdc.cam.ac.uk/data_request/cif, or by e-mailing
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.
Scheme 3. Large Scale Reaction and Control Experiments
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: hygao@sdu.edu.cn.
ORCID
́
́
Laszlo Kurti: 0000-0002-3412-5894
̈
Hongyin Gao: 0000-0003-4049-6832
Author Contributions
a
TEMPO = 2,2,6,6-tetramethylpiperidine 1-oxyl ; BHT = 2,6-di-tert-
butyl-4-methylphenol.
§These authors contributed equally.
Notes
1
Scheme 4. Synthetic Applications of Biaryl Products
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support of Shandong
University, the National Natural Science Foundation of China
(21702122), and the Natural Science Foundation of Shandong
Province (ZR2017MB002). We thank Prof. Di Sun at
Shandong University for the X-ray diffraction and data analysis.
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Detailed experimental procedures, characterization data,
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Accession Codes
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DOI: 10.1021/acs.orglett.9b00927
Org. Lett. XXXX, XXX, XXX−XXX