Enantioselective Copper-Catalyzed Electrophilic Dearomative Azidation of β-Naphthols

Article abstract of DOI:10.1021/acs.orglett.9b02604

The first example of copper-catalyzed enantioselective dearomative azidation of β-naphthols using a readily available N₃-transfer reagent is reported. A series of 2-hydroxy-1-naphthamides bearing a complex N-substituent were converted to the corresponding products in high yields with up to 96% ee, and chiral 1-azido-2-hydroxy-1-naphthoates were obtained with up to 90% ee under mild reaction conditions. The azides could be further transformed into the corresponding 1,2,3-triazoles smoothly via "click" reaction.

Full text of DOI:10.1021/acs.orglett.9b02604

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Letter
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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Enantioselective Copper-Catalyzed Electrophilic Dearomative
Azidation of β‑Naphthols
Chong-Ji Wang,† Jian Sun,† Wei Zhou, Jing Xue, Bing-Tao Ren, Guang-Yi Zhang, Yan-Le Mei,
and Qing-Hai Deng*
The Education Ministry Key Lab of Resource Chemistry and Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai
Normal University, Shanghai 200234, P. R. China
S
* Supporting Information
ABSTRACT: The first example of copper-catalyzed enantioselective dearomative azidation of β-naphthols using a readily
available N3-transfer reagent is reported. A series of 2-hydroxy-1-naphthamides bearing a complex N-substituent were converted
to the corresponding products in high yields with up to 96% ee, and chiral 1-azido-2-hydroxy-1-naphthoates were obtained with
up to 90% ee under mild reaction conditions. The azides could be further transformed into the corresponding 1,2,3-triazoles
smoothly via “click” reaction.
rganic azides1 are valuable and versatile molecules. The
azide group exists in some clinically approved drugs such
Scheme 1. (a) Asymmetric α-Azidation of Carbonyl
Compounds and (b, c) Dearomative Azidation of β-
O
a
as the anti-HIV azidonucleoside drug zidovudine AZT.2
Moreover, organic azides can be easily transformed into a
wide range of nitrogen-containing structural motifs via various
classic reactions3 such as the “click” reaction,4 aza-Wittig
reaction,5 Staudinger reduction,6 and Schmidt rearrangement.7
They are also currently considered as powerful precursors for
reactive species such as nitrenes and nitrenium ions.8
Therefore, numerous efforts have been devoted to develop
the synthetic strategies for organic azides.9 Among these
approaches, the catalytic stereoselective introduction of an
azido group into organic compounds is particularly attractive
but remains comparatively rare. Up to now, asymmetric direct
azidation10 has mainly focused on asymmetric nucleophilic
azido-functionalization of alkenes,11 allenes,12 aziridines,13 and
epoxides.14 However, the corresponding enantioselective
electrophilic variant has been rarely reported. In 2013,
Gade’s group15 developed the first asymmetric azidations of
β-keto esters and oxindoles applying the (boxmi)Fe(II)
complex as the catalyst, which provided the desired optically
pure azides with high enantioselectivities (Scheme 1a). In view
of the importance of chiral azides, developing new method-
ologies and broadening the substrate scope in this area are still
in great demand.
Naphthols
a
TBHP = tert-butyl hydroperoxide, PTAB = phenyl trimethyl
ammonium tribromide.
α-Azido β-naphthalenones are attractive targets because they
are transformed smoothly into the corresponding naphthale-
none derivatives, which are important structural motifs of
various biologically active natural products and therapeutic
reagents.16 Surprisingly, only two examples of preparing α-
azido-β-naphthalenones via oxidative azidation strategies were
reported by Prabhu’s group17 and Sarkar’s group,18 respec-
tively (Scheme 1b). To the best of our knowledge, the direct
Received: July 25, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02604
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
catalytic asymmetric azidation of β-naphthols remains unex-
plored. In this context, we report herein the first example of
copper-catalyzed asymmetric dearomative azidation of β-
naphthols for the synthesis of enantioenriched α-azido β-
naphthalenones in excellent yields and with moderate to high
enantioselectivities (Scheme 1c).
combination of Cu(OTf)2 with dbfox ligands21 (L4−L7,
dbfox = 4,6-dibenzofurandiyl-2,2′-bisoxazoline) was found to
provide very promising results (entries 4−7), and Cu(OTf)2/
L4 enabled the preparation of 3a in 86% yield with 66% ee
(entry 4).
The choice of solvent markedly influenced both the yield
and enantioselectivity. Among the solvents evaluated, dichloro-
methane gave both a higher yield and ee value (see the
Supporting Information). To further optimize the catalyst
performance in terms of yield and enantioselectivity, various
copper salts were tested in the reaction (entries 4 and 8−11).
Among these copper salts, Cu(CH3CN)4·PF6 is the best metal
salt and gave the product 3a in 90% yield with 73% ee (entry
11). Then, it is found that the kind of N-substituent is crucial
to improve the enantioselectivities (entries 11−14). 2-
Hydroxy-N,N-dibenzyl-1-naphthamide (1d) gave the desired
product 3d with excellent enantioselectivity (95% ee, entry
14). In addition, when azido-transfer reagent 2b was used in
place of 2a, the desired product was obtained in lower yield
and decreased enantioselectivity (entry 15). Finally, the
molecular sieve or lowering of the reaction temperature to 0
°C did not give better results (entries 16 and 17).
With the optimized reaction conditions in hand, the
substrate scope was first examined by varying the substituents
on the core of 2-hydroxy-N,N-dibenzyl-1-naphthamides
(Scheme 2). Aryl groups such as Ph, 4-Me-C6H4, 4-F-C6H4,
and 4-CF3-C6H4 on the 6-position or 7-position were well-
tolerated, and the corresponding products were all obtained in
high yields and enantioselectivities (3e−3i, 89−99% yields,
92−95% ee). Substrate 1j with 6-Br, substrate 1k with 6-Me,
and substrate 1l with 7-MeO could all be successfully
Amide structural motifs are omnipresent in natural products,
drugs, and biologically active molecules.19 However, inves-
tigations on dearomatization of arylamides are still rare.20
Thus, we began by using the reaction of 2-hydroxy-N,N-
diisopropyl-1-naphthamide (1a) with T-shaped iodine(III)
compound 2a as a model reaction to investigate different kinds
of metal salts combining with various chiral ligands and found
copper salts to be the most suitable catalyst precursors (see the
Supporting Information). We then chose Cu(OTf)2 as the
metal salt to screen a series of chiral bisoxazoline type ligands
(Table 1, entries 1−7). Tridentate bisoxazoline (pybox, L1)
was used as the ligand to generate the desired product 3a in
37% yield with low enantioselectivity (entry 1). The reaction
was promoted efficiently in the presence of bisoxazoline (box)
ligand L2, but only low enantioselectivity was observed (entry
2). Flexible tridentate bisoxazoline ligand L3 resulted in
slightly higher enantioselectivity (47%, entry 3). The
a
Table 1. Optimization of the Reaction Conditions
Scheme 2. Evaluation of the Substrate Scope of 2-Hydroxy-
a
1-naphthamides
b
c
time
(h)
yield
ee
entry
1 (R)
[Cu]
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OAc)2·H2O
(CuOTf)·0.5tol.
Cu(ClO4)2·6H2O
Cu(CH3CN)4·PF6
Cu(CH3CN)4·PF6
Cu(CH3CN)4·PF6
Cu(CH3CN)4·PF6
Cu(CH3CN)4·PF6
Cu(CH3CN)4·PF6
Cu(CH3CN)4·PF6
ligand
(%)
(%)
1
2
3
4
5
6
7
8
1a (iPr)
1a (iPr)
1a (iPr)
1a (iPr)
1a (iPr)
1a (iPr)
1a (iPr)
1a (iPr)
1a (iPr)
1a (iPr)
1a (iPr)
1b (Cy)
1c (Ph)
1d (Bn)
1d (Bn)
1d (Bn)
1d (Bn)
L1
L2
L3
L4
L5
L6
L7
L4
L4
L4
L4
L4
L4
L4
L4
L4
L4
48
24
12
20
10
10
12
23
25
14
13
11
15
19
24
15
45
37
92
94
86
80
84
86
64
90
91
90
75
94
86
45
90
72
9
−12
47
66
44
42
48
27
65
67
73
65
76
95
80
92
89
9
10
11
12
13
14
d
15
e
16
f
17
a
Reaction conditions: 1 (0.10 mmol), 2a (2 equiv), metal salt (10
mol %), ligand (12 mol %), CH2Cl2 (1.0 mL), rt (room temperature),
b
c
argon. The yields of isolated products. Determined by HPLC
d
e
a
analysis. 2a was replaced by 2b. 50 mg of a 4 Å molecular sieve was
Isolated yields of reactions performed on a 0.10 mmol scale; ee
f
added. The reaction was carried out under 0 °C.
values were determined by HPLC.
B
DOI: 10.1021/acs.orglett.9b02604
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
converted to their corresponding products in high yields and
enantioselectivities (3j−3l, 82−99% yields, 90−96% ee). To
our delight, 6-phenylethynyl-substituted substrate 1m was
compatible with the reaction conditions, and no “click”
reaction of the triple bond with azide occurred, giving the
dearomatization product 3m in 99% yield and 90% ee.
Furthermore, installation of substituents to the 4- and 3-
positions did not influence the reaction efficiency, and the
substrates 1n with 4-Br, 1o with 4-Ph, and 1p with 3-Me could
accomplish the asymmetric azidation with the outcomes of
67−99% yields and 90−93% ee (3n−3p). Notably, 2-hydroxy-
1-naphthamides bearing the complex N-substituent were also
well-tolerated. The substrates tethering indole (1q), isoindo-
line (1r), tetrahydroquinoline (1s), and tetrahydroisoquinoline
(1t) motifs achieved the corresponding products 3q−3t in
high yields with excellent enantioselectivities (91−99% yields,
91−94% ee); the substrates attaching pyrrolidine (1u),
piperidine (1v), morpholine (1w), and N-benzyl-N-(tert-
butyl) (1x) groups also were converted to the desired products
3u−3x in moderate to high ee values (74−87% ee).
on the 4-position of naphthols, such as 4i with 4-Ph and 4j
with 4-Br, could be successfully converted to their correspond-
ing products 5i (84% yield, 84% ee) and 5j (97% yield, 89%
ee), respectively. However, the substituents on the 3-position
obviously interfere with the enantiocontrol of the reaction. The
ee value was only moderate for product 5k with 3-Br (77% ee).
The substituent effect of the ester group (R3) was then
investigated (Scheme 3). The ester groups bearing an
unsaturated double bond or triple bond, like allyl, isoprenyl,
and propargyl, were tolerated, and the corresponding products
were obtained (5l−5n, 89−96% yields, 82−84% ee). The
bulkier groups, such as isopropyl and benzyl, could not
improve the enantioselectivity, and the corresponding
dearomatized products 5o−5p were obtained with good
enantioselectivities.
Notably, the absolute configurations of the enantiopure 3j
(96% ee) and 5j (97% ee) were both established to be S by
single-crystal X-ray structure analysis (Figure 1). The stereo-
chemical assignment of all other products was also made by
analogy.
Encouraged by the above results, we also broadened the
substrate scope to 2-hydroxy-1-naphthoates (Scheme 3).22
Scheme 3. Evaluation of the Substrate Scope of 2-Hydroxy-
a
1-naphthoates
Figure 1. X-ray structures of product 3j (a) and 5j (b).
When the radical inhibitors such as 2,2,6,6-tetramethyl-1-
piperidinyloxy (TEMPO), 1,1-diphenylethene, methyl acrylate,
and even (1-cyclopropylvinyl)benzene were added to the
reaction mixture, the reaction could still be carried out to
provide the corresponding product 3d smoothly (Scheme 4).
These results indicated that the reaction pathway probably
does not involve a radical process.
Scheme 4. Control Experiments for Mechanistic
Investigations
a
Isolated yields of reactions performed on the 0.10 mmol scale; ee
b
values were determined by HPLC. ee value of product after
recrystallization.
When methyl 2-hydroxy-1-naphthoate (4a) was utilized, the
desired product 5a was obtained in 98% yield and 88% ee. The
derivatives with aryl groups on the 6- or 7-position (4b−4d)
also performed well and provided the corresponding products
5b−5d in excellent yields and good enantioselectivities (97−
99% yields, 84−85% ee). Substrate 4e with 6-Br and substrate
4f with 7-MeO could all be successfully converted to their
corresponding products in excellent yields and good
enantioselectivities (5e−5f). It is noteworthy that substrate
4g with a phenylethynyl group as well as substrate 4h with
both a triple bond and a hydroxyl group were also well-
tolerated, and the corresponding products 5g and 5h were
obtained in 95% yield and 90% ee, and 83% yield and 89% ee,
respectively. Additionally, the substrates bearing substituents
On the basis of these data as shown above and in previous
related reports,23 we proposed a model for the key
intermediate to explain the stereochemistry of the reaction
(Figure 2). In this model, the catalyst functions as a chiral
Lewis acid, and the copper center coordinates with the
substrate by two-point binding, thus providing a rigid chiral
environment around the reaction site. The Re face of the
substrate is blocked by the phenyl group in the oxazolinyl unit,
C
DOI: 10.1021/acs.orglett.9b02604
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Accession Codes
CCDC 1813765 and 1845678 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.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: qinghaideng@shnu.edu.cn.
ORCID
Figure 2. Proposed spatial model for the key intermediate.
Qing-Hai Deng: 0000-0001-7793-1434
Author Contributions
†C.-J.W. and J.S. contributed equally.
Notes
and the azido species therefore preferentially approaches from
the Si face of the substrate, which is consistent with the
absolute configuration of products 3j and 5j.
To demonstrate the synthetic utility of this method, we
undertook further transformations of the resulting azides
(Scheme 5). The Cu-catalyzed azide−alkyne cycloaddition was
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Scheme 5. Further Transformations of the Azide Products
■
Financial support from the Program of the Professor of Special
Appointment (Eastern Scholar) at Shanghai Institutions of
Higher Learning, Shanghai Engineering Research Center of
Green Energy Chemical Engineering (18DZ2254200), the
National Natural Science Foundation of China (21402119,
21772122), and Shanghai Rising-Star Program
(16QA1403100) is greatly appreciated.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02604.
■
S
Experimental procedures, characterization data, copies of
NMR spectra for all new products, and HPLC data for
chiral compounds (PDF)
D
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Supporting Information.
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E
DOI: 10.1021/acs.orglett.9b02604
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