Enantioselective Synthesis of 3,3′-Disubstituted 2-Amino-2′-hydroxy-1,1′-binaphthyls by Copper-Catalyzed Aerobic Oxidative Cross-Coupling

Article abstract of DOI:10.1002/anie.202015001

A challenging direct asymmetric catalytic aerobic oxidative cross-coupling of 2-naphthylamine and 2-naphthol, using a novel Cu^I/SPDO system, has been successfully developed for the first time. Enantioenriched 3,3′-disubstituted NOBINs were achieved and could be readily derived to divergent chiral ligands and catalysts. This reaction features high enantioselectivities (up to 96 % ee) and good yields (up to 80 %). The DFT calculations suggest that the F–H interactions between CF₃ of L17 and H-1,8 of 2-naphthol, and the π–π stacking between the two coupling partners could play vital roles in the enantiocontrol of this cross-coupling reaction.

Full text of DOI:10.1002/anie.202015001

Angewandte
Communications
Chemie
How to cite: Angew. Chem. Int. Ed. 2021, 60, 7061–7065
International Edition: doi.org/10.1002/anie.202015001
German Edition: doi.org/10.1002/ange.202015001
Asymmetric Catalysis
Enantioselective Synthesis of 3,3’-Disubstituted 2-Amino-2’-hydroxy-
1,1’-binaphthyls by Copper-Catalyzed Aerobic Oxidative Cross-
Coupling
Xiao-Jing Zhao, Zi-Hao Li, Tong-Mei Ding, Jin-Miao Tian,* Yong-Qiang Tu,* Ai-Fang Wang,
and Yu-Yang Xie
Abstract: A challenging direct asymmetric catalytic aerobic
oxidative cross-coupling of 2-naphthylamine and 2-naphthol,
using a novel CuI/SPDO system, has been successfully
developed for the first time. Enantioenriched 3,3’-disubstituted
NOBINs were achieved and could be readily derived to
divergent chiral ligands and catalysts. This reaction features
high enantioselectivities (up to 96% ee) and good yields (up to
80%). The DFT calculations suggest that the F–H interactions
between CF3 of L17 and H-1,8 of 2-naphthol, and the p–p
stacking between the two coupling partners could play vital
roles in the enantiocontrol of this cross-coupling reaction.
A
tropisomerically enriched binaphthyl frameworks are
prevalent in numerous privileged chiral ligands and catalysts,
functional materials and bioactive natural products.[1] In the
past decades, the C2-symmetric BINOL-derived ligands and
catalysts have been widely used in asymmetric catalysis.[2] In
contrast, the C1-symmetric binaphthyl-derived species
receive fewer attention mainly due to the lack of efficient
preparation methods.[3] To our knowledge, more and more
C1-symmetric ligands and catalysts, such as NOBINs (2-
amino-2’-hydroxy-1,1’-binaphthyls)[4,5] and C1-symmetric
BINOLs[6,7] (Scheme 1a), have demonstrated the excellent
enantio-control in asymmetric catalysis. Notably, the bur-
geoning NOBINs have demonstrated usefulness in varied
asymmetric conversions including aldol reaction, phase trans-
fer catalysis, hydroamination and so on.[5] These promising
facts imply that the NOBINs, particularly those with 3,3’-
disubstituents, will exhibit more potential and even irreplace-
able usefulness in asymmetric transformation due to their
Scheme 1. Asymmetric catalyzed strategies towards enantioenriched
NOBINs.
special steric and electronic properties and some additional
advantageous properties of the NH2 group.
Though the racemic NOBIN was firstly reported early in
1991, optically pure NOBINs are mainly obtained by using
the transformation from other chiral binaphthyls,[8a,b] the
chiral substrate induction[8c] and the indirect asymmetric
catalyzed methods requiring protecting groups on the sub-
strates. Maruoka and Zhao synthesized enantioenriched
NOBINs by kinetic resolution requiring the NH2 and/or OH
group to be protected (Scheme 1b).[9] Tan developed the
enantioselective approaches toward optically pure NOBINs
via chiral acid-catalyzed addition of naphthylamine analogues
containing protecting groups (Scheme 1b).[10] Recently, Yang
prepared the protecting-group-free NOBINs also by the
kinetic resolution of the rac-NOBINs.[9h] Unfortunately, these
approaches are mainly confined to NOBINs without 3,3’-
disubstituents. However, particularly for synthesis of the
enantioenriched 3,3’-disubstituted NOBINs, the direct asym-
metric catalytic aerobic oxidative cross-coupling of the
unprotected 2-naphthylamine and 2-naphthol, which should
be the most straightforward and atom-economical method,
has not been successfully developed by far.[6,11] Therefore,
development of an effective and direct catalytic system
accessing to the enantioenriched 3,3’-disubstituted NOBINs
[*] X.-J. Zhao, Z.-H. Li, Dr. T.-M. Ding, Dr. J.-M. Tian, Prof. Y.-Q. Tu,
A.-F. Wang, Y.-Y. Xie
School of Chemistry and Chemical Engineering
Frontiers Science Center for Transformative Molecules, Shanghai Key
Laboratory for Molecular Engineering of Chiral Drugs, Shanghai Jiao
Tong University
Shanghai 200240 (P. R. China)
E-mail: tianjm13@lzu.edu.cn
tuyq@sjtu.edu.cn
Prof. Y.-Q. Tu
State Key Laboratory of Applied Organic Chemistry and College of
Chemistry and Chemical Engineering, Lanzhou University
Lanzhou 730000 (P. R. China)
E-mail: tuyq@lzu.edu.cn
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202015001.
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Table 1: Design of SPDO ligands for the asymmetric cross-coupling
via asymmetric oxidative cross-coupling remains a significant
task to be accomplished. Inspired by pioneerꢀs work together
with our successful SPD (spirocyclic pyrrolidine) catalysts in
asymmetric catalysis,[6f,12,13] herein, we present our efforts
toward this challenging cross-coupling of 2-naphthylamine
and 2-naphthol to synthesize the enantioenriched 3,3’-disub-
stituted NOBINs utilizing our novel CuI/SPDO system
(Scheme 1c).
reaction.[a]
Based on the reported literatures,[4d,6b,f] a large enough gap
of the electronic property between two coupling partners is
necessary to achieve this cross-coupling. Considering that the
NH2 group is more electron-rich than the OH group, 2-
naphthylamine with C3-EDG is proposed as the more
electron-rich partner to preferably form the radical species,
and the 2-naphthol with electron-deficient C3-EWG is
expected to be activated to couple with the radical.[6b]
Obviously, the NH2 group of 2-naphthylamine would lead to
some undesired side-reactions and catalyst deactivation.[4d,14]
Therefore, searching for a flexible ligand that more actively
coordinate with CuI ions than the NH2 group and reasonably
designing substrates are essential to efficiently achieve this
cross-coupling.
Inspired by our previous successful cross-coupling of 2-
naphthols, the 2-naphthylamine 1a and 2-naphthol 2a were
thus recommended as model substrates. From the results in
Table 1, we found that an initial test of this cross-coupling
with CuBr/L1, fortunately, could afford the desired NOBIN
3aa after careful examinations of experiment conditions (see
SI), but only with moderate outcome (65% yield, 54% ee)
and a 20% of homo-BINOL formed. Subsequent ligand
modification by varying the substituentꢀs property and
location was performed (Table 1).[15] Firstly, changing the
configuration of SPD-backbone or introducing gem-dimeth-
yls at C5 of oxazoline moiety reversed the absolute config-
uration of 3aa (L1 vs. L2 and L3). Delightedly, further
modification at the C4 of oxazoline part could largely
influence on the reaction outcome (L4–L17). Although the
aliphatic substituents would only lead to low ee data (0.5–
31% ee, L4–L6), the aromatic ones bearing mono electron-
withdrawing Cl-atom at ortho-, meta- or para-position of the
phenyl could slightly influence on the enantioselectivity, with
the m-Cl-Ph (L9) yielding the best 63% ee (L7–L9). Thus, we
speculated that the p···p interaction may be contribute to the
good result. The following examinations were focused on the
meta-positionꢀs substituents of phenyl. Notably, both EWG-
and EDG-substituents at C3 or C-3,5 could afford the better
enantioselectivities (60–81% ee, L10–L17), with L17 bearing
strong EWGs (3,5-diCF3) gave the best ee (81%). To reduce
the homo-coupling of 2a as much as possible, then 2a was
added in three batches in 20 h and the good result (86% ee,
75% yield) could be obtained, moreover the homo-BINOL
amount reduced to only < 9%.[6d,f]
Ligand
ee (Yield) [%][b]
Ligand
ee (Yield) [%][b]
L1
L2
L3
L4
L5
L6
L7
L8
L9
54 (65)
À27 (63)
À10 (56)
31 (55)
10 (58)
0.5 (52)
50 (55)
59 (62)
63 (64)
L10
L11
L12
L13
L14
L15
L16
L17
L17[c]
60 (61)
64 (55)
67 (65)
66 (65)
67 (64)
71 (67)
65 (61)
81 (65)
86 (75)
[a] The reactions were conducted using 1a (2.0 equiv), 2a (0.1 mmol,
1.0 equiv), CuBr (0.05 equiv), L1–L17 (0.06 equiv) in THF (1.0 mL) at r.t.
under air. [b] Yield of isolated product. [c] CuBr (0.1 equiv), L17
(0.12 equiv) were used.
and location of the substituent at phenyl could affect the
reaction efficiency. For the weak EWG or EDG substituent at
meta-position, the reactions gave the good enantioselectiv-
ities (84–85% ee) and yields (70–76%) (3ab–3ad). While the
strong EWG (CF3) substituent lead to quite different out-
comes (3ae–3ag), the meta-substituent yielded the pretty
good result (3ae, 91% ee, 73% yield), and reversely the
ortho-substituent gave the poor result (3af, 65% ee, 58%
yield) because of the steric hindrance. In the second group of
examples (3ah–3ap), investigation was focused on the 3,5-
disubstituents. These moieties gave the corresponding prod-
ucts in good results (90% ee, 70% yield). Interestingly, the
hetero disubstituents with EWG (CF3) and EDG (OMe)
(3ah) gave high ee value (91%) as well as good yield (74%),
while the homo di-EDG (3al) gave the better yield (75%)
and di-EWG (3an–3ap) switched to the better ee values
(91%). The absolute configuration of cross-coupling products
was assigned as (S) through the X-ray analysis of 3aj. For the
aliphatic methyl ester (3aq), though a medium 70% ee was
observed initially in THF, delightedly it could be promoted to
96% after the solvent was replaced with MeCN. However,
when other functional groups 3-Br and 3-PhCO substituents
were introduced at C3 position, the poor results were
obtained. The substrate containing -COPh at 3-substituted
position led to moderate outcome (3ar, 49% ee, 62% yield),
while the substrate 3-Bromo-2-naphthol afforded poor result
With the optimal conditions in hand, the substrates 2 was
firstly explored by varying the substituents at C3, C6 and C7.
The results are listed in Scheme 2 and showed that most
examples worked well to generate the NOBINs 3aa–3ax with
good to high enantioselectivities (up to 96% ee), moderate to
good yields (up to 80%).[16] In the first group of examples
(3ab–3ag) with C3-phenyl esters, both electronic property
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Scheme 3. Substrate scope: varying the substituent at C3, C6 and C7
of 2-naphthylamine.[a] [a] 1b–1r (2.0 equiv), 2e (0.1 mmol, 1.0 equiv),
CuBr (0.1 equiv), L17 (0.12 equiv) in THF (1.0 mL) at r.t. under air.
Scheme 2. Substrate scope: varying the substituent at C3, C6 and C7
of 2-naphthol.[a] [a] The reactions were conducted using 1a (2.0 equiv),
2a–2x (0.1 mmol, 1.0 equiv), CuBr (0.1 equiv), L17 (0.12 equiv) in
THF (1.0 mL) at r.t. under air. [b] THF was used instead of MeCN.
Deposition Number(s) 2027600 (for 3aj) contain(s) the supplemen-
tary crystallographic data for this paper. These data are provided free
of charge by the joint Cambridge Crystallographic Data Centre and
Fachinformationszentrum Karlsruhe Access Structures service www.
ccdc.cam.ac.uk/structures.
C6 and C7, both the strong EDG (OMe) and weak EWG (Br)
substituents gave high enantioselectivities, but the former
gave the moderate yield (3ep, 50%). However, the aliphatic
substituents (Me, Et) gave the good levels of enantioselectiv-
ities (79–87% ee), no matter they were located at C6 or C7
(3en, 3eo, 3er).
To shed light on this cross-coupling reaction mechanism,
some control experiments and DFT calculations were con-
ducted, and the reaction process was proposed as shown in
Figure 1 (also Figure 8-1 in SI). Firstly, a complex A of CuI/
L17 was oxidized by air to the real CuII/L17 catalyst B.[6a,c,f,17]
Then complex B underwent two different reaction paths with
1a and 2a, respectively: 1) Oxidation of 1a gave a radical
intermediate C, which could be caught by the DMPO and was
in line with the species detected by HRMS-ESI (Figure 7-3 in
SI); 2) Coordination and activation of 2a generated the
intermediate E, which was confirmed by 19F NMR spectra
analysis (Figure 7-5 in SI). It was the two well-differentiated
paths, that resulted in the good chemoselectivity. Also the
DFT calculations (Figure 1) suggested that the p···p inter-
action[18] between the phenyl of L17 and the naphthyl of 2a
was found in the intermediate E, which was in line with the
result of the ligand screening. Thirdly, based on the literature
reports about the aerobic oxidative cross-coupling, which
together with other supporting facts proposed that this cross-
coupling involved a radical-anion process.[4d,6d,17] Approach of
the radical C from both Si and Re sides of E were considered
and the corresponding transition states were designated as
TS2a (favored) and TS2b (unfavored), respectively. In
addition, some weak interactions were illustrated in TS2a,
such as the p···p interaction[18] between the two partners and
the F···H interaction[19] between the partner 2 and L17, which
(3as, 19% ee, 17% yield). The results suggested that the
structure of the ester group could also significantly affect
enantioselectivity and chemoselectivity of the cross-coupling.
At last, the fourth group of substrates 2 with substituent at C6
and C7 were evaluated, and a good level of reaction outcomes
(83–92% ee and 64–74% yield) were obtained. But a little
difference could be found: the Ph-substituent (3au, 3ax)
always gave better ee values than the aliphatic substituents
(3at, 3av, 3aw), no matter it was located at C6 or C7.
Next, we turned to investigate the scope of 1 by varying its
substituents at C3, C6 and C7 with 2e. As summarized in
Scheme 3, encouragingly, all examples also worked well to
give high enantioselectivities (up to 92% ee) and good yields.
Detailed investigations showed that: firstly, for the OBn-ether
at C3 (3eb–3ej), no matter meta-, para-monosubstituent or
3,5-di-substituents at the phenyl could gave the high enantio-
selectivities (88–91% ee), which were a little higher than the
ortho-substituent (84–85% ee). For the second kind of
substrates containing alkyl, methoxyl and EWG at C3
position, the alkyl and methoxyl substituents gave the
moderate outcomes (64–73% ee), while the substrate with
-COOPh substituent gave the moderate enantioselectivity
(3em, 79% ee) and low yield (23%). The results suggested
that 3-alkoxy is essential for the good chemoselectivity of the
cross coupling. For the third group of remote substituents at
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Figure 1. Energy profiles for the cross-coupling of 1a and 2a after formation of B and C. Bond distances are given in [ꢀ].
played vital roles in the enantio-controlling of the cross-
coupling. However, when radical C approached from the (Si)
sides of the intermediate E, there was steric hindrance
between L17 and E. Therefore, the DDG of TS2b is higher
than TS2a and attack from the Re side of E was favored.
Subsequently, the (S)-3aa was produced dominantly after
aromatization of G and releasing A. More detailed informa-
tion about the experimental investigations and the DFT
calculations are described in SI (S82–84).
advantages would be attributed to the character of the SPD-
backbone architecture together with a special 3,5-diCF3-
substituted oxazoline moiety of L17.
[Correction added on March 15, 2021, after first online
publication: In Figure 1 the labelling of the Si- and Re-face
was corrected for intermediates Fand F’. The same correction
was made in the text in relation to intermediate E.]
To further showcase the practical utility of the NOBINs
above, the gram-scale preparation of 3ae was carried out, and
a high 91% ee could be maintained under the optimum
conditions (Scheme 4a). In addition, considering the excel-
lent performance of these NOBIN-derived ligands in asym-
metric catalysis,[5] the useful 4 and 5 were efficiently
synthesized via simple conversions. (Scheme 4b).
Acknowledgements
We greatly thank the NSFC (Nos. 21871117, 21702136
21502080, 21772071), the “111” Program of MOE, the
Major project (2018ZX09711001-005-002) of MOST, and
the STCSM (19JC1430100) for their financial support.
In conclusion, we have successfully developed the asym-
metric catalytic aerobic oxidative cross-coupling of 2-naph-
thylamine and 2-naphthol utilizing our novel Cu/SPDO
catalytic system, achieving enantioenriched 3,3’-disubstituted
NOBINs with high enantioselectivity and good yield. These
Conflict of interest
The authors declare no conflict of interest.
Keywords: asymmetric catalysis · cross-coupling ·
3,3’- disubstituted NOBINs · enantioselectivity ·
mechanism investigation
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Manuscript received: November 11, 2020
Revised manuscript received: December 7, 2020
Accepted manuscript online: December 28, 2020
Version of record online: February 22, 2021
Angew. Chem. Int. Ed. 2021, 60, 7061 –7065
ꢀ 2020 Wiley-VCH GmbH
www.angewandte.org 7065