Quinine-catalyzed asymmetric synthesis of 2,2′-binaphthol-type biaryls under mild reaction conditions

Article abstract of DOI:10.1002/anie.201601660

Simple quinine as an organocatalyst mediates the addition of various naphthols to halogenated quinones to afford non-C₂-symmetrical, axially chiral biaryl products, which are promising compounds as chiral ligands and organocatalysts. The rotational barrier required to have two distinct atropisomers has been evaluated in the products generated from the addition of naphthols to various quinones by means of DFT calculations and HPLC. The use of halogenated quinones as reagents was necessary to have configurationally stable enantiomeric products which can be obtained in good yield and stereoselectivity. These compounds have also been prepared in gram quantities and recrystallized to near enantiopurity. Simple is better: Commercially available reagents such as quinones and naphthols are combined under mild reaction conditions in the presence of quinine to afford halogenated 2,2′-binaphthol-like biaryls in gram quantities. The products are isolated in good yield and stereoselectivity.

Full text of DOI:10.1002/anie.201601660

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International Edition: DOI: 10.1002/anie.201601660
German Edition: DOI: 10.1002/ange.201601660
Organocatalysis
Quinine-Catalyzed Asymmetric Synthesis of 2,2’-Binaphthol-Type
Biaryls under Mild Reaction Conditions
Mauro Moliterno, Riccardo Cari, Antonio Puglisi, Achille Antenucci, CØline Sperandio,
Erica Moretti, Antonio Di Sabato, Riccardo Salvio,* and Marco Bella*
Abstract: Simple quinine as an organocatalyst mediates the
addition of various naphthols to halogenated quinones to
afford non-C2-symmetrical, axially chiral biaryl products,
which are promising compounds as chiral ligands and organo-
catalysts. The rotational barrier required to have two distinct
atropisomers has been evaluated in the products generated
from the addition of naphthols to various quinones by means
of DFT calculations and HPLC. The use of halogenated
quinones as reagents was necessary to have configurationally
stable enantiomeric products which can be obtained in good
yield and stereoselectivity. These compounds have also been
prepared in gram quantities and recrystallized to near enan-
tiopurity.
A C2-symmetry axis was once considered a key feature for
obtaining high enantioselectivity in asymmetric catalysis,[3]
but over the years, several authors have also demonstrated the
efficiency of non-C2-symmetrical ligands and catalysts, such as
NOBIN.[4] Despite their wide popularity in asymmetric
catalysis, the large scale synthesis of these molecules suffers
À
from two critical issues. First, the formation of C C bonds in
an industrial process is a significant challenge, especially
2
2
À
C(sp ) C(sp ) couplings which are mediated by transition
metals and oxidants. The use of metals represents a technical
and economic issue for industry.[5a,b] Second, most scalable
syntheses only afford racemic products, which need to be
resolved by processes having a maximum theoretical yield of
50%.[2,5c] Therefore, an efficient organocatalytic (transition-
metal-free) approach to prepare these compounds could
represent a major breakthrough. Recently, the groups of
Miller[6a,b] and Matsubara[6c] demonstrated the possibility of
obtaining axially chiral biaryls through dynamic kinetic
resolution by using an organocatalytic bromination reaction.
Despite this process being of scientific value within academia,
the elaboration of the products into effective ligands and
catalysts does not appear straightforward and it has not been
demonstrated yet. Moreover, the synthesis of the achiral
precursors still requires a transition metal catalyzed reaction.
Therefore, a practical synthesis of a structure which closely
resembles BINOL, a structure already proven to be highly
effective as a chiral ligand and catalyst, might be preferable
and open new horizons in the field of asymmetric catalysis.
Prakash and co-workers reported that Cinchona alkaloid-
derived chiral bases could activate indoles, which would add
in an asymmetric way to ethyl trifluoropiruvate.[7a] Jørgensen,
Bella, and co-workers described a similar activation mode of
the corresponding 2-naphthols, which after addition to
azodicarboxylates, produced C–N axially chiral compounds
(Figure 2).[7b,c] We thought that this information could be
B
iaryl moieties bearing a chiral axis are present in a number
of natural products[1a] and are widely exploited in asymmetric
catalysis.[1b–f] A key molecule belonging to this class is 2,2’-
binaphthol (BINOL), which is produced on large scale by
oxidative coupling and resolved by Cinchona alkaloids.[2a,b]
Over the years, several chiral ligands and catalysts derived
from the privileged BINOL framework have been success-
fully employed in asymmetric synthesis (Figure 1).[2a–f]
Figure 1. Some widely exploited axially chiral ligands and catalysts.[2]
2
2
À
exploited to develop a C(sp ) C(sp ) coupling between
[*] M. Moliterno, R. Cari, A. Puglisi, A. Antenucci, C. Sperandio,
E. Moretti, A. Di Sabato, Dr. R. Salvio, Prof. M. Bella
Dipartimento di Chimica, Sapienza University of Roma
P. le Aldo Moro 5, 00185 Roma (IItaly)
E-mail: marco.bella@uniroma1.it
Dr. R. Salvio
IMC-CNR Sezione Meccanismi di Reazione
Università di Roma—Sapienza
P. le Aldo Moro 5, 00185 Roma (Italy)
E-mail: riccardo.salvio@uniroma1.it
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201601660.
Figure 2. Previous and current work.
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Scheme 1. Base-catalyzed addition of naphthols (2) to quinones (1) to afford biaryls (3) bearing a stereogenic axis (top). Energy of rotational
barrier calculated (b3lyp/6-311+g(d,p)// b3lyp/6-311+g(d,p)) using Gaussian09[9a] and determined by HPLC[9b] (bottom, see the Supporting
Information for details).
activated aromatic compounds and quinones, as electro-
philes,[8] to afford C–C axially chiral molecules which closely
resemble the skeleton of BINOLs. The configurational
stability of BINOLs stems from the hindered rotation along
compounds have a high rotational barrier and consequently
are configurationally stable. Also the presence of a chloro
substituent on the quinone moiety results in a significant
rotational barrier (3e), and we find a similar agreement
between the calculated and experimental data. Therefore, to
obtain configurationally stable enantiomers, it was necessary
to employ halogenated quinones such as 1b and 1c. The DFT
calculations turn out to be useful to predict whether the
enantiomers can have an interconversion rate comparable to
the HPLC timescale and therefore whether a plateau between
the peaks of the enantiomers can be observed.
À
the C C axis because of the presence of the hydroxy groups
(in the 2,2’-positions) and the peri protons (in the 8,8’-
positions). Since our compounds were missing this feature,
a preliminary aspect to be investigated was the substituent
pattern allowing a sufficient rotational barrier along the
newly formed chiral axis to obtain configurationally stable
atropisomers (Scheme 1).
While the rotational barriers of BINOL-like molecules
are known, we could not find data in the literature about the
specific compounds reported in our paper. The compound 3a
(Scheme 1) was separated into two enantiomers by HPLC
using a chiral stationary phase, but the shape of the chromato-
gram is anomalous because of the appearance of peaks with
a plateau between them (this is known as a “Batman”
profile).[10] Such peaks can be observed in the case of
atropisomers, with hindered rotation, which are interconvert-
ing on a time-scale comparable to the retention time of the
HPLC run. Indeed, analytical chiral separations can be used
to determine the rotational barriers of racemization.[9b] In
contrast, we could not get any separation for the enantiomers
of 3a’, thus pointing to a fast interconversion. Accordingly,
ab initio calculations indicate a low interconversion barrier
for 3a’, but the calculated energy barrier for the enantiomer
interconversion of 3a (22.9 kcalmolÀ1) is compatible with
a chromatogram of species interconverting on an HPLC time
scale and it is in reasonable agreement with that determined
by integration of the areas of HPLC peaks (22.0 kcalmolÀ1).[9]
We then prepared 3b and 3c, bearing a methoxy and a bromo
substituent, respectively, at the 7-position and the rotational
barrier still remains similar to that of 3a. Finally, we tested the
same reaction starting from 2,5-dibromo 1,4-benzoquinone
(1b), thus obtaining a mixture of 3d and its oxidized form 3d’.
Both in silico and experimental data indicate that the two
We then chose as a model the reaction of 7-methoxy 2-
naphthol (2b) and 2,6-dichloro 1,4-benzoquinone (1c), and
we investigated which reaction conditions could afford higher
yield and stereoselectivity. Previous attempts to run the
reaction under argon gave rise anyway to the formation of
a mixture of 3e and its oxidized quinonic form in variable
ratios. This oxidation process might be due to the presence of
1c as it can act as an oxidant. We then added sodium
borohydride to the reaction mixture to reduce any oxidized
quinone product, and then TFA to reprotonate the naphthol
sodium salts. We loaded the crude reaction mixture on a short
pad of silica gel and quick filtration afforded a substantially
pure sample of 3e. The combination of THF as the solvent
and quinine (I) as the catalyst (Table 1, entry 1) afforded 3e in
nearly quantitative yield and with 83:17 e.r. Chloroform or
toluene, which are common solvents employed for Cinchona
alkaloid-derived catalysis, were not as effective (entries 2 and
3). The temperature of 48C, which can be conveniently
achieved, appeared ideal so as to maximize the enantiomeric
ratio; the reaction run at À208C did not increase enantiose-
lectivity (entry 4) and the one conducted at room temperature
also produced products in less satisfactory enantioselectivity
(entry 5). A minor amount of catalyst and higher dilution
were instead beneficial and when we combined these findings
we achieved the highest enantioselectivity (entry 6). It is
important to stress that simple I was the most effective
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Table 1: Screening of reaction conditions for the organocatalyzed
to avoid chromatography to purify the products are aspects
addition of 2b to 1c to afford the axially chiral 3e (see the Supporting
Information for additional screening of reaction conditions and cata-
lysts).
which are more important than finding a catalyst which would
afford a higher enantiomeric ratio.
With our optimized reaction conditions, we explored the
scope of our reaction by testing several combinations of
halogenated 1,4-benzoquinones (1) and naphthols (2;
Scheme 2). To maximize the enantiomeric ratio, we ran the
reactions at high dilution. When compatible with reasonably
reaction times, we employed only a small excess of the
quinones 1 and ran the reactions at 48C. At first, we tested
carbomethoxy 1,4-benzoquinone (1d), which is the most
reactive quinone (reaction completed within a day and
quantitative yield), but it produced the corresponding biaryl
3 f only as a racemic mixture. The lack of stereocontrol might
Entry[a]
Cat
Solvent
t [h]
T [8C]
Yield [%][b]
e.r.[c]
1
2
3
4
5
I
I
I
I
I
I
II
III
IV
V
VI
I
THF
24
18
18
3
24
48
48
4
72
24
72
5
4
4
4
98
65
99
50
90
99
99
50
<10
99
83:17
63:37
63:37
83:17
79:21
86:14
21:79
83:17
50:50
85:15
56:44
87:13
CHCl3
toluene
THF
THF
THF
THF
THF
THF
THF
À20
RT
4
6[d,e]
7
8
9
10
11
12[d,e,f]
4
4
4
4
4
4
toluene
THF
99
99
[a] Reaction performed employing 50 mg (0.28 mmol,1.2 equiv) of 1c
and 40 mg (0.23 mmol, 1 equiv) of 2b with 4 mL of solvent (concen-
tration: 0.057m). [b] Yield of isolated product. [c] The e.r. value was
determined by HPLC on CSP using Chiralpack ID column and n-hexane/
isopropyl alcohol (88:12; flow 0.9 mLmin). [d] Concentration of
0.0275 M. [e] Used 15 mol% catalyst. [f] Used 2 equiv of 1c. TFA=tri-
fluoroacetic acid, THF=tetrahydrofuran.
catalyst, at least in our hands. None of the other natural and
synthetic Cinchona alkaloid derivatives we tested performed
better. In particular, the pseudoenantiomer quinidine (II)
gave rise to the formation of the enantiomer of 3e with similar
stereocontrol (entry 7). The hydroquinine III did not perform
better (entry 8) and hydrocupreine IV afforded essentially
racemic material (entry 9). The substitution of the 6-methoxy
group with other bulkier groups (V; entry 10 and VI;
entry 11) did not afford better results. The reaction employing
2 equivalents of 1c was faster (entry 12 versus entry 6).
Although the enantioselectivity was not the highest possible,
our experience regarding one of our asymmetric reactions,[11a]
which became a large-scale industrial process[11b] suggested
that for the wide applicability of a novel process this was not
a major issue. Specifically, mild reaction conditions (RT or
48C), high yield (above 95%), cheap and commercially
available catalysts (quinine) and reagents, plus the potential
Scheme 2. Scope of the quinine-catalyzed addition reaction of naph-
thols (2) to quinones (1) to produce axially chiral compounds (3) in
enantioenriched form. [a] Reaction performed employing 2.0 equiv of
quinones (1), 1.0 equiv of naphthol 2b (0.15 mmol), and with 5.5 mL
of solvent (concentration: 0.0275m). [b] Values within parentheses are
reaction time in days. [c] Used 1.2 equiv of 1. See the Supporting
Information for the determination of the absolute configuration.
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be due to the fast, nonselective reaction. The use of
dihalogenated benzoquinones [1b,c and 1e (2,5-dichloro
1,4-benzoquinone)] afforded the biaryls 3d and 3g–l in
good yield and moderate to good enantioselectivity (82–
99% yield, 80:20 to 89:11 e.r.). Chloro 1,4-benzoquinone (1 f)
presented a similar reactivity, even if the reactions required
a longer reaction time. However, the products 3m–r were
formed in better stereoselectivity (60–99% yield, 88.5:11.5 to
92:8 e.r.).
Finally, we tested bromo 1,4-benzoquinone (1g) and 2,6-
dibromo 1,4-benzoquinone (1h), which afforded the products
3s–u after a longer reaction time, but in comparable
stereoselectivity with that of the corresponding chloroqui-
nones 1c and 1 f (81–93% yield, 85:15 to 93:7 e.r.). For a full
report of all reaction conditions tested see Schemes S5 and S6
in the Supporting Information.
A possible rationalization regarding the formation of the
major enantiomer is depicted in Figure 3. According to this
scenario, the deprotonated hydroxy function on the naphthol
unit would interact with the protonated quinuclidine unit of
quinine and the quinone reagent would be activated by
hydrogen bonding at the 9-hydroxy functionality of the
Scheme 3. Preparative large-scale reactions for 3i and 3k.
several transformations. Studies are ongoing in our group as
well as others[12,13] to apply these compounds as useful ligands
and catalysts for highly enantioselective asymmetric reac-
tions.
Experimental Section
Typical experimental procedure: To a cooled (48C) suspension of the
naphthol 2 (0.15 mmol) and quinine I (7 mg, 0.0225 mmol, 15 mol %)
in THF (4.5 mL) was added
a cooled (48C) solution of the
halogenated 1,4 benzoquinone 1 (0.30 mmol, 2 equiv) dissolved in
1 mL of THF. The reaction mixture was stirred at 48C for 48 h, and
then the resulting dark solution was cooled to 08C (ice bath) and
NaBH4 (17 mg, 0.45 mmol) was added, followed by the dropwise
addition of 1 mL of MeOH. After 5 min to the solution was added
TFA (trifluoroacetic acid, 0.1 mL) and 1 g of silica gel, and the solvent
was removed by rotary evaporation. The resulting white solid was
loaded on the top of a column, and quickly filtered over a pad of silica
gel (20 gr) using diethyl ether/dichloromethane (1:20) as an eluent.
The solvent of the fraction containing the desired compound 3 was
evaporated, to give chemically pure material.
Figure 3. Proposed transition state leading to the major enantiomer
and corresponding DFT optimized structure (b3lyp/6-31g*//b3lyp/
6-31g*; some atoms are omitted for clarity).
catalyst. The p–p staking between the aromatic moiety of
the reagents and the quinoline part of the catalyst does not
seem to play any major role. DFT calculations and the
inversion of stereoinduction when employing catalysts lack-
ing the free 9-hydroxy group support this hypothesis.
To show the preparative usefulness of our transformation,
we performed two reactions on gram scale by employing
a higher concentration of reagents. The products 3i and 3k
were obtained in high yield and with minimal erosion of
enantiopurity, and have been recrystallized with high enan-
tiomeric excess (Scheme 3).
Acknowledgments
We wish to thank Sapienza Università di Roma for financial
support through “Progetto di Ateneo” 2014 and 2015. We are
indebted to Prof. Luca Bernardi and Prof. Luigi Mandolini for
several useful discussions, to Prof. Ruggero Caminiti for
providing computing time on the NARTEN Cluster HPC
Facility, to Dr. Antonello Alvino for mass spectra and to Dr.
Roberto Cirilli for the HPLC of compound 3m.
In conclusion, we have presented a new organocatalyzed
reaction for the synthesis of a class of novel biaryl compounds
and we have discussed the features necessary to prepare
configurationally stable atropisomers. We believe that the
mild reaction conditions of our reaction, coupled with the use
of a cheap and commercially available catalyst should render
it attractive for the large-scale preparation of these important
compounds, at the very least in a research laboratory.
Furthermore, the presence of various halogen atoms offers
the possibility to functionalize these compounds by using
Keywords: atropisomers · biaryls · chirality ·
density-functional calculations · organocatalysis
How to cite: Angew. Chem. Int. Ed. 2016, 55, 6525–6529
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Received: February 16, 2016
Published online: April 20, 2016
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