BINOL-fused maleimides - A new class of C2-symmetric chiral imides

Article abstract of DOI:10.1002/ejoc.201201525

The first synthesis of BINOL-fused maleimides has been developed. This new chiral scaffold is easily accessible from various BINOL analogues and 2,3-dihalomaleimide derivatives under basic conditions. It can be easily functionalized on the BINOL moiety an

Full text of DOI:10.1002/ejoc.201201525

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201201525
BINOL-Fused Maleimides – A New Class of C2-Symmetric Chiral Imides
Simon Brenet,[a] Benoit Baptiste,[a] Christian Philouze,[a] Florian Berthiol,*[a] and
Jacques Einhorn*[a]
Keywords: Ligand design / Organocatalysis / Chirality / Nucleophilic substitution / Cyclization / Oxygen heterocycles
The first synthesis of BINOL-fused maleimides has been de- tives under basic conditions. It can be easily functionalized
veloped. This new chiral scaffold is easily accessible from
various BINOL analogues and 2,3-dihalomaleimide deriva-
on the BINOL moiety and is an entry towards promising new
classes of chiral organocatalysts and ligands.
Introduction
Over the last two decades, N-hydroxyphthalimide
(NHPI) and its analogues have emerged as powerful
organocatalysts that allow selective and mild aerobic oxi-
dation of C–H bonds adjacent to activating moieties.[1,2]
However, enantioselective oxidation catalyzed by chiral
NHPI analogues remains highly challenging and to the best
of our knowledge only three examples have been reported.[3]
We reported the sole example of the oxidative kinetic reso-
lution of oxazolidines with high enantioselectivities by
using variable C2-symmetric atropisomeric NHPI ana-
logues[3b] after a few less successful early attempts,[3c]
whereas Tan et al. reported low enantioselectivities in the
kinetic resolution of typical alcohols by using an anthrone-
derived chiral NHPI analogue.[3a] Though efficient, our pre-
vious series of catalysts had two major drawbacks that nar-
rowed their scope: the synthesis was quite long and delicate
and included a resolution step,[4] and they have a high pro-
pensity for thermal isomerization as a result of their low
barriers of atropisomerization.[4,5] We have therefore been
in search for other types of variable chiral NHPI analogues
that are more readily available and less prone to thermal
isomerization. Herein, we report our first findings related
to the surprisingly easy and efficient synthesis of a new C2-
symmetric chiral scaffold of general structure 1 (Figure 1),
which is composed of a BINOL or BINOL derivative moi-
ety fused with a maleimide one. These findings open the
way to new C2-symmetric chiral NHPI analogues but also,
more generally, to an unprecedented type of chiral male-
imides that could find various applications, for example, as
enantioselective catalysts for asymmetric protonation[6] or
as chiral imidate ligands for transition metals.[7]
Figure 1. General structure of 1.
Results and Discussion
Our aim was to take advantage of the high reactivity of
2,3-dihalomaleimides and to react them with chiral nucleo-
philes to obtain new chiral maleimides. In particular, we
were intrigued about the possibility of reacting BINOL or
BINOL derivatives 3 with 2,3-dihalomaleimides 2 to obtain
cyclic compounds of general structure 1. Displacement of
one or two halogen atoms of the 2,3-dihalomaleimide deriv-
atives by various nucleophiles is well known. For example,
the reaction of nitrogen or sulfur nucleophiles with di-
bromomaleimides for the synthesis of biologically active
compounds[8] or for bioconjugation of cysteine-containing
proteins[9] is well documented. The reaction of alcohols or
phenols with N-substituted 2,3-chloromaleimides to give
mono- or disubstituted products according to the reaction
conditions has also been reported.[10] To the best of our
knowledge, however, no reactions with diphenols to obtain
oxygen heterocycles have been reported. Thus, we decided
to react catechol, as a model diphenol, with a series of 2,3-
dihalomaleimides under basic conditions (Scheme 1). The
protecting groups (PGs) on the nitrogen atom of 2,3-dihalo-
maleimides 2 were chosen in such a way so as to allow the
final obtention of either the free N-hydroxyimide or the N-
unsubstituted imide functional groups, that is, PG =
OBn,[3b] H,[11] tert-butyldimethylsilyl (TBS),[12] or p-nitro-
phenyl.[8a] Unfortunately, in most of the cases, only tars
were obtained with no trace of the expected six-membered
heterocyclic compounds. A well-defined compound was
nevertheless obtained in larger amounts when maleimide 2a
[a] Département de Chimie Moléculaire (SERCO), UMR-5250,
ICMG FR-2607, Université Joseph Fourier,
301 Rue de la Chimie, BP 53, 38041 Grenoble Cedex 9, France
Fax: +33-4-76635983
E-mail: Florian.Berthiol@ujf-grenoble.fr
Jacques.Einhorn@ujf-grenoble.fr
Homepage: dcm.ujf-grenoble.fr/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201525.
Eur. J. Org. Chem. 2013, 1041–1045
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. Brenet, B. Baptiste, C. Philouze, F. Berthiol, J. Einhorn
SHORT COMMUNICATION
was treated with catechol in the presence of K2CO3 in THF of (Ϯ)-1a, which was isolated in 48% yield (Table 1, en-
at 65 °C for 4 h. However, its spectroscopic data clearly try 1). An even better yield of 76% for (Ϯ)-1a was obtained
indicate that it corresponds to structure 4, resulting from starting from dibromomaleimide 2b, whereas diiodo ana-
nucleophilic attack of the two phenol functions of catechol logue 2c gave only 38% of (Ϯ)-1a (Table 1, entries 2 and 3).
at the same carbon atom of 2a, and one chlorine atom re- Expected N-unsubstituted heterocyclic compound (Ϯ)-1b
mains on the molecule. Such a reaction seems to be unprec- was also formed starting from 2,3-dibromomaleimide 2d,
edented starting from dihalomaleimide derivatives,[13] al- although in much lower yield (Table 1, entry 4). N-Protec-
though the formation of ketal- or dithioketal-type com- tion of the maleimide by a silyl group was not favorable, as
pounds has been reported from the reaction between 2,3-
Table 2. Substrate scope with various biphenols.[a]
dibromosuccinimide and ethylene glycol or 1,2-ethanedi-
thiol under basic conditions.[14] Ketal-type products are
also formed from 2,3-dibromosuccinic esters and catechol
under basic conditions.[15]
Scheme 1. Reaction of catechol with dihalomaleimides.
Although these results were not encouraging for our pro-
ject, we nevertheless attempted similar reactions with
BINOL (3a) in place of catechol. Contrastingly, we ob-
served the formation of the expected compounds in fair
yields: The reaction of (Ϯ)-3a with O-benzyl-2,3-dichloro-
N-hydroxymaleimide (2a) in anhydrous DMF in the pres-
ence of Na2CO3 at 60 °C for 1 h resulted in the formation
Table 1. Optimization of the condensation conditions.[a]
Entry
2
X
PG
Yield [%][b]
1
2
3
4
5
6
2a
2b
2c
2d
2e
2f
Cl
Br
I
Br
Br
Br
OBn
OBn
OBn
H
TBS
4-NO2C6H4
1a
1a
1a
1b
–
48
76
38
16[c]
[d]
–
1c
46, 73[e]
[a] Reaction conditions: BINOL (0.5 mmol), dihalomaleimide de-
rivative (1 equiv.), Na2CO3 (2 equiv.), DMF (0.1 m), 60 °C, 1 h.
[b] Isolated yield. [c] Calculated from a purified mixture containing
1b and 3a, see the Supporting Information for more details. [d] A
mixture of unreacted 3a, mono-O-silylated 3a, and other unidenti-
fied products was obtained. [e] Reaction conditions: BINOL
(0.5 mmol), dibromomaleimide derivative (1 equiv.), DIPEA
(10 equiv.), toluene (0.1 m), 50 °C, 1 h.
[a] Reaction conditions: biphenol
3 (0.5 mmol), 2b (1.1 to
1.3 equiv.), Na2CO3 (2 equiv.), DMF (0.1 m), 60 °C, 1 h. [b] Iso-
lated yield. [c] Biphenol (0.5 mmol), 2b (1.2 to 1.3 equiv.), KF
(15 equiv.), DMF (0.1 m), 45 °C, 24 h.
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BINOL-Fused Maleimides
the reaction of dibromomaleimide 2e did not lead to the ented towards the carbonyl groups of the imide function
expected product but only to a mixture of unreacted 3a, with one on each side of the maleimide ring.
mono-O-silylated 3a, and other unidentified compounds
By proper choice of those substituents, one should be
(Table 1, entry 5). Fair yields were restored starting from able to tune the local chiral environment around the imide
a dibromomaleimide bearing a p-nitrophenyl group on the moiety of this type of molecule, which should play a pivotal
nitrogen atom: thus, maleimide 2f gave compound (Ϯ)-1c role in the design of asymmetric catalysts based on this new
in 46% yield by using Na2CO3 in DMF or in 73% yield by chiral scaffold (Figure 3).
using diisopropyl(ethyl)amine (DIPEA) in toluene (Table 1,
entry 6).
Encouraged by these results, we decided to react a series
of 2,2Ј-biphenols with maleimide 2b to define the scope of
this reaction (Table 2). We first used enantiopure (S)-
BINOL and obtained, as expected, (S)-1a with no loss of
enantiopurity in 75% yield (Table 2, entry 1). We next tried
the reaction with 2,2Ј-biphenol 3b and obtained compound
1d in 82% isolated yield (Table 2, entry 2). With highly hin-
dered 2,2Ј-biphenol (Ϯ)-3c, however, the reaction did not
Figure 3. Tuning of the local chiral environment around the imide.
proceed at all, regardless of the reaction conditions
(Table 2, entry 3). With (R)-3,3Ј-dibromo-1,1Ј-bi-2-naph-
thol [(R)-3d], (R)-1e was obtained in 68% yield (Table 2,
entry 4). With diiodo analogue (S)-3e, the reaction was very
sluggish, probably as a result of steric hindrance: After 2 h,
only 25% conversion to expected product (S)-1f was ob-
We were therefore interested in the replacement of the
bromine or iodine atoms in compounds 1e, 1f, and 1g by
other substituents. As an example, we performed a double
Suzuki cross-coupling reaction between (Ϯ)-1f and p-tolyl-
boronic acid (Scheme 2) and obtained expected product
(Ϯ)-1h in 73% isolated yield.[19] This result opens a path to
the use of many other types of cross-coupling reactions and
should allow the straightforward synthesis of libraries of
catalysts with the ability to fine-tune their catalytic proper-
ties and enantioselectivities by modifying the chiral environ-
ment around the catalytic site.
1
served by H NMR spectroscopy. When the reaction was
prolonged, extensive decomposition of 2b was observed.[16]
Accordingly, we tested different sets of basic conditions to
minimize the decomposition of 2b. Finally, expected prod-
uct (S)-1f could be obtained in 76% isolated yield by using
KF as the base (Table 2, entry 5). With (R)-3,3Ј-dibromo-
octahydro BINOL [(R)-3f], we obtained (R)-1g in only 31%
yield under our standard conditions, but the yield could be
improved to 65% by using KF as the base (Table 2, en-
try 6).
BINOL-fused maleimides possess interesting structural
features, as can be seen from the X ray structure of (R)-
1e in Figure 2:[17] The central eight-membered 1,4-dioxocine
heterocycle adopts a chiral C2-symmetric twist-boat-chair
conformation, which is very close to the conformation en-
countered in other simple 1,4-dioxocine rings.[18] When em-
bedded in the present rigid scaffold, the dioxocine ring is
locked in one of its two possible enantiomeric configura-
tions because of the restrained rotation around the BINOL
Scheme 2. Suzuki cross-coupling reaction of 1f with p-tolylboronic
acid.
Enantiopure BINOL-fused maleimides proved to be
chiral axis. Conformational restrictions also dictate that quite robust toward racemization: no racemization occurred
substituents ortho to the oxygen atoms of the BINOL moi- when (S)-1a (Ͼ 99%ee) was heated at reflux in toluene for
ety (for example, the bromine atoms in Figure 2) are ori- 24 h. This contrasts favorably with our previous series of
chiral NHPI analogues based on a C2-symmetric scaffold
with two atropisomeric axes, which, when placed under the
same conditions, underwent rapid isomerization leading, af-
ter 24 h, to a thermodynamic equilibrium between the cis
and trans forms.[4,5] We also checked the final transforma-
tion into either the free N-hydroxyimide or the N-unsubsti-
tuted imide functional group. We first tried to remove the
benzyl protecting group in (Ϯ)-1a by catalytic hydrogena-
tion.[3b] In this case, however, the double bond of the male-
imide was reduced as well. Thus, we performed the depro-
tection by using boron tribromide and obtained free N-
hydroxyimide (Ϯ)-1i in 63% isolated yield (Scheme 3).
Figure 2. X-ray structure of 1e.
Eur. J. Org. Chem. 2013, 1041–1045
Interestingly, maleimides bearing a p-nitrophenyl group on
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S. Brenet, B. Baptiste, C. Philouze, F. Berthiol, J. Einhorn
SHORT COMMUNICATION
the nitrogen atom were also found to be suitable precursors in fair yields from BINOL, as well as from many other 2,2Ј-
of the corresponding N-hydroxyimides: when a solution of biphenols, and N-protected 2,3-dihalomaleimides under ba-
(Ϯ)-1c in acetonitrile was treated at room temperature with sic conditions. The new scaffold thus obtained can be read-
an aqueous solution of hydroxylamine for 18 h, (Ϯ)-1i was ily functionalized at the positions ortho to the oxygen atoms
isolated in 69% yield. As expected, p-nitrophenyl-substi- of the BINOL moiety, for example, through Suzuki cou-
tuted imide (Ϯ)-1c was also a suitable precursor of the N- pling. Both X-ray structure and conformational minimi-
unsubstituted imide:[8a] when a solution of (Ϯ)-1c in aceto- zation by MM2 calculations show that such substituents are
nitrile was treated at room temperature with aqueous am- oriented towards the imide moiety of the molecule, which
monia for 16 h, (Ϯ)-1b was isolated in 72% yield should allow the design of a proper local chiral environ-
(Scheme 3).
ment around it. We also determined that enantiopure com-
pounds do not racemize, even under harsh conditions and
that free N-hydroxyimides or N-unsubstituted imides can
be readily obtained from the N-protected compounds. Pre-
liminary experiments demonstrate that the new N-
hydroxyimides are aerobic oxidation catalysts and that the
N-unsubstituted imides constitute new chiral ligands of pal-
ladium. Further investigations of these compounds and
their use in asymmetric catalysis are now underway and will
be reported in due course.
Supporting Information (see footnote on the first page of this arti-
cle): General procedures and spectral data.
Acknowledgments
Scheme 3. Final transformations into free N-hydroxyimides and N-
unsubstituted imides.
Financial support from the Centre National de la Recherche Sci-
entifique (CNRS) and Université Joseph Fourier is gratefully ac-
knowledged. S. B. thanks the Université Joseph Fourier for a PhD
grant. The authors thank Dr. Yves Gimbert for preliminary DFT
calculations.
Some preliminary experiments were also performed to
outline potential applications of these new compounds.
Thus, (Ϯ)-1i was tested as an NHPI analogue in the aerobic
oxidation of isochroman.[1,2] Thus, isochromanone was ob-
tained in 70% yield after 12 h by using (Ϯ)-1i (5 mol-%)
and Mn(acac)2 (0.5 mol-%) in acetonitrile at 70 °C under an
atmosphere of O2 (10 bar). The blank reaction conducted
without 1i resulted in only unreacted isochroman. Recently,
N-unsubstituted imides have been reported to be interesting
ligands for transition metals. For example, Fairlamb pre-
pared an air-, light-, and moisture-stable palladium com-
plex bearing two triphenylphosphane and two succinimide
ligands simply by treating succinimide (2 equiv.) and tri-
phenylphosphane (2 equiv.) with Pd2(dba)3 (0.5 equiv.) at
room temperature in benzene. This complex was found to
be an active precatalyst for Suzuki coupling reactions.[7h]
In an analogous fashion, we reacted (R)-1b (2 equiv.) and
triphenylphosphane (2 equiv.) with Pd2(dba)3 (0.5 equiv.) at
room temperature in toluene. A white powder progressively
formed from the originally purple homogeneous mixture.
After 16 h at room temperature, the mixture was filtered,
washed with toluene, dissolved in dichloromethane, and
dried. All attempts to obtain single crystals suitable for X-
ray analysis failed. However, HRMS clearly indicates that it
corresponds to a complex of formula [(PPh3)2Pd{(R)-1b}2].
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In summary, we developed a straightforward synthesis of
new BINOL-fused maleimides. In sharp contrast with the
case of catechol, these compounds can be easily synthesized
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BINOL-Fused Maleimides
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Received: November 14, 2012
Published Online: January 24, 2013
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