Asymmetric hydrogenation of maleic acid diesters and anhydrides

Article abstract of DOI:10.1002/anie.201402034

Asymmetric hydrogenation of maleic and fumaric acid derivatives with iridium catalysts based on N,P ligands provides an efficient route to chiral enantioenriched succinates. A new catalyst derived from a 2,6-difluorophenyl- substituted pyridine-phosphinite ligand was developed and enables the conversion of a wide range of 2-alkyl and 2-arylmaleic acid diesters into the corresponding succinates in high enantiomeric purity. Mixtures of cis/trans substrates can be hydrogenated in an enantioconvergent fashion with high enantioselectivity, and further enhances the scope of this transformation. The products are valuable chiral building blocks with a structural motif found in many bioactive compounds, such as metalloproteinase inhibitors. An attractive enantioselective route to 2-alkyl- and 2-aryl-substituted succinic acid derivatives is opened up by the asymmetric hydrogenation of maleic and fumaric acid derivatives, using the new catalyst [Ir(cod)L]BAr_F, derived from a 2,6-difluorophenyl-substituted pyridine-phosphinite ligand. The products are valuable chiral building blocks having a structural motif found in many bioactive compounds. cod=1,5-cyclooctadiene.

Full text of DOI:10.1002/anie.201402034

Angewandte
Chemie
DOI: 10.1002/anie.201402034
Asymmetric Catalysis
Asymmetric Hydrogenation of Maleic Acid Diesters and Anhydrides**
Maurizio Bernasconi, Marc-Andrꢀ Mꢁller, and Andreas Pfaltz*
Abstract: Asymmetric hydrogenation of maleic and fumaric
acid derivatives with iridium catalysts based on N,P ligands
provides an efficient route to chiral enantioenriched succinates.
A new catalyst derived from a 2,6-difluorophenyl-substituted
pyridine-phosphinite ligand was developed and enables the
conversion of a wide range of 2-alkyl and 2-arylmaleic acid
diesters into the corresponding succinates in high enantiomeric
purity. Mixtures of cis/trans substrates can be hydrogenated in
an enantioconvergent fashion with high enantioselectivity, and
further enhances the scope of this transformation. The products
are valuable chiral building blocks with a structural motif
found in many bioactive compounds, such as metalloprotei-
nase inhibitors.
Figure 1. Structures of biologically active compounds containing a 2-
substituted succinic acid moiety.
I
ridium complexes derived from chiral heterobidentate
ligands have considerably enhanced the scope of asymmetric
hydrogenation.[1] Compared to rhodium and ruthenium
=
catalysts, which require a coordinating group near the C C
bond, iridium catalysts have been successfully used for the
asymmetric hydrogenation of a much wider range of func-
tionalized and unfunctionalized olefins. Nevertheless, there
are still important substrates classes, for which suitable chiral
catalysts are lacking.
Although highly enantioselective hydrogenations of a,b-
unsaturated carboxylic acids, esters, amides, and ketones have
been reported,[2,3] prochiral fumaric and maleic acid deriva-
tives remain a challenging substrate class.[4] The enantiose-
lective hydrogenation of substrates of this type would provide
access to synthetically valuable 2-substituted succinic acid
derivatives, which are versatile chiral building blocks and are
found in
a variety of biologically active molecules
(Figure 1).[5–8] Examples are the matrix metalloproteinase
(MMP) inhibitors 1 and 2, the caspase 1 inhibitor 3, and
mitiglinide 4 (a drug used to treat type 2 diabetes). Thus,
efficient methods for the enantioselective synthesis of sub-
stituted succinic acids and related compounds are of great
interest.
So far enantiomerically enriched 2-substituted succinic
acid derivatives have been synthesized by rhodium-catalyzed
asymmetric hydrogenation of the corresponding itaconic acid
precursors (Scheme 1a),[9] 1,4-addition of arylboronic acids to
Scheme 1. Enantioselective routes to chiral 2-substituted succinic acid
derivatives.
fumaric acid di-tert-butyl diester or maleimides (Sche-
me 1b),[10] or by organocatalytic parallel kinetic resolution
of monosubstituted succinic anhydrides (Scheme 1c).[11]
While the latter method is limited to a maximum yield of
50%, the hydrogenation of itaconic acid derivatives becomes
[*] Dr. M. Bernasconi, M.-A. Mꢀller, Prof. Dr. A. Pfaltz
University of Basel, Department of Chemistry
St. Johanns-Ring 19, 4056 Basel (Switzerland)
E-mail: andreas.pfaltz@unibas.ch
=
difficult when the C C bond is tetrasubstituted, as would be
required for the synthesis of compounds such as 2 or 3.
Moreover, 2-aryl-succinic acid derivatives are not accessible
in this way, whereas the rhodium-catalyzed 1,4-addition is
limited to arylboronic acids. Herein we report an efficient
alternative approach to this class of products based on
iridium-catalyzed asymmetric hydrogenation of prochiral
maleic and fumaric acid derivatives.
[**] We are thankful to R. Liffert for catalyst synthesis and Dr. Paolo
Tosatti for helpful discussions and help with the preparation of this
manuscript. Support by the Swiss National Science Foundation is
gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402034.
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were achieved with the catalyst prepared from the pyridine-
phosphinite L5. Apparently, the tBu2P group has a positive
effect on both the enantioselectivity and reactivity compared
to the Ph2P group in the analogous ligand L4. Further
variation of the ligand structure finally led to a virtually
perfect catalyst based on the pyridine-phosphinite L7, which
afforded the desired product 8a with full conversion and 99%
ee. The 2,6-difluorophenyl group in this new ligand was
introduced to examine the effect of an electron-withdrawing
substituent at this position and to prevent insertion of the
Scheme 2. Iridium-catalyzed asymmetric hydrogenation of maleic anhy-
drides. See Table 1 for ligand structure. BArF =tetrakis[3,5-bis(trifluoro-
methyl)phenyl]borate.
À
We first studied the hydrogenation of the maleic anhy-
drides 5a and 5b (Scheme 2). In an initial screening using 12
chiral iridium complexes and 5a as a test substrate, the
complex derived from the NeoPHOX ligand L2 emerged as
the best catalyst (see the Supporting Information). With this
catalyst 5a and 5b could be converted into the corresponding
succinic anhydrides 6a and 6b in good yield and high
enantioselectivity. However, the poor solubility of other
maleic anhydrides in dichloromethane prompted us to
examine the corresponding diesters, which are more soluble
and easily prepared in one step from dimethyl acetylenedi-
carboxylate.[12]
iridium atom into an adjacent aromatic C H bond, a process
which is facile for 2-phenylpyridines and can lead to catalyst
deactivation.[14]
The synthesis of this catalyst is summarized in Scheme 3.
The key step is a Suzuki–Miyaura cross-coupling of the
chloropyridine precursor 11[2b] with commercially available
(2,6-difluorophenyl)boronic acid using a procedure from
We first examined the asymmetric hydrogenation of
dimethyl 2-cyclohexylmaleate (7a) using a selection of
catalysts, based on N,P ligands (L1–L7), under standard
reaction conditions (Table 1). The catalyst derived from L2,
which had performed best with 5a and 5b, showed high
enantioselectivity but poor reactivity, thus resulting in low
conversion. Similar ee values but much higher conversions
Table 1: Ligand screening for the asymmetric hydrogenation of 2-
cyclohexyl dimethyl maleate (7a).[a]
Scheme 3. Synthesis of the ligand L7 and the corresponding [IrL7-
(cod)]BArF complex (for procedures, see the Supporting Information).
Entry
L
Conversion [%][b]
ee [%][c]
Buchwald and co-workers.[15] The reaction proceeded under
mild reaction conditions to afford the (2,6-difluorophenyl)-
pyridine 12 in excellent yield. Deprotection of the alcohol,
followed by nucleophilic substitution with di-tert-butylphos-
phoryl chloride, and subsequent complexation afforded the
complex [Ir(cod)L7]BArF in good overall yield.
1
2
3
4
5
6
7
L1
L2
L3
L4
L5
L6
L7
63
26
86
41
96
19 (À)
90 (+)
77 (À)
77 (+)
92 (+)
96 (+)
99 (+)
98
>99
To evaluate the scope of this catalyst for the hydro-
genation of maleic and fumaric acid esters, we examined
a series of 2-alkyl-substituted derivatives as substrates
(Table 2). Remarkably, the reactivity and enantioselectivity
of the catalyst proved to be rather insensitive to the steric
properties of the substrate. The ee values obtained from
methyl-, n-alkyl-, branched alkyl-, and even tert-butyl-sub-
stituted maleate were all within a narrow range of 95–99%.
Surprisingly, the fumarate derivative (E)-7b afforded the
same enantiomer of 8b with virtually the same ee value as the
corresponding maleate (Z)-7b (entries 1 and 2). This result
contrasts with the general experience that hydrogenation of
the cis and the trans isomer of a trisubstitued alkene leads to
opposite enantiomers with iridium catalysts of this type.[1c]
Accordingly, cis/trans mixtures can be hydrogenated in an
enantioconvergent fashion to give the corresponding succinic
[a] Reaction conditions: substrate (0.1 mmol), catalyst (1 mol%),
CH2Cl2 (0.5 mL, 0.2m). [b] Conversions were determined by GC analysis.
[c] Enantioselectivities were determined by GC analysis using a chiral
stationary phase.
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Table 2: Asymmetric hydrogenation of the alkyl-substituted dimethyl
maleates and fumarates 7b–f.[a]
Table 4: Asymmetric hydrogenation of the aryl- and heteroaryl-substi-
tuted dimethyl maleates 9b–m.[a]
Conversion [%][b]
ee [%][c]
Entry
R
Conversion [%][b]
ee [%][c]
Entry
Ar
1
2
3
4
5
6
7
(Z)-7b (R=Me)
(E)-7b (R=Me)
(E/Z)-7b (R=Me)[d]
(Z)-7c (R=nBu)
(Z)-7d (R=iBu)
(Z)-7e (R=iPr)
(Z)-7 f (R=tBu)
>99
>99
>99
97
>99
>99
99
97 (R, +)
98 (R, +)
98 (R, +)
96 (+)
95 (+)
99 (S, +)
98 (S, +)
1
2
4-MeC6H4 (9b)
3-MeC6H4 (9c)
3-MeC6H4 (9c)
2-MeC6H4 (9d)
4-OMeC6H4 (9e)
4-FC6H4 (9 f)
>99
52
>99
2
>99
>99
99
>99
>99
>99
30
97 (+)
95 (+)
96 (+)
n.d.
97 (+)
96 (+)
95 (+)
92 (+)
91 (+)
97 (+)
n.d.[e]
3[d]
4
5
6
7
8
3-FC6H4 (9g)
4-ClC6H4 (9h)
4-CF3C6H4 (9i)
4-C6H4C6H4 (9j)
4-TMSC6H4 (9k)
4-TMSC6H4 (9k)
4-SMeC6H4 (9l)
3-thiophenyl (9m)
[a] Reaction conditions: see Table 1. [b] Conversions were determined by
GC analysis. [c] Enantioselectivities were determined by GC analysis
using a chiral stationary phase. [d] A 1.5:1 mixture of (Z)-7b and (E)-7b
was used.
9
10
11
12[d]
13
14
>99
0
88
96 (+)
–
84 (+)
acid derivative with excellent enantioselectivity (entry 3),
thus making this transformation very attractive, especially for
substrates which are not easily obtained as pure E or
Z isomers.
We next examined 2-aryl-substituted dimethyl maleates as
substrates. In the hydrogenation of 2-phenylmaleate (9a), the
catalyst derived from L7 again showed the best performance,
thus affording the phenylsuccinate 10a with 96% ee and 95%
conversion (Table 3). The ThrePHOX complex [Ir-
[a] Reaction conditions: see Table 1. [b] Conversions were determined by
GC analysis. [c] Enantioselectivities were determined by GC or HPLC
analysis using a chiral stationary phase. [d] 2 mol% of catalyst were
used. [e] The ee value could not be determined because of overlapping
peaks of unreacted 9k and one of the enantiomers of the product in the
HPLC analysis. n.d.=not determined.
of 2 mol% for full conversion (entries 2 and 3). A trimethyl-
silyl group in the para position also reduced the reactivity, but
by increasing the catalyst loading to 2 mol% complete
conversion to the product was achieved (entries 11 and 12).
An ortho-methyl or a para-methylthio group, in contrast,
were not tolerated (entries 4 and 13). The 3-thiophenyl
derivative 9m showed somewhat lower reactivity than the
corresponding 9a, but still could be hydrogenated with 88%
conversion and 84% ee (entry 14). Overall, a diverse set of
highly enantioenriched succinates with an aromatic substitu-
ent in the 2-position is available in this way.
We also examined the hydrogenation of cis/trans mixtures
of 2-aryl-substituted fumarates and maleates (Table 5). In
contrast to the hydrogenation of the 2-methyl-substituted
analogues (Table 2, entries 2 and 3), the enantioselectivities
were lower than those obtained with the pure Z isomers.
Table 3: Ligand screening for the asymmetric hydrogenation of dimethyl
2-phenylmaleate (9a).[a]
Entry
L
Conversion [%][b]
ee [%][c]
1
2
3
4
L3
L5
L6
L7
>99
16
14
79 (R, À)
92 (S, +)
89 (S, +)
96 (S, +)
95
[a] Reaction conditions: see Table 1. [b] Conversions were determined by
GC analysis. [c] Enantioselectivities were determined by GC analysis
using a chiral stationary phase.
Table 5: Asymmetric hydrogenation of mixtures of fumarates and
maleates.[a]
(cod)L3]BArF exhibited somewhat higher reactivity, but
gave distinctly lower ee values. The complexes based on the
ligands L5 and L6 induced good enantioselectivities, but
conversions were below 20%.
Screening of
a series of 2-arylmaleates with [Ir-
Entry
R
E/Z
Conv. [%][b]
ee [%][c]
(cod)L7]BArF as the catalyst showed that both electron-rich
and electron-poor aryl groups were well tolerated (Table 4,
entries 5–12). A wide range of substrates with a meta or
para substituent on the aryl group was hydrogenated with
high enantioselectivies of 91–97% ee. While hydrogenation of
the para-substituted substrates 9b, 9e, 9 f, 9h, 9i, and 9j went
to completion under standard reaction conditions, the meta-
methylphenyl derivative 9c required a higher catalyst loading
1
2
3
4
(E/Z)-9a (R=H)
(E/Z)-9b (R=Me)
(E/Z)-9i (R=CF3)
(E/Z)-9j (R=Ph)
1:1.2
1:3.4
1:1.1
1:1.2
>99
>99
95
87 (S, +)
93 (+)
89 (+)
88 (+)
>99
[a] Reaction conditions: See Table 1. [b] Conversions were determined by
GC analysis. [c] Enantioselectivities were determined by GC or HPLC
analysis using a chiral stationary phase.
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In summary, asymmetric hydrogenation of maleic and
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Keywords: asymmetric catalysis · hydrogenation · iridium ·
N,P ligands · synthetic methods
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