Ruthenium NHC catalyzed highly asymmetric hydrogenation of benzofurans

Article abstract of DOI:10.1002/anie.201107811

H₂-O-T! Aromatic O-heterocycles are a challenging substrates for asymmetric hydrogenation (H₂). An in situ formed chiral N-heterocyclic carbene (NHC) ruthenium complex allows the high yielding, completely regioselective, and highly asymmetric hydrogenation of substituted benzofurans at room Temperature, giving valuable 2,3-dihydrobenzofurans (see scheme). Copyright

Full text of DOI:10.1002/anie.201107811

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DOI: 10.1002/anie.201107811
Asymmetric Hydrogenation
Ruthenium NHC Catalyzed Highly Asymmetric Hydrogenation of
Benzofurans**
Nuria Ortega, Slawomir Urban, Bernhard Beiring, and Frank Glorius*
Asymmetric hydrogenation of aromatic and heteroaromatic
compounds is one of the most straightforward ways for the
synthesis of saturated or partially saturated cyclic molecules,
which are present in many biologically active compounds.[1]
Starting from the pioneering work of Murata and co-workers
in 1987,[2] who obtained enantiomeric excess in the hydro-
genation of 2-methylquinoxaline, impressive advances have
been made in the asymmetric hydrogenation of certain
heterocycles. Mostly N-heterocycles, such as quinolines,[3]
quinoxalines,[2,4] and indoles,[5] were successfully hydrogen-
ated to the corresponding tetrahydroquinolines, tetrahydro-
quinoxalines, and indolines with more than 90% ee by using
homogeneous transition-metal catalysts or Brønsted acid
organocatalysts. However, despite the effort put into this area
in the last years, some valuable classes of substrates, especially
(non-N) heterocycles, such as furans, thiophenes, benzofur-
ans, and benzothiophenes, are much less explored and still
constitute a challenge for the field of asymmetric hydro-
genation. Therefore, new efficient and highly enantioselective
methodologies that permit access to the corresponding
reduced analogues are highly desirable.
There are numerous reports on the synthesis of 2,3-
dihydrobenzofurans.[6] However, even though hydrogenation
seems to be the most direct route for their synthesis, arguably,
it is more difficult compared to the hydrogenation of many
other heteroaromatic compounds.[7] Often partial decompo-
sition of the furan ring to 2-ethylcyclohexanol and b-cyclo-
hexylethyl alcohol is observed.[6,8] Regarding the asymmetric
hydrogenation of benzofurans, to date only two reports can be
found: In 2003, Baiker and co-workers used a combination of
Pd/Al2O3 with cinchonidine derivatives to obtain reduced 2-
benzofuran carboxylic acid in very low yield and only
50% ee.[9] In the field of homogeneous catalysis, Pfaltz and
co-workers applied pyridine phosphinite iridium complexes
obtaining a few 2,3-dihydrobenzofurans with excellent ee val-
ues (Scheme 1).[10] However, longer reaction times were
needed and only three of these substrates were described.
Herein we report an efficient, high yielding, highly regio- and
enantioselective hydrogenation of substituted benzofurans
proceeding under mild conditions.
Scheme 1. Asymmetric hydrogenation of benzofurans.
Recently, we have reported a new method for the
enantioselective hydrogenation of the aromatic carbocyclic
ring of substituted quinoxalines by using a chiral ruthenium
N-heterocyclic carbene[11] (NHC) complex.[12] This report was
the first example of a catalytic asymmetric hydrogenation of
the carbocyclic ring of aromatic compounds. In addition, the
choice of the NHC allowed a switch between the selective
hydrogenation of either the carbocyclic (using the NHC ICy)
or the heterocyclic ring (using the NHC SIPr). However, even
though the structure and mode of action of the catalysts are
not yet known, we decided to explore the reactivity of this
new catalytic system for other challenging substrates. To our
surprise, when we applied our Ru catalyst formed from ICy
for the hydrogenation of benzofuran, we selectively reduced
the heterocyclic ring, exclusively obtaining the corresponding
2,3-dihydrobenzofuran (Scheme 2).
[*] Dr. N. Ortega, S. Urban, B. Beiring, Prof. Dr. F. Glorius
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Organisch-Chemisches Institut
Corrensstrasse 40, 48149 Mꢁnster (Germany)
E-mail: glorius@uni-muenster.de
Homepage: http://www.uni-muenster.de/Chemie.oc/glorius/
index.htm
[**] Generous financial support by the Deutsche Forschungsgemein-
schaft (SFB 858) is gratefully acknowledged. The research of F.G.
has been supported by the Alfried Krupp Prize for Young University
Teachers of the Alfried Krupp von Bohlen und Halbach Foundation.
We also thank BASF (Prof. Dr. Klaus Ditrich) for the donation of
precious chiral amines (ChiPros). NHC=N-heterocyclic carbene.
Scheme 2. Switching between carbocycle and heterocycle hydrogena-
tion. Complete regioselectivity (>99:1) and quantitative yield was
obtained in each case.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107811.
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Table 2: Scope of the asymmetric hydrogenation of benzofurans 1a–q.[a]
Interestingly, using the substituted 2-phenyl benzofuran
(1a) as model substrate, hydrogenation resulted in the
formation of the corresponding racemic 2,3-dihydrobenzo-
furan 2a in quantitative yield and with complete regioselec-
tivity. Encouraged by these preliminary results, we tested to
see if our previously developed chiral ruthenium NHC
complex, formed from the imidazolium salt 3a (for formula,
see Table 1), could induce enantioselectivity in the hydro-
genation of substituted benzofurans.[13] When we submitted
1a to the hydrogenation process at 60 bar of H2 and 808C,
starting material was recovered, probably because of the
decomposition of the catalyst (Table 1, entry 1). Intriguingly,
upon decreasing the temperature to 408C, full conversion into
Table 1: Optimization of the reaction conditions for the asymmetric
hydrogenation of 2-phenyl benzofuran 1a.[a]
Entry NHC·HX Solvent T [8C] p(H2) [bar] Yield[b] [%] e.r.[c]
1
2
3
4
5
3a
3a
3b
3a
3a
toluene 80
toluene 40
toluene 40
toluene 25
60
60
60
10
10
n.d.
n.d.
>99
>99
>99
>99
96:4
94:6
97:3
99:1
[a] [Ru(cod)(2-methylallyl)2] (0.015 mmol), 3a (0.03 mmol), KOtBu
(0.045 mmol), n-hexane (2 mL) were stirred at 708C for 12 h, after which
it was added to substrate 1a–q (0.3 mmol). Hydrogenation was
performed at 10 bar H2, 258C, 16 h. Yields given are of isolated product.
Enantiomeric ratio was determined by HPLC on a chiral stationary phase.
The stereochemistry of the 2-alkyl and aryl-substituted products was
assigned in analogy to 2h and corsifuran A, respectively. [b] Reaction was
performed at 60 bar H2, 408C, 16 h. [c] The absolute configuration of 2h
was determined by comparison of optical rotation data with the literature
value (see the Supporting Information). In addition, we succeeded in
synthesizing corsifuran A,[15] a 2-aryl substituted 2,3-dihydrobenzofuran,
with this new hydrogenation method, again allowing the comparison of
optical rotation with the literature data. [d] Run with 0.5 mol% catalyst.
hexane
25
[a] General conditions: [Ru(cod)(2-methylallyl)2] (0.015 mmol), KOtBu
(0.045 mmol) and 3a or 3b (0.03 mmol) were stirred at 708C in the
shown solvent (2 mL) for 12 h, after which it was added to 1a
(0.30 mmol), and hydrogenation was performed under conditions
shown in each case for 16 h. The optimized conditions are shown in
italics. [b] Yields given are of isolated product. [c] E.r. was determined by
HPLC on a chiral stationary phase; n.d.=not detected.
the desired product 2a was obtained. Moreover, the enantio-
meric ratio reached surprisingly good 96:4. Furthermore, it
was possible to perform the reaction at even lower temper-
ature (258C) and lower hydrogen pressure (10 bar), resulting
in a reproducible slight increase of the enantiomeric ratio to
97:3 (Table 1, entry 4). Solvent screening revealed that non-
polar, aprotic solvents, such as n-hexane and toluene, were
best suited for this reaction, and the use of n-hexane gave an
increase of the enantiomeric ratio to excellent 99:1 (Table 1,
entry 5). Modifying the NHC derived from 3a by using its
unsaturated derivative 3b, led to similar results in conversion
and regioselectivity, but a slight decrease of enantiomeric
ratio of the product 2a (Table 1, entry 3).
Having established the optimized reaction conditions, we
tested a variety of substituted benzofurans to probe the
versatility of our catalytic system (Table 2). Interestingly, the
reactivity of the 2-phenyl-substituted benzofurans changes
significantly with the electronic properties of the substituents:
When the phenyl ring contains electron-withdrawing groups,
such as fluorine (1g) or trifluoromethyl (1 f) in a para
position, the reaction proceeds smoothly at 10 bar of hydro-
gen pressure and at room temperature, with full conversion
and very good enantiomeric ratio. However, when the phenyl
ring bears electron-donating substituents, such as a methoxy
group (1e), low conversion to the desired product was
observed. Fortunately, with 1e, full conversion was achieved
when the reaction was carried out at 60 bar of hydrogen and
408C, maintaining an excellent enantiomeric ratio of 99:1. We
also studied the effect of the substitution pattern of the 2-
phenyl ring on the reactivity. The reaction worked nicely for
the 2-(p-tolyl) (1d) and 2-(m-tolyl) benzofuran (1c) with full
conversion and enantiomeric ratio of 99:1, but in the case of 2-
(o-tolyl) benzofuran (1b) the reactivity dropped, resulting in
only 73% yield of isolated product, and 96:4 enantiomeric
ratio. To our knowledge, this is the first report of highly
asymmetric hydrogenations of aryl-substituted benzofurans.
In the case of alkyl-substituted benzofurans, the reaction
proceeds with perfect conversion and high enantioselectivity
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for many primary and secondary alkyl chains (1h–k). We
noticed that the enantiomeric ratio decreased slightly when
the length or substitution of the chain was increased, while
maintaining perfect conversion into the desired product.
Surprisingly, the reaction even works with the 2-(tert-butyl)-
benzofuran (1l) albeit with lower conversion and ee value.
The reaction also gave the desired product for 2-benzyl
benzofuran (1m) with an e.r. of 92:8. Changing the position of
the substituent to position 3 (1o) led to a slight drop in
enantioselectivity (93:7) compared to the regioisomer 1h, but
maintains perfect regioselectivity and conversion. By com-
parison of the optical rotation data of 2h, the absolute
configuration could be assigned to be R (see Table 2).
Experimental Section
General procedure: In a glove box, to a flame-dried screw-capped
tube equipped with magnetic stir bar was added the [Ru(cod)(2-
methylallyl)2] (4.8 mg, 0.015 mmol; cod = cyclooctadiene), imidazo-
lium salt 3a (14.1 mg, 0.03 mmol), and dry KOtBu (5.0 mg,
0.045 mmol). The mixture was suspended in hexane (2 mL) and
stirred at 708C for 12 h. Then the mixture was transferred under
argon to a glass vial containing benzofuran 1a–q (0.3 mmol) and a
magnetic stirring bar. The glass vial was placed in a 150 mL stainless-
steel reactor. The autoclave was pressurized with hydrogen gas
(10 bar or 60 bar) and depressurized three times before the indicated
reaction pressure was set. The reaction mixture was stirred at 258C or
408C for 16 h. After the autoclave was carefully depressurized, the
crude mixture was filtered through a plug of silica using a pentane/
EtOAc mixture (9:1), yielding analytically pure compounds 2a–q.
The enantiomeric ratio of all compounds was determined by HPLC
on a chiral stationary phase.
We also studied the influence of the substitution on the
carbocyclic ring of the benzofuran. When 6-(tert-butyl)-2-
phenylbenzofuran (1n) was submitted to hydrogenation
conditions, the corresponding 2,3-dihydrobenzofuran 2n was
obtained with no change on the enantiomeric ratio or
reactivity compared to the analogue 2a. Moreover, we
studied the influence of the presence of other aromatic
rings. When 2-(benzofuran-2-yl)pyridine (1p) was submitted
to hydrogenation conditions, the corresponding 2,3-dihydro-
benzofuran derivative (2p) was obtained smoothly without
any observed hydrogenation of the pyridine, but with a
considerable drop in the enantiomeric ratio compared to the
phenyl analogue 1a. The basis for this deterioration might be
the ability of 1p to form a bidentate chelate. To test this
hypothesis, we used the regioisomeric 3-(benzofuran-2-yl)pyr-
idine (1q), which led to the exclusive formation of 2,3-
dihydrobenzofuran with excellent 99:1 enantiomeric ratio.
To explore the kinetic behavior of this hydrogenation
process we chose 2-methyl benzofuran (1h) as model
substrate. First, we discovered that a significantly reduced
catalyst loading of 0.5 mol% was sufficient for full conversion
into 2h, providing a turnover number (TON) of 200 (Table 2).
Using 0.5 mol% of catalyst the reaction was stopped after
certain times. To our surprise, the reaction was already
complete after 2 h. However, when we reduced the reaction
time to 1 h all of the starting material was recovered
unchanged.[14] A closer inspection showed that most of the
substrate was converted into 2-methyl-2,3-dihydrobenzofuran
between 70 and 80 min, showing a turnover frequency (TOF)
of 1092 hÀ1. An induction period of 1 h seems to be required
to form the catalytically active species. Once formed, this
species provides most of the turnover within 10 min, demon-
strating a high efficiency of the catalyst.
Received: November 6, 2011
Published online: January 3, 2012
Keywords: 2,3-dihydrobenzofuran · asymmetric hydrogenation ·
.
heterocycles · N-heterocyclic carbenes · ruthenium
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ruthenium NHC complex in the high-yielding, regioselective,
and highly asymmetric hydrogenation of substituted benzo-
furans. Notably, the catalyst shows very high TOF and good
TON. We also present a very simple and straightforward
method for the asymmetric synthesis of valuable 2,3-dihy-
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[13] The catalyst was pre-formed before the hydrogenation reaction:
[Ru(cod)(2-methylallyl)2], KOtBu, and the imidazolium salt (3a
or 3b) were stirred at 708C for 12 h in hexane, after which it was
added to 1a and hydrogenation was performed under conditions
shown in Table 1.
[14] Yields given are determined by NMR spectroscopy. For more
details, see the Supporting Information.
[15] The synthesis of corsifuran A will be reported in due course.
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