Oxidative esterification, thioesterification, and amidation of aldehydes by a two-component organocatalyst system using a chiral N-heterocyclic carbene and redox-active riboflavin

Article abstract of DOI:10.1002/chem.201100737

Flavin of the month! Triazolium-derived N-heterocyclic carbenes (NHCs) and a flavin catalyzed the oxidative esterification, thioesterification, and amidation of aldehydes with various alcohols, thiols, and amines, respectively, with O₂ as the terminal oxidant (see scheme; R¹=aryl; R², R³=alkyl or aryl). By using a chiral NHC catalyst, the enantioselective acylation promoted the kinetic resolution of racemic alcohols and the desymmetrization of a meso-diol.

Full text of DOI:10.1002/chem.201100737

COMMUNICATION
DOI: 10.1002/chem.201100737
Oxidative Esterification, Thioesterification, and Amidation of Aldehydes by
a Two-Component Organocatalyst System Using a Chiral N-Heterocyclic
Carbene and Redox-Active Riboflavin
Soichiro Iwahana, Hiroki Iida,* and Eiji Yashima*[a]
There has been a dramatic increase in the research of or-
ganocatalysts for various transformations in the past decade
due to their environmental acceptability, high selectivity,
and efficiency.[1] Among them, N-heterocyclic carbenes
(NHCs) derived from azolium salts have been employed as
versatile organocatalysts in a broad range of chemical trans-
formations including the benzoin condensation, the Stetter
reaction, and the oxidative esterification or amidation of al-
dehydes, and a number of enantioselective versions of the
reactions have appeared.[2] In particular, the one-pot oxida-
tive esterification and amidation of aldehydes that consists
esterification of the aldehydes. Since molecular oxygen is an
inexpensive and environmentally friendly oxidant that ful-
fills the requirements of green chemistry, the development
of versatile and selective catalytic reactions using molecular
oxygen remains a great challenge in synthetic organic
chemistry.[13] However, to the best of our knowledge, the
NHC/flavin-catalyzed acylation with aldehydes is limited to
alcohols and the enantioselective approach has never been
reported. Herein, we report the first enantioselective oxida-
tive esterification of unfunctionalized aldehydes mediated
by a two-component organocatalyst, a chiral triazolium-de-
rived NHC, and a redox-active flavin derived from ribofla-
vin (vitamin B2). We also demonstrate that the present
system can be applied to the unprecedented thioesterifica-
tion and amidation of aldehydes with a thiol and an amine,
respectively.
We first investigated the oxidative esterification of benzal-
dehyde, with racemic 1-phenylethanol as a solvent, by using
a series of achiral and chiral azolium salts 1–5 as potential
catalysts in the presence of a riboflavin derivative 6 and trie-
thylamine (Et3N) under air (Scheme 1 and the Supporting
Information, Table S1). Among the azolium salts tested,
thiazoliums 1, 2, and an optically active triazolium (S,R)-5
produced only a trace amount of the desired 1-phenylethyl
benzoate. However, we found that the triazolium salts of
achiral 3 and chiral (S,R)-4 with an electron-withdrawing
perfluorophenyl substituent served as a precatalyst for the
À
À
of two separate reactions, an oxidation and C O and C N
bond formations, respectively, have recently received grow-
ing attention as an attractive and promising synthetic tool; it
can avoid the time consuming and costly multistep processes
involved in the purification of intermediates between the
steps, resulting in increased efficiency and decreased waste
generation.[3] Although the NHC-catalyzed oxidative
esterification of aldehydes has been reported,[4,5] the corre-
sponding thioesterification and amidation are limited.[6,7]
Moreover, only a few enantioselective esterifications have
been demonstrated that require a stoichiometric oxidation
[4a,d]
with an excess amount of MnO2
or internal redox reac-
tions of functionalized aldehydes, such as a-halo aldehydes
and a,b-unsaturated aldehydes.[5b,c,e,i,j]
In 1980, as an extension of the model studies on the coen-
zyme thiamine (vitamin B1), in which the catalytically active
species has been proposed to be a NHC,[8] Yano[9] and Shin-
kai[10] independently reported a biomimetic two-component
organocatalytic system for the oxidative esterification; a
combination of an achiral thiazolium-derived NHC and a
redox-active flavin were found to catalyze the direct esterifi-
cation of the unfunctionalized aldehydes under air.[11] Due
to the electron-transfer catalysis of flavins,[12] the molecular
oxygen (O2) could be used as the terminal oxidant for the
[a] S. Iwahana, Dr. H. Iida, Prof. Dr. E. Yashima
Department of Molecular Design and Engineering
Graduate School of Engineering, Nagoya University
Chikusa-ku, Nagoya 464-8603 (Japan)
Fax : (+81)52-789-3185
E-mail: iida@apchem.nagoya-u.ac.jp
yashima@apchem.nagoya-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100737.
Scheme 1. Oxidative esterification of benzaldehyde with various azolium
catalysts 1–5 and a riboflavin co-catalyst 6.
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oxidative esterification despite the moderate yield (24 and
21%, respectively). We note that the (S,R)-4 catalyzed the
esterification in an enantioselective fashion, giving the ester
with 40% enantiomeric excess (ee), whereas the achiral 3
combined with the optically active riboflavin 6 showed no
enantioselectivity, suggesting that the enantioselectivity ob-
served in the present oxidative esterification in esters is to-
tally controlled by the chirality of the NHC catalyst.
With these results in hand, we then applied the enantiose-
lective oxidative esterification, catalyzed by the two-compo-
nent organocatalyst system, to the kinetic resolution of vari-
ous racemic secondary alcohols (Table 1). The kinetic reso-
spectively, based on benzaldehyde) after 3 h (Table 1,
entry 7).
The two-component organocatalytic oxidation system de-
veloped for the kinetic resolution of the racemic alcohols
was then extended to the desymmetrization of a meso-diol.
The asymmetric benzoylation of cis-1,2-cyclohexanediol
with (S,R)-4 in the presence of 6 produced the (1S,2R)-mon-
obenzoate with 64% ee, although the generation of the
meso-dibenzoate caused a modest yield of isolated product
(30%, Scheme 2). In contrast, the desymmetrization cata-
lyzed by the corresponding enantiomeric triazolium salt
(R,S)-4 afforded the (1R,2S)-monobenzoate with a similar
enantioselectivity
(62% ee,
Scheme 2), supporting that the
chirality of 6 plays no role in
the enantioselectivity on the
acylation of the alcohol.
Table 1. Kinetic resolution of racemic alcohol.[a]
The chiral active species for
the enantioselective acylation of
alcohols can be proposed to be
Entry Alcohol
Aldehyde
[R1]
t
Conversion of ee value of
s[c]
[h] alcohol [%][b] unreacted alcohol [%][b]
1[d]
2
3
Ar=Ph
Ar=1-naphthyl Ph
Ar=2-naphthyl Ph
Ar=Ph
Ar=Ph
Ph
4
4
4
65
55
72
43 (R)
44 (R)
66 (R)
32 (R)
39 (R)
75 (1R,2R)
>99 (1R,2R)
90 (1R,2R)
–
2.3 (2.2)[e]
an
acyl
intermediate
9
(Scheme 3). The NHC 7, gener-
ated by the deprotonation of 4,
undergoes nucleophilic attack
on the benzaldehyde to afford
the Breslow intermediate 8,
which reacts again with
second benzaldehyde to yield
benzoin (benzoin condensation
3.2
3.1
2.6
3.7
5.6
–
6.0
–
–
4
1-naphthyl 19 50
2-naphthyl 19 47
Ph
5
6[f]
1
3
3
62
78
7[f]
8[i]
9[k]
Ph[g]
Ph
Ph
70[h]
a
Ar=Ph
Ar=Ph
24 ca. 0[j]
24 ca. 0[l]
–
pathway
in
Scheme 3).[2c,e,f]
[a] The kinetic resolution of alcohol (0.5 m) was carried out in the presence of (S,R)-4 (10 mol%), 6
(10 mol%), aldehyde (10 equiv), Et3N (0.5 equiv), diethylene glycol dimethyl ether (0.5 equiv, internal stan-
dard), and 4 ꢁ molecular sieves (MS 4ꢁ) in toluene at room temperature under air. [b] Determined by chiral
However, in the presence of the
flavin catalyst 6, the in-situ-gen-
erated 8 is immediately oxidized
to give the chiral acylation re-
agent 9 by the flavin-catalyzed
oxidation in which the reduced
flavin 6red catalytically regener-
GC. [c] The selectivity factor (s) was determined using the following equation; s=krel(fast/slow) =ln
(1Àee)]/ln[(1ÀC)(1+ee)], where C=conversion. [d] The values represent an average of four runs. [e] Deter-
mined using the yield and ee value of the ester produced; s=ln
ACHTUNGTRENNUNG
G
N
ACHTUNGTRENNUNG
[1ÀC(1+eeester)]/ln
ACHTUNGTRENNUNG
yield of ester. The value represents an average of three runs. [f] In CHCl3. [g] Benzaldehyde (2 equiv) was
used. [h] Benzoic acid and benzoin (45 and 6% yields based on benzaldehyde, respectively) were generated as
byproducts. [i] Without 6. [j] Benzoin (70% ee (R)) was isolated in 45% yield based on benzaldehyde.
[k] Under N2. [l] Benzoin (70% ee (R)) was isolated in 79% yield based on benzaldehyde.
ates
6 through the electron
lution of 1-phenylethanol, 1-(1-naphthyl)ethanol, and 1-(2-
naphthyl)ethanol provided modest, but promising selectivity
factors (s) of 2.3, 3.2, and 3.1, respectively (Table 1, en-
tries 1–3), although the enantioselectivity was lower than
that reported in the kinetic resolution based on the transes-
terification reaction catalyzed by chiral imidazolium-derived
NHCs.[14,15] Analogous aromatic alcohols and aldehydes
were subjected to the kinetic resolution, leading to similar
levels of selectivity (Table 1, entries 4 and 5). A remarkable
increase in the enantioselectivity was observed in the kinetic
resolution of trans-1,2-cyclohexanediol, in which the optical-
ly pure alcohol (>99% ee) was left at 78% conversion
(Table 1, entry 6). Although 10 equivalents of benzaldehyde
were used to accelerate the reaction, two equivalents were
enough to achieve the kinetic resolution, in which half the
amount of benzaldehyde was consumed for the side reaction
to give benzoic acid and benzoin (45% and 6% yields, re-
Scheme 2. Desymmetrization of cis-1,2-cyclohexanediol.
transfer process from molecular oxygen. To corroborate the
proposed mechanism, the enantioselective esterification was
performed without the flavin catalyst 6 (Table 1, entry 8).
As anticipated, the esterification did not proceed at all, but
instead the optically active benzoin was isolated in 45%
yield and with 70% ee. This result clearly indicates that the
oxidation of 8 with 6 plays an essential role in the oxidative
esterification. Interestingly, the reaction under nitrogen also
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Oxidative Esterification, Thioesterification, and Amidation of Aldehydes
COMMUNICATION
during the kinetic resolution,[17] the ee value of the unreact-
ed benzoin after the tandem reaction remains relatively
high, whereas the product shows a lower ee value.
The NHC-catalyzed thioesterification is of great impor-
tance in enzymatic reactions,[7] but only a few artificial
NHC-catalyzed thioesterification have been developed.[5g,7]
The thiol acts as a reactive nucleophile in the present two-
component organocatalyst system and the oxidative thioes-
terification of benzaldehyde with 1-phenylethanethiol in the
presence of (S,R)-4 and 6 took place to afford the corre-
sponding thioester in 58% yield with low enantioselectivity
(Table 2, entry 1). However, an organocatalytic highly enan-
tioselective thioesterification has recently been achieved.[18]
Table 2. Oxidative thioesterification and amidation.[a]
Entry
Nucleophile
Product
Yield
[%][b]
ee
[%][c]
Scheme 3. Plausible catalytic cycles of the two-component organocatalyst
system.
1
58
9
2
83
10
3
9
produced optically active benzoin (79% yield, 70% ee)
without any consumption of the alcohol (Table 1, entry 9),
whereas the oxidative esterification proceeded under the
aerobic conditions (Table 1, entry 1). As a consequence, we
were able to switch the reaction pathways either to the oxi-
dative esterification or the benzoin condensation by chang-
ing the reaction atmosphere, air, or nitrogen, which allows
ON/OFF switching of the flavin catalytic activity.
Taking advantage of the preset unique two-component
asymmetric organocatalyst system along with the ON/OFF
switching of the flavin activity, we then applied this system
to the one-pot oxidative tandem reaction of benzaldehyde
to benzoyl benzoin (11) (Scheme 4). First, the reaction of
3[d]
4
71
73
–
–
5[e]
PhNH2
[a] Thioesterification was carried out in the presence of a thiol (0.5m),
(S,R)-4 (10 mol%), (10 mol%), benzaldehyde (2 equiv), Et3N
(0.5 equiv), and 4ꢁ MS in CHCl3 at room temperature for 24 h under
air. Amidation was carried out in the presence of an amine (0.25m),
(S,R)-4 (30 mol%),
(2 equiv), DABCO (1.4 equiv), and 4 ꢁ MS in CH2Cl2 at room tempera-
ture for 24 h under air. [b] Yield of isolated products. [c] Determined by
chiral HPLC. [d] Without HOAt. [e] Reaction time was 57 h.
6
6 (30 mol%), HOAt (30 mol%), benzaldehyde
To further evaluate the substrate scope for the present
acylation system, we examined the amidation of benzalde-
hyde with alkyl and aryl amines in the presence of an achiral
co-catalyst, 1-hydroxyl-7-azabenzotriazole (HOAt).[6a] The
two-component organocatalytic amidation of 1-phenylethyl-
amine with HOAt successfully proceeded, yielding the cor-
responding amide in 83% yield, although a high catalyst
loading was required (Table 2, entry 2). On the other hand,
only a small amount of the amide (10% yield) was obtained
in the absence of HOAt (Table 2, entry 3). Rovis and co-
workers demonstrated that the co-catalyst HOAt probably
generated an activated ester, which was further converted to
the corresponding amide by the nucleophilic attack of the
amine.[6a] Therefore, the present acylation of amines might
proceed through an achiral activated carboxylate 10 generat-
ed by the acyl transfer reaction of HOAt with 9 (Scheme 3),
thus affording almost racemic amides. However, the direct
amidation without HOAt also took place in an enantioselec-
tive fashion as in the case for the esterification and thioes-
terification reactions (Table 2, entry 3), suggesting that a
more efficient enantioselective direct amidation may be pos-
Scheme 4. Tandem reaction of benzaldehyde.
benzaldehyde was performed in the presence of (S,R)-4 and
6 under nitrogen, resulting in the preferential formation of
optically active R-rich benzoin through the asymmetric ben-
zoin condensation. After 30 min, the reaction was continued
under air, which promoted the oxidative esterification of the
generated (R)-benzoin with benzaldehyde, thus finally pro-
ducing R-enriched 11 in 22% yield and with 66% ee togeth-
er with a trace amount of R-enriched benzoin (0.6% yield,
98% ee).[16] This tandem reaction consists of two enantiose-
lective reactions, that is, the asymmetric benzoin condensa-
tion followed by the enantioselective benzoylation of the
chiral benzoin produced. Because the in-situ-generated ben-
zoin was rich in the R-enantiomer and the subsequent ben-
zoylation preferentially takes place with the (S)-benzoin
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E. Yashima, H. Iida and S. Iwahana
lar Functions” of the Ministry of Education, Culture, Sports, Science, and
Technology, Japan.
sible. The two-component catalyst system using HOAt also
promoted the amidation of benzyl amine and aniline, yield-
ing the corresponding achiral amides in moderate yields
(Table 2, entries 4 and 5), whereas the triazolium salt 3 was
not an effective amidation catalyst.
Keywords: acylation · carbenes · enantioselectivity · flavins ·
In conclusion, we have developed the enantioselective ox-
idative esterification of aldehydes with alcohols based on
the two-component organocatalysts, chiral NHC, and ribo-
flavin-derived catalysts. This unique two-component organo-
catalytic system, employing molecular oxygen as an environ-
mentally benign terminal oxidant, mediated the enantiose-
lective acylation of alcohols, and this acylation promoted
the kinetic resolution of racemic alcohols and the desym-
metrization of a meso-diol. Moreover, the present acylation
could be applied to the oxidative thioesterification of thiols
and amidation of amines, although the selectivity for the
enantioselective processes (as well as the efficiency) is still
modest or low at this stage. However, we believe that NHC
catalysis coupled with the flavin-catalyzed aerobic oxidation
system will lead to the development of further unique and
more efficient tandem reactions promoted by multicatalyst
systems as observed in the cell.[19]
organocatalysis
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Experimental Section
Typical procedure for kinetic resolution of racemic alcohols: Compounds
(S,R)-4 (5.9 mg, 0.013 mmol), 6 (8.0 mg, 0.013 mmol), trans-1,2-cyclohexa-
nediol (14.6 mg, 0.125 mmol), and 4 ꢁ molecular sieves (MS 4ꢁ, ca.
9 mg) were placed in a 5 mL flask equipped with a stopcock. A balloon
filled with dry air, introduced through a calcium chloride tube, was at-
tached to the stopcock. The flask was then evacuated on a vacuum line
and flushed with air from the balloon. Anhydrous CHCl3 (0.25 mL), di-
ethylene glycol dimethyl ether (8.8 mL, 0.062 mmol), Et3N (8.7 mL,
0.063 mmol), and benzaldehyde (0.127 mL, 1.25 mmol) were then added
and the mixture was stirred at room temperature. The reaction was moni-
tored by GC to determine the yield of the ester and the conversion and
ee value of the unreacted alcohol. The results were summarized in
Table 1.
Typical procedure for oxidative amidation of benzaldehyde: Compounds
(S,R)-4 (17.5 mg, 0.037 mmol),
6 (23.8 mg, 0.038 mmol), DABCO
(19.6 mg, 0.174 mmol), HOAt (5.1 mg, 0.037 mmol), and MS 4ꢁ (ca.
9 mg) were added to a 5 mL flask equipped with a stopcock, and placed
under nitrogen. A balloon filled with dry air, introduced through a calci-
um chloride tube, was attached to the stopcock. The flask was then evac-
uated on a vacuum line and flushed with air from the balloon. Anhydrous
CH2Cl2 (0.5 mL), benzaldehyde (25.5 mL, 0.25 mmol), and rac-1-phenyle-
thylamine (15.8 mL, 0.125 mmol), were then added and the mixture was
stirred at room temperature. After 24 h, CHCl3 (20 mL) was added to
the mixture and the CHCl3 layer was washed with water (3ꢂ20 mL) and
then dried over anhydrous MgSO4. After filtration, the solvent was
evaporated to dryness. The residue was purified by column chromatogra-
phy (SiO2, hexane/AcOEt=1:0 to 3:17) to afford S-enriched N-(1-phe-
nylethyl)benzamide (23.4 mg, 83.1%, 3% ee) as a white solid. The results
are summarized in Table 2.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research (S)
from the Japan Society for the Promotion of Science (JSPS), and the
Global COE Program “Elucidation and Design of Materials and Molecu-
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Oxidative Esterification, Thioesterification, and Amidation of Aldehydes
COMMUNICATION
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Received: March 10, 2011
Published online: June 7, 2011
Chem. Eur. J. 2011, 17, 8009 – 8013
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8013