Minor enantiomer recycling: Metal catalyst, organocatalyst and biocatalyst working in concert

Article abstract of DOI:10.1002/chem.200901338

A minor enantiomer recycling one-pot procedure employing two reinforcing chiral catalysts has been developed. Continuous regeneration of the achiral starting material is effected via selective enzyme-catalyzed hydrolysis of the minor product enantiomer fr

Full text of DOI:10.1002/chem.200901338

FULL PAPER
DOI: 10.1002/chem.200901338
Minor Enantiomer Recycling: Metal Catalyst, Organocatalyst and Biocatalyst
Working in Concert
Erica Wingstrand,[a] Anna Laurell,[a] Linda Fransson,[b] Karl Hult,[b] and
Christina Moberg*[a]
Abstract: A minor enantiomer recy-
cling one-pot procedure employing two
reinforcing chiral catalysts has been de-
veloped. Continuous regeneration of
the achiral starting material is effected
via selective enzyme-catalyzed hydroly-
sis of the minor product enantiomer
from Lewis acid–Lewis base catalyzed
addition of acyl cyanides to prochiral
aldehydes in
a
two-phase solvent
obtained in the direct process, and in
high yields. A combination of a (S,S)-
salen Ti Lewis acid and Candida ant-
arctica lipase B provides the products
with R absolute configuration, whereas
the opposite enantiomer is obtained
from the (R,R)-salen Ti complex and
Candida rugosa lipase.
system. The process provides O-acylat-
ed cyanohydrins in close to perfect
enantioselectivities, higher than those
Keywords: asymmetric catalysis
enantioselectivity · enzyme cataly-
sis · Lewis acids · organocatalysis
·
Introduction
pose, but selective chemical transformation of the minor,
undesired enantiomer via a subsequent kinetic resolution to
a compound which easily can be separated from the desired
product has occasionally proven to be a useful option.[4] This
additional step may lead to high enantiopurity, but inevita-
bly results in loss of product, and consequently to lower
yields.
A process allowing the minor product enantiomer to be
recycled to the prochiral reactant, and thereby given anoth-
er chance in the catalytic reaction, would circumvent this
problem and would in principle allow quantitative conver-
sion of the prochiral compound to a single enantiomer
(Figure 1). However, for this to be realistic, a thermodynam-
ic driving force, allowing for a unidirectional cycle, is re-
quired.[5]
Major advances in asymmetric synthesis using metal cata-
lysts,[1] organocatalysts[2] or biocatalysts[3] have resulted in a
multitude of efficient methods for the preparation of chiral
enantiomerically enriched compounds with large structural
variations in both laboratory and industrial scale. The prod-
uct enantiomeric ratio obtained in the catalytic reactions is
determined by the difference in free energy of activation for
the formation of the two enantiomers. Each reactant has
one single chance to be transformed to the desired enantio-
mer, and reactions passing via the higher transition state
barrier lead to formation of the “wrong” enantiomer. Such
“mistakes” by the catalyst can be corrected by destroying
the undesired enantiomer in the subsequent purification;
this is often necessary in order to achieve the high enantio-
purity, often >99% ee, which is required for many applica-
tions. Crystallization is most commonly used for this pur-
[a] Dr. E. Wingstrand, A. Laurell, Prof. C. Moberg
KTH School of Chemical Science and Engineering
Organic Chemistry, 10044 Stockholm (Sweden)
Fax : (+46)8-791-2333
Figure 1. Minor enantiomer recycling.
E-mail: kimo@kth.se
From basic kinetic principles it follows that use of chiral
catalysts for both the transformation and the regeneration
of the reactant would lead to an enantioselectivity which is
higher than that of each individual step. Thus, the resulting
enantiomeric ratio e.r. of the product will benefit from the
two selective steps:
[b] Dr. L. Fransson, Prof. K. Hult
KTH School of Biotechnology, Department of Biochemistry
AlbaNova University Center, 10691 Stockholm (Sweden)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901338: Experimental proce-
dures and analytical data.
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Generally, the enantiomeric ratio e.r. achieved in a reac-
tion is defined as
chiral aldehydes, producing highly enantioenriched O-acy-
lated cyanohydrins [Scheme 1, Eq. (1)][6] which serve as both
synthetic intermediates[7] and versatile target compounds.[8]
Biocatalytic methods available for this transformation pro-
½Rꢀ
ð1Þ
e:r: ¼
½Sꢀ
and is dependent on the enan-
tioselectivity E of the catalyst
used. In a simple transforma-
tion of an achiral substrate the
enantiomeric ratio e.r. is con-
stant and equal to E.
e:r: ¼ E
ð2Þ
Scheme 1. Lewis acid-Lewis base catalyzed acylcyanation (1) and enzyme catalyzed hydrolysis (alcoholysis) of
O-acylated cyanohydrin (2).
When the selectivity of one
catalyst is reinforced by the
action of another, the resulting
enantiomeric ratio will be a function of the enantioselectivi-
ties of both catalysts. Let Echem and Eenz denote the enantio-
selectivities of a chemical catalyst and a reinforcing enzyme,
respectively, defined as:
ceed by dynamic kinetic resolution of in situ formed cyano-
hydrins, often with moderate yields and selectivities.[9] In
contrast, the reverse reaction, the hydrolysis or alcoholysis
of acylated cyanohydrins catalyzed by Candida antarctica
lipase B (CALB) is known to proceed with high selectivity
for the S enantiomer [Scheme 1, Eq. (2)],[10] a circumstance
that we exploited for the determination of enantiomeric
purity of several acylated cyanohydrins[11] and which was
subsequently used for the destruction of one enantiomer, re-
sulting in an increased enantiomeric purity of the product.[12]
We considered a process combining the reactions in
Scheme 1 to be a suitable candidate for the proof of concept
of Figure 1. A process with the forward Lewis acid–Lewis
base catalyzed reaction and backwards enzymatic hydrolysis
proceeding simultaneously would take advantage of the
atom efficiency and high yield of the former process to
induce chirality, and the selectivity of the latter to enhance
the enantiomeric ratio. It would constitute a dynamic kinetic
resolution of the product (Figure 1).
kchem;R
kchem;S
Echem
¼
ð3Þ
ð4Þ
ðkcat=KMÞ
ðkcat=KMÞRS
Eenz
¼
In a system utilizing both catalysts, the rate equations for R-
and S-product formation become (assuming the metal-cata-
lyzed reaction to follow second-order kinetics):
ꢀ
ꢁ
d½Rꢀ
dt
kcat
KM
¼ kchem;R½AcCNꢀ½aldehydeꢀ½Tiꢀ ꢁ
½Rꢀ½Enzꢀ
ð5Þ
ð6Þ
R
ꢀ
ꢁ
d½Sꢀ
dt
kcat
KM
¼ kchem;S½AcCNꢀ½aldehydeꢀ½Tiꢀ ꢁ
½Sꢀ½Enzꢀ
S
where [Enz] and [Ti] denote the concentration of free
enzyme and chemical catalyst, respectively. At steady state,
Results
d½Rꢀ
dt
d½Sꢀ
dt
where
¼
¼ 0,
To allow the forward reaction, catalyzed by a chiral titanium
salen complex[13] and a Lewis base, and the Candida antarcti-
ca lipase B catalyzed hydrolysis to proceed in concert, suita-
ble experimental conditions were needed. The optimal con-
ditions for the Lewis acid–Lewis base catalyzed reaction use
dichloromethane as solvent and a reaction temperature of
ꢁ408C,[6] whereas those for the enzymatic kinetic resolution
use propanol/toluene and 608C.[10] None of these conditions
is suitable for a cyclic process, since at ꢁ408C the enzyme
does not have the required reactivity, and at 608C the acyl-
cyanation is non-selective. Furthermore, in order to allow
unidirectional recycling, a mass flow through the system
providing a thermodynamic driving force is required, since a
closed reversible reaction network would lead to racemic
product. Replacement of propanol for water in the enzymat-
ic process leads to acetic acid in place of propyl acetate and,
½Rꢀ kchem;Rðkcat=KMÞ
½Sꢀ kchem;Sðkcat=KMÞRS
¼
ð7Þ
and thus
e:r: ¼ EchemEenz
ð8Þ
The selectivities of both catalysts will thus contribute to in-
creased enantiopurity and yield.
In order to realize a minor enantiomer recycling process,
catalytic reactions proceeding in both forward and backward
directions are needed. We recently described a catalytic
system consisting of a chiral Lewis acid and an achiral or
chiral Lewis base for the addition of a-ketonitriles to pro-
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Enantiomer Recycling
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under neutral or basic conditions, to acetate; formation of
acetate and HCN from acetyl cyanide and water should
ensure sufficient gain in energy to allow the desired recy-
cling (Scheme 2).
tially racemic product, 53:47 e.r., with 20 mol%) but, due to
the presence of the enzyme, the enantiomeric ratio of the
product increased gradually as the minor enantiomer was re-
cycled, at the same time as the total yield of the two enan-
tiomers remained constant due to the balancing forward re-
action. After complete addition of acetyl cyanide (a total of
3 equiv), the ratio of product enantiomers still increased
(from 96.1:3.9 to 99.9:0.1), while product formation gradual-
ly ceased due to the decomposition of acetyl cyanide, result-
ing in a final yield of 80%.
We decided to optimize the reaction conditions using ben-
zaldehyde as the substrate. We noted that the forward reac-
tion resumed upon continued slow addition of acetyl cya-
nide; after addition of a total of four equivalents of acetyl
cyanide the yield of product with enantiomeric ratio of
99.6:0.4 was 87%. Somewhat higher initial enantioselectivity
(79:21 e.r.) was observed when the forward reaction was run
at room temperature in the presence of 1.1 equivalents of
acetyl cyanide and 5 mol% each of the Ti dimer and 4-(di-
methylamino)pyridine (DMAP) without CALB, which was
added together with buffer only after 9 h. Further addition
of acetyl cyanide (up to a total of 3 equiv) during 16.5 h at
408C again resulted in both increased yield (from an initial
83 to 95%) and enantiomeric ratio (98:2). Stirring for an ad-
ditional 5 h resulted in a yield of 92% and 99.5:0.5 e.r. A
final yield of 97% of a product with 99.8:0.2 enantiomeric
ratio was obtained after the addition of a total of 4.5 equiva-
lents of acetyl cyanide and a total reaction time of 5.5 days
(Figure 2, Scheme 3). This can be compared with previous
results from the preparation of the same compound, 96%
yield and 94% ee from the Lewis acid–Lewis base catalyzed
acetylcyanation, 97% yield and 98% ee from enzymatic hy-
drolysis of enzyme catalyzed acetylation of the cyanohy-
drin,[9b] and 84% yield and 97% ee from enzymatic hydroly-
sis of a scalemic mixture of acetylated cyanohydrins.[12]
Scheme 2. Catalytic cycle for transformation and regeneration of reac-
tant.
The reaction between benzaldehyde (1a) and acetyl cya-
nide (2) was used in attempts to find reaction conditions al-
lowing the forward and backward reactions to proceed si-
multaneously (Scheme 2). By monitoring the ratio of prod-
uct enantiomers, (R)-3a and (S)-3a, and conversion with gas
chromatography, both processes could be followed over
time. After having screened a variety of solvents and solvent
mixtures, a two-phase system containing water and an or-
ganic solvent of lower density than water was found to allow
both processes to occur simultaneously. Toluene/buffer pH 8
(aq) was finally found to be the best choice. A buffer was
needed to neutralize the acetic
acid formed in the hydrolysis.
When simply mixing the re-
agents, the enzymatic hydroly-
sis proceeded well, as shown
by a high ratio of product en-
antiomers, but chemical yields
were low due to decomposition
of acetyl cyanide to acetic acid.
The problem was solved by
continuous addition of the acyl
cyanide. For the initial experi-
ments, a reaction temperature
of 408C was chosen in order to
obtain a reasonable rate for
the enzymatic reaction. Under
these conditions, with 1,8-
diazabicycloACHTUNGTRENNUNG[5.4.0]undec-7-ene
(DBU) as base, the enantiose-
lectivity of the metal-catalyzed
process was low (62:38 e.r.
with 10 mol% DBU and essen-
Figure 2. Enantiomeric excess (c) and total yield (a) of 3a as a function of time and amount of 2 (g)
added.
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C. Moberg et al.
(E=15). We were therefore pleased to find that the Lewis
acid–Lewis base–enzyme catalyzed recycling process pro-
ceeded with excellent results; the addition of a total amount
of 4.2 equivalents of acetyl cyanide over 110 h and a total
reaction time of 133 h gave acetylated cyanohydrin 3d in
88% yield and an enatiomeric ratio of 99.7:0.3. This can be
compared to the Lewis acid–Lewis base catalyzed process
which gave the same product in 89% yield but with an en-
antiomeric ratio of merely 95:5.[6b] Similar conditions for the
minor enantiomer recycling reaction with octanal, which un-
dergoes CALB-catalyzed hydrolysis with considerably
higher selectivity than hexanal (E > 1000) gave 3e in 84%
yield and 99.3:0.7 e.r.
The acetylated cyanohydrin from furfural, 3 f, was pre-
pared in racemic form in the same way as 3d and exposed
to CALB. The hydrolysis was only slightly more selective
than that of aliphatic derivative 3d, with hydrolysis of the R
enantiomer occurring at an appreciable rate (E=26). Even
in the absence of the enzyme, slow hydrolysis was observed
for racemic 3 f in toluene/buffer (aq, pH 8) and, to mimic
the reaction conditions, two equivalents of acetic acid; after
5 h at 408C 93% product remained, and after 60 h 84%.
The recycling procedure with 1 f provided the acylated cy-
anohydrin with R configuration with high enantiomeric
ratios of >97:3, but with moderate yields under conditions
employed for aldehyde 1c (addition of 3 equiv of acetyl cya-
nide over 22–25 h). Moreover, the yield decreased over
time, probably due to polymerization of furfural. Addition
over a shorter period of time, 5 h 45 min, followed by stir-
ring for an additional 2 h provided superior results, 81%
yield and 98.6:1.4 e.r. The optimal conditions found consist-
ed of 12 h reaction at ꢁ408C and 1 h at room temperature
before addition of CALB and buffer followed by addition of
acetyl cyanide over 6 h, and finally 4 h of stirring; under
these conditions (R)-3 f was obtained in 86% yield and
99.1:0.9 e.r. Addition of a larger excess of acetyl cyanide
may require a stronger buffer in order to avoid decomposi-
tion of the aldehyde.
Scheme 3. Minor enantiomer recycling of prochiral aldehydes. i) 14 h at
RT before addition of buffer and CALB; ii) 2 h at ꢁ408C and 1 h at RT
before addition of buffer and CALB.
Reactions with other aldehydes: In order to extend the
scope of the recycling process, we decided to subject a few
other aldehydes to the reaction conditions used for 1a. Ki-
netic resolution of several aromatic aldehydes, with elec-
tron-donating as well as electron-attracting substituents, is
known to proceed with high selectivity in the presence of
CALB in a propanol/toluene mixture.[10b] Slow addition of
acetyl cyanide to 4-methoxybenzene and 4-chlorobenzene in
the presence of the Lewis acid–Lewis base–enzyme catalytic
system under the non-optimized conditions initially used for
benzaldehyde (3 equiv of acetyl cyanide) gave the expected
products, (R)-3b and (R)-3c, with high selectivity, in enan-
tiomeric ratios 99.1:0.9 and 99.6:0.4, respectively
(Scheme 3).
In contrast, kinetic resolution of aliphatic analogues using
the same enzyme has been reported to result in low selectiv-
ity,[10] and, probably as a consequence of this low selectivity,
we had previously encountered problems with enzymatic de-
termination of products from aliphatic aldehydes.[11] Treat-
ment of a racemic mixture of 3d, prepared by reaction of
acetyl cyanide and hexanal at ambient temperature in the
presence of triethylamine, with CALB in a toluene/buffer
(aq, pH 8) system, that is, the same conditions as those used
in the recycling process, verified the lower selectivity in the
enzymatic hydrolysis of the aliphatic substrate; the hydroly-
sis of the S enantiomer proceeded rather slowly, at the same
time as slow hydrolysis of the R enantiomer was observed
Butanoyl cyanide: Due to the importance of acylated cyano-
hydrins with different acyl groups it was considered to be of
interest to study whether the recycling process could be per-
formed with some additional ketonitrile. Exchange of acetyl
cyanide for butanoyl cyanide did indeed give butanoyl ester
3g in good yield with high enantioselectivity (Scheme 3).
Opposite enantiomer: For a catalytic process to work satis-
factory, it should provide access to both enantiomers of the
product. Two enzyme preparations, known as Candida
rugosa lipase (CRL) and Candida cylindracea lipase (CCL),
were found to exhibit the opposite selectivity in the hydroly-
sis of 3a, thus preferentially transforming the R enantiomer
to the aldehyde, albeit with moderate selectivity (E=29 and
23 for CRL and CCL, respectively).[14] Having identified
these two enzymes, the recycling procedure was studied
using the Ti complex with opposite, that is, (R,R), absolute
configuration. Due to slow enzymatic hydrolysis, longer re-
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Enantiomer Recycling
FULL PAPER
action times were needed than in reactions using CALB.
Under non-optimized conditions 92% yield of O-acetyl-(S)-
2-hydroxy-2-phenylacetonitrile with 95.2:4.8 e.r. was ob-
tained using CRL and 93% yield of the same product with
94.0:6.0 e.r. using CCL, in both cases after 61 h. The enan-
tiomeric ratios were improved by continued stirring without
further addition of acetyl cyanide, but at the same time
gradually decreasing yields were observed; after stirring for
a further five days, the yields were 71 and 72% and enantio-
meric ratios 98.2:1.8 and 98.1:1.9, respectively.
Reactions with one chiral catalyst: At the high temperature
used (408C), the Lewis base can serve as the sole catalyst
for the acetylcyanation. Although racemic product is formed
initially (and the selectivity thus determined only by the
enzyme), the recycling process results in high final enantio-
selectivities (99:1 e.r. in reaction with benzaldehyde), albeit
in somewhat lower yield (77%) than that observed in the
presence of Lewis acid. A higher number of cycles and
more acyl cyanide are also required; a reaction run under
conditions where the metal catalyzed reaction results in an
e.r. of 80:20 and the enzymatic reaction proceeds with per-
fect selectivity gives a 99.8:0.2 ratio of the product enantio-
mers after four cycles and consumes 1.25 equivalents of acyl
cyanide, while nine cycles and 1.99 equivalents of acyl cya-
nide are required to achieve the same enantiomeric purity
using only the achiral Lewis base.
Figure 3. Enantioselective processes by combined use of synthetic re-
agents or catalysts and biocatalysts.
proach, one of the enantiomers of a racemic mixture can se-
lectively be reacted by an enzyme to an achiral compound,
which is then transformed to a racemic mixture of the origi-
nal compound by an achiral stoichiometric organic reagent.
This process, called cyclic deracemization,[16] results in dera-
cemization of the original racemic mixture (Figure 3C).[17]
In contrast to our procedure, which constitutes a chemical
transformation, in cyclic deracemization, as expressed by its
pioneer, “there is no product, but simply an optical enrich-
ment of the substrate”.[18] Even if the cycle in principle
could be entered via the prochiral intermediate, this possi-
bility has not been demonstrated and would, at least in the
examples reported so far, be hampered by the instability of
this intermediate. In this case, the selectivity relies on a
single chiral catalyst.
Discussion
Previous methodologies describe the quantitative transfor-
mation of racemic mixtures to enantioenriched products
using one enantioselective catalyst, while the present
method is a dynamic transformation using two reinforcing
enantioselective catalysts and starting from an achiral sub-
strate. We describe a conceptually new procedure in which
two reinforcing chiral catalysts are used. In the present ex-
ample the enantioselectivity of one catalyst is low (E=1.4)
whereas that of the other is high (E >500). The reinforcing
effect is therefore difficult to demonstrate experimentally in
this particular case, which however serves as a proof-of-con-
cept and shows that minor enantiomer recycling is experi-
mentally feasible.
Reinforced enantioselectivity by using two chiral catalysts:
In the present study, the selectivity of a chiral metal catalyst
is reinforced by an enantioselective enzyme, which recycles
the minor product enantiomer to starting material. The com-
bination of two catalysts in this recycling system results in
an enantiomeric ratio of the product that is the product of
the individual selectivities of the two catalysts. Further, the
recycling reaches a kinetic equilibrium. Without recycling,
the enzymatic resolution step inevitably consumes both the
major and the minor product. The recycling of product to
starting material therefore contributes to an increased yield.
Comparison to other processes: The process developed here
(Figure 3A) is different from established procedures in
which biocatalysts and metal catalysts or synthetic reagents
have been used in combination. In those cases an enzyme
catalyzes a chemical transformation of one of the enantio-
mers of a racemic mixture, whereas in the present case a
minor product enantiomer is recycled to achiral reactant. In
addition, in our minor enantiomer recycling procedure, two
chiral catalysts are employed. In dynamic kinetic resolu-
tions, an enzyme catalyzes a transformation of one enantio-
mer and the role of the metal complex is to racemize the re-
maining substrate (Figure 3B).[15] In contrast to our minor
enantiomer recycling, the selectivity in this process relies
only on the selectivity of the biocatalyst. In a different ap-
Conclusions
In conclusion, we have found a one-pot minor enantiomer
recycling procedure in which the enantioselectivity of a
metal-catalyzed reaction is improved by a biocatalyst with
concomitant increase in yield of the desired enantiomer.
The combined use of the two reinforcing catalysts results in
formation of close to enantiopure products in high yields,
even in reactions where the selectivity in both the Lewis
acid–Lewis base and the enzyme catalyzed reactions is not
ideal. By proper choice of the combination of metal catalyst
and biocatalyst both product enantiomers are accessible.
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C. Moberg et al.
Future studies will be devoted to further applications of this
new procedure, whereby mistakes by one catalyst are cor-
rected by a second catalyst.
Acknowledgements
This work was supported by grants from the Swedish Research Council
to C.M. and K.H. Valuable discussions with Professor Donna Blackmond,
Imperial College, London, are gratefully acknowledged. We thank Pro-
fessor Benjamin List, Max-Planck-Institut, Mꢂlheim, for suggesting a
modified title.
Experimental Section
General: All aldehydes were distilled (benzaldehyde from CaH2) prior to
use. Solvents were collected from a Glass-contour solvent dispensing
system. Internal standard, Candida antarctica lipase B (CALB, Novozyme
435), Candida rugosa lipase, and Candida cylindracea were purchased
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and used without further purification. (S,S)- and (R,R)-[(salen)TiACTHNUTRGENUG(N m-
O)]2,[13, 19] acetyl cyanide[20] and butanoyl cyanide[20] were prepared follow-
ing published procedures. Yields and enantiomeric ratios were deter-
mined by GC/MS using a chiral column (Chiraldex, G-TA (gamma cyclo-
dextrin trifluoroacetyl, 30 m ꢁ 0.25 mm) and n-undecane as internal stan-
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ACHTUNGTRENNUNG(m-O)]2 (6.7 mg, 0.0055 mmol), DBU (3.4 mL, 0.024 mmol), internal stan-
dard C11H24 (10 mL, 0.047 mmol), aldehyde (0.12 mmol), and acyl cyanide
(0.014 mmol) in toluene (0.25 mL) Acyl cyanide (0.35 mmol) diluted to
0.25 mL with toluene was then added over 5–22 h to the vigorously
stirred reaction mixture at 408C using a syringe pump. The reaction was
monitored by taking aliquots from the organic phase (ca 20 mL, which
were filtered through a plug of silica and eluted with diethyl ether),
which were analyzed by GC. The structure and absolute configuration of
the O-acylated cyanohydrins were verified by comparison with previously
reported NMR spectral data and with optical rotations.[6b,21]
Alternative procedure for Lewis acid–Lewis base–CALB catalyzed syn-
thesis of O-acylated cyanohydrins: A solution of (S,S)-[(salen)TiACTHNUTRGNE(UNG m-O)]2
(6.7 mg, 0.0055 mmol), DMAP (6.7 mg, 0.006 mmol), internal standard
C11H24 (10 mL, 0.047 mmol), benzaldehyde (12 mL, 0.12 mmol) and acetyl
cyanide (9.4 mL, 0.13 mmol) in toluene (0.3 mL) was stirred at room tem-
perature for 9 h. Immobilized CALB (10 mg) was then added followed
by 1m phosphate buffer pH 8 (0.5 mL). More acetyl cyanide (16.2 mL,
0.23 mmol) diluted to 0.2 mL with toluene was then added over 16.5 h to
the vigorously stirred reaction mixture at 408C using a syringe pump.
The reaction was monitored by GC while stirring was continued.
Preparative scale Lewis acid–Lewis base–CALB catalyzed synthesis and
purification of O-acetyl-(R)-2-hydroxy-2-phenylacetonitrile: Immobilized
CALB (100 mg) was added to
a solution of (S,S)-[(salen)TiACHTNUGTNRUEGN(m-O)]2
(70 mg, 0.06 mmol), DBU (34 mL, 0.24 mmol), benzaldehyde (122 mL,
1.2 mmol) and acetyl cyanide (5 mL, 0.071 mmol) in toluene (4.25 mL),
followed by 1m phosphate buffer pH 8 (5 mL). Acetyl cyanide (250 mL,
3.5 mmol) diluted to 1 mL with toluene was then added over 18 h to the
vigorously stirred reaction mixture at 408C using a syringe pump. Stirring
was continued for 3 h. After dilution with Et2O, the reaction mixture was
filtered through silica, the silica rinsed with ether, and the solvent was
evaporated under vacuum. The residue was purified with flash chroma-
tography on silica gel (hexane/ethyl acetate 7:1) to give O-acetyl-(R)-2-
hydroxy-2-phenylacetonitrile (180 mg, 86%, 99.67:0.33 e.r.).
[11] a) A. Hamberg, S. Lundgren, M. Penhoat, C. Moberg, K. Hult, J.
Am. Chem. Soc. 2006, 128, 2234–2235; b) A. Hamberg, S. Lundgren,
E. Wingstrand, C. Moberg, C. K. Hult, Chem. Eur. J. 2007, 13, 4334–
4341.
[12] Y. N. Belokon, A. J. Blacker, L. A. Clutterbuck, D. Hogg, M. North,
C. Reeve, Eur. J. Org. Chem. 2006, 4609–4617.
[13] Y. N. Belokon’, S. Caveda-Cepas, B. Green, N. S. Ikonnikov, V. N.
Khrustalev, V. S. Larichev, M. A. Moscalenko, M. North, C. Orizu,
V. I. Tararov, M. Tasinazzo, G. I. Timofeeva, L. V. Yashkina, J. Am.
Chem. Soc. 1999, 121, 3968–3973.
Lewis acid–Lewis base–CRL (or CCL) catalyzed synthesis of O-acylated
cyanohydrins: Acetyl cyanide (9.4 mL, 0.132 mmol) was added to a solu-
tion of (S,S)-[(salen)TiACHTUNGTRENNUNG(m-O)]2 (6.7 mg, 0.0055 mmol), DMAP (0.73 mg,
[14] It has been reported that the R enantiomers of several acetates of
cyanohydrins, although not that derived from benzaldehyde, are hy-
drolyzed by CRL: F. Effenberger, B. Gutterer, T. Ziegler, E. Eck-
hardt, R. Aichholz, Liebigs Ann. Chem. 1991, 47–54. Hydrolysis of
3a by CRL was later reported not to follow Kazlauskas et al.ꢆs rule:
U. Hanefeld, A. J. J. Straathof, J. J. Heijnen, J. Mol. Catal. B 2001,
11, 213–218.
[15] a) O. Pꢇmies, J.-E. Bꢃckvall, Chem. Rev. 2003, 103, 3247–3262;
b) H. Pellissier, Tetrahedron 2008, 64, 1563–1601; c) B. Martꢈn-
Matute, J.-E. Bꢃckvall in Asymmetric Organic Synthesis with En-
zymes (Eds.: V. Gotor, I. Alfonso, E. Garcꢈa-Urdiales), Wiley-VCH,
Weinheim, 2008, Chapter 4.
0.006 mmol), internal standard C11H24 (10 mL, 0.047 mmol) and benzalde-
hyde (12 mL, 0.12 mmol) in toluene (0.3 mL). The reaction mixture was
stirred at room temperature overnight. Enzyme (CRL or CCL) was
added to 1m phosphate buffer pH 8 (0.5 mL) until the solution was satu-
rated (ca 5 mg). The slurry was stirred for 10 min and then centrifuged.
Insoluble material was filtered off and the solution was added to the or-
ganic reaction mixture. Acetyl cyanide (17 mL, 0.228 mmol) diluted to
0.25 mL with toluene was then added over 20 h to the vigorously stirred
reaction mixture at 408C using a syringe pump. Additional acetyl cyanide
(8.5 mL, 0.12 mmol) diluted to 0.1 mL with toluene, was then added over
10 h. The reaction was monitored by GC.
[16] J. Steinreiber, K. Faber, H. Griengl, Chem. Eur. J. 2008, 14, 8060–
8072.
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Chem. Eur. J. 2009, 15, 12107 – 12113

Enantiomer Recycling
FULL PAPER
[17] C. J. Dunsmore, R. Carr, T. Fleming, N. J. Turner, J. Am. Chem. Soc.
2006, 128, 2224–2225.
[18] N. J. Turner in Asymmetric Organic Synthesis with Enzymes (Eds.:
V. Gotor, I. Alfonso, E. Garcꢈa-Urdiales), Wiley-VCH, Weinheim,
2008, Chapter 5.
[20] P. Roth, A. Hꢃdener, C. Tamm, Helv. Chim. Acta 1990, 73, 476–482.
[21] a) M. Inagaki, J. Hiratake, T. Nishioka, J. Oda, J. Org. Chem. 1992,
57, 5643–5649; b) H. S. Bevinakatti, A. A. Banerji, R. V. Newadkar,
J. Org. Chem. 1989, 54, 2453–2455.
[19] Y. N. Belokon, P. Carta, A. V. Gutnov, V. Maleev, M. A. Moskalen-
ko, L. V. Yashkina, N. S. Ikonnikov, N. V. Voskoboev, V. N. Khrusta-
lev, M. North, Helv. Chim. Acta 2002, 85, 3301–3312.
Received: May 19, 2009
Revised: July 9, 2009
Published online: September 18, 2009
Chem. Eur. J. 2009, 15, 12107 – 12113
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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