Biomimetic trifunctional organocatalyst showing a great acceleration for the transesterification between vinyl ester and alcohol

Article abstract of DOI:10.1039/b718763g

Trifunctional organocatalysts 1a and 1b mimicking the active site of serine hydrolases showed high catalytic activity with up to a 3 700 000-fold acceleration for the acyl-transfer reactions from vinyl trifluoroacetate to alcohol. The Royal Society of Chemistry.

Full text of DOI:10.1039/b718763g

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Biomimetic trifunctional organocatalyst showing a great acceleration for
the transesterification between vinyl ester and alcoholw
Tadashi Ema,* Daisuke Tanida, Tatsuya Matsukawa and Takashi Sakai*
Received (in Cambridge, UK) 4th December 2007, Accepted 2nd January 2008
First published as an Advance Article on the web 22nd January 2008
DOI: 10.1039/b718763g
Trifunctional organocatalysts 1a and 1b mimicking the active
site of serine hydrolases showed high catalytic activity with up to
a 3 700 000-fold acceleration for the acyl-transfer reactions
from vinyl trifluoroacetate to alcohol.
hydroxy group expected to act as a nucleophile, a pyridine
moiety as a base, and a (thio)urea group as an oxyanion hole
to stabilize the carbonyl oxyanion in the transition state. The
3,5-bis(trifluoromethyl)phenyl group was used to enhance the
hydrogen-bond donor ability of the (thio)urea group.4d,8a,8c
The carboxylate anion of the catalytic triad was omitted in this
study. Compounds 2–4 lack one of the three functional
groups, enabling us to investigate the role of each catalytic
group. We involved no binding sites for attracting a substrate
because we aimed at creation of a catalyst capable of accel-
erating the reaction by stabilizing the transition state, inde-
pendent of substrate binding. Synthetic procedures for 1–4 are
given in ESI.w
To evaluate the catalytic power of 1–4, we first examined
vinyl acetate as an acyl donor because it is used frequently in
lipase-catalyzed transesterifications.12,13 However, vinyl acet-
ate was not consumed at all even in the presence of 1 equiv. of
1a in CD3OD as measured by 1H NMR. We therefore
examined a more reactive acyl donor, vinyl trifluoroacetate.
When vinyl trifluoroacetate (100 mM) was added to a solution
of 1a (1 equiv.) and MeOH (500 mM) in CDCl3, to our delight,
the reaction was too fast to monitor by means of 19F NMR.
The creation of an organic compound with an enormous
catalytic power like an enzyme is an important and exciting
subject. Using useful synthetic methods and accumulated
mechanistic knowledge, chemists have tried to create artificial
enzymes with high catalytic activity.1,2 A rapidly growing area
related to this subject is asymmetric organocatalysis,3–10 which
is aimed at synthetic application. Although some of the
organocatalysts developed so far can work in very small
amounts,10 5–20 mol% of catalyst is used in many cases.3a
On the other hand, the catalytic activity and turnover number
of an enzyme are extremely high; for example, the apparent
second-order rate constant for an enzymatic reaction, ranging
from 107 to 108 Mꢀ1
controlled encounter frequency of an enzyme and a substrate
in water, 109 Mꢀ1 sꢀ1 11
Although various artificial enzymes
s
ꢀ1, is close to that for the diffusion-
.
and organocatalysts have been developed, they are still inferior
to natural enzymes in catalytic activity and turnover.1f There-
fore, it is worth constructing a highly active catalyst by
mimicking the active site of an enzyme, during which we can
learn various factors and clues to the dramatic improvement of
catalytic activity. Here we report highly active biomimetic
organocatalysts capable of catalyzing the acyl-transfer reac-
tions from vinyl trifluoroacetate to alcohol in an organic
solvent.
Serine hydrolases, such as lipases, esterases, and serine
proteases, are widely used as biocatalysts in organic synth-
esis.12,13 Because the reaction mechanisms together with their
crystal structures are well elucidated,13 and because they can
catalyze even transesterifications in organic solvents to find
valuable synthetic utility,12 we decided to design and synthe-
size an organocatalyst mimicking the active site of serine
hydrolases, particularly lipases. The candidates, searched by
performing MO calculations on the tetrahedral intermediates,
are shown in Scheme 1. Trifunctional compounds 1 bear a
Division of Chemistry and Biochemistry, Graduate School of Natural
Science and Technology, Okayama University, Tsushima, Okayama
700-8530, Japan. E-mail: ema@cc.okayama-u.ac.jp;
Fax: +81-86-251-8092; Tel: +81-86-251-8091
w Electronic supplementary information (ESI) available: Synthetic
procedures, copies of 1H, 13C, and 19F NMR spectra, evidence for
the generation of 1b-Ac, and the derivation of eqn (1) followed by the
determination of the rate constants. See DOI: 10.1039/b718763g
Scheme 1
ꢁc
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We therefore used a catalytic amount of 1a (1 mol%) under
otherwise the same reaction conditions, which allowed us to
monitor the progress of the reaction (Fig. 1). The signal for
methyl trifluoroacetate increased with a decrease in that for
vinyl trifluoroacetate (Fig. 1a). The time course of the reaction
is plotted in Fig. 1b. Fig. 1b clearly shows that the addition of
only 1 mol% of 1a accelerated the transesterification, which
reached almost 100% conversion in 1 h. We also examined the
corresponding urea derivative 1b. Surprisingly, 1b showed a
greater acceleration, completing the reaction within 30 min.
With a catalyst loading of 0.1 mol% of 1a and 1b, the
reactions were almost complete within 16 h and 4 h, respec-
tively (data not shown). We also found that 1a and 1b
(1 mol%) can accelerate the acylation of i-PrOH (Fig. 1b).
On the other hand, the control compounds 2–4, lacking either
the hydroxy, pyridine, or (thio)urea moiety, showed little or no
acceleration under the same reaction conditions (data not
shown in Fig. 1b). These results demonstrate that the three
functional groups work cooperatively to decrease the activa-
tion energy.
Scheme 2
We assumed the catalytic cycle shown in Scheme 2 for the
following reasons: (i) the fact that the hydroxy group in 1 is
essential to the catalysis strongly supports covalent catalysis
via the acyl-catalyst intermediate, 1-Ac, which was detected by
19F NMR; (ii) the use of vinyl ester makes the reaction
irreversible;12,13 (iii) the acyl-transfer reactions catalyzed by
1, which has no binding sites to form a catalyst–substrate
complex prior to the reaction, are likely to proceed via
collisional encounters; indeed, the binding constants were
too small to determine when NMR titrations were done to
estimate the binding affinity of 1 for vinyl acetate instead of
vinyl trifluoroacetate. We applied the steady-state approxima-
tion to the acyl-catalyst intermediate, 1-Ac, to derive eqn (1).
The rate constants were determined under pseudo-first-order
conditions and are listed in Table 1. The derivation of eqn (1)
and the procedure for the determination of the rate constants
are given in ESI.w
k1k2½1ꢂ½Aꢂ½Sꢂ
v ¼
þ kun½Aꢂ½Sꢂ
k1½Aꢂ þ k2½Sꢂ
ð1Þ
ðA : CF3CO2CHQCH2; S : ROHÞ
The great enhancements of the rate constants were confirmed
(Table 1). In the acyl-transfer reactions to MeOH, the k1
values for the acylations of 1a and 1b were 32 000- and
190 000-fold greater, respectively, than the kun value for the
uncatalyzed, background reaction, while the k2 values for the
subsequent deacylations of intermediates, 1a-Ac and 1b-Ac,
were 3800- and 8300-fold greater, respectively, than the kun
value. In the acyl-transfer reactions to i-PrOH, the k1 values
for the acylations of 1a and 1b were 650 000- and 3 700 000-
Table 1 Second-order rate constants for the 1-catalyzed trans-
esterifications between vinyl trifluoroacetate and alcohol in CDCl3
at 22 1Ca
1
ROH
k1/Mꢀ1 sꢀ1
k2/Mꢀ1 sꢀ1 k1/kun
k2/kun
Fig. 1 (a) Change of 565 MHz 19F NMR spectra of a mixture of 1a (1
mM), vinyl trifluoroacetate (100 mM) and MeOH (500 mM) in CDCl3
at 22 1C. (b) Time courses of the acyl-transfer reactions from vinyl
trifluoroacetate to MeOH (filled circles) and i-PrOH (open circles).
The reactions catalyzed by 1a and 1b (1 mol%) are shown in blue and
red, respectively, and the background reactions are shown in black.
The data for 2, 3, and 4, all of which were very close to the background
reactions, are omitted for clarity.
1a
1b
1a
1b
a
MeOH
MeOH
i-PrOH
2.4
14.4
2.6
0.29
0.63
0.12
0.61
32 000
3800
8300
30 000
150 000
190 000
650 000
3 700 000
i-PrOH 14.9
For the detailed procedure, see ESI.w The kun values for the acyla-
tions of MeOH and i-PrOH were 7.6 ꢃ 10ꢀ5 and 4.0 ꢃ 10ꢀ6 Mꢀ1 sꢀ1
,
respectively.
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fold greater, respectively, than the kun value, while the
k2 values for the deacylations of 1a-Ac and 1b-Ac were
30 000- and 150 000-fold greater, respectively, than the kun
value. These values are very high despite the lack of a
substrate-binding site in 1. The acceleration was greater for
i-PrOH than for MeOH, which is due to the lower background
level of the former.
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The results in Fig. 1b and Table 1 have three important
implications. First, the nucleophilicity of the hydroxy group of
a catalyst can be enhanced dramatically when a base, pyridine
in this case, is disposed in close proximity. Indeed, catalyst 3,
lacking the hydroxy group, cannot activate MeOH or i-PrOH
well enough to attack the acylating agent. These results are
consistent with the fact that serine hydrolases employed the
side chain of the serine residue, but not H2O, as a nucleophile
in the course of evolution.11 Second, the rate constant for the
acylation step (k1) is greater than that for the deacylation of
the acyl-catalyst intermediate (k2), and the latter reaction is the
rate-determining step. The same trend has been observed for
natural and artificial enzymes;1b,2f,9a,11 for example, when
p-nitrophenyl acetate was hydrolyzed, the acyl-enzyme/cata-
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taken place, giving off 1 equiv. of p-nitrophenolate anion.1b,2f
Third, Table 1 clearly indicates that urea catalyst 1b shows
higher activity in each step than thiourea catalyst 1a, which
suggests that the urea group is a better mimic of the oxyanion
hole of serine hydrolases.
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In summary, trifunctional organocatalysts 1 reminiscent of
the active site of serine hydrolases were designed and synthe-
sized. Up to a 3 700 000-fold acceleration with high catalytic
turnover was achieved by 1b at room temperature. Compar-
isons between 1 and 2–4 demonstrate that the three functional
groups in 1 work cooperatively to stabilize the transition state
and accelerate the reaction. This is, to the best of our knowl-
edge, the first example of a biomimetic organocatalyst bearing
a nucleophilic OH group, a base, and an oxyanion hole, which
can catalyze the transesterification between vinyl ester and
alcohol.14 Further work is in progress to optimize the structure
of the catalyst and apply the catalyst to asymmetric synthesis.
We thank Dr T. Nitoda (Okayama University) for the
measurement of mass spectra. We are grateful to the SC-
NMR Laboratory of Okayama University for the measure-
ment of NMR spectra.
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