Lipase/Oxovanadium Co-Catalyzed Dynamic Kinetic Resolution of Propargyl Alcohols: Competition between Racemization and Rearrangement

Article abstract of DOI:10.1021/acs.orglett.9b00334

Quantitative conversion of racemic propargyl alcohols into optically active propargyl esters with up to 99% ee has been achieved by lipase/oxovanadium co-catalyzed dynamic kinetic resolution, which combines the lipase-catalyzed enantioselective esterification of the racemic substrates and the in situ racemization of the remaining enantiomers. The success is owed to our discovery of a magic solvent, (trifluoromethyl)benzene, that accelerated the racemization while sufficiently suppressing the common oxovanadium-catalyzed rearrangement of propargyl alcohols to irreversibly produce enals.

Full text of DOI:10.1021/acs.orglett.9b00334

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Lipase/Oxovanadium Co-Catalyzed Dynamic Kinetic Resolution of
Propargyl Alcohols: Competition between Racemization and
Rearrangement
Shinji Kawanishi, Shinya Oki, Dhiman Kundu, and Shuji Akai*
Graduate School of Pharmaceutical Sciences, Osaka University, Yamadaoka 1-6, Suita, Osaka 565-0871, Japan
S
* Supporting Information
ABSTRACT: Quantitative conversion of racemic propargyl
alcohols into optically active propargyl esters with up to 99% ee
has been achieved by lipase/oxovanadium co-catalyzed dynamic
kinetic resolution, which combines the lipase-catalyzed enantiose-
lective esterification of the racemic substrates and the in situ
racemization of the remaining enantiomers. The success is owed to
our discovery of a magic solvent, (trifluoromethyl)benzene, that
accelerated the racemization while sufficiently suppressing the
common oxovanadium-catalyzed rearrangement of propargyl
alcohols to irreversibly produce enals.
he lipase-catalyzed kinetic resolution (KR) of racemic
secondary alcohols has been widely used for the
include the preparation of allenes7 and diverse cyclization
reactions such as the Huisgen reaction.8 Therefore, we have
been interested in applying the V-MPS4/lipase co-catalyzed
DKR to racemic propargyl alcohols 1 to obtain optically active
propargyl esters 2. However, it has been well-known that the
reactions of propargyl alcohols with oxovanadium moieties
bring about the 1,3-transposition of the hydroxyl group to form
conjugate enals, which is named Meyer−Schuster rearrange-
ment (Figure 1c).9,10 Thus, once the allenyl vanadates D are
formed via either the [3,3]-sigmatropic rearrangement of B or
C−O bond formation at the terminal carbon of a propargyl
cation C,10d the hydrolysis of D immediately forms 3 by
tautomerization of allenols E. This irreversible process is quite
different from the aforementioned racemization of allyl
alcohols (Figure 1b); the hampered DKR results in a
significant decrease in the yields of 2. We report herein how
we could effectively suppress the side reactions to achieve DKR
of propargyl alcohols ( )-1 leading to esters (R)-2 in high
chemical and optical yields.
To examine the feasibility of the racemization while
suppressing the Meyer−Schuster rearrangement, we studied
the time course of the V-MPS4 (5.0 mol %)-catalyzed
racemization of an optically pure propargyl alcohol (S)-1a as
a model substrate in a range of organic solvents at 50 °C
(Figure 2). The following results are noteworthy. First, the
racemization in acetonitrile, which was the most often used
solvent in our previous DKR,5 proceeded slowly to produce 1a
with 69% ee after 12 h. This result was very different from the
racemization of allyl alcohols in acetonitrile, where the
complete racemization was attained using only 1.0 mol % V-
MPS4 at 35 °C within 2 h.11 Second, iPr2O was another less
T
preparation of optically active compounds because of its
excellent chemo- and stereoselectivity, easy operation, and easy
purification of the products;1 however, it has an inherent
limitation that it can yield only 50% at maximum of each
optically pure enantiomer. This problem has been overcome by
combining KR and in situ racemization of the remaining less
reactive enantiomers using a racemization catalyst to produce
quantitative yields of optically pure products, and this protocol
is called dynamic kinetic resolution (DKR). The racemization
is mainly conducted by a redox process, for which more than a
dozen Ru/Rh complexes possessing different ligands have been
devised.2,3 Very recently, less expensive Fe complexes were also
reported to be useful for similar DKR.4 These DKR protocols
involving redox racemization have been applied to a wide range
of secondary alcohols.2−4 Around the same time, we
independently performed a novel DKR of allyl alcohols using
our original racemization catalyst V-MPS4 in which oxovana-
dium moieties are covalently bound to the inner surface of
mesoporous silica (MPS) with a pore diameter of 4 nm
(Figure 1a).5 Different from the aforementioned DKR, the V-
MPS4-catalyzed racemization proceeds along with 1,3-trans-
position of the hydroxyl group of allyl alcohols generating a
dynamic equilibrium between the two regioisomers [( )-I and
( )-II], and lipase catalyzes the chemo- and enantioselective
esterification of (R)-I (Figure 1b). Its small pore size has
dramatically improved the compatibility between the oxovana-
dium moiety and lipases to produce quantitative yields of
optically pure allyl esters (R)-III.
Enantiomerically pure propargyl alcohols and their esters are
found as partial structures of natural products and are
important building blocks of a range of natural products and
pharmaceutical compounds.6 Their synthetic applications
Received: January 26, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00334
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Organic Letters
Letter
Figure 2. Time course of the reaction of (S)-1a (99% ee) and V-
MPS4 (5.0 mol %) at 50 °C in different solvents. (a) Racemization of
1
(S)-1a and (b) formation of 3a based on H NMR analysis of the
◆
●
▲
reaction mixture: ( ) MeCN, ( ) iPr2O, (×) toluene, ( ) PhCF3,
○
and ( ) CH2Cl2 (in a sealed tube).
On the basis of these results, we next optimized the
conditions of V-MPS4/lipase co-catalyzed DKR of ( )-1a
(Table 1). First, DKR was conducted in (trifluoromethyl)-
benzene at 50 °C for 24 h using commercially available
immobilized Candida antarctica lipase B (CAL-B)12 (3 w/w)
and vinyl decanoate (2 equiv) with varying amounts of V-
MPS4 (entries 1−3). The corresponding ester (R)-2aC was
obtained in >80% yield with high enantioselectivity (97 to
>99% ee) in all cases, and the level of formation of 3a (11−
12% yield) was much lower than we expected from the results
depicted in Figure 2. A similar DKR of ( )-1a (1.4 mmol)
using 2.5 mol % V-MPS4 produced (R)-2aC (83% yield, 97%
ee) (entry 4), which was almost the same as DKR using 0.34
mmol of ( )-1a (entry 2). Second, almost the same result as
that in entry 3 was obtained by a similar reaction at 35 °C
(entry 5); however, another reaction at 10 °C did not reach
completion in 24 h (entry 6). The recovery of a 14% yield of
(S)-1a with 38% ee showed that both racemization and
enzymatic resolution became slower at 10 °C.
Figure 1. (a) Structure of V-MPS4. (b) Lipase/V-MPS4 co-catalyzed
DKR of racemic allyl alcohols (I and II). (c) Lipase/V-MPS4 co-
catalyzed DKR of racemic propargyl alcohols 1 and their Meyer−
Schuster rearrangement. OV(OSiR3)n (n = 2 or 3) describes a local
structure of the reactive site of V-MPS4.
suitable solvent for producing 1a with 65% ee after 12 h. Third,
the racemization in less polar solvents, such as toluene and
dichloromethane, proceeded faster (1a with 8% ee was
obtained in toluene after 12 h, and complete racemization
was observed in dichloromethane within 4 h). Fourth, the
faster the racemization proceeded, the higher the yield of 3a
(38% in toluene after 12 h and 62% in dichloromethane at 4
h). Further study led us to the discovery of (trifluoromethyl)-
benzene as the best solvent that promoted fast racemization
with relative suppression of the formation of 3a. In addition,
acetonitrile was the second choice of solvent because the level
of formation of 3a was much lower in acetonitrile than in
iPr2O, whereas the racemization rates were almost the same in
these solvents.
Third, the use of vinyl esters with a shorter acyl moiety
caused a slight decrease in the yield of esters (R)-2aB and (R)-
2aA due to the increase in the yield of 3 (compare entry 3 and
entries 7 and 8). Fourth, DKR in acetonitrile did not reach
completion in 24 h to give (R)-2aC (74% yield, >99% ee) and
(S)-1a (16% yield, 52% ee) mainly due to the slow
racemization, although the formation of 3 was suppressed
B
DOI: 10.1021/acs.orglett.9b00334
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization of DKR Conditions of ( )-1a
(R)-2a
(S)-1a
b
c
b
entry
solvent
V-MPS4 (mol %)
R
temp (°C) conversion (%)
yield (%)
ee (%) 3a yield (%)
yield (%)
ee (%)
1
2
3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
MeCN
CH2Cl2
toluene
1.0
2.5
5.0
2.5
5.0
5.0
5.0
5.0
5.0
5.0
5.0
n-C9H19
n-C9H19
n-C9H19
n-C9H19
n-C9H19
n-C9H19
n-C3H7
Me
50
50
50
50
35
10
50
50
50
50
50
93
100
100
100
100
86
100
100
84
(R)-2aC
(R)-2aC
(R)-2aC
(R)-2aC
(R)-2aC
(R)-2aC
(R)-2aB
(R)-2aA
(R)-2aC
(R)-2aC
(R)-2aC
81
83
85
83
82
81
81
77
74
81
97
99
>99
97
99
97
99
94
>99
>99
98
11
12
11
12
10
4
14
16
6
7
23
−
−
−
−
38
−
−
52
−
−
−
−
14
−
−
16
−
−
d
4
5
6
7
8
9
n-C9H19
n-C9H19
n-C9H19
e
10
11
100
100
17
10
−
−
f
77
a
b
c
1
Each reaction was conducted using 0.34 mmol of 1a except for entry 4. Isolated yield. Yield determined by H NMR. An approximate 3:1
d
e
f
mixture of (E)- and (Z)-3a was obtained. Conducted using 1.4 mmol of 1a. Conducted in a sealed tube. A mixture of two diastereomers of
bis[1-(4-tolyl)prop-2-yn-1-yl] ether (4a) was formed in 11% yield. The yield is calculated on the basis of the alcoholic moiety and corresponds to a
6% yield based on its molar amount.
(entry 9). However, DKR in dichloromethane produced (R)-
2aC (>99% ee) in an unsatisfactory yield (81%) due to the fast
formation of 3a (17% yield) (entry 10). Toluene was not a
favorable solvent because of the formation of another side
product 4a,13 resulting in a reduced yield of (R)-2aC (77%)
(entry 11). From these results, the DKR in (trifluoromethyl)-
benzene (entries 2−4) was chosen as the best among these
trials.
DKR of 4-methoxyl derivative ( )-1b under the same
conditions as entry 3 of Table 1 provided ester (R)-2bC in low
chemical and optical yields (67% yield, 83% ee) along with 3b
(18% yield) and 4b (7% yield) (Table 2, entry 1). Further
investigation by changing the amounts of V-MPS4 and CAL-B
and reaction temperature led to a dramatic improvement in the
yield and optical purity of (R)-2bC.11 For example, reducing
the amount of V-MPS4 (0.50 mol %) at 35 °C resulted in the
formation of (R)-2bC (96% yield, 97% ee), whereas the yields
of 3b and 4b were significantly decreased (entry 2). Because of
the fast racemization in (trifluoromethyl)benzene, the amount
of V-MPS4 was even reduced to 0.10 mol % to give (R)-2bC
(88% yield, 97% ee) despite the longer reaction time (48 h)
(entry 3). On the other hand, we had preliminarily reported
that (R)-2bB (96% yield, 99% ee) was obtained using vinyl
butyrate and V-MPS4 (1.0 mol %) in acetonitrile (entry 4),
even though we had little information about solvent effects.5a,14
Now, we have noticed that the high yield of (R)-2bB is mainly
due to efficient suppression of the Meyer−Schuster rearrange-
ment in acetonitrile.
moieties, such as ( )-1e and ( )-1f, to produce (R)-2eC and
(R)-2fC in around 90% yields with 92 and 98% ee, respectively
(entries 7 and 9, respectively). In addition, method II
produced a slightly better yield and a slightly better
enantioselectivity than method I for these alcohols (entries 8
and 10). Similarly, these two methods were applied to
substituted alcohols ( )-1g−i to give (R)-2gC, (R)-2hA,
and (R)-2iB, respectively, in good yields and enantioselectiv-
ities (entries 11−13, respectively). Moreover, the heterocyclic
alcohols, such as ( )-1j and ( )-1k, were converted into (R)-
2jB and (R)-2kC with high enantioselectivity (entries 14 and
15, respectively). However, alcohol ( )-1l having a methox-
ycarbonyl group at the para position was found to be a poor
substrate for this DKR to provide (R)-2lC (>99% ee) in 55%
yield along with recovered (S)-1l (38% yield, 96% ee) (entry
16), which was due to slow racemization of 1l.
These results show that method I is useful for many of the
substrates and allows the use of smaller amounts of V-MPS4
(0.10−0.50 mol %) due to faster racemization in
(trifluoromethyl)toluene than in acetonitrile. However,
method II generally forms products in chemical and optical
yields slightly higher than those produced by method I, even
though it is available for substrates having an electron-rich
aromatic moiety.
In conclusion, we have discovered that V-MPS4, in which
oxovanadium moieties are covalently bound to the inner
surface of mesoporous silica (MPS), preferentially catalyzes the
racemization of optically active propargyl alcohols in some
solvents, even though the oxovanadium compounds have been
mostly employed to catalyze the Meyer−Schuster rearrange-
ment of the propargyl alcohols to form the corresponding
conjugate enals.9,10 One of the most significant findings was
that the relative rates of rearrangement and racemization are
significantly dependent on solvents. In particular,
(trifluoromethyl)benzene was found to effectively suppress
the rearrangement while accelerating the racemization;
acetonitrile was another option for decelerating the rearrange-
On the basis of these studies, we set up two typical reaction
methods for DKR, the use of vinyl decanoate and V-MPS4 in
(trifluoromethyl)benzene (method I) and the use of vinyl
butyrate and V-MPS4 in acetonitrile (method II), and applied
them to various propargyl alcohols ( )-1. The results are
summarized in Table 2. ( )-1c and ( )-1d were converted to
(R)-2cC and (R)-2dC in high yields with high enantiose-
lectivities by method I (entries 5 and 6, respectively). Method I
was also useful for the alcohols having electron-rich aromatic
C
DOI: 10.1021/acs.orglett.9b00334
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Table 2. Substrate Scope of DKR of Propargyl Alcohols ( )-1
a
b
c
Method I: use of vinyl decanoate (R = n-C9H19) in PhCF3. Method II: use of vinyl butyrate (R = n-C3H7) in MeCN. Isolated yield. Determined
d
by 1H NMR analysis of the crude product. Bis[1-(4-methoxyphenyl)prop-2-yn-1-yl] ether (4b) was formed in 7% yield (for the calculation of the
yield, see footnote f of Table 1). Conducted using 0.10 mol % V-MPS4 and 0.6 w/w CAL-B for 48 h. 4b was formed in 3% yield (see footnote f of
Table 1). Cited from ref 5a. Vinyl acetate (R = Me) was used. Not detected. (S)-1l (38% yield, 96% ee) was recovered. CH2Cl2 was used in a
e
f
g
h
i
j
sealed tube instead of PhCF3.
D
DOI: 10.1021/acs.orglett.9b00334
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Letter
Kamiya, M.; Hanada, R.; Egi, M.; Akai, S. Bioorg. Med. Chem. 2018,
26, 1378−1386.
ment. By using these specific solvents, we could apply our V-
MPS4/lipase co-catalyzed DKR to racemic propargyl alcohols
to produce the corresponding optically active esters in >80%
yields with 90−99% ee in many cases. Further investigation to
clarify the unique nature of (trifluoromethyl)benzene and
acetonitrile and applications of our DKR of propargyl
alcohols15 are currently in progress in our laboratory.
(6) For some recent examples, see: (a) Prabhakar, P.; Rajaram, S.;
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Princival, J. L. Org. Biomol. Chem. 2015, 13, 6458−6462. (j) Princival,
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00334.
Full experimental data, including synthetic procedures,
characterization data, and NMR data (PDF)
(7) For some reviews on the construction of allenes from propargyl
alcohols and their derivatives, see: (a) Yu, S.; Ma, S. Chem. Commun.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: akai@phs.osaka-u.ac.jp.
ORCID
́
2011, 47, 5384−8418. (b) Tejedor, D.; Mendez-Abt, G.; Cotos, L.;
García-Tellado, F. Chem. Soc. Rev. 2013, 42, 458−471. (c) Zhu, Y.;
Sun, L.; Lu, P.; Wang, Y. ACS Catal. 2014, 4, 1911−1925. (d) Swamy,
K. C. K.; Anitha, M.; Gangadhararao, G.; Rama Suresh, R. Pure Appl.
Chem. 2017, 89, 367−377. (e) Sakata, K.; Nishibayashi, Y. Catal. Sci.
Technol. 2018, 8, 12−25.
Shuji Akai: 0000-0001-9149-8745
Notes
(8) For some reviews on Huisgen reactions, see: (a) Poonthiyil, V.;
Lindhorst, T. K.; Golovko, V. B.; Fairbanks, A. J. Beilstein J. Org.
The authors declare no competing financial interest.
́
́
̌
Chem. 2018, 14, 11−24. (b) Hladíkova, V.; Vana, J.; Hanusek, J.
Beilstein J. Org. Chem. 2018, 14, 1317−1348. (c) Rios-Gutierrez, M.;
Domingo, L. R. Eur. J. Org. Chem. 2019, 2019, 267−282.
(9) For some reviews related to Meyer−Schuster rearrangement,
see: (a) Swaminathan, S.; Narayanan, K. V. Chem. Rev. 1971, 71,
429−438. (b) Engel, D. A.; Dudley, G. B. Org. Biomol. Chem. 2009, 7,
4149−4158. (c) Cadierno, V.; Crochet, P.; García-Garrido, S. E.;
Gimeno, J. Dalton Trans 2010, 39, 4015−4031. (d) Roy, D.; Tharra,
P.; Baire, B. Asian J. Org. Chem. 2018, 7, 1015−1032. (e) Zhang, B.;
Wang, T. Asian J. Org. Chem. 2018, 7, 1758−1783.
ACKNOWLEDGMENTS
■
This work was financially supported by the JSPS KAKENHI
[16H01151/18HO4411 (Middle Molecular Strategy) and
18H02556] and AMED (Grant 18am0101084j).
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