In situ chiral modification of nickel catalysts for enantioselective hydrogenation of methyl acetoacetate

Article abstract of DOI:10.1135/cccc20051642

Chiral modifications of SiO₂-supported and Raney nickel catalysts were studied. The catalysts were employed in asymmetric hydrogenation of methyl acetoacetate (MAA) to (R)- and (S)-enantiomers of methyl 3-hydroxybutanoate. The effects of modification parameters such as type and concentration of modifier; presence of a co-modifier and other additives, pH of modification solution on the enantioselectivity of MAA hydrogenation were discussed. Characteristic features of the in situ modification of Ni/SiO ₂ were also evaluated and the results obtained were compared with the conventional (premodification) approach. Parameters for the conventional and in situ methods were optimised in a series of experiments for both types of catalysts. The in situ modified Ni/SiO₂ was found specifically suitable for repeated use due to virtually no decrease in selectivity and activity.

Full text of DOI:10.1135/cccc20051642

1642
Bartek, Klusoň, Červený:
In situ CHIRAL MODIFICATION OF NICKEL CATALYSTS FOR
ENANTIOSELECTIVE HYDROGENATION OF METHYL ACETOACETATE
1
2
3,
Lukáš BARTEK , Petr KLUSOŇ and Libor ČERVENÝ *
Department of Organic Technology, Institute of Chemical Technology, Prague, Technická 5,
1
2
166 28 Prague 6, Czech Republic; e-mail: lukas.bartek@vscht.cz, p.kluson@seznam.cz,
libor.cerveny@vscht.cz
3
Received April 8, 2005
Accepted May 26, 2005
Ch iral m odification s of SiO2-supported an d Ran ey n ickel catalysts were studied. Th e cata-
lysts were em ployed in asym m etric h ydrogen ation of m eth yl acetoacetate (MAA) to (R)- an d
(S)-en an tiom ers of m eth yl 3-h ydroxybutan oate. Th e effects of m odification param eters such
as type an d con cen tration of m odifier; presen ce of a co-m odifier an d oth er additives, pH of
m odification solution on th e en an tioselectivity of MAA h ydrogen ation were discussed. Ch ar-
acteristic features of th e in situ m odification of Ni/SiO2 were also evaluated an d th e results
obtain ed were com pared with th e con ven tion al (prem odification ) approach . Param eters for
th e con ven tion al an d in situ m eth ods were optim ised in a series of experim en ts for both
types of catalysts. Th e in situ m odified Ni/SiO2 was foun d specifically suitable for repeated
use due to virtually n o decrease in selectivity an d activity.
Keyw ord s: En an tioselective catalytic h ydrogen ation ; In situ m odification ; Tartaric acid m od-
ified Ran ey n ickel catalyst; Sol-gel; Asym m etric reduction s; Heterogen eous catalysis.
En an tioselective reaction s h ave becom e an essen tial part of m an y m an ufac-
turin g processes for fin e ch em icals, ph arm aceuticals, agroch em icals, fla-
vours an d fragran ces. Heterogen eous asym m etric catalysis as a prom isin g
tool for such purposes h as been growin g rapidly in recen t years1. Am on g
establish ed strategies for optically pure com poun ds, en an tiodifferen tiation
on h eterogen eous catalysts represen ts on e of th e m ost prom isin g option s.
Atten tion h as been paid to n oble m etals (typically Pt or Pd) ch irally m odi-
fied with cin ch on a alkaloids2–5 or to tran sition m etals (typically Ni 6–9
)
m odified with tartaric acid. Th e latter catalytic system h as been exten sively
tested in m odel h ydrogen ation s of β-ketoesters10–14 an d β-diketon es15,16
affordin g h igh optical yields (up to 98%). On e of th ese reaction s was asym -
m etric h ydrogen ation of m eth yl acetoacetate17–21 (MAA) as a m odel
β-ketoester to (R)- an d (S)-en an tiom ers of m eth yl 3-h ydroxybutan oate
(Fig. 1). Nickel catalysts m odified with tartaric acid usually required partici-
Collect. Czech. Chem. Commun. (Vol. 70) (2005)
doi:10.1135/cccc20051642

In situ Chiral Modification of Nickel Catalysts
1643
pation of oth er selectivity- or activity-en h an cin g factors. Th ose in cluded
e.g. specific form s of th e active ph ase (Ran ey n ickel22, supported n ickel cat-
alysts23, n ickel black24–26, bim etallic catalyst27,28), presen ce of m odifier an d
co-m odifier. Besides tartaric acid (TA), oth er m odifiers such as α-h ydroxy
acids29 or α-am in o acids30,31 were also tested. Th e presen ce of NaBr in th e
m odification solution in volvin g TA en h an ced th e ach ieved en an tio-
selectivity by 10–30% in com parison with experim en ts perform ed with out
th is co-m odifier. Addition of pivalic acid32, acetic acid33 or sodium 2-eth yl-
h exan oate34 in sm all quan tities furth er in creased optical yields. An oth er
possibility com prised utilisation of sodium tartrates in stead of TA. Th e role
of sodium ion s as a part of th e active species adsorbed35 on th e n ickel sur-
face h as n ot been fully elucidated36 yet. Th e effects of factors such as tem -
perature, pH, im pregn ation period, con cen tration an d volum e of a m odifier
(co-m odifier) solution were iden tified37–41 as essen tial. Th e m odification
step (in troduction of m odifiers or co-m odifiers) for obtain in g a catalytic
system with en h an ced en an tioselectivity could be arran ged eith er as
prem odification (n ickel catalyst, both Ran ey an d sol-gel, is im m ersed an d
m odified in an aqueous solution con tain in g TA an d NaBr before h ydroge-
n ation ) or as in situ42,43 m odification . Alth ough th e prem odification ap-
proach is sim ple an d leads to h igh en an tioselectivities of m an y types of
n ickel catalysts, it h as n ot been scaled up for in dustrial use. On e of th e
m ain reason s is th e gen eration of a large am oun t of waste solution in volv-
in g n ickel ion s durin g th e prem odification procedure. Th e latter approach
com prised direct addition of m odifiers (an d co-m odifiers) to th e reaction
m ixture already in cludin g MAA or oth er m odel reactan ts. Surprisin gly
en ough , th e in situ m odification provided a ch irally active catalytic system
with at least com parable optical yields to th ose ach ieved with stan dard im -
pregn ation . Th e in situ m odification also represen ts an en viron m en tally
frien dly process th at gen erates n o waste solution s con tain in g n ickel ion s.
Th e type44 of th e n ickel catalyst plays a key role. Ran ey n ickel is on e of
th e m ost con ven ien t m aterials for preparation of m odified Ni catalysts.
Ran ey n ickel m odified by ultrason ic irradiation 45 em bodied th e h igh est op-
tical yield ever attain ed (ee 98%). Modified supported Ni catalysts were also
O
H2
OH
H
OH
H
TA/NaBr-
modified Ni
H3C
CH2COOCH3
H3C
CH2COOCH3
H3C
CH2COOCH3
+
MAA
(R)-MHB
(S)-MHB
FIG. 1
Reaction sch em e of MAA h ydrogen ation over TA/NaBr-m odified Ni catalysts
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

1644
Bartek, Klusoň, Červený:
in ten sively studied46–48. It sh ould be n oted th at th e preparation of a m odi-
fied supported catalyst com prises m ore param eters affectin g th e optical
yield th an th at of a m odified RaNi. Besides th e stan dard preparation proce-
dure (im pregn ation of a support with an active species precursor) th e
sol-gel m eth od sh ould also be con sidered.
In th is paper we report on com parison of supported n ickel catalysts pre-
pared by th e sol-gel m eth od with Ran ey n ickel catalysts in term s of th eir
perform an ce in asym m etric h ydrogen ation of MAA. Ch aracteristic features
of th e catalytic system s produced by im pregn ation with tartaric acid (an d
its sodium salts – NaHTA, Na2TA) an d by th e in situ m odification are dis-
cussed.
EXPERIMENTAL
Catalysts
Ran ey n ickel was Actim et supplied by En gelh ard (SBET = 92 m 2 g–1, particle size 5–100 n m ).
Precursors of Ni/SiO2 (50 wt.% Ni) – tetraeth oxysilan e (0.3 m ol, Aldrich ) an d Ni(NO3)2·6H2O
(0.3 m ol, Aldrich ), were m ixed with 3 m ol of eth an ol an d boiled for 1 h . To th is m ixture
water was slowly added (160 m l) an d th e form ed gel was allowed to stan d for 14 h . Fin ally
th e m ixture was dried in vacuum an d decom posed in a furn ace at 573 K for 3 h . Th e
m aterial was reduced with H2 (120 m l m in –1) at 773 K for 4 h . Th e SBET of th e catalyst was
–1
450 m 2
g
an d th e evaluated m ean particle size of Ni was 100 n m .
Ch em icals
Meth an ol, eth an ol, an d tetrah ydrofuran were supplied by Fluka (HPLC grade); m eth yl
acetoacetate (MAA), HPLC grade water, NaBr, NaOH, (+)-(2R,3R)-tartaric acid (TA), sodium
h ydrogen -L-(+)-tartrate (NaHTA) an d sodium L-(+)-tartrate dih ydrate (Na2TA) were all of
Aldrich .
Prem odification Procedure
Ran ey n ickel (10 g) stored in alkalin e water solution was wash ed th ree tim es with 50 m l of
distilled water an d th en in troduced to th e m odification solution of TA, NaHTA or NaBr
(co-m odifier, Aldrich ). pH was adjusted by 20% NaOH usin g poten tiom etric in dication . Th e
suspen sion of th e catalyst an d m odification solution was stirred in a flask with a con den ser
at 373 K (90 m in ). Th e m odification solution was decan ted an d th e catalyst wash ed with
m eth an ol an d tetrah ydrofuran . Th e sol-gel catalyst (3 g) was in troduced to th e m odification
solution (100 m l) con tain in g TA or NaHTA or Na2TA an d NaBr (co-m odifier). Optim um pH
was foun d in a series of screen in g experim en ts. Th e suspen sion was stirred for 1 h , th en th e
solution was rem oved an d th e catalyst wash ed successively with 50 m l of water, m eth an ol
an d tetrah ydrofuran .
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

In situ Chiral Modification of Nickel Catalysts
In situ Modification
1645
Th e in situ m odification required direct in troduction of th e m odifier, co-m odifier an d addi-
tives m ixture (TA, NaBr, acetic acid) in to th e reaction vessel (already con tain in g MAA an d
th e catalyst) in solid form 26 (th e m eth od n oted h ere as “in situ A”) or dissolved in a sm all
quan tity of water30 (th e m eth od n oted h ere as “in situ B”).
Typical Experim en t
Reaction s were carried out in liquid ph ase in a stain less steel autoclave (300 m l) with MAA
(20 m l) in tetrah ydrofuran (70 m l) at 373 K, 10 MPa an d with 2–3 g of a catalyst. Sam ples of
a reaction m ixture were an alysed by GC on an HP5890 with a ch iral cyclodextrin colum n
BETA-DEX 325 (0.5 µm × 0.5 m m × 50 m , Supelco) an d on an MS/GC – Varian Saturn (th e
GC part was equipped with th e sam e GC colum n ). Optical yields (ee) were calculated usin g
th e followin g equation :
|[(R) − MHB]− [(S) − MHB]|
ee (%) =
× 100 .
|[(R) − MHB]+ [(S) − MHB]|
RESULTS AND DISCUSSION
Raney Nickel – Premodification
In itial screen in g experim en ts with Ran ey n ickel were design ed to elucidate
th e role of th e m odifier/co-m odifier ratio on th e en an tioselectivity (ee) of
MAA h ydrogen ation . En an tioselective properties of th e catalyst m odified
with NaHTA an d NaBr were studied first. Th e m axim um value of ee (71%)
was attain ed at th e NaHTA/NaBr weigh t ratio 1:2. Th is value was kept con -
stan t th rough out th e experim en ts (Fig. 2). Figure 3 sh ows th e effect of vari-
able NaHTA con cen tration . Th e h igh est ee (78%) was ach ieved at cNaHTA
=
0.19 m ol l–1. Th e con cen tration was in th e “com position al area” (0.15 to
0.22 m ol l–1), in wh ich th e catalytic surface was loaded with an optim um
am oun t of th e m odifier (with respect to ee) an d n o leach in g of n ickel was
observed. Th en th e TA/NaBr m odification system was tested in cludin g as-
sessm en t of th e effect of various m odifier/co-m odifier ratios. Th e optim al
TA/NaBr weigh t ratio was foun d again at 1:2. Th e in fluen ce of pH of th e
m odification solution is sh own in Fig. 4. Th e optim um pH was ~3 (ee 64%);
above th is value optical yields decreased rapidly, below pH 3 th e region of
Ni surface corrosion (h igh leach in g of Ni) was located. Prem odification us-
in g Na2TA revealed gen erally poor en an tioselectivity (~ee 5%).
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

1646
Bartek, Klusoň, Červený:
70
60
50
40
30
20
0.0
0.2
0.4
0.6
0.8
1.0
NaHTA/NaBr
FIG. 2
Th e effect of th e NaHTA/NaBr weigh t ratio on th e en an tioselectivity of MAA h ydrogen ation
(volum e of m odification solution 150 m l, cNaHTA = 0.16 m ol l–1; T = 293 K; t = 60 m in ; mcat
=
10 g of RaNi suspen sion )
80
70
60
50
40
30
20
10
0
0.0
0.1
0.2
0.3
0.4
–1
c
, mol l
(NaHTA)
FIG. 3
Th e effect of th e m odifier NaHTA con cen tration on th e en an tioselectivity of th e RaNi (ꢀ) an d
Ni/SiO2 (᭿) catalysts (volum e of m odification solution 150 m l (RaNi), 100 m l (Ni/SiO2),
NaHTA/NaBr = 0.5; T = 293 K; t = 60 m in ; mcat = 10 g of RaNi suspen sion , 3 g of Ni/SiO2)
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

In situ Chiral Modification of Nickel Catalysts
1647
Raney Nickel – in situ A
Catalytic perform an ce of in situ m odified Ran ey n ickel was studied in paral-
lel. Th is m odification approach h as n ot been previously applied to Ran ey
n ickel. Th e results obtain ed with direct addition of th e m odifier, co-
m odifier an d additive (TA, NaBr an d CH3COOH) in th eir crystallin e form s
(TA, NaBr) to th e reaction m ixture were n ot satisfactory. Addition of TA to
th e reaction m edia retarded th e h ydrogen ation rate an d provided very low
optical yields (~ee 2%).
Raney Nickel – in situ B
Th e m odification procedure with dissolved m odifier (TA), co-m odifier
(NaBr) an d additive (acetic acid) did n ot sh ow m ore prom isin g results th an
via previous m eth od (in situ A). Th e optical yields obtain ed were sligh tly
better (ee 8%) but low ee values were observed in a wide ran ge of m odifier
(co-m odifier) con cen tration s. Th e reason s for low en an tiodifferen tiation
ability of in situ m odified RaNi h ave n ot been clarified.
60
50
40
30
20
10
0
0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
pH
FIG. 4
Th e effect of pH of th e m odification solution on th e en an tioselectivity of th e RaNi (ꢀ) an d
Ni/SiO2 (᭹) catalysts (volum e of m odification solution 150 m l (RaNi), 100 m l (Ni/SiO2), cTA
=
0.2 m ol l–1; T= 293 K; t = 60 m in ; mcat = 10 g of RaNi suspen sion , 3 g of Ni/SiO2, pH adjusted
with 0.1 M NaOH)
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

1648
Catalysts Prepared by Sol-Gel Method – Premodification
Bartek, Klusoň, Červený:
Th e prem odification strategy for sol-gel catalysts was sim ilar to th at for
Ran ey n ickel. Effect of th e TA am oun t on ee was tested first. Its optim al
con cen tration was cTA = 0.056 m ol l–1 an d optim al TA/NaBr weigh t ratio
1:2. Th e effect of pH of th e m odification solution is sh own in Fig. 4. pH 3.2
represen ted th e region with th e best en an tiodifferen tiation sim ilarly as
for Ran ey n ickel (ee 46%). Th e optim al NaHTA/NaBr weigh t ratio was
foun d at 0.5; th e best en an tioselectivity (ee 55%) was ach ieved for cNaHTA
=
0.21 m ol l–1 (Fig. 3). In an alogous experim en ts with Na2TA, poor en an tio-
selectivity was revealed as observed also for Ran ey Ni (ee ~5%). Experim en ts
in volvin g TA at pH adjusted with NaOH an d with NaHTA proved an essen -
tial role of sodium ion s in stabilisation of a surface com plex. Sim ilarly posi-
tive effect of th e disodium salt as suggested elsewh ere49 was n ot con firm ed.
Catalysts Prepared Sol-Gel Method – in situ A
In situ m odification in volved direct addition s of th e m odifier, co-m odifier
an d additive (TA, NaBr an d CH3COOH) in th eir solid crystallin e form s (TA,
NaBr) to th e reaction system (m eth yl acetoacetate in tetrah ydrofuran ). Th e
effect of TA loadin g on ee is sh own in Fig. 5. Th is depen den ce revealed a
clear m axim um at 0.15 g of TA with th e weigh t ratios TA/NaBr = 50 an d
TA/CH3COOH = 1. It m ust be n oted th at on ly low en an tioselectivites were
ach ieved (~21%) likely due to th e lim ited solubility of TA an d NaBr in low-
polar tetrah ydrofuran .
Catalysts Prepared by Sol-Gel Method – in situ B
Th is approach was an alogous to in situ A regardin g th e com pon en ts (TA,
NaBr, CH3COOH); h owever, TA an d NaBr were dissolved in water first an d
th en togeth er with acetic acid in troduced to th e reactor with MAA. Th e
weigh t ratios were con stan t in all th e experim en ts (TA/NaBr = 6.25,
TA/CH3COOH = 0.05, TA/H2O = 0.1). Th e catalysts m odified by th e in situ
m eth od B were gen erally h igh ly active an d selective. Figure 5 clearly docu-
m en ts th e depen den ce of ee on th e am oun t of TA. Th e optim um was at
10.5 m g of th e m odifier. In th is case th e com pon en ts were well available on
th e catalytic surface un like in th e in situ m eth od A. Th e catalyst m odified as
described was used repeatedly in MAA h ydrogen ation with out an y sign ifi-
can t decrease in its activity an d en an tioselectivity (Fig. 6). In a sin gle run
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

In situ Chiral Modification of Nickel Catalysts
1649
60
22
20
18
16
14
55
50
45
12
10
8
40
35
6
0
0
0.2 0.4 0.6 0.8 1.0
TA, g
0
6
8
10 12 14 16
TA, mg
FIG. 5
Th e effect of TA am oun t in reaction m edia on th e en an tioselectivity for th e in situ m odified
Ni/SiO2 catalyst (mcat = 3 g of Ni/SiO2, in situ A (᭹): TA/NaBr = 50, TA/CH3COOH = 1; in situ B
(᭜): TA/NaBr = 6.25, TA/CH3COOH = 0.05, 100 µl of water)
60
58
56
54
52
50
1
2
3
4
5
6
7
8
9
10
reaction cycle
FIG. 6
Th e depen den ce of ee on th e n um ber of con secutive reaction cycles for th e in situ B m odified
Ni/SiO2 in a n um ber of con sequen t reaction cycles (mcat = 3 g of Ni/SiO2, TA/NaBr = 6.25,
TA/CH3COOH = 0.05, 100 µl of water; catalyst wash ed with 50 m l of THF after each run )
Collect. Czech. Chem. Commun. (Vol. 70) (2005)

1650
Bartek, Klusoň, Červený:
th e ee as h igh as 59% was ach ieved. After every cycle th e catalyst was kept
in th e reactor an d on ly wash ed with tetrah ydrofuran .
Experim en tal kin etic data for all catalytic system s are sum m arised in
Table I. It is eviden t th at of all th e catalysts Ran ey n ickel con ven tion ally
m odified with NaHTA could be con sidered as th e m ost suitable due to th e
h igh est en an tioselectivity. Con ven tion ally m odified sol-gel catalysts with
NaHTA were also m arkedly good. Th e sam e applied to th e in situ m eth od B
with dissolved TA an d NaBr. In th is specific case h igh resistan ce to deacti-
vation m ust n ot be om itted.
TABLE I
Th e effect of th e catalyst type an d m odification m eth od on th e en an tioselectivity an d reac-
tion rate
Modification
m eth od
Water
solution
ee
%
r
Catalyst
Modifiera
Additive
–1
m m ol gcat m in –1
TA
–
–
–
Yes
Yes
Yes
64
78
5
0.31
0.42
0.21
NaHTA
Na2TA
prem odification
RaNi
in situ A
in situ B
TA
TA
acetic acid
acetic acid
No
2
8
0.05
0.08
Yes
TA
–
–
–
Yes
Yes
Yes
46
55
5
0.47
0.55
0.09
prem odification
NaHTA
Na2TA
Ni/SiO2
in situ A
in situ B
TA
TA
acetic acid
acetic acid
No
12
59
0.43
0.58
Yes
a
NaBr as co-m odifier.
The authors wish to thank the Czech Science Foundation for supporting this project financially
(grant No. 104/03/0409). L. Bartek also thanks the Czech Science Foundation for funding partly his
Ph.D. studentship (grant No. 203/03/H140).
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