Kinetic study of asymmetric hydrogenation of methyl levulinate using the (COD)Ru(2-methylallyl)2-BINAP-HCl catalytic system

Article abstract of DOI:10.1016/j.molcata.2009.06.024

The kinetics of asymmetric hydrogenation of methyl levulinate in the presence of the (COD)Ru(2-methylallyl)₂-BINAP-HCl catalytic system was studied. The kinetic order in H₂, as well as in the catalyst, was found to be equal to 1, whe

Full text of DOI:10.1016/j.molcata.2009.06.024

Journal of Molecular Catalysis A: Chemical 311 (2009) 61–65
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Kinetic study of asymmetric hydrogenation of methyl levulinate using the
(
COD)Ru(2-methylallyl) –BINAP–HCl catalytic system
2
∗
Olga V. Turova, Eugenia V. Starodubtseva, Maxim G. Vinogradov , Vladimir A. Ferapontov
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky prosp., 119991 Moscow, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history:
The kinetics of asymmetric hydrogenation of methyl levulinate in the presence of the (COD)Ru(2-
methylallyl)2–BINAP–HCl catalytic system was studied. The kinetic order in H2, as well as in the catalyst,
was found to be equal to 1, whereas the kinetic order for the substrate and HCl falled from 1 to 0 with
their increasing starting concentration. The kinetic and P NMR data testify that the mechanism of the
reaction under consideration, most likely, involves the hydride transfer to the protonated substrate in the
dinuclear ruthenium complex as the rate-limiting step of the catalytic cycle.
Received 16 March 2009
Received in revised form 5 June 2009
Accepted 22 June 2009
31
Available online 30 June 2009
Keywords:
©
2009 Elsevier B.V. All rights reserved.
Asymmetric hydrogenation
Methyl levulinate
Catalysis
Ruthenium complexes
Kinetics
1. Introduction
bond hydrogenation [10], the mechanism of C O bond hydrogena-
tion was much less well known [11–20].
Ruthenium(II) complexes with chiral atropisomeric bispho-
ꢀ
sphine ligands, such as BINAP [2,2 -bis(diphenylphosphino)-
1
2. Experimental
ꢀ
,1 -binaphthyl] and its analogs, are efficient catalysts for the
asymmetric hydrogenation of ␣- and ␤-ketoesters [1]. At the same
time, there are only a few examples of the use of ␥-ketoesters
in this reaction [1–8] because these compounds are considerably
less active than ␣- and ␤-ketoesters. Among ruthenium-containing
catalytic systems, which we tested in the asymmetric hydro-
genation of ethyl levulinate used as a model ␥-ketoester, the
2.1. General
Commercial (Acros) levulinic acid, (R)-BINAP, and ruthenium
precatalyst 1 (both from Strem) were used without additional
purification. Methyl ester 2 was prepared by esterification of lev-
ulinic acid. Methanol was dehydrated and distilled under a stream
of argon before use. Argon was purified by passing through columns
(
1
COD)Ru(2-methylallyl)2 (1)–BINAP–HCl system (COD–cycloocta-
,5-diene) (Ru/HCl = 5–10) was found to be most efficient [7]. In this
system, the acid plays a dual role. Thus, HCl converts precatalyst 1
9] into the dihalogenide complex followed by the substitution of
containing a nickel/chromium catalyst, copper supported on Kiesel-
guhr (80 ◦C), and molecular sieves (4 Å). Hydrogen was purified
[
by passing through columns packed with a nickel/chromium cat-
COD with BINAP and serves as a promoter with respect to the in situ
alyst and molecular sieves. The 31P NMR spectra were recorded
prepared catalyst [(BINAP)RuCl ], resulting in a significant increase
on a Bruker AC-200 spectrometer (81.02 MHz) with respect to 85%
H PO .
2
inthehydrogenationrate. Theresultingchiral␥-hydroxyesterread-
ily undergoes cyclization into ␥-valerolactone in the course of the
reaction. The hydrogenation enantioselectivity of 98–99% ee with
complete conversion of ␥-ketoester was achieved [7] (Scheme 1).
In the present study, the hydrogenation kinetics of methyl lev-
ulinate (2) in the presence of the (1)–BINAP–HCl catalytic system
3
4
2.2. General procedure for hydrogenation
A solution of ketoester 2 (500 mg, 3.8 mmol) in a mixture of
MeOH (2 mL) and a 2 M solution of HCl in MeOH (95 ␮L, 0.19 mmol)
was degassed through three freezing–evacuation–thawing–argon
filling cycles. Then the degassed solution was poured into a tube
for hydrogenation containing precatalyst 1 (6 mg, 0.019 mmol) and
was investigated. As opposed to the transition metal-catalyzed C
C
(R)-BINAP (12 mg, 0.019 mmol). The tube was placed in a stainless-
∗ Corresponding author. Fax: +7 499 135 5328.
E-mail address: ving@ioc.ac.ru (M.G. Vinogradov).
steel autoclave (50 mL) which was purged with purified hydrogen
and pressured with H up to 4.3 atm. Then the reaction mixture was
2
1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.06.024

6
2
O.V. Turova et al. / Journal of Molecular Catalysis A: Chemical 311 (2009) 61–65
Scheme 1. Asymmetric catalytic hydrogenation of ␥-ketoesters.
◦
3.2. 31P NMR study of transformations of the ruthenium catalyst
magnetically stirred at 60 C. The hydrogenation experiments were
carried out in the kinetic region at a stirring rate of 700 rpm. In the
course of the reaction, the amount of absorbed H2 was determined
at intervals from the pressure decrease and the degree of conver-
sion. The hydrogenation enantioselectivity determined by GLC [7]
for lactone 4 was equal to 98–99% ee (R).
The catalyst [(BINAP)RuCl ] (A) was prepared in situ by the addi-
2
tion of two equivalents of HCl and one equivalent of BINAP to
complex 1 in acetone followed by removal of the solvent in vacuo.
The catalyst was directly used in the hydrogenation. The 31P NMR
spectrum of the catalyst thus prepared (orange powder) shows two
AB quartets, whose integral intensities are 1/2 (Fig. 3a). The main
signals in the 29–47 ppm region of the 31P NMR spectrum (a) may be
assigned to a mixture of neutral dinuclear ruthenium complexes B
and C (Scheme 2) containing two and three bridging chloride atoms,
respectively [21–23].
3. Results
3
.1. Kinetics of hydrogenation of ketoester 2
Fig. 1 shows that there is an induction period (20–30 min) for
After the introduction of HCl (in MeOH) into the NMR tube
(HCl/Ru = 7), the red-brown solution was allowed to stand at
the hydrogenation. This can be attributed to the fact that some
time is required for the formation of catalytically active particles,
after which extensive hydrogenation occurs. The latter period cor-
responding to the 15–20% conversion of 2 is characterized by the
highest hydrogenation rate (ωmax); and hydroxyester 3 and lactone
31
room temperature for 2 h, and the
P NMR spectrum was
again recorded. As evident from Fig. 3b, the signals in the
29–47 ppm region disappeared, and new signals at 51–59 ppm
were observed. Most probably, complexes B and C interact with
HCl to give an equilibrium mixture of similar anionic dinu-
clear complexes D and E involving bridging chloride atoms.
The synthesis of this type of anionic complexes from other
ruthenium precursors was documented, and these complexes
were characterized by 31P NMR spectroscopy [21,24,25]. As a
rule, these complexes contain the quaternary tetraalkylammo-
4
are produced in a ratio of 30/70. As can be seen from the plots in
Fig. 2, the ruthenium-catalyzed hydrogenation of ketoester 2 fol-
lows first order kinetics in H2 (a) and the catalyst (b) throughout
the range of their starting concentrations. At the same time, the
order in both the substrate (c) and HCl (d) changes from 1 to 0 with
increasing their starting concentrations.
Fig. 1. Kinetics of accumulation of hydrogenation products at different starting H2 pressures (a) and at different catalyst (b), substrate (c), and HCl (d) concentrations
◦
−1
in anhydrous MeOH at 60 C. Variable reaction conditions: p0 (H2) = 1.8 (1), 2.7 (2), 3.5 (3), 4.3 (4) atm, [Ru] = 1.41, [2]0 = 760, [HCl] = 16.2 mmol L (a); p0 (H2) = 4.3 atm,
−
1
−1
[
(
Ru] = 0.94 (1), 1.12 (2), 1.41 (3), 1.87 (4), [2]0 = 760, [HCl] = 16.2 mmol L (b); p0 (H2) = 4.3 atm, [Ru] = 1.41, [2]0 = 58 (1), 120 (2), 350 (3), 580 (4), 760 (5), [HCl] = 16.2 mmol L
c); p0 (H2) = 4.3 atm, [Ru] = 1.41, [2]0 = 760, [HCl] = 1.9 (1), 4.3 (2), 6.7 (3), 9.1 (4), 16.2 mmol L (5) (d).
−
1

O.V. Turova et al. / Journal of Molecular Catalysis A: Chemical 311 (2009) 61–65
63
Fig. 2. Dependence of ωmax on the starting H2 pressure (a) and the catalyst (b), substrate 2 (c), and HCl (d) concentrations.
nium counter-ion. To our knowledge, the only proton-containing
genation of ␥-ketoesters in the presence of the title catalytic system
can be proposed (Scheme 2).
2
dinuclear anionic Ru(II) complex, [{RuCl(tetra-Me-BITIOP)} (␮ -
2
−
+
ꢀ
ꢀ
ꢀ
Cl) ] H [tetra-Me-BITIOP-4,4 -bis(diphenylphosphino)-2,2 ,5,5 -
tetramethyl-3,3 -bithiophene], was reported until now. It was
3
ꢀ
4.1. Role of dinuclear ruthenium complexes
prepared by the reaction of Ru(CF COO) (tetra-Me-BITIOP) with
3
2
an HCl excess. The structure of the complex was estimated by X-
ray diffraction. As in the present study, the addition of HCl to the
It is known that only the halogenide-containing ruthenium
bisphosphine complexes are the active catalysts for the ketoester
hydrogenation [26]. Dinuclear halogenide complexes of the B and
C types have been studied in sufficient detail. In the absence of
neutral ruthenium complex caused a considerable shift of the 31
signals in the NMR spectrum [6].
P
superinduced donor ligands, such as PPh , the equilibrium between
3
4
. Discussion
mono- and dinuclear complexes in solution is virtually completely
shifted to the latter [27,28]. In dinuclear chloride complexes, the
phosphorus atoms of the bisphosphine ligand coordinated to the
ruthenium atom are often spectroscopically different (the AB sys-
Based on the results of the present study along with the data
available in the literature, the most probable mechanism of hydro-
Fig. 3. The 31 P NMR spectrum (CDCl3/MeOH = 3/1) of the catalyst prepared in situ from (COD)Ru(2-methylallyl)2, (R)-BINAP, and HCl (1/1/2) in acetone (a), and the NMR
spectrum of the same catalyst after the introduction of an acid excess ([HCl]/[Ru] = 7) into an NMR tube (b).

6
4
O.V. Turova et al. / Journal of Molecular Catalysis A: Chemical 311 (2009) 61–65
Scheme 2. Mechanism of ruthenium-catalyzed hydrogenation of ␥-ketoesters.
tem in the 31P NMR spectrum) even at room temperature [21]. The
P NMR spectrum assigned to dinuclear complexes B and C (Fig. 3a)
narily considered as the first step of catalytic hydrogenation [32].
Thus, in the course of ruthenium-catalyzed hydrogenation of 1,3-
pentanedione, the dinuclear ruthenium complex of type F was
shown (NMR) to be the main intermediate, whereas no mononu-
clear H2-containing ruthenium species was detected in the reaction
mixture [11]. It can be suggested that intermediate D having a free
vacancy adds the H2 molecule to give intermediate F.
Basically, the homogeneous catalytic hydrogenation of multiple
bonds, including the C O bond [32,33], may follow two alternative
pathways (Scheme 3).
31
is in accordance with this fact. Taking into account the results of the
kinetic study (Section 3.1), non-detectable mononuclear species A
cannot apparently be considered as catalytically active because the
half order kinetics in the catalyst would be expected in the case
of their reversible formation from the greatly predominant dinu-
clear complexes B or C. Earlier, the determining role of dinuclear
complexes was found for the ruthenium-catalyzed hydrogenation
of styrene [23]. Although two halogenide-containing anionic
dinuclear complexes D and E would be expected to be formed
by protonation of neutral intermediates B and C with HCl, only
complex D is most likely involved in the catalytic cycle since
complex E contains no free vacancy for the H2 coordination.
4.2. Coordination and heterolysis of H2
Beginning with the basic Kubas’ reports [29,30], a wide range
of transition metal complexes with coordinated molecular hydro-
gen, including ruthenium complexes [31,32], has been synthesized.
Coordination of molecular hydrogen by a metal complex is ordi-
Scheme 3. Principal routes of the catalytic C O bond hydrogenation through het-
erolysis of H2 (a and b) and the concerted pathway (c).

O.V. Turova et al. / Journal of Molecular Catalysis A: Chemical 311 (2009) 61–65
65
Any of the pathways postulated in most publications involves
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