Platinum-catalyzed enantioselective hydrogenation of α-ketoesters: An unprecedented surface reaction of methyl pyruvate [15]

Full text of DOI:10.1021/ja0017211

9864
J. Am. Chem. Soc. 2000, 122, 9864-9865
Platinum-Catalyzed Enantioselective Hydrogenation
of r-Ketoesters: An Unprecedented Surface Reaction
of Methyl Pyruvate
†
,†
Jonathan M. Bonello, Richard M. Lambert,*
‡
‡
Niklaus K u¨ nzle, and Alfons Baiker
Department of Chemistry, UniVersity of Cambridge
Cambridge CB2 1EW, United Kingdom
Laboratory of Technical Chemistry
Swiss Federal Institute of Technology
ETH-Zentrum, CH-8092 Z u¨ rich, Switzerland.
ReceiVed May 17, 2000
Figure 1. Fixed-bed reactor transient kinetic experiment of platinum-
catalyzed hydrogenation of methyl pyruvate (MP). Feed conditions during
Enantioselective heterogeneous catalysts are rarities although
their inherent technical importance is huge. Under appropriate
conditions they afford a high degree of stereochemical control
and large rate enhancement effects. Indeed, chiral heterogeneous
catalysis is a subject of indisputable importance in current chemi-
cal research. Such reactions constitute a relatively unexplored field
whose theoretical and practical implications are potentially far
reaching. Although the number of known systems is growing,
the subject as a whole remains, nevertheless, at a relatively early
different periods of time on stream are indicated. Conditions: Pt/Al
2 3
O
catalyst 500 mg; hydrogen pressure 50 bar; flow rates, hydrogen 5.2
-
1
-1
mL‚min , methyl pyruvate 0.5 mL‚min ; room temperature.
1,2
stage of development as is evident from recent reviews. This
is especially true in regard to fundamental studies of the surface
phenomena involved, even in the case of the most studied reac-
tions,includingtheonewhichisthesubjectofthiscommunications
the asymmetric hydrogenation of R-ketoesters on chirally modified
Pt surfaces.3 Although considerable effort has been expended
in the past decade to gain a detailed insight into the functioning
of this system, there are still a number of key issues requiring
clarification. Among these is the question of why the behavior
of this catalytic system depends on the sequence of introduction
of the reactants (methyl pyruvate, hydrogen) and modifier, as
-5
6
demonstrated earlier by transient kinetic measurements.
We have employed a combination of complementary methods
involving solution phase kinetic measurements on a practical
dispersed catalyst and studies on a Pt{111} single-crystal surface
by means of STM and NEXAFS. We show that in the absence
of the cinchona modifier and under conditions of hydrogen
starvation the catalyst deactivates due to blocking of the platinum
surface by self-condensation of the methyl pyruvate reactant.
Catalytic studies were performed using a 4 mm inner diameter
stainless steel tubular fixed-bed reactor system. Details of the
Figure 2. STM image (raw data) of methyl pyruvate on Pt{111} at
-5
2
98 K in the presence of a background pressure of 2 × 10 mbar
hydrogen [constant current mode, 1000 Å × 1000 Å, Ugap ) -1 V, I
T
)
10 nA].
reactor, analysis system, and experimental technique are given
7
elsewhere. The catalyst (5 wt % Pt/Al
2 3
O , Engelhard 4759) was
and a H pressure of 50 bar. STM experiments were carried out
2
pretreated before use in a separate reactor by flushing with 12
using an Omicron UHV STM-1 instrument operating under
-
1
-11
mL‚min
reductive treatment in H
temperature. After being cooled to room temperature in H
N
2
(99.995%) at 673 K for 30 min, followed by a
(99.999%) for 90 min at the same
, the
ultrahigh vacuum conditions (base pressure 5 × 10 mbar). This
2
apparatus incorporated LEED and Auger spectroscopy facilities
8
2
used for surface characterization prior to adsorption experiments.
catalyst was immediately transferred to the reactor and held under
nitrogen. Catalyst (500 mg) was applied, resulting in a bed length
of 30 mm. Methyl pyruvate (MP, Fluka, 97%) was used without
further purification. The reactor was operated at room temperature
Images were acquired in constant current mode and control
experiments indicated that there were no tip-induced artifacts:
neither molecular displacements, nor adsorbate decomposition.
The reaction was started by pre-flushing the reactor with hydro-
-
1
gen (5.2 mL‚min ) for 5 min and then adding methyl pyruvate
*
Address correspondence to this author.
University of Cambridge.
-
1
†
(0.5 mL‚min ). After 30 min the hydrogen flow was stopped
and the reactor was flushed with pure methyl pyruvate for 30
min. After this sequence the reaction was started again by adding
‡
Swiss Federal Institute of Technology.
(
1) Baiker, A.; Blaser, H. U. Handbook of Heterogeneous Catalysis; VCH-
Publishers: Weinheim, 1997; Vol. 5, pp 2422-2436.
-
1
H
2
(5.2 mL‚min ) for 30 min. Figure 1 illustrates the effect of
(
(
2) Baiker, A. Curr. Opin. Solid State Mater. Sci. 1998, 3(1), 86-93.
3) Blaser, H. U.; Jallet, H. P.; M u¨ ller, M.; Studer, M. Catal. Today 1997,
exposing the catalyst to methyl pyruvate in the absence of H . It
2
3
7, 441-463.
is apparent that catalytic activity was strongly suppressed as a
result of the hydrogen starvation step. Conversion of pyruvate to
methyl lactate fell from 80% to ∼ 40%. The cause of this
(8) Baddeley, C. J.; Stephenson, A. W.; Hardacre, C.; Tikhov, M.; Lambert,
R. M. Phys. ReV. B 1997, 56, 12589-12598.
(
(
(
4) Wells, P. B.; Wilkinson, A. G. Top. Catal. 1998, 5, 39-50.
5) Baiker, A. J. Mol. Catal. 1997, 115, 473-493.
6) Margitfalvi, J. L.; Minder, B.; Talas, E.; Botz, L.; Baiker, A. Stud. Surf.
Sci. Catal. 1993, 75, 2471-2474.
(
7) K u¨ nzle, N.; Hess, R.; Mallat, T.; Baiker, A. J. Catal. 1999, 186, 239-
2
41.
1
0.1021/ja0017211 CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/22/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 40, 2000 9865
Figure 3. Composite STM image (raw data and enhanced edge contrast)
of methyl pyruvate on Pt{111} at 298 K in the absence of hydrogen
showing the formation of polymeric chains [constant current mode, 1000
Å × 1000 Å, Ugap ) -1 V, I
T
) 10 nA].
deactivation was studied by STM, which yields the key informa-
9
tion; reference is also made to supporting NEXAFS and
XANES10 data, analyzed and presented in detail elsewhere.
STM images were also taken at room temperature, in the
presence and absence of coadsorbed hydrogen. The former were
acquired by first dosing the clean surface with hydrogen, then
-
5
pyruvate, and finally imaging in a standing pressure of 2 × 10
mbar of H . This procedure ensured that all “non-pyruvate” Pt
sites were always saturated with H adatoms. Figure 2 clearly
shows that, in the presence of H , methyl pyruvate monomers
2
a
Figure 4. Possible mechanisms for methyl pyruvate polymerization on
are distributed uniformly on the terraces of the Pt{111} surface.
Identification of these species as monomers is provided by the
measured molecular dimensions (8 ( 1 Å × 3 ( 0.5 Å) which
accord well with the van der Waals dimensions of methyl
pyruvate. Thus, the organic reactant has retained its integrity and
the whole of the metal surface should be available for catalytic
turnover. No long-range order is apparent (consistent with the
diffuse (1 × 1) LEED pattern) and there is a tendency for the
molecules to accumulate at step edges. In the absence of
coadsorbed hydrogen the picture is dramatically different, as
depicted in Figure 3 (in the upper half of the figure the edge
contrast has been enhanced). The monomers have undergone
reaction yielding polymeric structures, some of which exhibit
dendritic features. These polymeric species are up to ∼320 Å
long (∼ 40 monomer units) and are uniformly distributed over
the Pt{111} terraces. No step decoration is apparent. On such a
surface, only about half the initially available Pt sites would be
available for catalytic turnover, which happens to be com-
mensurate with our reactor data.
platinum.
the most acidic proton lies in the middle of the resulting
molecule: this provides the possibility of subsequent chain growth
taking place either linearly or in a branching fashion, in accord
with Figure 3. Furthermore, the resulting polymeric structures
containing only the CdO function are in excellent agreement with
9
10
our NEXAFS and XANES data.
We conclude that, under certain conditions, self-condensation
of the R-ketoester can be an important side reaction in the
enantioselective hydrogenation of methyl pyruvate on Pt surfaces.
This occurs in the absence of the cinchona modifier when the
ratio of H to adsorbed R-ketoester is too low. Our findings
a
emphasize the great importance of employing correctly chosen
start-up conditions and careful control of reactant concentrations
during catalyst operation, so as to avoid hydrogen starvation.
Catalyst deactivation due to self-condensation of the reactant as
shown here for methyl pyruvate hydrogenation is also likely to
occur with other R-ketoesters employed in this reaction.
The polymerization of aldehydes and ketones on platinum
11,12
group metal surfaces is not unknown.
How might it occur in
Acknowledgment. J.M.B. acknowledges the award of an Overseas
Research Studentship and additional support from the Cambridge
University Oppenheimer Fund. This work was supported in part by the
UK Engineering and Physical Sciences Research Council under Grant
GR/M76706 and by the Swiss National Science Foundation.
the case of methyl pyruvate on platinum? Two plausible mech-
anisms for this process are shown in Figure 4 (a and b). Both are
essentially aldol condensations. Mechanism (a) proceeds by
elimination of hydrogen and methoxy ()MeOH), mechanism (b)
via elimination of water. Both methanol and water are in fact
detected in the temperature-programmed reaction (TPR) of
JA0017211
9
(9) Bonello, J. M.; Williams, F. J.; Santra, A. K.; Lambert, R. M. J. Phys.
Chem. In press.
pyruvate decomposition on Pt{111}. However, mechanism (b)
results in the formation of CdC bonds in the polymeric chains
(
10) B u¨ rgi, T.; Atamny, F.; Knop-Gericke, A.; H a¨ vecker, M.; Schedel-
while our NEXAFS spectra of the polymer clearly show that such
Niedrig, T.; Schl o¨ gl, R.; Baiker, A. Catal. Lett. 2000, 66, 109.
9
(11) Henderson, M. A.; Zhou, Y.; White, J. M. J. Am. Chem. Soc. 1989,
bonds are not present. We therefore discount mechanism (b).
1
11, 1185-1193.
Mechanism (a) involves initial attack at the ester carbonyl (the
most electron deficient center). Once the first “dimer” is produced,
(
12) Henderson, M. A.; Mitchell, G. E.; White, J. M. Surf. Sci. 1987, 188,
206-218.