Lamar University Homogeneous Hydrogenation of Alkenes Catalyzed PPT I need a powerpoint with 10slides for presentation, with times new romanprepare it by u

Lamar University Homogeneous Hydrogenation of Alkenes Catalyzed PPT I need a powerpoint with 10slides for presentation, with times new romanprepare it by using the articles attatched 5152
Organometallics 1999, 18, 5152-5156
Homogeneous Hydrogenation of Alkenes Catalyzed by
the Ruthenium-Hydride Complex (PCy3)2(CO)(Cl)RuH:
Spectroscopic Observation of the Ruthenium-Ethyl and
Ruthenium-Ethylene-Hydride Intermediate Species
Chae S. Yi* and Do W. Lee
Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201-1881
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Received June 28, 1999
The ruthenium-hydride complex (PCy3)2(CO)(Cl)RuH (1a) was found to catalyze the
hydrogenation of terminal and cyclic alkenes. For example, the reaction of 1-hexene with 4
atm of H2 in the presence of 1a (1-hexene:1a ) 8300:1) produced the hydrogenation product
2 (TON ) 12 000 h-1). The treatment of 1a with excess ethylene led to the observation of
both the ruthenium-ethyl and ruthenium-ethylene-hydride species 4a and 5a, respectively.
The thermodynamic parameters for the equilibria among 1a, 4a, and 5a were estimated
from the VT NMR data. These results are consistent with a monohydride mechanistic
pathway.
Introduction
The mechanism of olefin hydrogenation reactions
catalyzed by Wilkinson’s catalyst, (PPh3)3RhCl, has been
well-known to occur via a so-called “dihydride” mechanism, in which a metal-dihydride species is formed
from an oxidative addition of H2 to the metal center.1
In contrast, the mechanism of the hydrogenation reactions catalyzed by complexes of the type (PPh3)3(CO)RhH and (PPh3)3RuHCl has been commonly regarded
to proceed via a “monohydride” pathway.1a The distinguishing features of the hydrogenation reaction catalyzed by monohydride metal complexes include (1) high
activity and selectivity toward terminal alkenes, (2) a
tendency to form isomerization products, and (3) the
generation of metal-alkyl intermediate species from an
insertion of olefin into a metal-hydride bond.1,2 Despite
experimental evidence of its existence, however, the
metal-alkyl intermediate species has been rarely observed under catalytically viable conditions.3 The need
for a better mechanistic understanding of the monohy(1) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987. (b) Spencer, A. In
Comprehensive Coordination Chemistry, Wilkinson, G., Ed.; Pergamon: New York, 1987; Vol. 6. (c) Parshall, G. W.; Ittel, S. D.
Homogeneous Catalysis, 2nd ed.; Wiley: New York, 1992. (d) Chalonger, P. A.; Esteruelas, M. A.; Joo?, F.; Oro, L. A. Homogeneous
Hydrogenation; Kluwer: Boston, 1994. (e) Bruner, H. In Applied
Homogeneous Catalysis with Organometallic Compounds; Cornils, B.,
Herrmann, W. A., Eds.; VCH: New York, 1996; Vol. 1.
(2) (a) Noyori, R. Asymmetric Catalysis in Organic Synthesis;
Wiley: New York, 1994. (b) Takaya, H.; Noyori, R. In Comprehensive
Organic Synthesis; Trost, B. M., Ed.; Pergamon: New York, 1990; Vol.
8. (c) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97-102. (d)
Giardello, M. A.; Conticello, V. P.; Brard, L.; Gagne?, M. R.; Marks, T.
J. J. Am. Chem. Soc. 1994, 116, 10241-10254. (e) Burk, M. J.; Harper,
T. G. P.; Kalberg, C. S. J. Am. Chem. Soc. 1995, 117, 4423-4424. (f)
Ohkuma, T.; Ooka, H.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1995,
117, 10417-10418. (g) Wiles, J. A.; Bergens, S. H.; Young, V. G. J.
Am. Chem. Soc. 1997, 119, 2940-2941. (h) Fehring, V.; Selke, R.
Angew. Chem., Int. Ed. Engl. 1998, 37, 1827-1830. (i) Troutman, M.
V.; Appella, D. H.; Buchwald, S. L. J. Am. Chem. Soc. 1999, 121, 49164917.
dride pathway has also been exemplified by recent
developments in metal-catalyzed asymmetric hydrogenation reactions, wherein the monohydride mechanistic
pathway has been suggested in some cases.2
We recently described an effective synthesis of the
previously inaccessible complex (PCy3)2(CO)(Cl)RuH
(1a).4 Since carbonyl-hydride complexes of the type (L)2(CO)(Cl)MH (M ) Ru, Os; L ) PPri3, PBut2Me) have
been shown to catalyze the hydrogenation and hydrosilylation reactions of alkenes and alkynes,5 we decided
to explore the catalytic activity of 1a toward these
reactions. Our initial hope was that the sterically
demanding PCy3-containing complex 1a might lead to
a higher activity and/or observation of reactive intermediate species. Herein, we wish to report that 1a is
an effective catalyst precursor for the homogeneous
hydrogenation of terminal and cyclic alkenes. We further disclose a direct spectroscopic observation of the
metal-alkyl and alkene-hydride species in the reaction
of 1a and ethylene.
(3) (a) Baird, M. C.; Mague, J. T.; Osborn, J. A.; Wilkinson, G. J.
Chem. Soc. A 1967, 1347-1359. (b) Jardine, F. H. Prog. Inorg. Chem.
1984, 31, 265-370 and references therein. (c) Andriollo, A.; Esteruelas,
M. A.; Meyer, U.; Oro, L. A.; Sa?nchez-Delgado, R. A.; Sola, E.; Valero,
C.; Werner, H. J. Am. Chem. Soc. 1989, 111, 7431-7437. (d) Ashby,
M. T.; Halpern, J. J. Am. Chem. Soc. 1991, 113, 589-594. (e) Ashby,
M. T.; Khan, M. A.; Halpern, J. Organometallics 1991, 10, 2011-2015.
(4) The synthesis of complex 1a was first reported without complete
characterization data and yields in: Mores, F. G.; Langhout, J. P. Recl.
Trav. Chim. Pays-Bas 1972, 91, 591-600. We recently reported a
reliable synthesis and characterization of 1a following a modified
literature procedure:5a,b Yi, C. S.; Lee, D. W.; Chen, Y. Organometallics
1999, 18, 2043-2045.
(5) For the synthesis and reactions of 1b, see: (a) Buil, M. L.; Elipe,
S.; Esteruelas, M. A.; On?ate, E.; Peinado, E.; Ruiz, N. Organometallics
1997, 16, 5748-5755 and references therein. (b) Esteruelas, M. A.;
Werner, H. J. Organomet. Chem. 1986, 303, 221-231. For hydrogenation and hydrosilation reactions catalyzed by 1b and related complexes,
see: (c) Esteruelas, M. A.; Valero, C.; Oro, L. A.; Meyer, U.; Werner,
H. Inorg. Chem. 1991, 30, 1159-1160. (d) Esteruelas, M. A.; Oro, L.
A.; Valero, C. Organometallics 1992, 11, 3362-3369. (e) Esteruelas,
M. A.; Herrero, J.; Oro, L. A. Organometallics 1993, 12, 2377-2379.
10.1021/om990493k CCC: $18.00 © 1999 American Chemical Society
Publication on Web 11/02/1999
Hydrogenation of Alkenes Catalyzed by an Ru-H Complex
Organometallics, Vol. 18, No. 24, 1999 5153
Table 1. Hydrogenation of Alkenes Catalyzed by 1aa
entry
alkene
product
amt of H2 (atm)
turnover rateb
1
2
3
4
5
6
7
8
9
1-hexene
1-hexene
allylbenzene
4-vinyl-1-cyclohexene
cyclopentene
cyclooctene
cyclooctene
trans-4-phenyl-3-buten-2-one
cyclopentenone
hexane
hexane
1-phenylpropane
4-ethylcyclohexene
cyclopentane
cyclooctane
cyclooctane
4-phenyl-2-butanone
cyclopentanone
1.0
4.0
2.0
2.0
1.0
1.0
2.0
4.0
4.0
1200 (180)
12000 (150)c
2000 (390)
1300
960
940
2700
15d
70d
a Reaction conditions: 5.7 mmol of alkene (1.0 M); 0.69 µmol of the catalyst (0.12 mM; alkene:1a ) 8300:1); 5 mL of C H ; room
6 6
temperature. b Turnover rate ) (mol of product) (mol of catalyst)-1 h-1. The values in parentheses correspond to isomerization turnovers.
c The turnover rate was estimated after 30 min of reaction time. d Reaction conditions: 2.0 mmol of alkene; 3.4 µmol of the catalyst
(alkene:1a ) 600:1); 55 °C.
Results and Discussion
We found that 1a is an effective catalyst precursor
for the hydrogenation of terminal and cyclic alkenes.
For example, the treatment of 1-hexene (0.48 g, 5.7
mmol) with 1.0 atm of H2 in the presence of 1a (0.5 mg,
0.69 µmol) at room temperature produced the hydrogenation product n-hexane (2) along with the isomerization products, 2-hexenes (3; TON ) 1200 h-1; 2:3 ) 6.7:
1) (eq 1). The turnover number (TON) for the hydro-
(1)
genation of 1-hexene reached 12 000 h-1 at 4.0 atm of
H2 with a virtually unchanged isomerization rate (Table
1, entry 2).6 The activity of the PPri3 analogue (PPri3)2(CO)(Cl)RuH (1b)5a was found to be considerably lower
than that of 1a under similar reaction conditions (TON
) 700 h-1 for 1-hexene at 1.0 atm of H2).
We next explored the catalytic activity of 1a toward
both terminal and cyclic alkenes. In general, high
activity/selectivity toward the hydrogenation of terminal
alkenes has been observed, as exemplified in the 4-vinyl1-cyclohexene case (entry 4). The hydrogenation rate of
strained cyclic alkenes was somewhat lower than that
of terminal alkenes (entries 5 and 6), while the hydrogenation of internal alkenes was found to be very
sluggish (entries 8 and 9).
In an attempt to better understand the mechanism
of the reaction, reactions of complex 1a with different
alkenes were followed by NMR. The most noticeable
change was observed in the reaction with ethylene. In
a heavy-walled NMR tube, excess ethylene (2.0 mmol)
was condensed into a CD2Cl2 solution containing complex 1a (6 mg, 8.3 µmol), and the tube was flame-sealed
at liquid-N2 temperature. When the mixture was warmed
to room temperature, the formation of a new species was
detected by 31P NMR (? 26.1), and this new species was
found to exist as an equilibrium mixture with 1a (1a:
(new complex) ) 1:1.7). Since no other peaks could be
assigned, due to the presence of PCy3 signals, the
structure of new species was established from an
analogous reaction with 13C-enriched ethylene. The 13C
NMR of the 13C-enriched sample showed two new ethyl
(6) Most of the isomerization reactions were found to occur during
the first 20 min before adding H2 (>90%).
Figure 1. van’t Hoff plot of ln K1 vs 1/T.
carbon resonances at ? 25.1 (qd, JCH ) 126.4, JCC ) 36.0
Hz) and 7.4 (ttd, JCH ) 126.4, JCC ) 36.0, JCP ) 6.9
Hz). The phosphorus signal at ? 26.1 turned into a
doublet (JCP ) 6.9 Hz) due to the coupling with the R-13C
atom. All of these spectroscopic data are consistent with
the ethyl complex 4a. The addition of H2 (1 atm) to this
reaction mixture slowly produced ethane (TON ? 5 h-1).
The equilibrium constants between 1a and 4a were
estimated from the VT NMR using the phosphorus
integration for both complexes in the temperature range
of -40 to +30 °C. The van’t Hoff analysis of these data
led to the thermodynamic parameters of ?H1° ) -7.5
( 1.0 kcal/mol and ?S1° ) -27 ( 4 eu for this
equilibrium reaction (Figure 1), which are comparable
to those for recently reported late-metal-mediated olefin
insertion reactions.7 Examples of low-valent rutheniumalkyl complexes having ?-hydrogens are relatively rare,8
(7) For selected examples on the energetics of late-metal-mediated
olefin insertions, see: (a) Brookhart, M.; Hauptman, E.; Lincoln, D.
M. J. Am. Chem. Soc. 1992, 114, 10394-10401. (b) Rix, F. C.;
Brookhart, M. J. Am. Chem. Soc. 1995, 117, 1137-1138. (c) Tanner,
M. J.; Brookhart, M.; DeSimone, J. M. J. Am. Chem. Soc. 1997, 119,
7617-7618.
5154
Organometallics, Vol. 18, No. 24, 1999
Yi and Lee
Figure 2. Metal-hydride region of the variable-temperature 1H NMR (CD2Cl2, 300 MHz) spectra of 1a and 5a from -80
to +40 °C.
and to the best of our knowledge, this is the first
example in which a ruthenium-alkyl species has been
directly observed under catalytically relevant conditions.
Another ruthenium species was identified in the
reaction of 1a with ethylene at low temperatures. Thus,
the new metal-hydride peak appeared at ? -5.20 (t,
JHP ) 24.0 Hz) at -80 °C, the structure of which was
established as the trans-ethylene-hydride species 5a
on the basis of the spectroscopic data. The hydride peak
of 5a was found to undergo temperature-dependent
chemical shift changes; the hydride peak of 5a at ?
-5.20 continuously moved toward the hydride peak of
1a at ? -23.0 ppm upon warming to room temperature
(Figure 2). The equilibrium constants (K2) between 1a
and 5a were determined by correlating the chemical
shift of the hydride peak in the temperature range of
-80 to +40 °C using a two-site chemical shift exchange
(8) (a) Hill, A. F. In Comprehensive Organometallic Chemistry II;
Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: New York,
1994; Vol. 12. (b) Fabre, S.; Kalck, P.; Lavigne, G. Angew. Chem., Int.
Ed. Engl. 1997, 36, 1092-1095. (c) Faure, M.; Maurette, L.; Donnadieu,
B.; Lavigne, G. Angew. Chem., Int. Ed. Engl. 1999, 38, 518-522. (d)
Edwards, A. J.; Elipe, S.; Esteruelas, M. A.; Lahoz, F. J.; Oro, L. A.;
Valero, C. Organometallics 1997, 16, 3828-3836.
Figure 3. van’t Hoff plot of ln K2 vs 1/T.
method.9 The van’t Hoff plot for this equilibrium reaction led to the thermodynamic parameters of ?H2° )
-12.0 ( 0.3 kcal/mol and ?S2° ) -45 ( 2 eu (Figure
3). In all of these measurements, ethylene concentration
in the solution was found to be in excess and remained
relatively constant throughout the temperature range
(ethylene:1a g 50:1; [ethylene] ) 1.7 M).10
We found that the phosphine ligand strongly inhibits
the catalytic reaction. For example, the addition of 0.7
mol % of PCy3 to the reaction mixture led to a drastically
(9) Hauger, B. E.; Gusev, D.; Caulton, K. G. J. Am. Chem. Soc. 1994,
116, 208-214.
Hydrogenation of Alkenes Catalyzed by an Ru-H Complex
Organometallics, Vol. 18, No. 24, 1999 5155
Scheme 1
reduced catalytic activity for 1-hexene (TON ) 20 h-1
at 1.0 atm of H2). Furthermore, the rate of the hydrogenation reaction was found to be considerably lower if
H2 was added to a cooled solution before adding an
alkene substrate (e.g., TON ) 5100 h-1 with 4.0 atm of
H2), compared to the reaction mixture stirred for about
20 min in the presence of an alkene substrate prior to
adding H2. In a separate experiment, the treatment of
1a with ca. 2 atm of H2 led to the formation of the ?2H2 complex 6, which was found to exist as a mixture
with 1a at low temperature. These results suggest that
the formation of ?2-H2 complex 6 might initially retard
the rate of hydrogenation by preventing the alkene
coordination to 1a. Previously, small molecules, including H2 and ethylene, have been shown to coordinate to
similar ruthenium and osmium complexes.11
We observed an inverse isotope effect for the hydrogenation reaction. For example, the hydrogenation of
cyclooctene with 2.0 atm of H2/D2 led to kH/kD ) 0.7 at
23 ( 0.5 °C. We also found that the hydrogenation of
allylbenzene using a 1:1 mixture of H2 and D2 (2 atm)
produced a mixture of d0, d1, and d2 hydrogenation
products. The formation of both d1 and d2 products is
consistent with the involvement of a monohydride
species. While it is risky to draw any conclusions for a
multistep catalytic reaction,12 these data suggest that
the C-H bond-forming process is the rate-determining
step. Similar inverse equilibrium isotope effects have
been commonly observed in metal-promoted hydrogen
transfer and alkene insertion reactions.13
These results are consistent with the generally accepted monohydride mechanistic pathway for the hydrogenation reaction (Scheme 1). One plausible way to
a catalytically active species is an alkene-induced dissociation of PCy3 ligand from 1a. The initial inhibition
of the reaction by H2 and the formation of 6 suggest that
(10) Ethylene concentration in the solution was found to vary ca.
20% in the temperature range -80 to +40 °C as measured against an
internal standard. We treated ethylene as a constant in calculating
equilibrium constants because ethylene concentration was sufficiently
in excess (>50-fold) compared to the ruthenium complexes in all cases.
An analogous measurement of equilibrium constants at a higher
pressure of ethylene ([ethylene] ) 2.2 M) was found to give virtually
identical thermodynamic parameter values.
(11) (a) Esteruelas, M. A.; Sola, E.; Oro, L. A.; Meyer, U.; Werner,
H. Angew. Chem., Int. Ed. Engl. 1988, 27, 1563-1564. (b) Gusev, D.
G.; Vymenits, A. B.; Bakhmutov, V. I. Inorg. Chem. 1992, 31, 2-4. (c)
Bakhmutov, V. I.; Bertra?n, J.; Esteruelas, M. A.; Lledo?s, A.; Maseras,
F.; Modrego, J.; Oro, L. A.; Sola, E. Chem. Eur. J. 1996, 2, 815-825.
(d) Gusev, D. G.; Kuhlman, R. L.; Renkema, K. B.; Eisenstein, O.;
Caulton, K. G. Inorg. Chem. 1996, 35, 6775-6783. (e) Huang, D.;
Huffman, J. C.; Bollinger, J. C.; Eisenstein, O.; Caulton, K. G. J. Am.
Chem. Soc. 1997, 119, 7398-7399.
(12) Halpern, J.; Okamoto, T.; Zakhariev, A. J. Mol. Catal. 1976, 2,
65-68.
(13) Bullock, R. M. In Transition Metal Hydrides, Dedieu, A., Ed.;
VCH: New York, 1992.
the coordination of an alkene substrate might facilitate
the dissociation of a phosphine ligand from the metal
center. The spectroscopic observation of 4a also implies
that the unsaturated alkyl species 7 could be formed
from 4 via the dissociation of a phosphine ligand. The
subsequent H2 addition/reductive elimination sequences
have been commonly proposed in the reactions catalyzed
by monohydride metal complexes.1,2
In summary, the ruthenium-hydride 1a was found
to catalyze the hydrogenation reaction of terminal and
cyclic alkenes. The spectroscopic observation of key
intermediate species, 4a and 5a, and preliminary
mechanistic studies suggest a monohydride mechanistic
pathway for this reaction. Efforts to increase the
catalytic activity of 1a and to determine the kinetics of
the hydrogenation reaction are currently underway.
Experimental Section
General Information. All reactions were carried out in
an inert-atmosphere glovebox or by using standard highvacuum and Schlenk-line techniques unless otherwise noted.
Benzene, hexanes, and Et2O were distilled from purple solutions of sodium and benzophenone immediately prior to use.
CH2Cl2 was distilled from CaH2. The NMR solvents were dried
from activated molecular sieves (4 Å). All organic alkene
substrates were vacuum-distilled from molecular sieves or
sodium prior to use. The 1H, 13C, and 31P NMR spectra were
recorded on a GE GN-Omega 300 MHz FT-NMR spectrometer.
Mass spectra were recorded from a Hewlett-Packard HP 5890
GC/MS spectrometer. Gas chromatographs were recorded from
a Hewlett-Packard HP 6890 GC spectrometer.
General Procedure of the Catalytic Hydrogenation
Reaction. In a glovebox, 1.0 mL of a predissolved 0.69 mM
C6H6 solution of 1a (0.5 mg, 0.69 µmol) was placed in a 100
mL Fisher-Porter pressure bottle equipped with a pressure
gauge and a stirring bar. The solution was diluted with 4.0
mL of C6H6. Excess alkene (5.7 mmol) was added, and the
bottle was attached to a vacuum line. The reaction bottle was
evacuated while it was cooled in a liquid-N2 bath. The reaction
bottle was warmed to room temperature with stirring over a
20 min period. An appropriate amount of H2 gas pressure was
applied to the reaction bottle, and the reaction mixture was
vigorously stirred at room temperature (22-23 °C) for 0.5-4
h. The product ratio was determined by GC.
VT NMR Study for the Reaction of 1a with Ethylene.
The ruthenium-hydride complex 1a (6 mg, 8.3 µmol) was
dissolved in 0.3 mL of CD2Cl2 in a heavy-walled NMR tube.
After three cycles of freeze-pump-thaw degassing, excess
ethylene gas (125 mL at 300 Torr, 2.0 mmol) was condensed
into the tube at liquid-N2 temperature, after which the tube
was flame-sealed. The tube was warmed to room temperature
and the solution was allowed to equilibrate for 30 min before
NMR analysis. The sample tube was inserted into the NMR
probe, and the probe was cooled by a cold stream of N2. The
equilibrium constants K1 were determined from the phospho-
5156
Organometallics, Vol. 18, No. 24, 1999
rus integration of 1a and 4a by 31P NMR in the temperature
range -40 to +30 °C (10 °C intervals). Similarly, the equilibrium constants K2 were determined by measuring the change
in chemical shift of the metal-hydride between 1a and 5a from
-80 to +40 °C (10 °C intervals). The sample was allowed to
equilibrate for 10-15 min before the data acquisition at each
temperature. The van’t Hoff plots using Cricket Graph v. 1.3
led to the thermodynamic values listed in the main text
(Figures 1 and 3). The stack plot of the variable-temperature
1H NMR spectra of 1a and 5a in the metal-hydride region is
shown in Figure 2. The ethylene concentration was assumed
to be constant thr…
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