The first structure determination of a possible intermediate in ruthenium 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl catalyzed hydrogenation with a prochiral group bound to ruthenium. Stoichiometric reaction of a chiral ruthenium-carbon bond with dihydrogen gas

Full text of DOI:10.1021/ja964014+

2940
J. Am. Chem. Soc. 1997, 119, 2940-2941
solution during hydrogenation of (Z)-methyl R-acetamidocin-
namate (MAC) catalyzed by a Ru-BINAP compound.We
recently reported the synthesis and the catalytic activity of [Ru-
((R)-BINAP)(H)(MeCN)(sol)2](BF4) (1, sol ) MeOH or THF).5
Compound 1 catalyzed the hydrogenation of MAC in methanol
solutions to generate N-acetylphenylalanine methyl ester (MACH2)
in 86% ee (R) (eq 1).5 This enantioselectivity is comparable to
those of other Ru-BINAP complexes.1a,4e
The First Structure Determination of a Possible
Intermediate in Ruthenium
2,2′-Bis(diphenylphosphino)-1,1′-binaphthyl
Catalyzed Hydrogenation with a Prochiral Group
Bound to Ruthenium. Stoichiometric Reaction of a
Chiral Ruthenium-Carbon Bond with Dihydrogen
Gas
Jason A. Wiles and Steven H. Bergens*
Department of Chemistry, UniVersity of Alberta
Edmonton, Alberta T6G 2G2, Canada
Victor G. Young
We found that the stoichiometric reaction between MAC and
1 in acetone at room temperature resulted in rapid formation of
a predominant species (2, >99%) in solution (eq 2). 31P NMR
spectra recorded under conditions similar to those of the catalytic
reaction (ambient temperature, 2 mol % of 1, MeOH solution,
pressure H2 ≈ 2 atm) showed that 2 was the predominant species
in solution during the catalytic hydrogenation.6 NMR suggested
X-ray Crystallographic Laboratory, 160 Kolthoff Hall
Chemistry Department, 207 Pleasant St. S.E.
The UniVersity of Minnesota
Minneapolis, Minnesota 55455
ReceiVed NoVember 20, 1996
Complexes of Ru(II) and 2,2′-bis(diphenylphosphino)-1,1′-
binaphthyl (BINAP) comprise the most effective catalyst
systems developed for the enantioselective hydrogenation of
prochiral olefins and ketones. Nearly 200 reports1 describing
these reactions, including several industrial syntheses, have
appeared since the first examples were disclosed in 1985 and
1986.2 Despite the intense study of these systems, there are no
reports of structural characterization (even using spectroscopy)3
of a species with a prochiral olefin or ketone bound to a Ru
center. The structures of the catalytic intermediates (and
therefore the origins of enantioselection) are speculative as they
have been inferred from indirect methodssisotopic labeling,
olefin isomerizations, and kinetic studies.4 We now report the
first isolation, structural characterization, and reaction with
dihydrogen gas of the major Ru-containing species present in
that 2 resulted from transfer of the hydride in 1 to the â-olefinic
carbon of MAC and transfer of Ru to the R-carbon to form a
5-membered metallacycle. Further, the signal in the 13C{1H}
NMR spectrum of 2 for the R-carbon showed cis- and trans-
2
2
coupling (δ 67.3, JCPcis ) 3.9 Hz, JCPtrans ) 42.2 Hz) to the
phosphorus nuclei, suggesting that the R-carbon was coordinated
to Ru in the plane containing the phosphine groups. The 13C
signals for the amido and the ester carbonyl groups were also
coupled to the phosphorus nuclei, suggesting that these groups
were coordinated to Ru as well. Rh(III) and Ir(III) alkyl hydride
compounds that are related to 2 have been spectroscopically
characterized by Halpern and Brown.7,8
* Author to whom correspondence should be addressed at (403)-492-
9703 (voice), (403)-492-8231 (fax), or Steve.Bergens@UAlberta.CA (e-
mail).
(1) (a) Takaya, H.; Ohta, T.; Noyori, R. In Catalytic Asymmetric
Synthesis; Ojima, I., Ed.;VCH Publishers: New York, 1993; p 1. (b) Noyori,
R. In Aymmetric Catalysis in Organic Synthesis; Wiley-Interscience, John
Wiley & Sons: New York, 1994; p 16. For recent examples, see: (c)
Kitamura, M.; Tokunaga, M.; Noyori, R. J. Am. Chem. Soc. 1993, 115,
144. (d) Mezzetti, A.; Costella, L.; Del Zotto, A.; Rigo, P. Gazz. Chim.
Ital. 1993, 123, 155. (e) Hoke, J. B.; Hollis, L. S.; Stern, E. W. J. Organomet.
Chem. 1993, 455, 193. (f) Manimaran, T.; Wu, T.-C.; Kolbucar, W. D.;
Kolich, C. H.; Stahly, G. P.; Fronczek, F. R.; Watkins, S. E. Organometallics
1993, 12, 1467. (g) Chiba, T.; Miyashita, A.; Nohira, H. Tetrahedron Lett.
1993, 34, 2351. (h) Wan, K.-t.; Davis, M. E. Tetrahedron Asymmetry 1993,
4, 2461. (i) Chan, A. C. S.; Laneman, S. Inorg. Chim. Acta 1994, 223,
165. (j) Kitamura, M.; Hsiao, Y.; Ohta, M.; Tsukamoto, M.; Ohta, T.;
Takaya, H.; Noyori, R. J. Org. Chem. 1994, 59, 297. (k) Geneˆt, J. P.; Pinel,
C.; Ratovelomanana-Vidal, V.; Mallart, S.; Pfister, X.; Can˜o De Andrade,
M. C.; Laffitte, J. A. Tetrahedron Asymmetry 1994, 5, 665. (l) Geneˆt, J. P.;
Pinel, C.; Ratovelomanana-Vidal, V.; Mallart, S.; Pfister, X.; Bischoff, L.;
Can˜o De Andrade, M. C.; Darses, S.; Galopin, C.; Laffitte, J. A. Tetrahedron
Asymmetry 1994, 5, 675. (m) Masima, K.; Kusano, K.-h.; Sato, N.;
Matsumura, Y.-i.; Nozaki, K.; Kumobayashi, H.; Sayo, N.; Hori, Y.;
Ishizaki, T.; Akutagawa, S.; Takaya, H. J. Org. Chem. 1994, 59, 3064. (n)
Chan, A. C. S.; Chen, C. C.; Yang, T. K.; Huang, J. H.; Lin, Y. C. Inorg.
Chim. Acta 1995, 234, 95. (o) Geneˆt, J. P.; Ratovelomanana-Vidal, V.; Can˜o
De Andrade Tetrahedron Lett. 1995, 36, 2063. (p) Ohkuma, T.; Ooka, H.;
Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1995, 117, 2675.
(q) Kitamura, M.; Tokunaga, M.; Noyori, R. J. Am. Chem. Soc. 1995, 117,
2931. (r) Burke, M. J.; Harper, G. P.; Kalberg, C. S. J. Am. Chem. Soc.
1995, 117, 4423. (s) Geneˆt, J. P.; Ratovelomanana-Vidal, V.; Can˜o De
Andrade, M. C.; Pfister, X.; Guerreiro, P.; Lenoir, J. Y. Tetrahedron Lett.
1995, 36, 4801. (t) Sun, Y.; LeBlond, C.; Wang, J.; Blackmond, D. G.;
Laquidara, J.; Sowa, J. R., Jr. J. Am. Chem. Soc. 1995, 117, 12647. (u)
Ohta, T.; Tonomura, Y.; Nozaki, K.; Takaya, H.; Mashima, K. Organo-
metallics 1996, 15, 1521.
(3) There have been several reports of observations of unidentified species
by NMR spectroscopy. (a) Saburi, M.; Takeuchi, H.; Ogasawara, M.;
Tsukahara, T.; Ishii, Y.; Ikariya, T.; Takahashi, T.; Uchida, Y. J. Organomet.
Chem. 1992, 428, 155. (b) King, S. A.; DiMichele, L. In Catalysis of
Organic Reactions, Chemical Industries; Scaros, M. G., Prunier, M. L.,
Eds.; Marcel Dekker, Inc.: New York, 1995; Vol. 62, p 157. For a crystal
structure of ∆-bis(tiglato)[(R)-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl]-
ruthenium(II), see: Ashby, M. T.; Khan, M. A.; Halpern, J. Organometallics
1991, 10, 2011. The prochiral olefins were not bound to the Ru center of
this complex.
(4) (a) Saburi, M.; Takeuchi, H.; Ogasawara, M.; Tsukahara, T.; Ishii,
Y.; Ikariya, T.; Takahashi, T.; Uchida, Y. J. Organomet. Chem. 1992, 428,
155. (b) Ashby, M. T.; Halpern, J. J. Am. Chem. Soc. 1991, 113, 589. (c)
Chan, A. S. C.; Chen, C. C.; Yang, T. K.; Huang, J. H.; Lin, Y. C. Inorg.
Chim. Acta 1995, 234, 95. (d) Ohta, T.; Takaya, H.; Noyori, R. Tetrahedron
Lett. 1990, 31, 7189. (e) Kawano, H.; Ikariya, T.; Ishii, Y.; Saburi, M.;
Yoshikawa, S.; Uchida, Y.; Kumobayashi, H. J. Chem. Soc., Perkin Trans.
1 1989, 1571. (f) Brown, J. M. Chem. Soc. ReV. 1993, 25. For mechanistic
studies attending use of other ruthenium-phosphine complexes as catalysts,
see: (g) James, B. R.; McMillan, R. S.; Morris, R. H.; Wang, D. K. W.
AdV. Chem. Ser. 1978, 167, 122. (h) James, B. R.; Wang, D. K. W. Can.
J. Chem. 1980, 58, 245. (i) Jardine, F. H. Prog. Inorg. Chem. 1984, 31,
265. (j) Dekleva, T. W.; Thorburn, I. S.; James, B. R. Inorg. Chim. Acta
1985, 100, 49.
(5) Wiles, J. A.; Lee, C. E.; McDonald, R.; Bergens, S. H. Organome-
tallics 1996, 15, 3782.
(6) Because of the low [Ru], 5000 scans were required to obtain a
reasonable 31P NMR spectrum. A spectrum recorded 1.5 h after mixing
(after approximately 8 turnovers) showed 2 as the predominant species in
solution.
(2) Ikariya, T.; Ishii, Y.; Kawano, H.; Arai, T.; Saburi, M.; Yoshikawa,
S.; Akutagawa, S. J. Chem. Soc., Chem. Commun. 1985, 992. Noyori, R.;
Ohta, M.; Hsiao, Y.; Kitamura, M.; Ohta, T.; Takaya, H. J. Am. Chem.
Soc. 1986, 108, 7117.
(7) Brown, J. M.; Chaloner, P. A. J. Chem. Soc., Chem. Commun. 1980,
344.
S0002-7863(96)04014-0 CCC: $14.00 © 1997 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 119, No. 12, 1997 2941
major enantiomer of the catalytic hydrogenation. The stoichio-
metric reaction of 2 with dihydrogen gas under conditions
similar11 to those of the catalytic hydrogenation resulted in
formation of MACH2 and [Ru((R)-BINAP)(H)(η6-MACH2)]+
(3), in which MACH2 was bonded to Ru as an η6-arene
ligand.12,13 MACH2 was liberated from 3 by refluxing in MeCN
solution (to generate [Ru((R)-BINAP)(H)(MeCN)3]+) (eq 3).
Figure 1. The solid state structure of 2 as determined by X-ray
diffraction. The positions of the hydrogen atoms are based on geometries
of the parent carbon atoms. Non-hydrogen atoms are represented at
the 20% probability level. Selected bond lengths (Å) are as follows:
Ru(1)-N(2), 2.009(9); Ru(1)-P(1), 2.269(3); Ru(1)-P(2), 2.369(3);
C(3)-C(6), 1.526(14).
The ee of the combined portions of MACH2 was 83% (R).14
Assuming that direct reaction of 2 with dihydrogen gas results
in a stereospecific replacement of Ru by hydrogen,15 these
results imply that formation of 2 was to some extent reversible
under the conditions of the catalytic hydrogenation.16,17
Hydrogenolysis of the bond between [Ru(BINAP)] and a
chiral carbon center has been proposed as a key step in several
catalytic hydrogenations and may be the enantioselective step
of the present catalytic hydrogenation.4 Further investigation
is required to determine if 2 is an actual intermediate in the
catalytic cycle and if the chiral interactions in 2 are relevant to
the origins of enantioselection. We note, however, that forma-
tion of compound 2 was rapid relative to the overall rate of the
catalytic hydrogenation, that 2 was likely the predominant
species in solution during catalysis, and that the rate of reaction
of 2 with dihydrogen gas was comparable to that of the catalytic
reaction.18
Figure 1 shows the solid state structure of 2 as determined
by X-ray diffraction.9 The positions of the signals in the 13C
CP/MAS NMR spectrum of 2 were nearly identical to those in
the solution spectrum, implying that the solid state structure
was representative of the solution structure. As predicted from
the NMR data, MACH was bonded to the Ru center via the
R-carbon and the amido and ester groups. Similar tridentate
bonding of MACH to a metal center was identified spectro-
scopically at low temperatures by Brown et al. for [Rh-
(DIPAMP)(MACH)H](BF4) (DIPAMP is (R,R)-1,2-bis[(o-
methoxyphenyl)phenylphosphino]ethane).7 We note that the
amido group of 2 occupied a coordination site cis to both
phosphines and that the ester group was coordinated in the plane
containing the phosphines. Molecular models indicated that
exchange of coordination sites by the ester and amide groups
would result in severe steric repulsions between an equatorial
phenyl group of (R)-BINAP and the benzyl group of MACH
for both absolute configurations around C(3). The bonding of
the enolate of MACH to Ru resembled that of an η3-oxo allyl
group in two ways. First, the geometry around the R-carbon
of MACH was intermediate between that of an sp3 and an sp2
carbon center.10 Second, the distance between Ru and the
carbonyl carbon of the ester group (Ru(1)-C(4) 2.353(9) Å)
was comparable to the ruthenium-oxygen (2.260(7)Å (Ru(1)-
O(2)), 2.075(7) Å (Ru(1)-O(1)) and to the ruthenium-carbon
(2.257 (10) Å (Ru(1)-C(3)) bond lengths.
Acknowledgment. This work was supported by the Natural
Sciences and Engineering Research Council of Canada and by the
University of Alberta. The expert assistance of Mr. G. Bigam of the
University of Alberta High Field NMR Laboratory in recording the
13C CP/MAS NMR spectrum of 2 is sincerely appreciated. We also
thank Dr. Robert McDonald, Faculty Service Officer, Structure De-
termination Laboratory, University of Alberta, for his assistance with
preparing this manuscript.
Supporting Information Available: Experimental and X-ray
crystallographic details (42 pages). See any current masthead page
for ordering and Internet access instructions.
JA964014+
(11) The dihydrogen gas pressure, temperature, and solvent were the same
as those for the catalytic reaction, but the [2] for the stoichiometric reaction
was ∼3 times higher than the initial [1] for the catalytic reaction.
(12) The remainder of the ruthenium compounds were unidentifiable,
and presumably resulted from decomposition of 1 under these conditions
in the absence of MAC.
The absolute configuration at the R-carbon was S. Stereospe-
cific replacement of Ru by a hydrogen atom would generate
(R)-MACH2, which has the same absolute configuration as the
(13) Compound 3 was identified by NMR spectroscopy. We synthesized
the (S)-MACH2 and the (rac)-MACH2 η6-arene adducts for comparison.
See the Supporting Information for NMR data.
(14) The ee was determined by separating MACH2 from [Ru((R)-
BINAP)(H)(MeCN)3]+ by chromatography in ethyl acetate over silica gel,
followed by use of Eu(tfc)3 shift reagent in chloroform-d1.
(15) We are investigating the reaction of 2 with acid in MeOH, with
dideuterium gas in MeOH, with dihydrogen gas in MeOH-d4, and with para-
enriched dihydrogen gas. We point out that solvolysis of the ruthenium-
carbon bond may also have occurred under these conditions.
(8) (a) Chan, A. S. C.; Halpern, J. J. Am. Chem. Soc. 1980, 102, 838.
(b) Brown, J. M.; Maddox, P. J. J. Chem. Soc., Chem. Commun. 1987,
1276. (c) Ramsden, J. A.; Claridge, T. D. W.; Brown, J. M. J. Chem. Soc.,
Chem. Commun. 1995, 2469.
(9) Crystals suitable for structure determination by X-ray diffraction were
obtained by liquid-liquid diffusion of diethyl ether into a 1,2-dichloroethane
solution of 3 at room temperature. Crystal data: M ) 1145.93; monoclinic,
space group P21; a ) 15.3846(1) Å, b ) 18.9635(3) Å, c ) 21.8056(1) Å,
â ) 101.856(1)°, V ) 6225.98(11) Å3, Z ) 4, dcalcd ) 1.223 g cm-3; crystal
size 0.45 × 0.25 × 0.12 mm; µ(Mo KR) ) 0.359 mm-1. Data were
measured using a Siemens SMART Platform CCD diffractometer with Mo
KR radiation (λ ) 0.710 73 Å; q range for data collection of 1.35-25.10°);
(16) A mechanism involving reversible formation of an intermediate
similar to 2 was proposed to account for the isomerization of (E)-2-
(benzoylamino)cinnamic acid to (Z)-2-(benzoylamino)cinnamic acid ob-
served during a hydrogenation catalyzed by Ru2Cl4[(R)-BINAP]2NEt3.4e
(17) Low-temperature reactions between 1 and MAC resulted in observa-
tion of two isomers of [Ru((R)-BINAP)(H)(η3-MAC)(MeCN)]+ (MAC is
bonded through the amide carbonyl group and through an olefin-ruthenium
π-bond) and one other compound. We are investigating the solution
structures and kinetics of these compounds in order to further understand
the mechanism of this hydrogenation.
2
16 214 independent reflections (10 610 with Fo2 > 2s(Fo )) were measured.
The structure was solved via direct methods (SHELXTL-V5.0). Full-matrix,
least-squares refinement on F2 (SHELXTL-V5.0) yielded R1 ) 0.0685 Fo
2
2
> 2s(Fo ), and wR2 ) 0.1733 (all data).
(10) Bond angles (deg) around C(3) with estimated standard deviations:
C(4)-C(3)-C(6), 118.1(8); N(1)-C(3)-C(4), 111.1(9); N(1)-C(3)-C(6),
114.6(8); Ru(1)-C(3)-N(1), 99.7(6); Ru(1)-C(3)-C(6), 132.7(7); Ru-
(1)-C(3)-C(4), 73.9(5).
(18) The stoichiometric reaction was 92% complete after 1.25 h.