Ruthenium dihydride ruh2(pph3)2((r,r)-cyclohexyldiamine) and ruthenium monohydride ruhcl(pph3)2(r,r)-cyclohexyldiamine: active catalyst and catalyst precursor for the hydrogenation of ketones and imines

Article abstract of DOI:10.1021/om000231e

The new monohydride RuHCl(PPh₃)₂(R,R-cydn), with base added, and dihydride RuH₂(PPh₃)₂-(R,R-cydn), in the absence of base, catalyze the hydrogenation of a wide variety of ketones and some imines at 3 atm of H₂ and 20°C with high turnover numbers. The mechanism is thought to involve the concerted dihydrogen transfer from cis hydride and N-H groups to the substrate followed by heterolytic dihydrogen splitting.

Full text of DOI:10.1021/om000231e

Organometallics 2000, 19, 2655-2657
2655
Ru th en iu m Dih yd r id e
Ru H (P P h ) ((R,R)-cycloh exyld ia m in e) a n d Ru th en iu m
2
3 2
Mon oh yd r id e Ru HCl(P P h ) ((R,R)-cycloh exyld ia m in e):
3
2
Active Ca ta lyst a n d Ca ta lyst P r ecu r sor for th e
Hyd r ogen a tion of Keton es a n d Im in es
Kamaluddin Abdur-Rashid,* Alan J . Lough, and Robert H. Morris*
Department of Chemistry, University of Toronto, 80 St. George Street,
Toronto, Ontario M5S 3H6, Canada
Received March 16, 2000
Summary: The new monohydride RuHCl(PPh3)2(R,R-
cydn), with base added, and dihydride RuH2(PPh3)2-
R,R-cydn), in the absence of base, catalyze the hydro-
Sch em e 1
(
genation of a wide variety of ketones and some imines
at 3 atm of H2 and 20 °C with high turnover numbers.
The mechanism is thought to involve the concerted
dihydrogen transfer from cis hydride and N-H groups
to the substrate followed by heterolytic dihydrogen
splitting.
Noyori and co-workers have reported RuCl2(PPh3)3/
diamine and RuCl2(diphosphine)(diamine) systems, which
are precursors for the generation of what appears to be
some of the most active catalysts for the homogeneous
and asymmetric hydrogenation of ketones in a 2-pro-
panol/base mixture.1
-3
They suspected that mono-
hydride or dihydride species were the active and selec-
tive catalysts in these systems, but they were unable
to isolate and characterize these (see footnote 16 of ref
This has led to the discovery that the dihydride Ru(H)2-
(PPh ) (cydn) (2; cydn ) (R,R)-cyclohexyldiamine) is a
3 2
very active ketone and imine hydrogenation catalyst,
while the monohydride complex Ru(H)(Cl)(PPh ) (cydn)
). In related work they4 and others have provided
,5
6
3
3
2
evidence that the bifunctional Ru-H/N-H motif, a
hydride and amine coordinated cis on ruthenium(II),
plays an important role in the hydrogenation of ketones
and imines by transfer of hydrogen from 2-propanol or
2 3 3
(1) is inactive. Ruthenium dihydrides RuH (L)(PPh )
1
2
13
(L ) H , N , vacant site ) are catalysts for the
2
2
hydrogenation of cyclohexanone in THF and the trans-
fer hydrogenation of cyclopentanone from 2-propanol
solvent. To the best of our knowledge, catalysts based
on ruthenium dihydride complexes with two amino
ligands are unprecedented.
[
HNEt3][HCO2]. Our interest in ruthenium hydride
7
-9
and dihydrogen chemistry
and in protonic-hydridic
NH‚‚‚HM bonds1
0,11
led us to study the nature of the
ruthenium hydrides that might be present in the Noyori
RuCl2(PPh3)3/diamine/KOH/2-propanol catalytic system.
When an equimolar mixture of RuHCl(PPh ) and
cydn in tetrahydrofuran is stirred at room temperature
under a nitrogen atmosphere, the substituted complex
3
3
(
1) Mikami, K.; Korenaga, T.; Terada, M.; Ohkuma, T.; Pham, T.;
RuHCl(PPh3)2(cydn) (1) is formed quantitatively (Scheme
Noyori, R. Angew. Chem., Int. Ed. 1999, 38, 495-497 and references
therein.
14
1
, eq 1). A similar reaction with ethylenediamine
yields RuHCl(PPh3)2(en), as will be described elsewhere.
(
2) Ohkuma, T.; Ooka, H.; Ikariya, T.; Noyori, R. J . Am. Chem. Soc.
995, 117, 10417-10418.
3) Ohkuma, T.; Doucet, H.; Pham, T.; Mikami, K.; Korenaga, T.;
1
31
1
1
The H and P{ H} NMR spectra are consistent with a
structure containing mutually cis hydride and phos-
phorus nuclei. The structure of 1 as shown in Scheme
(
Terada, M.; Noyori, R. J . Am. Chem. Soc. 1998, 120, 1086-1087.
(
(
4) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97-102.
5) Yamakawa, M.; Ito, H.; Noyori, R. J . Am. Chem. Soc. 2000, 122,
1
has been confirmed in a preliminary X-ray diffraction
1
466-1478.
6) Alonso, D. A.; Brandt, P.; Nordin, S. J . M.; Andersson, P. G. J .
Am. Chem. Soc. 1999, 121, 9580-9588.
7) Abdur-Rashid, K.; Gusev, D.; Lough, A. J .; Morris, R. H.
Organometallics 2000, 19, 834-843.
8) Fong, T. P.; Forde, C. E.; Lough, A. J .; Morris, R. H.; Rigo, P.;
Rocchini, E.; Stephan, T. J . Chem. Soc., Dalton Trans. 1999, 4475-
486.
9) Lough, A. J .; Morris, R. H.; Ricciuto, L.; Schleis, T. Inorg. Chim.
Acta 1998, 270, 238-246.
10) Lough, A. J .; Park, S.; Ramachandran, R.; Morris, R. H. J . Am.
Chem. Soc. 1994, 116, 8356-8357.
11) Abdur-Rashid, K.; Gusev, D. G.; Landau, S. E.; Lough, A. J .;
Morris, R. H. J . Am. Chem. Soc. 1998, 120, 11826-11827.
study.
(
(
(12) Linn, D. E.; Halpern, J . J . Am. Chem. Soc. 1987, 109, 2969-
2974.
(13) Aranyos, A.; Csjernyik, G.; Szabo, K. J .; Backvall, J . E. Chem.
Commun. 1999, 2131-2132.
3 3
(14) 1: THF (2 mL) was added to RuHCl(PPh ) (300 mg, 0.31 mmol)
(
4
(
and (R,R)-trans-cyclohexyldiamine (36 mg, 0.32 mmol) and the mixture
stirred for 6 h under nitrogen. The resulting solution was filtered and
hexanes (10 mL) added to the filtrate, precipitating a yellow solid.
(
1
2
Yield: 224 mg, 94%. H NMR (δ): -17.99 (t), J HP ) 25.7 Hz, 1H (RuH);
3
1
1
(
0.45 ppm (br), 2H (NH); 2.38 ppm (br), 2H (NH). P{ H} NMR (δ):
1
70.6 (s). IR: 1987 cm-
.
1
0.1021/om000231e CCC: $19.00 © 2000 American Chemical Society
Publication on Web 06/09/2000

2
656 Organometallics, Vol. 19, No. 14, 2000
Communications
Ta ble 1. Ca ta lytic Hyd r ogen a tion s Usin g
a
2 3 2 2
Ru H (P P h ) (R,R-cyd n ) (2) a n d H Ga s
F igu r e 1. Molecular structure and atomic numbering of
complex 2.
of Ru(1)-N(1) (2.225(3) Å). The RuH‚‚‚HN distances of
2
.8-3.4 Å are too long for significant hydrogen bonding
in the solid state. The solid state (Nujol) infrared
spectrum of 2 shows two sharp νRu-H peaks at 1833 and
-
1
1
3
844 cm
and four sharp νNH signals in the range
-
1
266-3337 cm , also in keeping with a lack of in-
tramolecular hydrogen bonding.
a
A sealed solution of complex 2 in benzene-d6 under
deuterium gas resulted in the rapid disappearance
(within 5 min) of both the NH (2.04 ppm) and hydride
Neat substrate except for entries 3, 9, and 10, where benzene
was used as the solvent. The product of entry 3 is exclusively the
allyl alcohol PhCHdCHCH(OH)Me.
(
-
2
-18.3 ppm) chemical shifts. A series of four triplets at
In the presence of catalytic amounts of a base such
as sodium hydroxide or sodium isopropoxide under H2
gas (3.5 atm) at 20 °C, complex 1 (1:base ) 1:10)
facilitates the efficient hydrogenation of neat ketones
and imines to alcohols and amines, respectively (results
similar to those of Table 1). In the absence of a base, no
hydrogenation was observed, clearly demonstrating that
the hydrido chloro species 1 is not the true catalyst.
When a mixture of 1 (50 mg, 65 µmol), acetone (4 mg,
18.25, -18.23, -18.22, and -18.20 ppm with J HP )
1
7 Hz in the hydride region of the H NMR spectrum is
observed after a 60% decrease in the intensity of the
hydride signal. These are due to some of the isotopomers
of 2 with one hydride, one deuteride, and with zero to
four deuterium atoms in place of the diamine N-H. The
rapid H/D exchange observed for both the hydrides and
NH moieties when a solution of 2 is exposed to D2 gas
is suggestive of an equilibrium between dihydride and
dihydrogen tautomers where the dihydrogen species is
too low in concentration to be detected (see eq 6, Scheme
6
7 µmol), and potassium isopropoxide (7 mg, 65 µmol)
in C6D6 (0.7 mL) was stirred under hydrogen gas (3.5
1
atm) for 30 min, the H NMR spectrum showed complete
2
). Such a mechanism has been proposed for H/D
conversion of the ketone to 2-propanol and the presence
of the novel dihydride species Ru(H)2(PPh3)2(cydn) (2).
The pure, bright yellow dihydride was prepared by
stirring equimolar amounts of 1 and potassium tri-sec-
butylborohydride in tetrahydrofuran for 12 h under a
nitrogen atmosphere at room temperature (Scheme 1,
eq 2) and then isolated.15 The triplet at -18.3 ppm with
1
7
exchange in iridium hydride-amine complexes and
has been observed directly for osmium hydride-thiol/
dihydrogen-thiolate complexes. No deuteration of
either the hydride or NH moieties is observed for com-
plex 1. The dihydrogen tautomer would be disfavored
in this case because the hydride in 1 is less basic, being
trans to chloride.
Complex 2 readily catalyzes the hydrogenation of neat
ketones to the alcohols under 3 atm of H2 gas at 20 °C
without the addition of a base. For example, neat
acetophenone (4.1 g, 34 mmol) was quantitatively
hydrogenated to (S)-phenethyl alcohol (60% ee) in less
than 8 h by use of catalyst 2 (5 mg, 0.0068 mmol).
1
8
1
J HP ) 27 Hz for the hydride in the H NMR spectrum
and the singlet at 67.2 ppm in the 31P{ H} NMR spectra
of this complex in C6D6 are consistent with trans phos-
phines and cis hydrides. The single-crystal X-ray struc-
ture of the complex is shown in Figure 1.1 The complex
is chiral and approximately C2 symmetric. The two
Ru-H bond lengths are 1.53(2) and 1.62(3) Å. The
Ru(1)-N(2) bond length of 2.284(2) Å is longer than that
1
6
(
16) Crystals of C42
diethyl ether solution of the complex with hexanes:
monoclinic, space group P2 , a ) 8.9595(2) Å, b ) 17.7941(6) Å, c )
112.1757(4) Å, â ) 103.688(2)°, V ) 1885.99(10) Å , d ) 1.306 g cm
Z ) 2, T ) 150(1) K, 17 210 reflections collected, R(F) ) 0.0369 and
R
H
46
N
2
P
2
Ru (2) were obtained by layering a
M
r
) 741.82,
(15) 2: THF (2 mL) was added to 1 (200 mg, 0.27 mmol) and
1
3
-3
potassium tri-sec-butylborohydride (200 mg of 1 M solution) and the
mixture stirred for 6 h under nitrogen. The resulting mixture was
filtered and evaporated to dryness, and the solids were extracted with
diethyl ether (10 mL). The ether solution was concentrated to 2 mL
and hexanes (5 mL) added, precipitating a bright yellow solid. Yield:
,
2
w
(F ) ) 0.0648 for 7972 independent reflections.
(17) Koelliker, R.; Milstein, D. J . Am. Chem. Soc. 1991, 113, 8524-
8525.
1
2
1
2
42 mg, 74%. H NMR (δ): -18.25 ppm (t), J HP ) 27.0 Hz, 2H (RuH);
(18) Schlaf, M.; Morris, R. H. J . Chem. Soc., Chem. Commun. 1995,
625-626.
3
1
1
.04 ppm (d), 2H (NH). P{ H} NMR (δ): 67.2 ppm (s).

Communications
Organometallics, Vol. 19, No. 14, 2000 2657
Sch em e 2
currently under study. When the resulting solution was
then exposed to H2 gas, the H NMR spectrum shows
1
the presence of phenethyl alcohol and regenerated 2.
These observations would be consistent with the
proposed mechanism shown in Scheme 2. The first steps
in the catalytic cycle (eqs 3 and 4) involves the concerted
transfer of the hydride to the carbonyl carbon and NH
proton to the oxygen as proposed also for the RuH-
6
4,5
(
η -arene)(NH2LNTs) catalysts.
Steps 5 and 6 are
proposed on the basis of the H/D exchange results
mentioned above.
This direct hydrogenation mechanism, in which the
dihydride catalyst is regenerated from H2 gas, differs
significantly from the transfer hydrogenation process
6
reported for the series of RuH(η -arene)(NH2LNTs)
complexes, in which a hydrogen-donor solvent such as
an alcohol or triethylammonium formate is required for
the regeneration of the monohydride catalyst. However,
in both of these classes of complexes, the presence of a
cis-M-H‚‚‚H-N bifunctional motif seems to be a key
feature for the activity of these catalysts.
Benzene was used as the solvent for the R,â-unsaturated
ketone, and the product was exclusively the allyl alcohol.
The same selectivity for the ketone over the olefin was
2
reported by Okhuma et al. Our study demonstrates
that deactivated and sterically congested ketones such
as tert-butyl methyl ketone (pinacolone) can also be
readily hydrogenated using this procedure. The high
conversions shown for the dialkyl ketones are unprec-
edented and attest to the enhanced activity of these
catalysts under the current conditions. The hydro-
genation of the aldimine and the ketimine in benzene
by use of 2 also proceeded under notably mild conditions
Ack n ow led gm en t. This work was supported by a
grant to R.H.M. from the NSERC of Canada and a loan
of ruthenium(III) chloride from J ohnson Matthey Ltd.
Su p p or tin g In for m a tion Ava ila ble: Text giving com-
plete characterization data for complexes 1 and 2 and X-ray
crystallographic tables for complex 2. This material is available
free of charge via the Internet at http://pubs.acs.org.
(Table 1).
One equivalent of acetophenone reacts with 2 in C6D6
in the absence of dihydrogen to give an intermediate
OM000231E