Ketone H2-hydrogenation catalysts: Ruthenium complexes with the headphone-like ligand bis(phosphaadamantyl)propane

Article abstract of DOI:10.1016/j.ica.2005.11.018

Treatment of 1,3-diphosphinopropane with acetylacetone in the presence of HCl gives the new chiral bis(phosphaadamantyl)propane ligand (bpap) (1) as a mixture of diastereoisomers. Recrystallization from ethanol gives a mixture enriched in rac diastereoisomer (90% rac/10% meso). The enriched mixture reacts with [RuHCl(PPh₃)₃] in refluxing THF to give [RuHCl(bpap)(PPh₃)] (2) in 73% yield. Compound 2 reacts readily with chiral diamines giving octahedral trans-[RuHCl(bpap)(diamine)] complexes 3 (diamine = (1R,2R)-1,2-diaminocyclohexane) and 4 (diamine = (1R,2R)-1,2-diphenylethylenediamine). Compounds 3 and 4 are very active catalysts for H₂-hydrogenation of neat acetophenone in the presence of KO^tBu as a strong base under mild conditions (room temperature, 3 atm of H₂). The low ee values for 1-phenethanol can be attributed to the similar shapes of two terminal adamantoid cages and the flexible backbone of the bpap ligand. The structures of complexes 2 and 3 have been determined by single-crystal X-ray diffraction.

Full text of DOI:10.1016/j.ica.2005.11.018

Inorganica Chimica Acta 359 (2006) 2864–2869
www.elsevier.com/locate/ica
Ketone H2-hydrogenation catalysts: Ruthenium complexes with
the headphone-like ligand bis(phosphaadamantyl)propane
a
a
a,
b
*
Alen Hadzovic , Alan J. Lough , Robert H. Morris
,
b
Paul G. Pringle , Damaris E. Zambrano-Williams
a
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ont., Canada M5S 3H6
b
School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK
Received 21 October 2005; accepted 20 November 2005
Available online 24 January 2006
Dedicated to Professor Brian James on the occasion of his 70th birthday.
Abstract
Treatment of 1,3-diphosphinopropane with acetylacetone in the presence of HCl gives the new chiral bis(phosphaadamantyl)propane
ligand (bpap) (1) as a mixture of diastereoisomers. Recrystallization from ethanol gives a mixture enriched in rac diastereoisomer (90%
rac/10% meso). The enriched mixture reacts with [RuHCl(PPh3)3] in refluxing THF to give [RuHCl(bpap)(PPh3)] (2) in 73% yield. Com-
pound 2 reacts readily with chiral diamines giving octahedral trans-[RuHCl(bpap)(diamine)] complexes 3 (diamine = (1R,2R)-1,2-diami-
nocyclohexane) and 4 (diamine = (1R,2R)-1,2-diphenylethylenediamine). Compounds 3 and 4 are very active catalysts for H2-
hydrogenation of neat acetophenone in the presence of KOtBu as a strong base under mild conditions (room temperature, 3 atm of
H2). The low ee values for 1-phenethanol can be attributed to the similar shapes of two terminal adamantoid cages and the flexible back-
bone of the bpap ligand. The structures of complexes 2 and 3 have been determined by single-crystal X-ray diffraction.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Chiral diphosphine; Ruthenium hydride; Ketone hydrogenation
1. Introduction
cance for asymmetric catalysis has been the advent of chiral
diphosphines such as DIOP [1], BINAP [2], DIPAMP [3]
and DUPHOS [4]. In this paper, we report the synthesis of
the chiral diphosphine bis(phosphaadamantyl)propane
(bpap) (1) and the synthesis, properties and preliminary cata-
lytic studies of the its chlorohydridoruthenium(II) complexes.
The development of new diphosphine ligands has been the
subject of much research in the past decades, stimulated
mostly by the potential use of these ligands in transition metal
complexes for homogeneous catalysis. Of particular signifi-
O
O
O
O
O
O
O
O
O
P
P
O
O
P
O
P
rac-1
meso-1
*
Corresponding author. Tel./fax: +1 416 978 6962.
E-mail address: rmorris@chem.utoronto.ca (R.H. Morris).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.11.018

A. Hadzovic et al. / Inorganica Chimica Acta 359 (2006) 2864–2869
2865
Diphosphine 1 can be prepared as a mixture of meso and
rac isomers in a one-pot procedure. Ligand 1 has previ-
ously been shown to have an unusually large cone angle
of 173ꢁ which cannot be reduced by intermeshing [5] and
an evaluation of its electronic properties from the m(CO)
of [Ni(CO)2(1)] showed that 1 has a r-basicity/p acidity
more akin to diphosphines containing P(aryl)2 than
P(alkyl)2 groups [6].
Some transition metal complexes having the meso/rac-
bpap ligand have already been reported [5] and Pd-catalyzed
tandem isomerisation-carbonylation of internal alkenes to
linear esters has been described [6]. Here, we report the first
use of 1 in an asymmetric catalytic transformation, the
hydrogenation of prochiral ketones.
the reaction mixture was stirred for a further 72 h. The
white solid product was then filtered off in air, washed with
water (2 · 20 ml) and then dried in vacuo to give 5.24 g
(60%) of 1. Recrystallisation of the approximately 1:1 mix-
ture of meso/rac isomers of 1 from boiling ethanol (0.5 g of
1 with 10 ml of ethanol) gave a solid containing predomi-
nantly (9:1) rac-1 with 76% recovery. EI mass spectrum:
m/z 472 (M+). Anal. Calc. for 1: C, 58.5; H, 8.1. Found:
C, 58.3; H, 8.4%. 1H NMR (CDCl3) d 1.50–2.00 (m,
14H, cage CH2, propane bridge), 1.25–1.48 (m, 24H, cage
CH3). 31P NMR (CDCl3) ꢀ30.2 (s, meso isomer), ꢀ31.0
(s, rac isomers).
2.3. Preparation of [RuHCl(bpap)(PPh3)] (2)
2. Experimental
Dry THF (8 ml) was added to 9:1 mixture of rac/meso-1
(0.177 g, 0.375 mmol) and [RuHCl(PPh3)3] (0.320 g,
0.35 mmol) and the mixture refluxed for 2 h under an
argon atmosphere giving a deep violet solution. The sol-
vent was removed in vacuo. The solid residue was extracted
with dry THF (2 ml) and filtered to remove insoluble impu-
rities. The resulting solution was concentrated in vacuo to
approximately 0.5 ml. Dry hexanes (15 ml) was added, pre-
cipitating a dark violet product. This mixture was left stir-
ring overnight. The pale violet liquid above the precipitate
was decanted and additional hexanes (approximately 5 ml)
added and stirring continued for 3 h. Finally, the violet
product was collected by filtration, washed with hexanes
and vacuum dried. Yield: 220 mg (73%). The product is
soluble in THF, ether and sparingly soluble in hexanes.
Crystals suitable for X-ray structural determination
were obtained spontaneously from a C6D6 solution without
evaporation. The structure coordinates have been depos-
ited at the Cambridge Crystallographic Data Centre and
allocated the deposition number CCDC 287142. 1H
2.1. General
All reactions were performed using either standard
Schlenk or glove box techniques with either an argon or a
dinitrogen atmosphere. Tetrahydrofuran, diethyl ether and
hexanes were purified and dried by reflux under an argon
atmosphere over sodium using benzophenone as an indica-
tor. Solvents for NMR were purchased from Cambridge
Isotope Laboratories: benzene-d6 was kept over molecular
sieves and THF-d8 (in ampoules) was used as received.
[RuHCl(PPh3)3] [7] and 1,3-diphosphinopropane [8] were
prepared as previously reported. (1R,2R)-1,2,-diaminocy-
clohexane (R,R-dach) and (1R,2R)-1,2-diphenyl-ethylene-
diamine (R,R-dpen) were purchased from Sigma–Aldrich
and used without further purification.
1H and 31P NMR spectra were recorded on a Varian
Gemini 300 spectrometer at 300 MHz (1H) and
121.5 MHz (31P). 1H NMR shifts are referred to tetrameth-
ylsilane and 31P to 85% H3PO4 as an external reference. IR
spectra were obtained on a PE Spectrum BX FT-IR spec-
trometer as Nujol mulls. Elemental analyses were done
on a Perkin–Elmer 2400 C/H/N/S analyzer. X-ray data
were collected on a Nonius Kappa-CCD diffractometer
using Mo Ka radiation and refined using SHELXTL-6.1 soft-
ware. Samples from catalytic tests were analyzed for con-
version and ee using a Perkin–Elmer Autosystem XL gas
chromatograph with a Chrompack capillary column Chira-
silDex CB (25 m · 0.25 mm). The carrier gas was H2 at a
column pressure of 5 psi, with an oven temperature of
130 ꢁC, injector temperature of 250 ꢁC, and FID tempera-
ture of 275 ꢁC. The retention times were: acetophenone
5.0 min, (R)-1-phenylethanol 8.5 min and (S)-1-phenyleth-
anol 9.1 min. The injected sample volume was 1 lL.
2
NMR (THF-d8) d ꢀ17.62 (ddd, 1H, RuH, JHP = 20.2,
22.1 and 35.7 Hz,), 0.50–4.56 (m, 60H, cage CH, propane
bridge), 6.73–7.09 (m, 15H, phenyl from PPh3); 31P
2
NMR (THF-d8) d 27.44 (ddd, JPP = 246.7 and 32.0 Hz,
2
2JPH = 20.2), 40.99 ppm (ddd, JPP = 246.7 and 7.8 Hz,
2
2JPH = 22.1 Hz), 80.96 ppm (ddd, JPP = 32.0 and 7.8 Hz,
2JPH = 35.7), IR (Nujol, cmꢀ1): 1858, 1881 (RuH). Anal.
Calc. for 2: C, 56.14; H, 6.24. Found: C, 55.88; H, 6.73%.
2.4. Preparation of trans-[RuHCl(bpap)(R,R-dach)] (3)
R,R-dach (0.015 g, 0.13 mmol) was added to a solution
of 2 (0.090 g, 0.10 mmol) in dry diethyl ether (1 ml). The
formation of a sparingly soluble, pale yellow green product
followed immediately. The resulting mixture was stirred for
additional 30 min. Dry hexanes was added (5 ml) and stir-
ring continued overnight. The liquid above the precipitate
was decanted and additional hexanes (2 ml) was added.
After 2 h the pale yellow-green product was collected by fil-
tration, washed with hexanes and vacuum dried. Yield:
51 mg (70%). The product is very soluble in THF and ben-
2.2. Synthesis of bpap ligand (1)
The phosphine H2P(CH2)3PH2 (2.0 ml, 18.5 mmol) was
added dropwise over 5 min to a stirred solution of 2,4-pen-
tanedione (11.40 g, 114 mmol) in aqueous HCl (40 ml, 5 M,
200 mmol). After 1 h a white precipitate began to form but
1
zene, poorly soluble in ether and insoluble in hexanes. H

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A. Hadzovic et al. / Inorganica Chimica Acta 359 (2006) 2864–2869
2
NMR (C6D6) d ꢀ19.08 (dd, 1H, RuH rac-1, JHP = 24.7,
to room temperature. The reactions were carried at
20 ꢁC. The sampling was done under a flow of hydrogen
when the H2 pressure would fall to atmospheric. After each
sample, the Schlenk bomb was cooled to the liquid nitro-
gen temperature to raise the pressure back. The samples
were diluted with ethanol and analyzed by GC.
2
29.4 Hz), ꢀ19.22 (dd, 1H, RuH rac-1, JHP = 23.7,
2
31.2 Hz), ꢀ18.01 (t, 1H, RuH meso-1, JHP = 25.8 Hz),
0.39–3.31 (m, CH in cage and propane bridge), 3.78 (s),
4.46 (s), 4.69 (s); 31P NMR (C6D6): d 46.74 (bd,
2JPP = 8.5 Hz), 47.85 (bd, 2JPP = 8.5), 48.1 (bd,
2JPP = 14 Hz), 50.6 (bd, 2JPP = 9.5 Hz), 51.6 (bd,
2JPP = 9.5), 51.65 (bd, JPP = 14 Hz). IR (Nujol, cmꢀ1):
3. Results and discussion
2
720 (m), 1373 (s), 1458 (s), 1996 (w, RuH), 3163, 3307,
3329, 3366 (w, NH). Anal. Calc. for 3: C, 48.05; H, 7.38;
N, 3.88. Found: C, 47.89; H, 7.45; N, 4.28%.
3.1. The synthesis of bpap (1)
Treatment of 1,3-diphosphinopropane with acetylace-
tone in the presence of HCl gave the desired diphosphine
1 in 60% yield as a mixture of diastereoisomers (Eq. (1)).
Samples rich in rac diastereoisomer (90%) are readily
obtained by recrystallisation of the mixture from ethanol
(see Section 2).
2.5. Preparation of trans-[RuHCl(bpap)(R,R-dpen)] (4)
A similar procedure as for 3 was used to prepare 4:
0.046 g (0.052 mmol) of 2 and 0.012 g (0.057 mmol) of
R,R-dpen gave 29 mg (82%) of the product. The product
O
O
O
O
HCl
P
O
O
P
H P
2
PH
2
+
meso-1
ð1Þ
2
O
O
O
O
O
O
O
P
O
P
rac-1
is pale yellow, soluble in THF and benzene and insoluble
The appearance of ligand 1 in crystal structures of its
compounds (see below for example) is reminiscent of a pair
of headphones. The terminal adamantanoid cages are the
source of the chirality but in the enantiomeric forms will
have very similar (and quite symmetrical) shapes and there-
fore might not be expected to give high enantioselectivities.
1
in hexanes. H NMR (C6D6) d ꢀ18.65 (dd, RuH rac-1,
2JHP = 25.2, 28.5 Hz), ꢀ18.79 (dd, RuH rac-1,
2JHP = 24.6, 30.6 Hz), ꢀ17.75 (t, RuH meso-1,
2JHP = 22.2 Hz) 1.03–5.53 (m), 6.70–7.16 (m, 2Ph); 31P
2
NMR (C6D6) d 48.7 (br d, JPP = 8.1 Hz), 51.5 (br d,
2
2JPP = 8.1 Hz), 52.1 (br d, JPP = 10.1 Hz), 53.0 (bd,
2
2JPP = 10.1 Hz), 54.2 (br d, JPP = 9.3), 54.3 (bd,
3.2. Synthesis and properties of [RuHCl(bpap)(PPh3)]
2JPP = 9.3 Hz). IR (Nujol, cmꢀ1): 1987 (m, RuH), 3151,
3252, 3306 (w, NH). Anal. Calc. for 4: C, 54.04; H, 6.74;
N, 3.4. Found: C, 53.50; H, 6.55; N, 2.87%.
Refluxing [RuHCl(PPh3)3] and a 9:1 mixture of rac/
meso-1 in dry THF under an argon atmosphere produces
the deep violet complex [RuHCl(bpap)(PPh3)] (2). Com-
plex 2 is soluble in most common solvents (THF, diethyl
ether, benzene, etc.) and crystallizes readily from concen-
trated solutions at room temperature or when cooled.
Crystals of 2, as a benzene solvate, suitable for X-ray crys-
tallography were grown from a concentrated C6D6 solu-
tion. The structure is shown in Fig. 1 and is that of a
distorted square pyramid with the hydride trans to chloride
ligand and the P(2) atom from the bpap ligand in the apical
2.6. Hydrogenation of acetophenone catalyzed by 3 or 4
To previously dried and degassed acetophenone (0.5 g)
in a Schlenk bomb, 5 mg of KOtBu and 5 mg of 3 or
5 mg of 4 were added under a dinitrogen atmosphere.
The bombs were evacuated and refilled with dihydrogen
gas at liquid nitrogen temperature, which gave approxi-
mately 3 atm of dihydrogen in the bombs when warmed

A. Hadzovic et al. / Inorganica Chimica Acta 359 (2006) 2864–2869
2867
Table 2
Selected distances and angles for complex 2
˚
Distances (A)
Ru(1)–H(1Ru)
Ru(1)–P(1)
Ru(1)–P(2)
Ru(1)–P(3)
Ru(1)–Cl(1)
1.58(2)
P(1)–C(1)
P(1)–C(8)
P(2)–C(3)
P(2)–C(14)
P(2)–C(19)
1.843(2)
1.887(2)
1.838(2)
1.896(3)
1.900(3)
2.3088(6)
2.2119(6)
2.3544(6)
2.4879(6)
Angles (ꢁ)
H(1Ru)–Ru(1)–Cl(1)
P(2)–Ru(1)–P(1)
P(2)–Ru(1)–P(3)
P(1)–Ru(1)–P(3)
H(1Ru)–Ru(1)–P(1)
H(1Ru)–Ru(1)–P(2)
168.8(8)
H(1Ru)–Ru(1)–P(3)
C(14)–P(2)–C(19)
C(8)–P(1)–C(4)
C(1)–C(2)–C(3)
C(1)–P(1)–Ru(1)
C(3)–P(2)–Ru(1)
86.5(8)
92.36(11)
92.42(11)
113.9(2)
118.51(8)
116.23(8)
93.54(2)
109.17(2)
152.26(2)
80.6(8)
80.6(8)
Fig. 1. ORTEP3 presentation and atom numbering scheme for 2. H atoms
are omitted for clarity (except the hydride), ellipsoid probability 50%.
position of the pyramid. The P(3), from the triphenylphos-
phine ligand, lies roughly trans to the P(1) of the diphos-
phine ligand with a P(1)–Ru–P(3) angle of 152.26(2)ꢁ.
Other significant distances and angles, along with selected
crystallographic data are given in Tables 1 and 2.
Fig. 2. The conformation of the six member Ru–bpap chelate is close to a
twist-boat. In this diagram, the PPh3 ligand is reduced to the P atom, the
cage is reduced to the C atoms bonded to P(1) and P(2) and the hydrogens
on C3 bridge are shown.
Out of several possible conformations that the six mem-
ber chelating ring can adopt [9], the one in 2 lies close to a
twist-boat (see Fig. 2). Ru(II) and C(2) located in the C3
bridge of bpap are in the flagpole positions of this confor-
mation. The rigid cage around each phosphorus atom and
the flexibility of the C3 backbone that lacks any substitu-
ents favor this conformation.
ꢀ17.43 ppm with 2JHP values of 20.2, 22.1 and 35.7 Hz sug-
gesting that the hydride is cis to all three phosphorus
atoms. The three phosphorus signals in the 31P NMR spec-
trum are also doublet of doublets of doublets. The small
The 1H and 31P NMR spectra of 2 suggest that the
geometry found in the crystal structure is retained in the
solution. The hydride region of the 1H NMR spectrum
shows a doublet of doublets of doublets centered at
2
2JP(1)P(2) and JP(2)P(3) values of 7.8 and 32 Hz are consis-
tent with P(1) being cis to P(2) and P(2) cis to P(3). The
2
large JP(1)P(3) of 248 Hz is consistent with P(1) being trans
to P(3). The IR spectrum (Nujol mull) of 2 shows two med-
ium bands at 1881 and 1858 cmꢀ1 assigned to m(RuH).
When dry and crystalline, the violet [RuHCl(b-
pap)(PPh3)] very slowly changes colour to brown when
exposed to air for longer than 3 days. However, in wet
and non-deaerated solvents it decomposes rapidly to give
brown solutions.
Table 1
Crystallographic data for complex [RuHCl(bpap)(PPh3)] Æ 2C6D6 (2)
Empirical formula
Formula weight
Crystal system
Space group
C53H65ClO6P3Ru
1028.49
monoclinic
P21/c
14.4810(2)
12.8960(2)
26.4460(5)
98.7670(7)
4881.01(14)
150(1)
˚
a (A)
˚
b (A)
3.3. Synthesis and properties of the trans-
[RuHCl(bpap)(diamine)] complexes
˚
c (A)
b (ꢁ)
3
˚
V (A )
T (K)
Z
[RuHCl(bpap)(PPh3)] reacts with (1R,2R)-1,2-diamino-
cyclohexane (R,R-dach) and (1R,2R)-1,2-diphenylethylen-
ediamine (R,R-dpen) in diethyl ether under a dinitrogen
atmosphere and at room temperature to give pale yellow-
4
Crystal size (mm)
Dcalc (Mg/m3)
R indices R1
0.30 · 0.25 · 0.16
1.400
0.0672
green,
ether-insoluble
complexes
trans-[RuHCl(b-
R indices (all data) wR2
Reflections collected
Independent reflections
0.0878
33671
11149
pap)(R,R-dach)] (3) and trans-[RuHCl(bpap)(R,R-dpen)]
(4).

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A. Hadzovic et al. / Inorganica Chimica Acta 359 (2006) 2864–2869
The crystals of 3 obtained from a benzene-d6 solution
and ꢀ19.22 ppm consistent with the two diastereomers.
There is also an additional (low intensity) triplet resonance
at ꢀ18.01 ppm due to the complex containing meso-1. The
2JPH coupling constant values (24.3, 29.7; 24.1, 31.2 Hz and
25.7 Hz) are consistent with the hydride ligand located cis
to the phosphorus atoms of the bpap ligand. The 31P
NMR spectrum consists of three broad overlapping pairs
of doublets. The 2JPP coupling constants values were estab-
lished from computer modeling since it was not feasible to
obtain them from the signals. As in the case of 2 the values
are small (8.5, 14, and 9.5 Hz).
layered with hexanes were of poor quality, which enabled
only an approximate establishment of the geometry of the
complex by X-ray diffraction. Fig. 3 shows two out of four
molecules of trans-[RuHCl(bpap)(R,R-dach)] found in the
unit cell. The molecules are diastereomers because of the
two diphosphine enantiomers present. The molecule
depicted on the left chelates in a d-skew conformation for
bpap and k for the diamine. The other, on the right, chelates
in a k-skew for bpap and k for the R,R-dach. The hydride
could not be located in the difference map but is expected
to be trans to the chloride ligand on the basis of the antici-
pated octahedral geometry. This is the first report on a dia-
stereomeric complex containing the bpap ligand.
Based on its 31P and 1H NMR spectra, we propose sim-
ilar structures for the complexes 4 and 3. The hydride
region in the 1H NMR spectrum of trans-[RuHCl(b-
pap)(R,R-dpen)] in C6D6 also has two doublets of doublets
centered at ꢀ18.65 and ꢀ18.79 ppm and a triplet at
ꢀ17.75 ppm with respective coupling constants of:
25.2 Hz, 28.5 Hz; 24.6, 30.6 Hz and 22.2 Hz.
As the coordination number of Ru(II) changes from five
in 2 to six in 3 and 4, the conformation of the six member
chelate ring also changes. In the complex 3 it lies between
that of a skew and a half-chair conformations (see
Fig. 4). The rigid structure of ligand 1 promotes this unu-
sual conformation.
Both IR spectra show the frequencies characteristic for
this type of compounds. They have a medium intensity
m(RuH) at 1997 cmꢀ1 and 1996 cmꢀ1 for 3 and 4, respec-
tively, and three weak m(NH) in the region 3367–
Both the 1H and 31P NMR spectra support the proposed
1
octahedral geometry. The hydride region of the H NMR
spectrum has two doublet of doublets centered at ꢀ19.08
3151 cmꢀ1
.
Fig. 3. Chem3D drawings of two diastereomers of 3.
Fig. 4. The conformation of six member ring in 3 (middle) lies between the skew (left) and half chair (right).

A. Hadzovic et al. / Inorganica Chimica Acta 359 (2006) 2864–2869
2869
RuHCl(bpap)(diamine)
KOtBu
Me
Ph
Me
Ph
H
+ H2
C
OH
C
O
+ H2
*
NH2
NH2
NH2
(R,R)-dach
O
O
O
O
(Complex 3)
O
Ph
O
P
P
(R,R)-dpen
(Complex 4)
NH2
bpap (rac)
Ph
diamine
Fig. 5. Hydrogenation (3 atm H2) of acetophenone catalyzed by complexes 3 and 4 in neat ketone at 20 ꢁC.
3.4. The catalytic activity of trans-
[RuHCl(bpap)(diamine)] complexes
This complex reacts easily with the diamines in dry ether
under a dinitrogen atmosphere giving pale yellow precipi-
tates of trans-[RuHCl(bpap)(diamine)] (diamine = R,R-
dach and R,R-dpen). These octahedral complexes are pre-
catalysts to very active catalysts for the hydrogenation of
acetophenone under mild conditions and high substrate/
precatalysts ratios. The very low ee of the product 1-phe-
nethanol is attributed to the symmetrical structure of the
phospha-adamantyl cages in bpap.
Complexes 3 and 4 were tested for their catalytic activity
for the hydrogenation at 3 atm of acetophenone and both
showed high activity in the presence of small amounts of
potassium tert-butoxide as base, in the neat ketone at
20 ꢁC (Fig. 5). Using 3, full conversion of ketone to 1-phe-
nethanol was achieved in less than 2 h with substrate:cata-
lyst:base = 600:1:6.5. Under the same conditions, 4 proved
to be less active converting the ketone fully in 4.5 h. How-
ever, neither complexes gave significant ee in the 1-pheneth-
anol produced: with 3, 3% (S) and with 4, 8% (S). This did
not change over the course of the catalytic hydrogenation
reactions, which were monitored over the reaction time.
The reactivity of these new Ru(II) chlorohydrido catalyst
precursors is high and comparable with other similar trans-
[RuHCl(diphosphine)(diamine)] precatalysts previously
reported by some of us [10,11]. However, only essentially
racemic alcohol is obtained. This might be a consequence
of bpap having rather symmetrical cages as the P-substitu-
ents and a flexible backbone. The fact that bpap is present in
the ruthenium diamine complex in both enantiomeric forms
does not necessarily explain the low ee. For example, the
complex with racemic tol-BINAP, trans-[RuCl2(tol-
BINAP)((S,S)-dpen)], is a precatalysts for the hydrogena-
tion of acetophenone to (S)-1-phenylethanol in 91% ee [12].
Acknowledgements
An NSERC Discovery Grant to R.H.M. supported this
work. We thank Johnson-Matthey for the loan of RuCl3
and Dr. K. Abdur-Rashid for a preliminary experiment.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.ica.2005.
11.018.
References
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