Scope, mechanism and diene inhibition of isomerization of allylic alcohols to saturated ketones catalyzed by ruthenium(II)-cyclopentadienyl complexes

Article abstract of DOI:10.1016/j.jorganchem.2004.11.017

The isomerization of 3-buten-2-ol to butanone catalyzed by Ru(II)Cp-complexes (Cp = η⁵-cyclopentadienyl) with phosphine and amine ligands is described. The reaction catalyzed by [RuCp(MeCN) ₃](PF₆) and two equivalents of triphenylphospine is first order in substrate with a k_ini of 0.43 h^-1 and an initial TOF of 13,000 h^-1. The catalyst precursor complex [RuClCp(dppb)] (dppb = bis(diphenylphosphino)butane) has been characterized by X-ray diffraction. This compound features a seven-membered ring incorporating the ruthenium centre and the dppb ligand. Combination of two equivalents of primary, secondary or tertiary amines and [RuCp(MeCN)₃](PF₆) results in active catalyst precursors. Within each group, increasing the bulk of the ligand gives lower isomerization rates. The combined effects of optimal pK_a, nucleophilicity and steric bulk make RuCp-complexes with secondary amines the most active precursors. With di-n-butylamine, 745 turnovers can be reached after 1 h. ³¹P NMR spectra indicate that the resting state in the catalytic cycle is a complex in which 3-buten-2-ol is η²-coordinated through the alkene moiety. This implies that coordination of the oxygen moiety and concomitant β-hydrogen abstraction is the rate-limiting step. A counterintuitive result is that allylic alcohols bind stronger to RuCp complexes with phosphine ligands than dienes. Inhibition of the catalyst appears to be a result of interaction of the diene with a ruthenium-allyl alcohol complex, which is sufficiently strong to prevent coordination of the oxygen moiety of the allylic alcohol. This hinders orientation of the allylic alcohol substrate in a suitable way to undergo β-hydrogen abstraction, thereby blocking isomerization catalysis.

Full text of DOI:10.1016/j.jorganchem.2004.11.017

Journal of Organometallic Chemistry 690 (2005) 1044–1055
www.elsevier.com/locate/jorganchem
Scope, mechanism and diene inhibition of isomerization of
allylic alcohols to saturated ketones catalyzed by
ruthenium(II)-cyclopentadienyl complexes
a
Robert C. van der Drift , Marcella Gagliardo , Huub Kooijman , Anthony L. Spek ,
a
b
b
a,
a
Elisabeth Bouwman *, Eite Drent
a
Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
b
Bijvoet Center for Biomolecular Research, Department of Crystal and Structural Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht,
The Netherlands
Received 11 October 2004; accepted 10 November 2004
Abstract
The isomerization of 3-buten-2-ol to butanone catalyzed by Ru(II)Cp-complexes (Cp = g5-cyclopentadienyl) with phosphine and
amine ligands is described. The reaction catalyzed by [RuCp(MeCN)3](PF6) and two equivalents of triphenylphospine is first order
in substrate with a kini of 0.43 hꢀ1 and an initial TOF of 13,000 hꢀ1. The catalyst precursor complex [RuClCp(dppb)]
(dppb = bis(diphenylphosphino)butane) has been characterized by X-ray diffraction. This compound features a seven-membered
ring incorporating the ruthenium centre and the dppb ligand.
Combination of two equivalents of primary, secondary or tertiary amines and [RuCp(MeCN)3](PF6) results in active catalyst
precursors. Within each group, increasing the bulk of the ligand gives lower isomerization rates. The combined effects of optimal
pKa, nucleophilicity and steric bulk make RuCp-complexes with secondary amines the most active precursors. With di-n-butyl-
amine, 745 turnovers can be reached after 1 h. 31P NMR spectra indicate that the resting state in the catalytic cycle is a complex
in which 3-buten-2-ol is g2-coordinated through the alkene moiety. This implies that coordination of the oxygen moiety and con-
comitant b-hydrogen abstraction is the rate-limiting step. A counterintuitive result is that allylic alcohols bind stronger to RuCp
complexes with phosphine ligands than dienes. Inhibition of the catalyst appears to be a result of interaction of the diene with a
ruthenium–allyl alcohol complex, which is sufficiently strong to prevent coordination of the oxygen moiety of the allylic alcohol.
This hinders orientation of the allylic alcohol substrate in a suitable way to undergo b-hydrogen abstraction, thereby blocking isom-
erization catalysis.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Isomerization; Allylic alcohols; Ruthenium; Cyclopentadienyl; Diene inhibition
1. Introduction
Isomerization of allylic alcohols forms an elegant short-
cut to carbonyl compounds [2,3] in a completely atom-
economical process that offers potentially several useful
applications in natural-product synthesis [4–6] and in
bulk chemical processes. As an example of the latter,
butanone (methyl ethyl ketone, MEK) is a Mton-
per-yearscalesolventtraditionallyproducedfrom butenes
[7,8]. This process requires the use of stoichiometric
With a growing demand for environmentally benign
organic reactions, homogeneously catalyzed reactions
that are atom efficient receive increasing attention [1].
*
Corresponding author. Tel.: +31 71 5274550; fax: +31 71 5274451.
E-mail address: bouwman@chem.leidenuniv.nl (E. Bouwman).
0022-328X/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.11.017

R.C. van der Drift et al. / Journal of Organometallic Chemistry 690 (2005) 1044–1055
1045
OSO3H
O
OH
H2SO4
-H2
H2O
Scheme 1. Present-day synthesis of MEK from a mixture of butenes.
amounts of concentrated sulfuric acid as shown in
Scheme 1.
mains active in the presence of dienes [18]. The discover-
ies of systems that are able to catalyze the isomerization
of allylic alcohols in the presence of dienes uptil now
have had a serendipitous character. The strong inhibi-
tory effect of dienes on most catalysts has been poorly
understood.
In this paper the results are described of a study on
the use of the ruthenium(II)-cyclopentadienyl moiety
in combination with various ligands as catalyst precur-
sors for the isomerization of 3-buten-2-ol to MEK. Next
to phosphine ligands, which have been discussed in the
literature previously [14,15,17], for the first time ligands
containing nitrogen-donor atoms are included in this
study intending to explore the scope of this catalytic sys-
tem. Furthermore, the mechanism of the isomerization
is investigated by 31P NMR spectroscopy both in the ab-
sence and the presence of dienes in an attempt to shed
light on the intriguing effect of dienes on the isomeriza-
tion reaction.
A viable alternative route starting from 1,3-butadiene
would need water as only other stoichiometric reagent.
After acid-catalyzed hydration of butadiene (Scheme
2(a)), the 1,2-addition product 3-buten-2-ol can be isom-
erized to MEK (Scheme 2(b)). As the equilibrium of
Scheme 2 a lies for 97% on the left-hand side, a process
in which both reactions take place in one reactor is pref-
erable in order to circumvent an extensive butadiene re-
cycle. The key reaction in this process is selective
isomerization of the secondary allylic alcohol to the sat-
urated ketone in the presence of dienes.
The feasibility of the one-pot synthesis of MEK from
1,3-butadiene was shown by Drent and co-workers [9].
The maximum conversion was obtained with a catalytic
system formed by in situ mixing of a ruthenium(III) salt
and 2,20-bipyridine (bpy) or 1,10-phenanthroline (phen)
[9,10]. A subsequent investigation in our group of possi-
ble deactivation routes for this type of catalyst [11] led to
the discovery of a significantly improved system [12].
Yet, the most active catalyst precursor thus far
for the overall hydration-isomerization reaction,
[RuCl3(dmso)(phen)], is only moderately active in isom-
erization of allylic alcohols in the absence of dienes [11].
This prompted us to investigate alternative catalyst pre-
cursors with higher intrinsic isomerization activities.
Ruthenium-cyclopentadienyl complexes with phosphine
ligands have been shown by Trost and co-workers
[13,14] and Slugovc et al. [15,16] to be active isomeriza-
tion catalysts. Indeed, [RuClCp(PPh3)2] in the presence
of silver(I) salts with non-coordinating anions proved
to be extremely active in the isomerization of 3-buten-
2-ol [17]. Likewise, application of didentate phosphine
ligands gave high turnover numbers to MEK [17].
Unfortunately, none of these RuCp-catalyst precur-
sors showed any isomerization activity in the presence
of dienes. Recently, a bis(allyl)-ruthenium(IV) complex
has been proved to catalyze the isomerization of allylic
alcohols efficiently and, interestingly, the complex re-
2. Results and discussion
2.1. RuCp-complexes containing monodentate or
didentate phosphine ligands
2.1.1. Isomerization of 3-buten-2-ol
The complex [RuClCp(PPh3)2] is synthetically readily
accessible [19] and forms a convenient precursor to anal-
ogous complexes with monodentate and didentate phos-
phine ligands [17]. In the publications of Trost on the
isomerization of allylic alcohols, catalyzed by
[RuClCp(PPh3)2], a low substrate-to-catalyst ratio of
100 was employed [13,14]. The catalytic efficiency of
the RuCp-moiety can be increased considerably by using
[RuCp(MeCN)2(PPh3)]+ as catalyst precursor [15,16].
This complex has been prepared quantitatively from
[RuCp(MeCN)3](PF6) (1) and triphenylphosphine by
Slugovc et al. [15,16]. To avoid tedious synthetic proce-
dures and allow for faster screening of catalytic activity
of various ligands in combination with the RuCp-moi-
ety, an in situ strategy is adopted here.
The presence of substitutionally labile acetonitrile li-
gands [20] makes 1 an ideal starting complex for in situ
testing. When 1 is used as catalyst precursor, only low
turnover numbers (TONs; 115 after 1 h; see Table 2)
are obtained. In the presence of 1 and two equivalents
of triphenylphosphine, a fast isomerization of 3-buten-
2-ol to MEK takes place, after a negligible induction
period (Fig. 1). A similar behaviour is observed if one
[H+], H2O
+
(a)
(b)
OH
OH
O
[cat.]
OH
Scheme 2. Acid-catalyzed hydration of 1,3-butadiene (a) followed by
isomerization (b) yields MEK in a complete atom-economical process.

1046
R.C. van der Drift et al. / Journal of Organometallic Chemistry 690 (2005) 1044–1055
chain (dppm ligand) to 18,000 hꢀ1 for a C4 chain (dppb
1500
1000
500
0
ligand) [17]. All complexes with didentate phosphine li-
gands are less active than [RuClCp(PPh3)2]. The pro-
posed mechanism of isomerization of allylic alcohols
involves the dissociation of one phosphine moiety (see
Section 2.4). The increased reactivity with increasing
chain length in the phosphine ligand has been ascribed
to the greater ease of ring opening of larger chelate
rings. Indeed, complexes with didentate phosphine li-
gands that are extremely rigid, such as with a phenylene
or ethene bridge, show no catalytic activity at all [17].
0
2
4
6
8
10
12
14
16
Time (min)
Fig. 1. Isomerization of 3-buten-2-ol to MEK, catalyzed by 1 with 2
equiv. of triphenylphosphine in diglyme. T = 100 ꢁC. Substrate/
catalyst precursor = 1640.
2.1.2. Crystal structure determination of
[RuClCp(dppb)]
Even though ruthenium(II)-cyclopentadienyl com-
plexes with didentate phosphine ligands are known for
quite some time and their use is widespread, only a
few solid-state structures have been reported. Notably,
of the catalyst-precursor series mentioned in the previ-
ous section, only the structures of the chloride com-
plexes containing the dppm [22] and the dppe [22,23]
ligands have been published. An interesting seven-mem-
bered ring is obtained with dppb as ligand. An ORTEP
drawing of [RuClCp(dppb)] is shown in Fig. 2. Some se-
lected bond lengths and angles are given in Table 1. The
solid-state structure shows that the dppb ligand coordi-
nates in a didentate fashion and this implies that isomer-
ization of allylic alcohols catalyzed by RuCp-complexes
with this ligand is only possible after opening up of the
chelate ring.
equivalent of triphenylphosphine is employed. The reac-
tion is first order in substrate with a kini of 0.43 hꢀ1 and
an initial TOF of 13,000 hꢀ1. This TOF is significantly
higher than obtained with 1 alone and comparable to
the activity reported for the isolated complex [RuCp-
(MeCN)2(PPh3)]+ [15].
As can be seen from Fig. 1, a full conversion of 3-bu-
ten-2-ol is not obtained. This is probably due to the for-
mation of small amounts of 1,3-butadiene through
reversion of the hydration reaction depicted in Scheme
2a under the reaction conditions, which inhibits the
isomerization reaction severely.
Several ruthenium(II)-cyclopentadienyl complexes
with didentate phosphine ligands [21], obtained by
reacting in each case the appropriate ligand with a solu-
tion of [RuClCp(PPh3)2] in toluene, have been used pre-
viously as catalyst precursor as well [17]. Upon
increasing the carbon-chain length between the phos-
phorus-donor atoms, the reactivity of the resultant
ruthenium complexes increases. Thus, initial turnover
frequencies in the series range from 530 hꢀ1 for a C1
In all respects, the structure of [RuClCp(dppb)] is
analogous to the previously reported complexes in the
series. The ruthenium centre may be considered to be
in a pseudo-octahedral environment (three-legged piano
˚
stool). The Ru(1)–Cl(2) bond length of 2.4404(4) A falls
˚
in the range of 2.430–2.453 A observed for similar ruthe-
nium(II)–cyclopentadienyl chloride complexes [22,24].
Fig. 2. ORTEP drawing and numbering scheme of [RuClCp(dppb)]. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the
50% probability level.

R.C. van der Drift et al. / Journal of Organometallic Chemistry 690 (2005) 1044–1055
1047
Table 1
Selected bond lengths and angles for [RuClCp(dppb)]
of the nitrogen donor atom. Addition of two equivalents
of 2-(diphenylphosphino)pyridine (2-PPh2py) to 1 gives
1
rise to two complexes as evidenced by H NMR, 31P
˚
Bond lengths (A)
Ru(1)–Cl(2)
Ru(1)–P(3)
Ru(1)–P(8)
Ru(1)–Cp
P(3)–C(4)
2.4404(4)
2.2810(5)
2.2809(6)
1.8508(9)
1.8503(19)
P(8)–C(7)
C(4)–C(5)
C(5)–C(6)
C(6)–C(7)
1.8330(17)
1.533(3)
1.530(3)
1.542(3)
NMR and mass spectrometry. The main product
(>90%) exhibits a singlet in the 31P NMR spectrum at
51.5 ppm, close to the singlet found for
[RuClCp(PPh3)2], thus suggesting monodentate coordi-
nation of the ligand through the phosphorus atom.
The other small singlet at 0.35 ppm may result from a
complex in which the ligand is monodentately coordi-
nated through nitrogen. From the facts that all signals
in the 31P NMR spectrum stem from symmetrical com-
plexes and that no free ligand (ꢀ3.5 ppm) is observed, it
can be concluded that 2-(diphenylphosphino)pyridine
primarily behaves as a monodentate phosphine, to
give [RuCp(MeCN)(2-PPh2py-jP)2]+. [RuCp(MeCN)-
(2-PPh2py-jP)2]+ catalyzes the isomerization of 3-
buten-2-ol to MEK as shown in Fig. 3(a). The reaction
is not simply first order in substrate and the catalyst is
considerably deactivated with time. The initial rate
(kini = 0.39 hꢀ1) is of the same order as for 1 with triphe-
nylphosphine (vide supra). However, the catalyst is
deactivated much faster and the total turnover number
is circa 30% lower.
Angles (ꢁ)
Ru(1)–P(3)–C(11)
Ru(1)–P(8)–C(31)
P(3)–Ru(1)–P(8)
P(3)–Ru(1)–Cl(2)
P(3)–C(4)–C(5)
118.46(7)
115.84(6)
93.64(2)
P(8)–Ru(1)–Cl(2)
P(8)–C(7)–C(6)
C(4)–C(5)–C(6)
C(5)–C(6)–C(7)
91.09(2)
112.81(12)
116.35(16)
113.93(15)
88.66(2)
118.95(15)
The Ru(1)–P(3) and Ru(1)–P(8) bond lengths are identi-
cal and consistent with distances found in a series of
complexes containing the Cp–Ru–Cl fragment [24]. As
expected, the P(3)–Ru(1)–P(8) bite angle (93.64(2)ꢁ) is
considerably larger than that found for complexes with
a C2 chain between the phosphorus atoms (82.37–
83.48ꢁ). The smallest bite angle for this series is found
with the dppm ligand (72.07ꢁ) [22]. The seven-membered
chelate ring is in a twist-chair conformation, with all
groups attached to this ring staggered with respect to
each other.
Addition of two equivalents of 2-(diphenylphosphi-
noethyl)pyridine (2-PPh2Etpy) to 1 results in the forma-
tion of two different species as evidenced most clearly by
31P NMR. One species gives rise to a singlet, indicating
two equivalent phosphorus atoms. This complex, which
is by far the dominant species (>80%), is analogous to
the complexes found with triphenylphosphine and
2-PPh2py, viz. monodentate coordination of the
phosphine with a pendant ethylpyridine group ([RuCp-
(MeCN)(2-PPh2Etpy-jP)2]+). The second species exhib-
its two doublets in the 31P NMR spectrum. These signals
suggest a minor fraction in which one ligand is coordi-
nated in a monodentate fashion, while another ligand
is coordinated through both the phosphorus atom and
the nitrogen atom, viz. [RuCp(2-PPh2Etpy-jP)(2-
PPh2Etpy-jP,jN)]+. This hypothesis is corroborated
The ruthenium phosphorus distances are slightly
longer than those found in the analogous hydride com-
plex [RuCpH(dppb)] (2.2809(6)–2.2810(5) vs 2.2451(8)–
˚
2.2463(8) A) [25]. The C4-bridge appears to be in the
same configuration in both complexes, since absolute
differences between the corresponding torsion angles
are less than 4ꢁ [25].
2.2. Isomerization of 3-buten-2-ol catalyzed by RuCp-
complexes containing didentate ligands with phosphorus
and nitrogen donor atoms
In several homogeneously catalyzed reactions, it
proved beneficial to replace one phenyl group in triphe-
nylphosphine by a pyridine moiety, either because of the
additional basic function or because of the coordination
1
by the H NMR spectrum, which contains a small dou-
blet at low field (9.3 ppm). Such a doublet can be as-
1000
800
600
400
200
0
120
80
40
0
0
50
100
0
2
4
6
8
10
14
16
Time (min1)2
Time (min)
(a)
(b)
Fig. 3. Isomerization of 3-buten-2-ol to MEK, catalyzed by 1 with (a) 2 equiv. of 2-PPh2py in diglyme and (b) 2 equiv. of 2-PPh2Etpy in diglyme.
T = 100 ꢁC. Substrate/catalyst precursor = 1640.

1048
R.C. van der Drift et al. / Journal of Organometallic Chemistry 690 (2005) 1044–1055
Table 2
Isomerization of 3-buten-2-ol to MEK, catalyzed by in situ mixtures of
cribed to the ortho hydrogen of a coordinated pyridine
moiety (see also Section 2.3). The catalytic activity in
the isomerization of 3-buten-2-ol of the ruthenium-
cyclopentadienyl complex with 2-PPh2Etpy is low (Fig.
3b).
1 with amine ligandsa
b
c
Entry
Ligand
Turnovers
pKa
1
2
MeCN
115
260
180
55
–
n-Propylamine
Isopropylamine
Aniline
10.61
10.60
4.62
The results indicate that both P,N-ligands behave
mainly as monodentate phosphines when reacted in situ
with 1. The presence of a pyridine moiety is not benefi-
cial to catalytic activity. Significant inhibition of the cat-
alysts is observed as can be derived from Figs. 3. The
inhibition is possibly caused by 1,3-butadiene, which
can be formed under the reaction conditions from the
substrate by dehydration. Indeed, in the presence of iso-
prene (2-methyl-1,3-butadiene), no activity is obtained
with either ligand. The different reaction profile that is
obtained when 2-PPh2Etpy is used as the ligand is prob-
ably in part due to oxidation of the ligand during isom-
erization. The free ligand 2-PPh2Etpy is more
susceptible to air oxidation than 2-PPh2py or triphenyl-
phosphine as can be seen from the facile oxidation of 2-
PPh2Etpy in the solid state upon exposure to air.
3
4
5
Piperidine
235
745
525
460
155
95
11.27
10.60
10.6
6
Di-n-butylamine
Di-n-hexylamine
Triethylamine
Tri-n-butylamine
Pyridine
7
8
10.74
10.75
5.2
9
10
11
12
13
a
1-Methylimidazole
2,20-Bipyridined
tmend
25
7.25
4.35e
10.01
0
25
Solvent: diglyme (15 ml), toluene (4.0 mmol) as internal standard;
ligand/ruthenium = 2; substrate/catalyst precursor = 1640; T = 100 ꢁC.
Turnovers determined by GLC after 1 h.
b
c
Value for the corresponding ammonium salt. Taken from [26].
Ligand/ruthenium = 1. tmen: N,N,N0,N0-tetramethylethylene
d
diamine.
e
Taken from [27].
2.3. Isomerization of 3-buten-2-ol catalyzed by
RuCp-complexes containing amines
(e.g., entry 6 vs. 7 and 8 vs. 9). Aromatic amines with rel-
atively low pKa-values all result in lower activity than of
1 alone (entries 4, 10–11). The combined effects of opti-
mal pKa, nucleophilicity and steric bulk make ruthe-
nium(II)–cyclopentadienyl complexes with secondary
amines the most active catalyst precursors. With its most
prominent exponent, di-n-butylamine, 745 turnovers can
be reached after one hour (entry 6). The application of
didentate ligands (entries 12–13) does not give active
catalysts. The strong chelate coordination and conse-
quent difficulty in opening of the ring to create a vacant
site hamper catalysis as was described for didentate
phosphines in Section 2.1.1.
To put the results with nitrogen-donor ligands in
perspective it should be noted that, although ruthe-
nium(II)–cyclopentadienyl complexes with phosphorous
ligands show initial activities of thousands per hour, the
majority of transition-metal complexes only reach TOFs
around 20 hꢀ1 and maximum TONs of ca. 100 in allyl
alcohol isomerization [2].
The catalyst precursors obtained from 1 and the
amines shown in Table 2 are stable and active in isomer-
ization catalysis for prolonged periods. This is exempli-
fied in Fig. 4, in which a profile of isomerization of
3-buten-2-ol catalyzed by 1 with two equivalents of pyr-
idine in time is depicted. From the figure, it becomes
apparent that at least two different catalytic species must
be present in solution in different stages of the reaction. In
the first 30 min, a relatively active species catalyzes the
isomerization (k = 0.16 hꢀ1, TOF = 215 hꢀ1), which then
changes to another species that is considerably less active
(kapparent = 0.044 molꢀ1 lꢀ1 hꢀ1, TOF = 25 hꢀ1), but that
retains its activity overnight (23 h) with only slight
2.3.1. Ligand type dependency
Literature on allylic-alcohol isomerization reactions
is dominated by complexes with phosphine ligands
[2,3]. Ligands with donor atoms from the same group
have been explored to a much lesser extent. In particu-
lar, isomerization catalyzed by RuCp-complexes with
amine ligands to the best of our knowledge has not been
reported.
An in situ strategy has been adopted in which two
equivalents of ligand are added to a solution of 1 and
3-buten-2-ol in diglyme. The results are shown in Table
2. The result of entry 1 demonstrates that 1 by itself is
only moderately active. Complex 1 in combination with
two equivalents of an amine in some cases results in the
formation of more active catalyst precursors for the
isomerization of 3-buten-2-ol to MEK. Addition of
two equivalents of primary (entries 2–4), secondary (en-
tries 5–7) and tertiary amines (entries 8–9) results in ac-
tive catalyst precursors. Within each group, increasing
the bulk of the ligand gives lower isomerization rates
200
150
100
50
0
0
100
200
300
Time (min)
Fig. 4. Isomerization of 3-buten-2-ol, catalyzed by 1 with 2 equiv. of
pyridine in diglyme. T = 100 ꢁC. Substrate/catalyst precursor = 1640.

R.C. van der Drift et al. / Journal of Organometallic Chemistry 690 (2005) 1044–1055
200
100
1049
deactivation. The relatively low activity of this catalytic
800
400
0
species in combination with the large excess of substrate
gives an apparent zeroth reaction-order in 3-buten-2-ol.
The proposed isomerization mechanism (see Section
2.4) contains a step in which an allylic alkoxide, ob-
tained by deprotonation of the substrate, is coordinated
to the ruthenium centre. Since amines are bases, it might
be expected that the initial rate acceleration when amine
ligands are used compared to 1 as catalyst precursor is
due to an increased alkoxide concentration. Ba¨ckvall re-
ported a significant rate increase with several ruthenium
catalyst precursors upon addition of catalytic amounts
of K2CO3 [28]. However, no rate enhancement was ob-
served for [RuClCp(PPh3)2] with K2CO3 [28]. When un-
der our experimental conditions [RuClCp(PPh3)2] is
used in combination with AgOTs and various amounts
of bases (K2CO3, NaOH) a lower rate is observed, pre-
sumably due to coordination of the bases to the ruthe-
nium centre. A large excess of potassium t-butoxide in
combination with 1 yields 510 turnovers after one hour,
demonstrating that deprotonation of the allylic alcohol
is beneficial, but this cannot be the only effect of amines.
No clear trend is observed with amine pKa. The use of
stronger bases (e.g., piperidine, tri-n-butylamine) does
not result in higher activity and amines with similar
pKa values may differ considerably in resulting activity
of the precursor complexes. If the rate-increasing effect
of amines were solely due to their basic properties, more
sterically hindered amines should result in higher activ-
ity. Yet, the opposite is observed.
0
0
50
100
0
100
200
300
(a)
(b)
Time (min)
Time (min)
Fig. 5. Effect of ligand to ruthenium ratio for catalyst precursors
formed in situ from 1 and (a) di-n-butylamine or (b) pyridine for the
isomerization of 3-buten-2-ol to MEK in diglyme. T = 100 ꢁC. (r) 10
equiv., (j) 5 equiv., (h) 4 equiv., ( ) 2 equiv., (m) 1 equiv., (d) no
*
ligand.
Addition of two equivalents of pyridine to 1 gives a
mixture of complexes, containing one, two and three
pyridine molecules coordinated to the ruthenium centre,
respectively (see Scheme 3). In the 1H NMR spectrum in
CDCl3 at 50 ꢁC, RuCp-complexes with coordinated pyr-
idine ligands can be observed, while MEK is being
formed. The pronounced effect of the ligand-to-ruthe-
nium ratio is most likely due to shifting of the equilibria
towards inactive species fully saturated with amine upon
use of more equivalents of ligand. The fact that an in-
creased amount of amine does not lead to an increased
rate, but that optimum activity is found at two equiva-
lents, again suggests that the positive effect of amine li-
gands is not exclusively due to their basic properties.
In concurrence with the results depicted in Fig. 4, it
appears from Fig. 5 that at least two different catalyti-
cally active species are present successively. This be-
comes most obvious from the curve with two
equivalents of di-n-butylamine. In the first 30 min, fast
isomerization takes place, thereafter MEK is steadily
formed but at much lower rate. All results are consistent
with a ruthenium(II)-cyclopentadienyl complex with one
amine ligand as the most active catalyst. Depending on
the amount of ligand and the ligand type, this species is
formed next to species with two and three amine ligands.
At higher ligand concentrations, the latter two species
dominate and isomerization rates go down. Deactiva-
tion may take place by ligand redistribution, which gives
rise to the initial complex 1 and species with two and
three amine ligands (Scheme 3). The concentrations of
the active species with one amine ligand and without
amine ligands thereby become lower and reach a steady
state. Ligand disproportionation has also proved to be a
major deactivation route in the synthesis of MEK from
1,3-butadiene catalyzed by ruthenium complexes with
phenanthroline ligands [12].
2.3.2. Influence of ligand-to-ruthenium ratio
Not only the type of amine, but also the relative
amount of ligand proves to be an important factor for
activity. In Fig. 5(a) are plotted the various activities
of 1 with different amounts of di-n-butylamine. Clearly,
two equivalents of ligand are most beneficial. Both less
than two equivalents and more than two equivalents
to 1 cause a steep drop in activity. At high ligand con-
centrations, even lower activity than with 1 alone is
observed.
A slightly different behaviour is observed with pyri-
dine as ligand, as shown in Fig. 5(b). Here, the highest
initial activity is found with one equivalent of pyridine.
However, the catalytically active species is deactivated
rapidly. With two equivalents of pyridine, slower deacti-
vation is observed and therefore a higher TON is at-
tained in the long run.
PF6
PF6
PF6
PF6
L
L
L
L
NCMe
NCMe
NCMe
Ru
Ru
Ru
Ru
L
L
L
L
L
MeCN
NCMe
NCMe
MeCN
MeCN
MeCN
Scheme 3. Equilibrium formation of several ruthenium complexes.

1050
R.C. van der Drift et al. / Journal of Organometallic Chemistry 690 (2005) 1044–1055
2.3.3. Inhibition by dienes
Cp
The results of isomerization experiments in the pres-
ence of isoprene are reported in Table 3. A much lower
isomerization rate is observed for all catalyst precursors.
However, it is interesting to note that 1 in combination
with nitrogen donor ligands is significantly less inhibited
by isoprene than if 1 is used in combination with triphe-
nylphosphine. If the activity of 1 with various ligands
under diene-free conditions is taken into account, it
can be deduced that ruthenium(II)-cyclopentadienyl
complexes with amines are more resistant to isoprene
compared to ruthenium(II)-cyclopentadienyl complexes
with triphenylphosphine by a factor of ca. 50. Yet, the
activities are all far too low to be attractive in the direct
hydration-isomerization reaction to produce MEK from
1,3-butadiene on an industrial scale.
Ru
P
P
OH
Cp
Ru
O
OH
P
P
H+
H+
OH
Cp
Ru
Cp
Ru
H
O
O
P
P
P
P
2.4. Mechanism and diene inhibition of the isomerization
of allylic alcohols catalyzed by RuCp-complexes with
phosphine ligands
Cp
Ru
O
H
P
P
All RuCp-complexes described in this paper are ex-
tremely effectively poisoned by isoprene, while some of
these complexes are under diene-free conditions among
the best catalyst precursors for allyl alcohol isomeriza-
tion. Several RuCp-complexes with didentate phosphine
ligands catalyze an entirely different reaction of 3-buten-
2-ol in the presence of isoprene [17]. The recent discov-
ery of a catalyst precursor that is much less affected by
isoprene [18] makes it even more interesting to study
the effect of dienes on the isomerization reaction.
An intramolecular mechanism has been proposed for
the isomerization of allylic alcohols by ruthenium(II)-
cyclopentadienyl complexes with phosphine ligands
[14,17]. This mechanism is depicted for complexes with
didentate ligands in Scheme 4. A 31P NMR study on
the behaviour of [RuClCp(dppe)] towards allylic alco-
hols showed that initial coordination of the allylic alco-
hol to the ruthenium centre takes place at the alkene
moiety and that the resultant species forms the resting
state of the catalytic cycle (top of the catalytic cycle in
Scheme 4) [17]. This implies that the next step, viz. coor-
Scheme 4. Mechanistic proposal for the isomerization of allylic
alcohols, catalyzed by ruthenium-cyclopentadienyl complexes with
didentate phosphorous ligands (PꢂP) [17].
dination of the oxygen moiety and concomitant b-
hydrogen abstraction is the rate-determining step.
[RuClCp(PPh3)2] reacted analogously; the first step in
the catalytic cycle after initial complexation is in this
case didentate coordination of the allyl alcohol with
concomitant dissociation of one triphenylphosphine li-
gand [17]. It is interesting to note that in these studies,
allylic alcohols with an internal double bond react much
slower and significant peaks for unreacted
[RuCp(dppe)]+ are still visible in the 31P NMR spectra
[17].
With monodentate triphenylphosphine ligands in
[RuClCp(PPh3)2], refluxing in isoprene leads to the for-
mation of a ruthenium–isoprene complex (presumably
g4-coordinated) as determined with mass spectrometry.
A strong g4-coordination of isoprene vs. a weaker g2-
coordination of allylic alcohols, preventing the required
initial coordination of the substrate as shown in Scheme
4, at first sight may be an explanation for the absence of
activity upon addition of isoprene. 31P NMR has been
used to study the effect of the addition of isoprene to a
mixture of allyl alcohol and ruthenium-cyclopentadienyl
complexes with phosphine ligands. Thus, the interaction
between [RuCp(dppe)]+, obtained after reaction of
[RuClCp(dppe)] with AgOTs, and isoprene and allyl
alcohol has been considered. The latter substrate is
isomerized to propanal, analogously to 3-buten-2-ol.
Surprisingly, addition of an excess of isoprene (ca.
Table 3
Isomerization of 3-buten-2-ol to MEK catalyzed by 1 with different
ligands in the presence of isoprenea
Entry
Ligand
Turnovers
1
2
3
4
5
a
MeCN
PPh3
3
2
Piperidine
Di-n-hexylamine
Di-n-butylamine
5
7
15
Solvent: diglyme (15 ml), isoprene (2 ml); ligand/ruthenium = 2;
T = 100 ꢁC, t = 1 h. Turnovers determined by GLC with toluene as
internal standard. Substrate/catalyst precursor = 1640. Isoprene/
ruthenium = 2000.
1000 equiv. to ruthenium) to
a
solution of

R.C. van der Drift et al. / Journal of Organometallic Chemistry 690 (2005) 1044–1055
1051
[RuCp(dppe)]+ in CDCl3 at room temperature results in
only partial coordination of the substrate. Around 50%
of the ruthenium complex remains ‘‘free’’ (Fig. 6(a)),
even after prolonged stirring. This seems to contradict
the idea of strong coordination. Moreover, both phos-
phorus atoms are bound to the ruthenium centre and
isoprene is therefore most likely coordinated in an g2-
fashion. The result of subsequent addition of allyl alco-
hol (ca. 10 equiv. to ruthenium) is depicted in Fig. 6(b).
Although a small amount of isoprene is still coordi-
nated, two doublets resulting from allyl alcohol coordi-
nation appear. Obviously, allyl alcohol coordinates first
to the ‘‘free’’ [RuCp(dppe)]+ and indeed the singlet at
77.5 ppm has disappeared. Comparison of the integrals
leads to the conclusion that in addition to complexation
to the ‘‘free’’ ruthenium centres, a significant amount of
isoprene is displaced. Thus, allyl alcohol binds more
strongly to [RuCp(dppe)]+ than does isoprene!
demonstrated. With other didentate phosphine ligands
and with 3-buten-2-ol as substrate, comparable results
are obtained. If [RuCp(PPh3)2]+ is used at room temper-
ature, no coordination of isoprene is observed with 31P
NMR (Figure S1). Addition of allyl alcohol leads to
instantaneous formation of the adduct (Figure S2),
again confirming the preferential coordination of the
substrate rather than the inhibitor. It should be noted
that at room temperature, isomerization occurs under
diene-free conditions, while in the presence of isoprene
no activity is observed.
Counter intuitively, the poisoning effect of isoprene is
not caused by preferential initial coordination. Still, iso-
prene is a strong poison and it must therefore interact
further in the catalytic cycle. In the presence of isoprene,
other ruthenium species (such as ruthenium–allyl) can-
not be detected with 31P NMR. The absence of methyl
vinyl ketone (MVK) in the GLC-chromatograms with
ruthenium-cyclopentadienyl complexes further suggests
that inhibition occurs before possible formation of a
ruthenium-hydride species (cf. Scheme 4). Apparently,
isoprene interacts with the ruthenium–allyl alcohol com-
plex sufficiently strong to prevent coordination of the
oxygen moiety of the allylic alcohol. This hinders
In Fig. 7 the results are shown of the reversed route.
Addition of a few drops of allyl alcohol gives instanta-
neous, complete formation of [RuCp(allyl alcohol)-
(dppe)]+ (Fig. 7(a)). Subsequent addition of 1 ml of
isoprene does not change the spectrum at all (Fig. 7(b)):
again the preferential coordination of allyl alcohol is
Fig. 6. 31P NMR spectra recorded at room temperature in CDCl3 of [RuCp(dppe)]+ (77.5 ppm) with (a) isoprene (ꢁ1000 equiv. to ruthenium) and
(b) after subsequent addition of allyl alcohol (ꢁ10 equiv. to ruthenium).

1052
R.C. van der Drift et al. / Journal of Organometallic Chemistry 690 (2005) 1044–1055
Fig. 7. 31P NMR spectra recorded at room temperature in CDCl3 of [RuCp(dppe)]+ with (a) allyl alcohol (ꢁ10 equiv. to ruthenium) and (b) after
subsequent addition of isoprene (ꢁ1000 equiv. to ruthenium).
orientation of the allylic alcohol substrate in a suitable
way to undergo b-hydrogen abstraction, thereby ham-
pering isomerization catalysis. The RuCp-complexes be-
have differently from ruthenium–phenanthroline
complexes [11]. With the latter type of complexes, signif-
icant amounts of MVK are formed and indirect evidence
exists for the formation of ruthenium–allyl species. Dif-
ferent inhibition pathways must exist for different types
of ruthenium complexes.
isomerization of allylic alcohols. The most active precur-
sor is obtained with di-n-butylamine (745 turnovers after
one hour). Amine ligands with relatively low pKa values
result in lower activity than if [RuCp(MeCN)3](PF6)
(115 turnovers after one hour) is used alone. A ligand
to metal ratio of two yields the most active precursors;
a steep drop in activity is observed with either more or
less equivalents of ligand. The results indicate that the
activity of [RuCp(MeCN)3](PF6) in the presence of
amines is not solely due to the basic function of the
ligands.
3. Conclusions
The mechanism of isomerization catalyzed by RuCp-
complexes has been studied in the presence and absence
of dienes. 31P NMR spectra indicate that the resting
state in the catalytic cycle is a complex in which 3-bu-
ten-2-ol is g2-coordinated through the alkene moiety.
This implies that coordination of the oxygen moiety
and concomitant b-hydrogen abstraction is the rate-
determining step. It has been shown that allylic alcohols
bind stronger to RuCp-complexes than dienes. Inhibi-
tion of the allyl alcohol isomerization reaction must be
a result of an interaction of the diene to a ruthenium–
A series of ligands have been tested in situ with
[RuCp(MeCN)3](PF6) for the isomerization of 3-bu-
ten-2-ol to MEK. The most active catalyst precursor is
obtained with triphenylphosphine (kini = 0.43 hꢀ1
TOF = 13,000 hꢀ1). Substitution of a phenyl moiety in
triphenylphosphine by a pyridine group, or introduction
of pyridine as a pendant group results in stronger deac-
tivation. Several amine ligands have for the first time
shown to result in active catalyst precursors for the
,

R.C. van der Drift et al. / Journal of Organometallic Chemistry 690 (2005) 1044–1055
1053
allyl alcohol complex sufficiently strong to prevent coor-
dination of the oxygen moiety of the allylic alcohol. This
hinders orientation of the allylic alcohol substrate in a
suitable way to undergo b-hydrogen abstraction, there-
by blocking isomerization catalysis.
minor, Cp), 4.4 (s, major, Cp), 2.12 (s, minor, MeCN),
2.0 (s, major, MeCN). 31P NMR (CDCl3): d = 51.5
(major), 0.35 (minor). ESI MS: m/z 733 ([RuCp-
(MeCN)(2-PPh2py)2]+), 692 ([RuCp(2-PPh2py)2]+), 512
([RuCp-(MeCN)2(2-PPh2py)]+), 471 ([RuCp(MeCN)-
(2-PPh2py)]+).
4. Experimental
4.4. Formation of [RuCp(MeCN)(2-PPh2Etpy)2](PF6)
by in situ mixing of 1 and 2-PPh2Etpy
4.1. General
In an NMR tube, 1 was mixed with two equivalents
of 2-PPh2Etpy at room temperature in CDCl3. 1H
NMR (CDCl3): a qualitative description is given here;
the different complexes have not been assigned nor have
the integrals been ascertained. d = 9.3 (d, major, ArH),
8.5 (d, minor, ArH), 7.8–7.1 (m, major, ArH), 4.45 (s,
minor, Cp), 4.4 (s, major, Cp), 2.0 (s, major, MeCN).
The signals for the bridge were observed in the range
3.5–0.8. 31P NMR (CDCl3): d = 49 (major), 42.5 (d,
2J(PP) = 49 Hz, minor), 38.2 (d, 2J(PP) = 49 Hz, minor).
ESI MS: m/z 748 ([RuCp(MeCN)(2-PPh2Etpy)2]+), 499
Silver(I) salts, allylic alcohols, isoprene, phosphines,
amines and other reagents were commercially available
and used as received. Solvents were of reagent grade.
The ligands 2-PPh2py [29] and 2-PPh2Etpy [30] and
the complex [RuCp(MeCN)3](PF6) [31] were prepared
according to the literature procedures. The syntheses
of [RuClCp(dppe)] and [RuClCp(dppb)] are described
elsewhere [17]. All syntheses and catalytic reactions were
performed in an argon atmosphere using standard
Schlenk techniques. Quantitative gas liquid chromatog-
raphy analyzes was carried out on a Chrompack appara-
tus equipped with a CP Sil 5CB column (50 m · 0.4 lm)
([RuCp(MeCN)2(2-
(MeCN)(2-PPh2Etpy)]+).
PPh2Etpy)]+),
458
([RuCp-
1
with toluene or anisole as internal standard. H NMR
spectra (199.5 MHz) were performed on a Jeol JNM
200 FTNMR spectrometer. Proton chemical shifts (d)
are reported in ppm relative to TMS (d = 0).
31P{1H}NMR spectra (121.5 MHz) were recorded on
a Bruker DPX 300 spectrometer. Chemical shifts (d)
are reported in ppm relative to external 85% aqueous
H3PO4 (d = 0). Mass spectra were recorded on a Finni-
gan MAT TSQ-70 equipped with a custom-made elec-
trospray interface (ESI) in acetonitrile/water or
methanol/water.
4.5. Reaction of 1 with two equivalents of pyridine and
3-buten-2-ol
In an NMR tube, 1 was mixed with two equivalents
of pyridine at room temperature in CDCl3. Next, 3-bu-
ten-2-ol was added and the tube was heated to 50 ꢁC. 1H
NMR (CDCl3): 8.77 (d, ArH), 8.57 (d, ArH), 8.24 (d,
ArH), 7.8–7.7 (m, ArH), 7.38 (t, ArH), 7.33 (t, ArH),
7.22 (t, ArH), 4.22 (s, Cp), 2.42 (br s, MeCN), 2.0 (br
s, MeCN). These peaks are in addition to the signals
of 3-buten-2-ol and MEK. The three doublets and three
triplets indicate the coexistence of three different pyri-
dine-containing complexes.
4.2. Formation of [RuCp(MeCN)(PPh3)2](PF6) by in
situ mixing of 1 and triphenylphosphine
In an NMR tube, 1 was mixed with two equivalents of
1
triphenylphosphine at room temperature in CDCl3. H
4.6. Mass spectrometry on reaction mixture of
[RuClCp(PPh3)2] with isoprene
NMR (CDCl3): d = 7.5–7.3 (m, 30H, PPh3), 4.43 (s, 5H,
Cp), 2.12 (s, 3H, MeCN). 31P NMR (CDCl3): d = 51.5.
The signal for the PF6 anion was outside the measured
range (d = ꢀ143.5 [20]). ESI-MS: m/z 732 ([RuCp-
(MeCN)(PPh3)2]+), 691 ([RuCp(PPh3)2]+), 510 ([RuCp-
(MeCN)2(PPh3)]+), 470 ([RuCp(MeCN)(PPh3)]+).
A solution of [RuClCp(PPh3)2] in isoprene was re-
fluxed for 2 h. ESI MS: m/z 497 ([RuCp(isoprene)-
(PPh3)]+).
4.7. NMR experiments with [RuClCp(dppe)] and
substrates with double bond moieties
4.3. Formation of [RuCp(MeCN)(2-PPh2py)2](PF6)
by in situ mixing of 1 and 2-PPh2py
To a solution of [RuClCp(dppe)] in CDCl3 was added
AgOTs (10% excess to ruthenium). Next, the substrate
was added and after stirring, a 31P NMR spectrum was re-
corded at room temperature. Recording at higher temper-
atures (up to 50 ꢁC) resulted in analogous spectra. No
other species were observed. 31P NMR (CDCl3):
[RuCp(dppe)]+: d = 77.5. [RuCp(allyl alcohol)-
In an NMR tube, 1 was mixed with two equivalents
1
of 2-PPh2py at room temperature in CDCl3. H NMR
(CDCl3): a qualitative description is given here; the dif-
ferent complexes have not been assigned nor have the
integrals been ascertained. d = 8.8 (d, minor, ArH),
8.7 (d, minor, ArH), 7.9–7.1 (m, major, ArH), 4.05 (s,
2
(dppe)]+: d = 85.3 (d, 1P, J (PP) = 21 Hz), 84.5 (d, 1P,

1054
R.C. van der Drift et al. / Journal of Organometallic Chemistry 690 (2005) 1044–1055
2J (PP) = 21 Hz). [RuCp(1-octene)(dppe)]+: d = 83.5 (d,
Acknowledgement
2
1P, 2J (PP) = 22 Hz), 82.9 (d, 1P, J (PP) = 22 Hz), 77.5
([RuCp(dppe)]+, ca. 5%). [RuCp(2-buten-1-ol)(dppe)]+:
d = 81.1 (d, 1P, 2J (PP) = 22 Hz), 80.5 (d, 1P, 2J (PP) = 22
Hz), 77.5 ([RuCp(dppe)]+, ca. 50%). [RuCp(iso-
This research was supported by the Technology
Foundation STW, applied science division of NWO
and the technology program of the Ministry of Eco-
nomic Affairs. This work was supported in part
(A.L.S.) by the Council for Chemical Sciences of the
Netherlands Organisation for Scientific Research (CW-
NWO). Dr. J.G. de Vries (DSM, The Netherlands)
and Mr. W.G. Reman (SRTCA, The Netherlands) are
thanked for stimulating discussions.
2
prene)(dppe)]+: d = 82.9 (d, 1P, J (PP) = 22 Hz), 74.9
(d, 1P, 2J (PP) = 22 Hz), 77.5 ([RuCp(dppe)]+, ca. 50%).
4.8. Isomerization of 3-buten-2-ol to MEK
In a typical experiment, 3-buten-2-ol (46 mmol), a
catalyst complex (0.008 mmol), a ligand if appropriate,
AgOTs (0.010 mmol) if appropriate and anisole or tolu-
ene (3.7 mmol) were brought into a two-neck round bot-
tom flask equipped with a reflux condenser and a
septum. Next, the flask was lowered into a pre-heated
oil bath of 100 ꢁC. After one hour a sample was taken
with an airtight syringe, and analyzed by 1H NMR
and/or GLC. In some cases, several samples were taken
at certain time intervals (see text). In cases where iso-
prene was used, 2 ml (20 mmol) was added to the reac-
tion mixture prior to heating.
Appendix A. Supplementary material
31P NMR spectra of the reaction of [RuCp(PPh3)2]+
with isoprene (Figure S1) and subsequently allyl alcohol
(Figure S2). Supplementary data associated with this
article can be found, in the online version, at
doi:10.1016/j.jorganchem.2004.11.017.
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[RuClCp(dppb)].
No.
CCDC-251670)
for
[21] In this paper, the following abbreviations for phosphine ligands
have been used: dppm = 1,1-bis(diphenylphosphino)methane;

R.C. van der Drift et al. / Journal of Organometallic Chemistry 690 (2005) 1044–1055
1055
dppe = 1,2-bis(diphenylphosphino)ethane; dppb = 1,4-bis(diphenyl-
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