Degradation of the ruthenium-based metathesis catalyst [RuCl2 (=CHPh)(H2IPr)(PCy3)] with primary alcohols

Article abstract of DOI:10.1016/j.jorganchem.2004.07.033

A new ruthenium hydride species, showing high catalytic isomerization and hydrogenation properties, was isolated via a ligand exchange reaction. The same species was also present in the mixture of degradation products of the monophosphinic complex RuCl(CO)(PCy)₃(IPr)CH(Ph), 3, after reaction with alcohols under basic conditions.

Full text of DOI:10.1016/j.jorganchem.2004.07.033

Journal of Organometallic Chemistry 689 (2004) 3113–3116
www.elsevier.com/locate/jorganchem
Short communication
Degradation of the ruthenium-based metathesis
catalyst [RuCl2(‚CHPh)(H2IPr)(PCy3)] with primary alcohols
1
Donatella Banti , Johannes C. Mol
*
Faculty of Science, Institute of Molecular Chemistry, Universiteit van Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
Received 14 July 2004; accepted 15 July 2004
Available online 23 August 2004
Abstract
A new ruthenium hydride species, showing high catalytic isomerization and hydrogenation properties, was isolated via a ligand
exchange reaction. The same species was also present in the mixture of degradation products of the monophosphinic complex
RuCl(CO)(PCy)3(IPr)CH(Ph), 3, after reaction with alcohols under basic conditions.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Metathesis; Isopropylidene ligand; Hydride; Isomerization; Hydrogenation
The last decade has seen an increasing exploitation
of olefin metathesis for synthetic purposes [1], mainly
due to the discovery and development of well-defined
PCy3
N
N
N
N
Cl
Cl
Ru
Cl
and stable ruthenium carbene species such as the clas-
sical first- and second-generation GrubbsÕ catalysts 1
and 2 [2] and the more recently described catalyst 3
[3a,b] bearing a modified version of the imidazolylid-
ene ligand (so-called MolÕs catalyst [3c]). The ruthe-
nium benzylidene species 3 was found to be very
active showing a particularly high turnover number
in self-metathesis reactions of terminal alkenes [3a]
and also exhibiting the highest initial ADMET (Acy-
clic Diene Metathesis) rate of any phosphine ligated
ruthenium complex [3c].
Ru
Cl
Ph
Ru
PCy3
Cl
Ph
Cl
Ph
PCy3
PCy3
1
2
3
Species 1 and 2 were recently found to be not only ac-
tive metathesis catalysts, but also precursors to isomeri-
zation and hydrogenation catalysts when treated with
alcohols in the presence of a base [4]. These findings
are rather intriguing since isomerization products are
sometimes found as by-products in metathesis reactions
[5]. It is therefore of primary importance that a complete
understanding of possible degradation pathways under
different conditions is resolved, in order to minimize or
exploit these side reactions for synthetic purposes [5d].
Given the high catalytic activity shown by the benzy-
lidene species 3, it was necessary to investigate first its
degradation process under basic conditions in the pres-
ence of a primary alcohol. The aim was to possibly
*
Corresponding author. Tel: +31 20 525 6490; fax: +31 20 525
6456.
E-mail addresses: donatella.banti@kcl.ac.uk (D. Banti), jcmol@
science.uva.nl (J.C. Mol).
1
Chemistry, KingÕs College London, Strand, WC2R 2LS London,
UK. Tel.: 020 7848 1147; fax: 020 7848 2810.
0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.07.033

3114
D. Banti, J.C. Mol / Journal of Organometallic Chemistry 689 (2004) 3113–3116
IPr
Ru
present in the mixture of degradation products of catalyst
3 when left to stand for 24 h under nitrogen in a CH2Cl2
solution.
IPr
Ru
Cl
Cl
H
ROH/Et3N
Toluene
Cl
C
Ph
O
PCy3
PCy3
When reacting the same species with benzyl alcohol
the same four major species were evident in the 31P
NMR having d = 51.02 (s), 47.04 (d, J = 24 Hz), 46.61
3
4
R = Me, Et, n-Pr, C9H19
1
IPr
(s) and 34.61 ppm (s). The H NMR spectrum showed
Cl
PhCH2OH/Et3N
Toluene
the formation of three hydride species. Along with the
major product (complex 4) displaying a doublet at
d = À24.32 ppm (J = 25 Hz) (ca. 80%), a minor product
with d = À7.09 ppm (s) (ca. 10%) and a new species giv-
ing a doublet at d = À13.31 ppm (d, J = 33 Hz) (ca. 5%)
were detected in the mixture. To our surprise there was
no signal in the phosphorous region at around 25 ppm
where we would anticipate the signal for the phenyl spe-
cies 5 [4].
Ru Ph
C
O
PCy3
5
IPr =
N
N
Scheme 1. Degradation of catalyst 3 in base/alcohol.
The formation of by-products could be due to the
predicted increased lability of the N-heterocyclic carbene
ligand H2IPr relative to H2IMes [6] though the same
mixture of products was obtained at lower temperatures
(r.t; 40 °C).
Since catalysts 4 and 5 were not obtained selectively
they must be formed and isolated via an independent
route. As for the mixed ligand species RuCl(CO)(P-
Cy3)(H2IMes)(X) (where X = H, 6 or Ph, 7) [4], ligand
exchange reactions between complexes 8 and 9 and the
free carbene 10 were attempted (Scheme 2).
The free carbene 10 was obtained in situ by reacting
the corresponding hydrochloride salt with a toluene sus-
pension of potassium tert-pentoxide. The solution con-
taining the free carbene 10 was then filtered by
cannula into a toluene solution of catalyst 8 or 9. The
two reactions were followed by 31P NMR spectroscopy.
The reaction of complex 8 proceeded to 95% conver-
sion. It was possible to isolate complex 4 from the mix-
ture via careful multiple precipitations [7].
In the case of the phenyl complex 9, the ligand ex-
change stopped at 70% conversion even upon addition
of subsequent batches of free carbene solution of the
substrate. Attempts to isolate the monophosphine spe-
cies 5 from 9 were unsuccessful.
obtain new isomerization or hydrogenation species from
catalyst 3 as a first step into selective tandem metathesis/
isomerization or tandem metathesis/hydrogenation
reactions.
In analogy with what was previously done for species
1 and 2 [4], catalyst 3 was reacted with primary aliphatic
alcohols and benzylic alcohol (Scheme 1). As for catalyst
2, these reactions with catalyst 3 did not give selectively
one product.
Upon reaction of catalyst 3 with an aliphatic alcohol at
80 °C two major hydride products were formed. The ma-
jor one showed a doublet at d = À24.34 ppm (JP–H = 21
Hz) and was identified as the monophosphine species 4
(Scheme 2). The phosphorous signal for this species gave
a doublet at d = 47.04 ppm (J = 24 Hz). It was not possi-
ble to obtain selectively this species through this route
1
even at low temperatures. The H NMR of the reaction
mixture showed the presence of a second hydride giving
a singlet at d = À7.07 ppm as a minor product (20–
25%) and in the 31P NMR a signal at d = 46.61 ppm (s).
Two more unidentified species were present in the 31P
NMR spectra, giving two signals at d = 51.03 ppm and
d = 34.61 ppm, respectively. This last species was also
Table 1
Isomerization activity of complex 4a
PCy3
Cl
N
N
Cl
Ru
H
Ru
PCy3
H
Entry
T (°C)
Conv. (%)
% Selectivity for
2-octene (cis:trans)
TON
–
C
N
N
C
O
PCy3
O
4
8
1
2
3
4
5
6
a
23
40
0
4.6
–
96 (30:70)
99 (26:74)
99 (29:31)
92 (28:72)
41 (27:73)
4600
Toluene
60
80
12.2
36.6
95.1
96.4
12,200
36,600
95,100
96,400
PCy3
Cl
Ru Ph
N
N
Cl
100
120
C
PCy3
Ru Ph
PCy3
O
Reaction procedure: After purification of the substrate (alumina
column), it was degassed and heated to the desired temperature. The
catalyst was then added and samples were taken at different times and
analyzed by GC–FID.
C
9
5
O
Scheme 2. Ligand exchange reaction of complexes 8 and 9.

D. Banti, J.C. Mol / Journal of Organometallic Chemistry 689 (2004) 3113–3116
3115
100
80
60
40
20
0
100
80
60
40
20
0
100
90
80
70
60
50
0
60 120 180 240 300 360 420 480
Reaction time (min)
20
40
60
80
100
120
Fig. 2. Conversion (full lines) and selectivity for 2-octene (dotted lines)
as functions of reaction time for the isomerization of neat 1-octene
(100,000 equivalents) at 100 °C in the presence of catalysts 4 ( ), 6 ( ),
8 ( ) and 9 ( ).
Temperature (˚C)
Fig. 1. Conversion as a function of reaction temperature for the
double-bond isomerization of neat 1-octene (100,000 equivalents) in
the presence of catalysts 4 ( ), 6 ( ), 8 ( ) and 9 ( ).
Table 2
Hydrogenation of 1-octene with hydride 4a
Entry
Equiv.
T (°C)
P (bar)
t (h)
Conv. (%)
TON
Isom. (%)
2-Octene (%)
cis:trans Ratio
1
2
3
4
a
100,000
100,000
250,000
350,000
23
100
100
100
4
4
1
4
22
3
56.0
98.1
97.0
43.0
56,000
98,100
2.8
19.6
67
64
72
75
72
56:64
31:69
33:67
39:61
3
242,000
151,000
2
31
Hydrogenation experiments were conducted in an autoclave fitted with a glass liner. The autoclave was filled with the substrate, the catalyst and
then quickly set to the desired H2 pressure and temperature.
The new complex 4 was tested for its hydrogenation
and isomerization properties for 1-octene; the results
were compared with the literature data [4].
Table 1 shows the isomerization activity of complex 4
and the graphs (Figs. 1 and 2) illustrate comparisons
with literature data.
genation catalyst. Such species also represent the main
degradation product of catalyst 3 in a basic alcoholic
solution, providing therefore a further step forward to-
wards the possibility to perform tandem metathesis/
isomerization and metathesis/hydrogenation reactions.
Hydrogenation experiments on 1-octene were con-
ducted in an autoclave that was filled with hydrogen
to the desired pressure and subsequently allowed to
come to the required temperature.
Acknowledgements
We thank Dr. Maarten Dinger for helpful
discussions.
As is evident from the results reported in Table 2 a
significant increase in the percentage of isomerization
products was recorded for 4 when compared to the
activity of similar catalysts in the literature [4]. This
could be due to the additional steric bulk of the carbene
ligand in 4 which impedes the hydrogenation of internal
olefins (once produced by the isomerization side reac-
tion). In order to ascertain whether the hydrogenation
selectivity for a-olefins was 100%, additional tests on
an internal olefin were also conducted.
Appendix A. Supplementary material
Supplementary data associated with this article can
be found, in the online version at doi:10.1016/
j.jorganchem.2004.07.033.
When 2-octene was used a mixture of octane and cis
and trans 2-octene was obtained (conversion: 74%)
revealing that the hydrogenation selectivity for a-ole-
fins is not 100% and that the hydrogenation of internal
olefins is slower than with previous hydride catalysts
[4].
In conclusion, the isopropylidene hydride 4 was iso-
lated and characterized for the first time. Reactivity tests
showed it to be a highly active isomerization and hydro-
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