Transfer hydrogenation of ketones and activated olefins using chelating NHC ruthenium complexes

Article abstract of DOI:10.1002/ejic.201100143

N-Heterocyclic carbene (NHC) ruthenium complexes consisting of different donor substituents attached to the NHC ligand efficiently catalyse the transfer hydrogenation of ketones and of activated olefins in α,β-unsaturated ketones to give saturated alcohols. The most active catalyst precursor contains a tethered olefin as a hemilabile donor site. This complex also converts nitriles and, depending on the reaction conditions, either benzylamines are produced by means of transfer hydrogenation, or amides from formal addition of H₂O. Kinetic analysis of the double hydrogenation of α,β-unsaturated ketones indicates fast isomerisation of the enol intermediate to its saturated ketone tautomer prior to the second hydrogenation. Copyright

Full text of DOI:10.1002/ejic.201100143

FULL PAPER
DOI: 10.1002/ejic.201100143
Transfer Hydrogenation of Ketones and Activated Olefins Using Chelating
NHC Ruthenium Complexes
Sabine Horn,[a] Claudio Gandolfi,[b] and Martin Albrecht*[a,b]
Keywords: Ruthenium / N-Heterocyclic carbene / Hydrogenation / Transfer hydrogenation / Ketones / Chemoselectivity
N-Heterocyclic carbene (NHC) ruthenium complexes con-
sisting of different donor substituents attached to the NHC
ligand efficiently catalyse the transfer hydrogenation of
ketones and of activated olefins in α,β-unsaturated ketones
to give saturated alcohols. The most active catalyst precursor
contains a tethered olefin as a hemilabile donor site. This
complex also converts nitriles and, depending on the reaction
conditions, either benzylamines are produced by means of
transfer hydrogenation, or amides from formal addition of
H2O. Kinetic analysis of the double hydrogenation of α,β-un-
saturated ketones indicates fast isomerisation of the enol in-
termediate to its saturated ketone tautomer prior to the sec-
ond hydrogenation.
homolytic dihydrogen activation, typically by oxidative ad-
dition and RuH2 formation, or heterolytic dihydrogen cleav-
age across the Ru–O bond may be surmised. Heterolytic
cleavage and involvement of a ruthenium monohydride in-
termediate should be facilitated if the source of dihydrogen
is strongly polarised. For example in iPrOH, hydrogen is
formally provided through a proton, bound to oxygen, and
a hydride-like carbinol hydrogen.[4] Based on this hypothesis
and considering the privileged role of Ru(arene) scaffolds
in (transfer) hydrogenation,[3b,3c,6] and specifically the suc-
cess of the corresponding NHC-containing complexes in
hydrogen transfer reactions,[7] we became interested in
probing the activity of complexes 1–4 in transfer hydrogen-
ation. A particularly intriguing aspect was the possibility of
using a single complex for either direct or transfer hydro-
genation of substrates. Despite the conceptual analogy of
these two hydrogenation processes, only few systems are
known that exhibit such dual activity.[8]
Introduction
Catalytic C–H bond making and breaking is one of the
most useful synthetic applications of organometallic chem-
istry. In most hydrogenation[1] and isomerisation reac-
tions,[2] the catalytically active species is a transition-metal
hydride which is often generated in situ. Various strategies
have been investigated to generate such reactive M–H inter-
mediates including the oxidative addition of molecular hy-
drogen, C–H bond activation of a substrate and hydride
abstraction from a hydrogen source such as a primary or
secondary alcohol, amine or formic acid.[3] This latter
method, viz. the abstraction of hydrogen from a donor mo-
lecule, constitutes a key step in transfer hydrogenation,[4] an
alternative approach to direct hydrogenation which avoids
the use of hazardous H2 gas.
Recently, we have shown that Ru(arene) complexes con-
taining a bidentate chelating N-heterocyclic carbene (NHC)
ligand are effective catalyst precursors for the direct hydro-
genation of olefins using H2 (complexes 1–4, Figure 1).[5]
The chelating group in these complexes has a pronounced
effect on the catalytic activity and the stability of the com-
plex. While the olefin donor group in 1 was rapidly hydro-
genated, thus inducing complex decomposition and pre-
dominantly heterogeneous hydrogenation, the carboxylato
group in 2 markedly increased the stability of the complex.
Owing to the presence of the carboxylato group, both
[a] School of Chemistry & Chemical Biology, University College
Dublin,
Belfield, Dublin 4, Ireland
Fax: +353-17162501
E-mail: martin.albrecht@ucd.ie
[b] Department of Chemistry, University of Fribourg,
Chemin du Musée 9, 1700 Fribourg, Switzerland
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100143.
Figure 1. Chelating NHC ruthenium complexes 1–4 and mono-
dentate carbene ruthenium complex 5.
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reaction conditions, complex 5 was a considerably less
active hydrogen transfer catalyst, achieving 36% conversion
after 10 min (Table 1, entry 6). This performance corre-
sponds to an estimated TOF50 of ca. 150 h–1. Attempts to
introduce a labile ligand into complex 5 in order to mimic
the hemilability of the olefin donor in complex 1 improved
the catalytic activity. For example, reaction of complex 5
with AgBF4 allowed for substitution of a ruthenium-bound
chloride in 5 by a solvent molecule and an essentially non-
coordinating anion. The resultant complex showed an ini-
tial activity in transfer hydrogenation that compares well
with that of complex 1 (entry 7), though completion is not
reached within 30 min. Functionalising the ruthenium–
carbene unit with an olefinic donor group seems to impart
a proper balance of lability, entailing swift activation of the
catalyst precursor, and stability of the catalytic resting states
towards undesired decomposition. These studies thus
underline the relevance of donor stabilisation as a concept
in NHC transition-metal catalysis.[14]
Results and Discussion
Transfer Hydrogenation of Ketones
Preliminary tests concentrated on evaluating the transfer
hydrogenation activity of complexes 1–4 by using benzo-
phenone as a model ketone. Standard transfer hydrogena-
tion conditions were used,[4] viz. refluxing iPrOH as a hy-
drogen source and KOH as an activator (substrate/base/
complex, 100:10:1; Table 1). Distinct differences in catalytic
hydrogen transfer activities were observed for these com-
plexes. While complete conversion was reached with all
complexes apart from 4 after extended reaction times, com-
plex 1 was most active (Ͼ 90% after 5 h). Using a carboxyl-
ato tether as in complex 2 decreased the conversion to 75%
and an even lower conversion (63%) was noted after 5 h
with the dicarbene complex 3. Changing the Ru(arene)Cl
scaffold in complex 2 to a RuCpPPh3 fragment was disad-
vantageous and complex 4, consisting of the same carboxyl-
ato-functionalised NHC ligand as 2, displayed poor ac-
tivity, reaching a modest 32% conversion after 24 h.
Table 1. Catalytic transfer hydrogenation of benzophenone.[a]
Entry Catalyst
Chelating group
Conversion (time)
1
2
3
4
1
2
3
4
1
5
olefin
COO–
NHC
COO–
olefin
Cl–
90% (5 h)
75% (5 h)
63% (5 h)
9% (5 h)
59% (10 min) 96% (30 min)
36% (10 min) 63% (30 min)
59% (10 min) 79% (30 min)
98% (24 h)
98% (24 h)
98% (24 h)
32% (24 h)
Figure 2. ORTEP representation of complex 5 (50% probability
ellipsoids, hydrogen atoms, cocrystallised CH2Cl2, and the second
crystallographically independent complex molecule omitted for
clarity). Selected bond lengths [Å] and angles [°]: Ru1–C11
2.063(2), Ru1–Cl1 2.4297(6), Ru1–Cl2 2.4437(6), C11–Ru1–Cl1
88.83(7), C11–Ru1–Cl2 88.68(7), Cl1–Ru1–Cl2 85.15(2).
5[b]
6[b]
7[b]
5 + AgBF4
(solvent)
[a] General conditions: substrate/KOH/catalyst, 100:10:1, conver-
sions determined by 1H NMR spectroscopy or GC–MS analysis.
[b] In sealed Schlenk tube with degassed solvent.
The appreciable catalytic activity of complex 1 strongly
suggests that the crucial ruthenium hydride intermediate is
not only accessible under direct hydrogenation conditions,
but also by transfer hydrogenation.[15] We therefore ex-
tended our studies to the hydrogenation of other functional
groups and activated C=C bonds, which were successfully
converted by direct hydrogenation using complexes 1–4.[5]
The reaction conditions were further optimised for com-
plex 1 (which showed the highest activity). The hydrogen
transfer rate increased substantially upon degassing the sol-
vent and upon performing the reaction in a gas-tight tube
under an inert atmosphere and at 90 °C. Under these condi-
tions, complete conversion was reached after less than one
hour (Table 1 entry 5). No induction time was noted. After
10 min, 59% conversion was achieved, which corresponds
to an approximate turnover frequency at 50% conversion
TOF50 of ca. 360 h–1. This rate is competitive with other
recently reported ruthenium-carbene complexes,[7,9] though
Transfer Hydrogenation of Other Functional Groups and
Activated Olefins
Transfer hydrogenation of esters, nitro, and cyano groups
considerably lower than the most active transfer hydrogena- met limited success (Table 2). The ester group in methyl
tion catalysts, which have TOF50 Ͼ 10,000 h–1.[10]
benzoate appeared to be unreactive and the conversion to
Since hydrogenation of the olefinic tether may constitute benzoic acid most likely ensued from saponification due to
a potential catalyst (de)activation,[11] complex 5, comprised the presence of aqueous KOH as an additive. Only low ac-
of an n-propyl wingtip group, was investigated as the satu- tivity was noted for the transfer hydrogenation of nitro-
rated version of 1 (cf. Figure 1). Complex 5 was synthesised benzene to aniline. With benzonitrile, however, clean forma-
by a transmetallation procedure and was characterised tion of benzamide occurred. Although this reaction typi-
spectroscopically as well as by X-ray diffraction (Fig- cally requires a large excess of H2O to achieve substantial
ure 2).[12] The pertinent bond lengths [Ru1–C11 2.063(2)] conversions,[16] complex 1 gave quantitative products after
and angles are within expected ranges.[13] Under identical 24 h in the presence of only 2.5 mol equiv. of H2O relative
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Transfer Hydrogenation of Ketones and Olefins with Ru Complexes
to the substrate. Hydrogenation of benzonitrile to benzyl-
amine was not observed under these conditions.[17] When
aqueous KOH was replaced by anhydrous tBuOK, transfer
hydrogenation to benzylamine took place, albeit only in low
yields. This low performance presumably originates from
strong substrate coordination to the ruthenium centre,
rather than catalyst inhibition by the intermediate imine.[10]
For example, N-benzylideneaniline is fully converted within
24 h, yielding predominantly the hydrogenated N-benzyl-
aniline. In addition to this major product, minor quantities
of aniline and benzylamine were also observed, both re-
sulting from C–N bond cleavage.[18] Obviously, nitriles bind
less reversibly to the ruthenium centre than imines, which
is in agreement with the high affinity of ruthenium(II) for
the cyano group.
Scheme 1. Transfer hydrogenation of benzylideneacetone (6). Rate
constants k refer to the observed rate constants kobs
.
and the singlet at δH = 2.15. Additionally, enol B was de-
tected as a minor component at an early stage of the reac-
tion.
Table 2. Catalytic transformation of functional groups under trans-
fer hydrogenation conditions using complex 1.[a]
A time profile of the reaction is depicted in Figure 3.
Accordingly, intermediate A reached a maximum concen-
tration of 68% after 5 h. After 24 h, the starting material
and intermediate B were completely consumed and the fully
hydrogenated alcohol 7 was the major product along with
traces of residue A (8%). While some catalysts have been
reported to be inactive in converting the enol intermediate
B and to yield mixtures,[19c,20] the catalyst derived from 1 is
active towards both intermediates A and B.[10c]
[a] General conditions as in Table 1 using complex 1 as catalyst
precursor and degassed solvents in sealed vessels; conversions after
24 h. [b] tBuOK instead of aq. KOH. [c] Final reaction mixture
also contained benzylamine (22%), aniline (18%) and unreacted
imine (1%).
Transfer hydrogenation has been widely used to reduce
not only carbonyl functions but also C=C double bonds
conjugated to an electron-withdrawing group such as a
carbonyl, ester, acid, nitro or cyano.[19] Hence, complex 1
was tested in the transfer hydrogenation of the α,β-unsatu-
rated ketone 6 as an activated olefin. Under optimised reac-
tion conditions, double transfer hydrogenation to 4-phen-
Figure 3. Time-dependent monitoring of the transfer hydrogena-
tion of 6 (᭹) by GC–MS and evolution of products A (᭜), B (᭡)
and 7 (O). Solid lines correspond to fitted rate constants using
Berkeley-Madonna software upon suppressing direct hydrogena-
tion of the enol intermediate B to 7.
The activity profile suggests that both intermediates A
ylbutan-2-ol (7) took place (91% after 5 h, Scheme 1). This and B are suitable substrates for a second transfer hydro-
product selectivity is in contrast to that observed in a study genation or, alternatively, that isomerisation between the
using a close analogue of complex 5, which revealed pre- two intermediates occurs and one substrate is preferentially
dominant formation of the saturated ketone A.[7b]
hydrogenated.[21] The latter model may be supported by the
In an attempt to investigate the pathway of the enone fact that the rate for the formation of A is comparable to
reduction in more detail, transfer hydrogenation was per- that of benzophenone hydrogenation (Table 1), thus sug-
formed in air to deliberately decelerate the catalyst activity gesting a direct formation of A from 6. Intermediate A may
(i.e. suboptimal conditions, Table 1). Time-dependent moni- further accumulate as a result of keto-enol isomerisation
toring of the reaction using GC–MS analysis and 1H NMR from enol B with an equilibrium constant that seems to
spectroscopy consistently revealed the presence of the largely favour the keto isomer.[2] The reaction profile would
monohydrogenated ketone A as the prevailing intermediate, be expected to be similar also in the former model, as-
indicated by the diagnostic multiplets at δH = 2.9 and 2.75 suming that formation of A is favoured over the formation
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S. Horn, C. Gandolfi, M. Albrecht
FULL PAPER
of B to such an extent that, in combination with faster hy- 18% was accomplished after 24 h with complex 1 as the
drogenation of B than A to the final product, the enol inter- catalyst precursor. While a remarkable TOF of 10 h–1 was
mediate concentration falls below detection limits after noted for the first 30 min, the overall reaction rate is much
some time. Kinetic modelling was therefore used to shed too low to account for a direct hydrogenation of intermedi-
further light on this double transfer hydrogenation process. ate B without prior isomerisation to the ketone A. Appar-
Taking into account the considerations outlined above, two ently, the electron-withdrawing nature of the phenyl substit-
different hypotheses were tested using the Berkeley-Ma- uent in styryl derivatives does not sufficiently polarise the
donna software.[22] In the first one, isomerisation between olefinic C=C bond. The lower catalytic activity of 1 towards
intermediates A and B was considered to be negligible (k5 styrene when compared with 6 may also suggest a critical
= 0 in Scheme 1),[23] implying that intermediates A and B role of the oxygen lone pair for the formation of a classical
were hydrogenated directly to the saturated product 7. In η3-allyl or a η3-oxoallyl intermediate.[26]
the second model, isomerisation was enforced and instead,
hydrogenation of the C=C bond in intermediate B was dis-
carded (k4 = 0), i.e. all enol intermediate isomerises to the
saturated ketone A prior to the second transfer hydrogena-
tion step. Only the second model succeeded in appropriately
reproducing the time-dependent concentrations of all four
components during the reaction (Figure 3). Upon sup-
pressing the isomerisation process, only the concentrations
for starting material and the final product were simulated
properly while the intermediates showed a poor match with
the observed data.[24]
Based on the best fitting kinetic model, the rate constants
for the hydrogenation of the keto functionality is only
slightly faster than that of the conjugated olefin (k1 = 0.299,
k3 = 0.222). Transfer hydrogenation of the ketone in A is,
however, substantially slower (k2 = 0.108) than in the conju-
gated system (viz. k3, formation of B from 6). By far the
fastest process is the isomerisation of the enol B into the
saturated ketone intermediate A (k5 = 0.672). Different
mechanisms have been proposed for this enol-to-ketone iso-
merisation,[2] including transient hydrogenation (reductive
process), transient dehydrogenation (oxidative process) and
direct isomerisation (probably by means of an allylic inter-
mediate). The former two processes seem unlikely in the
system studied here since the reaction conditions strongly
disfavour transfer dehydrogenation. The oxidative process
here would involve the rebuilding of 6, which is not compet-
itive to the oxidation of iPrOH, the latter being present in
large excess as a solvent.
Conclusions
We have demonstrated that ruthenium NHC complexes
that have previously shown activity in catalysing direct hy-
drogenation and the activation of H2 are also efficient cata-
lysts for the transfer hydrogenation of ketones. Depending
on the reaction conditions, nitriles and α,β-unsaturated
ketones were successfully converted as well. Kinetic analysis
of the enone reduction suggests that the ruthenium NHC
complexes not only catalyse transfer hydrogenation but also
induce a rapid keto-enol tautomerisation. Such isomerisa-
tions may become useful for H/D exchange reactions and
also for the transfer hydrogenation of less activated sub-
strates. Expansion of our results in these directions is cur-
rently in progress.
Experimental Section
General: The preparation of complexes 1–4[5] and 3-methyl-1-prop-
ylimidazolium bromide[27] was reported previously. CH2Cl2 was
dried with P2O5 and distilled before use. Anhydrous iPrOH was
purchased in 99.5% purity and used without further treatment. All
other reagents were commercially available and were used as re-
ceived. Column chromatography was carried out on Apollo Scien-
tific ZEOprep 60 (40–63 microns) column. All 1H NMR spectra
were recorded at 25 °C on Bruker or Varian spectrometers and ref-
erenced to residual proton solvent signals (δ in ppm, J in Hz). High
resolution mass spectrometry was carried out with a Micromass/
Waters Corp. (USA) liquid chromatography time-of-flight spec-
trometer equipped with an electrospray source. GC–MS analyses
were performed on a GCT Premier GC–MS instrument (Micro-
mass/Waters Corp. USA) using a temperature gradient.
Transfer hydrogenation of pure B[25] provided further
support for the proposed isomerisation process. Time-de-
pendent monitoring of the reaction indicated rapid con-
sumption of the allylic alcohol (75% after 10 min) and con-
comitant formation of the saturated ketone A (23%), while
the hydrogenated product 7 was present only in traces. The
concentration of 7 exceeded 10% after only about 1 h. The
Synthesis of Complex 5: A suspension of 3-methyl-1-propylimid-
azolium bromide (270 mg, 1.32 mmol) in CH2Cl2 (10 mL) was
placed under an N2 atmosphere and degassed by means of three
kinetic analyses are in agreement with a consecutive reac- freeze-pump-thaw cycles. Ag2O (153 mg, 0.66 mmol) was added
and the reaction mixture was stirred in the dark for 16 h. The crude
product was filtered through a pad of Celite. The filtrate was con-
centrated to 5 mL and added to a solution of [RuCl2(η6-p-
cymene)]2 (404 mg, 0.66 mmol) in degassed CH2Cl2 (10 mL). A so-
lid formed immediately and the reaction mixture was stirred for 4 h
in the dark. After filtration through Celite and evaporation of all
volatiles, the crude product was purified by column chromatog-
raphy on silica (CH3CN/H2O, 10:1) and recrystallisation from
tion involving isomerisation and subsequent hydrogenation.
The best fit revealed an approximate 2:1 ratio of the two
rate constants, which is slightly lower than that determined
for these two steps in the more complex reduction of 6,
a result that may be rationalised by the different starting
materials, i.e. free allyl alcohol vs. precoordinated (and per-
haps deprotonated) B when generated from 6.
In line with the kinetic model, transfer hydrogenation of
4-tert-butylstyrene as a nonisomerisable analogue of inter-
CH2Cl2/C5H12 to afford complex 5 as red crystals (155 mg, 27%).
3
1H NMR ([D6]acetone, 500 MHz, 25 °C): δ = 0.94 (t, JH,H
=
3
mediate B proceeded sluggishly. A moderate conversion of 7.4 Hz, 3 H, NCH2CH2CH3), 1.29 [d, JH,H = 6.9 Hz, 6 H,
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Transfer Hydrogenation of Ketones and Olefins with Ru Complexes
[4]
a) S. Gladiali, E. Alberico, in: Transition Metals for Organic
Synthesis (Eds.: M. Beller, C. Bolm), Wiley-VCH, Weinheim,
Germany, 2004, vol. 2, p. 145; b) D. Klomp, U. Hanefeld, J. A.
Peters, in: Handbook of Homogeneous Hydrogenation (Eds.:
J. G. de Vries, C. J. Elsevier), Wiley-VCH, Weinheim, Germany,
2007, vol. 2, p. 585; c) S. Gladiali, E. Alberico, Chem. Soc. Rev.
2006, 35, 226; d) J. S. M. Samec, J.-E. Bäckvall, P. G. An-
dersson, P. Brandt, Chem. Soc. Rev. 2006, 35, 237. For specific
examples relevant to this work, see, e.g.: e) J. Louie, C. W. Biel-
awski, R. H. Grubbs, J. Am. Chem. Soc. 2001, 123, 11312; f)
A. A. Danopoulos, S. Winston, W. B. Motherwell, Chem. Com-
mun. 2002, 1376; g) M. Poyatos, J. A. Mata, E. Falomir, R. H.
Crabtree, E. Peris, Organometallics 2003, 22, 1110.
CH(CH3)2], 1.84 (m, 2 H, NCH2CH2CH3), 1.96 (s, 3 H, Ccym
CH3), 2.97 (sept, JH,H = 6.9 Hz, 1 H, CHMe2), 4.00 (s, 3 H,
–
3
3
NCH3), 4.32 (br., 2 H, NCH2CH2CH3) 5.15, 5.47 (2ϫ d, JH,H
=
5.9 Hz, 2 H, Ccym–H), 7.31, 7.37 (2ϫ d, 3JH,H = 2.0 Hz, 2 H, Cimi
–
H) ppm. 13C{1H} NMR ([D6]acetone, 125 MHz, 25 °C): δ = 11.8
(NCH2CH2CH3), 19.2 (Ccym-CH3), 23.2 [CH(CH3)2], 26.2
(NCH2CH2CH3),
32.0
(CHMe2),
40.1
(NCH3),
53.9
(NCH2CH2CH3), 82.6, 88.1 (2ϫ Ccym-H), 99.7, 109.6 (2ϫ Ccym
-
C), 122.9, 125.5 (2ϫ Cimi-H), 175.9 (Cimi-Ru) ppm. HRMS (ES+):
calcd. for C17H26N2ClRu [M – Cl]+ 395.0828; found 395.0837.
Typical Procedure for Catalytic Transfer Hydrogenation: The cata-
lyst (20 μmol) was dissolved in iPrOH (10 mL).[28] KOH (0.10 mL
of 2 m solution in H2O, 0.2 mmol) was added and the mixture was
heated to reflux for 10 min. The substrate (2.0 mmol), containing
the internal standard 3,5-dimethylanisole (0.6 mmol), was then
added at once. Aliquots (0.2 mL) were taken at fixed times,
quenched with pentane (1 mL) and filtered through a short path
of silica. The silica was washed with Et2O (2ϫ 2 mL) and the com-
bined organic filtrates were analysed by GC–MS or carefully evap-
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Analysis of the product solution after catalytic runs was not
conclusive. Complex 1 was certainly not present anymore,
though the broadness of the signals precluded an unambiguous
identification of the allyl or n-propyl group and hence the pos-
tulation of wingtip group stability or hydrogenation. Re-
forming of the catalyst precursor after transfer hydrogenation
is rare and often limits the recycling of the catalyst. For a recent
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1
orated and analysed by H NMR spectroscopy.
Optimised Procedure for Catalytic Transfer Hydrogenation: A
10 mL oven-dried Schlenk-tube was placed under N2 and charged
with iPrOH (10 mL). The solvent was degassed by means of three
freeze-pump-thaw cycles and placed under N2 again. The catalyst
(20 μmol) was added and dissolved by using ultrasound (10 min,
40 °C). KOH (0.1 mL, 2 m in H2O, 0.2 mmol) was added and the
mixture preheated in a septum-sealed tube at 90 °C for 10 min.
Substrate (2.0 mmol) and the internal standard 3,5-dimethylanisole
(80 μL, 0.6 mmol) were added with a syringe. Aliquots (0.2 mL)
were taken at fixed times and analysed as outlined above.
[7]
Supporting Information (see footnote on the first page of this arti-
cle): Kinetic model including the transformation of B directly to 7
and crystallographic details.
[8]
[9]
Acknowledgments
We thank Prof. More O’Ferrall for fruitful discussions, Mr. Conboy
for technical assistance and Dr. Müller-Bunz for crystallographic
analyses. This work has been financially supported by the Swiss
National Science Foundation and the European Research Council
(ERC StG 208561).
[10]
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Crystal data for 5: Empirical formula [C17H26Cl2N2Ru]·
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0.5CH2Cl2, M 945.66, orange rod, triclinic, space group P1
(no. 2), a = 10.3524(4) Å, b = 13.0811(5) Å, c = 16.5139(7) Å,
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= 0.0535, R indices based on reflections with I Ͼ 2σ(I) (refine-
ment on F2), 434 parameters, 0 restraints. Analytical numeric
Eur. J. Inorg. Chem. 2011, 2863–2868
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2867

S. Horn, C. Gandolfi, M. Albrecht
FULL PAPER
absorption corrections applied, μ = 1.191 mm–1. CCDC-
810172 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
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No 1-phenylbut-2-ene-1-ol resulting from allylic isomerisation
was detected, indicating that the C–O bond is preserved under
the applied reaction conditions.
[22]
[23]
Berkeley-Madonna, version 8.3, University of California at
Berkeley, 2009.
All rate constants refer to observed rates, kobs, for each step,
which may be considered to be effective rate constants (apart
from k5) due to the large excess of iPrOH, which essentially
suppresses the reverse dehydrogenation reaction. An exception
to this approximation is the isomerisation rate k5, which pre-
sumably does not involve iPrOH.
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Received: February 11, 2011
Published Online: May 4, 2011
2868
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2863–2868