Synthesis and catalytic alcohol oxidation and ketone transfer hydrogenation activity of donor-functionalized mesoionic triazolylidene ruthenium(ii) complexes

Article abstract of DOI:10.1039/c3dt53052c

We report on the synthesis of a variety of C,E-bidentate triazolylidene ruthenium complexes that comprise different donor substituents E (E = C: phenyl anion; E = O: carboxylate, alkoxide; E = N: pyridine at heterocyclic carbon or nitrogen). Introduction of these donor functionalities is greatly facilitated by the synthetic versatility of triazoles, and their facile preparation routes. Five different complexes featuring a C,E-coordinated ruthenium center with chloride/cymene spectator ligands and three analogous solvento complexes with MeCN spectator ligands were prepared and evaluated as catalyst precursors for direct base- and oxidant-free alcohol dehydrogenation, and for transfer hydrogenation using basic iPrOH as a source of dihydrogen. In both catalytic reactions, the neutral/mono-cationic complexes with chloride/cymene spectator ligands performed better than the solvento ruthenium complexes. The donor functionality had a further profound impact on catalytic activity. For alcohol dehydrogenation, the C,C-bidentate phenyl-triazolylidene ligand induced highest conversions, while carboxylate or pyridine donor sites gave only moderate activity or none at all. In contrast, transfer hydrogenation is most efficient when a pyridyl donor group is linked to the triazolylidene via the heterocyclic carbon atom, providing turnover frequencies as high as 1400 h^-1 for cyclohexanone transfer hydrogenation. The role of the donor group is discussed in mechanistic terms.

Full text of DOI:10.1039/c3dt53052c

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Synthesis and catalytic alcohol oxidation and
ketone transfer hydrogenation activity of
donor-functionalized mesoionic triazolylidene
ruthenium(II) complexes†
Cite this: DOI: 10.1039/c3dt53052c
Manuela Delgado-Rebollo, Daniel Canseco-Gonzalez, Manuela Hollering,
Helge Mueller-Bunz and Martin Albrecht*
We report on the synthesis of a variety of C,E-bidentate triazolylidene ruthenium complexes that com-
prise different donor substituents E (E = C: phenyl anion; E = O: carboxylate, alkoxide; E = N: pyridine at
heterocyclic carbon or nitrogen). Introduction of these donor functionalities is greatly facilitated by the
synthetic versatility of triazoles, and their facile preparation routes. Five different complexes featuring a
C,E-coordinated ruthenium center with chloride/cymene spectator ligands and three analogous solvento
complexes with MeCN spectator ligands were prepared and evaluated as catalyst precursors for direct
base- and oxidant-free alcohol dehydrogenation, and for transfer hydrogenation using basic iPrOH as a
source of dihydrogen. In both catalytic reactions, the neutral/mono-cationic complexes with chloride/
cymene spectator ligands performed better than the solvento ruthenium complexes. The donor function-
ality had a further profound impact on catalytic activity. For alcohol dehydrogenation, the C,C-bidentate
phenyl-triazolylidene ligand induced highest conversions, while carboxylate or pyridine donor sites gave
only moderate activity or none at all. In contrast, transfer hydrogenation is most efficient when a pyridyl
donor group is linked to the triazolylidene via the heterocyclic carbon atom, providing turnover frequen-
cies as high as 1400 h−1 for cyclohexanone transfer hydrogenation. The role of the donor group is dis-
cussed in mechanistic terms.
Received 29th October 2013,
Accepted 27th November 2013
DOI: 10.1039/c3dt53052c
www.rsc.org/dalton
specific σ-donor properties, which are slightly stronger than
Introduction
classical imidazolium-derived systems,5a together with vast
The functionalization of key ligands with donor groups for synthetic flexibility of the ligand precursors.5c,6 The vast
application in catalytic reactions has been much explored in majority of mesoionic triazolylidene complexes reported thus
coordination and organometallic chemistry1 since the poten- far feature monodentate ligands. Even though ligand tuning
tially chelating nature of these donor groups offers electronic has been applied to tailor catalytic activity in various processes
and coordinative flexibility that may facilitate different steps of with such monodentate complexes,7 it is remarkable that the
a catalytic cycle.2 While robust chelation imparts stability to a synthetic flexibility of the triazole ligand synthesis has not
catalytically active species, hemilabile donor group coordi- been exploited more extensively up to now for the introduction of
nation may be highly valuable for accessing transient inter- chelating complexes. Examples with chelating triazolylidenes
mediates in a catalytic cycle.3 Accordingly, the concept of include ruthenium and iridium complexes featuring a chelat-
chelation has found widespread application in N-heterocyclic ing pyridyl-functionalized triazolylidene ligand for photosensi-
carbene (NHC) chemistry.4 As a subclass of NHCs, mesoionic tisation,8 water oxidation,9 and a series of rhodium and group
1,2,3-triazolylidene ligands have recently gained significant 10 complexes.10
attention,5 in part because they combine the benefits of
In light of the high versatility of [2 + 3] dipolar cyclo-
addition chemistry to access the corresponding 1,2,3-triazoles
as carbene precursors,11 we have been interested in expanding
the range of chelating triazolylidene ligands in order to gain a
better mechanistic understanding and eventually to rationally
enhance the catalytic activity of triazolylidene complexes in
ruthenium-catalyzed de- and transfer-hydrogenation reac-
tions.12 Therefore, we report here the synthesis and catalytic
School of Chemistry & Chemical Biology, University College Dublin, Belfield,
Dublin 4, Ireland. E-mail: martin.albrecht@ucd.ie; Fax: +353 17162501;
Tel: +353 17162504
†Electronic supplementary information (ESI) available. CCDC 968356–968359.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt53052c
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activity of a range of triazolylidene ruthenium complexes that (TBDMS = tert-butyldimethylsilyl) to avoid complications in
feature different donor functionalization. In an attempt to chemoselective alkylation and metallation. Notably, alkylation
maximize diversity, these donor-functionalized carbenes of the protected triazole with MeOTf induced cleavage of the
include (potentially) C,O-bidentate chelates featuring an alko- TBDMS group even under mild conditions (1 eq. at RT). Better
xide and a carboxylate donor group, C,N-bidentate, that are results were obtained when MeI was used as an alkylation
distinguished by their bonding to either the triazole nitrogen agent (3.5 molequiv. MeI, 80 °C, 5 h). Longer reaction times
or carbon, and an ortho-metalated phenyl substituent that should be avoided, however, as slow cleavage of the alcohol
imparts a C,C-bidentate coordination mode of the triazolyli- protecting group was also noted with MeI.17 The 1H NMR
dene. The catalytic activity of these ruthenium complexes in spectra of 1c and 1e show a low field resonance for the triazo-
hydrogen transfer reactions and alcohol oxidation indicates lium proton at C5 at δH 9.29 and 8.79 ppm for 1c and 1e
that tailoring of the donor group is critical and leads to high respectively, which are downfield from their corresponding
turnover frequencies.
triazole precursors.
Synthesis of the ruthenium cymene complexes
Ruthenation of triazolium salts 1c–f was accomplished by a
one-pot procedure mediated by Ag2O according to established
procedures9b in 43% to 74% yield (Scheme 1). All complexes
Results and discussion
Synthesis of ligand precursors
The new triazolium salts 1c–e were readily accessible via con- were stable towards moisture and air both in solution and in
ventional dipolar [2 + 3] cycloaddition11 of aryl azides and the the solid state. The neutral complexes 2c and 2e were better
corresponding alkyne, followed by selective N-functionali- soluble in chlorinated solvents and toluene than the cationic
zation. Due to the specific properties of the donor functionali- compound 2d. Successful complexation was indicated by the
ties in these three triazolium salts (i.e. ester, imine, and characteristic NMR data, specifically by the disappearance of
protected alcohol), each synthesis followed a distinct pathway. the aromatic triazolium proton around δH 8.8–9.7 ppm and by
Thus, methylation of the triazole precursor of 1c at the N3 the presence of resonances due to the cymene group and the
position with MeOTf gave a cleaner reaction than with MeI triazolylidene ligand in equimolar ratio. In the 13C NMR spec-
and proceeded in quantitative yield (98%). Elution over an ion trum, the carbenic carbon resonance appears around 170 ppm
exchange column gave the chloride analogue 1c′. Successful for complexes 2c–e, while the carboxylate group in 2c resonates
anion exchange was indicated by the characteristic downfield at δC 165.9 ppm.
shift of the acidic triazolium salt from δH 8.83 to 9.71 (CDCl3
While complexes 2e and 2f displayed the signature of a
solution), presumably as a consequence of better ion pairing monodentate carbene ligand such as symmetry-related mesityl
in 1c′.13 Both salts are unstable over time and decarboxylation and cymene protons, respectively, spectroscopic data indicate
of 1c was observed during recrystallization, cleanly yielding the chelating triazolylidene ligands in complexes 2c and 2d.
triazolium salt 1f. A similar decarboxylation was also observed Bidentate coordination was supported by the desymmetriza-
during the quaternization of the triazole precursor with MeI tion of the cymene ring, indicated by the four doublets
under microwave conditions. The decarboxylation was con- between 5.4 and 4.6 ppm (2c) and between 6.1 and 5.8 ppm
firmed spectroscopically by the absence of the carbonyl IR (2d). In addition, the two methyl groups of the isopropyl unit
stretch vibration at 1751 cm−1 and by the presence of two as well as the meta protons of the mesityl substituent of the
triazolium protons with equal intensity (δH 10.63 and 9.00) in triazolylidene ligand (in 2c) become inequivalent upon com-
the NMR spectrum, and by a single crystal X-ray diffraction plexation. Metal bonding of the pyridine unit in complex 2d
analysis (Fig. S1†). The lability of the ester moiety in 1c may was further indicated by the characteristic9b downfield shift
open new opportunities for metallation of triazolium salts via of the ortho proton of the pyridyl substituent from δH
decarboxylative methods.14
The triazolium salt 1d comprising a pyridine donor group complex.
8.72 ppm in the ligand precursor 1d to δH 9.24 ppm in the
at N3 was synthesized from the 1,5-disubstituted triazole pre-
Chelation of the carboxylate group in 2c is remarkable as it
cursor, which was prepared via an uncatalyzed cycloaddition implies loss of the methyl group from the ester precursor 1c.
of phenyl azide and an α-keto phosphorus ylide as the alkyne Presumably, ester hydrolysis is a consequence of the basic con-
equivalent (Scheme S1†).15 Subsequent quaternization of N3 ditions entailed by Ag2O and by the generation of OH– ions
with bromopyridine at elevated temperatures followed by an upon formation of the silver triazolylidene intermediate.18 In
anion exchange yielded 1d in high yields. This procedure the IR spectrum, the carbonyl vibration was observed at
is superior to the classic copper catalyzed pathway, as the sub- 1655 cm−1 as expected for a carboxylate group19 and similar to
sequent methylation of pyridyl-triazoles is unselective and pro- chelating imidazol-2-ylidine carboxylate ruthenium(II) com-
vides a mixture of pyridinium and triazolium products.9b,16
plexes (νCO = 1630 cm−1),20 yet at substantially lower energy
The triazolium salt 1e containing a masked alkoxide donor than the CO vibration in the ester precursor 1c (νCO
group was synthesized via classical copper-catalyzed click 1751 cm−1).
Unambiguous evidence for carboxylate chelation in 2c
alcohol functionality was subsequently TBDMS-protected was obtained by a single crystal X-ray diffraction analysis.
=
cycloaddition11 of mesityl azide and propargyl alcohol. The
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Scheme 1 Synthesis of the new complexes 2c–2f.
the carboxylate group produces a five-membered metallacycle
with an acute bite angle (C1–Ru–O1 76.84(7)°) and with Ru–C
and Ru–O bond distances of 2.0303(19) and 2.1444(15) Å,
respectively. The latter is slightly longer than in the related
imidazol-2-ylidine ruthenium(II) complex (Ru–O 2.079(8) Å),20
while the Ru–Ctrz bond is some 0.03–0.06
Å shorter
than in analogous monodentate triazolylidene ruthenium
complexes,7g yet in good agreement with other chelating com-
plexes, including the previously determined structures of 2a
and 2b (2.0372(15) and 2.0318(12) Å, respectively).9b,10c
Metallation of 1f may occur principally at two positions due
to the presence of two triazolium C–H sites. NMR spectro-
scopic analysis of the crude reaction product indicated,
however, selective metallation of only one position. NOE experi-
ments suggest a through-space interaction of the CH group of
the triazolylidene and the ortho methyl group of the mesityl
substituents, indicating metallation of the triazolium at the
site proximal to the methyl substituents. This selectivity, prob-
ably imparted by the steric shielding of the mesityl sub-
stituents, was unequivocally confirmed by a crystallographic
analysis (Fig. 2). Bond lengths and angles of this piano-stool
complex are unsurprising and in good agreement with related
monodentate ruthenium carbene complexes.7g,21
Fig. 1 ORTEP representation of 2c (50% probability, hydrogen atoms
omitted for clarity). The cymene ring is disordered. Selected bond
lengths (Å) and angles (°): Ru(1)–C(1) 2.0303(19), Ru(1)–O(1) 2.1444(15),
Ru(1)–Cl(1) 2.4170(1), Ru(1)–Ccentroid 1.679(5); C(1)–Ru(1)–O(1) 76.84(7),
C(1)–Ru(1)–Cl(1) 86.65(1), O(1)–Ru(1)–Cl(1) 83.95(5), C(1)–Ru(1)–Ccentroid
133.90(18), O(1)–Ru(1)–Ccentroid 132.87(15).
The molecular structure (Fig. 1) confirmed the expected con-
nectivity pattern as deduced from solution analysis and reveals
the classical three-legged piano-stool geometry. Chelation of
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Fig. 3 ORTEP representation of 3a (50% probability, hydrogen and
OTf − counterion omitted for clarity). Selected bond lengths (Å) and
angles (°): Ru–C(1) 2.0394(17), Ru–C(10) 2.0538(17), Ru–N(4) 2.1025(14),
Ru–N(5) 2.1140(14), Ru–N(6) 2.0093(14), Ru–N(7) 2.0231(14); C(1)–Ru–
C(10) 80.03(7), C(1)–Ru–N(4) 174.67(6), N(4)–Ru–N(5) 88.33(5), C10(1)–
Ru–N(7) 92.87(6), C10(1)–Ru–N(5) 176.68(6), N6(1)–Ru–N(7) 174.97(6).
Fig. 2 ORTEP representation of 2f (50% probability, hydrogen atoms
omitted for clarity and only one of the two independent molecules
shown). The cymene ring is disordered. Selected bond lengths (Å) and
angles (°): Ru(1)–C(1) 2.0573(17), Ru(1)–Cl(1) 2.4157(4), Ru(1)–Cl(2)
2.4274(4), Ru(1)–Ccentroid 1.6867(8); C(1)–Ru(1)–Cl(1) 84.59(5), C(1)–
Ru(1)–Cl(2) 87.78(5), C(1)–Ru(1)–Ccentroid 129.06(6), Cl(1)–Ru(1)–Cl(2)
88.00(2).
donor group. This pattern suggests that one MeCN ligand is
kinetically very labile, which may be a direct consequence of
the strong donor ability of the triazolylidene. The modification
of the spectator ligands induces less steric congestion, which
is demonstrated by the free rotation of the mesityl substituent
(cf. single resonance at δH 7.12 ppm for the aromatic protons).
The carbenic carbon resonance shifts to a lower field by
approximately 3–4 ppm upon transformation of 2 to 3 (e.g. δC
176.2 ppm in 3c vs. 172.8 ppm in 2c). No signs were observed
that would indicate metallacycle ring opening. For example in
complex 3a, four different proton signals for the N-bound
phenyl group suggest a Ru–CPh bond that is robust under the
applied reaction conditions. A single crystal X-ray diffraction
analysis of 3a is in agreement with this conclusion. The mole-
cular structure (Fig. 3) reveals the surmised metallacycle due
to C,C-bidentate chelation of the phenyl-substituted triazolyli-
dene ligand. The ruthenium center is located at the center of a
distorted octahedron. The bite angle C1–Ru–C10 of the chelat-
ing ligand is 80.03(7)°. The Ru–Ctrz bond is slightly but signifi-
cantly shorter than the Ru–CPh bond, 2.0394(17) vs. 2.0538(17)
Å. These bonds are shorter than those in the neutral ruthe-
nium complex 2a,10c presumably a direct consequence of the
lower electron density at ruthenium in a [Ru(NCMe)4]2+ con-
figuration as compared to the [RuCl(cym)]+ fragment. As
expected, the Ru–NMeCN bond lengths are strongly influenced
by their trans ligand. The Ru–N4 and Ru–N5 bonds featuring a
C-donor in trans position are substantially longer than the
Ru–N bond lengths of the mutually trans located MeCN
ligands. Interestingly, the trans influence of the mesoionic tri-
azolylidene ligand is almost identical to that of the anionic
phenyl ligand (Ru–N(4) 2.1025(14) Å vs. Ru–N(5) 2.1140(14) Å),
which suggests a high relevance of the mesoionic (rather than
a carbenic) limiting resonance structure.
Synthesis of solvento complexes
Selected solvento complexes were synthesized in order to
evaluate the role of the spectator ligands in catalysis (see
below). The complexes 3a–c were prepared from the ruthe-
nium(II) cymene precursors 2 by thermal replacement of the
cymene ancillary ligand with MeCN and simultaneous AgOTf-
mediated abstraction of the chloride ligand22 in 40–60% yield
(Scheme 2). These solvento complexes are more sensitive to air
than their cymene counterparts and gradual degradation was
indicated by a colour change from yellow to black over several
days. Complexes 3a–c are moderately soluble in chlorinated
solvents and toluene, and better soluble in polar solvents such
as MeCN, dichloroethane, 1,2-dichlorobenzene, and tBuOH.
Successful formation of the solvento complexes is deduced
from the absence of cymene resonances and the presence of
signals attributed to coordinated MeCN. Typically, only two
sets of MeCN resonances were observed in a 2 : 1 ratio,
assigned to the mutually trans positioned MeCN ligands and
one MeCN ligand trans to either the carbene or the chelating
Based on the established activity of N-heterocyclic carbene
ruthenium complexes in hydrogen transfer reactions23 and in
Scheme 2 Synthesis of complexes 3a–c.
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dehydrogenative alcohol oxidation,7g,24 we were interested in MeCN than the substrate ROH to the ruthenium center. An
exploring the effect of chelating groups in these types of cata- exception to this trend is the higher activity of 3b as compared
lytic transformations.
to 2b in DCB, though the activity is very moderate in either
case. Finally, chelation of a pyridine or a carboxylate group
(complexes 2b–d) consistently induces lower activity than
monodentate triazolylidenes, as demonstrated by the pyridine
containing ruthenium complexes 2d and 2b (entries 10–14).
The linkage of the pyridyl donor through either a triazolyli-
dene carbon or nitrogen, viz. 2b vs. 2d, has some impact
(entries 10 vs. 14), though conversions remained low with
either a catalyst precursor. Best results were obtained with the
C,C-bidentate triazolylidene complex 2a in DCB (entry 8). Turn-
over frequencies at 50% conversion reached 2.7 h−1 under opti-
mized conditions. Raising the temperature above 110 °C had
no beneficial effect, while lowering of the temperature resulted
in a substantial drop of activity (entries 17–19). Polar solvents
such as tBuOH inhibited catalytic activity (entry 20), presum-
ably because of competitive bonding with the substrate to
the ruthenium center in the active state (i.e. upon chloride
dissociation).7g
Catalytic alcohol oxidation
The ruthenium complexes 2a–e and the corresponding sol-
vento analogues 3a–c were evaluated as catalyst precursors for
the oxidant- and base-free oxidation of alcohols using benzyl
alcohol (BnOH) as a model substrate. The catalytic activity was
monitored in a first set of experiments in toluene and in 1,2-
dichlorobenzene (DCB; Table 1). Gradual formation of benz-
1
aldehyde was observed by H NMR spectroscopy upon heating
BnOH in the presence of 5 mol% of the triazolylidene ruthe-
nium complex in a closed system. Time-resolved monitoring of
the catalytic reaction suggests the pseudo-first order kinetics
for product formation as expected for homogeneous systems.†
The well-defined kinetic behavior provides a fairly accurate
estimate of the time required for 50% conversion and of the
turnover frequency at this conversion stage (TOF50).
Evaluation of the data in Table 1 reveals several trends.
Firstly, 1,2-dichlorobenzene is generally a better solvent for
alcohol oxidation than toluene (e.g. entry 2 vs. 8, entry 4 vs.
10). Secondly, in all examples investigated here, the cymene
complex 2 outperforms the analogous solvento complex 3 (e.g.
entry 8 vs. 9), presumably because of the stronger bonding of
The monodentate triazolylidene complex 2e provides up to
69% conversion (entry 6). We reasoned that the availability of
an alkoxy-functionality as opposed to a silylether would be
beneficial to catalytic activity for substrate deprotonation and
potential catalyst activation through formation of an Ru–OR
bond. Catalytic runs performed in the presence of NBu4F for
in situ alcohol deprotection25 indeed improved activity to some
extent (entries 7 and 15). However, NBu4NF has an accelerating
effect on its own:26 catalytic oxidation of BnOH with complex
2a in the presence of one equivalent of Bu4NF increased the
yield from 44% to 84% (entries 2 vs. 16).
Table 1 Oxidation of benzyl alcohol with triazolylidene ruthenium
complexesa
In addition to their activity in catalyzing alcohol oxidations,
the new ruthenium complexes 2 and 3 were investigated as
catalysts for ketone reduction via transfer hydrogenation. Stan-
dard protocols were applied consisting of basic isopropanol as
a formal dihydrogen donor and benzophenone as a model sub-
strate. Typical conditions for catalyst screening employed a
100 : 10 : 1 ratio substrate/base/catalyst (S/B/C), and results of
these experiments are compiled in Table 2. The impact of the
donor function is significantly different from that in alcohol
dehydrogenation, which is not unexpected when considering
the different modes of action. Alcohol dehydrogenation
requires the substitution of a chloride ligand in complexes
2a–e by a substrate alcohol molecule as the initial step,7g while
transfer hydrogenation likely involves the exchange of chloride
(or indeed a neutral ligand) by pre-formed isopropoxide.27 As a
consequence, the catalytic hydrogen transfer activity of the
solvento complexes is only marginally lower than that of the
analogous cymene complexes 2 (entries 4 vs. 5, and 6 vs. 7).
Complex 2b displays the highest performance and achieves
full conversion within less than 30 min. The monodentate tria-
zolylidene complex 2e is less active, followed by complexes 2c
Entry
[Ru]
Solvent
Yieldb (%)
TON
1
2
3
4
—
2a
3a
2b
3b
2e
2e
2a
3a
2b
3b
2c
3c
2d
2e
2a
2a
2a
2a
2a
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
DCB
DCB
DCB
DCB
DCB
DCB
DCB
DCB
Toluene
DCB
DCB
DCE
tBuOH
0
44
15
6
—
9
3
1
5
0
—
14
15
15
2
4
5
0.6
0.6
0.8
18
17
15
6
6
69
77
77
10
18
26
3
7c
8
9
10
11
12
13
14
15c
16c
17d
18e
19e
20e
3
4
90
84
77
30
26
0
5
—
a General reaction conditions: BnOH (0.2 mmol), [Ru] (0.01 mmol, and 2d, which do not achieve full conversion within 2 h
5 mol%), solvent (2 mL), 110 °C, 16 h, DCB = 1,2-dichlorobenzene,
(entries 6–9). Of note, the addition of a source of fluoride for
the in situ deprotection of the alcohol has a negative impact on
the catalytic activity and induces a drop in the catalytic activity
DCE = 1,2-dichloroethane. b Yields determined by 1H NMR integration
(anisole as a standard). c Addition of NBu4F (0.011 mmol). d At 130 °C.
e At 80 °C.
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Table 2 Catalytic transfer hydrogenation of benzophenone with ruthe-
nium(II) complexa
Table 3 Catalytic transfer hydrogenation of different substrates with
complex 2ba
Entrya
[Ru]
Mol%
Time (h)
Yieldb (%)
TON
Entry
R
R′
Time (h) Conversionb (%)
1
2
3
4
5
6
7
8
C6H5
p-ClC6H4
C6H5
(CH2)5
(CH2)4CHCH3
CH3
2-pyridyl
C6H5
C6H5
CH3
CH3
C2H5
0.5/2
0.5/2
0.5/2
86/98
63/97
83/98
1
2
3
4
5
6
7
8
—
2a
3a
2b
3b
2c
3c
2d
2e
2e
2b
3b
2b
3b
2b
3b
2b
3b
—
1
1
1
1
1
1
1
1
0.5/16
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
<1/11
<1/2
1/5
—
2
5
0.08/0.25 58/97
96/98
84/98
32/77
25/70
30/67
46/95
13/69
91/99
62/96
25/52
31/60
<1/<1
<1/<1
<1/1
98
98
77
70
67
95
69
198
192
520
600
—
—
2
0.5/2
CH2CHMe2 0.5/2
CH3
OCH3
H
H
H
H
H
H
H
73/>99
68/98
34/88
—
2/24
0.5/2
0.25/0.5
0.5/2
0.25/0.5
0.5/2
0.5/2
0.5/2
0.5/2
9
86 (12)/99 (0)c
31 (18)/44 (28)c
56 (27)/60 (32)c
64 (16)/83 (32)c
18/52
9
10d
11
12
13
14
15
C6H5
10c
11
12
13
14
15d
16d
17e
18e
1
p-ClC6H4
2-thiophenyl
2,4,6-(Me)3C6H2
3,4,5-(MeO)3C6H2
CH3(CH2)3
0.5
0.5
0.1
0.1
0.5
0.5
0.5
0.5
44/72
34/39
a Reaction conditions: substrate (1 mmol), KOH (0.1 mmol), 2b
(0.01 mmol) in iPrOH (5 mL), reflux temperature. b Conversion
selectively to the corresponding alcohol as determined by 1H NMR
spectroscopy. c Major product from aldol condensation (alcohol in
parentheses). d Catalyst pre-heated with a substrate and iPrOH for
10 min before addition of a base.
<1/<1
—
a General reaction conditions: benzophenone (1 mmol), KOH
(0.1 mmol), [Ru] (0.01 mmol, 1 mol%), iPrOH (5 mL), reflux temp-
erature. b Determined by 1H NMR spectroscopy. c With TBAF
(0.011 mmol). d Without base. e At 40 °C.
hydrogenation catalysts,29 yet substantially lower than the
most active species currently known for hydrogen transfer cata-
lysis.30 A further decrease of the concentration of the ruthe-
nium pre-catalyst to 0.1 mol% provided a maximum turnover
number of ca. 600, though the reaction does not proceed to
completion under these conditions (entries 13 and 14).
Complex 2b was used under optimized catalyst concen-
tration (0.5 mol%) for the transfer hydrogenation of a variety
of substrates (Table 3). Diaryl, aryl–alkyl, as well as dialkyl
ketones are readily hydrogenated under these conditions
(entries 1–7). Aryl chlorides are fully preserved and no pro-
ducts from hydro-dehalogenation were detected (entry 2).
Cyclohexanone is converted at a particularly high rate with
TOF50 up to 1400 h−1. Acylpyridine is generally considered to
be a difficult substrate because of potential N,O-bidentate
chelation of either the starting material or the product to the
catalytically active metal center, thus inhibiting catalytic turn-
overs.31 In line with this notion, this substrate is transfer
hydrogenated substantially slower (TOF50 25 h−1, entry 7). In
contrast, esters are not suitable substrates and no hydrogen-
ation was observed (entry 8).
to the level of the carboxylate-functionalized triazolylidene
complex 2c (cf. entries 6 and 10). Complexes 2a and 3a contain-
ing an anionic aryl ligand in addition to the mesoionic triazo-
lylidene are essentially inactive and yields are identical to
those observed in a blank experiment in the absence of any
ruthenium source (entries 1–3). This different behavior
demonstrates the strong donor ability of triazolylidene ligands,
which results in a formally charge-neutralized ruthenium
center and consequently in the preferred coordination of
neutral rather than anionic ligands. Accordingly, this complex
is better suited for alcohol oxidation, initiated by ROH coordi-
nation, rather than for transfer hydrogenation induced by RO−
coordination. In contrast, the pyridine chelated complex 2b
features a ruthenium center with relatively low electron
density, which facilitates coordination of the RO− anion, yet
does not favor chloride dissociation to enable ROH bonding
(cf. excellent performance in transfer hydrogenation vs. poor
performance in alcohol dehydrogenation).
Transfer hydrogenation with these ruthenium complexes
requires the presence of a base and reflux temperatures
(entries 15–18). Lowering of the catalyst loading to 0.5 mol% is
possible without significantly compromising yield or reaction
time (entries 11 and 12). Time-dependent monitoring of the
reaction did not reveal an induction period and showed
pseudo-first order kinetics for substrate consumption and
product formation.28† According to these analyses, turnover
frequencies at 50% conversion reach 680 h−1, a value that is
substantially higher than other carbene ruthenium transfer
Under the applied reaction conditions, aldehydes show a
distinct reactivity pattern. Rather than being reduced to the
primary alcohol, these substrates react predominantly with
acetone, generated in the dehydrogenative half-cycle of a clas-
sical transfer hydrogenation sequence. With benzyaldehyde as
the substrate, double aldol condensation was unambiguously
identified by the pertinent spectroscopic data of dibenzylidene
acetone (Scheme 3). Analogous reactivity patterns were
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Experimental section
General
The synthesis of 1-mesityl-4-methyl carboxylate-1,2,3-tri-
azole,34† 1-phenyl-5-methyl-1,2,3-triazole,15 mesityl azide,35
and of the ruthenium complexes 2a,10c 2b,9b and 3b9b was
described previously. Solvents used for the reactions were puri-
fied using an alumina/catalyst column system (Thermovac
Co.). All other reagents are commercially available and were
used as received. Microwave reactions were carried out using a
Biotage Initiator 2.5, operating at 100 W irradiation power.
Unless specified otherwise, NMR spectra were recorded at
25 °C on Varian Innova spectrometers operating at 300, 400 or
500 MHz (1H NMR) and 75, 100 or 125 MHz (13C{1H} NMR),
respectively. Chemical shifts (δ in ppm, coupling constants J in
Hz) were referenced to residual solvent resonances. Assign-
ments are based on homo- and heteronuclear shift correlation
spectroscopy. Microanalyses were performed by the Micro-
analytical Laboratory at University College Dublin, Ireland;
residual solvents were also identified by 1H NMR spectroscopy.
Scheme 3 Ruthenium-catalyzed transfer hydrogenation of aldehydes
with ketones and base-catalyzed aldol condensation of sterically unhin-
dered aryl aldehydes with acetone.
observed for chlorobenzaldehyde and thiophenyl aldehyde
(entries 9–12). In contrast, polysubstituted aryl aldehydes fail
to undergo such aldol condensation reactions and afford the
hydrogenation product (entries 13 and 14), though at lower
rates than observed for ketone conversion. Likewise, aliphatic
aldehydes are transfer hydrogenated to the corresponding
primary alcohol (entry 15). The fact that aldehyde hydrogen-
ation is partially reversible (cf. entry 9) suggests that complex
2b may also be effective for hydrogen borrowing reactions.32
However, reaction of BnOH as primary alcohol with
1-phenethylalcohol in the presence of KOH (10 mol%) did not
produce any coupled product in toluene or in 1,2-
dichlorobenzene.
Syntheses
Synthesis of 1-mesityl-1,2,3-triazol-4-yl methanol. Mesityl
azide (1.0 g, 6.20 mmol), and propargyl alcohol (278 mg,
4.96 mmol) were suspended in a mixture of water (7 mL) and
THF (7 mL). CuSO4 (22 mg, 0.01 mmol), and sodium ascorbate
(197 mg, 0.99 mmol) were added and the mixture was stirred
for 6 h at 100 °C under microwave irradiation. After cooling, all
volatiles were removed by evaporation and the residue was
extracted with MeCN (2 × 100 mL). The combined organic
phases were washed with water (2 × 60 mL), brine (2 × 50 mL),
dried over MgSO4, and evaporated to dryness. The residue was
washed with pentane (50 mL) purified by flash chromato-
graphy (SiO2; CH2Cl2–Et2O 6 : 1) to give the pure triazole as a
pale yellow powder (780 mg, 72%). 1H NMR (CDCl3, 500 MHz):
δ 7.60 (s, 1H, Htrz), 6.99 (s, 2H, Hmes), 4.91 (s, 2H, CH2OH),
2.58 (s, 1H, OH), 2.35 (s, 3H, ArCH3), 1.96 (s, 6H, ArCH3).
13C{1H} NMR (CDCl3, 125 MHz): δ 148.8 (Ctrz–CH2OH), 140.2,
Transfer hydrogenation of olefins33 was evaluated by using
cyclooctene and styrene as substrates. While the former was
unreactive and no cyclooctane was detected, styrene hydrogen-
ation was observed, albeit slowly, reaching about 15% conver-
sion to ethylbenzene after 24 h. Obviously, complex 2b is not
very reactive towards olefinic double bonds.
Conclusions
A straightforward access to donor-functionalized mesoionic 135.3, 133.7, 129.3 (4 × CMes), 123.9 (Ctrz–H), 56.8 (CH2OH),
triazolylidene ruthenium complexes was disclosed. Introduc- 21.3 (Ar–CH3), 17.5 (Ar–CH3). Anal. Calcd for C12H15N3O
tion of donor functional groups into the triazolylidene (217.27): C, 66.34; H, 6.96; N, 19.34. Found: C, 66.13; H, 6.91;
skeleton influences the activity on the ruthenium complexes N, 19.33.
as catalyst precursors in alcohol oxidation and in transfer
1-Mesityl-4-(tert-butyldimethylsilyloxymethylene)-1,2,3-triazole.
hydrogenation. The impact can be directly correlated with the 1-Mesityl-1,2,3-triazol-4-yl methanol (963 mg, 4.43 mmol),
propensity to form a reactive intermediate, i.e. with the poten- tBuMe2SiCl (750 mg, 4.87 mmol), and 1-methylimidazole
tial to coordinate neutral ROH or an ionic alkoxide, respect- (1.09 g, 13.3 mmol) were dissolved in dry DMF (50 mL) under
ively. These results highlight the potential of donor groups on a nitrogen atmosphere. The reaction mixture was allowed to
the triazolylidene ligand for catalytic applications and provide proceed for 18 h at room temperature, then quenched with
an attractive strategy for further catalyst improvement to water (25 mL) and extracted with Et2O (2 × 100 mL). The com-
mediate such redox transformations. In addition, it will be bined organic phases were washed with water (1 × 25 mL) and
attractive to combine both processes, ketone transfer hydro- brine (1 × 25 mL), dried over MgSO4 and evaporated to
genation and direct alcohol oxidation, into a single cell as a dryness. The residue was washed with pentane (50 mL) to
potential approach to generate a hydrogen fuel cell that is afford the pure product as a pale yellow semi-solid. Solidifica-
based on simple alcohols. Work is in progress along these tion of the pure triazole occurred after 18 hours under high
lines and aiming at replacing isopropanol with EtOH or vacuum as a pale off-white solid (1200 mg, 82%). 1H NMR
MeOH.
(CDCl3, 500 MHz): δ 7.52 (s, 1H, Htrz), 6.98 (s, 2H, Hmes), 4.97
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(s, 2H, CH2OH), 2.35 (s, 3H, ArCH3), 1.96 (s, 6H, ArCH3), 0.91 stirred for 6 h at 80 °C under microwave irradiation. After
(s, 9H, C–CH3), 0.11 (s, 6H, CH3, SiCH3). 13C{1H} NMR (CDCl3, cooling, all volatiles were removed by evaporation. The residue
125 MHz): δ 148.8 (Ctrz–CH2OH), 140.1, 135.3, 133.9, 129.2 (4 × was washed with copious amounts of pentane. Recrystalliza-
C
Mes), 123.6 (Ctrz–H), 58.3 (CH2OH), 25.9 (C–CH3), 21.3 tion of the residue from hot acetone gave 1e (70 mg, 88%).
(Ar–CH3), 18.4 (SiCMe3), 17.5 (Ar–CH3), −4.9 (Si–CH3). Anal. 1H NMR (CDCl3, 500 MHz): δ 8.79 (s, 1H, Htrz), 7.03 (s, 2H,
Calcd for C18H29N3OSi (331.53): C, 65.21; H, 8.82; N, 12.67.
Found: C, 64.99; H, 8.96; N, 12.41.
H
mes), 5.39 (s, 2H, CH2O), 4.56 (s, 3H, NCH3) 2.36 (s, 3H,
ArCH3), 2.11 (s, 6H, ArCH3), 0.90 (s, 9H, SiC–CH3, TBDMS),
Synthesis of 1c·OTf. To a solution of 1-mesityl-4-methyl 0.20 (s, 6H, SiCH3). 13C{1H} NMR (CDCl3, 125 MHz): δ 144.6
carboxylate-1,2,3-triazole (100 mg, 0.41 mmol) in Et2O (15 mL) (Ctrz–C), 142.8, 134.6, 131.3, (3 × Cmes) 131.2 (Ctrz–H), 130.1
was added MeOTf (0.74 g, 0.45 mmol) and the mixture was (CMes), 55.9 (CH2O), 40.6 (NCH3) 25.9 (C–CH3), 21.4 (Ar–CH3),
stirred in a closed microwave tube for 20 h at RT. A white pre- 18.3 (CMe3), 18.1 (Ar–CH3), −4.9 (Si–CH3). Anal. Calcd for
cipitate formed which was separated by filtration and washed C19H32IN3OSi (473.47): C, 48.20; H, 6.81; N, 8.87. Found:
with Et2O (2 × 30 mL). All volatiles were removed in vacuo to C, 47.73; H, 6.66; N, 8.67.
afford pure 1c·OTf as a white solid. Colourless needles were
Synthesis of 1f. The triazolium salt 1d is unstable and it
obtained after crystallization from CH2Cl2–Et2O (164 mg, decomposes over time to form 1f. Analytically pure beige
1
98%). H NMR (CDCl3, 300 MHz): δ 8.83 (s, 1H, Htrz), 7.05 (s, crystals of compound 1f were obtained from CH2Cl2–Et2O.
2H, Hmes), 4.70 (s, 3H, NCH3), 4.04 (s, 3H, OCH3), 2.37 (s, 3H, 1H NMR (400 MHz, CDCl3): δ 10.63 (s, 1H, Htrz), 9.00 (s, 1H,
ArCH3), 2.09 (s, 6H, ArCH3). 13C{1H} NMR (CDCl3, 125 MHz):
Htrz), 7.02 (s, 2H, Hmes), 4.75 (s, 3H, NCH3), 2.34 (s, 3H, CH3),
δ 155.4 (COO), 143.1 (Cmes), 134.8 (Ctrz–H), 134.7 (Cmes), 134.4 2.01 (s, 6H, CH3). 13C{1H} NMR (100 MHz, CDCl3): δ 142.8
(Ctrz–C), 131.0 (Cmes), 130.1 (Cmes), 54.3 (OCH3), 41.8 (NCH3), (Cmes), 134.8 (Cmes), 134.4 (Ctrz–H), 133.1 (Ctrz–H), 131.4 (Cmes),
21.3 (Ar–CH3), 17.4 (Ar–CH3). m/z: 260.1403 [M − OTf]+. Anal. 130.1 (CHmes), 41.2 (NCH3), 21.4 (Cmes–CH3), 17.6 (Cmes–CH3).
Calcd for C15H18F3N3O5S (409.38): C, 44.01; H, 4.43; N, 10.26. HRMS (ESI+, 30 V): m/z: 202.1057; calculated for [M − Cl]+
Found: C, 43.90; H, 4.34; N, 10.06.
202.2755.
Synthesis of 1c·Cl. Compound 1c·OTf (200 mg, 0.49 mmol)
Synthesis of 2c. A mixture of 1c·OTf or 1c·Cl (409 mg,
was dissolved in MeOH (5 mL) and filtered slowly through an 1.00 mmol), Ag2O (116 mg, 0.5 mmol) and [Ru(p-cymene)Cl2]2
ion exchange column (Dowex 1 × 8 200–400 MESH Cl). The (300 mg, 0.49 mmol) in CH2Cl2 (30 mL) was stirred in the dark
column was eluted with MeOH (2 × 10 mL). All volatiles of the at room temperature for 15 h and then filtered through Celite.
eluate were removed in vacuo to afford 1c·Cl as a highly hygro- The solvent was removed in vacuo and the red-orange residue
1
scopic solid (142 mg, 98%). H NMR (CDCl3, 300 MHz): δ 9.71 was purified by gradient column chromatography (SiO2,
(s, 1H, Htrz), 7.06 (s, 2H, Hmes), 4.81 (s, 3H, NCH3), 4.12 (s, 3H, CH2Cl2–acetone 4 : 1 to 4 : 3) to obtain 2c as a red-orange solid.
OCH3), 2.37 (s, 3H, ArCH3), 2.18 (s, 6H, ArCH3). 13C{1H} NMR Analytically pure crystals of 2c were obtained by recrystalliza-
(CDCl3, 125 MHz): δ 155.8 (COO), 143.0 (Cmes), 135.8 (Ctrz–H), tion from CH2Cl2–pentane (238 mg, 43%). 1H NMR (CDCl3,
134.7 (Cmes), 134.3 (Ctrz–C), 133.2 (Cmes), 131.1 (Cmes), 54.5 500 MHz): δ 7.14 (s, 1H, Hmes), 7.10 (s, 1H, Hmes), 5.42 (d, 3JHH
3
(OCH3), 42.2 (NCH3), 21.4 (Ar–CH3), 18.0 (Ar–CH3). HRMS = 6.0 Hz, 1H, Hcym), 5.36 (d, JHH = 6.0 Hz, 1H, Hcym), 5.07 (d,
(ESI) Calcd for C14H18N3O2 260.1399. Found 260.1410. 3JHH = 5.6 Hz, 1H, Hcym), 4.63 (d, 3JHH = 5.6 Hz, 1H, Hcym), 4.33
+
IR (KBr): 1751 cm−1 (s, CvO).
(s, 3H, NCH3), 2.42–2.40 (m, 4H, CHMe2 + ArCH3), 2.14 (s, 6H,
Synthesis of 1d. A mixture of 1-phenyl-5-methyl-1,2,3- ArCH3), 1.83 (s, 3H, Ccym–CH3), 1.01 (d, 3JHH = 7.1 Hz, 3H, CH–
triazole (200 mg, 1.26 mmol) and 2-bromopyridine (198 mg, CH3), 0.98 (d, JHH = 7.1 Hz, 3H, CH–CH3). 13C{1H} NMR
3
1.26 mmol) in a sealed vial was heated at 160 °C for 36 h. The (CDCl3, 125 MHz): δ 172.8 (Ctrz–Ru), 165.9 (COO), 141.4 (Cmes),
brown residue obtained after evaporation of all volatiles was 137.1 (Ctrz–C), 134.8 (Cmes), 134.7 (Cmes), 129.9 (Cmes), 128.9
dissolved in water (50 mL) and excess of KPF6 was added, (Cmes), 101.7, 93.3, 90.4, 85.7, 81.2, 81.0 (6 × Ccym), 36.3
resulting in an immediate precipitation of the compound. The (NCH3), 30.7 (CHMe2), 22.7 (CH–CH3), 21.5 (CH–CH3), 21.3
mixture was stirred for 20 min and filtered. The residue was (Ar–CH3), 18.5 (Ccym–CH3), 18.4 (Ar–CH3), 17.4 (Ar–CH3). IR
washed with water (2 × 10 mL) and Et2O (10 mL). Recrystalliza- (CDCl3):
ν =
1655 cm−1 (s, CvO). Anal. Calcd for
tion from MeCN–Et2O yielded analytically pure pale brown C23H28ClN3O2Ru (515.09) × 0.5 CH2Cl2: C, 50.63; H, 5.24; N,
crystals of 1d (250 mg, 52%). 1H NMR (CD3CN, 500 MHz): 7.54. Found: C, 50.65; H, 5.33; N, 7.23.
δ 9.13 (s, 1H, Htrz), 8.72 (ddd, JHH = 4.8, 1.6, 0.8, 1H, Hpy),
8.26–8.19 (m, 2H, Har), 7.88–7.77 (m, 6H, Har), 2.61 (d, JHH
Synthesis of 2d. Complex 1d (50 mg, 0.13 mmol), Ag2O
(30 mg, 0.13 mmol) and [Ru(p-cym)Cl2]2 (40 mg, 0.065 mmol)
=
0.64, 3H, CH3). 13C{1H} NMR (CD3CN, 100 MHz): δ 151.1 (Cpy), in dry CH2Cl2 (6 mL) were stirred in the dark at room tempera-
144.0 (Cpy), 142.4 (Car), 141.3 (Cpy), 134.0 (Ctrz–CH3), 132.9, ture for 40 h and then filtered through Celite. After evapor-
130.7, 127.8, 125.8, (4 × Car), 125.7 (Ctrz–H), 115.2 (Cpy), 10.8 ation of the solvent the residue was re-dissolved in CH2Cl2 and
(Ctrz–CH3). Anal. Calcd for C14H13F6N4P (382.24): C, 44.99; H, Et2O (50 mL) was added. Compound 2d was obtained as a
3.43; N, 14.66. Found: C, 44.00; H, 3.37; N, 14.46.
Synthesis of 1e. 1-Mesityl-4-(tert-butyldimethylsilyloxy- in vacuo (63 mg, 74%). H NMR (CD3CN, 400 MHz): δ 9.24 (d,
yellow solid which was collected by filtration and dried
1
methylene)-1,2,3-triazole (300 mg, 0.90 mmol) and MeI JHH = 6.0 1H, Hpy), 8.10 (m, 2H, Har), 7.72–7.60 (m, 6H, Har),
3
(348 mg, 2.71 mmol) were added to dry MeCN (10 mL) and 6.04, 6.00, 5.88, 5.80 (4 × d, JHH = 6.0 Hz, 1H, Hcym), 2.81 (s,
Dalton Trans.
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3
3H, Ctrz–CH3), 2.68 (sept, JHH = 7.0 Hz, 1H, CHMe2), 2.04 (s, 1H, Har), 7.63–7.45 (m, 6H, Har), 7.04 (td, JHH = 7.4, 1.2 Hz, 1H,
3
3H, Ccym–CH3), 1.15, 1.11 (2 × d, JHH = 7.0 Hz, 3H, CH–CH3). Har), 6.95 (td, JHH = 7.4, 1.2 Hz, 1H, Har), 4.02 (s, 3H, NCH3),
13C{1H} NMR (CD3CN, 100 MHz): δ 168.3 (Ctrz–Ru), 155.7 (Cpy), 2.50 (s, 3H, CH3CN), 1.96 (s, 6H, CH3CN). 13C{1H} NMR
145.1 (Cpy), 141.4 (Car), 141.3 (Cpy), 134.5 (Ctrz–Me), 132.1, (CD3CN, 125 MHz): δ 175.6 (Cph–Ru), 171.6 (Ctrz–Ru), 149.5
130.4, 126.8, 125.9, (4 × Car), 114.7 (Car), 89.6, 89.2, 87.9, 85.5 (Ctrz–Ph), 148.6, 141.4, 132.4, 130.7, 130.4 (5 × Car), 129.8
(4 × Ccym), 31.5 (CHMe2), 22.5, 22.3 (2 × CH–CH3), 18.5 (Ccym
CH3), 12.2 (Ctrz–CH3). HRMS (ESI) Calcd for C24H26N4ClRu+ 38.0 (NCH3), 4.6, 4.3 (2
507.0889. Found 507.0878. C24H24F3N7O3RuS (649.06) × 0.5CH2Cl2: C, 42.58; H, 3.65; N,
Synthesis of 2e. Compound 1e (120 mg, 0.25 mmol), Ag2O 14.19. Found: C, 42.31; H, 3.31; N, 13.84.
–
(MeCN), 127.2 (Car), 122.1, 118.7 (2 × MeCN), 113.8 (Car),
×
CH3CN). Anal. Calcd for
(30 mg, 0.13 mmol) and [Ru(p-cym)Cl2]2 (78 mg, 0.13 mmol)
Synthesis of 3c. Complex 2c (29 mg, 0.056 mmol) and
in dry CH2Cl2 (40 mL) were stirred in the dark at room temp- AgOTf (24 mg, 0.093 mmol) in dry MeCN (5 mL) were stirred at
erature for 20 h and then filtered through Celite. After evapor- reflux for 18 h. The mixture was filtered through Celite and all
ation of the solvent, a yellow residue was obtained which was volatiles were evaporated under reduced pressure. The residue
washed with pentane (100 mL). Redissolving the solid in 2 mL was washed with pentane yielding compound 3c as a yellow
of MeOH and filtration through a small pad of neutral solid which was collected and dried in vacuo (19 mg, 51%).
alumina afforded 2e as a pale yellow powder. Analytically pure Analytically pure 3c was obtained by recrystallization from
1
2e was obtained by recrystallization from CH2Cl2–Et2O (99 mg, CH2Cl2–Et2O. H NMR (CDCl3 400 MHz): δ 7.12 (s, 2H, Hmes),
1
60%). H NMR (CDCl3 500 MHz): δ 7.04 (s, 2H, Har), 5.08 (d, 4.36 (s, 3H, NCH3), 2.34 (s, 3H, ArCH3), 2.27 (s, 6H, CH3CN),
3JHH = 5.9 Hz, 2H, Hcym), 4.89 (s, 2H, CH2O), 4.74 (sbroad, 2H, 2.04 (s, 6H, ArCH3), 1.79 (s, 3H, CH3CN). 13C{1H} NMR
H
cym), 4.14 (s, 3H, NCH3), 2.39 (s, 3H, ArCH3), 2.25 (sept, (CD3CN, 100 MHz): δ 176.2 (Ctrz–Ru), 167.5 (COO), 141.5
3JHH = 6.9 Hz, 1H, CHMe2), 2.19 (s, 6H, ArCH3), 1.87 (s, 3H, (Cmes), 136.7 (Cmes), 136.5 (Ctrz–COO), 129.9 (Cmes), 125.9
3
Ccym–CH3), 1.11 (d, JHH = 6.9 Hz, 6H, CH–CH3), 0.94 (s, 9H, (Cmes), 123.7, 122.2 (2 × MeCN), 36.8 (NCH3), 21.2 (Ar–CH3),
C–CH3), 0.18 (s, 6H, SiCH3). 13C{1H} NMR (CDCl3, 125 MHz): 17.4 (Ar–CH3), 4.1, 3.7 (2 × CH3CN). HRMS (ESI+) Calcd for
δ 168.8 (Ctrz–Ru), 166.8 (Ctrz–CH2), 146.4, 140.5, 136.4, 129.0 (4 × C21H26N7O2Ru+: 510.1191. Found 510.1190.
C
Mes), 101.5, 94.0, 83.5 (3 × Ccym), 56.9 (CH2O), 37.9 (NCH3),
General procedure for alcohol oxidations
31.7 (CHMe2), 26.1 (C–CH3), 23.3 (CHCH3), 21.3 (ArCH3), 18.5
(Ccym–CH3), 18.3 (CMe3, TBDMS), 18.2 (ArCH3), −5.2 (Si–CH3). A mixture of alcohol (0.2 mmol) and the appropriate ruthe-
Anal. Calcd for C29H45Cl2N3RuSi × 0.5Et2O: (688.80): C, 54.05; nium complex 2 or 3 (0.01 mmol), and anisole (0.2 mmol) as
H, 7.32; N, 6.10. Found: C, 54.60; H, 6.88; N, 6.47.
an internal standard was refluxed in the selected solvent
Synthesis of 2f. A mixture of 1f (120.4 mg, 0.5 mmol), Ag2O (2 mL) in a closed vial. Aliquots were taken at specific
(50 mg, 0.21 mmol) and [Ru(p-cymene)Cl2]2 (128.6 mg, times, diluted with CDCl3 and analyzed by 1H NMR
0.21 mmol) in dry CH2Cl2 (10 mL) was stirred in the dark at spectroscopy.
room temperature for 15 h. After filtration through Celite and
solvent evaporation, the orange solid was washed with Et2O
General catalytic procedure for transfer hydrogenation
(2 × 25 mL) and dried in vacuo. Analytically pure orange The catalyst (1–0.1 mol%), iPrOH (5 mL) and a solution of
crystals of compound 2f were obtained in CH2Cl2–Et2O KOH (2 M, 0.05 mL, 0.1 mmol) were added to a one-neck
(186 mg, 73%). 1H NMR (400 MHz, CDCl3): δ 7.72 (s, 1H, Hprz), round-bottom flask equipped with a magnetic stirring bar.
3
6.96 (s, 2H, Hmes), 5.33 (d, JHH = 5.8 Hz, 2H, Hcym), 5.20 (d, The mixture was stirred at reflux for 10 minutes. Ketone
3
3JHH = 5.8 Hz, 2H, Hcym), 4.43 (s, 3H, NCH3), 2.71 (sept, JHH
=
(1.0 mmol) was added and heating was continued. Aliquots
7.0 Hz, 1H, CcymCHMe2) 2.32 (s, 3H, CmesCH3), 2.16 (s, 3H, (0.2 mL) were taken after 30 minutes and 2 h, poured into
3
C
C
cymCH3), 1.97 (s, 6H, CmesCH3), 1.20 (d, JHH = 7.0 Hz, 6H, cyclohexane or dichloromethane (2 mL) and filtered through
cymCHMe2). 13C{1H} NMR (100 MHz, CDCl3): δ 165.8 (Ctrz
–
Celite or SiO2 in a Pasteur pipette. All volatiles were removed
Ru), 141.3 (Cmes), 135.9 (Ctrz–H), 134.4 (Cmes), 132.6 (Cmes), under vacuum. Conversions were determined by 1H NMR
129.6 (CHmes), 104.1 (Ccym), 100.5 (Ccym), 84.6 (CHcym), spectroscopy using CDCl3 as a solvent.
84.1 (CHcym), 41.1 (NCH3), 31.0 (CHMe2), 22.6 (CH–CH3),
Crystallographic details
21.3 (Cmes–CH3), 18.8 (Ccym–CH3), 17.6 (Cmes–CH3).).
HRMS (ESI+, 30 V): m/z: 472.1096; calculated for [M − Cl]+ Crystal data for complexes 2c and 3a were collected using an
472.0088.
Agilent Technologies SuperNova. A diffractometer fitted with
Synthesis of 3a. Complex 2a (66 mg, 0.13 mmol) and AgOTf an Atlas detector that uses monochromated Mo-Kα radiation
(54 mg, 0.21 mmol) in dry MeCN (10 mL) were stirred at reflux (0.71073 Å). A complete dataset was collected, assuming that
for 18 h. The mixture was filtered through Celite and all vola- the Friedel pairs are not equivalent. An analytical numeric
tiles were evaporated under reduced pressure. The residue was absorption correction was performed.36 The structures were
washed with pentane yielding compound 3a as a yellow solid solved by direct methods using SHELXS-97 and refined by
which was collected and dried in vacuo (39 mg, 45%). Analyti- full-matrix least-squares fitting on F2 for all data using
cally pure 3a was obtained by recrystallization from CH2Cl2– SHELXL-97.37 Hydrogen atoms were added at calculated posi-
1
Et2O. H NMR (CDCl3 400 MHz): δ 7.92 (dd, JHH = 7.4, 1.2 Hz, tions and refined using a riding model. Anisotropic thermal
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displacement parameters were used for all non-hydrogen atoms.
Crystallographic details are summarized in Tables S3–S6.†
CCDC numbers 968356–968359 contain the supplementary
crystallographic data for this paper.
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Acknowledgements
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This work was financially supported by Science Foundation
Ireland, the European Research Council (ERC StG 208651). We
thank the DAAD (Erasmus travel support to M. H.) and the
Ministerio de Educación y Ciencia (MEC) for a FPU fellowship
(M. D.-R.) as well as support from COST Action CM1205.
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