Ester Hydrogenation Catalyzed by CNN-Pincer Complexes of Ruthenium

Article abstract of DOI:10.1021/acs.organomet.6b00009

Ruthenium complexes supported by two new CNN-pincer ligands were synthesized. Both were tested as catalysts for the hydrogenation of esters under mild conditions (105 °C, 6 bar H₂). A striking dependence on ligand structure was observed, as a dimethylamino-substituted ligand gave a nearly inactive catalyst, while a diethylamino-substituted variant gave up to 980 catalytic turnovers for the hydrogenation of benzyl benzoate. This system catalyzes the hydrogenation of various substrates including ethyl, benzyl, and hexyl esters, but is surprisingly unreactive toward methyl esters. Experiments demonstrate that base-catalyzed transesterification is rapid under the reaction conditions and that methyl esters are effectively hydrogenated when benzyl alcohol is added to the reaction mixture. The reverse reaction, dehydrogenation of primary alcohols to give esters, was tested as well; up to 920 catalytic turnovers were observed for the dehydrogenation of 1-hexanol to hexyl hexanoate.

Full text of DOI:10.1021/acs.organomet.6b00009

Article
pubs.acs.org/Organometallics
Ester Hydrogenation Catalyzed by CNN-Pincer Complexes of
Ruthenium
Daniel Kim, Linh Le, Myles J. Drance, Kelsey H. Jensen, Kristijan Bogdanovski, Tia N. Cervarich,
Melissa G. Barnard, Natalie J. Pudalov, Spring Melody M. Knapp, and Anthony R. Chianese*
Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, New York 13346, United States
S
* Supporting Information
ABSTRACT: Ruthenium complexes supported by two new
CNN-pincer ligands were synthesized. Both were tested as
catalysts for the hydrogenation of esters under mild conditions
(105 °C, 6 bar H2). A striking dependence on ligand structure
was observed, as a dimethylamino-substituted ligand gave a
nearly inactive catalyst, while a diethylamino-substituted variant
gave up to 980 catalytic turnovers for the hydrogenation of
benzyl benzoate. This system catalyzes the hydrogenation of various substrates including ethyl, benzyl, and hexyl esters, but is
surprisingly unreactive toward methyl esters. Experiments demonstrate that base-catalyzed transesterification is rapid under the
reaction conditions and that methyl esters are effectively hydrogenated when benzyl alcohol is added to the reaction mixture. The
reverse reaction, dehydrogenation of primary alcohols to give esters, was tested as well; up to 920 catalytic turnovers were
observed for the dehydrogenation of 1-hexanol to hexyl hexanoate.
hydrogenation or the reverse reaction, dehydrogenative
coupling of primary alcohols to esters (Figure 1). Many of
these catalysts feature an imidazole ring linked to a pyridine
ring by a CH2 group, which has the potential for reversible
deprotonation during catalysis. In each case,9a−d stoichiometric
reaction of the precatalyst with strong base led to a
dearomatized species where a methylene proton adjacent to
the NHC was removed, analogous to Milstein’s PNN-Ru
system.
INTRODUCTION
■
The reduction of esters to alcohols is an essential trans-
formation in organic synthesis. Classical methods for
laboratory-scale ester reduction with stoichiometric reagents
such as LiAlH4 are efficient and predictable, but suffer from low
atom economy.1 On the industrial scale, fatty acid esters are
reduced to alcohols by heterogeneous hydrogenation catalysis,
typically employing high temperature and pressure.2 The
development of efficient homogeneous catalysts for ester
hydrogenation that operate under milder conditions is
therefore of great interest, for both economic and environ-
mental purposes.
Although homogeneous catalysts for ester hydrogenation,
based primarily on ruthenium, have been known since the early
1980s,3 progress in the development of highly efficient catalysts
has greatly accelerated over the past decade.4 In 2006, Milstein
and co-workers reported a highly active ruthenium PNN-pincer
catalyst for ester hydrogenation, which is capable of reversible
protonation and deprotonation at the methylene carbon linking
the pyridine ring to the di-tert-butylphosphino moiety (Figure
1).5 This acid/base reactivity at a ligand site has become an
important design principle for catalytic hydrogenation of polar
bonds via the heterolytic activation of H2.6 Several structural
variations have since been reported as highly active ester-
hydrogenation catalysts, including Gusev’s Ru-PNN7a and
SNS7b pincer complexes, Kuriyama’s Ru-MACHO complex,7c
and Clarke’s Ru-PNN complex.7d
́
In the CNN-pincer systems developed by Sanchez and Song,
as well as Milstein’s PNN-Ru system, an alternative site of
deprotonation is the methylene group linking the pyridine ring
to the dialkylamine arm. DFT studies of Milstein’s complex
showed that the tautomer formed by deprotonation at the
amine side is less stable than the tautomer shown in Figure 1 by
6.6 kcal/mol, but is still a likely intermediate in photocatalytic
water splitting.10 Song and co-workers showed, also by DFT,
that the tautomer of their Ru-CNN complex deprotonated at
the amine side is less stable than the tautomer deprotonated at
the NHC side by 9.5 kcal/mol.9c Experimentally, they observed
complete deprotonation at the NHC side upon reaction with
strong bases, but exchange of the CH2 linker hydrogens with
deuterium from D2 indicated that deprotonation at the amine
side was kinetically accessible.
In an effort to probe the catalytic relevance of reversible
deprotonation at the different positions in cooperative pincer
ligands of this nature, we have synthesized two new Ru-CNN
pincer complexes that lack CH2 linkers on the NHC side,
N-Heterocyclic carbenes (NHCs) are now firmly established
as robust spectator ligands in transition-metal catalysis, due to
their exceptional σ-donating capability.8 Recently, several
ruthenium-pincer complexes containing NHC fragments9
have been developed as bifunctional catalysts for ester
contrasting the ligands developed by Song and San
́
chez (Figure
Received: January 7, 2016
© XXXX American Chemical Society
A
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RESULTS AND DISCUSSION
Ligand Synthesis and Metalation. We have previously
■
reported the synthesis of CCC-pincer ligands bearing two
benzimidazole groups, employing successive palladium-cata-
lyzed aminations to construct the ligand backbone followed by
cyclization with triethylorthoformate to give dicationic bis-
benzimidazolium salts.11,13 The two novel ligands 4-Me and 4-
Et were synthesized in a similar fashion, as shown in Scheme 1.
Scheme 1. Synthesis of CNN-Pincer Ligands 4-Me and 4-Et
The reductive amination of 6-bromopyridine-2-carboxaldehyde
with the appropriate secondary amines gave compounds 1-Me
and 1-Et. The known diamine 2, synthesized previously in two
steps,14 was prepared in one step via the palladium-catalyzed
coupling of 1,2-diaminobenzene and mesityl bromide.15
Compounds 1-Me and 1-Et were then coupled15 with diamine
2 to give the corresponding ligand precursors 3-Me and 3-Et.
Finally, the ligands 4-Me and 4-Et were obtained as
trihydrochloride salts via cyclization using HCl and triethylor-
thoformate.
Figure 1. Ruthenium-pincer complexes for ester hydrogenation.
As NHC complexes, especially those of multidentate NHC
ligands, are often challenging to synthesize,16 a wide variety of
methods for their metalation have been developed.17 Trans-
metalation from the silver-NHC complex18 was employed in
the synthesis of San
́
chez’s CNN-Ru complex,9a while complex-
Figure 2. Comparison of Song’s CNN-Ru complexes and the newly
reported complexes in this work.
ation of free carbenes generated by the reaction of the
imidazolium salt with strong base was successful in the
synthesis of Song’s CNN-Ru complex.9c For our CNN-pincer
ligands 4-Me and 4-Et, transmetalation from silver proved to be
the more reliable route (Scheme 2). For the formation of the
2). In addition to removing the possibility of deprotonation at
this position, an increase in the structural rigidity of the ligand
backbone is expected. Subtle ligand modifications of this nature
have previously been shown to have substantial effects on
reactivity in the context of alkane-dehydrogenation cataly-
sis.11,12
Scheme 2. Synthesis of Ruthenium-Pincer Complexes via
Transmetalation
Complexes 5-Me and 5-Et were tested as catalysts for ester
hydrogenation, showing turnover numbers up to 980 under
mild conditions (105 °C, 6 bar H2). Surprisingly, a subtle
change from NMe2 to NEt2 is associated with a drastic increase
in catalytic activity. Although 5-Et is unreactive toward methyl
esters, it is highly active for the hydrogenation of other esters
such as benzyl, ethyl, and hexyl, as well as lactones. The
dehydrogenation of primary alcoholsthe microscopic reverse
of ester hydrogenationwas briefly explored, and 5-Et showed
promise as a catalyst for this reaction as well.
B
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silver NHC complex, the presence of molecular sieves to absorb
the byproduct water was crucial, as the silver complex did not
form cleanly when they were omitted. Transmetalation to
ruthenium to give 5-Me and 5-Et occurred smoothly as
1
monitored by H NMR spectroscopy, but the isolation of the
pure product was challenging. Although silver halide often
precipitates upon transmetalation of NHCs from silver, in this
case the silver-containing byproduct showed a similar solubility
profile to the desired products and was identified by
crystallography as the known tetramer Ag4Cl4(PPh3)4.19
While the desired products 5-Me and 5-Et are moderately air
stable in solution, they decompose during column chromatog-
raphy in air, giving irreproducible yields. The ultimately
successful isolation strategy involved collecting the crude
precipitate from the THF reaction solution, performing air-
free column chromatography on neutral alumina, and finally
recrystallizing from CH2Cl2/toluene/pentane.
Figure 4. Crystal structure of 5-Et showing 50% probability ellipsoids.
Hydrogen atoms, except for the ruthenium-bound hydride, are
omitted for clarity. Selected metric data (bond lengths in Å and
angles in deg): Ru(1)−C(3), 1.854(3); Ru(1)−Cl(2), 2.5289(6);
Ru(1)−N(12), 2.057(2); Ru(1)−N(5), 2.315(2); Ru(1)−C(18),
1.933(2); Ru(1)−H(5), 1.591; C(3)−O(4), 1.152(3); C(18)−
N(19), 1.359(3); N(19)−C(26), 1.438(3); N(5)−Ru(1)−N(12),
75.99(8); N(12)−Ru(1)−C(18), 78.65(9); N(12)−Ru(1)−C(3),
175.63(10); Cl(2)−Ru(1)−C(3), 90.92(8); C(18)−N(19)−C(26)−
C(31), 95.17.
Both 5-Me and 5-Et were characterized by X-ray
crystallography, NMR spectroscopy, and elemental analysis.
Crystal structures of 5-Me (Figure 3) and 5-Et (Figure 4) show
Me and 5-Et. The hydrides of 5-Me and 5-Et were observed by
1H NMR at −14.62 and −14.73 ppm, respectively, in CD2Cl2,
while the hydride ligands in Song’s Ru-CNN complexes appear
at −15.34 ppm (Ar = 2,6-diisopropylphenyl) and −15.35 ppm
(Ar = mesityl) in CD2Cl2.9c
Ester Hydrogenation. Both 5-Me and 5-Et were tested as
catalysts for the hydrogenation of ethyl benzoate under mild
conditions (105 °C, 6 bar H2) in the presence of catalytic
amounts of NaOtBu. Yields were more reproducible when
several equivalents of base were used, relative to the ruthenium
complexes. As shown in Table 1, the substrate-to-catalyst ratio
Figure 3. Crystal structure of 5-Me showing 50% probability
ellipsoids. Hydrogen atoms, except for the ruthenium-bound hydride,
are omitted for clarity. Selected metric data (bond lengths in Å and
angles in deg): Ru(1)−C(31), 1.848(2); Ru(1)−Cl(2), 2.5365(6);
Ru(1)−N(6), 2.0540(19); Ru(1)−N(3), 2.245(2); Ru(1)−C(9),
1.932(2); Ru(1)−H(11), 1.601; C(31)−O(32), 1.154(3); C(9) −
N(10), 1.351(3); N(10)−C(11), 1.441(3); N(3)−Ru(1)−N(6),
78.09(8); N(6)−Ru(1)−C(9), 78.40(9); N(6)−Ru(1)−C(31),
176.75(9); Cl(2)−Ru(1)−C(31), 95.26(8); C(9)−N(10)−C(11)−
C(16), 88.19.
Table 1. Comparison of the Catalytic Activity of 5-Me and 5-
a
Et
that both are octahedral complexes with the chloride and
hydride ligands trans to each other. A correlation is observed
via NOESY between the ruthenium-hydride and an ortho-
methyl group on the mesityl ring for both 5-Me and 5-Et,
which is consistent with this geometry being retained in
solution. The ruthenium-containing bonds and angles, except
for the Ru−Namine bond length and Npyr−Ru-Namine angle, do
not differ significantly between 5-Me and 5-Et, indicating that
the additional steric bulk from the diethylamine group in 5-Et
does not result in a drastically different coordination environ-
ment around the metal compared with 5-Me. It is noteworthy
that the Ru−Namine bond length of 5-Et is longer than that of 5-
Me by 0.07 Å; this is likely due to the greater steric bulk of the
diethylamino group.
Substrate:
[Ru]
[Substrate]
(M)
[Ru]
yield, [Ru] = 5- yield, [Ru] = 5-
(mM)
Me
Et
125:1
250:1
250:1
500:1
0.25
0.25
0.50
0.50
2.0
1.0
2.0
1.0
20%
13%
3%
>99%
>99%
>99%
84%
2%
a
Reactions performed in toluene, with a total solution volume of 2.0
mL. In each case, the ratio of NaOtBu to [Ru] was 6:1. Yields were
1
measured by H NMR.
was altered, while keeping the ratio of base to ruthenium
constant at 6:1. Interestingly, 5-Et is highly active, producing
benzyl alcohol with a turnover number (TON) of up to 420,
while 5-Me is significantly less active, giving a maximum TON
of 33. Although we have limited evidence at this point, such a
stark difference in activity for such a subtle steric modification
suggests that dissociation of the amine arm during catalysis may
be important.20 In their seminal 2006 paper, Milstein and co-
When compared with Song’s Ru-CNN complexes9c shown in
Figure 2, the immediate coordination environment around
ruthenium in 5-Me and 5-Et is similar but shows a clear effect
of the lack of a methylene linker between the pyridine and
NHC rings. Most strikingly, the Npyr−Ru-CNHC bite angle is
88−89° in Song’s complexes and is approximately 78.5° in 5-
C
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workers proposed that dissociation of the NEt2 arm might be
important for the high activity of their PNN-Ru catalysts
compared to the lower activity seen with PNP ligands.5 For the
related hydrogenation of carbonate esters with this catalyst,
Wang and co-workers showed via DFT that a pathway
involving NEt2 dissociation is slightly less favorable than a
pathway not involving dechelation, with an energy difference of
2.4 kcal/mol.21
each substrate; Table 2 shows the lowest loading that gave full
or nearly full conversion. The catalyst tolerates a variety of
aromatic and aliphatic esters, including ethyl, benzyl, and hexyl
esters. A hydrogen pressure of 30 bar was required to fully
hydrogenate phthalide, as only 36% conversion was observed
under 6 bar. Because high H2 pressures have been necessary for
this substrate in every example we are aware of, the necessity is
likely of thermodynamic origin.9e,22
While 5-Et is a highly active catalyst, it is strikingly inactive
for the hydrogenation of a variety of methyl esters. For
example, ethyl benzoate and ethyl cyclohexanecarboxylate are
smoothly hydrogenated, while very low yields are obtained for
the corresponding methyl esters. To our knowledge, this
unique selectivity has not been reported to date for related
ester-hydrogenation catalysts. Compound 5-Et is generally
inactive in alcohol solvents, indicating that the product alcohols
may have an inhibiting effect on catalysis. A plausible
explanation for the lack of reactivity of methyl esters in our
system is that the byproduct methanol is a uniquely strong
catalyst poison. Alternatively, methyl esters may display an
intrinsically lower reactivity in our catalytic system.
As catalyst 5-Et was highly active for the hydrogenation of
ethyl benzoate, its activity was tested for a range of ester
substrates (Table 2). A range of catalyst loadings was tested for
Table 2. Substrate Scope of Ester Hydrogenation Catalyzed
a
by 5-Et
To distinguish between these alternative hypotheses, we
aimed to compare catalytic productivity in the presence of
different amounts of methanol and to compare the rates of
conversion of different esters (e.g., benzyl vs methyl) in the
same pot. Our initial efforts to conduct these experiments were
hampered by the observation that base-catalyzed transester-
ification occurs rapidly under our hydrogenation conditions.
While our hydrogenation experiments are conducted at 105 °C,
we observed that NaOtBu-catalyzed transesterification reaches
equilibrium within minutes at room temperature.23 Therefore,
an experiment examining the effect of added methanol might
show inhibition of catalysis because methanol is a poison or
because transesterification produces the less-reactive methyl
ester.
To cleanly distinguish between these hypotheses given that
transesterification occurs rapidly during hydrogenation, we
conducted an experiment where methyl decanoate was
hydrogenated in the presence of increasing amounts of benzyl
alcohol (Table 3). Due to rapid transesterification, an increase
in added benzyl alcohol will have two effects: (1) the
concentration of methanol will increase and (2) the methyl
decanoate reactant will be increasingly converted to benzyl
decanoate. If the primary cause of the low reactivity of methyl
esters is inhibition by methanol, we should observe slower
hydrogenation when more benzyl alcohol is added. If methyl
Table 3. Effect of Added Benzyl Alcohol on the
a
Hydrogenation of Methyl Decanoate
equiv BnOH added
yield 1-decanol
0
5%
a
0.2
0.5
1.0
25%
98%
99%
Reactions performed in toluene, with a total solution volume of 2.0
mL. In each case, the ratio of NaOtBu to [Ru] was 6:1. For
substrate:ruthenium ratios of 500 or 1000, the substrate concentration
was 0.50 M. For substrate:ruthenium ratios of 125 or 250, the
substrate concentration was 0.25 M. Yields were measured by 1H
NMR.
a
Reactions performed in toluene, with a total solution volume of 2.0
mL and 0.25 M methyl decanoate. Yields were measured by 1H NMR.
D
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residual solvent resonance (δ in parts per million, J in Hz). Elemental
analyses were performed by Robertson Microlit, Madison, NJ, USA.
High-resolution mass spectroscopic analysis was performed at the
University of Illinois Mass Spectroscopic Laboratory, Urbana, IL, USA.
Detailed NMR assignments for complexes 5-Me and 5-Et are given in
the Supporting Information.
esters are intrinsically less reactive than benzyl esters, we should
observe faster hydrogenation when more benzyl alcohol is
added. As the data in Table 3 show, we observed a greater yield
of 1-decanol as the amount of added benzyl alcohol was
increased, with 0.5 equiv leading to nearly full conversion. This
result is inconsistent with strong inhibition by methanol and
supports the alternative hypothesis that methyl esters are
intrinsically less reactive in our system. This result is particularly
surprising, and we do not have a clear explanation for it at this
stage. Stoichiometric reactions between 5-Et and NaOtBuin
the presence or absence of ester substratesproduced mixtures
of ruthenium complexes that we have not as yet been able to
characterize.
Dehydrogenation of 1-Hexanol. Many of the ruthenium-
based catalysts for ester hydrogenation have been shown to be
effective catalysts for the reverse reaction, dehydrogenative
coupling of primary alcohols to esters, under conditions where
hydrogen gas is removed from the system to shift the position
of equilibrium.7a,9a,24 An initial test for the dehydrogenation of
1-hexanol was conducted using 5-Et/NaOtBu, and good activity
was observed (Scheme 3). At a substrate-to-catalyst ratio of
Synthesis of (6-Bromopyridin-2-ylmethyl)dimethylamine (1-
Me). A Schlenk flask was flame-dried, and 6-bromopyridine-2-
carboxaldehyde (1.65 g, 8.87 mmol) was added. The flask was purged
and filled with argon. Dichloroethane (30 mL) and dimethylamine
(22.2 mL of a 2 M solution in THF, 44.4 mmol) were added, and the
mixture was stirred for 5 min. Then, sodium triacetoxyborohydride
(2.63 g, 12.4 mmol) was added, and the mixture was stirred overnight.
An additional portion of sodium triacetoxyborohydride (0.94 g, 4.4
mmol) was added the next day. After 1 h, saturated aqueous sodium
bicarbonate (15 mL) was added. The mixture was extracted twice with
30 mL of diethyl ether. The organic portions were combined and dried
over MgSO4. The solvent was evaporated, and the residue was purified
by column chromatography on silica gel, using a gradient of 0 to 30%
2-propanol in hexane. Yield: 1.16 g, 61%. 1H NMR (CDCl3): δ 7.50 (t,
1H, 3JHH = 7.7 Hz, CHpyridine), 7.38 (d, 1H, 3JHH = 7.4 Hz, CHpyridine),
3
7.34 (d, 1H, JHH = 7.8 Hz, CHpyridine), 3.55 (s, 2H, CH2NMe2), 2.27
(s, 6H, N(CH3)2). 13C NMR (CDCl3): δ 161.2, 141.4, 138.9, 126.4,
121.8, 65.2, 45.8. HRMS (ESI+): calcd for C8H12N2Br 215.0184,
found 215.0181.
Scheme 3. Alcohol Dehydrogenation Catalyzed by 5-Et
Synthesis of (6-Bromopyridin-2-ylmethyl)diethylamine (1-
Et). This compound was synthesized by a slight modification of a
known method.26 A Schlenk flask was flame-dried, and 6-
bromopyridine-2-carboxaldehyde (3.83 g, 20.6 mmol) was added.
The flask was purged and filled with argon. Dichloroethane (50 mL)
and diethylamine (7.5 mL, 106 mmol) were added, and the mixture
was stirred for a minute. Then, sodium triacetoxyborohydride (6.10 g,
28.8 mmol) was added, and the mixture was stirred. The reaction was
monitored for completion by TLC, using 30% ethyl acetate in hexanes.
After 1.5 h, 10% aqueous sodium hydroxide (40 mL) was added. The
mixture was extracted with 100 mL of diethyl ether, and this organic
fraction was washed with 2 × 50 mL of aqueous sodium hydroxide.
The organic portion was dried over MgSO4, the solvent was
1000:1, 1-hexanol was converted to hexyl hexanoate in 92%
yield in 45 h. A more detailed study of the dehydrogenative
coupling activity of our catalyst system is in progress.
CONCLUSION
■
1
evaporated, and a yellow oil (4.935 g, 99%) was obtained. H NMR
New ruthenium-CNN pincer complexes (5-Me and 5-Et) were
synthesized and tested as catalysts for ester hydrogenation. The
more active 5-Et catalyzes the hydrogenation of a variety of
substrates including ethyl, benzyl, and hexyl esters as well as
lactones. However, 5-Et is weakly active for the hydrogenation
methyl esters. Experiments were inconsistent with catalyst
poisoning by methanol and supported the hypothesis that
methyl esters are intrinsically less reactive than other esters
tested in our catalytic system. An initial screen of our catalyst
system for the acceptorless dehydrogenative coupling of
primary alcohols showed promising activity. Several extensions
to this work are currently under way, including mechanistic
characterization of the current system, modulation of the ligand
structure, and the application of first-row transition-metal
analogues.
(CDCl3): δ 7.50 (d, 2H, JHH = 4.7 Hz, CHpyridine), 7.32 (t, 1H, 3JHH
=
4.4 Hz, CHpyridine), 3.70 (s, 2H, CH2NEt2), 2.57 (q, 4H, 3JHH = 7.1 Hz,
N(CH2CH3)2), 1.04 (t, 6H, 3JHH = 7.1 Hz, N(CH2CH3)2). 13C NMR
(CDCl3): δ 163.2, 141.1, 138.8, 125.9, 121.4, 59.2, 47.6, 12.2.
Synthesis of 2-(Mesitylamino)aniline (2). Pd(OAc)2 (5 mg,
0.02 mmol), (R)-(−)-1-[(S)-2-(dicyclohexylphosphino)ferrocenyl]-
ethyldi-tert-butylphosphine (CyPF-tBu) (24.5 mg, 0.0442 mmol), a
stir bar, and 1,2-dimethoxyethane (40 mL) were added to a 100 mL
oven-dried round-bottom flask in the glovebox. After the mixture
became homogeneous, mesityl bromide (3.38 mL, 4.40 g, 22.1 mmol),
o-phenylenediamine (4.78 g, 44.2 mmol), and NaOtBu (6.37 g, 66.3
mmol) were added. The flask was sealed with a rubber septum and
heated at 80 °C overnight while stirring. The flask was brought out of
the glovebox and opened, and the mixture was filtered through a plug
of silica with 200 mL of ethyl acetate. The solvent was evaporated, and
the residue was isolated as a white solid by column chromatography on
silica gel, using a gradient of 0 to 25% ethyl acetate in hexanes. Yield:
3.75 g, 75%. NMR spectroscopic data were in agreement with those
reported in the literature.14
EXPERIMENTAL SECTION
■
General Methods. Unless stated otherwise, all reactions were
carried out either on a Schlenk line under argon or in an argon-filled
MBraun Labmaster 130 glovebox. Solvents were purified by sparging
with argon and passing through columns of activated alumina, using an
MBraun solvent purification system. All reagents and materials were
commercially available and were used as received, unless otherwise
noted. For the ruthenium precursor RuHCl(CO)(PPh3)3, both
commercially purchased samples (Alfa Aesar) and synthesized
material, prepared according to a literature method,25 were used.
Flash chromatography employing solvent gradients was performed by
using a Teledyne Isco Combiflash RF system. NMR spectra were
recorded at room temperature on a Bruker spectrometer (400 MHz
Synthesis of (6-((2-Mesitylamino)phenylamino)pyridin-2-
ylmethyl)dimethylamine (3-Me). A stir bar, (6-bromopyridin-2-
ylmethyl)dimethylamine (1-Me, 0.914 g, 4.25 mmol), 2-
(mesitylamino)aniline (2, 0.962 g, 4.25 mmol), NaOtBu (1.23 g,
12.7 mmol), and 1,2-dimethoxyethane (25 mL) were added to a 40
mL vial in the glovebox. Then, a solution of Pd(OAc)2 (1.91 mg, 8.50
μmol) and CyPF-tBu (9.43 mg, 17.0 μmol) in dimethoxyethane (1.5
mL) was made by stirring for 5 min and was added to the reaction vial.
The reaction mixture was heated at 80 °C overnight. The vial was
removed from the glovebox, and the mixture was filtered through a
plug of silica gel, eluting with 125 mL of ethyl acetate and 300 mL of
methanol. The volatiles were removed, and the residue was purified by
1
for H NMR and 100 MHz for 13C NMR) and referenced to the
E
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column chromatography, using a gradient of 0 to 25% methanol in
Synthesis of RuClH(CO)(CNNMe), 5-Me. In the glovebox, the
trihydrochloride salt 4-Me (200 mg, 0.417 mmol), a stir bar, molecular
sieves (ca. 700 mg), Ag2O (483 mg, 2.08 mmol), and THF (16 mL)
were added to a vial and stirred for 1.5 h in the dark. The mixture was
filtered through a PTFE filter disk into a vial with RuHCl(CO)(PPh3)3
(397 mg, 0.417 mmol). The reaction mixture was stirred and heated at
50 °C overnight. The reaction was then brought outside the glovebox,
and half of the solvent was evaporated. The mixture was filtered
through a medium frit and washed with THF (5 mL). The yellow solid
obtained was brought into the glovebox, and a short column was
performed on neutral alumina (10 mL) using a 20 mL plastic syringe
and a PTFE filter disk. The crude product was loaded with
dichloromethane, and impurities were washed away with dichloro-
methane (20 mL). The product was then eluted with 2:1 CH2Cl2/
THF (30 mL). Material purified in this manner was pure by NMR
spectroscopy, but catalytic trials were conducted using samples that
were further purified by recrystallization: a solution in dichloro-
methane was first layered with a small amount of toluene, then an
excess of pentane. Yield after recrystallization: 123 mg, 49% (corrected
1
dichloromethane. Yield: 1.19 g, 78%. H NMR (CDCl3): δ 7.43 (t,
3
3
4
1H, JHH = 7.8 Hz, CHarom), 7.24 (dd, 1H, JHH = 7.7 Hz, JHH = 1.5
Hz, CHarom), 7.00 (td, 1H, 3JHH = 7.9 Hz, JHH = 1.4 Hz, CHarom), 6.91
3
(s, 2H, CHarom), 6.77 (d, 1H, JHH = 7.2 Hz, CHarom), 6.71 (td, 1H
3JHH = 7.5 Hz, JHH = 1.4 Hz, CHarom), 6.42 (d, 1H, JHH = 8.4 Hz,
3
3
CHarom), 6.41 (br s, 1H, NH) 6.23 (dd, 1H, JHH = 8.2 Hz, JHH = 1.2
Hz, CHarom), 5.59 (br s, 1H, NH), 3.54 (s, 2H, CH2NMe2), 2.39 (s,
6H, CH2N(CH3)2), 2.28 (s, 3H, CH3‑mesityl), 2.10 (s, 6H, CH3‑mesityl).
13C NMR (CDCl3): δ 158.0, 156.2, 143.1, 138.4, 136.0, 135.6, 135.2,
129.3, 127.5, 127.1, 125.4, 117.8, 114.4, 112.3, 106.2, 65.3, 45.5, 21.0,
18.3. HRMS (ESI+): calcd for C23H29N4 361.2397, found 361.2392.
Synthesis of (6-((2-Mesitylamino)phenylamino)pyridin-2-
ylmethyl)diethylamine (3-Et). Pd(OAc)2 (2.11 mg, 9.39 μmol)
and CyPF-tBu (10.4 mg, 18.8 μmol) were combined in a 20 mL vial in
the glovebox with 1,2-dimethoxyethane (5 mL) and stirred for 5 min.
Then, (6-bromopyridin-2-ylmethyl)diethylamine (1-Et, 2.28 g, 9.39
mmol) and 2-(mesitylamino)aniline (2, 2.13 g, 9.39 mmol) were
added followed by the addition of NaOtBu (2.71 g, 28.2 mmol). After
adding more dimethoxyethane to nearly fill the vial, the reaction
mixture was heated at 80 °C overnight. The vial was removed from the
glovebox, and the mixture was filtered through a plug of silica gel,
eluting with 200 mL of ethyl acetate. The volatiles were removed, and
the residue was purified by column chromatography, using a gradient
1
for solvent impurities shown below). H NMR (CD2Cl2): δ 8.02 (t,
1H, 3JHH = 7.8 Hz, CHarom), 7.91 (d, 1H, 3JHH = 8.3 Hz, CHarom), 7.83
3
(d, 1H, JHH = 8.1 Hz, CHarom), 7.29 (m, 2H, CHarom), 7.21 (t, 1H,
3JHH = 7.8 Hz, CHarom), 7.14 (s, 1H, CHarom), 7.07 (s, 1H, CHarom),
3
2
6.73 (d, 1H, JHH = 8.0 Hz, CHarom), 4.86 (d, 1H, JHH = 14.6 Hz,
2
1
CH2NMe2), 3.57 (d, 1H, JHH = 14.8 Hz, CH2NMe2), 2.98 (s, 3H,
of 0 to 20% methanol in dichloromethane. Yield: 2.84 g, 78%. H
3
CH2N(CH3)2), 2.76 (s, 3H, CH2N(CH3)2), 2.41 (s, 3H, CH3‑mesityl),
2.17 (s, 3H, CH3‑mesityl), 1.92 (s, 3H, CH3‑mesityl), −14.62 (s, 1H, RuH).
13C NMR (CD2Cl2): δ 215.4, 205.0, 159.6, 151.2, 140.0, 139.0, 138.1,
136.8, 136.8, 133.1, 131.8, 129.9, 129.4, 124.0, 123.0, 116.9, 110.2,
109.8, 109.4, 69.4, 58.7, 51.9, 21.4, 18.4, 17.8. Recrystallized samples
contained disordered dichloromethane, pentane, and toluene that was
retained upon prolonged storage under vacuum at 50 °C and was
quantified by 1H NMR spectroscopy. Anal. Calcd for
C25H27ClN4ORu·0.51CH2Cl2·0.09CH3C6H5·0.14C5H12: C, 53.92; H,
5.12; N, 9.38. Found: C, 53.82; H, 5.11; N, 9.24.
NMR (CDCl3): δ 7.42 (t, 1H, JHH = 7.7 Hz, CHarom), 7.25 (dd, 1H,
3
3JHH = 7.8 Hz, JHH = 1.4 Hz, CHarom), 7.00 (t, 1H, JHH = 7.9 Hz,
3
CHarom), 6.91 (s, 2H, CHarom), 6.86 (d, 1H, JHH = 7.3 Hz, CHarom),
6.72 (td, 1H, 3JHH = 7.5 Hz, JHH = 1.4 Hz, CHarom), 6.39 (d, 1H, 3JHH
=
8.3 Hz, CHarom), 6.33 (br s, 1H, NH), 6.23 (dd, 1H, 3JHH = 8.2 Hz, JHH
= 1.3 Hz, CHarom), 5.59 (s, 1H NH), 3.63 (s, 2H, CH2NEt2), 2.65 (q,
3
4H, JHH = 7.1 Hz, N(CH2CH3)2), 2.29 (s, 3H, CH3‑mesityl), 2.11 (s,
6H, CH3‑mesityl), 1.10 (t, 6H, 3JHH = 7.1 Hz, N(CH2CH3)2). 13C NMR
(CDCl3): δ 158.7 (br) 157.8, 143.1, 138.3, 136.0, 135.5, 135.3, 129.3,
127.3, 127.0, 125.7, 117.8, 114.0, 112.3, 105.6, 59.3 (br), 47.4, 21.0,
18.3, 11.7. HRMS (ESI+): calcd for C25H33N4 389.2705, found
398.2711.
Synthesis of RuClH(CO)(CNNEt), 5-Et. The synthesis, chromato-
graphic purification, and recrystallization of 5-Et was performed in an
identical fashion to those of 5-Me, starting with 4-Et. Yield after
recrystallization: 116 mg, 53% (corrected for solvent impurities shown
Synthesis of Benzimidazolium Chloride 4-Me. To a flame-
dried 250 mL round-bottomed flask, 3-Me (1.19 g, 3.31 mmol), a stir
bar, triethylorthoformate (65 mL), and concentrated hydrochloric acid
(3.3 g, 33 mmol) were added. The reaction mixture was heated at 80
°C for 1 h, and then additional triethylorthoformate (10 mL) and
concentrated hydrochloric acid (2 mL) were added. The solvent was
evaporated under a stream of nitrogen while heating at 70 °C, and a
hygroscopic solid was obtained. The solid was washed with diethyl
ether and dried under vacuum. Yield: 1.44 g, 91%. 1H NMR (CDCl3):
δ 12.13 (br, 1H, CHbenzomidazolium), 8.64 (br, 1H, CHarom or NH), 8.50
1
3
below). H NMR (CD2Cl2): δ 7.90 (t, 1H, JHH = 8.0 Hz, CHarom),
3
3
7.85 (d, 1H, JHH = 8.3 Hz, CHarom), 7.79 (d, 1H, JHH = 8.1 Hz,
3
3
CHarom), 7.27 (t, 1H, JHH = 8.3 Hz, CHarom), 7.19 (t, 1H, JHH = 8.2
Hz, CHarom), 7.15 (s, 1H, CHarom), 7.09 (m, 2H, CHarom), 6.73 (d, 1H,
3JHH = 7.9 Hz, CHarom), 4.73 (d, 1H, 2JHH = 15.1 Hz, CH2NEt2), 3.96
(d, 1H, 2JHH = 15.0 Hz, CH2NEt2), 3.37 (m, 2H, N(CH2CH3)2), 2.87
(m, 2H, N(CH2CH3)2), 2.41 (s, 3H, CH3‑mesityl), 2.24 (s, 3H,
3
CH3‑mesityl), 1.92 (s, 3H, CH3‑mesityl), 1.12 (t, 3H, JHH = 7.0 Hz,
N(CH2CH3)2), 1.09 (t, 3H, 3JHH = 7.1 Hz, N(CH2CH3)2), −14.76 (s,
1H, RuH). 13C NMR (CD2Cl2): δ 215.3, 205.7, 160.3, 151.2, 140.2,
139.2, 138.3, 137.1, 136.9, 133.3, 131.9, 130.1, 129.6, 124.2, 123.2,
117.1, 110.4, 109.9, 109.3, 64.4, 55.0, 50.4, 21.6, 18.6, 18.0, 11.1, 9.9.
Recrystallized samples contained disordered pentane and toluene that
was retained upon prolonged storage under vacuum at 50 °C and were
quantified by 1H NMR spectroscopy. Anal. Calcd for
C27H31ClN4ORu·0.24CH3C6H5·0.19C5H12: C, 59.32; H, 5.91; N,
9.33. Found: C, 59.22; H, 5.88; N, 9.24.
(br, 1H, CHarom or NH), 8.22 (br, 1H, CHarom or NH), 8.07 (br, 1H,
3
CHarom or NH), 7.83 (br, 1H, CHarom or NH), 7.65 (br t, 1H, JHH
=
8.0 Hz, CHarom), 7.26 (br d, 1H, 3JHH = 8.0 Hz, CHarom), 7.04 (br, 2H,
CHarom), 4.81 (br, 2H, CH2NMe2), 2.92 (br, 6H, CH2N(CH3)2), 2.31
(s, 3H, CH3‑mesityl), 2.03 (s, 6H, CH3‑mesityl). 13C NMR (CDCl3): δ
152.5, 146.4, 143.6, 142.4, 141.9, 134.9, 132.2, 130.3, 129.4, 129.0,
128.7, 127.6, 127.0, 117.0, 116.4, 113.5, 60.8, 43.8, 21.2, 17.8. HRMS
(ESI+): calcd. for C24H27N4 (M − 2H − 3Cl) 371.2236, found
371.2231.
General Procedure for Ester Hydrogenation. In an argon-filled
glovebox, the appropriate ruthenium complex and NaOtBu were
dissolved in toluene. This mixture was stirred at 50 °C for 1 h, and a
homogeneous, orange solution formed. The appropriate ester
substrate was added as a solution in toluene, and this solution was
transferred to a glass-lined, stainless steel pressure reactor. The reactor
was sealed and brought out of the box. The reactor was pressurized
with 6 bar of hydrogen gas and vented three times. Next, the reactor
was pressurized with 6 bar of hydrogen gas and heated to an internal
temperature of 105 °C while stirring for 20 h. After this time, the
reactor was cooled for at least 30 min, vented carefully, and opened to
Synthesis of Benzimidazolium Chloride 4-Et. The synthesis
was performed in an analogous fashion to that of 4-Me, starting with
1
3-Et (2.85 g, 7.3 mmol). Yield: 3.70 g, 99%. H NMR (CDCl3): δ
12.38 (s, 1H, CHbenzomidazolium), 11.54 (br s, 1H, NH), 8.61 (d, 1H,
3JHH = 8.4 Hz, CHarom), 8.51 (br s, 1H, NH), 8.26 (br m, 2H, CHarom),
3
3
7.84 (t, 1H, JHH = 8.1 Hz, CHarom), 7.68 (t, 1H, JHH = 7.8 Hz,
3
CHarom), 7.30 (d, 1H, JHH = 8.4 Hz, CHarom), 7.08 (s, 2H, CHarom),
4.86 (br, 2H, CH2NEt2), 3.29 (br, 4H, N(CH2CH3)2), 2.35 (s, 3H,
CH3‑mesityl), 2.07 (s, 6H, CH3‑mesityl), 1.41 (br, 6H, N(CH2CH3)2). 13
C
NMR (CDCl3): δ 152.6 (br), 146.3, 144.0, 142.3, 142.0, 134.9, 132.4,
130.4, 129.3, 129.2, 128.7, 127.7, 127.3 (br) 117.1, 116.1 (br), 113.7,
56.0 (br), 48.3, 21.3, 17.9, 9.6 (br). HRMS (ESI+): calcd for C26H31N4
(M − 2H − 3Cl) 399.2549, found 399.2551.
1
the atmosphere. An aliquot was analyzed by H NMR spectroscopy.
Dehydrogenation of 1-Hexanol. In an argon-filled glovebox, 5-
Et (2.8 mg, 5.0 μmol) and NaOtBu (2.4 mg, 25 μmol) were dissolved
F
DOI: 10.1021/acs.organomet.6b00009
Organometallics XXXX, XXX, XXX−XXX

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Article
in 2.5 mL of toluene, and the solution was stirred at 50 °C for 1 h.
Then, 1-hexanol (5.0 mmol, 0.63 mL) was added, and the solution was
transferred to a flame-dried Schlenk flask. The flask was brought out of
the glovebox, and the system was refluxed using a J-Kem Scientific
aluminum heating/cooling block while stirring under a slight argon
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1
flow. Aliquots were removed via cannula and analyzed by H NMR
spectroscopy.
X-ray Crystallography, General Methods. Structure determi-
nations were performed on an Oxford Diffraction Gemini-R
diffractometer, using Mo Kα radiation. Single crystals were mounted
on Hampton Research Cryoloops using Paratone-N oil. Unit cell
determination, data collection and reduction, and empirical absorption
correction were performed using the CrysAlisPro software package.27
Direct methods structure solution was accomplished using SIR92,28
and full-matrix least-squares refinement was carried out using
CRYSTALS.29 All non-hydrogen atoms were refined anisotropically.
Unless otherwise noted, hydrogen atoms were placed in calculated
positions, and their positions were initially refined using distance and
angle restraints. All hydrogen positions were fixed in place for the final
refinement cycles.
X-ray Structure Determination of 5-Me. X-ray quality crystals
of 5-Me (yellow blocks) were grown by layering pentane over a
CH2Cl2 solution of 5-Me. The ruthenium-bound hydride was located
in the difference map, and its position was refined freely before being
fixed in place for the final refinement cycles. Highly disordered solvent
was present; correction for this residual density was performed using
the option SQUEEZE in the program package PLATON.30 A total of
113 electrons were removed from a total potential solvent-accessible
void of 499.4 Å3.
X-ray Structure Determination of 5-Et. X-ray quality crystals of
5-Et (yellow rods) were grown by layering pentane over a CH2Cl2
solution of 5-Et. The ruthenium-bound hydride was located in the
difference map, and its position was refined freely before being fixed in
place for the final refinement cycles. A disordered pentane molecule
was present close to a center of inversion, such that one of two
possible inversion-related orientations is adopted in each asymmetric
unit. The occupancy of all atoms in the pentane molecule was set to
0.5, the atoms (including carbon) were refined isotropically, and the
structure was refined using SAME restraints for the pentane molecule.
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109, 3612−3676. (c) Hopkinson, M. N.; Richter, C.; Schedler, M.;
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Aguila, M. J. B.; Schulpen, E. N.; van Putten, R.; Wiecko, J.; Muller, C.;
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Lefort, L.; Hensen, E. J. M.; Pidko, E. A. J. Am. Chem. Soc. 2015, 137,
7620−7623.
(10) Yang, X.; Hall, M. B. J. Am. Chem. Soc. 2010, 132, 120−130.
(11) Chianese, A. R.; Mo, A.; Lampland, N. L.; Swartz, R. L.; Bremer,
P. T. Organometallics 2010, 29, 3019−3026.
ASSOCIATED CONTENT
* Supporting Information
■
S
(12) Schultz, K. M.; Goldberg, K. I.; Gusev, D. G.; Heinekey, D. M.
Organometallics 2011, 30, 1429−1437.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00009.
(13) Chianese, A. R.; Shaner, S. E.; Tendler, J. A.; Pudalov, D. M.;
Shopov, D. Y.; Kim, D.; Rogers, S. L.; Mo, A. Organometallics 2012, 31,
7359−7367.
Images of NMR spectra and detailed NMR assignments
for complexes 5-Me and 5-Et (PDF)
CIF file giving crystallographic data for complexes 5-Me
(14) Despagnet-Ayoub, E.; Miqueu, K.; Sotiropoulos, J.-M.; Henling,
L. M.; Day, M. W.; Labinger, J. A.; Bercaw, J. E. Chem. Sci. 2013, 4,
2117−2121.
and 5-Et (CIF)
(15) Shen, Q.; Ogata, T.; Hartwig, J. F. J. Am. Chem. Soc. 2008, 130,
6586−6596.
AUTHOR INFORMATION
Corresponding Author
*E-mail: achianese@colgate.edu.
■
(16) Poyatos, M.; Mata, J. A.; Peris, E. Chem. Rev. 2009, 109, 3677−
3707.
(17) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47,
3122−3172.
Notes
(18) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972−975.
(19) Teo, B.-K.; Calabrese, J. C. Inorg. Chem. 1976, 15, 2467−2474.
(20) On the basis of the suggestion of a reviewer, we considered the
possibility that the dichloromethane impurity in recrystallized samples
of 5-Me, not present in 5-Et, was responsible for the reduced catalytic
activity of 5-Me. We dissolved a sample of 5-Me in toluene,
evaporated the solvent, then dried the solid under vacuum at 50 °C for
four days, after which time no dichloromethane impurity was observed
by NMR. Repeating the experiments in Table 1 with this
dichloromethane-free sample of 5-Me gave nearly identical results,
eliminating this possibility.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the National Science Foundation (CHE-1362501)
for financial support.
■
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