Donor-Flexible Bis(pyridylidene amide) Ligands for Highly Efficient Ruthenium-Catalyzed Olefin Oxidation

Article abstract of DOI:10.1002/anie.202002014

An exceptionally efficient ruthenium-based catalyst for olefin oxidation has been designed by exploiting N,N′-bis(pyridylidene)oxalamide (bisPYA) as a donor-flexible ligand. The dynamic donor ability of the bisPYA ligand, imparted by variable zwitterionic and neutral resonance structure contributions, paired with the redox activity of ruthenium provided catalytic activity for Lemieux–Johnson-type oxidative cleavage of olefins to efficiently prepare ketones and aldehydes. The ruthenium bisPYA complex significantly outperforms state-of-the-art systems and displays extraordinary catalytic activity in this oxidation, reaching turnover frequencies of 650 000 h^?1 and turnover numbers of several millions.

Full text of DOI:10.1002/anie.202002014

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Accepted Article
Title: Donor-flexible bis-pyridine amide ligands for highly efficient
ruthenium-catalyzed olefin oxidation
Authors: Kevin Salzmann, Candela Segarra, and Martin Albrecht
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202002014
Angew. Chem. 10.1002/ange.202002014
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Donor-flexible bis-pyridine amide ligands for highly efficient
ruthenium-catalyzed olefin oxidation
Kevin Salzmann,† Candela Segarra,† and Martin Albrecht*
Abstract. An exceptionally efficient ruthenium-based catalyst for
olefin oxidation has been designed by exploiting N,N’-
bis(pyridylidene)oxalamide (bisPYA) as donor-flexible ligand. The
dynamic donor ability of the bisPYA ligand, imparted by variable
zwitterionic and neutral resonance structure contributions, paired with
the redox-activity of ruthenium provided catalytic activity for Lemieux-
Johnson-type oxidative cleavage of olefins to efficiently prepare
ketones and aldehydes. The ruthenium bisPYA complex largely
outperforms state-of-the-art systems and displays extraordinary
catalytic activity in this oxidation, reaching turnover frequencies of
1,000,000 h–1 and turnover numbers of several millions.
representation with a p-acidic N-donor coordinated to the metal
center (B). We recently demonstrated that the contribution of the
two limiting resonance structures is flexible and dependent on
several factors such as the polarity of the solvent, the nature of
the spectator ligands, and the metal oxidation state.[9] This
behaviour provides unique opportunities for redox-catalysis, as
this ligand system has potential for promoting both reductive
elimination in its p-acidic form as well as oxidative additions in its
zwitterionic p-basic form on a putative catalytic cycle. As a first
proof of concept, a cyclometalated PYA iridium complex was
observed to outperform the corresponding cyclometalated
analogue containing a static pyridyl ligand in water oxidation,[10]
which requires the accessibility of iridium in several oxidation
states. Here we have exploited the synthetic flexibility of PYA
ligands[11] as easily accessible systems to generate ruthenium-
based olefin oxidation catalysts with ultra-high efficiency.
The carbonyl group is arguably the most versatile synthetic
functionality for organic transformations.[1] It is most conveniently
accessible by oxidation of alcohols,[2] or by the oxidative cleavage
of olefins, for example through ozonolysis.[3] Catalytic versions of
olefin oxidation are remarkably scarce. The most established
catalytic system is the osmium-based Lemieux-Johnson
catalyst,[4] as well as variations thereof based on osmium as well
as ruthenium,[5] which generally require high catalyst loading and
tend to suffer from rapid overoxidation to produce the acid
predominantly.[6] Recently, Bera and coworkers pioneered a more
efficient system based on an abnormal carbene for the oxidation
of C=C bonds into C=O functional groups which operates at
ambient temperature and with high selectivity.[7] Preliminary
mechanistic insights suggest a catalytic cycle involving toggling
between a low-valent RuII intermediate and a high-valent
RuIV(=O)2 species.
M
M
E
E
N
N
O
N
O
N
A
B
zwitterionic
π basic N-donor (amid)
stabilizes high-valent M
neutral
π acidic N-donor (imine)
stabilizes low-valent M
Scheme 1. Limiting resonance structures of the PYA ligand.
The bisPYA ligand precursor 1 was prepared starting from
cheap and readily available 4-aminopyridine by thermally induced
condensation with diethyl oxalate,[12] followed by pyridine N-
methylation with MeI and anion metathesis using KPF6 (Scheme
2). This procedure afforded the bis-pyridinium salt 1 in 81%
overall yield and at a cost of less than 3€ per gram (Table S1).
Subsequent reaction of this salt with [RuCl2(p-cym)]2, in the
presence of K2CO3 at 90 ºC afforded the cationic ruthenium
complex 2 containing a N,N-bidentate chelating bisPYA ligand, as
an air- and moisture-stable orange solid.
We hypothesized that switching between these two critical
intermediates is greatly facilitated when modifying the spectator
ligands from a static donor system to a donor-flexible scaffold
which has the potential to adapt its donor properties during a
catalytic cycle. Pyridylidene amides (PYAs; Scheme 1) are a
particularly attractive class of ligands[8] as they are characterized
by two limiting resonance structures that are comprised of either
a zwitterionic structure with a p-basic N-donor site (A), or a neutral
[*]
Mr. K. Salzmann, Dr. C. Segarra, Prof. Dr. M. Albrecht
Department of Chemistry & Biochemistry
University of Bern
Freiestrasse 3, 3012 Bern (Switzerland)
E-mail: martin.albrecht@dcb.unibe.ch
Successful complexation was indicated by NMR
spectroscopy through the loss of the amide protons and an upfield
shift of the pyridyl α proton resonance from dH = 8.85 in the ligand
precursor to 8.52 ppm in complex 2. In contrast, the resonance of
the β protons shifts slightly downfield from dH = 8.38 to 8.42 ppm
† These authors contributed equally
Supporting information for this article is given via a link at the end of
the document.
1
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solvent-dependent redox shifts, indicating much less
donor flexibility than the bisPYA ligand. This solvent-
induced change of resonance structure of the bisPYA
ligand corroborates the electronic flexibility of the
ligand as observed by NMR spectroscopy.
PF6
–
PF6
i)
N+
ii)
iii)
N
N
iv)
Ru
Cl
NH
N
H
N
–
NH2
PF6
N+
N
O
O
O
O
N
Complex 2 is an excellent catalyst for oxidative
cleavage of olefins using NaIO4 as sacrificial oxidant.
Initial catalytic runs were performed with styrene as
model substrate and with 2 equiv. NaIO4 in the solvent
mixture CH3CN/AcOEt/H2O (2:2:1 v/v) as described
by Bera and coworkers.[7] Under these conditions,
complex 2 afforded a moderate 53% conversion to
benzaldehyde along with small quantities of benzoic
acid. Optimization of the amount of oxidant to 3 equiv.
1
2
(Table
S7)
and
the
solvent
mixture
to
Scheme 2. Synthesis and ORTEP representation (50% probability elipsoids) of the
bisPYA ruthenium complex 2. Reagents and conditions: i) diethyl oxalate ii) MeI,
MeCN; iii) KPF6, H2O; iv) [RuCl2(cym)]2.K2CO3, MeCN. Selected bond lengths and
angles: Ru–N1 2.097(3) Å, Ru–N2 2.119(2) Å, N1–Ru–N2 76.94(10)°.
MeCN/CH2Cl2/H2O (1:1:3 v/v) (Table S8) resulted in
quantitative conversion of benzaldehyde in just 5 min
when using 1 mol% complex 2 as catalyst. Larger
quantities of sacrificial oxidant led to considerably
higher fractions of over-oxidized product and can in fact drive the
reaction completely towards the selective formation of the
carboxylic acid (> 5 equiv NaIO4), while deoxygenation of the
solvent and headspace suppressed over-oxidation markedly
(Table S9). It is interesting to note that complex 2 performs
significantly better under more polar conditions, viz. larger
quantities of H2O, which suggests a higher relevance of the polar
resonance structure A.
upon ruthenation. The single set of resonances for the pyridyl and
cymene resonances suggests Cs-symmetry in solution, in
agreement with N,N-bidentate chelation. Complexation also
induces an indicative 6 ppm downfield shift of the amide carbon
and was unambiguously confirmed by single-crystal X-ray
diffraction analysis (Table S3).
The donor flexibility of the bisPYA ligand in complex 2 was
established by NMR spectroscopic and electrochemical analysis.
Specifically, the NMR chemical shift difference of the doublet
resonances due to the pyridylidene α and β protons serves as a
diagnostic probe for the prevalence of the zwitterionic or neutral
resonance forms A and B.[13] NMR experiments in solvents of
different polarity revealed a gradual downfield shift of Ha from dH
7.98 to 8.18, and 8.52 when measured in DMSO-d6 (ε = 46.45),
CD3CN (ε = 35.94), and CD2Cl2 (ε = 8.93)[14] while the Hb
resonance drifts to higher field (from dH 8.72 to 8.53 and 8.42; Fig.
S3, Table S5). This behaviour is in agreement with a more
pronounced zwitterionic resonance structure contribution in polar
solvents.[13]
The catalytic system is ultra-active. At high catalyst loading
(1 mol%), the olefin oxidation is completed in 5 min. Gradual
lowering of the catalyst loading requires longer reaction times,
though activity is preserved (Table 1; Fig. S5). Even catalyst
loadings as low as 0.1 ppm afford full conversion within an
acceptable time range (24 h), leading to turnover numbers of
9,700,000. Under these dilute conditions, maximum turnover
frequencies of the catalyst TOFmax = 1,000,000 h–1 were noted.
Repetitive blank reactions in the absence of complex 2 indicate
less than 2% conversion in the same 24 h time frame, revealing
the high impact of complex 2 in this oxidation. The prolonged
reaction times led, however to a larger portion of overoxidation
(up to 24% acid). Nonetheless, these high activities at ambient
temperature provide excellent opportunities for large scale
transformations with catalyst loadings at sub-ppm level.
Likewise, electrochemical analysis of the RuII/RuIII oxidation
potential reveals a gradual shift to lower oxidation potential as the
solvent polarity is increased, with the anodic peak potential Epa of
complex 2 lowered from 1.00 V in CH2Cl2 to 0.89 V (MeNO2), 0.85
V (acetone), 0.71 V (MeCN), and to 0.55 V (DMSO; all potentials
vs ferrocene; Fig. S4, Table S6). This behavior is in good
agreement with a higher contribution of the neutral PYA
resonance structure with a p-acidic nitrogen in apolar solvents
(stabilization of the low-valent RuII), and a larger impact of the
zwitterionic PYA resonance structure with an anionic and p-basic
nitrogen donor in polar solvents and hence better stabilization of
the RuIII center. Of note, the analogous complex 3 containing a
bipyridine instead of a bisPYA ligand does not show similar
2
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Table 1. Effect of low catalyst loading.[a]
Entry
2 [mol%]
1
Time [h]
Conversion [%] [b]
Selectivity [%] [b, c]
TON
TOFmax [h-1] [d]
1
2
3
4
5
6
0.08
2
>99
>99
>99
>99
>99
>99
92
91
92
93
81
76
>99
900
1,800
0.1
>990
0.01
6
>9,900
11,000
31,000
160,000
650,000
0.001
0.0001
0.00001
8
>99,000
>990,000
10
24
>9,900,000
[a] Conditions: styrene (0.5 mmol), complex 2, NaIO4 (1.5 mmol), in MeCN/CH2Cl2/H2O (1:1:3; 5.0 mL) at 25 ºC. [b] Conversion/selectivity determined by 1H NMR
spectroscopy relative to external standard (anisole). [c] Selectivity towards benzaldehyde (in %) vs. overoxidation to benzoic acid. [d] maximum turnover frequency
TOFmax extracted from time-conversion profiles (Fig. S5).
In addition to styrene, also polysubstituted olefins are
successfully oxidized with complex 2 (Table 2). Geminally
substituted olefins are converted slower than trans disubstituted
olefins, while cis olefins are converted at the same rate as
monosubstituted olefins (entries 1–6). Of note, geminal olefin
oxidation leads to the formation of ketones that are not amenable
to overoxidation, while cis and trans olefins produced traces of
acid. Trisubstituted olefins are efficiently oxidized to the
corresponding ketone in short reaction time (entry 7), and even
tetrasubstituted olefins are fully converted, albeit only after
substantially longer reaction time (2 h vs 5 min; entry 8). These
different conversion rates suggest that complex 2 has potential to
discriminate tri- from tetrasubstituted olefins for conversion.[15]
Introduction of substituents on the aromatic portion of styrene had
a moderate effect on catalytic efficiency and require up to 30 min
for full conversion (entries 9–13), though activity is not correlated
with the electronic effects of the substituents (Hammett sp
parameters varying between –0.27 for OMe to 0.66 for CN).[16]
Halides and cyano groups are tolerated as functional groups and
are not affected. Allylic substrates are converted slightly slower
(entry 14), while the transformation of aliphatic substrates
requires considerably longer reaction times to reach completion
(entries 15 and 16). Significant steric shielding hampers catalytic
turnover markedly and conversion stalls at less than 30% (entry
17, cf full conversion if the tBu group is replaced by a methyl
group, entry 5).
Table 2. Scope of olefin oxidation with complex 2.[a]
PF6
N
cat
Ru
Cl
N
N
O
R
R
O
N
R
O
R
+
O
R’
R’
NaIO4
R’
R’
Entry
1
Substrate
Product
Time [min]
5
Temp. [°C]
25
Conversion [%][b]
100
Selectivity [%][b, c]
92
O
H
O
2
3
120
5
50
25
100
100
100
81
O
H
H
O
4
5
30
5
25
35
100
100
87
O
100
3
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O
6
5
5
25
35
35
100
100
100
96
H
O
7
O
8
100
100
H
O
O
120
O
O
9
5
30
5
25
25
35
100
100
100
80
85
76
H
O
10
H
O
O
O
11
H
C
N
C
N
O
12
15
15
15
35
35
25
100
100
100
73
75
64
H
Br
Br
O
13
F3C
14
H
F3C
O
H
O
15
16
17
180
240
18 h
50
50
35
100
100
25
67
100
25
H
O
O
[a] Conditions: substrate (0.5 mmol), Complex 2 (1 mol%), NaIO4 (1.5 mmol) in MeCN/CH2Cl2/H2O (1:1:3, 5 mL). [b] Conversion/selectivity determined by 1H NMR
spectroscopy or GC analysis relative to external standard (anisole or hexamethylbenzene). [c] Selectivity towards carbonyl product (in %) vs. overoxidation to the
corresponding acid.
Methyl oleate as abundant biomass feedstock[17] has been
particularly challenging substrate for oxidative
functionalization.[18] The bisPYA Ru(II) complex converts this
substrate with considerable activity, reaching up to 85% of
nonanal due to oxidative C=C cleavage.[19]
conversion with rates that are orders of magnitude lower than
those of complex 2 under identical reaction conditions. Bera’s
complex featuring a heteroaryl-stabilized mesoionic carbene is
slightly less active (30 min to reach full conversion), though
suppresses overoxidation more efficiently.[7] These data suggest
a highly beneficial effect of the donor flexibility of PYA and
mesoionic ligands for promoting catalytic oxidation activity,
presumably through imposing an electron relay mechanism of the
ligand similar to that observed in other electron transfer
processes.[20]
a
H
MeOOC
cat. 2
O
O
MeOOC
+
NaIO4
65 °C
H
85% (8 h)
Mechanistic insights obtained from experiments using lower
quantities of oxidant indicate
a reaction pathway that is
Scheme 3. Oxidative cleavage of methyl oleate.
reminiscent to the osmium-catalyzed Lemieux-oxidation,
involving the metal catalyzed dihydroxylation followed by gradual
oxidation of the diol to the carbonyl product with C–C bond
cleavage. Specifically, phenyl-1,2-diol as well as α-hydroxy
The impact of the bisPYA ligand is revealed when
considering ruthenium complexes with similar bidentate formally
neutral ligands (Fig. 1). Complex 3 containing a related N,N-
bidentate bipyridine ligand with less strong donor properties and
no donor flexibility is inactive in this oxidative reaction. Similarly,
the mixed PYA-carbene ligand in complex 4[9a] displays
essentially no activity in the first 30 min and then slowly induces
1
acetophenone were identified by H and 13C NMR spectroscopy
when styrene oxidation was performed with lower quantities of
NaIO4 (2 equiv.) and intercepted at partial conversion (Fig. S6–8).
Since diols are known to react with NaIO4 spontaneously with C–
C
bond cleavage, these experiments suggest that the
4
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Keywords: olefin oxidation • ruthenium • donor flexibility • N-
donor ligands
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Figure 1. Comparison of time-conversion profiles for the olefin oxidation of
styrene using complexes 2–4 reveals the benefit of the bisPYA ligand in
complex 2 for oxidation catalysis.
[7]
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ruthenium complex is predominantly catalyzing the diol formation.
Moreover, NaIO3 was identified as the product of the sacrificial
oxidant at the end of the reaction using X-ray diffraction analysis,
which suggests that NaIO4 serves as a formal 2e– oxidant that
releases one equivalent of oxygen. These observations are
consistent with a catalytic cycle involving the formation of a
bisPYA stabilized high-valent ruthenium dioxo species such as
[(bisPYA)RuIV(=O)2] for the activation and oxidation of the olefin
to produce the diol.[3c, 7] The exceptionally fast catalytic rates are
therefore attributed to a high stabilization of both, the RuIV(=O)2
intermediate through the zwitterionic resonance structure A of the
PYA ligands, as well as a high stabilization of the RuII(–OH)2
precursor via the neutral PYA resonance structure B (cf Scheme
1). This donor flexibility of the PYA units provides a rationale for a
flat energy surface of the catalytic cycle and hence efficient
turnover without catalyst deactivation.
[9]
a) K. F. Donnelly, C. Segarra, L. Shao, R. Suen, H. Müller-Bunz, M.
Albrecht, Organometallics 2015, 34, 4076–4084; b) V. Leigh, D. J.
Carleton, J. Olguin, H. Müller-Bunz, L. J. Wright, M. Albrecht Inorg.
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[17] A. Corma, S. Iborra, A. Velty, Chem. Rev. 2007, 107, 6, 2411-2502.
In conclusion, we present a readily accessible, cheap, and
ultra-efficient ruthenium bisPYA catalyst for olefin oxidation with
exceptionally high activity and life time, reaching TOFs and TONs
in the millions. The carbonyl products are obtained with high
selectivity, even when using challenging biomass-derived
substrates. Future work will focus on elucidating the mechanistic
role of the donor-flexible bisPYA ligand in order to use even milder
oxidants, as well as the evaluation of selectivity in the oxidation of
industrially relevant heteroatom-functionalized substrates.
[18] a) P. Spannring, I. Prat, M. Costas, M. Lutz, P. C. A. Bruijnincx, B. M.
Weckhuysen, R. J. M. Klein Gebbink, Catal. Sci. Technol. 2014, 4, 708-
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[19] The longer reaction time may be a result of the poor solubility of the
substrate in the polar phase of the reaction medium.
[20] Z.-Y. Cao, T. Ghosh, P. Melchiorre, Nat. Commun. 2018, 9, 3274.
Acknowledgements
We thank the Swiss National Science Foundation
(200020_182663) and the European Research Council (CoG
615653) for generous financial support of this work. We also thank
the crystallography service unit of the University of Bern for
analytical work (funded through SNSF R’equip 206021_128724).
5
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Entry for the Table of Contents
+
N
Ru
N
Cl
N
O
O
N
O
R
R
NaIO4
TOFmax: 1,000,000 h–1
TON: 9,700,000
Unstoppable: A ruthenium complex catalyzes Lemieux-type olefin oxidation at ppm level catalyst loading and with turnover frequencies
of up to a million per hour when coordinated to a donor-flexible bis-pyridylidene amide (PYA) ligand that can switch donor properties
and hence facilitates redox changes of the metal center.
Twitter usernames: @albrecht_lab
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