CNN pincer ruthenium complexes for efficient transfer hydrogenation of biomass-derived carbonyl compounds

Article abstract of DOI:10.1039/c9dt04292j

The ligand HCNN^OMe (6-(4-methoxyphenyl)-2-aminomethylpyridine) is easily prepared from the commercially available 6-(4-methoxyphenyl)pyridine-2-carbaldehyde by the reaction of hydroxylamine and hydrogenation (H₂, 1 atm) with Pd/C. The pincer complexes cis-[RuCl(CNN^OMe)(PPh₃)₂] (1) and [RuCl(CNN^OMe)(PP)] (PP = dppb, 2; and dppf, 3) are synthesized from [RuCl₂(PPh₃)₃], HCNN^OMe and PP (for 2 and 3) in 2-propanol with NEt₃ at reflux and are isolated in 85-93% yield. Carbonylation of 1 (CO, 1 atm) gives [RuCl(CNN^OMe)(CO)(PPh₃)] (4) (79% yield) which cleanly reacts with Na[BAr^f₄] and PCy₃, affording the cationic trans-[Ru(CNN^OMe)(CO)(PCy₃)(PPh₃)][BAr^f₄] (5) (92% yield). These robust pincer complexes display remarkably high catalytic activity in the transfer hydrogenation (TH) of lignocellulosic biomass carbonyl compounds, using 2-propanol at reflux in a basic medium (NaOiPr or K₂CO₃). Thus, furfural, 5-(hydroxymethyl)furfural and Cyrene are reduced to the corresponding alcohols with 2 and 3, at S/C in the range of 10 000-100 000, within minutes or hours (TOF up to 1 500 000 h^-1). The monocarbonyl complex 5 was found to be extremely active in the TH of cinnamaldehyde, vanillin derivatives and ethyl levulinate at S/C in the range of 10 000-50 000. Vanillyl alcohol is also obtained by the TH of vanillin with 5 (S/C = 500) in 2-propanol in the presence of K₂CO₃.

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CNN pincer ruthenium complexes for efficient
transfer hydrogenation of biomass-derived
carbonyl compounds†
Cite this: Dalton Trans., 2020, 49,
453
Rosario Figliolia,
Denise Lovison,
Paolo Cavigli, Clara Comuzzi,
Paolo Strazzolini,
Alessandro Del Zotto,
Daniele Zuccaccia,
Sabina Susmel,
Maurizio Ballico * and Walter Baratta
*
The ligand HCNNOMe (6-(4-methoxyphenyl)-2-aminomethylpyridine) is easily prepared from the com-
mercially available 6-(4-methoxyphenyl)pyridine-2-carbaldehyde by the reaction of hydroxylamine and
hydrogenation (H2,
1
atm) with Pd/C. The pincer complexes cis-[RuCl(CNNOMe)(PPh3)2] (1) and
[RuCl(CNNOMe)(PP)] (PP = dppb, 2; and dppf, 3) are synthesized from [RuCl2(PPh3)3], HCNNOMe and PP
(for 2 and 3) in 2-propanol with NEt3 at reflux and are isolated in 85–93% yield. Carbonylation of 1 (CO,
1 atm) gives [RuCl(CNNOMe)(CO)(PPh3)] (4) (79% yield) which cleanly reacts with Na[BArf4] and PCy3,
affording the cationic trans-[Ru(CNNOMe)(CO)(PCy3)(PPh3)][BArf4] (5) (92% yield). These robust pincer
complexes display remarkably high catalytic activity in the transfer hydrogenation (TH) of lignocellulosic
biomass carbonyl compounds, using 2-propanol at reflux in a basic medium (NaOiPr or K2CO3). Thus,
furfural, 5-(hydroxymethyl)furfural and Cyrene are reduced to the corresponding alcohols with 2 and 3, at
S/C in the range of 10 000–100 000, within minutes or hours (TOF up to 1 500 000 h−1). The mono-
carbonyl complex 5 was found to be extremely active in the TH of cinnamaldehyde, vanillin derivatives
and ethyl levulinate at S/C in the range of 10 000–50 000. Vanillyl alcohol is also obtained by the TH of
vanillin with 5 (S/C = 500) in 2-propanol in the presence of K2CO3.
Received 6th November 2019,
Accepted 2nd December 2019
DOI: 10.1039/c9dt04292j
rsc.li/dalton
reducing environmental impact, in agreement with the prin-
ciples of green chemistry.15,16 In this context, catalysis will play
Introduction
The development of simple and environmentally friendly pro- a leading role in the production of either bulky compounds
cesses for the production of valuable chemicals from bio- (biofuels), using heterogeneous catalysts, or biomass-derived
sourced compounds has become a necessity for a sustainable platform chemicals through selective transformations with
economy, as it will reduce the dependence on fossil well-defined homogeneous catalysts.17–20 Hydrogenation
resources.1–3 Carbohydrates and lignin are inexpensive and (HY)21–24 and transfer hydrogenation (TH)25–29 of carbonyl
globally accessible biomass feedstocks which can be employed compounds by means of ruthenium complexes30 are indust-
for the production of building blocks and biofuels1,4–10 via rially widely accepted processes for the synthesis of alcohols
hydrolysis, pyrolysis, defunctionalization and enzymatic degra- using H2 or 2-propanol as reducing agents, instead of NaBH4
dation reactions.1,11–14 In addition to the use of lignocellulosic and LiAlH4.31 The high control of selectivity imparted by the
biomass for conventional production, an emerging strategy is metal complexes, associated with the high atom economy with
the preparation of new biomass-based platform chemicals.3 respect to the classical methods, makes this approach a sus-
The search for clean and efficient processes that involve the tainable pathway for carbonyl reduction in organic synthesis. A
use of non-toxic renewable reagents and solvents, without the particularly successful outcome was the introduction of the
formation of side-products and waste, is a prerequisite for Noyori–Ikariya bifunctional amino ruthenium catalysts trans-
[RuCl2(PP)(diamine)] (PP = diphosphine) and [RuCl(arene)
(NN)] in HY and TH reactions, respectively (Fig. 1).32,33
The isolation of cis-[RuCl2(ampy)(PP)]34–37 and the related
Dipartimento DI4A – Università di Udine, Via del Cotonificio 108, I-33100 Udine,
pincer complexes [RuCl(CNN)(PP)]38–42 and [RuCl(CNN)(PPh3)
Italy. E-mail: maurizio.ballico@uniud.it, walter.baratta@uniud.it
†Electronic supplementary information (ESI) available: The NMR spectra of the
(CO)]43 containing the 2-(aminomethyl)pyridine (ampy)
isolated complexes and further data on the aldehyde and ketone TH reduction
catalyzed by the ruthenium derivatives. For ESI and other electronic format see
motif44 led to a class of extremely active TH and HY catalysts,
which are complementary to the renowned Noyori–Ikariya
DOI: 10.1039/C9DT04292J
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Fig. 1 The structure of amino- and ampy-type ruthenium complexes.
catalysts32,33 (Fig. 1). In addition to high selectivity, the pincer corresponding (E)-oxime, as inferred from the low field HCvN
complexes show a remarkably high productivity (S/C up to 1H NMR signal at δ 8.18 ppm, which is consistent with the
100 000), which is a critical parameter for the catalytic CvO related (E)-oxime pyridine derivatives.67,68 This intermediate is
reduction. In particular, the pincer complexes have allowed selectively hydrogenated with H2 (1 atm) using 10% Pd/C at
the clean reduction of commercial-grade ketones and even room temperature in ethanol (89% yield) (Scheme 1).
aldehydes,38,45,46 which can easily undergo aldol conden-
While the formation of the oxime occurs cleanly, the use of
sation,47 Claisen–Tishchenko,48–54 Cannizzaro55 and decarbo- a diluted solution of oxime in ethanol under low hydrogen
nylative side-reactions.56–61 Furthermore, the pincer CNN Ru pressure in the presence of a palladium catalyst is crucial to
complexes have been found to be active in dehydrogenation,62 avoiding the formation of both 6-(4-methoxyphenyl)-2-methyl-
racemization and deuteration of alcohols,63 as well as imine pyridine, via C–N cleavage, and the secondary amine bis
hydrogenation.64 It is worth pointing out that the HCNN ((6-(4-methoxyphenyl)pyridin-2-yl)methyl)amine, through the
ligands have been prepared through rather cumbersome pro- nucleophilic attack of the formed amine towards the inter-
cedures, namely starting from 6-arylpyridines and benzo[h] mediate imine. The reported synthesis represents a more
quinolones, via the formation of the N-oxide and cyano straightforward route to prepare HCNN pincer ligands on a
intermediates,41,65 entailing chemoenzymatic synthesis66 and g-scale, with respect to those previously reported, involving the
heterocyclization of 1-naphthylamine.38
formation of pyridine oxide and the use of Me3SiCN with di-
We report herein a straightforward synthesis of several methylcarbamoyl chloride as the cyanation agent in the 2 posi-
pincer Ru complexes, prepared from the easily accessible 6-(4- tion, followed by reduction with LiAlH4 or H2.41
methoxyphenyl)-2-aminomethylpyridine (HCNNOMe) ligand, in
combination with phosphines and carbon monoxide. The
robust pincer complexes [RuCl(CNNOMe)(PP)] and trans-[Ru
(CNNOMe)(CO)(PCy3)(PPh3)][BArf4] exhibit high rate and selecti-
vity in the TH of ketones and aldehydes available from ligno-
cellulosic biomass to the corresponding alcohols in 2-propa-
nol. High selectivity was attained for 5-HMF, Cyrene, ethyl
levulinate, cinnamaldehyde and vanillin derivatives with an
unprecedentedly high productivity (S/C up to 100 000).
Synthesis of the pincer ruthenium complexes
The treatment of [RuCl2(PPh3)3] with 1.2 equiv. of the ligand
HCNNOMe, in the presence of NEt3 (10 equiv.) in 2-propanol at
reflux (2 h), promptly affords the pincer complex cis-[RuCl
(CNNOMe)(PPh3)2] (1) in 93% yield, through the elimination of
one PPh3 and orthometalation reaction (Scheme 2).
The 31P{1H} NMR spectrum of 1 in CD2Cl2 shows two doub-
lets at δ 54.6 and 49.4 ppm, with a 2JPP of 32.8 Hz, whereas the
cyclometalated carbon atom appears in the 13C{1H} NMR spec-
2
trum at δ 183.7 ppm (dd with a JCP of 13.8 Hz and 8.2 Hz). In
1
addition, the CH2N proton signals of 1 appear in the H NMR
Results and discussion
2
spectrum as a doublet of doublets at δ 4.04 (dd, JHH = 16.2
Synthesis of the HCNNOMe ligand
3
Hz, JHH = 5.9 Hz) and a multiplet at δ 3.38, whereas the NH2
The 2,6 functionalized pyridine HCNNOMe ligand is easily signals are found at δ 3.65 and 1.87 ppm. The related dipho-
synthesized on a g-scale by treatment of the commercially sphine pincer derivative [RuCl(CNNOMe)(dppb)] (2) is isolated
available 6-(4-methoxyphenyl)pyridine-2-carbaldehyde with in 91% yield through a one-pot synthesis starting from
NH2OH·HCl, resulting in the quantitative formation of the [RuCl2(PPh3)3] and 1,4-bis(diphenylphosphino)butane (dppb)
Scheme 1 Synthesis of the ligand HCNNOMe
.
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Scheme 2 Synthesis of the complex cis-[RuCl(CNNOMe)(PPh3)2] (1).
Scheme 3 Synthesis of the diphosphine complexes [RuCl(CNNOMe)(PP)] (PP = dppb, 2; and dppf, 3).
in 2-propanol at reflux (2 h), via the intermediate [RuCl2(dppb) yield) has been obtained from [RuCl2(PPh3)3] and 1,1′-bis
(PPh3)], followed by reaction with HCNNOMe and NEt3 in 2-pro- (diphenylphosphino)ferrocene (dppf) in dichloromethane at
panol at reflux (2 h) (Scheme 3).
Alternatively,
RT (2 h), followed by reaction with the ligand HCNNOMe in the
2
is prepared by reacting [RuCl2(dppb) presence of NEt3 in 2-propanol at reflux, without the isolation
(PPh3)]69 with HCNNOMe and NEt3 in 2-propanol at reflux, and of the intermediate [RuCl2(dppf)(PPh3)m]n (Scheme 3).63,69 The
isolated in 88% yield. The 31P{1H} NMR spectrum of 2 shows spectroscopic data of 3 are similar to those of 2, with two
two doublets at δ = 56.9 and 41.0 ppm, with JPP = 37.7 Hz, 31P{1H} NMR doublets at δ = 61.5 and 44.1 ppm (2JPP
=
2
whereas the 13C{1H} NMR doublet at δ 52.3 ppm (3JCP = 2.7 Hz) 35.6 Hz) and the 13C{1H} NMR orthometalated signal at
2
corresponds to the CH2N group and the doublet of doublets at δ 182.5 (dd, JCP = 14.9 and 8.2 Hz). The monocarbonyl deriva-
δ 184.9 ppm (2JCP = 16.0 and 7.7 Hz) corresponds to the ortho- tive [RuCl(CNNOMe)(PPh3)(CO)] (4) has been isolated in 79%
1
metalated carbon atom. The H NMR spectrum of 2 shows a yield by the reaction of 1 with CO (1 atm) in CH2Cl2 at
doublet of doublets at δ 4.15 ppm and a triplet of doublets at δ RT (12 h), followed by a 48 h treatment of the crude product
3.75 ppm corresponding to the CH2N protons, whereas the with 2-propanol at reflux (Scheme 4).
NH2 signals are found at δ 3.45 and 2.03 ppm, as inferred
Control experiments show that during carbonylation of 1 at
from the 1H–15N HSQC 2D NMR spectrum (see ESI, Fig. S22†), RT both 4 and its isomer 4′ (δP 48.8 ppm, 2 : 1 ratio) are formed,
according to the related derivative [RuCl(CNN)(dppb)].41,42 the latter being completely converted into the thermodynamically
Similarly, the dppf complex [RuCl(CNNOMe)(dppf)] (3, 85% most stable complex 4 in 2-propanol at reflux. The 31P{1H} NMR
Scheme 4 Synthesis of the monocarbonyl complex [RuCl(CNNOMe)(CO)(PPh3)] (4).
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1
spectrum of 4 in CD2Cl2 shows a singlet at δ 56.7 ppm, the H and 8.9 Hz) in the 13C{1H} NMR spectrum, whereas the triplet
NMR signals for the diastereotopic methylene are found at δ 4.23 at δ 206.1 ppm (2JCP = 15.3 Hz) corresponds to the coordinated
and 3.41 and the NH2 protons are found at δ 3.87 and 2.79 ppm. CO, which exhibits an IR stretching absorption at 1919 cm−1
.
In the 13C{1H} NMR spectrum the cyclometalated carbon gives a All these data are consistent with those found for the related bis
doublet at δ 179.0 (2JCP = 12.6 Hz), whereas the doublet at δ 207.1 PPh3 complex trans-[Ru(CNN)(PPh3)2(CO)][BArf4].43 It is worth
(2JCP = 17.7 Hz) corresponds to the CO ligand, which exhibits an pointing out that although several homoleptic ruthenium(II)
IR νCO absorption band at 1913 cm−1 43
The treatment of 4 with complexes showing the trans-Ru(CO)(P)2 core have been
.
Na[BArf4] (Arf = 3,5-(CF3)2C6H3) in the presence of one equiv. of described, only few examples containing two phosphines, exert-
the bulky phosphine PCy3 in CH2Cl2 at RT quickly affords the cat- ing a different trans influence, have been reported so far.70–74
ionic complex trans-[Ru(CNNOMe)(CO)(PCy3)(PPh3)][BArf4] (5), iso-
Reduction of aldehydes and ketones via TH catalyzed by the
lated in 92% yield, via the substitution of Cl with the PCy3 ligand
(Scheme 5).
CNNOMe pincer ruthenium complexes
The 31P{1H} NMR spectrum of 5 in CD2Cl2 shows two doub-
The easily prepared pincer CNNOMe ruthenium complexes 1–5
have been investigated in the reduction of the model substrate
acetophenone a via TH with 2-propanol in the presence of a
base. These complexes have been found to be extremely pro-
ductive, affording quantitative conversion of a at S/C =
10 000–100 000 (S/C = substrate/catalyst molar ratio) and TOF
up to 1 100 000 h−1. This protocol has been subsequently
applied to the TH of carbonyl compounds obtained from
lignocellulosic biomass (Scheme 6).
2
lets at δ 34.2 and 22.0 ppm with JPP = 250.9 Hz, which is in
agreement with a trans arrangement of the two phosphorus
atoms. In the 1H NMR spectrum of 5 the two diastereotopic
CH2N protons appear as a doublet of doublets at δ 4.28 ppm
3
(2JHH = 16.3 Hz and JHH = 6.5 Hz) and as a multiplet in the
range δ 3.62–3.46 ppm, partially overlapped with one NH2
proton, while the other is at δ 2.83. The cyclometalated carbon
atom gives a doublet of doublets at δ 175.3 ppm (2JCP = 10.9 Hz
Scheme 5 Synthesis of the cationic carbonyl complex [Ru(CNNOMe)(CO)(PCy3)(PPh3)][BArf4] (5).
Scheme 6 Reduction of carbonyl compounds via TH catalyzed by complexes 1–5.
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Table 1 Catalytic TH of acetophenone a (0.1 M) with complexes 1–5
(S/C = 10 000–100 000) and NaOiPr (2 mol%) in 2-propanol at 82 °C
Therefore, it is likely that during the TH complex 5 undergoes
a substitution of PPh3, generating a robust catalytically active
hydride species H-Ru(CNN)(CO)(PCy3), which is less sensitive
towards catalyst deactivation.76–78
Entry
Complex
S/C
Time [min]
Conv.a [%]
TOFb [h−1
]
1
2
3
4
5
6
7
8
1
2
2
3
3
4
4
5
5
5
5
10 000
20 000
50 000
20 000
50 000
20 000
50 000
10 000
20 000
50 000
100 000
8 h
5
20
15
60
20
120
20
40
240
8 h
97
99
99
99
98
99
98
99
99
99
98
1200
On the basis of the results obtained with the model sub-
strate a, the most promising complexes 2, 3 and 5 have been
investigated in the TH of both aldehydes and ketones available
from the cellulose and lignin biomass, using 2-propanol as the
hydrogen donor, in the presence of NaOiPr or K2CO3
(Scheme 6). The reduction of furfural (FAL) b to furfuryl
alcohol has been achieved with 2 and 3 at S/C = 10 000 and
1000 with NaOiPr (2 mol%), affording selective reduction to
the corresponding alcohol (94 and 92%) in 30 and 2 min,
respectively (Table 2, entries 1 and 2). Conversely, with 5 at
S/C = 10 000 the formation of furfuryl alcohol (90%) has been
achieved in 10 min, with a small amount of uncharacterized
side products (4%) (entry 3). The use of potassium carbonate
as the weak base under the same catalytic conditions results in
1 100 000
450 000
260 000
100 000
95 000
60 000
150 000
110 000
92 000
9
10
11
42 000
a Conversions have been determined by GC analyses. b Turnover fre-
quency (moles of ketone converted to alcohol per mole of catalyst per
hour) at 50% conversion.
Complex 1 at a S/C of 10 000 shows poor activity in the TH poor conversion with the Ru catalysts (47% in 24 h with 2, see
of a, affording 97% conversion after 8 h with NaOiPr (2 mol%) ESI; Table S1,† entry 2). The catalytic HY of b has been
in 2-propanol at 82 °C (Table 1, entry 1). On the other hand, reported for Ru bis(diimine) complexes79 and Ru(III)–acetyl-
the diphosphine dppb derivative 2 leads to complete conver- acetonate/dppb at 100–140 °C with H2 under pressure.80
sion of a into 1-phenylethanol at S/C = 20 000 and 50 000 in 5
and 20 min with TOF values of 1 100 000 and 450 000 h−1
The substrate 5-(hydroxymethyl)furfural (5-HMF) c, avail-
able from the cellulose biomass (C6 sugars), can be hydrogen-
,
respectively (Table 1, entries 2 and 3), indicating that 2 is a ated to 2,5-bis(hydroxymethyl)furan (BHMF) which is a build-
highly productive catalytic TH system with an activity compar- ing block for the environment friendly polyurethanes and
able to that observed for the related CNN pincer complexes.42
polyesters81,82 and for the synthesis of 1,6-hexanediol used in
Complex 3, containing the robust dppf ligand, also allows adhesives.83 In spite of the vast literature on the heterogeneous
the reduction of a at S/C = 20 000 and 50 000, although with catalysis of c, only very recently has homogeneous TH emerged
longer reaction times (15 and 60 min) (Table 1, entries 4 as a viable and easy to apply route for the reduction of c to
and 5), which is in line with the analogous pincer 2-amino- BHMF.84 Complex 2 efficiently catalyzes the selective reduction
methylbenzo[h]quinoline derivatives [RuCl(CNNR)(dppf)].38 of c to BHMF at S/C = 10 000 and 20 000 in 5 and 10 min,
Interestingly, the monocarbonyl complex 4, obtained from 1 respectively (entries 4 and 5). With 3 and 5 complete conver-
by the substitution of one PPh3 with CO, shows a higher sion is observed at S/C = 10 000 and 50 000 (10 min–8 h)
activity with respect to 1, affording a quantitative reduction of (entries 6–9), whereas at S/C = 1000 the reduction occurs with
a in 20 and 120 min at S/C = 20 000 and 50 000, respectively, 3 in 1 min (see ESI; Table S1,† entry 3), indicating that these
even if the rate is slower than those for the diphosphine pincer complexes are among the most active and productive
derivatives 2 and 3 (entries 6 and 7). Finally, the cationic catalysts for the TH of c with 2-propanol.85 Using complex 2 as
complex 5, formed from 4 by the replacement of the chloride the catalyst (3.1 mg, S/C = 10 000), BHMF (4.57 g) has been pre-
ion with PCy3, shows complete reduction of a at S/C = pared in 90% yield (99% purity), starting from 5.00 g of c in
10 000–50 000, with a higher rate with respect to 4 (TOFs up to 2-propanol at reflux in 30 min. With 2 and 3 in the presence of
150 000 h−1, entries 8–10). It is worth noting that 5, unlike NaOiPr, incomplete reduction of 2,5-diformylfuran (DFF) d to
complexes 1–4, is highly soluble in alcohols, stable in diluted BHMF is observed, whereas with the weak base K2CO3
2-propanol solution for days and much less oxygen sensitive, (5 mol%) 3 affords only 4% conversion to the diol in 3 h (see
and allows the TH of a even at S/C = 100 000, showing that 5 is ESI; Table S1,† entry 4). Interestingly, complex 5 at S/C = 1000
a practical and productive catalyst for the carbonyl reduction with K2CO3 affords the quantitative formation of BHMF in
(entry 11). Control experiments, carried out under the same 60 min (entry 10). To the best of our knowledge, no examples
catalytic conditions and without ruthenium, show a small con- of TH of d with 2-propanol have been reported in the literature
version of a (9%) in 12 h, indicating that the base NaOiPr is a so far. The bicyclic ketone Cyrene (dihydrolevoglucosenone) e,
poor catalyst.75 The high performance of the monocarbonyl obtained by the HY of levoglucosenone from cellulosic materials
pincer 5, containing the PCy3 phosphine, is in line with our (e.g. Furacell process),86 is emerging as a green, non-toxic dipolar
studies on the systems [RuCl2(HCNN)(CO)2]/phosphines and aprotic solvent in place of N-methylpyrrolidone, DMF or sulfo-
the related pincer complex [Ru(CNN)(PPh3)2(CO)][BArf4].43 As a lane.87 The HY of e over the supported metal catalysts led to levo-
matter of fact, for this class of pincer carbonyl ruthenium com- glucosanol as a mixture of erythro and threo diastereoisomers88,89
plexes the use of the more basic phosphine PCy3, with respect and 1,6-hexanediol through ring opening.90 Conversely, no
to PPh3, resulted in an enhancement of the catalytic activity. examples of homogeneous catalysts for the TH of e have been
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Table 2 Catalytic TH of lignocellulosic biomass carbonyl compounds (0.1 M) to alcohols with complexes 2, 3, and 5 (S/C = 500–100 000) in 2-pro-
panol at 82 °C
Entry
Substrate
Complex
S/C
Basea
Time [min]
Conv.b [%]
Alcohol [%]
By-prod. [%]
1
2
3
4
5
6
7
8
b
b
b
c
c
c
c
c
c
d
e
e
e
e
e
e
f
f
f
f
f
g
g
g
g
g
h
h
h
i
2
3
5
2
2
3
3
5
5
5
2
2
3
3
5
5
2
2
3
3
5
2
3
5
5
5
3
5
5
3
5
10 000
1000
NaOiPr
NaOiPr
NaOiPr
NaOiPr
NaOiPr
NaOiPr
NaOiPr
NaOiPr
NaOiPr
K2CO3
NaOiPr
NaOiPr
NaOiPr
NaOiPr
NaOiPr
NaOiPr
NaOiPr
K2CO3
NaOiPr
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NaOiPr
NaOiPr
K2CO3
30
2
10
5
10
10
60
30
8 h
60
1
94
92
94
94
92
90
99
96
94
94
98
—
—
4
—
3
5
5
< 1
1
4
10 000
10 000
20 000
10 000
50 000
10 000
50 000
1000
99c
99c
99c
99c
99c
99c
99c
99
9
98
95
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
1000
99d
99d
99e
98e
98f
99f
92g
97g
58g
98g
96g
94
—
—
—
—
—
—
3h
2h
2h
1h
2h
4i
10 000
10 000
50 000
50 000
100 000
1000
1000
1000
1000
10 000
10 000
10 000
10 000
20 000
50 000
25 000
25 000
25 000
25 000
500
2
1
5
99
99
98
98
99
95
99
60
99
98
98
99
99
98
150
7 h
60
20
15
15
30
8 h
8 h
60
120
8 h
3
92
90
91
91
98
99
95
99
7i
9i
7i
99
8i
99c
99c
99c
99c
96c
1
—
4
—
—
30
15
4
j
36 h
96
a Base: NaOiPr (2 mol%) or K2CO3 (5 mol%). b Conversions have been determined by GC analyses. c Conversions have been determined by NMR
analyses. d erythro/threo ratio 1 : 1.2. e erythro/threo ratio 1.4 : 1. f erythro/threo ratio 1 : 5.7. g % of γ-valerolactone (GVL). h Isopropyl 4-hydroxypen-
tanoate. i 3-Phenylpropan-1-ol.
previously reported. Interestingly, we have found that e is easily pure threo alcohol has been recently isolated by selective
reduced via TH in 2-propanol using complexes 2, 3 and 5 with reduction of
NaOiPr (2 mol%) as the base (Scheme 7).
e
using baker’s yeast (Saccharomyces
cerevisiae).91,92 The HY of levulinic acid (LA) and levulinate
With 2 at S/C = 10 000 e is quantitatively reduced to alcohol esters,93–97 which are directly accessible from lignocellulosic
in 2 min with an erythro/threo ratio of 1/1.2. Conversely, 3 biomass,95 is an attractive process for the synthesis of
affords complete reduction at S/C = 10 000 and 50 000 in 1 and γ-valerolactone (GVL) which has wide applications as a solvent,
5 min, respectively, affording a TOF of 1 500 000 h−1 with fuel additive and monomer for polymer synthesis.98 Most of
erythro/threo = 1.4/1, as confirmed by 1H and 13C{1H} NMR the processes involve the HY of LA to GVL at high temperatures
measurements (entries 11–14).89,91 Although 5 shows a lower and H2 pressure.97,99–102 Recently, Ir and Ru complexes based
rate for the TH of e with respect to 2 and 3, complex 5 leads to on phosphine and dipyridylamine ligands have been reported
complete conversion at S/C = 50 000 and 100 000 in 150 min to efficiently hydrogenate LA to GVL with formic acid as the
and 7 h, respectively, with erythro/threo = 1/5.7 (entries 15 and hydrogen donor and in the presence of H2.103,104 In addition,
16, and see ESI, Fig. S46†). Notably, the diastereomerically the TH of LA and levulinate derivatives has been described
using the Shvo catalyst,105 Fe(OTf)2/tetraphos105 with formic
acid and the Casey’s iron catalyst94 with 2-propanol at S/C =
100, but no examples of Ru complexes for the TH of levulinate
esters with 2-propanol have been reported to date. The pincer
complex 2 (S/C = 1000) catalyzes efficiently the TH of ethyl
levulinate f to GVL (92%) in 60 min with NaOiPr (2 mol%)
(entry 17, and Scheme 8). Interestingly, the use of the weak
base K2CO3 (5 mol%) as the co-catalyst results in a higher con-
version and selectivity to GVL (97%) in a shorter reaction time
(20 min) (entry 18).
Scheme 7 Reduction of Cyrene e to levoglucosanol via TH catalyzed
by complexes 2, 3 and 5.
458 | Dalton Trans., 2020, 49, 453–465
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Scheme 8 Conversion of ethyl levulinate to GVL via TH catalyzed by complexes 2, 3 and 5.
A similar behavior has been observed for the dppf derivative (5 mol%) and NaOiPr (2 mol%), respectively (entries 28 and
3, leading to 99% conversion with K2CO3, whereas with 29). The dialdehyde i is rapidly and selectively reduced to the
NaOiPr only 60% of GVL was attained in 15 min (entries 19 corresponding dibenzyl alcohol in 99% yield with 3 (S/C =
and 20). Unlike 2 and 3, complex 5 has been proved to be 25 000) in the presence of NaOiPr in 4 min, with a remarkably
active at a lower loading, affording complete conversion of f at high TOF value of 500 000 h−1 and without the formation of
S/C = 10 000 in the presence of K2CO3 (5% mol) in 30 min, the monoalcohol or by-products of the aldol condensation,
with a TOF of 20 000 h−1. To the best of our knowledge, the due to the high reaction rate (entry 30). Interestingly, vanillin j,
cationic monocarbonyl PCy3 pincer complex 5 is one of the which is an acidic phenolic compound, is selectively reduced
most active systems for the TH of f (entry 21). Control 1H NMR to vanillyl alcohol (96%) with 5 (S/C = 500) in the presence of
experiments show that the reaction proceeds via the reduction K2CO3 (5 mol%) in 36 h at 82 °C, while complex 3 shows no
of f to a mixture of ethyl and isopropyl 4-hydroxyvalerate, gen- conversion under these catalytic conditions (entry 31). To the
erated in the 2-propanol basic medium, followed by an intra- best of our knowledge, complex 5 is the first example of a cata-
molecular cyclization to GVL and alcohol elimination, in lyst for the selective TH of vanillin to the corresponding alcohol.
accordance with the literature data (Scheme 8 and see ESI, The use of [RuCl2(PPh3)3] in the HY of j (S/C = 30 and 33 atm of
Fig. S40†).104 The HY of trans-cinnamaldehyde g, which can be H2) leads to the formation of vanillyl alcohol, 2-methoxy-4-
obtained from lignin defunctionalization, leads to 3-phenyl- methylphenol and 2-methoxy-phenol, as a result of hydrogen-
propanal, 3-phenylpropan-1-ol and 3-phenyl-2-propenol (cinna- ation and decarbonylative aldehyde reactions,110 whereas with
myl alcohol), which can find applications as feedstocks in the Shvo catalyst (S/C = 200 and 10 atm of H2) under acidic con-
pharmaceuticals, cosmetics and fine chemicals.106–108 The ditions the alcohol product is formed at 145 °C.111 Conversely,
chemoselective reduction of the CvO bond is more challen- the TH of j with formic acid on palladium results in the exclu-
ging because the HY of the CvC bond is thermodynamically sive formation of 2-methoxy-4-methylphenol.112
more favorable than that of the carbonyl group and many
efforts have been devoted to improve the selectivity towards
cinnamyl alcohol with metal based catalysts.109 Nevertheless,
we have found that g can be easily reduced via TH to the
Conclusions
corresponding allylic alcohol with complexes 2 and 3 (S/C = In summary, a straightforward synthesis of a series of pincer
10 000) leading to 94 and 92% conversion, respectively, in the CNNOMe ruthenium complexes is reported from a HCNNOMe
presence of K2CO3, with the formation of 3-phenylpropan-1-ol ligand prepared in high yield from the commercially available
as a by-product in 4 and 7% yield in 8 h, as a result of the con- 6-(4-methoxyphenyl)pyridine-2-carbaldehyde, by reaction with
comitant reduction of the CvC bond (entries 22 and 23). hydroxylamine and subsequent hydrogenation. These pincer
Control experiments with 2 at a higher loading (S/C = 1000) complexes are highly productive catalysts for the transfer
gives cinnamyl alcohol (94%) in 5 min, while after a prolonged hydrogenation (TH) of several biomass-derived carbonyl com-
reaction time (1 h) 3-phenylpropan-1-ol is formed in up to 35% pounds with 2-propanol as the hydrogen source, affording an
yield (see ESI; Table S1, entry 7 and Fig. S41†). Complex 5 has unprecedented activity (S/C ratio up to 100 000 and TOF up to
been proved to catalyze the TH of g at higher S/C ratios, 1 500 000 h−1). Interestingly, the derivatives [RuCl(CNNOMe
)
namely 10 000, 20 000 and 50 000, with the formation of 7–8% (PP)] (PP = dppb and dppf) display high catalytic activity in the
of 3-phenylpropan-1-ol (1–8 h, entries 24–26). The functiona- reduction of furfural, 5-HMF and Cyrene. Conversely, the
lized vanillin derivatives 3,4-dimethoxybenzaldehyde (veratral- monocarbonyl
trans-[Ru(CNNOMe)(CO)(PCy3)(PPh3)][BArf4]
dehyde) h, 4,4′-[ethane-1,2-diylbis(oxy)]bis(3-methoxybenzalde- shows an unprecedentedly high productivity in the TH of ethyl
hyde) i and also vanillin j have been reduced to the corres- levulinate, cinnamaldehyde and vanillin derivatives. These
ponding alcohols with the pincer ruthenium complexes. For results indicate that since no universal catalyst can be designed
these bio-derivatives, no examples of TH Ru catalysts have for the TH of carbonyl compounds, specific substrates can be
been previously reported. Veratraldehyde h is promptly con- efficiently reduced by pincer ruthenium complexes through a
verted to veratryl alcohol (3,4-dimethoxybenzyl alcohol) by suitable tuning of the ancillary ligands. Further studies on the
using complex 3 (S/C = 25 000) in 3 min (entry 27). By using development of highly active pincer catalysts for the C–H bond
the cationic complex
5 (S/C = 25 000) the quantitative formation reactions of biomass relevant products, including
reduction occurs in 30 and 15 min, in the presence of K2CO3 asymmetric transformations, are underway.
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Dalton Transactions
leading to complete conversion as found by TLC analysis. The
catalyst was removed from the reaction mixture by filtering
through a Celite pad and was thoroughly washed with absolute
Experimental
General
All reactions were carried out under an argon atmosphere EtOH. The resulting clear solution was concentrated to
using standard Schlenk techniques. The solvents were care- dryness under reduced pressure, affording 1.45 g of crude
fully dried by standard methods and distilled under argon product as an off-white solid. The residue was purified by
before use. The substrate i was synthetized following a pre- silica gel flash chromatography (eluent: Et2O/MeOH/NH4OH
viously reported method.113,114 The ruthenium complexes (94 : 5 : 1)), affording the pure compound as a colorless
[RuCl2(PPh3)3]115 and [RuCl2(dppb)(PPh3)]69 were prepared powder. Yield 1.25 g (89%). Elemental analysis calcd (%) for
according to literature procedures, whereas all other chemicals C13H14N2O: C 72.87, H 6.59, N 13.07; found: C 72.85, H 6.63, N
were purchased from Aldrich and Strem and used without 13.01. MS (m/z, ESI+): 215.20 [M + H]. 1H NMR (400.1 MHz,
further purification. NMR measurements were recorded on a CD2Cl2, 25 °C): δ = 8.05 (ddd, 3JHH = 8.9 Hz, 4JHH = 3.0 Hz, 5JHH
Bruker Avance III HD NMR 400 spectrometer. Chemical shifts = 2.0 Hz, 2H; aromatic protons), 7.72 (t, 3JHH = 7.7 Hz, 1H; aro-
3
(ppm) were relative to TMS for 1H and 13C{1H}, whereas H3PO4 matic proton), 7.59 (d, JHH = 7.8 Hz, 1H; aromatic proton),
3
3
was used for 31P{1H}. Infrared measurements were performed 7.20 (d, JHH = 7.6 Hz, 1H; aromatic proton), 7.03 (ddd, JHH
=
4
5
using a Bruker Vector 22 FTIR spectrometer. Elemental ana- 8.9 Hz, JHH = 3.0 Hz, JHH = 2.1 Hz, 2H; aromatic protons),
lyses (C, H, and N) were carried out with a Carlo Erba 1106 4.00 (s, 2H; CH2N), 3.89 (s, 3H; CH3O), 1.77 ppm (s, 2H; NH2).
analyzer, whereas GC analyses were performed with a Varian 13C{1H} NMR (100.6 MHz, CD2Cl2, 25 °C): δ = 162.0 (s; NCC),
CP-3380 gas chromatograph equipped with a 25 m length 160.5 (s; CCOCH3), 156.0 (s; NCCH2), 137.1–113.9 (m; aromatic
MEGADEX-ETTBDMS-β chiral column with hydrogen (5 psi) as carbon atoms), 55.3 (s; CH3O), 47.9 ppm (s; CH2N).
the carrier gas and a flame ionization detector (FID). ESI-MS
Synthesis of cis-[RuCl(CNNOMe)(PPh3)2] (1). The ligand
analysis and multi-stage mass spectrometry (MSn) experiments HCNNOMe (54 mg, 0.252 mmol, 1.2 equiv.) and NEt3 (291 μL,
were performed using a Finnigan LXQ Linear Ion Trap 2.09 mmol, 10 equiv.) were added to [RuCl2(PPh3)3] (200.0 mg,
(Thermo Scientific, San Jose, CA, USA) fitted with an ESI 0.209 mmol) in 2-propanol (5 mL) and the mixture was stirred
source operating in positive mode. The data acquisition was at reflux for 2 h. The resulting suspension was filtered, obtain-
under the control of Xcalibur software (Thermo Scientific).
ing a yellow solid, which was washed with 2-propanol (2 ×
Synthesis of (E)-6-(4-methoxyphenyl)pyridine-2-carbaldehyde 3 mL), methanol (2 × 3 mL), and n-heptane (2 × 5 mL) and
oxime. Hydroxylamine hydrochloride (0.57 g, 8.2 mmol) was dried under reduced pressure. Yield: 170 mg (93%). Elemental
carefully added to a solution of commercially available 6-(4- analysis calcd (%) for C49H43ClN2OP2Ru (874.36): C 67.31, H
methoxyphenyl)pyridine-2-carbaldehyde (1.50 g, 7.04 mmol) in 4.96, N 3.20; found: C 67.22, H 5.01, N 3.16. 1H NMR
3
4
a mixture of acetonitrile (47.5 mL), methanol (82.5 mL) and (400.1 MHz, CD2Cl2, 25 °C): δ = 8.02 (dt, JHH = 6.9 Hz, JHH
=
water (3.75 mL), which was heated up and turned yellow. The 1.6 Hz, 1H; aromatic proton), 7.77 (m, 1H; aromatic proton),
3
obtained clear solution was then stirred for 30 min at room 7.63 (d, JHH = 2.2 Hz, 1H; aromatic proton), 7.53–7.01 (m,
temperature. The solvents were evaporated off and the residue 13H; aromatic protons), 6.98–6.79 (m, 18H; aromatic protons),
3
3
was dissolved in 100 mL of ethyl acetate and extracted with 5% 6.61 (d, JHH = 7.5 Hz, 1H; aromatic proton), 6.41 (dd, JHH
=
4
2
aqueous NaHCO3 (2 × 50 mL). The organic phase was dried 8.4 Hz, JHH = 2.6, 1H; aromatic proton), 4.04 (dd, JHH = 16.1
3
with anhydrous Na2SO4 and evaporated under reduced Hz, JHH = 6.0 Hz, 1H; CH2N), 3.68 (s, 3H; CH3O), 3.74–3.60
pressure to obtain the oxime as a white solid. Yield 1.53 g (m, 1H; NH2), 3.38 (m, 1H; CH2N), 1.87 ppm (m, 1H; NH2). 13
C
=
(95%). Elemental analysis calcd (%) for C13H12N2O2: C 68.41, {1H} NMR (100.6 MHz, C2D2Cl4, 25 °C): δ = 183.7 (dd, JCP
2
2
H 5.30, N 12.27; found: C 68.37, H 5.39, N 12.21. MS (m/z, 13.2 Hz, JCP = 8.9 Hz; CRu), 163.0 (s; NCC), 157.2 (s;
ESI+): 251.16 [M + Na], 229.18 [M + H]. H NMR (400.1 MHz, CCOCH3), 156.8 (s; NCCH2), 142.2–109.0 (m; aromatic carbon
1
CD3OD, 25 °C): δ = 8.18 (s, 1H; HCvN), 7.95 (ddd, 3JHH = 8.9 Hz, atoms), 55.1 (s; CH3O), 50.8 ppm (d, JCP = 1.8 Hz; CH2N). 31P
2
4JHH = 2.9 Hz, 5JHH = 2.1 Hz, 2H; aromatic protons), 7.81 (pseudo- {1H} NMR (162.0 MHz, CD2Cl2, 25 °C): δ = 54.6 (d, JPP = 32.8
2
t, 3JHH = 7.9 Hz, 1H; aromatic proton), 7.75–7.72 (m, 2H; aromatic Hz), 49.4 (d, 2JPP = 32.8 Hz).
3
4
5
protons), 7.02 (ddd, JHH = 8.9 Hz, JHH = 2.9 Hz, JHH = 2.1 Hz,
Synthesis of [RuCl(CNNOMe)(dppb)] (2)
Method A. The complex [RuCl2(PPh3)3] (200 mg, 0.209 mmol)
2H; aromatic protons), 3.85 ppm (s, 3H; CH3O); 13C{1H} NMR
(100.6 MHz, CD3OD, 25 °C): δ = 160.9 (s; CCOCH3), 157.0 (s; and dppb (98 mg, 0.230 mmol, 1.10 equiv.) were suspended in
NCC), 152.1 (s; NCCHvN), 149.1 (s; CCHvN), 137.4–113.7 (m; 2-propanol (4.0 mL), and the mixture was stirred at reflux for
aromatic carbon atoms), 54.4 ppm (s; CH3O).
3 h. The ligand HCNNOMe (54 mg, 0.252 mmol, 1.21 equiv.)
Synthesis of 6-(4-methoxyphenyl)-2-aminomethylpyridine and NEt3 (291 μL, 2.09 mmol, 10 equiv.) were added and the
HCNNOMe. A solution of (E)-6-(4-methoxyphenyl)pyridine-2-car- mixture was refluxed for 2 h. The suspension was cooled to
baldehyde oxime (1.5 g, 6.58 mmol) in absolute ethanol room temperature, obtaining a yellow precipitate, which was
(450 mL) was introduced into a 500 mL three neck round bot- filtered, washed with 2-propanol (3 mL), MeOH (2 × 3 mL),
tomed flask under stirring and an inert atmosphere. After the and n-heptane (2 × 5 mL) and dried under reduced pressure.
addition of 10% Pd/C (150 mg), the reaction mixture was Yield: 147 mg (91%). Elemental analysis calcd (%) for
hydrogenated (1 atm of H2) at room temperature overnight, C41H41ClN2OP2Ru (776.26): C 63.39, H 5.32, N 3.61; found: C
460 | Dalton Trans., 2020, 49, 453–465
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3
63.34, H 5.27, N 3.65. 1H NMR (400.1 MHz, CD2Cl2, 25 °C): δ = 8.2 Hz, 1H; aromatic proton), 6.95 (d, JHH = 7.2 Hz, 1H; aro-
3
4
8.21 (tt, JHH = 9.2 Hz, JHH = 1.5 Hz, 2H; aromatic protons), matic proton), 6.75–6.69 (m, 4H; aromatic protons), 6.55 (dd,
3
4
7.78 (tt, JHH = 8.4 Hz, JHH = 1.3 Hz, 2H; aromatic protons), 3JHH = 8.6 Hz, 4JHH = 2.2 Hz, 1H; aromatic proton), 6.49 (t, 3JHH
7.58–7.41 (m, 7H; aromatic protons), 7.41–7.32 (m, 6H; aro- = 8.5 Hz, 2H; aromatic protons), 5.37 (s, 1H; C5H4), 4.90 (s, 1H;
3
matic protons), 7.30 (d, JHH = 8.5 Hz, 1H; aromatic proton), C5H4), 4.39 (s, 1H; C5H4), 4.27 (s, 1H; C5H4), 4.21 (s, 1H;
3
3
2
7.19 (t, JHH = 7.8 Hz, 1H; aromatic proton), 7.00 (d, JHH
=
=
=
C5H4), 4.14 (dd, JHH = 15.8 Hz, 3JHH = 4.8 Hz, 1H; CH2N), 4.00
3
4
8.1 Hz, 1H; aromatic proton), 6.88 (td, JHH = 7.6 Hz, JHH
1.2 Hz, 1H; aromatic proton), 6.67 (dd, JHH = 7.5 Hz, JHH
(s, 1H; C5H4), 3.90 (s, 1H; C5H4), 3.79 (s, 3H; CH3O), 3.67–3.57
(m, 1H; NH2), 3.47 (ddd, JHH = 16.7 Hz, JHH = 11.8 Hz, JHH
3
4
2
3
3
=
=
3
2
1.2 Hz, 1H; aromatic proton), 6.64 (d, JHH = 7.1 Hz, 1H; aro- 5.6 Hz, 1H; CH2N), 3.21 (s, 1H; C5H4), 2.06 ppm (dd, JHH
matic proton), 6.53 (dd, 3JHH = 8.5, 4JHH = 2.5 Hz, 1H; aromatic 8.9 Hz, JHH = 5.3 Hz, 1H; NH2). 13C{1H} NMR (100.6 MHz,
3
proton), 6.06 (t, 3JHH = 8.1 Hz, 2H; aromatic protons), 4.15 (dd, CD2Cl2, 25 °C): δ = 182.5 (dd, 2JCP = 14.9 Hz, 2JCP = 8.2 Hz; CRu),
2JHH = 15.5 Hz, JHH = 4.6 Hz, 1H; CH2N), 3.75 (td, JHH
=
163.1 (s; NCC), 156.7 (s; CCOCH3), 156.6 (s; NCCH2), 143.6–108.0
3
2
14.4 Hz, 3JHH = 4.7 Hz, 1H; CH2N), 3.61 (s, 3H; CH3O), 3.45 (td, (m; aromatic carbon atoms), 87.4 (dd, JCP = 38.3 Hz, JCP
=
1
3
2JHH = 11.6 Hz, JHH = 5.4 Hz, 1H; NH2), 3.16 (pseudo-q, JHH
=
4.3 Hz; ipso-C5H4), 86.3 (d, 1JCP = 49.6 Hz; ipso-C5H4), 77.4 (d, 2JCP
3
12.3 Hz, 1H; CH2P), 3.03 (tt, JHH = 13.4 Hz, 3JHH = 3.3 Hz, 1H; = 13.2 Hz; C5H4), 76.4 (d, JCP = 7.7 Hz; C5H4), 75.5 (d, JCP
=
2
2
3
CH2P), 2.33 (dd, 2JHH = 14.7 Hz, 3JHH = 9.0 Hz, 1H; CH2P), 2.24 2.5 Hz; C5H4), 73.4 (d, 2JCP = 6.8 Hz; C5H4), 73.1 (d, 3JCP = 4.8 Hz;
(t, JHH = 14.6 Hz, 1H; CH2P), 2.08–1.85 (m, 3H; CH2CH2P and C5H4), 69.2 (d, 3JCP = 1.3 Hz; C5H4), 69.1 (br s; C5H4), 68.7 (d, 2JCP
=
2
3
NH2), 1.79–1.65 (m, 1H; CH2CH2P), 1.65–1.56 (m, 1H; 5.0 Hz; C5H4), 54.9 (s; CH3O), 51.3 ppm (d, JCP = 2.1 Hz; CH2N).
CH2CH2P), 1.25–1.06 ppm (m, 1H; CH2CH2P). 13C{1H} NMR 31P{1H} NMR (162.0 MHz, CD2Cl2, 25 °C): δ = 61.5 (d, JPP
(100.6 MHz, CD2Cl2, 25 °C): δ = 184.9 (dd, 2JCP = 16.0 Hz, 2JCP 35.6 Hz), 44.1 ppm (d, 2JPP = 35.6 Hz).
7.7 Hz; CRu), 162.9 (s; NCC), 157.7 (s; CCOCH3), 155.9 (s;
Synthesis of [RuCl(CNNOMe)(CO)(PPh3)] (4). The complex cis-
NCCH2), 144.3–107.3 (m; aromatic carbon atoms), 54.6 (s; [RuCl(CNNOMe)(PPh3)2] (1) (251.9 mg, 0.29 mmol) was sus-
=
2
=
3
1
CH3O), 52.2 (d, JCP = 2.7 Hz; CH2N), 32.7 (dd, JCP = 24.7 Hz, pended in dichloromethane (5 mL) and the mixture was
3JCP = 1.5 Hz; CH2P), 30.7 (d, JCP = 31.5 Hz; CH2P), 26.5 (d, stirred under a CO atmosphere (1 atm) overnight at room
1
2JCP = 1.6 Hz; CH2CH2P), 21.8 ppm (d, JCP = 1.2 Hz; temperature. The obtained yellow solution was concentrated to
2
CH2CH2P). 31P{1H} NMR (162 MHz, CD2Cl2, 25 °C) δ 56.9 (d, about 1 mL by evaporation of the solvent under reduced
2JPP = 37.7 Hz), 41.0 ppm (d, 2JPP = 37.7 Hz).
pressure. The addition of n-heptane (10 mL) afforded a light-
Method B. To a suspension of [RuCl2(PPh3)(dppb)] (100 mg, yellow precipitate, which was washed with diethyl ether (3 ×
0.116 mmol) in 2-propanol (3.0 mL) the ligand HCNNOMe 5 mL) and n-heptane (3 × 10 mL) and dried under reduced
(28 mg, 0.131 mmol, 1.13 equiv.) and triethylamine (162 μL, pressure. The residue was suspended in 2-propanol (5 mL) and
1.16 mmol, 10 equiv.) were added. The mixture was stirred stirred at reflux for 48 h providing the product as a single
under reflux conditions for 2 h, obtaining a yellow precipitate, isomer. Yield: 115 mg (79%). Anal. calcd (%) for
which was filtered, washed with 2-propanol (2 × 2 mL), metha- C32H28ClN2O2PRu (640.08): C 60.05, H 4.41, N 4.38; found: C
nol (2 × 2 mL), and n-heptane (2 × 5 mL) and dried under 60.09, H 4.36, N 4.42. 1H NMR (400.1 MHz, CD2Cl2, 25 °C): δ =
3
reduced pressure. Yield: 80.1 mg (89%).
7.51 (t, JHH = 7.8 Hz, 1H; aromatic proton), 7.45–7.37 (m, 3H;
Synthesis of [RuCl(CNNOMe)(dppf)] (3). The complex aromatic protons), 7.35–7.16 (m, 14H; aromatic protons), 7.04
[RuCl2(PPh3)3] (200 mg, 0.209 mmol) and dppf (128 mg, (d, JHH = 1.8 Hz, 1H; aromatic proton), 6.65 (d, JHH = 7.5 Hz,
0.230 mmol, 1.10 equiv.) were dissolved in dichloromethane 1H; aromatic proton), 6.36 (dd, JHH = 8.5 Hz, JHH = 2.4 Hz,
(4.0 mL) and the solution was stirred for 3 h at room tempera- 1H; aromatic proton), 4.23 (dd, JHH = 16.4 Hz, JHH = 6.4 Hz,
4
3
3
4
2
3
2
3
ture. The solvent was evaporated under reduced pressure and 1H; CH2N), 3.87 (dd, JHH = 16.5 Hz, JHH = 9.0 Hz, 1H; NH2),
the ligand HCNNOMe (54 mg, 0.252 mmol, 1.21 equiv.) dis- 3.70 (s, 3H; CH3O), 3.41 (ddd, JHH = 16.5 Hz, JHH = 10.5 Hz,
2
3
solved in 2-propanol (4.0 mL) and NEt3 (291 μL, 2.09 mmol, 10 3JHH = 6.1 Hz, 1H; CH2N), 2.79 ppm (dd, JHH = 8.6 Hz, JHH
=
2
3
equiv.) were added. The mixture was refluxed for 3 h obtaining 6.8 Hz, 1H; NH2). 13C{1H} NMR (100.6 MHz, CD2Cl2, 25 °C):
a yellow precipitate, which was filtered, washed with 2-propa- δ = 207.1 (d, 2JCP = 17.7 Hz; CO), 179.0 (d, 2JCP = 12.6 Hz; CRu),
nol (2 × 4 mL), methanol (2 × 4 mL), and n-heptane (2 × 5 mL) 161.6 (s; NCC), 159.2 (s; CCOCH3), 156.6 (s; NCCH2),
and dried under reduced pressure. Yield: 161 mg (85%). 138.4–108.6 (m; aromatic carbon atoms), 54.7 (s; CH3O), 50.9
Elemental analysis calcd (%) for C47H41ClFeN2OP2Ru (904.17): (s; CH2N). 31P{1H} NMR (162.0 MHz, CD2Cl2, 25 °C): δ =
C 62.43, H 4.57, N 3.10; found: C 62.36, H 4.50, N 3.02. 1H 56.7 ppm (s). IR (Nujol): ˜ν = 1913 (s) (CuO) cm−1
.
NMR (400.1 MHz, CD2Cl2, 25 °C): δ = 8.51 (t, 3JHH = 8.3 Hz, 2H;
Synthesis of trans-[Ru(CNNOMe)(CO)(PCy3)(PPh3)][BArf4] (5).
aromatic protons), 8.06 (t, JHH = 7.4 Hz, 2H; aromatic Na[BArf4] (60.0 mg, 0.0677 mmol) and PCy3 (17.6 mg,
3
protons), 8.01 (d, JHH = 1.6 Hz, 1H; aromatic proton), 7.73 (t, 0.0628 mmol) were added to [RuCl(CNNOMe)(CO)(PPh3)] (4)
4
3JHH = 8.5 Hz, 1H; aromatic proton), 7.54 (d, 3JHH = 7.2 Hz, 1H; (40.0 mg, 0.0625 mmol) in dichloromethane (5 mL). The reac-
aromatic proton), 7.49 (t, 3JHH = 8.3 Hz, 2H; aromatic protons), tion mixture was stirred for 30 min at room temperature and
3
3
7.41 (d, JHH = 7.3 Hz, 1H; aromatic proton), 7.37 (t, JHH = 7.2 filtered to remove NaCl. The obtained solution was concen-
Hz, 2H; aromatic protons), 7.31 (d, 3JHH = 8.5 Hz, 1H; aromatic trated (1 mL) and the addition of n-heptane (5 mL) afforded a
3
proton), 7.28–7.15 (m, 4H; aromatic protons), 6.99 (d, JHH
=
light-yellow precipitate, which was filtered, washed with
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n-heptane (3 × 5 mL) and dried under reduced pressure. Yield reach a final volume of 10 mL. The addition of the Ru complex
100.5 mg (92%). Anal. calcd (%) for C82H73BF24N2O2P2Ru was considered the start time of the catalysis. The TH
(1748.29): C 56.34, H 4.21, N 1.60; found: C 56.38, H 4.26, N reductions were monitored analogously as described pre-
1.65. 1H NMR (400.1 MHz, CD2Cl2, 25 °C): δ = 7.79 (m, 8H; aro- viously by removal of an aliquot of the reaction mixture
3
matic protons), 7.66 (t, JHH = 8.0 Hz, 1H; aromatic proton), (approximately 0.5 mL) followed by the addition of diethyl
3
7.62 (br s, 4H; aromatic protons), 7.52 (d, JHH = 7.9 Hz, 1H; ether (1 : 1 v/v). After filtration through a short silica pad, the
3
aromatic proton), 7.50 (d, JHH = 8.7 Hz, 1H; aromatic proton), conversion was determined by GC analysis. The S/C molar
7.44–7.39 (m, 3H; aromatic protons), 7.34–7.26 (m, 6H; aro- ratio was 1000/1, whereas the base concentration was 5 mol%
matic protons), 7.25–7.17 (m, 6H; aromatic protons), 6.84 (m, with respect to the substrate (0.1 M). The same procedure was
3
1H; aromatic proton), 6.77 (d, JHH = 7.5 Hz, 1H; aromatic followed for TH reactions at other S/C ratios (in the range of
3
4
proton), 6.51 (dd, JHH = 8.6 Hz, JHH = 2.5 Hz, 1H; aromatic 500–50 000) using the appropriate amount of catalysts.
2
3
proton), 4.28 (dd, JHH = 16.3 Hz, JHH = 6.5 Hz, 1H; CH2N),
3.67 (s, 3H; CH3O), 3.62–3.46 (m, 2H; CH2N and NH2), 2.83
Preparation of 2,5-bis(hydroxymethyl)furan (BHMF)
(m, 1H; NH2), 2.15 (m, 3H; PCH), 2.00–1.72 (m, 10H; CH2 of 5-HMF (5.00 g, 39.6 mmol) and the complex 2 (3.1 mg,
Cy), 1.71–1.56 (m, 5H; CH2 of Cy), 1.55–1.27 (m, 4H; CH2 of 4.0 μmol) were dissolved in 2-propanol (388 mL) in a 1 L three-
Cy), 1.20 (q, JHH = 10.3 Hz, 4H; CH2 of Cy), 1.05 (q, JHH = 13.1 neck round bottom flask equipped with a magnetic stirrer and
Hz, 4H; CH2 of Cy), 0.81–0.64 ppm (m, 3H; CH2 of Cy). 13C{1H} a condenser. A 0.1 M solution of NaOiPr (7.9 mL, 0.79 mmol)
2
NMR (100.6 MHz, CD2Cl2, 25 °C): δ = 206.1 (t, JCP = 15.3 Hz; in 2-propanol was added and the resulting mixture was
2
2
CO), 175.3 (dd, JCP = 10.9 Hz, JCP = 8.9 Hz; CRu), 162.8 (s; refluxed for 30 min. The S/C molar ratio was 10 000/1, whereas
1
NCC), 161.8 (q, JCB = 50.1 Hz; CB), 160.0 (s; CCOCH3), 157.1 the base concentration was 2 mol% with respect to the sub-
(s; NCCH2), 137.9–109.2 (aromatic carbon atoms), 124.6 (q, strate (0.1 M). The obtained solution was cooled to room temp-
1JCF = 272.3 Hz; CF3), 54.7 (s; CH3O), 50.4 (s; CH2N), 35.6 (dd, erature and diethyl ether (300 mL) was added. After filtration
1JCP = 15.7 Hz, 3JCP = 1.5 Hz; PCH), 30.5 (s; CH2 of Cy), 28.7 (d, through a short silica pad, the solution was dried over anhy-
JCP = 2.9 Hz; CH2 of Cy), 27.5 (d, JCP = 12.3 Hz; CH2 of Cy), 27.4 drous Na2SO4 and the solvent was evaporated under reduced
(d, JCP = 9.5 Hz CH2 of Cy), 26.8 (d, JCP = 11.8 Hz; CH2 of Cy), pressure. The residue was dissolved in distilled water (25 mL),
26.0 ppm (s; CH2 of Cy). 31P{1H} NMR (162.0 MHz, CD2Cl2, extracted with ethyl acetate (2 × 100 mL) and diethyl ether
2
2
25 °C): δ = 34.2 (d, JPP = 250.9 Hz), 22.0 ppm (d, JPP
250.9 Hz). IR (Nujol): ˜ν = 1919 (s) (CuO) cm−1
=
(100 mL). The organic layers were combined, dried over anhy-
drous Na2SO4 and evaporated to dryness, affording the
product as a white solid with 99% purity, as inferred from
NMR analysis (see ESI, Fig. S44 and S45†). Yield 4.57 g (90%).
.
Catalytic TH of ketones and aldehydes in the presence of NaOiPr
The ruthenium catalyst solution used for TH was prepared by
dissolving the ruthenium complexes 1–5 (2.0 μmol) in 2 mL of
2-propanol. A 0.1 M solution of NaOiPr (200 μL, 20 μmol) in Conflicts of interest
2-propanol and the catalyst solution (1 mL, 1.0 μmol) were
The authors declare no competing financial interests.
added to the ketone or aldehyde solution (1.0 mmol) in 2-pro-
panol (final volume 10 mL) and the resulting mixture was
heated under reflux conditions. The reaction was sampled by
removing an aliquot of the reaction mixture (0.5 mL), which
was quenched by adding diethyl ether (1 : 1 v/v), filtered
through a short silica pad and subjected to GC analysis. The
addition of the base was considered the start time of the reac-
tion. The S/C molar ratio was 1000/1, whereas the base concen-
tration was 2 mol% with respect to the substrate (0.1 M). The
same procedure was followed for TH reactions at other S/C
ratios (in the range of 1000–100 000) using the appropriate
amount of catalysts. For solid and high-boiling compounds,
the solvent was evaporated under vacuum and the crude
mixture was dissolved in CDCl3 and analyzed by 1H and 13C
{1H} NMR spectroscopy.
Acknowledgements
This work was supported by the Ministero dell’Università e
della Ricerca (MIUR), PRIN 2015 program no 20154X9ATP_005
and the University of Udine, Research Quality Enhancement
Plan 2018, Action 3b Competitive research funding. The
authors thank Mr Francesco Cesaro for carrying out the trans-
fer hydrogenation tests, Mr Pierluigi Polese for performing the
elemental analyses and Dr Paolo Martinuzzi for NMR
assistance.
Notes and references
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