Selective and high yield transformation of glycerol to lactic acid using NNN pincer ruthenium catalysts

Article abstract of DOI:10.1039/d0cc02884c

The conversion of glycerol selectively to lactic acid has been accomplished in high yields (ca. 90%) by using a NNN pincer-Ru catalyst. DFT explains the role of the Ru-P bond and sterics in favoring the catalysis.

Full text of DOI:10.1039/d0cc02884c

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Conformational control of nonplanar free base porphyrins:
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DOI: 10.1039/D0CC02884C
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Selective and High Yield Transformation of Glycerol to Lactic Acid
Using NNN Pincer Ruthenium Catalysts
Moumita Dutta,a Kanu Das,a Siriyara Jagannatha Prathapa,b Hemant Kumar Srivastava*c and
Received 00th January 20xx,
Accepted 00th January 20xx
Akshai Kumar*a,d
DOI: 10.1039/x0xx00000x
The conversion of glycerol selectively to lactic acid has been synthesis of LA using pincerFe complexes.6g Tu and co-workers
accomplished in high yields (ca. 90 %) by a NNN pincerRu catalyst. have shown the applicability of NHCIr based copolymers in the
DFT explains the role of RuP bond and sterics in favoring catalysis. catalytic dehydrogenation of glycerol with good yield (98 % ca.
3266 TONs) and recyclability.6h In 2016, Williams achieved
Recent years have witnessed a surge in the utilization of
biodiesel1 which has significantly increased the formation of
glycerol that is one of its major by-products.2 Glycerol which is
nontoxic and biodegradable3a is also generated as a waste from
multiple processes that include hydrogenolysis of cellulose3b
and fermentation processes involving microorganisms.3c While
a small fraction of glycerol produced globally finds use as a
green non-volatile solvent,3d-e as a fuel3f and as a source of
hydrogen,3a,gk it can potentially be employed as a versatile
precursor to several synthetically useful transformations.3l
One such useful transformation of glycerol involves the
generation of hydrogen3h along with the production of lactic
acid (LA). Lactic acid is valuable synthetic feedstock and finds
wide application in food, pharmaceuticals, fine chemicals,
cosmetics and also in the synthesis of poly lactic acid (PLA).4
Lactic acid synthesis from glycerol via bioconversion3g involving
traditional fermentation and heterogeneous catalysis3a, 5 suffer
from several limitations. On the other hand, homogeneous
catalysis for glycerol to LA conversion have been reported to
demonstrate very good reactivity and selectivity.2, 3i, 6
conditions that provide high TONs (1057172) as well as good
yield (62%) of LA starting from glycerol using a Ir(I) complex at
145 C.6i Very recently, Voutchkova-Kostal have utilized Ir(I),
Ir(III) and Ru(II) NHC complexes for efficient transformation of
glycerol to LA.6j They have arrived at methods that provide high
TONs at 150 C and also under microwave conditions.
Recently, we have reported the synthesis7a of NNN pincerRu
complexes and their activity towards N-alkylation which
involved the dehydrogenation as the first step with 1a being
more efficient than 1b.7b We envisaged the utility of these
catalysts in other synthetically useful transformations that rely
on dehydrogenation such as glycerol to LA formation. Herein, a
systematic mechanistic insight has been obtained by comparing
the activity of pincerruthenium complexes (1ab) based on
bis(imino)pyridine with corresponding sterically less hindered
2,6bis(benzimidazole2yl) pyridine based complexes (1cd)
towards synthesis of LA from glycerol (Figure 1).
The complex (1c) based on 2,6bis(benzimidazole2yl) pyridine
was synthesised by improvising the procedure reported by us7a
and others7c,d (see SI). Notably Zhang and Peng have reported
similar complexes but with acetonitrile as ancillary ligands
which were shown to be active for transformation of benzyl
alcohols to corresponding acids.7e The 31P NMR revealed that
the major isomer in complex 1c had the two PPh3 groups trans
to each other. Treatment of 1c with NaPF6 in methanol resulted
in 1d‡ which was characterized by singlecrystal Xray analysis
(Figure 1d). We were fortunate to obtain the crystal structure
of 1a‡ (Figure 1c) which was elusive in our previous attempts.7a
In a typical experiment, to a flask containing 0.7 mol of 1a in
glycerol (2.56 mmol), KOH (1.48 mmol) was added followed by
addition of 2 mL ethanol (bp=78C) (entry 1, Table 1). The
reaction mixture was then heated under open vessel conditions
at 140 C under an argon atmosphere. As glycerol is very
viscous, ethanol is added to ensure proper homogeneity of the
In his pioneering work, Crabtree reported the acceptorless
dehydrogenation of glycerol using an Ir(I)NHC complex which
provided up to 30100 turnover numbers (TONs) in about 90%
yield with a selectivity of 95% towards LA.6e In 2015, the Beller
group demonstrated the exceptional activity of PNPRu that
resulted in 270000 TONs with about 67% yield of LA at 140 C
using NMP as a solvent.6f Hazari and Crabtree have reported the
a. Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati – 781039,
Assam, India. Email: akshaikumar@iitg.ac.in
b. Bruker India Scientific Pvt. Ltd., Bengaluru – 560092, Karnataka, India.
c. Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and
Research Guwahati, Guwahati – 781101, Assam, India. Email: hemantkrsri@gmail.com
d. Centre for Nanotechnology, Indian Institute of Technology Guwahati
Electronic Supplementary Information (ESI) available: [Xray crystallographic parameters, NMR
spectra and computational data (PDF) and Xray crystallographic data (CIF)]. See
DOI: 10.1039/x0xx00000x
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DOI: 10.1039/D0CC02884C
Figure 1. NNN pincerruthenium complexes based on (a) bis(imino)pyridine ligands and (b) 2,6bis(benzimidazole2yl) pyridine ligands investigated in the current study. ORTEP
diagram of (c) 1a and (d) 1d with ellipsoids drawn at 50% probability. The H atoms and Ph groups on P are omitted for sake of the clarity
reaction mixture. During the reaction, all the ethanol is lost
within a few minutes of reaction. The NMR spectra (Figure
S7S24) of the crude product does not show any evidence
either for presence of ethanol or its potential dehydrogenation
products. The reactions catalyzed by 1a (entry 1, Table 1) gave
moderate yield of LA. On the other hand, the performance of 1b
was relatively poor (entry 2, Table 1). The catalyst 1c based on
2,6bis(benzimidazole2yl) pyridine ligand exhibited good
activity towards the selective conversion of glycerol to LA (entry
3, Table 1). Its analog 1d with the PF6 counterion gave yields of
LA that was comparable with 1c. (entry 4, Table 1). This is
attributable to the fact that both 1c and 1d give rise to the same
active catalyst (See Scheme 1 and 2). Further optimization of
reaction conditions was carried out with 1c (Table 2). Under the
best reaction conditions, the observed high selectivity (Table 1
and 2) of LA clearly indicates that the decarbonylation of
glycerol6 and decarboxylation of lactic acid6 leading to ethylene
glycol and formic acid respectively are negligible.
Use of DMSO, DMF and benzonitrile leads to very low yield of
LA in the 1c catalyzed reactions (entries 2,3 and 5 Table 2).
Moderate yields of LA were obtained with either pxylene or
water as solvent (entries 4 and 6, Table 2). Upon increasing the
base loading, the 1c catalysed reaction gave very good yield of
LA (entry 8, Table 2). The yield however dropped to 59% (entry
9, Table 2) under solvent free conditions indicating the
importance of thorough premixing of the contents. In
particular, use of ethanol may assist in the easy generation of
the catalytically active RuH species7b(8ac) via hydride
elimination from Ru(ethoxide). As dioxane offers no such
possibility, not surprisingly, the yields are slightly lower in
comparison (compare entries 7 vs. 1 and 10 vs. 8, Table 2) to the
Table 2. Glycerol dehydrogenation to lactic acid catalysed by 1 under varying conditions
Entry
Base (Equiv.), Solvent
Yielda
5 (TONs)
693c% (2300)
9% (300)
2% (67)
46% (1533)
2% (67)
40% (1344)
68% (2267)
902c% (3000)
59% (1966)
76% (2557)
371c% (1233)
5% (1667)
3 Conversiona
(5 Selectivity)b
71% (97%)
26% (35%)
34% (6%)
5(5)
1
2
3
4
5
6
7
8
KOH (0.58), Ethanol
KOH (0.58), DMSO
KOH (0.58), DMF
KOH (0.58), pXylene
KOH (0.58), PhCH2CN
KOH (0.58), Water
KOH (0.58), Dioxane
KOH (1.0), Ethanol
KOH (1.0), 
KOH (1.0), Dioxane
NaOH (1.0), Ethanol
KOH (1.0), Ethanol
KOH (1.0), Ethanol
KOH (1.0), 
2%(1%)
5%(3%)
5%(25%)
2%()
1%()
3%()
3%()
1%()
2%()
1%(1%)
4%(5%)
1%(1%)
1%(1%)
1%(1%)
73% (63%)
3% (67%)
44% (98%)
72% (94%)
92% (98%)
61% (97%)
78% (97%)
52% (71%)
6% (83%)
9
10
11
12d
13d,e
14d,e
45% (15000)
44% (14666)
46% (98%)
46% (96%)
Reaction conditions: 0.7 mol (0.03 mol %) of 1c, 2.56 mmol of 3, 1.48 mmol KOH,
2 mL solvent in an open vessel under Ar at 140 C . aDetermined from 1H NMR using
sodium acetate as internal standard. bSelectivity = (yield of 5/conversion of 3)100.
c Reported as an average of two runs. d0.3 mol (0.003 mol %) of 1c, 10.24 mmol
e
of 3, 10.24 mmol KOH, 2 mL solvent in an open vessel under Ar at 140 C.
Performed in presence of 5.12 mmol of water.
yields obtained with ethanol under otherwise identical
conditions. Yields were drastically reduced either with use of
NaOH as base (entry 11, Table 2) or upon decreasing the loading
of 1c (entry 12, Table 2). However, in the presence of 0.5
equivalents water,6e 45% LA (15000 TONs) was obtained with
0.003 mol % of 1c (entries 13 and 14, Table 2). It is noteworthy
that at this lower catalyst loading it is easy to homogenize the
reaction mixture without the use of ethanol.
Table 1. Glycerol dehydrogenation to lactic acid catalysed by 1 under varying conditions
We have recently demonstrated the ability of catalysts (1ab)
to generate H2 when treated with primary alcohols at 140 C.7b
The plausible mechanism involved in the reaction is provided in
Scheme 1. The complexes (6ac), (7ac), (8ac) and (9ac)
depicted in Scheme 1 are either the intermediates or the
transition states (TSs) involved in the cycle catalyzed by the
corresponding complexes (1ad). For the sake of simplicity,
only the NNN ligating section of the pincer framework is
represented. While for (1ab), the generation of active species
(6ab) is fairly straightforward (eq. 1, Scheme 2), one could
anticipate, loss of two PPh3 molecules in the corresponding
Entry
1
3 Conversiona
5 Yielda
(TONs)
53% (1767)
26% (867)
693c% (2300)
66% (2200)
5(5)
Yielda
3%(2%)
1%(1%)
Selectivityb
towards 5
85%
1
2
1a
1b
62%
27%
96%
3
4
1c
1d
71%
67%
2%(1%)
1%()
97%
98%
Reaction conditions: 0.7 mol of 1, 2.56 mmol of 3, 1.48 mmol KOH, 2 mL solvent
in an open vessel under Ar at 140 C. aDetermined from 1H NMR using NaOAc as
internal standard. bSelectivity = (yield of 5/conversion of 3)100. c Average of 2 runs.
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Table 3. Interaction energy, free energy of formation and energy barriers involved in the
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DOI: 10.1039/D0CC02884C
transformation of glycerol to glyceraldehyde catalysed by 1ad
b
Entry
1
∆Einta (RuP)
∆G140 (68)
TS 7 / TS 9
TONc
(kcal/mol)
(kcal/mol)
(kcal/mol)
1
2
3
1a
1b
1c
39.82
44.35
8.62
31.78/21.39
21.89/23.90
1767
867
2300
2.51
39.00d
2.08d
16.17 / 21.39d
4
1d
2200
aInteraction energy is calculated as the difference between the energy of the
Scheme 1. Plausible mechanism involved in the (1ad) catalyzed transformation of
complex (1a–d) and the energy of the corresponding isolated PPh3 and
glycerol to lactic acid
(
R2NNN)RuCl2 (R=tBu, Ph) or [(Bim2NNN)RuCl]+ in the geometry of the complex (1a–
d).bFree energy of formation at 140 °C for hydride elimination 68. cValues
obtained from Table 1. dAs cation was computed, the values are same for 1c & 1d.
formation of the catalytically active species (6c) starting from
both 1c and 1d (equation 2, Scheme 2).
A hydride elimination from (6ac) results in the formation of
(8ac) and glyceraldehyde 4 via TS (7ac) (Scheme 1). The active
species (6ac) is regenerated via a bond metathesis of the
OH of 3 with the RuH of (8ac) through TS (9ac) (Scheme 1).
Spontaneous dehydration of 4 gives ,unsaturated aldehyde
4 and its tautomer 4. The Cannizzaro reaction of 4 leads to LA
5. It has been widely accepted that, in the transformation of
glycerol to LA, only the dehydrogenation step is
metalcatalyzed and the rest of the steps are spontaneous
organic processes in basic solution.2 The observed difference
between various catalysts (Table 1) can be attributed to the
variation in their relative dehydrogenation efficiencies. With an
intent to understand the relative ease of dehydrogenation, the
RuP interaction energy and the relative free energies at 140 °C
(G140) for the species involved in the catalysis were computed.
It has been shown7a,7f that the catalytic efficiency of Ru catalysts
with phosphine as ancillary ligands is dependent on the ease of
their release to generate the active catalyst (6 in this case). The
phosphine release can be quantified as the interaction energy
(Eint) of PPh3 either with (R2NNN)RuCl2 (R=tBu, Ph) arising
from 1a and 1b or with [(Bim2NNN)RuCl(PPh3)]+ arising from
both 1c and 1d (Table 3). The Eint followed the trend;
1b (44.35 kcal/mol) > 1a (39.82 kcal/mol) > (1c or 1d) (39.00
kcal/mol). Clearly, the complex 1b where the PPh3 is the hardest
to remove in comparison with 1a, 1c and 1d exhibited the
lowest activity (867 TONs). However, among 1a and 1c or 1d
with comparable RuP bond energy, 1a exhibited much lower
activity (1767 TONs) than 1c (2300 TONs) and 1d (2200 TONs).
This can be satisfactorily explained if one considers the
energetics of hydride elimination and H2 evolution (Figure 2).
The overall uphill reaction (6+36+4+H2; Figure 2) justifies the
need of high operating temperature (140 °C). While for the
reactions catalyzed by 1a, hydride elimination is the RDS, H2
evolution is the RDS for corresponding reactions catalyzed by 1b
and 1c/1d (Table 3, Figure S25). The barrier for the RDS followed
the trend; 1a (TS 7a; ΔG⧧140 = 31.78 kcal/mol) > 1b (TS 9b; ΔG⧧
140
= 23.90 kcal/mol) > (1c or 1d) (TS 9c; ΔG⧧ = 21.39 kcal/mol)
140
(Table 3, Figure 2, Figure S25). Though the energetics of the 1b
catalyzed reactions are more favourable than 1a (Table 3, Figure
S25), the reluctance of 1b to easily release the PPh3 (entry 2,
Table 3) contributes to its lower reactivity. On the other hand,
the reaction with catalyst 1c or 1d proceeds very efficiently
not only because the barrier for the hydride elimination
Figure 2. Free energy (140 °C) profile of the (1ad) catalyzed glycerol dehydrogenation
at PBEPBE functional using LANL2DZ basis set for Ru and 6-311G(d,p) for all other atoms.
Scheme 2. Generation of catalytically active (6ac) starting from (1ad)
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-4 | 3
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Javier, C. Pascale and C. Victorio, Current GreenVCiehweAmrti.c,le2O0n1lin4e,
1, 51-65; (e) A. E. Diaz-Alvarez and V. Cadierno, Appl. Sci.-
Basel, 2013, 3, 55-69. (f) B.Sims, BiodiDesOeI:l1M0.a10g3a9z/iDn0eC. CO0c2t8o8b4eCr
25, 2011. (g) S. J. Sarma, S. K. Brar, E. B. Sydney, Y. Le Bihan,
G. Buelna and C. R. Soccol, Int. J. Hydrogen Energ., 2012, 37,
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Figure 3. Space filling model of the TS 7 depicting the steric crowding around ruthenium.
(TS 7c; ΔG⧧140 = 16.17 kcal/mol) is about 1.96 folds (TS 7 barrier
in entries 1 and 3 Table 3) lower but also because the
hydride elimination is downhill by 6.54 kcal/mol (G140 for
entries 1 and 3 Table 3) in comparison with 1a (Figure 2). This
results in a significant build-up of glyceraldehyde in reactions
catalyzed by 1c/1d in stark contrast to 1a where the equilibrium
is more towards the left (6a + 3). The open vessel conditions
ensure that the equilibrium for the subsequent step (8c +3
6c+H2) is more towards the right. Furthermore, among 1c/1d
and 1a, the barrier for the RDS with 1c/1d is lower by about
10.39 kcal/mol (Ts:9c, TS:7a, Table 3). Despite the fact that the
generation of the active catalyst 6 starting from 1c and 1d is
comparable to 1a (Table 3), the favourable energetics that is
attributable to lower steric encumbrance (Figure 3) around the
Ru centre makes the catalysis with 1c/1d more conducive. It is
interesting to note that the inclusion of higher basis set or
dispersion corrections has minimal affect on the overall
energetics and no affect on the trend of the results (Table S3).
The utility of NNN pincerruthenium complexes (1ad) towards
synthesis of LA from glycerol has been demonstrated. Catalysts
based on 2,6bis(benzimidazole2yl) pyridine ligands not only
have optimal RuP bond energy that facilitates easy generation
of active catalyst, but also enjoys less steric crowding around
the Ru center that leads to favorable energetics making them
highly efficient catalysts.
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A.K. (CRG/2018/000607) and H.K.S (SB/S2/RJN/004-2015)
thanks the SERB, DST, India. Thanks to IITG (DSTFIST program,
ParamIshan, & CIF) and NIPER Guwahati for various facilities.
Conflicts of interest
There are no conflicts to declare
Notes and references
‡ CCDC 1998012 and CCDC 2009680 contains the supplementary
crystallographic data for this paper and can be obtained free of
charge via http://www.ccdc..cam.ac.uk/structures.
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