Cobalt Pincer Complexes for Catalytic Reduction of Carboxylic Acid Esters

Article abstract of DOI:10.1002/chem.201705201

A selection of cobalt(I) and cobalt(II) pincer type complexes with different substitution patterns was tested in the catalytic reduction of carboxylic acid esters to alcohols. The cobalt pincer type complex 4 is suitable for the hydrogenation of aromatic as well as aliphatic and cyclic esters. Mechanistic investigation indicated a metal ligand cooperated reaction pathway.

Full text of DOI:10.1002/chem.201705201

A Journal of
Accepted Article
Title: Cobalt pincer complexes for catalytic reduction of carboxylic acid
esters
Authors: Kathrin Junge, Bianca Wendt, Andrea Cingolani, Anke
Spannenberg, Zhihong Wei, Haijun Jiao, and Matthias Beller
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To be cited as: Chem. Eur. J. 10.1002/chem.201705201
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Cobalt pincer complexes for catalytic reduction of carboxylic acid
esters
Kathrin Junge,[a] Bianca Wendt,[a] Andrea Cingolani,[b] Anke Spannenberg,[a] Zhihong Wei,[a] Haijun
Jiao,[a] and Matthias Beller*[a]
Abstract:
A
selection of cobalt(I) and cobalt(II) pincer type
developed for a number of catalytic transformations such as
polymerization,[12] bond forming reactions,[13] hydrogenations,[14]
transfer hydrogenations,[15] hydrosilylations/hydroborations[16] or
Although cobalt based reductions of
aldehydes,[14b,15a] nitriles[14e,h,15d,16h]
or
alkenes[14a,b,g,15b,c] have been studied,[18] until now hydrogenation
of carboxylic acid esters in the presence of this metal has been
scarcely examined.[19]
complexes with different substitution patterns was tested in the
catalytic reduction of carboxylic acid esters to alcohols. The cobalt
pincer type complex 4 is suitable for the hydrogenation of aromatic
as well as aliphatic and cyclic esters. Mechanistic investigation
indicated a metal ligand cooperated reaction pathway.
17
]
N2 activation.[
ketones,[14b,c]
The reduction of carboxylic acid esters is a fundamental
method for the production of alcohols on industrial and
laboratory scale.[1] Traditionally, metal hydrides (LiAlH4, NaBH4)
are used for this transformation, which suffer from low functional
group tolerance and cause large amounts of waste stressing the
environment. Therefore, the development of novel catalytic
processes is of continuous interest. While conventional
heterogeneous systems for the reduction of fatty acid esters
require harsh reaction conditions (>200°C, 200 bar), for a long
time homogeneous catalysts have been limited regarding their
efficiency.[2]
Important milestones in this field have been reported by the
group of Milstein,[3] as well as by Firmenich SA[4] and Tagasako[5]
nearly ten years ago with the development of molecularly
defined ruthenium based catalysts allowing the hydrogenation of
esters under milder reaction conditions. Since then, tremendous
efforts have been reported for the catalytic reduction of
carboxylic acid esters and numerous homogeneous catalytic
systems based on noble metals have been developed.[ 6 ] In
recent years, the focus in catalyst development shifted more and
more to the application of cheap, abundant and low-toxic base
metals.[7] In line with this development, iron[8] and manganese[9]
derived catalysts for the reduction of carboxylic acid esters have
been presented demonstrating exciting catalytic performances
comparable to their precious metal based congeners.
Figure 1. Selected cobalt pincer complexes for catalytic hydrogenation.
More specifically, Milstein and co-workers reported the first
example of a Co NNP pincer catalyst C for the hydrogenation of
esters to alcohols (Figure 1).[19a] Interestingly, this cobalt catalyst
only reduces aliphatic esters while aromatic or fluorinated esters
are not affected. The unexpected catalytic behaviour is
explained by the hydrogenation of the enolate which is formed
from the aliphatic ester. Elsevier and de Bruin performed the
reduction of aromatic and aliphatic carboxylic acids as well as
for esters applying 5-10 mol% of Co(BF4)2/triphos.[19b] Very
Next to them also cobalt complexes can be regarded as
potential replacement for the classical noble metal catalysts due
to their low cost and availability.[10] Cobalt compounds, and here
recently, the cationic cobalt pincer complex
B originally
11
]
especially cobalt pincer type complexes,[
have been
developed by Hanson was reported by Jones and co-workers for
the reduction of esters requiring no additive.[19c] So far, all these
cobalt catalysts have been tested for a relatively small number
of esters with a limited tolerance of functional groups leaving
space for improvements. Here, based on our experiences in
non-noble metal catalyzed reductions[8b,e,9a] we report the
development of a more general applicable cobalt catalyst for the
hydrogenation of various carboxylic acid esters.
[a]
Dr. K. Junge, B. Wendt, Dr. A. Spannenberg, Z. Wei, Dr. H. Jiao,
Prof. M. Beller
Leibniz-Institut für Katalyse e.V. an der Universität Rostock
Albert-Einstein-Straße 29a
Rostock, 18059, Germany
E-mail: Matthias.Beller@catalysis.de
A. Cingolani
[b]
University of Bologna
Dipartiento di Chimica Industriale “Toso Montanari”
viale Risorgimento 4
40136 Bologna, Italy
Supporting information for this article is given via a link at the end of
the document.
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methyl benzoate took place (Table 1, entries 16, 17, 19). These
results are in agreement with the findings of Jones, who
observed an inhibition of the catalytic reaction, when cobalt
pincer complex B was used in the presence of 1 bar of CO.[19c]
Obviously, the coordination of CO to metal centre prevents the
formation of a catalytic active cobalt(I) species which was
postulated by Milstein.[19a] When the corresponding cobalt(I)
complex 7 was tested in the reduction of methyl benzoate, 89%
yield of benzyl alcohol are produced (Table 1, entry 18). This
result indicates the involvement of a cobalt(I) species in the
catalytic reaction pathway.
Table 1. Optimization of the reaction conditions for hydrogenation of 9a[a]
.
Scheme1. Synthesis of different Co pincer complexes 1-8.
Initially, a selection of aliphatic cobalt pincer complexes
bearing different substituents on the phosphorous atom of the
pincer ligand as well as on the metal centre was prepared
(Scheme 1). Compounds 1-4 are easily available by reaction of
CoX2 (X = Br, Cl) with the corresponding pincer ligand in good
yields.[20] Treatment of 1 and 2 with CO leads to the formation of
complexes 5 and 6. When the dichloro Co PNP pincer complex
2 is reduced with only 1 equivalent of NaBH4 the corresponding
Entry
Catalyst
Temp.
[°C]
Time
[h]
Conv.
[%]
Yield[b]
10a [%]
Yield[b]
11a [%]
1
1
2
3
4
4
4
4
4
-
140
140
48
48
48
48
48
48
48
48
48
24
24
6
48
65
67
>99
-
22
46
45
96
-
6
12
5
2
3
140
140
140
140
140
140
140
4
<1
-
monochloro Co(I) compound
7 is isolated as dark blue
5[c]
6[d]
7[e]
8[f]
9
crystals.[21] The cationic Co(I) dicarbonyl complex 8 is prepared
starting from 7 and an excess of CO.
3
-
-
2
-
-
Methyl benzoate was chosen as model substrate to test the
catalytic activity of the different cobalt PNP pincer complexes 1-
8. Only low to moderate yields of benzyl alcohol 10a and benzyl
benzoate 11a (product of transesterification of starting material
with benzyl alcohol) were detected, when 5 mol% of cobalt
pincer compounds 1-3 are applied in the presence of 20 mol% of
NaOMe, 50 bar of hydrogen gas, 140°C and 48 h in dioxane
(Table 1, entries 1-3). In the case of the phenyl substituted
cobalt pincer complex 4 quantitative conversion of methyl
benzoate and 96% yield of benzyl alcohol were obtained (Table
1, entry 4). Obviously, the amount and the kind of base play a
crucial role in this reaction. No hydrogenation of ester occurred
when KOH was used instead of NaOMe (Table 1, entry 5). A
further reduction of the quantity of NaOMe to 10, 5 or 0 mol%
led to a complete depletion of the catalytic activity (Table 1,
entries 6-8). In a blank experiment applying only NaOMe as
base 13% conversion but no formation of the corresponding
alcohol 10a were observed (entry 9). Shorter reaction time (24 h
or 6 h) and lower temperature (120°C-100°C) had no significant
influence on the reaction outcome (Table 1, entries 10-13).
Notably, applying 5 mol% of cobalt pincer complex 4 at 100°C
still produced nearly quantitative yield of benzyl alcohol in only 6
hours! A decrease of the catalyst loading to 1 or 2 mol% causes
lower product yields (Table 1, entries 14-15).
1
-
-
13
96
>99
>99
97
65
24
26
12
97
17
-
-
10
11
12
13
14[g]
15[h]
16
17
18
19
4
4
4
4
4
4
5
6
7
8
94
99
99
96
63
22
7
<1
-
140
120
120
100
120
120
120
120
120
120
-
6
-
6
<1
2
6
24
24
24
24
3
<1
89
-
<1
<1
-
[a] 9a (0.5 mmol), 1-8 (0.025 mmol), NaOMe (0.1 mmol), dioxane (2 mL), 50
bar H2. [b] Yield determined by GC analysis using hexadecane as an internal
standard. [c] KOH (0.1 mmol). [d] NaOMe (0.05 mmol). [e] NaOMe (0.01
mmol). [f] no base. [g] 4 (0.01 mmol), NaOMe (0.04 mmol). [h] 4 (0.005 mmol),
NaOMe (0.02 mmol).
Based on these preliminary tests cobalt pincer complex 4
was chosen to investigate the general applicability of this
catalytic system. Crystals of complex 4 which are suitable for X-
ray analysis were grown from THF/ethanol and the molecular
structure is shown in Figure 2.[22] The cobalt metal centre is five
coordinated in a distorted trigonal bipyramidal environment ( =
0.95).[23]
During our studies on the catalytic behavior of aliphatic
iron[8b,e] or manganese[9a] pincer complexes we observed the
beneficial effect of carbonyl ligands which are coordinated to the
metal centre. Therefore, we tested various, carbon monoxide
containing cobalt pincer species 5, 6 and 8 in this model reaction.
However, in none of these cases a considerable reduction of
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pincer complex 4. Interestingly, several fluorinated aromatic
esters (9f-h) are reduced with very high yields (entries 6-8),
while in case of chloro or bromo substituents partially
dehalogenation was observed. Also 4- (9i) and 2-methoxy (9j)
as well as 4-methylbenzoate (9k) are converted to the
corresponding alcohols in good to high yields (entries 9-11).
Next to methyl 2-naphthoate (9l), also the related diester (9m)
and methyl nicotinate (9o) produced the alcohol in 75-88% yield
(entries 12, 13 and 16). For terephthalic acid dimethylester (9n)
62% of the monoalcohol was formed with 5 mol% catalyst
loading (entry 14).
Figure 2. Molecular structure of 4. Displacement ellipsoids are drawn at the
30% probability level. Hydrogen atoms except of that attached to nitrogen are
omitted for clarity. Selected bond lengths (Å) and angles (°): Cl1-Co1
2.2884(8); Cl2-Co1 2.2933(8); Co1-N1 2.384(3); Co1-P2 2.3860(8); Co1-P1
2.3963(8); Cl1-Co1-N1 174.29(6); Cl2-Co1-P2 115.81(3); Cl2-Co1-P1
115.56(3); P2-Co1-P1 117.10(3); Cl1-Co1-P1 99.45(3); Cl1-Co1-P2 100.77(3).
Table 2. Aromatic substrate scope applying cobalt pincer complex 4 [a]
.
Entry
Ester
Alcohol
Conv.
[%][b]
Yield
[%][b]
1[c]
2[c]
3
4
5
9a R = Me
10a
10a
10a
10a
10a
>99
>99
90
37
30
99
99
89
35
26 (7)
9b R = Et
9c R = i-Pr
9d R = t-Bu
9e R = Bn
Figure 3. Molecular structure of 1. Displacement ellipsoids are drawn at the
30% probability level. Hydrogen atoms except of that attached to nitrogen are
omitted for clarity. Selected bond lengths (Å) and angles (°): Br1-Co1
2.3906(4); Br2-Co1 2.6090(4); Co1-N1 2.0111(16); Co1-P2 2.2468(7); Co1-P1
2.2473(7); Br1-Co1-N1 163.61(5); Br2-Co1-P2 92.367(16); Br2-Co1-P1
97.387(18); P2-Co1-P1 166.24(2); Br1-Co1-P1 93.771(18); Br1-Co1-P2
93.690(17).
6[c]
7[c]
8[c]
9
10
11
9f R = 4-F, R`= Me
10f R = 4-F
10g R = 2-F
10h R = 4-CF3
10i R = 4-MeO
10j R = 2-MeO
10k R = 4-Me
99
99
99
99
81
99
85
89
90
85
65 (5)
90 (2)
9g R = 2-F, R`= Me
9h R = 4-CF3, R`= Me
9i R = 4-MeO, R`= Me
9j R = 2-MeO, R`= Me
9k R = 4-Me, R´= Et
Besides, the molecular structure of complex 1 could be
determined by X-ray analysis (Figure 3).[22] The crystals are
obtained from CH2Cl2/pentane and the structure shows the five
coordinated cobalt metal centre in a distorted square-pyramidal
geometry ( = 0.04).[23] While in Co pincer complex 1 the
ethylene bridges between N and P are orientated almost
symmetrically with respect to the plane defined by N, Co and the
halogen atoms, this was not observed in Co pincer complex 4.[24]
In Table 2 the results for the reduction of several aromatic
esters with cobalt complex 4 under optimized reaction conditions
(5 mol% 4, 20 mol% NaOMe, 50 bar H2, 120°C, 6 or 24 h,
dioxane) are summarized. First, we investigated the influence of
the different ester moieties on the reactivity. Methyl, ethyl and i-
propyl benzoates could be reduced to benzyl alcohol 10a with
very high yields (Table 2, entries 1-3). Benzoates bearing
sterically more demanding ester groups such as t-Bu or Bn show
reduced reactivity with good to moderate yields (entries 4 and 5).
For these substrates minor amounts (< 10%) of the
corresponding transesterification products were detected. As
discussed above the reported cobalt catalysts B, C and
CoBF4/triphos were not active for aromatic esters and/or showed
a very limited substrate scope regarding functional groups.
Therefore, methyl benzoates with substituents in different
position of the aromatic ring were applied with the cobalt PNP
12
99
95
76
75
13[d]
14
85
62
15[d]
16
70
99
33[e]
88
[a] 7 (0.5 mmol), 4 (0.025 mmol), NaOMe (0.1 mmol), dioxane (2 mL), 24 h,
120 °C, 50 bar H2. [b] Conversion and yield determined by GC analysis using
hexadecane as an internal standard. Yield of 11 is given in parenthesis. [c]
120°C, 6 h. [d] 4 (0.05 mmol), NaOMe (0.2 mmol), 48 h. [e] 36% yield of the
monoalcohol 10n`was detected.
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Furthermore, cobalt complex
hydrogenation of various aliphatic and cyclic carboxylic acid
esters (Table 3).
4
was also tested for
ene-1-carboxylate 9v is not affected during the reduction
producing cyclohexenylmethanol 10v in 55% yield (entry 7).
Next to the aliphatic diester dimethyl succinate 9cc (entry 14)
also a selection of different lactones was successfully reduced
to the respective diols (entries 10-13).
Table 3. Reduction of aliphatic and cyclic esters applying cobalt pincer
complex 4[a]
.
Finally, several control experiments were done to learn more
about the possible catalytic reaction mechanism. During the
initial optimization of the reaction parameters the formation of a
black precipitate was observed at 140°C when the benchmark
reaction was stopped after 24 h. Obviously, the cobalt pincer
complex 4 tends to decompose at higher temperature forming
metallic nanoparticles (magnetically active). In order to prevent
such decomposition of the cobalt pincer complex 4 further
catalytic experiments were carried out at 120°C showing a
brown clear solution after the catalytic reaction. Nevertheless,
the partial formation of very small cobalt nanoparticles cannot be
excluded. To distinguish between a possible homogeneous or
heterogeneous catalysis poisoning experiments with Hg(0) or
PPh3 were realized (Scheme 2).[ 25 ] An excess of Hg(0) or
substoichiometric amounts of PPh3 (2.5 or 1 mol%) were added
to the reaction mixture containing 5 mol% catalyst 4, 20 mol%
NaOMe and methyl benzoate before the catalysis was started.
In all three experiments no suppression of the catalytic activity
was observed and benzyl alcohol was obtained in 96-99% yield.
This is clear evidence that cobalt nanoparticles do not act as
catalyst under these conditions and a homogeneous reaction
pathway seems to be preferred.
Entry
Ester
Alcohol
Conv.
[%][b]
Yield
[%][b]
1
2
95
99
82 (3)
89
3
4
87
63
65 (7)
60 (3)
5
6
85
69
79
52 (7)
7[c]
79
55 (10)
8
9
92
65
75 (9)
62
Scheme 2. Poisoning experiments with Hg(0) and PPh3.
10
90
81[d]
83
11
12
88
90
Notably, Milstein discussed the formation of a Co(I) hydride
complex as catalytic active species.[19a] In our case, we observed
90
67[d]
the formation of
a
[(PNP)Co(I)Cl] (7) starting from
[(PNP)Co(II)Cl2] (2) in the presence of base as described by the
group of Chirik.[26] When Co(I) pincer complex 7 was tested for
the reduction of methyl benzoate and methyl octanoate the
respective alcohols are produced in high yields underlining this
assumption (Scheme 3). Different to the catalytic system of
Milstein C the Co PNP pincer catalyst 4 is able to reduce
aromatic next to aliphatic carboxylic acid esters. Milstein
postulated a hydrogenation mechanism via the enolate form of
the aliphatic ester which is not possible for aromatic substrates
and can be excluded for this kind of substrates.
13
96
92
94
87
14[e]
[a] 9 (0.5 mmol), 4 (0.025 mmol), NaOMe (0.1 mmol), dioxane (2 mL), 24 h,
120 °C, 50 bar H2. [b] Conversion and yield determined by GC analysis using
hexadecane as an internal standard. Yield of 11 is given in parenthesis. [c]
120°C, 6 h. [d] Isolated yield. [e] 4 (0.05 mmol), NaOMe (0.2 mmol), 48 h.
An important advantage of pincer based metal complexes in
catalysis is the metal-ligand cooperation concept (MLC) where
the ligands play an active role during the reaction.[27] In our
previous studies on iron[8b] or manganese[9a] pincer type
catalyzed ester reduction we observed MLC where the NH
moiety of the aliphatic pincer ligand was involved in the
Linear carboxylic acid esters (entries 1-4) are smoothly
reduced to alcohols with cobalt PNP pincer complex 4 in yields
between 60-89%. In case of several cyclic esters (entries 5-9)
the corresponding alcohols are obtained in moderate to good
yields. Interestingly, the C-C double bond in methyl cyclohex-3-
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hydrogen transfer step during the catalysis. Whenever the NH
position was blocked by a substituent no reaction occurred.
Therefore we also investigated this aspect for the Co catalyzed
ester reduction applying Co PNP pincer based complexes. The
Co pincer complex 12 bearing a methyl substituent on the
nitrogen of the pincer ligand was prepared starting from CoCl2
and bis(2-diphenylphosphinoethyl)methylamine.[28]
By measuring the magnetic moments they underlined the
paramagnetic properties of these complexes.[29] Here, our DFT
computations show that the quartet state of complex 2 is 13.92
kcal/mol more stable than the double state, while in case of
complex 7 the triplet state is 22.13 kcal/mol more stable than the
singlet state in enthalpy.[30] Furthermore, Arnold and coworkers
showed that H2 can be oxidatively added to complex 7 to form
complex 14,[21c] which is only stable under an H2 atmosphere. A
quick removal of H2 regenerates 7 even in the solid state. In the
molecular structure of 14 the N-H and Co-Cl moieties are
orientated cis to each other; which might explain why a H/D
exchange was not observed under non-basic condition. In
addition, Arnold showed that complex 13 can be reduced by KC8
under an H2 atmosphere to give the trihydride complex 15,
where two H2 molecules were added to the Co centre. Complex
15 is stable under N2 atmosphere in solid state. Actually,
complex 15 is similar with the homologous trihydride iridium
complex,[31] which has been used as an effective catalyst in the
reduction of esters. Starting from 15, we computed the
dehydrogenated complexes, which are formed via heterolytic
dehydrogenation across the N-H and the Co-H bond (16) and
via homolytic dehydrogenation at the Co centre (17). Complex
16 can be considered as the intermediate involved in an outer
sphere mechanism, while complex 17 should be associated with
an inner sphere mechanism.
Scheme 3. Reaction of cobalt pincer complex 7 and 12 with methyl benzoate
and methyl hexanoate.
When 5 mol% of 12 are used for the catalytic reduction of methyl
benzoate and methyl octanoate no reaction takes place
(Scheme 3). Based on these results we can conclude that for
the Co catalyzed ester reduction applying this type of Co PNP
pincer complexes the NH moiety is mandatory which can be
regarded as a clue that an outer sphere mechanism takes place.
These findings differ from the observations of Jones for Co
pincer complex B where different ester substrates are also
reduced with the corresponding N-Me Co complex and an inner
sphere mechanism is postulated.[19c]
Scheme 5. Potential H2 elimination routes of trihydride complex 15.
In addition to these experimental studies, results from
literature can be used to discuss the mechanisms.[21] Arnold et
al.,[21b] reported that reaction of 2 with n-BuLi in toluene yields
the reduced complex 7 and the deprotonated complex 13
(Scheme 4).
For R = Ph, it is found that the singlet state of 16 is much
more stable than the corresponding triplet state by 12.55
kcal/mol; while the triplet state of complex 17 is more stable than
the corresponding singlet state by 4.91 kcal/mol. It is very
interesting to note that the singlet state of 16 is only 0.90
kcal/mol higher in energy than the triplet state of 17; indicating
that both heterolytic and homolytic H2 elimination are thermo-
dynamically possible and competitive. Similar results are also
found for R = iPr, where complex 16 has a singlet ground state,
while complex 17 has a triplet ground state. In this case the
singlet state of 16 is slightly more stable than the triplet state of
17 by 2.86 kcal/mol. Such energetic properties between 16 and
17 provide the assumption that depending on the applied
reaction conditions both types of mechanisms (outer or inner
sphere) are possible. During our mechanistic research, we
collected hints that an outer sphere mechanism might take place,
but based on DFT computation an inner sphere cannot be
excluded. Therefore, further investigations are required.
Scheme 4. Reaction network of complex 2.[21b,c]
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In summary, we developed a homogeneous reduction of
carboxylic acid esters catalyzed by cobalt PNP pincer
complexes. Co pincer type catalyst 4 efficiently reduces a large
number of aromatic, aliphatic and cyclic esters to alcohols.
Further investigations on the reaction mechanisms are currently
on way.
[5]
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ChemCatChem 2016, 8, 3431- 3435.
Catalytic transformations were performed in a 300 mL Parr autoclave
equipped with an internal aluminum plate to include eight uniform
reaction glass vials (4 mL) equipped with a stirring bar and sealed with
cap, septum and needle. The autoclave was placed into an aluminum
block as heating system to perform the reactions.
General procedure: In a reaction vial (4 mL), cobalt complex 4 (0.05
equiv.; 0.025 mmol) and NaOMe (0.2 equiv.; 0.1 mmol) are mixed with
2.0 mL of dioxane containing a defined amount of hexadecane as
standard. After short time of stirring, the liquid ester (0.5 mmol) was
added under argon. Solid esters were added directly to the reaction vial.
The vial was placed into the alloy plate in the autoclave. The apparatus
was purged five times with hydrogen gas, pressurized to 50 bar H2, and
stirred for 6-24 h at 120°C. Afterwards, the autoclave was cooled to room
temperature, depressurized and the reaction mixture was analyzed by
[8]
GC. Selected products from Table
chromatography.
3 were isolated by column
Acknowledgements
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for their excellent analytical support as well as the State of
Mecklenburg-Western Pomerania for financial support.
Especially, the authors thank Dr. Elisabetta Alberico for the
synthesis of N-methyl bis(2-chloroethyl)amine hydrochloride.
Keywords: cobalt • pincer complexes • hydrogenation • ester •
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Entry for the Table of Contents
COMMUNICATION
Kathrin Junge, Bianca Wendt, Andrea
Cingolani, Anke Spannenberg, Zhihong
Wei, Haijun Jiao, and Matthias Beller*
Page No. – Page No.
Cobalt pincer catalyst for the
reduction of carboxylic acid esters: A
study of selectivity
An efficient cobalt PNP pincer type catalyst has been developed for the catalytic
reduction of carboxylic acid esters to alcohols. The cobalt pincer type complex 4 is
suitable for the hydrogenation of numerous aromatic as well as aliphatic and cyclic
esters. Mechanistic investigation indicated a metal ligand cooperated reaction
pathway.
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