Formic acid disproportionation into formaldehyde triggered by vanadium complexes with iridium catalysis under mild conditions inN-methylation

Article abstract of DOI:10.1039/d1gc00275a

Formaldehyde (CH₂O) has been used as a key platform reagent in the chemical industry for many decades. Currently, the industrial production of CH₂O mainly depends on fossil resources, involving a highly energetic three-step process (200-1100 °C). Herein, we describe renewable formic acid (HCO₂H) disproportionation into CH₂O triggered by vanadium complexes with iridium catalysis under mild conditions at 30-50 °C inN-methylation. The gram-scale application ofin situgenerated CH₂O by HCO₂H disproportionation is demonstrated.

Full text of DOI:10.1039/d1gc00275a

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Formic acid disproportionation into formaldehyde
triggered by vanadium complexes with iridium
catalysis under mild conditions in N-methylation†
a
Cite this: Green Chem., 2021, 23,
2918
Received 25th January 2021,
Accepted 22nd March 2021
Chao-Zheng Zhou,a Yu-Rou Zhao,a Yan-Jun Guo,a Ping Zhanga,b and Yang Li
*
DOI: 10.1039/d1gc00275a
rsc.li/greenchem
Formaldehyde (CH2O) has been used as a key platform reagent in Gambarotta and co-workers reported an efficient reduction of
the chemical industry for many decades. Currently, the industrial HCO2H to CH2O at 350 °C with the consumption of a large
production of CH2O mainly depends on fossil resources, involving amount of zinc as a reductant. This is the state of the art
a highly energetic three-step process (200–1100 °C). Herein, we method for CH2O production from HCO2H.46 In addition, util-
describe renewable formic acid (HCO2H) disproportionation into ization of HCO2H for the synthesis of the formaldehyde surro-
CH2O triggered by vanadium complexes with iridium catalysis gate dialkoxymethane was developed, via formate ester and
under mild conditions at 30–50 °C in N-methylation. The gram- methoxy methanol as the possible intermediates with the
scale application of in situ generated CH2O by HCO2H dispropor- requirement of H2 (80 bar) and methanol as the reactants at
tionation is demonstrated.
80 °C.48
Herein, we report the discovery of HCO2H disproportiona-
tion into CH2O triggered by vanadium complexes with iridium
catalysis at 30–50 °C in N-methylation. Specifically, the active
vanadium complexes are determined to be [(VO)2+(xH2O)L].
According to the control experiments, the ligand (L) should be
the generated intermediates during renewable HCO2H pro-
duction by hydrolysis–oxidation of biomass,49–53 the analogues
of glyoxal in aqueous solutions.54 The vanadium complex may
act as a Lewis acid to activate HCO2H. As the feature of dispro-
portionation, extra reductants can be avoided (Fig. 1).
Introduction
For many decades, formaldehyde (CH2O) has been used as a
key platform reagent in the chemical industry. It is commonly
used in the preparation of many materials, including resins
and plastics, with an annual global demand of over 30 mega-
tons per year.1 Currently, the industrial production of CH2O
mainly depends on a highly energetic three-step process con-
sisting of steam reforming of natural gas to syngas
(700–1100 °C), methanol synthesis (200–300 °C), and finally
partial
oxidation/dehydrogenation
of
methanol
Results and discussion
(300–400 °C).1–5 Selective CH2O generation, by reduction of
carbon monoxide6–12 or carbon dioxide13–27 and oxidation of
methane,28–39 is being developed.
Recently, we demonstrated HCO2H production in up to quanti-
tative yield by hydrolysis–oxidation of various types of ligno-
cellulosic biomass, including wheat straw, corn straw, rice
straw, etc even in a near 10-gram-scale reaction.55 In the trans-
Due to the dwindling supply of fossil resources, CH2O pro-
duction from renewable resources, such as renewable formic
acid (HCO2H),40–43 is highly desirable.1 To date, only a few
methods of CH2O production by reduction of HCO2H have
been reported,44–46 probably due to the fact that the reduction
potential is similar to that of HCO2H reduction to methanol.47
aCenter for Organic Chemistry, Frontier Institute of Science and Technology and
State Key Laboratory of Multiphase Flow in Power Engineering. Xi’an Jiaotong
University, Xi’an, Shaanxi, 710054, P. R. China. E-mail: liyang79@mail.xjtu.edu.cn
bCollege of Chemistry and Chemical Engineering, Xianyang Normal University,
Xianyang, Shaanxi, 712000, P. R. China
Fig. 1 HCO2H disproportionation into CH2O triggered by vanadium
complexes with iridium catalysis in the N-methylation. a L should be the
analogues of glyoxal in the aqueous solution.
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1gc00275a
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formation, air/oxygen (3–5 MPa) was used as an oxidant and
Meanwhile, the iridium complex, containing Cp* and 2,4-
sodium metavanadate (NaVO3, 3.5 mol%) was used as a cata- dihydoxypyrimidine connected with an imidazoline moiety,
lyst precursor. The reaction was carried out in 0.7 wt% H2SO4 which displayed the best efficiency for hydrogen production
aqueous solution with 1 v% DMSO at 160 °C for 3 h.55 from the hydrolysis–oxidation solution of biomass,55 only
Generally, the industrial production of HCO2H mainly resulted in 40% yield of 3 (Table S4,† entry 1). Other catalysis
depends on CO as the starting material, which is normally pro- systems of [RhCp*Cl2]2, rhodium complex containing Cp* and
duced in an unsustainable way from coal, oil or natural gas by 4,4′-dihydroxy-2,2′-bipyridine, [RuCl2(benzene)]2/dppe, RuCl3/
gasification at high temperatures (>900 K), through a multistep m-TPPTS, and Fe(BF4)2/(Ph2PCH2CH2)3P, which displayed
process via methyl formate or formate as intermediates.56
lower efficiencies for the hydrogen production from the oxi-
In comparison with industrial HCO2H production, our dation of glucose solution,55 were also detected. No reactivity
method displayed some advantages, such as renewable start- was observed (Table S4,† entries 2–6).
ing materials, comparatively mild conditions, etc. The pro-
The influence of pH values on the transformation was also
duced renewable HCO2H was applied to the hydrogenation of investigated (Table S5†). The results of control experiments
nitroarenes by utilization of its hydrogen source.57 Thus we demonstrated that there was no obvious influence on the cata-
envisioned the feasibility of utilization of the hydrogen source lytic activity during the change in pH value from 0.9 to 2.5
as well as the carbon source of the renewable HCO2H58 for the (Table S5,† entries 1–3). The catalytic activity was sharply
N-methylation of quinolines to afford the corresponding decreased in the pH range from 3.0 to 3.5 (Table S5,† entries
N-methyl tetrahydroquinolines using tetrahydroquinolines as 4–6). The reaction activity was inhibited at the pH value of 4.0
the intermediates.59–61
(Table S5,† entry 7).
Quinoline (1) was selected as a model substrate for
To gain insight into the reaction mechanism of the
N-methylation (Table S2†). Initially, the reaction was carried N-methylation, a series of experiments were conducted. As the
out using renewable HCO2H (8 equiv.) contained in wheat reaction should involve the hydrogenation of quinoline fol-
straw hydrolysis–oxidation aqueous solution (HOAS).55 lowed by the N-methylation, sequential studies of the hydro-
N-Methyl tetrahydroquinoline 3 in 30% yield and tetrahydro- genation and the N-methylation were designed.
quinoline 4 in 60% yield were obtained in the presence of
Firstly, hydrogenation of quinoline 1 in the presence of 2.5
iridium catalyst 2 (Table S2,† entry 1). As the comparatively equiv. of HCO2H or the renewable HCO2H contained in the
easily available renewable HCO2H from wheat straw, after a wheat straw HOAS was studied (Fig. 2, eqn (2) and (3)). In both
series of experiments, it was found that using renewable cases, the hydrogenation product 4 was obtained in excellent
HCO2H (30 equiv.) with 2 (1 mol%), 3 was obtained in 91% yields (90% and 89%, respectively). In addition, in comparison
yield, by a total conversion of 4, after 20 h at 50 °C (Fig. 2, eqn with the no N-methylation product 3 in the former case, 5%
(1)), with the utilization rate of HCO2H of 15% (Table S3†).
yield of 3 was detected in the latter case. These results suggest
that the HCO2H contained in the wheat straw HOAS is the
major active component in the hydrogenation. For the
N-methylation, there should be other factors contained in the
wheat straw HOAS, besides HCO2H.
Next, N-methylation was investigated using tetrahydroqui-
noline 4 with 2.5 equiv. of the renewable HCO2H, and 3 was
obtained in 25% yield (Fig. 2, eqn (4)). Furthermore, with 30
equiv. of the renewable HCO2H, 3 was obtained in 88% yield
(Fig. 2, eqn (5)). However, when using 30 equiv. of HCO2H, 3
was not detected (Fig. 2, eqn (6)). These results further indicate
that other factors contained in the wheat straw HOAS, besides
HCO2H, are essential for N-methylation.
Furthermore, possible intermediates of amide 5 58,60,61 and
urea 6 62–65 for the N-methylation were tested under the opti-
mized conditions (Fig. 2, eqn (7) and (8)), respectively. In both
cases,
3 was not observed, which indicates that the
N-methylation does not involve the pathways of the amide or
the urea formation.
Then, the possibility of N-methylation via the Eschweiler–
Clarke reaction pathway was tested with 4 (Fig. 2, eqn (9)–(11)).
The Eschweiler–Clarke reaction pathway involves using CH2O
as the carbon source with HCO2H as a reductant to afford the
hydrogen source.66,67 In the presence of 1.1 equiv. of CH2O
and 1.5 equiv. of HCO2H with 2 under similar conditions, 4
was converted to 3 in 46% yield (Fig. 2, eqn (9)). Increasing the
Fig. 2 Control experiments for the Eschweiler–Clarke reaction
pathway of the N-methylation. a The renewable HCO2H was prepared by
the hydrolysis–oxidation of the wheat straw.55 1, 4, 5, or 6 (0.25 mmol),
2 (1.0 mol%). Yields were determined by 1H NMR using Cl2CHCHCl2 as
an internal standard.
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amount of HCO2H to 30 equiv., while keeping the amount of pathway required at least 0.25 equiv. of CH2O or CH2(OH)2.
CH2O constant, did not induce an obvious increase of the Thus the lacking CH2O should be in situ generated from
yield of 3 (49%, Fig. 2, eqn (10)). However, increasing the HCO2H. Second, a 41% capture rate of CH2O was obtained
amount of CH2O to 2 equiv., with 1.5 equiv. of HCO2H, under similar standard conditions with the deduction of the
afforded 3 in 64% yield (Fig. 2, eqn (11)). Comparing these amount of CH2(OH)2 contained in the wheat straw HOAS
results with no observation of 3 in the presence of 30 equiv. of (Fig. 3, eqn (1)).
HCO2H (Fig. 2, eqn (6)), we deduced the in situ CH2O gene-
The generation of MeOH during the transformation was
ration by HCO2H disproportionation, triggered by the [V] com- also detected. There was about 3% MeOH in the renewable
plexes contained in the wheat straw HOAS, under optimized HCO2H from the hydrolysis–oxidation of biomass, which is
standard conditions (Fig. 2, eqn (1)).
derived from the oxidation of DMSO.55 After the reaction, the
1
To prove the deduction, the typical CH2O capture experi- aqueous mixture was detected by crude H NMR, the content
ment, in which ammonia and acetoacetic ester acted as of MeOH was still determined to be about 3% (Fig. S3†).
capture reagents,68 was conducted using renewable HCO2H Therefore, we deduce that further reduction of in situ gener-
and normal HCO2H under similar optimized conditions ated CH2O to MeOH is difficult in the aqueous reaction
(Fig. 3, eqn (1) and (2)). In the transformation, the CH2O mixture. The reason should be the major form of in situ gener-
capture rate, the ratio of in situ generated CH2O to the con- ated CH2O as CH2(OH)2 in the aqueous reaction mixture.
sumed HCO2H, was determined to be 41% or 21%, respect-
After that, many experiments were performed using the [V]
ively. The captured product 7 was obtained in 77% or 23% complexes related with the in situ CH2O generation (Table 1).
yield, respectively. Notably, only in the presence of HCO2H The amount of [V] complexes in the wheat straw HOAS used
without the [V] complexes, 7 could not be obtained. These for the N-methylation was about 1.1 equiv., accompanied by 30
results demonstrate in situ CH2O generation by HCO2H dispro- equiv. of renewable HCO2H, as 3.5 mol% of NaVO3 was used
portionation under optimized standard conditions.
as the catalyst precursor for one mole of HCO2H production.
As a supplementary specification, with an understanding of
The N-methylation was conducted in the presence of NaVO3
the Eschweiler–Clarke reaction pathway of N-methylation, we (1.1 equiv.) or VOSO4 (1.1 equiv.), both of which can be used
realized
that
the
minor
component
CH2(OH)2 as good catalyst precursors for the hydrolysis–oxidation of
(3.4–3.6 mol%),55 the major form of CH2O in aqueous solu- biomass into HCO2H.55 However, no observation of 3 using
tion, with an accumulation of about 1.1 equiv. accompanied NaVO3 or 7% yield of 3 using VOSO4 was detected (Table 1,
by 30 equiv. of HCO2H in the wheat straw HOAS, should con- entries 2 and 3).
tribute to the transformation to some extent.
Meanwhile, the same amount of [V] complexes from wheat
However, our experiments clearly proved the in situ CH2O straw HOAS by removal of HCO2H and other minor organic
generation from HCO2H under the standard conditions. First, components, including HOAc, MeOH, 1,4-dioxane, DMSO, and
25% of 3 was obtained from 4 using 2.5 equiv. of renewable DMSO2,55 was used for the N-methylation to give 3 in 45%
HCO2H contained in the wheat straw HOAS, accompanied by yield (Table 1, entry 4). It was further proved that these minor
no more than 0.09 equiv. of CH2(OH)2 (Fig. 2, eqn (4)). organic components in wheat straw HOAS had no influence on
Production of 3 in 25% yield via the Eschweiler–Clarke the in situ CH2O generation in the N-methylation (Table S6†).
Furthermore, it was found that 5 mol% of [V] complexes,
simply prepared by a 10 minute procedure of hydrolysis–oxi-
dation of glucose (0.24 equiv.), induced the N-methylation in
83% yield (Table 1, entry 5). To avoid some uncertain factors
for the mechanism insight, herein, glucose was used instead
of the wheat straw. When 10 mol% of [V] complexes was used,
the yield of N-methylation reached 86% (Table 1, entry 6).
Notably, with the lack of any one factor of the hydrolysis–oxi-
dation procedure of glucose, the resulted [V] complexes could
not afford product 3 (Table S7†).
In comparison with the standard reaction conditions to
obtain 91% yield of 3 (Fig. 2, eqn (1)), these results suggest
that the active [V] complexes should contain the vanadium
element as well as the possible intermediates generated
Fig. 3 CH2O capture experiments. Reaction conditions of eqn (1): acet-
oacetic ester (2 mmol), renewable HCO2H (8 mmol, 4.0 equiv.) con-
tained in the wheat straw HOAS, NH3 aqueous solution (16 mmol, 8.0
equiv.), 2 (1.0 mol%), 50 °C, 20 h. a The renewable HCO2H was prepared
by the hydrolysis–oxidation of the wheat straw.55 b Isolated yield is
reported based on the average of two experiments. Reaction conditions
of eqn (2): acetoacetic ester (2 mmol), HCO2H (8 mmol, 4.0 equiv.), NH3
aqueous solution (16 mmol, 8.0 equiv.), 2 (1.0 mol%), and the [V] solu-
tion prepared as shown in note c, 50 °C, 20 h. c The wheat straw HOAS
was extracted by EtOAc to remove HCO2H and the other minor com-
ponents to afford the [V] solution.
during the hydrolysis–oxidation of biomass.
First, the valence of the [V] complexes was detected. The [V]
complexes from the wheat straw HOAS and the simply pre-
pared [V] complexes by the hydrolysis–oxidation of glucose dis-
played the blue color of [V4+] in an aqueous solution.69 These
[V] complexes were further confirmed using the EPR spectrum
(Fig. 4, 2b and 2c).51 It confirmed that [V4+] is contained in the
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Table 1 Control experiments for in situ CH2O generation triggered by
[V] complexes
3 (Yield
%)
Entrya Reactants and additives
1b
2
3
HCO2H (30 equiv.)
—
—
7
HCO2H (30 equiv.), NaVO3 (1.1 equiv.)
HCO2H (30 equiv.), VOSO4 (1.1 equiv.)
HCO2H (30 equiv.), [V] (1.1 equiv.) from wheat
straw HOAS with the removal of HCO2H and other
minor organic componentsc
HCO2H (30 equiv.), prepared [V] (5 mol%) by a
10 min hydrolysis–oxidation of glucose (0.24
equiv.) with NaVO3 as a catalyst precursor
HCO2H (30 equiv.), prepared [V] (10 mol%) by a
10 min hydrolysis–oxidation of glucose (0.48
equiv.) with NaVO3 as a catalyst precursor
HCO2H (30 equiv.), VOSO4 (0.1 equiv.), 1,3-dihy-
droxyacetone (0.1 equiv.)
4
45
Fig. 4 The 51V NMR and EPR spectra of vanadium. (a) The same initial
concentration of standard NaVO3, pH 2.25 (the pH was adjusted by
0.7 wt% H2SO4 aqueous solution). (b) Wheat straw hydrolysis–oxidation
aqueous solution, pH 2.25. (c) Glucose hydrolysis–oxidation aqueous
solution, pH 2.25. (d) After the standard reaction of N-methylation, the
reaction mixture was detected. (e) The same concentration of standard
VOSO4, pH 2.25 (the pH was adjusted by 0.7 wt% H2SO4 aqueous
solution).
5d
6e
83
86
7
7
8
HCO2H (30 equiv.), VOSO4 (0.1 equiv.), glyceralde- 13
hyde (0.1 equiv.)
9
HCO2H (30 equiv.), VOSO4 (0.1 equiv.), glycolalde-
—
mixture of glyoxal with VOSO4 induced 42% yield of 3 (Table 1,
entry 10), which displayed an obvious higher efficiency than
other possible intermediates (Table 1, entries 7–9, 11). These
results indicate that the [V4+] or [V5+] complexes, with the ana-
logues of glyoxal in aqueous solution, form the major active
[V] complexes to trigger in situ CH2O generation for the
N-methylation.
In addition, the experiment of NaVO3 instead of VOSO4
with glyoxal gave 35% yield of 3 (Table 1, entry 12). During the
procedure, after mixing glyoxal with the yellow aqueous solu-
hyde (0.1 equiv.)f
10
11
12
HCO2H (30 equiv.), VOSO4 (0.1 equiv.), glyoxal (0.1 42
equiv.)g
HCO2H (30 equiv.), VOSO4 (0.1 equiv.), glycolic
acid (0.1 equiv.)
12
HCO2H (30 equiv.), NaVO3 (0.1 equiv.), glyoxal (0.1 35
equiv.)g
13h
14h
15h
16h
HCO2H (30 equiv.), Zn(OTf)2 (0.1 equiv.)
HCO2H (30 equiv.), Zn(OTf)2 (1.1 equiv.)
HCO2H (30 equiv.), Cu(OTf)2 (0.1 equiv.)
HCO2H (30 equiv.), Cu(OTf)2 (1.1 equiv.)
3
5
5
10
a Reaction conditions: 4 (0.25 mmol), 2 (1.0 mol%), reactants and addi-
tives, 50 °C, 20 h. Yields were determined by 1H NMR using
tion of NaVO3 for several minutes, the color of the reaction
Cl2CHCHCl2 as an internal standard. b Herein, the experiment of eqn changed to blue due to [VO2 (xH2O)].69–72 Then mixing glyoxal
+
(6) in Fig. 2 was given for obvious comparison. c The [V] complexes
with the aqueous solution of NaVO3 was further detected
using the in situ UV-visible absorption spectra. The initial
were obtained from wheat straw HOAS by removal of HCO2H and the
other minor organic components by extraction with EtOAc. d The reac-
tion was conducted at 30 °C, and the amount of CH2(OH)2 was deter- spectra and the spectra within 3 min, 5 min and 30 min are
mined to be less than 0.02 mmol from glucose HOAS. e The reaction
shown in Fig. 5(a–d). They demonstrate that the UV-visible
absorption spectra of mixing glyoxal with NaVO3 are identical
was conducted at 30 °C, and the amount of CH2(OH)2 was determined
to be less than 0.05 mmol from glucose HOAS. f The dimer of glycolal-
dehyde was used. g Glyoxal aqueous solution was used, the major form to those of mixing glyoxal with VOSO4 within 30 min (Fig. 5e)
of glyoxal aqueous solution was ethane-1,1,2,2-tetraol.54 h The reaction
and VOSO4 in acidic aqueous solution (Fig. 5f), which exist as
was conducted at 70 °C for 30 h.
[VO2 (xH2O)] in aqueous solution.71 These results indicate that
+
NaVO3 is reduced to [V4+] as [VO2 (xH2O)] in the presence of
+
glyoxal.
[V] complexes besides [V5+], which was detected by 51V NMR
Furthermore, after the standard reaction of N-methylation,
(Fig. 4, 1b and 1c).55 According to the influence of the pH the reaction mixture was detected by EPR and 51V NMR again
values on the existing form of [V4+] complexes,69–72 the [V4+ (Fig. 4, 1d and 2d). The results indicate that only the [V4+] com-
complexes should exist as [VO2+(xH2O)] related complexes plexes remained after the reaction.
]
when the pH value of the reaction mixture was about 2.25.
Based on these experiments, the active vanadium com-
Next, the possible intermediates generated during the hydro- plexes that triggered the HCO2H disproportionation into CH2O
lysis–oxidation of biomass contained in the active [V] com- are determined to be [(VO)2+(xH2O)L], in which the ligand (L)
plexes were investigated. VOSO4 (10 mol%) aqueous solution should be the analogues of glyoxal, generated during renew-
with the possible intermediates (10 mol%) of hydrolysis–oxi- able HCO2H production by hydrolysis–oxidation of biomass.
dation of biomass, including 1,3-dihydroxyacetone, glyceralde-
To further investigate the function of the active
hyde, glycolaldehyde, glyoxal, and glycolic acid,49–53 respect- [(VO)2+(xH2O)L] complexes, the transformation was conducted
ively, was stirred for 30 min at room temperature, and then the in the presence of typical Lewis acids, such as Zn(OTf)2 or Cu
N-methylation was conducted (Table 1, entries 7–11). The (OTf)2 both in 10 mol% or 1 equiv. (Table 1, entries 13–16).
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Fig. 6 Proposed in situ CH2O generation by disproportionation of
HCO2H during the N-methylation. a L should be the analogues of glyoxal
in aqueous solution.
Fig. 5 The in situ UV-visible absorption spectra of mixing glyoxal with
the solution of NaVO3. (a) The initial UV-visible absorption spectra of
mixing glyoxal with NaVO3 in 0.7 wt% H2SO4 aqueous solution under an
Ar atmosphere. (b) The UV-visible absorption spectra of mixing glyoxal
with NaVO3 in 0.7 wt% H2SO4 aqueous solution under an Ar atmosphere
over 3 min. (c) The UV-visible absorption spectra of mixing glyoxal with
NaVO3 in 0.7 wt% H2SO4 aqueous solution under an Ar atmosphere over
5 min. (d) The UV-visible absorption spectra of mixing glyoxal with
NaVO3 in 0.7 wt% H2SO4 aqueous solution under an Ar atmosphere over
30 min. (e) The UV-visible absorption spectra of mixing glyoxal with
VOSO4 in 0.7 wt% H2SO4 aqueous solution under an Ar atmosphere. (f)
The UV-visible absorption spectra of VOSO4 in 0.7 wt% H2SO4 aqueous
solution under an Ar atmosphere.
by the hydrogenation of 7 with the [Ir]–H complex to afford the
N-methylation product 3.
Synthetic application
To make the transformation more applicable, the gram-scale
reaction of N-methylation of quinoline 1 with HCO2H in the
presence of 2 (1 mol%), as well as the prepared [V] complexes
(10 mol%), was conducted. 0.85 g of 3 was obtained in 75%
yield at 30 °C. (Fig. 7, eqn (1)).
Furthermore, the gram-scale reaction of N-methylation of
quinoline 1 with renewable HCO2H, produced from 10.5 g of
the wheat straw, was also conducted. 1.0 g of 3 was obtained in
91% yield with 0.5 mol% of catalyst 2 (Fig. 7, eqn (2)). Product
3 was obtained in 72% yield at 30 °C.
Product 3 was obtained in 3–10% yields. Based on these
results, it was deduced that the active [(VO)2+(xH2O)L] com-
plexes may act as a Lewis acid to activate HCO2H,71 by coordi-
nation of the carbonyl group, during the in situ CH2O
generation.
As 1.1 equiv. of [V] was contained by using renewable
HCO2H from the wheat straw HOAS in the N-methylation, the
Furthermore, in the standard mercury poisoning test,73
3
was obtained in a comparable yield of 87%, in comparison
with 91% under standard conditions. This result indicates
homogeneous catalysis for the in situ CH2O generation in the
N-methylation. The quantitative concentration of the
vanadium complex is the same as the concentration of 0.02 M
in the original wheat straw HOAS.
According to the results of these studies, the in situ CH2O
generation by disproportionation of HCO2H during the
N-methylation is proposed as shown in Fig. 6. The iridium
catalyst precursor 2 releases H3O+ to afford the [Cp*L′′IrH]+
complex (L′′ = the anion of 4,4′-dihydroxy-2,2′-bipyridine),
which is followed by coordination with formate (HCO2−) and
decarboxylation to generate the [Ir]–H complex of [Cp*L″
IrH]+.55,74,75 The active vanadium complexes, which were deter-
Fig. 7 Gram-scale application of in situ generated CH2O. a [V]
(10 mol%) prepared by a 10 min hydrolysis–oxidation of glucose (0.24
equiv.) with NaVO3 as a catalyst precursor. b [V] contained during the
mined to be [(VO)2+(xH2O)L] (L should be the analogues of wheat straw HOAS. Reaction conditions of eqn (1): quinoline
1
(7.74 mmol, 1.0 g), 2 (0.0774 mmol, 1 mol%, 48.8 mg), 10 mol% [V] solu-
tion from glucose HOAS, HCO2H (232.2 mmol, 30 equiv., 10.7 g,
8.8 mL), H2O (250 mL), 30 °C, 65 h. Reaction conditions of eqn (2): qui-
noline 1 (7.74 mmol, 1.0 g), 2 (0.0387 mmol, 0.5 mol%, 24.4 mg), renew-
able HCO2H (185.7 mmol, 24 equiv.) contained in the wheat straw HOAS
glyoxal in the aqueous solution), promote the reduction of
HCO2H to CH2(OH)2 by the [Ir]–H complex by coordination of
the carbonyl group. An equilibrium exists between CH2(OH)2
and CH2O. Meanwhile, compound 1 is hydrogenated to 4 by
the [Ir]–H complex. Condensation of 4 with CH2O is followed (334.5 mL), 50 °C, 35 h. c 30 °C, 65 h.
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Acknowledgements
This work was supported by the National Key R&D Program of
China (No: 2018YFB1501601) and the Instrument Analysis
Center of the Xi’an Jiaotong University.
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C.-Z. Z. carried out the experimental and data-analysis work.
Y.-R. Z., Y.-J. G., and Prof. P. Z. contributed to some experi-
ments and the discussion. Y. L. designed the reaction and
directed the project. Y. L. wrote the paper with the assistance
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Conflicts of interest
There are no conflicts to declare.
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