B(C6F5)3: a robust catalyst for the activation of CO2 and dimethylamine borane for the N-formylation reactions

Article abstract of DOI:10.1039/c8nj02816h

In this work, B(C₆F₅)₃ is utilized as an organocatalyst for the transition-metal-free N-formylation of amines using carbon dioxide (CO₂) as a C1 source and dimethylamine borane (Me₂NH·BH₃) as a green hydrogen transfer source at 80 °C. Most reported works utilize silane and hydrogen for the N-formylation reactions using CO₂ which have thus far been limited by low atom economy, high cost or the use of harsh reaction conditions. This catalytic protocol affords a broad range of formylated products in moderate to excellent yields under mild reaction conditions with a high TON and TOF. The bulky boron (B(C₆F₅)₃) catalyst reacts with amines and forms a Frustrated Lewis Pair (FLP) and activates CO₂ and Me₂NH·BH₃ molecules. Additionally, this boron catalyst shows high catalytic activity for the cyclization of o-phenylenediamines using CO₂ and Me₂NH·BH₃ to synthesize benzimidazoles.

Full text of DOI:10.1039/c8nj02816h

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B(C6F5)3: a robust catalyst for the activation of
CO2 and dimethylamine borane for the
N-formylation reactions†
Cite this:DOI: 10.1039/c8nj02816h
Vitthal B. Saptal, Gaurav Juneja and Bhalchandra M. Bhanage
*
In this work, B(C6F5)3 is utilized as an organocatalyst for the transition-metal-free N-formylation of
amines using carbon dioxide (CO2) as a C1 source and dimethylamine borane (Me2NHÁBH3) as a green
hydrogen transfer source at 80 1C. Most reported works utilize silane and hydrogen for the
N-formylation reactions using CO2 which have thus far been limited by low atom economy, high cost or
the use of harsh reaction conditions. This catalytic protocol affords a broad range of formylated
products in moderate to excellent yields under mild reaction conditions with a high TON and TOF. The
bulky boron (B(C6F5)3) catalyst reacts with amines and forms a Frustrated Lewis Pair (FLP) and activates
CO2 and Me2NHÁBH3 molecules. Additionally, this boron catalyst shows high catalytic activity for the
cyclization of o-phenylenediamines using CO2 and Me2NHÁBH3 to synthesize benzimidazoles.
Received 6th June 2018,
Accepted 20th August 2018
DOI: 10.1039/c8nj02816h
rsc.li/njc
The boron-based Lewis acids, especially B(C6F5)3 and its
derivatives showed efficient reactivity for the activation of
Introduction
Carbon dioxide is non-toxic, cost-effective, renewable in nature silanes and are used as the most powerful metal-free catalyst
and considered as a desirable carbon feedstock for the synthesis of for the hydrosilylation of a range of carbonyl compounds11 as
various value-added chemicals by replacing other toxic harmful C1 well as imines.12 Apart from the range of other reactions,13
sources such as carbon monoxide or phosgene.1 However, the B(C6F5)3 and corresponding electrophilic boranes have a tendency
reaction of CO2 is not easily favored, since it is thermodynamically to activate Si–H as well as stronger H–H bonds by acting as
and kinetically stable. Thus, the utilization of CO2 is still very frustrated Lewis pairs (FLP) in combination with sterically
limited. The utilization of CO2 as a starting material provides the hindered electron donors.14b–d The combination of bulky phos-
important carbonyl products such as cyclic carbonates,2 formic phines/amines with B(C6F5)3 forms the FLP and activates CO2
acids,3 oxazolidinones,4 formamides,5 urea6 and carbamates.7
molecule in the presence of hydrosilanes or H2 for the reductive
Among the various transformations, the reductive functionaliza- conversion.14 Additionally, the presence of the highly electron-
tion of CO2 has various academic as well as industrial applications. deficient B(C6F5)3 species activates the amine molecule through
Conventionally, several types of reductants such as dihydrogen, strong electronic interactions as demonstrated using NMR spectro-
hydrosilanes and hydroboranes have been intensively studied for scopy by Liu et al. (Scheme 1a).15 The CO2 adsorption capacity of
the aminative reductive functionalization with CO2.8 Among the fluorous material is always greater than that of the non-fluorous
them, the N-formylation of amines using CO2 is an important material and hence the fluorous material shows high catalytic
reaction since the N-formyl substrates have wide applications in activity for CO2 fixation reactions and solubility of CO2.16
pharmaceuticals, are extensively used as polar solvents and have
The amine borane (AB) and its derivatives are of intense
many uses as synthetic intermediates and reagents in functional interest in this research due to their interesting characteristics
group transformation.9 Previously, various methods were reported such as being cost efficient, non-toxic, easy to handle, and
for the reductive functionalization of CO2 with amines to synthe- having high H2 storage capacity and high atom economy as
size N-formamides.10 However, these reported methods have one
or more disadvantages such as use of expensive transition metals
or requirement of high pressure and temperature conditions.
Department of Chemistry, Institute of Chemical Technology (Autonomous),
Matunga, Mumbai, India. E-mail: bm.bhanage@gmail.com; Fax: +91 22 2414 5614;
Scheme 1 Reactivity of B(C6F5)3 towards the (a) amine15 and (b) formation
of FLP intermediate with 1,2-diamine.21a
Tel: +91 22 3361 1111, +91 22 3361 2222
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj02816h
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a
Table 1 N-Formylation of morpholine (1a) using CO2 and Me2NHÁBH3
Scheme 2 B(C6F5)3 promoted N-formylation of amines using CO2 and
Me2NHÁBH3.
Entry
Solvent
Temp. (1C)
Yieldb (%)
TON
TOF (hÀ1
)
compared to those of silane.17 Among them, dimethylamine
borane (DMAB) is a hydrogen surrogate of AB and has not been
greatly explored in the literature. Me2NHÁBH3 also has high
volume/mass hydrogen density, high stability, easy accessibility,
non-flammability and water-solubility.18 Previously, a large number
of transition metal-based homogeneous catalysts, as well as
heterogeneous catalysts, have been repeatedly studied for the
dehydrogenation of AB.19 Meanwhile, there are very few reports
available for the organocatalyzed transfer hydrogenation of AB
via the FLP mechanism.20 Shi et al. recently utilized B(C6F5)3 as
catalyst for the hydrogenation of N-heterocycles with AB.20a
By considering the high catalytic activity of B(C6F5)3 towards
the amine and CO2 herein, we utilized B(C6F5)3 as a highly
efficient, metal-free catalyst for the tandem dehydrogenation of
Me2NHÁBH3 and reductive functionalization of amines with
CO2 to synthesize formamides and benzimidazoles under mild
reaction conditions (Scheme 2).
1
2
3
4
5
6
7
8
CH3CN
Toluene
1,4-Dioxane
THF
80
80
80
80
80
80
100
50
RT
80
80
80
80
80
80
99
78
64
63
45
53
99
70
33
99
21
99
99
38
58
507
399
327
322
230
271
507
358
169
1237
—
127
100
82
80
57
68
127
90
42
309
—
—
CH2Cl2
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
9
10c
11d
12e
13f
14g
15h
1237
1237
475
725
309
309
119
362
a
Reaction conditions: 1a (5 mmol), Me2NHÁBH3 (10 mmol), B(C6F5)3
b
(0.2 mol%), CH3CN (3 mL), CO2 (3.0 MPa), time (4 h). Determined by
c
d
e
using the GC. B(C6F5)3 (0.08 mol%,) Without B(C6F5)3. 2 MPa.
f
g
h
1 MPa. Me2NHÁBH3 (1 mmol). 2 h. TON/TOF (hÀ1).
it was found that the use of 1 MPa pressure also provided 99%
yield of 2a (Table 1, entry 13). Then, we decreased the amount
of Me2NHÁBH3 and a lower yield of 2a was noted (Table 1, entry
14). Next, we studied the effect of time on reaction conditions
and it was found that 4 h was the optimum time to attain the
highest yield (Fig. 1). Although 2 h of reaction resulted in good
TOF, only 58% yield of 2a was noted.
After the optimization of reaction conditions, various aliphatic
and aromatic primary and secondary amines with different
functional groups were examined (Table 2). Initially, a series
of cyclic aliphatic secondary amines were tested for the formylation
reaction and it was observed that they provided excellent yields
(Table 2, 2a–5a). The cyclic primary amine, cyclohexane amine, also
provided good yields (Table 2, 6a). Next, simple symmetric aliphatic
amines such as dibutyl amine, diethylamine and dimethylamine
were tested under the optimized reaction conditions and good
Results and discussion
The formylation of N-morpholine 1a to 4-formylmorpholine 2a
with CO2 as a C1 source and Me2NHÁBH3 as a mild reducing
agent under 3 MPa of CO2 pressure for 4 h was chosen as a
model reaction system for the optimization of reaction conditions at
80 1C. Initially, we investigated the effect of solvents on the
reaction conditions in the presence of 0.2 mol% catalyst. The
use of acetonitrile as the solvent provided an excellent yield of
N-formylated product (99%) (Table 1, entry 1). Toluene as a non-
polar solvent also provided a good yield of 2a (Table 1, entry 2).
Other solvents, 1,4-dioxane and THF, gave a moderate yield of
2a (Table 1, entries 3 and 4). However, the reaction with solvent-
free condition provided only 45% yield of 2a (Table 1, entry 5). A
chlorinated solvent such as CH2Cl2 provided 45% yield of 2a
(Table 1, entry 6).
Next, we studied the effect of temperature on the reaction
system (Table 1, entries 7–9). The use of higher temperature i.e.
100 1C provided a similar yield to that at 80 1C (Table 1, entry 7),
while decreasing the temperature of the reaction system to
50 1C resulted in a lower yield of 2a (Table 1, entry 8). At room
temperature, a low yield of 2a was noted (Table 1, entry 9). Next,
we studied the effect of catalyst loading and it was found that
the use of only 0.08 mol% B(C6F5)3 also provided a similar yield
to that when 0.2 mol% B(C6F5)3 was used with a high TON and
TOF (Table 1, entry 10). The control experiment without catalyst
resulted in 21% yield of 2a (Table 1, entry 11).
Next, the effect of CO2 pressure was studied (Table 1, entries
12 and 13). The pressure was lowered from 3 MPa to 1 MPa and
Fig. 1 Effect of time on reaction conditions for the formylation reactions.
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Table 2 Substrate scope for the formylation of aminesa
Table 3 Cyclization of 1,2-diaminobenzene using CO2
a
a
Reaction conditions: 1,2-diaminobenzene (5 mmol), Me2NHÁBH3
(10 mmol), CO2 (1 MPa), B(C6F5)3 (0.08 mol%), CH3CN (3 mL), 4 h,
100 1C. TON/TOF (hÀ1). Determined by GC/MS. N.D = not detected.
interest due to use of CO2 as a C1 source.21 The B(C6F5)3
catalyzed reaction of o-phenylenediamine with CO2 and
Me2NHÁBH3 was utilized for the N-cyclization reaction and an
excellent yield of benzimidazole was noted (Table 3, 3b). Then,
N-substituted diamine, N-methyl-1,2-diaminobenzene, also provided
an excellent yield of substituted benzimidazole (Table 3, 4b). Then,
fluoro-substituted diamine, 4-fluoro-1,2-diaminobenzene, was tested
for the cyclization reaction and about 60% yield was noted
(Table 3, 5b). Next, methyl-substituted 1,2-diaminobenzene was
also tested and good yield was noted (Table 3, 6b). No cyclized
product was obtained in the case of 2,3-diaminophenol (Table 3, 7b).
Furthermore, a tentative reaction mechanism for the B(C6F5)3
catalyzed N-formylation and N-cyclization reactions via FLP
a
Reaction conditions: amines (5 mmol), Me2NHÁBH3 (10 mmol), CO2
(1 MPa), B(C6F5)3 (0.08 mol%), CH3CN (3 mL), 4 h, 80 1C. Yield
determined by using the GC/MS. TON/TOF (hÀ1).
yields of formylated products were noted (Table 2, 7a, 8a and 9a).
Then, a range of aromatic substrates such as aniline, substituted
anilines and N-substituted anilines were tested and good to
excellent yields of the corresponding N-formylated products
were noted (Table 2, 10a, 11a, 12a and 13a). Next, methyl
substituted benzylamine and benzylamine were tested for the
formylation reactions and good were noted (Table 2, 14a and 15a).
After the successful synthesis of various substrates, we moved to
cyclization of 1,2-diaminobenzenes with CO2 and Me2NHÁBH3 to
synthesize benzimidazoles (Table 3). 1,2-Diaminobenzenes also
react with B(C6F5)3 and form the FLP as shown spectroscopically
by Sun et al. (Scheme 1b).21a Benzimidazoles are important scaffolds
present in various biologically active molecules and natural
products. Among the various reported methods, the synthesis
of benzimidazoles from CO2 and 1,2-diamines are of intense
Scheme 3 Proposed reaction mechanism for the formylation of amines
using CO2 and Me2NHÁBH3.
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Experimental
For more details please see the ESI† section.
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the N-formylation of amines using B(C6F5)3 as a highly efficient
metal-free catalyst for the tandem dehydrogenation of Me2NHÁBH3
and reductive functionalization of CO2 under mild reaction
conditions. A wide variety of substrates with a range of functional
groups were synthesized under mild conditions with high TON
and TOF values. Me2NHÁBH3 provides high atom economy and
after completion of the reaction, Me2NHÁBH3 was easily removed
using water. Additionally, this catalytic protocol was successfully
able to cyclize 1,2-diaminobenzene and various substituted 1,2-
diaminobenzenes to provide the corresponding benzimidazoles
using CO2 and Me2NHÁBH3.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The author Vitthal Saptal acknowledges the University
Grant Commission (UGC) for providing the senior research
fellowship (SRF).
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