Hydrophosphination of CO2 and Subsequent Formate Transfer in the 1,3,2-Diazaphospholene-Catalyzed N-Formylation of Amines

Article abstract of DOI:10.1002/anie.201505244

Hydrophosphination of CO₂ with 1,3,2-Diazaphospholene (NHP-H; 1) afforded phosphorus formate (NHP-OCOH; 2) through the formation of a bond between the electrophilic phosphorus atom in 1 and the oxygen atom from CO₂, along with hydride transfer to the carbon atom of CO₂. Transfer of the formate from 2 to Ph₂SiH₂ produced Ph₂Si(OCHO)₂ (3) in a reaction that could be carried out in a catalytic manner by using 5 mol % of 1. These elementary reactions were applied to the metal-free catalytic N-formylation of amine derivatives with CO₂ in one pot under ambient conditions.

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International Edition: DOI: 10.1002/anie.201505244
German Edition: DOI: 10.1002/ange.201505244
CO2 Activation
Hot Paper
Hydrophosphination of CO2 and Subsequent Formate Transfer in the
1,3,2-Diazaphospholene-Catalyzed N-Formylation of Amines
Che Chang Chong and Rei Kinjo*
Abstract: Hydrophosphination of CO2 with 1,3,2-Diazaphos-
pholene (NHP-H; 1) afforded phosphorus formate (NHP-
OCOH; 2) through the formation of a bond between the
electrophilic phosphorus atom in 1 and the oxygen atom from
CO2, along with hydride transfer to the carbon atom of CO2.
Transfer of the formate from 2 to Ph2SiH2 produced Ph2Si-
(OCHO)2 (3) in a reaction that could be carried out in
a catalytic manner by using 5 mol% of 1. These elementary
reactions were applied to the metal-free catalytic N-formyla-
tion of amine derivatives with CO2 in one pot under ambient
conditions.
T
he valorization of carbon dioxide (CO2) has attracted much
attention in synthetic chemistry because of the potential use
of CO2 as a nontoxic, abundant, and inexpensive C1 source
for the production of useful chemicals.[1,2] However, the
considerable thermodynamic stability of CO2 hampers facile
activation. To address this issue, several metal catalysts have
been employed for the transformation of CO2, which has
afforded various commodity chemicals such as formic acid
derivatives, methane, methanol, CO, and formaldehyde.[3]
Metal-free catalytic systems for CO2 valorization present
an alternative method to the prevalent use of conventional
metal catalysts.[4,5] The seminal work by Grimme, Stephan,
Erker, et al. in 2009 demonstrated the potential of frustrated
Lewis pairs (FLPs), consisting of a Lewis base (LB) and Lewis
acid (LA), for CO2 capture.[6,7] Since then, a variety of systems
based on main-group compounds have been developed for
the activation of CO2.[8] Depending on the binding modes to
CO2, these systems are mainly classified into three types: one
LB adduct (I), one LB and one LA adduct (II), and one LB
and two LA adducts (III; Figure 1a).[9] These are also
recognized as key structures of initial intermediates in the
functionalization of CO2 by main-group catalysts. For
instance, hydroboration of CO2 catalyzed by main-group
compounds is inferred to proceed via the activation of CO2 to
Figure 1. a) Three coordination modes in the activation of CO2 by
À
main-group compounds. b) Direct insertion of [E] H bonds into CO2.
c) Examples of products and an intermediate from direct insertion of
V
À
the P N bond into CO2. d) Generic structure of 2-H-1,3,2-diazaphos-
pholene (1).
block hydrides [E]-H (IV) gives rise to formate derivatives
À
(V) via insertion of the E H bond (Figure 1b). In this process,
the main-group fragment [E] and the H atom of IV bond to an
oxygen atom and the carbon atom of CO2, respectively. The
À
À
E H bond thus has to be polarized as E(d +) H(d-) so that it
concomitantly displays hydride-donor ability and electro-
philic nature at the [E] moiety. As such, extant examples are
limited to group 13–14 elements such as B,[11] Al,[12] Ga,[13]
Ge,[14] and Sn.[15]
With group 15 elements (Figure 1c), Cavell et al. reported
V
À
À
the insertion of CO2 into the P N bond of Me(F3C)3P NMe2
over two days, which afforded the hexacoordinate phosphorus
derivative VI.[16] Stephan et al. demonstrated that amido-
phosphoranes containing strained four-membered rings read-
ily reacted with CO2 to produce PV heterocycles (VIIa–b).[17]
Recently, Fontaine et al. described the activation of CO2 with
phosphazenes, in which an intermediate (VIII) involving CO2
À
=
give insertion of the B H bond into the activated C O
bond.[8g–h,10a] However, recent studies by Fontaine and Bour-
issou suggest that such a mechanistic pathway often involves
a high activation barrier.[10b–d] Meanwhile, it has been
reported that the direct reduction of CO2 by main-group p-
insertion into the PV N bond was proposed.
In those
[18]
À
reactions, the P centre in the + V oxidation state acted as an
electrophile to form a bond with an O atom of CO2. Relevant
sequestration of CO2 with phosphines (R3PIII) is extremely
[*] C.-C. Chong, Prof. Dr. R. Kinjo
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University
Nanyang Link 21, Singapore 637371 (Singapore)
E-mail: rkinjo@ntu.edu.sg
rare,[19,20] and to the best of our knowledge, formation of
a phosphorus(III) formate (R2PIII OCOH) via P H bond
insertion into CO2, that is, hydrophosphination, has not thus
far been achieved. This is presumably because phosphines are
typically nucleophilic, and therefore the interaction with CO2
III
À
À
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201505244.
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should predominantly generate adduct I (Figure 1a) rather
We postulated that the downfield 31P NMR chemical shift
[2d,8p–q]
than the PIII H bond insertion product.
Given the
and the long P O bond in 2 indicate an ionic property of the
À
À
significance of CO2 valorization, it is highly desirable to
develop diverse strategies for the capture of CO2 and its
transformation into value-added compounds.
bond that induces zwitterionic character (2’; Scheme 1a),
from which the formate group should be readily transferred to
the appropriate acceptor. To bear out our hypothesis, we
examined the reaction of 2 with a hydrosilane because the
Recently, we reported that 1,3,2-diazaphospholene (1),
which exhibits zwitterionic properties (1’; Figure 1d),[21]
effectively promotes the hydroboration of carbonyl com-
pounds.[22] The reaction mechanism involved a facile insertion
À
expected formation of a strong Si O bond could be a driving
force for the reaction. Under CO2 atmosphere, treatment of 2
with a half equivalent of Ph2SiH2 afforded Ph2Si(OCHO)2 (3)
as the major product, concomitant with the formation of the
siloxane 4 (3/4 = 2.3:1 after 15 min; Scheme 1c). We also
observed that 3 was converted into 4 after 18 h under the
reaction conditions (Figures S1,S2 in the Supporting Infor-
mation). The formation of 3 demonstrates the transfer of
formate from 2 to Ph2SiH2, along with regeneration of 1.
During the reaction, however, only a peak for 2 was detected
in the 31P NMR spectrum, probably owing to a rapid reaction
between the in situ regenerated 1 and CO2. Therefore, we
À
=
of the exocyclic P H bond of 1 into the C O bond at the
initial step. This result prompted us to investigate the
reactivity of 1 towards CO2 since it possesses consecutive
=
C O bonds. Herein, we report a facile CO2 insertion into the
À
P H bond of 1 to form the phosphorus formate 2. Transfer of
the formate from 2 to silane, and its application in the
catalytic N-formylation of amines are also described.
A CD3CN solution of 1 was exposed to 1 atm of CO2 at
ambient temperature. Instantaneously, the yellow solution
darkened to a deep brown color, showing a singlet at
111.5 ppm in the 31P NMR spectrum, which is shifted down-
field compared to that (57.8 ppm) of 1. After workup, product
2 was isolated as a brown solid in 93% yield (Scheme 1a). The
À
examined the subsequent addition insertion of the P H bond
of 1 into CO2 followed by formate transfer from 2 to silane in
a catalytic process.
CO2 was introduced into a J-young NMR tube filled with
a CD3CN solution of Ph2SiH2 and 5 mol% of 1, and the
reaction was monitored by NMR spectroscopy. Within 1 h at
room temperature, the conversion of Ph2SiH2 into 3 (> 95%)
was confirmed, and only small amounts of siloxane 4 (< 5%)
were detected (Scheme 2, Figure S3-1). We also performed
Scheme 2. Catalytic hydrosilylation of CO2 with 5 mol% of 1.
Scheme 1. a) Reaction of 1 with CO2 (13CO2). b) Solid-state structure of
2. Hydrogen atoms, except for H11, are omitted for clarity. c) Formate
transfer from 2 to Ph2SiH2 under CO2 atmosphere.
a
13C-labeling experiment for the catalytic reaction under
13CO2 atmosphere and confirmed the clean formation of
Ph2Si(O13CHO)2 (3-13C; 1JH-C = 232.5 Hz; Figures S3,S4).
Recently, catalytic N-formylation and N-methylation of
amines using CO2 as the carbon source were reported with
various organocatalysts.[23,24] In these reactions, formate
derivatives are considered to be key intermediates for the
formation of formamides.[23–25] Therefore, we attempted the
one-pot N-formylation of amines with 1 as a catalyst. To
investigate the reaction, N-methylaniline was utilized as a test
substrate. After a brief screening of the reaction conditions
(Table S3–1), we obtained the optimized conditions as shown
in Scheme 3. Significantly, when 13CO2 was used, 13C-labelled
N-methyl-N-phenylformamide was obtained (see the Sup-
porting Information).
1H NMR spectrum of 2 shows a characteristic peak at
8.10 ppm while the 13C{1H} NMR spectrum shows a singlet
at 164.2 ppm. We also carried out a 13C-labelling study with
13CO2, which produced 2-13C (90% yield). The 1H NMR
spectrum of 2-13C displays a doublet at 8.13 ppm that is due to
coupling (1JH-C = 210.8 Hz) with the 13C atom arising from
13CO2 (see the Supporting Information). In the solid-state IR
=
spectrum of 2, a peak corresponding to the C O stretching
vibration was detected at 1720 cmÀ1. These results indicate
the presence of a formate group (OCHO) in 2, which was
decisively confirmed by X-ray diffractometry (Scheme 1b).
À
The P1 O1 distance (1.808(1) Š) is 11% longer than a typical
À
P O single bond (1.63 Š). The formation of phosphorus
formate 2 rather than adduct I (Figure 1a) demonstrates that
one of the O atoms in CO2 attacks the electrophilic P atom,
whereas the C atom in CO2 accepts the H atom as a hydride
from 1. Note that this result presents the first example of
hydrophosphination of CO2.
Scheme 3. N-formylation of PhMeNH with CO2 catalyzed by 1.
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Table 1: Scope of catalytic N-formylation of secondary amines with CO2.
Table 2: Scope of the catalytic N-formylation of primary amines with
CO2.
5
t [h]
0.5 6a
7b
Product Yield [%][a] TON TOF [hrÀ1
]
8
t [h] Product Yield [%][a] TON TOF [hrÀ1
]
5a
5b
95
61
19
12
38
6
8a
2
9a
9b
84
97
17
19
9
1
2
8b
18
5c
5d
5e
5 f
0.5 6c
94
53
94
87
19
11
19
17
38
2
8c
8d
8e
24
10
10
9c
9d
9e
85
>99
72
17
20
14
0.7
2
7
2
5
5
7d
6e
6 f
6g
10
4
1
8 f
8g
8h
4
2
2
9 f
97
91
99
19
18
20
5
9
5g
5h
93
78
19
16
4
3
9g
9h
6
6h
10
8i
8j
2
2
9i
9j
82
90
16
18
8
9
5i
5j
5k
18
6
6i
6j
6k
71
94
64
14
19
13
1
3
[a] Yields were determined by 1H NMR spectroscopy with 1,3,5-trime-
thoxybenzene as an internal standard.
23
0.6
[a] Yields were determined by 1H NMR spectroscopy with 1,3,5-trime-
thoxybenzene as an internal standard.
With the optimized conditions in hand, we examined the
scope of the catalytic reaction with various secondary amines
(Table 1). Sterically less-hindered aliphatic amines (5a, 5c,
5e) afforded the corresponding N-formylamines in excellent
yields (6a 95%, 6c 94%, 6e 94%). Meanwhile, reactions with
sterically hindered and highly basic amines such as diisopro-
pylamine (5b) and 2,2,4,4,-tetramethylpiperidine (5d),
afforded N-methylated amines (7b: 61%, 7d: 53%), which
were probably formed by subsequent reduction of the
corresponding N-formylamines (6b, 6d).[24,26] Secondary
amines containing aryl substituents (5 f–k) were well toler-
ated and the corresponding formamides (6 f–k) were pro-
duced in good to excellent yields (94–64%). To expand the
scope of the reaction, we also tested primary amines
(Table 2). All of the amines employed (8a–j) were well
tolerated and the corresponding formamides (9a–j) were
produced in 72–99% yields.
To gain insight into the reaction mechanism, we per-
formed control reactions. In the absence of silane, a mixture
of N-methylaniline and 1 under CO2 atmosphere only
afforded 2, and no further reaction between 2 and N-
methylaniline was observed. We also confirmed that N-
methylaniline does not react with Ph2SiH2. Similarly, no
apparent reaction was observed between 1 and silane. Mean-
Scheme 4. Proposed mechanism for the N-formylation of amines with
Ph2SiH2 and CO2 catalyzed by 1.
while, addition of N-methylaniline to Ph2Si(OCHO)2 (3) gave
N-formyl-N-methylaniline after 2 h at room temperature in
the absence of 1 and 2. Therefore, we propose a mechanism
(Scheme 4) for the N-formylation of amines that involves the
À
following three steps: 1) insertion of the P H bond of 1 into
CO2 to generate 2, 2) transfer of a formate from 2 to Ph2SiH2
to form Ph2Si(OCOH)2 (3) along with the regeneration of 1,
3) formation of formamides and also silane byproducts such
as silanols and (Ph2SiO)3 through the reaction of 3 with
amines. Catalyst 1 thus acts as a reducing agent for CO2 to
form 2, which subsequently transfers its formate moiety to
silane. This mechanism differs from that proposed for the
NHC–silane system in the N-formylation of amines,[23,24]
where an initial activation of silane by NHC is energetically
more favorable than the mechanism encompassing the
formation of an NHC–CO2 adduct.[26]
In summary, we have demonstrated the first hydrophos-
phination of CO2 with 1, subsequent transfer of formate from
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Acknowledgements
We are grateful to the Singapore Ministry of Education
(MOE2013-T2-1-005) and Nanyang Technological University
(NTU) for financial support. We thank to Dr. Rakesh
Ganguly (NTU) for assistance with X-ray crystallographic
analysis.
Keywords: CO2 activation · hydrophosphination ·
hydrosilylation · metal-free catalysis · N-formylation
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Received: June 10, 2015
Revised: July 5, 2015
Published online: August 14, 2015
12120 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 12116 –12120