Effective Formylation of Amines with Carbon Dioxide and Diphenylsilane Catalyzed by Chelating bis(tzNHC) Rhodium Complexes

Article abstract of DOI:10.1002/anie.201504072

The reductive formylation of amines using CO₂ and hydrosilanes is an attractive method for incorporating CO₂ into valuable organic compounds. However, previous systems required either high catalyst loadings or high temperatures to achieve high efficiency, and the substrate scope was mostly limited to simple amines. To address these problems, a series of alkyl bridged chelating bis(NHC) rhodium complexes (NHC=N-heterocyclic carbene) have been synthesized and applied to the reductive formylation of amines using CO₂ and Ph₂SiH₂. A rhodium-based bis(tzNHC) complex (tz=1,2,3-triazol-5-ylidene) was identified to be highly effective at a low catalyst loading and ambient temperature, and a wide substrate scope, including amines with reducible functional groups, were compatible. Beyond the norm: Rhodium complexes bearing a strong electron-donating bis(1,2,3-triazol-5-ylidene) ligand were found to be excellent catalysts for the reductive formylation of amines with CO₂ and Ph₂SiH₂ at ambient temperature. The catalyst system possesses a broad substrate scope which tolerates a variety of reducible functional groups and is suitable for the synthesis of bioactive compounds. Tf=trifuoromethanesulfonyl.

Full text of DOI:10.1002/anie.201504072

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201504072
German Edition: DOI: 10.1002/ange.201504072
Synthetic Methods
Effective Formylation of Amines with Carbon Dioxide and
Diphenylsilane Catalyzed by Chelating bis(tzNHC) Rhodium
Complexes**
Thanh V. Q. Nguyen, Woo-Jin Yoo, and Shu¯ Kobayashi*
Abstract: The reductive formylation of amines using CO2 and
hydrosilanes is an attractive method for incorporating CO2 into
valuable organic compounds. However, previous systems
required either high catalyst loadings or high temperatures to
achieve high efficiency, and the substrate scope was mostly
limited to simple amines. To address these problems, a series of
alkyl bridged chelating bis(NHC) rhodium complexes
(NHC = N-heterocyclic carbene) have been synthesized and
applied to the reductive formylation of amines using CO2 and
Ph2SiH2. A rhodium-based bis(tzNHC) complex (tz = 1,2,3-
triazol-5-ylidene) was identified to be highly effective at a low
catalyst loading and ambient temperature, and a wide substrate
scope, including amines with reducible functional groups, were
compatible.
iron[6e]) as active catalysts. However, in all cases, a high
catalyst loading and/or high temperature were necessary for
this reductive coupling process. Furthermore, the substrate
scope for this transformation is mostly limited to simple
amines. Therefore, the development of new types of catalysts,
which can facilitate the reductive formylation reaction both at
a low catalyst loading and ambient temperature, is of great
interest.
To achieve this goal, we turned our attention to the
application of N-heterocyclic carbene (NHC)/metal com-
plexes as potential catalysts. Based on the fact that the
common intermediate in metal-catalyzed hydrosilylation
reactions is the metal hydride complex,[7] we rationalized
that the strong electron-donating ability of NHC ligands may
increase the nucleophilicity of the metal hydride species and
facilitate the reduction of the weakly electrophilic CO2.
Moreover, the ability of NHCs to form strong bonds with
various transition metals would lead to more robust catalysts
and would permit lower catalyst loadings without causing
catalyst decomposition.[8] Although the use of electron-rich
NHC/metal complexes to facilitate nucleophilic additions to
CO2, to afford carboxylic acids or esters, is well known,[1]
there are few examples of its use for catalytic hydrosilylation
reactions of CO2.[6i,9] To the best of our knowledge, a selective
NHC/metal-catalyzed formylation of amines with CO2 has
not been reported.
Recently, bis(NHC) complexes of rhodium were found to
be effective catalysts for the hydrosilylation of ketones at
ambient temperature.[10] These bis(carbene) ligands not only
improved the stability of the rhodium complexes against
decomposition, but also enabled better control of the steric
and electronic properties around the metal center.[10a] We
hypothesized that these types of metal complexes could be
promising catalysts for hydrosilylation reactions of CO2.
Furthermore, since the high electron density of NHC/metal
complexes may be crucial for the favorable interaction
between CO2 and the key metal hydride intermediate, we
envisioned that modifying the ligands from normal to
abnormal NHCs should generate better catalysts because of
the superior electron-donating ability of the latter.[11] Such an
effect was previously observed for the copper-catalyzed
carboxylation reaction of benzoxazoles and benzothiazoles
with CO2,[12] and the iridium-catalyzed transfer hydrogenation
of CO2 with 2-propanol.[13] Among various types of abnormal
NHCs, tzNHCs (tz = 1,2,3-triazol-5-ylidene) stand out as
excellent candidates because they are easily accessed and
diversified by the [3+2] cycloaddition of azides and
alkynes.[14] Herein, we report the modular synthesis of alkyl-
bridged bis(tzNHC) rhodium complexes and their application
T
he incorporation of CO2 into organic compounds is highly
desirable since CO2 is a low-cost, abundant, and nontoxic raw
material. However, CO2 represents the highest oxidation
state of carbon and this limits its synthetic utility to the
formation of low-energy synthetic targets by CO2 insertion
reactions or to multielectron chemical reductive processes to
generate energy-rich small molecules such as formic acid,
carbon monoxide, methanol, and methane.[1,2] With respect to
catalytic reduction of CO2, hydrogen gas is the cleanest and
most atom-economical reductant, but harsh reaction condi-
tions, such as a high pressure of a mixture of gases and high
reaction temperature, prevents broad application of this
reducing agent. In contrast, the use of boranes[3] and hydro-
silanes[4] as reductants facilitates the catalytic reduction of
CO2 under much milder reaction conditions. In a related
process, the reduction of CO2 in the presence of amines
ultimately affords formamides or methylamines, which are
value-added bulk and fine chemicals.[5,6] Since the initial
report by Cantat and co-workers on the catalytic reductive
formylation of amines with CO2 and silanes,[6a] rapid develop-
ment in this field has led to the disclosure of various
organocatalyts[6a,b] and organometallic complexes (copper,[6c,d]
[*] T. V. Q. Nguyen, Dr. W.-J. Yoo, Prof. Dr. S. Kobayashi
Department of Chemistry, School of Science
The University of Tokyo
7-3-1 Hongo, Bunkyo, Tokyo (Japan)
E-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp
[**] This work was partially supported by a Grant-in-Aid for Scientific
Research from the Japan Society for the Promotion of Science
(JSPS), the Japan Science and Technology Agency (JST), and the
Ministry of Education, Culture, Sports, Science and Technology
(MEXT). tzNHC=1,2,3-triazol-5-ylidene N-heterocyclic carbene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504072.
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as effective catalysts for the reductive formylation of amines
with CO2 and Ph2SiH2.
With the optimized reaction conditions in hand,[16] we
began to investigate the substrate scope of this reaction
(Table 2). The use of dibenzylamine (4a) provided the desired
formamide 5a in 95% yield upon isolation (entry 1). In
addition, the reaction could be conveniently scaled up to
a gram scale without any loss of activity. Similarly, substitu-
tion of a benzyl group with an alkyl group, such as methyl (4b)
or ethyl (4c) did not affect the reactivity (entries 2–3).
Dioctylamine (4d), possessing two long alkyl chains, was
also found to be a suitable substrate for this reaction (entry 4).
Various cyclic aliphatic amines (4e–h) were tested and
excellent yields of the cyclic formamides 5e–h were obtained
(entries 5–8). When the primary aliphatic amines 4i–o were
used, the desired reactions proceeded selectively to provide
the monoformylated products 5i–o (entries 9–15). Notably,
the formylation of (R)-4j proceeded without any racemiza-
tion. The use of trans-1,2-cyclohexanediamine (4p) provided
To access the desired bis(tzNHC)-ligated rhodium com-
plexes, the standard method of utilizing silver NHCs as
carbene transmetallation agents, a method reported for the
synthesis of the bis(imNHC) rhodium complexes 1 and 2
(im = imidazole-2-ylidene), was applied (Figure 1).[15] In
addition to KPF6, different anion sources were examined to
generate a series of rhodium complexes (3a–f).[16]
The catalytic activity of the newly synthesized metal
complexes was investigated for the formylation of dibenzyl-
amine (4a) with CO2 as a C1 source and Ph2SiH2 as the
reductant (Table 1). Rhodium complexes containing the
imNHC backbone (1 and 2) were initially examined as
catalysts, but the desired formamide 5a was not observed
(entries 1 and 2). In contrast, the complex 3a, bearing
a tzNHC backbone, showed high activity, even at lower
catalyst loadings (entries 3 and 4). Encouraged by these
results, we began to screen various bis(tzNHC) rhodium
complexes (3b–f) bearing a trimethylene linker (entries 5–9).
By varying the catalyst structure, we found that the counter-
anions influenced the activity of these catalysts. Although no
clear trend was uncovered, we found that the rhodium
complexes possessing either ÀOTf, ÀNTf2, or ÀBPh4 counter-
anions enjoyed the highest catalytic activity (entries 6–8).
Moreover, since NHCs are known as effective organocata-
lysts for this type of reaction,[6b] we also examined the use of
the tzNHC precursor as a catalyst (entry 10). In this case, 5a
was not observed, thus indicating that the real catalyst species
in our system was not the dissociated free carbene. Based on
these results, 3c was chosen as the best catalyst for the
reductive formylation reaction of dibenzylamine (4a) with
CO2 and Ph2SiH2.
Table 2: Substrate scope for the reductive formylation of amines (4a–v)
with CO2 and Ph2SiH2.[a]
Entry
Substrate
Product
Yield [%][b]
1
2
3
4a R=Bn
4b R=Me
4c R=Et
95 (quant.)[c]
quant.
92
4
5
4d
4e
88
99
6
4 f X=CH2
4g X=O
4h X=NPh
99
89
97
7
8[d]
9
4i R=H
96
10
4j R=Me[e]
4k R=Ph
quant.[f]
quant.
11[d]
12
4l R=nHex
quant.
85
82
13
4m R=cHex
4n R=TBSO(CH2)3
4o R=tBu
Figure 1. Chelating bis(NHC) rhodium complexes used in this work.
Tf =trifuoromethanesulfonyl.
14[d]
15[d]
60
16[d,g]
4p (rac)
75
Table 1: Optimization of the reductive formylation of dibenzylamine (4a)
with CO2 and Ph2SiH2.[a]
17[d]
18[d]
19[d]
20[d,h]
21[d]
22[d]
23[d]
4q R=4-Me
4r R=4-tBu
4s R=4-OMe
4t R=4-Br
4u R=3-Me
4v R=2-Me
4w R=2,6-Me2
89
quant.
95
40
80
Entry [Rh] (mol%) Yield [%][b]
Entry [Rh] (mol%) Yield [%][b]
1
2
3
4
5
1 (0.5)
2 (0.5)
3a (0.5)
3a (0.1)
3b (0.1)
n.d.
n.d.
>95
80
83
6
7
8
3c (0.1)
3d (0.1)
3e (0.1)
3 f (0.5)
>95
>95
>95
85
30
n.d.
9
[a] Reaction conditions: Amines 4a–v (0.50 mmol), Ph2SiH2
(1.25 mmol), and 3c (0.50 mmol) in CH2Cl2 (2 mL) under 25 atm of CO2.
[b] Yields of the isolated products 5a–v were based on 4a–v. [c] Reaction
using 5.03 mmol of 4a. [d] 24 h. [e] Enantiopurity of 4j (96.8% ee) was
determined after formylation with ethyl formate. [f] 98.7% ee. [g] Used
0.25 mol% of 3c and 5 equiv of Ph2SiH2. [h] Used 0.5 mol% of 3c.
TBS=tert-butyldimethylsilyl.
[c]
10
–
n.d.
[a] Reaction conditions: 4a (0.25 mmol), Ph2SiH2 (0.625 mmol), and
[Rh] (0.25–1.25 mmol) in CH2Cl2 (1 mL) under 25 atm of CO2. [b] Yields
were determined by 1H NMR analysis using 1,1,2,2-tetrachloroethane as
an internal standard. [c] Bis(triazolium) salt (5 mol%) and KOtBu
(10 mol%) was used as the catalyst. n.d.=not detected.
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the diformamide product 5p in good yield (entry 16). Next,
we evaluated aniline derivatives (4q–w) as substrates
(entries 17–23). Reactions of anilines bearing electron-donat-
ing groups on the para position (4q–s) reacted, albeit with
a prolonged reaction time because of the lower nucleophi-
licity of anilines when compared to aliphatic amines
(entries 17–19). The importance of the nucleophilicity of the
aniline derivatives was highlighted when we examined 4-
bromoaniline (4t) as a substrate. In this case, the effectiveness
of the formylation reaction was decreased, even under higher
catalyst loading (entry 20). In addition, sterically hindered
anilines, such as 2-methylaniline (4v) and 2,6-dimethylani-
lines (4w), were found to be poor substrates (entries 22–23).
Since the newly developed bis(tzNHC) rhodium complex
3c was found to be an exceptionally effective catalyst for the
reduction of CO2, we then examined the possibility of
expanding the substrate scope (Table 3). We hypothesized
that under high pressure CO2, preferential reduction of the
large excess of CO2 over other functional groups should occur
and that our catalytic system could tolerate amines which
possess reducible functional groups. In spite of the fact that
NHC rhodium complexes are well known to be excellent
catalysts for the hydrosilylation of ketones,[7b] the piperazine
derivative 4x, which contains a ketone moiety, was success-
fully formylated to afford 5x in high yield (entry 1). Similarly,
the piperazine derivatives 4y–aa, bearing amide functional
groups, also reacted smoothly to generate the corresponding
formamides 5y–aa in good to excellent yields (entries 2–4).
Furthermore, it was found that amines bearing alkenyl (4ab),
alkynyl (4ac), and ester (4ad) moieties were successfully
formylated in moderate to excellent yields (entries 5–7). In
addition, the amino esters 4ae and 4af were found to be
viable substrates for this reaction to provide the monoformy-
lated products 5ae–af (entries 8 and 9). In the case of 4ae, the
reaction occurred with the retention of the original config-
uration. Finally, the synthetic utility of this methodology was
demonstrated by the formylation of the a-aminoketone 4ag
to afford 5ag, which is a key intermediate in the total
synthesis of Iguratimod (an anti-inflammatory drug),[17] in
quantitative yield (entry 10).
Several possible reaction pathways were considered for
the reductive formylation reaction of amines with CO2 and
Ph2SiH2 as catalyzed by the rhodium complex 3c. One
possibility is that CO2 may initially be hydrosilylated to
generate a silyl formate, which could be intercepted by
a nucleophilic amine to furnish the expected formamide.
However, this pathway was ruled out since an attempted
reduction of CO2 by Ph2SiH2, in the absence of an amine,
failed.[6i,16] An alternative reaction pathway may involve the
rapid formation of an ammonium carbamate, derived from
the amine and CO2, followed by the reduction of this salt by
hydrosilane and 3c.[6c,d] This possibility was examined by using
a benzylamine/CO2 adduct[18] as a substrate. Since the
formamide 5i was obtained only under high pressure of
CO2,[16] it is unclear if this adduct could be directly reduced.
However, since CO2 and the amine/CO2 adduct coexist in
equilibrium and the reduction of CO2 should be favored over
the adduct because of its higher electrophilicity,[19] we
assumed that CO2 was reduced by the rhodium hydride
intermediate.
Based on these control studies and the previously reports
on rhodium-catalyzed hydrosilylation reactions of carbonyl
compounds,[7b] a plausible mechanism is shown in Scheme 1.
Initially, dissociation of the auxiliary ligand (cod) from 3c,
followed by oxidative addition of Ph2SiH2 and subsequent
hydride transfer from the silyl ligand to the rhodium center
would generate the rhodium silylene intermediate A. Such an
intermediate was previously proposed in the Hofmann–Gade-
type mechanism,[20] and indirect evidence for its formation
was obtained by silylene trapping experiments:[10b] the
reaction of Ph2SiH2 with tBuMe2SiOH occurred smoothly in
the presence of 3c, while the use of monohydrosilane, such as
Ph2MeSiH, resulted in no reaction.[16] Consequently, no
Table 3: Reductive formylation of amines (4x–ag) with reducible func-
tional groups.[a]
Entry
1
Substrate
Product
Yield [%][b]
90
2
3
4
4y R=C6H5
4z R=4-BrC6H4
4aa R=OtBu
85
quant.
79
=
5
6
7
4ab R=CH CH2
quant.
63
90
ꢀ
4ac R=C CH
4ad X=CO2Me
8[c,d]
4ae
40[e]
9
4af
quant.
10[f,g]
quant.
[a] Reaction conditions: Amines (4x–ag; 0.50 mmol), Ph2SiH2
(1.25 mmol), and 3c (0.50 mmol) in CH2Cl2 (2 mL) under 25 atm of CO2.
[b] Yields of the isolated products 5x–ag were based on 4x–ag. [c] Et3N
(0.50 mmol) was added. [d] Enantiopurity of 4ae (98.2% ee) was
determined after formylation with trimethyl orthoformate. [e] 96.1% ee.
[f] 4ag (0.125 mmol), 3c (1 mol%). [g] Et3N (0.25 mmol) was added.
Scheme 1. Plausible reaction mechanism.
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[4] F. J. Fernµndez-Alvarez, A. M. Aitani, L. A. Oro, Catal. Sci.
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catalytic formylation of 4a, to give 5a, with CO2 was observed
when different monohydrosilanes, which cannot form the key
rhodium silylene intermediate A, were used as the reductants.
Next, coordination of CO2 to A followed by a pseudo-
intramolecular hydride transfer from the rhodium to CO2
would generate the rhodium silyl formate B. The nucleophilic
attack of the amine on B would afford C and subsequent
reductive elimination of C would furnish the silyl intermedi-
ate D and close the catalytic cycle. Finally, elimination of the
silanol E would result in the formation of the desired
formamide. Although E was not directly observed, indirect
evidence for its formation was supported by the detection of
the disiloxane F, which could be obtained from the reaction
between E and A.
In conclusion, we successfully synthesized a series of
chelating bis(NHC) rhodium complexes and found that
a rhodium complex derived from a bis(tzNHC) bearing
a trimethylene linker possesses excellent catalytic activity
towards the reductive formylation of amines with CO2 and
Ph2SiH2 at ambient temperature with low catalyst loading
under an elevated pressure of CO2. The mild reaction
conditions of this catalyst system allowed a broad substrate
scope and excellent functional-group tolerance.
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Experimental Section
General procedure: 3c (0.50 mmol, taken as 1 mL from 0.50 mm
freshly prepared stock solution in CH2Cl2) and a solution of freshly
purified amines 4a–g (0.50 mmol) in 0.5 mL of CH2Cl2 were added to
a 50 mL stainless steel high pressure reactor. The resulting solution
was then stirred (400 rpm) under 25 atm of CO2 for 15 min. Then,
a solution of Ph2SiH2 (0.23 g, 1.25 mmol) in CH2Cl2 (0.5 mL) was
added, and the reaction mixture was stirred under 25 atm of CO2 for
the indicated time. After the reaction, excess CO2 was vented. The
reaction mixture was purified by silica-gel column chromatography
eluting with MeOH/CH2Cl2 (5:95) or EtOAc/n-hexane (3:7) to afford
the desired formamides 5a–ag.
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Keywords: amides · carbon dioxide · formylation ·
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Received: May 4, 2015
Published online: June 19, 2015
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