Production of Formamides from CO and Amines Induced by Porphyrin Rhodium(II) Metalloradical

Article abstract of DOI:10.1021/jacs.8b03029

It is of fundamental importance to transform carbon monoxide (CO) to petrochemical feedstocks and fine chemicals. Many strategies built on the activation of C≡O bond by π-back bonding from the transition metal center were developed during the past decades. Herein, a new CO activation method, in which the CO was converted to the active acyl-like metalloradical, [(por)Rh(CO)]^? (por = porphyrin), was reported. The reactivity of [(por)Rh(CO)]^? and other rhodium porphyrin compounds, such as (por)RhCHO and (por)RhC(O)NHⁿPr, and corresponding mechanism studies were conducted experimentally and computationally and inspired the design of a new conversion system featuring 100% atom economy that promotes carbonylation of amines to formamides using porphyrin rhodium(II) metalloradical. Following this radical based pathway, the carbonylations of a series of primary and secondary aliphatic amines were examined, and turnover numbers up to 224 were obtained.

Full text of DOI:10.1021/jacs.8b03029

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Article
Production of Formamides from CO and Amines
Induced by Porphyrin Rhodium(II) Metallo-radical
Jiajing Zhang, Wentao Zhang, Minghui Xu, Yang Zhang, Xuefeng Fu, and Huayi Fang
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 11 May 2018
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Production of Formamides from CO and Amines Induced by Por-
phyrin Rhodium(II) Metallo-radical
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‡
a
‡ a
a
a
a
b,
Jiajing Zhang , Wentao Zhang , Minghui Xu, Yang Zhang, Xuefeng Fu, and Huayi Fang *
a
Beijing National Laboratory for Molecular Sciences, State Key Lab of Rare Earth Materials Chemistry and Applica-
tions, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
b
Department of Chemistry, Fudan University, Shanghai 200433, China
ABSTRACT: It is of fundamental importance to transform carbon monoxide (CO) to petrochemical feedstocks and fine
chemicals. Many strategies built on the activation of C≡O bond by π-back bonding from the transition metal center were
developed during the past decades. Herein, a new CO activation method, in which the CO was converted to the active
•
•
acyl-like metallo-radical [(por)Rh(CO)] (por = porphyrin), was reported. The reactivity of [(por)Rh(CO)] and other rho-
n
dium porphyrin compounds, such as (por)RhCHO and (por)RhC(O)NH Pr, and corresponding mechanism studies were
conducted experimentally and computationally, and inspired the design of a new conversion system featuring 100% atom-
ic economy that promotes carbonylation of amines to formamides using porphyrin rhodium(II) metallo-radical. Following
this radical based pathway, the carbonylations of a series of primary and secondary aliphatic amines were examined and
turnover numbers up to 224 were obtained.
small molecule activation and CO coupling.12 The
•
INTRODUCTION
[(por)Rh(CO)] (por = porphyrin) radicals, featuring bent
1
3
The synthetic utilization of carbon monoxide (CO) in
the construction of carbonyl containing organic materials
is of great significance in sustainable chemistry.1-3 The
mechanism that predominates over a myriad of CO acti-
vation strategies starts from the linear coordination of CO
to the metal center, from which the strong C≡O bond is
activated by the π-back bonding (Figure 1a). The final
products are almost universally formed via nucleophilic
attack on the weakened C≡O bond, which reduces the
scope of substrates eligible for the carbonylation and re-
stricts the design of new CO conversion pathways. Alter-
native CO activation strategies aiming at breaking this
limitation can be envisioned by transforming CO to more
reactive intermediates, such as carbonyl cations, anions or
radicals (Figure 1b),9, 10 which can further activate other
substrates with low activation energy barriers. However,
very few examples have been reported till now as a conse-
quence of the lack of suitable approaches to convert free
Rh–CO geometries, were reported by Wayland via the
•
coordination of CO to [(por)Rh(II)] metallo-radical14.
Considerable spin density was delocalized onto the bent
13, 15
CO ligand as confirmed by EPR measurements.
The
bent CO ligand showed radical-like reactivity with trior-
ganostannanes to form rhodium formyl complexes (Rh–
•
CHO), with [(por)Rh(II)] to form dirhodium ketone (Rh–
C(O)–Rh), with alcohols to form alkoxyl carbonyl rhodi-
4
-8
um (Rh–C(O)–OR), and dimerized to form rhodium α-
13, 16-25
diketone (Rh–C(O)–C(O)–Rh).
However, further
transformations of the activation products are disfavored
under thermal conditions because of the rigid planar por-
phyrin ligand, which blocked the requisite vacant coordi-
nation sites for migration-insertion and reductive elimi-
nation. Recent progresses in our lab revealed that photol-
ysis of Rh–C bonds might be a productive route to over-
come their thermal stability and obtain catalytic turnover
in systems involving those novel porphyrin rhodium
1
1
CO to such reactive intermediates.
26-28
complexes.
Formamides are important intermediates and precur-
29, 30
sors in organic syntheses of drugs,
basic industrial
31
chemicals and reagents in the Vilsmeier formylation
reaction. A few homogeneous systems based on dehy-
drogenative coupling of alcohols and amines showed
broad applicability among diverse categories of sub-
3
2
3
3
3
4-38
strates,
yet atom-economical synthesis under mild
Figure 1. Activated (a) and active (b) “CO” moiety.
conditions for amides, especially formamides, remains a
Among all these intermediates, carbonyl radical is of
particular interest due to its potential applications in
39-42
challenge.
Synthesis of formamides directly from CO
and amines represents a straightforward method. Never-
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theless, an efficient realization would require new meth- of the electron deficient acyl radical character of 2, as re-
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43-47
ods of activation of CO and insertion of amines.
Here-
vealed by preceding computational studies (Mulliken
atomic charges calculated for the carbonyl carbon atoms
in free CO and [(por)Rh(CO)] are -0.013 and 0.12, respec-
tively). At the same time, the bent geometry further re-
duces steric hindrance around the carbonyl carbon atom
in comparison with that for the linear CO.
(a)
in, by taking advantage of the unusual reactivity of the
•
•
bent CO metal complex [(por)Rh(CO)] with N‒H bonds
in amines, we report a new route for photocatalytic con-
version of CO to formamides with 100% atomic economy.
RESULTS AND DISCUSSION
The Rh–C bond in (TMP)Rh–R (TMP = 5,10,15,20-
tetramesitylporphyrin, R = alkyl and acyl groups such as
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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9
0
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9
0
1
2
3
4
5
6
7
8
9
0
o
CH
3
and C(O)OEt) could be easily photolyzed at 0 C
•
within 2 hours to obtain [(TMP)Rh(II)] (1) (Scheme 1).
Pressuring 1 atm of CO into the benzene solution of
(
b)
•
[(TMP)Rh(II)] results in near-quantitative formation of
•
[
(TMP)Rh(CO)] (2) (Scheme 1). Both paramagnetic com-
pounds are identified by EPR measurements (Figure S15, g
= 2.820, g = 1.990 for 1 ; Figure S16, g = 2.176, g = 2.147,
= 1.998 for 2 ), agreeing well with previously reported
⊥
‖
1
2
g
3
15, 19
results.
Figure 2. Calculated spin density on model rhodium porphy-
•
•
Scheme 1. Synthesis of 1 and 2.
rin complexes [(por)Rh(II)] (a) and [(por)Rh(CO)] (b).
Scheme 2. Activation of N–H bond in amines by 2,
taking PrNH
n
2
(n-propylamine) as example.
DFT calculations based on the model rhodium porphy-
rins, for which all meso-substituents are replaced by hy-
drogens, showed that the unpaired electron is mainly lo-
calized on the Rh metal center (Mulliken spin density:
n
With these in mind, the reaction of 2 with PrNH
examined. After addition of excess of PrNH
lents) to the benzene solution of 2, a set of characteristic
multiplets were detected in the high field of H NMR
spectrum after 2 hours at room temperature (δ 0.60 ppm
2 H, NHCH CH CH ), -0.60 ppm (m, 2 H,
NHCH CH CH ), -0.67 ppm (t, 3 H, J(H,H) = 6.8 Hz;
2
was
n
2
(18 equiva-
1
•
0.99, Figure 2a) in [(por)Rh(II)] . The optimized structure
•
(m,
2
2
3
of [(por)Rh(CO)] revealed a bent-coordinated CO (Figure
3
o
2
b) with a Rh-C-O angle of 143.7 . Considerable spin pop-
2
2
3
NHCH
2
CH
2
CH
3
), -1.82 ppm (m, 1 H, NHCH
2
CH
2
CH ),
3
ulation was found on the carbonyl carbon atom (Mulliken
spin density: 0.24), while the Mulliken spin density on Rh
metal center is reduced to 0.72, accordingly. In compari-
son, only a minor fraction of the total spin is delocalized
to the oxygen atom in CO (Mulliken spin density: 0.03).
The calculated length (1.14 Å ) and Mayer bond order (2.29)
of the C–O bond in the bent CO ligand is comparable to
that of free CO (1.13 Å and 2.44 Å , respectively), indicating
a slightly activated C≡O bond as a result of the weak π
back-bonding from Rh metal center. Therefore, the ge-
which is identical to that of the independently prepared
(TMP)RhC(O)NH Pr (3) (Figure S5). The solid state struc-
ture of 3 was determined by X-ray diffraction, as shown in
n
Figure 3a.
The assumed coproduct 4 was not directly observed
due to its instability. Instead, (TMP)RhH (5) and PrN-
HCHO (6) were found. Complexes 6 and 5 are likely gen-
erated by the reaction of 4 with PrNH as implied by the
independent model reaction between (TPP)Rh–CHO
(TPP = 5,10,15,20-tetraphenylporphyrin, 7, which is an
analogue of 4 and can be readily prepared from (TPP)RhH
(8), and CO) and PrNH (Scheme 3). After vacuum
transfer of 1 μL of PrNH
ing an equilibrium distribution of 7 (0.00116 M) and 8
0.00101 M), which was obtained under 1 atm of CO/H (1:1
v/v), at room temperature in dark, the characteristic high-
field H NMR doublet of Rh–CHO disappeared, accompa-
nied by the formation of 6 after 2 h. The turnover number
TON) of formamide 6 was 42 when a large excess of
was used. During the reaction, the formation of
TPP)RhC(O)NH Pr (9) was also observed using H NMR
n
n
2
•
ometry and electronic analysis of [(por)Rh(CO)] are con-
sistent with the presence of the acyl radical like CO ligand,
4
9
n
13
which was also suggested by previous EPR studies.
2
n
2
to a benzene solution contain-
Inspired by the E–H (H, C, O) bond cleavage of H
2
, CH
4
•
and ethanol by the reactive [(por)Rh(CO)] , which lead to
(
2
the
stoichiometric
and (por)RhC(O)OC
we speculate that 2 might activate N–H bonds in
amines via a similar manner (Scheme 2), forming
TMP)RhC(O)NHR and (TMP)RhCHO (4). Interaction of
formation
of
(por)RhCHO,
, respectively,
21,
(
por)RhC(O)CH
3
2
H
5
1
2
5, 48
(
(
n
PrNH
2
the amine nitrogen atom with 2 is expected to be even
more favored than the aforementioned substrates because
n
1
(
which was stable at this reaction condition thus impeding
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further catalytic turnover. The solid state structure of 9
was also determined by X-ray diffraction (Figure 3b).
(TMP)Rh–C(O)NHR could also be photolyzed using high
voltage mercury lamp as the light source. Photolysis of
1
2
3
4
5
6
7
8
9
1
3
n
benzene solution of (TMP)Rh C(O)NH Pr led to the
A straightforward strategy for improving the TON of
this thermal catalytic carbonylation of amines could be
proposed as blocking the formation of 9. Addition of ex-
1
3
n
formation of • C(O)NH Pr radical, which was trapped in
situ by 2,2,6,6-tetramethylpiperidin-1-yl (TEMPO) to form
TEMPO– C(O)NH Pr as detected by high resolution mass
1
3
n
cess amount of Me EtSiH, which is a hydrogen atom do-
2
+
spectroscopy ([M+H] , m/z 244.20997). Meanwhile,
nor, was found to effectively increase the thermal catalyt-
ic performance (Table S1). The concentration of 2 was
significantly lowered due to the hydrogen atom abstrac-
•
generation of [(TMP)Rh(II)] was also demonstrated by
EPR measurement. The active acyl radical could abstract
the axial hydrogen atom in 5, which was a good hydrogen
atom donor, thus leading to the formation of target
formamide 6 (Scheme 4). The hydrogen source for the
product was further confirmed by the isotope-labelling
tion from Me EtSiH by 2, which led to the generation of 7,
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
thus hindered the formation of 9 (Scheme S1). Similar
reactivity was also seen for other metallocarbonyl radi-
5
0, 51
cals.
With the presence of 30 equiv. of Me EtSiH,
2
1
3
n
TONs ranged from 14 (for cyclohexylamine) to 470 (for n-
hexylamine) were obtained at room temperature within 7
days (Table S2).
experiment in which D– C(O)NH Pr was observed during
the light irradiation of the benzene solution containing
1
3
n
both (TMP)Rh C(O)NH Pr and (TMP)Rh–D.
(a)
Scheme 4. Formation of formamide 6 by light irradia-
tion of the mixture of 3 and 5.
Scheme 5. Catalytic cycle of photo production of
formamides from CO and amines catalyzed by
•
[
(por)Rh(II)] .
(b)
n
Figure 3. Solid state structures of (TMP)RhC(O)NH Pr·THF
n
(a) and (TPP)RhC(O)NH Pr·THF (b). Thermal ellipsoids set
at 50% probability; hydrogen atoms are omitted for clarity.
Scheme 3. Formation of formamide 6 from the reac-
tion of 7 with n-propylamine.
The abovementioned elementary steps constitute the
observed catalytic production cycle of 6 as illustrated in
Scheme 5. The formal carbonyl insertion reaction of CO
with amine is envisioned to proceed through: a)
•
•
[
(por)Rh(II)] binds with CO to form [(por)Rh(CO)] ; b)
•
the resulting [(por)Rh(CO)] intermediate reacts with
1
2
1
2
R R N–H to produce (por)RhCHO and (por)RhC(O)NR R ;
c) nucleophilic attack of amine to (por)RhCHO releases
formamide product and generates (por)RhH; d) photo-
Previously, our lab had reported a photocatalytic hy-
1
2
cleavage of Rh–C bond in (por)RhC(O)NR R regenerates
3
droxylation protocol for Si–C(sp ) bonds, in which homo-
•
[
(
(por)Rh(II)] , and e) abstraction of H atom from
lytic cleavage of (por)Rh–CH
3
bond by photolysis played a
1
2
por)RhH by •C(O)NR R acyl radical leads to the for-
vital role in regenerating the active porphyrin rhodium
28
mation of formamide product and the regeneration of
catalyst. We hypothesize that the Rh–C bond in
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2
3
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7
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9
1
1
1
1
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1
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2
2
2
2
2
2
2
2
2
3
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3
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0
1
2
3
4
5
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0
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3
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5
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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0
1
2
3
4
5
6
7
8
9
0
n
Figure 4. Calculated potential energy surface (PES) for rhodium porphyrin catalyzed production of PrNHCHO from CO and
PrNH .
2
n
•
strongly exoergic hydrogen atom abstraction in (por)RhH
by •C(O) PrNH radical (Figure 4, step 5), which is pro-
duced by the photocleavage of the Rh–C bond in
por)RhC(O)NH Pr, and consequently leads to the for-
[(por)Rh(II)] .
n
In order to get more insights about the aforementioned
catalytic formamide productions, the proposed mecha-
nism was further investigated by DFT calculations based
on the model rhodium porphyrin. The calculated poten-
tial energy surface (PES) shows that the whole catalytic
process is significantly exothermic (Figure 4). The catalyt-
n
(
n
mation of the PrNHCHO product accompanied by the
•
regeneration of the initial [(por)Rh(II)] catalyst.
Encouraged and inspired by these mechanistic insights,
we optimized the reaction conditions for the catalytic
ic process is initiated by the coordination of CO to the
•
n
transformation of PrNH
2
to 6 using the less toxic toluene
[(por)Rh(II)] catalyst (Figure 4, step 1) with a calculated
solvent, and a maximum TON of 224 was obtained as de-
termined by GC (see SI for details). Control experiments
showed that there was no conversion without light or
catalyst 1. The source of the carbonyl carbon in 6 was
exothermicity of 14.5 kcal/mol, which is in good accord-
ance with the experimentally observed ease of binding of
•
•
CO to [(TPP)Rh(II)] or [(TMP)Rh(II)] . The formal overall
•
ter-molecular reaction of [(por)Rh(CO)] with nPrNH
2
13
identified to be the CO gas by a C-labelling reaction, in
(
Scheme 5, step b) is achieved by means of two individual
13
n
which CO and PrNH were used as reactants and H–
C(O)NH Pr was observed as the product. When PrND
was used, the formyl hydrogen in product 6 was accord-
ingly changed to D, which demonstrated that the hydro-
gen of the formyl group in the product came from one of
2
elementary reactions to avoid the unfavorable entropy
13
n
n
2
change for the ter-molecular pathway: the interaction of
•
nPrNH
step
to the carbonyl group in [(por)Rh(CO)] (Figure 4,
2
2),
and
reaction
of
the
resulting
•
8
nPr)]
the amino hydrogens in the substrate. And when d -
[(por)Rh(C(O)NH
2
with another molecule of
to the formation of
(por)RhC(O)NH Pr and (por)RhCHO (Figure 4, step 3).
Although the former reaction is endothermic, the latter
reaction is strongly exoergic and provides the major driv-
•
toluene was used as the solvent, no D was found in 6,
thus excluded the possibility that toluene could act as the
hydrogen source. Other substrates, including several pri-
mary and secondary amines, were also examined under
identical condition with corresponding TONs ranging
from 17 (for benzyl amine) to 107 (for n-butylamine) (Ta-
ble S3).
[(por)Rh(CO)] ,
leading
n
ing force for this formal ter-molecular reaction. The fol-
n
lowing reaction of (por)RhCHO with PrNH
2
producing
n
the final product PrNHCHO and (por)RhH (Figure 4,
step 4), traverses a moderate activation energy barrier
Rhodium(I) porphyrin, which could be generated as a
result of σ-donor ligand induced disproportionation of
por)Rh(II), was reported to react with CO to form
(por)RhCHO. To further distinguish the presented radi-
cal based pathway from the close-shell pathway, the cata-
lytic conversion of piperidine and CO to piperidine-1-
carbaldehyde by 2 was monitored using electron para-
magnetic resonance. A strong isotropic resonance at
g=1.982, which could be tentatively assigned to the
(
22.8 kcal/mol). Remarkably, the computed energy barrier
1
9
(
for step 4 well matches previously reported β-H elimina-
4
8
5
2-
tion processes of rhodium porphyrin based compounds.
5
5
In comparison, no observable energy barriers are identi-
fied for other fundamental steps. Step 4 is most likely in
equilibrium as indicated by the very small free energy
change (~ -2.0 kcal/mol). However, this equilibrium is
driven to the right side when coupled with the sequential
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•
paramagnetic resonance; TON, turnover number; PES, po-
tential energy surface; DFT, density functional theory.
[
(TMP)Rh(piperidine)
and its intensity kept increasing during the measurement
Figure S23). Therefore, although the close-shell pathway
2
]
metallo-radical, was observed
1
2
3
4
5
6
7
8
9
(
REFERENCES
could not be completely ruled out at the present stage,
the radical based mechanism is thought to account for
the production of formamides from amines and CO.
(
1) Bipp, H.; Kieczka, H., Formamides. In Ullmann's
Encyclopedia of Industrial Chemistry, Wiley-VCH Ver-
lag GmbH & Co. KGaA: 2000.
CONCLUSIONS
(
2) Maitlis, P. M.; Haynes, A.; Sunley, G. J.; Howard, M.
In conclusion, the reactivity and corresponding mecha-
J., Dalton Trans. 1996, 2187.
nism of
a
series rhodium porphyrins, such as
(3) Thomas, C. M.; Süss-Fink, G., Coord. Chem. Rev.
2003, 243, 125.
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1
1
1
1
1
1
1
1
1
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2
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2
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2
2
2
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n
[(por)Rh(CO)] , (por)RhCHO and (por)RhC(O)NH Pr,
were investigated in detail. The combination of these el-
ementary reactions led to the selective production of
formamides from CO and amines without the formation
of any by-product, thus illustrating a novel metallo-
radical based strategy for CO activation and conversion
exhibiting 100% atomic economy. As a benefit of the ac-
tive metallo-radicals and the photochemistry of rhodium
porphyrins, the whole conversion proceeded under mild
conditions without any sacrificial reagents. This protocol
provides new mechanistic insight into conversion of CO
to value-added compounds. The tandem reactions de-
signed in this article that activates CO through one elec-
tron process may shed light into the design of new cata-
lytic processes in transformation of metal-carbonyl com-
plexes into useful chemicals. Further applications of this
system to other substrates are underway.
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ASSOCIATED CONTENT
Supporting Information. Additional experimental details,
spectral data for related compounds and crystallographic
data. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Chem. 2001, 79, 854.
Corresponding Author
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* E-mail: hfang@fudan.edu.cn
(
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(
Author Contributions
‡ These authors contributed equally.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENT
We gratefully acknowledge financial support from Shanghai
Sailing Program (18YF1401800) and National Natural Science
Foundation of China (21571009).
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ABBREVIATIONS
por, porphyrin; TMP, 5,10,15,20-tetramesitylporphyrin; TPP,
5,10,15,20-tetraphenylporphyrin;
TEMPO, 2,2,6,6-tetramethylpiperidin-1-yl; EPR, electron
(
THF,
tetrahydrofuran;
(
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Production of Formamides from CO and Amines Induced by Porphyrin Rhodium(II) Metallo-radical
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