Dioxygen activation with stable N-heterocyclic carbenes

Article abstract of DOI:10.1039/c8cc08911f

Dioxygen activation with both cyclic (amino)(alkyl)carbenes and di(amino)carbenes at ambient temperature is described. Theoretical studies suggest that electron rearrangement from the doubly filled σ orbital of carbene carbon to its vacant p orbital through the π? antibonding orbital of ³O₂ can be considered for C_carbene-O bond formation.

Full text of DOI:10.1039/c8cc08911f

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Dioxygen activation with stable N-heterocyclic
carbenes†
ab
Cite this:DOI: 10.1039/c8cc08911f
c
b
ab
Jinghua Tang,‡ Xuejiao J. Gao, ‡ Huarong Tang* and Xiaoming Zeng
*
Received 8th November 2018,
Accepted 10th January 2019
DOI: 10.1039/c8cc08911f
rsc.li/chemcomm
Dioxygen activation with both cyclic (amino)(alkyl)carbenes and
di(amino)carbenes at ambient temperature is described. Theoretical
studies suggest that electron rearrangement from the doubly filled
r orbital of carbene carbon to its vacant p orbital through the
3
p* antibonding orbital of
2
O can be considered for Ccarbene–O
bond formation.
Dioxygen is essential for life. Its reaction plays a crucial role
in various fundamental biological processes and industrial
chemistry. Dioxygen activation in nature is mainly achieved
1
,2
by metalloenzyme complexes.
Artificial chemical mimics
using electronically and coordinatively unsaturated transition
metal complexes have been widely adopted, and readily proved Fig. 1 Selected examples of activation of molecular dioxygen with main-
group element compounds.
their usefulness as reactants in applications throughout academia
3
–5
and the chemical industry.
Although main-group elements
18–20
possess different electronic properties as compared to transi- and the relevant reactivity remains largely unknown,
despite
tion metals, these organic motifs show appealing reactivities the fact that a thermal reaction with a prototypic persistent carbene
like transition metal complexes in the activation of small has been observed by irradiation of phenylchlorodiazirine under an
6
–9
3
21
molecules such as H , NH , C H and CO . In contrast, the argon atmosphere containing O
2
at low temperature (35 K).
2
3
2
4
2
3
2
interaction of ground-state molecular dioxygen ( O ) with main-
Stable NHCs possess a lone pair of valence electrons and an
group compounds has rarely been explored, although it is well accessible vacant p orbital, which endow them with unique
known that many organic structural motifs are sensitive to an reactivity in reactions with small molecules serving as good
2
2–25
air atmosphere. Striking results recently highlighted N-hetero- s-electron-donors and p-electron-acceptors.
Remarkable
1
0
cyclic carbene (NHC)-stabilized disilicon II, diphosphorus achievements include the activation of hydrogen and ammonia
1
1
12
26,27
0
III and silylenes I showing unique reactivity in the activation using cyclic (amino)(alkyl)carbenes (CAACs)
and N,N -diamido-
3
13–16
28
of O , as well as using siloxysilylene IV
and frustrated carbenes. In contrast to the treatment with excited dioxygen
Lewis pair V (Fig. 1). To our knowledge, there are no reports ( O ), the reaction between O and singlet carbene suffers from
2 2
2
1
7
1
3
3
of reactions of
O
2
with stable, and readily available NHCs, a formidable spin-forbidden obstacle. Mechanistic exploration
for interpreting plausible pathways in the reaction would be
theoretically appealing. Herein, we report that dioxygen activa-
a
Frontier Institute of Science and Technology, Xi’an Jiaotong University,
Xi’an 710054, China. E-mail: zengxiaoming@scu.edu.cn
tion was enabled by stable, singlet and readily available cyclic
2
9
b
c
mono(amino)carbenes (CAACs) and 1,3-bis(2,6-diisopropyl-
phenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IPr) at ambient
temperature, forming the related lactam and cyclic urea deri-
vatives through a formal oxygenation (Fig. 1).
College of Chemistry and School of International Studies, Sichuan University,
Chengdu 610064, China. E-mail: tanghuarong@scu.edu.cn
College of Chemistry and Chemical Engineering, Jiangxi Normal University,
Nanchang 330022, China
†
Electronic supplementary information (ESI) available. CCDC 1539794. For ESI
By bubbling dioxygen (99.999% purity) in a tetrahydrofuran
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8cc08911f
(
THF) solution containing CAACs (1 with 1-(2,6-diisopropylphenyl)-
Me
‡
J. Tang and X. J. Gao contributed equally.
3,3,5,5-tetramethylpyrrolidine-2-ylidene
(
CAAC),
2
with
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Fig. 2 The reactions between stable and readily available N-heterocyclic
carbenes with molecular dioxygen.
2
-(2,6-diisopropylphenyl)-3,3-dimethyl-2-azaspiro[4,5]decan-1-
Cy
ylidene ( CAAC), and 3 with 2-(2,6-diisopropylphenyl)-3,3,6,8-
tetramethyl-2-azaspiro[4,5]dec-7-ene-1-ylidene
cHex
30
(
CAAC) ),
the lactams of 1, 2, and 3 were formed in 20–45% yields
at 35 1C (Fig. 2a). Their compositions were corroborated by
1
3
carbon nuclear magnetic resonance ( C NMR) singlet reso-
nances with the appearance of characteristic chemical signals
at 179.5 ppm (1), 179.1 ppm (2), and 177.1 ppm (3) for the
corresponding carbon signals of the CQO moieties. These
were shifted dramatically upfield by Dd of 124–143 ppm
compared with that of free carbenes. The characteristic
18
Scheme 1
O-Labelling experiments.
To gain mechanistic insight into the bonding model
absorption bands in the infrared spectra for the CQO groups involving dioxygen activation with singlet NHCs and plot the
ꢀ1
were observed at 1677, 1676, and 1675 cm , respectively. The energy profiles of the possible pathways, we performed density
molecular structure of lactam 2 was confirmed by crystallo- functional theory (DFT) calculations by choosing the reaction of
cHex
3
graphic analysis (Fig. 2b). The pyrrole ring in 2 adopts a
slightly twisted conformation (twist angle t = 14.21) with an
CAAC with O2 as a model. As shown in Fig. 3a and b,
cHex
CAAC initially employs its lone pair to interact with the
3
3
2
out-of-plane C2–C3–C4 fragment on the five-membered ring frontier orbitals of O , forming an intermediate Int1 through
3
backbone.
the transition state TS1 by overcoming the activation energy
Inspired by these results, we then turned our attention to barrier of 21.9 kcal mol . Subsequently, an intersystem
examining the possibility of the activation of O with cyclic crossing (ISC) process can be considered with an exothermicity
2
di(amino)carbene. We were delighted to find that oxygenation of 2.5 kcal mol to give the intermediate Int1. Such an ISC
ꢀ
1
3
ꢀ
1
1
using stable, commercially available IPr proceeded smoothly by process is rationalized by a potential energy surface across S0
3
bubbling O into a THF solution, resulting in the formation of and T1 (please see ESI† for details). Without the ISC pathway,
2
cyclic urea 4 in 34% isolated yield (Fig. 2). It showed a the following cleavage of the oxygen–oxygen bond by the
ꢀ1
cHex
characteristic absorption band for the CQO group at 1676 cm
reaction with another molecule of
CAAC is proved to be
in the infrared spectrum and the related carbon signals at thermodynamically unfavorable in the formation of the lactam
1
3
ꢀ1
1
52.1 ppm in the
C NMR spectrum. When saturated because of the high activation barrier (68.9 kcal mol ).
1
,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene (SIPr) was, Compared with a continuous reaction with another molecular
3
cHex
1
instead, conducted by bubbling
O
2
into THF, the related
CAAC, the homo-interaction with Int1 itself overcomes a
ꢀ
1
carbene-oxo adduct was not detected by in situ NMR and relatively low reaction energy (21.9 versus 27.9 kcal mol ) in
GC-MS analyses (see ESI†). the formation of the related lactam product combined with
To confirm that the oxygen moiety on the lactam and cyclic a release of O
urea derivatives was produced from molecular dioxygen, with excited state dioxygen ( O
1
cHex
2
. On the other hand, the reaction of
CAAC
) allows the straightforward
O labeling experiments were performed by vigorously stirring formation of Int1 by overcoming a low activation energy of
1
2
1
8
1
cHex
ꢀ1
a THF solution containing free carbenes of
under an atmosphere of
CAAC or IPr 5.7 kcal mol
.
1
8
3
O (499% purity). The corres- Fig. 4a shows the geometric structure of Ts1. For clarity, the
2
ponding products were isolated and characterized by a high- schematic orbital interactions between the two singly occupied
resolution mass spectrometer, showing major peaks at molecular orbitals (SOMO), i.e. the p * and p2py* orbitals of
2
pz
3
2
3
70.2988 and 407.2921 m/z, which are consistent with the
O , the empty 2p and the doubly occupied sp orbitals of
2 z
1
8
18
cHex
molecular weights of 3- O and 4- O, respectively (Scheme 1). carbene carbon of
CAAC are shown in Fig. 4b. The p2pz* and
2
are vertically arranged, as well as the empty
CAAC 2p and doubly occupied sp orbitals of the carbene carbon of
1
8
3
This suggests that the
introduced into the scaffolds of the free carbenes (
and IPr).
O of molecular dioxygen was
p
2py* orbitals of O
cHex
2
z
cHex
3
2
CAAC. The p2pz* and p2py* orbitals of O overlap with the
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3
Fig. 4 (a) The geometric structure of Ts1 with key bond lengths, angles
and dihedrals that are labelled. The schematic orbital interactions between
3
cHex
cHex
3
O
2
and
CAAC. (b) Calculated
CAAC– O
2
interaction frontier
3
molecular orbitals, SOMO and LUMO+2, of TS1. Computed energies
(
(
in parentheses) are given in electronvolt. (c) Plot of deformation densities
cHex
3
3
Dr
1
and Dr
2
) of the orbital interaction between
2
CAAC and O in TS1
ꢀ1
with the corresponding interaction energies (DE (kcal mol )).
Fig. 3 Relative energy profiles for possible pathways of the reaction of
3
cHex
O
2
with
CAAC (represented by CAAC), and selected frontier molecular
3
orbitals for transition states of TS1. (a) Energy profiles for the oxygenation
of
unoccupied p orbital of the carbene carbon, contributing to the
formation of the lactam p bond.
cHex
CAAC. Bond distances (Å) are given in the structure drawings.
cHex
(b) Possible pathways for the oxygenation of
CAAC.
In conclusion, we developed an oxygenation reaction by the
activation of molecular dioxygen with stable, readily available
NHCs. The strong nucleophilicity and electrophilicity of these
2
2
z
p and sp orbitals of carbene carbon, respectively, which singlet carbenes possessing electron-donating and electron-
corresponds to the interactions illustrated by the SOMO and accepting frontier orbitals endow them with the unique ability
3
32–34
LUMO+2 of Ts1 (Fig. 4c).
to break the oxygen–oxygen bonds at ambient temperature.
CAAC This finding explains why such stable NHCs are sensitive to air
cHex
The nature of the electronic interaction between
3
and
2
O was further analysed by extended transition state in organic media and provides an entry to form pyrrolidinones
analysis combined with natural orbitals for chemical valence and imidazolones by the use of molecular dioxygen as a
3
1
(
ETS-NOCV). Fig. 4b shows that the differential densities ‘‘green’’ oxidant. It demonstrates that NHCs are able to serve
corresponding to the interactions contribute the most to the as mimics of complexes toward the activation of molecular
3
cHex
3
6
bonding between O
2
and
CAAC in Ts1. It was found that dioxygen, just as a transition metal function.
the charge flows from the red to the blue area. This can be
Support for this work by NSFC (No. 21202128 and 21572175),
ascribed to electron density transfer from the doubly occupied the State Key Laboratory of Elemento-organic Chemistry
cHex
s orbital of
CAAC to one of the p* antibonding orbitals of (Nankai University), the Beijing National Laboratory for
3
O
O
2
2
and back donation from the p* antibonding orbitals of Molecular Sciences and the SCU is gratefully acknowledged.
to the unoccupied p orbital of
3
cHex
CAAC, respectively. The We thank Prof. Xingfa Gao (JXNU) for the help in calculations
former is the dominating interaction, having the interaction and Prof. Gernot Frenking (PUM) for valuable suggestions.
ꢀ
1
energy of ꢀ31.2 kcal mol . The results of these theoretical
studies indicate that electron rearrangement from the s orbital
of the carbene centre into one of the p* antibonding orbitals of
Conflicts of interest
3
2
O essentially contributes to the formation of the lactam
3
s bond in Int1. The electron density may then flow into the There are no conflicts to declare.
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Notes and references
18 A. Igau, H. Gr u¨ tzmacher, A. Baceiredo and G. Bertrand, J. Am. Chem.
Soc., 1988, 110, 6463–6466.
19 A. J. Arduengo, R. L. Harlow and M. Kline, J. Am. Chem. Soc., 1991,
113, 361–363.
1
2
3
B. J. Wallar and J. D. Lipscomb, Chem. Rev., 1996, 96, 2625–2657.
W. Nam, Acc. Chem. Res., 2007, 40, 465–634.
M. Costas, M. P. Mehn, M. P. Jensen and L. Que Jr., Chem. Rev., 20 Y. Wang and G. H. Robinson, Inorg. Chem., 2014, 53, 11815–11832.
2
004, 104, 939–986.
21 G. A. Ganzer, R. S. Sheridan and M. T. H. Liu, J. Am. Chem. Soc.,
1986, 108, 1517–1520.
22 A. J. Arduengo and G. Bertrand, Chem. Rev., 2009, 109, 3209–3210.
4
5
C. E. MacBeth, A. P. Golombek, V. G. Young Jr, C. Yang, K. Kuczera,
M. P. Hendrich and A. S. Borovik, Science, 2000, 289, 938–941.
M. T. Kieber-Emmons and C. G. Riordan, Acc. Chem. Res., 2007, 40, 23 M. N. Hopkinson, C. Richter, M. Schedler and F. Glorius, Nature,
6
18–625.
2014, 510, 485–496.
24 M. Soleilhavoup and G. Bertrand, Acc. Chem. Res., 2015, 48, 256–266.
D. Martin, M. Soleilhavoup and G. Bertrand, Chem. Sci., 2011, 2, 25 J. P. Moerdyk, D. Schilter and C. W. Bielawski, Acc. Chem. Res., 2016,
6
7
P. P. Power, Nature, 2010, 463, 171–177.
3
89–399.
D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2015, 54, 6400–6441. 26 G. D. Frey, V. Lavallo, B. Donnadieu, W. W. Schoeller and
S. K. Mandal and H. W. Roesky, Acc. Chem. Res., 2012, 45, 298–307.
G. Bertrand, Science, 2007, 316, 439–441.
49, 1458–1468.
8
9
1
1
1
1
1
0 Y. Wang, M. Chen, Y. Xie, P. Wei, H. F. Schaefer III, P. V. R. Schleyer 27 M. Melaimi, R. Jazzar, M. Soleilhavoup and G. Bertrand, Angew.
and G. H. Robinson, Nat. Chem., 2015, 7, 509–513.
Chem., Int. Ed., 2017, 56, 10046–10068.
1 Y. Wang, Y. Xie, P. Wei, H. F. Schaefer III, P. V. R. Schleyer and 28 T. W. Hudnall, J. P. Moerdyk and C. W. Bielawski, Chem. Commun.,
G. H. Robinson, J. Am. Chem. Soc., 2013, 135, 19139–19142.
2010, 46, 4288–4290.
2 Y. Xiong, S. Yao, R. M u¨ ller, M. Kaupp and M. Driess, Nat. Chem., 29 V. Lavallo, Y. Canac, C. Pr ¨a sang, B. Donnadieu and G. Bertrand,
010, 2, 577–580.
Angew. Chem., Int. Ed., 2005, 44, 5705–5709.
3 S. Yao, Y. Xiong, M. Brym and M. Driess, J. Am. Chem. Soc., 2007, 30 X. Zeng, G. D. Frey, R. Kinjo, B. Donnadieu and G. Bertrand, J. Am.
29, 7268–7269.
Chem. Soc., 2009, 131, 8690–8696.
4 H. Bornemann and W. Sander, J. Am. Chem. Soc., 2000, 122, 31 S. Grimme, S. Ehrilich and L. Goerigk, J. Comput. Chem., 2001, 32,
2
1
6
727–6734.
1456–1465.
1
1
5 H. Cui, J. Zhang, Y. Tao and C. Cui, Inorg. Chem., 2016, 55, 46–50.
6 R. Rodriguez, D. Gau, T. Troadec, N. Saffon-Merceron, V. Branchadell,
32 D. Martin, N. Lassauque, B. Donnadieu and G. Bertrand, Angew.
Chem., Int. Ed., 2012, 51, 6172–6175.
A. Baceiredo and T. A. Kato, Angew. Chem., Int. Ed., 2013, 52, 33 A. F u¨ rstner, M. Alcarazo, H. Krause and C. W. Lehmann, J. Am.
980–8983.
Chem. Soc., 2007, 129, 12676–12677.
7 T. Wang, G. Kehr, L. Liu, S. Grimme, C. G. Daniliuc and G. Erker, 34 B. Rao, H. Tang, X. Zeng, L. Liu, M. Melaimi and G. Bertrand, Angew.
J. Am. Chem. Soc., 2016, 138, 4302–4305.
Chem., Int. Ed., 2015, 54, 14915–14919.
8
1
Chem. Commun.
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