DFT studies on the reaction mechanisms of 1,4-dilithio 1,3-dienes with nitriles

Article abstract of DOI:10.1021/om300869t

Mechanisms for the reactions of 1,4-dilithio-1,3-butadienes and nitriles are explored through both experiments and DFT calculations. The computational results suggest that the selectivity of these reaction systems is strongly affected by the structures of the substrates. As the first step of all reaction pathways, the addition intermediate of one C-Li bond to the nitrile is formed. When tetraalkyl-substituted 1,4-dilithio 1,3-dienes and 2-cyanopyridine are used, the intermediate gives the cyclopentadienyl amine product as the kinetic product because of the coordination of the pyridyl N atom to the lithium atoms (system B). This addition intermediate also undergoes a second nitrile insertion into the C-Li bond, giving the dilithio ketimine intermediate. When tetraalkyl-substituted 1,4-dilithio 1,3-dienes and aryl nitriles are used, the dilithio ketimine intermediate undergoes a 1,6-cyclization, generating pyridine and triazine products through thermodynamically favored pathways (systems A and B). When cyclic 1,4-dilithiobutadiene and tertiary aliphatic nitriles are used, the dilithio ketimine intermediate undergoes two sequential 1,5-cyclization steps with a lower energy barrier, generating tricyclic Δ¹- bipyrrolines (system C). Experimentally, the lithium-containing triazine intermediate 10 and Δ¹-bipyrroline intermediate 19 have been isolated and their structures investigated through NMR and quenching experiments. The calculation results clearly show the mechanism details of these reactions and are in good agreement with the experimental observations.

Full text of DOI:10.1021/om300869t

Article
pubs.acs.org/Organometallics
DFT Studies on the Reaction Mechanisms of 1,4-Dilithio 1,3-Dienes
with Nitriles
Fei Zhao,† Ming Zhan,† Wen-Xiong Zhang,† and Zhenfeng Xi*,†,‡
†Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, People’s Republic of China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Shanghai 200032, People’s Republic of
China
S
* Supporting Information
ABSTRACT: Mechanisms for the reactions of 1,4-dilithio-
1,3-butadienes and nitriles are explored through both experi-
ments and DFT calculations. The computational results
suggest that the selectivity of these reaction systems is strongly
affected by the structures of the substrates. As the first step of
all reaction pathways, the addition intermediate of one C−Li
bond to the nitrile is formed. When tetraalkyl-substituted 1,4-
dilithio 1,3-dienes and 2-cyanopyridine are used, the
intermediate gives the cyclopentadienyl amine product as the
kinetic product because of the coordination of the pyridyl N
atom to the lithium atoms (system B). This addition intermediate also undergoes a second nitrile insertion into the C−Li bond,
giving the dilithio ketimine intermediate. When tetraalkyl-substituted 1,4-dilithio 1,3-dienes and aryl nitriles are used, the dilithio
ketimine intermediate undergoes a 1,6-cyclization, generating pyridine and triazine products through thermodynamically favored
pathways (systems A and B). When cyclic 1,4-dilithiobutadiene and tertiary aliphatic nitriles are used, the dilithio ketimine
intermediate undergoes two sequential 1,5-cyclization steps with a lower energy barrier, generating tricyclic Δ1-bipyrrolines
(system C). Experimentally, the lithium-containing triazine intermediate 10 and Δ1-bipyrroline intermediate 19 have been
isolated and their structures investigated through NMR and quenching experiments. The calculation results clearly show the
mechanism details of these reactions and are in good agreement with the experimental observations.
reactions with 2 equiv of various nitriles resulted in the
formation of a series of tricyclic Δ1-bipyrrolines 6 in good
yields (Scheme 1, type III). No formation of the triazine
derivative 3c was observed even with 4 equiv of nitrile,
indicating the reaction mechanism for the formation of 6
should be different from that for the pyridine derivatives 2a,b.
The observed diverse reaction patterns of the dilithio
reagents with nitriles prompted us to investigate the reaction
mechanisms in depth. In this paper, we report our experimental
and computational analysis on possible reaction pathways.
INTRODUCTION
■
The reaction of organolithium reagents with nitriles is of
fundamental interest in organometallic chemistry and organic
synthesis.1,2 In recent years, we have systematically studied the
reactions of 1,4-dilithio 1,3-dienes.3,4 Among these, the reaction
of 1,4-dilithio 1,3-dienes with nitriles have interested us very
much.4c,j It was found that, depending on the substitution
pattern of the dilithio skeleton and the nitrile used, three types
of reactions took place: the reaction of 1,2,3,4-tetrasubstituted
1,4-dilithio 1,3-diene 1a with various nitriles (e.g., benzonitrile)
afforded the pyridine derivative 2a in good yields. Along with
the formation of 2a (Scheme 1, type I), the triazine 3a was also
formed with 1, 2, or 3 equiv of benzonitrile (PhCN).4c A 4
equiv amount of benzonitrile should be used to ensure high
yields of product 2a. When 2-cyanopyridine was used and the
reaction was carried out at a lower temperature (−60 °C), the
cyclopentadienyl amine 4a was isolated in 23% yield, along with
a 66% yield of 2b (Scheme 1, type II). Increasing the reaction
temperature from −60 °C to room temperature or even 50 °C
did not change the yields of products 4a and 2b, indicating that
the formation of 2b might be independent of the precursor of
4a. In this case 4 equiv of 2-cyanopyridine was also required
and the triazine 3b was obtained in over 70% yield. However,
when cyclic 1,4-dilithio 1,3-dienes 5 were applied, their
RESULTS AND DISCUSSION
■
Experimental Results. In 1980, Schleyer studied the
structure of 1,4-dilithio 1,3-dienes via computation.5 It is
revealed that a symmetrically bridged structure should be the
most stable. The existence of the double lithium bridge has
been proved by several crystal structures.6 A series of
experiments have been carried out to explore the mechanism
of the reaction of 1,4-dilithio 1,3-dienes with nitriles.
The 1,4-dilithio 1,3-diene derivative 1b contains two C−Li
bonds; thus, it could react with one or two molecules of nitriles,
Received: September 10, 2012
Published: March 19, 2013
© 2013 American Chemical Society
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Scheme 1. Three Types of Reactions of 1,4-Dilithio-1,3-
dienes with Various Nitriles
Scheme 5. Proposed Mechanism for the Formation of the
Pyridines and Triazines
Scheme 2. Trapping of Intermediate 9b
Scheme 6. Proposed Mechanism for the Formation of the
Cyclopentadienyl Amine
Scheme 3. Formation and Isolation of Intermediate 10
Scheme 7. Mechanism for the Formation of the Tricyclic Δ1-
Bipyrrolines
Scheme 4. Quenching Reactions of 10
Scheme 8. Substrates of Three Reaction Systems
generating one or two N-lithio ketimine groups, respectively.
To obtain the information about the intermediates, the reaction
of benzonitrile with 1b was performed at −78 °C for 30 min.
After quenching, the reaction mixture was analyzed by GC-MS
and a new product 8 was observed with an m/z value of 269.
These results supported the formation of the N-lithio ketimine
intermediate 9b with one C−Li bond unreacted (Scheme 2).
To obtain the organolithium intermediates before quenching,
1b was allowed to react with benzonitrile in diethyl ether for 3
h at room temperature. Workup of this reaction mixture in a
glovebox separated the pyridine derivative 2c and the unknown
product 10. The 13C NMR spectrum of 10 showed only a
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a
Scheme 9. Nitrile Insertion Steps and Relative Energies
Scheme 10. Proposed Reaction Pathway from IMA2a to
IMA5
a
Coordinated solvent molecules are omitted for clarity.
broad peak at 120−130 ppm, indicating that the product 10
should contain only sp2-C bonds. According to the in situ NMR
spectra, pyridines were formed in the reaction process, which
indicates that product 10 is an organolithium compound.
Quenching of 10 afforded triazine 3a as the only organic
product.7 On the basis of all these observations, the structures
of 10 are proposed and shown in Scheme 3.
We had tried our best to obtain the crystal structure of 10 in
the glovebox many times. However, only quenched product
triazine 3a was obtained, probably because 10 is highly reactive.
We then turned to explore 10 via quenching with different
electrophiles.
Several commonly used electrophiles, such as MeI, Et3OBF4,
and alkyl bromides, have been tested, affording 11a−e as the
corresponding products. However, these products were not
stable and quickly turned to triazine 3a (Scheme 4). The
driving force to form the aromatic ring might explain the
instability of 11a−e.
On the basis of the experimental results above, a plausible
mechanism is proposed (Scheme 5). Insertion of the nitrile into
one C−Li bond of 1 generates N-lithio ketimine intermediate
9. Then a similar process between 9 and a second molecule of
nitrile results in the 1,8-dilithium intermediate 11. 6π
cyclization of 11 affords 12. After 12 is formed, another two
molecules of nitriles participate in the reaction to give 14 with a
long side chain. Cyclization of this side chain generates another
six-membered ring in the molecule and gives 15a or 15b.
Finally, the key intermediate 10 is eliminated from 15a or 15b,
generating the aromatic pyridine derivative 2. Quenching of 10
with water affords triazine 3.
Figure 1. DFT-optimized structures of doubly lithium bridged
intermediates IMA1, IMA2a, and IMA3 and singly lithium bridged
intermediate IMA2c. Coordinated solvent molecules are omitted for
clarity. Selected atom distances (in Å): IMA2a, C1−C5 = 3.260;
IMA2c, C1−C5 = 2.924.
Figure 2. DFT-calculated potential energy surfaces of pathways a and b in system A. Coordinated solvent molecules are omitted for clarity.
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Figure 3. DFT-calculated potential energy surfaces of pathway c in system A. Coordinated solvent molecules are omitted for clarity.
Figure 4. DFT-optimized structures of transition states in system A. Coordinated solvent molecules are omitted for clarity.
Using 2-cyanopyridine instead of benzonitrile would generate
multisubstituted pyridine, triazine, and cyclopentadienyl amine
as products (reaction type II). The proposed mechanism of the
formation of cyclopentadienyl amine was reported by our group
in 2008 (Scheme 6).4a
Computational Results. To further investigate mecha-
nisms of these processes, DFT calculations were carried out to
find these reaction pathways leading to three different types of
products. According to the experimental results, the reaction
between 1,4-dilithio-1,3-butadienes and nitriles would result in
three pathways after quenching: (a) pyridine derivatives, (b)
bipyrrolines, and/or (c) cyclopentadienyl amine. To our
knowledge, one of the most dominant factors affecting the
chemoselectivity is the structure of substrates. Thus, we focused
on three structurally different substrate systems (Scheme 8).
The calculations were carried out at the PBE1PBE/6-31G(d)
level,8 and the results are discussed in the following sections.
At first, we tried to explore the potential energy surfaces of
the reaction pathways using 1,2,3,4-tetramethyl-1,4-dilithio-1,3-
The reaction of cyclic 1,4-dilithio 1,3-dienes 5 and 2 equiv of
trimethylacetonitrile forms Δ1-bipyrroline 6a in good yields
after quenching. We tried to get the corresponding lithium-
containing intermediate. After washing with Et2O, a solid
product was collected. Quenching this solid with DCl yielded
1
6a-D as the sole product. The H NMR spectrum of this solid
showed no alkenyl hydrogen, indicating lithium atoms are
adjacent to carbon atoms. As the precursor of 6a, the structure
of 19 is proposed as shown in Scheme 7.
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Figure 5. DFT-calculated potential energy surfaces of pathways a and b in system B. Coordinated solvent molecules are omitted for clarity.
Figure 6. DFT-calculated potential energy surfaces of pathway c in
system B. Coordinated solvent molecules are omitted for clarity.
butadiene (with two diethyl ethers as coordinated solvent
Figure 7. DFT-optimized structures of IMB2a, IMB2c, IMB4, and
TSB4. Coordinated solvent molecules are omitted for clarity. Selected
molecules) and benzonitrile as the substrates (Scheme 8,
system A). Previously, both experimental6 and computational5,9
atom distances (in Å): IMB2a:, C1−C5 = 3.170; IMA2c, C1−C5 =
studies have showed that the doubly lithium bridged structure
IMA1 is the most stable configuration. Insertion of the first
2.924, C1−Li1 = 2.081, N1−Li1 = 2.006, N1−Li2 = 1.916, N2−Li2 =
2.021; TSB4, C1−C5 = 2.084, C1−Li1 = 2.124, N1−Li1 = 2.126,
nitrile into one C−Li bond generates the 1,6-dilithium
intermediate IMA2a. A similar process between IMA2a and
the second nitrile results in the 1,8-dilithium intermediate
IMA3 (Scheme 9). Our calculations suggest that IMA2a and
IMA3 also show doubly lithium bridged configurations (Figure
1). Each insertion of a nitrile molecule decreases the free
energy of the whole system, suggesting that these nitrile
insertion processes are thermodynamically favored.
The two cyclization types of IMA3 lead to two different
pathways: 1,6-cyclization results in the six-membered-ring
product IMA7, and 1,5-cyclization results in the bicyclic
product IMA9. Experimentally, the pyridine derivative IMA6 is
formed before quenching, and 1,3,5-triazine was found after
workup. On the basis of this result, intermediate IMA7 is
proposed to undergo further reaction. A PhC(Li)NLi moiety
is eliminated from IMA7 and reacts with benzonitrile to form
N1−Li2 = 1.864, N2−Li2 = 2.008.
triazine derivative IMA10. This process decreases the Gibbs
free energy even more (Figure 2, pathway a).
It is too difficult for IMA2a to transform to pyridine
derivative IMA5 via 6π cyclization, as lithium-adjacent carbon
and nitrogen atoms both contain a negative charge. The
transition state of 6π cyclization of IMA2a is also not found
(Scheme 10).
On the other hand, 1,5-cyclization of IMA3 gives
intermediate IMA8, which undergoes another 1,5-cyclization
process to generate bicyclic intermediate IMA9 (Figure 2,
pathway b). In this system, the pyridine pathway gives more
stable products, although the energy barrier is higher than for
the bipyrroline pathway. The calculation results above are in
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Figure 8. DFT-calculated potential energy surfaces of pathways a and b in system C. Coordinated solvent molecules are omitted for clarity.
clearly show that pathway a is more favored than pathway b and
are in agreement with the experiments.
In the experiments, system B is the only case to afford the
cyclopentadienyl amine product. This result is also supported
by DFT calculations. The result indicates that IMB2c is more
stable than IMB2a in free energy (Figure 6). Although the C1−
C5 distances in these two structures are almost the same (3.170
Å for IMB2a and 3.195 Å for IMB2c), the N2−Li2 distances in
IMB2c and TSB4 clearly show the strong coordination effect of
the pyridyl group to the lithium atom (Figure 7). This could
explain the stabilization of IMB2c. For the same reason, the
free energy of IMB4c is lower than that of either IMB2c or
IMB2a. The barrier of each step toward IMB4 is reasonable.
The results above show that IMB4 could be formed under the
appropriate conditions as the kinetic product.
In system C, the pathways leading to the pyridine derivative
Figure 9. DFT-calculated potential energy surface of pathway c in
and the bipyrroline derivative also diverge at the stationary
point IMC3. The energy barriers of pathway b starting from
IMC3 to IMC9 (13.1 kcal/mol) are much lower than that of
pathway a starting from IMC3 to IMC7 (21.4 kcal/mol).
Consideration of the entire pathways gives a similar result: for
pathway a, the rate-determining step is the cyclization step (via
TSC3) with an energy barrier of 21.4 kcal/mol; for pathway b,
the rate-determining step is the addition of the first tBuCN (via
TSC1) with an energy barrier of 15.4 kcal/mol. These results
show that the bipyrroline product is kinetically favored in
system C and are in good agreement with the experimental
results (Figure 8).
system C. Coordinated solvent molecules are omitted for clarity.
agreement with the fact that only pyridine and triazine products
are formed in the type I reaction.
The potential energy surface of pathway c is shown in Figure
3. The intermediate IMA2a changes to the isomer IMA2c
before the formation of cyclopentadienyl amine IMA4. Since
one of the lithium bridges is broken, the energy of IMA2c is
higher than that of IMA2a. The C1−C5 distances in IMA2a
and IMA2c are 3.366 and 2.924 Å, respectively (Figure 1).
These data clearly indicate that IMA2c is more sterically
favored for the intramolecular addition, despite the lack of
stabilization from the double lithium bridge. However, the
Gibbs free energy of IMA4 is similar to that of IMA2a and is
much higher than that of IMA3. This is probably because of the
destruction of the double lithium bridge in IMA2c. As a result,
IMA4 cannot be formed as the final product. Optimized
structures of all transition states of system A are shown in
Figure 4.
In system B, the pathways to the pyridine and the bipyrroline
products are found to be similar to those in system A. The
Gibbs free energy in solution of TSB3 is −28.3 kcal/mol, 6.5
kcal/mol lower than TSB6, indicating that pathway a has a
lower energy barrier than pathway b. The final products of
pathway a (IMB6 and IMB10) are also thermodynamically
more stable than that of pathway b (Figure 5). These results
The energy profiles of system C with cyclic 2,3-disubstituted
1,4-dilithiobutadiene and tert-butyl cyanide as the starting
materials is shown in Figure 9. Pathway c leading to
cyclopentadienyl amine is strongly thermodynamically disfa-
vored, since the free energy of IMC4 is too high. The large
C1−C5 distance in IMC2a (3.306 Å) also shows that
transformation of IMC2a to IMC4 is disfavored. It should be
noted that the structure of IMC2c does not exist as a stationary
t
point, probably due to the steric hindrance of the Bu group in
IMC2a.
In DFT calculations, it is found that the six-membered ring
could have either a chair or a boat configuration. In the
structure of IMC1, the cyclic ring exhibits a boat configuration,
because the boat configuration could keep the butadiene
moiety coplanar and thus minimize the energy of the lithium
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Figure 10. DFT-optimized structures of stationary points in system C. Coordinated solvent molecules are omitted for clarity. Selected atom distance
(in Å): IMC2, C1−C5 = 3.306.
bridges. In other stationary points, it is not necessary for the
butadiene moiety to be coplanar; thus, the chair configuration is
most favored (Figure 10).
systems A and C, IM10 is more stable than IM11. For system
B, IMB11 has a lower free energy than IMB10. Still, the
structures with higher free energy could not be ruled out.
Taking the case of system A, calculations show that IMA10 is
16.4 kcal/mol more stable than IMA11. In the structure of
IMA10, the two lithium atoms are chelated by three nitrogen
atoms. In the structure of IMA11, it seems that the lithium
atoms are coordinated with “aza-allylic” systems (Figure 11).
IMA11 is more in line with the experimental fact that the
intermediate “lithium triazine” is highly sensitive to air and
moisture and is difficult to test by XRD. Hence, both structure
types are possible and the real structures of the “lithium
triazine” need further crystallographic work.
Although triazines are obtained along with pyridines after
quenching, the structures of the precursors of triazines,
however, are still unknown in the experiments. We have
proposed two types of precursors: one involves three nitriles,
and the other involves four. Both types of structures should
involve two lithium atoms. After full optimization, IMA10,
IMB10, and IMC10 show similar configurations, while IMA11,
IMB11, and IMC11 look much different: IMA11 involves a
dilithio bridge moiety; the configuration of IMB11 is almost
planar with all rings and lithium atoms involved; IMC11 does
not have any lithium bridge, and the triazine ring shows boat
configuration (this configuration is similar to IMA5). For
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Figure 11. Structures and potential energies of IM10 and IM11. Coordinated solvent molecules are omitted for clarity. ΔGsol, ΔGgas (in
parentheses), and ΔHgas values (in brackets) are given in kcal/mol.
purification. Solvents were purified by an Mbraun SPS-800 Solvent
CONCLUSIONS
■
Purification System and dried over fresh Na chips in the glovebox.
In summary, we have investigated mechanisms for the reactions
of 1,4-dilithio-1,3-butadienes and nitriles through both experi-
t
nBuLi and BuLi were obtained from Acros.
Organometallic samples for NMR spectroscopic measurements
ments and DFT calculations. In the type I reaction, the pyridine
derivatives are formed before quenching. The lithium-
containing triazine intermediates have been isolated and their
structures explored through various quenching experiments.
The computational results suggest that the selectivity of these
reaction systems is strongly affected by substrates: tetraalkyl-
substituted 1,4-dilithio-1,3-butadienes and aryl nitriles (e.g.,
benzonitrile and pyridylnitrile) give pyridine and triazine
products (systems A and B), while cyclic 2,3-disubstituted
1,4-dilithiobutadiene 5 and tertiary aliphatic nitriles generate
Δ1-bipyrrolines (system C). In system B, the cyclopentadienyl
amine product is formed as the kinetic product due to the
coordination of the pyridyl group to a lithium atom. These
calculation results are in good agreement with experimental
observations.
were prepared in the glovebox by use of J. Young valve NMR tubes
(Wilmad 528-JY). 1H and 13C NMR spectra were recorded on a
Bruker-400 spectrometer (FT, 400 MHz for 1H; 100 MHz for 13C) or
a JEOL-AL300 spectrometer (FT, 300 MHz for 1H; 75 MHz for 13C)
at room temperature, unless otherwise noted.
Synthesis and Isolation of Dilithium Triazine (10). 1,2,3,4-
Tetraethyl-1,4-dilithiobuta-1,3-diene (1b) was prepared according to
the literature.3a To a solution of 1b (1.78 g, 10.0 mmol) in 50.0 mL of
Et2O in a 200 mL Schlenk tube was added PhCN (4.12 g, 40.0 mmol)
at −78 °C (dry ice/acetone); the mixture was stirred for 5 min and
then was warmed to room temperature. After the mixture was stirred
for 3 h, the Schlenk tube was transferred to the glovebox. Then 175.0
mL of Et2O was added to the mixture with vigorous stirring. Filtration
separated the mixture to the filtrate and a solid part. The solid part was
washed with a solvent mixture (petroleum ether/ethyl ether 4/1). The
solvent was evaporated under vacuum to give product 10.
Procedure for the Trapping of 9b. To a solution of 1b (178 mg,
1.0 mmol) in 5.0 mL of Et2O in a 50 mL Schlenk tube was added
PhCN (412 mg, 4.0 mmol) at −78 °C (dry ice/acetone). After it was
stirred for 30 min at −78 °C, the mixture was quenched with saturated
aqueous NaHCO3. When the sample was analyzed by GC-MS, the
new product 8 was found with an m/z value of 269 except for PhCN
and 1,2,3,4-tetraethylbuta-1,3-diene. These results indicated the
formation of the N-lithio ketimine intermediate 9b with one C−Li
bond left.
EXPERIMENTAL SECTION
■
General Methods. All reactions were conducted under a slightly
positive pressure of dry nitrogen using standard Schlenk line
techniques or under a nitrogen atmosphere in a Mikrouna Super
(1220/750) glovebox. The nitrogen in the glovebox was constantly
circulated through a copper/molecular sieves catalyst unit. The oxygen
and moisture concentrations in the glovebox atmosphere were
monitored by an O2/H2O Combi-Analyzer to ensure both were
always below 1 ppm. Unless otherwise noted, all starting materials
were commercially available and were used without further
Procedure for Quenching Reactions of 10. To a solution of 10
(338 mg, 1.0 mmol) in 5.0 mL of Et2O in a 50 mL Schlenk tube was
added an electrophile (MeI, Et3OBF4, RBr, etc.) at −78 °C (dry ice/
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acetone). Then the mixture was warmed to room temperature and
stirred for 2 h. Because the product is easily decomposed into triazine
3, the solvent mixture was evaporated under vacuum directly. The
crude products were characterized by NMR.
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Computational Methods. All calculations were carried out with
the GAUSSIAN 03 program package.10 All the minima and transition
states were fully optimized at the PBE1PBE level11 using the 6-31G(d)
basis set. Harmonic frequency calculations were performed at the same
level for every structure to confirm it as a local minimum or transition
state and to derive the thermochemical corrections for enthalpies and
free energies. The intrinsic reaction coordinate (IRC) analysis12 was
carried out throughout the pathways to confirm that all stationary
points are smoothly connected to each other. Solvent effects in Et2O
(ε = 4.335) were evaluated by single-point calculations using the PCM
model13 for all the stationary points. All enthalpies and Gibbs free
energies in the text are given in kcal/mol and were calculated under
standard conditions (298 K, 1 atm). All distances are given in Å.
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ASSOCIATED CONTENT
* Supporting Information
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S
Text, figures, and tables giving experimental procedures and
computational details. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for Z.X.: zfxi@pku.edu.cn.
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Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NSFC and the 973 Program
(2011CB808700). We thank Prof. Zhi-Xiang Yu of Peking
University and Prof. Yuxue Li of Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, for insightful
discussions and comments.
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