Homolytic Cleavage of a B-B Bond by the Cooperative Catalysis of Two Lewis Bases: Computational Design and Experimental Verification

Article abstract of DOI:10.1002/anie.201511917

Density functional theory (DFT) investigations revealed that 4-cyanopyridine was capable of homolytically cleaving the B-B σ bond of diborane via the cooperative coordination to the two boron atoms of the diborane to generate pyridine boryl radicals. Our experimental verification provides supportive evidence for this new B-B activation mode. With this novel activation strategy, we have experimentally realized the catalytic reduction of azo-compounds to hydrazine derivatives, deoxygenation of sulfoxides to sulfides, and reduction of quinones with B₂(pin)₂ at mild conditions. Breaking good: The diborane B-B bond can be homolytically cleaved via the cooperative catalysis of two 4-cyanopyridine molecules. Using this combination of a diborane (B₂(pin)₂) and 4-cyanopyridine also allows the catalytic reduction of the N=N double bond of azo-compounds to hydrazine derivatives, deoxygenation of sulfoxides to sulfides, and reduction of quinones under mild conditions.

Full text of DOI:10.1002/anie.201511917

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Communications
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International Edition: DOI: 10.1002/anie.201511917
German Edition: DOI: 10.1002/ange.201511917
BÀB Bond Activation
À
Homolytic Cleavage of a B B Bond by the Cooperative Catalysis of Two
Lewis Bases: Computational Design and Experimental Verification
Guoqiang Wang, Honglin Zhang, Jiyang Zhao, Wei Li, Jia Cao, Chengjian Zhu,* and
Shuhua Li*
[
11]
Abstract: Density functional theory (DFT) investigations
revealed that 4-cyanopyridine was capable of homolytically
cleaving the BÀB s bond of diborane via the cooperative
derivatives, strong bases such as N-heterocyclic carbenes
[
6]
[7b]
(NHC), Cs CO , t-BuOK are usually used in the activa-
2
3
tion of diborane compounds. Given the considerable utility of
diboranes in synthetic chemistry, it is highly desirable to
develop new strategies for the activation of BÀB bond.
coordination to the two boron atoms of the diborane to
generate pyridine boryl radicals. Our experimental verification
provides supportive evidence for this new BÀB activation
2
3
Note that in the Lewis adducts of sp –sp diborane
2
mode. With this novel activation strategy, we have experimen-
tally realized the catalytic reduction of azo-compounds to
hydrazine derivatives, deoxygenation of sulfoxides to sulfides,
and reduction of quinones with B (pin) at mild conditions.
compounds, there is another sp hybridized boron atom. We
envisioned that if the empty p orbital of the uncoordinated
boron atom is associated with a second Lewis base, the BÀB
bond would be further weakened, and might be cleaved in
2
2
D
iborane compounds, R BÀBR , are highly useful
2
2
synthetic modules for concise synthesis of organo-
[
1]
boronates, which are versatile building blocks for
the generation of new chemical bonds using the
[2]
Suzuki–Miyaura crossing-coupling reactions. Many
transition-metal complexes are known to be able to
effectively cleave the BÀB bond of diborane and
catalyze the borylation or diboration of unsaturated
[
3,4]
organic substrates.
On the other hand, transition-
metal-free catalytic systems for activating the BÀB
bond of diborane compounds have also attracted
[
5]
much attention.
2
3
Recently, the Lewis adducts of sp –sp diborane
compounds have been reported for the borylation or
[5–9]
diboration of unsaturated substrates.
It has been
shown that diborane compounds became a source of
nucleophilic boryl moiety, when one of the boron
atoms of the diborane was tetracoordinated to
a strong base (Scheme 1a). In such cases, the B-
Scheme 1. Representative transition-metal-free BÀB bond activation modes.
a) Heterolytic BÀB bond cleavage using strong bases. b) Homolytic BÀB
bond cleavage with 4-cyanopyridine.
2
3
(
ly.
sp )ÀB(sp ) bonds are always cleaved heterolytical-
[6c,7b]
In addition, reductive addition of diboranes to steri-
a homolytic manner. With appropriate Lewis bases, the boryl
radical generated from the homolytic cleavage of diborane
cally hindered pyrazines in the presence of bipyridine
[10]
derivatives has also been reported.
Nevertheless, the
BÀB bond could be sufficiently stable, through the possible
[12]
mechanistic details of this reaction are unclear yet. Owing
captodative effect.
Pyridines with electron-withdrawing
to the high BÀB bond dissociation energy (BDE) of diborane
substituents may act as appropriate Lewis bases to activate
the BÀB bond of diborane compounds as a result of their high
boryl radical stabilization energies (see Scheme S1 in the
Supporting Information). Herein, we report a computation-
ally designed and experimentally verified catalyst which can
homolytically cleave the BÀB bond of B (pin) via the
[
*] G. Wang, J. Zhao, W. Li, J. Cao, Prof. Dr. S. Li
Key Laboratory of Mesoscopic Chemistry of Ministry of Education,
Institute of Theoretical and Computational Chemistry, School of
Chemistry and Chemical Engineering, Nanjing University
Nanjing, 210093 (P.R. China)
2
2
cooperative Lewis base mechanism (Scheme 1b).
E-mail: shuhua@nju.edu.cn
Before verifying our hypothesis experimentally, we per-
H. Zhang, Prof. Dr. C. Zhu
formed density functional theory (DFT) calculations with the
State Key Laboratory of Coordination Chemistry, School of Chemistry
and Chemical Engineering, Nanjing University
Nanjing 210093 (P.R. China)
[13]
M06-2X functional to determine whether the BÀB bond of
B (pin) could be activated in this way. Computational details
2
2
E-mail: cjzhu@nju.edu.cn
are given in the Supporting Information. The 4-cyanopyridine
1) was first selected to be the Lewis base. The free-energy
profile of the reaction between 4-cyanopyridine and B (pin)
(
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201511917.
2
2
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Scheme 2. Free-radical trapping experiments.
cyanopyridine gave phenylboronate in 35% yield at the same
reaction conditions (Scheme 2b). Our DFT calculations
suggested that the boryl radical (INT4) could induce the
homolytic OÀO bond cleavage of BPO to form the benzoy-
loxy radical (see Figure S5). This experiment provides
a supportive evidence for the homolytic cleavage of the BÀ
B bond of B (pin) .
2
2
We also carried out electron paramagnetic resonance
EPR) experiments to examine the possibility of directly
(
detecting the boryl radical (INT4). When B (pin) (2) was
2
2
added to a THF solution of 4-cyanopyridine, a spectrum at
g = 2.00319 (298 K) was obtained, as shown in Figure 2. The
À1
Figure 1. Computed Gibbs free energy (in kcalmol ) profile for the 4-
cyanopyridine catalyzed homolytic BÀB bond cleavage in B (pin) in
2
2
the solvent (benzene). Hydrogen atoms and the methyl group of
pinacol boron are omitted for clarity. Interatomic distances are in Š.
(2) is shown in Figure 1 (see Supporting Information for
details). First, the coordination of 4-cyanopyridine to one
2
3
boron atom of B (pin)2 generates the sp –sp diborane
2
(
INT1). Then, a second 4-cyanopyridine molecule is coordi-
2
nated to the sp boron atom of INT1 to form a possible
intermediate (INT2) via TS1, with a barrier of 13.5 kcalmol .
À1
Figure 2. X Band EPR spectrum obtained in THF at 298 K. a) Spectrum
of 0.1m solutions of 4-cyanopyridine. b) Spectrum of a 1:1 mixture of
Owing to the formation of two strong BÀN bonds, this
À1
intermediate is only 13.5 kcalmol in free energy above the
4
-cyanopyridine and B (pin) .
2 2
separated reactants. Then, the homolytic cleavage of the BÀB
À1
bond in INT2 occurs via TS2 with a barrier of 17.9 kcalmol
(
relative to the separated reactants) to generate a diradical
hyperfine splittings (hfs) together with the spin-density
distribution suggested that the unpaired electron of INT4
was mainly delocalized over the 4-cyanopyridine moiety. The
direct observation of the EPR signal of the boryl radical
(INT4) provides a strong support for the proposed mecha-
nism in Figure 1. Interestingly, in the chemistry of frustrated
species (INT3), in which two radicals are loosely bound via
the p–p stacking interaction. Our local correlation post-
Hartree–Fock calculations (see supporting information for
details) also lead to a barrier of 18.2 kcalmol , being very
close to the M06-2X result (17.9 kcalmol ). Finally, this
[14]
À1
À1
[15]
diradical species may readily dissociate into two boryl radicals
Lewis pairs (FLP) or chemistry containing Lewis adduct of
2
3
[5–9]
(INT4). The rate-determining step in this process is the
sp –sp diboranes,
the Lewis bases are always regarded as
homolytic BÀB bond cleavage, and the whole process is
an electron donor. On the contrary, 4-cyanopyridine behaves
À1
exothermic by 20.4 kcalmol (with respect to the separated
as a single-electron acceptor in the homolytic BÀB bond
reactants). The reactions of B (pin) and other substituted
cleavage of B (pin) .
2
2
2
2
pyridines were also investigated theoretically. Our calcula-
tions suggested that 4-cyanopyridine is the most effective
Lewis base for BÀB bond activation, which has the lowest
Intrigued by the above computational and experimental
results, we further attempt to explore whether this novel BÀB
activation mode can be employed in some chemical trans-
formations. First, we focused our attention on the catalytic
reduction of the N=N double bond of the azobenzene with
B (pin) . The reductive diboration of N=N double bond has
activation barrier.
Subsequently, we performed free-radical trapping experi-
ments to verify whether the BÀB bond of B (pin) could be
2
2
2
2
homolytically cleaved with 4-cyanopyridine to generate the
boryl radical (INT4). In the absence of 4-cyanopyridine, a 1:1
mixture of benzoyl peroxide (BPO) and B (pin) was stirred
only been reported with rather specific diboron com-
[16]
pounds. Herein, we demonstrated that the catalytic reduc-
tion of azoarenes can be achieved with B (pin) using 4-
2
2
2
2
at room temperature for 4 h, and no desired product was
detected (Scheme 2a). However, the addition of 20 mol% 4-
cyanopyridine as the catalyst. In the presence of 20 mol% of
4-cyanopyridine, product 4a was obtained at 708C in 73%
5
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[a]
yield (Table 1, entry 4). After further screening, the highest
Table 2: Scope of the catalytic reduction of azo-compounds.
yield of 4a (99%) was obtained with 1.5 equivalent of
B (pin) in pentane (entry 7). Some solvents (CH Cl and
2
2
2
2
CH CN) were found to be not suitable for this reaction,
3
probably because of their greater stabilization for the free-
radical intermediate (INT4).
[b]
Substrates
Products
Yield [%]
[
a]
Table 1: Optimization of the reaction conditions.
3b
4b
77
99
3
c
4c
4d
[b]
3d
3e
96
Entry
Cat. [mol%]
B (pin) [equiv]
Solvent
Yield [%]
2
2
1
2
3
4
5
6
7
none
20
20
20
20
1.2
1.2
1.2
1.2
1.2
1.2
1.5
pentane
CH Cl
CH CN
n.d.
trace
trace
73
85
88
2
2
4e
4 f
4g
4h
4i
87
98
99
99
75
99
3
THF
3
f
hexane
pentane
pentane
20
20
99
3g
3h
[
a] Reaction conditions: azobenzene (0.20 mmol), solvent (1.0 mL),
4 h. Catalyst loading relative to azobenzene. [b] Yields were determined
by H NMR analysis of the crude reaction mixture with CH Br as an
2
1
2
2
internal standard.
3
3
i
j
4j
Under the optimal reaction conditions, we further exam-
ined the substrate scope of this catalytic reaction with various
azo-compounds (Table 2). The azoarenes with electron-
donating groups (3b–d) as well as electron-withdrawing
groups (3e–j) on the phenyl were reduced in the reaction,
affording the corresponding diaryl hydrazines in good to
excellent yields (75–99%). Meanwhile, the position of
substituents on the phenyl showed little influence on the
reactivity (3b–g). Interestingly, for the substrate with ester
groups (3j), only the N=N double bond of azobenzene was
[
a] Reaction conditions: azoarenes (0.20 mmol) with B (pin)
2 2
(0.30 mmol) in pentane (1.0 mL) at 708C for 24 h. Catalyst loading
1
relative to azobenzene. [b] Yields were determined by H NMR analysis
of the crude reaction mixture with CH Br as an internal standard.
2
2
reduced. The halide (F, Cl, Br) groups were also well tolerated
(3e–i).
The possible mechanism for the reduction of azobenzene
was also investigated using DFT calculations. The most
probable pathway is shown in Scheme 3 (the structures of
all stationary points are listed in Figure S4 of the Supporting
Information). As shown in Scheme 3, the reduction of
azobenzenes occurs via a successive boryl radical migration
mechanism. The rate-determining step in this reaction (via
À1
TS3) has a barrier of 18.9 kcalmol , and the whole process is
À1
exothermic by 19.2 kcalmol (with respect to the reactants
3
a and INT4). The calculated results are consistent with the
mild experiment conditions (at 708C). The experimental
results and DFT calculations showed that the reduction of the
N=N double bond of azo-compounds could be achieved at
mild conditions via the proposed new BÀB bond activation
Scheme 3. The suggested catalytic cycle for the reduction of the
azobenzene with B (pin) using 4-cyanopyridine as catalyst. The
2
2
relative Gibbs free energies (relative to 3a and INT4) are given in
mode.
À1
parentheses (in kcalmol ).
With the 4-cyanopyridine/B (pin) system, we also inves-
2
2
tigated the reductive deoxygenation of sulfoxides to sulfides
Scheme 4). In the presence of the catalytic amount of 4-
cyanopyridine, diphenyl or phenyl methyl sulfoxides bearing
electron-withdrawing groups (Br, Cl, CN) were reduced at
(
(50–69%). Although many methods have been developed for
[17]
deoxygenation of sulfoxides, some of these processes have
[17a]
[17b]
used moisture-sensitive reagents,
or transition metals.
8
08C to afford the desired sulfides in moderate isolated yields
The possible reaction pathway for this type of reaction was
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compounds and quinones, and deoxygenation of sulfoxides
to sulfides, with 4-cyanopyridine and B (pin)2 at mild
2
conditions. Further studies to find more efficient Lewis
bases for homolytic BÀB bond activation, and explore other
diborane involved transformations are underway.
Acknowledgements
Scheme 4. 4-Cyanopyridine-catalyzed deoxygenation of sulfoxides to
sulfides with B (pin) . [a] The reaction was performed in toluene.
2
2
This work was supported by the National Natural Science
Foundation of China (Grants No. 21333004 and No.
21361140376) and the National Basic Research Program
(Grant No. 2011CB808501).
also explored with DFT calculations, with the diphenyl
sulfoxide as the substrate. Our calculations suggest that this
reaction occurs via a stepwise mechanism (see Figure S6). The
rate-limit step of the whole reaction has a barrier of
Keywords: BÀB bond activation · computational design ·
À1
1
7
6.0 kcalmol , and the whole process is exothermic by
cooperative activation · homolytic cleavage · Lewis base
À1
9.8 kcalmol . Therefore, the use of B (pin) as reducing
2
2
How to cite: Angew. Chem. Int. Ed. 2016, 55, 5985–5989
Angew. Chem. 2016, 128, 6089–6093
reagents may represent a practical method for deoxygenation
of sulfoxides to sulfides, owing to the low toxicity, high
stability of B (pin) .
2
2
Furthermore, the 4-cyanopyridine/B (pin)2 system was
applied to achieve the catalytic reduction of quinones. The
reduction of 1,4-naphthoquinone and 2-methylnaphthalene-
2
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2
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[
2
2
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2
2
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5
988
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Communications
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