Reversible [4 + 2] cycloaddition reaction of 1,3,2,5-diazadiborinine with ethylene

Article abstract of DOI:10.1039/c5sc03174e

Under ambient conditions, a [4 + 2] cycloaddition reaction of 1,3,2,5-diazadiborinine 1 with ethylene afforded a bicyclo[2.2.2] derivative 2, which was structurally characterized. The cyclization process was found to be reversible, and thus retro-[4 + 2] cycloaddition reproduced 1 quantitatively, concomitant with the release of ethylene. Compound 1 reacted regio-selectively and stereo-selectively with styrene derivatives and norbornene, respectively, and these processes were found to be reversible too. Computational studies determined the reaction pathways which were consistent with the regio-selectivity observed in the reaction of styrene, and the reaction was suggested to be essentially concerted but highly asynchronous.

Full text of DOI:10.1039/c5sc03174e

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DOI: 10.1039/C5SC03174E
Journal Name
ARTICLE
Reversible [4+2] Cycloaddition Reaction of 1,3,2,5-Diazadiborinine
with Ethylene
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
b
a
,a
,a
Di Wu, Rakesh Ganguly, Yongxin Li, Sin Ni Hoo, Hajime Hirao* and Rei Kinjo*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Under ambient conditions, a [4+2] cycloaddition reaction of 1,3,2,5-diazadiborine 1 with ethylene afforded a bicyclo[2.2.2]
derivative 2, which was structurally characterized. The cyclization process was found to be reversible, and thus retro-[4+2]
cycloaddition reproduced 1 quantitatively, concomitant with the release of ethylene. Compound 1 reacted regio-
selectively and stereo-selectively with styrene derivatives and norbornene, respectively, and these processes were found
to be reversible too. Computational studies determined reaction pathways consistent with the regio-selectivity observed
in the reaction of styrene, and the reaction was suggested to be essentially concerted but highly asynchronous.
demonstrated that ethylene could be incorporated onto
9
Introduction
phosphonium sila-ylide, giving rise to silirane IIb
.
The
formation of IIb depended on the ethylene pressure, and
indeed, IIb reverted to IIa after a decrease in ethylene
pressure. Recently, Tuononen and Power et al. reported
reversible binding of ethylene to two-coordinate acyclic
silylene IIIa under ambient conditions, which generated sililane
Reversible coordination processes between a metal center and
olefins are of paramount significance in transition metal
1
catalysis. The labile propensity of metal-olefin interactions
2
can be accounted for by the Dewar-Chatt-Duncanson model;
this model rationalizes, for example, the bonding mode in
10
2
3
IIIb with a tetra-coordinate silicon atom. Very recently,
Tokitoh and Sasamori et al. have reported that 1,2-
digermacyclobutene IVa underwent sequestration of two
Zeise’s salt [Cl Pt(η -C H )]K, in which donation of

-electrons
from ethylene to an unoccupied d-orbital of the metal and
backdonation of d-electrons from the metal to the *-orbital
3
2 4

-

11
4
ethylene molecules to furnish IVb. Retro-conversion of IVb to
of ethylene occur concurrently. Such simultaneous donor-
acceptor interactions confer unique chemical properties on
transition metal complexes.
IVa was observed by reducing ethylene pressure. In these
reactions, formation of the cycloadducts is inferred to involve
a [1+2] interaction between the heavier group 14 element
center and the C=C bond of ethylene in the initial step. With a
group 13 compound, Jordan et al. reported that cationic
In recent years, it has been amply demonstrated that main
5
group elements can behave as transition metals, e.g., in
6
unique activation reactions of ethylene as well as other
7
aluminum
-diketiminate derivative Va and ethylene
olefins where several systems were found to bind to olefins
underwent a [4+2] cycloaddition reaction, yielding cycloadduct
reversibly. In 2009, Power et al. reported a cycloaddition
12
Vb, which was characterized only by NMR spectroscopy.
reaction of distannyne Ia with two equivalents of ethylene,
8a
Although Vb did not release ethylene even under vacuum,
addition of N,N-dimethylaniline to a solution of Vb allowed
ethylene elimination concomitant with the formation of Vc at
room temperature. Additive-free reversible [4+2] cyclization
which afforded a bicycle[2.2.0] derivative Ib
. Remarkably, Ib
readily released ethylene molecules via thermal retro-
cycloaddition under reduced pressure or upon standing at 25
o
C, to reproduce Ia. Kato, Baceiredo, and co-workers
between a 6-compound and ethylene still remains highly
13
challenging.
Recently, we have reported the preparation of aromatic
a
Division of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, Singapore 637371,
Singapore. E-mail: rkinjo@ntu.edu.sg; hirao@ntu.edu.sg
b NTU-CBC Crystallography Facility, Nanyang Technological University, Singapore
1
4a
1,3,2,5-diazadiborinine
1
,
which formally involves a neutral
5a,14b-
tricoordinate boron center surrounded by eight electrons.
Preliminary studies indicated the unique reactivity of
6
37371, Singapore
h
1
†
Electronic supplementary information (ESI) available: Experimental and
calculation details, and crystallographic information for
2
,
4a
,
4b
,
4d
,
6
. CCDC
toward unsaturated compounds such as carbon dioxide,
forming bicyclo[2.2.2] derivatives, which prompted us to
1
418724, 1418725, 1418726, 1418727, and 1418728. For ESI and crystallographic
data in CIF or other electronic format see. See DOI: 10.1039/x0xx00000x
investigate the reactions of
1
with ethylene and other non-
This journal is © The Royal Society of Chemistry 20xx
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ARTICLE
Journal Name
activated olefins in detail. Herein, we report reversible shortening of the C3–N1 (1.302(4) Å) bond, compared to those
View Article Online
14
DOI: 10.1039/C5SC03174E
cycloaddition reactions of
1
with various alkenes. We also of 1. Compound 2 is stable at room temperature, both in the
describe single-crystal X-ray diffraction analysis of the solid state and in solution. However, upon heating a C D
6
6
o
cycloadducts, kinetic experiments, and computational studies.
solution of
rise to
between benzene and ethylene as well as the retro Diels-Alder
2
at 150 C, a clean ring opening occurred, giving
1
quantitatively. Note that the Diels-Alder reaction
15
reaction have scarcely described thus far, despite the
extensive application of the elementary reactions in organic
16
synthesis.
Scheme 1 Reversible [4+2] cycloaddition reaction between 1 and
ethylene.
Fig. 1 Examples of reversible cycloaddition reactions between main
group compounds and ethylene.
Fig. 2 Solid-state structure of 2. Thermal ellipsoids are set at the
5
0% probability level. Hydrogen atoms except for those on the C1
and C2 atoms are omitted for clarity. Selected bond lengths [Å] and
angles [ ]: B1–C1 1.653(5), B1–N1 1.588(5), B2–C2 1.671(5), B2–C3
o
Results and discussion, Experimental
Reaction of 1 with ethylene
1.593(5), C1–C2 1.550(4), C3–N1 1.302(4); N1–B1–C1 104.0(3), B1–
C1–C2 110.4(3), C1–C2–B2 113.3(3), C2–B2–C3 100.7(3).
Ethylene was introduced into a benzene solution of
and the reaction mixture was stirred at ambient temperature
for 3 h. After removal of all volatiles in vacuo, compound was
obtained as a white power in 97% yield (Scheme 1). In the
NMR spectrum of , two peaks were detected at ‒16.0 ppm
and 1.6 ppm that are shifted upfield with respect to those (7.3
1 at 1 bar
2
1
1
B
2
Reaction of 1 with styrene derivatives
To examine the scope of the cycloaddition reaction, we
14
ppm and 24.9 ppm) for the corresponding boron atoms in
Recrystallization from a saturated benzene solution at room
temperature afforded single crystals of , and the solid-state
structure was confirmed by X-ray diffraction analysis (Figure
). Both of the boron centers are tetra-coordinated, indicating
1.
employed styrene derivatives
3
as alkene substrates (Scheme
2
). A slight excess amount (2.0 eq) of styrene 3a was added to
2
a C D solution of
1 in a J-Young NMR tube, and the reaction
6
6
was monitored by NMR spectroscopy. This reaction was
drastically slower than that of ethylene, with full conversion of
2
3
sp hybridization. The B1–C1 (1.653(5) Å) and B2–C2 (1.671(5)
Å) distances are significantly longer than the typical B–C single
bond (1.59 Å). The long C1–C2 (1.550(4) Å) distance is an
1
4
observed only after 12 h at room temperature. After workup,
11
a
was isolated as a white powder in 90% yield. In the B NMR
spectrum of 4a, two singlets appeared at ‒14.3 ppm and 0.8
ppm, which are comparable to those (‒16.0 ppm and 1.6 ppm)
3
3
indication of a C(sp )–C(sp ) single bond. There is a lengthening
of the B1–N1 (1.588(5) Å) and B2–C3 (1.593(5) Å) bonds and a
of
2
. X-ray diffraction analysis revealed the bicyclo[2.2.2]
2
| J. Name., 2012, 00, 1-3
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structure of 4a and confirmed that the CH and the CHPh
ARTICLE
Interestingly, we noted that the cyclizatiVoienw ArrtieclaecOtniolinne
2
carbons of 3a selectively bind to the boron between two proceeded faster with styrenes be
D
a
O
ri
I
n
: 1
g
0.10
a
3
n
9/C5
e
S
l
C
e
0
ct
3
r
1
o
74
n
E
-
nitrogen atoms and the boron between two carbon atoms in withdrawing group under the same reaction conditions. Thus,
1
,
respectively (Figure 3a). Remarkably, retro [4+2] cycloaddition while the reactions with 3a-c required more than 12 h for the
of 4a was observed at ambient conditions. Thus, when isolated full conversion into the cycloadducts 4a-c, the reactions with
4
a was re-dissolved in C D and the solution was kept at 3d and 3e completed within 6 h and 3 h, respectively. Indeed,
6 6
ambient temperature, partial regeneration of
1
and 3a was the equilibrium constant for the reversible cycloaddition highly
detected in the H NMR spectrum. Meanwhile, a full depends on the para-substituent of the styrene derivatives
conversion of 4a into and 3a was confirmed upon heating (Table 1). To evaluate the substituent effect on the
the solution at 110 C. Cyclization reactions of with para- cycloaddition reaction, we checked the Hammett correlation
substituted styrene derivatives 3b-e also proceeded cleanly at for the reaction of
room temperature, and the corresponding [4+2] cycloadducts measured based on the conversion rate of
1
1
o
1
1
7
1
with 3a-e
.
The rate constants k were
using its integral
b-e were obtained. Analogous to 4a, the bicyclo[2.2.2] values in H NMR spectra (see the Supporting Information).
structures of 4b and 4d formed via regio-selective [4+2] Figure 4 shows a linear correlation between log k and
cycloaddition were confirmed by X-ray diffraction studies substituent constants with
of +1.43, which is in line with the
Figures 3b and 3c). We also confirmed that 4b-e underwent sensitivity of the cycloaddition reaction to the inductive effect
1
1
4

(
o
retro [4+2] cycloaddition reactions at 110-150 C, to afford
1
of the substituents.
and 3b-e in 80-100% yields (see the Supporting Information).
Table 1. Equilibrium constants (K4-1) at room temperature.
styrene
3a
3b
3c
3d
3e
1.148 x 103 0.601 x 103 0.457 x 103 8.741 x 103 2.011 x 104
K4-1
Scheme 2. Regio-selective [4+2] cycloaddition reactions between 1
and styrenes 3.
(a)
(b)
(c)
Fig. 4. Hammett plot for the reactions of 1 with para-substituted
styrenes 3a-e in THF-d at room temperature.
8
Computational study on reaction pathway
To gain insight into the reaction mechanism, pathways for
the cycloaddition reaction between
explored theoretically using DFT calculations at the M06-
X(SCRF, solvent = benzene)/Def2-TZVP//M06-2X/6-31G* level
of theory, where the M06-2X functional accounts for
1 and styrene 3a were
2
18
dispersion effects. We attempted to obtain pathways for the
concerted and stepwise mechanisms. However, only concerted
Fig. 3. Solid-state structures of 4a (a), 4b (b) and 4d (c). Thermal pathways could be determined (Figure 5a), which suggests
ellipsoids are set at the 50% probability level. Most of the hydrogen that the reaction proceeds in a concerted fashion. We could
atoms are omitted for clarity.
determine two possible pathways for the concerted reaction,
namely, a favored pathway (path A) leading to the
experimentally observed product and a less favored pathway
(path B). Stephan et al. demonstrated a stepwise activation of
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19
olefin with an intermolecular frustrated Lewis pair system.
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Thus, by employing a Lewis acidic borane tethered to a vinyl (a)
group through an alkyl chain, they experimentally confirmed
that a unique borane-olefin van der Waals (vdW) complex was
formed prior to nucleophilic attack by a Lewis base. In our
system, a similar vdW interaction could not be observed at the
stage of a reactant complex (RC). An important steric reason
appears to be that the phenyl ring attached to the Lewis acidic
boron atom in the N-B-N moiety is perpendicular to the
diazadiborinine ring, and thus the phenyl ring hinders the full
coordination of an olefin onto the boron. Moreover, both
DOI: 10.1039/C5SC03174E
Lewis acidic and Lewis basic boron centers are present in
interact with each other in the 6 -system system, which could
allow for cooperative activation of olefin.
1 and (b)

Consistent with the experimental observation, path A has a
lower energy barrier. The computationally estimated
activation parameters for the [4+2] cycloaddition process on
‡
‒1
‡
‡
path A (

H = 6.6 kcal mol ,

S = ‒48.9 e.u.,

G
= 21.2
298)
(
‒
1
kcal mol ) agreed reasonably with the experimental values
‡
‒1
‡
‡
(
H
= 11.6±0.9 kcal mol ,

S
= ‒37.6±3.0 e.u.,

G (
=
298)
‒1
2
2.8±1.8 kcal mol ). The reaction on path A was calculated as
slightly exergonic, consistent with the fact that heating is
necessary for the retro-conversion (Scheme 2). It is interesting
to note that although the reaction is essentially concerted, the
transition state (TS) geometries shown in Figure 5b suggest
that the reaction is highly asynchronous. On path A, one of the
two forming B–C bonds has a distance of 2.18 Å, whereas the
other B–C bond is longer (2.69 Å). The same trend was also
‒
1
Fig. 5 (a) DFT-calculated free energy profiles (kcal mol ) for the
proposed mechanism of the [4+2] cycloaddition reaction between 1
and ethylene. The optimized structures were obtained at the M06-
2
X/6-31G* level. The G values in C H at 298 K were obtained from
6 6
the M06-2X/6-31G* gas-phase harmonic frequencies and the
energy in C6H calculated with the Def2-TZVP basis set. (b)
6
20
Transition-state geometries.
observed recently by Liu et al. The short distance of the
former B–C bond reflects an efficient orbital interaction
(a)
between the LUMO+3 of
1 and the HOMO of styrene 3a which
has large amplitude on the terminal carbon (Figure 6). In
addition to the orbital interaction, the effects of exchange and
electrostatic interactions should play important roles in
determining the regioselectivity. The exchange repulsion
between the HOMOs of styrene 3a and
this asynchronous configuration, because the HOMO of
1
may be reduced in
has
1
significant amplitude on the C-B-C moiety and the phenyl ring
substituted on the boron atom. Furthermore, as shown in
Figure S4-2, the boron atoms at the N-B-N and C-B-C moieties
in
respectively, whereas the terminal carbon (‒0.39) in styrene
is more electronegative than the internal carbon (‒0.12).
1 have positive (+0.35) and negative (‒0.18) charges,
HOMO
LUMO+3
3
a
(b)
Therefore, the TS for path B will feel a larger electrostatic
repulsion. In terms of orbital interaction, path B should be
more favorable because it can form an efficient HOMO (1)-
LUMO (3a) interaction (Figure 6). However, the stabilization
arising from this orbital interaction is significantly diminished
because of the above-mentioned repulsive effects. It is
inferred that in addition to the repulsion at the reaction
centers, the steric repulsion between the phenyl ring of 3a and
HOMO
LUMO
the two dimethyl moiety (-CMe -) of
1
in the TS of path B may
also contribute to the distinct regio-selectivity in this reaction
Figure 5b).
2
Fig. 6 Key frontier orbitals of 1 (a) and styrene 3a (b).
(
4
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Reaction of 1 with norbornene
We also investigated the limitation of the substrate scope.
Cyclization reactions of with cyclic and acyclic internal
Notes and references
View Article Online
DOI: 10.1039/C5SC03174E
1
(a) N. Winterton, G. J. Leigh, Modern Coordination Chemistry:
The Legacy of Joseph Chatt (RSC, Cambridge, UK, 2002); (b)
R. Jones, Chem. Rev. 1968, 68, 785 – 806.
1
alkenes such as 1,1-bis(trimethylsilyl)ethene, trans-
2
(a) J. Chatt, L. A. Duncanson, L. M. Venanzi, J. Chem. Soc.
diphenylethene, cyclohexene, cyclohexadiene did not proceed
1
955, 4456 – 4460; (b) J. Chatt, L. A. Duncanson, J. Chem.
o
even under heating conditions (r.t. ~ 150 C), presumably
Soc. 1953, 2939 – 2947; (c) M. Dewar, Bull. Soc. Chim. Fr.
1951, 18, C79.
because of steric hindrance. By contrast, the reaction of
norbornene , which features a strained bicyclic skeleton,
underwent stereo-selective [4+2] cycloaddition at 90 C, and
1 with
3
4
5
(a) W. C. Zeise, Poggendorff’s Ann. Phys. 1837, 40, 234 – 252;
5
(
(
b) W. C. Zeise, Poggendorff’s Ann. Phys. 1831, 21, 497 – 541;
o
6
c) W. C. Zeise, Poggendorff’s Ann. Phys. 1827, 9, 632 – 633;
was obtained as a solo product in 83 % yield (Scheme 3). The
stereoselectivity of the reaction originated probably from the
(d) W. C. Zeise, Overs. K. Dan. Vidensk. Selsk. Forh. 1825, 13.
(a) A. Fürstner, P. W. Davies, Angew. Chem. Int. Ed. 2007, 46
,
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1
6
410 – 3449; (b) G. Frenking, N. Fröhlich, Chem. Rev. 2000,
fact that the steric hindrance of the CH moiety is smaller than
2
00, 717 – 774; (c) A. Dedieu, Chem. Rev. 2000, 100, 543 –
that of the CH CH part of 5. Retro-[4+2] cycloaddition of 6
2
2
00.
o
proceeded thermally by heating at 150 C, which cleanly
reproduced and
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Radacki, C. W. Tate, A. Vargas, Q. Ye, Nature 2015, 522, 327
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(
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Scheme 3. Stereo-selective [4+2] cycloaddition reaction between 1
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1
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7
In summary, we have demonstrated a [4+2] cycloaddition
2
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reaction between
compound at room temperature. We also showed regio-
selective and stereo-selective [4+2] cycloaddition of with
1
and ethylene which afforded bicyclo[2.2.2]
8,
2
2
1
7
styrene derivatives and norbornene, respectively. All of the
cycloadducts underwent retro-[4+2] cycloaddition reactions
Organometallics 2010, 29, 6594 – 6607; (e) T. Voss, C. Chen,
G. Kehr, E. Nauha, G. Erker, D. W. Stephan, Chem. Eur. J.
2
010, 16, 3005 – 3008; (f) J.-B. Sortaiste, T. Voss, G. Kehr, R.
that reproduced
1
and the corresponding alkenes
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50, 10414 – 10416.
quantitatively. Computational studies using DFT calculations
revealed that the transition state for the cycloaddition is
concerted, but highly asynchronous. Studies on reversible
8
9
cycloaddition of
bonds, and its application are currently under investigation in
our laboratory.
1 with substrates featuring other unsaturated
20
1
1
0 F. Lips, J. C. Fettinger, A. Mansikkamäki, H. M. Tuononen, P. P.
Power, J. Am. Chem. Soc. 2014, 136, 634 – 637.
Acknowledgements
1 T. Sasamori, T. Sugahara, T. Agou, K. Sugamata, J.-D. Guo, S.
We are grateful to Nanyang Technological University (NTU)
and PSF/A*STAR (SERC 1321202066) of Singapore, for financial
support. H.H. thanks the High Performance Computing Centre
of NTU for computer resources.
Nagase, N. Tokitoh, Chem. Sci. 2015,
6
,
DOI:
1
0.1039/C5SC01266J.
1
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3 For recent reports on reversible [4+2] reaction with activated
View Article Online
DOI: 10.1039/C5SC03174E
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