Bicyclic (amino)(borata)carbene derived from diazadiborinine and isonitrile

Article abstract of DOI:10.1039/c9cc06453b

The reaction of 1,4,2,5-diazadiborinine (1) with two equivalents of an aryl isonitrile afforded a bicyclic product containing an indole unit (2) or ketenimine moiety (3), suggesting the generation of a B,N-carbene intermediate formed via a [4+2] cycloaddi

Full text of DOI:10.1039/c9cc06453b

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Bicyclic (amino)(borata)carbene derived from
diazadiborinine and isonitrile†
Cite this: Chem. Commun., 2019,
5, 13012
5
Baolin Wang, Kota Koshino and Rei Kinjo
*
Received 20th August 2019,
Accepted 4th October 2019
DOI: 10.1039/c9cc06453b
rsc.li/chemcomm
The reaction of 1,4,2,5-diazadiborinine (1) with two equivalents of an
aryl isonitrile afforded a bicyclic product containing an indole unit (2)
or ketenimine moiety (3), suggesting the generation of a B,N-carbene
intermediate formed via a [4+2] cycloaddition reaction in the initial
step. The employment of the tolyl(phenyl isonitrile)gold complex
(
PhNCAuTol) as the substrate allowed the bicyclic (amino)(borata)-
carbene gold complexes (4, 5) to be accessed.
1
2
Since the seminal works of Bertrand and Arduengo three
decades ago, the chemistry of isolable carbenes has dramatically
3
advanced. By tuning the steric and electronic properties around
Fig. 1 (a) A schematic drawing of NHC, cAAC and B,N-carbene. (b) B,N-
carbene Zn complex reported by Marder. (c) Previous results (EE = alkene,
allene, alkyne and nitrile). (d) Target molecule in this work.
the carbene centre, a number of carbenes featuring various
s-donating and p-accepting nature have been developed to date.
As a representative example, in 2005 the group of Bertrand reported
0
4
the synthesis of five-membered cyclic (alkyl)(amino)carbene (cAAC),
Computationally, a brief inspection of the frontier molecular
orbitals of a series of representative carbenes (A–E) indicated
that the LUMO of bicyclic B,N-carbene (E) is comparable in
energy to those of other carbenes (A–D) whereas E possesses the
highest HOMO among them (Fig. 2).
which exhibits superior s-donating and p-accepting ability with
5
respect to N-heterocyclic carbenes (NHCs) (Fig. 1a). While a
3
,6–13
variety of cyclic carbenes are available nowadays,
an isolable
B,N-carbene derivative (Fig. 1a) involving boron and nitrogen-
based substituents adjacent to the carbene centre has never been
described thus far. In 2014, Marder et al. reported that a ring-
Experimentally, we began our investigation with the reaction
between 1 and phenyl isonitrile (PhNC). When a stoichiometric
expansion reaction of NHC with ZnCl
zinc complex supported by B,N-carbene ligands (Fig. 1b).
2 2 2
and B pin afforded a
1
4
In our previous study, we have shown that 1,4,2,5-diaza-
diborinine 1 undergoes a [4+2] cycloaddition reaction with
unsaturated bonds, such as those in alkenes, allenes, alkynes
and nitriles, to furnish the corresponding bicyclo[2.2.2] mole-
1
5
cules (Fig. 1c). We envisaged that a similar [4+2] cycloaddition
reaction would proceed between 1,4,2,5-diazadiborinine and an
isonitrile, which may generate a bicyclic B,N-carbene derivative
(Fig. 1d). Herein, we report the reactions of 1,4,2,5-diazadiborinine
with isonitriles.
Division of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, Nanyang Link 21,
Singapore 637371, Singapore. E-mail: rkinjo@ntu.edu.sg
†
Electronic supplementary information (ESI) available. CCDC 1948002–1948005.
For ESI and crystallographic data in CIF or other electronic format see DOI:
0.1039/c9cc06453b
Fig. 2 Energy (eV) of the frontier orbitals of representative carbenes A–E,
calculated at the M05-2X/6-311G(d,p) level of theory.
1
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Scheme 2 A proposed mechanism for the formation of 2.
afford ketenimine intermediate II. The subsequent insertion of
a C–H bond at the ortho-position in the phenyl group into the
CQN moiety of ketenimine leads to 2. To gain deep insight into
the reaction pathway, we next employed 2,6-dimethyl phenyl
isonitrile (2,6-Me C H NC) as the substrate. The treatment of 1
2
6
3
Scheme 1 Reactions of 1 with PhNC and 2,6-Me
2
C
6
H
3
NC.
and 2,6-Me C H NC in 1 : 2 ratio readily afforded the product 3
2 6 3
(Scheme 1, bottom). An X-ray diffraction study revealed the
ketenimine structure of 3 in the solid-state (Fig. 3, right), which
supports the generation of an intermediate II for the formation
of 2 (Scheme 2).
amount of PhNC was added into the benzene solution of 1 at room
temperature, the orange colour of 1 did not fade completely.
The addition of another equivalent of PhNC led to a colourless
16
While the isolation of compounds 2 and 3 implies that the
reaction of 1 with an isonitrile would yield a B,N-carbene as an
initial transient intermediate, neither its direct observation by
low-temperature NMR measurement nor chemical trapping
reactions were successful. Accordingly, our attention turned to
1
solution, the H NMR spectrum of which showed the generation of
a major product (Fig. S1 and S2, ESI†). This initial product slowly
isomerized to more stable compound 2 over 48 hours at room
temperature (Scheme 1, top) (Fig. S3 and S4, ESI†). 2 was char-
acterized by standard crystallographic, spectroscopic and analytical
methods. The solid-state structure of 2 (Fig. 3, left) revealed that an
indole unit derived from two PhNC molecules is formed during the
reaction, which is in line with the fact that the reaction requires two
equivalents of PhNC.
A plausible mechanism for the formation of 2 is shown in
Scheme 2. The initial step may involve the [4+2] cycloaddition
reaction between 1 and PhNC to yield a B,N-carbene species I,
which can be quickly trapped by a second PhNC molecule to
17
the use of an isonitrile-metal complex as the substrate. We
reasoned that the presence of the metal at the isonitrile carbon
should prevent subsequent reaction with the second isonitrile,
which may allow access to the B,N-carbene metal complex. To bear
out this hypothesis, a stoichiometric amount of tolyl(phenyl iso-
18
nitrile)gold complex (PhNCAuTol) was reacted with 1 in THF at
room temperature (Scheme 3). The immediate disappearance of
the orange colour indicated the complete consumption of 1. After
workup, a B,N-carbene gold complex 4 was isolated in 60% yield,
which was fully characterized. We also confirmed that the treat-
ment of complex 4 with HCl (1 eq.) readily afforded the corres-
ponding gold chloride complex 5 (in 65% yield), concomitant with
the elimination of toluene.
An X-ray diffraction analysis of 4 and 5 confirms the formation
of bicyclic (amino)(barata)carbene at the gold sphere (Fig. 4). The
C15–Au1–C22 (176.96(15)1) in 4 and C15–Au1–Cl1 (178.83(15)1)
in 5 are nearly linear, and the carbene–gold distance in 5
(
Au1–C15 2.003(5) Å) is shorter than that (Au1–C15 2.057(4) Å) in 4,
19
and comparable to those reported for NHC–gold complexes. The
Au1–Cl1 bond length (2.3061(14) Å) in 5 is in the range 2.2758(12) Å
1
9
to 2.3183(15) Å reported for NHC–gold complexes.
We have shown that two isonitrile molecules undergo a
coupling reaction over 1,4,2,5-diazadiborinine 1 to afford bicyclic
Fig. 3 Solid-state structures of 2 (left) and 3 (right) (all hydrogen atoms
except for the H atom on the N6 in 2 are omitted for clarity. Thermal ellipsoids
are set at the 50% probability level). Selected bond lengths [Å] and angles [1]:
2
B1–C21 1.623(3), C21–N5 1.410(3), B2–N5 1.568(3), C21–C22 1.367(3),
C22–N6 1.427(3), C29–N6 1.393(3), C22–C21–B1 134.0(2), C21–N5–B2
1
1
15.78(18), C22–C21–N5 109.2(2), N5–C21–B1114.91(19), C21–C22–N6
26.5(2), C29–N6–C22 123.0(2); 3 B1–C15 1.650(2), C15–N5 1.427(2),
B2–N5 1.543(2), C15–C16 1.312(2), C16–N6 1.244(2), N5–C15–B1 113.93(14),
C16–C15–B1 123.83(15), C16–C15–N5 121.93(15), C15–N5–B2 115.56(13),
N6–C16–C15 173.89(18).
Scheme 3 Synthesis of B,N-carbene-supported gold complexes 4 and 5.
Chem. Commun., 2019, 55, 13012--13014 | 13013
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0,21
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We are grateful to Nanyang Technological University (NTU)
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for financial support.
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21 For the preliminary result of our catalytic studies, see the ESI†.
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