Trapping of a Borirane Intermediate in the Reductive Coupling of an Arylborane to a Diborene

Article abstract of DOI:10.1021/jacs.0c02306

The reductive coupling of a N-heterocyclic carbene (NHC)-stabilized aryldibromoborane yields a mixture of trans- and cis-diborenes in which the aryl groups are coplanar with the diborene core. Under dilute reduction conditions two diastereomers of a borirane-borane intermediate are isolated, which upon further reduction give rise to the aforementioned diborene mixture. DFT calculations suggest a mechanism proceeding via nucleophilic attack of a dicoordinate borylene intermediate on the aryl ring and subsequent intramolecular B-B bond formation.

Full text of DOI:10.1021/jacs.0c02306

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Communication
Trapping of a Borirane Intermediate in the
Reductive Coupling of an Arylborane to a Diborene
Alexander Hermann, Merle Arrowsmith, Daniel E. Trujillo-Gonzalez, Jose
Oscar Carlos Jimenez-Halla, Alfredo Vargas, and Holger Braunschweig
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c02306 • Publication Date (Web): 09 Mar 2020
Downloaded from pubs.acs.org on March 10, 2020
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Trapping of a Borirane Intermediate in the Reductive Coupling of
an Arylborane to a Diborene
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Alexander Hermann,† Merle Arrowsmith,† Daniel E. Trujillo-Gonzalez,‡ J. Oscar C. Jiménez-
Halla,‡ Alfredo Vargas,§ and Holger Braunschweig*,†
† Institute for Inorganic Chemistry and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-
Maximilians-Universität Wüzburg, Am Hubland, 97074 Würzburg, Germany
‡ Departamento de Química, División de Ciencias Naturales y Exactas, Universidad de Guanajuato, Noria Alta S/N,
Col. Noria Alta, 36050 Guanajuato, GTO, Mexico
§ Department of Chemistry, School of Life Sciences, University of Sussex, Brighton BN1 9QJ, Sussex, United
Kingdom
Supporting Information Placeholder
ABSTRACT: The reductive coupling of an NHC-stabilized
aryldibromoborane yields a mixture of trans- and cis-diborenes in
which the aryl groups are coplanar with the diborene core. Under
dilute reduction conditions two diastereomers of a borirane-borane
intermediate are isolated, which upon further reduction give rise to
the aforementioned diborene mixture. DFT calculations suggest a
mechanism proceeding via nucleophilic attack of a dicoordinate
borylene intermediate on the aryl ring and subsequent
intramolecular B-B bond formation.
a)
L
R
B
2 e
[L XR]
X
Path 1:
boryl radical
dimerization
B
B
X
R
(B)
L
L
L
R
B
LR
1 e
1 e
(C)
B
B
B B
B
[L XR]
LR
X
R
(A)
R
X
Path 2:
borylene
dimerization
L
1 e
R = Ar
L = NHC
2 e
B
LX2
B
LX2
(C)
B
LAr
Since the isolation of the first N-heterocyclic carbene (NHC)-
stabilized diborenes, (NHC)2B2H2, from the reduction of
(NHC)BBr3 coupled with radical hydrogen abstraction,1,2 much
progress has been made in the synthesis of doubly base-stabilized
diborenes.3 Whereas N-heterocyclic carbene (NHC)-stabilized
diborenes, (NHC)2B2R2 (R = alkyl, (hetero)aryl), are generally
obtained from the reductive coupling of (NHC)BRX2 (X =
halogen),4-7 the synthesis of phosphine-stabilized8-11 and
unsymmetrical diborenes12-14 relies on the reduction of
dihalodiborane precursors containing a preexisting B–B single
bond. While these latter reductions proceed via a relatively
straightforward (consecutive or concerted) halogen abstraction
process, the reductive coupling of (NHC)BRX2 to diborenes
presents far more complex mechanistic possibilities. In a first step,
a one-electron reduction would generate a boryl radical (A),15
which could undergo either homocoupling to the diborane (B), the
two-electron reduction of which yields the diborene (Scheme 1a,
path 1), or a second one-electron reduction to generate a transient
dicoordinate borylene (C),16 the dimerization of which leads to the
diborene (Scheme 1a, path 2), or a complex combination of both
pathways.
Path 3 (this work):
borylene dimerization
via borirene-borane
(D)
B
L
Ar
b)
IMe
2 Na
B
Cl
Cl
B
I
H
_ 2 NaCl , _Naph
H
+
IMe
IMe
IMe
(A)
via
(C)
or
+
B
B
H
Cl
H
Scheme 1. a) Possible pathways in the reductive coupling of
(NHC)BRX2 to diborenes. b)Reduction of (IMe)BHCl2 to borirane
I with sodium naphthalenide. IMe = 1,3-dimethylimidazol-2-
ylidene; Naph = naphthalene.
the coupling of boryl radical (A) with [Naph]•– (Scheme 1b),
followed by one-electron reduction.20 The authors based their
mechanism on the reduction of (IMe)BH2Cl with NaNaph, which
yielded the diborane (IMe)2B2H4 through radical homocoupling of
[(IMe)BH2]•, as well as a product of heteroradical coupling
between [(IMe)BH2]• and [Naph]•–. While their study did not refute
the possibility of a borylene-based reaction directly, other studies
have shown that transient dicoordinate borylenes (C) tend to
undergo intramolecular ligand C–H or C–C bond activations rather
than borylene dimerization.21-25 However, (C) may be trapped and
In 2011 we reported evidence for a borylene-based mechanism:
upon reducing (IMe)BHCl2 (IMe = 1,3-dimethylimidazol-2-
ylidene) with sodium naphthalenide (NaNaph), borirane I was
isolated (Scheme 1b), which may be viewed as the trapping product
of borylene (C) with naphthalene.17 Curran and Lacôte, who were
the first to report transient NHC-stabilized boryl radicals,18,19
questioned this hypothesis, stating that I could equally result from
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d)
C6D6, 70 °C
2
3
4
5
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7
8
9
irreversible
CF3
F3C
CF3
F3C
N
N
N
N
N
iPr
iPr
iPr
iPr
a)
10 equiv KC8
iPr
N
N
+
B
CF3
B
B
F3C
F3C
B
B
Br
Br
CF3
1
[ ] = 0.15 M
iPr
iPr
iPr
iPr
iPr
in C6H6, rt
N
N
N
F3C
1
2
diborene
CF3
(53% isolated)
2b
2a
(observed in solution)
3:1
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N
N
iPr
iPr
e)
1
2 equiv
c)
10 equiv KC8
3a/3b
CF3
CF3
h C6H6, rt
ca. 30% conversion to
[
] = 0.6 M
Br
Br
iPr
B
3a/3b
H
in C6H6, rt
b)
10 equiv KC8
*
*
+ 70% various byproducts
B
*
N
1
[ ] = 0.015 M
CF3
3
borirane-borane
3b
in C6H6, rt
N
iPr
3a
(RRR/SSS) /
(RRS/SSR) 3:1
(43% isolated)
F3C
Scheme 2. Direct and stepwise reduction of 1 to diborene 2a/2b via borirane-borane intermediate 3a/3b.
isolated with an additional small donor ligand (NHC, PR3, CO,
CNR, N2).25-29 In this work we show an alternative pathway in the
reductive coupling of a NHC-stabilized aryldihaloborane to a
diborene via an intermediate borirane-borane species (Scheme 1a,
path 3), the configuration of which determines whether the trans-
or the cis-diborene is obtained, and probe the reduction mechanism
via DFT calculations.
quite well (λmax = 600 nm), albeit with red-shifted features, a
common result of qualitative TDDFT. The principal absorption
a)
N2
C9
N1
F1
F2
F3
B1
C1
C1'
B1'
The reduction of a 0.15 M solution of (IiPr)BArFBr2 (1, IiPr =
1,3-diisopropylimidazol-2-ylidene; ArF = 3,5-bis(trifluoromethyl)-
phenyl)30 in C6H6 with 10 equiv. KC8 resulted in a deep purple
suspension. Recrystallization from benzene afforded purple
crystals of diborene 2 (δ11B = 26 ppm) in 53% isolated yield
(Scheme 2a). X-ray crystallographic analysis of 2 revealed a
diborene in which the ArF rings are trans to each other and coplanar
with the >B=B< core (Fig. 1b).31 This coplanar arrangement is also
found in NHC-stabilized diborenes with unsubstituted or meta-
substituted heteroaromatic substituents,4,7 while diborenes with
ortho-substituted (hetero)aromatic substituents tend towards a
perpendicular arrangement.5,7 Consequently, the coplanarity of the
diborene unit and the (hetero)aromatic substituents in the solid state
is governed by steric rather than electronic factors. However, since
2 displays only a single 19F NMR resonance at –62.9 ppm the aryl
substituents are likely freely rotating in solution.
DFT calculations in the gas phase at the OLYP/ZORA/TZ2P
level of theory surprisingly provided an optimized structure in
which the aryl rings are not coplanar with the >B=B< core (see
Supporting Information for further details). Taking into account
short-range dispersion forces, calculations at the ω-B97XD/6-
31G(d) level of theory provided a planar structure in agreement
with the X-ray crystallographic data, showing once more the
importance of dispersion forces in enforcing the solid-state
geometry of low-valent compounds.32 Independent of the level of
calculation chosen, the HOMO of 2a is mainly located at the B=B
double bond whereas the LUMO is delocalized over the π* system
of both ArF rings with a strong B-Caryl π-bonding component (Fig.
2, compare Fig. S55 in the Supporting Information). In accordance
with its intense purple color, 2a shows a strong absorption
maximum at λmax = 576 nm, in the same region as other NHC-
stabilized diborenes,7 with a shoulder at λ2 = 480 nm. The
calculated spectrum of 2a reproduces the experimental absorption
C9'
b)
C1
B1
C1'
B1'
Figure 1. a) Crystallographically-derived molecular structure of
2a. b) View showing the coplanarity of the diborene and aryl
moieties. Thermal ellipsoids set at 50% probability. Thermal
ellipsoids of ligand periphery and hydrogen atoms omitted for
clarity.30
corresponds mainly to the HOMO→LUMO transition (2.067 eV)
and the shoulder at lower energy to the HOMO→LUMO+3
transition (Fig. 2).
Fractional crystallization of the filtrate from the isolation of 2
(Scheme 2a, see SI for details) also yielded a small crop of colorless
crystals (7%), the NMR spectrum of which revealed the presence
of two diastereomers, 3a (75%, δ11B = –3.1, –27.8 ppm) and 3b
(25%, δ11B = –3.1, –26.4 ppm). The solid-state structure of the
major isomer, 3a (Fig. 3), shows the trapping of a formal
(IiPr)ArFB: borylene moiety by the aryl ring of 1, akin to that
shown in Scheme 1b.17 While 3 presents three stereogenic centers
in C18, C23 and B1, the syn-selectivity of the trapping reaction
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leads to only two possible diastereomers: 3a (RRR/SSS), depicted
in Fig. 3 and 3b (RRS/SSR). The identity of each diastereomeric
We carried out DFT calculations aiming to understand both the
mechanism of formation of 3 and its subsequent conversion to 2
(see SI Figs. S58–S61 for details). Following the one-electron
1
2
3
0.4
4
5
6
7
N4
0.2
F10
LUMO
N3
C26
500
600
700
F12
F11
HOMO
Br1
F7
8
LUMO+3
B2
C22
H23
9
C18
C23
B1
HOMO
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F9
Br2
F8
C9
N2
C2
N1
F4
C1
H2
F2
F1
F6
F3
F5
Figure 3. Crystallographically-derived molecular structures of 3a,
showing the H23···H2 spatial interaction with a dotted line.
Thermal ellipsoids set at 50% probability. Thermal ellipsoids of
ligand periphery and hydrogen atoms omitted for clarity. Selected
bond lengths (Å) and angles (°): B1-C1 1.598(6), B1-C9 1.597(6),
B1-C18 1.637(6), B1-C23 1.610(6), C18-C23 1.581(5), C18-C19
1.454(5), C22-C23 1.488(5), C18-C23-B1 61.6(2), C23-B1-C18
58.2(2), B1-C18-C23 60.2(2).
HOMO
LUMO
LUMO+3
Figure 2. Experimental absorption spectrum of 2a with calculated
spectrum at the OLYP/ZORA/TZ2P level of theory in inset, and
plots of the frontier MOs corresponding to the main transitions.
pair was ascertained by a ROESY experiment (see Fig. S21 in the
SI). For 3a, the proton at C23 (δ1H = 2.99 ppm) shows a cross-peak
to the ortho-protons of the ArF group bound to B1 (δ1H = 8.90 ppm),
suggesting their spatial proximity, as would only occur in the
(RRR/SSS) configuration (H23···H2 2.83(2) Å, Fig. 3). For 3b, the
proton at C23 (δ1H = 3.26 ppm) displays no such correlation, and
therefore likely belongs to the (RRS/SSR) diastereomeric pair.
Compound 3 was isolated in higher yield (43%) by carrying out
the reduction of 1 with KC8 under dilute conditions (0.015 M,
Scheme 2b). This observation led us to postulate that 3 may be an
intermediate in the formation of diborene 2. Indeed the reduction
of isolated 3a/3b (3:1) with 10 equiv. KC8 under concentrated
conditions (0.6 M in C6D6) resulted in complete conversion to 2 as
reduction of 1 to a transient boryl radical, A (ΔGR = –21.5 kcal mol–
1) several pathways were considered. In the first proposal (Scheme
3a), homocoupling of two molecules of A yields dibromodiborane
B (ΔGR = –31.1 kcal mol–1), which undergoes twofold reduction to
diborene 2 (ΔGR = –69.2 kcal mol–1). While thermodynamically
viable, this proposal circumvents the isolated intermediate 3, which
is why two more pathways involving 3 were investigated. The
attack of radical A at the ArF ring of 1 to generate the aryl radical
C (Scheme 3b), in analogy to the mechanism proposed by Curran
and Lacôte for the formation of I (Scheme 1b),20 was rejected as
being too endergonic (ΔGR = +21.1 kcal mol–1). In contrast, further
reduction of A yields a transient but energetically accessible
borylene D (ΔGR = +6.8 kcal mol–1),33 which is stabilized by strong
π backbonding from the borylene lone pair to the NHC carbene
carbon (depicted in Fig. 4 as a donor-acceptor bonding). Such
dicoordinate NHC-stabilized singlet borylenes have been studied
theoretically by Lu et al, who also found a strong π backbonding
determined by 11B NMR spectroscopy (Scheme 2c). H and 19F
1
NMR spectra of the filtered reaction mixture, however, evidenced
the formation of a second minor species (ca. 25%) beside the
resonances of 2. 1H and 19F NMR DOSY experiments yielded near-
identical diffusion coefficients for the two species (2: 7.24x10–10
m2 s–1; minor product: 6.92x10–10 m2 s–1, see Figs. S33–S43 in the
SI), leading us to identify the minor product as the cis isomer of the
diborene (Scheme 2). Re-examination of the NMR spectra of the
crude reaction mixture from Scheme 2a also showed the formation
of both trans-2 (2a) and cis-2 (2b) in a similar ratio. Above 70 °C
in C6D6, 2b, which was calculated to be 2.3 kcal mol–1 higher in
energy than 2a, converted irreversibly to 2a (Scheme 2d, see Fig.
S44 in the SI). This also fits with the calculated rotational barrier
of 22.3 kcal mol–1 between the two diborene isomers. With 2a and
2b being formed in the same ratio as 3a and 3b, respectively, their
formation may be rationalized as resulting from the reduction of
one of the two diastereomers, respectively. This hints at an
intramolecular reduction process with strong geometric constraints
preventing isomerization. It is noteworthy that until now, cis-
diborenes were only accessible through the reduction of 1,2-
dihalodiboranes stabilized by 1,2-bridging ligands, which enforce
the cis geometry.6,10
a) Radical homocoupling
L
L
L
ArF
Br
ArF
L
2 K
2
B
B
B
B
B
ArF
Br
ArF
Br
_ 2 KBr
ArF
L
A
2
B
b) Radical attack at ArF
L
L
L
Br
Br
B
L
CF3
CF3
B
+
B
ArF
Br
Br
CF3
Br
B
ArF
C
1
A
F3C
Br
K
_ KBr
c) Nucleophilic attack of borylene at ArF
L
L
L
CF3
CF3
Br
Br
B
H
B
+
B
ArF
Br
Br
CF3
2 K
_ 2 KBr
B
ArF
2
1
D
3
F3C
L
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Scheme 3. Mechanisms investigated for the reduction of 1 to 2. L
= IiPr.
1
2
3
4
5
6
7
8
9
10
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1
2
iPr
N
3
4
5
6
N
CF3
B
iPr
iPr
N
H
CF3
B
N
CF3
CF3
IiPr
iPr
B
N
CF3
iPr
ArF
7
N
8
9
TS1
(18.9)
B
iPr
H
IiPr
B
CF3
Int2
(2.9)
ArF
H
Int1
(0.0)
10
11
12
13
14
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19
20
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B
ArF
IiPr
TS2
(3.7)
iPr
N
iPr
N
TS3
(_3.4)
Int3
(_15.2)
N
CF3
N
CF3
CF3
B
iPr
B
H
iPr
iPr
N
CF3
B
CF3
H
IiPr
N
B
ArF
B
ArF
iPr
IiPr
CF3
H
B
CF3
IiPr
ArF
N N
B
iPr
iPr
2a
(_48.0)
F3C
B
CF3
iPr
iPr
N N
Reaction coordinate
CF3
Figure 4. Mechanism of the rearrangement of borirane-borylene intermediate Int1 to diborene 2a calculated at the (PCM:benzene)M06-
2X/6-311+G(2d,p)//M06-2X/6-31+G(d) level of theory. Relative Gibbs free energies in parentheses (kcal mol–1).
component.34 The nucleophilic attack of D at the ArF ring of 1
generates 3 (Scheme 3c) in an overall highly exothermic reaction
(ΔGR = –58.0 kcal mol–1).35 Diastereomer 3a was calculated to be
1.7 kcal mol–1 more stable than 3b, thereby accounting for the 3:1
diastereomeric ratio of 3a and 3b observed experimentally. The
twofold reduction of 3a results in the borirane-borylene Int1 (ΔGR
= –25.6 kcal mol–1). From Int1 the stepwise haptotropic migration
of the B2 fragment, which eventually yields diborene 2, is highly
exergonic (ΔGR = –48.0 kcal mol–1, Fig. 4). In the first rate-limiting
step (ΔG≠ = 18.9 kcal mol–1), the borirane ring migrates from the
2,3-position of the dearomatized ArF ring to the thermodynamically
slightly less favored 1,2-position (Int2, ΔG1 = +2.9 kcal mol–1).
This is followed by a quasi barrierless (ΔG≠ = 0.8 kcal mol–1)
migration of B2 from the ipso-ArF carbon to B1 to form a 1,2-
diborete ring fused with ArF (Int2, ΔG2 = –2.9 kcal mol–1).
Subsequent breaking of the B2-C23 bond leads to the highly
exergonic re-aromatization of the ArF ring and selective formation
of 2a (ΔG2 = –32.8 kcal mol–1).36
trapping of a transient dicoordinate borylene, (NHC)=BAr, by the
aryl ring of the remaining borane (NHC)BX2Ar. The trans- or cis-
arrangement of the resulting diborene is governed by the
stereochemistry of this borirane-borane intermediate. Given that
the vast majority of NHC-stabilized diborenes result from the
reductive coupling of (hetero)aryldihaloborane precursors,4-7 this
borirane pathway may well be more broadly relevant, in which case
NMR-spectroscopic examination of the crude products of these
reductions could reveal the hitherto unknown formation of other
cis-diborenes as minor reduction products.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website and includes synthetic procedures, NMR
spectra of isolated compounds and reaction mixtures, UV-vis
spectra, cyclic voltammetry data, crystallographic and
computational details (PDF), as well as cif files for all
crystallographically characterized compounds.
Cyclic voltammetry of diborene 2a in THF showed a reversible
oxidation wave at –0.61 V (vs. the Fc/Fc+ couple), suggesting the
possibility of chemical oxidation. We therefore wondered if a
comproportionation of diborene 2 and borane 1 would lead back to
borirane-borane 3. While a mixture of 2 and two equivalents of 1
in C6D6 showed no reactivity up to 80 °C, irradiation with UV-light
over a period of three days at room temperature resulted in full
consumption of 2 and one equivalent of 1 with formation of ca.
30% of 3a/3b in a 3:1 ratio (Scheme 2e), besides various
AUTHOR INFORMATION
Corresponding Author
*h.braunschweig@uni-wuerzburg.de
Notes
1
The authors declare no competing financial interest.
unidentified products, as determined by H NMR spectroscopy.
The reduction of 3 to 2 is thereby at least partially reversible.
To conclude, we have identified a new possible mechanism for
the reductive coupling of a (NHC)BX2Ar precursor to the
corresponding diborene via a borirane-borane intermediate. DFT
calculations show that the latter is likely to result from the formal
ACKNOWLEDGMENT
This project was funded by the European Research Council (ERC)
under the European Union Horizon 2020 Research and Innovation
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Heterocyclic Carbene Borane Radicals. J. Am. Chem. Soc. 2010, 132,
2350-2358.
Program (grant agreement no. 669054). D.E.T.-J. and J.O.C.J.-H.
acknowledge 822937 CONACyT fellowship. A.V. acknowledges
the University of Sussex for financial support.
1
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(19) Ueng, S.-H.; Solovyev, A.; Yuan, X.; Geib, S. J.; Fensterbank, L.;
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P. N-Heterocyclic Carbene Boryl Radicals: A New Class of Boron-
Centered Radical. J. Am. Chem. Soc. 2009, 131, 11256-11262.
(20) Curran, D. P.; Boussonnière, A.; Geib, S. J.; Lacôte, E. The Parent
Borylene: Betwixt and Between. Angew. Chem. Int. Ed. 2012, 51,
1602-1605.
(21) Grigsby, W. J.; Power, P. P. Isolation and Reduction of Sterically
Encumbered Arylboron Dihalides: Novel Boranediyl Insertion into
C−C σ-Bonds. J. Am. Chem. Soc. 1996, 118, 7981-7988.
(22) Bissinger, P.; Braunschweig, H.; Damme, A.; Dewhurst, R. D.;
Kupfer, T.; Radacki, K.; Wagner, K. Generation of a Carbene-
Stabilized Bora-borylene and its Insertion into a C–H Bond. J. Am.
Chem. Soc. 2011, 133, 19044-19047.
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Radacki, K.; Ye, Q. Main-Group Metallomimetics: Transition Metal-
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H.; Bringmann, G.; Celik, M. A.; Dewhurst, R. D.; Finze, M.; Grüne,
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(30) Compound 1 was synthesized by bromide exchange of the precursor
(IiPr)BArFCat (Cat = catecholato) with BBr3 at –78 °C in hexanes.
See Figs. S48 and S49 in the SI for the crystallographically-derived
molecular structures of (IiPr)BArFCat and 1.
(31) The entire molecule 2a crystallized as twofold disordered in a 89:11
ratio via a mirror plane perpendicular to the ArBBAr plane,
traversing the center of the B=B bond and including N1 and N2.
Since 1,2- and 1,3-distance restraints had to be applied throughout,
bond lengths and angles of 2a may not be discussed further.
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Dellermann, T.; Dewhurst, R. D.; Kelch, H.; Krummenacher, I.;
Mattock, J. D.; Müssig, J. H.; Thiess, T.; Vargas, A.; Zhang, J.
Engineering a Small HOMO-LUMO Gap and Intramolecular B—B
Hydroarylation by Diborene/Anthracene Orbital Intercalation.
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(12) Lu, W.; Li, Y.; Ganguly, R.; Kinjo, R. Alkene-Carbene Isomerization
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Chem. Soc. 2017, 139, 5047-5050.
(33) The formation of a more stable boryl anion, [(IiPr)BBrArF]K, is
likely to be more favorable but was not computed here due to
uncertainty about the nuclearity of such a species in non-coordinating
benzene.
(13) Lu, W.; Li, Y.; Ganguly, R.; Kinjo, R. Crystalline Neutral Allenic
Diborene. Angew. Chem. Int. Ed. 2017, 56, 9829–9832
(14) Stennett, T. E.; Mattock, J. D.; Vollert, I.; Vargas, A.; Braunschweig,
H. Unsymmetrical, Cyclic Diborenes and Thermal Rearrangement to
a Borylborylene. Angew. Chem. Int. Ed. 2018, 57, 4098-4102.
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(16) Soleilhavoup, M.; Bertrand, G. Borylenes: An Emerging Class of
Compounds. Angew. Chem. Int. Ed. 2017, 56, 10282-10292.
(17) Bissinger, P.; Braunschweig, H.; Kraft, K.; Kupfer, T. Trapping the
elusive parent borylene. Angew. Chem. Int. Ed. 2011, 50, 4704–4707.
(18) Walton, J. C.; Makhlouf Brahmi, M.; Fensterbank, L.; Lacôte, E.;
Malacria, M.; Chu, Q.; Ueng, S.-H.; Solovyev, A.; Curran, D. P. EPR
Studies of the Generation, Structure, and Reactivity of N-
(34) Lu, D.; Hea, Y.; Wu C. Electronic structure of mono(Lewis
base)stabilized borylenes. Phys. Chem. Chem. Phys. 2019, 21,
23533-23540.
(35) Unfortunately, attempts to trap borylene D by performing the
reduction of 1 in the presence of a cyclic (alkyl)(amino)carbene
(CAAC) failed, although the corresponding tricoordinate borylene
(CAAC)(IiPr)BArF (5) could be synthesized from the reduction of
(CAAC)BCl2ArF (4) in the presence of IiPr. See Supporting
information for synthetic details and Figs. S50-S52 for the
crystallographically-derived
molecular
structures
of
the
(CAAC)BArFCat precursor, and compounds 4 and 5, respectively.
(36) By a similar mechanism the reduction of 3b leads selectively to the
cis-diborene 2b.
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SYNOPSIS TOC (Word Style “SN_Synopsis_TOC”).
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2
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L
ArF
B
B
B
B
L
L
ArF
ArF
2 e
2 e
L
ArF
CF3
CF3
+
B
X
X
B
H
X
CF3
X
via
ArF
*
*
L
B
F3C
B
*
borylene
ArF
two diastereomers
L
L
L
trans- and
cis-diborene!
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