Synthesis and structure of a carbene-stabilized π-boryl anion

Article abstract of DOI:10.1002/anie.200906884

(Chemical Equation Presented) Attack with π electrons: Reduction of a chloroborole coordinated by an N-heterocyclic carbene results in the formation of a carbene-stabilized borole monoanion (see scheme; Mes=mesityl), the molecular structure of which has b

Full text of DOI:10.1002/anie.200906884

Angewandte
Chemie
DOI: 10.1002/anie.200906884
Boron Heterocycles
Synthesis and Structure of a Carbene-Stabilized p-Boryl Anion**
Holger Braunschweig,* Ching-Wen Chiu, Krzysztof Radacki, and Thomas Kupfer
Dedicated to Professor Peter Paetzold on the occasion of his 75th birthday
One of the vital research areas in modern main-group
chemistry is focused on the synthesis of Group 13 carbene
analogues. In contrast to the facile synthesis of anionic gallyl
species,[1] the isolation of a boryl anion was only reported
recently.[2] Chemical reduction of haloboranes with KC8 or
we describe the synthesis of a carbene-stabilized borole
monoanion and its reactivity toward electrophiles. Structural
and computational (DFT) data of the title compound are also
discussed.
The SIMes (1,3-dimesitylimidazolidin-2-ylidene) adduct
of 1-chloro-2,3,4,5-tetraphenylborole[10] was obtained by
mixing equimolar amounts of the reagents in benzene and
was isolated as a yellow solid after purification (Scheme 1).
Na/K alloy generally yields products from radical reactions,
[3]
À
that is, C H bond insertion, homocoupling, and rearrange-
ment.[4,5] In 2006, the groups of Nozaki and Yamashita
isolated the first structurally characterized boryllithium
reagent of type A by reduction of the corresponding 2-halo-
1,3,2-diazaborole.[2a] In addition to these NHC-analogous
(NHC = N-heterocyclic carbene) boryl anions, reduction of
the chloroborylene complex [((h5-C5H5)Mn(CO)2)2(BCl)][6]
resulted in the formation of the corresponding linear anionic
dimetalloborylene complex
B
(DME = 1,2-dimethoxy-
ethane), which reacts as a boron-centered nucleophile.[7]
Scheme 1. Synthesis and reactivity of SIMes-stabilized borole monoan-
ion 2. Mes=mesityl.
Prior to the isolation of A, a few studies suggested that
base-stabilized sp3 nucleophilic boryl anions (type C; Cy =
cyclohexyl) could be generated by reduction of the corre-
sponding triethylamine- or tricyclohexylphosphine-coordi-
nated haloboranes.[8] However, definitive characterization of
the base-stabilized boryl anion has never been reported.
In recent studies, NHCs have proven useful in stabilizing
low-valent boron-containing molecules, such as neutral dibor-
enes, borenium cations, and neutral 9-boraanthracenes.[9]
Therefore, we decided to investigate whether an NHC is
effective in stabilizing the borole-based boryl anion. Herein
The resonance in the 11B NMR spectrum of 1 detected at d =
À3.1 ppm as a sharp singlet indicates the formation of a
1
tetracoordinate boron center. The H NMR spectrum of 1
exhibits only one set of signals for the methyl protons,
À
suggesting free rotation of SIMes about the C B bond in
solution at ambient temperature.
Reduction of 1 with excess KC8 in diethyl ether affords the
monoanionic derivative 2 in 37% yield. Upon reduction, a
deep reddish-purple color developed immediately and the
completeness of reaction was monitored by 11B NMR spec-
troscopy. The 11B resonance of 2 observed at d = 12 ppm rules
out the formulation of 2 as a borataalkene derivative, for
which the 11B NMR spectroscopy signals generally appear
around d = 40 ppm.[11] The aforementioned 11B chemical shift
of 2 is also shifted considerable upfield compared to that for
[K2(Ph4C4BPh)] (d = 26 ppm), thus emphasizing the differ-
ence in the electronic structure of 2 and borole dianions.[12]
Therefore, compound 2 is better described as an NHC-
stabilized boryl anion, in which the lone-pair electrons reside
in the p-bonding orbital of the boron atom and are stabilized
by delocalization over the C4B ring (Figure 1).
[*] Prof. Dr. H. Braunschweig, Dr. C.-W. Chiu, Dr. K. Radacki,
Dr. T. Kupfer
Institut fꢀr Anorganische Chemie
Julius-Maximilians-Universitꢁt Wꢀrzburg
Am Hubland, 97074 Wꢀrzburg (Germany)
Fax: (+49)931-888-4623
E-mail: h.braunschweig@mail.uni-wuerzburg.de
Homepage: http://www-anorganik.chemie.uni-wuerzburg.de/
Braunschweig/index.html
[**] C.-W.C. thanks the Alexander von Humboldt Foundation for a
postdoctoral fellowship.
Dark-purple crystals of 2 suitable for X-ray analysis were
obtained from cooling a diethyl ether/pentane solution to
À308C. Compound 2 crystallized as a potassium-bridged
Supporting information for this article, including full experimental
details and X-ray crystal data, is available on the WWW under http://
dx.doi.org/10.1002/anie.200906884.
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2041

Communications
Figure 1. Crystal structure of 2. Ellipsoids are set at the 50% proba-
bility level. Hydrogen atoms are omitted for clarity. Selected bond
lengths [ꢂ] and angles [8]: B–C5 1.541(2), B–C1 1.535(2), B–C4
1.541(2), C1–C2 1.431(1), C2–C3 1.446(1), C3–C4 1.41(1), C5–N1
1.371(1), C5–N2 1.374(1); C1-B-C5 128.55(10), C1-B-C4 104.95(9), C4-
B-C5 125.79(10), B-C1-C2 107.21(9), C1-C2-C3 109.57(9), C2-C3-C4
111.36(9), C3-C4-B 106.86(9).
Figure 2. Depiction of the HOMO of 2’.
interaction. This electronic situation of 2 is somewhat inverse
to that of boryl anions of type A, in which the lone-pair
electrons occupy an exocyclic sp2 orbital, while the pz orbital
of boron is stabilized by p bonding to the adjacent nitrogen
atoms.[2d,16] In compound 2, however, the electron density
preferentially resides in the pz orbital of the boron atom,
while the NHC donates its lone pair into the equatorial sp2
orbital. The NHC ligand serves not only as a strong s donor
but also as a p acceptor in delocalizing the electrons from the
boron atom.
For probing the nucleophilicity of the boron atom,
compound 2 was treated with an excess of MeI in Et2O at
ambient temperature. After workup, the NHC-coordinated 1-
methyl-2,3,4,5-tetraphenylborole (3) was isolated as a color-
less solid in 71% yield. The resonance of 3 in the 11B NMR
spectrum detected at d = À10.8 ppm is significantly shifted
upfield compared to that detected for 2, in agreement with the
¯
dimer in the space group P1 (Figure 1; only one half is
displayed; the dimer is depicted in Figure S1). Within the
dimer unit, the potassium cation resides above one of the
borole rings and coordinates to the phenyl substituent at the
C2 atom of the other borole molecule through an intermo-
lecular cation–p interaction (Figure S1 in the Supporting
Information). The borole ring is essentially planar with an
average displacement of the ring atoms above the C4B plane
of 0.0085 ꢀ. The boron atom adopts the trigonal-planar
geometry, as indicated by the sum of angles of 359.38. In
À
comparison with the neutral boroles, the intraannular C C
bond-length alternation within the C4B ring is significantly
decreased.[13] The B C1 and B C4 bonds are also shorter
than those observed in neutral boroles.[12b] In fact, the
structural features of 2 within the borole ring resemble
those of borole dianions, thus emphasizing the aromatic
À
À
1
formation of a tetracoordinate boron center. The H reso-
character of the C4B ring in 2.[12b] The B C5 bond of
nance of the methyl (B Me) group at d = À0.4 ppm is also
À
À
1.5406(15) ꢀ is also significantly shorter than that of the
NHC-coordinated borabenzenes (1.596(2) ꢀ)[14] or 9-boraan-
thracenes (1.607(4) ꢀ).[9d] This finding together with a
relatively small dihedral angle of 36.38 between the carbene
ligand and borole moiety suggests non-negligible p bonding
between the boron atom and the exocyclic carbene ligand.[15]
To better understand the electronic structure of 2,
computations of the model complex 2’ were performed with
DFT methods at the B3LYP/6-31g* level of theory. Complex
2’, in which the mesityl groups are replaced by 2,6-dimethyl-
phenyl groups, was optimized as a monomer in the absence of
the potassium ion coordination. The energy-optimized geom-
etry of 2’ is in good agreement with that determined
experimentally, thus indicating that the critical bonding
features are captured in the model complex. Examination of
the electron density distribution of the HOMO of 2’ indeed
reveals a p-like bonding orbital between the boron atom and
carbene carbon center with significant contribution from
boron atom (14.6%), suggesting the presence of a p-
nucleophilic boron atom (Figure 2). This finding also implies
the existence of a considerable p-back-bonding contribution
from boron to carbon to the overall borole–carbene bonding
shifted upfield with respect to those of the methyl protons of
the mesityl groups. As a result of the quadrupolar coupling
with the boron nuclei, the signal of the boron-bound methyl
group in the 13C NMR spectrum detected at d = 8.2 ppm is
very weak and was only identified by HMQC methods.
Single crystals suitable for X-ray analysis were obtained
from slow evaporation of a Et2O/n-hexane solution of 3 at
room temperature. Compound 3 crystallizes in the monoclinic
P21/c space group and displays two independent molecules in
the asymmetric unit (Figure 3). These two independent
molecules feature similar structural parameters and are
arbitrarily denoted as molecule I and molecule II. The
propeller-like arrangement of the phenyl groups with average
dihedral angles of 53.28 (I) and 56.88 (II) represents the
common structural feature of tetraphenyl-substituted boroles.
The boron atom adopts a distorted tetrahedral environment,
À
and all B C bond lengths fall within the expected range for
the corresponding single bonds. The significant elongation of
À
À
the B C1 [1.649(2) ꢀ (I); 1.650(2) ꢀ (II)] and B C4 bonds
[1.648(2) ꢀ (I); 1.650(2) ꢀ (II)] compared to those observed
in 2 indicates the loss of p-electron delocalization over the
C4B ring mediated by the pz orbital of the boron atom.
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Angewandte
Chemie
substituted borole derivatives, which are otherwise difficult to
obtain. More detailed reactivity studies of 2 are currently
underway.
Experimental Section
2: Compound 1 (20 mg, 0.028 mmol) and KC8 (20 mg, 0.15 mmol)
were mixed in a Young NMR tube and dissolved in diethyl ether
(0.5 mL). The reaction was monitored with 11B NMR spectroscopy
until the total consumption of compound 1 was detected. After
standing at ambient temperature for overnight, the dark reddish-
purple solution was filtered through a filter pipette in a glovebox.
Additional diethyl ether (2 ꢁ 0.5 mL) was used to wash the product
from the solid residue. The combined filtrate was layered with one-
tenth volume of pentane and kept at À308C for 10 days to give dark
purple crystals. Yield 7.5 mg (37%). 1H NMR (500 MHz, C5D5N,
297 K): d = 2.06 (s, 6H, p-MeMes), 2.30 (s, 12H, o-MeMes), 3.61 (s,
4H, NCH2CH2N), 6.49 (s, 4H, MesCH), 6.58–6.62 (m, 2H), 6.68–6.74
(m, 4H), 6.87–6.91 (m, 6H), 6.96–7.03 ppm (m, 8H). 13C{1H} NMR
(126 MHz, C5D5N, 297 K): d = 20.4 (o-MeMes), 20.7 (p-MeMes), 51.3
(NCH2CH2N), 120.2 (Ph), 120.8 (Ph), 126.2 (Ph), 126.5 (Ph), 129.5
(Mes), 131.4 (Ph), 132.3 (Ph), 134.5, 140.0, 145.2, 152.7 ppm. 11B{1H}
NMR (160 MHz, C5D5N, 297 K): d = 12.7 ppm. Elemental analysis
(%) calcd. for C51H51BKN2O0.5: C 81.69, H 6.86, N 3.74; found:
C 80.63, H 6.72, N 4.22.
Figure 3. Crystal structure of one of the independent molecules of 3.
Ellipsoids are set at the 50% probability level. Hydrogen atoms are
omitted for clarity. Selected bond lengths [ꢂ]: molecule I: B–C6
1.633(2), B–C5 1.647(2), B–C1 1.649(2), B–C4 1.648(2), C1–C2
1.355(2), C2–C3 1.481(2), C3–C4 1.363(2), C5–N1 1.352(2), C5–N2
1.346(2); molecule II: B–C6 1.633(2), B–C5 1.649(2), B–C1 1.650(2),
B–C4 1.650(2), C1–C2 1.364(2), C2–C3 1.479(2), C3–C4 1.359(2), C5–
N1 1.342(2), C5–N2 1.357(2)
À
Lengthening of the B C5 bond [1.647(2) ꢀ (I); 1.649(2) ꢀ
À
À
(II)] in addition to the contraction of both C5 N bonds [C5
À
À
N1: 1.3524(17), C5 N2: 1.3460(18) ꢀ (I); C5 N1: 1.3417(18),
C5 N2: 1.3569(17) ꢀ (II)] indicates canceling of p-electron
À
back-donation from the boron atom to the carbene carbon
center upon methylation.
Received: December 7, 2009
Published online: February 15, 2010
The nucleophilic character of the boron atom in 2 as
expressed in its reactivity towards MeI is highly unusual and
unique in comparison with all other relevant six-p-electron
boracycles (Scheme 2). Thus, in borabenzenes 4, the boron
atom acts as an electrophile and prompts nucleophilic
addition.[17] Likewise, electrophilic quenching of borataben-
zene 5 only results in the formation of a 2-substituted
boracyclohexadiene.[18] Accordingly, the borataalkene 6 also
represents a carbon-centered nucleophile and displays no
nucleophilicity of the boron atom.[19]
Keywords: boron · carbenes · heterocycles · nucleophilicity ·
reduction
.
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