NHC-stabilized 1-hydro-1 H-borole and its nondegenerate sigmatropic isomers

Article abstract of DOI:10.1021/ic200533e

Electrophilic quenching of a carbene-stabilized π-boryl anion with the Bronsted acid Et₃NHCl provides a convenient synthetic route to the parent NHC-stabilized 1-hydroborole, which isomerizes to the corresponding 3-hydroborole via two successiv

Full text of DOI:10.1021/ic200533e

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pubs.acs.org/IC
NHC-Stabilized 1-Hydro-1H-borole and Its Nondegenerate
Sigmatropic Isomers
Holger Braunschweig,* Ching-Wen Chiu, Thomas Kupfer, and Krzysztof Radacki
Institut f€ur Anorganische Chemie, Julius-Maximilians Universit€at W€urzburg, Am Hubland, 97074 W€urzburg, Germany
S Supporting Information
b
Scheme 1. Syntheses of η5-(C4R4BH)-Containing Metal
ABSTRACT: Electrophilic quenching of a carbene-stabi-
lized π-boryl anion with the Brønsted acid Et3NHCl provides
a convenient synthetic route to the parent NHC-stabilized
1-hydroborole, which isomerizes to the corresponding 3-hy-
droborole via two successive nondegenerate [1,5]-sigmatro-
pic hydrogen migrations. Molecular structures of both
isomers have been determined by X-ray crystallography.
Complexes
eing the smallest neutral antiaromatic boracycles, boroles
B
have recently attracted considerable attention because of
their potential application in optoelectronics.1 The antiaromatic
character of boroles entails strong electrophilicity of the boron
center in combination with the presence of a highly activated
conjugated carbon backbone, which readily participates in
DielsꢀAlder reactions.2 Accordingly, it is imperative to protect
the C4B core with bulky substituents in order to enable the
isolation of monomeric, nonannulated borole derivatives.2,3 In
the past few years, various substituents such as halide, aryl, and
amino functionalities, as well as ferrocenyl and platinum complex
fragments, have been successfully attached to the boron atom.4
By contrast, little is known for the parent unsubstituted 1-hydro-
borole. Apart from those generated within the coordination
sphere of transition metals, the 1-hydro-1H-boroles were only
studied by computational methods.5 However, preparation of the
η5-(C4R4BH)-containing metal complexes is not straightforward
either. These species were obtained via (i) AlH3-induced ring
contraction of the 1,2-diboratabenzene in [Cp*Rh(η6-C4H4-
(BNMe2)2)],6 (ii) BH group insertion to the metalꢀC bonds
of {Cp(PPh3)Co(κ2-C4Ph4)},7 or (iii) reduction of η5-halobor-
ole metal complexes with hydride reagents (Scheme 1).8
Scheme 2. Synthesis and Electrophilic Quench with MeI of 1
synthesis of a NHC-stabilized 1-hydro-1H-borole via proton-
ation of the corresponding π-boryl anion. In addition, the non-
degenerate [1,5]-sigmatropic isomers of 1-hydro-1H-borole are
also studied experimentally and computationally.
Protonation of 1 with excess triethylammonium chloride in
Et2O at room temperature resulted in a substantial color change
from dark reddish purple to light orange within 10 min. After
removal of the solvent under vacuum, 2-BH (Figure 1) was iso-
lated via hexane extraction of the reaction residue. The 11B NMR
resonance of 2-BH is detected at ꢀ18 ppm with a 1JBH coupling
of 85 Hz, which is comparable to that observed for trialkyl
borohydrides14 and stable carbeneꢀborane adducts.15 Conse-
quently, the corresponding 1H NMR signal of the BꢀH proton
(δ 3.12) can only be identified in the 11B-decoupled spectrum,
thus confirming the direct bonding between boron and hydro-
gen. The symmetric 1H NMR resonances of the mesityl groups
suggest a free rotation of the SIMes ligand about the Bꢀcarbene
bond in solution at room temperature.
Prior to the isolation of boron nucleophiles,9 the generation
of BꢀH bonds generally relied on the hydride reduction of
haloboranes,10 chemical reduction of boron halides,11 or reaction
of (C6F5)3B and triethylsilane.12 With the isolation of nucleo-
philic boryl anions, an alternative approach to the BꢀH bond was
made accessible by means of protonation of the boron atom with
weak acids.9a In our previous study, chemical reduction of a
SIMesꢀchloroborole adduct (SIMes: 1,3-dimesitylimidazolidin-
2-ylidene) afforded the π-boryl anion 1, which readily reacted
with CH3I to yield the carbene-stabilized 1-methylborole
(Scheme 2).13 Because of the nucleophilicity of the boron center
in 1, this monoanionic borollide represents a unique precursor
to various B-substituted borole derivatives, which are difficult to
prepare by conventional methods. In this paper, we report on the
Received: March 15, 2011
Published: April 11, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ic200533e Inorg. Chem. 2011, 50, 4247–4249
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Scheme 3. Synthesis of 2-BH and Its [1,5]-Sigmatropic
Isomers, 2-CH0 and 2-CH
characteristics of 2-BH have been detected in solution. In addition,
all attempts to separate the crystals of 2-BH and 2-CH mechani-
cally failed, and we were not able to obtain analytical data for 2-CH
by variable-temperature NMR spectroscopy or solid state UVꢀvi-
sible spectroscopy so far.
Even though sigmatropic rearrangements of this type are
pretty common in cyclopentadienyl chemistry, only a few reports
of a related reactivity have appeared in the literature for
boracycles.18 In order to elucidate the relative stabilities of 2-
BH and 2-CH, theoretical computations of the two isomers
and its putative third isomer, namely, 2-hydroborole 2-CH0, were
performed with density functional theory (DFT) methods
applying the B3LYP functional and 6-31G* basis sets. The
optimized geometries of 2-BH and 2-CH are in fairly good
agreement with those determined by X-ray diffraction. Examina-
tion of the enthalpies of the molecules in the gas phase revealed
that 2-BH is thermodynamically favored over 2-CH and 2-CH0
by 9.0 and 4.6 kcal molꢀ1, respectively. This result is consistent
with the absence of any detectable quantities of 2-CH and 2-CH0
in solution; i.e., the equilibrium between the three different
isomers, as depicted in Scheme 3, most likely lies completely
on the side of 2-BH. Nevertheless, the fact that it has been
possible to isolate single crystals of 2-CH might be connected to
its favorable crystallization properties, which are capable of shift-
ing the equilibrium toward 2-CH during the crystallization
process. Although 2-CH0 is more stable than the 2-CH isomer,
all efforts toward the isolation and structural characterization of
2-CH0 remained unsuccessful.
Figure 1. Molecular structures of 2-BH (top) and 2-CH with hydrogen
atoms omitted for clarity. Thermal ellipsoids are set at the 50%
probability level.
Interestingly, slow evaporation of a 2-BH solution in CH2Cl2/
hexane afforded two types of crystalline materials, i.e., (i) pale-
yellow blocks and (ii) a very few orange needles, which were both
suitable for X-ray diffraction.16 Analysis of the yellow blocks
verified the formation of a 1-hydro-2,3,4,5-tetraphenyl-1H-bor-
ole SIMes adduct (2-BH). The presence of a tetracoordinate
boron center is confirmed by the sum of the CꢀBꢀC angles
around B1 of 326.8°. Rehybridization of the boron atom results
in a significant elongation of the Bꢀcarbene bond distance
[B1ꢀC5 1.628(2) Å] with respect to that in 1 [1.541(2) Å], as
well as a pronounced CꢀC bond length alternation within the
butadiene backbone [C1ꢀC2 1.359(2) Å; C2ꢀC3 1.479(2) Å;
C3ꢀC4 1.363(2) Å], which strongly resembles those observed
in NHC-stabilized 1-methyl-2,3,4,5-tetraphenylborole.13
X-ray diffraction of the orange needles, on the other hand,
allowed for the structural characterization of the 3-hydroborole
2-CH, one of the two possible nondegenerate [1,5]-sigmatropic
isomers of 2-BH. The shift of the hydrogen atom from B1 to C2 is
authenticated by the presence of a tetrahedral C2 center (Σ =
331.1°), whereas the remaining ring atoms still display their trigonal-
planar geometry. The B1ꢀC1 bond distance [1.463(4) Å]
is noticeably shorter than the BꢀC bonds in 1 [B1ꢀC1
1.535(2) Å; B1ꢀC4 1.541(2) Å], which indicates a strong
bonding interaction between B1 and C1 in 2-CH. In fact, the
B1ꢀC1 bond distance is analogous to that reported for borataalk-
ene compounds, in which the formal bond order for the BꢀC
linkage is 2.17 Hence, 2-CH can also be considered a carbene-
stabilized boraalkene. Despite its structural elucidation in the solid
state, 2-CH consistently eluded spectroscopic characterization in
solution, even if the crystalline mixture of 2-BH and 2-CH was
used in the measurements. In any case, only the spectroscopic
In summary, the NHC-stabilized borole anion 1 represents a
unique synthon for the generation of 1-hydro-1H-borole 2-BH,
which is related to 2-CH by a sequence of two nondegenerate [1,5]-
sigmatropic rearrangements of the boron-bound hydrogen atom.
Two of three possible isomers were isolated and characterized in the
solid state by single-crystal X-ray analysis (2-BH and 2-CH). DFT
studies indicate that the equilibrium between the three isomers lies
completely on the side of 2-BH in solution due to thermodynamics.
Further reactivity studies of the π-boryl anion are currently ongoing.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures,
b
spectroscopic and crystallographic data, details on the theoretical
calculations, and crystallographic material in CIF format. This
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material is available free of charge via the Internet at http://pubs.
acs.org.
(13) Braunschweig, H.; Chiu, C.-W.; Radacki, K.; Kupfer, T. Angew.
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’ AUTHOR INFORMATION
(15) Kuhn, N.; Henkel, G.; Kratz, T.; Kreutzberg, J.; Boese, R.;
Maulitz, A. H. Chem. Ber. 1993, 126, 2041.
Corresponding Author
*E-mail: h.braunschweig@mail.uni-wuerzburg.de.
(16) Crystal data for 2-BH: C49H47BN2, Mr = 674.70, yellow block,
0.49 ꢁ 0.38 ꢁ 0.29 mm3, monoclinic space group P21/n, a = 12.5450(6) Å,
b = 13.9877(7) Å, c = 22.8093(11) Å, β = 103.996(2)°, V = 3883.7(3) Å3,
Z = 4, Fcalcd = 1.154 g cmꢀ3, μ = 0.066 mmꢀ1, F(000) = 1440, T = 100(2)
K, R1 = 0.0738, wR2 = 0.1402, 9578 independent reflections [2θ e 56.64°]
(Rint = 0.0532) and 475 parameters. Crystal data for 2-CH: C49H47BN2,
Mr = 674.70, orange needles, 0.32 ꢁ 0.09 ꢁ 0.07 mm3, monoclinic space
group P21/c, a = 9.8006(8) Å, b = 18.2005(13) Å, c = 21.5781(17) Å,
β = 95.482(4)°, V = 3831.4(5) Å3, Z = 4, Fcalcd = 1.170 g cmꢀ3, μ =
0.067 mmꢀ1, F(000) = 1440, T = 100(2) K, R1 = 0.1029, wR2 = 0.1454,
8857 independent reflections [2θ e 56.7°] (Rint = 0.0000) and 552
parameters.
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’ ACKNOWLEDGMENT
C.-W.C. is grateful for a postdoctoral research fellowship from
the Alexander von Humboldt Foundation.
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