Generation of a carbene-stabilized bora-borylene and its insertion into a C-H bond

Article abstract of DOI:10.1021/ja208372k

A novel NHC adduct of a dihalodiborane(4), 1, is reduced by KC₈ with formation of the five-membered boracycle 2. The reaction most likely proceeds via C-H insertion of an intermediate NHC-stabilized free bora-borylene species.

Full text of DOI:10.1021/ja208372k

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Generation of a Carbene-Stabilized Bora-borylene and its Insertion
into a CÀH Bond
Philipp Bissinger, Holger Braunschweig,* Alexander Damme, Rian D. Dewhurst, Thomas Kupfer,
Krzysztof Radacki, and Katharina Wagner
Institut f€ur Anorganische Chemie, Julius-Maximilians-Universit€at W€urzburg, Am Hubland, 97074 W€urzburg, Germany
S Supporting Information
b
and in possession of a boron-based lone pair of electrons. They
ABSTRACT: A novel NHC adduct of a dihalodiborane(4),
1, is reduced by KC8 with formation of the five-membered
boracycle 2. The reaction most likely proceeds via CÀH
insertion of an intermediate NHC-stabilized free bora-
borylene species.
also demonstrated the ability of this compound to undergo one-
electron oxidation or add as a Lewis base to H+ (X).15
In stark contrast to the aforementioned NHCÀmonoborane
chemistry, the corresponding diborane(4)Àmono-NHC species
are restricted to only two partially characterized (11B NMR)
examples.16a,b However, sp2Àsp3-hybridized mixed diboranes
featuring other Lewis donors have been prepared,16c,d and some
have been used for the copper-catalyzed β-boration of α,
β-unsaturated conjugated compounds.16eÀg Thus, on the basis of
our borylene-trapping reaction and the work of Power, we set out
to combine the concepts of NHC stabilization and diboranes
with the reduction of BX2 fragments to form borylenes. In this
communication, we initially describe an unexpected rearrange-
ment of the substituents on the diborane(4) framework upon
addition of a carbene (see below). This fortuitous result provided
compounds with a boron atom bearing two halides and a carbene
donor. The unprecedented 1,1-dihalodiborane(4) connectivity
pattern gave us a unique opportunity to probe the reduction
chemistry of a BX2 fragment directly connected to another boron
atom. Electronic stabilization of the BX2 fragment by the carbene
donor and steric shielding by the surrounding mesityl substitu-
ents provided further encouragement. The results of the sub-
sequent reduction attempts are also presented herein.
We became interested in the compound 1,2-dichloro-1,
2-dimesityldiborane(4) (1; Scheme 1), as its mesityl groups impart
considerable steric protection to the boron atoms while provid-
ing negligible electronic stabilization, thus making 1 much more
reactive than traditional 1,2-dihalo-1,2-diaminodiboranes(4).17
We reasoned that Lewis base adducts of 1 with one or two bulky
NHCs would be promising precursors for reduction chemistry.
Treatment of 1 with 1,3-bis(2,4,6-trimethylphenyl)-imidazo-
lin-2-ylidene (SIMes) in toluene resulted in new 11B NMR
signals at δ À0.17 and 87.4. The strong high-field shift of one
boron nucleus relative to the starting material 1 (δ 85.2) clearly
indicated the formation of a four-coordinate boron center in the
product 2 (Scheme 1).
n 1967, at a time when few were focused on the element boron,
P. L. Timms generated the free borylene “:BF” by passing BF3
I
over solid boron at high temperature and low pressure.1 Later,
West provided more evidence for borylenes by photolyzing
disilaboranes at low temperatures.2 Timms’ work was so far
ahead of its time that it took another 31 years before a borylene
could be stabilized as a terminal ligand on a transition metal3,4
and a further 11 years before the elusive fluoroborylene in
particular could be similarly tamed.5 Although borylene chem-
istry took decades to gain a foothold, research interest in
synthesizing or generating low-valent and reactive boron com-
pounds, such as boron-centered anions, radicals, and multiply
bound species, was undeterred.
In 2006, Yamashita and Nozaki turned on its head many
years of unsuccessful attempts at generating a boryl anion
(I, Figure 1),6 spurring new interest in the stabilization of reactive
boron compounds and inspiring us to synthesize two new
families of formal boryl anions (II and III).7,8 Recently, an
interdisciplinary consortium comprising the groups of Curran,
Malacria, Newcomb, Walton, Fensterbank, and Lac^ote has
revived the study of boryl radicals, using state-of-the-art techni-
ques for their characterization (IV).9 Their work has shown that
by adding an N-heterocyclic carbene (NHC) to a boron center,
radical abstraction of H• can result in stable boron-centered
radicals. Likewise, Power's group has a long history of reducing
boron-containing compounds, including diboranes (V, leading
to multiply bound BÀB radical anions)10 and dihaloboranes
(VII, resulting in insertion of the boron fragment into a CÀC
bond, presumably via a two-coordinate borylene intermediate).11
Similarly, Robinson's group has also exploited NHC stabiliza-
tion to synthesize a diborene (VI, Figure 1).12 We recently
reduced an NHC-stabilized dihaloborane and trapped the pre-
sumed borylene intermediate as its naphthalene adduct (VIII),13
and related work by Robinson on the reduction of NHC-stabilized
dihaloboranes and monohaloboronium cations showed hydrogen
abstraction/boron coupling and CÀH bond insertion reactivity
(IX).14 As we were preparing this manuscript, Bertrand and co-
workers published the synthesis of a doubly carbene-stabilized
terminal borylene (:BH), a compound isoelectronic to an amine
The identity of 2 was unambiguously determined by NMR
spectroscopy, elemental analysis, and X-ray diffraction to be an
NHC-stabilized 1,1-dichlorodiborane(4). This result was rather
surprising because this rearrangement reaction necessarily re-
quires the cleavage of at least one BÀCl and one BÀC bond.
However, we note that a similar rearrangement has been
observed in the transhalogenation reactions of various
Received: September 9, 2011
Published: October 27, 2011
r
2011 American Chemical Society
19044
dx.doi.org/10.1021/ja208372k J. Am. Chem. Soc. 2011, 133, 19044–19047
|

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Figure 2. Structure of 2 in the solid state. Thermal ellipsoids are shown
at 50% probability. H atoms have been omitted for clarity. Selected bond
lengths (Å) and angles (deg): B1ÀB2, 1.774(3); B1ÀC1, 1.653(2);
B1ÀCl1, 1.889(2); B1ÀCl2, 1.906(2); ∑—B2, 359.4; B2ÀB1ÀCl1,
112.5(2); B2ÀB1ÀCl2, 101.2(1); B2ÀB1ÀC1, 124.2(2).
Figure 3. Structure of 3 in the solid state. Thermal ellipsoids are shown
at 50% probability. Apart from that bound to B1, H atoms have been
omitted for clarity. Selected bond lengths (Å) and angles (deg): B1ÀB2,
1.670(3); B1ÀC1, 1.553(3); B1ÀH1, 1.25(2); B2ÀH1, 1.51(2); ∑—B2
,
359.9; ∑—B1, 357.5; H1ÀB1ÀB2, 60.1(9); H1ÀB2ÀB1, 45.8(8);
B2ÀB1ÀC 128.1(2).
judged by 11B NMR spectroscopy. The 11B NMR spectrum
shows a more upfield-shifted signal (δ 57.5) for the three-
coordinate boron center than in 2 (δ 87.4), while the second
signal at δ 0.15 lies in the expected range for four-coordinate
1
boron nuclei. A broad signal at 0.81 ppm in the H NMR
spectrum, which sharpened considerably upon 11B decoupling,
was assigned to a boron-bound hydrogen atom. Although these
data were insufficient to determine the connectivity of 3, the
molecular structure provided by X-ray crystallography allowed us
to assign the highly unusual structure of 3 depicted in Figure 3.
The protons of the CH2 group bound to boron give rise to two
doublet signals each (2J = 19 Hz) at δ 1.82 and 2.57. The protons
in the backbone of the NHC ligand show a complex multiplet,
which may be an indication of hindered rotation of the NHC
group. The most downfield 13C NMR signal (δ 161.3) was
assigned to the C3 nucleus connected to the CH2 group at boron.
The signals for the quaternary carbon atoms bound to boron
(141.9, 145.5 ppm) were significantly broadened. The signal for
the carbene center, however, could not be located.
Figure 1. Relevant anionic and radical boron compounds or trapping
products thereof.
Scheme 1. Carbene-Induced Rearrangement of 1,
2-Dichlorodiborane(4) 1 To Form the Unsymmetrical
Carbene-Stabilized 1,1-Dichlorodiborane(4) 2
The solid-state structure of 3 shows a B1ÀC1 distance of
1.553(3) Å, which is considerably shorter than that in 2
[1.653(2) Å]. The BÀB distance [1.670(3) Å] in 3 is also
shortened relative to that in the starting material [1.774(3) Å].
However, the most striking feature of the structure of 3 is the
position of the boron-bound hydrogen atom, which was reliably
located. This hydrogen appears to be strongly distorted toward
1,2-dichloro-1,2-diaryldiboranes(4) with LiF.18 The molecular
structure of 2 in the solid state is shown in Figure 2. The BÀB
bond in 2 has a length of 1.774(3) Å, which is notably elongated
relative to the BÀB bond in 1 [1.680(6) Å].19
Subsequent reduction of NHC adduct 2 with 2 equiv of KC8
resulted in the quantitative conversion to the new compound 3 as
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dx.doi.org/10.1021/ja208372k |J. Am. Chem. Soc. 2011, 133, 19044–19047

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COMMUNICATION
Scheme 2. Proposed Mechanism for the Formation of 3
Figure 4. Molecular structure of 4 in the solid state.20 Thermal
ellipsoids are shown at 50% probability. H atoms have been omitted
for clarity. Selected bond lengths (Å) and angles (deg): B1ÀC1, 1.608(6);
BÀBr1, 2.041(5); BÀBr2, 2.093(5); B1ÀC4, 1.671(6); ∑—C1, 360.0;
N1ÀC1ÀN2, 106.4(3); N2ÀC1ÀB1, 107.1(3); B1ÀC1ÀN1, 146.5(3).
Scheme 3. Synthesis of 4
different from that of 4 (δ À6.85).12 The 1H, 11B, and 13C{1H}
NMR spectroscopic data for 4 are unobtrusive and in full agree-
ment with its determined solid-state structure (Figure 4; see the
Supporting Information for full experimental, spectroscopic, and
structural details).
the second boron atom (B1ÀH1, 1.25(2) Å; B2ÀH1, 1.51(2) Å).
B2 lies in a trigonal-planar environment (∑—B2 = 359.9ꢀ), while
the three non-hydrogen substituents of B1 are also bound in a planar
fashion (∑—B1 = 357.5ꢀ). The hydrogen atom sits above the
plane defined by the bicyclic rings, making a triangular HÀBÀB
system with extremely acute HÀBÀB angles (H1ÀB1ÀB2,
60.1ꢀ; H1ÀB2ÀB1, 45.8ꢀ). Interestingly, the B2ÀB1ÀC1 angle
[128.1(2)ꢀ] is significantly larger than that in 2 [124.2(2)ꢀ],
although this distortion from true trigonal planarity is probably
the result of steric interactions between the bulky adjacent B-Mes
and N-Mes groups.
Thus, the solid-state structure of 3 includes a highly unusual
planar R2BÀBR(NHC) system with a semibridging boron-
bound hydrogen atom above this plane, making the compound
difficult to classify. We are unaware of any diborane compound
with comparable structural features. The high-field 11B NMR
signal of 3 (δ 0.15) is consistent with a tetracoordinate bor-
aneÀbase adduct or borate description, suggesting that a more
conventional tetrahedral geometry of the NHC-bound boron
atom prevails in solution. A 11B,1H HMQC NMR spectrum of 3
permitted the analysis of the cross-peak profile to determine the
BÀH coupling constant (1JB,H = 160 Hz). The absence of a
similar correlation for the second boron atom indicates that the
proton is in this case terminally bound in solution.
To ensure that the formation of 3 was not the result of a
comparable elimination process, we extensively heated solutions
of the precursor 2 in various solvents. However, the NHCÀdi-
borane adduct 2 proved to be perfectly stable even under
relatively harsh thermal conditions, thus providing further sup-
port for the proposed borylene mechanism (see above).
In conclusion, we have described the formation of a hitherto
unknown NHC adduct of the 1,2-diaryl-1,2-dihalodiborane(4) 1.
Interestingly, an unprecedented rearrangement occurred upon
adduct formation, furnishing the NHC-stabilized 1,1-dichloro-
diborane(4) 2. Its reaction with KC8 afforded compound 3,
which is presumably formed by CÀH insertion of an intermedi-
ate NHC-stabilized borylene. In contrast, the reaction of ItBu
with BBr3 led to the formation of a ring-closure product upon
heating in solution. This behavior was not observed for 2 upon
heating in solution, thus supporting the formation 3 via an NHC-
stabilized bora-borylene intermediate.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details and crys-
b
tallographic data (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
Bicyclic diborane 3 might be considered the product of the
formal insertion of an NHC-stabilized borylene into a methyl
CÀH bond of one mesityl group. Given the strongly reducing
conditions of the reaction and previously observed “free” bor-
ylene reactivity patterns, we are inclined to propose a mechanism
involving two-electron reduction of the dichloroboryl group
to form a reactive bora-borylene intermediate, as shown in
Scheme 2.
’ AUTHOR INFORMATION
Corresponding Author
h.braunschweig@uni-wuerzburg.de
’ ACKNOWLEDGMENT
We are grateful to the German Science Foundation (DFG) for
financial support and to Dr. Florian Bauer for valuable discus-
sions and his help in setting up this manuscript.
It should be noted that in our laboratories certain NHC-
stabilized haloboranes have been shown to undergo thermally
induced ring-closure reactions, which to the best of our knowledge
have no precedent in NHCÀborane chemistry. An example of this
is the reaction of 1,3-di-tert-butylimidazol-2-ylidene (ItBu) with
BBr3, which affords the cyclic species 4 in high yield with
spontaneous elimination of HBr (Scheme 3). However, the
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