Synthesis, coupling, and condensation reactions of 1,2-diborylated benzenes: An experimental and quantum-chemical study

Article abstract of DOI:10.1002/chem.201201547

1,2-Bis(pinacolboryl)benzene (1,2-C₆H₄(Bpin) ₂, 2) was synthesized in preparatively useful yields from 1,2-C ₆H₄Br₂, iPrO-Bpin, and Mg turnings in the presence of 1,2-C₂H₄Br₂ as an entrainer. Compound 2 is a versatile starting material for the synthesis of (un)symmetrically substituted benzenes (i.e., 1,2-C₆H ₄(Ar¹)(Ar²)) through sequential Suzuki-Miyaura coupling reactions. Alternatively, it can be transformed into bis-borate Li ₂[1,2-C₆H₄(BH₃)₂] (3) through reduction with Li[AlH₄]. In the crystal lattice, the diethyl ether solvate 3·OEt₂ establishes a columnar structure that is reinforced by an intricate network of B-(μ-H)-Li interactions. Hydride-abstraction from compound 3 with Me₃SiCl leads to the transient ditopic borane 1,2-C₆H₄(BH₂) ₂, which can either be used in situ for subsequent hydroboration reactions or trapped as its stable NMe₂Et diadduct (6). In SMe ₂ solution, the putative diadduct 1,2-C₆H ₄(BH₂·SMe₂)₂ is not long-term stable but rather undergoes a condensation reaction to give 9,10-dihydro-9,10-diboraanthracene, HB(μ-C₆H₄) ₂BH, and BH₃. 9,10-Dihydro-9,10-diboraanthracene was isolated from the reaction mixture as its SMe₂ monoadduct (7), which dimerizes in the solid state through two B-H-B bridges ((7)₂, elucidated by X-ray crystallography). In contrast, hydride-abstraction from compound 3 in THF or CH₂Cl₂ provides the unique exo-adduct H₂B(μ-H)₂B(μ-C₆H₄) ₂B(μ-H)₂BH₂ (8, elucidated by X-ray crystallography). Quantum-chemical calculations on various conceivable isomers of [1,2-C₆H₄(BH₂)₂]₂ revealed that compound 8 was the most stable of these species. Moreover, the calculations confirmed the experimental findings that the NMe₂Et diadduct of 1,2-C₆H₄(BH₂)₂ is significantly more stable than the corresponding SMe₂ complex and that the latter complex is not able to compete successfully with borane-dimerization and -condensation. The reaction cascade in SMe₂, which proceeds from 1,2-C₆H₄(BH₂)₂ to the observed adducts of HB(μ-C₆H₄)₂BH, has been elucidated in detail and the important role of B-C-B-bridged intermediates has been firmly established. Dynamic covalent chemistry: Bis-borate Li₂[1,2-C₆H₄(BH₃) ₂] is a useful source of bis-borane 1,2-C₆H ₄(BH₂)₂, which can be trapped or used in hydroboration reactions; in the absence of a trapping reagent, redistribution affords 9,10-dihydro-9,10-diboraanthracene. Copyright

Full text of DOI:10.1002/chem.201201547

FULL PAPER
DOI: 10.1002/chem.201201547
Synthesis, Coupling, and Condensation Reactions of 1,2-Diborylated
Benzenes: An Experimental and Quantum-Chemical Study
ꢀmer Seven, Zheng-Wang Qu, Hui Zhu, Michael Bolte, Hans-Wolfram Lerner,
Max C. Holthausen,* and Matthias Wagner*[a]
Abstract: 1,2-Bis(pinacolboryl)benzene
(1,2-C6H4A(Bpin)2, 2) was synthesized in
preparatively useful yields from 1,2-
for subsequent hydroboration reactions
or trapped as its stable NMe2Et diad-
duct (6). In SMe2 solution, the putative
diadduct 1,2-C6H4A(BH2·SMe2)2 is not
long-term stable but rather undergoes
a condensation reaction to give 9,10-di-
H2BAHCNUTGRTEGU(NNN m-H)2BACHTUNTREGGN(NUN m-C6H4)2BACHTNUGTREN(NUGN m-H)2BH2 (8,
N
G
elucidated by X-ray crystallography).
Quantum-chemical calculations on var-
ious conceivable isomers of [1,2-C6H4-
ACHTUNGTRENNUNG
ꢀ
C6H4Br2, iPrO Bpin, and Mg turnings
CHTUNGTRENNUNG
in the presence of 1,2-C2H4Br2 as an
entrainer. Compound 2 is a versatile
starting material for the synthesis of
(un)symmetrically substituted benzenes
ACHTNUGTERN(NUGN BH2)2]2 revealed that compound 8 was
the most stable of these species. More-
over, the calculations confirmed the ex-
perimental findings that the NMe2Et
hydro-9,10-diboraanthracene,
HBACTHNGUTRENNU(G m-
C6H4)2BH, and BH3. 9,10-Dihydro-
9,10-diboraanthracene was isolated
from the reaction mixture as its SMe2
monoadduct (7), which dimerizes in
(i.e., 1,2-C6H
G
N
diadduct of 1,2-C6H4ACHTUGNTERNNU(G BH2)2 is signifi-
quential Suzuki–Miyaura coupling re-
actions. Alternatively, it can be trans-
cantly more stable than the corre-
sponding SMe2 complex and that the
latter complex is not able to compete
successfully with borane-dimerization
and -condensation. The reaction cas-
cade in SMe2, which proceeds from 1,2-
ꢀ ꢀ
formed into bis-borate Li
N
the solid state through two B H B
G
bridges ((7)2, elucidated by X-ray crys-
tallography). In contrast, hydride-ab-
straction from compound 3 in THF or
CH2Cl2 provides the unique exo-adduct
Li[AlH4]. In the crystal lattice, the di-
ethyl ether solvate 3·OEt2 establishes a
columnar structure that is reinforced
C6H
HB(m-C6H4)2BH, has been elucidated
4ACHTNUGTREN(NUGN BH2)2 to the observed adducts of
ꢀ
ꢀ
by an intricate network of B
G
ACHTUNGTRENNUNG
interactions. Hydride-abstraction from
compound 3 with Me3SiCl leads to the
transient ditopic borane 1,2-C6H4-
in detail and the important role of
Keywords: boranes · borates · con-
ꢀ ꢀ
B C B-bridged intermediates has
densation
· Grignard reaction ·
been firmly established.
quantum chemistry
ACHTUNGTRENNUNG(BH2)2, which can either be used in situ
Introduction
The scope of arylboranes can be further expanded when two
(or more) boryl substituents are introduced into the same
arene system: With respect to synthesis, such compounds
offer the possibility of performing double cross-coupling re-
actions with aromatic (di)halides as a synthetic route to
functionalized PAHs (e.g., oligophenyls, phenanthrenes, and
Borylated arenes are key starting materials for Suzuki–
Miyaura coupling reactions[1] and, therefore, have applica-
tions that range from the synthesis of natural products[2] to
the preparation of polycyclic aromatic hydrocarbons
(PAHs),[3,4] which are relevant for organic electronics.[5–11]
Moreover, arylboranes are also interesting in their own right
because they can serve as metallocene-catalyst activators,[12]
catalysts,[13–17] chemosensors,[18,19] or as luminescent dyes.[20–25]
dibenzoACHTUNGTRNEUNG
[g,p]chrysenes).[26–29] In the case of boron-based (co)-
catalysts, cooperative binding can lead to improved catalytic
performance[30–36] and, in the case of chemosensors, signal-
amplification has been reported.[37–41]
Therefore, efficient synthetic routes to oligoborylated
arenes are in great demand, especially to afford arenes that
contain boryl groups at adjacent positions on the ring. 1,2-
Diborylbenzenes are prototypical examples of vicinal di-
borylated arenes and various different strategies for their
preparation have been developed; however, many of these
syntheses suffer from distinct disadvantages, such as: 1) The
[a] Dipl.-Chem. ꢀ. Seven, Dr. Z.-W. Qu, Dr. H. Zhu, Dr. M. Bolte,
Dr. H.-W. Lerner, Prof. Dr. M. C. Holthausen, Prof. Dr. M. Wagner
Institut fꢁr Anorganische und Analytische Chemie
Goethe-Universitꢂt Frankfurt, Max-von-Laue-Straße 7
60438 Frankfurt am Main (Germany)
Fax : (+49)69-798-29260
E-mail: Max.Holthausen@chemie.uni-frankfurt.de
Matthias.Wagner@chemie.uni-frankfurt.de
reaction of 1,2-C6H
route to 1,2-bis(dichloroboryl)benzene (1,2-C6H4-
(BCl2)2);[42] however, BCl3 is a hazardous, highly corrosive
chemical and 9,10-dichloro-9,10-dihydro-9,10-diboraanthra-
cene (ClB(m-C6H4)2BCl), as well as partially methylated bor-
anes, are often formed as major byproducts of the Si/B-ex-
4ACTHNUGTREN(NUGN EMe3)2 (E=Si, Sn) with BCl3 provides
Supporting information for this article, including single-crystal X-ray
a
AHCTUNGTRENNUNG
analysis of compound 2 and RB
C(H)=C(H)tBu), quantum-chemical data, and X-ray crystallographic
data of compounds 2, 3·OEt2, (7)2, 8, and RB(m-C6H4)(m-pzSiMe3)2BR
ACHTNUGTRNEN(UGN m-C6H4)(m-pzSiMe3)2BR (R:
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(R: C(H)=C(H)tBu), is available on the WWW under http://
dx.doi.org/10.1002/chem.201201547.
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change reaction.[43] 2) The electrochemical reduction of 1,2-
dihalobenzenes (1,2-C6H4X2; X=Cl, Br, I) at a consumable
Mg anode in the presence of pinacolborane (HBpin) pro-
vides a mixture of 1,2-C6H4ACHTNUGTREN(NUGN Bpin)2 (<20%), 1,2-C6H4-
A
CHTUNGTRENNUNG
trimerization of alkynylboranes gives mixtures of the 1,2,4-
and the 1,3,5-isomers;[45,46] one remarkable exception is the
ꢀ ꢁ ꢀ
reaction of catB C C Bcat with [CpCo(CO)2], which leads
to hexaborylbenzene C6ACTHNUTRGNEUNG
(Bcat)6 (H2cat=catechol).[47] 4) The
Pd-catalyzed diborylation of benzyne, which starts from 1,2-
C6H4A(OTf)(SiMe3) and pinB-Bpin, is costly and requires the
C
ACHTUNGTRENNUNG
purification of the crude product by recycling gel-permea-
tion chromatography; therefore, it has only been applied on
a milligram scale.[48,49] Suginome and co-workers have re-
ported another Pd-catalyzed 1,2-borylation reaction, which
leads to 1,2-C6H
scale, but also uses expensive starting materials (i.e., 1,2-
C6H4ACHTUNGTRENNUNG(Bdans)(Br) and pinB-Bpin; H2dans=1,8-diaminonaph-
4ACHTUNGTRENNUNG(Bdans)ACHTUNTGREN(NUGN Bpin) in 75% yield on a gram
thalene).[26] A more-economically viable approach has been
developed by Percec and co-workers, which relies on a Zn-
activated Ni0 catalytic system that transforms 1,2-C6H4(X)2
with neopentylglycolborane (HBnpg) into 1,2-C6H4ACTHNUTRGEN(UNG Bnpg)2
in yields of 76% (X=Cl) and 83% (X=Br).[50]
Herein, we report a time- and cost-efficient uncatalyzed
Grignard route to 1,2-bis(pinacolboryl)benzene (1,2-C6H4-
ACHTUNGTRENNUNG(Bpin)2, 2; Scheme 1). This route uses the “entrainment
method”[51–53] and provides a convenient access to multigram
quantities of the product. With the aim of developing new
building blocks for boron-containing optoelectronic materi-
als,[24,25] we have subsequently used compound 2 to synthe-
size the ditopic trihydroborate Li
2ACHTUTNGRENNUG[1,2-C6H4ACHTUNTGERN(NUGN BH3)2] (3;
Scheme 1) and we have investigated the chemical properties
Scheme 1. Synthesis of compounds 2–5: a) THF, 20–408C, until the addi-
tion of the entrainer was complete, then reflux, 4 h; b) Et2O/n-pentane,
08C (2 h) to room temperature (16 h); c) Me2S, room temperature, 36 h.
of the corresponding ditopic borane 1,2-C6H
perimental and quantum-chemical means.
4ACHTUNGERTN(NUNG BH2)2 by ex-
Results and Discussion
symmetrically substituted benzenes (i.e., 1,2-C6H4ACHTUNGTRENNUNG
(Ar1)-
Synthesis and characterization of 1,2-C6H
analogy to our recently reported synthesis of 1,2-bis(tri
ylsilyl)benzene,[53] compound 2 was prepared from 1,2-dibro-
G
(Ar2)) through (sequential) Suzuki–Miyaura coupling reac-
ACHTUNGTRENNUNG
tions.[48,49,54] To confirm that compound 2 obtained by using
our Grignard procedure could be used for similar purposes,
we tested it in the Pd-catalyzed double-arylation reaction
with 2,2’-dibromobiphenyl and obtained triphenylene in
ACHTUNGTRENNUNG
mobenzene (1), isopropoxypinacolborane, and Mg turnings
in the presence of 1,2-dibromoethane as an entrainer
(Scheme 1). This entrainment method[51] ensured a continu-
ous activation of the Mg surface by a dropwise addition of
the entrainer compound into the reaction mixture. In this
case, a stoichiometric ratio of 1,2-dibromoethane/1,2-dibro-
mobenzene was required for optimum conversion into the
target compound (2). After aqueous workup, analytically
pure compound 2 was obtained by recrystallization of the
crude product from n-pentane in 40% yield. All of the
NMR spectroscopic data of compound 2 were in full agree-
ment with published data;[48] X-ray structural analysis of
compound 2 is provided in the Supporting Information.
Yoshida et al. have previously demonstrated that com-
pound 2 is a valuable starting material for the synthesis of
symmetrically (i.e., 1,2-C6H4(Ar)2; Ar=aryl) as well as un-
75% yield (THF, 608C, [Pd
ACHTUNGTRENNUNG
Synthesis and characterization of Li
C
4ACHTUNGERTN(NUNG BH3)2] (3):
Bis-borate 3 is accessible by the treatment of compound 2
with Li[AlH4] in Et2O/n-pentane (Scheme 1). Crystals of
compound 3·OEt2 that were suitable for X-ray crystallogra-
phy were grown by the gas-phase diffusion of n-hexane into
a solution of the bis-borate in Et2O/n-pentane.
The 11B NMR spectrum of compound 3 is characterized
1
by a quartet at d=ꢀ26.6 ppm with a J
(11B,H) coupling con-
ACHTUNGTRENNUNG
stant of 79 Hz (cf. Li[PhBH3]:
dACHTUNGTRENNUNG
ACTHNUTRGNEUNG
1J(11B,H) =76 Hz[55]). The corresponding boron-bound hy-
dride substituents give rise to a 1:1:1:1 quartet at d=
1
1.03 ppm in the H NMR spectrum. This value is in perfect
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Synthesis and Reactions of 1,2-Diborylated Benzenes
FULL PAPER
agreement with those of the related p- and m-phenylene-
bridged derivatives, that is, Li
G
2ACHTUNGTREN(NUG n-hexyl)2ACHUTNTGREN(NUGN BH3)2]
(d(1H)=1.02 ppm[56]
)
and Li3[1,3,5-C6H CHTUNGTRENNNUG
(BH3)3] (d(1H)=
1.07 ppm[57]). Therefore, we note a surprisingly low influence
of the distance between the two negatively charged trihydro-
borate groups on their NMR properties.
The crystal lattice of compound 3·OEt2 (Figure 1; Table 1)
contains parallel columns {Li
ACHUTGTNERN[NUG 1,2-C6H4ACHTUGNTRENNNUG
(BH3)2]}1, which con-
sist of alternating Li+ ions and [1,2-C6H
4A
each Li1 atom is coordinated by four trihydroborate moiet-
ies in a double-chelating manner[58,59] and each [1,2-C6H4-
ACHTUNGTRENNUNG ACHTUNGTRENNUNG
(BH3)2]2ꢀ ligand bridges two Li1 atoms. All {Li
ACHTUNGTRENNUNG
adjacent phenylene rings within the same column point in
opposite directions, with dihedral angles of 5.7(1)8. Each Li2
atom binds to three different trihydroborate substituents
and to one Et2O ligand. The Li2···B1 and Li2···B2C interac-
tions reinforce the {LiACHTUNGRTENNUG[1,2-C6H4ACHTUNGTERN(NUNG BH3)2]}1 stacks, whilst the
Figure 1. Crystal packing of compound 3·OEt2; H atoms (except on
boron) and Et2O ligands are omitted for clarity. Selected distances [ꢄ]:
Li1···B1 2.480(4), Li1···B2 2.666(4), Li1···B1A 2.594(4), Li1···B2A
2.573(4), Li2···B1 2.479(4), Li2···B2B 2.496(4), Li2···B2C 2.671(4),
Li1···C1 2.408(3), Li1···C2 2.547(4), Li1···Li2A 3.381(5), Li2···Li1C
3.381(5), Li2···Li2D 3.313(6). Symmetry transformations that were used
to generate equivalent atoms: A: ꢀx+1, yꢀ1/2, ꢀz+1/2; B: x, ꢀy+3/2,
z+1/2; C: ꢀx+1, y+1/2, ꢀz+1/2; D: ꢀx+1, ꢀy+2, ꢀz+1.
Li2···B2B contacts connect two columns. The four Li1···B
distances are in the range 2.480(4)–2.666(4) ꢄ and the three
Li2···B distances are between 2.479(4)–2.671(4) ꢄ. Accord-
ing to Edelstein’s[60] correlation of the metal–boron distances
as a measure of the denticity of a trihydroborate group,
B···Li distances of about 2.50 ꢄ are typical of an h2-RBH3-
Li coordination mode.[56,57] However, an inspection of the
geometric constraints within the crystal structure of
Table 1. Crystallographic data for compounds 3·OEt2, (7)2, and 8.
compound 3·OEt2 reveals that some of these bonds
3·OEt2
(7)2
8
are better characterized as h1-RBH3-Li (Figure 2).
formula
Mw
color, shape
T [K]
C10H20B2Li2O
191.76
C28H32B4S2
475.90
C12H16B4
203.49
ꢀ
Additional Li1 p interactions are indicated by
short Li1···C1 and Li1···C2 distances of 2.408(3) ꢄ
and 2.547(4) ꢄ, respectively.
colorless, plate
173(2)
colorless, block
173(2)
colorless, needle
173(2)
radiation, l [ꢄ]
crystal system
space group
a [ꢀ]
b [ꢀ]
c [ꢀ]
MoKa, 0.71073
monoclinic
P21/c
12.058(2)
8.0725(11)
13.621(3)
90
112.194(14)
90
1227.6(4)
4
1.038
416
0.058
0.18ꢅ0.16ꢅ0.09
9122
MoKa, 0.71073
triclinic
P1
MoKa, 0.71073
monoclinic
C2/m
11.3511(16)
15.0339(16)
7.0878(11)
90
105.470(11)
90
1165.7(3)
4
1.159
432
0.060
0.46ꢅ0.08ꢅ0.08
6315
Reactivity of 1,2-C6H
absence of olefins or Lewis bases: To test the reac-
tivity of 1,2-C6H4A(BH2)2 toward hydroboration, we
generated the free ditopic borane in situ by treating
a suspension of compound 3·OEt2 in Me2S with
4ACHTUNGTREN(NUNG BH2)2 in the presence and
¯
6.9647(4)
9.5417(6)
12.0420(7)
107.517(5)
97.549(4)
111.210(5)
684.97(7)
1
1.154
252
0.209
0.33ꢅ0.32ꢅ0.27
22256
3516 (0.0457)
3516/0/164
1.107
0.0317, 0.0835
0.0335, 0.0847
0.312, ꢀ0.264
CHTUNGTRENNUNG
a [8]
b [8]
ꢁ
Me3SiCl in the presence of 15 equivalents of tBuC
g [8]
CH (Scheme 1). An NMR spectroscopic investiga-
tion revealed the formation of cyclic hydroboration
product 5 as the dominant species. The initially ex-
pected 1,2-bis(divinylboryl)benzene (4) could not
be isolated; it was not even unequivocally identifia-
ble in the reaction mixture. For a targeted synthesis
of compound 5, the experiment was repeated with
V [ꢄ3]
Z
Dcalcd [gcmꢀ3
]
F
A
]
crystal size [mm]
total reflns
unique reflns (Rint
)
2156 (0.1010)
2156/0/160
0.927
0.0492, 0.1048
0.0863, 0.1189
0.170, ꢀ0.137
1073 (0.0783)
1073/0/95
0.999
0.0473, 0.1102
0.0631, 0.1164
0.233, ꢀ0.260
data/restraints/parameters
ꢁ
3.5 equivalents of tBuC CH. The compound shows
GOF on F2
a single 11B{1H} NMR resonance at d=71.6 ppm,
R1, wR2 [I>2s(I)]
R1, wR2 (all data)
which is in the typical region for triorganylbor-
1
anes.[61] The H NMR spectrum of compound 5 fea-
largest diff peak and hole
[eꢄꢀ3
]
tures two doublets at d=6.73 (2H) and 7.12 ppm
3
(2H; J
U
two E-alkenyl substituents.
A
A
can be assigned to the pyrazolide adduct of compound 5
(for more information on the use of pyrazolides in MS in-
vestigations of ditopic boron Lewis acids, see refer-
ACHTUNGTRENNUNG
ence [62]). Attempts to crystallize compound
5
or
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M. C. Holthausen, M. Wagner et al.
Figure 2. Plot of the asymmetric unit of compound 3·OEt2 in the solid
state; ellipsoids are set at 50% probability. Selected bond lengths [ꢄ]
and angles [8]: B1–C1 1.626(3), B2–C2 1.618(3), Li2–O1 1.940(4); B1-C1-
C2 123.1(2), B2-C2-C1 122.1(2).
K[5·pzSiMe3] failed. However, leaving
a solution of
K[5·pzSiMe3] in C6D6 to slowly evaporate at room tempera-
ture in an open NMR tube afforded crystals of the 1,2-phe-
nylene-bridged pyrazabole RBACTHNUTRGNE(UNG m-C6H4)(m-pzSiMe3)2BR (R:
C(H)=C(H)tBu). Thus, under these (hydrolytic) conditions,
the methanediide bridge in compound 5 was replaced by a
Scheme 2. Synthesis of compounds 6–8 and insertion of BH3 into 9H-9-
borafluorene (9) to give compound 10: a) toluene, room temperature,
12 h; b) Me2S, room temperature, 14 h; c) CH2Cl2, room temperature,
19 h.
ꢀ
pyrazolide linker, thereby generating an uncharged B N an-
alogue of triptycene (for X-ray structure analysis, see the
Supporting Information).[63]
Next, we tried to trap the transient borane, 1,2-C6H4-
ACHTUNGTRENNUNG(BH2)2, by adding selected Lewis bases. The amine adduct
(6) is readily accessible through hydride-abstraction from
compound 3·OEt2 with Me3SiCl in the presence of two
equivalents of NMe2Et (Scheme 2). Under an inert atmos-
phere, a solution of compound 6 in CD2Cl2 is stable for days
at room temperature (monitored by NMR spectroscopy).
The 11B NMR spectrum of compound 6 shows one poorly
tween compound 3·OEt2 and Me3SiCl in SMe2, we repeated
it on a preparative scale. After removal of the formed LiCl,
the colorless filtrate was stored at 48C until single crystals
had grown. X-ray analysis revealed a Ci-symmetric dimer of
the SMe2 monoadduct of DBA (i.e., compound (7)2;
Scheme 2, Figure 3, top). The dimer is held together by two
central B-H-B two-electron-three-center bonds. The result-
ing interdimer B2···B2# distance is 1.817(2) ꢄ, which com-
pares well with the respective intermonomer B···B distances
in polymeric DBA ((DBA)n; Figure 3, bottom; average
value: 1.818(12) ꢄ[64]). Furthermore, similar to (DBA)n, the
monomeric units of compound (7)2 deviate significantly
from planarity with a dihedral angle of 135.88 between the
two phenylene rings (cf. (DBA)n: 133.88). It has been sug-
gested that monomer-folding in (DBA)n is the result of oth-
erwise-unfavorable steric interactions between adjacent
DBA moieties and the same reasoning is probably true for
resolved triplet at d=ꢀ1.3 ppm (1J
ACHTUNGTRENNUNG
BH2 protons give a broad signal at d(1H)=2.66 ppm and the
ratio of the integrals of the aromatic/aliphatic resonances in
1
the H NMR spectrum is in agreement with the proposed di-
adduct structure.
In contrast to NMe2Et, SMe2 is not able to stabilize 1,2-
C6H4ACHTUNGTRENNUNG(BH2)2 in solution for extended periods of time. Moni-
toring a solution of in situ generated 1,2-C6H4ACHTNUTRGENNG(U BH2)2 in
SMe2 by 11B NMR spectroscopy reveals a broadened dou-
blet at d=2.1 ppm and a quartet at d=ꢀ19.1 ppm (1J-
ACHTUNGTRENNUNG
(11B,H) =105 Hz), thus indicating the formation of the
known SMe2 diadduct (Me2S)(H)B
G
N
compound (7)2.[64,65] Compound (7)2 features B1 S1 bond
ꢀ
of 9,10-dihydro-9,10-diboraanthracene (DBA), together with
BH3·SMe2.[61] Moreover, we observe two poorly resolved
triplets at d=ꢀ6.9 and ꢀ8.4 ppm. At the start of the investi-
gation, the two triplets are the dominant signals; however,
their relative intensities decrease over several hours, thus
suggesting that these resonances belong to short-lived aryl-
BH2·SMe2 species. To shed more light on the reaction be-
lengths of 1.997(1) ꢄ, which are somewhat shorter than in
the corresponding syn-diadduct of DBA, that is,
(Me2S)(H)B
(m-C6H4)2B(H)
E
(average
value:
2.031(2) ꢄ[62]).
In an attempt to record NMR spectra of compound (7)2,
one large crystal was treated with [D8]THF in an NMR
tube. The volume of the crystal gradually decreased, whilst
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Synthesis and Reactions of 1,2-Diborylated Benzenes
FULL PAPER
Me3SiCl in either THF or CH2Cl2 (8; Scheme 2, Table 1).
The same compound is obtained when the solution in SMe2
(see above) is evaporated almost to dryness and the residue
is recrystallized from C6H6. Compound 8 is insoluble at
room temperature in all common non-donating solvents,
which precludes its NMR spectroscopic investigation. A sol-
ution of compound 8 in pyridine exclusively revealed reso-
nances of the known pyridine diadduct of DBA[64] and of
BH3·pyridine.[61] The crystal lattice of compound 8 contains
two crystallographically independent molecules in the asym-
metric unit (8A and 8B). Both molecules possess an inversion
center, a mirror plane, and a 2-fold rotation axis, which lead
to a crystallographically imposed planar 9,10-diboraanthra-
cene core. Because all key metric parameters of molecules
8A and 8B are very similar, only structure 8A will be dis-
cussed herein (Figure 3, middle). Structure 8A can be regard-
ed as a low-molecular-weight analogue of (DBA)n:[64–66] We
observe a B1···B2 distance of 1.793(3) ꢄ and a C1-B1-C1#
bond angle of 120.6(2)8, as compared to an average B···B
distance of 1.818(12) ꢄ and an average C-B-C bond angle of
114.8(4)8 in (DBA)n.
Compound 8 represents the first crystallographically char-
acterized organyldiborane(6) with the unsymmetrical R2B-
AHCTUNGTRENNG(UN m-H)2BH2 structure. The fact that, in compound 8, a BH3
molecule coordinates in an exocyclic manner to the DBA
framework is in striking contrast to the behavior of B2H6
toward the in situ generated 9H-9-borafluorene (9): In this
ꢀ
latter case, the insertion of BH3 into one B C bond is the
preferred reaction pathway and gives endocyclic adduct
10[67,68] (Scheme 2).[59,68,69]
The generation of compound 8 from two equivalents of
1,2-C6H4ACHTNURTGNEUNG(BH2)2 formally represents a substituent-redistribu-
tion reaction with complete conservation of the number of
atoms. As mentioned above, monitoring a solution of 1,2-
C6H4ACHTNUGRTNEUNG
(BH2)2 in SMe2 by 11B NMR spectroscopy indicated
that an excess of the Lewis base can stabilize the separated
DBA and BH3 species as their respective thioether adducts.
However, B-H-B two-electron-three-center bonding, assist-
ed by the poor solubility of compound 8, is obviously able
to successfully compete with s-adduct formation. The as-
Figure 3. Molecular structures of compounds (7)2 (top), 8A (middle), and
(DBA)n (bottom) in the solid state; ellipsoids in structures (7)2 and 8A
are set at 50% probability. Selected distances [ꢄ] and angles [8] for com-
pound (7)2: B1–S1 1.997(1), B1–C1 1.602(2), B1–C11 1.604(1), B2···B2#
sembly of a DBA scaffold from 1,2-C6H4ACTHNUTRGNE(UGN BH2)2 proceeds
1.817(2); C1-B1-C11 112.5(1), Ar(C1)//ArACTHNUTRGNE(NUG C11) 135.8. Symmetry trans-
within minutes at room temperature. In contrast, the con-
ventional synthesis of 9,10-dibromo-9,10-dihydro-9,10-di-
ACHUTNGRENUbNG oraAHCUTNGTRENaNNGU nthracene requires heating of 1,2-bis(trimethylsilyl)-
formation that was used to generate equivalent atoms: #: ꢀx+1, ꢀy+1,
ꢀz+1. 8A: B1–C1 1.577(2), B1···B2 1.793(3); C1-B1-C1# 120.6(2). Sym-
metry transformation that was used to generate equivalent atoms: #: x,
ꢀy, z.
benzene with excess BBr3 in a sealed glass ampoule (1208C,
6 d, 54% yield).[65]
part of it went into the liquid phase and another part was
transformed into a colorless powder. The supernatant gave
Furthermore, the related condensation reaction of 1,1’-fc-
AHCTNUGTERNG(NNU BH2)2 in SMe2 provides the BH-bridged poly(ferrocen-
ꢀ ꢀ
ꢀ
characteristic NMR spectra of (Me2S)(H)B
E
N
ACHTUNGTERNNyUNG lene) [ fc B(H) ]n with the liberation of BH3·SMe2 (fc=
(C5H4)2Fe).[58,70,71] Despite numerous efforts, we have so far
not been able to provide evidence for the formation of 8-
type cyclic dimer 11, even though [1.1]diborataferroceno-
phanes (12) are known (Figure 4).[72,73] Thus, the preorgan-
This result leads to the conclusion that, upon dissolution,
compound (7)2 undergoes a redistribution of its SMe2 li-
gands so that two of them are attached to the same DBA
molecule, whilst the other molecule stabilizes itself by poly-
merization through B-H-B two-electron-three-center bonds.
We isolated crystals of the adduct between DBA and two
equivalents of BH3 from mixtures of compound 3·OEt2 and
ized rigid structure of 1,2-C6H4ACTHNUTRGNEUNG(BH2)2 appears to be an im-
portant factor in promoting the formation of cyclic dimers
rather than polymerization.
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M. C. Holthausen, M. Wagner et al.
first investigated the relative stabilities of several dimers of
structure A and of its mono- and diadducts with the Lewis
bases NMe2Et and SMe2 by using quantum-chemical meth-
ods. The structures of the resulting minima are shown in
Figure 5.
In the absence of coordinating donor bases, compound A
compensates for its inherent electron deficiency by undergo-
ing dimerization; thus, we explored the relative free energies
ꢀ
ꢀ
of various conceivable species. Initial B H/B H interac-
tions[68] between two monomers (A) lead to the formation
of two isomeric dimers (B and C), both of which feature
two classical B-H-B bridges. Compared to the dimerization
of BH3 to B2H6, for which DGR =ꢀ27.9 kcalmolꢀ1, the exer-
gonicity of these steps is only moderate (B: DGR =
ꢀ16.0 kcalmolꢀ1, C: DGR =ꢀ17.0 kcalmolꢀ1). Clearly, this
Figure 4. Hypothetical cyclic dimer (11) of the diborylated ferrocene 1,1’-
fc(BH2)2 and the crystallographically characterized [1.1]diborataferroce-
nophane (12).
ACHTUNGTRENNUNG
ꢀ
DFT calculations: To establish the underlying thermochem-
result is a consequence of significant intramolecular aryl
istry in the reactivity of 1,2-C6H
N
BH2 p-conjugation in the planar structure A moderating the
Figure 5. Optimized molecular structures of dimeric isomers (B–K) of structure A and the mono- and diadducts of NMe2Et (AN, AN2) and SMe2 (AS,
AS2); non-trivial symmetry point groups are given in parentheses. Gibbs free energies (in kcalmolꢀ1) of dimers B–K refer to two molecules of A; relative
Gibbs free energies for the base adducts refer to A and the uncoordinated bases.
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Synthesis and Reactions of 1,2-Diborylated Benzenes
FULL PAPER
Lewis acidity of the boron atoms. Whereas further stabiliza-
tion of the two dangling BH2 groups in the trans isomer (B)
is only possible through oligomerization (which we did not
investigate further herein), the slightly more stable
8); we will discuss potential reaction pathways that lead to
isomer I further below. Finally, mixed exo/endo-isomer J and
the exo-analogue of isomer H (i.e., isomer K) also represent
rather stable species.
A
In the presence of the N-donor base NMe2Et, adduct for-
mation (AN, Figure 5) is energetically favored over the di-
merization of structure A. However, the entropic penalty
that is associated with the reaction AN+NMe2Et!AN2
lowers the stability of AN2 to a similar extent that the ring
strain destabilizes the intramolecular second B2H4 bridge in
isomer D. We note in passing that the optimized structure of
adduct AN2 validates the proposed structure for compound
6 (Scheme 2).
The sequential binding of two SMe2 molecules to struc-
ture A is substantially less exergonic than NMe2Et-coordina-
tion (AS: DGR =ꢀ9.5 kcalmolꢀ1, AS2: DGR =ꢀ17.0 kcal
molꢀ1, with reference to fully separated molecules) and is
fully in line with the experimental observations described
above; also, the formation of an SMe2 adduct cannot com-
pete thermodynamically with the dimerization of compound
A. Whilst it is reasonable to assume that the in situ genera-
tion of structure A in SMe2 leads to an AS2 intermediate,
this species is unstable under the experimental conditions
and, accordingly, only transient NMR signatures that are as-
lar B H/B H addition to form a second cis-B2H4 bridge. Al-
though this intramolecular step does not suffer the loss in
entropy that accompanies the intermolecular formation of
isomer C, closure of the second B2H4 bridge only stabilizes
the resulting dimer (D) by an additional ꢀ13.7 kcalmolꢀ1.
Clearly, the substantial amount of strain energy that is inher-
ent in the rather stiff, bowl-shaped molecular structure of
dimer D partially outweighs the stabilization that is brought
about by the second B2H4 linker. Our attempts to localize
other, potentially less strained conformers of dimer D, or
isomers that contain trans-B2H4 bridges, failed. Instead, we
localized a D2d-symmetric isomer (E), which is bound
through a distributed B-H-B bridging network. However,
isomer E is significantly higher in energy than all of the
other isomers identified so far and, hence, is not considered
further.
Alternative isomers are accessible through B H/B C ad-
dition between two monomers (A). In a previous work on
the mechanisms for the oligomerization of 9H-9-borafluo-
ꢀ
rene, we showed that this elementary step results in the for-
mation of dimers that are joined together by the remarkable
signable to aryl BH2·SMe2 species, such as AS2, have been
detected. Furthermore, NMR evidence for the formation of
combination of classical B-H-B bridges and B-C-B linkages
DBA·ACTHGNUETRN(NUG SMe2)2 and BH3·SMe2 in solution, together with the
[68]
ꢀ
through B B-bridging aryl rings. For the systems studied
isolation of compounds (7)2 and 8, indicate the lability of
SMe2-donor adducts and the existence of intriguing rear-
rangement processes.
herein, we investigated the possibility of realizing doubly
phenyl-bridged dimers, which are conceivable owing to the
particular ortho-substitution pattern that is present in struc-
ture A. Indeed, we identified two corresponding isomers
that are thermochemically relevant despite their seemingly
strained structures: Dimer F shows two singly bridging
To shed further light on the origins of these findings, we
investigated potential mechanisms for the formation of
DBA in the presence of SMe2 solvent molecules. As an ini-
tial step in the reaction sequence that commences from AS2,
we identified an SN2-like transition state (TS1): The boron-
bound hydride atom of one monomer (AS2) attacks the
BH2·SMe2 group of a second monomer with the synchronous
liberation of the SMe2 donor, thereby yielding a B-H-B-
bridged dimeric intermediate (L, Scheme 3). This step has
phenACHTUNGTRENNUNGylene rings and its strongly exergonic formation
(DGR =ꢀ31.1 kcalmolꢀ1) is comparable to that of dimer D
(DGR =ꢀ30.7 kcalmolꢀ1). In contrast, dimer G features a
hitherto-unknown doubly bridging phenylene ring that con-
tributes two adjacent carbon atoms to two B-C-B linkages.
Notwithstanding its lower thermodynamic stability (DGR =
ꢀ20.4 kcalmolꢀ1) than dimer F, structure G still represents a
sufficiently exergonic species to render the molecular design
of kinetically stabilized derivatives a rewarding synthetic
goal. We note in passing that a combination of both binding
motifs in the two energetically most favorable isomers (D
and F) results in an equally stable isomer (H; DGR =
an appreciable activation barrier (DG° =23.7 kcalmolꢀ1)
ꢀ1
ꢀ
and is endoergic by 16.7 kcalmol . Subsequent B C bond
formation occurs with the concomitant loss of a second
ꢀ
SMe2 molecule via transition state TS2 and leads to a B B
phenyl-bridged intermediate (M) with a negligible activation
barrier of DG° =7.2 kcalmolꢀ1. The second half of the reac-
tion sequence that leads to donor-free structure
F
ꢀ31.4 kcalmolꢀ1). The rearrangement of isomer F into exo-
(Scheme 3) is comprised of identical elementary steps with
lower barrier heights.
ꢀ1
ꢀ
ꢀ
B H/B H adduct I liberates another 11 kcalmol
and
yields the thermodynamically most favored species that was
identified in our computational study. The plausible alterna-
tive scenario, that is, the dimerization of BH3 (DGR =
ꢀ27.9 kcalmolꢀ1) and the polymerization of DBA, which is
essentially thermoneutral,[64] is thermodynamically disfa-
vored compared to the formation of isomer I (DGR =
ꢀ41.9 kcalmolꢀ1). In line with these results, compound I was
isolated from aged solutions of in situ generated structure A
in CH2Cl2 (cf. crystallographically characterized compound
As alluded to in our general evaluation of conceivable
isomers (Figure 5), the primary product (F) is less stable
than isomer I (which corresponds to the experimentally
characterized product 8; Scheme 2, Figure 3). Indeed, we
found that SMe2 mediates the exergonic rearrangement of
isomer F into isomer I with low barriers (Scheme 4). Attack
of SMe2 on isomer F yields isomer O and BH3·SMe2. This
step is essentially driven by the exergonicity of the latter
ꢀ1
ꢀ
B S adduct (DGR =ꢀ14.8 kcalmol ). The three-coordinate
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M. C. Holthausen, M. Wagner et al.
the observation of crystalline
(Q)2
(=(7)2;
Scheme 2,
Figure 3) precipitating from
cooled reaction mixtures. With
lower amounts of SMe2, by
evaporation of the reaction
mixture and redissolution of the
residue in C6H6, path b be-
comes increasingly relevant.
Here, a BH3-exchange cascade
between BH3·SMe2/SMe2 and
DBA rearranges isomer O into
isomer
Figure 3) under
I
(=8; Scheme 2,
kinetic
a
regime that is comparable to
that of path a.
Conclusion
The Grignard reaction between
ꢀ
1,2-C6H4Br2 and iPrO Bpin
with 1,2-C2H4Br2 as an entrain-
er is an exceptionally conven-
ient method for the preparation
of the versatile synthetic build-
ing block 1,2-C6H
HBpin=pinacolborane).
Compound can be em-
4ACHTUNGTERN(NUNG Bpin)2 (2;
2
ployed in double cross-coupling
reactions for the synthesis of
functionalized polycyclic aro-
matic hydrocarbons. However,
we focused on its transforma-
tion into the vicinal bis-borate
Li2ACHTUNGTRNENUG[1,2-C6H4CAHTUNGTRENN(GUN BH3)2] (3), which
was crystallographically charac-
terized as its diethyl ether sol-
vate, 3·OEt2. The intricate coor-
dination polymer network that
is established by solvate 3·OEt2
in the solid state holds promise
Scheme 3. Reaction sequence from AS2 to F; relative Gibbs free energies are given in kcalmolꢀ1 and the imag-
inary modes of the transition states are given in parentheses.
boron atom in isomer O can either bind one SMe2 molecule
from the solvent (i.e., Pcis/Ptrans; Scheme 4a) or reaccept
BH3 in an exocyclic fashion from BH3·SMe2 that was liberat-
ed in the preceding step (K; Scheme 4b). Along path a,
both Pcis and Ptrans are efficiently funneled into isomer Q
with low activation barriers. Under the experimental condi-
tions, isomer Q is generated in neat SMe2 and, therefore, is
for various applications because the well-developed chemis-
try of the [BH4]ꢀ ion as a terminal or bridging ligand in a
wide range of (transition)-metal complexes[74–79] can now be
expanded by using bis-borates like [1,2-C6H
chelating variant.
(BH3)2]2ꢀ as a
4ACHTUNGTRENNUNG
Hydride-abstraction from compound 3 yields the free di-
topic borane 1,2-C6H4A(BH2)2. Its properties as a hydrobora-
CTHUNGTRENNUNG
likely to add a second SMe2 molecule. The resulting di
G
tion reagent are governed by the proximity of the two boryl
A
groups, which undergo a cyclic double-hydroboration reac-
sents a major component of the reaction mixture, as indicat-
ed by in situ 11B NMR spectroscopic analysis (see above).
Within the assumed limits of the accuracy of our calcula-
tions, the dimerization of isomer Q to afford dimer (Q)2 is
equally favorable. Thus, in SMe2, structures R and (Q)2 will
readily interconvert through Q, which is in agreement with
tion with the formation of a benzannelated 1,3-diborolene
(5).
In the presence of the strong Lewis base NMe2Et, 1,2-
ꢀ
C6H
G
such as SMe2 or THF, are not able to stabilize the 1,2-C6H4-
AHCTUNGTREN(GNUN BH2)2 scaffold. Instead, the compound undergoes scram-
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Synthesis and Reactions of 1,2-Diborylated Benzenes
FULL PAPER
Scheme 4. Reaction sequence from F to: a) base adducts R and (Q)2 and b) BH3 adduct I; relative Gibbs free energies are given in kcalmolꢀ1 and the
imaginary modes of the transition states are given in parentheses.
bling of the substituents with the formation of 9,10-dihydro-
9,10-diboraanthracene within a short period of time at room
temperature. For comparison, the conventional synthesis of
9,10-dibromo-9,10-dihydro-9,10-diboraanthracene requires
prolonged heating of 1,2-bis(trimethylsilyl)benzene with
excess BBr3 in a sealed glass ampoule (1208C, 6 d). There-
fore, we conclude that the process of substituent-redistribu-
tion in arylACTHNUGRTNEUNG(hydro)boranes, which has mainly been regarded
as a nuisance in the past, can be turned into a versatile
method for the construction of sophisticated boron-contain-
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M. C. Holthausen, M. Wagner et al.
331.2 (100) [M+H]+; elemental analysis calcd (%) for C18H28B2O4:
C 65.51, H 8.55; found: C 65.88, H 8.49.
ing p sysACHTUNGTRENNUNGtems. Modern density functional theory has provid-
ed a detailed insight into the underlying reaction mecha-
nisms. We compared theoretical and experimental findings
wherever possible and, pleasingly, found a good agreement
between the two. Therefore, we are confident that our theo-
retical approach represents a powerful tool for predicting
key facets of the dynamic covalent chemistry of aryl-
Synthesis of compound 3: Compound 2 (1.01 g, 3.06 mmol) was dissolved
in a mixture of Et2O (16 mL) and n-pentane (4 mL) and the solution was
cooled to 08C in an ice bath. A solution of Li[AlH4] (1m in Et2O, 9.2 mL,
9.2 mmol) was added by using a syringe. The resulting suspension was
stirred for 2 h at 08C and for a further 16 h at RT. The colorless precipi-
tate was filtered off and the filtrate was concentrated to a volume of ap-
proximately 10 mL. Gas-diffusion of n-hexane into the filtrate gave color-
less crystals of compound 3·OEt2. Yield: 0.48 g (81%); 1H NMR
(300.0 MHz, [D8]THF): d=7.30 (br, 2H; H3,6), 6.68–6.62 (m, 2H;
ACHTUNGTRENNUNG
H4,5), 3.40 (q, 3J
CH3), 1.03 ppm (q, 1J
AHCTUNGTRENNUNG
N
ACHTUNGTREN(NUNG H,H)=7 Hz, 6H;
CHTUNGTRENNUNG
123.2 (C4,5), 66.1 (CH2), 15.5 ppm (CH3); 11B{1H} NMR (96.3 MHz,
moderate donor properties, such as thioethers, also form ad-
ducts but do not entirely block the reactive borane sites,
which undergo base-assisted rearrangement processes that
ultimately lead to DBA with remarkable selectivity.
[D8]THF): d=ꢀ26.6 ppm (h1/2 =15 Hz); 11B NMR (96.3 MHz, [D8]THF):
d=ꢀ26.6 (q, 1J
(11B,H)=79 Hz); elemental analysis calcd (%) for
ACHTUNGTRENNUNG
C10H20B2Li2O: C 62.63, H 10.51; found: C 62.70, H 10.49.
Synthesis of compound 5: Compound 3·OEt2 (0.27 g, 1.4 mmol) was sus-
ꢁ
pended in dry Me2S (20 mL) at RT. Neat tBuC CH (0.6 mL, 0.4 g,
5 mmol) and neat Me3SiCl (0.4 mL, 0.3 g, 3 mmol) were added to the sus-
pension by using a syringe. The mixture was stirred at RT for 36 h, fil-
tered, and the filtrate was evaporated to dryness under vacuum to give a
colorless oil. The crude product was treated with C6H6 (15 mL) and the
resulting suspension was filtered again. The filtrate was evaporated to
dryness under vacuum to give a highly viscous, colorless oil. Yield: 0.41 g
(84%); 1H NMR (300.0 MHz, C6D6): d=8.18–8.11 (m, 2H; H3,6), 7.45–
Experimental Section
Unless otherwise specified, all reactions were carried out in rigorously
dried solvents under dry nitrogen/argon atmospheres by using Schlenk or
glove-box techniques. n-Pentane, n-hexane, C6H6, C6D6, toluene, Et2O,
THF, and [D8]THF were dried over Na/benzophenone; CDCl3, CH2Cl2,
CD2Cl2, Me2S, Me3SiCl, and NMe2Et were dried over CaH2. NMR spec-
tra were recorded on Bruker AM 250, DPX 250, Avance 300, and
Avance 400 spectrometers at RT if not otherwise specified. Chemical
shifts were referenced to (residual) solvent peaks (1H/13C{1H}: C6D6 7.16/
128.06, CDCl3 7.26/77.16, CD2Cl2 5.32/53.84, [D8]THF 3.58/67.21) or to
external BF3·Et2O (11B,11B{1H}). Abbreviations: s=singlet, t=triplet, q=
quartet, m=multiplet, br=broad, n.o.=not observed. Combustion analy-
sis was performed by the Microanalytical Laboratory of the University of
Frankfurt and by the Microanalytical Laboratory Pascher, Remagen
(Germany).
7.39 (m, 2H; H4,5), 7.12 (d, 3J
(d, 3J(H,H)=17.8 Hz, 2H; CH=CHtBu), 2.14 (t, 3J
B(CH)B), 2.03 (d, J
1.03 ppm (s, 18H; C
(CH=CHtBu), 157.3 (C1,2)*, 132.5 (C3,6), 131.0 (C4,5), 125.2 (CH=
CHtBu)*, 43.6 (CH2tBu), 35.6 (C(CH3)3), 32.6 (CH2C(CH3)3), 30.0
(CH2C(CH3)3), 29.0 ppm (C
(CH3)3), n.o. (B(CH)B); 11B{1H} NMR
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
3
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
(96.3 MHz, C6D6): d=71.6 ppm (h1/2 =1500 Hz); MS (ESIꢀ): m/z (%):
487.6 (100) [M+pzSiMe3]ꢀ. *: This signal was not observed in the 1D
13C{1H} NMR spectrum but was located by HMBC experiments.
Synthesis of compound 6: Compound 3·OEt2 (0.58 g, 3.0 mmol) was sus-
pended in toluene (20 mL) at RT. Neat, dry NMe2Et (0.7 mL, 0.5 g,
6 mmol) and neat Me3SiCl (1.0 mL, 0.86 g, 7.9 mmol) were added to the
suspension by using a syringe. The mixture was stirred at RT for 12 h, fil-
tered, and the filtrate was evaporated to dryness under vacuum to give a
colorless solid. The crude product was treated with C6H6 (15 mL) and the
resulting suspension was filtered again. The filtrate was evaporated to
dryness under vacuum to give a colorless solid. Yield: 0.41 g (55%);
1H NMR (300.0 MHz, CD2Cl2): d=7.47–7.41 (m, 2H; H3,6), 7.04–6.98
Synthesis of compound 2: Mg turnings (2.45 g, 101 mmol) were heated
with a heat gun in a Schlenk flask that was equipped with a reflux con-
denser and a dropping funnel under vacuum for 15 min. After the flask
had cooled back to RT, THF (100 mL), neat isopropoxypinacolborane
(16.0 mL, 14.6 g, 78.4 mmol), and neat 1,2-dibromobenzene (3.0 mL,
5.9 g, 25 mmol) were added. After an induction period of approximately
5 min, the color of the mixture changed from colorless to yellow and the
exothermic reaction started. Therefore, the flask was cooled in a water
bath to maintain the temperatures between 20–408C throughout the
entire reaction period. After 15 min, a solution of 1,2-dibromoethane
(2.1 mL, 4.6 g, 24 mmol) in THF (15 mL) was added dropwise to the vig-
orously stirred slurry over 1 h. After the addition was complete, the
yellow reaction mixture was heated at reflux for 4 h before being allowed
to cool back to RT. All of the volatile compounds were removed under
reduced pressure. The remaining solid residue was treated at 08C with n-
hexane (50 mL) and with a saturated aqueous solution of NaHCO3
(50 mL). After stirring for 15 min, the two liquid phases were separated
and the aqueous phase was extracted with n-hexane (2ꢅ20 mL). The
combined organic phases were extracted with H2O (2ꢅ20 mL), dried
over anhydrous MgSO4, and filtered. All of the volatile compounds were
removed from the filtrate under reduced pressure and the remaining
yellow oil was dissolved in n-pentane (20 mL). Slow evaporation of the
solvent at RT over 2 d gave colorless crystals that were immersed in a
pale-yellow oil. On storing the sample at 48C, large crystals formed,
which were collected on a frit (G2) and dried between two sheets of pulp
to remove any residual oil. Yield: 3.3 g (40%); 1H NMR (300.0 MHz,
CDCl3): d=7.68–7.62 (m, 2H; ArH), 7.40–7.34 (m, 2H; ArH), 1.37 ppm
(s, 24H; CH3); 13C{1H} NMR (75.5 MHz, CDCl3): d=133.6 (ArC), 129.3
(ArC), 84.0 (CCH3), 25.0 ppm (CH3), n.o. (CB); 11B{1H} NMR
(96.3 MHz, CDCl3): d=31.6 ppm (h1/2 =280 Hz); MS (ESI+): m/z (%):
(m, 2H; H4,5), 2.84 (q, 3J
BH2), 2.37 (s, 12H; CH3), 1.14 ppm (t, 3J
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
13C{1H} NMR (75.5 MHz, CD2Cl2): d=154.6 (C1,2)*, 139.6 (C3,6), 124.6
(C4,5), 56.9 (CH2CH3), 47.9 (CH3), 8.5 ppm (CH2CH3); 11B{1H} NMR
(96.3 MHz, CD2Cl2): d=ꢀ1.3 ppm (h1/2 =120 Hz); 11B NMR (96.3 MHz,
CD2Cl2): d=ꢀ1.3 ppm (t, 1J
ACHTUNGTRENNUNG
(%) for C14H30B2N2: C 67.80, H 12.19, N 11.29; found: C 68.01, H 12.10,
N 10.70. *: This signal was not observed in the 1D 13C{1H} NMR spec-
trum but was located by HMBC experiments.
Synthesis of compound (7)2: Compound 3·OEt2 (0.05 g, 0.3 mmol) was
suspended in dry Me2S (5 mL). Neat Me3SiCl (0.1 mL, 0.09 g, 0.8 mmol)
was added at RT by using a syringe. After the mixture was stirred for
14 h, a colorless precipitate had formed, which was removed by filtration,
and the filtrate was stored at 48C to grow colorless single crystals of com-
pound (7)2. Yield: 9 mg (25%); 1H NMR (250.1 MHz, [D8]THF): d=
7.62–7.55 (m, 4H; H1,4,5,8), 7.19–7.13 (m, 4H; H2,3,6,7), 4.42 (br, 2H;
BH), 1.95 ppm (s, 12H; S
ACHTUNGTRENNUNG
152.5 (CB)*, 136.3 (C1,4,5,8), 127.2 (C2,3,6,7), 18.6 ppm (SACHTUNGTRENNUNG
11B{1H} NMR (80.3 MHz, [D8]THF): d=19.9 ppm (h1/2 =360 Hz). *: This
signal was not observed in the 1D 13C{1H} NMR spectrum but was locat-
ed by HMBC experiments. In addition to the solution, a colorless solid
was present in the NMR tube.
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ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Synthesis and Reactions of 1,2-Diborylated Benzenes
FULL PAPER
Synthesis of compound 8: Method A. Compound 3·OEt2 (0.08 g,
0.4 mmol) was suspended in dry Me2S (5 mL). Neat Me3SiCl (0.5 mL,
0.4 g, 4 mmol) was added at RT by using a syringe. After the mixture was
stirred for 12 h, a colorless precipitate had formed, which was removed
by filtration, and the filtrate was evaporated to dryness under vacuum to
give a colorless oil. The oil was redissolved in C6H6 (5 mL) and the solu-
tion was stored at 48C for 5 d to afford needle-shaped crystals of com-
pound 8. Method B. Compound 3·OEt2 (0.08 g, 0.4 mmol) was placed in a
narrow Schlenk tube, dissolved in THF (5 mL), and the solution was
carefully layered with neat Me3SiCl (0.3 mL, 0.3 g, 2 mmol) at RT. After
the tube had been stored at RT for 2 h, a colorless solid had formed, to-
gether with colorless, needle-shaped crystals of compound 8, which were
isolated by manual crystal picking. Method C. Compound 3·OEt2 (0.66 g,
3.4 mmol) was suspended in dry CH2Cl2 (20 mL). Neat Me3SiCl (1.0 mL,
0.86 g, 7.9 mmol) was added at RT by using a syringe. After the mixture
was stirred for 19 h, a colorless precipitate had formed, which was re-
moved by filtration, and the filtrate was stored at 48C for 1 d to obtain
colorless needle-shaped crystals of compound 8. All three methods repro-
ducibly gave yields of about 10%.
Acknowledgements
This work was supported by the Beilstein-Institut, Frankfurt am Main
(Germany), within the research collaboration NanoBiC, through a PhD
grant for ꢀ.S. and postdoctoral grants for Z.-W.Q. and H.Z. Computer
time and excellent support was provided by the Center for Scientific
Computing (CSC), and the LOEWE CSC, Frankfurt.
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a
STOE IPDS-II diffractometer with graphite-monochromated Mo Ka ra-
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PLATON[80] for compounds (7)2 and tBuC(H)=C(H)B
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were disordered over two positions with site-occupancy factors of 0.56(2)
and 0.51(1) for the major occupied site. The disordered atoms were re-
ꢀ
fined isotropically. The C C distances in the C6D6 ring of tBuC(H)=
C(H)B
1.39(1) ꢄ.
CCDC-878339 (2), CCDC-878340 (3·OEt2), CCDC-878341 ((7)2), CCDC-
878342 (8), CCDC-878343 (tBuC(H)=C(H)B(m-C6H4)(m-pzSiMe3)2-
BC(H)=C(H)tBu) contain the supplementary crystallographic data for
A
were
restrained
to
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Computational details: Geometry optimizations and harmonic frequency
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dispersion-corrected B97D[84] GGA-functional in combination with the
6–311++GACHTUNGTRENNUNG
(d,p)[85,86] basis set. All structures were optimized within the
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CBSACHTUNGTRENNUNG(2/3)//B97D/6–311++GACHTUNGTREN(NUNG d,p) level of theory.
Chem. Eur. J. 2012, 00, 0 – 0
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Received: May 3, 2012
Published online: && &&, 0000
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Chem. Eur. J. 0000, 00, 0 – 0
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Synthesis and Reactions of 1,2-Diborylated Benzenes
FULL PAPER
Boranes
ꢁ. Seven, Z.-W. Qu, H. Zhu, M. Bolte,
H.-W. Lerner, M. C. Holthausen,*
M. Wagner* ................... &&&& — &&&&
Synthesis, Coupling, and Condensation
Reactions of 1,2-Diborylated
Benzenes: An Experimental and
Quantum-Chemical Study
Dynamic covalent chemistry: Bis-
borate Li2A[1,2-C6H4A(BH3)2] is a useful
source of bis-borane 1,2-C6H4A(BH2)2,
which can be trapped or used in hydro-
boration reactions; in the absence of a
trapping reagent, redistribution affords
9,10-dihydro-9,10-diboraanthracene.
N
G
CTHUNGTRENNUNG
Chem. Eur. J. 2012, 00, 0 – 0
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!