High-yield syntheses and reactivity studies of 1,2-diborylated and 1,2,4,5-tetraborylated benzenes

Article abstract of DOI:10.1021/om500104w

Treatment of 1,2-dibromobenzene (1,2-C₆H₄Br ₂) or 1,2,4,5-tetrabromobenzene (1,2,4,5-C₆H ₂Br₄) with 2 equiv or 4 equiv of n-BuLi in the presence of excess iso-propoxy(pinacol)borane ((i-PrO)Bpin) furnishes 1,2-C ₆H₄(Br)(Bpin) (1) or 1,4,2,5-C₆H ₂(Br)₂(Bpin)₂ (3) with excellent selectivity. The subsequent reaction of 1 or 3 with Mg turnings and more (i-PrO)Bpin gives the di- and tetraborylated benzenes 1,2-C₆H₄(Bpin) ₂ (2) and 1,2,4,5-C₆H₂(Bpin)₄ (4) in overall yields of about 65%. For the Grignard transformation step, it is essential to continuously add 1 equiv (1) or 2 equiv (3) of 1,2-dibromoethane as an entrainer over a period of 1 h. Compounds 1 and 2 have been transformed into the ortho-functionalized trihydroborates Li[1,2-C₆H ₄(Br)(BH₃)] (Li[7]) and Li[1,2-C₆H ₄(Bpin)(BH₃)] (Li[8]) by means of 1 equiv of Li[AlH ₄]. Using 3 equiv of Li[AlH₄], 2 can also be converted into the ditopic lithium trihydroborate Li₂[1,2-C₆H ₄(BH₃)₂] (Li₂[9]); even the tetratopic derivative Li₄[1,2,4,5-C₆H₂(BH ₃)₄] (Li₄[10]) is accessible from 4 and 4 equiv of Li[AlH₄]. The compounds Li[7], Li[8], Li₂[9], and Li₄[10] have been crystallographically characterized as ether solvates, but still show Ar-BH₃-η²-Li interactions as the dominant mode of coordination. In the cases of Li₂[9] and Li ₄[10] an intricate three-dimensional network and a zigzag polymer, respectively, are established by the contact ion pairs in the crystal lattice.

Full text of DOI:10.1021/om500104w

Article
pubs.acs.org/Organometallics
High-Yield Syntheses and Reactivity Studies of 1,2-Diborylated and
1,2,4,5-Tetraborylated Benzenes
̈
Omer Seven, Michael Bolte, Hans-Wolfram Lerner, and Matthias Wagner*
Institut fur Anorganische und Analytische Chemie, J. W. Goethe-Universitat Frankfurt, Max-von-Laue-Strasse 7, D-60438 Frankfurt
̈
̈
(Main), Germany
S
* Supporting Information
ABSTRACT: Treatment of 1,2-dibromobenzene (1,2-C6H4Br2) or 1,2,4,5-tetrabromobenzene (1,2,4,5-C6H2Br4) with 2 equiv
or 4 equiv of n-BuLi in the presence of excess iso-propoxy(pinacol)borane ((i-PrO)Bpin) furnishes 1,2-C6H4(Br)(Bpin) (1) or
1,4,2,5-C6H2(Br)2(Bpin)2 (3) with excellent selectivity. The subsequent reaction of 1 or 3 with Mg turnings and more (i-
PrO)Bpin gives the di- and tetraborylated benzenes 1,2-C6H4(Bpin)2 (2) and 1,2,4,5-C6H2(Bpin)4 (4) in overall yields of about
65%. For the Grignard transformation step, it is essential to continuously add 1 equiv (1) or 2 equiv (3) of 1,2-dibromoethane as
an entrainer over a period of 1 h. Compounds 1 and 2 have been transformed into the ortho-functionalized trihydroborates
Li[1,2-C6H4(Br)(BH3)] (Li[7]) and Li[1,2-C6H4(Bpin)(BH3)] (Li[8]) by means of 1 equiv of Li[AlH4]. Using 3 equiv of
Li[AlH4], 2 can also be converted into the ditopic lithium trihydroborate Li2[1,2-C6H4(BH3)2] (Li2[9]); even the tetratopic
derivative Li4[1,2,4,5-C6H2(BH3)4] (Li4[10]) is accessible from 4 and 4 equiv of Li[AlH4]. The compounds Li[7], Li[8], Li2[9],
and Li4[10] have been crystallographically characterized as ether solvates, but still show Ar−BH3-η2-Li interactions as the
dominant mode of coordination. In the cases of Li2[9] and Li4[10] an intricate three-dimensional network and a zigzag polymer,
respectively, are established by the contact ion pairs in the crystal lattice.
the BX2 group, the introduction of further BX2 substituents into
an already monoborylated arene scaffold requires increasingly
forcing conditions. Most of these problems do not exist in the
case of compounds Ar−B(OR′)2, which are much more
convenient to prepare and to isolate.10 Because of the low
Lewis acidity of the B(OR′)2 group, even exhaustively borylated
arenes such as hexakis(catecholboryl)benzene, C6(Bcat)6, can
be made.11,12
The latter compound is remarkable, especially because it
features the structural motif of a 1,2-diborylated benzene ring.
1,2-Diborylated arenes have recently been shown to be useful
starting materials for the synthesis of polycyclic aromatic
hydrocarbons via Pd-catalyzed double cross-coupling reac-
tions.13 Yet, C6(Bcat)6 is a special case, because it was obtained
through the Co-catalyzed cyclotrimerization of catB−CC−
Bcat. In spite of numerous efforts, the attachment of B(OR′)2
substituents at vicinal positions of an already existing benzene
ring still remains a challenging task (Scheme 1): Either the
yields are low, or expensive reagents/catalysts are required, or
tedious purification of the reaction product (e.g., via preparative
recycling gel permeation chromatography) is necessary;
moreover, some of the reactions shown in Scheme 1 have
been performed only on a milligram scale.
INTRODUCTION
■
Borylated arenes are valuable building blocks for various
synthetic purposes. On the one hand, they can be employed to
prepare highly functionalized aromatics through Suzuki-type
C−C-coupling protocols.1 On the other hand, they provide
access to extended boron-doped π-electron systems in which
the boron atoms are integral parts of the conjugation pathways.
Such compounds are of great interest as fluorescent and
electron-transport materials for optoelectronic applications.2−7
The arylboranes, Ar−BR2, employed for individual purposes,
differ fundamentally in their chemical nature: Suzuki reactions
require derivatives of arylboronic acids in which the Ar−B
bonds are the weakest links and the BR2 groups remain
essentially inert (e.g., B(OH)2, pinacolboryl (Bpin), neo-
pentylglycolboryl (Bneop)); consequently, the BR2 fragment
is lost in the course of the transformation. For the assembly of
extended boron-containing π-systems, however, the Ar−B bond
must stay intact, while the B−R bonds need to be reactive (e.g.,
BCl2, BBr2).
Access to compounds Ar−BX2 (X = Cl, Br) is limited
because highly corrosive, air- and moisture-sensitive starting
materials have to be used (BCl3, BBr3) and the reaction
products cannot be easily purified by column chromatography.
The syntheses are generally carried out either as direct
electrophilic aromatic borylations8 or as Si/B or Sn/B exchange
reactions.9 Moreover, due to the strong π-acceptor influence of
Received: January 28, 2014
Published: February 18, 2014
© 2014 American Chemical Society
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a
a
Scheme 1. Synthesis of 1,2-C6H4(B(OR′)2)2
Scheme 2. Synthesis of Compounds 1 and 2
a
Conditions: (i) 2 n-BuLi, 2.2 (i-PrO)Bpin, toluene/THF (4:1), −78
°C, 7 h → room temperature, overnight; (ii) 3 Mg, 1.9 (i-PrO)Bpin, 1
1,2-C2H4Br2, THF, reflux temperature, 5 h → room temperature,
overnight; (iii) 4 Mg, 3.1 (i-PrO)Bpin, 1 1,2-C2H4Br2, THF, 20 °C−
40 °C, 1 h → reflux temperature, 4 h.24
a
Scheme 3. Synthesis of Compounds 3 and 4
a
Conditions: (i) 3.9 n-BuLi, 4.9 (i-PrO)Bpin, toluene/THF (4:1),
−78 °C, 7 h → room temperature, overnight; (ii) 6.8 Mg, 8.1 (i-
PrO)Bpin, 2.3 1,2-C2H4Br2, THF, reflux temperature, 5 h → room
temperature, overnight; (iii) 5.9 Mg, 7.5 (i-PrO)Bpin, 1.8 1,2-C2H4Br2,
THF, reflux temperature, 5 h.
a
Scheme 4. Synthesis of Compounds 3, 5, and 6
Conditions: (i) THF, room temperature;14 (ii) 1,4-dioxane, 110 °C,
24 h (SPhos = 2-dicyclohexylphosphino-2′,6′-diisopropoxybiphen-
yl);15 (iii) toluene, 100 °C, 2 h (dppp = 1,3-bis(diphenylphosphino)-
propane; dppf = 1,1′-bis(diphenylphosphino)ferrocene);16 (iv) 1,4-
dioxane, 80 °C;17 (v) 1,2-diethoxyethane, 100 °C, 7 h;18 (vi) 1,2-
dimethoxyethane, 80 °C, 23 h (dba = dibenzylideneacetone; 1-AdNC
= 1-adamantyl isocyanide);19 (vii) CHCl3, 0 °C, 2 h → room
temperature, 16 h;20 (viii) (1) THF, between −90 and −75 °C, 20
min; (2) −85 °C, 20 min, then aqueous HCl at −30 °C.21
a
a
Conditions: (i) 5.9 Mg, 7.5 (i-PrO)Bpin, 1.8 1,2-C2H4Br2, THF,
reflux temperature, 5 h; (ii) 7 Mg, 5.4 (i-PrO)Bpin, 2.5 1,2-C2H4Br2,
THF, reflux temperature, 4 h.
polycondensation reactions7,24,29−35 to prepare extended
boron-containing π-electron materials.
Given this background, we have elaborated time- and cost-
efficient protocols for the preparation of 1,2-C6H4(Bpin)2 (2)
and 1,2,4,5-C6H2(Bpin)4 (4; Schemes 2 and 3), which are now
available for Suzuki coupling reactions. Since the focus of our
research lies on the development of unusual and sophisticated
arylborane frameworks, we have subsequently optimized
procedures to transform the Bpin substituents into trihydro-
borate groups.22−24 The products obtained, Li2[1,2-
C6H4(BH3)2] (Li2[9]) and Li4[1,2,4,5-C6H2(BH3)4] (Li4[10];
Scheme 5), are interesting novel chelating ligands for metal
ions.25 As a long-term perspective, we intend to employ the
corresponding free boranes in hydroboration26−28 and
RESULTS AND DISCUSSION
■
Synthesis of vic-Diborylbenzenes. Our initial attempts at
the synthesis of 1,2-C6H4(Bpin)2 (2) were inspired by an
economically viable method giving facile access to 1,2-
C6H4(SiMe3)2. The disilylbenzene is best prepared from 1,2-
C6H4Br2, Mg turnings, and Me3SiCl in THF. Key to success is
the dropwise addition of 1,2-dibromoethane (1,2-C2H4Br2) to
the reaction mixture in order to continuously activate the Mg
surface (“entrainment method”).36,37 The most important
advantages of the entrainment method in this case lie in the
cheap reagents, the short reaction times (30 min), the mild
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protocol compared to the published one-step Grignard route24
is most likely gained from the fact that essentially no benzyne
intermediates are created; for example, in contrast to the one-
step procedure, we find no biphenylene in the reaction
mixtures.
Scheme 5. Synthesis of Compounds Li[7], Li[8], Li2[9], and
Li4[10]
a
Applying the general strategies developed for the preparation
of 1 and 2, even the tetraborylated benzene 1,2,4,5-
C6H2(Bpin)4 (4) is within easy reach (Scheme 3). Starting
from a mixture of 1,2,4,5-C6H2Br4 and (i-PrO)Bpin, a 2-fold
lithium−bromine exchange with n-BuLi can be performed
selectively in the para-positions of the benzene ring to afford 3
in 77% yield. The subsequent Grignard reaction with 1,2-
C2H4Br2 as the entrainer gives 1,2,4,5-C6H2(Bpin)4 (4) in a
clean conversion (83%). Compounds 3 and 4 show 11B{1H}
NMR resonances at 30.2 and 31.7 ppm, respectively, which lie
in the typical shift range of Ar−Bpin species (cf. 2: δ(11B) =
31.6).24 Each of the compounds gives rise to only one 1H NMR
signal in the aromatic region of the spectrum; all the proton
integral values are consistent with the presence of two (3) and
four (4) Bpin substituents in the molecules. The molecular
structures of 3 and 4 have further been confirmed by X-ray
crystallography (cf. the SI for more details).
We also tested the reactivity of 1,2,4,5-C6H2Br4 or C6Br6
toward (i-PrO)Bpin under the conditions of the entrained
Grignard reaction (Schemes 3 and 4). From the complex
product blends formed, the diborylated benzenes 3 and 6 as
well as the triborylated benzene 5 were isolated, albeit not in
preparatively useful yields (cf. the SI for NMR data and X-ray
crystal structure analyses of 5 and 6).
Synthesis of (Oligotopic) Trihydroborate Anions.
Lithium trihydroborates Li[Ar−BH3] were prepared by treat-
ment of the appropriate Ar−Bpin precursors with Li[AlH4] in
Et2O or THF (Scheme 5). One Bpin unit generally consumes 1
equiv of Li[AlH4].43 This finding was exploited in the following
to selectively reduce only one of the two Bpin substituents in 2
and to obtain species Li[8] (NMR spectra of the crude reaction
mixture indicate Li[8] to be by far the major organoboron
product; the yield of 62% refers to isolated, single-crystalline
material). We finally note that for reasons of solubility it is
essential to perform the synthesis of Li4[10] in THF rather
than in Et2O.
a
Conditions: (i) 1 Li[AlH4], Et2O, −78 °C, 4 h → room temperature,
overnight; (ii) 3 Li[AlH4], Et2O/n-pentane (4:1), 0 °C, 2 h → room
temperature, 16 h;24 (iii) 4.2 Li[AlH4], THF, −78 °C, 5 h → room
temperature, overnight.
reaction conditions (room temperature), and the fact that the
cancerogenic solvent hexamethylphosphoramide38 is avoided.
We have already shown that the entrainment method can be
successfully adapted also for the preparation of 1,2-
C6H4(Bpin)2 (2; 1,2-C6H4Br2, Mg turnings, (i-PrO)Bpin, 1,2-
C2H4Br2); yields were obtained in the range of 40% when the
reaction was carried out on a multigram scale.24
With the aim to further increase the efficiency of the
synthesis of 2, a two-step protocol was now developed, which
not only furnishes 2 in overall yields of 70% but also provides
the compound 1,2-C6H4(Br)(Bpin) (1)39 as a potentially useful
reagent for Stille/Suzuki tandem coupling reactions (Scheme
2): In the first step, a mixture of 1,2-C6H4Br2 and (i-PrO)Bpin
(2.2 equiv) in toluene/THF (4:1) was treated with n-BuLi in n-
hexane (2 equiv) at −78 °C to obtain 1 in yields of 86% after
workup. It is advantageous to perform the lithium−bromine
exchange reaction already in the presence of the boron
electrophile in order to minimize unwanted side reactions
due to benzyne formation. In turn, the yields of 1 increase
significantly when an excess amount of (i-PrO)Bpin and n-BuLi
is used, because these reagents are partly consumed by the
nucleophilic attack of the alkyllithium species on the boric acid
ester leading to (n-Bu)Bpin.40,41 It is conceivable that the
formation of (n-Bu)Bpin can be suppressed by substantially
lowering the reaction temperature; however, we found that the
convenience of working at −78 °C outweighs the extra costs for
the starting materials. It is important to note that no
diborylbenzene 2 was ever detectable in the reaction mixtures,
even though the amounts of n-BuLi and (i-PrO)Bpin would
have been sufficient for a second borylation. Accordingly, all
attempts at a targeted synthesis of 2 through a 2-fold lithium−
bromine exchange reaction failed. A Grignard reaction under
entrainment conditions, however, cleanly transforms the
compound 1,2-C6H4(Br)(Bpin) (1) into 1,2-C6H4(Bpin)2 (2)
in 80% yield (Scheme 2).42 The major benefit of the two-step
The 11B NMR spectra of Li[7], Li[8], and Li4[10] are
characterized by resonances at −28.3 ppm (1JBH = 79 Hz), 32.2
ppm/−26.7 ppm (1JBH = 79 Hz), and −27.3 ppm (1JBH = 75
Hz), respectively (cf. Li2[9]: −26.6 ppm (1JBH = 79 Hz)).24
The signals of the corresponding BH protons appear as 1:1:1:1
quartets at δ(1H) = 1.11, 1.19, and 0.95, respectively (cf. Li2[9]:
1.03 ppm).24 The aryl proton H-c of Li[7] gives rise to a
broadened multiplet, likely due to coupling to the adjacent BH3
group. Apart from C-a and C-e, all other phenylene carbon
atoms show coupling to the boron nucleus of the BH3
substituent in the 13C{1H} NMR spectrum; in the case of C-
1
b, a well-resolved 1:1:1:1 quartet is visible (159.7 ppm, JBC
=
56 Hz). The same general features are found for the aryl
resonances of compound Li[8] and therefore do not merit
1
further discussion. In the H NMR spectrum of Li4[10], we
assign a broadened signal at 7.54 ppm to the aryl protons. The
corresponding CH signal does not possess a resolved multiplet
structure; an extremely broad hump centered at approximately
δ(13C) = 152 is tentatively attributed to the CB nuclei.
Solid-State Structures and Reactivity of (Oligotopic)
Trihydroborate Anions. For the discussion and a comparison
of the crystal structures of Li[8]−Li4[10], the mode of
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coordination of the [Ar−BH3]− anions to the Li+ cations is
particularly relevant. Given that the precise position of
hydrogen atoms is often difficult to locate by X-ray
crystallography, we will employ the individual B···Li distances
as an alternative diagnostic tool. According to Edelstein’s
correlation, radii of 1.6 0.1 and 1.36 0.06 Å are estimated
for bidentate and tridentate trihydroborate ligands, respec-
tively.44 Thus, B···Li distances of about 2.50 and 2.26 Å can be
expected for Ar−BH3-η2-Li and Ar−BH3-η3-Li coordination
modes;31 relevant B···Li distances are compiled in the figure
captions of the corresponding crystal structure plots.
We first compare the solid-state structures of Li[7] and
Li[8], which are both lithium phenyl(trihydro)borate deriva-
tives bearing an additional substituent in the ortho-position
(Table 1S). The bromo derivative crystallizes from Et2O/
toluene together with 0.5 equiv of Li+-coordinated Et2O in the
form of a coordination polymer ({Li(Et2O)[7]Li[7]}∞). Under
similar conditions, the pinacolboryl derivative crystallizes with 1
equiv of Li+-coordinated Et2O to form dimers (Li(Et2O)[8])2.
The crystal lattice contains two crystallographically independ-
A
ent molecules in the asymmetric unit ((Li(Et2O)[8])2 ,
B
(Li(Et2O)[8])2 ). Since all key metrical parameters of the
two molecules are the same within the experimental error
margins, only the structure of molecule A will be considered in
the paragraph to follow.
Figure 1 compares a dimeric section (Li(Et2O)[7])2 of the
polymer chain {Li(Et2O)[7]Li[7]}∞ with the discrete dimer
(Li(Et2O)[8])2 in order to illustrate similarities and differences
in the coordination behavior of the [Ar−BH3]− ligands, leading
to the different solid-state structures observed. Each Li(1)+ ion
of the centrosymmetric fragment (Li(Et2O)[7])2 coordinates
to the BH3 substituents of two [7]− ligands in η2 fashions and
also to one Et2O molecule. Consequently, the dimer contains a
planar four-membered Li2B2 ring, very similar to the case of
(Li(Et2O)[8])2.
Fundamental variations between the two species, however,
arise from different degrees of rotation about their ligands’
B(1)−C(1) bonds: In the case of (Li(Et2O)[8])2, the Bpin
group binds to Li(1) through O(1) and thereby completes a
tetrahedral coordination sphere. In contrast, the Br(1) atom of
[7]− points away from Li(1), which therefore remains three-
coordinate. Instead of acting as an intramolecular chelator of
Li(1), Br(1) binds to another lithium ion, Li(2)+, and thereby
induces polymer chain formation (Figure 2). Li(2) is part of a
second centrosymmetric dimer, (Li[7])2, and located in a
tetrahedral ligand environment consisting of Br(1), η2-H3B(1),
η2-H3B(2), and η2-H3B(2B).
The crystal structure of the Et2O solvate {Li(Et2O)Li[9]}∞
has previously been published,24 but will briefly be discussed
again in the context of the solid-state structure of the tetratopic
trihydroborate Li4[1,2,4,5-C6H2(BH3)4] (Li4[10]; Table 1S).
Crystals of the latter compound were grown from THF/
toluene; an X-ray analysis revealed a composition according to
the formula {(Li(thf)2)3Li[10]·THF}∞. While {Li(Et2O)Li-
[9]}∞ forms interconnected stacks of parallel columns in the
solid state, the crystal lattice of {(Li(thf)2)3Li[10]·THF}∞
features discrete zigzag coordination polymer chains (Figure 3).
Important symmetry elements of each column and each
zigzag chain are a 2-fold screw axis and a mirror plane,
respectively. In order to reveal important connectivity path-
ways, details of the two crystal structures are shown in Figure 4.
In the crystal lattice of {Li(Et2O)Li[9]}∞, each Li(1)+ cation
is tetracoordinated by two chelating [9]2− anions, which results
Figure 1. Molecular structures of the dimeric unit (Li(Et2O)[7])2 of
{Li(Et2O)[7]Li[7]}∞ (top) and of (Li(Et2O)[8])2 (bottom; molecule
A). Hydrogen atoms (except on boron) and carbon atoms of the Et2O
ligands have been omitted for clarity. Selected bond lengths [Å],
atom···atom distances [Å], bond angles [deg], and torsion angles
[deg]: (Li(Et2O)[7])2: B(1)···Li(1) = 2.499(6), B(1A)···Li(1) =
2.493(7), O(1)−Li(1) = 1.886(6), B(1)−C(1) = 1.602(4), Br(1)−
C(2) = 1.921(3); B(1)−C(1)−C(2) = 125.0(3), Br(1)−C(2)−C(1)
= 119.3(2). (Li(Et2O)[8])2: B(1)···Li(1) = 2.454(3), B(1A)···Li(1) =
2.423(3), O(1)−Li(1) = 1.997(3), O(21)−Li(1) = 1.993(3), B(1)−
C(1) = 1.610(2), B(2)−C(2) = 1.554(2), B(2)−O(1) = 1.390(2),
B(2)−O(2) = 1.366(2); B(1)−C(1)−C(2) = 123.8(1), B(2)−C(2)−
C(1) = 124.7(1), O(1)−B(2)−C(2) = 127.9(1), O(2)−B(2)−C(2) =
120.4(1), O(1)−B(2)−O(2) = 111.7(1); B(1)−C(1)−C(2)−B(2) =
−7.4(2), O(1)−B(2)−C(2)−C(1) = 22.4(3). Symmetry trans-
formation used to generate equivalent atoms: A: −x+1, −y+1, −z+1.
in an unusual square-planar ligand environment. On the other
hand, each [9]2− anion bridges two Li(1)+ cations. Two
neighboring phenylene rings within the same column point in
opposite directions, with a dihedral angle of 5.7(1)°. With
respect to Li(1), {(Li(thf)2)3Li[10]·THF}∞ exhibits a bonding
situation similar to {Li(Et2O)Li[9]}∞ (H3B(1), H3B(2A)).
Analogous to [9]2−, [10]4− also bridges two Li+ cations (Li(1)
and Li(3)); however, Li(3) carries two thf ligands and therefore
acts as an end-cap rather than as a connecting element. Thus,
the polymeric structure of {(Li(thf)2)3Li[10]·THF}∞ is
brought about by its third and fourth trihydroborate moieties.
The dihedral angle of 67.5(1)° between adjacent phenylene
rings is also significantly different from that in {Li(Et2O)Li-
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Figure 2. Molecular structure of {Li(Et2O)[7]Li[7]}∞ in the solid state. All hydrogen atoms and also the carbon atoms of the Et2O ligands have
been omitted for clarity. Selected bond lengths [Å], atom···atom distances [Å], bond angles [deg], and dihedral angle [deg]: Br(1)−Li(2) =
2.889(6), B(1)···Li(2) = 2.511(6), B(2)···Li(2) = 2.438(7), B(2B)···Li(2) = 2.473(6), B(2)−C(11) = 1.604(5), Br(2)−C(12) = 1.915(3); B(2)−
C(11)−C(12) = 123.8(3), Br(2)−C(12)−C(11) = 119.5(2); ph(C(1))//ph(C(11)) = 75.0(1); ph(C(X)) = phenylene ring containing C(X).
Symmetry transformations used to generate equivalent atoms: A: −x+1, −y+1, −z+1; B: −x+1, −y, −z+1.
[9]}∞. In addition to the central Li(1)+ ion, two further
[Li(2)(thf)2]+ cations adopt bridging positions between [10]4−
anions (η2-H3B(1), η3-H3B(2A)). We find a comparable
situation in {Li(Et2O)Li[9]}∞ (cf. [Li(2)(Et2O)]+); yet here,
the Li+ ion binds only one Et2O ligand and therefore establishes
a short contact to the trihydroborate moiety of an adjacent
column, whereas in {(Li(thf)2)3Li[10]·THF}∞, each zigzag
chain remains a largely isolated entity. All key bond lengths and
bond angles of [10]4− are largely the same as in the previously
described dianion [9]2− and will therefore not be considered
further.
BH3·thf46 were visible in the NMR spectra of the extract.
This finding indicates that, indeed, a polycondensation reaction
had taken place. Moreover, the bathochromic shift of the
absorption wavelength of the solid material indicates a more
extended conjugation length than in the starting material. Our
preliminary results thus indicate that compounds like Li4[10]
are potentially useful precursors for the preparation of boron-
containing polycyclic aromatic hydrocarbons. However, a
thorough characterization of these species is only possible if
their solubility in inert solvents can be increased in the future
by the introduction of solubilizing side chains at the positions 3
and 6 of the benzene ring.
Aryl(hydro)boranes are prone to substituent scrambling. In
the case of corresponding ditopic species, this characteristic
reactivity pattern can be used as the basis of a polycondensation
protocol furnishing extended main-chain boron-containing π-
electron systems.7,29−32,35,45 In the case of Li2[1,2-
C6H4(BH3)2] (Li2[9]), addition of Me3SiCl as a hydride
scavenger leads to the formation of the highly reactive free
borane 1,2-C6H4(BH2)2, which can be trapped as its amine
adduct 1,2-C6H4(BH2·NMe2Et)2.24 In the absence of a strong
Lewis base, however, the borane undergoes a cyclocondensa-
tion reaction to yield 9,10-dihydro-9,10-diboraanthracene and
BH3.24 We therefore reckoned that hydride abstraction from
Li4[10] should ultimately result in related ribbon-like macro-
molecular species. For solubility reasons the reaction between
the lithium salt Li4[10] and Me3SiCl was carried out in THF.
Irrespective of whether we added the hydride scavenger rapidly
to a stirred solution of Li4[10] or whether we layered the
solution of Li4[10] with Me3SiCl to slow the reaction rate, the
outcome of the experiments was the same: A yellow
microcrystalline solid precipitated from the initially clear and
colorless solution and did not redissolve in any common
aprotic solvent. An NMR spectroscopic investigation of the
supernatant revealed that all starting material had been
consumed in a quantitative transformation. After the precipitate
had been agitated with THF-d8, only the resonances of
CONCLUSION
■
vic-Diborylated benzenes are valuable synthetic building blocks
for various purposes. We have developed an economically
viable and convenient access route to 1,2-bis(pinacolboryl)-
benzene (1,2-C6H4(Bpin)2; 2) and 1,2,4,5-tetrakis-
(pinacolboryl)benzene (1,2,4,5-C6H2(Bpin)4; 4), starting from
1,2-C6H4Br2 or 1,2,4,5-C6H2Br4 and (i-PrO)Bpin. The syn-
thesis protocols are based on two-step sequences consisting of a
lithium−bromine exchange reaction and a subsequent en-
trained Grignard reaction. In terms of product selectivity and
yield, the two-step approach is by far superior to alternative
protocols relying exclusively on either of the two methods,
because unwanted side reactions due to benzyne formation are
suppressed.
Treatment of the primary products, 1,2-C6H4(Bpin)2 (2) and
1,2,4,5-C6H2(Bpin)4 (4), with Li[AlH4] furnishes the corre-
sponding oligotopic lithium trihydroborates Li2[1,2-
C6H4(BH3)2] (Li2[9]) and Li4[1,2,4,5-C6H2(BH3)4]
(Li4[10]). In the solid state, the vicinal trihydroborate
substituents act as chelating ligands toward some of their Li+
counterions, Ar−BH3-η2-Li being the dominant mode of
coordination. Irrespective of the accumulated negative charge
on the [9]2− and [10]4− anions, both lithium trihydroborates
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Figure 4. Sections of the molecular structures of {Li(Et2O)Li[9]}∞
(top) and {(Li(thf)2)3Li[10]·THF}∞ (bottom) in the solid state.
Hydrogen atoms (except on boron), the carbon atoms of the Et2O and
the thf molecules, and the noncoordinating THF molecules have been
omitted for clarity. Selected bond lengths [Å], atom···atom distances
[Å], bond angles [deg], and dihedral angle [deg] for {(Li(thf)2)3Li-
[10]·THF}∞: B(1)···Li(1) = 2.564(6), B(2A)···Li(1) = 2.468(6),
B(1)···Li(2) = 2.538(7), B(2A)···Li(2) = 2.291(6), O(11)−Li(2) =
1.961(6), O(21)−Li(2) = 1.991(7), B(1)···Li(3) = 2.523(6), O(31)−
Li(3) = 2.039(8), O(41)−Li(3) = 1.968(8), B(1)−C(1) = 1.626(4),
B(2)−C(3) = 1.610(4); B(1)−C(1)−C(2) = 119.8(3), B(2)−C(3)−
C(2) = 119.7(3); ph(C(1))//ph(C(1A)) = 67.5(1); ph(C(X)) =
phenylene ring containing C(X). Symmetry transformations used for
{(Li(thf)2)3Li[10]·THF}∞ to generate equivalent atoms: A: x−1/2, y,
−z+3/2; B: x, −y+3/2, z; C: x−1/2, −y+3/2, −z+3/2.
Figure 3. Molecular structures of {Li(Et2O)Li[9]}∞ (top) and
{(Li(thf)2)3Li[10]·THF}∞ (bottom) in the solid state. All hydrogen
atoms, the carbon atoms of the Et2O and the thf molecules, and the
noncoordinating THF molecules have been omitted for clarity.
Symmetry transformations used for {(Li(thf)2)3Li[10]·THF}∞ to
generate equivalent atoms: A: x−1/2, y, −z+3/2; B: x, −y+3/2, z; C:
x−1/2, −y+3/2, −z+3/2.
are stable under inert conditions and do not tend to
spontaneously eliminate LiH. In the case of 1,2-C6H4(Bpin)2
(2), we succeeded in the selective reduction of only one of the
two Bpin substituents to obtain the species Li[1,2-C6H4(Bpin)-
(BH3)] (Li[8]) in excellent yields. We therefore conclude that
hydride transfer from Li[AlH4] proceeds exhaustively at one
boron atom before a second Bpin moiety is attacked. Work is in
progress in our laboratories to employ these oligotopic
trihydroborates for the synthesis of boron-doped extended π-
conjugated materials.
3.58/67.21) or external BF3·OEt2 (11B{1H}). Abbreviations: s =
singlet, d = doublet, t = triplet, vt = virtual triplet, m = multiplet, q =
quartet, sept = septet, br = broad, n.o. = not observed. Combustion
analyses were performed by the Microanalytical Laboratory of the
Goethe University Frankfurt or by the Microanalytical Laboratory
Pascher (Remagen, Germany).
Synthesis of 1. 1,2-C6H4Br2 (10.0 mL, 19.6 g, 82.9 mmol) and (i-
PrO)Bpin (37.0 mL, 33.7 g, 181 mmol) were dissolved in a mixture of
toluene (120 mL) and THF (30 mL), and the solution was cooled to
−78 °C in an i-PrOH/dry ice bath. A solution of n-BuLi in n-hexane
(1.53 M, 110 mL, 168 mmol) was added dropwise with stirring over 3
h. After the addition was complete, the turbid yellow solution was
stirred for another 4 h at −78 °C and then slowly warmed to room
temperature overnight. The resulting suspension was treated with a
saturated aqueous solution of NaHCO3 (50 mL). After stirring for 10
min, the resulting two liquid phases were separated and the aqueous
phase was extracted with n-hexane (2 × 20 mL) and ethyl acetate (2 ×
EXPERIMENTAL SECTION
■
General Considerations. All reactions and manipulations were
carried out in an argon-filled glovebox or by applying standard Schlenk
techniques under a nitrogen atmosphere. Toluene, Et2O, THF, and
THF-d8 were carefully dried over Na/benzophenone; CH2Cl2 and
CDCl3 were dried over CaH2 and freshly distilled prior to use. Column
chromatography was performed using silica gel 60 (Macherey-Nagel).
NMR spectra were recorded at room temperature using a Bruker
Avance II 300 spectrometer. Chemical shifts are referenced to
(residual) solvent signals (1H/13C{1H}; CDCl3: 7.26/77.16; THF-d8:
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20 mL). The combined organic phases were washed with deionized
H2O (50 mL), dried over anhydrous MgSO4, and filtered. All volatiles
were removed from the filtrate under vacuum, and the pale yellow, oily
residue was vacuum distilled to give a colorless, viscous liquid. Yield:
20.3 g (86%).
1H NMR (300.0 MHz, CDCl3): δ 7.74 (s, 2H; CH), 1.37 (s, 24H;
CH3). 11B{1H} NMR (96.3 MHz, CDCl3): δ 30.2 (h1/2 = 380 Hz).
13C{1H} NMR (75.5 MHz, CDCl3): δ 140.1 (CH), 126.4 (CBr), 84.8
(CCH3), 24.9 (CH3), n.o. (CB). Anal. Calcd for C18H26B2Br2O4
(487.83): C, 44.32; H, 5.37. Found: C, 44.50; H, 5.44.
1H and 13C{1H} NMR spectra were in accord with published data
of a sample of 1 obtained through a two-step synthesis sequence.39
Synthesis of 2. Mg turnings (1.52 g, 62.5 mmol) were heated with
a heat gun under vacuum for 15 min in a two-necked Schlenk flask
equipped with a reflux condenser and a dropping funnel. After the flask
had cooled to room temperature again, 1 (5.9 g, 21 mmol), (i-
PrO)Bpin (8.0 mL, 7.3 g, 39 mmol), and THF (100 mL) were added.
The mixture was heated by means of a heating mantle to start the
Grignard reaction. As soon as the solvent had reached reflux
temperature, the heating mantle was switched off and a solution of
1,2-C2H4Br2 (1.8 mL, 3.9 g, 21 mmol) in THF (10 mL) was added
continuously to the vigorously stirred reaction mixture over a period of
1 h. After the addition was complete, the resulting yellow slurry was
heated to reflux temperature for 4 h. The reaction mixture was allowed
to cool to room temperature again and stirred overnight. It was treated
with a saturated aqueous solution of NaHCO3 (50 mL) and stirred for
30 min, the two liquid phases were separated, and the aqueous phase
was extracted with n-hexane (50 mL) and ethyl acetate (50 mL). The
combined organic phases were washed with H2O (50 mL), dried over
anhydrous MgSO4, and filtered, and all volatiles were removed from
the filtrate under vacuum. The flask containing the crude product was
placed in a silicone oil bath, and impurities were sublimed off in an oil-
pump vacuum at temperatures up to 80 °C. In a second run,
compound 2 itself was sublimed at temperatures T ≥ 100 °C; in order
to speed up the process, the flask can be heated to 200 °C without
thermolysis of 2. Compound 2 was obtained as a colorless, analytically
pure solid. Yield: 5.48 g (80%). All NMR spectra (1H, 11B{1H},
13C{1H}) were in accord with published data.24
Synthesis and Characterization of 3. Method A. Mg turnings
(1.86 g, 76.5 mmol) were heated with a heat gun under vacuum for 15
min in a two-necked Schlenk flask equipped with a reflux condenser
and a dropping funnel. After the flask had cooled to room temperature
again, 1,2,4,5-C6H2Br4 (5.0 g, 13 mmol), (i-PrO)Bpin (20.0 mL, 18.2
g, 98.0 mmol), and THF (100 mL) were added. The mixture was
heated to reflux temperature by means of a heating mantle to start the
Grignard reaction. While external heating was continued, a solution of
1,2-C2H4Br2 (2.0 mL, 4.4 g, 23 mmol) in THF (15 mL) was added
dropwise with stirring over 1 h. Heating was continued for a further 4
h, the mixture was allowed to cool to room temperature, and all
volatiles were removed under reduced pressure. n-Hexane (50 mL),
ethyl acetate (50 mL), and a saturated aqueous solution of NaHCO3
(50 mL) were added to the crude product. After separation of the two
liquid phases, the organic phase was washed with H2O (50 mL), dried
over anhydrous MgSO4, and filtered, and the filtrate was evaporated to
dryness under reduced pressure. The resulting yellow oil was purified
by column chromatography (n-hexane/ethyl acetate, 3:1, Rf = 0.79) to
finally obtain 3 as a colorless solid. Colorless block-shaped single
crystals were grown by slow evaporation of a solution of 3 in CHCl3
over a period of 7 d. Yield: 0.82 g (13%).
Synthesis and Characterization of 4. Mg turnings (512 mg,
21.1 mmol) were heated with a heat gun under vacuum for 15 min in a
two-necked Schlenk flask equipped with a reflux condenser and a
dropping funnel. After the flask had cooled to room temperature again,
3 (1.5 g, 3.1 mmol), (i-PrO)Bpin (5.0 mL, 4.6 g, 25 mmol), and THF
(100 mL) were added. The mixture was heated to reflux temperature
by means of a heating mantle to start the Grignard reaction. While
external heating was continued, a solution of 1,2-C2H4Br2 (0.6 mL, 1 g,
7 mmol) in THF (10 mL) was added dropwise with stirring over 1 h.
Heating of the resulting greenish slurry was continued for a further 4 h
before the mixture was allowed to cool to room temperature overnight.
The resulting turbid solution was treated with a saturated aqueous
solution of NaHCO3 (50 mL). After stirring for 30 min, the two liquid
phases were separated and the aqueous phase was washed with n-
hexane (2 × 30 mL) and ethyl acetate (2 × 30 mL). The combined
organic phases were washed with H2O (50 mL), dried over anhydrous
MgSO4, and filtered, and the filtrate was evaporated under vacuum.
The solid residue was heated to 150 °C (silicone oil bath) in an oil-
pump vacuum to remove impurities by sublimation and to obtain 4 as
a colorless solid. Yield: 1.49 g (83%). Colorless blocks suitable for X-
ray crystallography were obtained by slow evaporation of a solution of
4 in CH2Cl2.
1H NMR (300.0 MHz, CDCl3): δ 7.87 (s, 2H; CH), 1.33 (s, 48H;
CH3). 11B{1H} NMR (96.3 MHz, CDCl3): δ 31.7 (h1/2 = 800 Hz).
13C{1H} NMR (75.5 MHz, CDCl3): δ 137.9 (CH), 83.8 (CCH3), 25.0
(CH3), n.o. (CB). Anal. Calcd for C30H50B4O8 (581.94): C, 61.92; H,
8.66. Found: C, 61.90; H, 8.74.
Synthesis and Characterization of Li[7]. A solution of 1 (2.0 g,
7.1 mmol) in Et2O (150 mL) was cooled to −78 °C in an i-PrOH/dry
ice bath. A solution of Li[AlH4] in Et2O (1.0 M, 7.1 mL, 7.1 mmol)
was added by using a syringe. The resulting slurry was stirred at −78
°C for 4 h and then allowed to warm to room temperature overnight.
Insoluble material was collected on a frit (G4) and washed with Et2O
(2 × 30 mL). The flask containing the combined filtrates was
connected to a flask containing toluene, and a gentle vacuum was
applied to grow colorless crystals of {Li(Et2O)[7]Li[7]}∞ by vapor
diffusion. Yield: 0.66 g (44%).
1H NMR (300.0 MHz, THF-d8): δ 7.37 (br m, 1H; H-c), 7.18 (d,
3
3JHH = 7.8 Hz, 1H; H-f), 6.82 (dvt, JHH = 7.2 Hz, 4JHH = 1.2 Hz, 1H;
H-d), 6.62 (dvt, 3JHH = 7.5 Hz, 4JHH = 1.6 Hz, 1H; H-e), 3.37 (q, 3JHH
= 7.0 Hz, 2H; OCH2CH3), 1.11 (q, 1J(11)BH = 79 Hz/sept, 1J(10)BH = 27
Hz, 3H; BH), 1.09 (t, 3JHH = 7.0 Hz, 3H; OCH2CH3). 11B{1H} NMR
(96.3 MHz, THF-d8): δ −28.3 (h1/2 = 5 Hz). 11B NMR (96.3 MHz,
1
THF-d8): δ −28.3 (q, JBH = 79 Hz). 13C{1H} NMR (75.4 MHz,
THF-d8): δ 159.7 (q, 1JBC = 56 Hz; C-b), 138.0 (m; C-c), 133.3 (C-a),
3
130.7 (m; C-f), 125.1 (q, JBC = 3.0 Hz; C-d), 124.1 (C-e), 66.0
(OCH2CH3), 15.4 (OCH2CH3). Anal. Calcd for C12H14B2Br2Li2·
C4H10O (427.67): C, 44.93; H, 5.66. Found: C, 44.33; H, 5.81.
Synthesis and Characterization of Li[8]. A solution of 2 (2.0 g,
6.1 mmol) in Et2O (150 mL) was cooled to −78 °C in an i-PrOH/dry
ice bath. A solution of Li[AlH4] in Et2O (1.0 M, 6.1 mL, 6.1 mmol)
was added by using a syringe. The suspension was stirred at −78 °C
for 4 h and then allowed to warm to room temperature overnight.
Insoluble material was collected on a frit (G4) and washed with Et2O
(2 × 30 mL). The flask containing the combined filtrates was
connected to a flask containing toluene, and a gentle vacuum was
applied to grow colorless block-shaped crystals of (Li(Et2O)[8])2 by
vapor diffusion. Yield: 1.12 g (62%).
Method B. A Schlenk flask was charged with 1,2,4,5-C6H2Br4 (5.0 g,
13 mmol), (i-PrO)Bpin (13.0 mL, 11.9 g, 63.7 mmol), toluene (120
mL), and THF (30 mL). The resulting solution was cooled to −78 °C
in an i-PrOH/dry ice bath, and a solution of n-BuLi in n-hexane (1.38
M, 37 mL, 51 mmol) was added dropwise with stirring over 3 h. The
yellow reaction mixture was stirred for another 4 h at −78 °C and then
slowly warmed to room temperature overnight. The resulting turbid
solution was treated with a saturated aqueous solution of NaHCO3
(50 mL). After stirring for 10 min, the two liquid phases were
separated and the aqueous phase was extracted with n-hexane (2 × 30
mL) and ethyl acetate (2 × 30 mL). The combined organic phases
were washed with H2O (50 mL), dried over anhydrous MgSO4, and
filtered, and the filtrate was evaporated under vacuum. The oily residue
was heated to 100 °C under vacuum to remove impurities by
distillation/sublimation and to obtain 3 as a colorless solid. Yield: 4.9 g
(77%).
1H NMR (300.0 MHz, THF-d8): δ 7.45 (dd, 3JHH = 7.4 Hz, 4JHH
=
1.3 Hz, 1H; H-f), 7.40 (br m, 1H; H-c), 6.95 (dvt, 3JHH = 7.4 Hz, 4JHH
= 1.3 Hz, 1H; H-d), 6.74 (dvt, 3JHH = 7.4 Hz, 4JHH = 0.9 Hz, 1H; H-e),
3.38 (q, 3JHH = 7.0 Hz, 4H; OCH2CH3), 1.35 (s, 12H; CH3), 1.19 (q,
3
1JBH = 79 Hz, 3H; BH), 1.11 (t, JHH = 7.0 Hz, 6H; OCH2CH3).
11B{1H} NMR (96.3 MHz, THF-d8): δ 32.2 (h1/2 = 500 Hz; Bpin),
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−26.7 (h1/2 = 10 Hz; BH3). 11B NMR (96.3 MHz, THF-d8): δ 32.2 (s;
((Li(Et2O)[8])2), 983109 ({(Li(thf)2)3Li[10]·THF}∞), and 983110
(1-bromo-2-(catecholboryl)benzene).
1
Bpin), −26.7 (q, JBH = 79 Hz; BH3). 13C{1H} NMR (75.4 MHz,
1
THF-d8): δ 167.3 (q, JBC = 53 Hz; C-b), 137.4 (m; C-c), 134.6 (m;
C-f), 128.8 (m; C-d), 121.5 (C-e), 84.1 (CCH3), 66.1 (OCH2CH3),
24.6 (CH3), 15.5 (OCH2CH3), n.o. (C-a). Anal. Calcd for
C12H19B2LiO2·C4H10O (297.95): C, 64.50; H, 9.81. Found: C,
64.56; H, 9.76.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthesis and characterization of 1-bromo-2-(catecholboryl)-
benzene, 1,2-bis(catecholboryl)benzene, 5, and 6; plots of the
1H and 13C{1H} NMR spectra of 3, 4, 5, 6, Li[7], Li[8],
Li4[10], and 1-bromo-2-(catecholboryl)benzene; details of the
X-ray crystal structure analyses of 3, 4, 5, 6, and 1-bromo-2-
(catecholboryl)benzene. Crystallographic data of 3, 4, 5, 6,
{Li(Et2O)[7]Li[7]}∞, (Li(Et2O)[8])2, {(Li(thf)2)3Li[10]·
THF}∞, and 1-bromo-2-(catecholboryl)benzene in crystallo-
graphic information file (CIF) format. This material is available
free of charge via the Internet at http://pubs.acs.org.
Synthesis and Characterization of Li4[10]. 4 (0.50 g, 0.86
mmol) was dissolved in THF (50 mL) and cooled to −78 °C in an i-
PrOH/dry ice bath. A solution of Li[AlH4] in Et2O (1.0 M, 3.6 mL,
3.6 mmol) was added by using a syringe. The suspension was stirred
for 5 h at −78 °C and then allowed to warm to room temperature
overnight. All insolubles were collected on a frit (G4), and the filtrate
was evaporated to dryness under vacuum to give Li4[10]·3thf as a
1
colorless solid (the number of thf ligands was estimated by H NMR
spectroscopy and verified by combustion analysis). Yield of Li4[10]·
3thf: 0.25 g (78%). Block-shaped crystals of {(Li(thf)2)3Li[10]·
THF}∞ were grown by connecting a flask containing Li4[10]·3thf in
THF with a flask containing toluene and applying a gentle vacuum to
induce vapor diffusion.
AUTHOR INFORMATION
Corresponding Author
*Fax: +49 69 798 29260. E-mail: Matthias.Wagner@chemie.
uni-frankfurt.de.
■
1H NMR (300.0 MHz, THF-d8): δ 7.54 (br s, 2H; CH), 3.62 (m,
1
12H; THF-H2,5), 1.77 (m, 12H; THF-H3,4), 0.95 (q, JBH = 75 Hz,
12H; BH). 11B{1H} NMR (96.3 MHz, THF-d8): δ −27.3 (h1/2 = 30
Notes
1
Hz). 11B NMR (96.3 MHz, THF-d8): δ −27.3 (q, JBH = 75 Hz).
13C{1H} NMR (75.4 MHz, THF-d8): δ ca. 152 (very br; CB), 147.6
(br; CH), 68.0 (THF-C2,5), 26.2 (THF-C3,4). Anal. Calcd for
C6H14B4Li4·3C4H8O (373.50): C, 57.88; H, 10.25. Found: C, 57.77;
H, 10.06.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Beilstein-Institut, Frankfurt am
Main (Germany), within the research collaboration NanoBiC,
X-ray Crystal Structure Determinations. Data for 3 were
collected on a STOE IPDS II two-circle diffractometer with graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å) and corrected for
absorption with an empirical absorption correction using the program
PLATON.47 Data for all other structures were collected on a STOE
IPDS II two-circle diffractometer with a Genix Microfocus tube with
mirror optics also using Mo Kα radiation and were scaled using the
frame-scaling procedure in the X-AREA program system.48 All
structures were solved by direct methods using the program
SHELXS49 and refined against F2 with full-matrix least-squares
techniques using the program SHELXL-97.49
In 4, the absolute structure could not be determined. Four C atoms
are disordered with a site occupation factor of 0.62(4), four C atoms
are disordered with a site occupation factor of 0.62(3), and one C
atom is disordered with a site occupation factor of 0.5(1) for the
respective major occupied sites. The disordered atoms were
isotropically refined. Geometric parameters involving the disordered
atoms were restrained to be equal.
In 5, five C atoms are disordered with a site occupation factor of
0.572(7) for the major occupied sites and six C atoms are disordered
with a site occupation factor of 0.68(1) for the major occupied sites.
The disordered atoms were isotropically refined. Geometric
parameters involving the disordered atoms were restrained to be
equal. Compound 5 crystallizes with two crystallographically
independent molecules A and B in the asymmetric unit; since all
key metrical parameters of the two molecules are the same within the
experimental error margins, only the structure of molecule A will be
considered in the SI.
In 6, all methyl groups are disordered with a site occupation factor
of 0.68(1) for the major occupied sites. The disordered atoms were
isotropically refined.
In {Li(Et2O)[7]Li[7]}∞ and (Li(Et2O)[8])2, H atoms bonded to
boron atoms were isotropically refined; compound (Li(Et2O)[8])2
crystallizes with two crystallographically independent molecules A and
B in the asymmetric unit.
In {(Li(thf)2)3Li[10]·THF}∞, the H atoms bonded to boron atoms
were isotropically refined. One thf ring is disordered about a mirror
plane with equal occupancies for both sites.
For 1-bromo-2-(catecholboryl)benzene, the absolute structure was
determined (Flack parameter: x = 0.001(17)).
̈
through a Ph.D. grant for O.S.
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