Syntheses and properties of triborane(5)s possessing bulky diamino substituents on terminal boron atoms

Article abstract of DOI:10.1039/c1cc11334h

The present communication reports the chemistry of three linear triborane(5) compounds possessing bulky diamino substituents on the terminal boron atoms. Fluorotriborane 2 was synthesized by a reaction of boryllithium and BF₃·OEt₂. H

Full text of DOI:10.1039/c1cc11334h

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COMMUNICATION
Syntheses and properties of triborane(5)s possessing bulky diamino
substituents on terminal boron atomsw
Yumi Hayashi, Yasutomo Segawa, Makoto Yamashita* and Kyoko Nozaki*
Received 8th March 2011, Accepted 22nd March 2011
DOI: 10.1039/c1cc11334h
The present communication reports the chemistry of three linear
triborane(5) compounds possessing bulky diamino substituents on
the terminal boron atoms. Fluorotriborane 2 was synthesized by a
reaction of boryllithium and BF3ÁOEt2. Halogen exchange reaction
of 2 took place by a treatment with ClSiMe3 to give the corres-
ponding chlorotriborane(5) 3. Addition of silver tetraarylborate to 3
in ether afforded a hydroxylated triborane(5) compound 5 probably
via an unstable cationic boron intermediate 4.
Scheme 1 Reported examples of linear borane.
anion equivalents.12–15 They could react with organic electro-
‘‘Catenation’’ is a word describing the ability of an element to
construct a long chain structure involving covalent bonds.1
philes to form a B–C bond in a nucleophilic manner. The high
Many examples of catenation can be found as consecutive
C–C bonds in organic chemistry and Si–Si bonds in polysilane
chemistry. In contrast, boron, one of the closest neighbors to
carbon in the periodic table, does not show catenation due
to the following reason. In general, compounds containing
multiple boron atoms tend to form a multinuclear structure
through sharing the electrons by three or more orbitals as
found in the B2H6 molecule and the polyborane structure
because of the electron deficiency of the boron atom.2,3 In the
literature, only three types of examples for catenated boranes
have been reported as triborane(5), tetraborane(6), and
hexaborane(8).4–6 These catenated boranes were synthesized
by a reductive coupling of dimethylamino-substituted chloro-
borane and a similar chlorodiborane(4) by using a sodium/
potassium alloy (Scheme 1).7 Dimethylamino groups on the
terminal boron atom of boranes could be converted to
halogeno substituents (Cl, Br and I). The obtained terminal-
halogenated boranes could also accept nucleophiles such as
alkoxy, amino, alkyl, alkynyl, phosphino, and alkylthio
groups onto their terminal boron atom.
nucleophilicity of the boron center in boryl anion equivalents
prompted us to explore new methods to synthesize catenated
borane derivatives. Herein, we report syntheses and properties
of triborane(5) derivatives using boryllithium 1. In the reaction
of chlorinated triborane(5) 3 with the silver tetraarylborate
salt, the formation of the transient cationic boron compound 4
was suggested.
Simple mixing of boryllithium 1, generated in THF, with
BF3ÁOEt2 in hexane gave fluorotriborane(5) 2 in 58% yield
(Scheme 2). Resonances in the 1H and 13C NMR spectrum of 2
showed a highly symmetrical pattern with two distinct methyl,
one methine, one backbone CH, and two aromatic signals. In
the 11B NMR spectrum of 2, two signals at dB 83 and 25 were
observed. The former signal could be assigned to the central
boron atom from the integral ratio of two signals, although no
11B–19F coupling was observed probably due to significant
broadening of the signal. It should be noted that the signal of
the central boron atom appeared at a significantly lower-field
compared to that (dB 60.2) observed in the reported pentakis-
(dimethylamino)triborane(5) compound.5 This remarkable
shift of 11B resonance may be due to a less-effective overlap
of p orbitals along the B–F bond in 2 compared to the B–N
bond in the previously reported triborane(5) compounds.
The subsequent halogen exchange reaction of 2 could be
achieved by a treatment of 2 with ClSiMe3 to give the
chlorinated triborane(5) 3 in 60% yield (Scheme 2). The features
On the other hand, we recently reported a new boron-
containing reagent, boryllithium 1,8–10 having an anionic
charge and nucleophilicity on the boron center.11 The anionic
boron moiety in 1 could also be transferred to the other metals
such as magnesium, copper, and zinc to form a series of boryl
7-3-1 Hongo, Bunkyo-ku, 113-8656 Tokyo, Japan.
E-mail: makotoy@chembio.t.u-tokyo.ac.jp,
nozaki@chembio.t.u-tokyo.ac.jp; Fax: +3-81-5841-7263;
Tel: +81-5841-7262
w Electronic supplementary information (ESI) available: All experi-
mental procedures and spectroscopic data. CCDC 810989. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1cc11334h
Scheme 2 Syntheses of triborane(5)s 2 and 3 (Dip = 2,6-iPr2C6H3).
c
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Fig. 2 ORTEP drawing of 5 (hydrogen atoms except for the hydroxy
group, one of two independent molecules, and the co-crystallized
n-hexane molecule are omitted for clarity).
Fig. 1 ORTEP drawing of 3 (hydrogen atoms, minor parts of
disordered isopropyl groups, and the co-crystallized THF molecule
are omitted for clarity).
1
of 3 in its H and 13C NMR spectra are similar to those of 2.
smaller. The B–O bond distance was in the range of typical
values for those in trialkoxyborane and tetraalkoxy-
diborane(4) derivatives as were observed in Cambridge
crystallographic database. Existence of a hydroxy group in 5
In the 11B NMR spectrum of 3, a lower-field shifted signal
(dB 103) of the central boron atom compared to that in 2 was
observed and it is probably due to less-overlapped
p-orbitals on boron and chlorine atoms. Recrystallization of
3 from THF/n-hexane gave single crystals suitable for X-ray
1
was confirmed by the D2O test in its H NMR spectrum and
OH at 3524 cmÀ1 in its IR spectrum. It may be assumed that a
n
cationic intermediate 4 was generated by an absorption of
crystallographic analysis. The crystal structure of
3
is
illustrated in Fig. 1. In the solid state, B–B lengths are
1.707(6) and 1.729(6) A, which are in the range of B–B lengths
in the reported triborane(5) and tetraborane(6) derivatives.4–6
In contrast, the B–B–B angle of 141.51 in 3 is larger than those
(120 Æ 81) in the reported linear boranes.4–6 Despite the
distortion around the central boron atom, the sum of the
angles around each boron center is 3601, indicating their
planarity. The B–Cl bond of 1.797(4) A is similar to the typical
values for B–Cl bonds.1 Dihedral angles of Cl–B–B–N are
21.31 and 73.41, showing that one boron-containing five-
membered ring is close to being coplanar to the plane of the
central boron atom and the other five-membered ring is almost
perpendicular. This configuration may result from the steric
repulsion between four bulky Dip groups.
chloride on the silver cation to precipitate as AgCl. The
diborylboryl cation
4 may be distinguished from the
previously reported example of isolated boryl cations,17,18
where the cationic boron center has one or two heteroatom(s)
to gain a stabilization by pp–pp interaction between the boron
and heteroatom(s). The lack of pp–pp stabilization may give
anomalous instability to 4 to induce the further reaction of 4
with ether.
The possible reaction mechanism regarding the previously
reported gas-phase reaction of the dimethylboryl cation with
ether19 is illustrated in Scheme 4. After the formation of 4, the
solvent ether immediately attacks the cationic boron center to
form the diethylboryloxonium cation 6.20 Elimination of
two ethylene molecules (detected by GC) from 6, via two
subsequent reactions including the formation of an ethyl
cation and transfer of a proton,21 induces the formation of
dihydroboryloxonium 8. Loss of a proton from 8 gives the
hydroxylated product 5.
Treatment of chlorotriborane(5) 3 with silver tetrakis-
(pentafluorophenyl)borate in ether led to formation of a white
precipitate and hydroxytriborane(5) 5 in quantitative yield as
was observed in 1H NMR (C6D6) of the crude product
(Scheme 3).16 Hydroxytriborane(5)
5
was independently
In conclusion, boryllithium 1 reacted with BF3ÁOEt2 to give
the corresponding fluorotriborane(5) 2 via double nucleophilic
substitution by the boryl anion at the boron center of BF3. The
fluorine atom in 2 could be replaced with a chlorine atom by
synthesized and structurally characterized by X-ray crystallo-
graphy (Fig. 2). Compared with 3, the B–B lengths (av. 1.732 A)
in 5 are longer and the B–B–B angles (av. 135.51) in 5 are
Scheme 4 Plausible mechanism for the formation of 5 [B =
Scheme 3 Reaction of 3 with Ag[B(C6F5)4].
B(NDipCH)2].
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¨
R. Muller and S. Ostreicher, Angew. Chem., Int. Ed., 2009, 48,
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chlorotriborane(5) 3. Treatment of 3 with the silver salt in
ether gave the hydroxytriborane(5) 5. Formation of 5 was
explained by assuming an unstable cationic intermediate 4 and
subsequent decomposition of the ether molecule.
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c
5890 Chem. Commun., 2011, 47, 5888–5890
This journal is The Royal Society of Chemistry 2011