One-pot synthesis of [(C6F5)2BH 2]- from C6F5MgBr/BH 3·SMe2 and its in situ transformation to Piers borane

Article abstract of DOI:10.1021/om200186w

Depending on the crystallization procedure, the one-pot reaction between 2 C₆F₅MgBr, 1 BH₃·SMe₂, and 1 Me₃SiCl furnishes the hydridoborate salts [Mg₂(Et ₂O)₃Br₂Cl][(C₆F₅) ₂BH₂] and [Mg(Et₂O)₂][(C ₆F₅)₂BH₂]₂, both of which are convenient starting materials for the in situ generation of Piers borane (C₆F₅)₂BH.

Full text of DOI:10.1021/om200186w

ARTICLE
pubs.acs.org/Organometallics
One-Pot Synthesis of [(C6F5)2BH2]ꢀ from C6F5MgBr/BH3 SMe2
and Its in Situ Transformation to Piers’ Borane
3
Anna Schnurr, Kamil Samigullin, Jens M. Breunig, Michael Bolte, Hans-Wolfram Lerner, and
Matthias Wagner*
Institut f€ur Anorganische und Analytische Chemie, J. W. Goethe-Universit€at Frankfurt, Max-von-Laue-Strasse 7, D-60438 Frankfurt
(Main), Germany.
S Supporting Information
b
ABSTRACT: Depending on the crystallization procedure, the one-
pot reaction between 2 C6F5MgBr, 1 BH3 SMe2, and 1 Me3SiCl
3
furnishes the hydridoborate salts [Mg2(Et2O)3Br2Cl][(C6F5)2BH2]
and [Mg(Et2O)2][(C6F5)2BH2]2, both of which are convenient
starting materials for the in situ generation of Piers’ borane
(C6F5)2BH.
erfluoroarylboranes find applications as anion sensors, as
cocatalysts in metallocene-based homogeneous olefin poly-
The currently best established synthesis protocols for
P
(C6F5)2BH and for its thioether adduct (C6F5)2BH SMe2 are
3
outlined in Scheme 1.
merizations, and as Lewis acid catalysts in a range of organic
transformations (e.g., addition of silyl enol ethers to aldehydes,
alkyl chlorides, and R,β-unsaturated ketones; hydrosilylation
of carbonyl functions; DielsꢀAlder reactions).1 Together with
sterically encumbered Lewis bases, they can form so-called
“frustrated Lewis pairs (FLPs)”, which have been employed for
the activation of various small molecules, including H2.2
Route 1. (C6F5)2BH can be prepared from (C6F5)2BCl13 by
treatment with Me2Si(H)Cl.9,11 The synthesis of (C6F5)2BCl, in
turn, starts from C6F5Li and Cl2SnMe2 and proceeds via the
reaction of the resulting stannane (C6F5)2SnMe214 with BCl3.11
By this method, (C6F5)2BH is obtained in a very satisfactory
overall yield of 62%. However, the procedure has to be regarded
as experimentally demanding: (i) C6F5Li is a thermolabile,
potentially hazardous compound, (ii) Cl2SnMe2 and BCl3 are
toxic/corrosive, and (iii) C6F5 transfer from tin to boron has to
be carried out at 120 °C (thick-walled glass bomb; boiling point
of BCl3 at ambient pressure: 12.6 °C) and takes 48 h.
Even though tris(pentafluorophenyl)borane, (C6F5)3B,3,4 is still
the most widely used perfluoroarylborane, much of the progress
witnessed in the aforementioned application fields was due to the
development of more sophisticated derivativesꢀamong them bulky
moleculessuchastris(2,20,200-perfluorobiphenyl)borane,5 bifunctional
species,6 and even polymeric7 and dendritic8 perfluoroarylboranes.
The incorporation of boryl functionalities into molecular
frameworks can conveniently be achieved via the hydroboration
reaction. Thus, in the present context the secondary borane
(C6F5)2BH, which has been developed by Piers et al., is a key
reagent for the synthesis of compounds (C6F5)2BR, in which R
(i) provides the set-screw for a fine tuning of the Lewis acidity at
the boron center or (ii) bears an additional functional group as in
Route 2. (C6F5)2BH is also accessible in a one-step procedure
by heating commercially available (C6F5)3B and Et3SiH in
benzene to 60 °C for 3 days (69% yield).11 In comparison to
route 1, route 2 is less labor intensive but nevertheless suffers
from certain disadvantages: (i) (C6F5)3B is high-priced and the
commercial samples tend to be contaminated with the water
adduct (C6F5)3B OH2, (ii) the reaction requires a rather long
3
time and shows a tendency to over-reduce the borane, thereby
contaminating the product with the dimer [(C6F5)2B(μ-H)2B-
(H)(C6F5)], and (iii) one-third of the C6F5 rings is transferred to
silicon and therefore wasted.
9
the chelating Lewis acid (C6F5)2B(Me3Si)C(H)ꢀC(H)2B(C6F5)2
or the bridged FLP (C6F5)2BC(H)2ꢀC(H)2PMes2 (Mes =
mesityl).10 Hydroboration reactions with (C6F5)2BH generally
occur with high rates and regio- and chemoselectivity compar-
able to or better than those with other reagents. Most impor-
tantly, terminal alkynes can selectively be monohydroborated to
the corresponding vinylboranes (C6F5)2BC(H)dC(H)R.9,11
This offers a convenient tool to modulate the optoelectronic
properties of extended conjugated π-electron systems and is
therefore interesting for the design of novel boron-doped photo-
and electroluminescent organic materials.12
Route 3. An (in principle) more atom-economic variant of
route 2 has been reported by Lancaster et al., who replaced
Et3SiH by BH3 SMe2, which reacts with (C6F5)3B OEt2 in light
3
3
petroleum at room temperature within minutes to give
(C6F5)2BH SMe2 in 54% yield.15 In a related study, Hoshi
et al. recently reported that a solution of (C6F5)2BH SMe2 in
hexane can be generated by substituent redistribution between
3
3
(C6F5)2BH adopts a dimeric structure in the solid state and
Received: February 28, 2011
Published: April 29, 2011
undergoes a monomer/dimer equilibrium in aromatic solvents.11
r
2011 American Chemical Society
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ARTICLE
Scheme 1. Literature Syntheses of (C6F5)2BH and
(C6F5)2BH SMe2
3
Figure 1. 11B NMR spectra of mixtures of C6F5MgBr and BH3 SMe2
3
(C6D6): (a) stoichiometric ratio 2:1, Et2O/toluene, room temperature,
12 h; (b) stoichiometric ratio 3:1, benzene, 75 °C, 18 h; (c) stoichio-
metric ratio 1:1, Et2O/toluene, room temperature, 3 h; (d) stoichio-
metric ratio 2:1, þ1 Me3SiCl, Et2O/toluene, room temperature, 3 h.
Scheme 2. Reaction of BH3 (donor) with 2 equiv of
3
C6F5MgBra
long as room temperature was maintained (Scheme 2). However,
replacement of the solvents with benzene, addition of a third
equivalent of C6F5MgBr,21 and the application of elevated tem-
peratures (75°C; 14h) led toan increase inthe relative proportion
of [(C6F5)2BH2]ꢀ. The formation of this borohydride was further
confirmed by the isolation of [Mg2(THF)6Br3][(C6F5)2BH2],
which was structurally characterized by X-ray crystallography (cf.
the Supporting Information for details). From these results we
conclude that the hydride adduct [(C6F5)2BH2]ꢀ of the target
molecule (C6F5)2BH can indeed be generated by this methodol-
ogy, albeit in low yields (e10%).
a Legend: (i) donor = THF, Et2O/THF, room temperature, 12 h; donor =
SMe2, Et2O/toluene, room temperature, 12 h. Note that the composi-
tion of the (complex) cations is unknown.
In the next step, the series of experiments was repeated with
BH3 SMe2 (solution in toluene) in order to study the influence
3
(C6F5)3B and BH3 SMe2 and that (C6F5)2BH SMe2 catalyzes
of the Lewis base on the reaction outcome. As shown in
Scheme 2, the reaction with 2 equiv of C6F5MgBr at room
temperature effected a higher proportion of [(C6F5)2BH2]ꢀ
3
3
the hydroboration of alk-1-ynes with pinacolborane.16
Our interest in (C6F5)2BH mainly arises from applications in
materials sciences: i.e., from its reactivity toward terminal alkynes
and from the possibility to modulate the optoelectronic proper-
ties of extended conjugated π-systems. Since the results of
Lancaster and Hoshi indicate that there is facile exchange
between hydride and C6F5 groups in the corresponding boranes
and their adducts, we decided to investigate the reaction between
than in the case of BH3 THF (cf. Figure 1a for the 11B NMR
3
spectrum of the mixture). When the transformation was carried
out in benzene at reflux temperature, [(C6F5)2BH2]ꢀ became
the major product; however, significant amounts of other species
were still present, as was confirmed by 11B (Figure 1b) and
19F{1H} NMR spectroscopy.
C6F5MgBr and BH3 SMe2 in order to develop a safe time- and
In an attempt to achieve an exchange between hydride and
C6F5 groups under milder conditions and thereby to avoid
unwanted side products, we decided to generate three-coordi-
nate borane intermediates by means of Me3SiCl as a hydride
acceptor. In a three-step sequence (Scheme 3), a mixture of
[(C6F5)2BH2]ꢀ, [(C6F5)BH3]ꢀ, and [BH4]ꢀ was first gener-
3
cost-efficient method for the synthesis of (C6F5)2BH itself and of
its adduct (C6F5)2BH SMe2.
3
’ RESULTS AND DISCUSSION
One-Pot Synthesis of the [(C6F5)2BH2]ꢀ Anion. In all studies
reported herein, the Grignard reagent C6F5MgBr was employed,
because it is thermally much more stable than the corresponding
lithium compound C6F5Li and can therefore conveniently be
prepared and handled at room temperature and above.17
ated from BH3 SMe2 and 1 equiv of C6F5MgBr in Et2O/toluene
3
at room temperature (cf. Figure 1c for the 11B NMR spectrum),
followed by the addition of 1 equiv of neat Me3SiCl and a further
1 equiv of the Grignard reagent (cf. the Supporting Information
for the detailed synthesis protocol). According to 11B (Figure 1d)
and 19F{1H} NMR spectroscopy,18 this procedure resulted in the
essentially quantitative formation of [(C6F5)2BH2]ꢀ.
In a first exploratory experiment, BH3 THF in THF was
3
treated at room temperature with 1 equiv of C6F5MgBr in Et2O.
The mixture was stirred for 12 h and then investigated by 11B
NMR spectroscopy (C6D6). Three signals at ꢀ30.2 ppm (triplet,
1JBH = 82 Hz), ꢀ34.1 ppm (quartet, 1JBH = 79 Hz), and ꢀ39.7
ppm (quintet, 1JBH = 81 Hz) were observed (integral ratio 1:3:1),
which agree well with the reported shift values of
[(C6F5)2BH2]ꢀ,18 [(C6F5)BH3]ꢀ,19 and [BH4]ꢀ.20 We note
that an increase in the amount of added C6F5MgBr to 2 equiv did
not result in a significant change of the product distribution as
Finally, benefitting from the fact that C6F5MgBr does not react
with Me3SiCl in Et2O at room temperature,22 the aforemen-
tioned procedure could be developed into a one-step synthesis
protocol: 2 equiv of C6F5MgBr were treated at room tem-
perature with 1 equiv of BH3 SMe2 and immediately after
3
with 1 equiv of neat Me3SiCl. After workup, the dihydrido-
borate can be isolated by crystallization from either CHCl3/
Et2O or benzene.
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Scheme 3. Three-Step and One-Step Syntheses of
[Mg2(Et2O)3Br2Cl][(C6F5)2BH2] and
[Mg(Et2O)2][(C6F5)2BH2]2
Table 1. Selected Crystallographic Data for
{[Mg2(Et2O)3Br2.4Cl0.6][(C6F5)2BH2]}2 (toluene)
3
3
a
2CHCl3
formula
C48H64B2Br4.8Cl1.2F20Mg4O6
3
C7H8 2CHCl3
3
fw
1992.83
color, shape
temp (K)
radiation (λ (Å))
cryst syst
space group
a (Å)
colorless, block
173(2)
Mo KR (0.710 73)
orthorhombic
Pbca
17.371(2)
21.4852(19)
21.5176(18)
90
b (Å)
c (Å)
R (deg)
β (deg)
γ (deg)
V (Å3)
90
a Legend: (i) Et2O/toluene, room temperature, 3 h.
90
8030.8(13)
4
Z
From CHCl3/Et2O, the salt [Mg2(Et2O)3Br2Cl][(C6F5)2BH2]
was obtained; its composition was confirmed by elemental analysis
(C, H, Cl, Br) and X-ray crystallography. Crystal data and structure
refinement details for the compound are compiled in Table 1. The
salt crystallizes together with 1 equiv of toluene and 2 equiv of
CHCl3 as a centrosymmetric dimer with a complex cationic core
and two peripheral [(C6F5)2BH2]ꢀ anions (Figure 2). The cation
can be described as consisting of two face-sharing heterocubanes,
two opposite corners of which are unoccupied. The positions X(2)
and X(3) are shared between Brꢀ and Clꢀ with relative occupancy
factors of 0.704(5):0.296(5) and 0.681(5):0.319(5), respectively. If
both ratios are artificially forced to 0.5:0.5, the R values of the
structure solution become only slightly worse (i.e., R1 (all data) =
0.1209 vs 0.1124, wR2 (all data) = 0.1252 vs 0.1061). Given this
background, we used the fully refined structure for the discussion of
bond lengths and angles but find it adequate to employ the idealized
formula sum of [Mg2(Et2O)3Br2Cl][(C6F5)2BH2] to refer to the
compound and for calculations of its molecular mass.
Dcalcd (g cmꢀ3
)
1.648
F(000)
3970
μ (mmꢀ1
)
2.763
cryst size (mm3)
0.35 ꢁ 0.32 ꢁ 0.27
42 315
no. of rflns collected
no. of indep rflns (Rint
)
7552 (0.1151)
7552/0/446
0.869
no. of data/restraints/params
GOF on F2
R1, wR2 (I > 2σ(I))
0.0505, 0.0893
0.1124, 0.1061
0.860, ꢀ0.860
R1, wR2 (all data)
largest diff peak, hole (e Åꢀ3
)
X-ray powder diffractogram of the solid hydrolysate. In a sub-
sequent purification step, we applied a static vacuum to a
suspension of [Mg(Et2O)2][(C6F5)2BH2]2 in benzene
(350 Torr; reflux temperature; 24 h), thereby driving off excess
Et2O and reducing the solubility of magnesium halide contaminants.
After filtration, crystals of [Mg(Et2O)2][(C6F5)2BH2]2 were grown
from the filtrate, which, according to EDX spectroscopy, now
contained e1 atom % of Clꢀ/Brꢀ ions.
In the case of Mg(2), an octahedral coordination sphere is
completed by two Et2O ligands, whereas Mg(1) bears one Et2O
ligand and has one hydridoborate ion in close proximity. Using
Edelstein’s23 correlation of metalꢀboron distances as a measure
of the denticity of a hydridoborate group, a value of 1.6 ( 0.1 Å is
estimated for the ionic radius of a bidentate [R2BH2]ꢀ ligand. In
A critical overview of our synthesis protocol to [(C6F5)2BH2]ꢀ
and its suitability for the further preparation of Piers’ borane has to
consider the following facts: (i) The actual synthesis took an
overall time of 5 h; crystallization required an additional 5 days
(CHCl3/Et2O; yield: 68%) or 7 days (benzene; yield: 57%). The
new access route has therefore considerable advantages over the
literature-known method18 based on (C6F5)2BCl. (ii) The salt
[Mg(Et2O)2][(C6F5)2BH2]2 contains a lower proportion of Et2O
than the salt [Mg2(Et2O)3Br2Cl][(C6F5)2BH2]. Both species
proved to be suitable starting materials for the hydroboration of
alk-1-ynes with in situ generated (C6F5)2BH (described below are
the syntheses using [Mg2(Et2O)3Br2Cl][(C6F5)2BH2]). (iii) The
new protocol cannot be applied to the synthesis and isolation of
the free borane (C6F5)2BH, most likely because Et2O reacts with
(C6F5)2BH in the absence of more powerful trapping agents.
turn, a B Mg distance of about 2.26 Å would be indicative for a
3 3 3
R2BH2-η2-Mg coordination mode (effective ionic radius of
pentacoordinate Mg2þ: 0.66 Å24). In [Mg2(Et2O)3Br2Cl]-
[(C6F5)2BH2], the B(1) Mg(1) distance is stretched to
3 3 3
2.498(7) Å, most likely as a result of steric repulsion. We refrain
from a detailed discussion of the geometric parameters of the anion
[(C6F5)2BH2]ꢀ, because the crystal structure of the related com-
pound [Li(Et2O)][(C6F5)2BH2] has already been described and no
significant differences were observed.18
From benzene, we obtained crystals of the salt
[Mg(Et2O)2][(C6F5)2BH2]2, which features two dihydridobo-
rate ligands bonded to the Mg2þ ion in an η2 fashion (B Mg =
3 3 3
2.416(3) Å; the compound crystallizes together with 2 equiv of
benzene; cf. the Supporting Information for details of the X-ray
crystal structure analysis). However, EDX measurements revealed
the presence of minor amounts of Clꢀ and Brꢀ (e3 atom %).
Moreover, after the addition of water to a representative sample of
Synthesis of (C6F5)2BH SMe2 from [Mg2(Et2O)3Br2Cl]-
[(C6F5)2BH2]. {[Mg2(Et2O)3Br2Cl][(C6F5)2BH2]}2 (toluene)
2CHCl3 was freed from toluene and CHCl3 under a dynamic
3
3
3
the crystal crop, the signature of MgBr2 6H2O was visible in the
vacuum, dissolved in SMe2, and treated with 1 equiv of Me3SiCl.
3
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reactions at a boron center. In previous work, we have shown
that ferrocenylborane (FcBH2), generated in situ from Li-
[FcBH3], immediately undergoes a condensation reaction to
Fc2BH and B2H6,25,26 whereas the corresponding cymantrenyl-
borane dimer ((CymBH2)2) can readily be isolated and structu-
rally characterized by X-ray crystallography.27 So far, we have
attributed the different reactivities to differences in the electron
densities on the organometallic fragments. The observations
described in this paper now suggest that other factors are also
likely to play a role. (ii) In our one-pot protocol, the C6F5/
hydride scrambling reaction does not lead to aꢀstatistical mixture
of all conceivable products [(C6F5)xBH4ꢀx
]
(x = 0ꢀ4) but
selectively gives [(C6F5)2BH2]ꢀ in preparatively useful yields.
’ EXPERIMENTAL SECTION
General Remarks. All manipulations were carried out under a
nitrogen atmosphere using Schlenk tube techniques and rigorously dried
solvents. NMR spectra were recorded on a Bruker Avance 300 spectro-
meter. Chemical shifts are referenced to residual solvent signals (1H),
Figure 2. Molecular structure and numbering scheme of compound
{[Mg2(Et2O)3Br2.4Cl0.6][(C6F5)2BH2]}2 (displacement ellipsoids are
drawn at the 30% probability level; H atoms except on boron and ethyl
groups on coordinated Et2O molecules are omitted for clarity; positions
external BF3 Et2O (11B, 11B{1H}), or external CFCl3 (19F{1H}).
3
Abbreviations: s = singlet, d = doublet, tr = triplet, q = quartet, quin =
quintet, br = broad, n.r. = multiplet expected in the 1H NMR spectrum
but not resolved. Me3SiCl was stirred with CaH2 (30 min) and vacuum-
transferred into a Schlenk storage vessel. SMe2 was dried over LiAlH4
(12 h at room temperature) and vacuum-transferred into a Schlenk
storage vessel. BH3 THF (1.0 M in THF; Acros), BH3 SMe2 (2.0 M in
X are shared between Br and Cl). Selected bond lengths (Å), atom
3 3 3
atom distances (Å), and angles (deg): Mg(1)ꢀBr(1) = 2.711(2),
Mg(2)ꢀBr(1) = 2.768(2), Mg(2A)ꢀBr(1) = 2.688(2), B(1)ꢀC(1) =
1.625(8), B(1)ꢀC(11) = 1.610(8), B(1) Mg(1) = 2.498(7); Mg-
3 3 3
(1)ꢀBr(1)ꢀMg(2) = 88.7(1), Mg(1)ꢀBr(1)ꢀMg(2A) = 90.2(1),
Mg(2)ꢀBr(1)ꢀMg(2A) = 95.7(1), C(1)ꢀB(1)ꢀC(11) = 112.7(4).
Symmetry transformation used to generate equivalent atoms: (A) ꢀx þ
1, ꢀy þ 1, ꢀz þ 1.
3
3
toluene; Aldrich), and C6F5Br (fluorochem) are commercially available
and were used as received.
Synthesis of {[Mg2(Et2O)3Br2Cl][(C6F5)2BH2]}2. At room
temperature, a stirred solution of freshly prepared C6F5MgBr
(8.0 mmol) in Et2O (10 mL) was treated first with a solution of
After 90 min, the 11B NMR spectrum of the mixture revealed
exclusively one doublet at ꢀ12.1 ppm; in the 1H NMR spectrum, a
broad multiplet at 3.58 ppm and a singlet at 1.10 ppm (integral ratio
1:6) testified to the presence of one boron-bonded H atom and one
coordinated SMe2 ligand, respectively. The adduct (C6F5)2BH
BH3 SMe2 in toluene (2.0 M; 2.0 mL, 4.0 mmol) and then with neat
3
Me3SiCl (0.52 mL, 4.1 mmol). The mixture was stirred for 3 h, the
volatiles were driven off in vacuo, and the solid pale brown residue was
dissolved in CHCl3 (40 mL) and Et2O (5 mL). After filtration from a
small amount of insoluble material (mainly unconsumed Mg turnings),
the light yellow filtrate was concentrated under reduced pressure until it
turned slightly turbid and then stored at 5 °C. Crystallization of
{[Mg2(Et2O)3Br2Cl][(C6F5)2BH2]}2 (toluene) 2CHCl3 started after
SMe2 was isolated in 90% yield as a colorless flaky solid; all
3
NMR shift values were in full agreement with published data15 for
this compound.
In Situ Generation of (C6F5)2BH and Hydroboration of Alk-
1-ynes. tert-Butylacetylene or phenylacetylene in benzene (2 equiv)
was added at room temperature to a suspension of {[Mg2(Et2O)3-
Br2Cl][(C6F5)2BH2]}2 in benzene/hexane (1:1). The mixture was
treated with Me3SiCl (2 equiv) and stirred for 2 h. Reaction control
by NMR spectroscopy (1H, 11B, 19F{1H}; C6D6) revealed quanti-
tative conversion to the monohydroboration products (C6F5)2-
BC(H)dC(H)tBu11 and (C6F5)2BC(H)dC(H)Ph;11 after work-
up, yields of 70% and 79% were respectively obtained.
3
3
several hours and continued for another 5 days. Crystals suitable for
X-ray diffraction were isolated by decantation; concentration of the
mother liquor in vacuo and further storage at 5 °C yielded a second crop.
Samples used for reactions were washed with a mixture of benzene
(2 mL) and hexane (4 mL) and dried in vacuo over a period of 2 h to
remove cocrystallized toluene and CHCl3. Yield of {[Mg2(Et2O)3-
Br2Cl][(C6F5)2BH2]}2: 2.21 g (68%). For the three-step synthesis of
{[Mg2(Et2O)3Br2Cl][(C6F5)2BH2]}2 and for the synthesis of [Mg(Et2-
O)2][(C6F5)2BH2]2, see the Supporting Information.
3
1H NMR (300.0 MHz, CD2Cl2): δ 1.25 (tr, 18H, JHH = 7.0 Hz;
OCH2CH3), 2.03 (br q, 2H, 1JBH = 70 Hz; BH2), 3.81 (q, 12H, 3JHH
=
’ CONCLUSION
The hydride adduct [(C6F5)2BH2]ꢀ of Piers’ borane
(C6F5)2BH is conveniently accessible in a one-pot procedure
7.0 Hz; OCH2CH3). 11B{1H} NMR (96.3 MHz, CD2Cl2): δ ꢀ30.5
(h1/2 = 40 Hz). 11B NMR (96.3 MHz, CD2Cl2): δ ꢀ30.5 (tr, 1JBH = 70
Hz). 19F{1H} NMR (282.3 MHz, CD2Cl2): δ ꢀ164.8 (br, 4F; Fm),
ꢀ159.6 (br, 2F; Fp), ꢀ133.1 (br, 4F; Fo). MS (ESIꢀ): m/z (%) 347.3
(100) [(C6F5)2BH2]ꢀ. Anal. Calcd for C24H32BBr2ClF10Mg2O3
[813.18]: C, 35.45; H, 3.97; Br, 19.65; Cl, 4.36. Found: C, 35.07; H,
3.58; Br, 19.7; Cl, 4.87.
from C6F5MgBr, BH3 SMe2, and Me3SiCl. Starting from this
3
compound, (C6F5)2BH can be generated in situ and trapped with
SMe2 or tert-butylacetylene/phenylacetylene. We therefore sug-
gest our method as a viable alternative to the established
syntheses of (C6F5)2BH whenever the hydroboration of alk-1-
ynes is the focus of application.
Synthesis of (C6F5)2BH SMe2. Me3SiCl in SMe2 (0.75 M;
3
0.50 mL, 0.38 mmol) was added at room temperature via syringe to a
stirred solution of {[Mg2(Et2O)3Br2Cl][(C6F5)2BH2]}2 (0.28 g,
0.17 mmol) in SMe2 (5 mL), whereupon a colorless precipitate imme-
diately formed. The reaction mixture was stirred for 90 min, all volatiles were
With regard to the underlying reaction mechanism, the
following results are noteworthy: (i) The electron-poor C6F5
substituent readily takes part in aryl/hydride redistribution
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removed under reduced pressure, and the colorless solid residue was
suspended in benzene (5 mL). After filtration, the insoluble material was
extracted with benzene (2 ꢁ 5 mL) and the combined organic phases were
Selected crystallographic data, plots of the molecular structures, and
selected geometric parameters of [Mg2(THF)6Br3][(C6F5)2BH2]
and [Mg(Et2O)2][(C6F5)2BH2]2 2(benzene) and ORTEP plot of
3
freeze-dried in vacuo to obtain (C6F5)2BH SMe2 as a colorless flaky solid.
{[Mg2(Et2O)3Br2.4Cl0.6][(C6F5)2BH2]}2. CIF files for [Mg2-
(THF)6Br3][(C6F5)2BH2], {[Mg2(Et2O)3Br2.4Cl0.6][(C6F5)2-
BH2]}2 (toluene) 2CHCl3, and [Mg(Et2O)2][(C6F5)2BH2]2
3
Yield: 0.13 g (90%).
All 1H, 11B, and 19F{1H} NMR shift values are in full agreement with
3
3
3
the published data for (C6F5)2BH SMe2 (cf. the Supporting Informa-
2(benzene). This material is available free of charge via the Internet
at http://pubs.acs.org.
3
tion for details).15
General Procedure for the Hydroboration of Alk-1-ynes.
{[Mg2(Et2O)3Br2Cl][(C6F5)2BH2]}2 was suspended in a mixture of
benzene (2 mL) and hexane (2 mL). The alk-1-yne (solution in
benzene) and Me3SiCl (solution in hexane) were added at room
temperature via syringe. After the mixture was stirred for 2 h, the
conversion was complete (NMR spectroscopic control). The reaction
mixture was filtered, the insoluble material was extracted with benzene
(2 ꢁ 1 mL), and the combined organic phases were evaporated to
dryness under reduced pressure to obtain the hydroboration product in
pure form.
’ AUTHOR INFORMATION
Corresponding Author
*Fax: þ49 69 798 29260. E-mail: Matthias.Wagner@
chemie.uni-frankfurt.de.
’ ACKNOWLEDGMENT
M.W. acknowledges financial support by the Beilstein Institut,
Frankfurt/Main, Germany, within the research collaboration
NanoBiC.
(C6F5)2BC(H)dC(H)tBu: {[Mg2(Et2O)3Br2Cl][(C6F5)2BH2]}2
(0.10 g, 0.06 mmol), tert-butylacetylene (0.13 M in benzene; 0.95 mL,
0.12 mmol), and Me3SiCl (0.27 M in hexane; 0.50 mL, 0.14 mmol);
yield: 0.036 g (70%). (C6F5)2BC(H)dC(H)Ph: {[Mg2(Et2O)3Br2Cl]-
[(C6F5)2BH2]}2 (0.027 g, 0.017 mmol), phenylacetylene (0.17 M in
benzene; 0.20 mL, 0.033 mmol), and Me3SiCl (0.18 M in hexane;
0.20 mL, 0.037 mmol); yield: 0.012 g (79%). All 1H, 11B, and 19F{1H}
NMR shift values are in full agreement with the published data for these
compounds (cf. the Supporting Information for details).11
’ REFERENCES
(1) For selected reviews, see: (a) Piers, W. E.; Chivers, T. Chem. Soc.
Rev. 1997, 26, 345–354. (b) Ishihara, K.; Yamamoto, H. Eur. J. Org.
Chem. 1999, 527–538. (c) Chen, E. Y.-X.; Marks, T. J. Chem. Rev. 2000,
100, 1391–1434. (d) Chivers, T. J. Fluorine Chem. 2002, 115, 1–8. (e)
Piers, W. E. Adv. Organomet. Chem. 2005, 52, 1–76. (f) Focante, F.;
Mercandelli, P.; Sironi, A.; Resconi, L. Coord. Chem. Rev. 2006,
250, 170–188. (g) Wade, C. R.; Broomsgrove, A. E. J.; Aldridge, S.;
Gabbaï, F. P. Chem. Rev. 2010, 110, 3958–3984.
X-ray Crystal Structure Analysis of [Mg2(THF)6Br3]-
[(C6F5)2BH2], {[Mg2(Et2O)3Br2.4Cl0.6][(C6F5)2BH2]}2 (toluene)
3
3
2CHCl3, and [Mg(Et2O)2][(C6F5)2BH2]2 2(benzene). Data were
3
collected on a STOE IPDS II two-circle diffractometer with graphite-
monochromated Mo KR radiation. Empirical absorption corrections were
performed using the MULABS28 option in PLATON.29 The structures
were solved by direct methods using the program SHELXS30 and refined
against F2 with full-matrix least-squares techniques using the program
SHELXL-97.31 Five of the six coordinating THF molecules in
[Mg2(THF)6Br3][(C6F5)2BH2] are disordered over two positions with
occupancy factors of 0.51(1), 0.53(3), 0.54(2), 0.60(4), and 0.53(2) for
the major occupied sites. The C atoms of the disordered THF molecules
were isotropically refined. Bond lengths and angles of the disordered THF
molecules were restrained to be equal to those of the nondisordered THF
molecule. In {[Mg2(Et2O)3Br2.4Cl0.6][(C6F5)2BH2]}2 (toluene) 2CHCl3,
the toluene molecule is disordered over two equally occupied positions.
One of the Br atoms (Br(1)) was refined as being fully occupied, the
remaining two halogen positions were refined as being disordered with Cl
but sharing the same coordinates and the same displacement parameters.
The respective site occupation factors refined to 0.704(5) for Br(2) and
0.681(5) for Br(3). If both ratios are artificially forced to 0.5:0.5, several
peaks appear in the residual electron density map slightly below 1 e Åꢀ3 at
distances of ca. 0.5 Å from X(2) and X(3) (cf. Figure 2). If the ratio Brꢀ:Clꢀ
is refined freely, no significant peak arises close to X(2) and X(3). In
(2) For reviews on FLPs, see: (a) Stephan, D. W. Org. Biomol. Chem.
2008, 6, 1535–1539. (b) Stephan, D. W. Dalton Trans.
2009, 3129–3136. (c) Stephan, D. W.; Erker, G. Angew. Chem., Int.
Ed. 2010, 49, 46–76.
(3) Massey, A. G.; Park, A. J.; Stone, F. G. A. Proc. Chem. Soc.
1963, 212.
(4) Sundararaman, A.; J€akle, F. J. Organomet. Chem. 2003,
681, 134–142.
(5) Chen, Y.-X.; Stern, C. L.; Yang, S.; Marks, T. J. J. Am. Chem. Soc.
1996, 118, 12451–12452.
(6) Piers, W. E.; Irvine, G. J.; Williams, V. C. Eur. J. Inorg. Chem.
2000, 2131–2142.
(7) Qin, Y.; Cheng, G.; Sundararaman, A.; J€akle, F. J. Am. Chem. Soc.
2002, 124, 12672–12673.
(8) Roesler, R.; Har, B. J. N.; Piers, W. E. Organometallics 2002,
21, 4300–4302.
(9) Parks, D. J.; Spence, R. E. v. H.; Piers, W. E. Angew. Chem., Int. Ed.
3
3
Engl. 1995, 34, 809–811.
(10) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Fr€ohlich, R.;
Grimme, S.; Stephan, D. W. Chem. Commun. 2007, 5072–5074.
(11) Parks, D. J.; Piers, W. E.; Yap, G. P. A. Organometallics 1998,
17, 5492–5503.
(12) J€akle, F. Chem. Rev. 2010, 110, 3985–4022.
(13) Chambers, R. D.; Chivers, T. J. Chem. Soc. 1965, 3933–3939.
(14) (C6F5)2SnMe2 is also accessible from C6F5MgBr and
Br2SnMe2. However, use of C6F5Li and the less expensive Cl2SnMe2
(which can, moreover, be recycled) gives the target compound in higher
purity and better yield.11
[Mg(Et2O)2][(C6F5)2BH2]2 2(benzene), the H atoms bonded to B were
3
freely refined.
CCDC reference numbers: 813324 ([Mg2(THF)6Br3][(C6F5)2BH2]),
813325 ({[Mg2(Et2O)3Br2.4Cl0.6][(C6F5)2BH2]}2 (toluene) 2CHCl3),
3
3
and 813326 ([Mg(Et2O)2][(C6F5)2BH2]2 2(benzene)).
3
(15) Fuller, A.-M.; Hughes, D. L.; Lancaster, S. J.; White, C. M.
Organometallics 2010, 29, 2194–2197.
’ ASSOCIATED CONTENT
(16) Hoshi, M.; Shirakawa, K.; Okimoto, M. Tetrahedron Lett. 2007,
48, 8475–8478.
(17) Tamborski, C.; Soloski, E. J.; Ward, J. P. J. Org. Chem. 1966,
31, 4230–4232.
S
Supporting Information. Synthesis and crystallization of
b
[Mg2(THF)6Br3][(C6F5)2BH2], three-step synthesis of {[Mg2-
(Et2O)3Br2Cl][(C6F5)2BH2]}2, and synthesis of [Mg(Et2O)2]-
[(C6F5)2BH2]2. 1H, 11B, and 19F{1H} NMR spectra of (C6F5)2BH
(18) Douthwaite, R. E. Polyhedron 2000, 19, 1579–1583. The anion
3
SMe2, (C6F5)2BC(H)dC(H)tBu, and (C6F5)2BC(H)dC(H)Ph.
[(C6F5)2BH2]ꢀ has also been obtained by Berke et al. upon reaction of
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Organometallics
ARTICLE
the FLP Mes3P/(C6F5)2BH with H2. The primary product [Mes3-
PH][(C6F5)2BH2] undergoes further reaction to [Mes3PH][(C6F5)3-
BH] and [Mes3PH][(C6F5)BH3], which represents a C6F5/hydride
redistribution similar to that exploited in our synthesis: Jiang, C.;
Blacque, O.; Berke, H. Organometallics 2009, 28, 5233–5239.
(19) Biscoe, M. R.; Breslow, R. J. Am. Chem. Soc. 2003, 125,
12718–12719.
(20) Lorbach, A.; Nadj, A.; T€ullmann, S.; Dornhaus, F.; Sch€odel, F.;
S€anger, I.; Margraf, G.; Bats, J. W.; Bolte, M.; Holthausen, M. C.;
Wagner, M.; Lerner, H.-W. Inorg. Chem. 2009, 48, 1005–1017.
(21) The use of excess C6F5MgBr was necessary, because the
compound gradually decomposes at elevated temperatures.17
(22) Edmondson, R. C.; Jukes, A. E.; Gilman, H. J. Organomet. Chem.
1970, 25, 273–276. We have reinvestigated the reactivity of C6F5MgBr
toward Me3SiCl in different solvents. As reported in this reference,
C6F5SiMe3 is only formed in THF, not in Et2O.
(23) Edelstein, N. Inorg. Chem. 1981, 20, 297–299.
(24) Shannon, R. D. Acta Crystallogr. 1976, A32, 751–767.
(25) Scheibitz, M.; Bats, J. W.; Bolte, M.; Lerner, H.-W.; Wagner, M.
Organometallics 2004, 23, 940–942.
(26) Scheibitz, M.; Li, H.; Schnorr, J.; Sꢀanchez Perucha, A.; Bolte,
M.; Lerner, H.-W.; J€akle, F.; Wagner, M. J. Am. Chem. Soc. 2009,
131, 16319–16329.
(27) Eckensberger, U. D.; Weber, M.; Wildt, J.; Bolte, M.; Lerner,
H.-W.; Wagner, M. Organometallics 2010, 29, 5301–5309.
(28) Blessing, R. H. Acta Crystallogr. 1995, A51, 33–38.
(29) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.
(30) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467–473.
(31) Sheldrick, G. M. SHELXL-97. A Program for the Refinement of
Crystal Structures; Universit€at G€ottingen, G€ottingen, Germany, 1997.
2843
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