Preparation of dihydroborole derivatives by a simple 1,1-carboboration route

Article abstract of DOI:10.1021/om300065m

2,3-Dihydroboroles are readily formed by treatment of dicyclopropylacetylene with the strongly electrophilic borane B(C ₆F₅)₃. The reaction involves a sequence of 1,1-carboboration reactions and proceeds via dienylborane intermediates. Consistent with this, the reaction of the conjugated enyne 15c with B(C ₆F₅)₃ leads to the formation of a dihydroborole.

Full text of DOI:10.1021/om300065m

Article
pubs.acs.org/Organometallics
Preparation of Dihydroborole Derivatives by a Simple 1,1-
Carboboration Route
Andreas Feldmann,† Azusa Iida,†,‡ Roland Frohlich,†,§ Shigehiro Yamaguchi,‡ Gerald Kehr,†
̈
and Gerhard Erker*,†
†Organisch-Chemisches Institut der Universitat Munster, Corrensstrasse 40, 48149 Munster, Germany
̈
̈
̈
‡Department of Chemistry, Graduate School of Science, Nagoya University Furo, Chikusa, Nagoya 464-8602, Japan
S
* Supporting Information
ABSTRACT: 2,3-Dihydroboroles are readily formed by
treatment of dicyclopropylacetylene with the strongly electro-
philic borane B(C6F5)3. The reaction involves a sequence of
1,1-carboboration reactions and proceeds via dienylborane
intermediates. Consistent with this, the reaction of the
conjugated enyne 15c with B(C6F5)3 leads to the formation
of a dihydroborole.
Chart 2. Syntheses of Dihydroborole Derivatives by Zweifel9
INTRODUCTION
There has been a recent revived interest in the chemistry of
unsaturated five-membered boron heterocycles, the boroles
(1), the 2,3-dihydroboroles (2), and the 2,5-dihydroboroles
(3), respectively (see Chart 1). The potentially antiaromatic
■
(top) and Herberich10 (bottom)
Chart 1. Boron Heterocycles: Borole (1), 2,3-Dihydro- (2),
and 2,5-Dihydroborole (3) Derivatives
RESULTS AND DISCUSSION
■
In this study we reacted the strong boron Lewis acid B(C6F5)3
(7)15 with cyclopropylacetylene (8a) and dicyclopropylacetyl-
ene (8b),16 respectively. The former reaction resulted in a clean
boroles can be synthesized and isolated by variants of the
1,1-carboboration reaction of the acetylenic substrate; the latter
original route established by J. Eisch et al. if stabilized by
sufficiently bulky aromatic substituents.1 Eisch used the
reaction opened an easy pathway to 2,3-dihydroborole systems.
Treatment of cyclopropylacetylene (8a) with B(C6F5)3 (7) in
respective highly substituted 1,4-dilithio(tetraphenyl)butadiene
[D6]-benzene at ambient temperature gave a 2:1 mixture of the
1,1-carboboration products Z-9a and E-9a (characterized by
NMR; for details see the Supporting Information). Subsequent
photolysis (HPK 125, Pyrex filter) rapidly led to an almost
complete isomerization of the E- to the Z-9a isomer. On a
preparative scale a 1:1 mixture of 8a and the borane 7 in
pentane was stirred for 4 h at room temperature, followed by
irradiation overnight and workup to give the pure Z-9a product
in 63% yield as a colorless solid. The compound shows typical
reagent for the reaction with RBCl2. Later variants employed,
for example, the corresponding substituted zirconacyclopenta-
dienes, as shown by Fagan et al.,2 or the respective
stannacyclopentadiene systems.3−8
2,5-Dihydroborole derivatives 4 were originally prepared by
Zweifel from conjugated enynes in a two-step reaction
sequence involving hydroboration and subsequent photolysis
(see Chart 2).9 Herberich synthesized the 2,5-dihydroborole
system 5 by treatment of the Cl2BNR2 substrate with the
oligomeric “butadiene-magnesium” reagent.10 The Rh(I)-
catalyzed isomerization of 5 represents one of the rare
examples of a rather convenient 2,3-dihydroborole syn-
thesis11,12 (see Chart 2).
1
cyclopropyl H NMR features at δ 0.42, 0.66 (2 H each) and
1.12 (1 H) and an olefinic CH resonance at δ 6.36 (d, 3JHH
of 19F NMR signals in an overall 2:1 intensity ratio originating
=
11.0 Hz) (see Figure 1), a 11B NMR signal at δ 61, and two sets
We have now found examples where 2,3-dihydroborole
systems can be obtained in a very convenient way under mild
reaction conditions by a 1,1-carboboration route13,14 starting
from simple substituted alkynes.
Received: January 26, 2012
Published: March 14, 2012
© 2012 American Chemical Society
2445
dx.doi.org/10.1021/om300065m | Organometallics 2012, 31, 2445−2451

Organometallics
Article
1
Figure 1. H NMR spectrum (500 MHz, [D6]-benzene*, 298 K) of the 1,1-carboboration product Z-9a.
from the −B(C6F5)2 group and the adjacent (migrated) −C6F5
substituent.
lines the emerging synthetic potential of these advanced 1,1-
carboboration reactions.14,17 The obtained alkenylborane Z-9a
is a powerful Lewis acid itself. It forms a frustrated Lewis pair18
t
with the bulky Bu3P phosphane Lewis base. This FLP rapidly
Scheme 1. Synthesis of Z-9a in a 1,1-Carboboration Reaction
activates dihydrogen under mild conditions to form the
hydridoborate/phosphonium salt 10 (for details, including
the X-ray crystal structure analysis of 10, see the Supporting
Information).
Scheme 2. Activation of Dihydrogen by the FLP Z-9a/PtBu3
Compound Z-9a was characterized by X-ray diffraction (see
Figure 2). The X-ray crystal structure analysis shows that the
The reaction of dicyclopropylacetylene (8b) with B(C6F5)3
(7) takes a slightly different course. The mixture of 8b and 7
was dissolved in dichloromethane and stirred at room
temperature overnight. This resulted in a 1:1 mixture of the
products 11 and 12b (see Scheme 3).19 From the mixture we
Scheme 3. Preparation of 11, 13, and 14
Figure 2. View of the molecular structure of the 1,1-carboboration
product Z-9a.
boron Lewis acid 7 had added to a former acetylene carbon
atom and shifted one of its C6F5 substituents to the same
carbon center (B1−C2: 1.529(3) Å, C2−C31: 1.488(3) Å,
angle B1−C2−C31 117.2(2)°). In order to allow the 1,1-
carboboration sequence to take place, one of the former
acetylene substituents (probably H) had to undergo a 1,2-
migration along the acetylene −CC− backbone. Con-
sequently, we find both the H and the cyclopropyl substituent
bonded to the other olefinic carbon atom (C3) of the product
(C2−C3: 1.354(3) Å). The C6F5 and the cyclopropyl groups
are found in a Z-1,2-arrangement at the central olefinic bond
(angle C31−C2−C3: 121.8(2)°, C2−C3−C4: 127.4(2)°).
The product Z-9a could formally be regarded as a
hydroboration product of 1-cyclopropyl-2-pentafluorophenyl-
acetylene. Its straightforward formation from 8a by the 1,1-
carboboration/photochemical isomerization sequence under-
identified the open product 12b spectroscopically. It features
the H NMR signals of the terminal trans-propenyl group at δ
1
5.50/5.88 (3JHH = 13.9 Hz) and δ 1.10 (CH3) in addition to the
typical cyclopropyl resonances. We find 19F NMR signals of the
single carbon-bound C6F5 group and of the −B(C6F5)2
substituent. The 11B NMR signal of 12b occurs at ca. δ 66.
The dihydroborole product 11 features a 11B NMR signal at δ
70. It shows the 19F NMR signals of three different C6F5 groups
and 1H NMR signals of the saturated endocyclic −CH(CH3)−
CH(C6F5)− unit at δ 2.43/3.11 and 0.95 (CH3). The 1,2-trans-
arrangement of the CH3/C6F5 substituents was secured by the
X-ray crystal structure analysis of a derivative (14, see below).
2446
dx.doi.org/10.1021/om300065m | Organometallics 2012, 31, 2445−2451

Organometallics
Article
We found that heating of this mixture resulted in a clean
rearrangement of the open product 12b to a dihydroborole
isomer 13. Therefore, we carried out the reaction of 8b with the
borane 7 in toluene solution at 80 °C (20 h). After removal of
the toluene solvent the pure dihydroborole product 11 was
precipitated with pentane and isolated in 28% yield. The
isomeric dihydroborole 13 was recovered from the pentane
solution, admixed with a minor amount of 11 (∼30%, 13: 11 ≈
3:1). Compound 13 was spectroscopically characterized (for
details see the Experimental Section and the Supporting
Information).
Scheme 4. Possible Pathways for the Formation of 11 and 13
We treated the isolated pure dihydroborole 11 with a slight
excess of tert-butylisocyanide (CH2Cl2, rt, overnight) to get the
1:1 adduct 14 in 96% yield. Single crystals of this compound
suitable for X-ray crystal structure analysis (see Figure 3) were
in toluene to give the 2,3-dihydroborole product 16. Single
crystals of 16 were obtained from pentane. The X-ray crystal
structure analysis confirmed the formation of the five-
membered boron-heterocycle by a consecutive 2-fold C6F5
migration from boron to carbon. Consequently, the −SiMe3
has undergone a 1,2-shift along the C1−C2 vector (1.362(3)
Å). The boron atom is planar tricoordinate (sum of the C−B−
C bond angles: 359.8°). In contrast to the related compound
14, the 2,3-dihydroborole 16 features the pair of vicinal CH3
and C6F5 substituents at the C3−C4 single bond in cis
positions (see Scheme 5 and Figure 4).
Scheme 5. Synthesis of 2,3-Dihydroborole 16 Starting from
Enyne 15c
Figure 3. View of the molecular structure of the dihydroborole/
isocyanide adduct 14.
obtained from CD2Cl2/cyclopentane by the diffusion method.
t
Compound 14 features a single diastereomer of the BuNC
adduct of the dihydroborole 11 in the crystal. It exhibits a
slightly envelope-shaped unsaturated five-membered hetero-
cyclic framework with bond lengths B1−C1 = 1.621(3) Å and
C1−C2 = 1.335(3) Å, a C1−B1−C4 angle of 98.2(1)°, and
dihedral angles B1−C1−C2−C3 = 1.4(2)° and C1−C2−C3−
C4 = 16.9(2)°. The 3-CH3, 4-C6F5, and B-C6F5 substituents are
each oriented 1,2-trans to each other. The C1−C2 carbon−
carbon double bond bears a C6F5 substituent (at C1 adjacent to
boron) and an intact cyclopropyl group (at C2). The linear
tBuNC ligand is attached at the endocyclic dihydroborole
boron atom (B1−C5: 1.611(3) Å, C5−N1: 1.137(2) Å, angles
B1−C5−N1: 177.3(2)°, C5−N1−C6: 172.7(2)°).
In solution compound 16 features the 19F NMR signals of
three different C6F5 groups as well as H NMR signals of the
1
methyl (δ 0.99, 3 H) and −SiMe3 substituents (δ 0.10, 9 H,
2JSiH = 6.7 Hz). We obtained some information about the
The observation of the formation of the dienylborane
product 12b indicates that one of the cyclopropyl substituents
of the substrate 8b was opened during the reaction (see
Scheme 4).19 The isomer 12a was not observed; it apparently
reacted further to the eventually isolated dihydroborole product
11. This reaction represents a rare example of a 1,1-
carboboration reaction of an alkene substrate.9,20 The open
product 12b eventually undergoes the analogous cyclization
reaction by thermally induced intramolecular alkene 1,1-
carboboration upon heating.
These results suggested that it should be possible to prepare
2,3-dihydroboroles by reacting a conjugated enyne with, for
example, B(C6F5)3. This is indeed the case. The reaction of the
substituted enyne starting material 15c with one molar
equivalent of B(C6F5)3 (7) was carried out at 70 °C overnight
Figure 4. Molecular structure of 2,3-dihydroborole 16.
2447
dx.doi.org/10.1021/om300065m | Organometallics 2012, 31, 2445−2451

Organometallics
Article
overnight (UV light; HPK 125, Pyrex filter). When concentrating the
reaction mixture (ca. 10 mL), the borane Z-9a precipitated as a white
solid. The product was isolated after 10 min at −78 °C via cannula
filtration. Finally the product was washed with cold n-pentane (5 mL)
and dried under vacuum. (246 mg, 0.43 mmol, 63%, colorless solid).
Colorless crystals suitable for X-ray crystal structure analysis were
obtained by slow evaporation of a solution of compound Z-9a in n-
pentane. Anal. Calcd for C23H6BF15 (M = 578.08 g/mol): C, 47.79; H,
alleged intermediate 12c by carrying out the reaction of 15c
with B(C6F5)3 (7) in [D6]-benzene at ambient temperature in
an NMR experiment. 12c is characterized by H NMR signals
of the olefinic CH2 group at δ 4.62, the methyl group (δ
1.50), and the −SiMe3 moiety (δ −0.04). It features a 11B NMR
signal at δ 61 and the 19F NMR signals for two different C6F5
groups in the ratio of 2:1. Eventually heating of the solution
containing 12c to 70 °C resulted in the formation of 16.
1
1
1.05. Found: C, 47.28; H, 0.21. Mp: 137 °C. H NMR (500 MHz,
3
[D6]-benzene, 298 K): δ 6.36 (d, JHH = 11.0 Hz, 1H, CH), 1.12
CONCLUSIONS
(br, 1H, CH), 0.66 (m, 2H, CH2(trans)), 0.42 (m, 2H, CH2(cis)).
■
13C{1H} NMR (126 MHz, [D6]-benzene, 298 K): δ 178.2 (CH),
Our study shows that 2,3-dihydroboroles can rather easily
become available by the advanced variants of the 1,1-
carboboration reaction. The use of strongly electrophilic
RB(C6F5)2-type boranes has made this reaction type very
attractive for carrying out reaction sequences that require
specific C−C bond formation. In addition, our example shows
that carbon−carbon σ-bond activation,14a like that in the
reaction sequence leading to 12a,b, is observed more often in
these advanced 1,1-carboboration reactions. This indicates that
1,1-carboboration may be of increasing importance in achieving
progress in the selective rupture of nonactivated C−C single
bonds and their utilization as “reactive groups” in synthesis, like
that taking place in the described dihydroborole synthesis.
1
1
146.2 (dm, JFC ≈ 246 Hz, m-BC6F5), 144.0 (dm, JFC ≈ 245 Hz,
m-CC6F5), 143.3 (dm, 1JFC ≈ 258 Hz, p-BC6F5), 140.7 (dm, 1JFC ≈ 255
Hz, p-CC6F5), 137.8 (dm, 1JFC ≈ 251 Hz, o-CC6F5), 137.6 (dm, 1JFC
≈
254 Hz, o-BC6F5), 130.8 (br, CB), 114.9 (tm, JFC = 19.4 Hz,
i-CC6F5), 113.5 (br, i-BC6F5), 17.9 (CH), 12.4 (CH2). 11B{1H} NMR
(160 MHz, [D6]-benzene, 298 K): δ 61 (ν1/2 ≈ 1000 Hz). 19F NMR
(470 MHz, [D6]-benzene, 298 K): δ −130.5 (m, 4F, o-BC6F5), −139.2
2
(m, 2F, o-CC6F5), −147.2 (t, JFF = 20.7 Hz, 2F, p-BC6F5), −154.4 (t,
3
3JFF = 21.4 Hz, 1F, p-CC6F5), −160.4 (m, 4F, m-BC6F5), −161.6 (m,
2F, m-CC6F5), [Δδ19Fm,p = 13.2].
X-ray crystal structure of Z-9a: formula C23H6BF15, M = 578.09,
colorless crystal 0.45 × 0.20 × 0.15 mm, a = 8.8154(3) Å, b =
12.9050(6) Å, c = 19.3331(5) Å, V = 2199.39(14) Å3, ρcalc = 1.746 g
cm−3, μ = 1.743 mm−1, empirical absorption correction (0.508 ≤ T ≤
0.780), Z = 4, orthorhombic, space group P212121 (No. 19), λ =
1.54178 Å, T = 223(2) K, ω and φ scans, 11 561 reflections collected
( h, k, l), [(sin θ)/λ] = 0.60 Å−1, 3764 independent (Rint = 0.036)
and 3604 observed reflections [I ≥ 2σ(I)], 352 refined parameters, R =
0.032, wR2 = 0.087, Flack parameter 0.00(10), max. (min.) residual
electron density 0.12 (−0.17) e Å−3, hydrogen atoms calculated and
refined as riding atoms.
EXPERIMENTAL SECTION
■
General Information. All reactions were carried out under an
argon atmosphere with Schlenk-type glassware or in a glovebox.
Solvents (including deuterated solvents used for NMR spectroscopy)
were dried and distilled under argon prior to use (for more
information see the Supporting Information). The following instru-
ments were used for physical characterization of the compounds.
Elemental analyses: Foss-Heraeus CHNO-Rapid. NMR: Bruker AC
200P (11B: 64 MHz), Bruker AV300 (1H: 300 MHz, 19F: 282 MHz),
Varian 500 MHz INOVA (1H: 500 MHz, 13C: 126 MHz, 11B: 160
MHz, 19F: 470 MHz, 29Si: 99 MHz), Varian UNITY plus NMR
spectrometer (1H: 600 MHz, 13C: 151 MHz, 11B: 193 MHz, 19F: 564
MHz, 31P: 243 MHz). Assignments of the resonances are supported by
2D experiments. All NMR spectra were recorded at room temperature
unless otherwise noted. Melting points: DSC 2010 (TA-Instruments)
apparatus, determined by the baseline method. Infrared spectroscopy:
Varian 3100 FT-infrared spectroscopy (Excalibur Series) spectrometer.
X-ray diffraction: Data sets were collected with a Nonius KappaCCD
diffractometer. Programs used: data collection COLLECT (Nonius
B.V., 1998), data reduction Denzo-SMN (Z. Otwinowski, W. Minor,
Methods Enzymol. 1997, 276, 307−326), absorption correction Denzo
(Z. Otwinowski, D. Borek, W. Majewski, W. Minor, Acta Crystallogr.
2003, A59, 228−234), structure solution SHELXS-97 (G. M.
Sheldrick, Acta Crystallogr. 1990, A46, 467−473), structure refinement
SHELXL-97 (G. M. Sheldrick, Acta Crystallogr. 2008, A64, 112−122),
graphics XP (BrukerAXS, 2000). Graphics show thermal ellipsoids
with 50% probability, R values are given for the observed reflections,
and wR2 values for all independent ones.
Characterization of Vinylborane E-9a. NMR Scale. The E-
isomer E-9a was characterized by NMR spectroscopy of a
mixture of isomers Z-9a and E-9a. Therefore tris-
(pentafluorophenyl)borane (7, 26 mg, 0.050 mmol, 1.0
equiv) was dissolved in [D6]-benzene (0.7 mL), and cyclo-
propylacetylene (8a, 3.8 μL, 3.0 mg, 0.050 mmol, 1.0 equiv)
was added in the dark. The reaction mixture was transferred
into an NMR tube, which was sealed immediately in an argon
atmosphere. Z-9a and E-9a were obtained in a ratio of Z-9a:E-
1
9a = 2:1 (determined from the H NMR spectrum; admixed
with a compound not identified yet). H NMR (500 MHz,
1
3
[D6]-benzene, 299 K): δ 5.83 (d, JHH = 11.2 Hz, 1H, CH),
1.16 (br, 1H, CH), 0.38 (m, 2H, CH2(trans)), 0.31 (m, 2H,
CH2(cis)). 13C{1H} NMR (126 MHz, [D6]-benzene, 299 K): δ
168.3 (CH), 127.5 (CB)1, 16.8 (CH), 11.0 (CH2), [C6F5
1
not listed; from the GHMBC experiment]. 19F NMR (470
MHz, [D6]-benzene, 299 K): δ −129.3 (m, 4F, o-BC6F5),
−142.9 (m, 2F, o-CC6F5), −145.1 (m, 2F, p-BC6F5), −156.0 (t,
3JFF = 21.4 Hz, 1F, p-CC6F5), −160.5 (m, 4F, m-BC6F5), −162.2
19
(m, 2F, m-CC6F5), [Δδ
F
m,p
= 15.4].
Preparation of Compound 10. Both Z-9a (28.9 mg, 50.0 μmol,
1.00 equiv) and tri(tert-butyl)phosphane (10.1 mg, 50.0 μmol, 1.00
equiv) were dissolved in n-pentane (each 2 mL) separately and then
mixed together. The turbid reaction mixture was stirred for 30 min at
ambient temperature until the mixture became clear again.
Subsequently, the flask was evacuated, flushed with dihydrogen
(p(H2) = 2.5 bar), and stirred for 24 h. The colorless precipitate was
isolated via cannula filtration and washed with n-pentane (2 × 2 mL).
After drying under high vacuum, compound 10 was isolated as a
colorless solid (31 mg, 40 μmol, 80%). Single crystals suitable for an
X-ray diffraction analysis were obtained by slow diffusion of n-pentane
Materials. All reagents were obtained commercially and used
without further purification unless otherwise noted. Cyclopropylace-
tylene (8a) was obtained from Aldrich and dried over 4 Å molecular
sieves. B(C6F5)3 (7) was prepared according to procedures reported in
the literature (caution: the LiC6F5 intermediate involved is
explosive).15 Following a modified literature procedure, dicyclopropyl-
acetylene (8b) was prepared using commercially available 5-chloro-1-
pentyne.16 Also, trimethyl-(3-methylbut-3-en-1-ynyl)silane (15c) was
obtained from commercially available 2-methyl-1-buten-3-yne follow-
ing a literature procedure.21
Preparation of Vinylborane Z-9a. Tris(pentafluorophenyl)-
borane (7, 300 mg, 0.59 mmol, 1.0 equiv) was dissolved in n-pentane
(25 mL). After the addition of cyclopropylacetylene (8a, 50 μL, 39 mg,
0.59 mmol, 1.0 equiv) the yellow reaction mixture was stirred at
ambient temperature for 4 h. Subsequently, the mixture was irradiated
1
into a solution of 10 in dichloromethane at −35 °C. H NMR (600
MHz, CD2Cl2, 298 K): δ 5.32 (d, 1JPH = 433.8 Hz, 1H, PH), 4.77 (d,
3JHH = 9.3 Hz, 1H, CH), 3.20 (br 1:1:1:1 q, 1JBH ≈ 94 Hz, 1H, BH),
1.64 (d, 3JPH = 15.7 Hz, 27H, CH3), 0.98 (m, 1H, CH), 0.50 (m, 2H,
CH2(trans)), 0.22 (m, 2H, CH2(cis)). 13C{1H} NMR (151 MHz, CD2Cl2,
2448
dx.doi.org/10.1021/om300065m | Organometallics 2012, 31, 2445−2451

Organometallics
Article
3
298 K): δ 211.8 (CB), 138.1 (CH), 38.0 (d, 1JPC = 27.0 Hz, CtBu),
(m, 2F, o-CC6F5), −149.0 (t, JFF = 19.2 Hz, 2F, p-BC6F5), −155.2 (t,
3JFF = 21.5 Hz, 1F, p-CC6F5), −162.4 (m, 4F, m-BC6F5), −163.1 (m,
2F, m-CC6F5), [Δδ19Fm,p = 13.4].
30.3 (CH3), 13.1 (CH), 6.4 (CH2), [C6F5 not listed]. 11B NMR (192
1
MHz, CD2Cl2, 298 K): δ −40.0 (br d, JBH ≈ 92 Hz). 11B{1H} NMR
(192 MHz, CD2Cl2, 298 K): δ −40.0 (ν1/2 ≈ 50 Hz). 19F NMR (564
MHz, CD2Cl2, 298 K): δ −131.3 (m, 4F, o-BC6F5), −140.8 (m, 2F,
Characterization of 2,3-Dihydroborole 13. Both tris-
(pentafluorophenyl)borane (7, 512 mg, 1.00 mmol, 1.00 equiv) and
dicyclopropylacetylene (8b, 106 mg, 1.00 mmol, 1.00 equiv) were
dissolved in toluene (each 3 mL) separately and then mixed together.
Subsequently, the orange solution was heated at 80 °C for 20 h. After
removal of the solvent in vacuo, the residue was dissolved in n-pentane
(5 mL) and the mixture was dried again. This procedure was repeated
four times. Thereupon, n-pentane (5 mL) was added and a colorless
precipitate was formed. At −10 °C the solution was filtered off and the
reddish residue was washed with cold n-pentane (4 × 5 mL). After
evaporation of the solvent, a mixture of 13 and 11 in a ratio of 3:1 was
obtained as an orange, highly viscous oil (192 mg, 0.311 mmol, 31%).
NMR scale: 2,3-Dihydroborole 13 was characterized by NMR
spectroscopy of a mixture of the 2,3-dihydroboroles 11 and 13.
Therefore tris(pentafluorophenyl)borane (7, 51.2 mg, 0.100 mmol,
1.00 equiv) was dissolved in [D6]-benzene (0.7 mL) and
dicyclopropylacetylene (8b, 10.6 mg, 0.100 mmol, 1.00 equiv) was
added. The reaction mixture was transferred into an NMR tube, which
was sealed immediately in an argon atmosphere. 11 and 13 were
obtained in a ratio of 11:13 = 1:1 (determined in 1H NMR). 1H NMR
(500 MHz, [D6]-benzene, 298 K): δ 2.36, 2.12 (AB, each d, each 2JHH
3
3
o-CC6F5), −164.2 (t, JFF = 20.9 Hz, 1F, p-CC6F5), −164.9 (t, JFF
=
20.4 Hz, 2F, p-BC6F5), −166.6 (m, 2F, m-CC6F5), −167.7 (m, 4F,
m-BC6F5), [Δδ
F
m,p
= 2.8]. 31P NMR (243 MHz, CD2Cl2, 298 K): δ
19
58.7 (d28tet, JPH = 433.8 Hz, JPH = 15.7 Hz). 31P{1H} NMR (243
1
3
MHz, CD2Cl2, 298 K): δ 58.7 (ν1/2 ≈ 5 Hz).
X-ray crystal structure analysis of 10: formula C35H35BF15P, M
= 782.41, colorless crystal 0.25 × 0.20 × 0.17 mm, a = 9.9747(8) Å, b
= 12.2460(13) Å, c = 16.1369(17) Å, α = 87.714(12)°, β = 75.686(9)
°, γ = 72.078(8)°, V = 1815.9(3) Å3, ρcalc = 1.431 g cm−3, μ = 1.604
mm−1, empirical absorption correction (0.690 ≤ T ≤ 0.772), Z = 2,
triclinic, space group P1 (No. 2), λ = 1.54178 Å, T = 223(2) K, ω and
̅
φ scans, 19 519 reflections collected ( h, k, l), [(sin θ)/λ] = 0.60
Å−1, 6210 independent (Rint = 0.039) and 5596 observed reflections [I
≥ 2σ(I)], 484 refined parameters, R = 0.076, wR2 = 0.229, max. (min.)
residual electron density 1.31 (−0.46) e Å−3, cation disordered and
refined with thermal restraints, hydrogen atoms at P and B from
difference Fourier calculations, others calculated and refined as riding
atoms.
Preparation of 2,3-Dihydroborole 11. Both tris-
(pentafluorophenyl)borane (7, 512 mg, 1.00 mmol, 1.00 equiv) and
dicyclopropylacetylene (8b, 106 mg, 1.00 mmol, 1.00 equiv) were
dissolved in toluene (each 3 mL) separately and then mixed together.
Subsequently, the orange solution was heated at 80 °C for 20 h. After
removal of the solvent in vacuo, the residue was dissolved in n-pentane
(5 mL) and the mixture was dried again. This procedure was repeated
four times. Thereupon, n-pentane (5 mL) was added and a colorless
precipitate was formed, which was isolated via cannula filtration at −10
°C and thoroughly washed with n-pentane (5 × 3−5 mL). Finally, the
obtained product 11 was dried under vacuum (173 mg, 0.280 mmol,
28%, colorless solid). Single crystals suitable for an X-ray diffraction
analysis were obtained by slow evaporation of a mixture of 11 and 13
in n-pentane (for details see the Supporting Information). Anal. Calcd
for C26H10BF15 (M = 618.14 g/mol): C, 50.52; H, 1.63. Found: C,
C
= 18.5 Hz, each 1H, CH2), 1.44 (t, J = 2.3 Hz, 3H, CH3), 1.42 (m,
1H, CH), 0.64, 0.58 (each m, Σ 4H, CH2). 13C{1H} NMR (126 MHz,
[D6]-benzene, 298 K): δ 192.9 (C), 131.3 (CB), 51.1 (CCH2),
37.6 (br, MeCB), 20.7 (m, CH3), 16.7 (CH), 10.7, 10.5 (CH2), [C6F5
not listed]. 19F NMR (470 MHz, [D6]-benzene, 298 K): δ −128.9 (m,
a
b
2F, o-BC6F5), −139.4 (m, 2F, o-CC6F5 ), −141.0 (m, 1F, o-CC6F5 ),
b
−141.1 (m, 1F, o′-CC6F5 ), −148.3 (m, 1F, p-BC6F5), −156.9 (t, 3JFF
=
b
3
a
21.4 Hz, 1F, p-CC6F5 ), −158.2 (t, JFF = 21.6 Hz, 1F, p-CC6F5 ),
−161.4 (m, 2F, m-BC6F5), −163.3 (m, 2F, m-CC6F5 ), −163.3 (m, 1F,
a
b
b
m-CC6F5 ), −163.6 (m, 1F, m′-CC6F5 ), [Δδ19Fm,p = 13.1].
Preparation of Dihydroborole/Isocyanide Adduct 14. Both
the 2,3-dihydroborole 11 (68.5 mg, 0.110 mmol, 1.00 equiv) and tert-
butyl isocyanide (11 mg, 0.13 mmol, 1.2 equiv) were dissolved in
dichloromethane (each 2 mL) separately and then mixed together.
Subsequently, the colorless solution was stirred overnight at ambient
temperature. After removal of the solvent in vacuo, the colorless
residue was washed with n-pentane (3 mL). Finally, product 14 was
dried under vacuum (74.0 mg, 0.106 mmol, 96%, colorless solid).
Colorless crystals suitable for an X-ray crystal structure analysis were
obtained by slow diffusion of cyclopentane into a solution of
compound 14 in deuterated dichloromethane. Mp: 171 °C. 1H
NMR (600 MHz, [D6]-benzene, 298 K): δ 3.43 (br m, 1H, CHMe),
3.24 (d, 3JHH = 9.5 Hz, 1H, CHC6F5), 1.44 (m, 1H, CH), 1.27 (d, 3JHH
= 6.9 Hz, 3H, CH3), 0.80 (s, 9H, CH3tBu), 0.46 (trans), 0.13 (cis)
(each m, each 1H, CH2), 0.33 (trans), −0.03 (cis) (each m, each 1H,
CH2′). 13C{1H} NMR (151 MHz, [D6]-benzene, 298 K): δ 162.5 (
1
50.00; H, 1.98. Mp: 151 °C. H NMR (500 MHz, [D6]-benzene, 298
3
4
K): δ 3.11 (br, 1H, CHC6F5), 2.43 (qd, JHH = 7.2 Hz, JHH = 2.0 Hz,
1H, CHMe), 1.27 (m, 1H, CH), 0.95 (d, 3JHH = 7.2 Hz, 3H, CH3), 0.65
(trans), 0.44 (cis) (each m, each 1H, CH2), 0.56 (trans), 0.47 (cis)
(each m, each 1H, CH2′). 13C{1H} NMR (126 MHz, [D6]-benzene,
2
298 K): δ 200.1 (CC), 132.9 (br, CB), 116.0 (tm, JFC ≈ 19 Hz,
a
b
i-CC6F5 ), 114.3 (tm, 2JFC ≈ 20 Hz, i-CC6F5 ), 109.5 (br, i-BC6F5), 52.5
(CHMe), 41.7 (br, CHC6F5), 19.9 (CH3), 15.9 (CH), 10.7 (CH2), 8.7
(CH2′), [C6F5 not listed]. 11B{1H} NMR (160 MHz, [D6]-benzene,
298 K): δ 70 (ν1/2 ≈ 1000 Hz). 19F NMR (470 MHz, [D6]-benzene,
b
298 K): δ −130.2 (m, 2F, o-BC6F5), −138.4 (m, 1F, o-CC6F5 ), −142.5
(m, 1F, o′-CC6F5 ), −143.2 (m, 2F, o-CC6F5 ), −146.5 (m, 1F,
b
a
3
b
3
p-BC6F5), −155.6 (t, JFF = 21.6 Hz, 1F, p-CC6F5 ), −156.8 (t, JFF
=
C), 129.3 (CB)1, 127.3 (CN)1, 119.7 (tm, JFC ≈ 17 Hz,
2
a
21.4 Hz, 1F, p-CC6F5 ), −160.0 (m, 2F, m-BC6F5), −161.8 (m, 2F,
i-CC6F5 ), 117.6 (tm, 2JFC ≈ 20 Hz, i-CC6F5 ), 116.2 (br, i-BC6F5), 59.3
(CtBu), 46.7 (br, CHMe), 41.5 (br, CHC6F5), 28.1 (m, CH3tBu), 20.0 (m,
CH3), 12.7 (CH), 6.1 (CH2′), 5.1 (CH2), [not listed: o-, m-, p-C6F5;
1from the GHMBC experiment]. 11B{1H} NMR (192 MHz, [D6]-
benzene, 298 K): δ −13.4 (ν1/2 ≈ 150 Hz). 19F NMR (564 MHz,
[D6]-benzene, 298 K): δ −133.9 (m, 2F, o-BC6F5), −136.9 (m, 1F,
a
b
a
b
b
m-CC6F5 ), −162.3 (m, 1F, m-CC6F5 ), −162.6 (m, 1F, m′-CC6F5 ),
[Δδ19Fm,p = 13.5].
Characterization of Butadienylborane 12b. Both tris-
(pentafluorophenyl)borane (7, 512 mg, 1.00 mmol, 1.00 equiv) and
dicyclopropylacetylene (8a, 106 mg, 1.00 mmol, 1.00 equiv) were
dissolved in dichloromethane (each 3 mL) separately and then mixed
together. Subsequently, the orange solution was stirred overnight at
ambient temperature. After removal of the solvent in vacuo a 1:1
b
a
b
o-CC6F5 ), −140.9 (br, 2F, o-CC6F5 ), −142.0 (m, 1F, o′-CC6F5 ),
−156.2 (t, 3JFF = 20.9 Hz, 1F, p-BC6F5), −158.2 (t, 3JFF = 21.5 Hz, 1F,
b
3
a
p-CC6F5 ), −159.9 (t, JFF = 21.5 Hz, 1F, p-CC6F5 ), −162.7 (m, 1F,
1
1
b
a
m-CC6F5 ), −162.9 (m, 2F, m-BC6F5), −163.4 (br, 2F, m-CC6F5 ),
mixture (determined in H NMR) of 12b and 11 was obtained. H
3
b
−165.1 (m, 1F, m′-CC6F5 ), [Δδ19Fm,p = 6.7].
NMR (500 MHz, [D6]-benzene, 298 K): δ 5.88 (dq, JHH = 13.9 Hz,
3JHH = 6.6 Hz, 1H, CHMe), 5.50 (dm, 3JHH = 13.9 Hz, 1H, CH),
1.24 (m, 1H, CH), 1.10 (dd, 3JHH = 6.6 Hz, 4JHH = 1.5 Hz, 3H, CH3),
0.55 (m, 2H, CH2(trans)), 0.48 (m, 2H, CH2(cis)). 13C{1H} NMR (126
MHz, [D6]-benzene, 298 K): δ 174.1 (C), 143.3 (CMe), 133.6
(CH), 130.6 (CB)1, 17.9 (CH3), 17.6 (CH), 9.7 (CH2), [C6F5
X-ray crystal structure analysis of 14: formula C31H19BF15N, M
= 701.28, colorless crystal 0.30 × 0.30 × 0.27 mm, a = 13.1586(5) Å, b
= 10.5956(3) Å, c = 21.6978(9) Å, β = 94.760(4)°, V = 3014.74(19)
Å3, ρcalc = 1.545 g cm−3, μ = 1.391 mm−1, empirical absorption
correction (0.680 ≤ T ≤ 0.705), Z = 4, monoclinic, space group P21/c
(No. 14), λ = 1.54178 Å, T = 223(2) K, ω and φ scans, 25 924
reflections collected ( h, k, l), [(sin θ)/λ] = 0.60 Å−1, 5240
independent (Rint = 0.042) and 4615 observed reflections [I ≥ 2σ(I)],
not listed; from the GHMBC experiment]. 11B{1H} NMR (64 MHz,
1
[D6]-benzene, 298 K) [mixture]: δ ∼66 (ν1/2 ≈ 2000 Hz). 19F NMR
(470 MHz, [D6]-benzene, 298 K): δ −131.4 (m, 4F, o-BC6F5), −139.3
2449
dx.doi.org/10.1021/om300065m | Organometallics 2012, 31, 2445−2451

Organometallics
Article
437 refined parameters, R = 0.044, wR2 = 0.121, max. (min.) residual
electron density 0.20 (−0.23) e Å−3, hydrogen atoms calculated and
refined as riding atoms.
223(2) K, ω and φ scans, 21 478 reflections collected ( h, k, l),
[(sin θ)/λ] = 0.60 Å−1, 4930 independent (Rint = 0.036) and 4763
observed reflections [I ≥ 2σ(I)], 439 refined parameters, R = 0.041,
wR2 = 0.114, max. (min.) residual electron density 0.29 (−0.32) e Å−3,
hydrogen atoms calculated and refined as riding atoms.
Characterization of Vinylborane 12c. NMR Scale. Both the
enyne 15c (13.8 mg, 0.100 mmol, 1.00 equiv) and tris-
(pentafluorophenyl)borane (7, 51.2 mg, 0.100 mmol, 1.00
equiv) were dissolved in [D6]-benzene (each ca. 0.4 mL)
separately and then mixed together. The yellow reaction
mixture was transferred into an NMR tube, which was sealed in
an argon atmosphere immediately. 12c was obtained in 100%
conversion of the starting materials. 1H NMR (500 MHz, [D6]-
benzene, 299 K): δ 4.62 (m, 2H, CH2), 1.50 (s, 3H, CH3),
−0.04 (s, 2JSiH = 2.5 Hz, 9H, SiMe3). 13C{1H} NMR (126 MHz,
ASSOCIATED CONTENT
* Supporting Information
■
S
Text and figures giving further experimental and spectroscopic
details and crystallographic data. This material is available free
of charge via the Internet at http://pubs.acs.org.
1
[D6]-benzene, 299 K): δ 173.4 (CSi), 148.5 (dm, JFC ≈ 250
AUTHOR INFORMATION
Corresponding Author
*E-mail: erker@uni-muenster.de.
■
1
Hz, C6F5), 147.1 (CMe), 144.6 (dm, JFC ≈ 260 Hz, C6F5),
1
1
144.0 (dm, JFC ≈ 245 Hz, C6F5), 140.7 (dm, JFC ≈ 255 Hz,
C6F5), 138.6 (CB), 137.8 (dm, JFC ≈ 255 Hz, C6F5), 137.7
1
(dm, JFC ≈ 255 Hz, C6F5), 116.0 (m,2JFC ≈ 22 Hz, i-CC6F5),
1
Author Contributions
114.1 (br m, i-BC6F5), 112.4 (CH2), 22.8 (CH3), 0.4 (1JSiC
=
§X-ray crystal structure analyses.
53.2 Hz, SiMe3). 11B{1H} NMR (160 MHz, [D6]-benzene, 299
K): δ ∼61 (ν1/2 ≈ 1400 Hz). 19F NMR (470 MHz, [D6]-
benzene, 299 K): δ −126.7 (m, 4F, o-BC6F5), −136.8 (m, 2F,
Notes
The authors declare no competing financial interest.
o-CC6F5), −143.6 (tm, JFF = 20.9 Hz, 2F, p-BC6F5), −153.5 (t,
3
3JFF = 21.4 Hz, 1F, p-CC6F5), −160.5 (m, 4F, m-BC6F5), −162.0
ACKNOWLEDGMENTS
■
(m, 2F, m-CC6F5), [Δδ
F
m,p
= 16.9]. 29Si{1H} DEPT (99
19
Financial support from the Deutsche Forschungsgemeinschaft
is gratefully acknowledged. We thank Boulder Scientific
Company for a donation of B(C6F5)3.
MHz, [D6]-benzene, 299 K): δ −4.8 (SiMe3).
Preparation of 2,3-Dihydroborole 16. Both enyne 15c (138
mg, 1.00 mmol, 1.00 equiv) and tris(pentafluorophenyl)borane (7,
512 mg, 1.00 mmol, 1.00 equiv) were dissolved in toluene (each 5
mL) separately and then mixed together. Subsequently, the yellow
solution was stirred overnight at 70 °C before the orange reaction
mixture was evaporated. After removal of the solvent in vacuo, the
residue was dissolved in n-pentane (5 mL) and the mixture was dried
again. This procedure was repeated four times. Finally, n-pentane (5
mL) was added, and the solution was cooled to −78 °C and kept for
30 min. During this time the product precipitated and the solution was
removed via cannula. The product was washed with n-pentane (3 × 3
mL) at −78 °C and eventually dried under vacuum (211 mg, 0.320
mmol, 32%, colorless solid). Colorless crystals suitable for X-ray crystal
structure analysis were obtained by slow evaporation of a solution of
compound 16 in n-pentane at ambient temperature. Anal. Calcd for
C26H14BF15Si (M = 650.26 g/mol): C, 48.02; H, 2.17. Found: C,
DEDICATION
■
Dedicated to Professor Friedrich Bickelhaupt on the occasion
of his 80th birthday.
REFERENCES
■
(1) Eisch, J. J.; Hota, N. K.; Kozima, S. J. Am. Chem. Soc. 1969, 91,
4574.
(2) (a) Fagan, P. J.; Burns, E. G.; Calabrese, J. C. J. Am. Chem. Soc.
1988, 110, 2979. (b) Fagan, P. J.; Nugent, W. A.; Calabrese, J. C. J.
Am. Chem. Soc. 1994, 116, 1880.
(3) Killian, L.; Wrackmeyer, B. J. Organomet. Chem. 1978, 148, 137.
́
(4) Braunschweig, H.; Fernandez, I.; Frenking, G.; Kupfer, T. Angew.
Chem. 2008, 120, 1977; Angew. Chem., Int. Ed. 2008, 47, 1951.
(5) (a) So, C.-W.; Watanabe, D.; Wakamiya, A.; Yamaguchi, S.
Organometallics 2008, 27, 3496. (b) Wakamiya, A.; Mishima, K.;
Ekawa, K.; Yamaguchi, S. Chem. Commun. 2008, 579.
(6) Fan, C.; Piers, W. E.; Parvez, M. Angew. Chem. 2009, 121, 2999;
Angew. Chem., Int. Ed. 2009, 48, 2955.
(7) (a) Fan, C.; Piers, W. E.; Parvez, M.; McDonald, R.
Organometallics 2010, 29, 5132. (b) Fan, C.; Mercier, L. G.; Piers,
W. E.; Tuononen, H. M.; Parvez, M. J. Am. Chem. Soc. 2010, 132,
9604.
1
48.14; H, 2.45. Mp: 95 °C. H NMR (500 MHz, CD2Cl2, 298 K): δ
3.83 (br m, 1H, CHMe), 3.66 (br m, 1H, CHC6F5), 0.99 (d, 3JHH = 7.3
2
Hz, 3H, CH3), 0.10 (d, JSiH = 6.7 Hz, 9H, SiMe3). 19F NMR (470
MHz, CD2Cl2, 298 K): δ −130.6 (m, 2F, o-BC6F5), −138.4, −143.0
a
(each br, each 1F, o-CC6F5 ), −140.5, −142.0 (each m, each 1F,
b
3
o-CC6F5b), −149.2 (m, 1F, p-BC6F5), −157.0 (t, JFF = 20.7 Hz, 1F,
3
a
p-CC6F5 ), −157.4 (t, JFF = 20.7 Hz, 1F, p-CC6F5 ), −161.3 (m, 2F,
m-BC6F5), −162.9 (br, 2F, m-CC6F5 ), −163.0, −163.5 (each m, each
a
b
19
1F, m-CC6F5 ), [Δδ
F
= 12.1]. 1H NMR (500 MHz, CD2Cl2, 253
m,p
K): δ 3.79 (br m, 1H, CHMe), 3.63 (br m, 1H, CHC6F5), 0.95 (d, 3JHH
= 7.2 Hz, 3H, CH3), 0.06 (s, JSiH = 6.8 Hz, 9H, SiMe3). 13C{1H}
2
(8) Borole-metal complexes: (a) Herberich, G. E.; Buschges, U.;
̈
NMR (126 MHz, CD2Cl2, 253 K): δ 208.7 (CSi), 145.5 (CB),
Hessner, B.; Luthe, H. J. Organomet. Chem. 1986, 312, 13.
̈
b
a
115.7 (tm, 2JFC ≈ 20 Hz, i-CC6F5 ), 114.9 (tm, 2JFC ≈ 20 Hz, i-CC6F5 ),
109.4 (br m, i-BC6F5), 52.9 (CHMe), 40.5 (br, CHC6F5), 19.2 (CH3),
−1.6 (1JSiC = 52.8 Hz, SiMe3), [not listed o-, p-, m-C6F5]. 19F NMR
(470 MHz, CD2Cl2, 253 K): δ −130.5 (m, 2F, o-BC6F5), −138.4,
(b) Herberich, G. E.; Boveleth, W.; Hessner, B.; Koffer, D. P. J;
̈
Negele, M.; Saive, R. J. Organomet. Chem. 1986, 308, 153.
(c) Herberich, G. E.; Hausmann, I.; Hessner, B.; Negele, M. J.
Organomet. Chem. 1989, 362, 259. (d) Braunschweig, H; Chiu, C.-W.;
Radacki, K.; Brenner, P. Chem. Commun. 2010, 46, 916. See also:
(e) Braunschweig, H.; Kupfer, T. Chem. Commun. 2011, 47, 10903.
(9) (a) Clark, G. M.; Hancock, K. G.; Zweifel, G. J. Am. Chem. Soc.
a
−142.9 (each m, each 1F, o-CC6F5 ), −140.7, −142.0 (each m, each
b
1F, o-CC6F5 ), −149.0 (m, 1F, p-BC6F5), −156.7 (t, 3JFF = 21.0 Hz, 1F,
b
3
a
p-CC6F5 ), −157.3 (t, JFF = 21.0 Hz, 1F, p-CC6F5 ), −161.0 (m, 2F,
a
m-BC6F5), −162.5, −162.9 (each m, each 1F, m-CC6F5 ), −162.7,
1971, 93, 1308. See also: (b) Herberich, G. E.; Hessner, B.; Sohnen,
̈
b
19
−163.2 (each m, each 1F, m-CC6F5 ), [Δδ
F
= 12.0]. 29Si{1H}
D. J. Organomet. Chem. 1982, 233, C35.
m,p
(10) (a) Fujita, K.; Ohnuma, Y; Yasuda, H.; Tani, H. J. Organomet.
Chem. 1976, 113, 201. (b) Wreford, S. S.; Whitney, J. F. Inorg. Chem.
1981, 20, 3918. (c) Yasuda, H.; Kajihara, Y.; Mashima, K.; Nagasuna,
K.; Lee, K.; Nakamura, A. Organometallics 1982, 1, 388. (d) Dorf, U.;
Engel, K; Erker, G. Organometallics 1983, 2, 462. (e) Herberich, G. E.;
DEPT (99 MHz, CD2Cl2, 253 K): δ −5.6 (SiMe3).
X-ray crystal structure analysis of 16: formula C26H14BF15Si·1/
2C5H12, M = 686.35, colorless crystal 0.35 × 0.28 × 0.27 mm, a =
7.4428(2) Å, b = 12.5739(10) Å, c = 16.1379(5) Å, α = 92.161(3)°, β
= 102.917(2)°, γ = 100.476(4)°, V = 1442.67(13) Å3, ρcalc = 1.580 g
cm−3, μ = 1.810 mm−1, empirical absorption correction (0.570 ≤ T ≤
Boveleth, W.; Heßner, B.; Hostalek, M.; Koffer, D. P. J; Ohst, H.;
̈
0.641), Z = 2, triclinic, space group P1 (No. 2), λ = 1.54178 Å, T =
Sohnen, D. Chem. Ber. 1986, 119, 420. (f) Herberich, G. E.; Marx, H.-
̈
̅
2450
dx.doi.org/10.1021/om300065m | Organometallics 2012, 31, 2445−2451

Organometallics
Article
W.; Wagner, T. Chem. Ber. 1994, 127, 2135. (g) Herberich, G. E.;
Wagner, T.; Marx, H.-W. J. Organomet. Chem. 1995, 502, 67. See also:
(h) Bromm, D.; Stalke, D.; Heine, A.; Meller, A.; Sheldrick, G. M. J.
Organomet. Chem. 1990, 386, 1.
(11) Herberich, G. E.; Hessner, B.; Sohnen, D. J. Organomet. Chem.
̈
1983, 256, C23.
(12) For more complicated examples see e.g.: (a) Wrackmeyer, B.
Organometallics 1984, 3, 1. (b) Khan, E.; Kempe, R.; Wrackmeyer, B.
Appl. Organometal. Chem. 2009, 23, 204.
(13) Wrackmeyer, B. Heteroat. Chem. 2006, 17, 188 , and references
therein.
(14) (a) Chen, C.; Kehr, G.; Frohlich, R.; Erker, G. J. Am. Chem. Soc.
̈
2010, 132, 13594. , and references therein. (b) Chen, C.; Eweiner, F.;
Wibbeling, B.; Frohlich, R.; Senda, S.; Ohki, Y.; Tatsumi, K.; Grimme,
̈
S.; Kehr, G.; Erker, G. Chem. Asian J. 2010, 5, 2199. (c) Chen, C.;
Frohlich, R.; Kehr, G.; Erker, G. Chem. Commun. 2010, 46, 3580.
̈
(d) Chen, C.; Voss, T.; Frohlich, R.; Kehr, G.; Erker, G. Org. Lett.
̈
2011, 13, 62.
(15) (a) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245.
(b) Wang, C.; Erker, G.; Kehr, G.; Wedeking, K.; Frohlich, R.
̈
Organometallics 2005, 24, 4760.
(16) (a) Militzer, H.-C.; Schomenauer, S.; Otte, C.; Puls, C.; Hain, J.;
̈
Brase, S.; de Meijere, A. Synthesis 1993, 998. (b) Scott, L. T.; Cooney,
̈
M. J.; Otte, C.; Puls, C.; Haumann, T.; Boese, R.; Carroll, P. J.; Smith
III, A. B.; de Meijere, A. J. Am. Chem. Soc. 1994, 116, 10275.
(17) Jiang, C.; Blacque, O.; Berke, H. Organometallics 2010, 29, 125.
(18) Stephan, D. W.; Erker, G. Angew. Chem. 2010, 122, 50; Angew.
Chem., Int. Ed. 2010, 49, 46.
(19) For comparison: Tamaki, T.; Nagata, M.; Ohashi, M.; Ogoshi,
S. Chem.Eur. J. 2009, 15, 10083 , and references therein.
(20) (a) Wrackmeyer, B.; Tok, O. L. Appl. Organomet. Chem. 2007,
21, 531. (b) Xu, B.-H.; Kehr, G.; Frohlich, R.; Erker, G. Chem.Eur. J.
̈
2010, 16, 12538.
(21) Waser, J.; Gonzal
Carreira, E. M. Org. Lett. 2005, 7, 4249.
́ ́
ez-Gomez, J. C.; Nambu, H.; Huber, P.;
2451
dx.doi.org/10.1021/om300065m | Organometallics 2012, 31, 2445−2451