Reactions of bis(alkynyl)silanes with HB(C6F5) 2: Formation of boryl-substituted silacyclobutene derivatives

Article abstract of DOI:10.1016/j.jorganchem.2010.09.024

Treatment of R₂Si(CC-SiMe₃)₂ [1a (Me), 1b (Ph)] with HB(C₆F₅)₂ at low temperature (253 K (a), 273 K (b)) gives the -B(C₆F₅)₂ substituted silacyclobutene products (4a,b) under kinetic control. Upon warming to room temperature they disappear to form the thermodynamically favoured isomeric silole derivatives (2a,b). Similar treatment of Me₂Si(CC-R ¹)₂ [5a (R¹ = Ph), 5b (R¹ = tert-butyl) with HB(C₆F₅)₂ at room temperature gave the stable -B(C₆F₅)₂ substituted silacyclobutene derivatives 6 and 7, respectively. Subsequent photolysis resulted in a Z- to E-isomerization of the substituted exocyclic CC double bonds in these products. The silacyclobutene derivative E-6 was characterized by an X-ray crystal structure analysis.

Full text of DOI:10.1016/j.jorganchem.2010.09.024

Journal of Organometallic Chemistry 696 (2011) 1184e1188
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Reactions of bis(alkynyl)silanes with HB(C6F5)2: Formation of boryl-substituted
silacyclobutene derivatives
*
Juri Ugolotti, Gereon Dierker, Roland Fröhlich, Gerald Kehr, Gerhard Erker
Organisch-Chemisches Institut der Universität Münster, Corrensstrasse 40, 48149 Münster, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Treatment of R2Si(C^C-SiMe3)2 [1a (Me), 1b (Ph)] with HB(C6F5)2 at low temperature (253 K (a), 273 K
(b)) gives the eB(C6F5)2 substituted silacyclobutene products (4a,b) under kinetic control. Upon warming
to room temperature they disappear to form the thermodynamically favoured isomeric silole derivatives
(2a,b). Similar treatment of Me2Si(C^CeR1)2 [5a (R1 ¼ Ph), 5b (R1 ¼ tert-butyl) with HB(C6F5)2 at room
temperature gave the stable eB(C6F5)2 substituted silacyclobutene derivatives 6 and 7, respectively.
Subsequent photolysis resulted in a Z- to E-isomerization of the substituted exocyclic C]C double bonds
in these products. The silacyclobutene derivative E-6 was characterized by an X-ray crystal structure
analysis.
Received 27 July 2010
Received in revised form
7 September 2010
Accepted 7 September 2010
Keywords:
Silicon
Boron
Ó 2010 Published by Elsevier B.V.
Hydroboration
Carboboration
Photolysis
1. Introduction
2. Results and discussion
We had recently shown that the bis(trimethylsilylethynyl)
silanes 1a (R ¼ CH3) and 1b (R ¼ Ph) reacted readily with Piers’
borane [HB(C6F5)2] at room temperature to yield the eB(C6F5)2
substituted silole derivatives 2a and 2b, respectively (see Chart 1)
[1]. These products are probably formed by means of a reaction
sequence formally involving a 1,1-hydroboration followed by 1,1-
vinylboration after a subsequent alkynyl shift from silicon to boron,
similarly as it was previously proposed by Wrackmeyer et al. for
many related examples employing less electrophilic boranes [2,3].
We could then show that the silole derivatives 2a,b undergo a clean
rearrangement reaction to yield the isomeric products 3a,b upon
photolysis (HPK 125, Pyrex filter). This photoreaction follows
Bis(trimethylsilylethynyl)dimethylsilane 1a [1.3b] was mixed
with 1 M equivalent of HB(C6F5)2 [5] in d8-toluene at ꢀ78 ꢁC. The
sample was sealed in an NMR tube. The reaction mixture was then
slowly warmed to ꢀ20 ꢁC and the progress of the reaction was
monitored by NMR spectroscopy. The NMR spectra show the
formation of the silacyclobutene derivative 4a under kinetic
control. The assignment was supported by the NMR analysis of the
system (for details see the Supporting information), and its
comparison with the data of the isomers 2a and 3a and a compar-
ison with a differently substituted analogous borylsilacyclobutene
derivative [6] that was characterized by X-ray diffraction (see
below) (Scheme 1).
a pattern reminiscent of the di-
p
-methane rearrangement [4]. We
The eB(C6F5)2 substituted silacyclobutene product 4a features
have now found that the formation of the silole derivatives 2a,b is
preceded by the formation of their boryl-substituted silacyclobu-
tene isomers which can be observed when the reaction of the bis
(alkynyl)silanes (1a,b) with HB(C6F5)2 is performed under kinetic
control at low temperatures. We have also found that formation of
stable silacyclobutene derivatives can become dominant in the
reaction of differently substituted bis(alkynyl)silanes with HB
(C6F5)2. We will describe several such examples in this account.
a set of four 13C NMR resonances of the core at
d 188.6 (br, C3), 173.6
(br, C2), 168.4 (1JSi,C ¼ 52.1 Hz, C4) and 131.5 (1JSi,C ¼ 68.1 Hz, C5)
that are distinctly different from those of both silole isomers 2a [
d
174.2 (br, C3), 159.8 (C2), 150.9 (C4) and 149.9 (C5) in d6-benzene]
and 3a [
d
171.5 (1JSi,C ¼ 61.7 Hz, C3), 181.1 (1JSi,C ¼ 58.3 Hz,
1JSi,C ¼ 42.2 Hz, C2), 172.1 (C4) and 151.6 (br, C5)]. Compound 4a
shows the 1H NMR signal of the olefinic 5-H proton at
d 5.94 with
3
2
a coupling constant of JSi,H ¼ 22.1 Hz and JSi,H ¼ 5.3 Hz (298 K),
respectively [cf. 2a:
d
7.42 (3JSi,H ¼ 16.9 Hz and 7.5 Hz)]. Compound
4a shows three 29Si NMR signals [
d
ꢀ7.7 (5-Si), ꢀ11.1 (2-Si) and 21.8
(SiMe2); cf. 2a:
4a is broad (n1/2 z 2500 Hz) and occurs at
d
ꢀ7.1, ꢀ9.6, 25.6]. The 11B NMR signal of compound
* Corresponding author. Fax: þ49 251 83 36503.
E-mail address: erker@uni-muenster.de (G. Erker).
d
64 and the 19F NMR
0022-328X/$ e see front matter Ó 2010 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2010.09.024

J. Ugolotti et al. / Journal of Organometallic Chemistry 696 (2011) 1184e1188
1185
SiMe3
SiMe3
Me3Si
HB(C6F5)2
R2Si
R2Si
R2Si
B(C6F5)2
H
B(C6F5)2
H
Me3Si
Me3Si
SiMe3
1
a
b
1,1-vinylboration
Me3Si
R = CH3 ( ), Ph ( )
Me3Si
R2Si
SiMe3
H
B(C6F5)2
H
hν
R2Si
(C6F5)2B
Me3Si
3a b
,
2a b
,
Chart 1.
resonances are located at
d
ꢀ126.9 (o), ꢀ143.2 (p) and ꢀ160.7 (m).
The X-ray crystal structure analysis (see Fig. 1) confirms the
formation of the four-membered heterocyclic product. The silacy-
clobutene type core structure contains three sp2-hybridized carbon
atoms. The C2eC3 linkage (1.362(4) Å) is in the typical C]C double
bond range. The C3eC4 C(sp2)-C(sp2) single bond is much longer
(1.495(4) Å) and the exocyclic C4eC5 carbonecarbon double bond
is again short (1.334(4) Å). The central C3Si four-membered ring
unit has a “kite shaped” structure. The Si1eC2 (1.883(3) Å) and
Si1eC4 (1.862(3) Å) vectors comprise an internal bonding angle of
74.6(1)ꢁ (i.e. C2eSi1eC4). Consequently the external C11eSi1eC12
angle is much larger (111.6(2)ꢁ). The remaining angles inside the
central C3Si heterocycle were found at 105.1(2)ꢁ (C2eC3eC4), 88.5
(2)ꢁ (C3eC4eSi1) and 91.7(2)ꢁ (C3eC2eSi1).
Both the chemical shift of the 11B and the observed large 19F NMR
Dd(p, m) shift difference (17.5 ppm) of the C6F5 groups at boron are
typical of a Lewis acidic tricoordinate boron.
The corresponding bis(trimethylsilylethynyl)diphenylsilane
starting material (1b) reacts analogously with HB(C6F5)2 under
kinetic control. In this case we can warm the in situ prepared
sample to 0 ꢁC to observe the formation of the respective bor-
ylsilacyclobutene derivative 4b, which was characterized spectro-
scopically (for details see the Experimental section).
Upon warming the samples containing either 4a or 4b to room
temperature their characteristic NMR signals disappear and we
observed the formation of the previously characterized boryl-silole
derivatives 2a and 2b, respectively [1].
Changing the substituents at the alkynyl groups of the starting
materials allowed us to shift the reaction course completely to the
formation of stable eB(C6F5)2 substituted silacyclobutene products
[7]. We reacted bis(phenylethynyl)dimethylsilane (5a) with HB
(C6F5)2 in toluene. After 1 day at room temperature a 2: 1 mixture of
the products Z-6 and E-6 was isolated in a combined yield of 89%.
We obtained single crystals of the minor isomer E-6 from the
mixture by crystallization from pentane at ꢀ30 ꢁC.
The phenyl substituent at C5 is found trans-oriented to silicon at
the exocyclic C4 ¼ C5 carbonecarbon double bond, which denotes
the product as the E-6 isomer. The plane of the 5-phenyl
SiMe3
Me3Si
R2Si
B(C6F5)2
H
HB(C6F5)2
R2Si
r.t.
toluene
SiMe3
Me3Si
1a b
,
2a b
,
R = CH3 (a)
253K
5
H
r.t. Me3Si
R2Si
r.t.
R = Ph (b)
273K
4
B(C6F5)2
3
2
4a b SiMe3
,
Scheme 1.
Fig. 1. Molecular structure of compound E-6.

1186
J. Ugolotti et al. / Journal of Organometallic Chemistry 696 (2011) 1184e1188
Table 1
a 78% yield of the eB(C6F5)2 substituted silacyclobutene derivative
Z-7 (see Scheme 2, see Table 1 for the NMR data). Photolysis of the
pure Z-7 isomer (toluene, 2 h, r.t.) resulted in Z-7eE-7 isomeriza-
tion. From the workup procedure we isolated an E-7/Z-7 product
mixture of ca. 3.5:1. The spectroscopic characterization of the major
component, the photoproduct E-7, was carried out from this
product mixture (see Table 1). We note that all the compounds 6
Selected NMR data of the eB(C6F5)2 substituted silacyclobutene derivatives 6 and 7.a
E-6
Z-6
E-7
Z-7
13C NMR
172.4
170.9
C2
C3 (br)
C4
168.8
172.0
149.1
129.9
181.0
164.0
139.7
136.0
176.8
166.4
143.4
140.9
149.9
C5
127.8
1H NMR
6.53
and
parameters.
The borylsilacyclobutene product formation by treatment of the
7 have very similar characteristic NMR spectroscopic
5-H
Si1
B1
6.72
13.8
65
5.36
15.9
e
5.47
11.2
64
29Si NMR
14.2
substituted bis(alkynyl)silane starting materials with Piers’ borane
can be rationalized along the lines as previously proposed by
Wrackmeyer et al. for the analogous four-membered ring product
formation of bis(alkynyl)silanes with BR3 reagents [2,6]. In our case
we propose initial 1,2-hydroboration which would lead to the
intermediate {8} (see Scheme 2) followed by intermolecular
alkynyl-transfer from silicon to boron. Subsequent 1,1-vinyl-
boration at the state of {9} would then lead to the observed four-
membered ring products [6].
11B NMR
58
19F NMR
ꢀ126.7
ꢀ146.2
ꢀ162.1
o-F
p-F
m-F
ꢀ127.8
ꢀ143.5
ꢀ160.7
ꢀ125.6
ꢀ144.6
ꢀ161.4
ꢀ127.6
ꢀ144.8
ꢀ161.1
a
NMR spectra in d6-benzene at 298 K.
substituent is markedly rotated away from the adjacent olefinic
plane. The second phenyl substituent is found attached at the ring
carbon C2. This phenyl group is rotated out of the plane of the
central four-membered heterocyclic core. The eB(C6F5)2 substit-
uent is attached at C3 (C3eB1: 1.554(4) Å). The boron atom in
compound E-6 is trigonal planar (sum of CeBeC angles: 360.1(3)ꢁ).
The plane of one of the C6F5 rings (C41eC46) at the boron and the
phenyl substituent at C5 (C51eC56) are oriented close to parallel
(distance between the centroids of theses two rings: 3.411 Å). This
This work shows that reactions of alkynes with main-group
metal substituents with suitable ReB(C6F5)2 reagents [here HB
(C6F5)2] principally follows some of the reaction pathways that
were established or proposed to proceed with the “classical” R3B
trialkylborane reagents, only that the special C6F5 substituted
borane reagents seem to often undergo such reactions much more
easily and under very mild reaction conditions.
3. Experimental section
may possibly indicate the presence of a
between these units [8].
p,p-stacking interaction
3.1. General information
Compounds E-6 and Z-6 show very typical NMR data (see Table 1).
They both feature 11B NMR signals in the typical ReB(C6F5)2
tricoordinate boron range [9]. Consequently, the corresponding 19F
NMR spectra show large Dd(p,m) [16.6 ꢂ 0.7] differences [10]. Both
isomers show similar 13C NMR and 29Si NMR values of the central
C3Si core (see Table 1) [6]. Photolysis of the initially formed Z-6/E-6
mixture (HPK 125, toluene, r.t., 2 h) with UV light leads to a rather
efficient Z-6 to E-6 isomerization. We have isolated a product in 75%
yield that consists of the almost pure E-6 isomer (E-6/Z-6 z 10:1).
The reaction of bis(3,3-dimethyl-1-butynyl)dimethylsilane (5b)
with HB(C6F5)2 in toluene at room temperature (overnight) gave
Reactions with air- and moisture-sensitive compounds were
carried out under an argon atmosphere using Schlenk-type glass-
ware or a glove-box. Solvents were dried with the procedure
reported by Grubbs [11] or, particularly deuterated solvents, were
distilled from appropriate drying agents and stored under an argon
atmosphere. The following instruments were used for physical
characterization of the compounds: NMR: Varian Inova 500 (1H:
500 MHz,
126 MHz,
d
1H(TMS) ¼ 0; 11B: 161 MHz,
d
11B(BF3 ꢃ OEt2) ¼ 0; 13C:
d
13C(TMS) ¼ 0; 19F: 470 MHz,
d
19F(CFCl3) ¼ 0; 29Si:
99 MHz {using DEPT pulse sequence based on JSi,H ¼ 7.0 Hz},
d
29Si
R1
R1
R1
H
H
HB(C6F5)2
B(C6F5)2
B(C6F5)2
Me2Si
Me2Si
Me2Si
r.t.
toluene
R1
R1
R1
5
{8}
{9}
1,1-vinylboration
a
b
R = Ph ( ), tBu ( )
R1
R1
H
H
hν
Me2Si
B(C6F5)2
Me2Si
B(C6F5)2
R1
R1
6
6
R = Ph
R = tBu
E-
(Δ ~ 1 : 5)
Z-
(hν ~ 10 : 1)
(Δ ~ 0 : 1)
E-7
Z-7
(hν ~ 3.5 : 1)
Scheme 2.

J. Ugolotti et al. / Journal of Organometallic Chemistry 696 (2011) 1184e1188
1187
(TMS) ¼ 0). Most NMR assignments were supported by additional
2D experiments. The IR spectra were obtained with a Varian 3100
FT-IR (EXCALIBUR Series) spectrometer, the MS GC-EI with Triple-
quad TSQ (Thermo-Finnigan-MAT, Bremen). Elemental analyses
were performed with a Foss-Heraeus CHN-O-Rapid instrument.
Data set was collected with a Nonius KappaCCD diffractometer,
equipped with a rotating anode generator. Programs used: data
collection COLLECT (Nonius B.V., 1998), data reduction Denzo-SMN
[12], absorption correction Denzo [13], structure solution SHELXS-
97 [14], structure refinement SHELXL-97 [15], graphics SCHAKAL (E.
Keller, Univ. Freiburg, 1997).
1). Crystals of the E isomer suitable for X-ray diffraction were
obtained from a concentrated pentane solution at ꢀ30 ꢁC.
Z-6: 1H NMR (C6D6, 298 K):
d
7.28 (m, 2 H, o-Ph5), 7.11 (m, 2 H,
m-Ph5), 7.04 (m, 4 H, o, m-Ph2), 7.02 (m, 1 H, p-Ph5) 6.91 (m, 1 H, p-
Ph2) 6.90 (m, 2 H, p-Ph2), 6.72 (s, 3JSi,H ¼ 16.3 Hz, 1 H, 5-H), 0.54 (s,
2JSi,H ¼ 6.8 Hz, 6 H, SiMe2). 13C{1H} NMR (C6D6, 298 K):
d 172.0 (br,
C3),168.8 (1JSi,C ¼ 53.8 Hz, C2),149.1 (1JSi,C ¼ 52.9 Hz, C4),147.8 (dm,
1JF,C z 254 Hz, C6F5), 144.7 (dm, 1JF,C z 261 Hz, C6F5), 139.6 (i-Ph5),
138.7 (i-Ph2), 137.7 (dm, 1JF,C z 253 Hz, C6F5), 129.9 (1JSi,C ¼ 66.6 Hz,
C5), 129.0 (m-Ph5), 128.9 (o-Ph2), 128.5 (m-Ph2), 128.4 (p-Ph2), 127.5
(p-Ph5), 126.5 (o-Ph5), 113.3 (br, i-C6F5), ꢀ0.8 (1JSi,C ¼ 47.3 Hz,
SiMe2). 11B{1H} NMR (C6D6, 298 K):
d 65 (br, n
½ z 1900 Hz). 19F
3.1.1. NMR-scale determination of the intermediate of Z-4a and Z-
4b
NMR (C6D6, 298 K):
d
ꢀ127.8 (m, 2 F, o-F), ꢀ143.5 (tt, 3JF,F ¼ 20.7 Hz,
4JF,F ¼ 6.4 Hz, 2 F, p-F), ꢀ160.7 (m, 4 F, m-F) [Ddm,p ¼ 17.2]. 29Si
Z-4a: Bis(trimethylsilylethynyl)dimethylsilane (1a) (20.0 mg,
0.08 mmol), and bis(pentafluorophenyl)borane (27.4 mg,
0.08 mmol) were weighed in an argon-filled glove-box and put
inside an NMR tube. The tube was then cooled to ꢀ78 ꢁC and d8-
toluene (0.8 mL) was added by a syringe under argon. The tube was
then sealed under vacuum and kept at low temperature until the
{DEPT} NMR (C6D6, 298 K): d 13.8 (SiMe2).
E-6: 1H NMR (C6D6, 298 K):
d
7.01 (m, 4 H, o-Ph5), 6.94 (m, 2 H,
m-Ph2) 6.90 (m, 2 H, o-Ph2), 6.82 (m, 4 H, p-Ph2), 6.81 (m, 4 H, m-
Ph5), 6.79 (m, 1 H, p-Ph5), 6.53 (s, JSi,H ¼ 9.6 Hz, H, 5-H), 0.44 (s,
3
2JSi,H ¼ 6.9 Hz, 6 H, SiMe2). 13C{1H} NMR (C6D6, 298 K):
d 172.4 (C2),
170.9 (br, C3), 149.9 (C4), 148.8 (dm, 1JF,C z 252 Hz, C6F5), 143.9 (dm,
1JF,C z 260 Hz, C6F5), 143.5 (i-Ph5), 139.3 (i-Ph2), 137.5 (dm,
1JF,C z 251 Hz, C6F5), 129.7 (m-Ph5), 128.4 (m-Ph2), 127.9 (p-Ph2),
127.8 (C5), 127.7 (o-Ph2), 126.7 (p-Ph5), 126.3 (o-Ph5), 113.4 (br, i-
measurement, performed at ꢀ20 ꢁC in
a pre-cooled NMR
spectrometer.
1H NMR (C7D8, 253 K):
d
5.94 (s, JSi,H ¼ 22.1 Hz, 1H, 5-H), 0.42 (s,
6 H, SiMe2), 0.03 (s, 9 H, 5-SiMe3), ꢀ0.11 (s, 9 H, 2-SiMe3). 13C{1H}
C6F5), ꢀ1.8 (1JSi,C ¼ 47.0 Hz, SiMe2). 11B{1H} NMR (C6D6, 298 K):
d 58
NMR (C7D8, 253 K):
d
188.6 (br, C3), 173.6 (br, C2), 168.4
(br,
n
½ z 2100 Hz). 19F NMR (C6D6, 298 K):
d
ꢀ126.7 (br m, 2 F, o-F),
(1JSi,C ¼ 52.1 Hz, C4), 148.8 (dm, JF,C z 254 Hz, C6F5), 144.7 (dm,
ꢀ146.2 (tt, 3JF,F ¼ 20.7 Hz, 4JF,F ¼ 6.2 Hz,1 F, p-F), ꢀ162.1 (br, 2 F, m-F)
1
1JF,C z 263 Hz, C6F5), 137.6 (dm, JF,C z 253 Hz, C6F5), 131.5
[
Ddm,p ¼ 15.9]. 29Si{1H} NMR (C6D6, 298 K):
d 14.2 (SiMe2).
1
(1JSi,C ¼ 68.1 Hz, C5), 113.6 (br, i-C6F5), 0.4 (1JSi,C ¼ 44.7 Hz, SiMe2),
X-ray crystal structure analysis of E-6: formula C30H17BF10Si,
ꢀ0.3 (1JSi,C ¼ 51.8 Hz, 2-SiMe3), ꢀ0.6 (1JSi,C ¼ 52.1 Hz, 5-SiMe3). 11
B
M ¼ 606.34, yellow crystal 0.35 ꢃ 0.30 ꢃ 0.15 mm, a ¼ 19.0604(3),
{1H} NMR (C7D8, 243 K):
d
64 (n1/2 z 2500 Hz). 19F NMR (C7D8,
b ¼ 7.1624(2), c ¼ 22.4928(5) Å,
b
¼ 115.392(1)o, V ¼ 2774.03
3
253 K):
d
ꢀ126.9 (m, 2 F, o-F), ꢀ143.2 (t, JF,F ¼ 20.8 Hz, 1 F, p-F),
(11) Å3, rcalc ¼ 1.452 g cmꢀ3
,
m
¼ 0.171 mmꢀ1, empirical absorption
ꢀ160.7 (m, 2 F, m-F) [Ddm,p ¼ 17.5]. 29Si{1H} NMR (C7D8, 253 K):
correction (0.942 ꢄ T ꢄ 0.975), Z ¼ 4, monoclinic, space group P21/c
d
21.8 (SiMe2), ꢀ7.7 (5-Si), ꢀ11.1 (2-Si).
(No. 14),
l
¼ 0.71073 Å, T ¼ 223(2) K,
)/
u and 4 scans, 23002
Z-4b: Bis(trimethylsilylethynyl)diphenylsilane (1b) (25.0 mg,
reflections collected (ꢂh, ꢂk, ꢂl), [(sin
q
l
] ¼ 0.67 Åꢀ1, 6516 inde-
0.07 mmol), and bis(pentafluorophenyl)borane (22.6 mg,
0.07 mmol) were weighed under argon and placed inside an NMR
tube. The tube was then cooled to ꢀ78 ꢁC and d8-toluene (0.8 mL)
was added by a syringe. The tube was then sealed under vacuum
and kept at low temperature until the measurement, performed at
pendent (Rint ¼ 0.054) and 4399 observed reflections [I ꢅ 2
s(I)],
381 refined parameters, R ¼ 0.069, wR2 ¼ 0.149, max. (min.)
residual electron density 0.20 (ꢀ0.28) e Åꢀ3, hydrogen atoms
calculated and refined as riding atoms.
0
ꢁC in a pre-cooled NMR instrumentation.
3.1.3. Irradiation of E-6/Z-6
1H NMR (C7D8, 273 K):
d
7.88 (m, 4 H, o-Ph), 7.19 (m, 4 H, m-Ph),
Bis(phenylethynyl)dimethylsilane (5a) (300 mg, 1.15 mmol), and
bis(pentafluorophenyl)-borane (398 mg, 1.15 mmol), were weighed
in an argon-filled glove-box into a Schlenk flask (200 mL). Toluene
(30 mL) was added, and the bright red solution was stirred for 4 h.
After this time, the solution was irradiated with UV lamp for 2 h
with stirring at room temperature. After irradiation the colour of
the solution was still red but darker than before. The volatiles were
then removed and the product was washed with hexane
(2 ꢃ 20 mL). A dark-red powder was obtained with yield 75%
(525 mg). The two isomers were present in photochemical equi-
librium of E-6: Z-6 z 10: 1, which did not change after several days
at environmental light. For NMR characterization, see above.
7.13 (m, 2 H, p-Ph), 6.13 (s, JSi,H ¼ 23.1 Hz, 5-H), ꢀ0.11 (s,
2JSi,H ¼ 6.5 Hz, 9 H, 5-SiMe3), ꢀ0.18 (s, 2JSi,H ¼ 6.6 Hz, 9 H, 2-SiMe3).
13C{1H} NMR (C7D8, 273 K):
d 193.9 (br, C3), 174.2 (br, C2), 166.2
(C4), 148.9 (dm, 1JF,C z 253 Hz, C6F5), 145.0 (dm, 1JF,Ce262 Hz, C6F5),
137.7 (dm, 1JF,C z 254 Hz, C6F5), 135.9 (o-Ph), 133.8 (1JSi,C ¼ 63.5 Hz,
i-Ph), 132.7 (1JSi,C ¼ 65.5 Hz, C5), 130.9 (p-Ph), 128.5 (m-Ph), 113.7
(br, i-C6F5), ꢀ0.1 (2-SiMe3), ꢀ0.7 (5-SiMe3). 11B{1H} NMR (C7D8,
273 K):
d
64 (br,
n
½ z 2400 Hz). 19F NMR (C7D8, 273 K):
d
ꢀ126.5
(m, 2 F, o-F), ꢀ143.1 (m, 1 F, p-F), ꢀ160.6 (m, 2 F, m-F) [Ddm,p ¼ 17.5].
29Si{DEPT} NMR (C7D8, 273 K):
Si).
d
7.9 (SiPh2), ꢀ6.5 (5-Si), ꢀ10.7 (2-
3.1.2. 3-Bis(pentafluorophenyl)boryl-2-phenyl-4-benzylidene-
dimethylsilacyclobut-2-ene (Z-6, E-6)
3.1.4. 3-Bis(pentafluorophenyl)boryl-2-(1,1-dimethylethyl)-4-(Z)-
(2,2-dimethylpropylidene)-dimethylsilacyclobut-2-ene (Z-7)
Representative experiment: Bis(phenylethynyl)dimethylsilane
(5a) (400 mg, 1.54 mmol), and bis(pentafluorophenyl)borane
(531 mg, 1.54 mmol), were weighed in an argon-filled glove-box
and put into a Schlenk flask (200 mL). Toluene (50 mL) was added
by cannula under argon and the resulting bright red solution was
stirred overnight. The day after, the volatiles were removed under
high vacuum leaving a yellow oily product that was washed with
pentane (1 ꢃ 20 mL). A yellow powder was obtained after removing
the volatiles by high vacuum. Yield 89% (825 mg). The NMR spectra
showed the presence of both Z-6 and E-6 isomers (here: Z: E z 5:
Bis(3,3-dimethyl-1-butynyl)dimethylsilane (5b) (400 mg,
1.81 mmol), and bis(pentafluorophenyl)borane (628 mg,1.81 mmol),
were weighed in an argon-filled glove-box and put into a Schlenk
flask (100 mL). Toluene (30 mL) was added by cannula under argon
and the resulting bright yellow solution was stirred overnight. The
day after, the volatiles were removed under high vacuum leaving
a yellow oil. The product was washed with pentane (1 ꢃ 50 mL), and
no further purification was performed. Yield 78% (805 mg).
1H NMR (C6D6, 298 K):
d
5.47 (s, 3JSi,H ¼ 17.2 Hz, 1H, 5-H), 0.97 (s,
9 H, 2-C(CH3)3), 0.93 (s, 9 H, 5-C(CH3)3), 0.44 (s, 2JSi,H ¼ 6.9 Hz, 6 H,

1188
J. Ugolotti et al. / Journal of Organometallic Chemistry 696 (2011) 1184e1188
SiMe2). 13C{1H} NMR (C6D6, 298 K):
d
176.8 (1JSi,C ¼ 53.2 Hz, C2),
(c) R. Köster, G. Seidel, B. Wrackmeyer, Chem. Ber. 122 (1989) 1825e1850 see
also:;
(d) B. Wrackmeyer, S.V. Gruener, A.S. Zolotareva, Z. Naturforsch. 58b (2003)
1035e1040;
(e) B. Wrackmeyer, E.V. Klimkina, Y.N. Bubnov, J. Organomet. Chem. 620 (2001)
51e59;
1
166.4 (br, C3), 148.6 (dm, JF,C z 252 Hz, C6F5), 144.6 (dm,
1JF,C
1JF,C z 253 Hz, C6F5), 114.4 (br, i-C6F5), 36.1 (Cquat, 2-C(CH3)3), 34.7
z 261 Hz, C6F5), 143.4 (C4), 140.9 (C5), 137.7 (dm,
(Cquat
,
5-C(CH3)3), 31.0 (2-C(CH3)3), 30.0 (5-C(CH3)3), 0.6
(f) R. Köster, G. Seidel, I. Klopp, C. Krüger, G. Kehr, J. Süß, B. Wrackmeyer, Chem.
Ber. 126 (1993) 1385e1396;
(1JSi,C ¼ 46.3 Hz, SiMe2). 11B{1H} NMR (C6D6, 298 K):
d 64 (br,
½ z 1500 Hz). 19F NMR (C6D6, 298 K):
d
ꢀ127.6 (m, 2 F, o-F), ꢀ144.8
(g) B. Wrackmeyer, K. Horchler, Organometallics 9 (1990) 1881e1886.
[4] (a) H.E. Zimmerman, Pure Appl. Chem. 78 (2006) 2193e2203 (and references
cited therein);
n
3
4
(tt, JF,F ¼ 20.9 Hz, JF,F ¼ 5.9 Hz, 1 F, p-F), ꢀ161.1 (m, 2 F, m-F)
[
Ddm,p ¼ 16.3]. 29Si{1H} NMR (C6D6, 298 K):
d 11.2 (SiMe2).
(b) H.E. Zimmerman, D. Armesto, Chem. Rev. 96 (1996) 3065e3112;
(c) H.E. Zimmerman, Acc. Chem. Res. 15 (1982) 312e317;
(d) H.E. Zimmerman, Science 191 (1976) 523e528;
(e) S.S. Hixson, P.S. Mariano, H.E. Zimmerman, Chem. Rev. 73 (1973) 531e551;
(f) H.E. Zimmerman, R.W. Binkley, R.S. Givens, M.A. Sherwin, J. Am. Chem. Soc.
89 (1967) 3932e3933 See also:;
3.1.4.1. Irradiation of Z-7: preparation of E-7. 3-Bis(penta-
fluorophenyl)boryl-2-(1,1-dimethylethyl)-4-(Z)-(2,2-dimethylpro-
pylidene)-dimethylsilacyclobut-2-ene (Z-7) (420 mg, 0.741 mmol)
was dissolved by toluene (10 mL) in a Schlenk flask (50 mL) under
argon with stirring, and the bright yellow solution was irradiated
with UV lamp for 2 h. The volatiles were then removed and the
product was washed by pentane (2 ꢃ 10 mL). The isomer E-7 was
formed in photochemical equilibrium of E-7: Z-7e3.5: 1, which did
not change after several days at environmental light. Isolated yield
for the isomers mixture 68% (285 mg).
(g) J.D. Wilkey, G.B. Schuster, J. Am. Chem. Soc. 113 (1991) 2149e2155;
(h) J.J. Eisch, B. Shafii, M.P. Boleslawski, Pure Appl. Chem. 63 (1991) 365e368;
(i) U. Burger, R. Etienne, Helv. Chim. Acta 67 (1984) 2057e2062.
[5] (a) D.J. Parks, W.E. Piers, G.P.A. Yap, Organometallics 17 (1998) 5492e5503;
(b) R.E.v.H. Spence, W.E. Piers, Y. Sun, M. Parvez, L.R. MacGillivray,
M.J. Zaworotko, Organometallics 17 (1998) 2459e2469;
(c) W.E. Piers, T. Chivers, Chem. Soc. Rev. 26 (1997) 345e354;
(d) R.E.v.H. Spence, D.J. Parks, W.E. Piers, M.-A. MacDonald, M.J. Zaworotko,
S.J. Rettig, Angew. Chem. 107 (1995) 1337e1340;
1H NMR (C6D6, 298 K):
d
5.36 (s, 3JSi,H ¼ 13.8 Hz,1 H, 5-H), 0.90 (s,
Angew. Chem., Int. Ed. 34 (1995) 1230e1233;
(e) D.J. Parks, R.E.v.H. Spence, W.E. Piers, Angew. Chem. 107 (1995) 895e897;
Angew Chem. Int. Ed. 34 (1995) 809e811.
9 H, 2-C(CH3)3), 0.81 (s, 9 H, 5-C(CH3)3), 0.37 (s, 2JSi,H ¼ 6.9 Hz, 6 H,
SiMe2). 13C{1H} NMR (C6D6, 298 K):
d
181.0 (1JSi,C ¼ 50.3 Hz, C2),
[6] (a) E. Khan, S. Bayer, B. Wrackmeyer, Z. Naturforsch. 64b (2009) 47e57;
(b) B. Wrackmeyer, E. Khan, R. Kempe, Appl. Organomet. Chem. 22 (2008)
383e388;
1
164.0 (br, C3), 149.5 (dm, JF,C z 253 Hz, C6F5), 144.7 (dm,
1JF,C z 261 Hz, C6F5), 139.7 (1JSi,C ¼ 55.2 Hz, C4), 137.8 (dm,
1JF,C z 252 Hz, C6F5), 136.0 (C5), 114.4 (br, i-C6F5), 36.1 (Cquat, 2-C
(CH3)3), 32.7 (Cquat, 5-C(CH3)3), 31.6 (2-C(CH3)3), 30.3 (5-C(CH3)3),
(c) B. Wrackmeyer, E. Khan, S. Bayer, K. Shahid, Z. Naturforsch. 62b (2007)
1174e1182;
(d) B. Wrackmeyer, E. Khan, R. Kempe, Appl. Organomet. Chem. 21 (2007)
39e45;
ꢀ1.2 (1JSi,C ¼ 45.1 Hz, SiMe2). 19F NMR (C6D6, 298 K):
ꢀ125.6 (br s,
d
(e) B. Wrackmeyer, H.E. Maisel, E. Molla, A. Mottalib, A. Badshah, M.H. Bhatti,
S. Ali, Appl. Organomet. Chem. 17 (2003) 465e472.
[7] For related systems see: (a) J. Liu, S. Zhang, W.-X. Zhang, Z. Xi, Organometallics
28 (2009) 413e417;
2 F, o-F), ꢀ144.6 (tt, 3JF,F ¼ 20.9 Hz, 4JF,F ¼ 6.6 Hz, 1 F, p-F), ꢀ161.4 (m,
2 F, m-F) [Ddm,p ¼ 16.8]. 29Si{1H} NMR (C6D6, 298 K):
d 15.9 (SiMe2).
(b) D. Yan, M. Bolte, N. Auner, J. Organomet. Chem. 693 (2008) 908e916;
(c) J. Liu, X. Sun, M. Miyazaki, L. Liu, C. Wang, Z. Xi, J. Org. Chem. 72 (2007)
3137e3140;
Acknowledgment
(d) D. Yan, J. Mohsseni-Ala, N. Auner, M. Bolte, J.W. Bate, Chem. Eur. J. 13
(2007) 7204e7214;
(e) Review: J. Mohsseni-Ala, N. Auner Inorg. Chim. Acta 359 (2006)
4677e4697.
Financial support from the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie is gratefully acknowl-
edged. We thank the BASF for a generous gift of solvents.
[8] (a) S. Grimme, Angew. Chem. 120 (2008) 3478e3483;
Angew. Chem. Int. Ed. 47 (2008) 3430e3434;
Appendix A. Supplementary material
(b) Y. El-azizi, A. Schmitzer, S.K. Collins, Angew. Chem. 118 (2006) 982e987;
Angew. Chem. Int. Ed. Engl. 45 (2006) 968e973;
(c) S.L. Cockroft, C.A. Hunter, K.R. Lawson, J. Perkins, C.J. Urch, J. Am. Chem. Soc.
127 (2005) 8594e8595;
(d) J.H. Williams, Acc. Chem. Res. 26 (1993) 593e598 Selected examples:;
(e) S.L. Cockroft, J. Perkins, C. Zonta, H. Adams, S.E. Spey, C.M.R. Low,
J.G. Vinter, K.R. Lawson, C.J. Urch, C.A. Hunter, Org. Biomol. Chem. 5 (2007)
1062e1080;
(f) J.M. Blackwell, W.E. Piers, M. Parvez, R. McDonald, Organometallics 21
(2002) 1400e1407;
(g) D.J. Parks, W.E. Piers, M. Parvez, R. Atencio, M.J. Zaworotko, Organome-
tallics 17 (1998) 1369e1377.
CCDC 783505 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge at www.ccdc.
cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(internat.) þ44(1223)336-033, E-mail: deposit@ccdc.cam.ac.uk].
Appendix. Supplementary material
[9] B. Wrackmeyer, R. Köster, “NMR-spektroskopie von organobor-verbindun-
gen” , In: Houben-Weyl, Methoden der organischen Chemie, Band 13. Thieme,
Stuttgart, 1984, Teil 3c,pp. 406e408.
Supplementary data related to this article can be found online at
doi:10.1016/j.jorganchem.2010.09.024.
[10] T. Beringhelli, D. Donghi, D. Maggioni, G. D’Alfonso, Coord. Chem. Rev. 252
(2008) 2292e2313 (and references cited therein);
W.E. Piers, Adv. Organomet. Chem. 52 (2005) 1e76.
[11] A.B. Pangborn, M.A. Giardello, R.H. Grubbs, R.K. Rosen, F.J. Timmers, Organo-
metallics 15 (1996) 1518e1520.
[12] Z. Otwinowski, W. Minor, Meth. Enzymol. 276 (1997) 307e326.
[13] Z. Otwinowski, D. Borek, W. Majewski, W. Minor, Acta Crystallogr. A59 (2003)
228e234.
[14] G.M. Sheldrick, Acta Cryst A46 (1990) 467e473.
[15] G.M. Sheldrick, Acta Cryst A64 (2008) 112e122.
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