Photochemical isomerisation of boryl-substituted silole derivatives

Article abstract of DOI:10.1039/b927221f

Bis(alkynyl)silanes (1a,b) react with bis(pentafluorophenyl)-borane to 3-boryl-2,5-bis(trimethylsilyl) substituted siloles 3a,b. Subsequent irradiation of 3 with Pyrex filtered UV light resulted in the complete conversion to the new 5-boryl-2,3-bis(trimet

Full text of DOI:10.1039/b927221f

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Photochemical isomerisation of boryl-substituted silole derivativeswz
¨
Juri Ugolotti, Gerald Kehr, Roland Frohlichy and Gerhard Erker*
Received (in Cambridge, UK) 23rd December 2009, Accepted 4th March 2010
First published as an Advance Article on the web 25th March 2010
DOI: 10.1039/b927221f
Bis(alkynyl)silanes (1a,b) react with bis(pentafluorophenyl)-
borane to 3-boryl-2,5-bis(trimethylsilyl) substituted siloles 3a,b.
Subsequent irradiation of 3 with Pyrex filtered UV light resulted
in the complete conversion to the new 5-boryl-2,3-bis(trimethyl-
silyl) substituted siloles 4a,b.
from its characteristic spectroscopic features and
a
comparison with 2 (see above). Compound 3a shows 13C
NMR signals of the C4 framework at d 174.2 (br, C3),
159.8 (C2), 150.9 (C4) and 149.9 (C5). The C(4)–H
1H NMR signal was located at d 7.42 (3JSi,H = 16.9 Hz,
3JSi,H = 7.5 Hz). The compound features a pair of 29Si NMR
resonances of the SiMe3 substituents at d ꢀ7.1 and ꢀ9.6
(with corresponding 1H NMR signals at d 0.16 and ꢀ0.02)
in addition to the Me2Si 29Si NMR feature at d 25.6.
The broad 11B NMR resonance indicates the presence of a
tricoordinate boron center (d 64) which is supported by the
typical large m/p separation of the C6F5 19F NMR signals of
compound 3a [d ꢀ126.4 (o), ꢀ143.6 (p), ꢀ160.7 (m)].5
The reaction of the bis(trimethylsilylethynyl)diphenylsilane
starting material (1b) with HB(C6F5)2 proceeded analogously
and gave the silole derivative 3b as a yellow oil in >70% yield.
For both compounds we assume a reaction pathway involving
a sequence of a formal 1,1-hydroboration followed by a
1,1-alkenylboration reaction (see Scheme 2).6
Siloles are increasingly important heterocycles. Suitably
substituted or annelated derivatives are playing an increasingly
significant role in materials chemistry.1 Among these, boryl
substituted derivatives are of special interest.2 We had recently
reported about the unconventional synthesis of the boryl silole
system 2 that was prepared by means of treatment of the silyl
acetylene derivative 1b with the strong boron Lewis acid
B(C6F5)3 under ambient conditions (see Scheme 1).2 This
reaction involves a 1,1-carboboration reaction.3 The resulting
characteristic substitution pattern was secured by an X-ray
crystal structure analysis. We now wish to report about an
interesting new isomerisation reaction of such boryl functio-
nalized siloles, bearing bulky silyl place holder groups at the
a-positions, which proceeds rapidly upon photolysis. This
reaction makes boryl substituted siloles with a different
substitution pattern readily and cleanly available. This new
silole rearrangement will here be described and discussed using
a small series of novel boryl-silole derivatives that we have
prepared by treatment of the parent silyl acetylenes (1) with
‘‘Piers’ borane’’ [HB(C6F5)2].4
The 1,1-dimethyl-2,5-bis(trimethylsilyl)-3-borylsilole 3a
rearranged cleanly to the corresponding 2,3-bis(trimethylsilyl)-
5-boryl isomer (4a) upon photolysis. Irradiation of compound
3a with Pyrex filtered UV light (HPK 125) in toluene solution
at room temperature for 4 h resulted in a practically complete
conversion to 4a that was isolated as a solid in close to
80% yield. The new compound was characterized by C,H-
elemental analysis, spectroscopically and by X-ray diffraction
We treated bis(trimethylsilylethynyl)dimethylsilane (1a)
with one molar equivalent of HB(C6F5)2 in toluene solution
at room temperature. Workup after one day gave the product
(3a) as a viscous oil (isolated in >70% yield). Due to the
consistency of the product we were not able to get it
completely solvent free and analytically pure, but its identifi-
cation as the b-boryl substituted silole derivative followed
Scheme 1
Organisch-Chemisches Institut, Westfalische Wilhelms-Universitat,
¨ ¨
Corrensstrasse 40, 48149 Munster, Germany.
¨
E-mail: erker@uni-muenster.de; Fax: +49 251 83 36503
w Dedicated to Professor Karl-Heinz Thiele on the occasion of his
80th birthday.
z Electronic supplementary information (ESI) available: Text and
figures giving further experimental and spectroscopic details. CCDC
765606 (1a) and 765607 (4a). For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/b927221f
y X-Ray structure analysis.
Scheme 2
ꢁc
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Scheme 4
of the di-p-methane rearrangement.8,9 Scheme 4 shows a
schematic representation of the necessary bond breaking
and forming processes without implying that the depicted
formulae represent actual intermediates involved in these
rearrangements.
This study shows that the boryl-functionalized siloles of the
types 2 and 3, bearing the conjugated boryl substituent in the
b-position, which are readily available from corresponding
sila-acetylene and borane precursors by the unique 1,1-hydro-
(or -carbo-)boration/1,1-vinylboration sequence, rearrange
readily and completely to their a-boryl regioisomers upon
photolysis. This combination of easily performed reactions
will probably make a variety of siloles with useful substituent
patterns readily available for applications in e.g. materials
science and elsewhere.
Fig. 1 Molecular structure of the photoproduct 4a (50% probability
of thermal ellipsoids).
Financial support from the Fonds der Chemischen Industrie
and the Deutsche Forschungsgemeinschaft is gratefully
acknowledged. We thank the BASF for a gift of solvents.
(single crystals were obtained from pentane at ꢀ30 1C).7 In the
crystal compound 4a features a planar five-membered hetero-
cyclic framework [Si1–C2: 1.892(3) A, Si1–C5: 1.866(3) A,
angle C2–Si1–C5: 93.5(2)1] with an alternating C4-unit
[C2–C3: 1.368(4) A, C3–C4: 1.494(4) A, C4–C5: 1.357(5) A].
Carbon atoms C2 and C3 carry the bulky trimethylsilyl
substituents [C2–Si21: 1.876(3) A, angle Si21–C2–C3:
130.9(3)1, C3–Si31: 1.899(3) A, angle Si31–C3–C2: 133.7(3)1]
and the –B(C6F5)2 substituent is attached at the a-carbon
atom C5 [C5–B1: 1.523(5) A]. The boron atom in compound
4a is planar tricoordinate (sum of the bonding angles: 360.01)
(Fig. 1). Compound 4a shows a 11B NMR resonance at d 61
and 29Si NMR signals at d 24.3 (SiMe2) and d ꢀ7.2 and ꢀ10.1
(SiMe3). The 13C NMR resonances of the central carbon
framework occur at d 181.1 (1JSi,C = 58.3 Hz, 1JSi,C = 42.2 Hz,
Notes and references
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3 R. Koster, G. Seidel, J. Suß and B. Wrackmeyer, Chem. Ber., 1993,
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3
C2), 172.1 (C4, 1H NMR: d 8.11, JSi,H = 14.3 Hz), 171.5
(1JSi,C = 61.7 Hz, C3) and 151.6 (br, C5).
Photolysis of the Ph2Si-containing compound 3b proceeded
analogously. After 2 h irradiation time workup gave the
rearranged product 4b in ca. 71% yield (Scheme 2, for details
see the ESIz). We have also photolyzed the previously reported
C6F5 substituted analogue 2 (see Scheme 1). The photolysis
reaction required 6 h irradiation time to go to completion and
we isolated the isomer 5 in 58% yield from the workup
procedure (see Scheme 3).
These photolytic isomerisation reactions formally proceed
by mutual exchange of the C3-[B] and C5-[Si] moieties at the
five-membered heterocyclic framework. Although the mecha-
nistic details of this reaction still have to be elucidated, this
framework reorganization formally corresponds to the pattern
6 B. Wrackmeyer, H. E. Maisel, W. Milius, M. H. Bhatti and S. Ali,
Z. Naturforsch., 2003, 58b, 543; See also: E. Khan, S. Bayer and
B. Wrackmeyer, Z. Naturforsch., 2009, 64b, 47; E. Khan, S. Bayer
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7 4a: the bright yellow solution of bis(trimethylsilylethynyl)dimethyl-
silane (1a) (365 mg, 1.445 mmol) and bis(pentafluorophenyl)borane
(500 mg, 1.445 mmol) in toluene (50 mL) was stirred overnight.
Subsequently the solution was irradiated for 4 hours to finally get 4a
as a yellow-brown solid (79%, 685 mg). Crystals suitable for X-ray
diffraction were obtained from a concentrated pentane solution
Scheme 3
at ꢀ30 1C. C24H25BF10Si3 (598.5): calcd.
C 48.16, H 4.21;
ꢁc
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found C 47.69, H 4.25. Crystal data for C24H25BF10Si3 (4a), M =
598.52, monoclinic, space group C2/c (No. 15), a = 26.7841(7), b =
6.5724(2), c = 33.4388(9) A, b = 107.519(1)1, V = 5613.4(3) A3,
Dc = 1.416 g cmꢀ3, m = 0.248 mmꢀ1, Z = 8, l = 0.71073 A,
T = 223(2) K, 21 439 reflections collected (ꢂh, ꢂk, ꢂl), [(siny)/l] =
0.66 Aꢀ1, 6379 independent (Rint = 0.071), and 4407 observed
reflections [I Z 2s(I)], 351 refined parameters, R = 0.068, wR2 =
0.157. CCDC 765607 [for details see the ESIz].
S. S. Hixson, P. S. Mariano and H. E. Zimmerman, Chem. Rev.,
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cited therein.
9 See also: U. Burger and R. Etienne, Helv. Chim. Acta, 1984, 67,
2057; J. J. Eisch, B. Shafii and M. P. Boleslawski, Pure Appl. Chem.,
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8 See for example: H. E. Zimmerman, R. W. Binkley, R. S. Givens
and M. A. Sherwin, J. Am. Chem. Soc., 1967, 89, 3932;
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3018 | Chem. Commun., 2010, 46, 3016–3018