Unexpected photochemical reactions of 2-aryl- 2-ethynyltrisilane derivative

Article abstract of DOI:10.1515/mgmc-2014-0042

The synthesis of cyclic disilene 10 was attempted by photolysis of 2-alkynyltrisilane 9. As a result, silylsilylene 14 was generated, along with the elimination of the alkynylsilane moiety, in the photolysis of 2-alkynyltrisilane 9, similar to the photoly

Full text of DOI:10.1515/mgmc-2014-0042

Main Group Met. Chem. 2015; 38(1-2): 51–55
Short Communication
Yasunobu Egawa, Yoshiyuki Mizuhata, Takahiro Sasamori and Norihiro Tokitoh*
Unexpected photochemical reactions of 2-aryl-
2-ethynyltrisilane derivative
Abstract: The synthesis of cyclic disilene 10 was attempted are expected to show through-space interaction between
by photolysis of 2-alkynyltrisilane 9. As a result, silyl- the two alkynyl groups, resulting in electronic proper-
silylene 14 was generated, along with the elimination of ties different from those of (E)-isomers. To synthesize the
the alkynylsilane moiety, in the photolysis of 2-alkynyl- (Z)-1,2-dialkynyldisilenes selectively, we attempted the
trisilane 9, similar to the photolysis of alkynyldisilane. In reduction of compound 3, the two ethynylsilane moie-
addition, the product 14 trapped by triethylsilane under- ties of which were linked by tether (for cyclic disilenes,
went further photolysis to produce hydrosilylene 16.
see Sakurai et al., 2002; Tanaka et al., 2006; Abe et al.,
2010), but the desired disilene 4 could not be synthesized
(Scheme 2) (Mizuhata et al., 2015). The reaction afforded
the cyclic 1,2-dihydrodisilane 5 in low yield, where the
chlorine atoms on the silicon atoms were fully replaced
by hydrogen atoms derived from solvent and/or benzylic
protons of 3. This result suggests that hydrogen abstrac-
tion occurred by radical species generated in the stepwise
reduction of chlorosilane moieties.
Keywords: photolysis; silylene; trisilane.
DOI 10.1515/mgmc-2014-0042
Received November 27, 2014; accepted February 19, 2015; previously
published online March 24, 2015
Disilenes are the silicon analogue of carbon-carbon dou-
ble-bond compounds, and the first isolation of tetramesi-
tyldisilene was reported by West et al. (1981). Since then,
various types of disilenes have been synthesized, and their
characters have been revealed in detail (for reviews, see
Okazaki and West, 1996; Kira and Iwamoto, 2006; Lee and
Sekiguchi, 2010). There remains an ever-increasing inter-
est in the challenging study of the interaction of organic
π-conjugated systems and the silicon-silicon double-bond
units (Bejan and Scheschkewitz, 2007; Fukazawa et al.,
2007; Kinjo et al., 2007; Sasamori et al., 2008; Iwamoto
et al., 2009). In that field, we have succeeded in the syn-
thesis and isolation of the first example of a silicon ana-
logue of (E)-endiyne, (E)-1,2-dialkynyldisilenes 1, and
revealed that conjugative interaction exists between the
Siꢀ=ꢀSi and C≡C units (Sato et al., 2010).
In this particular case, the reduction of the corre-
sponding dichlorosilane, the most general method for the
generation of a silylene (or a disilene), was found to be not
appropriate. We then decided to adopt another synthetic
method for a disilene or a silylene without using a reduc-
ing agent, i.e., the photolysis of the corresponding trisilane
derivative. By using this method, many disilenes, includ-
ing the first disilene, tetramesityldisilene, have been suc-
cessfully synthesized (for reviews, see Okazaki and West,
1996; Kira and Iwamoto, 2006; Lee and Sekiguchi, 2010).
The photolysis of linear-trisilanes giving disilenes pro-
ceeds via the initial formation of a silylene, followed by its
dimerization (Ishikawa and Kumada, 1981). In addition,
Apeloig et al. (1994) generated alkynylsilylenes 6–8 in a
low-temperature glass matrix using photolysis of linear-
trisilane bearing an alkynyl group, and its formation was
suggested by the trapping reaction with triethylsilane
Those compounds were synthesized by the reduction
of dihalosilanes 2 (Scheme 1). In the reactions, however,
only (E)-isomer was generated, and the formation of (Z)-
isomers could not be detected. (Z)-1,2-Dialkynyldisilenes
(Et
3SiH) (Scheme 3). Based on these results, we designed
and synthesized compound 9, the 2-alkynyltrisilane moie-
ties of which were linked, and attempted the photolysis of
9 to synthesize cyclic disilene 10 (Scheme 4). In this paper,
we discuss the unexpected elimination of alkynylsilane
rather than disilane in the photoreaction of 9.
*Corresponding author: Norihiro Tokitoh, Institute for Chemical
Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan,
e-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp
Yasunobu Egawa, Yoshiyuki Mizuhata and Takahiro Sasamori:
Institute for Chemical Research, Kyoto University, Gokasho, Uji,
Kyoto 611-0011, Japan
The synthesis of the precursor, compound 9, was per-
formed. 1,2-Bis(ethynyldimethylsilyl)benzene 11 was pre-
pared by a reaction of 1,2-bis(bromodimethylsilyl)benzene
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52ꢀ ꢀY. Egawa et al.: Photochemical reactions of 2-aryl-2-ethynyltrisilane derivative
Scheme 4:ꢀStrategy for the synthesis of 10.
Scheme 1:ꢀSyntheses of (E)-1,2-dialkynyldisilenes 1.
(Cavelier-Frontin et al., 1991) with ethynylmagnesiumbro-
mide in THF (11% yield) as a pale yellow viscous liquid.
The synthesis of 9 was achieved by the reaction of the
dilithio compound generated by the treatment of 11 with
n-butyllithium followed by treatment with TipSi(SiMe3)2Cl
(Tipꢀ=ꢀ2,4,6-triisopropylphenyl) (Archibald et al., 1992) in
23% yield as a viscous liquid (Scheme 5). Compounds 9
and 11 were characterized by NMR spectroscopy together
with elemental analysis.
Scheme 5:ꢀSynthesis of 9.
To check the generation of silylene from 9 via the
elimination of disilane described in Scheme 3, we per-
formed the photolysis of compound 9 in the presence of
Et3SiH, which is commonly used as a trapping reagent for
silylenes. A n-hexane solution of precursor 9 and Et3SiH
was placed in a quartz sealed tube and irradiated with a
low-pressure mercury lamp at -78°C for 24 h (Scheme 6).
As a result, compound 12 bearing trimethylsilylethynyl
moieties and triethylsilyldihydrosilane 13 were obtained
at 39% and 27%, respectively. Both compounds were
Scheme 6:ꢀPhotolysis of 9 in the presence of triethylsilane.
characterized by NMR spectroscopy together with ele- 13. Whereas hydrosilylenes are important intermediates in
mental analyses. The formation of compound 13 suggests view of bearing a primitive substituent, hydrogen, there
the generation of silylsilylene 14 via the elimination of are few examples of the generation (Hong et al., 1994;
alkynylsilane (Scheme 6). That is, the generated silylene Sasamori et al., 2007; Agou et al., 2012). As far as we know,
14 was trapped by Et3SiH to afford trisilane 15, and it was only one example was reported for the generation of
photolyzed again to give hydrosilylene 16 in the next step. hydrosilylene by the photolysis of oligosilane (Hong et al.,
Silylene 16 was trapped by Et3SiH to afford dihydrosilane 1994). In that report, photolysis of MeSi(SiMe3)3 in the
Scheme 2:ꢀAttempted synthesis of cyclic 1,2-dialkynyldislene 4.
Scheme 3:ꢀGeneration of alkynylsilylenes 6–8.
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Y. Egawa et al.: Photochemical reactions of 2-aryl-2-ethynyltrisilane derivativeꢂ
ꢂ53
presence of Et3SiH afforded Me(H)Si(SiMe3)(SiEt3) mainly
(36%), together with 1,1-dihydrodisilane, H2(Me)SiSiEt3, in
1.5% yield, which suggests the generation of hydromethyl-
silylene from the initial product, Me(H)Si(SiMe3)(SiEt3). In
the example using compound 9, monohydrosilane 15 was
not observed, and the yield of dihydrosilane 13 was much
higher than that in the previous report. This result suggests
that the Tip-substitution is effective for the generation of
hydrosilylene. Actually, photoreaction of TipSi(SiMe3)2H
(Archibald et al., 1992) in the presence of Et3SiH proceeded
cleanly and afforded dihydrosilane 13 in 39% yield. When
compound 9 was irradiated under the same conditions
in the absence of Et3SiH, only a complicated mixture
was obtained. Its 29Si NMR spectrum indicated the exist-
ence of (Z)-Tip(Me3Si)Siꢀ=ꢀSi(SiMe3)Tip (at 97.8 ppm)
(Archibald et al., 1992), but no other signal was observed
in the lower field region other than this signal.
In the photolysis of compound 9, alkynylsilane, not
disilane, was eliminated from the central silicon atom
of trisilane moieties. This result marked a sharp contrast
with the results reported by Apeloig et al. (1994) described
above. Neither the trapped compound of alkynylsilylene
nor the compound bearing triethylsilylacetylene terminus
were observed in these experiments.
A possible mechanism for the generation of 12 is pro-
posed in Scheme 7. Compound 17 containing silacyclo-
propene moieties is generated, and then the photolysis
of compound 17 may give silylene 14 and compound 12.
A similar reaction mechanism is reported in the photore-
action of alkynyldisilane 18 in the presence of methanol
by Ishikawa, Kumada, and their co-workers (Scheme 8)
(Ishikawa et al., 1980; Ishikawa and Kumada, 1981). Sila-
cyclopropene 19 could be isolated, and the following
photolysis gave bis(trimethylsilyl)acetylene 20 and dimesi-
tylsilylene 21, the generation of which was suggested by
the isolation of 22. In this reaction, generation of 1-silaal-
lene 23 was suggested as a side reaction by the formation
of 24. It is likely due to this type of side-reaction that the
yields of 12 and 13 were low, similar to the photolysis of 9.
In summary, silylsilylene 14 was observed to generate
in the photolysis of 2-alkynyltrisilane 9 via the elimina-
tion of alkynylsilane rather than disilane. Judging from
Scheme 8:ꢀPhotolysis of alkynyldisilane 18 in the presence of
methanol.
the results obtained in this work and those shown in
Scheme 3, the elimination of alkynylsilane from 2-alkynyl-
trisilane is competitive with that of disilane, and the selec-
tivity could change depending on the reaction conditions
or small differences in the substituents. For the synthesis
of dialkynyldisilene using photolysis, protection of the
alkynyl moieties is considered to be necessary. In addition,
hydrosilylene 16 was observed to generate via compound
15. The yield of the trapped compound, dihydrosilane 13,
was still low, but much higher than the previous result,
indicating this methodology possibly presents an alterna-
tive way to generate a hydrosilylene.
Experimental section
General
All experiments were performed under an argon atmosphere unless
otherwise noted. All solvents were purified by standard methods and/
or The Ultimate Solvent System (Pure Process Technology, Nashua,
1
New Hampshire, USA) (Pangborn et al., 1996) prior to use. H-NMR
(300 MHz), 13C-NMR (75 MHz), and 29Si-NMR (59 MHz) spectra were
measured in C6D6 (Cambridge Isotope Laboratories, Inc., Tewksbury,
Massachusetts, USA) with a JNM AL-300 spectrometer (JEOL resonance
Inc., Tokyo, Japan). A signal due to C6D5H (7.15 ppm) was used as an
internal standard in 1H-NMR, and a signal due to C6D6 (128 ppm) was
used in 13C-NMR. Multiplicity of signals in 13C-NMR spectra was deter-
mined by DEPT technique. 29Si-NMR (59 MHz) spectra were measured
in C6D6 with a JEOL JNM AL-300 spectrometer using the signal for
SiMe4 (0 ppm) in C6D6 as an external standard. Gel permeation liq-
uid chromatography (GPLC) was performed on an LC-908, LC-918,
or LC-908-C60 (Japan Analytical Industry Co., Ltd., Tokyo, Japan)
equipped with JAIGEL 1H and 2H columns (eluent: chloroform). All
melting points were determined on a micro-melting-point apparatus
(Yanaco, Kyoto, Japan). Elemental analyses were carried out at the
Microanalytical Laboratory of the Institute of Chemical Research,
Kyoto University. Preparative thin-layer chromatography (PTLC)
Scheme 7:ꢀPossible mechanism for the generation of 12 and 14.
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54ꢀ ꢀY. Egawa et al.: Photochemical reactions of 2-aryl-2-ethynyltrisilane derivative
was performed with Kieselgel 60 PF254 (Merck KGaA, Darmstadt, contained in a quartz sealed tube was exposed to a low-pressure mer-
Germany, Art. No. 7747).
cury lamp (SUV110PL-10 with UVB-110, SEN LIGHTS CORPORATION,
Osaka, Japan) while immersed in a -78°C bath for 24 h. All of the volatile
materials were removed under reduced pressure. The reaction mixture
was separated by GPLC (eluent: toluene) to afford oligomeric products
and the mixture of 12 and 13. The following PTLC (eluent: n-hexane)
gave 12 (51 mg, 0.13 mmol, 39%) and 13 (62 mg, 0.18 mmol, 27%). 12:
viscous liquid; 1H-NMR (300 MHz, C6D6, rt, δ in ppm) 0.16 (s, 18H), 0.66
(s, 12H), 7.17–7.20 (m, 2H), 7.99–8.02 (m, 2H); 13C-NMR (75 MHz, C6D6, rt,
δ in ppm) 0.0 (CH3), 2.5 (CH3), 114.7 (C), 116.8 (C), 128.8 (CH), 136.3 (CH),
142.7 (C); 29Si-NMR (59 MHz, C6D6, rt, δ in ppm) -18.9, -21.4; HRMS (FAB)
Synthesis of 1,2-bis(ethynyldimethylsilyl)benzene (11)
A solution of 1,2-bis(hydrodimethylsilyl)benzene (12.6 g, 64.8 mmol,
Tokyo Chemical Industry Co., Ltd., Tokyo, Japan) in CCl4 (50 mL) was
added to a solution of bromine (7.6 mL, 150 mmol, NACALAI TESQUE,
Inc., Kyoto, Japan) in CCl4 (50 mL) and cooled by ice-bath while main-
taining an argon atmosphere. Argon gas was bubbled through the
mixture for a further 30 min before removing the solvent in vacuum.
The residue was used for the next reaction without further purifi-
cation. To that compound was added ethynylmagnesiumbromide
solution (0.5 m THF solution, 370 mL, 185 mmol, Sigma-Aldrich Co.
LLC, St. Louis, Missouri, USA) at room temperature, and the mixture
was stirred at room temperature for 3 days. The reaction mixture
was treated with aqueous sodium hydrogen carbonate (Wako Pure
Chemical Industries, Ltd., Osaka, Japan), and the organic layer was
extracted with n-hexane. Afer the removal of volatile substances,
the residue was subjected to silica gel chromatography (eluent:
n-hexane) to afford 11 (1.75 g, 7.22 mmol, 11%). 11: pale yellow viscous
+
m/z calcd for C20H34Si4 ([M] ): 386.1738, found: 386.1737; Anal. calcd for
C20H34Si4: C, 62.10; H, 8.86%, found: C, 61.97; H, 8.77%. 13: viscous liq-
uid; 1H-NMR (300 MHz, C6D6, rt, δ in ppm) 0.72 (q, Jꢀ=ꢀ8.4 Hz, 6H), 0.98 (t,
Jꢀ=ꢀ8.4 Hz, 9H), 1.20 (d, Jꢀ=ꢀ6.9 Hz, 6H), 1.33 (d, Jꢀ=ꢀ6.7 Hz, 12H), 2.77 (sept,
Jꢀ=ꢀ6.9 Hz, 1H), 3.33 (sept, Jꢀ=ꢀ6.7 Hz, 2H), 4.52 (s, 2H), 7.12 (s, 2H); 13C-NMR
(75 MHz, C6D6, rt, δ in ppm) 4.6 (CH2), 8.2 (CH3), 24.1 (CH3), 24.5 (CH3),
34.7 (CH), 35.3 (CH), 121.0 (CH), 124.7 (C), 150.4 (C), 155.7 (C); 29Si-NMR
(119 MHz, C6D6, rt, δ in ppm) -5.7, -82.8; HRMS (DART) m/z calcd for
+
C21H34Si4 ([M+H] ): 349.2741, found: 349.2725; Anal. calcd for C20H34Si4:
C, 72.33; H, 11.56%, found: C, 72.08; H, 11.56%.
1
liquid; H-NMR (300 MHz, C6D6, rt, δ in ppm) 0.57 (s, 12H), 2.15 (s,
Photolysis of TipSi(SiMe3)2H
2H), 7.17–7.20 (m, 2H), 7.95–8.00 (m, 2H); 13C-NMR (75 MHz, C6D6, rt, δ
in ppm) 2.1 (CH3), 90.4 (C), 96.6 (C), 128.9 (CH), 136.4 (CH), 142.3 (C);
29Si-NMR (59 MHz, C6D6, rt, δ in ppm) -59.2 (SiMe2); HRMS (EI) m/z A solution of TipSi(SiMe3)2H (239 mg, 0.632 mmol) and triethylsilane
+
calcd for C14H18Si2 ([M] ): 242.0947, found: 242.0946; Anal. calcd for (2.0 mL, 12 mmol) in n-hexane (6.0 mL) contained in a quartz sealed
C14H18Si2: C, 69.35; H, 7.48%, found: C, 69.34; H, 7.45%.
tube was exposed to a low-pressure mercury lamp while immersed
in a -78°C bath for 24 h. All of the volatile materials were removed
under reduced pressure. The following PTLC (eluent: n-hexane) gave
13 (86 mg, 0.25 mmol, 39%).
Synthesis of 1,2-bis(((1,1,1,3,3,3-hexamethyl-2-(2,4,6-
triisopropylphenyl)trisilan-2-yl)ethynyl)dimethylsilyl)
benzene (9)
Acknowledgments: This work was supported by Grants-
in-Aid for Scientific Research on Innovative Areas
(Nos. 24109013, 23108707), Scientific Research (B)
(Nos. 22350017, 25288021), Scientific Research (C) (No.
26410044), Young Scientist (B) (No. 21750042), Global COE
Program (B09, ‘Integrated Materials Science’), and Excel-
lent Graduate Schools from the Ministry of Education, Cul-
ture, Sports, Science and Technology of Japan.
To a THF solution (10 mL) of 11 (328 mg, 1.35 mmol) was added
n-butyllithium (1.60 m in n-hexane, 0.850 mL, 1.36 mmol, KANTO
chemical co., Inc., Tokyo, Japan) at -78°C, and the mixture was
stirred at -78°C for 30 min. The resulting solution was added to a
THF solution (10 mL) of TipSi(SiMe3)2Cl (1.12 g, 2.71 mmol) at room
temperature, and the mixture was stirred at room temperature for
12 h. The solvents were removed under reduced pressure. n-Hexane
was added to the residue, and the resulting suspension was filtered
®
through Celite to remove inorganic salts. The reaction mixture was
purified by GPLC (eluent: CHCl3) to afford 9 (308 mg, 0.309 mmol,
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