Fourfold Diels-Alder Reaction of Tetraethynylsilane

Article abstract of DOI:10.1002/chem.201404799

A series of ethynylated silanes, including tetraethynylsilane was treated with tetraphenylcyclopentadienone at 300 8C under microwave irradiation to give the aromatized Diels-Alder adducts as sterically encumbered mini-dendrimers with up to 20 benzene rings. The sterically most congested adducts display red-shifted emission through intramolecular p-p interactions in the excited state.

Full text of DOI:10.1002/chem.201404799

DOI: 10.1002/chem.201404799
Communication
&
Diels–Alder Reaction |Hot Paper|
Fourfold Diels–Alder Reaction of Tetraethynylsilane
Florian L. Geyer, Alexander Rode, and Uwe H. F. Bunz*[a]
Abstract: A series of ethynylated silanes, including tetrae-
thynylsilane, was treated with tetraphenylcyclopentadie-
none at 3008C under microwave irradiation to give the ar-
omatized Diels–Alder adducts as sterically encumbered
mini-dendrimers with up to 20 benzene rings. The sterical-
ly most congested adducts display red-shifted emission
through intramolecular p–p interactions in the excited
state.
Scheme 1. Preparation of the phenylethynyl silanes 8–10 and their reaction
with tetraphenylcyclopentadienone 12.
Tetraethynylsilane (11) is now an easily produced and surpris-
ingly stable, non-explosive material, accessible on gram
scale.[1a] It should be a powerful core for dendrimers or other
spherical assemblies. Functional molecules with fourfold tetra-
hedral reactivity are known, but often not easily prepared, in
contrast to 11.[1b–f] The tetrayne 11 has been functionalized
using organometallic fragments to give its dicobalt hexacar-
bonyl and similar complexes.[2a] In addition a number of its de-
rivatives have been studied in detail[2b,c] while the parent com-
pound 11 has been neglected so far—probably due to the ex-
plosive side products from the original syntheses.[2d,e]
for purification, similar to 11, the synthesis of which is de-
scribed elsewhere;[1a] the alkynes are colorless and crystalline
solids. While all of these are literature known, they have rarely
been investigated or used as substrates for further functionali-
zation. The ethynylated silanes were isolated and character-
ized. Compound 14 was claimed (1968) to form by boiling 9 in
xylene with 12[4]—in our hands we did not observe formation
of any product under these conditions.
We treated 8–10 with tetraphenylcyclopentadienone[5a] to
successfully isolate unsymmetrical, truncated Mꢀllen-type[5]
mini-dendrimers 13–15. While 13 and 14 formed in reasonable
yields, the already sterically overcrowded mini-dendrimer 15
formed in only 5% yield. While the cycloaddition and decar-
bonylative aromatization of tetraphenylcyclopentadienone 12
is known, usually less drastic conditions are required.[5] Howev-
er, the cycloadditions work only at extreme temperature with
microwave irradiation (3008C) in sealed tubes and were carried
out in diphenyl ether for that very reason (b.p. 2588C).
However, when we tried to perform more mainstream cou-
pling reactions with 11, such as Sonogashira or Hay reactions,
the core collapsed. Attempts to metalate 11 using Lappert’s re-
agent (Me2NSnMe3) or nBuLi followed by reaction with classic
electrophiles, such as methyl iodide or tributylchlorostannane,
also destroyed the scaffold. While nucleophilic exchange reac-
tions of diethynylsilanes were observed in the past,[3] the situa-
tion significantly worsens for 11. Partially desilylated deriva-
tives, even though now easily obtained, did not fare better in
attempts for functionalization.
At that point we surmised that both electrophilic metal cen-
ters, but also bases induced catastrophic core failure. Conse-
quently, thermal reactions such as Diels–Alder cycloadditions
might work for 11. To test our hypothesis, we prepared the
thermally equally stable, but less sensitive silanes 8–10 with an
increasing number of free alkyne units, employing reaction
conditions successful for the preparation of 11 (Scheme 1).
Separation by chromatographic columns is only necessary
after deprotection and only for 8 and 9, while 10 is sublimed
The first approach to prepare 16a is depicted in Scheme 2.
We initially hoped to carry out the cycloaddition of 7, as 11
[a] F. L. Geyer, A. Rode, Prof. U. H. F. Bunz
Organisch-Chemisches Institut, Ruprecht-Karls-Universitꢀt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax: (+49)6221-54-8401
Scheme 2. Thermal reaction of tetraethynylsilane derivative 7 (prepared ac-
cording to reference [1a]) with four equivalents tetraphenylcyclopentadie-
none 12 under ambient atmosphere results in cleavage of the central SiÀC
bonds and formation of 17.
E-mail: uwe.bunz@oci.uni-heidelberg.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404799.
Chem. Eur. J. 2014, 20, 1 – 7
1
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Communication
cannot be used—it is too volatile at these extreme tempera-
tures. We anticipated that the TMS-groups would be cleaved
eventually, but neither 16a nor 16c could be isolated. Reaction
with sub-stoichiometric amounts of 12 (e.g., 1–3 equiv) did
not show conversion to the expected products. Instead, the
central SiÀC bonds are cleaved. Compound 7 acts as a substi-
tute for TMS-acetylene, and 17 is the only isolated product.
Using sealed tubes 11 reacted with 12 (Scheme 3) for 2 h
under microwave irradiation (3008C). The main product is the
Figure 1. Molecular structure of tetrakis(1,2,3,4-tetraphenylphenyl)silane
(16a) according to DFT (B3LYP 6-31G*//B3LYP 6-31G*) geometry optimiza-
tion. Left: Van der Waals model, right: stick model. The central SiÀC bond
length is 1.93 ꢂ. In tetraphenylsilane the analogous bond is calculated to be
1.90 ꢂ (DFT, B3LYP 6-31G*//B3LYP 6-31G*).
Scheme 3. Reaction of tetraethynylsilane 11 (prepared according to referen-
ce [1a]) with tetraphenylcyclopentadienone 12.
alkyne 16b (27%) and 16a (4%). The final cycloaddition is
most demanding from a steric point of view. A longer reaction
time does not increase but rather lowers the yield. Thermal de-
composition is an issue especially for this last addition, as it ap-
parently requires a high activation energy. Separation of 16a
and 16b is difficult and could only be achieved using a di-
chloromethane (CH2Cl2) in petroleum ether (PE) gradient for
chromatography, followed by a second purification column.
The crude yield of 16a is 14%, but this fraction shows impuri-
ties in the alkane region. Repeated precipitation gave pure
16a at cost of the yield. Alkyne 16b itself is quite interesting
as a bulky alkyne substituent and could be used to tune the
film-forming properties of various organic electronic materials
that are prone to crystallization, which is unfavorable in organ-
ic solar cells or for matrix materials.[6] In further experiments,
we could increase the raw yield (column) of 16a to 34%, while
16b was only formed as a trace product. Experiments to in-
crease the yield of 16b by reacting 11 with only three equiva-
lents of 12 failed—the yield remained almost constant com-
pared to our initial synthesis. Compound 16a is crowded ac-
cording to DFT calculations (see Figure 1), probably more than
other oligophenyl compounds known so far. Figure 1 shows
the crowding at the silicon center testified to by the elongated
central SiÀCAr bond that stretches from 1.90 to 1.93 ꢂ. This
makes 16a fundamentally interesting as a core for dendrimers,
if a more sophisticated tetracyclone derivative is used.
Figure 2. Aromatic region of the oligophenyl dendrimers (600 MHz, CD2Cl2).
Spectra are normalized on the number of protons of the respective com-
pound.
7.44 ppm (16a), well explained by the hydrogen atom being
increasingly magnetically shielded when surrounded by bulky
tetraphenylbenzene groups. This goes hand in hand with the
broadening of the signal from 13–16a—another observation
that is to be expected for increasing steric crowding. From
Figure 1, we can see that the geometry optimized structure
shows that the single hydrogen atom rests deep within the oli-
gophenylene core. The broadening of this signal also suggests
that the rotation slows down, at least in 16a. To test this hy-
1
pothesis, we performed variable-temperature H NMR spectra.
In dichloromethane, when going from ambient temperature to
À788C one can see broadening and split of the signals due to
further restriction of rotation. If one heats the solutions of 16a
in [D2]tetrachloroethane, up to 1008C, a significant sharpening
of the signals is observed. Interestingly, the two NMR spectra
taken at ambient temperature, one in dichloromethane and
the other in tetrachloroethane, are surprisingly different, with
the latter showing broadened signals (Figure 3). We assume
that this is due the size of the solvent and its interaction with
16a. Overall, 16a must experience severe and solvent-depen-
dent restricted rotation around its many phenyl–phenyl bonds.
1
Figure 2 shows the aromatic regions of the H NMR spectra
of oligophenyl compounds 13–16a. The single proton at the
tetraphenylbenzene ring shows as a singlet between 7.4 and
7.8 ppm for each compound. This single hydrogen atom is di-
agnostic for the increasing steric demand of the compounds,
while the phenyl rings are successively substituted with tetra-
phenylbenzene groups. One notes a significant upfield shift
when going from 13 to 16a: 7.70 (13), 7.65 (14), 7.52 (15),
&
&
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
2
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Communication
Figure 5. Left: calculated DFT ground state structure of 14 (B3LYP 6-31G*).
Well visible (circle) is the overlap of a phenyl ring positioned on the Si
center with the phenyl group of a tetraphenylbenzene unit. Right: 14 dis-
solved in 1,3-dichlorobenzene and irradiated with 365 nm light. Top: room
temperature, bottom: 1708C.
from that of the other oligomers—quantum chemical calcula-
tions give a hint why this could be (Figure 5). A phenyl group
attached to the Si center lies almost parallel to a second
phenyl ring, part of a tetraphenylbenzene dendrimer arm with
the ring planes showing distances of 3.3–3.8 ꢂ between them
according to ground state quantum chemical calculations. In
the excited state, we presume that an exciton which is local-
ized on the interacting tetraphenylbenzene ring is also stabi-
lized by the nearby phenyl ring, therefore leading to some-
thing akin to an intramolecular heteroexcimer, stabilizing the
excited state of 14 significantly, and leading to the observed,
broadened, red-shifted emission feature. Following this hy-
pothesis, the fluorescence intensity should decrease upon
heating due to the thermal activation of the constricted rota-
tions—thus destabilizing the exited state. This feature that can
indeed be observed when heating a solution of 14 in 1,3-di-
chlorobenzene from ambient temperature to 1708C. Following
image analysis (see Supporting Information), the fluorescence
decreased to approximately half of its initial value (Figure 5).
In the solid state (Figure 6) compound 13 is non fluorescent,
but the sterically more congested derivatives 14, 15, and 16a
display a moderately intense, blue fluorescence. The fluores-
cence is similar to that observed for 14–16a in solution. We
assume that in the solid state the conformation of the phenyl
rings is frozen and that—in the excited state—there are signifi-
1
Figure 3. Temperature dependent H NMR spectroscopy (500 MHz) of the ar-
omatic reagion of compound 16a.
The absorption spectra of all of the cycloadducts are broad
and featureless (see Supporting Information). The emission of
13 and 15 resemble that of o-terphenyl and p-terphenyl, the
innate chromophores of the structures (Figure 4); the bands of
16a and particularly 14 are broadened and spread well into
the visible (blue). This is unexpected when looking at the mo-
lecular structure. Particularly the emission of 14 looks different
Figure 4. Normalized emission spectra of oligophenyl compounds 13–16a.
For comparison, the spectra of biphenyl and o/p-terphenyl are included. Ex-
citation wavelength 275 nm (for the absorption spectra see Supporting In-
formation).
Figure 6. Appearance of 13–16a. Top row: normal light. Bottom row: Fluo-
rescence color of the materials while irradiated with 365 nm UV light.
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
3
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Communication
hexane; 1.05 equiv) was slowly added and the solution warmed up
to 08C. After being stirred for 1 h, the respective phenylchlorosi-
lane (1 equiv) was added to the solution followed by the reaction
turning opaque. The mixture was slowly allowed to warm to ambi-
ent temperture, while being stirred overnight. Deionized water
(50 mL) was added, the mixture stirred for an additional 5 min. The
now clear organic layer was separated from the aqueous layer,
which was extracted with diethyl ether (3ꢃ100 mL). The combined
organic layers were dried over anhydrous magnesium sulfate, the
solvent removed under reduced pressure. The product was crystal-
lized from hot hexane.
cant enforced p–p interactions which stabilize the excited
state and lead to the observed blue emission. The phenyl
groups will also suppress intermolecular interactions, which
otherwise would deactivate the excited state through several
different decay pathways. The exact mechanism, however, is
difficult to unravel at the moment.
In conclusion, multiply ethynylated silanes react with tetra-
phenylcyclopentadienone to give stable and isolable yet steri-
cally encumbered mini-dendrimers such as 16a or 14. Thermal
reactions are the only way to functionalize compounds such as
11; other reactions failed in our hands. This is the first time
that tetraethynylsilane (11) was fully functionalized. Apparently,
even drastic conditions can be applied to force 11 and related
derivatives into reactions. The steric overcrowding leads to un-
usual red-shifted emission spectra in 16a and 14; in the excit-
ed state, the substituents separated by the silicon center must
interact to a significant extent. In the future we aim to build
up more complex mini-dendrimers using 11 with substituted
and functional tetraarylcyclopentadienones. Simplicity of syn-
thesis and, therefore, accessibility make 11 a promising core
for the construction of further materials.
(Trimethylsilyl)ethynyltriphenylsilane (4): Prepared according to GP1.
n-Butyllithium (1.42 mL, 3.56 mmol, 1.05 equiv 2.5m hexane solu-
tion) was added to trimethylsilyl acetylene in diethyl ether (531 mL,
3.37 mmol, 1.10 equiv). A solution of chlorotriphenylsilane (1.00 g,
3.39 mmol, 1.00 equiv) in dry diethyl ether (10 mL) was added to
the acetylide solution. After crystallization, 4 was obtained (color-
less crystals, 1.18 g, 4.00 mmol, 98%). Rf (petroleum ether/ethyl
1
acetate 2:1)=0.79; m.p. 678C; H NMR (300.51 MHz, CDCl3, 258C):
d=7.70–7.63 (m, 6H), 7.51–7.35 (m, 10H), 0.27 ppm (s, 9H);
13C NMR (75.76 MHz, CDCl3, 258C): d=135.56, 133.48, 129.86,
127.92, À0.16 ppm; IR (neat): n˜ =3067 (w), 3049 (w), 2961 (w),
2898 (w), 2103 (w), 1428 (m), 1248 (m), 1112 (s), 839 (s), 768 (s), 761
(s), 739 (s), 707 (s), 694 (s), 677 (m), 502 cmÀ1 (s). Spectral data in
agreement to reported literature.[7,8]
Bis((trimethylsilyl)ethynyl)diphenylsilane (5): Prepared according to
GP1. n-Butyllithium (3.24 mL, 8.10 mmol, 2.10 equiv, 2.5m hexane
solution) was added to trimethylsilyl acetylene in diethyl ether
(1.18 mL, 8.29 mmol, 2.20 equiv). Dichlorodiphenylsilane (831 mL,
3.95 mmol, 1.00 equiv) was added to the acetylide solution. After
crystallization, 5 was obtained (colorless crystals, 1.30 g, 5.14 mmol,
87%). Rf (petroleum ether/ethyl acetate 2:1)=0.81; m.p. 648C,
1H NMR (300.51 MHz, CDCl3, 258C): d=7.78–7.72 (m, 4H), 7.45–7.35
(m, 6H), 0.24 ppm (s, 9H); IR (neat): n˜ =3067 (w), 3049 (w), 2961
(w), 2899 (w), 2109 (w), 1429 (m), 1249 (m), 1110 (m), 840 (m), 759
(m), 740 (m), 694 (s), 501 (m), 495 (m), 489 (m), 484 cmÀ1 (m). Spec-
tral data in agreement to reported literature.[9,10]
Experimental Section
All reagents and solvents were obtained from ABCR, Sigma–Al-
drich, VWR or Grꢀssing GmbH and were used without further pu-
rification. Preparation of air- and moisture-sensitive materials was
carried out in flame-dried flasks under an atmosphere of nitrogen
by using Schlenk techniques. Thin-layer chromatograms were ob-
tained using polygram-TLC-plates (40ꢃ80 mm, SIL G/UV254,
0.2 mm layer thickness, Macherey–Nagel). For reactions carried out
in a microwave, an Anton Paar Microwave Synthesis Reactor Mono-
wave 300 was used. For column chromatography the Isolera Prime
flash system with self-packed columns (SiO2, grain size 0.04–
0.063 mm, Macherey–Nagel) was used. Melting points were deter-
mined with a Melting Point Apparatus MEL-TEMP (Electrothermal,
Tris((trimethylsilyl)ethynyl)phenylsilane (6): Prepared according to
GP1. n-Butyllithium in hexane (5.96 mL, 14.9 mmol, 3.15 equiv,
2.50m hexane solution) was added to trimethylsilyl acetylenein di-
ethyl ether (2.22 mL, 15.6 mmol, 3.30 equiv). Chlorotriphenylsilane
(755 mL, 4.73 mmol, 1.00 equiv) was added to the acetylide solu-
tion. After crystallization, 6 was obtained (colorless crystals, 1.78 g,
4.49 mmol, 95%). Rf (petroleum ether/ethyl acetate 2:1)=0.83;
m.p. 1148C; 1H NMR (500.13 MHz, CDCl3, 258C): d=7.85–7.78 (m,
2H), 7.50–7.39 (m, 3H), 0.22 ppm (s, 27H); 13C NMR (125.77 MHz,
CDCl3): d=134.48, 131.55, 130.44, 128.00, 117.85, 105.05,
À0.45 ppm; MS (EI+): m/z calcd for C48H36Si: 396.1581 [M]+; found:
396.1569, correct isotope distribution; IR (neat): n˜ =3072 (w), 3056
(w), 2962 (w), 2898 (w), 2115 (w), 1428 (w), 1251 (m), 1112 (m), 836
(m), 780 (m), 757 (s), 740 (m), 694 (s), 497 (s), 494 cmÀ1 (s). Spectral
data in agreement to reported literature.[9]
Rochford, UK) and reported uncorrected. H and 13C spectra were
1
recorded on Bruker Avance 300 (300 MHz), Bruker Avance 400
(400 MHz), Bruker Avance 500 (500 MHz), or Bruker Avance 600
(600 MHz) spectrometer. Chemical shifts (d) are reported in parts
per million (ppm) relative to traces of CHCl3 or
[D2]dichloromethane in the deuterated solvent. Coupling constants
(J) are reported in Hz. All NMR spectra were integrated and pro-
cessed by using TopSpin 3.0 (Bruker). MS spectra were recorded on
a Vacuum Generators ZAB-2F, Finnigan MAT TSQ 700 or JEOL JMS-
700 spectrometers. Infrared (IR) spectra are reported in wavenum-
bers (cmÀ1) and were recorded on a Jasco FT/IR-4100 spectrometer.
Semi-empirical calculations were carried out using Spartan 10, Ver-
sion 1.1.0. 7 and 11 were prepared according to literature.[1]
For Figure 5a solution of 14 (2.7 mg) in 1,3-dichlorobenzene (2 mL)
was prepared and irradiated with 365 nm UV light. Photographs
were acquired with a Canon EOS 7D fitted with a Canon Macro
Lens EF-S 60 mm, both images were acquired with a shutter speed
of 0.25 s, aperture F2.8 and ISO100 (color space sRGB).
Ethynyltriphenylsilane (8): Compound 4 (684 mg, 1.92 mmol) was
dissolved in methanol/THF (1:1, 100 mL). Potassium carbonate
(663 mg, 4.80 mmol, 2.50 equiv) was added and the solution stirred
for 15 min before water (100 mL) was added. The aqueous phase
was extracted with dichloromethane (5ꢃ100 mL); the combined
organic layers were dried over anhydrous magnesium sulfate the
solvent evaporated. Crude 8 was purified by column chromatogra-
phy (SiO2, petroleum ether/ethyl acetate), Rf (petroleum ether/ethyl
acetate 3:1)=0.69, to yield a colorless solid (343 mg, 1.21 mmol,
63%). 1H NMR (400.33 MHz, CDCl3, 258C): d=7.68–7.63 (m, 6H),
7.49–7.38 (m, 9H), 2.50 ppm (s, 1H); 13C NMR (100.64 MHz, CDCl3,
Synthesis and characterization
General procedure (GP1) for the synthesis of 4–6: A solution of
trimethylsilyl acetylene (1.10 equiv per chlorine) in dry diethyl
ether (50 mL) was cooled to À788C. n-Butyllithium (2.5m in
&
&
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
4
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Communication
258C) d=135.17, 134.99, 130.12, 127.93 ppm; IR (neat): n˜ =3287,
3266, 3074, 3056, 2039, 1430, 1113, 690, 668, 650, 573 cmÀ1. Spec-
tral data in agreement to reported literature.[10]
6.80–6.75 (m, 3H), 6.71–6.40 (m, 2H), 6.68–6.64 (m, 1H), 6.56–6.53
(m, 2H), 6.49–6.44 ppm (m, 2H); 13C NMR (150.95 MHz, CD2Cl2,
258C): d=149.22 (Cq), 152.58 (Cq), 142.48 (Cq), 142.06 (Cq), 141.54
(Cq), 141.00 (Cq), 140.81 (Cq), 139.79 (CH), 139.77 (Cq), 136.69 (CH),
135.85 (CH), 133.12 (CH), 131.91 (CH), 131.87 (CH), 131.51 (CH),
130.43 (CH), 129.48 (CH), 128.07 (CH), 128.02 (CH), 127.34 (CH),
126.82 (CH), 126.82 (CH), 126.78 (CH), 126.59 (CH), 126.31 (CH),
126.13 (CH), 125.55 ppm (CH); HR-MS (DART+): m/z calcd for
C48H36Si: 640.2586 [M]+; found: 640.2594, correct isotope distribu-
tion; m/z calcd for C48H40NSi: 658.2925 [M+NH4]+; found:
658.2934, correct isotope distribution; m/z calcd for C96H76NSi2:
1299.5544 [2M+NH4]+; found: 1299.5516, correct isotope distribu-
tion; IR (neat): n˜ =3058 (w), 3020 (w), 1428 (m), 1111 (m), 740 (m),
694 (s), 502 cmÀ1 (s).
Diethynyldiphenylsilane (9): Compound 5 (500 mg, 1.33 mmol)
was dissolved in dry dichloromethane (5 mL) in a flame dried
Schlenk flask under nitrogen atmosphere. The solution was cooled
to 08C and triflic acid (234 mL, 2.65 mmol, 2.00 equiv) was added
slowly through a syringe. The brownish solution was allowed to
warm up to ambient and stirred under light protection for 16 h.
After addition of deionized water (100 mL) and extraction with
pentane (3ꢃ100 mL), the combined organic layers were dried over
anhydrous magnesium sulfate. Solvent removal reduced pressure
is followed by Kugelrohr distillation. A heat gradient from 1908C to
À788C at 1.1 mbar was applied. A slightly reddish liquid con-
densed as the room temperature fraction, which was purified by
column chromatography (SiO2, petroleum ether/ethyl acetate)
yielding 9 (colorless solid, 163 mg, 705 mmol, 53%). Rf (petroleum
Bis(1,2,3,4-tetraphenylphenyl)diphenylsilane (14): Prepared according
to GP2. Compounds 9 (95.0 mg, 409 mmol, 1.00 equiv) and 12
(314 mg, 818 mmol, 2.00 equiv) were reacted in diphenyl ether
(7 mL). Column chromatography yielded a yellowish solid (Rf (pe-
troleum ether/dichloromethane 5:1)=0.52), which was then puri-
fied by repeated precipitation (3ꢃ) by dissolving the material in di-
chloromethane (2 mL) and adding methanol (10 mL) to give a col-
orless, blue fluorescent (irradiated with 365 nm) solid (107 mg,
114 mmol, 28%). m.p. 2058C 1H NMR (600.25 MHz, CD2Cl2, 258C):
d=7.65 (s, 2H), 7.36–7.33 (m, 4H), 7.27–7.23 (m, 2H), 7.20–7.16 (m,
4H), 7.15–7.09 (m, 6H), 7.07–7.04 (m, 4H), 6.95–6.92 (ml, 6H), 6.88–
6.84 (m, 4H), 6.82–6.76 (m, 8H), 6.73–6.69 (m, 8H), 6.66–6.62 ppm
(m, 4H); 13C NMR (150.95 MHz, CD2Cl2, 258C): d=147.73 (Cq),
142.63 (Cq), 142.35 (Cq), 142.01 (Cq), 141.54 (Cq), 141.24 (Cq),
140.92 (Cq), 139.39 (Cq), 139.33 (CH), 137.12 (CH), 136.63 (Cq),
134.16 (Cq), 131.98 (CH), 131.92 (CH), 131.85 (CH), 130.56 (CH),
128.98 (CH), 127.93 (CH), 127.64 (CH), 127.32 (CH), 126.83 (CH),
126.59 (CH), 126.50 (CH), 126.05 (CH), 125.47 ppm (CH); HR-MS
(DART+):m/z calcd for C72H52Si: 944.3838 [M]+; found: 944.3894,
correct isotope distribution; m/z calcd for C72H56NSi: 962.4177 [M+
NH4]+; found: 962.4208, correct isotope distribution; IR (neat): n˜ =
3052 (w), 3022 (w), 1600 (w), 1491 (w), 1441 (m), 1428 (m), 1098
(m), 1071 (m), 1019 (m), 795 (m), 760 (m), 693 (s), 635 (m), 514 (m),
500 (s), 489 (m), 485 cmÀ1 (m).
1
ether/ethyl acetate 2:1)=0.63; H NMR (300.51 MHz, CDCl3, 258C):
d=7.89–7.80 (m, 4H), 7.55–7.40 (m, 6H), 2.79 ppm (s, 2H); IR
(neat): n˜ =3281 (m), 3072 (w), 3036 (w), 2049 (s), 1255 (m), 1080 (s),
1060 (s), 842 (s), 714 (s), 680 (s), 615 (s), 610 cmÀ1 (s); Spectral data
in agreement to reported literature.[3,6,9]
Triethynylphenylsilane (10): Compound 6 (500 mg, 1.26 mmol)
was dissolved in dry dichloromethane (5 mL) in a flame-dried
Schlenk flask under nitrogen atmosphere. The solution was cooled
to 08C and triflic acid (334 mL 3.78 mmol, 3.00 equiv) was slowly
added through a syringe. The now brownish solution brought to
ambient and stirred under light protection for 16 h. Deionized
water (100 mL) was added and the mixture intensively shaken in
an extraction funnel, upon which the organic phase lost its brown
color but remained slightly beige. The aqueous phase was extract-
ed with pentane (3ꢃ100 mL) and the combined organic layers
were dried over anhydrous magnesium sulfate. The solvent was re-
moved under reduced pressure to give an off-white crystalline
solid, purified by Kugelrohr distillation applying a heat gradient
from 858C to À788C at 7.4ꢃ10À1 mbar. Compound 10 condensed
at the ambient fraction (colorless needles, 143 mg, 793 mmol,
59%). 1H NMR (300.51 MHz, CDCl3, 258C): d=7.87–7.80 (m, 2H),
7.56–7.42 (m, 3H), 2.70 ppm (s, 3H); 13C NMR (75.57 MHz, CDCl3,
258C): d=134.30, 131.19, 128.35, 96.75, 81.82 ppm; MS (EI+): m/z
calcd for C12H8Si: 180.0395 [M]+; found: 396.1581, correct isotope
distribution. Spectral data in agreement to reported literature.[2e,9]
Tris(1,2,3,4-tetraphenylphenyl)phenylsilane (15): Prepared according
to GP2. Compounds 10 (100 mg, 555 mmol, 1.00 equiv) and 12
(640 mg, 1.66 mmol, 3.00 equiv) were reacted in diphenyl ether
(15 mL). Column chromatography yielded a yellowish solid (Rf (pe-
troleum ether/dichloromethane 5:1)=0.68) which was then puri-
fied by repeated precipitation (3ꢃ) by dissolving the material in di-
chloromethane (2 mL) and adding methanol (10 mL) to give a col-
orless, blue fluorescent (irradiated with 365 nm) solid (35.9 mg,
27.8 mmol, 5%). M.p. 2748C; 1H NMR (600.25 MHz, CD2Cl2, 258C):
d=7.52 (s, 3H), 7.22–7.18 (m, 1H), 7.12–7.09 (m, 9H), 7.09–7.06 (m,
2H), 7.04–7.01 (m, 2H), 6.95–6.92 (m, 6H), 6.92–6.89 (m, 9H), 6.82–
6.78 (m, 6H), 6.78–6.73 (m, 9H), 6.69–6.66 (m, 6H), 6.66–6.63 (m,
9H), 6.60–6.56 ppm (m, 6H); 13C NMR (150.95 MHz, CD2Cl2, 258C):
d=142.65 (Cq), 142.41 (Cq), 141.99 (Cq), 141.48 (Cq), 141.06 (Cq),
140.97 (Cq), 139.66 (CH), 138.71 (Cq), 137.77 (CH), 136.72 (Cq),
134.91 (Cq), 132.16 (CH), 132.02 (CH), 131.89 (CH), 130.78 (CH),
128.78 (CH), 127.85 (CH), 127.32 (CH), 127.24 (CH), 126.73 (CH),
126.64 (CH), 126.35 (CH), 125.96 (CH), 125.34 ppm (CH); HR-MS
(DART+): m/z calcd for C96H68Si: 1249.5124 [M]+; found: 1249.5115,
correct isotope distribution; m/z calcd for C96H72NSi: 1267.5426
[M+NH4]+; found: 1267.5412, correct isotope distribution; m/z
calcd for C96H72NOSi: 1283.5411 [M+NH4 +O]+; found: 1283.5401;
IR (neat): n˜ =3052 (w), 3017 (w), 1598 (w), 1491 (w), 1441 (w), 1426
(w), 1364 (w), 1072 (w), 1017 (w), 759 (m), 693 (s), 506 cmÀ1 (m).
General procedure (GP2) for the cycloaddition of 12 with ethy-
nylphenylsilanes 8–10 and 11: Compounds 8–10 or 11 and
1.00 equiv 12 per alkyne unit were dissolved in diphenyl ether
under nitrogen atmosphere. While being stirred for 1 h (8–10) or
2 h (11) in a microwave, the reaction mixture turned from magenta
to brown. After cooling to ambient, the solvent was removed
using a Kugelrohr apparatus. The crude product was purified by
column chromatography (SiO2, petroleum ether/dichloromethane)
and, for further purification, precipitated several times from di-
chloromethane by addition of methanol.
1,2,3,4-(Tetraphenylphenyl)triphenylsilane (13): Prepared according
to GP2. Compounds 8 (150 mg, 527 mmol) and 12 (203 mg,
527 mmol, 1.00 equiv) were reacted in diphenyl ether (5 mL).
Column chromatography yielded a yellowish solid (Rf (petroleum
ether/dichloromethane 5:1)=0.46), further purified by repeated
precipitation (3ꢃ) by dissolving the material in dichloromethane
(2 mL) and adding methanol (10 mL) to give a colorless, blue fluo-
rescent (irradiated with 365 nm) solid (181 mg, 279 mmol, 53%).
M.p. 1898C, 1H NMR (600.25 MHz, CD2Cl2, 258C): d=7.70 (s, 1H),
7.51–7.48 (m, 6H), 7.35–7.31 (m, 3H), 7.27–7–25 (m, 6H), 7.11–7.06
(m, 3H), 7.06–7.02 (m, 2H), 6.94–6.91 (m, 3H), 6.90–6.86 (m, 2H),
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
5
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Communication
Tetrakis(1,2,3,4-tetraphenylphenyl)silane (16a) and ethynyltris(1,2,3,4-
tetraphenylphenyl)silane (16b): Prepared according to GP2.Com-
pounds 11 (8.25 mg, 64.3 mmol, 1.00 equiv) and 12 (99.0 mg,
257 mmol, 4.00 equiv) were dissolved in diphenyl ether (5 mL).
Column chromatography yielded two yellowish solids (Rf (petrole-
um ether/dichloromethane 1:1)=0.64 (16b), 0.71 (16a)). Each frac-
tion was subjected to chromatography again (same eluent). Crude
16a was obtained as off-white solid (28 mg, 18.3 mmol, 14%). To
obtain pure material, 16a was repeatedly precipitated (3ꢃ) by dis-
solving in dichloromethane (2 mL) and adding methanol (10 mL)
to a give a colorless, blue fluorescent (irradiated with 365 nm) 16a:
(4.00 mg, 2.58 mmol, 4%). Alkyne 16b: (26.0 mg, 17.4 mmol, 26%).
distribution; m/z calcd for C92H68NOSi: 1231.5099 [M+NH4 +O]+;
found: 1231.5092, correct isotope distribution; IR (neat): n˜ =3281
(w), 3053 (w), 3023 (w), 2036 (w), 694 cmÀ1 (s).
Acknowledgements
This work was supported by a grant from the Deutsche For-
schungsgemeinschaft (DFG) (Bu771/7-1).
Keywords: dendrimer
oligophenyl · silane
· Diels–Alder reaction · ethynyl ·
Optimized conditions for 16a: Compounds 11 (51 mg, 398 mmol,
1.00 equiv) and 12 (612 mg, 1.59 mmol, 4.00 equiv) were dissolved
in diphenyl ether (2 mL) and heated in a sealed tube to 3008C for
2 h in a reaction microwave Crude 16a was obtained as off-white
solid (213 mg, 137 mmol, 34%). Traces of 16b were removed by
a single column (eluent as above). Purification by repeated precipi-
tation from dichloromethane with methanol yielded 16a (107 mg,
[1] a) F. L. Geyer, F. Rominger, U. H. F. Bunz, Chem. Eur. J. 2014, 20, 3600–
3605; b) U. Bunz, K. P. C. Vollhardt, J. S. Ho, Angew. Chem. 1992, 31,
1648–1651; c) K. S. Feldman, C. M. Kraebel, M. Parvez, J. Am. Chem. Soc.
1993, 115, 3846–3847; d) F. Diederich, Nature 1994, 369, 199–207;
e) U. M. Wiesler, A. J. Berresheim, F. Morgenroth, G. Lieser, K. Mꢀllen,
Macromolecules 2001, 34, 187–199.
1
97 mmol). Rf =0.71; m.p. 1758C; H NMR (600.25 MHz, CD2Cl2, 258C):
[2] a) M. A. Brook, B. Ramacher, C. Dallaire, H. K. Gupta, D. Ulbrich, R. Ruffo-
lo, Inorg. Chim. Acta 1996, 250, 49–57; b) B. Wrackmeyer, E. Khan, S.
Bayer, O. L. Tok, E. V. Klimkina, W. Milius, R. Kempe, Z. Naturforsch. B
2010, 65, 725–744; c) B. Wrackmeyer, E. Khan, A. Badshah, E. Molla, P.
Thoma, O. L. Tok, W. Milius, R. Kempe, J. Senker, Z. Naturforsch. B 2010,
65, 119–127; d) W. Davidsohn, M. C. Henry, J. Organomet. Chem. 1966,
5, 29–34; e) U. Krꢀerke, J. Organomet. Chem. 1970, 21, 83–90.
[3] R. Bortolin, B. Oarbhoo, S. S. D. Brown, J. Chem. Soc. Chem. Commun.
1988, 1079–1081.
[4] C. Kraihanzel, M. L. Losee, J. Org. Chem. 1968, 33, 1983–1986.
[5] a) W. Dilthey, F. Quint, J. Prakt. Chem. 1930, 128, 139–149; b) M. A.
Ogliaruso, M. G. Romanelli, E. I. Becker, Chem. Rev. 1965, 65, 261–367;
c) M. Wind, U. Wiesler, K. Saalwꢄchter, K. Mꢀllen, H. W. Spiess, Adv.
Mater. 2001, 13, 752–756; d) T. Qin, G. Zhou, H. Schreiber, R. E. Bauer,
M. Baumgarten, C. E. Anson, E. J. W. List, K. Mꢀllen, Angew. Chem. Int. Ed.
2008, 47, 8292–8296; Angew. Chem. 2008, 120, 8416–8420; e) M. D.
Watson, A. Fechtenkçtter, K. Mꢀllen, Chem. Rev. 2001, 101, 1267–1300;
f) C. Li, M. Y. Liu, N. G. Pschirer, M. Baumgarten, K. Mꢀllen, Chem. Rev.
2010, 110, 6817–6855; g) B. C. Englert, M. D. Smith, K. I. Hardcastle,
U. H. F. Bunz, Macromolecules 2004, 37, 8212–8221; h) G. Shao, A. Orita,
K. Nishijima, K. Ishimaru, M. Takezaki, K. Wakamatsu, R. Gleiter, J. Otera,
Chem. Asian J. 2007, 2, 489–498.
d=7.44 (s, 4H), 7.20–7.10 (m, 12H), 6.95–6.87 (m, 21H), 6.86–6.82
(m, 8H), 6.74–6.68 (m, 12H), 6.65–6.57 (m, 19H), 6.51–6.46 ppm (m,
8H); 13C NMR (150.95 MHz, CD2Cl2, 258C): d=146.73 (Cq), 142.84
(Cq), 142.54 (Cq), 141.62 (Cq), 141.30 (Cq), 141.22 (Cq), 141.00 (Cq),
138.59 (Cq), 132.62 (CH), 131.99 (CH), 131.92 (CH), 130.82 (CH),
127.82 (CH), 127.22 (CH), 126.64 (CH), 126.32 (CH), 126.15 (CH),
125.93 (CH), 125.20 ppm (CH); HR-MS (DART+): m/z calcd for
C120H84Si: 1553.6376 [M]+; found:1553.6375, correct isotope distri-
bution; m/z calcd for
C
120H88NSi: 1571.6714 [M]+; found:
1571.6636, correct isotope distribution; m/z calcd for C120H88NOSi:
1587.6663 [M+NH4 +O]+; found: 1587.6625, correct isotope distri-
bution; m/z calcd for C120H88NO2Si: 1603.6612 [M+NH4 +2O]+;
found: 1603.6584, correct isotope distribution; m/z calcd for
C114H84NSi: 1495.6401 [M+NH4ÀPh]+; found: 1495.6407, correct
isotope distribution; IR (neat): n˜ =3052 (w), 3022 (w), 1071 (m),
695 cmÀ1 (s).
Data for ethinyltris(1,2,3,4-tetraphenylphenyl)silane (16b): Rf =0.64.
1
m.p. 1808C (decomp); H NMR (600.25 MHz, CD2Cl2, 258C): d=7.50
(s, 3H), 7.20–7.10 (m, 10H), 7.04–6.96 (m, 6H), 6.95–6.87 (m, 16H),
6.85–6.73 (m, 23), 6.73–6.67 (m, 6H), 2.08 ppm (s, 1H); 13C NMR
(150.95 MHz, CD2Cl2, 258C): d=147.35 (Cq), 142.622 (Cq), 141.88
(Cq), 141.66 (Cq), 141.60 (Cq), 141.16 (Cq), 140.89 (Cq), 139.28 (Cq),
139.17 (CH), 133.40 (Cq), 132.15 (CH), 131.98 (CH), 131.81 (CH),
130.58 (CH), 127.90 (CH), 127.27 (CH), 126.83 (CH), 126.74 (CH),
126.68 (CH), 126.50 (CH), 126.04 (CH), 125.51 (CH), 97.83 (CH),
86.73 ppm (Cq); HR-MS (DART+): m/z calcd for C92H64Si: 1196.4777
[M]+, found: 1196.4755, correct isotope distribution; m/z calcd for
C92H68NSi: 1214.5116 [M+NH4]+; found: 1214.5067, correct isotope
[6] B. Geffroy, P. le Roy, C. Prat, Polym. Int. 2006, 55, 572–582.
[7] C. S. Kraihanzel, M. L. Losee, J. Organomet. Chem. 1967, 10, 427–437.
[8] C. Dallaire, M. A. Brook, Organometallics 1993, 12, 2332–2338.
[9] I. Kownacki, B. Marciniec, B. Dudziec, M. Kubicki, Organometallics 2011,
30, 2539–2545.
[10] A. Naka, M. Ishikawa, J. Organomet. Chem. 2000, 611, 248–255.
Received: August 10, 2014
Published online on && &&, 0000
&
&
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
6
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Communication
COMMUNICATION
Thermal enforcement: Naked tetraethy-
nylsilane reacts with tetracyclone at
3008C under microwave addition to
give the sterically hindered fourfold cy-
cloaddition product (see figure).
&
Diels–Alder Reaction
F. L. Geyer, A. Rode, U. H. F. Bunz*
&& – &&
Fourfold Diels–Alder Reaction of
Tetraethynylsilane
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
7
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ