Reduction of phenyl silyl acetylenes with lithium: Unexpected formation of a dilithium dibenzopentalenide

Article abstract of DOI:10.1002/anie.200604067

(Chemical Equation Presented) The degree of bulkiness of the silyl substituent strongly affects the reduction mode of phenyl silyl acetylenes. For example, the treatment of phenyl(triisopropylsilyl)acetylene with lithium led to the unprecedented formation

Full text of DOI:10.1002/anie.200604067

Communications
DOI: 10.1002/anie.200604067
Lithiated Compounds
Reduction of Phenyl Silyl Acetylenes with Lithium: Unexpected
Formation of a Dilithium Dibenzopentalenide**
Masaichi Saito,* Michio Nakamura, Tomoyuki Tajima, and Michikazu Yoshioka
Dedicated to Professor Renji Okazaki on the occasion of his retirement
The 1,4-dilithio-1,3-butadiene that is obtained from phenyl-
acetylene with lithium[1] is often employed as the starting
compound for the synthesis of metal-containing carbocyclic
p systems, for example, Group 14 metalloles.[2] X-ray crystal
structure analyses of 1,4-dithio-1,3-butadienes have shown
that these compounds are monomeric with two bridging
lithium ions.[3] The pentalene dianion, on the other hand, has
received considerable attention as a ligand of sandwich-type
transition-metal complexes.[4,5] However, the dibenzopenta-
lene dianion, which is p extended to a greater degree than the
pentalene dianion, has received less attention in spite of its
potential usefulness as a building block for ladder-type
p-conjugated molecules of growing interest.[6–8]
During our studies on the synthesis of reactive species
with a 1,3-butadiene skeleton,[9] the reduction of phenyl-
(trimethylsilyl)acetylene to give the corresponding 1,4-di-
lithio-1,3-butadiene[1b] prompted us to investigate the reduc-
tion of phenylacetylenes with a bulky silyl substituent. We
report herein the reduction of phenyl(triisopropylsilyl)acety-
lene with lithium, which led to the unexpected formation of a
dilithium dibenzopentalenide together with a 1,4-dilithio-1,3-
butadiene, the normal reaction product. The first X-ray
crystallographic characterization of a dilithium dibenzopen-
talenide, and a novel dimeric structure of a 1,4-dilithio-1,3-
butadiene with an Li4 tetrahedron, are also described.
Scheme 1. Reduction of phenyl silyl acetylenes 1 and 3.
propylsilyl)acetylene (3)[12] with excess lithium gave a deep-
red solution, which suggested the formation of an anionic
species. Treatment of the resulting mixture with iodine
afforded the unpredicted dibenzopentalene 4 (7%) as well
as the Z,Z configured 1,4-diiodo-1,3-butadiene 5 (28%;
Scheme 1), the structures of which were established by
X-ray crystallographic analysis.[11,13] The C C bonds in the
À
six-membered rings of 4 are nearly equal in length, whereas
À
remarkable bond alternation of the C C bonds (1.357–
In an attempt to synthesize the corresponding extremely
encumbered 1,4-dilithio-1,3-butadiene, we first carried out a
reduction of phenyl(tri-tert-butylsilyl)acetylene (1). The treat-
ment of 1 with excess lithium, however, led to cleavage of the
1.512Š) is found in the five-membered rings, as was observed
in dibenzopentalenes described previously.[14]
As the formation of a dibenzopentalene from a phenyl-
acetylene is unprecedented, the reaction of 3 with excess
lithium was monitored by NMR spectroscopy. The dark-red
solution resulting from the reaction of 3 with lithium was
placed in an NMR tube with C6D6 for NMR lock. In the
1H NMR spectrum, the major signals could be assigned to 1,4-
dilithio-1,3-butadiene 6, although minor signals were
observed in the aliphatic and aromatic regions. In the
13C NMR spectrum, a low-field resonance was observed at
d = 205.95 ppm, characteristic for lithiated vinyl carbon atoms
of 1,4-dilithio-1,3-butadiene.[3a] After recrystallization of the
reaction mixture from hexane in a glovebox at À308C,
however, the dilithium dibenzopentalenide 7 was isolated in
8% yield. Compound 7 could be recrystallized easily from the
crude product in spite of its low yield. The remarkable high-
field resonance of the 7Li nuclei at d = À8.4 ppm is caused by
the strong shielding effect of the aromatic ring current.
The X-ray crystal structure analysis of 7 reveals that each
lithium atom above or below each central five-membered ring
is coordinated to the five-membered ring in an h5 fashion and
is also coordinated to one molecule of diethyl ether
À
C Ph bond, as evidenced by the formation of tri-tert-
[10,11]
butylsilylacetylene (2; 92 %)
when the reaction was
quenched with H2O (Scheme 1). Steric congestion apparently
prevents dimerization of the radical intermediate derived
from 1. Next, the reduction of less bulky phenyl silyl
acetylenes was investigated. The reaction of phenyl(triiso-
[*] Prof. Dr. M. Saito, M. Nakamura, Dr. T. Tajima, Prof. Dr. M. Yoshioka
Department of Chemistry
Graduate School of Science and Engineering
Saitama University
Shimo-okubo, Saitama-city, Saitama, 338-8570 (Japan)
Fax: (+81)48-858-3700
E-mail: masaichi@chem.saitama-u.ac.jp
[**] This research was partially supported by a Grant-in-Aid for Young
Scientists (B), No. 17750032 (to M.S.) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan. M.S.
acknowledges a research grant from Toray Science Foundation.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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À
(Figure 1). The lengths of the C C bonds in the five-
isomerization of cyclooctatetraene to pentalene—similar to
membered rings are nearly equal (1.444–1.468 Š), whereas
a slight bond alternation is found for the bonds in the six-
membered ring (1.372–1.420 Š). The preference for aromatic
the proposed isomerization of C to 7—has been reported
previously.[17]
The reduction of less bulky phenyl(tert-butyldimethyl-
silyl)acetylene (8) was also examined.[18] In this case, however,
1,4-dilithio-1,3-butadiene 9 was generated as the main
product without the formation of the corresponding dibenzo-
pentalene dianion, as evidenced by the absence of any high-
field 7Li NMR signals characteristic of dilithium dibenzopen-
talenides (d = À8.4 ppm for 7). The configuration of 9 was
determined to be Z,Z, as Z,Z configured 1,4-diiodo-1,3-
butadiene 10 was formed in 27% yield[11,13,19] when the
reaction was quenched with iodine (Scheme 3). Recrystalli-
zation of the reaction mixture from hexane in a glovebox at
Figure 1. ORTEPdrawing of the structure of 7 (thermal ellipsoids with
40% probability for non-hydrogen atoms). All hydrogen atoms are
À
omitted for clarity. Selected bond lengths [Š]: C(1) C(2) 1.4681(15),
À
À
À
C(2) C(3) 1.4434(16), C(3) C(4) 1.4124(16), C(4) C(5) 1.3765(17),
À
À
À
C(5) C(6) 1.4089(18), C(6) C(7) 1.3715(18), C(7) C(8) 1.4203(16),
#
#
À
À
C(8) C(1) 1.4460(16), C(2) C(2) 1.451(2).
delocalization in the five-membered ring over benzenoid
delocalization has been found previously for some benzannu-
lated anions.[15] Although a dibenzopentalene dianion has
been synthesized by deprotonation of the parent dihydrodi-
benzopentalene or reduction of the parent dibenzopenta-
lene,[16] the formation of a dibenzopentalene dianion from a
phenylacetylene is unprecedented, and compound 7 is the
first dibenzopentalene dianion to have been characterized by
X-ray crystallography. The dilithium dibenzopentalenide 7
was oxidized by iodine to afford dibenzopentalene 4.
A tentative mechanism is proposed in Scheme 2, although
attempts to trap intermediates with external reagents failed.
The reaction route to 6 and 7 could involve the radical anion
intermediate A/B, with A being the resonance form that leads
to 7 through intermolecular cyclization followed by extrusion
of dihydrogen to give the dianionic intermediate C, and B
being the one that leads to 6. The conversion of 6 into 7 at
room temperature can be ruled out on the basis of evidence
gained by monitoring the reaction by NMR spectroscopy. The
Scheme 3. Reduction of the phenyl silyl acetylene 8.
À308C afforded 9 in 18% yield. In the 7Li NMR spectrum, a
single resonance was observed at d = 1.1 ppm, which suggests
that 9 has a non-aromatic character. In the 13C NMR
spectrum, a signal assignable to the a carbon atom appeared
at d = 201.98 ppm as a septet with a 13C–7Li coupling constant
of 16 Hz. This coupling pattern suggests that the a carbon
atom interacts strongly with two lithium atoms in solution;
dilithium bridging was observed in the X-ray structures of
Z,Z configured 1,4-dilithio-1,3-butadienes.[3]
The X-ray structure analysis of 9 revealed a novel dimeric
structure with an Li4 tetrahedron, although the structure of
the monomer unit is roughly similar to those of dilithium-
bridged Z,Z configured 1,4-dilithio-1,3-butadienes.[3] The
À
average length of the Li Li bonds is 2.533 Š, which is
comparable to bond lengths observed for some Li4 tetrahe-
drons (2.56 Š for MeLi,[20] 2.55 Š for EtLi,[21]
and 2.52 Š for 2-Me2NC6H4Li).[22] Each lith-
ium atom is coordinated by two butadiene
moieties in different ways: through
h4 coordination (Li(2) and Li(4) in Figure 2)
or h2 coordination (Li(1) and Li(3) in
Figure 2) by one butadiene moiety and
h1 coordination by a terminal carbon atom of
the other butadiene moiety. Thus, each buta-
diene moiety is associated with an h4-coordi-
nated, an h2-coordinated, and two h1-coordi-
À
nated lithium atoms. The average Li C bond
lengths in the case of h4 and h1 coordination
are 2.22 and 2.26 Š, respectively. These values
are also comparable to those observed for
some Li4 tetrahedrons (2.27 Š for MeLi,[20]
Scheme 2. Possible mechanism for the formation of 6 and 7.
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Communications
1
from hexane at À308C gave 9 (238 mg, 25%). H NMR (400 MHz,
diethyl ether/C6D6): d = À0.52(s, 12H), 0.93 (s, 18H), 6.63–6.66 (m,
6H), 6.74–6.77 ppm (m, 4H); 13C NMR (101 MHz, diethyl ether/
C6D6): d = À3.5 (q), 18.3 (s), 28.2 (q), 123.5 (d), 126.6 (d), 129.1 (d),
152.3 (s), 163.6 (s), 202.0 ppm (sept, J(7Li,13C) = 16 Hz); 7Li NMR
(156 MHz, diethyl ether/C6D6): d = 1.1 ppm.
Crystallographic data for 7:[13] C42H70Li2O2Si2, Mr = 677.04, 0.50 ”
0.25 ” 0.25 mm3, triclinic, a = 9.7607(6), b = 10.2083(6), c =
11.5575(7) Š, a = 97.0870(10), b = 105.3280(10), g = 105.0580(10)8,
V= 1049.44(11) Š3, T= 153 K, 1calcd = 1.071 gcmÀ3
, Z = 2, space
¯
group P1, R1 = 0.042( I > 2s(I), 4315 reflections), wR2 = 0.126 (for
all reflections) for 4981 reflections and 225 parameters, GOF = 1.005.
Crystallographic data for 9:[13] C56H80Li4Si4, Mr = 893.32, 0.35 ”
0.20 ” 0.20 mm3, monoclinic, a = 11.7703(10), b = 20.2245(16), c =
Figure 2. ORTEPdrawing of the structure of 9 (thermal ellipsoids with
23.4790(19) Š, b = 96.203(2)8, V= 5556.4(8) Š3, T= 103 K, 1calcd
=
40% probability for non-hydrogen atoms). All hydrogen atoms and the
atom positions of a disordered minor part are omitted for clarity.
1.068 gcmÀ3, Z = 4, space group P21/n, R1 = 0.048 (I > 2s(I), 7294
reflections), wR2 = 0.133 (for all reflections) for 9764 reflections and
681 parameters, GOF = 1.019. Disorder around one of the tert-butyl
groups was found. The occupancies of the disordered tert-butyl groups
were refined to be 0.59:0.41.
À
À
Selected bond lengths [Š]: C(1) C(2) 1.366(3), C(2) C(3) 1.548(3),
À
À
À
À
C(3) C(4) 1.369(3), C(1) Li(2) 2.222(4), C(2) Li(2) 2.299(4), C(3)
À
À
À
Li(2) 2.327(4), C(4) Li(2) 2.124(4), C(1) Li(1) 2.165(4), C(2) Li(1)
2.602(4), C(3)- Li(1) 2.558(4), C(4) Li(1) 2.185(4).
À
À
Received: October 3, 2006
Published online: January 24, 2007
2.25 Š for EtLi,[21] and 2.27 Š for 2-Me2NC6H4Li).[22] The
2
À
average Li C bond length in the case of h coordination is
2.18 Š, whereas the average distance between the h2-coordi-
nated lithium center and the C2and C3 atoms is 2.62Š, which
is significantly longer than the other Li–C distances. Similar
h2 coordination of a lithium atom was also reported for other
Z,Z configured 1,4-dilithio-1,3-butadienes.[3]
Keywords: alkynes · aromatic systems · intermolecular
.
cyclization · metalation · reduction
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In summary, the reduction of phenyl(triisopropylsilyl)-
acetylene (3) with lithium led to the unprecedented formation
of the dilithium dibenzopentalenide 7 together with 1,4-
dilithio-1,3-butadiene 6, whereas the reduction of phenyl(tert-
butyldimethylsilyl)acetylene (8) with lithium gave 1,4-dili-
thio-1,3-butadiene 9. Thus, the bulkiness of the silyl substitu-
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mode. Although the yield of the dilithium dibenzopentalenide
is not high, its ready crystallization from the crude mixture
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Experimental Section
Isolation of 7: A solution of 3 (4.010 g, 15.5 mmol) in diethyl ether
(16 mL) was added to lithium (401 mg, 57.8 mmol) at room temper-
ature, and the reaction mixture was stirred overnight. In a glovebox,
insoluble material was removed by filtration and the filtrate was
concentrated. Repeated recrystallization of the residue from hexane
1
at À308C gave 7 (377 mg, 7%). H NMR (400 MHz, diethyl ether/
C6D6): d = 1.18 (d, J = 8 Hz, 36H), 1.98 (sept, J = 8 Hz, 6H), 6.59–6.65
(m, 2H), 6.78–6.83 (m, 2H), 7.81 (d, J = 9 Hz, 2H), 8.12ppm (d, J =
8 Hz, 2H); 1H NMR (400 MHz, C6D6): d = À0.01 (t, J = 7 Hz, 12H),
1.45 (d, J = 7 Hz, 36H), 2.24–2.32 (m, 14H), 7.08–7.11 (m, 2H), 7.24–
7.28 (m, 2H), 8.21 (d, J = 9 Hz, 2H), 8.57 ppm (d, J = 8 Hz, 2H);
13C NMR (101 MHz, diethyl ether/C6D6): d = 14.8 (d), 20.1 (q), 112.8
(d), 117.6 (s), 119.6 (d), 121.8 (d), 122.8 (d), 141.7 ppm (s);
7Li NMR(156 MHz, diethyl ether/C6D6): d = À8.4 ppm.
Isolation of 9: A solution of 8 (931 mg, 4.30 mmol) in diethyl
ether (4.3 mL) was added to lithium (92mg, 13.3 mmol) at room
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filtrate was concentrated. Repeated recrystallization of the residue
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[11] For experimental details, see the Supporting Information.
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