Synthesis and isolation of the first germacyclopropabenzene: A study to elucidate the intrinsic factor for the ring deformation of cyclopropabenzene skeletons

Article abstract of DOI:10.1021/om020545x

A study was performed to elucidate the intrinsic factor for the ring deformation of cyclopropabenzene skeletons. The treatment of an overcrowded diaryldilithiogermane, Tbt(Dip)GeLi₂ (Tbt = 2,4,6,-tris[bis(trim-ethylsilyl)methyl]phenyl; Dip = 2,6-diisopropylphenyl), generated by exhaustive reduction of the corresponding dibromogermane Tbt(Dip)GeBr₂, with 1,2-dibromobenzene resulted in the isolation of the germacyclopropabenzene. The experimental results were in good agreement with those obtained by theoretical calculations.

Full text of DOI:10.1021/om020545x

Organometallics 2002, 21, 4309-4311
4309
Syn th esis a n d Isola tion of th e F ir st
Ger m a cyclop r op a ben zen e: A Stu d y to Elu cid a te th e
In tr in sic F a ctor for th e Rin g Defor m a tion of
Cyclop r op a ben zen e Sk eleton s
Norihiro Tokitoh,*,† Ken Hatano,‡ Takayo Sasaki,† Takahiro Sasamori,†
Nobuhiro Takeda,† Nozomi Takagi,§ and Shigeru Nagase§
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, J apan,
Institute for Fundamental Research of Organic Chemistry, Kyushu University,
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, J apan, and Institute for Molecular Science,
Myodaiji, Okazaki 444-8585, J apan
Received J uly 9, 2002
Ch a r t 1
Summary: The treatment of an overcrowded diaryldil-
ithiogermane, Tbt(Dip)GeLi2 (Tbt ) 2,4,6-tris[bis(trim-
ethylsilyl)methyl]phenyl; Dip ) 2,6-diisopropylphenyl),
generated by exhaustive reduction of the corresponding
dibromogermane Tbt(Dip)GeBr2, with 1,2-dibromoben-
zene resulted in the isolation of the first stable germa-
cyclopropabenzene, which was fully characterized by 1H
and 13C NMR spectra, FAB-MS, and X-ray structural
analysis. As for the ring deformation of cyclopropaben-
zene and its heavier group 14 element analogues, the
experimental results are in good agreement with those
obtained by theoretical calculations.
heteracyclopropabenzene by taking advantage of the
characteristic reactivity of the sterically hindered dil-
ithiosilane toward 1,2-dibromobenzene.5 An X-ray crys-
tallographic analysis revealed that the central benzene
ring of 2a is much less perturbed by annelation than
that of cyclopropabenzene (1) (Chart 1). The successful
isolation of silacyclopropabenzene 2a naturally prompted
us to examine the systematic structural comparison of
metallacyclopropabenzenes of heavier group 14 ele-
ments. In this paper, we present the synthesis and
crystallographic structural analysis of the first germa-
cyclopropabenzene 3a together with theoretical calcula-
tions on the molecular geometries of all heteracyclo-
propabenzenes containing a heavier group 14 element.
Similarly to the generation of dilithiosilane Tbt(Dip)-
SiLi2, the overcrowded dibromogermane Tbt(Dip)GeBr2
(4) was subjected to exhaustive reduction using an
excess amount of lithium naphthalenide (5 molar equiv)
at -78 °C in tetrahydrofuran in the hope of obtaining
the dilithiogermane Tbt(Dip)GeLi2 (5).6 The almost
Since the finding of significant deformation for the
fused aromatic rings in the series of benzocycloalkanes,1
one of the most important subjects assigned to organic
chemists has been to solve the riddle of such deforma-
tion.2 Cyclopropabenzene (1) has attracted special at-
tention because it has the most severely enforced
deformation in this series. Although the systematic
comparison of the structural features of cyclopropaben-
zene with those of its heteroatom analogues would be
very useful and informative in elucidating the intrinsic
factor for the deformation of the aromatic ring in this
fused-ring system, there has been no example of a stable
heteracyclopropabenzene so far.3
Meanwhile, we have already succeeded in the genera-
tion of the overcrowded diaryldilithiosilane Tbt(Dip)-
SiLi2 (Tbt ) 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl,
Dip ) 2,6-diisopropylphenyl) and found that this new
organosilicon building block is useful for the synthesis
of a variety of cyclic organosilicon compounds by the
reaction with bifunctional electrophiles.4 Recently, we
have synthesized and isolated silacyclopropabenzene 2a
(R ) Tbt, R′ ) Dip) as the first example of a stable
(5) Hatano, K.; Tokitoh, N.; Takagi, N.; Nagase, S. J . Am. Chem.
Soc. 2000, 122, 4829-4830.
(6) (a) Generation of Ar2GeM2 (Ar ) C6H5, p-CH3C6H4; M ) Li, Na,
K) by the reduction of the corresponding Ar2GeH2 with alkali metals
in HMPA or THF has already been reported. Although the dimetalated
germanes thus generated were treated with alkyl halides to produce
the corresponding substitution products in moderate yields, no coupling
product was reported in the reactions with p-halotoluenes: Mochida,
K.; Tatsushige, N.; Hamashima, M. Bull. Chem. Soc. J pn. 1985, 58,
1443-1447. (b) We have preliminarily reported the reaction of the
diaryldilithiogermane 5 with TeCl2, giving the corresponding germane-
tellone, a germanium-tellurium double-bond compound: Tokitoh, N.;
Sadahiro, T.; Hatano, K.; Sasaki, T.; Takeda, N.; Okazaki, R. Chem.
Lett. 2002, 34-35. (c) Very recently, silyl-substituted dilithiogermanes
have been isolated as stable compounds: Sekiguchi, A.; Izumi, R.;
Ihara, S.; Ichinohe, M.; Lee, V. Y. Angew. Chem., Int. Ed. 2002, 41,
1598-1600.
* To whom correspondence should be addressed. E-mail: tokitoh@
boc.kuicr.kyoto-u.ac.jp. Tel: +81-774-38-3200. Fax: +81-774-38-3209.
† Kyoto University.
‡ Kyushu University.
§ Institute for Molecular Science.
(1) Mills, W. H.; Nixson, I. G. J . Chem. Soc. 1930, 2510-2524.
(2) For example: (a) Halton, B. Chem. Rev. 1989, 89, 1161-1185.
(b) Halton, B. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 349-364.
(3) (a) The´taz, C.; Wentrup, C. J . Am. Chem. Soc. 1976, 98, 1258-
1259. (b) Schulz, R.; Schweig, A. Tetrahedron Lett. 1984, 25, 2337-
2340. (c) White, R. C.; Scoby, J .; Roberts, T. D. Tetrahedron Lett. 1979,
30, 2785-2788.
(4) Tokitoh, N.; Hatano, K.; Sadahiro, T.; Okazaki, R. Chem. Lett.
1999, 931-932.
10.1021/om020545x CCC: $22.00 © 2002 American Chemical Society
Publication on Web 09/13/2002

4310 Organometallics, Vol. 21, No. 21, 2002
Communications
Sch em e 1
Sch em e 2
quantitative generation of 5 was confirmed by the
trapping experiments with deuterium oxide and io-
domethane to give the corresponding trapping reaction
products 6 and 7, respectively (Scheme 1). Interestingly,
a considerable amount of dilithiogermane 5 can be
trapped with these reagents without the migration of
lithium atom from the germanium center to the ortho
benzyl position of the Tbt group, even at room temper-
ature. The much higher thermal stability of 5 observed
here is in sharp contrast to the ready lithium migration
of its silicon analogue at -50 °C.4
The dilithiogermane 5 thus generated was allowed to
react with 1 equiv of 1,2-dibromobenzene at -78 °C, and
the following purification by HPLC and recrystallization
from hexane/EtOH resulted in the isolation of the first
stable germacyclopropabenzene, 3a , in 40% yield.7
Interestingly, 3a was isolated as stable colorless crystals
in the air but slowly decomposed on silica gel, giving
the ring-opened hydrolyzed product 8, in contrast to the
high stability of its silicon analogue previously reported
(Scheme 2).8 Anyhow, it should be noted that the same
synthetic approach as that for silacyclopropabenzene 2a
can be applied to its heavier congener, i.e., germacyclo-
propabenzene 3a .
F igu r e 1. ORTEP drawing of germacyclopropabenzene 3a
with thermal ellipsoid plots (50% probability). Selected
bond lengths (Å) and angles (deg): Ge(1)-C(1), 1.9403(18);
Ge(1)-C(2), 1.9318(18); Ge(1)-C(7), 1.9666(17); Ge(1)-
C(34), 1.9761(18); C(1)-C(2), 1.391(3); C(1)-C(6), 1.385-
(3); C(2)-C(3), 1.388(3); C(3)-C(4), 1.392(3); C(4)-C(5),
1.397(3); C(5)-C(6), 1.391(3); C(1)-Ge(1)-C(2), 42.11(8);
C(1)-Ge(1)-C(7), 121.54(7); C(1)-Ge(1)-C(34), 113.13(7);
C(2)-Ge(1)-C(7), 119.36(7); C(2)-Ge(1)-C(34), 117.07(7);
C(7)-Ge(1)-C(34), 120.05(7); Ge(1)-C(1)-C(2), 68.62(11);
Ge(1)-C(1)-C(6), 169.47(16); C(2)-C(1)-C(6), 121.88(17);
Ge(1)-C(2)-C(1), 69.27(10); Ge(1)-C(2)-C(3), 168.68(16);
C(1)-C(2)-C(3), 122.00(17); C(2)-C(3)-C(4), 116.28(18);
C(3)-C(4)-C(5), 121.72(18); C(4)-C(5)-C(6), 121.61(18);
C(1)-C(6)-C(5), 116.51(18).
The molecular structure of 3a determined by X-ray
crystallographic analysis is shown in Figure 1 together
with some selected bond lengths and angles.9 The
germacyclopropabenzene skeleton was found to have a
completely planar geometry. The sums of bond angles
at C1 and C2 are both almost 360°, and the sum of
interior bond angles in the benzene ring is 720.0°. All
six C-C lengths in the central benzene ring of 3a are
F igu r e 2. Comparison of bond angles in cyclopropaben-
zenes 1, 2a , and 3a . Legend for footnotes: adata were
collected at -153 °C;13 bdata were collected at -180 °C;5
cthis work.
(7) 3a : colorless crystals; mp 235-237 dec; 1H NMR (CDCl3, 400
MHz) δ -0.10 (s, 36H), 0.02 (s, 18H), 0.92 (d, 6H, 3J HH ) 6.5 Hz), 1.28
(s, 1H), 1.33 (d, 6H, 3J HH ) 6.5 Hz), 2.49 (brs, 1H), 2.72 (brs, 1H), 3.70
in the range of usual C-C distances reported for a
nonperturbed benzene ring (1.39-1.40 Å).12 To clarify
the structural differences in the benzene moiety of the
series of cyclopropabenzenes (1, 2a , and 3a ) character-
ized by X-ray analyses, the bond angles and lengths for
1, 2a , and 3a are shown in Figures 2 and 3, respectively.
Germacyclopropabenzene 3a has a slightly squashed
benzene moiety, in which all bond angles deviate from
the ideal sp2 bond angle (120°). As can be seen in Figure
3
(sept, 2H, J HH ) 6.5 Hz), 6.23 (br s, 1H), 6.38 (brs, 1H), 7.10 (d, 2H,
4
3
3J HH ) 7.7 Hz), 7.24 (AA′BB′, 2H, J HH ) 2.8, J HH ) 5.0 Hz), 7.25 (t,
1H, J HH ) 7.7 Hz), 7.61 (AA′BB′, 2H, J HH ) 2.8, J HH ) 5.0 Hz); 13
NMR (CDCl3, 100 MHz) δ 1.04 (q), 1.08 (q), 1.25 (q), 22.7 (q), 27.9 (q),
27.9 (d), 28.3 (d), 30.6 (d), 35.0 (d), 122.8 (d), 123.1 (d), 126.2 (d), 128.0
(d), 129.1 (d), 129.6 (s), 129.7 (d), 139.0 (s), 143.9 (s), 149.8 (s), 150.3
C
3
4
3
(s), 150.5 (s), 153.4 (s); high-resolution FAB-MS m/z calcd for C45H81
-
Si672Ge 861.4174 ([M + H]+), found 861.4178 ([M + H]+). Anal. Calcd
for C45H80Si6Ge: C, 62.68; H, 9.35. Found: C, 62.38; H, 9.51.
(8) The surprising stability of the ring-opening reaction of silicon
analogue 2a was already reported in ref 5. Compound 2a can be
purified by preparative thin-layer chromatography on silica gel without
any decomposition. The reactivity of germacyclopropabenzene 3a will
be described elsewhere.
(10) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R.
J . Appl. Crystallogr. 1999, 32, 115-119.
(11) Sheldrick, G. M. SHELX-97, Program for the Refinement of
Crystal Structures; University of Go¨ttingen, Go¨ttingen, Germany,
1997.
(9) Crystallographic data for 3a : the structure was solved by direct
methods (SIR-97)10 and refined by full-matrix least-squares procedures
on F2 for all reflections (SHELX-97);11 C45H80Si6Ge, mol wt 862.22,
triclinic, space group P1h (No. 2), a ) 9.8813(3) Å, b ) 12.0099(11) Å,
c ) 23.125(4) Å, r ) 76.709(3)°, â ) 84.638(3)°, γ ) 72.6833(13)°, V )
2548.8(5) Å3, Z ) 2, Dcalcd ) 1.123 Mg/m3, µ ) 0.770 mm-1, R1(I >
2σ(I)) ) 0.0309, wR2(all data) ) 0.0789, T )103(2) K, GOF ) 1.045.
(12) Allen, F. K.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G. J . Chem. Soc., Perkin Trans. 2 1987, S1-S19.

Communications
Organometallics, Vol. 21, No. 21, 2002 4311
carbons by the expanded three-membered ring to the
direction of their outer apex.4 The X-ray analyses for
2a and 3a strongly suggest that the distinct molecular
distortion and bond localization observed in cyclopropa-
benzene 1 is a characteristic property due to tight
annelation and also are in good agreement with theo-
retical calculations for parent sila- and germacyclopr-
opabenzenes.
For a systematic study of heteracyclopropabenzenes
containing a group 14 element, theoretical calculations
were carried out for the heavier congeners of cyclo-
propabenzene, i.e., 3b (R ) R′ ) H), 9 (E ) Sn), and 10
(E ) Pb).14 All of them were found to have a planar
heteracyclopropabenzene skeleton as an energy mini-
mum, as in the case of silacyclopropabenzene 2b (R )
R′ ) H),5 and no remarkable geometrical difference was
observed among the benzene moieties of heavier cyclo-
propabenzenes (2b, 3b, 9, and 10). Our conclusion on
the structures of sila- and germacyclopropabenzenes (2
and 3) is that the three-membered ring containing a
heavier group 14 element can enjoy annelation with
much less perturbation.
F igu r e 3. Comparison of bond lengths in cyclopropaben-
zenes 1, 2a , and 3a . Legend for footnotes: adata were
collected at -153 °C;13 bdata were collected at -180 °C;5
cthis work.
2, the same tendency (R, γ > 120° > â) was observed
for the series of cyclopropabenzenes 1-3, and this can
be interpreted in terms of the influence of the three-
membered-ring annelation. As for the bond lengths,
cyclopropabenzene 1, the most tightly annelated system,
is most significantly perturbed as compared with other
heavier congeners 2a and 3a . The obvious C-C bond
shortening compared with the typical aromatic C-C
length is reported for the juncture C-C bond a and its
neighboring bonds b and b′ of 1 (Figure 3).13 However,
such C-C shortening was not recognized for germacy-
clopropabenzene 3a (a ) 1.391(3) Å and b, b′ ) 1.388-
(3), 1.385(3) Å) (Figure 1). Thus, the structural features
of benzene nuclei in germacyclopropabenzne 3a are very
similar to those of the previously reported silicon
analogue 2a .5
The most extreme structural difference between the
hydrocarbon system 1 and its heteroatom analogues (2a
and 3a ) is the length of the juncture bond a , which is
most probably due to their disparate ways of releasing
of the strain energy.5 Annelation of a three-membered
ring to a benzene ring naturally enforces severe distor-
tion upon the juncture carbons (C1 and C2), leading to
deviation from the ideal sp2 geometry. The extreme bond
shortening observed in cyclopropabenzene 1 is one of
the modes that compensates for this localized distortion.
On the other hand, such bond shortening is not neces-
sary for the heavier congeners, since 2a and 3a have
longer C-M bonds (M ) Si, Ge) than the C-C bonds in
1. They can release the strain energy at juncture
Further studies on the preparations of unprecedented
ring systems, i.e., other heavier cyclopropabenzene
analogues and bis(heteracyclopropa)benzenes, are cur-
rently in progress.
Ack n ow led gm en t. This work was supported by a
Grant-in-Aid for COE Research on Elements Science
(No. 12CE2005), a Grant-in-Aid for Scientific Research
on Priority Areas (A) (No. 11166250) from the Ministry
of Education, Culture, Sports, Science, and Technology
of J apan, and Grants-in-Aid for Scientific Research (A)
(Nos. 11304045 and 14204064) from the J apan Society
for the Promotion of Science.
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
data with complete tables of bond lengths, bond angles, torsion
angles, and thermal and positional parameters for 3a . This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM020545X
(14) Calculated at the B3LYP/6-311G(2d, p) level (TZ(2d for Ge, Sn,
and Pd)): 3b (E ) Ge), a ) 1.385 Å, b ) 1.390 Å, c ) 1.392 Å, d )
1.403 Å, R ) 122.14°, â ) 116.21°, γ ) 121.65°; 9 (E ) Sn), a ) 1.390
Å, b ) 1.391 Å, c ) 1.392 Å, d ) 1.401 Å, R ) 121.71°, â ) 116.87°, γ
) 121.42°; 10 (E ) Pb), a ) 1.389 Å, b ) 1.388 Å, c ) 1.394 Å, d )
1.399 Å, R ) 121.82°, â ) 116.75°, γ ) 121.43°.
(13) Neidlein, R.; Christen, D.; Poigne´e, V.; Boese, R.; Bla¨ser, D.;
Gieren, A.; Ruiz-Pe´rez, C.; Hu¨bner, T. Angew. Chem., Int. Ed. Engl.
1975, 27, 294-295.