Synthesis of bis(germacyclopropa)benzenes and structures of their annelated benzene rings

Article abstract of DOI:10.1021/om0507629

Extremely hindered bis(germacyclopropa)benzenes (3a,b; the IUPAC name is 4,8-digermatricyclo[5.1.0.0^3,5]octa-1,3(5),6-triene) were synthesized as stable crystalline compounds by the reaction of the corresponding dilithiogermane Tbt(Dip)GeLi

Full text of DOI:10.1021/om0507629

230
Organometallics 2006, 25, 230-235
Synthesis of Bis(germacyclopropa)benzenes and Structures of Their
Annelated Benzene Rings
Tomoyuki Tajima, Takahiro Sasamori, Nobuhiro Takeda, Norihiro Tokitoh,*
Ken Yoshida, and Masaru Nakahara
Institute for Chemical Research, Kyoto UniVersity, Gokasho, Uji, Kyoto 611-0011, Japan
ReceiVed September 2, 2005
Extremely hindered bis(germacyclopropa)benzenes (3a,b; the IUPAC name is 4,8-digermatricyclo-
[5.1.0.03,5]octa-1,3(5),6-triene) were synthesized as stable crystalline compounds by the reaction of the
corresponding dilithiogermane Tbt(Dip)GeLi2 (8; Tbt ) 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Dip
) 2,6-diisopropylphenyl) with 1,2,4,5-tetrabromobenzene. The structures of the two stereoisomeric bis-
(germacyclopropa)benzenes (3a, cis isomer; 3b, trans isomer) were definitively determined by X-ray
crystallographic analysis. The central benzene ring of 3a was found to be folded, in contrast to the planar
benzene ring of 3b. The X-ray crystallographic analyses of 3a and 3b and the theoretical calculations for
some model molecules revealed that the annelated benzene rings have no distinct bond alternation.
Chart 1. Bis(cyclopropa)benzene (1) and Related Ring
Systems
Introduction
Cycloproparenes,1 i.e., strained aromatic hydrocarbon com-
pounds fused with a three-membered ring, have attracted
theoretical and experimental interest due to their unique
structures and reactivities. Nevertheless, the synthesis of bis-
(cyclopropa)benzene (1; Chart 1), in which two cyclopropene
rings are fused to the central benzene ring, has not been achieved
yet,2 probably due to its high strain energy and the lack of
suitable synthetic methods. The attention to the structure of such
a strained ring system has been drawn mainly from the
viewpoint of the Mills-Nixon effect.3 In contrast to this effect,
it is well-known that the lengths of the juncture bonds of many
types of cycloproparenes are found to be shorter than that of a
benzene, as evidenced by computational works4a-e and experi-
mental results.4f-m On the other hand, no experimental evidence
has been obtained for the benzene nucleus annelated with
multiple small hydrocarbon rings thus far, though the hypothesis
itself is stated also for such multiring annelated systems that
they are expected to have considerable bond alternation.5 In this
context, elucidation of the structural features of benzene
derivatives annelated with multiple small rings might be
important to understand the strains of small-ring compounds
systematically.
of the corresponding overcrowded dilithiometallanes7-10 Tbt-
(Dip)ELi2 (Tbt ) 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl;
Dip ) 2,6-diisopropylphenyl; E ) Si, Ge), with o-dibromoben-
zene. The new isolated metallacyclopropabenzenes 6b,c were
fully characterized to show their unique molecular structures
and reactivities.11 More recently, we succeeded in the synthesis
of the first stable bis(silacyclopropa)benzenes 2a,b by the
(4) Many groups have cast serious doubts on the Mills-Nixon effect;
see: (a) Stanger, A. J. Am. Chem. Soc. 1991, 113, 8277-8280. (b)
Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 1992, 114, 9583-9587.
(c) Bu¨rgi, H. B.; Baldridge, K. K.; Hardcastle, K.; Frank, N. L.; Gantzel,
P.; Siegel, J. S.; Ziller, J. Amgew. Chem., Int. Ed. Engl. 1995, 34, 1454-
1456. (d) Stanger, A. J. Am. Chem. Soc. 1998, 120, 12034-12040. (e)
Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 2001, 123, 1755-1759.
(f) Bachrach, S. M. J. Organomet. Chem. 2002, 634, 39-46. (g) Billups,
W. E.; Arnry, B. E.; Lin, L. J. Org. Chem. 1984, 39, 3436-3437. (h)
Diercks, R.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1986, 108, 3150-3152.
(i) Boese, R.; Bla¨ser, D. Angew. Chem., Int. Ed. Engl. 1988, 27, 304-305.
(j) Neidlein, R.; Christie, D.; Poignee, V.; Boese, R.; Bla¨ser, D.; Gieren,
A.; Ruiz-Perez, C.; Huebner, T. Angew. Chem., Int. Ed. Engl. 1988, 27,
294-295. (k) Bla¨ser, D.; Boese, R.; Brett, W. A.; Rademacher, P.;
Schwager, H.; Stanger, A.; Vollhardt, K. P. C. Angew. Chem., Int. Ed. Engl.
1989, 28, 206-209. (l) Boese, R.; Bla¨ser, D.; Billiups, W. E.; Haley, M.
M.; Maulitz, A. H.; Mohler, D. L.; Vollhardt, K. P. C. Angew. Chem., Int.
Ed. Engl. 1994, 33, 313-317. (m) Holmes, D.; Kumaraswamy, S.; Matzger,
A. J.; Vollhardt, K. P. C. Chem. Eur. J. 1999, 5, 3399-3412. (n) Beckhaus,
H.-D.; Faust, R.; Matzger, A. J.; Mohler, D. L.; Rogers, D. W.; Ru¨chart,
C.; Sawhney, A. K.; Verevkin, S. P.; Vollhardt, C.; Wolff, S. J. Am. Chem.
Soc. 2000, 122, 7819-7820.
Although no stable example had been known for heteracy-
clopropabenzenes, i.e., cyclopropabenzene derivatives having
a bridging heteroatom instead of a carbon atom, except for some
transient, intermediary species reported so far,6 we have recently
reported the synthesis of the first stable silacyclopropabenzene
(6b)7a,d and germacyclopropabenzene (6c),7b i.e., heavier element
analogues of a cyclopropabenzene derivative, by the reactions
* To whom correspondence should be addressed. Tel: +81-774-38-3200.
Fax: +81-774-38-3209. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp.
(1) For reviews, see: (a) Halton, B. Chem. ReV. 2003, 103, 1327-1370.
(b) Halton, B. Chem. ReV. 1989, 89, 1161-1185. (c) Halton, B. Ind. Eng.
Chem. Prod. Res. DeV. 1980, 19, 349-364.
(2) For the attempted synthesis of a bis(cyclopropa)benzene, see: (a)
Banwell, M. G.; Halton, B. Aust. J. Chem. 1979, 32, 2689-2699. (b)
Banwell, M. G.; Halton, B. Aust. J. Chem. 1979, 32, 849-858. (c) Banwell,
M. G.; Halton, B. Aust. J. Chem. 1980, 33, 2277-2290. For the attempted
synthesis of a bis(cyclopropa)[a.c]naphthalene, see: (d) Brinker, U. H.;
Wu¨ster, H.; Maas, G. Angew. Chem., Int. Ed. Engl. 1987, 26, 577-578.
(3) Mills, W. H.; Nixon, I. G. J. Chem. Soc. 1930, 2510-2524.
(5) For recent theoretical calculations on 1, see: (a) Soncini, A.; Havenith,
R. W. A.; Fowler, P. W.; Jenneskens, L. W.; Steiner, E. J. Org. Chem.
2002, 67, 4753-4758. (b) Bachrach, S. M. J. Organomet. Chem. 2002,
643, 39-46.
10.1021/om0507629 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 11/08/2005

Tajima et al.
Organometallics, Vol. 25, No. 1, 2006 231
Scheme 1. Synthesis of 3a,b
reaction of an overcrowded dilithiosilane, Tbt(Dip)SiLi2, with
1,2,4,5-tetrabromobenzene.7c,d Meanwhile, the bis(metallacy-
clopropa)benzenes bis(nickelacyclopropa)benzene (4) and bis-
(zirconacyclopropa)benzene (5) have been prepared and their
crystal structures were reported by Buchwald and Bennett,
respectively.12 Interestingly, 4 and 5 feature not bis(metallacy-
clopropa)benzene character but transition-metal-benzyne com-
plex character, as judged by the results of their X-ray crystal-
lographic analysis.
Here, we describe the synthesis of the first stable bis-
(germacyclopropa)benzenes (3a, cis isomer; 3b, trans isomer),
the heavier congeners of 1 and 2, by the reaction of the
corresponding overcrowded dilithiogermane Tbt(Dip)GeLi2 (8)
with 1,2,4,5-tetrabromobenzene. The structural features of 3a,b
were revealed by X-ray crystallographic analyses along with
theoretical studies on the structures of some model molecules.
The aromaticity of the central benzene rings of 3a,b is also
evaluated on the basis of their structures, NMR studies, and
NICS calculations together with those of related compounds.
Scheme 2. Possible Mechanism for the Formation of 3a,b
Results and Discussion
Synthesis of Bis(germacyclopropa)benzenes 3a,b. Treat-
ment of the diaryldilithiogermane Tbt(Dip)GeLi2 (8), which was
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generated by the exhaustive reduction of Tbt(Dip)GeBr2 (7) with
an excess amount of lithium naphthalenide (THF solution, 5
mol equiv) at -78 °C in THF, with 0.6 equiv of 1,2,4,5-
tetrabromobenzene at -40 °C gave the bis(germacyclopropa)-
benzenes 3a (cis isomer) and 3b (trans isomer) as colorless
crystals in 13 and 7% yields, respectively (Scheme 1). Although
bis(germacyclopropa)benzenes 3a,b were stable enough to be
handled in the open air and/or in hydrocarbon solvents such as
hexane and benzene, they underwent decomposition in halo-
genated solvents such as CHCl3 or on treatment with silica gel.13
The formation of bis(germacyclopropa)benzenes 3a,b can be
explained by a reaction mechanism similar to that for the
previously reported metallacyclopropabenzenes 6b,c.7 At first,
dilithiogermane 8 reacts with 1,2,4,5-tetrabromobenzene to give
Tbt(Dip)GeBrLi (10)7b and 1,2,4-tribromo-5-lithiobenzene via
a Li-Br exchange reaction. Then, the intermediary 4,5-
dibromobenzyne, which should be generated by the elimination
of LiBr from 1,2,4-tribromo-5-lithiobenzene, may react with
germylenoid 107b to afford the o-germylated phenyllithium 11.
The intramolecular cyclization of 11 followed by a subsequent
similar condensation at the 3,4-positions of the resulting 3,4-
dibromo-1-germacyclopropabenzene gives 3a,b (Scheme 2). In
addition, Tbt(Dip)GeBr2 (7; 12%) and Tbt(Dip)GeH2 (9; 4%)
were obtained as other reaction products in this reaction.
Dibromogermane 7 was most likely obtained by the competing
Li-Br exchange reaction of Tbt(Dip)GeBrLi (10) with 1,2,4,5-
tetrabromobenzene.7b Tbt(Dip)GeH2 (9) was probably generated
by the protonation of 8. The formation of bis(germacyclopropa)-
benzenes 3a,b should be interpreted in terms of the concurrent
generation of different reactive species such as the germylenoid
10 and benzyne species in the reaction under the mild conditions.
Molecular Structures of 3a,b in the Solid State. The
molecular structures of 3a,b were definitively determined by
X-ray crystallographic analysis at -170 °C (Figures 1 and 2).
The selected bond lengths and angles of 3a,b are listed in Table
1, together with the optimized structural parameters of model
(11) (a) Tajima, T.; Sasaki, T.; Sasamori, T.; Takeda, N.; Tokitoh, N.
Chem. Commun. 2004, 402-403. (b) Tajima, T.; Sasaki, T.; Sasamori, T.;
Takeda, N.; Tokitoh, N. Appl. Organomet. Chem. 2005. 19, 570-577. (c)
Sasamori, T.; Sasaki, T.; Takeda, N.; Tokitoh, N. Organometallics 2005,
24, 612-618.
molecules 3c-e (3c, R1-4 ) H; 3d, R1-4 ) Me; 3e, R1-4
)
(12) (a) Bennett, M. A.; Grage, J. S.; Griffiths, K. D.; Roberts, N. K.;
Robertson, G. B.; Wickramasinghe, W. A. Angew. Chem., Int. Ed. Engl.
1988, 27, 941-942. (b) Buchwald, S. L.; Lucas, E. A.; Dewan. J. C. J.
Am. Chem. Soc. 1987, 109, 4396-4397.
(13) The bis(silacyclopropa)benzenes 2a,b were surprisingly stable as
compared with 3a,b. Compounds 2a,b can be purified by preparative thin-
layer chromatography on silica gel without any decomposition.7c,d

232 Organometallics, Vol. 25, No. 1, 2006
Bis(germacyclopropa)benzenes
differences of the bond lengths. Although there may be some
possibility for the existence of bond alternation in 3a, it can be
concluded that the bis(germacyclopropa)benzenes 3a,b showed
no distinct bond alternation and no obvious Mills-Nixon effect
was observed in 3a,b at this stage.
Interestingly, the central benzene ring skeletons of 3a,b have
bond angles undoubtedly deviating from those of the usual
benzene rings (120°); that is, the interior angles at C3 and C6
are 111.9-112.5° and those of the C1, C2, C4, and C5 juncture
carbons are 123.1-124.2°. The deviations in the interior angles
of 3a and 3b are slightly larger than those of the previously
reported germacyclopropabenzene 6c.8c
Figure 1. ORTEP drawing of 3a with thermal ellipsoid plots (50%
probability). Hydrogen atoms are omitted for clarity.
The observed structural parameters of the central benzene
rings of bis(germacyclopropa)benzenes 3a,b are supported by
the calculated values for some model molecules (3c-e).
Experimental and theoretical studies on the structural features
of the bis(germacyclopropa)benzenes suggest that the structural
features of the central benzene rings of 3a,b are different from
those of hydrocarbon systems: i.e., the bis(cyclopropa)benzene
derivatives 1.
1H and 13C NMR Spectroscopic Studies of 3a,b. Bis-
(germacyclopropa)benzenes 3a,b exhibited very complicated 1H
and 13C NMR spectra at room temperature. To solve this
1
problem, we have measured their H and 13C NMR spectra at
various temperatures. The NMR chemical shifts for the protons
and carbons of the central benzene rings of 3a,b were found in
the aromatic region. In the 1H NMR spectrum of 3a in
tetrachloroethane-d2 at 30 °C, two protons of the central benzene
ring appeared as a singlet signal at 7.77 ppm.15 In contrast to
the case of 3a, the aromatic protons of bis(germacyclopeopa)-
benzene 3b were observed nonequivalently as two signals at
7.87 and 7.98 ppm in C6D12 at 30 °C. At 50 °C, the coalesced
signal assignable to the aromatic protons of the bis(germacy-
clopropa)benzene ring was observed as one broad signal at 7.93
ppm (Figure 4). This dynamic behavior was observed as fully
reversible processes.
The 13C NMR spectrum of 3b in C6D12 at 80 °C showed
signals at 126.3 (Câ) and 150.0 ppm (CR),16 which can be
assigned to the 13C NMR chemical shifts for the carbon atoms
of the central benzene ring, in good agreement with those
calculated in some model molecules (Table 2). The 13C NMR
chemical shifts of 3b observed here are in the region similar to
those of silacyclopropabenzene 6b (150.0 (CR) and 126.6 ppm
(Câ)) and germacyclopropabenzene 6c (153.4 (CR) and 126.6
ppm (Câ)).
Signals in the 13C NMR spectra of 3a could not be assigned,
due to their being much more complicated than those of 3b
even at higher temperatures. Thus, measurements of the 13C
NMR spectra of 3a were performed under several conditions
(for example, the measurement at 135 °C in tetrachloroethane-
d2), but the spectra were still complicated and 3a slowly
decomposed under such severe conditions. Such complication
of the 13C NMR spectra of 3a is probably due to the highly
restricted rotation of the some C-C bonds in 3a.
Figure 2. ORTEP drawing of 3b with thermal ellipsoid plots (50%
probability). Hydrogen atoms and a solvated hexane molecule are
omitted for clarity.
Ph). The structural analysis for 3b revealed the planarity of the
bis(germacyclopropa)benzene ring. The sums of the bond angles
around the juncture carbon atoms (C1, C2, C4, and C5) and
the sum of the interior bond angles of the central benzene ring
are almost 360 and 720°, respectively. On the other hand, the
central benzene ring of 3a was found to be slightly folded.
The folded benzene ring of 3a is depicted in Figure 3 together
with that of 2a. The dihedral angle between the Ge1-C1-C2
and C6-C1-C2-C3 planes (δ) and that between the C3-C4-
C5-C6 and C4-Ge2-C5 planes (ú) are 13.69 and 11.73°,
respectively. Moreover, the benzene ring is bent and the dihedral
angle between the C6-C1-C2-C3 and C3-C4-C5-C6
planes (ꢀ) is 1.88°. Previous works showed that the central
benzene ring of bis(silacyclopropa)benzene 2a had a similar bent
structure, probably due to the steric repulsion between the two
bulky Tbt groups (the corresponding dihedral angles are δ )
11.95°, ú ) 10.31° and ꢀ ) 4.73°, respectively).7c,d The bent
structure of 3a might also be due to the large steric repulsion
provided by the extremely bulky steric protection groups as in
the case of 2a, in consideration of the planar geometries of the
central benzene ring in the theoretically optimized structures
of 3c-e.
Calculations of Nucleus-Independent Chemical Shift
(NICS)17 Values for Bis(metallacyclopropa)benzenes. It is
The computational works on bis(cyclopropa)benzene 1 (R
) H) by Jenneskens et al. indicated the relatively short length
(1.336 Å) for the juncture C-C bonds in its benzene ring.5 In
the case of 3a, the C-C bond lengths in the central benzene
ring vary to a small extent (between 1.416(9) and 1.364(9) Å),
while those of 3b showed almost no variation (1.389(6)-1.403-
(6) Å).14 The existence of bond alternation in 3a cannot be
argued, due to relatively large standard deviations and the subtle
(14) Typical C-C bond lengths of benzene are 1.39-1.40 Å: Allen, F.
H.; Kennard, O.; Watson, D. C.; Brammer, L.; Orpen, A. G. J. Chem. Soc.,
Perkin Trans. 2 1987, S1-S19.
(15) In the 1H NMR of 3a in C6D12 at 30 °C, the singlet observed at
7.84 ppm can be assigned to the two protons of the central benzene ring.
1
The aliphatic region of the H NMR spectrum of 3a was too complicated
to be assigned.
(16) Variable-temperature 13C NMR spectra of 3b were measured in the
range of 30-80 °C. See the Supporting Information.

Tajima et al.
Organometallics, Vol. 25, No. 1, 2006 233
Table 1. Observed and Calculated Bond Lengths (Å) and Angles (deg) of Bis(germacyclopropa)benzenes
obsd
calcda
3a (cis isomer)
R1 ) R3 ) Dip
R2 ) R4 ) Tbt
3b (trans isomer)
R1 ) R4 ) Dip
R2 ) R3 ) Tbt
3c
3d
3e
R1-4 ) H
R1-4 ) Me
R1-4 ) Ph
Bond Lengths
Ge1-C1
Ge1-C2
Ge2-C4
Ge2-C5
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C6-C1
1.928(7)
1.934(6)
1.939(7)
1.908(6)
1.408(10)
1.381(8)
1.396(9)
1.364(9)
1.416(9)
1.403(9)
1.937(4)
1.944(4)
1.935(4)
1.938(4)
1.394(6)
1.394(6)
1.397(6)
1.389(6)
1.391(6)
1.403(6)
1.929
1.929
1.929
1.929
1.391
1.392
1.392
1.391
1.392
1.392
1.931
1.931
1.931
1.931
1.400
1.392
1.392
1.400
1.392
1.392
1.932
1.932
1.932
1.932
1.400
1.393
1.393
1.400
1.393
1.393
Bond Angles
C1-C2-C3
123.1(6)
113.3(6)
124.2(6)
123.9(6)
111.9(6)
123.5(6)
68.8(4)
68.4(4)
42.8(3)
68.0(4)
70.4(4)
41.5(3)
123.8(4)
112.5(4)
123.7(4)
124.2(4)
112.3(4)
123.6(4)
69.2(2)
68.7(3)
42.10(17)
69.1(3)
123.8
112.3
123.8
123.8
112.3
123.8
68.9
68.9
42.2
68.9
68.9
123.2
113.6
123.2
123.2
113.6
123.2
68.7
68.7
42.5
68.7
68.7
123.4
113.3
123.4
123.4
113.3
123.4
68.8
68.8
42.5
68.8
68.8
C2-C3-C4
C3-C4-C5
C4-C5-C6
C5-C6-C1
C6-C1-C2
Ge1-C1-C2
Ge1-C2-C1
C1-Ge1-C2
Ge2-C4-C5
Ge2-C5-C4
C4-Ge2-C5
68.9(3)
42.05(18)
42.2
42.5
42.5
a B3LYP/6-311+G(2d,p) for C and H and LANL2DZ for Ge.
1
Table 2. Observed and Calculated H and 13C NMR
Chemical Shifts (ppm) of Bis(germacyclopropa)benzenes and
Related Compounds
Figure 3. Dihedral angles of 2a (E ) Si) and 3a (E ) Ge).
δ(Hâ)
δ(CR)
δ(Câ)
δ(Cγ)
3a (R1, R3 ) Dip, R2, R4 ) Tbt)a
3b (R1, R4 ) Dip, R2, R3 ) Tbt)c
3c (R1-4 ) H)d
7.83
7.93
8.17
8.08
b
b
150.0
145.8
155.6
125.4
150.0
153.4
126.3
131.5
130.2
114.7
126.6
126.6
3d (R1-4 ) Me)d
6a (E ) C, R1,2 ) H)e
128.8
130.1
129.7
6b (E ) Si, R1 ) Dip, R2 ) Tbt)f
6c (E ) Ge, R1 ) Dip, R2 ) Tbt)f
a Measured in tetrachloroethane-d2 at 90 °C. b Not assigned. c Measured
in cyclohexane-d12 at 80 °C. d Calculated at GIAO-B3LYP/6-311++G(2d,p)//
B3LYP/6-311G(2d,p) for C and H and LANL2DZ for Ge. e Measured in
CCl4/CDCl3 (3:1). f Measured in CDCl3 at room temperature.
for the NICS indicate a greater aromaticity for the ring. The
NICS(1) values of the related compounds are listed in Figure
5. The computation of the NICS(1) value of bis(germacyclo-
propa)benzene 3c showed a large negative value (-13.2 ppm),
as did that of benzene (-12.8 ppm). It is suggested that bis-
(germacyclopropa)benzene 3c has a ring current effect similar
to that of the monogermacyclopropabenzene on the basis of
NICS values of cyclopropabenzenes and metallacyclopropa-
benzenes. It should be noted that the aromaticity of the benzene
ring is not dramatically changed, even in the case of an arene
ring fused with two three-membered rings.
Figure 4. Variable-temperature 1H NMR spectra for the aromatic
region of 3b in cyclohexane-d12.
important to compare the aromaticity of the central benzene
ring of the bis(metallacyclopropa)benzenes with that of the
related hydrocarbon systems and monometallacyclopropaben-
zenes. Recently, Schleyer has advocated the use of the NICS
as a probe for aromaticity.17 Negative larger absolute values
(17) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes,
N. J. R. v. E. J. Am. Chem. Soc. 1996, 118, 6317-6318.

234 Organometallics, Vol. 25, No. 1, 2006
Bis(germacyclopropa)benzenes
spectrometer. Elemental analyses were carried out at the Micro-
analytical Laboratory of Institute for Chemical Research, Kyoto
University. Melting points were determined on a Yanaco micro
melting point apparatus and were uncorrected. Tbt(Dip)GeBr2 (7)
was prepared according to the reported procedures.7b
Synthesis of Bis(germacyclopropa)benzenes 3a,b. A THF
solution of lithium naphthalenide (2.36 M, 0.53 mL, 1.25 mmol, 5
mol equiv) was added to a THF solution (3 mL) of 7 (227 mg,
0.25 mmol) at -78 °C. After the mixture was stirred for 1 h at the
same temperature, 1,2,4,5-tetrabromobenzene powder (58 mg, 0.15
mmol) was added to the reaction mixture at -40 °C. After the
removal of the solvent, the mixture was extracted with hexane
several times. The extracts were combined, filtered with Celite,
and evaporated. The residue was subjected to GPLC (toluene)
followed by gel permeation optical resolution chromatography19
using hexane as an eluent to give 3a (34.9 mg, 21.3 µmol, 13%)
and 3b (14.4 mg, 8.8 µmol, 7%), respectively. The other fraction
of GPLC was purified by wet column chromatography (SiO2/
hexane) to give Tbt(Dip)GeH2 (7.9 mg, 1.0 µmol, 4%) and Tbt-
(Dip)GeBr2 (7; 27.2 mg, 30 µmol, 12%), respectively. Spectral data
for Tbt(Dip)GeH2 (9) and Tbt(Dip)GeBr2 (7) have already been
reported.7b
Figure 5. NICS(0) and NICS(1) values (ppm) for the annelated
ring systems calculated at the GIAO-HF/6-31G*//B3LYP/6-31G*
level.
Conclusion
In summary, we have succeeded in the syntheses of bis-
(germacyclopropa)benzenes 3a,b as stable crystalline com-
pounds for the first time by taking advantage of dilithiogermane
8 bearing two bulky substituents. Since the synthesis of a stable
bis(cyclopropa)benzene has not been achieved yet,2 the stable
examples of bis(metallacyclopropa)benzenes are important from
the viewpoints of structural chemistry and organoelement
chemistry. In addition, dilithiometalanes were proved to be
useful reagents for the synthesis of the metallacyclopropaben-
zene derivatives. The crystallographic analyses of the bis-
(metallacyclopropa)benzenes revealed that the annelated benzene
rings showed almost no distinct bond alternation. It can be
concluded that annelation of metallacyclopropene rings18 to a
benzene ring does not cause significant bond alternation but
has very little influence on the aromaticity of the central benzene
ring. We believe that these results provide very important
information on the chemistry of cycloproparenes and heavier
group 14 elements.
1
3a: colorless crystals; mp 156 °C dec; H NMR (C2D2Cl4, 90
°C) δ -0.94 (s, 54H), 0.19 (s, 54H), 0.62 (br s, 4H), 1.08-1.34
(m, 24H), 1.38 (s, 2H), 2.81 (s, 2H), 3.67 (br s, 2H), 6.43 (br s,
3
3
4H), 6.93 (d, JHH ) 7.8 Hz, 4H), 7.08 (t, JHH ) 7.8 Hz, 2H),
7.83 (s, 2H); HRMS (FAB) m/z calcd for C84H15474Ge2Si12
1646.7705 [M+], found 1646.7753 [M+]. Anal. Calcd for C84H154
-
Ge2Si12: C, 61.28; H, 9.43, Found: C, 61.27; H, 9.54.
1
3b: colorless crystals; mp 150 °C dec; H NMR (C6D12, 100
°C) δ -0.11 (s, 36H), 0.05 (s, 72H), 0.87 (br s, 2H), 1.09 (br s,
4H), 1.35 (br s, 24H), 2.83 (s, 2H), 2.81 (br s, 2H), 6.40 (brs, 4H),
3
3
7.03 (d, JHH ) 7.8 Hz, 4H), 7.15 (t, JHH ) 7.8 Hz, 2H), 7.92 (s,
2H); 13C NMR (C6D12, 80 °C) δ 1.3 (q), 1.9 (q), 23.4 (q), 28.1 (q),
30.1 (d), 31.7 (d), 35.4 (d), 37.5 (d), 124.1 (d), 126.3 (d), 129.7
(d), 130.3 (d), 131.5 (s), 131.4 (s), 141.6 (s), 144.6 (s), 152.0 (s),
152.7 (s), 154.5 (s); HRMS (FAB) m/z calcd for C84H15474Ge2Si12
Experimental Section
1646.7705 [M+], found 1646.7761 [M+]. Anal. Calcd for C84H154
Ge2Si12: C, 61.28; H, 9.43, Found: C, 61.25; H, 9.45.
-
General Procedure. All experiments were performed under an
argon atmosphere unless otherwise noted. Solvents were dried by
standard methods and freshly distilled prior to use. Cyclohexane-
d12 and tetrachloroethane-d2 were purchased from Cambridge
Isotopes. All NMR spectra were collected on JEOL JNM AL-300
and ECA-600 spectrometers. 1H and 13C NMR chemical shifts are
recorded in ppm relative to SiMe4 (δ 0) and were referenced
internally with respect to the residual proton impurity (cyclohexane-
d12, δ 1.38; tetrachloroethane-d2, δ 5.98) and the 13C resonance of
the solvent (cyclohexane, δ 26.4), respectively. Wet column
chromatography was performed with Wakogel C-200. Preparative
gel permeation liquid chromatography (GPLC) was performed on
a LC-908 or LC-918 instrument (Japan Analytical Industry Co.,
Ltd.) equipped with JAIGEL 1H and 2H columns (eluent toluene)
and JAIGEL-OA2000 (eluent hexane). Fast atom bombardment
(FAB) mass spectral data were obtained on a JEOL JMS-700
Theoretical Calculations. Theoretical calculations for model
molecules 3c-e were carried out using the Gaussian 98W program20
with density functional theory at the B3LYP level.21 It was
confirmed by frequency calculations that the optimized structures
have minimum energies. The 6-311G+(2d,p) basis sets were used
for C and H. The Lanl2DZ basis sets were used with effective core
potential for Ge. The GIAO calculations were carried out at the
6-311G++(2d,p) levels for the optimized geometries, and the
results of calculations for tetramethylsilane at the same level were
used for the standard of chemical shifts.
X-ray Structural Determinations. Crystallographic data of 3a,b
are summarized in Table 3. Single crystals of 3a were grown by
(19) Although 3a,b featured no chirality character, the separation could
only be achieved using optical resolution chromatography with (R)-
phenylglycine as packing.
(18) For germacyclopropenes: (a) Egorov, M. P.; Kolesnikov, S. P.;
Struchkov, Yu. T.; Antipin, M. Y.; Sereda, S. V.; Nefedov, O. M. J.
Organomet. Chem. 1985, 290, C27-C30. (b) Ando, W.; Ohgaki, H.; Kabe,
Y. Angew. Chem., Int. Ed. Engl. 1994, 33, 659-661. (c) Tokitoh, N.;
Kishikawa, K.; Matsumoto, T.; Okazaki, R. Chem. Lett. 1995, 827-828.
(d) Fukaya, N.; Ichinohe, M.; Kabe, Y.; Sekiguchi, A. Organometallics
2001, 20, 3364-3366. (e) Meiners, F.; Saak, W.; Weidenbruch, M. Z.
Anorg. Allg. Chem. 2002, 628, 2821-2822. For other heavy cyclopropanes
and cyclopropenes, see: (f) Sekiguchi, A.; Lee, V. Y. Chem. ReV. 2003,
103, 1429-1447. (g) Sekiguchi, A.; Lee, V. Y. In Organosilicon Chemistry
V, From Molecules to Materials; Auner, N., Weis, J., Eds.; Wiley-VCH:
Weinheim, Germany, 2003; pp 92-100. (h) Lee, V. Y.; Sekiguchi, A. In
The Chemistry of Organic Germanium, Tin and Lead Compounds; Rap-
poport, Z., Apeloig, Y., Eds.; Wiley: New York, 2002; Vol. 2, Chapter
14, pp 903-933. (i) Roskamp, E.; Rpskamp, C. In ComprehensiVe
Heterocyclic Chemistry II; Katritzky, A. R., Rees, C. W., Scriven, E. F.
V., Eds.; Pergamon: New York, 1996; Vol. 1A, pp 305-331.
(20) All calculations were carried out using Gaussian 98W: Frisch, M.
J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman,
J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant,
J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M.
C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci,
B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.;
Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson,
B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle,
E. S.; Pople, J. A. Gaussian 98, revision A.9; Gaussian, Inc.: Pittsburgh,
PA, 1998.
(21) (a) Becke, A. D. Phys. ReV. 1988, A38, 3098-3100. (b) Becke, A.
D. J. Chem. Phys. 1993, 98, 5648-5652. (c) Lee, W. Y.; Parr, R. G. Phys.
ReV. B 1988, 37, 785-789.

Tajima et al.
Organometallics, Vol. 25, No. 1, 2006 235
and refined by full-matrix least-squares procedures on F2 for all
reflections (SHELXL-97).23 All hydrogen atoms were placed using
AFIX instructions, while all of the other atoms were refined
anisotropically. In the case of 3a, the disordered trimethylsilyl
groups at the para position of the Tbt group were restrained to be
identical with each other using the DFIX and SIMU instructions.
The occupancies of disordered fragments were refined (0.59:0.41).
In the case of 3b, the solvated hexane molecule was disordered
and the occupancies were restrained to be identical with each other
using the DFIX and SIMU instructions. The occupancies of the
disordered fragments were refined (0.62:0.38). Crystallographic data
for the structural analysis have been deposited with the Cambridge
Crystallographic Data Center: CCDC No. 277662 and No. 277663
for 3a,b, respectively.
Table 3. Crystallographic Data for 3a and 3b‚(hexane)
3a
3b‚(hexane)
formula
mol wt
C84H154Ge2Si12
1646.33
C90H168Ge2Si12
1732.50
cryst color
cryst dimens/mm
cryst syst
space group
unit cell syst
a/Å
colorless
colorless
0.20 × 0.20 × 0.20 0.50 × 0.30 × 0.03
orthorhombic
monoclinic
P21/n (No. 14)
P212121 (No. 19)
12.833(2)
16.0045(17)
48.473(7)
90
90
90
9956(2)
4
1.098
0.785
3560
15.2827(14)
43.085(2)
17.3703(15)
90
112.659(4)
90
10554.8(15)
4
1.090
b/Å
c/Å
R/deg
â/deg
γ/deg
V/Å3
Z
Dc/g cm-3
µ/mm-1
F(000)
Acknowledgment. This work was partially supported by
Grants-in-Aid for the COE Research on Elements Science (No.
12CE2005), Scientific Researches (Nos. 14078213, 14204064,
and 15655011), and the 21st Century COE on Kyoto University
Alliance for Chemistry (Novel Organic Materials Creation &
Transformation Project) and Nanotechnology Support Project
from the Ministry of Education, Culture, Sports, Science, and
Technology of Japan. This manuscript was written at TU
Braunschweig during the tenure of a von Humboldt Senior
Research Award of N. Tokitoh, who is grateful to the Alexander
von Humboldt Stiftung for their generosity and to Professors
Reinhard Schmutzler and Wolf-Walther du Mont for their warm
hospitality. T.T. acknowledges Research Fellowships of the
Japan Society for the Promotion of Science for Young Scientists.
We are grateful to Japan Analytical Industry Co., Ltd., for active
support for the preparative HPLC.
0.744
3760
radiation
temperature (K)
θ range/deg
hkl range
Mo KR (λ ) 0.710 70 Å)
103(2)
1.73-25.00
2.39-25.00
-15 e h e 15
-19 e k e 19
-57 e l e 57
84 168
-18 e h e 18
-50 e k e 51
-20 e l e 20
89 731
no. of measd rflns
no. of unique rflns
19 532 (Rint
0.1116)
99.8
)
18 478 (Rint
0.0744)
99.3
)
completeness to θ/%
max and min transmn
refinement method
no. of data/restraints/
params
0.8588 and 0.8588
0.9780 and 0.7075
full-matrix least squares on F2
17 490/65/965
18 478/100/1009
goodness of fit on F2
1.153
1.218
final R indices (I > 2σ(I)) R1 ) 0.0809
R1 ) 0.0785
wR2 ) 0.1252
R1 ) 0.0947
wR2 ) 0.1312
0.732 and -0.582
wR2 ) 0.1729
Supporting Information Available: CIF files giving crystal-
lographic data for 3a,b, including all refinement parameters, bond
lengths and angles, and positional and thermal parameters, and text
and a figure giving details of variable-temperature 13C NMR studies
of 3b. This material is available free of charge via the Internet at
http://pubs.acs.org.
R indices (all data)
R1 ) 0.0967
wR2 ) 0.1799
0.490 and -0.359
largest diff peak and
hole/e Å-3
the slow evaporation of its saturated solution in hexane and EtOH
in the refrigerator, while single crystals of 3b were grown in hexane,
respectively. Sample preparation consisted of coating the crystal
with hydrocarbon oil, mounting it on a glass fiber, and placing it
under a cold stream of N2 on the diffractometer. The intensity data
of 3a,b were collected on a Rigaku/MSC Mercury CCD diffrac-
tometer with graphite-monochromated Mo KR radiation (λ )
0.710 71 Å). The structures were solved by direct methods (SIR97)22
OM0507629
(22) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115-119.
(23) Sheldrick, G. M. SHELXL-97, Program for the Refinement of
Crystal Structures; University of Go¨ttingen, Go¨ttingen, Germany, 1997.