Ring contraction of 1,2-digermacyclohexa-1,4-diene to (germacyclopent-3-enyl)germylene

Article abstract of DOI:10.1021/om010369c

Thermolysis of cis,cis-1,6,7-trigerma-1,6,7-tris(tri-tert-butylsilyl)-7-mesityl-3, 4-dimethylbicyclo[4.1.0]- hept-3-ene (1a) and cis,cis-1,6,7-trigerma-1,6,7-tris(tri-tert-butylsilyl)-7-mesityl-3-methylbicyclo [4.1.0]hept-3-ene (1b) at 70°C in benzene in the presence of diphenylacetylene and triethylsilane yielded products arising from (germacyclopent-3-enyl)germylene, which is formed by the ring contraction of 1,2-digermacyclohexa-1,4-diene. Theoretical calculations on this contraction reaction leading from a six-membered ring with a Ge=Ge double bond to a five-membered ring have also been performed.

Full text of DOI:10.1021/om010369c

3364
Organometallics 2001, 20, 3364-3366
Rin g Con tr a ction of 1,2-Diger m a cycloh exa -1,4-d ien e to
(Ger m a cyclop en t-3-en yl)ger m ylen e
Norihisa Fukaya, Masaaki Ichinohe, Yoshio Kabe, and Akira Sekiguchi*
Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571, J apan
Received May 7, 2001
Sch em e 1
Summary: Thermolysis of cis,cis-1,6,7-trigerma-1,6,7-
tris(tri-tert-butylsilyl)-7-mesityl-3,4-dimethylbicyclo[4.1.0]-
hept-3-ene (1a ) and cis,cis-1,6,7-trigerma-1,6,7-tris(tri-
tert-butylsilyl)-7-mesityl-3-methylbicyclo[4.1.0]hept-3-
ene (1b) at 70 °C in benzene in the presence of
diphenylacetylene and triethylsilane yielded products
arising from (germacyclopent-3-enyl)germylene, which is
formed by the ring contraction of 1,2-digermacyclohexa-
1,4-diene. Theoretical calculations on this contraction
reaction leading from a six-membered ring with a
GedGe double bond to a five-membered ring have also
been performed.
The chemistry of unsaturated cyclic compounds con-
taining group 14 elements heavier than carbon is a
recent subject of considerable interest.1,2 Synthetic
methodologies and studies of the structural features of
such compounds have developed quickly during the past
5 years.2 Ring contraction and expansion reactions of
unsaturated cyclic compounds are very important and
well-established in organic chemistry; however, such
reactions are very rare in the chemistry of heavier group
14 elements.3 Recently, we have succeeded in synthesiz-
ing bicyclic 1,6,7-trigermabicyclo[4.1.0]hept-3-ene and
1,4,5-trigermabicyclo[2.1.0]pent-2-ene derivatives by a
cycloaddition reaction of 2,3-dimethyl-1,3-butadiene or
isoprene and phenylacetylene to the endocyclic GedGe
double bond of mesityl-substituted cyclotrigermene.4 We
presumed that the present fused bicyclic cyclotriger-
manes could be suitable precursors of the unsaturated
germanium compounds by the cycloelimination reaction,
since the thermal and photochemical generation of
digermenes and germylenes from the cyclotrigermanes
is well-established.1 Thus, we examined the thermal
reaction of the 1,6,7-trigermabicyclo[4.1.0]hept-3-ene
derivative in the presence of diphenylacetylene and
triethylsilane and found an unprecedented ring contrac-
tion of a 1,2-digermacyclohexa-1,4-diene to a (germa-
cyclopent-3-enyl)germylene derivative. We here report
the first example of a facile ring contraction of unsatur-
ated cyclic germanium compounds, studied from both
the experimental and theoretical points of view.
Thermolysis of a benzene solution of 1,6,7-trigermabi-
cyclo[4.1.0]hept-3-ene (1a )4 for 6 h at 70 °C in a sealed
tube in the presence of diphenylacetylene afforded
colorless crystals of the germacyclopropene derivative
2 in 82% yield and pale yellow crystals of the germa-
cyclopropenylgermacyclopentene derivative 3a in 53%
yield (Scheme 1).5 Thermolysis of 1b with diphenyl-
acetylene also gave 2 (88%) and 3b (58%). However, no
evidence for the [2 + 2] reaction of the 1,2-digermacy-
clohexa-1,4-diene species 5 with diphenylacetylene to
form the 1,6-digermabicyclo[4.2.0]octa-3,7-diene deriva-
tive was found. Interestingly, thermal reaction of 1a in
the presence of phenylacetylene proceeded in a different
way to give colorless crystals of 3,4-dimethyl-7-phenyl-
1,6-bis[tris(tri-tert-butylsilyl)]-1,6-digermabicyclo[4.2.0]-
octa-3,7-diene (7) in 77% yield, arising from [2 + 2]
cycloaddition of 5a with phenylacetylene.6
(1) (a) Tsumuraya, T.; Batcheller, S. A.; Masamune, S. Angew.
Chem., Int. Ed. Engl. 1991, 30, 902. (b) Mackay, K. M. The Chemistry
of Organic Germanium, Tin and Lead Compounds; Patai, S., Ed.;
Wiley: New York, 1995; Chapter 2. (c) Driess, M.; Gru¨tzmacher, H.
Angew. Chem., Int. Ed. Engl. 1996, 35, 828. (d) Baines, K. M.; Stibbs,
W. G. Adv. Organomet. Chem. 1996, 39, 275. (e) Weidenbruch, M. Eur.
J . Inorg. Chem. 1999, 373. (f) Power, P. P. Chem. Rev. 1999, 99, 3463.
(g) Escudie´, J .; Ranaivonjatovo, H. Adv. Organomet. Chem. 1999, 44,
113.
(2) (a) Cyclotrigermenes: Sekiguchi, A.; Yamazaki, H.; Kabuto, C.;
Sakurai, H.; Nagase, S. J . Am. Chem. Soc. 1995, 117, 8025. (b)
Sekiguchi, A.; Fukaya, N.; Ichinohe, M.; Takagi, N.; Nagase, S. J . Am.
Chem. Soc. 1999, 121, 11587. Cyclotetrasilenes: (c) Kira, M.; Iwamoto,
T.; Kabuto, C. J . Am. Chem. Soc. 1996, 118, 10303. (d) Wiberg, N.;
Auner, H.; No¨th, H.; Knizek, J .; Polborn, K. Angew. Chem., Int. Ed.
1998, 36, 2869. Cyclotrisilenes: (e) Iwamoto, T.; Kabuto, C.; Kira, M.
J . Am. Chem. Soc. 1999, 121, 886. (f) Ichinohe, M.; Matsuno, T.;
Sekiguchi, A. Angew. Chem., Int. Ed. 1999, 38, 2194. Disilager-
mirenes: (g) Lee, V. Ya.; Ichinohe, M.; Sekiguchi, A.; Takagi, N.;
Nagase, J . Am. Chem. Soc. 2000, 122, 9034. Spiropentasiladiene: (h)
Iwamoto, T.; Tamura, M.; Kabuto. C.; Kira, M. Science 2000, 290, 504.
Chalcogenatetrasilacyclopentenes: (i) Grybat, A.; Boomgaarden. S.;
Saak, W.; Marsmann, H.; Weidenbruch, M. Angew. Chem., Int. Ed.
1999, 38, 2010. Cyclostannene: (j) Wiberg, N.; Lerner, H.-W.; Vasisht,
S.-K.; Wagner, S.; Karaghiosoff, K.; No¨th, H.; Ponikwar, W. Eur. J .
Inorg. Chem. 1999, 1211. Disilagermacyclopentadiene: (k) Lee, V. Ya.;
Ichinohe, M.; Sekiguchi, A. J . Am. Chem. Soc. 2000, 122, 12604.
(3) Isomerization of a 4-silamethylenecyclopropene derivative to a
silacyclobutadiene derivative was reported; see: (a) Sakamoto, K.;
Ogasawara, J .; Sakurai, H.; Kira, M. J . Am. Chem. Soc. 1997, 119,
3405. (b) Veszpre´mi, T.; Takahashi, M.; Ogasawara, J .; Sakamoto, K.;
Kira, M. J . Am. Chem. Soc. 1998, 120, 2408.
(4) Fukaya, N.; Ichinohe, M.; Sekiguchi, A. Angew. Chem., Int. Ed.
2000, 39, 3881.
10.1021/om010369c CCC: $20.00 © 2001 American Chemical Society
Publication on Web 07/13/2001

Communications
Organometallics, Vol. 20, No. 16, 2001 3365
Sch em e 2
The formation of products 2 and 3 clearly shows the
existence of intermediate germylenes 4 and 6 during the
thermolysis of 1 (Scheme 2). It seems that the 1,2-
digermacyclohexa-1,4-diene 5, which could be initially
formed by the cycloelimination of 1, does not undergo
[2 + 2] cycloaddition with diphenylacetylene, probably
because of steric reasons. Alternatively, the resulting 5
undergoes a ring contraction to give the germylene 6,
which then reacts with diphenylacetylene to form
compound 3.
Figure 1.7 The five-membered ring is nearly planar (sum
of interior angles 538.9°). Although the Ge-C bond
lengths of the three-membered ring (1.980(3) and 1.985-
(3) Å) slightly exceed the upper limit of literature values
of germacyclopropenes (typically 1.88-1.94 Å), the
CdC double bond (1.328(5) Å) is relatively short (typi-
cally 1.33-1.39 Å).8 The Ge-Ge bond (2.4922(5) Å) and
Ge-Si bonds (2.449(1) and 2.439(1) Å) are elongated by
the steric repulsion.
To confirm the formation of the germylene 6, we have
also examined thermolysis of 1 in the presence of
triethylsilane, which can react exclusively with the
divalent species. Thus, thermolysis of 1a with an excess
of triethylsilane for 6 h at 70 °C yielded (tri-tert-
butylsilyl)(triethylsilyl)(mesityl)germane (8) in 86% yield
and digermane derivative 9 in 83% yield (Scheme 3).9
These compounds appear to be derived from the inser-
tion of the resulting germylene 4 and (germacyclopen-
tenyl)germylene 6a into the Si-H bond of triethylsilane,
respectively.
The molecular structure of 3b was unambiguously
characterized by X-ray crystallography, as shown in
(5) Procedure of thermolysis of 1a in the presence of diphenylacety-
lene. The yellow crystals of 1a (50 mg, 0.049 mmol) and diphenylacety-
lene (40 mg, 0.22 mmol) were placed in a reaction tube with a magnetic
stirrer. Dry degassed benzene (0.6 mL) was introduced by vacuum
transfer, and then the tube was sealed. The mixture was heated for 6
h at 70 °C. The solvent was removed in vacuo, and the resulting residue
was separated by gel permeation chromatography to give 2 (23 mg,
82%) and 3a (21 mg, 53%). 2: colorless crystals; mp 145-148 °C; 1H
NMR (C6D6, δ) 1.25 (s, 27 H), 2.06 (s, 3 H), 2.85 (s, 6 H), 6.72 (s, 2 H),
7.02 (t, J ) 7 Hz, 2 H), 7.16 (t, J ) 7 Hz, 4 H), 7.68 (d, J ) 7 Hz, 4 H);
13C NMR (C6D6 , δ) 21.0, 25.1, 26.5, 31.8, 127.1, 128.4, 128.5, 128.7,
136.4, 137.8, 142.5, 143.4, 148.9; 29Si NMR (C6D6, δ) 25.3. Anal. Calcd
for C35H48GeSi: C, 73.82; H, 8.50. Found: C, 73.35; H, 8.48. 3a : pale
yellow crystals; mp 194-197 °C; 1H NMR (C6D6, δ) 1.13 (s, 27 H), 1.25
(s, 27 H), 1.87 (s, 6 H), 2.34 (d, J ) 17 Hz 2 H), 2.40 (d, J ) 17 Hz, 2
H), 7.05 (t, J ) 7 Hz, 2 H), 7.24 (t, J ) 7 Hz, 4 H), 7.92 (d, J ) 7 Hz,
4H); 13C NMR (C6D6 , δ) 19.8, 24.7 (2C), 31.9, 32.1, 32.4, 127.6, 128.5,
130.2, 132.4, 135.3, 144.0; 29Si NMR (C6D6, δ) 27.7, 36.5. Anal. Calcd
for C44H74Ge2Si2: C, 65.70; H, 9.27. Found: C, 65.32; H, 9.14.
Baines et al. reported the facile digermene-to-germyl-
germylene and germasilene-to-silylgermylene rearrange-
ments by 1,2-mesityl shifts of tetramesityldigermene
(7) Single crystals of 3b were obtained by recrystallization from
hexane. Crystal data for 3b at 120 K: molecular formula C43H72Ge2-
Si2, mol wt 790.37, monoclinic, a ) 14.6250(8) Å, b ) 14.6190(7) Å, c
) 20.1150(11) Å, â ) 91.077(4)°, V ) 4299.9(4) Å3, space group P21/n,
Z ) 4, Dcalcd ) 1.221 g/cm3. The final R factor was 0.0559 (Rw ) 0.1631
for all data) for 10 018 reflections with I > 2σ(I). GOF ) 1.091. The
structure was solved by the direct method and refined by the full-
matrix least-squares method using the SHELXL-97 program. The
position of the carbon atom of the methyl group attached to the CdC
double bond of the five-membered ring is disordered between the C40
and C41 atoms, and the molecular structure with the largest occupancy
(70%, C43 bonded to C40) is shown in Figure 1.
(8) (a) Egorov, M. P.; Kolesnikov, S. P.; Struchkov, Y. T.; Antipin,
M. Y.; Sereda, S. V.; Nefedov, O. M. J . Organomet. Chem. 1985, 290,
C27. (b) Ando, W.; Ohgaki, H.; Kabe, Y. Angew. Chem., Int. Ed. Engl.
1994, 33, 659. (c) Tokitoh, N.; Kishikawa, K.; Matsumoto, T.; Okazaki,
R. Chem. Lett. 1995, 827.
(6) Spectroscopic data for 7: mp 179-182 °C; 1H NMR (C6D6, δ)
1.19 (s, 27 H), 1.30 (s, 27 H), 1.88 (s, 3 H), 2.04 (s, 3 H), 2.11-2.18 (m,
2 H), 2.32-2.41 (m, 1 H), 2.69-2.74 (m, 1 H), 7.00 (t, J ) 9 Hz, 2 H),
7.12 (t, J ) 9 Hz, 1 H), 7.25 (s, 1 H), 7.32 (d, J ) 9 Hz, 2 H); 13C NMR
(C6D6, δ) 22.1, 22.5, 24.5, 24.6, 30.6, 31.5, 31.8, 31.9, 123.1, 125.5, 126.4,
126.7, 128.1, 146.1, 151.4, 170.3; 29Si NMR (C6D6, δ) 27.9, 29.7. Anal.
Calcd for C38H70Ge2Si2: C, 62.67; H, 9.69. Found: C, 63.16; H, 9.32.
Crystal data for 7‚0.5(toluene) at 120 K: molecular formula C38H70
-
)
Ge2Si2‚0.5C7H8, mol wt 774.37, triclinic, 9.3000(8) Å,
a
)
b
12.303(1) Å, c )19.992(1) Å, R ) 75.719(5)°, â ) 85.135(5)°, γ )
74.544(4)°, V ) 2136.1(3) Å3, space group P1h, Z ) 2, Dcalcd ) 1.204
g/cm3. The final R factor was 0.0499 (Rw ) 0.1301 for all data) for 9484
reflections with I > 2σ(I). GOF ) 1.042.

3366 Organometallics, Vol. 20, No. 16, 2001
Communications
Ta ble 1. Rela tive En er gies (in k ca l/m ol) of th e Diger m en es, th e Cor r esp on d in g Ger m ylger m ylen e Isom er s,
a n d th e Tr a n sition Sta tes a t th e B3LYP /6-31G* Level Ca lcu la tion s
Sch em e 3
the 6-31G* basis set (Table 1). In H2GedGeH2, di-
germene-to-germylgermylene isomerization is endo-
thermic by 1.7 kcal mol-1 with a barrier of 9.5 kcal
F igu r e 1. Molecular structure of 3b with thermal el-
lipsoids drawn at the 30% level (hydrogen atoms are
omitted for clarity). Selected bond distances (Å): Ge1-Ge2
mol-1. For Me(H)GedGeH2, the isomerization barrier
(19.8 kcal mol-1) is much higher than that of H2Ged
GeH2, although the energy difference between Me(H)-
GedGeH2 and HG¨ e-Ge(H2)Me (2.8 kcal mol-1 endo-
thermic) is similar to that of H2GedGeH2 and HG¨ e-
GeH3. This result suggests that alkyl groups are much
more reluctant to migrate than hydrogen atoms. How-
ever, the isomerization of the parent 1,2-digermacyclo-
hexa-1,4-diene to (germacyclopent-3-enyl)germylene is
3.2 kcal mol-1 exothermic, and the barrier is only 8.1
kcal mol-1, in sharp contrast to the Me group migration.
Thus, this theoretical study shows that the ring con-
traction of 1,2-digermacyclohexa-1,4-diene is the favored
reaction both kinetically and thermodynamically.13
) 2.4922(5), Ge1-Si1 ) 2.449(1), Ge2-Si2 ) 2.439(1),
Ge1-C25 ) 1.980(3), Ge1-C26 ) 1.985(3), C25-C26 )
1.328(5). Selected bond angles (deg): C25-Ge1-C26 )
39.2(1), C26-C25-Ge1 ) 70.7(2), C25-C26-Ge1 ) 70.2-
(2), Si1-Ge1-Ge2 ) 134.0(0), Si2-Ge2-Ge1 ) 121.6(0),
C39-Ge2-C42 ) 91.3(1).
and tetramesitylgermasilene, respectively.10,11 Since the
present system is quite interesting from the viewpoint
of the unusual contraction reaction leading from a six-
membered ring with a GedGe double bond to a five-
membered ring, we have performed theoretical calcu-
lations on this system.12 Barriers to isomerization of the
three prototype digermenes, H2GedGeH2, Me(H)Ged
GeH2, and the parent 1,2-digermacyclohexa-1,4-diene,
to the corresponding germylgermylenes have been de-
termined by DFT calculations at the B3LYP level with
Ack n ow led gm en t. This work was supported by
Grants-in-Aid for Scientific Research (Nos. 12042213,
13029015, and 13440185) from the Ministry of Educa-
tion, Science and Culture of J apan and the TARA
(Tsukuba Advanced Research Alliance) fund.
(9) Spectroscopic data for 8 and 9: 8: colorless oil; 1H NMR (C6D6,
δ) 0.84-0.90 (m, 6 H), 0.95-0.98 (m, 9 H), 1.23 (s, 27 H), 2.10 (s, 3 H),
2.55 (s, 3 H), 2.65 (s, 3 H), 4.23 (s, 1 H), 6.78 (s, 1 H), 6.87 (s, 1 H); 13
C
Su p p or tin g In for m a tion Ava ila ble: Tables giving the
details of the X-ray structure determination, and bond angles
and figures giving thermal ellipsoid plots for 3b and 7 and
figures and tables giving the calculated geometrical param-
eters of the transition state structure for the cyclic digermene
and IRC results. This material is available free of charge via
the Internet at http://pubs.acs.org.
NMR (C6D6, δ) 6.8, 8.5, 21.0, 24.8, 27.5 (2C), 31.8, 128.2, 129.7, 136.8,
137.9, 142.8, 143.9; 29Si NMR (C6D6, δ) 4.6, 28.1. Anal. Calcd for C27H54
-
GeSi2: C, 63.90; H, 10.72. Found: C, 63.56; H, 10.44. 9: colorless solid;
mp 96-99 °C; 1H NMR (C6D6, δ) 1.05-1.09 (m, 6 H), 1.13-1.18 (m, 9
H), 1.29 (s, 27 H), 1.30 (s, 27 H), 1.78 (s, 6 H), 2.49 (d, J ) 17 Hz, 2 H),
2.60 (d, J ) 17 Hz, 2 H), 3.29 (s, 1 H); 13C NMR (C6D6, δ) 8.2, 8.9, 19.4
(2C), 24.3, 26.0, 32.0, 32.4, 32.7, 36.9, 131.9, 132.3; 29Si NMR (C6D6,
δ) 9.1, 25.5, 40.2. Anal. Calcd for C36H80Ge2Si3: C, 58.24; H, 10.86.
Found: C, 58.04; H, 10.54.
OM010369C
(10) (a) Baines, K. M.; Cooke, J . A. Organometallics 1992, 11, 3487.
(b) Baines, K. M.; Cooke, J . A.; Vittal, J . J . J . Chem. Soc., Chem.
Commun. 1992, 1484.
(11) Baines, K. M.; Cooke, J . A.; Dixon, C. E.; Liu, H. W.; Netherton,
M. R. Organometallics 1994, 13, 631.
(12) A theoretical calculation on the isomerization of H2GedGeH2
to HG¨ eGeH3 at the CCSD(T)/ANO level has been reported; see: Grev,
R. S.; Schaefer, H. F., III. Organometallics 1992, 11, 3489.
(13) The migratory aptitude of silyl groups would be much larger
than that of H and alkyl, as reported in disilene-silylene isomerization
calculated by Nagase et al.; see: Nagase, S.; Kudo, T. Organometallics
1984, 3, 1320. However, the migration of the Bu3Si group in the real
molecule was not observed due to steric reasons.
t