Synthesis of fused 1-sila-, 1-germa-, and 1-selenacyclohepta-2,4,6-trienes

Article abstract of DOI:10.1021/om8003114

A concise route to 1-sila-, 1-germa-, and 1-selenacyclohepta-2,4,6-trienes containing two fused benzo[b]thiophene units is described. Metalation of the ethylene acetal of 3-bromobenzo[b]thiophene-2-carboxaldehyde and subsequent quenching with Me₂SiCl₂, Me₂GeCl₂, or (PhSO₂)₂Se gave the corresponding bis(benzo[b]thiophen- 3-yl)silane, -germane, or -selenide, respectively, which were subjected to deprotection, followed by McMurry coupling, eventually affording the target compounds in good overall yields. It was also concluded that application of this approach to synthesis of related benzo-fused metallacyclohepta-2,4,6-trienes is limited to electron-deficient targets, as attempts involving electronrich substrates failed or gave only low yields at the final McMurry coupling stage.

Full text of DOI:10.1021/om8003114

3960
Organometallics 2008, 27, 3960–3963
Synthesis of Fused 1-Sila-, 1-Germa-, and
1-Selenacyclohepta-2,4,6-trienes
Hamid Shirani and Tomasz Janosik*
Unit for Organic Chemistry, Department of Biosciences and Nutrition, Karolinska Institute, NoVum
Research Park, SE-141 57 Huddinge, Sweden
ReceiVed April 7, 2008
A concise route to 1-sila-, 1-germa-, and 1-selenacyclohepta-2,4,6-trienes containing two fused
benzo[b]thiophene units is described. Metalation of the ethylene acetal of 3-bromobenzo[b]thiophene-
2-carboxaldehyde and subsequent quenching with Me2SiCl2, Me2GeCl2, or (PhSO2)2Se gave the
corresponding bis(benzo[b]thiophen-3-yl)silane, -germane, or -selenide, respectively, which were subjected
to deprotection, followed by McMurry coupling, eventually affording the target compounds in good overall
yields. It was also concluded that application of this approach to synthesis of related benzo-fused
metallacyclohepta-2,4,6-trienes is limited to electron-deficient targets, as attempts involving electron-
rich substrates failed or gave only low yields at the final McMurry coupling stage.
1-germacyclohepta-2,4,6-trienes,11–15,23 or derivatives of 1-se-
Introduction
lenacyclohepta-2,4,6-triene (4).14,15,24,25 Despite all progress in
Derivatives of the thiepin ring system (1) (Figure 1) have
been intensely studied over the years, with particular emphasis
on aspects such as pharmacological properties, structure, and
aromaticity.1–3 Likewise, the structurally related 1-metallacy-
cloheptatrienes incorporating the elements silicon, germanium,
selenium, or tellurium have also received certain treatment. In
particular, the 1-silacyclohepta-2,4,6-triene (silepin) 2 and related
structures featuring the 1-silacyclohepta-2,4,6-triene ring system
have been subjected to detailed investigations, resulting in
several synthetic approaches,4–16 as well as deeper understanding
of their properties,9,10,13,15,17–20 but only relatively few studies
have been devoted to the parent ring system 321,22 and other
this area, there is still need for development of new synthetic
approaches to these systems, as the existing routes often require
time-consuming multistep preparation of rather complex starting
materials, giving only low overall yields of the desired products.
Therefore, it was envisaged that implementation of a variant of
our recently reported approach to dibenzo[b,f]thiepins26 would
offer a new convenient access to fused derivatives of the
1-metallacyclohepta-2,4,6-trienes 2-4 from readily available
and inexpensive starting materials.
Results and Discussion
Our synthetic efforts commenced with development of a
feasible procedure for preparation of bis(phenylsulfonyl) se-
lenide, a crucial reagent for construction the 1-selenacyclohep-
tatrienes in our planned series of compounds. As the reported
procedures involved reaction of selenium oxychloride or
selenium tetrachloride with sodium benzenesulfinate in ben-
zene,27 we sought an alternative method involving a more
readily available and inexpensive selenium source. Hence, it
was established that generation of selenium dichloride by
reaction of elemental selenium with sulfuryl chloride in THF
as previously reported,28 followed by addition of the resulting
solution to a suspension of sodium benzenesulfinate in anhy-
* Corresponding author. Fax: +46 8 6081501. E-mail: Tomasz.Janosik@
ki.se.
(1) Protiva, M. J. Heterocycl. Chem. 1996, 33, 497–521.
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G. R., Eds.; Elsevier Science: Oxford, 1996; Vol. 9, pp 67-111.
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10.1021/om8003114 CCC: $40.75
2008 American Chemical Society
Publication on Web 07/03/2008

Fused 1-Sila-, 1-Germa-, and 1-Selenacyclohepta-2,4,6-trienes
Organometallics, Vol. 27, No. 15, 2008 3961
Scheme 2a
Figure 1
Scheme 1a
a
Reagents and conditions: (i) n-BuLi, THF, -78 °C, 0.5 h; then
(PhSO2)2Se (for 9a) or Me2GeCl2 (for 9b), -78 °C to rt, 16 h; (ii) aq
HClO4, 1,4-dioxane, rt, 3-16 h; (iii) TiCl4, Zn, pyridine, THF, reflux,
2.5 h; then 11a-b, rt, 16 h; reflux, 4 h; then aq K2CO3, rt, 18 h.
Application of this strategy for synthesis of benzo-fused
metallacyclohepta-2,4,6-trienes proved to be more problematic,
particularly in cases involving electron-rich substrates. The
starting 2-bromobenzaldehyde acetal 9a36 was lithiated and
treated with (PhSO2)2Se, giving the bis(aryl) selenide 10a
(Scheme 2). As anticipated, deacetalization of 10a afforded the
dialdehyde 11a, which could eventually be annulated to the
desired 1-selenacyclohepta-2,4,6-triene 12a. However, a similar
series of reactions starting from 9a involving Me2SiCl2 or
Me2GeCl2 as the electrophiles proceeded as expected only until
the final McMurry coupling, which failed to produce the desired
compounds, presumably due to decomposition of the products
or the starting materials during the reaction. On the other hand,
a similar sequence involving initial metalation and reaction of
the less electron-rich acetal 9b37,38 with Me2GeCl2 afforded the
intermediate 10b, which could next be deprotected, giving 11b,
a substrate for a final annulation to the known product 12b23
under McMurry conditions. The modest yield (36%) of the final
step in this series of reactions could also in this case be attributed
to partial decomposition of the formed electron-rich 1-germa-
cyclohepta-2,4,6-triene or its precursor. Although there are
examples of some relatively stable fused 1-germacyclohepta-
2,4,6-trienes,15 it has for instance recently been demonstrated
that the parent molecule 3 undergoes complete thermolysis at
80 °C in toluene-d8 within 30 min, giving benzene and
dimethylgermylene, which can be trapped by reaction with 2,3-
dimethyl-1,3-butadiene.22 It is also noteworthy that attempted
preparation of the known compound 5,5-dimethyl-5H-5-sila-
[2,3:6,7]dibenzocyclohepta-2,4,6-triene5–7 from 9b using our
route resulted in isolation of only small amounts of impure
product. This outcome, combined with fact that this molecule
is rather stable and has previously been purified by distillation,5
illustrates one of the limitations of our approach. However, the
stability of condensed 1-sila- or 1-germacyclohepta-2,4,6-trienes
appears to improve upon fusion to more electron-deficient
aromatics or heteroaromatics, rendering such targets compatible
with the McMurry conditions, as illustrated by the successful
preparation of the extended systems 8a,b described above.
In conclusion, efficient syntheses of several fused 1-metal-
lacyclohepta-2,4,6-trienes have been achieved from simple
a
Reagents and conditions: (i) n-BuLi, THF, -78 °C, 0.5 h; then
Me2SiCl2/Me2GeCl2/(PhSO2)2Se, -78 °C to rt, 16 h; (ii) aq HClO4,
1,4-dioxane, rt, 3-16 h; (iii) TiCl4, Zn, pyridine, THF, reflux, 2.5 h;
then 7a-c, rt, 16 h; reflux, 4 h; then aq K2CO3, rt, 18 h.
drous benzene, provided practical and convenient access to
useful quantities of bis(phenylsulfonyl) selenide. The crude
material obtained in this fashion could be conveniently purified
by column chromatography or just a simple trituration with
diethyl ether and, if necessary, by a subsequent recrystallization
from acetonitrile.
With all required reagents in hand, we next embarked upon
preparation of a set of extended 1-metallacyclohepta-2,4,6-
trienes. The benzo[b]thiophene carboxaldehyde acetal 5,29 which
was readily prepared by protection of 3-bromobenzo[b]thioph-
ene-2-carboxaldehyde,29,30 was selected as the starting material
for this purpose. Hence, metalation31–33 of 5 using n-BuLi as
described prevously,26 followed by treatment of the resulting
organometallic intermediate with 0.5 equiv of either Me2SiCl2,
Me2GeCl2, or (PhSO2)2Se, gave the expected products 6a-c,
which could thereafter be deprotected, giving the dialdehydes
7a-c by treatment with aqueous perchloric acid (Scheme 1).
The intermediates 7a-c were eventually subjected to McMurry
coupling,34,35 producing the novel extended metallacyclohep-
tatrienes 8a-c in good overall yields. It was also noted that
the products 8a,b deteriorate slowly when stored at room
temperature for prolonged periods of time and should therefore
be kept in a freezer.
(29) Dore, G.; Bonhomme, M.; Robba, M. Tetrahedron 1972, 28, 2553–
2573.
(30) Bjo¨rk, M.; Grivas, S. J. Heterocycl. Chem. 2006, 43, 101–109.
(31) Snieckus, V. Chem. ReV. 1990, 90, 879–933.
(32) Schlosser, M. Angew. Chem., Int. Ed. 2005, 44, 376–393.
(33) Schlosser, M. Synlett 2007, 3096–3102.
(36) Charlton, J. L.; Alauddin, M. M. J. Org. Chem. 1986, 51, 3490–
3493.
(34) McMurry, J. E. Acc. Chem. Res. 1983, 16, 405–411.
(35) McMurry, J. E. Chem. ReV. 1989, 89, 1513–1524.
(37) Watanabe, T.; Soma, N. Chem. Pharm. Bull. 1971, 19, 2215–2221.
(38) Newman, M. S.; Lee, L.-F. J. Org. Chem. 1975, 40, 2650–2652.

3962 Organometallics, Vol. 27, No. 15, 2008
Shirani and Janosik
Bis(2-[1,3]dioxolan-2-yl-benzo[b]thiophen-3-yl)-dimethylger-
mane (6b). Purification by column chromatography using n-
heptane/EtOAc (6:1 f 3:1) gave 6b (0.75 g, 73%) as a white solid;
mp 149-151.5 °C. IR (neat): 2884, 1349, 1182, 1153, 1085, 1061,
1023, 955, 940, 914, 836, 813, 754, 730 cm-1. 1H NMR (DMSO-
d6): δ 7.95 (d, J ) 7.9 Hz, 2 H), 7.64 (d, J ) 8.0 Hz, 2 H),
7.29-7.24 (m, 2 H), 7.18-7.13 (m, 2 H), 4.15-3.90 (m, 8 H),
0.97 (s, 6 H). 13C NMR (DMSO-d6): δ 148.5 (C), 143.4 (C), 139.1
(C), 133.1 (C), 124.5 (CH), 124.2 (CH), 124.2 (CH), 122.6 (CH),
99.1 (CH), 65.2 (CH2), 1.5 (CH3). Anal. Calcd for C24H24GeO4S2:
C, 56.17; H, 4.71. Found: C, 56.22; H, 4.67.
masked aromatic or heteroaromatic aldehydes, giving access to
hitherto unknown ring systems. The findings suggest that the
strategy used in this work may be implemented for preparation
of further related systems containing other elements, except in
cases involving electron-rich substrates.
Experimental Section
General Information. 1H and 13C NMR spectra were recorded
on a Bruker DPX 300 instrument operating at 300.13 MHz for 1H
and 75.47 MHz for 13C, respectively, using the residual solvent
resonances as reference. The IR spectra were performed on an
Avatar 330 FT-IR spectrometer (Thermo Nicolet). Elemental
analyses were performed by H. Kolbe Mikroanalytisches Labora-
torium, Mu¨lheim an der Ruhr, Germany. The melting points were
measured in open capillary tubes using a Bu¨chi B-545 melting point
apparatus. All chemicals originated from commercial sources and
were used as received, except THF, which was distilled from
sodium and benzophenone, and benzene, which was stored over
sodium wire. Chromatography was performed using silica gel
(particle size 40-63 µm).
Bis(2-[1,3]dioxolan-2-yl-benzo[b]thiophen-3-yl) selenide (6c).
Purification by column chromatography using n-heptane/EtOAc (5:1
f 2:1) gave 6c (0.76 g, 78%) as a yellow solid; mp 174-177.5
°C. IR (neat): 2885, 1363, 1194, 1076, 1055, 1017, 992, 941, 755,
727 cm-1. 1H NMR (DMSO-d6): δ 7.98-7.95 (m, 2 H), 7.70-7.68
(m, 2 H), 7.37-7.26 (m, 4 H), 4.19-4.00 (m, 8 H). 13C NMR
(DMSO-d6): δ 144.4 (C), 140.2 (C), 137.9 (C), 125.7 (CH), 125.0
(CH), 123.2 (CH), 123.2 (CH), 117.8 (C), 99.3 (CH), 65.4 (CH2).
Anal. Calcd for C22H18O4S2Se: C, 53.98; H, 3.71. Found: C, 54.02;
H, 3.75.
Bis(phenylsulfonyl) selenide [(PhSO2)2Se]. SeCl2 [generated by
reaction of selenium powder (2.0 g, 25.3 mmol) and sulfuryl
chloride (3.4 g, 25.3 mmol) in anhydrous THF (60 mL)]28 was
added dropwise to a suspension of sodium benzenesulfinate (8.3
g, 50.6 mmol) in anhydrous benzene (100 mL) at room temperature
under nitrogen atmosphere. The resulting mixture was stirred at
room temperature for 19 h and was thereafter passed through a
pad of Celite. The solvents were evaporated in vacuo, and the
residue was subjected to column chromatography using n-heptane/
EtOAc (4:1 f 2:1) to give bis(phenylsulfonyl) selenide27 (4.9 g,
54%) as a yellow crystalline solid, which decomposes slowly if
exposed to ambient temperature and light. If necessary, this material
can be recrystallized from acetonitrile, giving a final yield of ∼30%.
Alternatively, material of good quality could also be obtained by
initial trituration of the crude product with Et2O, followed by
crystallization from acetonitrile. IR (neat): 1447, 1341, 1324, 1306,
1146, 1066, 746, 708, 676 cm-1. 1H NMR (CDCl3): δ 8.03-7.99
(m, 4 H), 7.74-7.68 (m, 2 H), 7.62-7.56 (m, 4 H). 13C NMR
(CDCl3): δ 146.9 (C), 135.0 (CH), 129.6 (CH), 127.8 (CH). HRMS
(FAB): m/z 362.9267 [M + H]+, C12H10O4S2Se + H requires
362.9264.
Bis(2-[1,3]dioxolan-2-yl-4,5-dimethoxyphenyl) selenide (10a).
Purification by column chromatography using n-heptane/EtOAc (4:1
f 1:1) gave 10a (0.63 g, 63%) as a white solid; mp 116-118 °C.
IR (neat): 2960, 2865, 1596, 1505, 1398, 1378, 1263, 1251, 1198,
1
1163, 1079, 1027, 983, 961, 938, 865, 840, 766 cm-1. H NMR
(DMSO-d6): δ 7.08 (s, 2 H), 6.87 (s, 2 H), 6.00 (s, 2 H), 4.13-4.04
(m, 4 H), 4.00-3.93 (m, 4 H), 3.76 (s, 6 H), 3.60 (s, 6 H). 13C
NMR (DMSO-d6): δ 149.5 (C), 148.6 (C), 131.1 (C), 121.4 (C),
117.2 (CH), 110.1 (CH), 102.4 (CH), 64.9 (CH2), 55.5 (CH3), 55.5
(CH3). Anal. Calcd for C22H26O8Se: C, 53.12; H, 5.27. Found: C,
53.07; H, 5.24.
Bis(2-[1,3]dioxolan-2-yl-phenyl)dimethylgermane (10b). Pu-
rification by column chromatography using n-heptane/EtOAc (6:1
f 4:1) gave 10b (0.61 g, 76%) as a white solid; mp 81-84.5 °C.
IR (neat): 2895, 1398, 1215, 1121, 1079, 1049, 1018, 985, 968,
940, 803, 763, 731 cm-1. 1H NMR (DMSO-d6): δ 7.55-7.52 (m,
2 H), 7.44-7.33 (m, 6 H), 5.56 (s, 2 H), 3.98-3.86 (m, 4 H),
3.80-3.68 (m, 4 H), 0.70 (s, 6 H). 13C NMR (DMSO-d6): δ 142.4
(C), 139.3 (C), 133.7 (CH), 128.7 (CH), 128.5 (CH), 126.1 (CH),
102.2 (CH), 64.5 (CH2), 0.7 (CH3). Anal. Calcd for C20H24GeO4:
C, 59.90; H, 6.03. Found: C 59.93; H, 5.96.
General Procedure for Synthesis of Diacetals 6a-c and 10a,b.
A solution of n-BuLi (1.6 M in hexanes, 3.0 mL, 4.8 mmol) was
added dropwise to a solution of the acetal 5,29 9a,36 or 9b37,38 (4.0
mmol) in anhydrous THF (30 mL) at -78 °C under nitrogen
atmosphere. The mixture was stirred for 30 min at -78 °C, followed
by addition of a solution of Me2SiCl2, Me2GeCl2, or (PhSO2)2Se
(2.0 mmol) in THF (10 mL) during 10 min at -78 °C. The resulting
mixture was allowed to warm to room temperature over 16-18 h,
thereafter treated with saturated aqueous NH4Cl (30 mL), and
extracted with Et2O (2 × 30 mL). The combined organic extracts
were washed with water (2 × 30 mL) and brine (30 mL) and dried
over Na2SO4. Evaporation of the solvents in vacuo gave a residue,
which was subjected to column chromatography as indicated for
each specific example, giving the diacetals 6a-c or 10a,b.
General Procedure for Synthesis of Dialdehydes 7a-c and
11a,b. A solution of aqueous HClO4 (70%, 0.24 mL) in water (1
mL) was added to a solution of the diacetals 6a-c or 10a,b (0.85
mmol) in acetone (15-20 mL) at room temperature. The reaction
mixture was stirred at room temperature for 16 h (3 h for 11b),
and was thereafter treated with saturated aqueous NaHCO3 (10 mL).
The resulting mixture was extracted with EtOAc/CH2Cl2 (30 mL),
washed with water (2 × 30 mL) and brine (30 mL), and dried over
Na2SO4. Evaporation of the solvents in vacuo gave a residue, which
was either triturated with Et2O or subjected to column chromatog-
raphy as indicated below, to afford the desired products 7a-c or
11a,b.
Bis(2-formylbenzo[b]thiophen-3-yl)dimethylsilane (7a). The
crude product was triturated with Et2O to give compound 7a (0.31
g, 96%) as a white solid; mp 158-161.5 °C. IR (neat): 1652, 1476,
Bis(2-[1,3]dioxolan-2-yl-benzo[b]thiophen-3-yl)dimethylsi-
lane (6a). Column chromatography using n-heptane/EtOAc (5:1
f 3:1) gave 6a (0.68 g, 73%) as white solid; mp 153-156 °C. IR
(neat): 2888, 1514, 1506, 1365, 1345, 1251, 1191, 1179, 1157, 1138,
1426, 1244, 1184, 1152, 930, 836, 786, 777, 758, 748, 725 cm-1
.
1H NMR (DMSO-d6): δ 10.09 (s, 2 H), 8.15-8.12 (m, 2 H),
7.87-7.84 (m, 2 H), 7.53-7.47 (m, 2 H), 7.37-7.31 (m, 2 H),
1.09 (s, 6 H). 13C NMR (DMSO-d6): δ 186.3 (CH), 150.3 (C),
144.8 (C), 143.4 (C), 141.7 (C), 127.8 (CH), 126.9 (CH), 125.4
(CH), 123.6 (CH), 2.4 (CH3). Anal. Calcd for C20H16O2S2Si: C,
63.12; H, 4.24. Found: C, 63.11; H, 4.21.
1093, 1076, 1055, 1024, 994, 957, 931, 829, 787, 756, 732 cm-1
.
1H NMR (DMSO-d6): δ 7.93 (br d, J ) 8.0 Hz, 2 H), 7.66 (br d,
J ) 8.1 Hz, 2 H), 7.28-7.22 (m, 2 H), 7.16-7.10 (m, 2 H),
4.18-3.94 (m, 8 H), 0.83 (s, 6 H). 13C NMR (DMSO-d6): δ 151.0
(C), 143.8 (C), 139.1 (C), 132.2 (C), 124.4 (CH), 124.3 (CH), 124.1
(CH), 122.4 (CH), 99.0 (CH), 65.2 (CH2), 1.1 (CH3). Anal. Calcd
for C24H24O4S2Si: C, 61.51; H, 5.16. Found: C, 61.65; H, 5.12.
Bis(2-formylbenzo[b]thiophen-3-yl)dimethylgermane (7b). The
crude product was triturated with Et2O to give compound 7b (0.35
g, 97%) as a white solid; mp 155-157 °C. IR (neat): 1656, 1487,

Fused 1-Sila-, 1-Germa-, and 1-Selenacyclohepta-2,4,6-trienes
Organometallics, Vol. 27, No. 15, 2008 3963
1475, 1246, 1182, 1159, 1110, 914, 824, 756, 732, 712 cm-1; H
NMR (DMSO-d6): δ 10.01 (s, 2 H), 8.16-8.13 (m, 2 H), 7.86-7.83
(m, 2 H), 7.55-7.49 (m, 2 H), 7.39-7.33 (m, 2 H), 1.21 (s, 6 H).
13C NMR (DMSO-d6): δ 186.1 (CH), 148.3 (C), 147.0 (C), 142.9
(C), 141.6 (C), 128.0 (CH), 126.8 (CH), 125.5 (CH), 123.7 (CH),
3.1 (CH3). Anal. Calcd for C20H16GeO2S2: C, 56.51; H, 3.79. Found:
C, 56.44; H, 3.84.
Bis(2-formylbenzo[b]thiophen-3-yl) selenide (7c). The crude
product was triturated with Et2O to give compound 7c (0.33 g, 97%)
as a light yellow solid; mp 211-214 °C. IR (neat): 1661, 1587,
1492, 1425, 1288, 1244, 1191, 1165, 1129, 907, 752, 726, 712
cm-1. 1H NMR (DMSO-d6): δ 10.56 (s, 2 H), 8.12 (ddd, J ) 8.2,
1.1, 0.7 Hz, 2 H), 7.79 (ddd, J ) 8.2, 1.2, 0.7 Hz, 2 H), 7.5 (ddd,
J ) 8.2, 7.1, 1.2 Hz, 2 H), 7.40 (ddd, J ) 8.2, 7.1, 1.1 Hz, 2 H).
13C NMR (DMSO-d6): δ 186.0 (CH), 142.2 (C), 140.2 (C), 140.1
(C), 130.3 (C), 128.9 (CH), 126.2 (CH), 124.9 (CH), 124.1 (CH).
Anal. Calcd for C18H10O2S2Se: C, 53.86; H, 2.51. Found: C, 53.76;
H, 2.42.
Evaporation of the solvents in vacuo, followed by purification of
the crude product as indicated for each specific example, yielded
the metallacycloheptatrienes 8a-c and 12a,b.
1
Compound 8a. Purification by column chromatography using
n-heptane gave 8a (0.13 g, 75%) as a yellow solid; mp 149-151
°C. IR (neat): 2921, 1505, 1425, 1311, 1298, 1270, 1248, 1126,
1091, 1048, 943, 843, 803, 768, 749, 725 cm-1. 1H NMR (DMSO-
d6): δ 8.11-8.04 (m, 4 H), 7.52-7.41 (m, 4 H), 6.89 (s, 2 H), 0.85
(s, 6 H). 13C NMR (DMSO-d6): δ 146.3 (C), 144.9 (C), 141.0 (C),
130.5 (C), 125.1 (CH), 124.9 (CH), 124.8 (CH), 123.3 (CH), 122.3
(CH), 0.6 (CH3). HRMS (FAB): m/z 348.0456 [M+], C20H16S2Si
requires 348.0463.
Compound 8b. Purification by column chromatography using
n-heptane gave 8b (0.16 g, 81%) as a yellow solid; mp 168-170
°C. IR (neat): 2921, 1426, 1308, 1048, 1043, 939, 798, 773, 750,
723 cm-1. 1H NMR (DMSO-d6): δ 8.08-7.97 (m, 4 H), 7.52-7.41
(m, 4 H), 6.79 (s, 2 H), 0.93 (s, 6 H). 13C NMR (DMSO-d6): δ
144.5 (C), 144.4 (C), 140.8 (C), 131.1 (C), 125.1 (CH), 124.9 (CH),
124.5 (CH), 122.9 (CH), 122.4 (CH), 0.7 (CH3). HRMS (FAB):
m/z 393.9912 [M+], C20H16GeS2 requires 393.9905.
Compound 8c. Purification by column chromatography using
n-heptane/EtOAc (9:1 f 7:1) gave 8c (0.18 g, 97%) as a yellow
solid; mp 187.5-188.5 °C. IR (neat): 1431, 1251, 1085, 891, 857,
810, 759, 744, 724 cm-1. 1H NMR (DMSO-d6): δ 8.01-7.95 (m,
4 H), 7.53-7.40 (m, 4 H), 7.26 (s, 2 H). 13C NMR (DMSO-d6): δ
141.2 (C), 140.4 (C), 139.1 (C), 127.8 (CH), 125.7 (CH), 125.3
(CH), 123.1 (CH), 122.9 (CH), 117.5 (C). Anal. Calcd for
C18H10S2Se: C, 58.53; H, 2.73. Found: C, 58.46; H, 2.85.
Bis(2-formyl-4,5-dimethoxyphenyl) selenide (11a). The crude
product was triturated with Et2O to give compound 11a (0.30 g,
86%) as an off-white solid; mp 160-163.5 °C. IR (neat): 1668,
1579, 1549, 1501, 1458, 1436, 1362, 1336, 1260, 1227, 1215, 1189,
1
1155, 1040, 1021, 870, 807, 788, 738 cm-1. H NMR (DMSO-
d6): δ 10.07 (s, 2 H), 7.59 (s, 2 H), 6.79 (s, 2 H), 3.85 (s, 6 H),
3.65 (s, 6 H). 13C NMR (DMSO-d6): δ 191.9 (CH), 153.9 (C),
148.6 (C), 128.5 (C), 128.1 (C), 116.0 (CH), 113.9 (CH), 55.8
(CH3), 55.8 (CH3). Anal. Calcd for C18H18O6Se: C, 52.82; H, 4.43.
Found: C, 52.78; H, 4.47.
Bis(2-formylphenyl)dimethylgermane (11b). Purification by
column chromatography using n-heptane/EtOAc (7:1 f 4:1) gave
11b (0.23 g, 86%) as a white solid; mp 95-99 °C. IR (neat): 1693,
1676, 1559, 1292, 1194, 1117, 1065, 839, 816, 750 cm-1. 1H NMR
(DMSO-d6): δ 9.94 (s, 2 H), 7.99-7.97 (m, 2 H), 7.67-7.58 (m,
6 H), 0.68 (s, 6 H). 13C NMR (DMSO-d6): δ 193.6 (CH), 142.6
(C), 139.7 (C), 135.2 (CH), 133.4 (CH), 133.2 (CH), 129.2 (CH),
-0.1 (CH3). The molecular ion peak was not detected in the FAB
mass spectrum of 11b, which displayed a fragment ion (base peak)
with m/z 299.0127, resulting from loss of a CH3 group.
2,3,7,8-Tetramethoxy-5H-5-selena[2,3:6,7]dibenzocyclohepta-
2,4,6-triene (12a). Purification by column chromatography using
n-heptane/EtOAc (4:1 f 2:1) gave 12a (0.15 g, 80%) as a white
solid; mp 199-201.5 °C. IR (neat): 1587, 1496, 1464, 1346, 1331,
1
1237, 1197, 1147, 1037, 871, 859, 792 cm-1. H NMR (DMSO-
d6): δ 7.05 (s, 2 H), 6.92 (s, 2 H), 6.87 (s, 2 H), 3.76 (s, 6 H), 3.73
(s, 6 H). 13C NMR (DMSO-d6): δ 149.5 (C), 148.7 (C), 133.2 (C),
132.8 (CH), 119.8 (C), 116.2 (CH), 112.5 (CH), 55.6 (CH3), 55.6
(CH3). Anal. Calcd for C18H18O4Se: C, 57.30; H, 4.81. Found: C,
57.19; H, 4.87.
General Procedure for Synthesis of Metallacycloheptatrienes
8a-c and 12a,b. TiCl4 (2.7 mL, 25 mmol) was added dropwise
during 10 min to anhydrous THF (100 mL) at -78 °C under argon
atmosphere, and the resulting solution was stirred for 5 min. The
yellow slurry was allowed to warm to room temperature, followed
by addition of zinc powder (3.3 g, 50 mmol) and pyridine (0.5
mL). This suspension was heated at reflux for 2.5 h and was then
allowed to cool to room temperature. The appropriate dialdehyde
(7a-c or 11a,b) (0.50 mmol) was added slowly as a dilute solution
in anhydrous THF (100 mL) over 4 h at room temperature. After
complete addition, the reaction mixture was allowed to stir at room
temperature for 16 h and was thereafter heated at reflux for an
additional period of 4 h. After cooling to room temperature, a 50%
aqueous solution of K2CO3 (50 mL) was added. The resulting
mixture was stirred vigorously for 18 h and thereafter passed
through a pad of Celite, which was washed with EtOAc (40 mL).
The layers were separated and the aqueous phase was extracted
with EtOAc (50 mL). The combined organic layers were washed
with water (2 × 50 mL) and brine (50 mL) and dried (Na2SO4).
5,5-Dimethyl-5H-5-germa[2,3:6,7]dibenzocyclohepta-2,4,6-
triene (12b). Purification by column chromatography using n-
heptane gave 12b (0.05 g, 36%) as a colorless viscous oil. IR (neat):
2921, 2852, 829, 798, 769, 733 cm1. 1H NMR (DMSO-d6): δ
7.49-7.46 (m, 2 H), 7.42-7.31 (m, 6 H), 6.94 (s, 2 H), 0.56 (s, 6
H). 13C NMR (DMSO-d6): δ 140.7 (C), 138.9 (C), 133.0 (CH),
131.9 (CH), 129.3 (CH), 128.6 (CH), 127.6 (CH), -5.4 (CH3).
HRMS (FAB) m/z 283.0535 [M + H]+, C16H16Ge + H requires
283.0542.
Acknowledgment. The Magnus Bergvall Foundation is
gratefully acknowledged for financial support.
Supporting Information Available: 1H and 13C NMR spectra
for compounds 8a, 8b, 11b, and 12b. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM8003114