Synthesis of Polysubstituted Germoles and Benzogermoles Using a Substoichiometric Amount of Diisobutylaluminum Hydride

Article abstract of DOI:10.1021/acs.orglett.1c01314

We developed a synthetic route to unsymmetrically polysubstituted germoles bearing different substituents from 1-hydrogermyl-4-silyl-1,3-enynes. The reaction proceeded with 0.5 equiv of diisobutylaluminum hydride. Various 2-silylgermoles including benzogermoles were obtained in good to excellent yields. 2-Germylbenzogermoles could be also successfully synthesized from 1-hydrogermyl-4-germyl-1,3-enynes under the same reaction conditions.

Full text of DOI:10.1021/acs.orglett.1c01314

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Letter
Synthesis of Polysubstituted Germoles and Benzogermoles Using a
Substoichiometric Amount of Diisobutylaluminum Hydride
Ko Kojima, Seiya Uchida, Hidenori Kinoshita,* and Katsukiyo Miura
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ABSTRACT: We developed a synthetic route to unsymmetrically
polysubstituted germoles bearing different substituents from 1-
hydrogermyl-4-silyl-1,3-enynes. The reaction proceeded with 0.5
equiv of diisobutylaluminum hydride. Various 2-silylgermoles
including benzogermoles were obtained in good to excellent
yields. 2-Germylbenzogermoles could be also successfully synthe-
sized from 1-hydrogermyl-4-germyl-1,3-enynes under the same
reaction conditions.
6a
based reductive cyclization of alkynes.6b,c,e,f The Cp2TiCl2
he synthetic methods for organogermanium compounds
Thave been less studied than those of other group 14 and Cp2ZrCl26d mediated syntheses of germoles have also been
homologues, such as organosilicon1 and organotin2 com-
reported. Another approach to construct germoles was
achieved by Ru catalyzed double trans-hydrogermylation of
1,3-diynes.4b The synthesis of 3-borylgermoles has also been
conducted.6g Additionally, transition metal catalyzed syntheses
of benzogermoles (Scheme 1b)7 and dibenzogermoles
(Scheme 1c)8 have been reported. Except for their distinctive
advantages and applications, these methods are not suitable for
the synthesis of unsymmetrically substituted germoles bearing
different substituents.
pounds, although the organogermanium compounds are
potentially useful synthetic reagents3 and organic molecular
materials.4 One of the promising germanium-based organic
molecular materials is a germole, germacyclopentadiene.
Germoles are molecules that have an energetically low-lying
LUMO similar to siloles.4 Therefore, they are expected to
exhibit properties that either equal or surpass those of siloles.5
To date there have been several reports on the preparation of
germoles (Scheme 1a). The conventional method used to
prepare symmetrically substituted germoles is the lithium-
Recently, we reported a novel method for the preparation of
unsymmetrically substituted siloles and benzosiloles from 1,4-
bissilyl-1,3-enynes and o-silylethynylsilylbenzenes using
DIBAL-H (Scheme 2a).9 The silyl group at the alkyne
terminus is crucial, and the cyclization reaction did not
proceed when the substrates bearing a terminal or internal
alkyne moiety instead of the silylalkyne moiety was used.9a To
expand the synthetic utility of this method and develop the
synthetic organic chemistry of organogermanium compounds,
we applied these reaction conditions to germole synthesis
(Scheme 2b). Herein we disclose the germole synthesis by the
DIBAL-H-promoted intramolecular Ge−C bond forming
reaction. Consequently, the DIBAL-H-promoted cyclization
of 1-germyl-4-silyl-1,3-enynes 1 proceeded smoothly to form
the corresponding unsymmetrically polysubstituted germoles 2
bearing different substituents in moderate to excellent yields as
a single regioisomer. In the synthesis of benzogermoles, it was
Scheme 1. Previously Reported Methods Used to Prepare
Germoles, Benzogermoles, and Dibenzogermoles
Received: April 16, 2021
Published: June 1, 2021
https://doi.org/10.1021/acs.orglett.1c01314
© 2021 American Chemical Society
Org. Lett. 2021, 23, 4598−4602
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With the optimized conditions in hand, we investigated the
substrate scope for the DIBAL-H-promoted cyclization. As
shown in Figure 1, unsymmetrically polysubstituted germoles
2b−2f, which cannot be synthesized using other methods,
were obtained in good to excellent yields.
Scheme 2. Previous Work and This Work
revealed that modified reaction conditions were required to
complete the transformation. Furthermore, a novel germole, 2-
germylbenzo[b]germole, was successfully obtained under the
same reaction conditions.
We commenced a feasibility study for the synthesis of
germoles using 1a as a model substrate (Table 1). The optimal
Figure 1. Substrate scope for synthesis of germoles. Conditions: 1
(0.25 mmol), DIBAL-H (0.125 mmol), and octane (0.75 mL).
Isolated product yields are reported.
Table 1. Optimization of Reaction Conditions for Synthesis
a
of Germoles
Subsequently, we moved onto the synthesis of benzoger-
moles (Table 2). Contrary to our expectations, the optimized
Table 2. Optimization of Reaction Conditions for Synthesis
a
of Benzogermoles
b
entry
time (h)
DIBAL-H (equiv)
yield of 2a (%)
1
2
3
4
5
6
7
0.5
1
2
1
1
1.2
1.2
1.2
0.5
0.3
0.25
0
78
77
68
91
b
b
c
entry time (h) DIBAL-H (equiv) yield of 4a (%) yield of 5a (%)
92
87
d
c
c
c
1
1
1
2
3
4
5
6
7
1
1
0.5
1.2
1.2
1.2
1.2
0.5
0.5
22
28
0
48
42
71
6
0
d
e
a
2
All reactions were carried out using 1a (0.25 mmol) in octane (0.75
mL) at 80 °C. Isolated yield. Poor reproducibility was observed. 1a
was recovered quantitatively.
b
c
d
6
24
6
88
82
33
89
0
17
0
f
24
conditions used for the synthesis of siloles9 were adapted to
the reaction of 1a (entry 1), and the desired germole 2a was
obtained in 78% yield. The reaction time was then examined
(entries 1−3). However, an extended reaction time did not
improve the product yield (entry 1 vs entries 2−3). Then, the
amount of DIBAL-H used in the reaction was investigated
(entries 4−6). The reaction with a reduced amount of DIBAL-
H (0.5 equiv) gave a better result, and 2a was isolated in 91%
yield (entry 4). The use of an even lesser amount of DIBAL-H
was also successful; however, poor reproducibility was
observed (entries 5 and 6). The prolonged reaction time
with 1.2 equiv of DIBAL-H (entry 1 vs 3) and the excess
amount of DIBAL-H (entry 2 vs 4) lowered the yield of 2a,
because DIBAL-H gradually decomposed 2a. This phenom-
enon was also observed in the synthesis of siloles.9a DIBAL-H
was crucial in this transformation and its absence did not afford
2a (entry 7). This observation showed that the possibility of a
simple thermal cyclization could be excluded. We concluded
that the optimal conditions for the cyclization of 1a to germole
2a were those described in entry 4.
a
All reactions were carried out using 3a (0.25 mmol) in octane (0.75
b
c
mL) at 80 °C. Isolated yield. NMR yield. 3a (0.050 mmol, 20%)
d
e
was recovered. The reaction was conducted at 50 °C. 3a (0.069
f
mmol, 28%) was recovered. 3a (0.048 mmol, 19%) was recovered.
reaction conditions used for the synthesis of germoles did not
work well (Table 2, entry 1). This result was different from
that previously observed during the synthesis of benzosiloles
reported by our group.9 Therefore, we reoptimized the
reaction conditions using 3a as a model substrate (Table 2).
The reaction of 3a under the optimized conditions used for the
synthesis of 2a gave the desired benzogermole 4a along with
(E)-β-silyl styrene 5a (entry 1). The reaction temperature is
crucial to achieve the cyclization reaction. The reaction
performed at 50 °C for 2 h using 1.2 equiv of DIBAL-H
gave 5a in 71% yield along with 3a (entry 3). Therefore, we
performed the reaction at 80 °C to investigate the effect of
reaction time (entries 2−5). It was revealed that 3a was
completely transformed into 4a after 24 h reaction without the
formation of 5a (entry 5). The reduced amount of DIBAL-H
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used in the reaction improved the yield of 4a (entry 7). A
large-scale reaction using 3a (1.0 g, 2.5 mmol) successfully
gave 4a (0.91 g) in 91% yield without any loss of reactivity
under the conditions described in entry 7.
Scheme 3. Plausible Mechanism
The generality of this substoichiometric amount of DIBAL-
H-promoted synthesis of benzogermoles was investigated
under the optimized reaction conditions (Table 2, entry 7),
and the results are shown in Figure 2. 2-Silylated
to indicate the presence of these intermediates. It also shows
that the C−Ge bond-forming step (3) is relatively slow in the
cyclization of 3a, which lowers the total reaction rate of 3a.
DIBAL-H itself is not stable at high temperatures, and the
decomposition of the original and/or regenerated DIBAL-H
gradually occurs during the reaction under heating conditions.
Consequently, the reproducibility of the reaction using less
than 0.5 equiv of DIBAL-H was poor.
Next, we focused on the synthesis of 2-germylgermoles from
their corresponding germylacetylene derivatives. Silicon and
germanium belong to group 14, and organosilyl and organo-
germyl groups exhibit similar directing effects. For example,
hyperconjugation between the σ* orbitals of C−Si and C−Ge
bonds and adjacent occupied orbitals provides the α-effect, or a
stabilizing effect on adjacent α-anionic species.13 Therefore, we
expected that the germyl group on the sp carbon atom would
also promote the cyclization to form germoles. To test this
hypothesis, we carried out the reaction using 6a under the
same reaction conditions as those described in entry 7 of Table
2. The desired 2-germylbenzogermole 7a was obtained in 87%
isolated yield (Scheme 4). A benzogermole bearing a
Figure 2. Substrate scope for synthesis of benzogermoles. Conditions:
1 (0.25 mmol), DIBAL-H (0.125 mmol), and octane (0.75 mL).
a
Isolated yields are shown. 1,2-Dichloroethane (0.75 mL) was used
b
instead of octane. DIBAL-H (0.25 mmol) was used. The reaction
time was 72 h.
benzogermoles 4b−4d were obtained in good yields. The
reactions of the substrates bearing halo group (F or Cl)
provided 4e and 4f in 91% and 54% yield, respectively, without
loss of the halogen group. The structure of 4e was clearly
confirmed by X-ray crystallographic analysis.10 The cyclization
of 3g and 3h,11 bearing a bulky silyl group on the sp-carbon
atom, gave 2-silylated benzogermoles 4g and 4h in moderate
yields along with trace amounts of (E)-β-silylstyrenes 5g and
5h. Non-negligible amounts of 3g and 3h were also recovered.
These results show that the steric hindrance of the
silylacetylene moiety inhibits the hydroalumination step
triggering the formation of germoles 4g and 4h.
Scheme 4. Synthesis of 2-Germylbenzogermoles
It is noteworthy that the double cyclization proceeded
smoothly to form germolo[5,4-f]benzogermole 4i in excellent
yield, although an elongated reaction time was required.
A plausible reaction mechanism and rationale for the
successful cyclization using a substoichiometric amount of
DIBAL-H are shown in Scheme 3. The DIBAL-H-promoted
transformation consists of three steps: (1) regioselective
hydroalumination of the silylalkyne moiety in 1 to form (Z)-
alkenylaluminum I with the α-effect of the silyl group,12 (2)
geometrical isomerization of I into the thermodynamically
favored (E)-isomer II by steric and electronic effects of the
aluminum and silyl moieties,12 and (3) intramolecular
nucleophilic substitution of II between the hydrogermyl
moiety and the sp2-carbon bound to Al to form the cyclized
product 2 and regenerate DIBAL-H. The DIBAL-H generated
in situ promotes the cyclization of 1 to 2 via III and IV.9 The
formation of 5a (Table 2) can be rationalized by protode-
alumination of intermediates II and IV on aqueous quenching
triphenylgermyl group 7b was also synthesized in 32% yield
from 6b.11 These yields were comparable with those observed
for 4a and 4h (Figure 2). As expected, the germyl groups on
the sp carbon atom of 6 was found to have similar directing
effects to those of the silyl groups, which promote
regioselective hydroalumination of 6 to V and the subsequent
isomerization of V to VII via VI for the cyclization to 7.12,13a
The iododesilylation of 4a was investigated for further
modification of the obtained germoles (Scheme 5). In our
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Notes
Scheme 5. Iododesilylation of 4a
The authors declare no competing financial interest.
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01314.
Experimental procedures and characterization data; X-
ray crystallographic data for 4e (PDF)
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tion of 1,3-Diynes. Org. Lett. 2010, 12, 1056−1058. (c) Dong, Z.;
Accession Codes
CCDC 2075032 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
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AUTHOR INFORMATION
Corresponding Author
■
Hidenori Kinoshita − Department of Applied Chemistry,
Graduate School of Science and Engineering, Saitama
University, Saitama 338-8570, Japan; orcid.org/0000-
0002-9761-9015; Email: hkino@mail.saitama-u.ac.jp
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Authors
Ko Kojima − Department of Applied Chemistry, Graduate
School of Science and Engineering, Saitama University,
Saitama 338-8570, Japan
Seiya Uchida − Department of Applied Chemistry, Graduate
School of Science and Engineering, Saitama University,
Saitama 338-8570, Japan
Katsukiyo Miura − Department of Applied Chemistry,
Graduate School of Science and Engineering, Saitama
University, Saitama 338-8570, Japan; orcid.org/0000-
0002-4535-8677
Complete contact information is available at:
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