Transition-Metal-Free, Formal C-H Germylation of Arenes and Styrenes via Dibenzothiophenium Salts

Article abstract of DOI:10.1021/acs.orglett.1c01505

We report an operationally simple, selective, and transition-metal-free germylation of arenes and styrenes at room temperature, using a robust and bench-stable Ge source (R3Ge-SiR3) and dibenzothiophenium salts as enabling intermediates. The first direct engagement in cross-coupling of the newly made E-alkenyl germanes is also presented, allowing the chemoselective arylation under air-tolerant nanoparticle catalysis.

Full text of DOI:10.1021/acs.orglett.1c01505

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Letter
Transition-Metal-Free, Formal C−H Germylation of Arenes and
Styrenes via Dibenzothiophenium Salts
Aymane Selmani and Franziska Schoenebeck*
Cite This: Org. Lett. 2021, 23, 4779−4784
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ABSTRACT: We report an operationally simple, selective, and transition-
metal-free germylation of arenes and styrenes at room temperature, using a
robust and bench-stable Ge source (R3Ge-SiR3) and dibenzothiophenium salts
as enabling intermediates. The first direct engagement in cross-coupling of the
newly made E-alkenyl germanes is also presented, allowing the chemoselective
arylation under air-tolerant nanoparticle catalysis.
tions in drug discovery1 and material science.2 More recently,
the potential of aryl germanes as alternative and orthogonal
coupling partners in synthesis and catalysis has also been
showcased,3,4 which allows their selective functionalization in
the presence of C-halogen, C-BPin, or C-silanes and therefore
adds a dimension of versatility in the modular assembly of
densely functionalized polyarenes. While alkylations of closely
related germatranes could also recently be achieved,5 the direct
functionalization of alkenyl germanes remains unprecedented.
In this context, the broader impact of organogermanes is
directly correlated to their synthesizability, and as such, there is
a significant demand for an efficient, selective, and practical
methodology to install the Ge functionality into organic
molecules. Ideally, this methodology is amenable to advanced
building blocks to meet the need for late-stage diversification
in a medicinal and material context on one hand and to be able
to access diverse building blocks for modular synthesis on the
wing to their relatively high robustness, hydrophobicity,
Oand low toxicity, organogermanes have found applica-
other hand.3 Classical syntheses of Csp2-germanes rely on Csp2
-
halides via stoichiometric organometallic reactivity3,6 or
transition metal catalysis,7 albeit of limited scope owing to
the basicity of reagents and/or the need for harsh reaction
conditions. Moreover, the C-halogen sites would be of utmost
value for modular building blocks (see Figure 1), and a
synthetic strategy to introduce the Ge functionality in the
presence of C-halogen would therefore be highly enabling.
While direct C−H functionalization would be desired, such
protocols suffer from the need of a directing group and high
temperature (>90 °C), resulting in only a handful of ortho-
germylated examples, and the employed transition metal is
Received: May 2, 2021
Published: June 4, 2021
Figure 1. Significance and state-of-the-art of the aryl germane
synthesis (top) and this work (bottom).
© 2021 The Authors. Published by
American Chemical Society
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incompatible with, e.g., C−I bonds.8 By contrast, a mild and
directing-group-free C−H germylation would greatly improve
the diversity of accessible Csp2-germanes. In this context, we
recently reported a Pd(I)-dimer-catalyzed9 germylation via
formal C−H functionalization of nonactivated and directing-
group-free arenes, enabled by (tetrafluoro)thianthrenium salts
as key intermediates and using triethylgermane (Et3GeH) as a
reagent (Figure 1).10 While the method was mild, it was
limited to arenes, and a competitive side reaction with the air-
sensitive Et3GeH was encountered in some cases, which
generates Ar−H as the byproduct.7a,b,11
aryl germane did not result in the absence of a transition metal
(see Figure 2). Instead, one of the C−S bonds of the
thianthrene moiety was cleaved instead of ipso-substitution.26
With a view to develop a more efficient, practical, and
selective method that expands the accessible substrates also to
olefins, we set out to develop a transition-metal-free
germylation of Csp2-sites using a robust and air-stable Ge
reagent. Moreover, we showcase the first direct coupling of
alkenyl germanes.
Figure 2. Rationalization of the regioselectivity in the germylation of
4-fluorophenyl dibenzothiophenium salt. Free energy differences are
given in kcal/mol.
To this end, we further focused on Csp2-SR2 salts, which
were shown to be powerful linchpins. Recently, Lewis,12
Procter,13 and Ritter14 demonstrated that aryl and alkenyl
sulfonium salts (such as (tetrafluoro)thianthrenium and
dibenzothiophenium) were readily introduced by formal C−
H functionalization of arenes or olefins in a site-selective
manner, providing a versatile platform for functionalizations in
C−C, C−N, C−O, C−Hal, SiR3, or BR2 bond formations with
transition metal and/or photoredox catalysis.12−17 Under
metal-free conditions,18 the borylation of thiol-derived
We therefore embarked on investigating alternative
sulfonium salts that can readily be made from arenes directly
via C−H functionalization. In this context, dibenzothiophe-
nium salts have previously been shown to give high reactivity
toward direct ipso-substitution in aromatic [18F]-fluorinatio-
n.14e,18b Our computational studies25 of this salt indicated that
the Et3Ge− anion would initially add to the sulfur to adopt a
trigonal bipyramidal sulfurane adduct,27 which subsequently
undergoes ligand coupling13b with an activation free energy
barrier of ΔG‡ = 0.8 kcal/mol to yield the desired ArGeEt3
(see Figure 2 and SI for a complete profile). This process is
ΔΔG‡ = 33.9 kcal/mol favored over the alternative bond
cleavage to break the dibenzothiophene bond. These
observations parallel previous computational studies of the
functionalization of sulfonium salts.18b Encouraged by these
data, we set out to experimentally explore this transformation.
Upon testing a range of fluoride sources and solvents, to our
delight, we identified that germylation of 4-fluorophenyl
dibenzothiophenium salt with PhMe2Si-GeEt3 proceeded
very efficiently at room temperature in MeCN with CsF,
giving the desired 4-fluorophenyl triethylgermane in 90% yield
(Figure 2). No side products arising from silylation were
detected; PhMe2Si-F was observed by GC-MS analysis,
whereas Et3Ge-F was not detected, showcasing the high
chemoselectivity in the activation of the [Ge−Si] reagent.
Notably, our tests with the alternative digermane Et6Ge2
reagent in place of PhMe2Si-GeEt3 under otherwise identical
conditions did not give rise to conversion, indicating that
Et6Ge2 cannot be activated in this manner with a fluoride
source. We also tested alternative electrophiles for the
germylation; however, no germylation occurred with the
corresponding aryl iodide, bromide, triflate, dimethylsulfo-
nium, or tetrafluorothianthrenium salts, and only the
dibenzothiophenium salt gave rise to the aryl germane.
+
sulfonium salts, i.e., ArSMe2 , under UV light with limited
functional group tolerance has been reported,19 and for highly
+
activated ArSMe2 salts that bear an electron-withdrawing
substituent in the para-position, silylation and stannylation
using R3Si-SiR3 or R3Sn-SnR3 under CsF activation could
recently be achieved without an added transition metal.20a
However, our attempts to extend this methodology to the
+
+
corresponding germylation of PhSMe2 (or alkenyl-SMe2 )
failed to produce the corresponding organogermanes.20b
With the aim to identify a suitable nucleophilic [Ge] source
and move away from the unstable [Ge]−H reagent, we
envisioned that a [Ge−Si] reagent might allow us to liberate a
[R3Ge]− nucleophile in situ and as such circumvent the need
for any transition metal catalyst. We focused on PhMe2Si-
GeEt3,21,22 which presents high air and moisture stability
paired with storability at room temperature23 and has been
shown to liberate the desired [Ge] anion over the [Si] anion
under TBAF activation, which allowed us to substitute aryl
bromides to the corresponding aryl germanes, albeit under
moderate yield (approximately 35−40%) and under Ar−H
byproduct formation.24 Indeed, our computational studies at
the CPCM (MeCN) M06-2X/6-311++G(d,p) level of theory
confirmed that attack of a fluoride anion at Si over the
alternative attack at Ge is both kinetically (by ΔΔG‡ = 10.1
kcal/mol) and thermodynamically (by ΔΔGrxn = 15.2 kcal/
mol) favored (see SI for a complete profile).25
The reagent PhMe2Si-GeEt3 can be readily prepared from
the chlorosilane PhMe2SiCl, in the presence of lithium
naphthalenide, and Et3GeCl in high yield (4.48 g, 76%
yield).21 In this manner, analogous [Ge−Si] compounds are
also accessible by varying the employed chlorogermane
R3GeCl. For example, we synthesized PhMe2Si-GenBu3 in
63% yield.
However, when we subjected PhMe2Si-GeEt3 along with
CsF to a (tetrafluoro)thianthrenium salt, which we previously
germylated under Pd(I) dimer catalysis,10 the corresponding
With the reaction conditions in hand, we next examined the
scope of this two-step germylation process. The protocol
proved to be effective for a range of compounds; there was no
marked influence of steric or electronic effects on the reaction
efficiency. Access to halogenated aryl germanes was possible,
leading to valuable scaffolds for further diversification
opportunities (1−6). Electron-rich (7) and -poor arenes (8−
9) reacted well, in both dibenzothiophenylation and
germylation reactions, to afford the corresponding aryl
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a
Scheme 1. Substrate Scope of the Transition-Metal-Free and Site-Selective Germylation of Arenes
a
Reaction conditions for the first step: arene (1.0 equiv), dibenzothiophene-S-oxide (1.1 equiv), Tf2O (1.2 equiv) in DCM (0.25 M), −40 °C to rt.
Yields are given for isolated products in parentheses. Reaction conditions for the second step: aryl dibenzothiophenium salt (1.0 equiv), PhMe2Si-
GeEt3 (1.2 equiv), and CsF (1.2 equiv) in MeCN (0.1 M), rt. Yields are given for isolated products.
triethylgermanes in high yields. Several anisole derivatives, with
trifluoromethyl (11), nitro (12), nitrile (13), or fluorine (14)
as substituents in various positions, were successfully
converted, showcasing the excellent site selectivity, as no
other regioisomers were observed. Aniline-containing triethyl-
germanes could also be synthesized (15−16). The process is
also compatible with heterocycles (17−19).
Within the realm of late-stage functionalization, we also
investigated the possibility for germylation of drug-like
molecules and bioactive compounds. The formal C−H
germylation of fenofibrate (21) and clofibrate (22) was
selectively achieved in good yields. It is important to note
that reduction of the aryl dibenzothiophenium salt to Ar−H
was fully prevented using the PhMe2Si-GeEt3 reagent, which
contrasts the previous metal-catalyzed development with
Et3GeH.10 Moreover, the introduction of alternative R3Ge
groups is also possible through modification of the R3Ge-
SiMe2Ph reagent.28 To illustrate this, we synthesized tri-n-
butylgermanes (23−25) in this manner (Scheme 1).
However, to date, there has not been a precedence of direct
C−C coupling of alkenyl germanes. In fact, in attempts to
couple alkenyl germanes under Pd catalysis, their halogenation
was instead conducted (under harsh conditions using I−Cl,
neat at 85 °C).30d
We therefore next set out to investigate the germylation of
the corresponding alkenyl dibenzothiophenium salts. While the
synthesis and Csp2−Csp2 coupling of styryl sulfonium salts have
previously been established,13b,c their germylation (as well as
conceptually similar silylation or borylation) has not been
demonstrated. To our delight, we observed smooth germyla-
tion of styrenes (after initial C−H dibenzothiophenylation),
using the same conditions as for the arenes. We successfully
prepared a variety of diversely substituted styrenes in high
yield, including examples that contained sensitive C-halogen
sites (27−28). Notably, the (E)-isomer was exclusively formed
in all cases (Figure 3, top). With these styryl triethylgermanes
in hand, we subsequently explored their reactivity with the aim
to establish their first direct engagement in C−C cross-
coupling reactivity with aryl halides.
With the efficient formal C−H germylation of arenes
demonstrated, we subsequently set out to investigate the C−
H germylation of alkenes. Alkenyl germanes are currently
accessible from the corresponding vinyl halides (via metalation
and quenching with Ge-electrophile)29 or the hydrogermyla-
tion of alkynes.30
Building on our previous findings of orthogonal nanoparticle
catalysis with aryl germanes,3f we found that the employment
of Pd-nanoparticle conditions allowed the direct cross-coupling
of aryl iodides with our newly synthesized alkenyl germanes.
Notably, the coupling was fully E-selective and privileged at the
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initial C−H dibenzothiophenylation from DCM to MeCN was
sufficient to ultimately obtain aryl or alkenyl germanes,13c
albeit in moderate yield. An added filtration of the
dibenzothiophenylation reaction mixture (over a pad of
NaHCO3 and MgSO4) along with the solvent switch increased
the yields of the germylated products drastically (Figure 4).
Importantly, this telescoping approach was found to be
applicable to the synthesis of both styryl as well as aryl
germanes.
As the dibenzothiophene is regenerated in the trans-
formation, it can readily be recovered by column chromatog-
raphy in pentane. For the synthesis of 39 (187 mg, 65% yield)
from 1-phenyl-2-pyrrolidinone (0.09 mmol scale), we were
able to recover dibenzothiophene (168 mg) in 92% yield.
In conclusion, we developed an operationally simple,
selective, and transition-metal-free germylation of arenes and
styrenes at room temperature via formal C−H functionaliza-
tion, enabled by dibenzothiophenium salts as key intermedi-
ates. The protocol makes use of a robust, bench-stable, and
readily accessible R3Ge-SiR3 reagent and is applicable to
various arenes and styrenes, delivering the corresponding E-
alkenes exclusively. Moreover, the first direct arylation of the
alkenyl germanes was also showcased, which proceeds
efficiently under Pd-nanoparticle catalysis with aryl iodides
(also in air) in a fully orthogonal manner to established
coupling regimes, as competing C-BPin or C-halogen sites are
tolerated in the coupling process. Owing to the potential of
C
sp2-germanes as functional molecules as well as orthogonal
Figure 3. Synthesis of styryl germanes from styrenes (top) and Pd-
catalyzed cross-coupling of styryl germanes with aryl iodides
(bottom). (See SI for reaction conditions).
coupling partners in synthesis and catalysis paired with the
simplicity of the presented method (mild conditions,
recoverable dibenzothiophene, and bench-stable reagents),
we anticipate widespread interest in the described method-
ology in academia and industry.
Ge site, leaving the competing C−BPin or C−Cl sites
completely untouched (Figure 3, bottom). This constitutes
the first example of direct cross-coupling of alkenyl
trialkylgermane and emphasizes their wider potential in
chemoselective cross-couplings for the rapid and modular
buildup of molecules.
For operational simplicity of the protocol, we also tested the
possibility of a telescoped reaction sequence starting from the
arene or styrene directly,31 which would avoid the separate
isolation and purification of the dibenzothiophenium salt
intermediates. We found that a simple solvent switch after the
ASSOCIATED CONTENT
■
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01505.
1
Experimental procedures, characterization data, and H
and 13C NMR spectra of the new compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Franziska Schoenebeck − Institute of Organic Chemistry,
RWTH Aachen University, 52074 Aachen, Germany;
orcid.org/0000-0003-0047-0929;
Email: franziska.schoenebeck@rwth-aachen.de
Author
Aymane Selmani − Institute of Organic Chemistry, RWTH
Aachen University, 52074 Aachen, Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01505
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Calculations were performed with computing resources
granted by JARA-HPC from RWTH Aachen University
under project “jara0091”.
Figure 4. Straightforward germylation of arenes and styrenes via a
telescoped reaction without isolation of intermediate salts and
recovery of dibenzothiophene.
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Letter
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