Synthesis of vinylgermanes via the Au/TiO2-catalyzed cis-1,2-digermylation of alkynes and the regioselective hydrogermylation of allenes

Article abstract of DOI:10.1021/acs.orglett.1c00997

In the presence of Au/TiO2 (1 mol %), terminal alkynes react quantitatively with stoichiometric amounts of the unactivated digermane Me3Ge-GeMe3, forming exclusively cis-1,2-digermylated alkenes. We also establish the Au/TiO2-catalyzed hydrogermylation of terminal allenes with Et3GeH, which exhibits a highly regioselective mode of addition on the more substituted double bond forming vinylgermanes. Additionally, we provide preliminary results regarding the Pd nanoparticle-catalyzed C-C coupling of 1,2-digermyl alkenes with aryl iodides.

Full text of DOI:10.1021/acs.orglett.1c00997

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Letter
Synthesis of Vinylgermanes via the Au/TiO2‑Catalyzed cis-1,2-
Digermylation of Alkynes and the Regioselective Hydrogermylation
of Allenes
Anastasia Louka and Manolis Stratakis*
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ABSTRACT: In the presence of Au/TiO2 (1 mol %), terminal alkynes react
quantitatively with stoichiometric amounts of the unactivated digermane
Me3Ge-GeMe3, forming exclusively cis-1,2-digermylated alkenes. We also
establish the Au/TiO2-catalyzed hydrogermylation of terminal allenes with
Et3GeH, which exhibits a highly regioselective mode of addition on the more
substituted double bond forming vinylgermanes. Additionally, we provide
preliminary results regarding the Pd nanoparticle-catalyzed C−C coupling of
1,2-digermyl alkenes with aryl iodides.
n contrast to organosilicon compounds whose synthesis and
that subsequently adds to alkynes.12 A few other reports
IC−C cross coupling reactions have been extensively regarding the Pd- or Pt-catalyzed reaction of the rather exotic
cyclic oligogermanes with alkynes are also known.13 As per
linear 1,2-digermanes (Scheme 1), the Pd-catalyzed (5 mol %)
studied,1 organogermanes have not received significant
attention.2 Very recently, however, the field of organo-
germanium chemistry has been revived, particularly by the
findings by the group of Schoenebeck, who have uncovered a
series of novel and selective metal-catalyzed C−C cross
coupling protocols of C(sp2)-germyl linkages.3 The main
innovation in these cross coupling protocols is that simple
trialkylgermyl substituents are being used,4 in contrast to the
previously known cases where the germyl substituents should
be activated either by internal coordination/chelation forming
hypervalent species, or if bearing labile heteroatom ligands and
Si−Ge linkages.5 As the C(sp2)-GeR3 functionality (R = alkyl)
appears to be very promising in a palette of synthetic
applications, there is need for the development of efficient
protocols regarding the selective preparation of vinylger-
manes.6 So far the majority of the existing protocols deal
with the hydrogermylation of alkynes.
Scheme 1. Catalytic 1,2-Digermylation of Alkynes
Our group has in recent years a continuous interest in
studying the Au nanoparticle (Au NP)-catalyzed reactions of σ
heteroelement linkages (Si−Si, B−B, B−Si) with π systems
forming alkenyl silanes or boranes.7 Thus, in the presence of
Au/TiO2, alkynes react smoothly with 1,2-disilanes (directly or
indirectly),8 silylboranes,9 or 1,2-diboranes,10 forming the
corresponding cis-1,2-adducts. Urged by the recent awakening
of organogermanium chemistry,3 we examined the possible
digermylation of alkynes as a practical route for the synthesis of
vinylgermanes using Au NPs as the catalyst. The 1,2-
digermylation of alkynes is a transformation that has not
been extensively studied in the past, and there are only a few
synthetically useful protocols in the literature.11 In the first
reported example, a highly strained 3-membered ring
digermirane was used, which undergoes insertion on Pd(0)
digermylation of excess of phenylacetylene with the activated
ClMe2GeGeMe2Cl yields at 120 °C the cis-adducts in
moderate to good yields, while under the same conditions,
the unactivated hexamethyldigermane (Me3GeGeMe3) pro-
vides trace amounts of products.14 The conversion yield of the
digermylation with Me3GeGeMe3 can increase to 60% using
Received: March 23, 2021
Published: April 9, 2021
https://doi.org/10.1021/acs.orglett.1c00997
© 2021 American Chemical Society
Org. Lett. 2021, 23, 3599−3603
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Letter
Pt(acac)2 as catalyst (5 mol %) at 120 °C but with a 5-fold
molar excess of alkyne.13a
To our delight, in the presence of Au/TiO2 (1 mol % in Au),
the reaction of a series of terminal alkynes with Me3GeGeMe3
(2, 1.2 equiv) proceeds smoothly in dry 1,2-dichloroethane as
the optimum solvent (70 °C), yielding after 2 h the 1,2-cis-
digermylated adducts in very high isolated yields (Table 1).
2-fold excess of hexamethyldigermane is required for the
reaction to go to completion within 4 h. Also, with
stoichiometric amounts of (Me3Ge)2, propargyl alcohol (1q)
provides the digermyl adduct 3q, but with 2.0−2.5 equiv, the
trimethylgermyl protected alcohol 3q (compound 3q′, see
Supporting Information) is also formed in ∼50% yield,
indicative of a Au/TiO2-catalyzed O-germylation.15 Given
that in the presence of a Pt catalyst alkynes react poorly with
hexamethyldigermane and under rather forcing conditions13a
and are almost unreactive in the presence of Pd catalysts,14 our
catalytic system is superior as much lower catalyst loading is
required, the reaction conditions are milder, the reaction time
is relatively short, the reactants are almost stoichiometric, and
finally, the isolated yields are excellent. Unfortunately, internal
alkynes appear to be completely unreactive, as has also been
observed in their attempted Au/TiO2-catalyzed 1,2-disilylatio-
n.8a While Au(I) can formally catalyze the addition of
digermane to alkynes via an oxidative addition/reductive
elimination catalytic cycle,16 in our hands, a series of Au(I)
substances were inefficient catalysts (page S3, Supporting
Information) with some reactivity (5% yield) observed in the
presence of 5 mol % of Ph3PAuNTf2 at 70 °C after 24 h.
Perhaps this transformation can be optimized after extensive
screening of various Au(I)-based catalysts.
Table 1. cis-1,2-Digermylation of Terminal Alkynes with
a
Me3GeGeMe3 Catalyzed by Au/TiO2
Following the very simple catalytic digermylation protocol of
alkynes, we turned our attention to the hydrogermylation of
allenes as a new method for the synthesis of vinylgermanes
using the readily available Et3GeH. Our group has shown some
years ago that the analogous Au NP-catalyzed hydrosilylation
of allenes is regioselective, with predominant addition on the
more substituted double bond,17 while with dihydrosilanes the
main pathway is dehydrogenative disilylation on the terminal
double bond.18 In the literature, there is only one example
dealing with the hydrogermylation of allenes.19 In this specific
report, Ph3GeH had been used under catalysis either by Pd(0)
or Et3B (radical conditions). The selectivity is peculiar,
providing allylic germanes in moderate yields. After opti-
mization of reactants, stoichiometry, and catalyst support (see
Supporting Information), we found that the reaction between
Et3GeH (5, 1.5 equiv) and a series of terminal allenes (Table
2) occurs readily in DCE within 2−6 h at 70 °C in the
presence of Au/TiO2 (1 mol %) and yields vinylgermanes. For
the case of monosubstituted allenes the reaction is faster, and
the addition occurs predominately on the internal double bond
in 92−99% selectivity. For 1,1-disubstituted which require 6 h
to react, the selectivity is >99%. Allenene 4j provided an
interesting result. The anticipated hydrogermylation product 6j
was formed as a minor one, while the major was the
conjugated 6j′. Product 6j′ does not arise from isomerization
of 6j under the reaction conditions. Additionally, vinyl-
idenecyclohexane (4k) yielded apart from the anticipated 6k,
the dehydrogenated product 6k′ (in 34% relative ratio), a
product selectivity similar to what has been observed in its
analogous Au/TiO2-catalyzed hydrosilylation.14 The product
distributions from 4j and 4k are analyzed in the accompanying
mechanistic discussion.
a
b
c
d
Isolated yields. Two equiv of (Me3Ge)2, 4 h. Z/E = 75/25. With
2.5 equiv of (Me3Ge)2, a mixture of 3q and its O-trimethylgermyl
protected derivative 3q′ is formed in ∼1/1 ratio (see Supporting
Information).
Au/Al2O3 and Au/ZnO exhibited lower efficiency (page S3 of
the Supporting Information). In the majority of the cases, there
was no essential need to perform any chromatographic
purification of the adducts, indicative that no side products
were formed. The syn-1,2-addition of the digermane was
confirmed by 2D NMR experiments. The reaction is
stoichiometric, as in many cases, the same outcome may be
observed using strictly 1.0 equiv of digermane. Given, however,
that Me3GeGeMe3 suffers from partial destruction in the
presence of traces of moisture, we used a 20% molar excess in
all cases to secure the completion of the digermylation.
Notably, the efficiency of our catalytic system is still substantial
at 0.1 mol % loading of Au/TiO2, obtaining ∼40−70% lower
yields within 2 h (page S3 of the Supporting Information).
Among the examples of Table 1, for reasons that are not fully
understood, p-bromophenylacetylene (1f) reacts slowly, and a
From the mechanistic point of view, we consider in Scheme
2 analogous intermediates to those that have been invoked in
the corresponding Au/TiO2-catalyzed disilylation of alkynes8a
or hydrosilylation17 of allenes. In the first case, after insertion
of the Ge−Ge linkage of digermane on the Au cluster (Aun)
occurs coordination of the alkyne on the Me3Ge−Aun-GeMe3
species, followed by addition of the trimethylgermyl moieties
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Table 2. Regioselective Hydrogermylation of Allenes
Catalyzed by Au/TiO2
Scheme 2. Proposed Mechanisms in the Au/TiO2-Catalyzed
cis-1,2-Digermylation of Alkynes and Hydrogermylation of
Allenes
a
a
For intermediates IV−VI, full charges are shown for clarity.
a
For monosubstituted allenes, the reaction time is 2 h, while for 1,1-
disubstituted allenes, it is 6 h. Internal/terminal double bond
addition: 94/6; 92/8; 98/2; 96/4; 99/1.
b
c
d
e
f
Scheme 3. Pd-Catalyzed Cross Coupling of cis-1,2-
Digermylated Adducts with Aryl Iodides
on the triple bond via an inner sphere mechanism
(intermediate I). Then, the cis-1,2-digermylated product 3 is
released from the surface of the nanoparticle. Intermediate I
for the case of the trimethylsilylacetylene is more likely having
a dipolar character, as also suggested in the Au/TiO2-catalyzed
disilylation8a and diborylation10 of silylacetylenes, providing a
mixture of E/Z isomers (products 3oE/Z). In the hydro-
germylation of allenes, we consider an η-1-like complex II as
intermediate, similar to what has been suggested in the
addition of X-Y heteroelement linkages to allenes,17,18,20 which
delivers the hydride on the more electrophilic site (more
substituted C) to form III. This mechanism is consonant to
the hydrogermylation of conjugated allene 4j, for which the
anticipated product 6j is a minor one. Through charge
delocalization (structures IV and V for which fully developed
charges are shown for clarity), hydride transfer occurs on the
former sp2-CH of the cyclohexyl ring, leading to the major
germyl-bearing conjugated diene 6j′. The type II intermediate
complex also rationalizes the formation of the side product 6k′
in the hydrogermylation of vinylidenecyclohexane (4k). There,
intermediate VI may undergo intramolecular elimination of H2
via a six-membered transition state (bottom part of Scheme 2).
To examine the synthetic utility of the 1,2-digermylated
adducts (3), as a preliminary study, we attempted their Pd-
catalyzed C−C cross-coupling reactions with aryl iodides
(Scheme 3) based on the protocol that has recently been
developed by the group of Schoenebeck4a for the case of aryl
triethylgermanes (Pd2dba3/AgBF4). The first reaction which
was examined was that of the digermylated adduct 3b with 3
equiv of p-iodotoluene. To our delight, the diarylated cross
coupling product 8a (trip-tolylethylene) was formed in 72%
yield. However, with different aryl combinations (aryl group in
the starting digermylated compound and aryl iodide), a
mixture of three isomeric products were seen in the cases
that were examined. The major product is diarylated 8-gem in
a relative yield of ∼55%, accompanied by a mixture of 8-cis
and 8-trans (Scheme 3). In terms of conventional cross
coupling aspects of the C−Ge functionalities, the major 8-gem,
which has two newly introduced aryl substituents on the less
substituted sp2-carbon atom of digermyl precursor, is an
unexpected one. Despite this interesting observation, the Pd-
catalyzed coupling protocol does not appear to be synthetically
useful for these substrates. Yet, the accompanying mechanistic
analysis for the formation of the three possible isomers is quite
interesting. Using 1.5 equiv of aryl iodides instead of 3.0, it was
possible to isolate the intermediate monoarylated cross
coupling products that were seen in approximately 25−30%
relative yield in the crude reaction mixture. Thus, in the
reaction between 3b and p-iodotoluene, the only appearing
monoarylated germyl product was characterized based on the
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combination of NOESY and HMBC experiments as compound
9a having an E-configuration (Scheme 4 and Supporting
which may find potent and novel applications in organic
synthesis, and there are limited examples of stereoselective and
regioselective methods for their preparation. Our work
contributes significantly toward this direction.
Scheme 4. Proposed Mechanism in the Pd-Catalyzed
Diarylation of cis-1,2-Digermylated Alkenes with Aryl
Iodides
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00997.
Copies of 1H, 13C NMR, and MS spectra of all products
(PDF)
AUTHOR INFORMATION
Corresponding Author
■
Manolis Stratakis − Department of Chemistry, University of
Crete, 71003 Heraklion, Greece; orcid.org/0000-0002-
0962-8297; Email: stratakis@uoc.gr
Author
Anastasia Louka − Department of Chemistry, University of
Crete, 71003 Heraklion, Greece
Information).21 Similarly, in the reaction between 3c and p-
fluoroiodobenzene, the only monoarylated intermediate 9c was
also isolated and characterized as a single E-isomer. Treatment
of 9c with Pd2dba3/AgBF4 and 1.5 equiv of p-fluoroiodo-
benzene gave the three diarylated coupling products 8c-gem/
8c-cis/8c-trans in a relative ratio identical to what is reported
in Scheme 3. This result tells us that the E-monogermylated 9c
is the only intermediate of the diarylation.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00997
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The research work was supported by the Hellenic Foundation
for Research and Innovation (HFRI) and the General
Secretariat for Research and Technology (GSRT) under the
HFRI PhD Fellowship grant GA no. 31448.
In Scheme 4, we provide some mechanistic clues regarding
the product distribution of the three diarylated coupling
products, which is based on the mechanism proposed in the
Pd-catalyzed Schoenebeck’s arylgermane/aryl iodide cross
coupling reaction. The in situ formed electrophilic Pdn
nanoparticles (we consider herein a hypothetical Pd3 cluster4a)
undergo oxidative addition by aryl iodides, and the resulting
DEDICATION
■
This paper is dedicated to the memory of Professor Zvi
Rappoport (The Hebrew University of Jerusalem).
+
electrophilic arylated cationic species I−Pd3Ar2 react with
digermylated alkene forming intermediate VII, which via
fluoride ion capture of the trimethylgermyl group yields
monoarylated germyl alkene 9 as a single E-geometrical isomer.
The addition of the electrophilic aryl group occurs exclusively
on the less substituted alkene carbon of 3 because it leads to
the more stable aryl-stabilized carbocation. Intermediate 9 may
subsequently undergo two possible modes of a second aryl
addition from the electrophilic aryl-bearing cationic Pd species
with predominant C−C bond formation on its less substituted
alkene C atom to form VIII. Through H-shift on VIII,
intermediate X is derived which, after Me3GeF elimination,
leads to the major diarylated product 8-gem. We propose that
intermediate VIII is more stable compared to its alternative IX
due to the extra stabilization of the carbocation by the germyl
group.22 Intermediate IX can lead to the E/Z isomeric
products 8-cis and 8-trans. Despite the moderate selectivity
in those attempted cross coupling experiments, very useful
mechanistic conclusions were drawn that may help in the
development of novel cross coupling reactions.
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