Iridium complex catalyzed germylative coupling reaction between alkynes and iodogermanes-a new route to alkynylgermanium and alkynylgermasilicon compounds

Article abstract of DOI:10.1039/c4dt02290d

The new reaction of terminal alkynes with iodogermanes proceeding in the presence of an Ir^I-complex [{Ir(μ-Cl)(CO)₂}₂] (I) and NEt(ⁱPr)₂, as a hydrogen iodide acceptor, leads to the formation of functionalized alkynylgermanes. This reaction occurs via direct activation of the C_sp-H bond in the starting alkyne. Detailed stoichiometric experiments using [Ir(cod)(CCPh)(PCy₃)] (IVa) and iodogermane were performed and resulted in a proposal of a reasonable mechanism for the germylative coupling reaction between alkynes and iodogermanes.

Full text of DOI:10.1039/c4dt02290d

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Iridium complex catalyzed germylative coupling
reaction between alkynes and iodogermanes –
a new route to alkynylgermanium and
alkynylgermasilicon compounds†
Cite this: Dalton Trans., 2014, 43,
16795
Monika Rzonsowska,a Ireneusz Kownacki,a Beata Dudzieca and
Bogdan Marciniec*a,b
The new reaction of terminal alkynes with iodogermanes proceeding in the presence of an IrI-complex
[{Ir(µ-Cl)(CO)2}2] (I) and NEt(iPr)2, as a hydrogen iodide acceptor, leads to the formation of functionalized
alkynylgermanes. This reaction occurs via direct activation of the Csp–H bond in the starting alkyne.
Detailed stoichiometric experiments using [Ir(cod)(CCPh)(PCy3)] (IVa) and iodogermane were performed
and resulted in a proposal of a reasonable mechanism for the germylative coupling reaction between
alkynes and iodogermanes.
Received 28th July 2014,
Accepted 12th September 2014
DOI: 10.1039/c4dt02290d
www.rsc.org/dalton
Introduction
The unique properties of unsaturated organogermanes make
them an interesting group of compounds. Due to their low
toxicity, organogermanes can be used as reagents in organic
syntheses, where they can substitute toxic organotin compounds,
i.e. in the formation of a wide range of molecular and macro-
molecular organic compounds of complex structure and interest-
ing physicochemical properties.1 Germanium-based compounds
possessing π-conjugated systems (along with their respective
silicon compounds) are also considered to be potential candi-
dates for electronic devices as a result of their photo- and electro-
luminescent properties,1,2 while the discovery of their biological
activities has enabled their application, e.g. in anticancer
therapy, and hypotensive and immunomodulating medicines.3
The literature offers some examples of syntheses of ethynyl-
or alkynylgermanes. The methods of their syntheses can be
divided into two groups. The first includes classical stoichio-
metric reactions with organometallic reagents, i.e. organo-
germanium halides with metal acetylides or germylation of
terminal alkynes,4 or redistribution reactions using tin
reagents.5 Catalytic routes to alkynylgermane synthesis are very
limited. Recently, we have reported a method for the prepa-
Scheme 1 Ruthenium-catalyzed germylative coupling of terminal
alkynes with vinyl-substituted organogermane compounds.
Scheme 2 Iridium catalyzed coupling of terminal alkynes with iodo-
trimethylsilane (Me3SiI).
ration of alkynylgermanes via germylative cross-coupling of
selected terminal alkynes with various vinylgermanes catalyzed
by complexes containing [Ru]–H and/or [Ru]–Si bonds and
occurring according to the following equation (Scheme 1).6
Unfortunately, the germylation of phenylacetylene with
vinylgermane does not occur due to the competitive dimeriza-
tion of phenylacetylene under catalytic conditions. In this
process, vinylgermane compounds function as germylating
agents and hydrogen acceptors.7 A process of great practical
significance is the reaction of the silylation of terminal alkynes
with iodosilanes catalyzed by iridium complexes, which has
also been developed by our group (Scheme 2).8
aFaculty of Chemistry, Department of Organometallic Chemistry, Adam Mickiewicz
University in Poznan, Umultowska 89b, Poznan 61-614, Poland.
E-mail: Bogdan.Marciniec@amu.edu.pl
bCenter for Advanced Technologies, Adam Mickiewicz University in Poznan,
Umultowska 89c, Poznań 61-614, Poland
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4dt02290d
In this study, this method is applied for the synthesis of
new and well-known organogermanium compounds.
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with iodosilanes, which proposes as an initial step the gene-
ration of the hexacoordinate IrIII (IIIa) complex via oxidative
addition of terminal alkyne into the tetracoordinate IrI precur-
sor (II) (Scheme 4).8 The structure of complex IIIa was solved
Scheme 3 Coupling of terminal alkynes with iodogermanes (R13GeI).
by X-ray analysis (for R = Si(i-Pr)3 and NR′ = pyridine) provid-
3
ing direct evidence of the activation of the Csp–H by the IrI
carbonyl complex. The spatial configuration of the ligand
attached to the iridium center assumed the elimination of the
respective amine hydrochlorides (dehydrohalogenation occur-
ring in the presence of amines is a well-known process10) and
formation of iridium complex IV.
Results and discussion
The coupling of terminal alkynes with iodotrimethylgermanes
in the presence of an iridium(I) chlorocarbonyl complex
[{Ir(µ-Cl)(CO)2}2] (I) and NEt(iPr)2 as a hydrogen iodide accep-
tor leads to the formation of alkynyl-substituted germanes
(Scheme 3).9
In view of these mechanistic observations for silylative
coupling, detailed studies were performed to determine the
mechanism of the germylative coupling of terminal alkynes
with iodogermane involving oxidative addition/reductive elim-
ination or σ-bond metathesis. For further stoichiometric investi-
gations, the tetracoordinate IrI complex [Ir(cod)(CCPh)(PCy3)]
(IVa) was chosen, as previously used in a study of oxidative
addition by Oro et al.11 Detailed stoichiometric experiments
The optimization of reaction conditions was performed
successfully for the model reaction, i.e. germylation of phenyl-
acetylene with Me3GeI. Several parameters were tested, e.g.
first of all, the type of solvent (toluene, DMF, THF), which
showed that the highest conversion of alkyne was achieved in
toluene (98%) and an unacceptably low conversion in THF and
DMF. Other factors affecting the yield of alkynylgermane
included the amount of catalyst, temperature and the type of
base. A small loading of (I), i.e. <1 × 10−2, and low temperature
(lower than 80 °C) require the reaction time of 48 hours
for complete phenylacetylene consumption. However, a temp-
erature increase to 110 °C results in a drop in the yield of the
targeted product due to side dimerization and trimerization of
the initial alkyne. We also speculate that the iridium species
(IV) (see Scheme 6) is thermally unstable. Further tests per-
formed for model amines, e.g. NEt(iPr)2 (98% phenylacetylene
conversion), NEt3 (78%), C5H11N (40%), and C5H5N (0%),
proved that less hindered amines efficiently blocked the cata-
lytic activity of the initial iridium complex and the best choice
was NEt(iPr)2. The steric and electronic effect of the R1 substi-
tuent at iodogermane was also tested. Several attempts for
application of n-Bu and Ph germanium derivatives revealed
their lower activity resulting in longer reaction time, i.e. 48 h,
to obtain very good product yield.
The application of the [{Ir(μ-Cl)(CO)2}2] (I) catalyst for the
germylative coupling of selected terminal alkynes, i.e. alkyl,
cycloalkyl, aryl and silyl, and germylethynes with R13GeI,
under the optimum conditions enables the respective alkynyl-
silanes to be generated as exclusive products (see the data pre-
sented in Table 1).
The reaction was successfully performed under an argon
atmosphere (Schlenk flask) in a closed system, in toluene at
80 °C. Most of the new alkynylgermane derivatives were iso-
lated and fully characterized by spectroscopic methods
(GC-MS, 1H, 13C NMR, HRMS (see ESI†)). The proposed pro-
cedure appears to be universal, as it can be used for both non-
functionalized and functionalized alkynes. The iridium cata-
lyst (I) seems to be resistant to terminal alkynes with initial
functional groups, e.g. –OH, and no preliminary germylation
of alcohol is observed (in contrast to the results of the ana-
logous silylation8).
1
using IVa and iodogermane were performed with H, 13C, 31P
NMR and GC-MS monitoring – the results are presented
below.
The synthesized and isolated tetracoordinate IrI complex
(IVa) was fully characterized by NMR spectroscopy. There is
only one singlet at 19.54 ppm in the 31P NMR spectrum,
coming from coordinated tricyclohexylphosphine. The 13C
NMR spectrum reveals a specific resonance line at 67.30 ppm
assigned to vCH of the cyclooctadiene coordinated to the
IrI square planar coordination area. Two doublets assigned to
the triple bond of the phenylethynyl moiety are present
2
at 126.41 ppm for –CC-Ph with a coupling constant JC–P
=
13.6 Hz and 83.25 ppm for –CC-Ph with a coupling constant
3JC–P = 12.7 Hz. There are also five additional signals assigned
to the phenyl group: 131.21, 129.85, 128.39, 125.18, 121.63.
The 1H NMR spectrum reveals groups of signals corresponding
to the phenyl ring (7.68 ppm (dd), 7.16 ppm (tr), 6.97 ppm
(dd)), two multiplets at 5.27 ppm and 3.76 ppm assigned
to the vCH of the coordinated cyclooctadiene and
2.44–1.10 ppm of the cyclohexyl substituent on phosphorus.
Direct addition of iodogermane to complex IVa was performed
in benzene-d6 in a J. Young NMR tube (Scheme 5).
The system was maintained at room temperature for
12 hours and then the reaction temperature was increased to
45 °C for a further 24 hours. After each change in the process
conditions, a series of NMR analyses were carried out.
After 12 hours at room temperature, the 31P NMR spectrum
revealed (besides the peak at 19.55 ppm assigned to the
tetracoordinate IrI complex (IVa)) an additional resonance line
at 14.69 ppm coming from a new coordinative species,
probably hexacoordinate IrIII complex (Va), i.e. a product of the
oxidative addition of iodogermane to IVa (Fig. 1 ESI†). The
13C NMR analysis also disclosed changes in the resonance
lines assigned to the carbon-containing ligands, confirming a
new coordination sphere, i.e. formation of Va. A new resonance
line assigned to vCH cyclooctadiene coordinated to the IrIII
octahedral coordination area appeared at 53.9 ppm. There are
The available literature presents a scheme for the mechan-
ism of iridium-catalyzed silylative coupling of terminal alkynes
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Table 1 Iridium(I)-catalyzed germylation of terminal alkynesa
Entry
1
R1
R2
Isolated yield [%]
94
Entry
12
R1
R2
Isolated yield [%]
97
–CH3
–Ph
–CH3
2
3
–CH2CH2Ph
92
13
14
–Si(iPr)3
–Ge(iPr)3
93
34
79b
4
5
65b
80b
15
16
–n-C4H9
–Ph
99b
97b
–CH2CH2Ph
6
7
8
–n-C6H13
–SiMe2Ph
95
95
96
17
18
19
–SiEt3
99b
90b
95b
–SiMe2Ph
–n-C6H13
9
65b
20
21
70b
76b
10
11
65
93
Ph
–SiEt3
a Reaction conditions: [Ir] : [alkyne] : [R13GeI] : [base] = 10−2 : 1 : 1.2 : 1.8, 24 h, 80 °C. b 48 h, closed system.
Scheme 5 Stoichiometric reaction of IVa with iodotrimethylgermane
Scheme 4 Activation of the spC–H bond in terminal alkynes.
with respective products (observed in NMR and GC-MS spectra).
also new, additional doublets for the triple bond of the phenyl-
cyclooctadiene in a new hexacoordinate IrIII complex (Va)
(Fig. 3 – see ESI,† p. S-29). After heating at 45 °C for an
additional 24 hours, the changes in the 31P, 13C and 1H spectra
become even more intense. The resonance lines coming from
complex IVa are less intense, in contrast to the strong signals
from the hexacoordinate IrIII complex (Va). There are also new
resonance lines in the 13C NMR spectrum at 105.42 ppm and
96.12 ppm that can be assigned to trimethyl(phenylethynyl)-
germane, i.e. the product of the reductive elimination from
complex Va. The GC-MS analysis of the resulting reaction
ethynyl moiety at 128.65 ppm for –CC-Ph with a coupling
2
constant JC–P = 29.4 Hz and 88.01 ppm for –CC-Ph with a
3
coupling constant JC–P = 13.1 Hz, which is consistent with a
position adjacent (cis) to the trimethylgermyl group (Fig. 2 see
ESI,† p. S-28).
Also, the 1H NMR data confirm the appearance of a new
coordinative species of IrIII. There are additional resonance
lines assigned to the phenyl ring at 7.50 ppm (m), 7.14 ppm
(m), 6.95 ppm (m) as well as three multiplets at 5.58 ppm,
5.44 ppm and 3.28 ppm assigned to the vCH of coordinated
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and 29Si NMR spectra were recorded on a Bruker Ultra Shield
spectrometer (600 MHz) and a Varian Mercury spectrometer
(300 HMz), and Bruker Avance II (400 MHz) and Bruker Avance
III (500 MHz) spectrometers in benzene-d6 and CDCl3. High
resolution mass spectra (HRMS) were obtained by electron
impact ionization (EI) using an AMD Intectra Mass AMD 402
instrument. Some of the products gave significant fragmenta-
tion which made them difficult to analyze by the HRMS
method. For a few of them HRMS analysis was impossible to
perform or have been obtained for fragmented ions. The
chemicals were purchased from the following sources: IrCl3 ×
33H2O from Pressure Chemicals; cod, C6D6, CDCl3, toluene
from Sigma Aldrich; Me3GeI, NEt(iPr)2 from ABCR; CO from
Linde Gas. The complex [{Ir(μ-Cl)(cod)}2],13 Bu3GeI and
Ph3GeI,14 ethynyltriisopropylgermane,15 (1-trimethylsiloxy-
cyclohexyl)ethyne16 were synthesized according to published
methods. All solvents and liquid reagents were purchased
from commercial sources, dried and distilled under an argon
atmosphere prior to use (in case of iodogermanes, they were
additionally kept at 4–8 °C).
Scheme 6 Mechanism of the germylative coupling of terminal alkynes
with iodogermane.
General synthetic procedure for germylative coupling of
terminal alkynes
mixture verifies this observation and simultaneously suggests
the formation of a new tetracoordinate IrI complex (VIa)
(Scheme 5).
The glass Schlenk reactor equipped with a magnetic stirring
bar was evacuated and flushed with argon. A given amount of
the complex [{Ir(μ-Cl)(CO)2}2] was placed in the reactor under a
flow of argon; then toluene and a precise amount of NEt(iPr)2
were added. The mixture was then stirred for 10 min. Next,
Detailed stoichiometric studies were also performed for the
[Ir(CO)2(CCPh)(PCy3)] complex and similar results were
obtained.12 However, this complex seemed to exhibit higher
reactivity. The presence of the hexacoordinate IrIII complex
(produced via oxidative addition of iodogermane) was much
less visible (less intense resonance lines in the NMR spectra)
due to a rapid, consecutive reductive elimination of the product,
i.e. trimethyl(phenylethynyl)germane (GS-MS detection). This
result may suggest further application of the complex as an
effective catalyst in germylative coupling reactions.
appropriate amounts of terminal alkyne and R1 GeI were
3
added. The reaction was conducted in a closed system at 80 °C
upon stirring for 24 hours (or 48 hours). After the reaction was
complete, toluene and any excess of other reagents were
removed under reduced pressure. The crude product was iso-
lated by purification using column chromatography (SiO2)
with hexane as an eluent.
The results of the above experiments allow us to propose a
reasonable mechanism for the germylative coupling of term-
inal alkynes with iodogermanes proceeding in the presence of
the active species IV (Scheme 6).
Stoichiometric reaction of complex IVa with
iodotrimethylgermane
Identification of complexes IVa, Va and VIa enabled the
proposal of the respective intermediates IV, V and VI in this
catalytic mechanism. Germylative coupling of terminal alkynes
with iodogermanes involves the oxidative addition of iodo-
germane to tetracoordinate complex IV (generated analogously
to a documented reaction path for silylative coupling – see
Scheme 4) followed by reductive elimination of the product, i.e.
trimethyl(alkynyl)germane. The next step is the known process
of consecutive oxidative addition of terminal alkyne to VI, gen-
erating hexacoordinate species III, undergoing reductive elim-
ination, recovering IV and closing the whole catalytic cycle.
An amount of IV (0.073 g, 11 mmol) was introduced into an
NMR tube along with 0.6 mL benzene-d6 at room temperature
under
a
dry argon atmosphere. Iodotrimethylgermane
(0.031–0.034 g, 13–14 mmol) was added. The reaction was
carried out at room temperature for 12 h with NMR detection;
after that, it was continued for an additional 24 h at 45 °C, fol-
lowed by subsequent NMR analysis.
Conclusions
We have reported a new and universal (considering iodo-
germane compounds) catalytic method for the synthesis of un-
saturated organogermanium compounds, analogous to that of
silicon compound synthesis, involving the activation of Csp–H
bonds in terminal alkynes in the presence of an iridium cata-
Experimental
General methods
All synthesis and manipulations were carried out under an lyst. Our tests have shown the formation of products that are
1
argon atmosphere using regular Schlenk techniques. H, 13C, mostly new compounds which could play a very important role
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