Cobalt-Catalyzed (E)-β-Selective Hydrogermylation of Terminal Alkynes

Article abstract of DOI:10.1021/acs.orglett.1c00928

A cobalt-catalyzed method for the hydrogermylation of alkynes is reported, providing a selective and accessible route to (E)-β-vinyl(trialkyl)germanes from terminal alkynes and HGeBu3. As shown in multiple examples, the developed method demonstrates a broad functional group tolerance an practical utility for late-stage hydrogermylation of natural products. The method is compatible with alkynes bearing both aryl and alkyl substituents, providing unrivaled selectivity for previously challenging 1° alkyl-substituted alkynes. Moreover, the catalyst used in this method, Co2(CO)8, is a cheap and commercially available reagent. Conducted mechanistic studies supported the syn-addition of Bu3GeH to an alkyne π-complex.

Full text of DOI:10.1021/acs.orglett.1c00928

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Letter
Cobalt-Catalyzed (E)‑β-Selective Hydrogermylation of Terminal
Alkynes
Maxim R. Radzhabov and Neal P. Mankad*
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ABSTRACT: A cobalt-catalyzed method for the hydrogermylation
of alkynes is reported, providing a selective and accessible route to
(E)-β-vinyl(trialkyl)germanes from terminal alkynes and HGeBu3.
As shown in multiple examples, the developed method
demonstrates a broad functional group tolerance an practical
utility for late-stage hydrogermylation of natural products. The
method is compatible with alkynes bearing both aryl and alkyl
substituents, providing unrivaled selectivity for previously challeng-
ing 1° alkyl-substituted alkynes. Moreover, the catalyst used in this
method, Co2(CO)8, is a cheap and commercially available reagent.
Conducted mechanistic studies supported the syn-addition of
Bu3GeH to an alkyne π-complex.
iven the prominence of Pd-catalyzed cross-coupling for
C−C bond formation, there is a continued demand for
β- and (E)-β-products.20 Among precious metals, Pd catalysts
have been studied especially well. Oshima and Utimoto21
demonstrated that the Pd(PPh3)4 catalyst yields (E)-β-isomers
selectively with Ph3GeH, but with trialkylgermanes the
selectivity was reported to be only 4:1 (β/α) for primary
alkylacetylenes (Figure 1b). In 2009, Maleczka13 reported that
this Pd catalyst affords only the (E)-β-isomer for tertiary
alkylacetylenes, but for unhindered alkynes the selectivity
remained no more than 4:1. The (E)-β- and α-selective
hydrogermylation of phenylacetylene with tri-n-butylgermane,
Bu3GeH, by Rh catalysts was reported by Wada in 1991,
though this reaction requires 10 equiv of PhCCH.22 (E)-β-
Vinylgermanes were also formed by the Ru-catalyzed
dehydrogenative germylation of styrene with Bu3GeH but
again required 10 equiv of styrene for a high selectivity.23
Given the limitations of precious metal-catalyzed hydro-
germylations, there exists an opportunity for earth-abundant
metal catalysis; however, this has been studied to a lesser
extent and is only represented by Mn and Fe (Figure 1c). In
2011, Nakazawa demonstrated that Fe catalysts yield (Z)-β-
isomers for both Bu3GeH and Ph3GeH as well as aryl- and
alkylacetylenes with excellent yields and selectivities.24 Very
recently, Zhang and Zhang reported the visible light-initiated,
G
new coupling partners that enable advanced synthetic
methods. Recently, organogermanes1−3 have emerged as
prospective coupling partners to circumvent the limitations
of traditional organozinc and organomagnesium reagents4,5
(low functional group tolerance), organosilanes6 (low
reactivity), organostannanes7 (acute toxicity), and organo-
boronic acids8 (acid and base sensitivity). In this regard,
vinylgermanes are of particular interest as versatile synthetic
building blocks due to their low toxicity,9 resistance to
protonolysis,10 and benchtop stability.11 Vinylgermanes have
proven to be useful reagents that can be converted to
vinylhalides with retention of the double-bond geometry12
and used as partners in Pd-catalyzed coupling.13,14 Thus,
catalytic methods for producing vinylgermanes with high regio-
and stereoselectivities are desirable, and the most atom-
economical approach would be the hydrogermylation of
alkynes.
Although a variety of catalysts has been studied to selectively
perform the hydrogermylation of alkynes since the middle of
the 20th century,15 control over regio- and stereoselectivity still
remains a challenging problem that limits the development of
vinylgermane chemistry. In many cases, the selectivity is
controlled by specific reaction conditions or functional
groups14,16−19 but is moderate in the absence thereof. Studied
hydrogermylation catalysts include Lewis acids; precious
metals such as Pd, Ru, Rh, and Pt;10 and earth-abundant
metals such as Mn and Fe (Figure 1). Lewis acids (Figure 1a),
represented by boron compounds, tend to yield only (Z)-β-
isomers (α for propiolates) if trialkylgermanes are used12 but
can generate germyl radicals from Ph3GeH and yield both (Z)-
Received: March 22, 2021
Published: April 6, 2021
https://doi.org/10.1021/acs.orglett.1c00928
© 2021 American Chemical Society
Org. Lett. 2021, 23, 3221−3226
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Letter
control experiments indicated that the presence of (NHC)-
CuCl was not required to observe hydrogermylation activity in
some cases. In particular, catalytic Na[Co(CO)4] provided the
isomers α (2a) and (E)-β (3a) with reasonable conversions.
Alas, the isomers were in a ∼1:1 ratio with no formation of the
(Z)-β-product 4a (Table 1, entry 1). To date, this is the first
example of a reported Co-catalyzed hydrogermylation reaction.
Other metal carbonyl anions, such as NaCrCp(CO)3,
NaWCp(CO)3, and NaMoCp(CO)3, yielded no products at
all. An extensive investigation of Na[Co(CO)3L] (L = CO or
PPh3) catalysts did not provide a selectivity greater than 4:1 for
3a/2a (Table 1, entry 2).
It was recently reported that Mn2(CO)10 can catalyze
visible-light-induced radical hydrogermylation of arylalkynes,
though results with alkylalkynes were not included.25 There-
fore, we decided to try the reported conditions with
alkylalkynes using Mn2(CO)10 and a related Co catalyst,
Co2(CO)8 (Table 1, entries 3 and 4). We found that the Mn-
catalyzed reaction mostly yields the (Z)-β-product 4a with a
high selectivity (Z/E = 10:1) but a low conversion (∼22%).
Under the same conditions, Co2(CO)8 demonstrated a
somewhat better conversion (∼55%) but essentially no
selectivity, giving all three isomers 2a, 3a, and 4a. The same
set of reagents, being activated thermally (Table 1, entry 5),
performed much better and mostly yielded the desired (E)-β-
isomer (3a) with a better selectivity (β/α = 5.5:1) and overall
conversion (∼70%). Changing the Bu3GeH:1-decyne stoichio-
metric ratio from 1.2:1.0 to 1.0:1.1 significantly boosted the
reaction selectivity and conversion (Table 1, entry 6).
Lowering temperature and the reaction time did not
dramatically diminish the results (1, entries 7 and 8,
respectively). A lower catalyst loading slightly decreases the
selectivity of the reaction, but greater loading does not have
any noticeable effect (Table 1, entries 9 and 10). Thus, we
decided to proceed with the optimized conditions of entry 8.
Under the optimal conditions, we tested the hydro-
germylation for a range of substituted alkynes with various
functional groups (Figure 2). The vast majority of tested
alkynes smoothly underwent anti-Markovnikov hydrogermyla-
tion to afford (E)-β-vinylgermanes with good regioselectivities
and excellent yields. Apparently, ester (3c), amide (phthali-
mide 3e), silyl (3j), and silyl ether (3m) are very well tolerated
Figure 1. Hydrogermylation of terminal alkynes.
Mn-catalyzed, (Z)-β-selective hydrogermylation of arylalkynes
with Bu3GeH in 2019.25
Herein, we report a Co-catalyzed, (E)-β-selective hydro-
germylation of alkyl- and arylacetylenes with Bu3GeH (Figure
1d), a new method that allowed us to obtain (E)-β-isomers
from primary alkylacetylenes with selectivities by far surpassing
all existing methods, including those that employ precious-
metal catalysts.
Initially, we believed that an approach similar to that
previously reported by our group for hydrostannylation26
could be developed for trialkylgermanium hydrides and thus
began our investigation using (NHC)CuCl and metal carbonyl
anion cocatalysts to catalyze the reaction between 1-decyne
and Bu3GeH (NHC = N-heterocyclic carbene). However,
a
Table 1. Optimization of the Reaction Conditions
entry
catalyst
T (°C)
1a (equiv)
Bu3GeH (equiv)
time (h)
solvent
2a (%)
3a (%)
4a (%)
1
2
3
4
5
6
7
8
9
10
NaCo(CO)4 (10%)
NaCo(CO)4 (10%)
Mn2(CO)10 (10%)
Co2(CO)8 (10%)
Co2(CO)8 (10%)
Co2(CO)8 (10%)
Co2(CO)8 (10%)
Co2(CO)8 (10%)
Co2(CO)8 (5%)
100
85
RT, hν
RT, hν
85
85
60
60
60
1.0
1.1
1.0
1.0
1.0
1.1
1.1
1.1
1.1
1.1
1.2
1.0
1.2
1.2
1.2
1.0
1.0
1.0
1.0
1.0
15
15
15
15
15
15
15
4
toluene
DCE
DCM
DCM
DCE
DCE
DCE
DCE
DCE
DCE
56
20
traces
16
10
8
9
9
12
9
44
80
2
24
55
90
91
86
85
87
0
0
20
16
6
0
0
0
0
b
b
15
15
Co2(CO)8 (15%)
60
0
a
1
The reactions were performed on a 0.2 mmol scale. Yields were determined by the H NMR integration of isolated mixtures against an internal
b
standard. The light source was blue LED strip lights (DC 12 V 2A power supply).
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Figure 2. Hydrogermylation of various alkynes using the Co2(CO)8 catalyst (0.2 mmol scale). Reported values correspond to isolated mixtures (α
+ (E)-β). Values in parentheses are NMR yields for the (E)-β isomer only. (*) For 3t, the isolated yield is >100% because some 5-decyne remained
in the mixture.
Figure 3. Plausible mechanism for the Co-catalyzed hydrogermylation.
among alkyl-substituted vital functional groups. Halide groups
(3f), strongly coordinating alkyl nitrile (3l), and even an
unprotected alcohol group (3k) are totally compatible with the
reaction conditions. Bulky tertiary and secondary alkyl
substituents (such as 3h and 3i) greatly increase the selectivity
of the reaction (with the unfortunate exception of cyclo-
propylacetylene 3g, which gave a low selectivity). It is also
interesting to note that a low selectivity and yield were
observed for methyl propiolate (3d), but propiolates generally
tend to yield α-products.12 Internal alkynes do react under the
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a
given conditions, though with somewhat lower yields (3t),
probably due to substantial steric hindrance. Remarkably, the
reaction proved to be regiospecific for silyl acetylene (3j) and
especially Mestranol (3s), as no traces of the undesired α-
isomer were observed in the corresponding NMR spectra.
Therefore, our method demonstrated its practical utility for the
late-stage hydrogermylation of natural compounds and
pharmaceuticals.
Aryl-substituted alkynes were also tested under the same
conditions (Figure 2), giving (E)-β-vinylgermanes as the major
products with good yields. Activated arylalkynes with electron-
donating or electron-withdrawing groups have higher selectiv-
ities for (E)-β isomers (3o and 3p), compared to nonactivated
phenyl acetylene (3n). However, overly strong electron-
withdrawing groups can decrease the selectivity (3r). Poor
selectivity was also observed for ethynyl naphthalene (3q).
We also investigated the reactivity of triphenylgermanium
hydride, Ph3GeH, under the studied conditions (Figure 2), but
unfortunately only moderate yields and a poor selectivity were
obtained with 1-decyne and phenyl acetylene (3u and 3v,
respectively). The reaction mechanism for Ph3GeH is probably
different from Bu3GeH, as the former is prone to form
radicals.20
Table 2. Mechanistic Studies
T
2a
3a
4a
entry
catalytic system
(°C)
solvent (%) (%) (%)
1
2
3
4
5
NaCo(CO)4 + PhCOOH
NaCo(CO)4 + tBuONa
Co2(CO)8 + PhCOOH
Ph3Si−Co(CO)4
85
100
60
100
100
DCE
toluene
DCE
toluene
toluene
9
5
9
15
8
82
3
91
68
66
0
2
0
2
1
Ph3Si−Co(CO)4 + MeOH
a
The reaction was performed on a 0.2 mmol scale. Yields were
determined by the H NMR integration of isolated mixtures against
1
an internal standard.
selectivity for the (E)-β-product in 1,2-dichloroethane
(DCE) than in other solvents (see the Supporting
Information). Based on precedents from Chatani and
Murai,31,32 it is reasonable to speculate that NaCo(CO)4
generates HCo(CO)4 in situ from DCE by a SN2/β-hydride
elimination sequence.
To further probe the cobalt hydride mechanism, we
synthesized Ph3Si−Co(CO)4 for the subsequent alcoholysis
of the cobalt−silicon bond (to generate HCo(CO)4 in situ
according to a known procedure).33 We found that, in toluene,
the selectivity for the (E)-β isomer is significantly lower than
under the optimized conditions, but the selectivity is restored
upon the addition of MeOH to the mixture (Table 2, entries
4−5).
Radical pathways can be ruled out, since the direct visible-
light-induced generation of radicals from Co2(CO)8 proved to
be unselective (Table 1, entry 10). We can also conclude that
the studied reaction does not proceed via η1-vinylidene Co
complex; such a pathway would yield a mixture of (E/Z)-β
isomers,34 whereas for most Co2(CO)8-catalyzed reactions we
observed a mixture of α- and (E)-β-isomers with little to no
(Z)-β-product.
In summary, Co complexes were demonstrated for the first
time as viable catalysts of a hydrogermylation reaction,
enabling a selective and accessible protocol for the synthesis
of previously limited (E)-β-vinyl(trialkyl)germanes from
terminal alkynes and Bu3GeH. Tertiary, secondary, and most
importantly primary alkylacetylenes react well and with high
selectivities under the optimized conditions. Arylacetylenes are
also applicable under the reaction conditions. Obtained data
and mechanistic studies supported the syn-addition of Bu3GeH
to a π-alkyne complex and demonstrated the crucial role of
HCo(CO)4 in the catalytic cycle.
We would like to propose the following mechanism (Figure
3) for our (E)-β-selective hydrogermylation, which is based on
analogies found in the literature.27,28 The reaction between
Co2(CO)8 and Bu3GeH most likely generates cobalt
tetracarbonyl hydride, HCo(CO)4, and Bu3Ge−Co(CO)4
(analogous to that studied by Jeannin),29 and then both
cores can probably catalyze the hydrogermylation reaction
(albeit with different selectivities and efficiencies).
Just like the well-known hydroformylation, the cycles begin
with the thermally induced dissociation of the carbonyl CO
from original HCo(CO)4 A and Bu3Ge−Co(CO)4
F
complexes to generate active 16-electron species B and G,
respectively (Figure 3). Then, the addition of an alkyne
generates the corresponding π-complexes C and H. For the
HCo(CO)4 pathway, a hydride complex C easily undergoes
1,2-migratory insertion to form a new 16-electron alkenyl
tricarbonyl complex D. Then, complex D undergoes oxidative
addition with Bu3GeH to give a new 18-electron germylvinyl-
(tricarbonyl)cobalt E, which releases the desired alkene by
reductive elimination. It is quite probable that Bu3Ge−
Co(CO)4 F actively takes part in the studied reaction, since
higher amounts of Bu3GeH significantly change the reaction
yields and selectivity (please see the Supporting Information).
In this pathway, a tributylgermyl group in H can undergo
migratory insertion to form a new 16-electron I. That new
species can undergo oxidative addition with another molecule
of Bu3GeH to give J, which then releases the product via
reductive elimination. An analogous hydrosilylation of ethylene
by Et3Si−Co(CO)4 was studied by Wrighton under photo-
chemical conditions.30
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00928.
We conducted some experiments to obtain additional
support for the proposed mechanism (Table 2). The first
test was to study the susceptibility of the reaction to acids and
bases present in the reaction mixture, since we believe the key
intermediate is cobalt tetracarbonyl hydride, HCo(CO)4,
which is known to be acidic. While acids have little to no
effect on the reaction for both NaCo(CO)4 and Co2(CO)8
(Table 2, entries 1 and 3, respectively), strong bases like
tBuONa inhibit the reaction (Table 2, entry 2). It is worth
noting that NaCo(CO)4 demonstrated a much higher
Experimental details, additional experimental results,
characterization data, and NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Neal P. Mankad − Department of Chemistry, University of
Illinois at Chicago, Chicago, Illinois 60607, United States;
orcid.org/0000-0001-6923-5164; Email: npm@uic.edu
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Author
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Letter
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Maxim R. Radzhabov − Department of Chemistry, University
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00928
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This material is based on work supported by the U.S.
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Organic Letters
pubs.acs.org/OrgLett
Letter
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Org. Lett. 2021, 23, 3221−3226