Access to Fully Alkylated Germanes by B(C6F5)3-Catalyzed Transfer Hydrogermylation of Alkenes

Article abstract of DOI:10.1021/acs.orglett.7b00672

Various cyclohexa-2,5-dien-1-yl-substituted germanes are shown to serve as easy-to-handle surrogates of hydrogermanes, including gaseous MeGeH₃ and Me₂GeH₂. The Ge-H functional group is liberated by treatment with catalytic amounts of B(C₆F₅)₃ and participates in situ in the B(C₆F₅)₃-catalyzed hydrogermylation of alkenes. The range of suitable alkenes is broad, and the overall procedure provides a convenient access to tetraalkyl-substituted germanes at room temperature. Transfer hydrogermylation of internal alkynes works equally well and selectively forms the trans or cis diastereomer depending on the electronic bias of the C≡C bond.

Full text of DOI:10.1021/acs.orglett.7b00672

Letter
pubs.acs.org/OrgLett
Access to Fully Alkylated Germanes by B(C6F5)3‑Catalyzed Transfer
Hydrogermylation of Alkenes
Sebastian Keess and Martin Oestreich*
Institut fur Chemie, Technische Universitat Berlin, Strasse des 17. Juni 115, 10623 Berlin, Germany
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̈
S
* Supporting Information
ABSTRACT: Various cyclohexa-2,5-dien-1-yl-substituted germanes are
shown to serve as easy-to-handle surrogates of hydrogermanes,
including gaseous MeGeH3 and Me2GeH2. The Ge−H functional
group is liberated by treatment with catalytic amounts of B(C6F5)3 and
participates in situ in the B(C6F5)3-catalyzed hydrogermylation of
alkenes. The range of suitable alkenes is broad, and the overall
procedure provides a convenient access to tetraalkyl-substituted
germanes at room temperature. Transfer hydrogermylation of internal
alkynes works equally well and selectively forms the trans or cis
diastereomer depending on the electronic bias of the CC bond.
ydrogermylation1 of alkenes is a prevalent method for the
catalytic synthesis of alkyl-substituted germanes.2 The
Scheme 1. Access to Alkyl-Substituted Germanes by Alkene
Hydrogermylation (Top) and Surrogates for Hydrosilanes
and Hydrogermanes (Bottom)
H
addition of hydrogermanes across the CC bond typically
relies on the activation of the Ge−H bond by transition-metal
4
complexes3 or radical initiators such as Et3B/O2 and AIBN.5,6
Previously reported radical-based hydrogermylation protocols
have been limited to triarylgermanes and are therefore not
suitable for the preparation of tetraalkyl-substituted ger-
manes.4,5 Conversely, transition-metal catalysts such as
H2PtCl6 have been shown to activate alkyl-substituted hydro-
germanes RnGeH4−n (R = alkyl, n = 1−3) for alkene
hydrogermylation.1 However, these protocols are usually
constrained by the need for elevated reaction temperatures
and terminal alkenes, i.e., α-olefins (Scheme 1, top).7 Also,
applications of volatile or even gaseous hydrogermanes such as
MeGeH3 (bp −35 °C) or Me2GeH2 (bp 3 °C) in hydro-
germylation reactions are rare, as handling of these flammable
hydrogermanes is hazardous and inconvenient.8
Our group recently demonstrated the potential of adequately
substituted cyclohexa-1,4-dienes to serve as easy-to-handle
synthetic equivalents of H−/Si+ (I, Scheme 1, bottom),9 H−/
11
H+,10 and H−/tBu+
when activated by the Lewis acid
B(C6F5)3.12 As part of this research program, we anticipated
that this approach could provide an entry into the mild
preparation of fully alkylated germanes. The hydrogermylation
of alkenes catalyzed by B(C6F5)3 is in fact unprecedented.13 By
analogy to the ionic transfer hydrosilylation using cyclohexa-
2,5-dien-1-ylsilanes I,9,14 we imagined that the related bisallylic
germanes II would be equally able to liberate the corresponding
hydrogermanes by treatment with catalytic amounts of
B(C6F5)3 (Scheme 1, bottom).
We then prepared cyclohexa-2,5-dien-1-ylgermanes 1a−1c as
surrogates of Et3GeH, Me2GeH2, and MeGeH3, respectively,
from lithiated cyclohexa-1,4-diene and the requisite chloro-
germane (Figure 1; see the Supporting Information for details).
Attempts to synthesize surrogates of the parent monogermane,
GeH4, either fully decorated with cyclohexa-2,5-dien-1-yl
substituents (as in 1d) or with three cyclohexa-1,4-diene
units and one hydride (as in 1e) were unsuccessful.
To demonstrate the feasibility of our proposal, we subjected
surrogate 1a to the typical protocol of the alkene transfer
Received: March 6, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00672
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Letter
hindered position at C2 (9 → 22); this is in agreement with the
regioselectivity previously found in the B(C6F5)3-catalyzed
hydrosilylation of 9,9b again emphasizing that a benzylic
(secondary) carbocation is favored over a tertiary. An allylic
silyl group as in 10 was tolerated, and 1,3-difunctionalized 23
was formed quantitatively. α,β-Unsaturated ester 11 reacted
selectively to yield 24 with incorporation of the germyl group in
the α-position and no reduction of the carboxyl group.15 In
contrast, chemoselective 1,4-reduction of chalcone occurred to
yield the corresponding ketone after hydrolysis (12 → 25).
We also probed electronically unbiased and biased CC
bonds in the transfer hydrogermylation, as Gevorgyan and co-
workers had reported the stereodivergent addition of Et3GeH
across internal alkynes (trans-hydrogermylation) and propio-
lates (cis-hydrogermylation) catalyzed by B(C6F5)3.13a Accord-
ingly, 13 and 14 converted diastereoselectively into (Z)-26 and
(E)-27, respectively, under the setup of our transfer hydro-
germylation (Scheme 2, bottom).
Figure 1. Cyclohexa-2,5-dien-1-ylgermanes as surrogates of hydro-
germanes Et3GeH, Me2GeH2, and MeGeH3 (left) and GeH4 (right).
CHD = cyclohexa-2,5-dien-1-yl.
hydrosilylation9 using 5.0 mol % of B(C6F5)3 and 1,1-
diphenylethylene (2) as the model substrate (Scheme 2).
Quantitative conversion of 2 and 94% isolated yield of 15 were
obtained in 1,2-F2C6H4 after 5 h at a substrate concentration of
1.0 M (see the Supporting Information for a screening of
solvents). A practical aspect of this protocol is the convenient
removal of the byproducts benzene and unreacted Et3GeH
from the crude reaction mixture under reduced pressure.
Encouraged by this result, we subjected differently substituted
alkenes 3−12 to this protocol (Scheme 2, top). Electronically
modified 1,1-diphenylethylene derivatives 3 and 4 showed the
expected reactivity trend, giving full conversion for methoxy-
substituted 3 in 3 h, while more electron-deficient 4 required
double the amount of B(C6F5)3 and more of the surrogate to
secure quantitative conversion. Styrenes 5 and 6 participated
well with an excellent 95% yield of 18 and 19, respectively.
Likewise, α-olefin 7 and 1,1-dialkyl-substituted 8 furnished 20
and 21 in near-quantitative yields. Trisubstituted 2-methyl-
indene favored C−Ge bond formation in the sterically more
We also investigated cyclic dienes 28−30 (Scheme 3). When
using 0.6 equiv of surrogate 1a, cyclohexa-1,3-diene reacted
Scheme 3. Transfer Hydrogermylation of Cyclic Dienes
Scheme 2. Transfer Hydrogermylation of Alkenes and
Alkynes Using Et3GeH Surrogate 1a
a
Isolated as a mixture with trans-32 and trans-33 in a ratio of 31:
(trans-32+trans-33) = 53:47.
completely regioselectively to afford the homoallylic germane
(28 → 31). The same adduct was obtained using cyclohexa-1,4-
diene (29 → 31) along with significant amounts of fully
hydrogermylated cyclohexanes trans-32 and trans-33.16 These
outcomes are particularly noteworthy, as the related H2PtCl6-
catalyzed hydrogermylation of 28 and 29 yields the
corresponding vinylic germane due to migration of the
unreacted double bond.17 Interestingly, cyclohexa-1,4-diene
(29) was reported to engage in transfer hydrogenation of
alkenes under this setup,10b but we did not observe any
reduction. Treatment of either diene with 2.4 equiv of surrogate
1a led to an equimolar mixture of trans-1,3- and trans-1,4-
bis(triethylgermyl)cyclohexane (28 or 29 → trans-32 and trans-
33).16 Transfer hydrogermylation of norborna-2,5-diene
proceeded with an exo/endo ratio of 87:13 (30 → 34).
We then continued with 1b (surrogate for Me2GeH2) and 1c
(surrogate for MeGeH3) in the transfer hydrogermylation of
representative alkenes (Scheme 4). 1,1-Diphenylethylene
derivatives 3 and 2 required a longer reaction time (72 h) or
higher catalyst loading (10 mol %), respectively, to form
diadduct 35 and triadduct 36 in good yields. Conversely,
styrenes 5 and 6 gave high yields with surrogate 1b or 1c in the
a
b
10 mol % of B(C6F5)3 and 1.6 equiv of 1a used. 24 h. Reaction run
c
d
on a 1.0 mmol scale. Performed at 90 °C. 2.0 equiv of 1a used.
Reaction run on a 0.1 mmol scale.
B
DOI: 10.1021/acs.orglett.7b00672
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Letter
Scheme 4. Transfer Hydrogermylation of Selected Alkenes
Using Me2GeH2 and MeGeH3 Surrogates 1b and 1c
Scheme 6. Proposed Catalytic Cycles for the B(C6F5)3-
Mediated Hydrogermane Release (Left) and the B(C6F5)3-
Catalyzed Alkene Hydrogermylation (Right)
a
72 h. 72% conversion of the substrate determined by 1H NMR
b
analysis. Obtained as a 1:1 mixture of meso and C2-symmetric
compounds. 10 mol % of the catalyst used. Obtained as a 3:1
c
d
[HB(C6F5)3]− (1 → III → IV). IV eventually collapses to
release the hydrogermane along with stoichiometric benzene
(left cycle). Subsequent activation of the hydrogermane by
B(C6F5)3 and transfer of the electrophilic germyl moiety to π-
basic substrate VI forms ion pair VII (V + VI → VII)21 that,
after hydride transfer from [HB(C6F5)3]−, gives adduct VIII
concomitant with regeneration of B(C6F5)3 (right cycle).22
We have introduced here three cyclohexa-2,5-dien-1-yl-
substituted germanes to serve as synthetic equivalents of
hydrogermanes. These engage in ionic transfer hydrogermyla-
tion reactions of a broad range of differently substituted alkenes
(and alkynes) by catalytic treatment with the strong boron
Lewis acid B(C6F5)3. This enables the metal-free synthesis of
alkyl-substituted germanes at room temperature.
mixture of C1- and C3-symmetric compounds.
presence of a lower catalyst loading (5.0 mol %). Likewise,
terminal alkenes 7 and 8 furnished the diadducts 41 and 43 in
excellent yields whereas slightly lower yields were obtained for
triadducts 42 and 44. Allylic silane 10 was prone to
decomposition in combination with surrogates 1b and 1c,
and only diadduct 45 was isolated in satisfactory yield.
We also performed control experiments (1) to distinguish
between hydrogermane release from surrogate 1 followed by
Ge−H bond activation and direct transfer of the electrofuge (=
germylium ion) onto the π-basic substrate as well as (2) to
exclude the involvement of radical intermediates.18 1H NMR
analysis confirmed for all three surrogates 1 that treatment of 1
with catalytic amounts of B(C6F5)3 in CD2Cl2 triggers the
liberation of the hydrogermane together with benzene even in
the absence of any alkene (see the Supporting Information for
details). Further, acyclic diene 46 cleanly afforded 48 under the
standard setup, and 47 was not detected (Scheme 5).19
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b00672.
General procedures, experimental details, a screening of
solvents, time-dependent NMR experiments, character-
ization data, and NMR spectra for all compounds (PDF)
Scheme 5. Mechanistic Control Experiment to Exclude
Radical Intermediates
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: martin.oestreich@tu-berlin.de.
ORCID
Sebastian Keess: 0000-0002-0791-6877
Martin Oestreich: 0000-0002-1487-9218
Notes
The authors declare no competing financial interest.
These observations support the stepwise ionic mechanism
described earlier for the related transfer hydrosilylation
(Scheme 6).9,20 Liberation of the hydrogermane commences
with B(C6F5)3-triggered hydride abstraction from the bisallylic
methylene group of surrogate 1,9e thereby forming ion pair IV
consisting of a germanium-stabilized Wheland complex or a
benzene-stabilized germylium ion together with borohydride
ACKNOWLEDGMENTS
■
This research was supported by the Deutsche Forschungsge-
meinschaft (Oe 249/11-1). M.O. is indebted to the Einstein
Foundation (Berlin) for an endowed professorship. We thank
Dr. Sebastian Kemper (TU Berlin) for expert advice with the
NMR analyses.
C
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Letter
Science 2016, 351, 832−836. (d) Fang, X.; Yu, P.; Prina Cerai, G.;
Morandi, B. Chem.−Eur. J. 2016, 22, 15629−15633.
(15) Comparable regioselectivity was observed by Gevorgyan and co-
workers in the B(C6F5)3-catalyzed hydrogermylation of propiolates.
See ref 13a.
(16) The relative configuration of trans-32 was verified by NMR
spectroscopy, and we assume the same for trans-33, for which
unambiguous assignment by NMR spectroscopic analysis was not
possible.
(17) Fish, R. H.; Kuivila, M. G. J. Org. Chem. 1966, 31, 2445−2450.
(18) B(C6F5)3 has been shown to function as a one-electron oxidant.
For the seminal report, see: Harlan, C. J.; Hascall, T.; Fujita, E.;
Norton, J. R. J. Am. Chem. Soc. 1999, 121, 7274−7275.
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mechanism for their B(C6F5)3-catalyzed hydrogermylation using
hydrogermanes. See ref 13a.
(20) Conversely, transfer hydrogenation and transfer hydro-tert-
butylation from these cyclohexa-1,4-dienes were shown to proceed by
direct transfer of the electrofuge. See refs 10b and 11.
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J. B.; Zhao, Y.; Wu, H. J. Org. Chem. 1999, 64, 2729−2736.
(22) For the hydride donor abilities of various main-group hydrides,
see: Heiden, Z. M.; Lathem, A. P. Organometallics 2015, 34, 1818−
1827.
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D
DOI: 10.1021/acs.orglett.7b00672
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