Mechanism and Selectivity of Aryltrimethylgermane Cation Radical Fragmentations

Article abstract of DOI:10.1021/acs.joc.0c01032

Aryltrimethylgermane cation radicals were generated by nanosecond transient absorption spectroscopy. Transient kinetics experiments show that the aryltrimethylgermane cation radicals react with added nucleophiles in reactions that are first-order in both

Full text of DOI:10.1021/acs.joc.0c01032

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Mechanism and Selectivity of Aryltrimethylgermane Cation Radical
Fragmentations
Adam M. Feinberg and Joseph P. Dinnocenzo*
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ABSTRACT: Aryltrimethylgermane cation radicals were generated by nanosecond
transient absorption spectroscopy. Transient kinetics experiments show that the
aryltrimethylgermane cation radicals react with added nucleophiles in reactions that are
first-order in both the cation radicals and the nucleophiles. Preparative photo-oxidation
experiments demonstrate that the intermediate cation radicals react with nucleophiles,
resulting in aryl−Ge or Me−Ge nucleophile-assisted fragmentations. The aryltrimethylgermane cation radicals were found to react
more slowly than analogous stannane cation radicals; however, loss of the thermodynamically disfavored aryl radicals remains
competitive with methyl radical loss.
INTRODUCTION
RESULTS AND DISCUSSION
■
■
Aryltrialkylstannane cation radicals generated in solution have
recently been shown to undergo C−Sn bond fragmentation by
a nucleophile-assisted mechanism (SN2).1,2 An interesting
feature of these reactions is that the stannane cation radicals
show a remarkable preference for loss of the (less stable) aryl
radicals over methyl radicals, a result hitherto unprecedented
for cation radical bond fragmentations. These curious results
prompted us to study the fragmentations of analogous germane
cation radicals to determine if they showed fragmentation
selectivities similar to their stannane cation radical analogues.
The work described herein seeks answers to three key
questions. First, do aryltrialkylgermane cation radicals
generated in solution undergo efficient C−Ge bond
fragmentation? Second, if so, are these fragmentations
unimolecular or nucleophile-assisted processes? Third, are
the unusual aryl/methyl fragmentation selectivities observed
for aryltrimethylstannane cation radicals also observed for
related germane cation radicals?
Fragmentation reactions of unsymmetrically substituted
germane cation radicals in solution have rarely been studied
before. Allyl- or benzyl-trialkylgermane cation radicals are
known to have a strong bias for fragmentation to form the
more stable allyl or benzyl radicals.3 The oxidation of several
aryltrimethylgermanes using AlCl3 in CH2Cl2 monitored by
EPR spectroscopy has previously been shown to result in the
formation of substituted biphenyl cation radicals.4 Although
the mechanism of these latter reactions and the identification
of EPR-silent products have not been investigated, the results
could be consistent with aryl radical formation followed by
dimerization and subsequent one-electron oxidation. In the gas
phase, not surprisingly, fragmentation of phenyltrimethylger-
mane cation radical leads to nearly exclusive loss of methyl
over phenyl radical.5
In analogy to the previously studied aryltrimethylstannane
cation radicals,1,2 we first chose to attempt the generation of
the (4-methoxyphenyl)trimethylgermane cation radical (1+•).
Nanosecond transient absorption spectroscopy (NTAS) using
a cosensitized photo-oxidation method described previously6
was used to generate 1+•. Briefly, a dioxygen-saturated
dichloromethane (CH2Cl2) solution containing 1 mM N-
methylquinolinium hexafluorophosphate (NMQ+), toluene
(0.5 M), and 10 mM 1 was excited with a nanosecond laser
(10 ns, 343 nm). Excitation of NMQ+ produces its singlet
excited state (1NMQ+*), which is rapidly intercepted by
toluene to efficiently produce toluene+• and the N-methyl-
quinolinyl radical (NMQ•) by photoinduced electron transfer.
NMQ• is scavenged by dioxygen in <100 ns leading to O2
−•
,
which does not have interfering absorptions in the visible
region where aromatic radical cations typically absorb.
Interception of toluene+• by good electron donors such as 1
efficiently produces their cation radicals. The transient
absorption spectrum collected 500 ns after the laser pulse is
shown in Figure 1a. This absorption spectrum was tentatively
assigned to 1+• by analogy to the absorption spectrum of
anisole cation radical (λmax ≈ 430 nm).7 The transient was
found to rapidly react (k = 6.7 × 109 M−1 s−1) with the good
electron donor 4,4′-dimethoxybiphenyl (2) to give 2+• (λmax
≈
420 nm)8 by monitoring the growth of 2+• at 415 nm or the
decay of 1+• at 460 nm [see Figure 1b]. The reaction of the
Received: April 27, 2020
https://dx.doi.org/10.1021/acs.joc.0c01032
© XXXX American Chemical Society
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Fragmentation of 1+• was anticipated to produce nucleophile-
germylium cation adducts. The reaction workup was designed
to convert these adducts to germyl chlorides by treatment with
LiCl, following the analogous procedure previously used for
aryltrialkylstannane photo-oxidations (Scheme 1).1,2 The two
Scheme 1. Conditions for Photo-oxidation of
Aryltrimethylgermanes
Figure 1. (a) Transient absorption spectra assigned to 1+• (λmax
≈
470 nm) recorded in CH2Cl2 500 ns after the laser pulse. (b)
Transient spectrum of 2+• generated by the reaction of 1+• and 2 in
CH2Cl2. The inset shows a plot of the pseudo-first-order rate constant
for the reaction of 1+• with 2 vs [2], determined by monitoring the
decay of 1+• at 460 nm.
expected products are (4-methoxyphenyl)dimethylgermyl
chloride (3), indicative of methyl radical loss, and
trimethylgermyl chloride (4), indicative of aryl radical loss.
In practice, these were the only products formed as revealed by
1H NMR spectroscopy. The product ratios were determined by
integrating the germyl-methyl protons of 3 versus 4, and the
overall mass balance was determined using an internal
standard.
The preparative photo-oxidations of 1 in the presence of
nucleophiles resulted in the formation of 3 and 4 with typical
mass balances of >95%. The statistically corrected aryl/Me loss
ratios determined from the product ratios are summarized in
Table 1. Remarkably, the selectivity for the aryl/Me loss ratio,
transient with 2 to produce 2+• provides strong support to the
assignment of the transient spectrum in Figure 1a to 1+•.
Possible reactions of 1+• in the presence of added
nucleophiles were next probed by NTAS. The kinetics of 1+•
decay were monitored at 460 nm in the presence of varying
concentrations of methanol (HOMe), tert-butyl alcohol
(HOBut), and acetonitrile (MeCN). 1+• was found to react
with all of the nucleophiles in reactions that were first-order in
both 1+• and the nucleophile (Figure 2). Bimolecular rate
Table 1. Comparison of Statistically Corrected Aryl/Me
Loss Ratios from Photo-oxidation of 1 in CH2Cl2 with
Methanol, tert-Butyl Alcohol, and Acetonitrile as
Nucleophiles without and with Added Base (K2CO3) to
Aryl/Me Loss Ratios from Photo-oxidation of p-
MeOPhSnMe3
p-MeOPh/Me loss ratios
a
1
b
nucleophile
without base
with base
p-MeOPhSnMe3
methanol
t-butyl alcohol
acetonitrile
60
360
2.1
53
370
1.9
13.6
6.8
0.5
a
b
Errors 10%. Reference 1.
Figure 2. Pseudo-first-order rate constant plots for the reactions of
1+• with added methanol (red squares), tert-butyl alcohol (black
circles), and acetonitrile (blue triangles).
indicative of aryl versus methyl radical loss, was ∼55:1 for the
reaction with HOMe and ∼365:1 for the reaction with
HOButsignificantly greater selectivities than those observed
in analogous stannane cation radical fragmentations (see Table
1).1,2 Addition of acetonitrile resulted in more competitive aryl
and methyl radical cleavages but still showed a small preference
for aryl radical loss. To test whether aryl loss could, in part, be
due to protodegermylation, the photo-oxidations were also
conducted in the presence of anhydrous K2CO3 as an acid
scavenger. The addition of the base did not affect the product
ratios within the experimental error, supporting the conclusion
that the products are the result of nucleophile-assisted cation
radical fragmentations.
constants for the reaction of 1+• with HOMe and HOBut are
1.6 × 106 and 8.2 × 105 M−1 s−1, respectively. That the more
sterically encumbered HOBut reacts less rapidly than HOMe is
consistent with nucleophile-assisted fragmentation. While
MeCN was observed to react with 1+•, the rate constant is
comparable to the background decay of 1+•, and the
bimolecular rate constant can only be estimated to be ∼1 ×
105 M−1 s−1.
The NTAS kinetics for 1+• strongly suggest a nucleophile-
assisted decay pathway but do not establish the exact nature of
the reaction. Toward this end, preparative photo-oxidations of
1 were conducted to identify the products for the reaction of
1+• in the presence of nucleophiles. Preparative photo-
oxidations (343 nm) of 1 were carried out in CH2Cl2
containing 1 mM NMQ+, 0.5 M toluene, and the nucleophiles.
Products were determined at partial conversion (≤25%).
The combined NTAS and product studies provide strong
support for the hypothesis that the C−Ge bond fragmentations
of 1+• proceed by a nucleophile-assisted mechanism that shows
a higher preference for substitution to form the less stable aryl
radical than aryltrimethylstannane cation radials.1,2 A Bent’s
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Figure 3. (a) Red trace: transient spectrum recorded 500 ns after laser excitation (343 nm) of an argon-saturated HFIP solution containing 5 (10
mM), NMQ+ (1 mM), and benzene (0.5 M). Green trace: transient spectrum recorded 500 ns after laser excitation (343 nm) of an argon-saturated
HFIP solution containing 5 (10 mM), NMQ+ (1 mM), PhCH2SiMe3 (0.5 mM), and benzene (0.5 M). Black trace: subtraction of the green trace
from the red trace (spectrum assigned to 5+•). (b) Plot of the pseudo-first-order rate constant for the reaction of 5+• with 2 vs [2], determined by
monitoring the appearance of 2+• at 415 nm. The inset shows typical rise of 2+• (black line) and the first-order fit to the experimental data (gray
line).
rule explanation has previously been proposed to explain the
fragmentation selectivities for the reaction of aryltrimethyl-
stannane cation radicals with nucleophiles, where the more
electronegative aryl substituent prefers to be in an apical,
leaving group position of a trigonal bipyramidal transition state
for nucleophilic substitution.1 If the same effect applies to the
fragmentation of germane cation radicals, the lower reactivity
of 1+• versus its stannane cation radical analogue could result
in a more advanced (later) transition state for the reaction of
1+• with nucleophiles, which could lead to the difference in
electronegativities of the departing radicals playing a greater
role in determining the fragmentation selectivities for 1+•.
1+• is ∼20−30 times less reactive with nucleophiles than its
comparably substituted stannane cation radical counterpart.1
In search of a more reactive germane cation radical, we next
chose to examine that of (4-tert-butylphenyl)trimethyl
germane (5), which was expected to be significantly more
reactive than 1+•. Indeed, 5+• proved to be too reactive to
study in CH2Cl2 solution by NTAS. Even in the absence of
added nucleophiles, 5 rapidly reacts with toluene+•, but no
follow-up transient attributable to 5+• could be detected in
CH2Cl2.
the red spectrum leads to the black spectrum in Figure 3a,
which was tentatively assigned to 5+•. This hypothesis was
tested by determining if the transient assigned to 5+• reacted
with a good electron donor by electron transfer. As shown in
Figure 3b, the transient species was found to rapidly react with
4,4′-dimethoxybiphenyl (2) (k = 4.7 × 109 M−1 s−1) to
produce 2+•, supporting the assignment of the transient
spectrum attributed to 5+•. The spectrum of 5+• in HFIP
shows some similarities and some differences relative to that of
the tert-butylbenzene cation radical, previously determined in
CH3CN (λmax ≈ 450 nm).7 The short wavelength band for 5+•
in HFIP (λmax ≈ 420 nm) is somewhat hypsochromically
shifted relative to the tert-butylbenzene cation radical. The
long wavelength band at ∼640 nm is not present in the tert-
butylbenzene cation radical and is presumably associated with
an electronic transition involving the trimethylgermyl group.
Although the assignment of the spectrum for 5+• seems secure,
further work will be required to determine the precise nature of
these spectral transitions.
Having confirmed the generation of 5+• in HFIP, reactions
of the cation radical with nucleophiles were next investigated.
As with 1+•, 5+• was found to react with added nucleophiles in
reactions that are first-order in both 5+• and the nucleophiles
(Figure 4). The rate constants for the reaction of CH3CN,
Fortunately, it was possible to generate and observe 5+• in
1,1,1,3,3,3-hexafluoroisopropanol (HFIP), a solvent known to
stabilize cation radicals in comparison to other polar solvents.9
Optimal conditions for recording the absorption spectrum of
5+• used a solution of 5/NMQ+/benzene in argon-saturated
HFIP. Under these conditions, the transient spectrum shown
in red in Figure 3a is observed. This spectrum contains
contributions from the NMQ• radical (λmax ≈ 540 nm),10
which is long-lived in the absence of dioxygen. The
contribution of NMQ• was readily determined by performing
the transient absorption experiment in the presence of 0.5 mM
benzyltrimethylsilane (6). Under these conditions, 5+• is
expected to rapidly react with 6 by electron transfer to produce
6+•, which is known to rapidly undergo the benzylic C−Si
bond to give benzyl radicals and a trimethylsilyl cation/
nucleophile adduct,6 both of which are spectroscopically silent
in the wavelength region examined here. Thus, 6 acts as a
cation radical scavenger, effectively removing 5+• from the
transient spectrum. The transient absorption spectrum
recorded in the presence of 6 is shown in green in Figure
3a. Subtraction of the green spectrum (due to NMQ•) from
Figure 4. Pseudo-first-order rate constant plots for the reactions of
5+• with added methanol (red squares), tert-butyl alcohol (black
circles), and acetonitrile (blue triangles).
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HOMe, and HOBut are 6.0 × 105, 3.9 × 105, and 4.0 × 104
M−1 s−1, respectively. The HOMe/HOBut rate constant ratio
(≈10) shows a clear steric effect on the nucleophile, consistent
with a nucleophile-assisted fragmentation mechanism. Despite
the high reactivity of 5+• in CH2Cl2, the rate constants for the
reaction of 5+• with nucleophiles in HFIP are lower than those
of 1+• in CH2Cl2. This can be accounted for by the differences
in reactivity between these solvents for nucleophilic sub-
stitutions on cation radicals. For example, the rate constants for
the SN2 reactions of benzyltrialkylsilane cation radicals with
methanol are ∼100 times lower in HFIP than in CH2Cl2.6 We
note that the enhanced reactivity of CH3CN versus HOMe
with 5+• in HFIP versus the reversed reactivity of these
nucleophiles with 1+• in CH2Cl2 is consistent with earlier
results on the reactions of benzylsilane cation radicals, where
the decreased relative reactivity of HOMe versus CH3CN in
HFIP was attributed to differential effects of hydrogen
bonding.6
CONCLUSIONS
■
Aryltrimethylgermane cation radicals have been generated by
NTAS. Kinetic studies show that the germane cation radicals
react with nucleophiles in reactions that are first-order in both
the cation radicals and the nucleophiles. Product studies show
that the reactions of the germane cation radicals lead to aryl−
Ge and Me−Ge bond fragmentations. The kinetic and product
studies are consistent with a nucleophile-assisted (SN2)
fragmentation process, in direct analogy to the previous
reported reactions of aryltrimethylstannane cation radicals.
Nucleophile-assisted fragmentations of the (4-
methoxyphenyl)trimethylgermane cation radical occur with
lower bimolecular rate constants than those for the
corresponding stannane cation radical in CH2Cl2 but with
higher aryl versus methyl radical cleavage selectivities. The
more reactive cation radical of (4-tert-butylphenyl)trimethyl-
germane also exhibits competitive aryl/methyl radical
fragmentation but with lower aryl/methyl selectivities in
HFIP as a solvent. The distinctly different reactivity and
selectivity resulting from simple substitution of the aryl ring
affords potentially tunable reaction parameters for future
development.
As described for 1, preparative photolyses of 5 in the
presence of nucleophiles were used to determine if the
reactions of 5+• observed by NTAS were due to cation radical
fragmentations. The conditions for the photolysis with 5 were
similar to those used for 1, with the exception that HFIP was
used as the solvent and benzene as the co-donor in order to
match the NTAS conditions with 5. The products of photo-
oxidation of 5 with CH3CN and HOMe were studied. The
relatively low bimolecular rate constant for the reaction of 5+•
with HOBut precluded product studies for this nucleophile
because a sufficiently high concentration of HOBut could not
be used to ensure that the majority of cation radical decay was
due to the reaction with the nucleophile. The aryl/methyl loss
ratios were determined as described above for 1. Stirring of the
photo-oxidized samples of 5 with LiCl resulted in the
formation of the expected germyl chlorides, (4-tert-
butylphenyl)dimethylgermyl chloride (7), and trimethylgermyl
chloride (4), corresponding to methyl and aryl radical
EXPERIMENTAL SECTION
■
General Experimental Procedures and Techniques. Unless
otherwise noted, the following conditions were used for all
nonaqueous reactions. Reactions were conducted at room temper-
ature in oven-dried glassware (125 °C) under a nitrogen atmosphere,
and the solutions were stirred magnetically using Teflon-coated
magnetic stir-bars. Air- and moisture-sensitive reagents and solutions
were transferred via a syringe or cannula and were introduced to the
apparatus through rubber septa or three-way stopcocks under a
vigorous nitrogen purge.
Routine 1H NMR spectra were recorded at 400 or 500 MHz.
Chemical shifts (δ) are reported in ppm relative to tetramethylsilane
using the residual proton in the solvent as an internal standard.
Proton−proton coupling constants are measured using line spacings.
The following abbreviations were used: s (singlet), d (doublet), t
(triplet), pent (pentet), and m (multiplet). 13C NMR spectra were
recorded at 100 MHz; chemical shifts were referenced to the solvent
as an internal standard.
1
cleavage, respectively. Product ratios were quantified by H
NMR integration of the germyl-methyl proton signals in 4 and
7. As shown in Table 2, photo-oxidation of 5+• in the presence
Steady state photolysis reactions at 334 nm were performed using a
200 W or 500 W mercury arc lamp equipped with a liquid filter filled
with deionized water to absorb IR light. The emitted light was
successively filtered through an Oriel 56521 cutoff filter and an Oriel
56421 interference filter (334 nm).
Materials. Unless otherwise noted, all materials were obtained
from commercial sources. Bis(4-methoxyphenyl)dimethylgermane
was prepared by a literature method.11
Table 2. Statistically Corrected Aryl/Me Loss Ratios for
Photo-oxidation of 5 in HFIP with Methanol and
Acetonitrile as Nucleophiles without and with Added Base
a
t-BuPh/Me loss ratios
nucleophile
without base
with base
methanol
acetonitrile
0.57
0.77
0.50
0.60
Nanosecond Transient Absorption Spectroscopy. A XeCl
excimer laser (308 nm) was used to pump a dye laser containing p-
terphenyl laser dye for 343 nm excitation. Transient spectral
absorptions were monitored at a right angle to the laser excitation
by using a home-built, xenon flashlamp system equipped with a
Perkin-Elmer FX-193 flashlamp to generate the analyzing light. The
analyzing light was focused into the end of a fiber optic cable and onto
the entrance slit of a monochromator equipped with an intensified
CCD. A pulsed xenon arc lamp was used as the monitoring light
source for kinetics analyses. The monitoring light was passed through
a monochromator and detected using a photomultiplier tube (PMT).
The signal from the PMT was directed into a digitizing oscilloscope
and then to a computer for viewing, storage, and data analysis. All
transient experiments were conducted at room temperature (22 °C).
Preparation of (4-Methoxyphenyl)trimethylgermane (1). 1 is a
a
Errors 10%.
of CH3CN or HOMe resulted in competitive aryl and methyl
radical cleavage. As with 1, the addition of a base did not
substantially alter the product ratios within the experimental
error.
The aryl/Me fragmentation selectivities for 5 are both lower
than those observed with 1, with the HOMe selectivity being
substantially more affected. At present, the reasons for the
relative changes are not clear. Further work will be required to
determine whether the differences are dependent on the
structures of the germane cation radicals and/or the reaction
solvents. It was not possible to determine the fragmentation
selectivites for 1+• in HFIP for comparison with 5+• because in
this solvent 1+• is too unreactive with nucleophiles.
1
known compound; its H NMR spectral data were compared to the
literature values.12 A three-neck 100 mL round-bottomed flask was
charged with magnesium turnings (0.68 g, 27.8 mmol), followed by a
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small crystal of I2 and 5 mL of tetrahydrofuran (THF). The mixture
was heated to reflux using a heat gun and allowed to stir until reflux
subsided. After heating again to reflux, a solution of 4-bromoanisole
(3.33 mL, 26.6 mmol) in THF (20 mL) was added dropwise at a rate
sufficient to maintain reflux. After 2 h, bromotrimethylgermane (3.25
mL, 25.3 mmol) in THF (10 mL) was added dropwise, and the
mixture was heated to reflux with an oil bath for an additional 8 h.
After cooling to room temperature, a saturated aqueous ammonium
chloride solution (25 mL) was added to the reaction mixture, and the
organic layer was extracted with diethyl ether (3 × 50 mL). The
combined organic layers were washed with water (3 × 15 mL), dried
over anhydrous Na2SO4, and concentrated to give a clear, yellow oil.
Column chromatography (hexanes, SiO2) followed by vacuum
distillation (bp 98 °C, 8 torr) afforded a clear, colorless oil (5.1 g,
78%).
isopropanol bath, and dichlorodimethylgermane (0.15 mL, 1.3 mmol)
was added via a syringe in one portion. The reaction was warmed to
room temperature and stirred for 12 h. Distilled water (10 mL) was
added, and the reaction mixture was extracted with diethyl ether (3 ×
10 mL). The combined organic layers were washed with water (2 ×
10 mL) and saturated brine (25 mL), dried over anhydrous Na2SO4,
and concentrated under reduced pressure to give a colorless oil (0.30
g, 73%). The material was used without further purification.
1H NMR (CDCl3): δ 7.43 (d, J = 8.0 Hz, 3.7 H), 7.38 (d, J = 8.0
Hz, 4.0 H), 1.32 (s, 18.4 H), 0.62 (s, 5.9 H).
13C{1H} NMR (CDCl3): δ 151.2, 136.7, 133.3, 124.9, 34.4, 31.1,
−3.2.
HRMS (ESI-TOF) m/z: [M]+ calcd for C22H32Ge, 370.1716;
found, 370.1702.
Preparation of Chloro(4-tert-butylphenyl)dimethylgermane (7).
A 5 mL round-bottomed flask was charged with bis(4-tert-
butylphenyl)dimethylgermane (50 mg, 0.135 mmol) and dry
CH2Cl2 (2.0 mL). The flask was cooled in a dry-ice/isopropanol
bath. Methanesulfonic acid (9 μL, 0.135 mmol) was added via a
syringe in one portion, and the reaction flask was warmed to room
temperature. After stirring for 1 h, the reaction flask was again cooled
in a dry-ice/isopropanol bath, and lithium chloride (42 mg, 1 mmol)
was added under a vigorous flow of nitrogen. The resulting mixture
was stirred for 3 h. After warming to room temperature, the mixture
was filtered and concentrated under reduced pressure to afford a clear
oil (14 mg, 38%) that contained ∼10% of the starting material. The
crude product did not require further purification to be used as a
chemical shift reference for product studies.
1H NMR (CDCl3): δ 7.39 (d, J = 8.4 Hz, 2.0 H), 6.92 (d, J = 8.4
Hz, 2.0 H), 3.81 (s, 3.0 H), 0.36 (s, 9.0 H).
Lit:12 1H NMR (acetone-d6): δ 7.12 (m, 4 H), 3.46 (s, 3 H), 0.38
(s, 9 H).
HRMS (ESI-TOF) m/z: [M]+ calcd for C10H16OGe, 226.0413;
found, 226.0411.
Preparation of Chloro(4-methoxyphenyl)dimethylgermane (3).
A 5 mL round-bottomed flask containing bis(4-methoxyphenyl)-
dimethylgermane (50 mg, 0.15 mmol) and dry CH2Cl2 (1.0 mL) was
cooled in a dry-ice/isopropanol bath. Methanesulfonic acid (10 μL,
0.15 mmol) was added via a syringe in one portion, and the reaction
was warmed to room temperature. After stirring for 1 h, the reaction
solution was again cooled in a dry-ice/isopropanol bath. Lithium
chloride (42 mg, 1 mmol) was added under a vigorous flow of
nitrogen, and the mixture was stirred for 3 h. After warming to room
temperature, the mixture was filtered and concentrated under reduced
pressure to give a clear oil (26 mg, 71%) that contained ∼10% of the
starting material. The crude product did not require further
purification to be used as a chemical shift reference for product
studies.
1H NMR (CDCl3): δ 7.54 (d, J = 8.4 Hz 1.9 H), 7.46 (d, J = 8.4
Hz, 1.9 H), 1.33 (s, 9.5 H), 0.93 (s, 5.8 H).
13C{1H} NMR (CD2Cl2): δ 133.7, 132.5, 125.9, 125.4, 35.1, 31.3,
3.3.
HRMS (ESI-TOF) m/z: [M]+ calcd for C12H19GeCl, 272.0387;
found, 272.0378.
Representative Procedure for Photo-oxidations of (4-Methox-
yphenyl)-trimethylgermane (1). A stopcocked cuvette was charged
with 4.0 mL of dichloromethane solution containing 0.020 M 1, 1
mM N-methylquinolinium hexafluorophosphate6, 0.5 M toluene, and
1.0 M methanol. The reaction solution was purged for 20 min with
dioxygen that was passed through a bubbler containing a frit
immersed in CH2Cl2. After removal of a 0.5 mL of aliquot, the cuvette
was then irradiated for 60 min at 334 nm. After irradiation, a second
0.5 mL of aliquot was removed and stirred over powdered LiCl (∼25
mg) for 3 h. To each aliquot was added 0.5 mL of 10 mM 1,4-dioxane
standard solution in dichloromethane. The samples were dried over
Na2SO4, filtered through glass wool into an NMR tube, and doped
with 200 μL of CD2Cl2. Samples were analyzed by 1H NMR
spectroscopy. The germyl-methyl group hydrogens of the starting
material and germyl chlorides (3 and 4) were integrated against the
methylene proton resonance of the 1,4-dioxane internal standard to
determine the mass balance and aryl/Me fragmentation ratio.
Representative Procedure for Photo-oxidations of (4-tert-
Butylphenyl)-trimethylgermane (5). A stopcocked cuvette was
charged with 4.0 mL of an HFIP solution containing 0.020 M 5, 1
mM N-methylquinolinium hexafluorophosphate, 0.5 M benzene, and
1.0 M methanol. The reaction solution was purged with dioxygen for
20 min, and a 0.5 mL of aliquot was removed. The cuvette was then
irradiated for 15 min at 334 nm. After irradiation, a second 0.5 mL of
aliquot was removed and stirred over powdered LiCl (∼25 mg) for 3
h. To each aliquot was added 1.0 mL of a solution of 5 mM
hexamethyldisiloxane (HMDS) in dichloromethane. The samples
were dried over Na2SO4, filtered through glass wool into an NMR
tube, and doped with 200 μL of CD2Cl2. Samples were analyzed by
1H NMR spectroscopy. The germyl-methyl group hydrogens of the
1H NMR (CDCl3): δ 7.51 (d, J = 8.4 Hz 1.9 H), 6.97 (d, J = 8.4
Hz, 1.9 H), 3.83 (s, 3.0 H), 0.92 (s, 6.2 H).
13C{1H} NMR (CDCl3): δ 161.2, 133.8, 129.5, 114.2, 55.2, 3.4.
HRMS (ESI-TOF) m/z: [M]+ calcd for C9H13ClGeO, 245.9876;
found, 245.9876.
Preparation of (4-tert-Butylphenyl)trimethylgermane (5). A
three-neck, 100 mL round-bottomed flask was charged with
magnesium turnings (0.67 g, 27.5 mmol) followed by a small crystal
of I2 and 5 mL of THF. The mixture was heated to reflux using a heat
gun and allowed to stir until reflux subsided. After heating to reflux
with an oil bath, a solution of 4-tert-butylbromobenzene (4.34 mL, 25
mmol) in THF (20 mL) was added dropwise at a rate sufficient to
maintain reflux. After 2 h, bromotrimethylgermane (3.21 mL, 25
mmol) in THF (10 mL) was added dropwise, and the mixture was
heated to reflux with an oil bath for an additional 12 h. After cooling
to room temperature, a saturated aqueous ammonium chloride
solution (25 mL) was added to the reaction mixture, and the organic
layer was extracted with diethyl ether (3 × 50 mL). The combined
organic layers were washed with water (3 × 15 mL), dried over
anhydrous Na2SO4, and concentrated to give a clear, yellow oil.
Column chromatography (hexanes, SiO2) afforded a clear, colorless
oil (5.8 g, 93%).
1H NMR (CDCl3): δ 7.43 + 7.38 (d + d, J = 8.2 Hz, 3.9 H), 1.33
(s, 9.2 H), 0.37 (s, 8.9 H).
13C{1H} NMR (CDCl3): δ 151.1, 139.0, 132.8, 124.9, 34.6, 31.3,
−1.8.
HRMS (ESI-TOF) m/z: [M]+ calcd for C13H22Ge, 252.0933;
found, 252.0931.
Preparation of Bis(4-tert-butylphenyl)dimethylgermane. A 50
mL round-bottomed flask was charged with diethyl ether (15 mL)
and 4-tert-butylbromobenzene (0.511 mL, 3.0 mmol). The flask was
cooled in a dry-ice/isopropanol bath. A 1.7 M solution of tert-
butyllithium in pentane (3.82 mL, 6.5 mmol) was added via a syringe
pump over 30 min, and the reaction mixture was warmed to room
temperature for 1 h. The flask was again cooled in a dry-ice/
starting material and resulting germyl chlorides (4 and 7) were
integrated against the methylene proton resonance of the HMDS
internal standard to determine the mass balance and aryl/Me
fragmentation ratio.
Representative Procedure for the Generation and Observation
of Cation Radicals. A 10 mL volumetric flask was charged with 0.54
E
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Article
mL of toluene (0.5 M) and 23 mg of (4-methoxyphenyl)-
trimethylgermane (0.01 M). The flask was filled to the mark with a
solution of N-methylquinolinium hexafluorophosphate (1.0 mM) in
CH2Cl2. Using a volumetric pipette, 3 mL of this solution was
transferred to a stopcocked cuvette and gently purged for 15 min with
dioxygen that was passed through a bubbler containing a frit
immersed in CH2Cl2. The cuvette was then photolyzed on the
nanosecond transient absorption system. The resulting transient UV−
vis spectrum was collected at least 100 ns after the laser pulse; the
time required to ensure that NMQ• is scavenged by O2.
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Hexafluoropropan-2-ol as a Solvent for the Generation of Highly
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Method for Determining Oxidation Potentials in Organic Media. J.
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Poly(arylmethylgermanes) by Demethanative Coupling: A Mild
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.joc.0c01032.
NMR spectra (1H and 13C); representative kinetic plots
1
for bimolecular rate constants; and representative H
NMR spectra from preparative photo-oxidations (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Joseph P. Dinnocenzo − Department of Chemistry, University
of Rochester, Rochester, New York 14627, United States;
orcid.org/0000-0003-0206-3497; Email: jpd@
chem.rochester.edu
Author
Adam M. Feinberg − Department of Chemistry, University of
Rochester, Rochester, New York 14627, United States;
orcid.org/0000-0001-5551-3725
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.joc.0c01032
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The NSF (CHE-1762365) is gratefully acknowledged for
financial support.
■
REFERENCES
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