SN2 and E2 Branching of Main-Group-Metal Alkyl Intermediates in Alkane CH Oxidation: Mechanistic Investigation Using Isotopically Labeled Main-Group-Metal Alkyls

Article abstract of DOI:10.1021/acs.organomet.0c00120

The main-group-metal alkyl compounds trialkyltin and dialkylthallium have been utilized to investigate the mechanism of functionalization of monoalkyl thallium and lead species, proposed to be putative intermediates in alkane (RH) functionalization, formed via CH activation of alkanes (methane, ethane, and propane) using electrophilic Tl(III) and Pb(IV) in trifluoroacetic acid (HTFA). Two different organometallic transalkylation methods were used to generate the putative intermediates in situ. The results herein strongly support a mechanism of CH activation to generate a main-group-metal alkyl intermediate which undergoes reductive functionalization to generate the products, R-TFA, and the reduced metal salt. In the case of ethane there are two products, ethyl trifluoroacetate (EtTFA) and 1,2-bis(trifluoroacetoxy)ethylene glycol (EG(TFA)₂), observed in the reaction mixture that are proposed to form in parallel from a common intermediate, EtTl(TFA)₂. The alkyl transfer studies herein strongly support the simultaneous formation of both species from this intermediate. Furthermore, studies conducted using regiospecifically isotopically labeled diethylthallium salts strongly support an S_N2 functionalization from EtTl(TFA)₂ to give EtTFA (and reduced Tl(TFA)) and an E2 elimination (also from EtTl(TFA)₂) to generate ethylene, which instantly reacts with an additional 1 equiv of Tl(TFA)₃ to generate EG(TFA)₂.

Full text of DOI:10.1021/acs.organomet.0c00120

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Article
SN2 and E2 Branching of Main-Group-Metal Alkyl Intermediates in
Alkane CH Oxidation: Mechanistic Investigation Using Isotopically
Labeled Main-Group-Metal Alkyls
Niles Jensen Gunsalus, Anjaneyulu Koppaka, Brian G. Hashiguchi, Michael M. Konnick, Sae Hume Park,
Daniel H. Ess, and Roy A. Periana*
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ABSTRACT: The main-group-metal alkyl compounds trialkyltin
and dialkylthallium have been utilized to investigate the
mechanism of functionalization of monoalkyl thallium and lead
species, proposed to be putative intermediates in alkane (RH)
functionalization, formed via CH activation of alkanes (methane,
ethane, and propane) using electrophilic Tl(III) and Pb(IV) in
trifluoroacetic acid (HTFA). Two different organometallic trans-
alkylation methods were used to generate the putative
intermediates in situ. The results herein strongly support a
mechanism of CH activation to generate a main-group-metal
alkyl intermediate which undergoes reductive functionalization to
generate the products, R-TFA, and the reduced metal salt. In the
case of ethane there are two products, ethyl trifluoroacetate (EtTFA) and 1,2-bis(trifluoroacetoxy)ethylene glycol (EG(TFA)2),
observed in the reaction mixture that are proposed to form in parallel from a common intermediate, EtTl(TFA)2. The alkyl transfer
studies herein strongly support the simultaneous formation of both species from this intermediate. Furthermore, studies conducted
using regiospecifically isotopically labeled diethylthallium salts strongly support an SN2 functionalization from EtTl(TFA)2 to give
EtTFA (and reduced Tl(TFA)) and an E2 elimination (also from EtTl(TFA)2) to generate ethylene, which instantly reacts with an
additional 1 equiv of Tl(TFA)3 to generate EG(TFA)2.
One approach to this challenge is electrophilic CH
activation (CHA) (Scheme 1), whereby soft, highly polar-
izable, electrophilic metal centers (MnX2) are utilized that can
favorably interact with the very low energy electrons in the
HOMO of alkanes, leading to heterolytic cleavage of the C−H
bond and formation of a metal alkyl intermediate (R−Mn−X).
Subsequent reductive functionalization (MRF) from this
intermediate results in the desired product (R−X) and a
reduced form of the metal electrophile (Mn−2). Using this
strategy, systems have been developed based on Hg,4,5 Pt,6−8
Pd,9,10 Ir,11,12 Sb,13 Ag, and Au14 as well as other electrophilic
metal centers.2
INTRODUCTION
■
Selective conversion of light alkanes (methane, ethane,
propane) to functionalized products remains one of the
longstanding, unsolved challenges in chemistry. This is an
important problem because of the vast, underutilized reserves
of natural gas (mostly methane with some ethane and
propane) present within the US and worldwide.1 Technology
for the selective conversion of natural gas could lead to the
augmentation or replacement of oil as the primary feedstock
for chemicals, materials, and fuels. The selective conversion of
alkanes (particularly methane) has proven to be particularly
challenging, given the lack of functional groups on alkanes and
the intrinsically lower reactivity of the CH bonds relative to the
bonds in the products. Indeed, by almost every quantitative
measure of reactivity, alkanes (i.e., methane) are less reactive
than their desired products (i.e., methanol), making the
prevention of product overoxidation (i.e., to CO2) challeng-
ing.2 This is why current, commercially available technologies
for the conversion of natural gas to chemicals and fuels utilize
multistep, indirect, high-temperature (>800 °C) processes.3 A
process that could convert natural gas at lower temperatures
(∼200 °C), selectively and directly, to chemicals and fuels
would be simpler and more economical (vide infra).
We recently reported,15 to our knowledge, the first main-
group systems, Tl(TFA)3 and Pb(TFA)4, that selectively and
rapidly functionalize all of the alkanes in natural gas (methane,
Received: February 19, 2020
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Scheme 1. Two-Step Mechanism for the Functionalization
of Unactivated Alkanes (R−H) to Functionalized Products
a
(R−X)
a
Step one involves CHA of R−H by an electrophilic metal species,
MnX2, resulting in cleavage of the C−H bond and formation of a
metal alkyl, R−Mn−X. Step two involves reductive functionalization
of R−Mn−X (MRF) to give the oxidized product, R−X, and the
reduced metal species, Mn−2
.
ethane, and propane) to the corresponding mono- and diol
esters in a non-superacid medium, HTFA (F3CC(O)OH) at
180 °C (Figures 1 and 2). On the basis of initial studies with
1
Figure 2. Overlay of H NMR spectra of crude reaction mixtures of
Tl(TFA)3 with (A) methane, (B) ethane, (C) propane, and (D) a
100/8/1 methane/ethane/propane mixture.15
support a CH activation, M−R functionalization mechanism
and offer a fundamental understanding of the basis for the
parallel formation of the mono- and diol esters from the higher
alkanes (i.e., ethane and propane) with these main-group
oxidants.
The reaction of ethane with Tl(TFA)3 in HTFA generated
only the mono- and difunctionalized oxygenated products,
ethanol trifluoroacetate ester (EtTFA) and ethylene glycol
trifluoroacetate diester (EG(TFA)2), respectively.15 No other
organic products such as ethylene or transient alkyl−Tl
intermediates are observed. Consistent with rate-determining
C−H cleavage reactions, studies with CD3CH3 showed a
normal H/D kinetic isotope effect of 3.4/1 for the formation of
EtTFA. The reactions of ethane in DTFA solvent showed no D
incorporation in ethane (d-ethane), suggesting irreversible CH
activation. Our expectation was that the products of this
reaction would be generated from the sequential reactions
ethane → EtTFA → EG(TFA)2. Surprisingly, however, time
course studies showed that EtTFA and EG(TFA)2 were
generated in parallel at a constant ∼2:1 ratio throughout the
course of the reaction: i.e., the selectivity was independent of
the concentrations of both ethane and Tl(TFA)3 (which was
the limiting reagent and was consumed as the reaction
procdeded). Control studies with 13CH3CH2TFA further
confirmed that EtTFA is stable and not converted to ethane,
ethylene, or EG(TFA)2 under the reaction conditions.
EG(TFA)2 is similarly stable. Consistent with the lack of
observation of ethylene, control experiments showed that
addition of ethylene to solutions of Tl(TFA)3 at 180 °C
generated EG(TFA)2, essentially on mixing, without observa-
tion of any Tl−R intermediates. Thus, considering the separate
reaction in DTFA, the selectivity of the reaction of ethane with
Tl(TFA)3 in DFTA to the three expected products d-ethane,
EtTFA, and EG(TFA)2 is ∼0:2:1 within experimental error.15
The observations discussed above for the reaction between
ethane and Tl(TFA)3 led us to propose the overall mechanism
for the reaction shown in Scheme 2. As can be seen, the three
products are proposed to result from three fundamentally
different branching reactions from the putative intermediate,
EtTl(TFA)2, generated from CH activation of ethane. Thus,
Figure 1. Main-group-metal complex mediated, stoichiometric CH
activation to generate metal alkyl intermediates that functionalize to
give oxidized products. With higher alkanes (ethane and propane),
four-electron-oxidation products are also observed in addition to the
expected two-electron-oxidation products. Because the corresponding
alcohol products are esterified with HTFA and the CH activation
mechanism is electrophilic, the products are effectively protected from
undergoing subsequent oxidations with the main-group oxidants. A
mixture of alkanes (as in natural gas) can also be functionalized
selectively (see Figure 2).
methane, the reaction was proposed to proceed by
stoichiometric C−H activation and M−R functionalization
pathways (Figure 1). We were particularly intrigued by parallel
and selective formation of both the mono- and diol esters from
ethane and propane (Figures 1 and 2). Previous work with
higher alkanes would predict16 poorer selectivity for the higher
alkanes relative to methane due to the increased number of C−
H bonds in the substrates and products and the relatively harsh
reaction conditions. The surprisingly high efficiency, selectiv-
ity, and parallel formation of multiple products in reactions of
Tl (or Pb) with ethane (or propane) provided an excellent
rationale for mechanistic study to understand the basis for this
high selectivity. The work herein describes the detailed
experimental studies of the independently generated putative
M−Et intermediate predicted from the electrophilic CH
activation of ethane with Mn+ electrophile. The data further
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monoalkylthallium(III) compounds in the literature and the
high reactivity attributed to these types of species.17,18 In order
to examine the chemistry of this intermediate in more detail,
we focused on in situ generation by independent syntheses.
The literature shows that alkyl transfers between the same, as
well as different, main-group elements are facile, selective,
predictable, and well-defined. Thus, dialkyl HgR2 complexes
have been shown to readily react with HgX2 salts to selectively
and reproducibly generate the expected monoalkyl species,
RHgX, with well-defined kinetics.19−22 Similar observations
Scheme 2. Proposed Mechanism for the Functionalization
of Ethane with Tl(TFA)3
a
have been made with alkyltin species23−25 RnSnX4−n
,
a
CH activation generates a metal alkyl, EtTl(TFA)2 (k1). Function-
alization occurs either via SN2-type attack by a TFA anion to give
EtTFA (k2) or E2-type elimination with a TFA anion to generate
ethylene (k3), which further reacts with Tl(TFA)3 to give EG(TFA)2
(k4 and k5). Controls demonstrate that EtTFA, once formed, does not
convert to EG(TFA)2 in the presence or absence of Tl(TFA)3 under
the reaction conditions (k6). Formation of the metal alkyl
intermediate via k1 is irreversible, meaning that protonolysis (the
microscopic reverse of CH activation) of EtTl(TFA)2 to generate
ethane does not occur (k−1).
trialkyllead species20,21 R3PbX, and dialkylthallium species17
R2TlX. We hypothesized that we could independently generate
EtTl(TFA)2 in situ by similar alkyl transfer reactions. Scheme 3
a
Scheme 3. Routes to Access EtTl(TFA)2
when the reaction is run in DTFA, d-ethane would generally be
expected to form from k−1 (the microscopic reverse of C−H
activation). However, as no d-ethane was observed, if the
reaction proceeds by the proposed mechanism, then formation
of the intermediate, EtTl(TFA)2, via k1 must be irreversible
(i.e., k2 and k3 ≫ k−1). EtTFA is formed via an irreversible
SN2-type functionalization (step k2) and EG(TFA)2 is formed
via an irreversible E2-type functionalization (k3, k4, and k5).
The lack of observation of ethylene or any Tl−R intermediates
(such as 1 in Scheme 2) along with the observed rapid reaction
of ethylene to selectively generate EG(TFA)2 would suggest
that k4 and k5 > k3. Controls show that the subsequent
formation of EG(TFA)2 from EtTFA does not occur at an
appreciable rate under the reaction conditions (k6).15
On the basis of the mechanism proposed in Scheme 2, the
above observations suggest that the selectivity of the reaction
for d-ethane (when reaction is run in DTFA), EtTFA, and
EG(TFA)2 should be independent of the concentration of the
putative intermediate, EtTl(TFA)2, and should be set by the
ratio of the branching rates k−1/k2/k3 for the formation of
ethane, EtTFA, and EG(TFA)2, respectively. This correspond-
ence between the observed ratio of products and the predicted
ratio of rates for the three branching steps provided a good
rationale for the study of this reaction mechanism. The key
hypothesis is that, if the reaction of ethane with Tl(TFA)3
proceeds as shown in Scheme 2, then the stoichiometric
reaction of independently synthesized EtTl(TFA)2 should,
under similar reaction conditions, give the same ratio of
products (ethane/ EtTFA/EG(TFA)2 ≈ 0:2:1) as observed in
the stoichiometric reaction between ethane and Tl(TFA)3 in
DTFA (d-ethane/EtTFA/EG(TFA)2 = 0:2:1).
a
Ethane can react with Tl(TFA)3 via k1 to give the putative
intermediate, or the species can be accessed via organometallic
transalkylation with Et3Sn(TFA) via k7 or with Et2Tl(TFA) via k8. In
any case, functionalization of the formed intermediate should occur as
described above in Scheme 2 regardless of the alkyl source used.
shows the CH activation route for generating the putative
EtTl(TFA)2 intermediate from ethane as well as two
alternative routes for generating the same intermediate in situ
in the absence of ethane.
Alkyl and aryl transfer from organotin (RnSnX4−n) reagents
are well-known, facile, well-defined, selective reactions that
have been extensively utilized in organic and organometallic
synthesis.23 As shown in Scheme 3, we utilized this
methodology to independently generate EtTl(TFA)2 by in
situ alkylation of Tl(TFA)3 with Et3Sn(TFA) (k7). Et3Sn-
(TFA) was generated in situ by addition of Et4Sn to HTFA.26
The resulting solution of Et3Sn(TFA) was stable for a few
hours and only underwent slow protonolysis to generate
Et2Sn(TFA)2). An important advantage of this approach, in
addition to generating EtTl(TFA)2, is the ease of synthesis of a
wide range of organotin (RnSnX4−n) reagents that give ready
access to a wide range of RTl(TFA)2 intermediates for future
study. However, an important disadvantage of this method is
introduction of tin reagents that are not present in the ethane
CH functionalization reaction, which could influence the
course of these studies.
Due to concerns of handling alkyltin and alkyl thallium
compounds at elevated temperatures, the majority of the
studies were carried out at room temperature in sealed, glass
NMR tubes. Controls indicate that the presence or exclusion of
O2 has no effect on these reactions. The addition of
Et3Sn(TFA) to excess Tl(TFA)3 (3.3 equiv relative to the
ethyl groups on Sn) in HTFA at room temperature resulted in
the generation of EtTFA and EG(TFA)2 in an ∼2:1 ratio,
which is the same as that observed in the reaction between
RESULTS AND DISCUSSION
■
The mechanistic studies were carried out in HTFA to ensure
relevance to the reported conditions for reactions with ethane
and to facilitate studies based on 1H NMR spectroscopy. Initial
efforts focused on detection of the putative intermediate,
EtTl(TFA)2, shown in Scheme 2. However, various attempts,
such as carrying out reactions of ethane and Tl(TFA)3 in
HTFA at lower temperatures, in other solvents, and with more
reactive alkanes such as isobutane failed to show any R−TlX2
intermediates. This is consistent with the rarity of
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Tl(TFA)3 and ethane. Furthermore, ethane was not observed
by H NMR, consistent with k2 and k3 (shown in Scheme 2)
being greater than k−1.
ambient pressure in glass NMR tubes to minimize the handling
of Et2Tl(TFA) at elevated temperatures and pressures (the
compound is likely highly toxic). Control experiments indicate
that there is no difference in reactivity in the presence or
absence of O2 and that the selectivity is unaffected by the order
of addition of reagents.
1
Interestingly, the alkyl transfer between Et3Sn(TFA) and
Tl(TFA)3 to generate Et2Sn(TFA)2 and EtTl(TFA)2 (eq 1)
seems to occur upon mixing at room temperature, but the
transfer between Et2Sn(TFA)2 and Tl(TFA)3 seems to be
much slower (eq 2). Thus, despite the presence of excess
Tl(TFA)3, only one of the ethyl groups on Et3Sn(TFA) was
transferred and Et2Sn(TFA)2 remained in the reaction mixture
and was only slowly converted to products, but most of the
organotin was consumed if the reaction mixtures were left to
The reactions were carried out by addition of 0.5 mL of a
homogeneous, 250 mM solution of Tl(TFA)3 to 0.5 mL of a
25 mM solution of Et2Tl(TFA) in HTFA in a J. Young NMR
1
tube. The reaction was monitored by H NMR. On the time
scale of the NMR experiment, all of the Et2Tl(TFA) was
1
consumed and three products were observed by H NMR:
1
sit for extended periods of time. The H NMR spectrum for
EtTFA, EG(TFA)2 and HO-CH2CH2-TFA (Figure 4,
the functionalization reaction between Et3Sn(TFA) and
Tl(TFA)3 at room temperature, after 18 h, is compared to
that for the reaction between Tl(TFA)3 and ethane in Figure 3
(spectra A and B, respectively).
Et3Sn(TFA) + TI(TFA)3
fast
⎯⎯→ Et2Sn(TFA)2 + EtTI(TFA)2
(1)
(2)
RT
Et2Sn(TFA)2 + TI(TFA)3
fast
⎯⎯→ EtSn(TFA)3 + EtTI(TFA)2
RT
1
Figure 4. H NMR spectra of the reaction between Et2Tl(TFA) and
Tl(TFA)3 (A) 30 min after mixing showing the formation of EtTFA,
some EG(TFA)2, and HO-CH2CH2TFA and (B) the same reaction
after the mixture sat overnight, showing that the intermediate species
quantitatively converts to EG(TFA)2. The final ratio of EtTFA to
EG(TFA)2 is 2:1.
spectrum A). This third species, HO-CH2CH2-TFA, was
slowly esterified at room temperature and, after 18 h, was
quantitatively converted to EG(TFA)2 with a final ratio of
EtTFA:EG(TFA)2 of ∼2.1 (Figure 4, spectrum B). In addition
to TFA-CH2CH2-TFA, HO-CH2CH2-TFA is also observed to
form in the reaction of Tl(TFA)3 with ethylene in HTFA. This
35
is because our analyses suggest that the Tl(TFA)3 is a
1
Figure 3. H NMR spectra of (A) the reaction between Et3Sn(TFA)
and Tl(TFA)3 at RT 18 h after mixing and (B) the reaction between
Tl(TFA)3 and ethane at 180 °C. In both cases the ratio of EtTFA to
EG(TFA)2 is ∼2:1.
hydrated species, consistent with Tl(III) having one of the
highest known hydration energies. The synthesis of Tl(TFA)3
requires heating Tl2O3 in the presence of HTFA with up to
20% water by volume, and despite our repeated efforts, we
have no evidence that our attempts to prepare anhydrous
Tl(TFA)3 (such as by recrystallization from trifluoracetic
anhydride) were ever successful. As such, in the reaction
between Tl(TFA)3 and ethylene (or ethane), water is just as
likely (or perhaps more likely) to be the attacking nucleophile
and generates an alcohol (or protonated alcohol) product
which is then esterified (quickly in the case of EtOH and HO-
CH2CH2-OH and more slowly for the second OH in TFA-
CH2CH2-OH). This monoesterified glycol product then slowly
converts to TFA-CH2CH2-TFA in HTFA at room temper-
ature. Alternatively, heating the solution to 100 °C resulted in
very rapid conversion of HO-CH2CH2-TFA to EG(TFA)2. No
ethane was detected in the reaction mixture, and the mass
balance for EtTFA and EG(TFA)2 was >95% on the basis of
the amount of added Et2Tl(TFA). The intermediate species,
HO-CH2CH2TFA, is also observed in the reaction between
ethylene and Tl(TFA)3 in HTFA when the reaction is carried
out at room temperature and slowly converts to EG(TFA)2.
The intermediate is also observed upon addition of ethylene
glycol, HOCH2CH2OH, to trifluoroacetic acid.29
Given that the EtTl(TFA)2 generated via alkyl transfer was
only transiently generated and not physically observed, we
wanted to access it via a separate route. Interestingly,
dialkylthallium species, R2TlX, are well-known and stable
unlike the monoalkyl species, RTlX2.27,28 We postulated that
Et2Tl(TFA) could be used analogously to Et3Sn(TFA) as a
second route to generate EtTl(TFA)2 in situ (Scheme 3, k8).
This route was particularly appealing, as the alkyl transfer from
Et2Tl(TFA) to Tl(TFA)3 would be expected to generate 2
equiv of EtTl(TFA)2 without the generation of any other non-
Tl species that could interfere with the selectivity of the
reaction. Control studies demonstrated that solutions of
Et2Tl(TFA) in HTFA were stable up to 100 °C in the
absence of Tl(TFA)3 and did not generate any products such
as ethane and butane. Interestingly, the temperature was
observed to have little effect on the selectivity for EtTFA and
EG(TFA)2 (from RT to 125 °C; see the Supporting
Information) and so most of the reactions between Tl(TFA)3
and Et2Tl(TFA) were carried out at room temperature and
D
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The observation of an identical intermediate in the reaction
between ethylene and Tl(TFA)3 as well as in the reaction of
Et2Tl(TFA) and Tl(TFA)3 compellingly suggests that free
ethylene is formed in the functionalization reaction of
Et2Tl(TFA), but we wanted to provide further, direct support
for its formation as well as evidence that the functionalization
of Et2Tl(TFA) was proceeding by alkyl transfer to generate 2
equiv of EtTl(TFA)2 and not by a different mechanism such as
outer-sphere electron transfer etc. In order to investigate this,
we decided to make regiospecifically labeled diethylthallium
compounds 2 and 8 (Scheme 4) and investigate their reactivity
Scheme 4. Functionalization of Deuterium-Labeled Diethyl
a
Thallium Compounds 2 and 8
Figure 5. 1H NMR stack plot for the functionalization of
regiospecifically deuterated Et2Tl(TFA) species 2 and 8. (A)
Functionalization of 2 taken after 10 h at room temperature. Species
4 is present while 10 is not, showing retention of the deuterium label
in the SN2 functionalization. (B) Functionalization of 2 taken
immediately upon mixing, showing the presence of 5 and 6 in a 1:1
ratio and suggesting the formation of free ethylene. Both 5 and 6 were
seen to quantitatively convert to 7. (C) Functionalization of 8 taken
after 10 h at room temperature. Species 10 is present while 4 is not,
showing retention of the deuterium label in the SN2 functionalization.
(D) Functionalization of 8 taken immediately upon mixing, showing
the presence of 5 and 6 in a 1:1 ratio and suggesting formation of free
ethylene. Both 5 and 6 were observed to quantitatively convert to 7.
could be possible on both the SN2 and E2-type pathways.30,31
The SKIEs observed with 2 and 8 are consistent with literature
reports for SN2 and E2 reactions from alkyl halides.31−33 When
these functionalizations are carried out at room temperature,
the intermediate species 5 and 6 are observed immediately
after mixing, in both cases, in an ∼1:1 ratio (Figure 5,
spectrum B for 2 and spectrum D for 8). This is evidence for
the formation of free ethylene, as CH2CD2 generated from
either 2 or 8 should add to Tl(TFA)3 in a 1:1 ratio as the SKIE
for addition would be expected to be small. Esterification from
both 5 and 6 would generate 7, as observed.
a
Alkyl transfer is expected to generate monoethyl thallium
compounds 3 and 9, respectively. As shown, the deuterium label is
expected to be preserved in the SN2 products, 4 and 10. Evidence for
free ethylene, resulting from an E2 mechanism, would be predicted to
result in observation of both 2-hydroxyethyltrifluoroacetate inter-
mediates, 5 and 6, from both 2 and 8.
with Tl(TFA)3. As shown in Scheme 4, transalkylation with 2
or 8 would be expected to generate monoalkylthallium
compound 3 or 9, respectively. SN2 functionalization from 3
would be expected to result in the formation of 4 with the
deuterium label preserved in the product. Likewise, SN2
functionalization from intermediate 9 should result in the
formation of 10. Elimination from either intermediate should
lead to the observation of both 5 and 6 due to rapid reaction of
the generated CD2CH2, and these species would be expected
to slowly convert to 7.
Fully consistent with the expected reactions shown in
Scheme 4 (by extension from Schemes 2 and 3) addition of 2
to excess Tl(TFA)3 was found to generate, after 10 h, only 4
and 7 in an ∼3:1 ratio (Figure 5, spectrum A). In contrast,
reactions with 8 lead only to 10 and 7, now in a closer to ∼2:1
ratio (Figure 5, spectrum C) observed in the ethane reactions.
This higher 3:1 ratio with 2 in comparison to the unlabeled
Et2Tl(TFA) and 8 is expected. The E2-type reaction would be
expected to show a primary KIE from 2 where a C−D bond is
cleaved. In contrast, from 8, the E2-type reaction involves
cleavage of a C−H bond and the ratio of 4 to 7 would be
expected to be more similar to that from the putative
intermediate, EtTl(TFA)2. No attempts at highly accurate
quantitative analyses of the difference among 4, 10, and 7 from
2 or 8 were made, as secondary kinetic isotope effects (SKIEs)
Similarly to ethane, propane was also found to generate two
products, isopropyl trifluoroacetate (iPrTFA) and 1,2-propy-
lene glycol bis(trifluoroacetate) (PG(TFA)2) (Table 1, entry
7). No n-propyltrifluoroacetate was ever observed. As such, the
Table 1. Products Obtained from Various Alkyl Sources
a
with Tl(III) and Pb(IV)
entry
oxidant
alkyl source
product (ratio)
b c
,
1
Tl(TFA)3
Tl(TFA)3
Tl(TFA)3
Tl(TFA)3
Tl(TFA)3
Tl(TFA)3
Tl(TFA)3
Tl(TFA)3
Tl(TFA)3
Pb(TFA)4
Pb(TFA)4
Pb(TFA)4
Pb(TFA)4
methane
Me2Tl(TFA)
ethane
Et2Tl(TFA)
Et3Sn(TFA)
Et3Sn(TFA)
propane
iPr2Tl(TFA)
iPr3Sn(TFA)
methane
MeTFA
MeTFA
b
2
b c
,
3
EtTFA and EG(TFA)2 (2:1)
EtTFA and EG(TFA)2 (2:1)
EtTFA and EG(TFA)2 (2:1)
EtTFA and EG(TFA)2 (3:1)
iPrTFA and PG(TFA)2 (3:1)
iPrTFA and PG(TFA)2 (6:1)
iPrTFA and PG(TFA)2 (6:1)
MeTFA
4
5
c
6
b c
,
7
8
9
b c
,
10
11
12
13
b c
,
ethane
Me4Sn
Et3Sn(TFA)
EtTFA and EG(TFA)2 (2:1)
MeTFA
EtTFA and EG(TFA)2 (2.6:1)
a
b
See the Experimental Section for details. See ref 15 for experimental
c
details. Reaction at 180 °C.
E
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proposed mechanism involved C−H activation of a secondary
Tl(III). Similar to the case for Tl(III), Pb(IV) also reacts with
ethylene in HTFA at RT to give EG(TFA)2 (see the
Supporting Information). These similarities in the reactivities
of Pb(IV) and Tl(III) suggest that both systems proceed
through an analogous CH activation and M−R functionaliza-
tion mechanism in alkane oxidation reactions as discussed
above. These observations persuaded us not to carry out any
further regiospecific isotopic labeling studies with the Pb(IV)
system.
i
C−H bond of propane by Tl(TFA)3 to generate the PrTl-
(TFA)2 intermediate. Functionalization of this intermediate by
an SN2 attack would generate iPrTFA. An E2 elimination from
this intermediate would generate propylene, which would be
expected to generate the glycol species, analogously to the
i
reaction with ethylene. Interestingly, unlike EtTFA, PrTFA
was found to be unstable under the reaction conditions used in
the reaction between Tl(TFA)3 and propane. It was observed
to slowly convert to PG(TFA)2, as shown in eq 3.
CONCLUSIONS
■
The main-group electrophilic metal cations Tl(III) and Pb(IV)
selectively oxidize mixtures of methane, ethane, and propane to
alcohol esters.15 Experimental and theoretical studies15 taken
together strongly support the proposed mechanism for alkane
functionalization involving slow, irreversible electrophilic CH
activation of alkanes, R−H, with the metal complex MnX2 to
generate an XMn−R intermediate, followed by fast M−R
i
The observed ratio of the two products PrTFA to
PG(TFA)2 under the actual reaction conditions was found to
be ∼3:1 (Table 1, entry 7). Although there was found to be
reductive functionalization to generate RX and Mn−2
.
i
some conversion of PrTFA to PG(TFA)2, we still wanted to
Interestingly, in the reaction with ethane and propane two
products have been observed to form simultaneously. In this
report we have presented the mechanistic investigations of
isotopically labeled compounds to support the proposed
mechanism using alkylthallium compounds to demonstrate
the product distribution. Facile alkyl transfer studies were used
to generate the Mn−R intermediate (proposed to be forming in
the reaction of alkane with Tl(III)) in situ using two
independent routes. Most importantly, regiospecific labeling
of the R group provided strong evidence for SN2 and E2
pathways to generate EtTFA and ethylene products,
respectively, from the XMn−R intermediate. Ethylene further
undergoes fast reaction with another 1 equiv of MnX2 to give
the final product EG(TFA)2. These mechanistic studies can
also be extended to the reactions involving Pb(IV), as similar
product selectivity/distribution was seen in reactions with
methane, ethane, and propane.
study the functionalization from the putative intermediate,
iPrTl(TFA)2. While the ratio of products would be predicted
to be slightly higher than 3:1 (vide infra), we would not expect
to see other products (i.e., n-propyltrifluoroacetate or 1,3-
propylene glycol bis(trifluoroacetate)) and observation of
these types of products would suggest an alternative reaction
mechanism. As such, we thought that this would be a good
system to apply our alkyl transfer methodology to study the
functionalization behavior of the proposed intermediate
(Scheme 5).
Scheme 5. Functionalization of (iPr)2Tl(TFA) with
a
Tl(TFA)3
More recently we have also shown that Sb(V)13 and
hypervalent I(III)34 functionalize methane and ethane to
alcohol esters in HTFA medium. These were also shown to
proceed via a CH activation/MR functionalization mechanism.
However, in the case of Sb(V), the EG(TFA)2 product was
shown to form not via a parallel E2 pathway from the Et−SbV
intermediate but from the reaction of EtTFA with Sb(V) to
generate ethylene. In the case of oxidation reaction of alkanes
involving I(III), although SN2 and E2 branching was observed,
the SN2 pathway was predominant.
In general, CH oxidation reactions of higher alkanes with
electrophilic main-group-metal oxidants seem to be adopting a
similar mechanism (Scheme 1) involving CH activation to
generate the XMn−R intermediate, which then undergoes
functionalization to generate the RX product. However, the
relative rates of the parallel SN2 and E2 reactions of the XMn−
R intermediate that lead to difunctionalized products can vary
substantially with the Mn+ electrophiles.
a
Alkyl transfer generates iPrTl(TFA)2, which functionalizes via SN2 to
generate iPrTFA or via E2 to generate propylene. Propylene
subsequently reacts with excess Tl(TFA)3 to generate PG(TFA)2.
No other products, such as n-propyltrifluoroacetate or 1,3-propylene
glycol bis(trifluoroacetate), would be expected.
i
As expected, addition of 10 equiv of Tl(TFA)3 to Pr2Tl-
i
(TFA) gave only Pr-TFA and PG(TFA)2 in a ratio of ∼6:1.
No other products were observed. This provides very strong
evidence for the reaction with propane also proceeding via a
Tl−alkyl intermediate and, more specifically, a Tl−iPr
intermediate resulting from C−H activation of a secondary
C−H bond in propane. In addition to these systems, we have
also used transalkylation techniques to generate the putative
intermediates for the reactions between alkanes and Pb(TFA)4
following an experimental procedure similar to that used in the
case of Tl(III) systems. We found these reactions to be very
similar to reactions involving Tl(III). The results of these
studies are summarized in Table 1 along with the results of
EXPERIMENTAL SECTION
■
All air- and water-sensitive procedures were carried out either in a
MBraun inert-atmosphere glovebox or using standard Schlenk
techniques under argon. All deuterated solvents (Cambridge
Isotopes) were used as received. 13CH4 and 13C2H6 were purchased
from Isotech and used as received. Methane, ethane, and propane
gases were purchased from Praxair and used as received. Tl(TFA)3,35
Pb(TFA)4,36 and Me2TlCl28 were synthesized according to previously
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published procedures. Methyl 2,2,2-trifluoroacetate (MeTFA), ethyl
2,2,2-trifluoroacetate (EtTFA), isopropyl trifluoroacetate (iPrTFA),
1-13C-ethanol, ethylene glycol, and 1,2-propylene glycol were
purchased (Aldrich or Oakwood Chemicals) and used as received.
CH313CH2TFA, ethylene glycol bis(trifluoroactate) (EG(TFA)2), and
1,2-propanediol bis(trifluoroacetate) (PG(TFA)2) were synthesized
via addition of the corresponding alcohol to neat trifuoroacetic
anhydride (TFA2O) followed by evaporation of the solvent. Et2TlCl
was synthesized according to the procedure reported by Gilman and
Jones.37 Triethyltin trifluoroacetate, Et3Sn(TFA), was prepared by
addition of a known amount of tetraethyltin to trifluoroacetic acid, as
has been previously described, and used in situ.26 Ethyl iodide-1,1-d2
(H3C-CD2I) and isopropyl iodide were purchased from Sigma-
Aldrich and used as received. Ethyl iodide-2,2,2-d3 (D3C-CH2I) was
purchased from Cambridge Isotopes and used as received.
Tetraethylstannane (Et4Sn), tetraisopropylstannane (i-Pr4Sn), and
tetrapropylstannane (n-Pr4Sn) were purchased from Sigma-Aldrich
and used as received. Lithium reagents were purchased from Sigma-
Aldrich and were all titrated by reported methods, prior to use.38
NMR spectra were obtained on a Bruker Digital Avance III 400
temperature and stirred overnight. After it was stirred overnight, the
solution was cooled to 0 °C in an ice bath and 1 M HCl was added
slowly until the pH of the reaction mixture was ∼1−2. The
precipitated (D3C-CH2)2TlCl was collected in a frit, the filter cake
was washed first with three portions of 1 M HCl (25 mL each) and
then three portions of deionized water (25 mL each). Air was pulled
over the filter cake until it was sufficiently dry, and the product was
1
collected and further dried in vacuo. The yield was 1.16 g (65%). H
NMR (400 MHz, DMSO-d6, δ/ppm): 1.54 (bd, 4H JTl−H = 360.8
Hz). 13C{1H} NMR (400 MHz, DMSO-d6, δ/ppm): 41.7 (bd, JTl−C
=
2483.6 Hz), 12.2 (bs). HRMS (ESI): calcd for C4H4D6Tl+ 269.0903,
found 269.0906. 205Tl NMR (400 MHz, DMSO-d6, δ/ppm): 3506.99
(bs).
Synthesis of (H3C-CD2)2TlCl. An oven-dried Schlenk flask,
containing a magnetic stir bar, that had been evacuated and backfilled
with argon, was charged with 1.3 mL (2.5 g, 15.8 mmol) of H3C-CD2I
and 30 mL of dry diethyl ether. With stirring, this solution was cooled
to −78 °C in a dry ice/acetone bath and 14.9 mL (30.9 mmol, 1.95
equiv) of a 2.07 M stock solution of t-BuLi in heptane was added
dropwise via syringe. Upon complete addition, the solution was
stirred for 5 min at −78 °C and then it was removed from the bath
and stirred at room temperature for an additional 1 h to ensure
complete conversion to H3C-CD2-Li. A separate oven-dried Schlenk
flask was charged with 2 g (6 mmol) of thallous iodide. To this was
added 25 mL of dry diethyl ether and 1.3 mL of H3C-CD2I (15.8
mmol, 2.6 equiv). The solution of alkyllithium reagent was transferred
into a dropping funnel and added slowly over about 45 min to the
rapidly stirred suspension of thallous iodide at −20 °C. Upon
addition, the reaction mixture was warmed to room temperature and
stirred overnight. After the mixture was stirred overnight, the solution
was cooled to 0 °C in an ice bath and 1 M HCl was added slowly until
the pH of the reaction mixture was ∼1−2. The precipitated (H3C-
CD2)2TlCl was collected in a frit, and the filter cake was washed first
with three portions of 1 M HCl (25 mL each) and then three portions
of deionized water (25 mL each). Air was pulled over the filter cake
until it was sufficiently dry, and the product was collected and further
1
(400.132 MHz for H, 100.623 MHz for 13C, and 230.908 for 205Tl)
1
spectrometer equipped with a BBOF probe. All H NMR chemical
shifts are reported in units of ppm and referenced to residual proton
signals in deuterated solvent (for compound characterization) or an
internal reference (dichloromethane or dimethyl sulfone) in the
solvent. 13C NMR spectra were referenced to 13C signals for the
solvent and 205Tl NMR spectra were recorded using thallous nitrate in
D2O as a calibration standard (referenced at 0 ppm).39 Gas
measurements were analyzed with a Shimadzu GC-MS QP2010S
instrument equipped with an Agilent HP-Plot Q column. All high-
resolution mass spectra were obtained by the University of Illinois,
Department of Chemistry, Mass Spectrometer Services on an ESI
mass spectrometer. Elemental analysis was performed by Atlantic
Microlab, Inc. of Norcross, GA. See the Supporting Information for
the NMR spectra of the new compounds and reaction mixtures
mentioned herein.
1
Synthesis of Et2Tl(TFA). An amberized vial with a Teflon cap and
a magnetic stir bar was charged with 1 g (3.36 mmol) of Et2TlCl and
560 mg (3.36 mmol, 1 equiv) of silver trifluoroacetate. Approximately
20 mL of methanol was added, and the suspension was stirred
overnight. The reaction mixture was passed through a plug of Celite
to get rid of precipitated silver chloride, and the solvent was removed
in vacuo. The crude reaction mixture was then taken into
dichloromethane containing 10% methanol, and it was passed
through a small plug of alumina with dichloromethane containing
10% methanol as eluent. The solvent was removed in vacuo to give the
dried in vacuo. The yield was 1.16 g (64%). H NMR (400 MHz,
DMSO-d6, δ/ppm): 1.49 (bd, 6H, JTl−H = 619.8 Hz). 13C{1H} NMR
(400 MHz, DMSO-d6, δ/ppm): 41.5 (bd, JTl−C = 2460.0 Hz), 13.0
(bd, JTl−C = 114.6 Hz). HRMS (ESI): calcd for C4H6D4Tl+ 267.0778,
found 267.0778. 205Tl NMR (400 MHz, DMSO-d6, δ/ppm) 3531.22
(bm).
i
Synthesis of Pr2TlCl. The procedure was analogous to the
preparation of Et2TlCl, except that isopropyl iodide and isopropyl-
lithium were used in place of ethyl iodide and ethyllithium,
respectively. The yield was 685 mg (35%). 1H NMR (400 MHz,
CD3OD, δ/ppm): 2.32 (bd, 2H, JTl−H = 261.2 Hz), 1.65 (bd, 12H,
1
title compound in approximately 95% yield. H NMR (400 MHz,
CD3OD, δ/ppm): 1.84 (bd, 4H, JTl−H = 376.9 Hz), 1.53 (bd, 6H,
JTl−H = 623.4 Hz). 13C{1H} NMR (400 MHz, CD3OD, δ/ppm):
163.1 (q, JC−F = 34.5 Hz), 118.4 (q, JC−F = 293.3 Hz), 40.7 (bd, JTl−C
= 2232.1 Hz), 11.1 (bd, JTl−C = 144.2 Hz). 205Tl NMR (400 MHz,
CD3OD, δ/ppm): 3290.52 (m). HRMS (ESI): calcd for C4H10Tl+
261.0506, found 261.0518. Anal. Calcd for C6H10F3O2Tl: C, 19.19;
H, 2.68; N, 0.00. Found: C, 19.56; H, 2.66; N, 0.00.
Synthesis of (D3C-CH2)2TlCl. An oven-dried Schlenk flask,
containing a magnetic stir bar, that had been evacuated and backfilled
with argon was charged with 1.2 mL (2.38 g, 14.9 mmol) of D3C-
CH2I and 30 mL of dry diethyl ether. With stirring, this solution was
cooled to −78 °C in a dry ice/acetone bath and 17 mL (28.31 mmol,
1.9 equiv) of a 1.66 M stock solution of t-BuLi in pentane was added
dropwise via syringe. Upon complete addition, the solution was
stirred for 5 min at −78 °C, and then it was removed from the bath
and stirred at room temperature for an additional 1 h to ensure
complete conversion to D3C-CH2-Li. A separate oven-dried Schlenk
flask was charged with 2 g (6 mmol) of thallous iodide. To this was
added 25 mL of dry diethyl ether and 1.2 mL of D3C-CH2I (14.9
mmol, 2.5 equiv). The solution of the alkyllithium reagent was
transferred into a dropping funnel and added slowly over about 45
min to the rapidly stirred suspension of thallous iodide at −20 °C.
Upon addition, the reaction mixture was warmed to room
J
Tl−H = 581.7 Hz). 13C{1H} NMR (400 MHz, CD3OD, δ/ppm): 60.1
(bd, JTl−C = 1868.0 Hz), 22.84 (bs). 205Tl NMR (400 MHz, CD3OD,
δ/ppm): 3307.45 (bs).
Synthesis of (D3C-CH2)2Tl(TFA) (2). A procedure analogous to
the synthesis of Et2Tl(TFA) was used, except that (D3C-CH2)2TlCl
was used in place of Et2TlCl. 1H NMR (400 MHz, CD3OD, δ/ppm):
1.78 (bd, 4H, JTl−H = 364.1 Hz). 13C{1H} NMR (400 MHz, CD3OD,
δ/ppm): 163.2 (q, JC−F = 34.6 Hz), 118.4 (q, JC−F = 293.3 Hz), 40.7
(bd, JTl−C = 2232.1 Hz), 11.1 (bd, JTl−C = 144.2 Hz). 19F{1H} NMR
(400 MHz, CD3OD, δ/ppm): −76.91. 205Tl NMR (400 MHz,
CD3OD, δ/ppm): 3291.37 (bs). HRMS (ESI): calcd for C4H4D6Tl+
269.0898, found 269.0929.
Synthesis of (H3C-CD2)2Tl(TFA) (8). A procedure analogous to
the synthesis of Et2Tl(TFA) was used, except that (H3C-CD2)2TlCl
was used in place of Et2TlCl. 1H NMR (400 MHz, CD3OD, δ/ppm):
1.52 (bd, 6H, JTl−H = 628.6 Hz). 13C{1H} NMR (400 MHz, CD3OD,
δ/ppm): 163.2 (q, JC−F = 34.6 Hz), 118.4 (q, JC−F = 292.7 Hz), 40.3
(bd, JTl−C = 2233.9 Hz), 11.9 (bd, JTl−C = 148.9 Hz). 19F{1H} NMR
(400 MHz, CD3OD, δ/ppm): −76.90. 205Tl NMR (400 MHz,
CD3OD, δ/ppm): 3294.85 (m). HRMS (ESI): calcd for C4H6D4Tl+
267.0772, found 267.0777.
Synthesis of iPr2Tl(TFA). A procedure analogous to the synthesis
of Et2Tl(TFA) was used, except that i-Pr2TlCl was used in place of
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1
Et2TlCl. H NMR (400 MHz, CD3OD, δ/ppm): 2.36 (bd, 2H, JTl−H
= 274.3 Hz), 1.61 (bd, 12H, JTl−H = 572.3 Hz). 13C{1H} NMR (400
(C2H4) was utilized as the gas. 1H NMR analysis indicated
quantitative generation of EG(TFA)2 post reaction (250 mM).
Room-Temperature Oxidation of Ethylene Mediated by
Tl(TFA)3. A glass-lined, stainless steel, high-pressure reactor was
charged with 2 mL of a 250 mM solution of Tl(TFA)3 in HTFA. The
reactor was sealed and pressure degassed five times with 500 psig of
argon. It was subsequently charged with 500 psig of ethylene with
stirring and sealed. The reaction mixture was stirred at room
temperature for 20 min, at which point the pressure was released. The
reactor was opened, and 0.2 mL of a 250 mM solution of methylene
chloride in HTFA was added as an internal standard. The reaction
MHz, CD3OD, δ/ppm): 163.3 (q, JC−F = 34.9 Hz), 118.6 (q, JC−F
=
292.7 Hz), 59.7 (bd, JTl−C = 1857.2 Hz), 22.3 (bs). 19F{1H} NMR
(400 MHz, CD3OD, δ/ppm): −76.88. 205Tl NMR (400 MHz,
CD3OD, δ/ppm): 3125.96 (m). HRMS (ESI): calcd for C6H14Tl+
291.0834, found 291.0842. Anal. Calcd for C8H14F3O2Tl: C, 23.81;
H, 3.50; N, 0.00. Found: C, 24.13; H, 3.72; N, 0.00.
General Protocol for Metal− Alkyl Functionalization
Studies. Unless otherwise mentioned here, in a typical study, a J.
Young NMR tube (containing a C6D6 coaxial for locking and
shimming) was charged, in an open atmosphere, with 0.5 mL of a 25
mM solution of metal alkyl in HTFA (see Table 1) and an equimolar
amount of methylene chloride or dimethyl sulfone (as an internal
standard). A T0 1H NMR of the reaction mixture was taken.
Subsequently, 0.5 mL of a 250 mM solution of oxidant (Tl(TFA)3 or
Pb(TFA)4) was added (10 equiv based on metal alkyl) and the J.
1
mixture was analyzed by H NMR using a C6D6 coaxial for locking
and shimming. The results revealed a species analogous to that seen in
the functionalization of Et2Tl(TFA), and this species was observed to
quantitatively collapse to EG(TFA)2.
Room-Temperature Oxidation of Ethylene Mediated by
Pb(TFA)4. A procedure analogous to that one above was used, with
the exception of using Pb(TFA)4 in place of Tl(TFA)3. An analogous
species that quantitatively collapsed to EG(TFA)2 was observed in
this case as well.
1
Young tube was resealed. Upon initial mixing, a H NMR spectrum
was taken. Product formation was monitored as a function of time.
Reported product distributions are an average of three functionaliza-
tion studies. See Table 1 for the results. The metal alkyls used in these
studies are all presumed to be highly toxic. It is an absolute necessity
to avoid exposure in all circumstances. Proper PPE should be worn at
all times and proper sanitation procedures followed carefully.
Et2Tl(TFA) Functionalization Oxygen Dependence Study.
This study was carried out analogously to the functionalization study
of Et2Tl(TFA) with Tl(TFA)3, except that all solutions were degassed
by a series of three freeze−pump−thaw cycles prior to use. Schlenk
techniques were used to charge all reagents to a J. Young tube that
was under argon to ensure that the environment was oxygen-free. It
was found that the functionalization reaction did not display a
dependence on O2.
General Considerations for High-Temperature Functional-
ization Studies. The metal alkyls used in all of these studies are
highly toxic, and extreme care should be taken to avoid contact in all
circumstances. With that said, extra precautions must be taken when
performing chemistry with these molecules when the solvent
temperature is substantially higher than its boiling point. The use of
proper PPE is absolutely a requirement, and it is necessary to use
equipment that can handle the high pressures associated with boiling
solvents in a sealed vessel. As such, a specialized reactor had to be
designed to conduct these high-temperature functionalization studies.
Functionalization of Et2Tl(TFA) at 75 °C. A 0.5 mL portion of a
20 mM stock solution of Et2Tl(TFA) in HTFA was placed in a
microwave vial containing a stir bar and an additional 0.5 mL of
HTFA. The vial was sealed and heated to 75 °C with stirring. When
the temperature was reached, 0.2 mL of a 1 M stock solution of
Tl(TFA)3 in HTFA was added. The reaction mixture was stirred for
15 min at 75 °C and then removed from the heat and cooled to RT. A
0.1 mL portion of a 300 mM solution of AcOH in HTFA was added
as an internal standard. Approximately 0.3 mL was transferred to an
NMR tube, and the reaction mixture was analyzed by 1H NMR using
a benzene-d6 coaxial for locking and shimming. The ratio of EtTFA to
EG(TFA)2 was found to be 2.1:1.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00120.
NMR spectral data of new compounds and crude
reaction mixtures (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Roy A. Periana − The Scripps Energy and Materials Center,
Department of Chemistry, The Scripps Research Institute,
Jupiter, Florida 33458, United States; orcid.org/0000-
0001-7838-257X; Email: rperiana@scripps.edu
Authors
Niles Jensen Gunsalus − The Scripps Energy and Materials
Center, Department of Chemistry, The Scripps Research
Institute, Jupiter, Florida 33458, United States; orcid.org/
0000-0003-3219-2011
Anjaneyulu Koppaka − The Scripps Energy and Materials
Center, Department of Chemistry, The Scripps Research
Institute, Jupiter, Florida 33458, United States; orcid.org/
0000-0002-8884-2215
Brian G. Hashiguchi − The Scripps Energy and Materials
Center, Department of Chemistry, The Scripps Research
Institute, Jupiter, Florida 33458, United States
Michael M. Konnick − The Scripps Energy and Materials
Center, Department of Chemistry, The Scripps Research
Institute, Jupiter, Florida 33458, United States
Sae Hume Park − The Scripps Energy and Materials Center,
Department of Chemistry, The Scripps Research Institute,
Jupiter, Florida 33458, United States
Functionalization of Et2Tl(TFA) at 100 °C. A 0.5 mL portion of
a 20 mM stock solution of Et2Tl(TFA) in HTFA was added to a
microwave vial containing a stir bar and an additional 0.5 mL of
HTFA. The vial was sealed and heated to 100 °C with stirring. When
the temperature was reached, 0.2 mL of a 1 M stock solution of
Tl(TFA)3 in HTFA was added. The reaction mixture was stirred for
15 min at 100 °C and then removed from the heat and cooled to RT.
A 0.1 mL portion of a 300 mM solution of AcOH in HTFA was
added as an internal standard. Approximately 0.3 mL was transferred
Daniel H. Ess − Department of Chemistry and Biochemistry,
Brigham Young University (BYU), Provo, Utah 84602, United
States; orcid.org/0000-0001-5689-9762
1
to an NMR tube, and the reaction was analyzed by H NMR using a
benzene-d6 coaxial for locking and shimming. The ratio of EtTFA to
EG(TFA)2 was found to be 2.0:1.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00120
Reaction of Tl(TFA)3 with Ethylene (C2H4). Ethylene reactions
were conducted at 180 °C following a procedure described
previously15, with the only difference being that 500 psig of ethylene
Notes
The authors declare no competing financial interest.
H
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(18) Uemura, S.; Toshimitsu, A.; Okano, M. Preparation of Some
Alkylthallium(III) Dichlorides and Their Reactions with Potassium
and Copper Halides. Bull. Chem. Soc. Jpn. 1976, 49 (10), 2762−2764.
(19) Gardner, H. C.; Kochi, J. K. Charge-Transfer Spectra and
Alkylation of Tetracyanoethylene with Organometallic Derivatives of
Lead, Tin, and Mercury. J. Am. Chem. Soc. 1976, 98 (9), 2460−2469.
(20) Ingold, C. K. Organo-Metal Substitutions. Helv. Chim. Acta
1964, 47 (5), 1191−1203.
ACKNOWLEDGMENTS
■
We thank Hyconix Inc. for support of this work at Scripps
Florida and the opportunity to publish this work. D.H.E.
acknowledges support of this work by the U.S. Department of
Energy, Office of Science, Basic Energy Sciences, Catalysis
Science Program, under Award # DE-SC0018329.
(21) Kochi, J. K. Electron-Transfer Mechanisms for Organometallic
Intermediates in Catalytic Reactions. Acc. Chem. Res. 1974, 7 (10),
351−360.
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