Rhodium-Catalyzed Arene Alkenylation Using only Dioxygen as the Oxidant

Article abstract of DOI:10.1021/acscatal.0c03439

We report the oxidative conversion of unactivated arenes and alkenes to alkenyl arenes using unpurified air or purified dioxygen as the only oxidant. This method uses RhCl3 salt as the catalyst precursor and avoids the use of co-oxidants such as Cu(II). The use of dioxygen as the in situ oxidant gives water as the only byproduct of the alkenylation reaction. Conditions to achieve >1000 turnovers of alkenyl benzene products have been developed. As the catalysis progresses, oxidation of styrene product to form benzaldehyde becomes competitive. Compared to the Rh catalysis using Cu(II) oxidants, the aerobic reactions give decreased reaction rate and reduced anti-Markovnikov/Markovnikov selectivity when using α-olefins. For styrene formation, the reaction rate shows a first-order dependence on catalyst concentration, ethylene concentration (with saturation at higher ethylene concentrations), and dioxygen. An intermolecular kinetic isotope effect value of 2.7(6) was determined from parallel reactions with C6H6 versus C6D6. Synthesis of trans-stilbene and pentenyltoluenes has been demonstrated using this Rh-catalyzed aerobic alkenylation reaction.

Full text of DOI:10.1021/acscatal.0c03439

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Rhodium-Catalyzed Arene Alkenylation Using Only Dioxygen as the
Oxidant
Weihao Zhu and T. Brent Gunnoe*
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ABSTRACT: We report the oxidative conversion of unactivated arenes and
alkenes to alkenyl arenes using unpurified air or purified dioxygen as the only
oxidant. This method uses RhCl3 salt as the catalyst precursor and avoids the
use of co-oxidants such as Cu(II). The use of dioxygen as the in situ oxidant
gives water as the only byproduct of the alkenylation reaction. Conditions to
achieve >1000 turnovers of alkenyl benzene products have been developed.
As the catalysis progresses, oxidation of styrene product to form
benzaldehyde becomes competitive. Compared to the Rh catalysis using
Cu(II) oxidants, the aerobic reactions give decreased reaction rate and
reduced anti-Markovnikov/Markovnikov selectivity when using α-olefins. For styrene formation, the reaction rate shows a first-order
dependence on catalyst concentration, ethylene concentration (with saturation at higher ethylene concentrations), and dioxygen. An
intermolecular kinetic isotope effect value of 2.7(6) was determined from parallel reactions with C6H6 versus C6D6. Synthesis of
trans-stilbene and pentenyltoluenes has been demonstrated using this Rh-catalyzed aerobic alkenylation reaction.
KEYWORDS: C−H activation, rhodium, aerobic, styrene, arene alkenylation
Markovnikov products (and, in some cases, selectivity for
Markovnikov products), limited catalyst longevity, and an
inability to selectively produce alkenyl arenes. The recent Ni
catalyst reported by Hartwig and co-workers gives exception-
ally high selectivity for anti-Markovnikov alkyl arenes (>50:1 in
most cases) when using α-olefins.16
INTRODUCTION
■
Alkyl and alkenyl arenes are produced on a large scale for use
in the manufacture of commodity and fine chemicals.1−4 For
example, in 2018, the global consumption of ethylbenzene was
∼40 million tons with styrene production from ethylbenzene
on a scale of ∼38 million tons.5−8 Commercial synthesis of
alkyl arenes, which are often precursors to alkenyl arenes,
usually involves either acid-catalyzed Friedel−Crafts or zeolite-
mediated arene alkylation.1−7 Both of these processes involve
acidic catalysis, which can result in the generation of
polyalkylated arene products.9 As a result, energy-intensive
distillation and a transalkylation process are often required to
increase the yield of monoalkylated products.5−7 Additionally,
when using α-olefins, acid catalysts do not produce 1-aryl
alkanes (i.e., anti-Markovnikov products) since the formation
of carbocationic intermediates from the olefin leads to the
exclusive formation of n-aryl alkanes (n > 1; i.e., Markovnikov
products). Palladium-catalyzed aryl-carbon coupling reactions
can produce alkyl or alkenyl arenes using aryl halides or pseudo
halides, but they require arene halogenation and often a
stoichiometric amount of metal-containing reagent.10−15
Olefin hydroarylation with unfunctionalized hydrocarbons
catalyzed by molecular Ni,16,17 Ir,18−23 Ru,24−31 or Pt32−40
complexes has been demonstrated for arene alkylation using
olefins. In contrast to acid-catalyzed arene alkylation, these
transition metal-catalyzed olefin hydroarylation reactions
generate both 1-aryl and n-aryl alkanes (n > 1) when using
α-olefins such as propene and 1-hexene. However, these
catalysts have drawbacks including low selectivity for anti-
Late transition-metal catalysts have been developed to
convert unactivated arenes and olefins to alkenyl arenes
using hydrocarbons in the presence of an oxidant (Scheme
1A). Examples of Pd- and Rh-catalyzed arene alkenylation
under anaerobic conditions that require stoichiometric
consumption of oxidants such as (NH4)2S2O8,41 AgOAc,42
44
V2O5,43 PhCO3 Bu, tBuOOH,45 or copper(II) carboxy-
lates46−50 have been reported. In addition, we previously
reported that a cationic Ru(II) complex [(MeOTTM)Ru(P-
(OCH2)3CEt)(NCMe)Ph][BAr′4] (MeOTTM = 4,4′,4″-
(methoxymethanetriyl)-tris(1-benzyl-1H-1,2,3-triazole; BAr′4
= tetrakis[3,5-bis(trifluoromethyl)phenyl]borate)) catalyzes
the conversion of ethylene and benzene with 53 turnovers
(TOs) of styrene with ethylene serving as an oxidant to form
ethane (Scheme 1B).51 Hong and Yamazaki reported that
Rh4(CO)12 catalyzes the conversion of benzene and ethylene
t
Received: August 6, 2020
Revised: September 6, 2020
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oxidant in the presence of air-recyclable copper(II) pivalate co-
oxidant. Unfortunately, the limited stability of the Cu(II)
carboxylate co-oxidant gives rise to the formation of side
products (e.g., phenyl carboxylate) that result from its thermal
degradation.
Scheme 1. Arene Alkenylation Consuming Stoichiometric
Oxidants (A), Ethylene (B), and Dioxygen in the Presence
(C) or Absence (D) of Co-Oxidants
Pd- and Ru-catalyzed arene alkenylation using electron-
deficient acrylates as the alkene source with dioxygen as the
direct oxidant was reported (Schemes 1D and2).67−70 Only a
Scheme 2. Metal-Catalyzed Arene Alkenylation Using Only
Air or Dioxygen as Oxidant in the Absence of Co-oxidants
in the presence of carbon monoxide to give 472 TOs of styrene
and 809 TOs of 3-pentanone.52 Recently, we demonstrated the
Rh-catalyzed production of alkenyl arenes from benzene and
olefins using [Rh(μ-OAc)(η2-C2H4)2]2 as the catalyst
precursor with high TOs (>800 TOs of styrene and >1470
TOs of propenylbenzene) and near-quantitative yield (>95%)
relative to the Cu(II) oxidant.53−55
While the above listed reactions are important advance-
ments, the use of dioxygen as the in situ and sole oxidant offers
potential advantages since dioxygen is inexpensive (especially
when the source is unpurified air) and the only waste is water.
However, direct use of dioxygen presents challenges including
the following: (1) the radical nature of dioxygen can lead to
undesirable odd-electron reactions; (2) dioxygen can react
with low-valent metals to generate metal oxo or peroxo
products that are incapable of catalysis; and (3) mechanisms
for the reaction of dioxygen to sequester reducing equivalents,
such as hydrogen in this case, are not well understood.
few examples exist for aerobic styrene production from
benzene and ethylene in the absence of a co-oxidant (Scheme
2C). Shue demonstrated that Pd(OAc)2 can catalyze benzene
ethylation under oxygen to yield six TOs of styrene at 80 °C.71
Matsumoto, Periana, and co-workers reported that Pd and Rh
catalysts with the addition of acetylacetone afford 47 and 23
TOs of styrene, respectively.56,57 Milstein and co-workers
reported a Ru(II) catalyst that yields 19 TOs of styrene under
dioxygen and carbon monoxide.72 Despite the significant
advancement of these catalytic processes, they give low TOs
and require high pressure of pure dioxygen and/or the use of
carbon monoxide. Herein, we demonstrate a co-oxidant free
aerobic benzene alkenylation with ethylene or propylene using
unpurified air (and purified dioxygen) as the only oxidant
(Scheme 2D) along with a discussion of limitations of the
aerobic process compared to the use of Cu(II) as a co-oxidant.
The newly reported aerobic method uses RhCl3 as the catalyst
precursor and does not require additional additives or ligands,
and >1000 TOs of alkenyl benzene products are achieved
under optimized conditions.
The Matsumoto, Periana, and Fujiwara groups studied Pd-
and Rh-catalyzed aerobic benzene ethenylation to form styrene
in which Cu(II) or Ag(I) is present as a co-oxidant and is
regenerated in situ using air or dioxygen (Scheme 1C).56−58
The overall process is akin to the commercialized Wacker−
Hoechst process for ethylene oxidation in which the CuCl2
oxidant is regenerated either in situ by dioxygen or ex situ in a
separate reactor by air.4,59 Subsequently, examples of aerobic
benzene alkenylation using benzoquinone60 or polyoxometa-
lates61−63 as the co-oxidant were reported. We reported
conditions for the formation of styrene from benzene and
ethylene catalyzed by Pd(OAc)2 using copper(II) pivalate
[Cu(OPiv)2] in the presence of purified dioxygen to yield
∼2400 TOs of styrene with >85% selectivity.64 The keys to
increase selectivity for styrene are applying low ethylene
pressure (20 psig) to reduce the production of undesired vinyl
pivalate and high reaction temperature (180 °C) to facilitate
the conversion of vinyl pivalate and benzene to styrene.64 Also,
we recently demonstrated that under optimized conditions, (5-
FP)Rh(TFA)(η2-C2H4) (TFA = trifluoroacetate, 5-FP = 1,2-
bis(N-7-azaindolyl)benzene) can catalyze aerobic benzene
alkenylation in the presence of Cu(OPiv)2 with air as the in
situ oxidant to produce >13,000 TOs of propenylbenzene
products with no evidence of catalyst deactivation.65 We
developed a Rh-catalyzed aerobic alkenylation reaction for
synthesis of stilbene derivatives from simple arenes and
styrenes in good to high yields with good functional-group
tolerance.66 The reaction proceeds with air as a terminal
RESULTS AND DISCUSSION
■
We initially investigated RhCl3 as a catalyst precursor for the
aerobic oxidation of benzene ethenylation (Table 1). For all
catalytic reactions shown in Table 1, the TOs are the average
of a minimum of three experiments. Heating a 10 mL benzene
solution of RhCl3 (0.112 mM) under 70 psig of ethylene with
1 atm of air at 150 °C reveals no styrene production after 60 h
(Table 1, entry1). Under these conditions, the formation of a
black precipitate, presumed to be elemental Rh,73 is observed.
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Table 1. Optimization of the Rhodium-Catalyzed Aerobic
a
Benzene Alkenylation Using air as an oxidant
volume of benzene
b
entry
(mL)
additives
TOs
1
2
3
4
5
6
7
8
10
10
10
10
10
10
10
10
10
10
10
5
0
2000 equiv HOAc
2000 equiv HOPiv
2000 equiv acetic anhydride
2000 equiv HTFA
2000 equiv NaOAc
2000 equiv KOAc
2000 equiv Na2CO3
2000 equiv Na2SO4
2000 equiv NaCl
2000 equiv KCl
5 mL HOAc
8 mL HOAc
2 mL HOAc
5 mL HOAc
5 mL HOAc
5 mL HOAc + 2000 equiv
NaOAc
5 mL HOAc + 2000 equiv
KOAc
5 mL HOAc + 50 equiv
acetylacetone
70(8)
40(6)
46(5)
0
10(4)
8(3)
11(3)
3(1)
0
0
Figure 1. Comparison of TOs vs time for aerobic benzene
alkenylation with ethylene in presence of acids. Reaction conditions:
10 mL of benzene, 0.112 mM RhCl3, 70 psig ethylene, 1 atm air, and
150 °C. Equivalents of acid or acetic anhydride are relative to RhCl3.
Each data point represents the average of three separate experiments.
Error bars represent the standard deviations based on a minimum of
three independent experiments.
c
c
c
c
9
c
10
11
12
13
14
15
16
17
c
165(17)
186(21)
159(29)
153(22)
deprotonation (CMD) mechanism that is facilitated by the
presence of Rh−OCOR groups.74−78 Increasing the loading of
acid by using benzene/acetic acid in a 1:1 (v/v) mixture
increases catalytic rate (Figure 2) with 165(17) TOs of styrene
2
8
5
5
d
e
0
5
84(19)
96(25)
134(8)
18
19
5
5
20
21
5
5
5 mL HOAc + 0.05 mL H2O
5 mL HOAc + 1 mL H2O
170(18)
151(35)
a
Unless otherwise noted, reaction conditions are 0.112 mM RhCl3, 1
atm air, 70 psig C2H4, 60 h, and 150 °C. The reactions were sampled
every 12 h until 60 h. Each data point represents the average of three
separate experiments. Values in parentheses represent the standard
deviations based on a minimum of three independent experiments.
b
c
d
TOs are obtained by GC−FID. TO is obtained at 96 h. [Rh(μ-
e
OAc)(η2-C2H4)2]2 (0.112 mM Rh) used. Without Rh source.
Figure 2. Comparison of TOs vs time for aerobic benzene
alkenylation using different volume ratios of benzene and acetic
acid. Reaction conditions: 10 mL of a benzene/acetic acid solution,
0.112 mM RhCl3, 70 psig ethylene, 1 atm air, and 150 °C. Each data
point represents the average of three separate experiments. Error bars
represent the standard deviations based on a minimum of three
independent experiments.
However, the addition of 2000 equiv of HOAc (relative to the
Rh) under otherwise identical conditions produces 70(8) TOs
of styrene at a constant rate for 60 h (Figure 1 and Table 1,
entry 2) coupled with formation of trace amounts of phenyl
acetate, vinyl acetate, benzaldehyde, and stilbene (<1 TO for
each product) as well as 5(1) TOs of biphenyl (see Table S1).
We previously reported that in the absence of a Cu(II) oxidant,
the [Rh(μ-OAc)(η2-C2H4)2]2 catalyst precursor decomposes
to Rh(0) in benzene at 90 °C under ethylene pressure.73
Although the details are unknown, we speculate that HOAc
suppresses reductive decomposition of the Rh catalyst to form
Rh(s) by serving as a reactive oxidant. Replacing HOAc with
the same amount of other carboxylic acids afforded less
satisfactory results (Figure 1 and Table 1, entries 3−5).
Substituting the carboxylic acid with 2000 equiv of carboxylate
salts is partially effective, providing less than 12 TOs of styrene
product after 96 h (Table 1, entries 6−9), whereas the use of
NaCl or KCl under the identical conditions revealed no
styrene production after 96 h (Table 1, entries 10 and 11).
These results suggest that a carboxylate group is required for
successful reaction, which we attribute to Rh-mediated
benzene C−H activation via a concerted metalation−
production after 60 h (Table 1, entry 12). For example, the
apparent turnover frequency (TOF) after 12 h for the catalysis
with 2000 equiv of HOAc is 2.5 × 10−3 s−1, while the apparent
TOF at the same time point for the catalysis using a 1:1 (v/v)
mixture of benzene/acetic acid is 9.7 × 10−3 s−1. Small
quantities of the byproducts phenyl acetate [11(2) TOs],
benzaldehyde [22(2) TOs], stilbene [4(2) TOs], and biphenyl
[7(1) TOs] were also detected (Table S1). Changing the
benzene/HOAc ratio (v/v) from 1:1 to 1:3 and 3:1 did not
influence the catalytic performance (Figure 2 and Table 1,
entries 13 and 14). Using the Rh(I) complex [Rh(μ-OAc)(η2-
C2H4)2]2 (0.112 mM Rh) as the catalyst precursor revealed
statistically identical TOs versus time plots compared to RhCl3
catalysis under the same conditions (Table 1, entry 15, see the
Supporting Information, Figure S3). This suggests that the two
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Rh catalyst precursors, [Rh(μ-OAc)(η2-C2H4)2]2 and RhCl3,
are possibly converted into the same catalytically active Rh
species under catalytic conditions. Control reactions in the
absence of rhodium reveal no reaction (Table 1, entry 16).
Using 0.112 mM RhCl3, 1:1 benzene/HOAc (v/v) solution,
70 psig C2H4, and 1 atm of air at 150 °C as a standard set of
conditions, the inclusion of additives such as acetylacetone or
carboxylate salts does not result in an increased catalytic rate or
yield (Figure 3 and Table 1, entries 17−19). Notably, adding
Figure 4. Effect of reaction temperature on aerobic benzene
alkenylation reaction. Reaction conditions: 5 mL C6H6, 5 mL
HOAc, 0.112 mM RhCl3, and 1 atm air. Each data point represents
the average of three separate experiments. Error bars represent the
standard deviations based on a minimum of three independent
experiments.
Figure 3. Comparison of TOs vs time for aerobic benzene
alkenylation with ethylene in the absence or presence of additives.
Reaction conditions: 5 mL C6H6, 5 mL HOAc, 0.112 mM RhCl3, 70
psig ethylene, 1 atm air, and 150 °C. Each data point represents the
average of three separate experiments. Equivalents of additives are
relative to RhCl3. Error bars represent the standard deviations based
on a minimum of three independent experiments.
0.05 or 1.00 mL of distilled water has negligible impact on the
rate of styrene production (Figure S5 and Table 1, entries 20
and 21). These results suggest that the catalytic reaction is not
significantly influenced by the formation of water as a
byproduct.
The effect of temperature on catalysis was investigated under
the standard set of conditions (Figure 4). Generally, catalyst
activity is increased by raising the temperature from 120 to 170
°C. However, further increasing the temperature to 180 °C
(ethylene pressure was decreased to 50 psig due to the
pressure limit of the Fisher−Porter reactor) led to reduced
catalyst efficacy [39(9) TOs at 180 °C vs 226(31) TOs at 150
°C after 36 h]. This likely results from low solubility of
ethylene in benzene at high temperature. Thus, the optimal
temperature appears to be 170 °C.
Under the standard set of conditions, we probed aerobic
benzene alkenylation with propylene at 150 and 170 °C
(Figure 5). The catalyst is more active at 170 °C than at 150
°C, showing a similar trend to that observed in aerobic
catalysis with ethylene. The catalyst is selective for anti-
Markovnikov (herein, called linear) 1-phenylpropylene prod-
ucts over Markovnikov (branched) 2-phenylpropylene prod-
Figure 5. Plots of TOs of total products vs time (A) and L/B ratio vs
time (B) for aerobic benzene alkenylation with propylene at 150 and
170 °C. Reaction conditions: 5 mL C6H6, 5 mL HOAc, 0.112 mM
RhCl3, 25 psig propylene, 1 atm air, and 170 °C. Each data point
represents the average of three separate experiments. Error bars
represent the standard deviations based on a minimum of three
independent experiments.
ucts, indicating a non-acid-catalyzed process. The linear/
branched product (L/B) ratio increases with reaction time, and
the rate of increase is more rapid at 170 °C than at 150 °C. We
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speculated that the increase of the L/B ratio results from
different rates of reaction of the four phenylpropylene products
instead of the change of intrinsic catalyst L/B selectivity. To
test this hypothesis, we added the four phenylpropylene
products (the concentrations are approximately those of
phenylpropylene products after 24 h of catalysis at 170 °C)
at the beginning of a catalytic reaction using benzene and
ethylene to produce styrene. As catalytic styrene production
proceeds, the L/B ratio increased from an initial 5.5:1 to
8.3(3):1 after 12 h and was determined to be 35(3):1 after 36
h (Figure 6). This trend is similar to the observations in
We speculate that the observed increase in L/B ratio is
controlled by the different rates of reaction of the four
phenylpropylene products with H2O2 (kobs1 and kobs2 in
Scheme 3). If kobs1 is smaller than kobs2, α-methylstyrene will
Scheme 3. Explanation for the Increase of Observed L/B
Ratio during the Aerobic Benzene Alkenylation with
Propylene Based on Product Reaction with Hydrogen
Peroxide
be consumed more rapidly than the linear products once H2O2
is formed. In that case, the unreacted phenylpropylene
products give rise to a higher observed L/B ratio than the
intrinsic catalyst L/B selectivity, and the observed L/B ratio
will increase over time.
When the catalyst loading is reduced to 0.022 mM RhCl3,
277(25) TOs of styrene can be reached accompanied by
93(12) TOs of benzaldehyde after 72 h under the standard set
of conditions (Figures 7 and S6). Continued reaction for an
Figure 6. Plots of L/B ratio of four phenylpropylene products vs time
under catalytic conditions. Reaction conditions: 5 mL C6H6, 5 mL
HOAc, 0.112 mM RhCl3, 70 psig ethylene, 1 atm air, 23 equiv of
allylbenzene, 9 equiv of α-methylstyrene, 3 equiv of cis-β-
methylstyrene, 23 equiv of trans-β-methylstyrene, and 170 °C. Each
data point represents the average of three separate experiments. Error
bars represent the standard deviations based on a minimum of three
independent experiments.
aerobic benzene alkenylation with propylene. Next, we
speculated that the aerobic benzene alkenylation possibly
produces H2O2 as a byproduct.79−81 Thus, we studied the
influence of H2O2 on the rate of phenylpropylene con-
sumption. The L/B ratio increases more rapidly in the
presence of added H2O2 with the L/B ratio increasing from
an initial 5.5:1 to 45(4):1 and then 144(27):1 after 36 h when
100 and 500 equiv of H2O2 (relative to Rh) was added at the
start of the reaction (Figure 6). The isomerization of branched
α-methylstyrene to linear products catalyzed by Rh could
increase the observed L/B ratio. To study this possible
isomerization, we added 50 equiv of α-methylstyrene (relative
to Rh) at the beginning of a catalytic reaction using benzene
and ethylene to produce styrene. After 36 h of reaction, linear
phenylpropylene products were not observed (eq 1), indicating
that the isomerization of branched α-methylstyrene to linear
phenylpropylene products does not occur under the catalytic
conditions.
Figure 7. Comparison of TOs of styrene vs time for aerobic benzene
alkenylation with ethylene using different volume ratios of benzene
and acetic acid. Reaction conditions: 10 mL of a benzene/acetic acid
solution, 0.022 mM RhCl3, 70 psig ethylene, 1 atm air, and 170 °C.
Each data point represents the average of three separate experiments.
Error bars represent the standard deviations based on a minimum of
three independent experiments.
additional 24 h does not increase the TOs of styrene [268(37)
TOs], but higher TOs of benzaldehyde [169(20) TOs] are
observed. Increasing the benzene/HOAc ratio (v/v) from 1:1
to 4:1 and 9:1 increases the production of styrene. Performing
catalysis with a 9:1 benzene/HOAc ratio (v/v) yields
1031(74) TOs of styrene and 353(37) TOs of benzaldehyde
after 192 h (Figures 7 and S6). The formation of 341(45) TOs
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of biphenyl, 72(12) TOs of PhOAc, and 119(28) TOs of
stilbene were also observed (Table S1). Benzaldehyde likely
forms from non-metal-catalyzed oxidative cleavage of styrene
by dioxygen.82 We believe that as the reaction progresses,
under some conditions the benzene alkenylation becomes slow
likely due to catalyst deactivation, whereas benzaldehyde
production from styrene becomes competitive as the amount
of styrene increases.
We performed a series of kinetic studies on three reaction
components (ethylene, Rh catalyst, and dioxygen; Figure 8).
Under the standard set of conditions at 170 °C, a first-order
dependence on ethylene concentration was revealed under
ethylene pressure ranging from 25 to 50 psig (Figure 8A), and
saturation kinetics were observed when the catalysis was run at
ethylene pressure >50 psig (Figure 9). This is in contrast to the
previously reported inverse first-order dependence on ethylene
concentration for Ir(III)-,19 Pt(II)-36 or Ru(II)-catalyzed25
ethylene hydroarylation. For the Ir(III), Pt(II), and Ru(II)
catalysts, the inverse dependence on ethylene concentration
resulted from the formation of off-cycle M(CH2CH2Ph)(η2-
C2H4) (M = cis-(acac-O,O)2Ir, TpRu(CO) or [(tbpy)Pt]+,
acac = acetylacetonate, Tp = hydridotris(pyrazolyl)borate, tbpy
= 4,4′-di-tert-butyl-2,2′-bipyridine) resting states, which block
benzene coordination that is required for C−H bond
activation.21,29,40 The rate dependence on Rh concentration
is near first-order using 0.022−1.121 mM of RhCl3 (Figure
8B), which is inconsistent with a catalytic pathway that
proceeds through a binuclear Rh intermediate or transition
state. To study the dependence on dioxygen pressure, we
decreased the reaction temperature to 150 °C, which provides
a direct comparison to previously reported Rh-catalyzed
anaerobic benzene alkenylation using Cu(II) as the oxidant
at 150 °C.83 A first-order dependence on dioxygen pressure
using 1.0−2.5 atm of dioxygen was observed (Figure 8C). This
suggests that the reaction of a Rh intermediate with the
dioxygen oxidant is a kinetically relevant step. Performing
aerobic catalysis in C6H6 and C6D6 independently in separate
reactors shows a primary kinetic isotope effect (KIE) of 2.7(6)
(Scheme 4), which is consistent with the reported values for
Rh-mediated C−H activation reactions.83−85 The KIE result
indicates that benzene C−H bond activation likely occurs
before or during the rate-limiting step. Catalysis using
CD3CO2D reveals a statistically identical rate to the catalysis
using CH3CO2H within 12 h (Figure S11), suggesting that the
proton from acetic acid is not involved in kinetically relevant
steps.
Based on the experimental results, a plausible catalytic cycle
for Rh-catalyzed aerobic styrene production is illustrated in
Scheme 5. First, a Rh acetate species is generated through the
reaction of a RhCl3 precursor with HOAc, followed by benzene
C−H activation via a CMD mechanism pathway using an
acetate ligand. Ethylene coordination and migratory insertion
into the Rh−Ph bond forms a Rh−CH2CH2Ph intermediate,
which undergoes β-hydride elimination to provide styrene and
a Rh−H intermediate. Next, dioxygen reacts with the Rh−H
intermediate, most likely to form a Rh−OOH complex.
Mechanisms for the insertion of dioxygen into late metal-
hydrides, such as Pd,87−94 Ir,95−99 Rh,79−81,100,101 and Pt,102,103
to generate a metal-hydroperoxo species have been demon-
strated. Protonation of Rh−OOH regenerates the catalytically
active Rh−OAc intermediate and releases hydrogen peroxide.
More detailed studies are needed to probe the oxidation state
of the Rh catalyst and characterize possible catalytic
Figure 8. Log−log plots of (A) observed rate constants (kobs) vs
concentration of C2H4. Reaction conditions: 5 mL C6H6, 5 mL
HOAc, 0.112 mM RhCl3, 1 atm air, and 170 °C. (B) kobs vs
concentration of RhCl3. Reaction conditions: 5 mL C6H6, 5 mL
HOAc, 70 psig C2H4, 1 atm air, and 170 °C. (C) kobs vs pressure of
O2. Reaction conditions: 5 mL C6H6, 5 mL HOAc, 70 psig C2H4,
0.112 mM RhCl3, and 150 °C. Each reaction was run in triplicate. The
reactions were sampled every 4 h until 12 h. Each trial produces
styrene at a constant rate within 12 h (Figures S7−S9). Observed rate
constants (kobs) were extracted from the initial rate regime (from 0 to
4 h) of styrene TOs vs time plots (Figures S7−S9). The concentration
of ethylene dissolved in benzene under various ethylene pressures at
170 °C was determined according to a reported method.86 Each data
point represents the average of three separate experiments. Error bars
represent the standard deviations based on a minimum of three
independent experiments.
intermediates, although under the oxidizing conditions, it
seems likely that the Rh(III) oxidation state is maintained.
Under anaerobic conditions for Rh-catalyzed styrene
production, catalysis using Cu(II) as an oxidant shows a
first-order dependence on ethylene concentration over a
concentration ranging from 79 to 237 mM and a near zero-
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likely a Rh−H, with Cu(II) is kinetically irrelevant. This also
explains why the rate of catalysis is independent of the
concentration of Cu(II). Similar to catalysis using Cu(II)
oxidant under anaerobic conditions, Rh catalysis under aerobic
conditions without Cu(II) shows a first-order dependence on
ethylene concentration under ethylene pressure below 50 psig
and primary KIE when comparing reaction rate using of C6H6
and C6D6. However, a first-order dependence on the
concentration of dioxygen is observed, which is in contrast
with a near zero-order dependence on the concentration of
Cu(II) oxidant under anaerobic conditions. These observa-
tions indicate that benzene C−H activation and the reaction of
a Rh intermediate with dioxygen and with ethylene are all
kinetically relevant steps. The kinetic studies are in agreement
with our proposed mechanism (Scheme 6) in which C−H
Figure 9. Plot of kobs vs [C2H4]. Reaction conditions: 5 mL C6H6, 5
mL HOAc, 0.112 mM RhCl3, 1 atm air, and 170 °C. Each data point
represents the average of three separate experiments. Error bars
represent the standard deviations based on a minimum of three
independent experiments.
Scheme 6. Comparison of Mechanistic Pathway (Top) and
Reaction Coordinate (Bottom) of Rh-Catalyzed Benzene
Ethylation under Aerobic and Anaerobic Conditions
Scheme 4. Kinetic Isotope Experiment Using Two Parallel
a
Reactions with C6H6 vs C6D6
a
Each reaction was run in triplicate. The reactions were sampled every
4 h until 12 h. Each trial produces styrene at a constant rate within 12
h (Figure S10). kH and kD values were determined using average initial
rates (from 0 to 4 h) of three separate experiments. The reported
error represents the propagated standard deviation of all values.
Scheme 5. Proposed Mechanism for Aerobic Styrene
Production Using RhCl3 as a Catalyst Precursor in the
Presence of HOAc
activation and olefin insertion occur before the rate-
determining step, which is proposed to be the reaction of
Rh−H with O2. The different kinetic dependence on the
oxidant under aerobic conditions using O2 and anaerobic
conditions using Cu(II) are consistent with the possibility of a
slower reaction of a Rh−H intermediate with dioxygen than
the analogous reaction using Cu(II) under anaerobic
conditions. This could lead to a shift of rate-limiting steps
from olefin insertion (under anaerobic conditions)83 to net
dioxygen insertion (regardless of specific mechanism) into a
Rh−H bond after olefin insertion (under aerobic conditions)
(Scheme 6). Assuming that changing the oxidant from Cu(II)
to O2 does not affect the rate of C−H activation, olefin
insertion, and β-hydride elimination (which is likely a
simplification), a reduced TOF when using O2 as the sole
oxidant is expected. Consistent with this expectation, at 150 °C
order kinetic dependence on the concentration of Cu(II).83 In
addition, for comparative conversion of C6H6 and C6D6, a
primary KIE value >2.5 was observed. Based on these kinetic
data, for the Cu(II) process we proposed that benzene C−H
activation occurs before the rate-determining olefin insertion
step, whereas the reaction of Rh with Cu(II) likely occurs after
the olefin insertion step. Hence, the C−H bond activation and
the reaction with ethylene are kinetically relevant to the overall
catalytic process, whereas the reaction of a Rh intermediate,
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in the presence of 0.112 mM Rh catalyst, the maximum
apparent TOF (2.1 × 10−3 s−1, calculated based on TO after
12 h) achieved under aerobic conditions using 2.5 atm O2 is
lower than the optimized TOF (2.8 × 10−3 s−1, calculated
when maximum apparent TOF is achieved after the induction
period) under anaerobic conditions using a Cu(OAc)2
oxidant.73
using a sub-stoichiometric amount of Cu(II) carboxylate as a
co-oxidant, which is regenerated by air.66 Using this aerobic
alkenylation protocol, trans-stilbene can be prepared from
benzene and styrene in 43(1)% yield after 60 h (Scheme 8).
Scheme 8. Rh-Catalyzed Aerobic Benzene Alkenylation with
a
Styrene to Produce trans-Stilbene
We recently demonstrated that anaerobic benzene prope-
nylation catalyzed by [Rh(μ-OAc)(η2-C2H4)2]2 produces
phenylpropylene products with ∼8:1 ratio of linear 1-
phenylpropylene products to branched α-methylstyrene when
Cu(OAc)2 or Cu(OPiv)2 is used as the in situ oxidant,54 which
is a higher L/B selectivity than Rh-catalyzed aerobic benzene
propenylation presented herein. Under Curtin−Hammett
conditions, the equilibrium constant between two interconvert-
ing insertion products and the relative rates of product
formation after the olefin insertion step determine the
selectivity between linear and branched products.55 Based on
our hypothesis that the reaction involves the dioxygen oxidant
after the olefin insertion step is the rate-limiting step under
aerobic conditions, we speculate that L/B selectivity is
controlled by Keq in Scheme 7 and the relative ΔG⧧ for the
a
Reaction conditions: 9 mL benzene, 1 mL acetic acid, 0.5 mmol
styrene, 0.560 mM RhCl3, 1 atm air, and 170 °C. Yields, which were
determined by GC, represent the average of three separate
experiments based on the amount of styrene. The standard deviations
given in parentheses are based on three independent experiments.
For this aerobic reaction, 24(1)% of styrene was converted to
benzaldehyde, which limits the yield of the stilbene product.
Also, we have demonstrated Rh-catalyzed alkenylation of
toluene with 1-pentene using Cu(II) oxidant to form meta- and
para-substituted 1-pentenyltoluenes and 2-pentenyltoluenes.48
Under the aerobic alkenylation conditions, 1-pentene and
toluene were transformed into the desired 1-pentenyltoluenes
and 2-pentenyltoluenes. Upon hydrogenation, 108(8) TOs of
pentyltoluenes with L/B ratio (i.e., 1-pentyltoluenes/2-
pentyltoluenes) of 7(1):1 and a meta/para/ortho ratio of
∼15:7:1 (Scheme 9) were obtained.
Scheme 7. Possible Explanation for Different Linear/
Branched Product Selectivity of Aerobic and Anaerobic Rh
Catalysis
Scheme 9. Rh-Catalyzed Aerobic Alkenylation of Toluene
a
with 1-Pentene
a
Reaction conditions: 9 mL toluene, 1 mL acetic acid, 2000 equiv of
1-pentene relative to Rh, 0.112 mM RhCl3, 1 atm air, and 170 °C.
Hydrogenation was achieved using 10% Pd/C under hydrogen
atmosphere. The reaction was run in triplicate. The standard
deviations given in parentheses are based on three independent
experiments. The L/B ratio in the graphic is the ratio of anti-
Markovnikov to Markovnikov products.
SUMMARY AND CONCLUSIONS
■
In conclusion, we have developed a Rh-catalyzed step-
economical aerobic benzene alkenylation reaction that
produces H2O as the sole byproduct from the oxidation step.
Notable hallmarks include the following: (1) the catalysis uses
commercially available RhCl3 salt as catalyst precursor in the
absence of additional additives and ligand, (2) the catalysis can
use unpurified air or oxygen as the sole oxidant and does not
require other co-oxidants, (3) the Rh catalysts can achieve a
TON >1000 under optimized conditions. A first-order
dependence on O2 oxidant was observed, which is indicative
of a kinetically relevant O2 oxidation step. Based on this
hypothesis, the reduced L/B selectivity relative to the
anaerobic Rh catalysis using Cu(II) oxidants is likely a result
of Curtin−Hammett conditions. The aerobic alkenylation
protocol is successfully applied to synthesis of trans-stilbene
from benzene and styrene and conversion of toluene and 1-
rates of subsequent steps (Scheme 7; L/B ratio = k1Keq/k2). In
contrast, under anaerobic conditions, L/B ratio is likely
governed by the kinetic selectivity of rate-limiting olefin
insertion (Scheme 7; L/B ratio = k3/k4). Therefore, the change
in rate-limiting steps is likely responsible for the decreased L/B
selectivity under aerobic conditions.
We explored the versatility of the Rh-catalyzed aerobic
alkenylation by probing substituted olefins and arenes. We
have reported a Rh-catalyzed alkenylation reaction for
synthesis of stilbene derivatives from arenes and styrenes
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pentene to pentenyltoluenes. Targets for future catalyst
improvement include enhancing catalyst reactivity, which
would allow a lower reaction temperature to potentially inhibit
benzaldehyde formation.
cyclooctane as the internal standard. 1-Tolyl-1-pentane
production was quantified using linear regression analysis of
gas chromatograms of standard samples of authentic products.
The slope and correlation coefficient of the regression lines
were 1.34 and 0.998 (1-m-tolyl-1-pentane), 1.85 and 0.997 (1-
p-tolyl-1-pentane), and 1.84 and 0.998 (1-o-tolyl-1-pentane),
respectively. The production of 2-tolyl-2-pentenes was
quantified by using the slope and correlation coefficient for a
fit of cumene versus n-propylbenzene, which enabled an
approximation of the ratio of 1-tolyl-1-pentenes to 2-tolyl-2-
pentenes. The slope and correlation coefficient of the
regression line were 1.24 and 0.98 for cumene versus n-
propylbenzene, respectively.
EXPERIMENTAL SECTION
■
General Considerations. Glovebox purity was maintained
by periodic nitrogen purges and was monitored by an oxygen
analyzer (O2 concentration was <15 ppm for all reactions).
Benzene was dried by passage through columns of activated
alumina. Benzene-d6 was used as received and stored under a
N2 atmosphere over 4 Å molecular sieves. Ethylene, propylene,
hydrogen, and dioxygen were purchased in gas cylinders from
GTS-Welco and used as received. Rhodium trichloride
trihydrate was purchased from Pressure Chemical and used
as received. All other reagents were purchased from
commercial sources and used as received. [Rh(μ-OAc)(η2-
C2H4)2]2 was prepared according to a literature procedure.54
GC−FID was performed using a Shimadzu GC-2014 system
with a 30 m × 90.25 mm HP5 column with 0.25 μm film
thickness. Gas chromatography−mass spectrometry (GC−
MS) was performed using a Shimadzu GCMS-QP2010 Plus
instrument with a 30 m × 0.25 mm SHRXI-5MS column with
a 0.25 μm thickness using electron impact ionization was used.
The mixture of hydrocarbons and air is flammable and
potentially explosive. We advise taking proper safety
precaution (e.g., blast shield, burst valves for pressure relief)
and avoiding any possible ignition source.
Catalytic Alkenylation of Benzene with Ethylene and
Propylene Using RhCl3 as the Catalyst Precursor and
Air or Dioxygen as the Oxidant. A representative catalytic
reaction is described here. Under air, a stock solution
containing rhodium chloride trihydrate (0.0118 g, 0.0449
mmol) and distilled water (2 mL) was prepared in a
volumetric flask. Fisher−Porter reactors were charged with
0.05 mL of stock solution (contains 0.0011 mmol Rh) and
placed under dynamic vacuum at 40 °C for 2 h to remove
water. The Fisher−Porter reactors were then brought into the
glovebox and charged with hexamethylbenzene (0.0182 g,
0.1121 mmol), additives (2000 equiv of relative to per Rh),
and benzene (10 mL) or a benzene (5 mL)/acetic acid (5 mL)
mixture. The vessels were sealed, purged with air or dioxygen,
and pressurized with ethylene (70 psig) or propylene (25
psig). For the reaction under 60, 50, 40, 30, or 20 psig
ethylene, after being charged with air and ethylene, the vessels
were subsequently pressurized with N2 up to 80 psig as the top
pressure. Then the reactors were stirred and heated to 150 or
170 °C. The reaction mixtures without the internal standard
hexamethylbenzene required addition of external hexamethyl-
benzene while sampling. Since the mixture of hydrocarbon gas
and air/O2 is flammable and explosive, it is important to be
aware of explosive limits. The lower and upper explosive limits
(LEL and UEL) of ethylene and propylene in air are 2.7 and
36.0 for ethylene and 2.0 and 11.1 for propylene (concen-
trations in percent by volume), respectively.104 In our reactions
with air, ethylene in the reactors is at a concentration in air
higher than the UEL. Thus, the mixture of hydrocarbon gas
and air under our conditions will be outside the explosive
region. We advise taking proper safety precaution (e.g., blast
shield, burst valves for pressure relief) and avoiding any
possible ignition source in reactions with air or O2. The
reactions were sampled every 12 h until 60 h or every 4 h until
12 h. The resulting product mixture was analyzed by GC−FID.
Catalytic Alkenylation of Benzene with Ethylene and
Propylene Using [Rh(μ-OAc)(η2-C2H4)2]2 as the Catalyst
Precursor and Air or Dioxygen as the Oxidant. In the
glovebox, a stock solution containing [Rh(μ-OAc)(η2-
C2H4)2]2 (0.0111 g, 0.0224 mmol), hexamethylbenzene
(0.3640 g, 2.2430 mmol), and benzene (100 mL) was
prepared in a volumetric flask. Fisher−Porter reactors were
charged with the stock solution (5 mL) and acetic acid (5 mL).
The vessels were sealed, purged with air (1 atm) and
pressurized with ethylene (70 psig), and subsequently stirred
and heated to 150 °C. The mixture of hydrocarbon gas and air
is flammable and explosive; we advise taking proper safety
precaution (e.g., blast shield, burst valves for pressure relief)
and avoiding any possible ignition source. The reactions were
To analyze reaction mixture samples by GC−FID, the
reactors were cooled to room temperature, sampled under N2,
recharged with gases, and reheated. For reaction mixtures
containing the internal standard hexamethylbenzene, aliquots
of the reaction mixture (<100 μL) were sampled, diluted with
benzene (0.25 mL), and washed with saturated sodium
carbonate solution (0.25 mL). The aqueous and organic layers
were separated. The resulting organic layers were analyzed by
GC−FID. The reaction mixtures without the internal standard
hexamethylbenzene required addition of external hexamethyl-
benzene while sampling. A stock solution containing
hexamethylbenzene (0.0364 g, 0.2243 mmol) and benzene
(100 mL) was prepared in a volumetric flask. 100 μL of the
reaction mixture and 0.5 mL of stock solution were measured
by microsyringes and combined together. Aliquots of the
resulting solution (<100 μL) were washed with saturated
sodium carbonate solution (0.25 mL). The aqueous and
organic layers were separated. The resulting organic layers
were analyzed by GC−FID.
Vinyl acetate, styrene, benzaldehyde, phenyl acetate,
biphenyl, trans-stilbene, allylbenzene, α-methylstyrene, cis-β-
methylstyrene, and trans-β-methylstyrene production were
quantified using linear regression analysis of gas chromato-
grams of standard samples of authentic products. A plot of
peak area ratios versus molar ratios gave a regression line using
hexamethylbenzene as the internal standard. The slopes and
correlation coefficients of the regression lines were 5.83 and
0.99 (vinyl acetate), 1.78 and 0.99 (styrene), 2.24 and 0.99
(benzaldehyde), 1.67 and 0.99 (phenyl acetate), 1.07 and 0.99
(biphenyl), 0.87 and 0.99 (trans-stilbene), 1.40 and 0.99
(allylbenzene), 1.23 and 0.99 (α-methylstyrene), 1.47 and 0.99
(cis-β-methylstyrene), and 1.38 and 0.99 (trans-β-methylstyr-
ene), respectively.
To analyze reaction mixture samples by GC−MS, plots of
peak areas versus molar ratios gave regression lines using
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sampled every 12 h until 60 h. The resulting product mixture
was analyzed by GC−FID.
https://pubs.acs.org/10.1021/acscatal.0c03439
Notes
Catalytic Alkenylation of Benzene with Styrene
Using RhCl3 as the Catalyst Precursor and Air as the
Oxidant. Under air, a stock solution containing rhodium(III)
chloride trihydrate (0.059 g, 0.224 mmol) and distilled water
(2 mL) was prepared in a volumetric flask. Fisher−Porter
reactors were charged with 0.05 mL of the stock solution
(0.0056 mmol Rh) and placed under dynamic vacuum at 40
°C for 2 h to remove water. The Fisher−Porter reactors were
then brought into the glovebox and charged with benzene (9
mL), styrene (0.5 mmol), and acetic acid (1 mL). The vessels
were sealed, purged with air, and pressurized with dinitrogen.
The mixtures were stirred and heated to 170 °C. The reactions
were sampled at 60 h. The resulting product mixture was
analyzed by GC−FID.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the U.S. Department of Energy,
Office of Basic Energy Sciences, Chemical Sciences, Geo-
sciences, and Biosciences Division (DE-SC0000776).
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AUTHOR INFORMATION
Corresponding Author
■
T. Brent Gunnoe − Department of Chemistry, University of
Virginia, Charlottesville, Virginia 22904, United States;
orcid.org/0000-0001-5714-3887; Email: tbg7h@
virginia.edu
Author
Weihao Zhu − Department of Chemistry, University of Virginia,
Charlottesville, Virginia 22904, United States
Complete contact information is available at:
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