Catalytic Synthesis of Superlinear Alkenyl Arenes Using a Rh(I) Catalyst Supported by a "capping Arene" Ligand: Access to Aerobic Catalysis

Article abstract of DOI:10.1021/jacs.8b07728

Alkyl and alkenyl arenes are used in a wide range of products. However, the synthesis of 1-phenylalkanes or their alkenyl variants from arenes and alkenes is not accessible with current commercial acid-based catalytic processes. Here, it is reported that an air-stable Rh(I) complex, (5-FP)Rh(TFA)(η²-C₂H₄) (5-FP = 1,2-bis(N-7-azaindolyl)benzene; TFA = trifluoroacetate), serves as a catalyst precursor for the oxidative conversion of arenes and alkenes to alkenyl arenes that are precursors to 1-phenylalkanes upon hydrogenation. It has been demonstrated that coordination of the 5-FP ligand enhances catalyst longevity compared to unligated Rh(I) catalyst precursors, and the 5-FP-ligated catalyst permits in situ recycling of the Cu(II) oxidant using air. The 5-FP ligand provides a Rh catalyst that can maintain activity for arene alkenylation over at least 2 weeks in reactions at 150 °C that involve multiple Cu(II) regeneration steps using air. Conditions to achieve >13 000 catalytic turnovers with an 8:1 linear:branched (L:B) ratio have been demonstrated. In addition, the catalyst is active under aerobic conditions using air as the sole oxidant. At 80 °C, an 18:1 L:B ratio of alkenyl arenes has been observed, but the reaction rate is substantially reduced compared to 150 °C. Quantum mechanics (QM) calculations compare two predicted reaction pathways with the experimental data, showing that an oxidative addition/reductive elimination pathway is energetically favored over a pathway that involves C-H activation by concerted metalation-deprotonation. In addition, our QM computations are consistent with the observed selectivity (11:1) for linear alkenyl arene products.

Full text of DOI:10.1021/jacs.8b07728

Article
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
pubs.acs.org/JACS
Catalytic Synthesis of Superlinear Alkenyl Arenes Using a Rh(I)
Catalyst Supported by a “Capping Arene” Ligand: Access to Aerobic
Catalysis
Junqi Chen, Robert J. Nielsen,* William A. Goddard, III, Bradley A. McKeown,†
†
,‡
‡
Diane A. Dickie,† and T. Brent Gunnoe*
,†
†
Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States
‡
Materials and Process Simulation Center, California Institute of Technology, Pasadena, California 91125, United States
*
S Supporting Information
ABSTRACT: Alkyl and alkenyl arenes are used in a wide
range of products. However, the synthesis of 1-phenylalkanes
or their alkenyl variants from arenes and alkenes is not
accessible with current commercial acid-based catalytic
processes. Here, it is reported that an air-stable Rh(I) complex,
2
(
5-FP)Rh(TFA)(η -C H ) (5-FP = 1,2-bis(N-7-azaindolyl)-
2
4
benzene; TFA = trifluoroacetate), serves as a catalyst precursor
for the oxidative conversion of arenes and alkenes to alkenyl
arenes that are precursors to 1-phenylalkanes upon hydrogenation. It has been demonstrated that coordination of the 5-FP
ligand enhances catalyst longevity compared to unligated Rh(I) catalyst precursors, and the 5-FP-ligated catalyst permits in situ
recycling of the Cu(II) oxidant using air. The 5-FP ligand provides a Rh catalyst that can maintain activity for arene alkenylation
over at least 2 weeks in reactions at 150 °C that involve multiple Cu(II) regeneration steps using air. Conditions to achieve
>
13 000 catalytic turnovers with an 8:1 linear:branched (L:B) ratio have been demonstrated. In addition, the catalyst is active
under aerobic conditions using air as the sole oxidant. At 80 °C, an 18:1 L:B ratio of alkenyl arenes has been observed, but the
reaction rate is substantially reduced compared to 150 °C. Quantum mechanics (QM) calculations compare two predicted
reaction pathways with the experimental data, showing that an oxidative addition/reductive elimination pathway is energetically
favored over a pathway that involves C−H activation by concerted metalation−deprotonation. In addition, our QM
computations are consistent with the observed selectivity (11:1) for linear alkenyl arene products.
INTRODUCTION
mental benefit of more facile biodegradations. Since then,
■
LASs have gradually replaced BASs and have become the main
Billions of pounds of alkyl and alkenyl arenes are produced
each year, and they serve as precursors for plastics, elastomers,
detergents, and pharmaceuticals.
11,12
component in modern detergents.
Currently, LABs are
1
−5
produced from benzene alkylation with terminal olefins using
For example, the global
1
1
acidic catalysts. These acid−based methods generate
production of ethylbenzene is over 20 million tons annually
with approximately 98% of ethylbenzene converted to
carbocationic intermediates that undergo electrophilic aro-
1
,6−9
matic substitution. As a result, the major fractions of LABs are
styrene.
Currently, ethylbenzene is produced from
5
2
- and 3-phenyl alkanes. The production of 1-phenyl alkanes,
benzene alkylation with ethylene either using Friedel−Crafts
or the alkenyl analogs, is not possible with current commercial
catalysis with AlCl and a Brønsted acid or using acidic zeolite
3
13
5,10
acid−based catalysts. To differentiate the 1-phenyl alkanes
catalysts.
These acid-based catalysts often produce poly-
from the predominantly 2-substituted LABs, we have labeled
the former superlinear alkyl benzenes (SLABs, Chart 1) or
superlinear alkenyl benzenes for their unsaturated variants.
Transition metal mediated olefin hydroarylation or oxidative
olefin hydroarylation can circumvent the formation of
carbocationic intermediates and presents a potential alternative
to acid-based catalysis for alkyl or alkenyl arene synthesis.
Transition metal catalyzed olefin hydroarylation reactions
often combine olefin insertion into metal−aryl bonds with
alkylated products, which results in the need for distillation
followed by catalytic transalkylation of the polyalkyl arenes
with arene to optimize the yield of monoalkyl arene product
1
(
for example, processes for ethylbenzene production). Long-
chain alkylbenzenes are converted to alkylbenzenesulfonates,
which are the active component in detergents, through alkyl
benzenesulfonation. In the 1960s, branched alkylbenzenesul-
fonates (BASs), produced by sulfonation of branched alkyl
benzenes (Chart 1), were used widely as raw materials for
domestic detergents. However, their low rate of biodegradation
1
4−28
metal-mediated C−H activation
and offer possible
1
1
led to pollution. Linear alkylbenzenes (LABs) were
introduced in the mid-1960s as a precursor for making linear
alkylbenzenesulfonates (LASs), which provided the environ-
Received: July 21, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.8b07728
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Chart 1. Examples of Branched Alkyl Benzenes (BABs), Linear Alkyl Benzenes (LABs), and Super Linear Alkyl Benzenes
(
SLABs)
Scheme 1. Current Processes for the Synthesis of Styrene, Linear Alkyl Benzenes (LABs) and Reported Processes for the
Synthesis of Alkane Benzenes That Can Be Hydrogenated to Form Superlinear Alkyl Benzenes (SLABs)
Scheme 2. Illustration of Our Concept of Using the “Capping Arene” Ligand 5-FP to Inhibit the Air Oxidation of Rh(I) to
Rh(III)
advantages over acid-based processes including (1) direct
oxidative alkenylation (as opposed to alkylation), (2)
production of 1-aryl alkanes or alkene precursors, and (3)
control over the regioselectivity for alkylation or alkenylation
of substituted arenes. By controlling the regioselectivity of the
olefin insertion step (i.e., 1,2- vs 2,1-insertion), it is possible to
selectively produce 1-phenyl alkanes or their unsaturated
alkenyl variants (Scheme 1). For example, Ru(II) and Ir(III)
catalysts have been reported to convert benzene and α-olefins
such as propylene or 1-hexene to alkyl arenes with ∼1.6:1
L:B selectivity for catalytic benzene alkylation when using
simple α-olefins (e.g., propene, 1-hexene) either favors
branched products or is only modestly selective for linear
products with at best 1.6:1 L:B ratios.
FI
2
Recently, our group reported that ( DAB)Rh(TFA)(η -
FI
C H ) ( DAB = N,N′-bis(pentafluorophenyl)-2,3-dimethyl-
2
4
1,4-diaza-1,3-butadiene; TFA = trifluoroacetate) serves as a
catalyst precursor for the oxidative hydrophenylation of
ethylene to produce styrene using Cu(II) salts as the in situ
3
9,40
oxidant.
We also reported that the simple Rh complex
16,18,29,30
2
linear:branched (L:B) ratios.
In contrast, Pt com-
[Rh(μ-OAc)(η -C H )] is a catalyst precursor to convert α-
2
4
2
plexes have been shown to catalyze olefin hydroarylation
slightly favoring the generation of branched products.
olefins and arenes to alkenyl arenes with high anti-
Markovnikov selectivity (up to a 10:1 L:B ratio). An
31−34
41
However, similar to the Ru(II) and Ir(III) catalysts, one
example of a Pt catalyst that achieves a L:B ratio of 1.6:1 with
advantage of these processes is that the reduced Cu(I) product
can be easily regenerated to Cu(II) upon mixing with air, a
transformation that is well known and has been industrially
3
2
1
-hexene as olefin has been reported.
It has been
42−45
demonstrated that a Ni(II) complex can mediate olefin
hydroarylation with trifluoromethyl-substituted arenes with a
L:B ratio up to 19:1, but these transformations are
used for Pd-catalyzed ethylene oxidation.
However, under
our previously reported conditions, these two Rh catalysts can
only maintain catalytic activity under an inert atmosphere that
avoids air or oxygen. Thus, in situ air regeneration of the
Cu(II) oxidant was not demonstrated. More ideal would be
air-tolerant catalysts for which the Cu(II) oxidant could be
easily regenerated in situ or the Cu(II) could be replaced with
air or oxygen. Yet, accessing air-stable Rh catalysts is a
challenge since the conversion of Rh(I) to Rh(III) peroxide
35
stoichiometric in Ni. In a different approach, 1-aryl alkanes
have been prepared through dehydrogenation of alkyl arenes
36,37
followed by olefin cross-metathesis.
In addition, Kim and
co-workers showed that a Rh complex could generate linear
products using heterofunctionalized 2-phenylpyridine and
3
8
terminal alkenes. Thus, to the best of our knowledge, the
B
DOI: 10.1021/jacs.8b07728
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
46−48
complexes is often facile.
We speculated that issues
regarding oxidative degradation of previously reported Rh
catalysts were a result of this facile aerobic oxidation. Thus, we
sought to design a ligand that would thermodynamically and/
or kinetically suppress the oxidation of Rh(I). Oxidation from
Rh(I) to Rh(III) is generally favored by formation of
octahedral products that provide electronically saturated 18-
electron complexes. We speculated that the ligand 5-FP [5-FP
=
1,2-bis(N-7-azaindoly)benzene], which we have termed a
“capping arene” ligand, would prevent the coordination of a
sixth strongly donating ligand to the metal center upon its
oxidation to Rh(III) and, hence, stabilize Rh(I) against
oxidation by oxygen and permit catalysis under aerobic
2
49−51
Figure 1. OPTEP of (5-FP)Rh(TFA)(η -C H ) (1). Ellipsoids are
2 4
conditions (Scheme 2).
Using this design, we report a
drawn at the 50% probability level, and hydrogen atoms are omitted
new Rh catalyst precursor that achieves efficient oxidative
olefin hydroarylation using air either to recycle Cu(II) oxidants
for clarity. Selected bond lengths (Å): Rh1−C21 2.101(3), Rh1−C22
2
.096(3), C8−Rh1 3.002(3), C13−Rh1 3.008(3), N1−Rh1 2.134(2),
or as the sole oxidant. The new Rh complex, (5-FP)Rh-
N4−Rh1 2.022(2). Selected bond angles (deg): C5−N2−C8
127.0(2), C17−N3−C13 128.4(2), N1−Rh1−N4 86.94(9), C22−
Rh1−C21 38.8(1).
2
(
TFA)(η -C H ) (1), catalyzes the conversion of benzene and
2
4
propylene to linear alkenyl arenes in the presence of oxygen or
using air as the in situ oxidant. Turnovers (TOs) > 13 000 with
an 8:1 L:B ratio of alkenyl arenes have been achieved with no
evidence of catalyst decomposition after 2 weeks at 150 °C. At
important to future efforts to control Rh-based redox
chemistry.
We investigated complex 1 as a catalyst precursor for the
oxidative hydrophenylation of ethylene to probe reactivity and
optimize the reaction conditions. Table 1 summarizes the
results. Heating a 10 mL benzene solution of complex 1 (0.001
mol % relative to benzene) with 50 psig ethylene and
8
0 °C, a L:B ratio of ∼18:1 has been observed. Although the
source of the catalytic enhancement is not experimentally
demonstrated, our studies indicate that this approach has led
to an improved catalytic process.
RESULT AND DISCUSSION
The ligand 1,2-bis(N-7-azaindolyl)benzene was synthesized
according to a published procedure. The Rh(I) complex 1
was synthesized by mixing 5-FP and [Rh(μ-TFA)(η -C H )]
■
Cu(OPiv) (240 equiv relative to 1, OPiv = pivalate) at 150
2
°C affords 114(1) TOs of styrene after 1 h (for all catalytic
reactions the average TOs and standard deviations based on at
least three separate experiments are given). The percent yields
52
2
2
4
2
in tetrahydrofuran (THF). Complex 1 has been characterized
in Table 1 are based on Cu(OPiv) as the limiting regent with
2
by multinuclear NMR spectroscopy, elemental analysis, and a
two equivalents of Cu(II) required per equivalent of styrene.
Trace amounts of phenyl pivalate (PhOPiv) and biphenyl were
observed as side products but were not quantified. Biphenyl
likely forms from a Rh-mediated oxidative benzene coupling
reaction, and the generation of PhOPiv results from a slow
1
3
single-crystal X-ray diffraction study. As anticipated, the
C
1
and H NMR data of 1 are consistent with an asymmetric
complex. For coordinated ethylene, the H NMR spectrum
1
reveals two broad peaks, which is consistent with rapid rotation
53
of ethylene on the time scale of the NMR experiment.
A
background reaction that occurs upon heating Cu(OPiv) to
2
62
small coupling constant (∼1.5 Hz) is observed between Rh
150 °C in benzene. The conversion of benzene, ethylene,
and Cu(OPiv) to styrene using complex 1 can achieve a >90%
1
and all hydrogens on the C6 arene ring of the ligand in the H
2
NMR spectrum, which indicates weak coordination of the
arene moiety to Rh. The structure of complex 1 was confirmed
by single-crystal X-ray diffraction. Figure 1 depicts an ORTEP
of complex 1. The coordinated ethylene is oriented
approximately perpendicular to the Rh square plane, as is
yield with high selectivity for styrene. Under most conditions,
small amounts of trans-stilbene from the hydrophenylation of
styrene were also detected as shown in Table 1.
We investigated the influence of temperature on the reaction
(Table 1, entries 1−4). The reaction time is reduced by raising
the temperature from 120 °C to 180 °C; however, the yield
decreases at 180 °C with an increase in production of trans-
8
2
53−58
typical for four-coordinate and d η -olefin complexes.
The arene ring of the ligand is positioned with Rh−C8 and
Rh−C13 distances of 3.002(3) and 3.008(3) Å, respectively.
As expected, these distances are longer than a typical Rh−C
63
stilbene (0.6 TOs vs 5.2 TOs) and PhOPiv. Under the
conditions studied, the optimal temperature is 150 °C, which
provides a 95% yield of styrene. Higher ethylene pressure can
facilitate the reaction (Table 1, entries 3, 5−7), as indicated by
a reduction in time to achieve a >90% yield from 4 h at 15 psig
of ethylene (entry 5) to 1 h at 70 psig of ethylene (entry 7).
Entries 8−10 in Table 1 demonstrate that the addition of
pivalic acid can benefit the reaction. The reaction rate is
enhanced, and the over 90% yield can still be achieved. For
example, at 50 psig of ethylene at 150 °C without added pivalic
acid, the reaction requires about 1 h to reach >90% yield
(entry 3), but with added pivalic acid the time to achieve a
>90% yield is reduced to 0.5 h (entries 8−10). We suggest that
pivalic acid has two possible benefits. First, acid increases the
5
4,59−61
single bond,
which indicates weak coordination
between Rh and the arene moiety. When compared with a
similar crystal structure reported by our group in which the
50
ligand has a quinoline other than the 7-azaindole group, the
distance between Rh and the benzene ring center is longer for
complex 1 (3.354 vs 3.019 Å, Figure 2). The distance is
measured by calculating a centroid with six carbon atoms of
the benzene ring and then measuring the distance between the
Rh atom and the centroid. Thus, the five-membered nitrogen
heterocycle of the 7-azaindole group likely weakens the Rh−
arene bonding interaction by positioning the arene group
farther from the Rh center than the six-membered ring of the
quinoline-based ligand. This feature seemingly provides an
ability to fine-tune the Rh−arene interaction, which could be
solubility of the Cu(OPiv) in benzene. Second, it is known
2
that Cu(OAc) and water undergo reaction to generate acetic
C
DOI: 10.1021/jacs.8b07728
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
2
50
Figure 2. Structure comparison between (5-FP)Rh(TFA)(η -C H ) (1) and former reported “capping arene” complex.
2
4
2
Table 1. Results for Oxidative Hydrophenylation of Ethylene Using (5-FP)Rh(TFA)(η -C H ) (1) as Catalyst Precursor
2
4
products/TOs
trans-stilbene
a
b
temp/°C
acid /equiv
C H /psig
time/h
styrene
% yield styrene
2
4
1
2
3
4
5
6
7
8
9
1
1
1
1
120
130
150
180
150
150
150
150
150
150
150
150
150
50
50
50
50
15
25
70
50
50
50
50
500
50
16
10
1
0.5
4
2
1
0.5
0.5
0.5
7
109(9)
107(3)
114(1)
83(8)
113(7)
108(3)
109(4)
109(2)
108(1)
108(6)
671(42)
632(5)
96(5)
0.6
0.2
1.3
5.2
0.4
1.0
0.2
1.6
2.0
2.2
124(12)
4
91
89
95
69
94
90
91
91
90
90
56
53
80
240
480
2400
2400
0
1
2
3
c
c
e
,
d
6
40
480
1.4
a
b
c
Acid additive is HOPiv for entries 8−11, HOAc for entry 13. Yields are relative to the limiting reagent Cu(II) oxidant. 2400 equiv of Cu(OPiv)
2
d
e
was used in the reaction. The reaction is carried out in a stainless steel Parr reactor. 240 equiv of Cu(OAc) was used in the reaction.
2
6
4
acid and a Cu oxide. Oxidation of Cu(I) salts and acid
pivalic or acetic acid) to Cu(II) produces water. Thus, the
trations can effectively suppress the formation of “double”
arylation products. However, only a ∼50% yield is observed
with high ethylene pressure. This could be due to the
production of vinyl pivalate at higher ethylene concentrations.
In addition, the reaction in entry 12 used stainless steel
reactors, which gives rise to a difference in heating compared
to the glass Fischer−Porter vessels. The use of Cu(II) acetate
(
Cu(I)/H O reaction becomes important to the longevity of
2
the catalytic process, as the addition of acid (e.g., pivalic or
acetic) can inhibit the water-induced decomposition of
Cu(OAc) or Cu(OPiv) (see below for studies of catalyst
longevity).
Using a large amount of Cu(II) (2400 equiv relative to 1,
entry 11) results in a decreased yield of styrene (∼56%) due to
the formation of trans-stilbene. Importantly, when using 500
psig of ethylene in order to promote the formation of styrene,
the reaction can achieve a >99:1 selectivity for styrene over
trans-stilbene (entry 12). Thus, using high olefin concen-
reduces the yield from ∼90% with Cu(OPiv) to ∼80%
2
(entries 9 and 13, Table 1).
Next, we probed the use of 1 for the oxidative hydro-
phenylation of propylene (Table 2). Heating 10 mL of a
benzene solution of complex 1 (0.001 mol % relative to
benzene) to 150 °C with 30 psig of propylene with Cu(OPiv)2
D
DOI: 10.1021/jacs.8b07728
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Table 2. Catalytic Oxidative Hydrophenylation of Propylene Using 1
a
b
temp/°C
acid /equiv
time/h
products/TOs
L:B ratio
yield/%
1
2
3
4
5
6
7
150
120
100
80
60
150
150
480
480
480
480
480
2400
480
0.5
4
16
144
110
10
86(7)
77(1)
66(3)
38(10)
0
11
13
15
18
N/A
11
9
72
64
55
32
0
c
953(48)
49(2)
79
41
d
24
a
b
c
d
Acid additive is HOPiv for entries 1−6, HOAc for entry 7. Yield is relative to Cu(II). 2400 equiv of Cu(OPiv) was added to the reaction. 240
2
equiv of Cu(OAc) was added to the reaction.
2
2
(
240 equiv relative to 1) and HOPiv (480 equiv relative to 1)
OAc)(η -C H )] were tested to provide a direct comparison
2 4 2
over 0.5 h affords 86 TOs with a L:B ratio of ∼11:1. The L:B
ratio is determined based on the n-propylbenzene to cumene
ratio that would result from hydrogenation of the alkenyl
products (i.e., it is the ratio of allylbenzene and β-
methylstyrenes vs α-methylstyrene; see Table 2). The L:B
ratio of products increases to 18:1 when the temperature is
decreased from 150 °C to 80 °C, but catalyst activity is
decreased (Table 2, entries 1−5). No alkenyl benzene
products were observed after 110 h when the temperature is
of the impact of the capping arene ligand to the [Rh(μ-
2
OAc)(η -C H )] catalyst precursor (Figure 3). Complex 1
2
4
2
6
0 °C.
In order to test the catalyst longevity, a large amount of
Cu(OPiv)2 (2400 equiv relative to 1) was used in the
conversion of benzene and propylene. This resulted in 900
TOs (79% yield) after 10 h. The yield is similar to lower
Cu(II) oxidant loading (Table 2, entries 1 and 6). For the
oxidative hydrophenylation of ethylene to produce styrene
under similar conditions (Table 1, entry 11), only 671 TOs are
produced with >120 TOs of stilbene production. But, for the
oxidative hydrophenylation of propylene, only ∼20 TOs of
Figure 3. Oxidative hydrophenylation of propylene with in situ Cu
1
,1′-(E)-1-propene-1,3-diylbis[benzene] are formed compared
oxidant regeneration. Plot of turnovers versus time, reaction
to the amount of trans-stilbene from the reaction with
ethylene. It may that the steric bulk of propenyl arenes
inhibits a second hydrophenylation reaction compared to the
rate of styrene hydroarylation; however, electronic influences
might also be important.
2
conditions: [Rh] = 0.001 mol % of complex 1 or [Rh(μ-OAc)(η -
C H )] in 10 mL of benzene, 240 equiv of Cu(OPiv) , 480 equiv of
2
4
2
2
HOPiv, 30 psig of propylene. The reactor is refilled with air and 50
psig of N at every sampling point for regenerating Cu oxidant at 120
2
°
C, and then all air is removed and the reactor is pressurized with 30
psig of propylene.
One of the drawbacks of our previously reported Rh
catalysts is that under the reported conditions catalytic activity
39−41
is only maintained under an inert atmosphere.
Since the
“capping arene” ligand was selected to protect the Rh complex
maintains activity through 10 air cycling procedures, achieving
against oxidative degradation or suppression of catalysis in the
presence of air, reaction with in situ aerobic Cu(II)
regeneration was probed with 0.001 mol % complex 1, 30
∼800 TOs, and complex 1 clearly outperforms [Rh(μ-
2
OAc)(η -C H )] in both activity and longevity. As can be
2
4
2
seen in Figure 2, after ∼800 TOs have been reached, catalytic
activity decreases; however, upon addition of pivalic acid and
more benzene, catalyst activity resumes (see Supporting
Information, Figure S4). Thus, the decrease in activity of 1
in Figure 2 is not due to catalyst degradation. Rather, it is likely
due to the equilibrium between Cu(OPiv) and water with
pivalic acid and Cu oxide (see brief discussion above), and the
addition of pivalic acid can shift the equilibrium to permit
facile air recycling to form the Cu(II) oxidant Cu(OPiv)2.
Next, we tested the ability of complex 1 to catalyze benzene
alkenylation under aerobic conditions during catalysis through
psig of propylene, 240 equiv of Cu(OPiv) and 480 equiv of
2
HOPiv at 150 °C. Without regeneration of Cu(II), the
maximum TOs are 120. For these experiments, air was added
to the reactor after consumption of Cu(II) oxidant, and the
reaction mixture was heated at 120 °C for in situ Cu(OPiv)2
regeneration. After regeneration of Cu(II) (indicated by a
color change from bronze to blue; see Supporting
Information), air was removed with a propylene purge, and
the mixture was again heated at 150 °C to continue oxidative
propylene hydrophenylation. Both complex 1 and [Rh(μ-
E
DOI: 10.1021/jacs.8b07728
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
the combination of benzene, propylene, Cu(OPiv) , and
2
HOPiv in the presence of air. That is, rather than recycling
Cu(II) with air and then removing the air, we performed
catalysis in the presence of air. It is noteworthy that these
experiments used unpurified air and not purified oxygen.
Figure 4A shows TOs and L:B ratios versus time for 0.001 mol
%
complex 1, 30 psig of propylene with 1 atm of air, 240 equiv
of Cu(OPiv) , and 2400 equiv of HOPiv at 150 °C. In the
2
absence of air, the maximum TOs are 120. Catalysis continues
over a period of at least 2 days with no evidence of catalyst
deactivation, as shown by the linear TOs versus time plot to
give >1200 TOs after 48 h. The L:B ratio is ∼7 (Figure 4B),
which is reduced compared to catalysis under anaerobic
conditions (L:B ratio is ∼11). As catalysis progresses, the L:B
ratio increased from 6:1 (4 h plot) to 8:1 (48 h plot), but this
is due to the slow reaction of the branched product α-
methylstyrene in the presence of air (see Figure S5). The
observed TOF (turnover frequency, calculated from Table 1,
entry 8, and the first data point in Figure 4A) is decreased from
0
.06 TOs/s for catalysis under anaerobic conditions to 0.01
TOs/s for catalysis under aerobic conditions. When the
catalyst loading is reduced to 0.0001 mol % of 1 relative to
benzene, greater than 13 000 TOs can be reached after 336 h
with no evidence of catalyst deactivation (Figure 4C). The
results suggest that complex 1 is quite stable under aerobic
conditions using unpurified air at 150 °C. Since it has been
reported that Rh and Ru catalysts can produce alkenyl arenes
65−67
using purified oxygen as the oxidant,
complex 1 was
probed using 1 atm of unpurified air as the sole oxidant in the
absence of Cu(II). The catalysis is successful with >500 TOs
after 240 h with a L:B ratio of ∼5 (see Figure S6). Thus,
complex 1 can catalyze oxidative alkenyl arene formation using
only oxygen as the oxidant, but the observed TOF is decreased
−
4
to 6 × 10 TOs/s (calculated with the 240 h time plot), and
the L:B ratio is also decreased.
Increasing the Cu(II) amount (relative to complex 1) for the
oxidative hydrophenylation of ethylene resulted in a decrease
in the yield compared to lower Cu(II) amounts. For example,
the yield drops from 90% to 56% when 2400 equiv of
Cu(OPiv)2 was applied compared with 240 equiv of
Cu(OPiv)2 (Table 1, entries 10 and 11). However, for
oxidative hydrophenylation of propylene a corresponding
decrease in yield is not observed upon increasing the Cu(II)
amount. For example, the yield increased from 72% to 79%
when 2400 equiv of Cu(OPiv) was used compared to 240
2
equiv of Cu(OPiv) (Table 2, entries 1 and 6). Based on these
2
observations, we studied possible product inhibition by styrene
and allylbenzene, separately, for the hydrophenylation of
ethylene and propylene. To do so, we added different amounts
of styrene or allylbenzene at the beginning of the reaction to
determine the influence of the products on the rate of catalysis.
Styrene inhibited the reaction substantially with percent yield
decreasing by slightly more than 2-fold from 95% to 44% after
Figure 4. Hydrophenylation of propylene under aerobic conditions.
(A) Plot of turnovers versus time, reaction conditions: [Rh] = 0.001
1
h when 480 equiv of styrene (relative to 1) was added at the
mol % 1 in 10 mL of benzene, 240 equiv of Cu(OPiv) , 2400 equiv of
start of the reaction (Table 3, entries 1 and 3). Conversely, we
found that addition of pivalic acid mitigates the product
inhibition; the percent yield increased from 44% to 58% after 1
h when 480 equiv of pivalic acid (relative to 1) is added to the
reaction (Table 3, entries 3 and 5). However, the yield still
decreases ∼40% compared with the non-styrene addition trial
2
HOPiv, 30 psig of propylene with 1 atm of air at 150 °C. The reactor
is refilled with air at every sampling point. (B) L:B ratio versus time
plot; reaction conditions are the same as in A. (C) Plot of turnovers
versus time; reaction conditions: 0.0001 mol % Rh, 2400 equiv of
Cu(OPiv) , 48 000 equiv of HOPiv, 1 atm of air, 30 psig of propylene,
2
at 150 °C. Fresh air was purged to the reactor every 24 h, and the
reaction mixture was sampled every 48 h at every sample point. The
data are from four separate experiments with standard deviations
given.
(Table 3, entries 1 and 5). Allylbenzene exhibits less inhibition
than styrene; the percent yield decreased from 72% to 57%
when 480 equiv of allylbenzene is added at the beginning of
F
DOI: 10.1021/jacs.8b07728
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Table 3. Study of Product Inhibition of Rh-Catalyzed
Oxidative Hydrophenylation of Ethylene or Propylene
Using Styrene and Allylbenzene
activation by a concerted metalation−deprotonation (CMD)
mechanism has a DFT-predicted barrier of 30.6 kcal/mol.
Alternatively, oxidative insertion of Rh(I) into a C−H bond
followed by reductive elimination of pivalic acid leads to
barriers that are lower for both the C−H oxidative addition
step (24.4 kcal/mol) and the O−H reductive elimination (26.6
kcal/mol). Thus, despite the observation for many other
reactions that the CMD pathway is favored for C−H
a
products/TOs
amount of added
HOPiv/
equiv
alkenyl
trans-
L:B
b
vinyl arene/equiv
time/h benzene stilbene ratio
1
2
3
4
5
6
7
1
1
1
1
1
0.5
0.5
114(1)
78(3)
53(3)
3(13)
69(7)
86(4)
68(2)
1.3
240
480
960
480
5(1)
9(1)
22(4)
7(1)
70
activation, the DFT calculations predict that for this system
the oxidative addition/reductive elimination process is favored
by 4 kcal/mol.
480
480
480
Liberation of HOPiv and coordination of ethylene generates
a Rh(I) phenyl species (5) that can form a C−C bond via
olefin insertion. The DFT-predicted transition state for
ethylene insertion (TS3) lies 26.4 kcal/mol above starting
complex 2 and 22.6 kcal/mol above the precursor 5. The
three-coordinate Rh(I) alkyl complex 6 leads to styrene
formation via a facile β-hydride elimination through TS4,
although this can be inhibited by the favorable coordination of
an additional ethylene to form 6′. The resulting Rh(I) hydride
with styrene coordinated (7) can undergo ligand exchange
with ethylene to form free styrene and complex 8. Either
complex 7 or 8 can potentially react with Cu(II) to regenerate
complex 2.
Minor products can result from branches off this pathway.
Protonolysis of the Rh−C bond in 6 would divert the reaction
to produce ethylbenzene. However, using pivalic acid or
benzene as a proton source, the barriers for this reaction are
predicted to be 8 or 18 kcal/mol higher, respectively, than the
β-hydride elimination reaction that leads to the observed
product styrene. A more competitive detour is the generation
of stilbene by the insertion of styrene in 5′ via TS3′. The
barrier for the insertion of styrene is only 5 kcal/mol higher
than the insertion of ethylene, which is consistent with the
experimental observation of stilbene formation once styrene is
formed by catalysis. Within the accuracy of DFT calculations
we cannot distinguish whether C−H activation (specifically the
reductive elimination transition state TS2 at 26.6 kcal/mol) or
subsequent olefin insertion limits the rate of the overall
reaction. Furthermore, the relative rates of these branches are
11
10
480
a
Reaction conditions: 0.001 mol % complex 1 relative to benzene,
40 equiv of Cu(OPiv) , 50 psig of ethylene or 30 psig of propylene
2
2
b
reacted at 150 °C. Styrene is added for entries 1−5 and allylbenzene
is added for entry 7. Loading is relative to complex 1.
the reaction (Table 3, entries 5 and 7). At this point we cannot
assign the increased influence of styrene relative to
allylbenzene to either steric or electronic effects. But, it
might be expected that other products from hydrophenylation
of propylene (α-methylstyrene, cis-β-methylstyrene, and trans-
β-methylstyrene), which have less sterically accessible double
bonds, will exhibit less product inhibition on the rate of
catalysis.
Density functional theory (DFT) calculations were used to
predict the elementary steps that could be responsible for the
reactions of ethylene and propene catalyzed by complex 1.
69
The predicted free energy surfaces at experimental conditions
suggest an oxidative arylation mechanism comprising benzene
C−H activation, olefin insertion, β-hydride elimination, and
regeneration by O or Cu(II). Our predicted L:B ratio of 11:1
2
from the relative free energies of propene insertion transition
states is consistent with the experimentally observed ratio of
1
1:1 (Table 2), which supports the mechanism from the DFT
calculations.
At concentrations mimicking reaction conditions, displace-
ment of ethylene from 2 by benzene is predicted from the DFT
to be endergonic by 9.2 kcal/mol (Figure 5). Benzene C−H
Figure 5. Free energies (kcal/mol) at 423 K from DFT calculations {M06, including PBF solvation (see Supporting Information)} of the proposed
catalytic cycle for conversion of ethylene and benzene to styrene. Conditions: [Rh] = 0.11 mM, [HOPiv] = 0.053 M, 4.4 atm of C H , 1 atm of O ,
2
4
2
[
styrene] = [ethylbenzene] = 0.1 M, in benzene.
G
DOI: 10.1021/jacs.8b07728
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Figure 6. Free energies (kcal/mol) at 423 K from DFT calculations of our proposed catalytic cycle for the oxidative hydroarylation of propene
(
conditions as in Figure 5). The inset displays isomers of the olefin insertion transition state, with the propene and phenyl units drawn as cylinders.
II
Figure 7. Free energies (kcal/mol) at 423 K from DFT calculations for reactions of oxidants with the rhodium hydride 8. Conditions: [Cu ] =
I
•
[
Cu ] = [Rh] = [ OOH] = 0.11 mM, [HOPiv] = 0.053 M, 4.4 atm of C H , 1 atm of O , in benzene. Superscripts denote spin multiplicities.
2 4 2
sensitive to the evolving concentrations of ethylene and styrene
over the course of reaction.
calculated L:B ratio of 11, which is consistent with the value
(11:1) observed from experimental observations (see Table 2),
derives from a factor of ∼2 from the entropic difference and a
factor of ∼5 from the enthalpy differences. The activation
energies for β-hydride elimination transition states producing
branched or linear olefin are more than 10 kcal/mol lower than
the olefin insertion barriers. Therefore, the selectivity
determined in the insertion step is not expected to be
obscured by β-hydride elimination or product dissociation.
As a first step toward understanding the regeneration of 1 by
Figure 6 shows the DFT results for the conversion of
propene to linear and branched products. Since propene is
easier to displace by benzene than ethylene, the computed C−
H activation barriers are effectively reduced. The resulting
phenyl complex lies about 4 kcal/mol uphill and precedes
olefin insertion transition states that lead to linear or branched
products. The four distinct arrangements of the C -symmetric
s
ligand, propene, and phenyl group were predicted by DFT to
have free energies of activation within 3 kcal/mol. Both
transition states with linear regiochemistry lead to lower
activation barriers (24.0 and 25.2 kcal/mol) than those leading
to the branched product (26.2 and 26.8 kcal/mol). The ratio
of the summed rates through the linear and branched paths
predicted using transition state theory at 150 °C predicts a
linear:branched ratio of 11.2:1. The lowest-energy transition
state also has a higher vibrational entropy, suggesting the
fragments in this arrangement are less tightly packed. The
O and Cu(II), intermediates that might result from the
2
reaction of these with the most exergonic, and likely reducing,
state in Figure 5 were considered (Figure 7). One feasible
route involves the Cu(II)/Cu(I) couple, represented computa-
tionally by Cu(OPiv) /Cu(OPiv)(C H ). The hydride com-
2
6
6
plex 8 undergoes an exergonic single-electron oxidation by
Cu(OPiv)2 via a net carboxyl group transfer. Reductive
elimination of pivalic acid to form the Rh(0) complex 10 is
also exergonic. A second oxidation to regenerate 2 is highly
H
DOI: 10.1021/jacs.8b07728
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
1
13
exergonic. Direct complexation of O to 8 is unlikely: no
frequency 150 MHz). All H and C spectra are referenced against
2
1
13
13
residual proton signals ( H NMR) or C resonances ( C NMR) of
complex forms on the triplet surface and the singlet 11 is
highly exergonic. A role for the capping arene in activating O2
was considered, but a singlet Rh(III) complex 12 with O2
bridging the Rh(III) and an arene carbon lies 26 kcal/mol
uphill. Dissociation of ethylene from 8 to create a vacancy is
1
9
the deuterated solvents. F spectra are referenced with hexafluor-
obenzene (used as an external standard).
Ethylbenzene, cumene, styrene, phenylacetate, phenyl pivalate,
cumene, allylbenzene, α-methylstyrene, trans-β-methylstyrene, cis-β-
methylstyrene, and biphenyl production was quantified using linear
regression analysis of gas chromatograms of standard samples of an
authentic product. A plot of peak area ratios versus molar ratios gave a
regression line using hexamethylbenzene as the internal standard. For
the GC/FID instrument, the slope and correlation coefficient of the
regression lines were 1.72 and 0.99 (ethylbenzene), 1.67 and 0.99
also unlikely (ΔG = 27.7 kcal/mol, 13). The transition state
3
for an associative substitution of O for ethylene was found
2
(TS5), which could be the entry to rearrangements that
exergonically produce rhodium peroxide 18. The activation
⧧
barrier (ΔG = 25.5 kcal/mol) is high but comparable to those
(
styrene), 0.87 and 0.99 (trans-stilbene), 1.67 and 0.99 (phenyl-
earlier in the mechanism. Slightly more favorable is a direct
3
⧧
acetate), 1.22 and 0.99 (phenyl pivalate), 1.99 and 0.99 (cumene),
1.40 and 0.99 (allylbenzene), 1.23 and 0.99 (α-methylstyrene), 1.47
and 0.99 (cis-β-methylstyrene), 1.38 and 0.99 (trans-β-methylstyr-
ene), and 0.96 and 0.99 (biphenyl), respectively.
Propylene and ethylene were purchased in gas cylinders from GTS-
Welco and used as received. [Rh(η -C H ) (μ-TFA)] and 1,2-bis(N-
hydrogen atom abstraction by O (TS6, ΔG = 22.2 kcal/
2
7
1−75
mol).
the hydride is perhaps more likely than the direct involvement
of O . For example, reaction of 8 with a hydroperoxy radical
However, a radical chain mechanism for converting
2
2
(
generated photochemically or via decomposition of a
2
4
2
2
peroxide intermediate) yields the Rh(II) complex 14. The
7-azaindoly)benzene were prepared according to literature proce-
3
52,68
weakened Rh−H bond can easily be broken by O to yield 18,
dures.
sources and used as received.
Synthesis of (5-FP)Rh(TFA)(η -C H ) (1). A THF solution (10 mL)
All other reagents were purchased from commercial
2
I
effecting a net insertion of O2 into the Rh −H bond.
2
Alternatively, the Rh(I) superoxide complex 19 is able to
abstract a hydrogen atom from 8 exergonically to yield the
Rh(0) 10, which regenerates 19 spontaneously via reaction
2 4
−
4
of 1,2-bis(N-7-azaindolyl)benzene (0.0810 g, 2.61 × 10 mol) was
added to a THF solution (10 mL) of [Rh(η -C H ) (μ-TFA)]
2
2
4
2
2
−
4
7
6
(0.0800 g, 1.47 × 10 mol) dropwise and stirred for 48 h. The
reaction mixture was dried under vacuum. The resulting solid was
dissolved in minimal THF (2 mL), and pentane (30 mL) was added
to the solution to give a yellow precipitate. The solid was collected by
filtration, washed with pentane (3 × 10 mL), and dried under vacuum
to afford the analytically pure 1 (0.110 g, yield = 76%). X-ray quality
crystals of 1 were grown by the vapor diffusion method using THF
and n-pentane. Note: see Supporting Information for ligand labeling
with O2. A full mechanistic study including ligands that do
not facilitate aerobic oxidation is beyond the scope of this
paper, but these DFT calculations indicate that both anaerobic
and aerobic pathways are viable.
CONCLUSIONS
■
1
3
To our knowledge, there are no previous examples of catalytic
conversion of benzene and propylene to linear alkylbenzene
with high selectivity for anti-Markovnikov products under
aerobic conditions. We have reported that a “capping arene”-
scheme. H NMR (CD Cl , 600 MHz): δ = 9.27 (dd, J = 5.3 Hz,
2
2
HH
3
1.4 Hz, 1H, 5 or 5′), 8.37 (d, J = 5.5 Hz, 1H, 5 or 5′), 7.91 (td,
HH
3
3
J
HH
= 7.7 Hz, 1.5 Hz, 1H, 7 or 7′), 7.86 (dd, J = 7.9 Hz, 1.6 Hz,
HH
1H, 6 or 6′), 7.77−7.81 (m, 2H, 1H of 6 or 6′ and 1H of 7 or 7′),
2
3
3
7
.58 (dd, J = 7.8 Hz, 1.3 Hz, 1H, 3 or 3′), 7.45 (dd, J = 7.9, 1.4
supported Rh(I) complex (5-FP)Rh(TFA)(η -C H ) serves as
HH
HH
2
4
3
3
Hz, 1H, 3 or 3′), 7.31 (d, J = 3.6 Hz, 1H, 1 or 1′), 7.23 (d, J
=
a catalyst precursor under anaerobic or aerobic conditions.
These results represent the following hallmarks: (1) the Rh
catalyst is quite stable in air at 150 °C, (2) the catalysis can use
air as the direct oxidant without any Cu(II) salts, (3) the Rh
catalyst shows a remarkable longevity that can reach a
significant turnover number (>13 000) without obvious
catalyst deactivation at 150 °C for at least 2 weeks of reaction,
and (4) the catalyst can achieve a linear to branched ratio up to
HH
HH
3
3
6
1
.5 Hz, 1H, 1 or 1′), 7.08 (dd, J = 7.9 Hz, 5.3 Hz, 1H, 4 or 4′),
HH
3
3
.73 (dd, J = 7.8 Hz, 5.5 Hz, 1H, 4 or 4′), 6.48 (d, J = 3.6 Hz,
HH
HH
3
H, 2 or 2′), 6.46 (d, J = 3.6 Hz, 1H, 2 or 2′), 2.75 (br, 2H,
HH
19
ethylene-H), and 2.54 (br, 2H, ethylene-H) ppm. F NMR (CD Cl ,
2
2
13
1
6
00 MHz): δ = −74.0 (s, TFA) ppm. C{ H} NMR (CD Cl , 150
2
2
2
MHz): δ = 161.9 (q, OC(O)CF , J = 34 Hz, 151.9 (s, Ar−C),
3
CF
150.4 (s, Ar−C), 148.8 (s, Ar−C), 146.2 (s, Ar−C), 13l.2 (s, Ar−C),
134.8 (s, Ar−C), 131.9 (s, Ar−C), 131.1 (s, Ar−C), 131.0 (s, Ar−C),
130.6 (s, Ar−C), 130.5 (s, Ar−C), 130.4 (s, Ar−C), 130.0 (s, Ar−C),
1
8:1, which is to our knowledge the highest ratio for
1
1
28.8 (s, Ar−C), 121.8 (s, Ar−C), 121.1 (s, Ar−C), 117.4 (s, Ar−C),
16.6 (s, Ar−C), 103.0 (s, ethylene−C), 102.7 (s, ethylene−C) ppm.
conversion of hydrocarbon substrates.
Peak of OC(O)CF is overlapped with some Ar−C peaks. Anal. Calcd
3
EXPERIMENTAL SECTION
■
for C24
H: 3.26; N: 10.00.
H F N O Rh: C: 52.00; H: 3.27; N: 10.11. Found: C: 51.21;
18 3 4 2
General Considerations. All manipulations were performed
under an atmosphere of dry nitrogen using standard Schlenk
techniques and/or in a glovebox. Glovebox purity was maintained
by periodic nitrogen purges and was monitored by an oxygen analyzer
Catalytic Oxidative Hydrophenylation of Ethylene. A
representative catalytic reaction is described (Table 1, entry 9). A
stock solution containing 1 (0.0156 g, 0.0280 mmol, 0.001 mol % of
rhodium catalyst), hexamethylbenzene (0.0911 g, 0.561 mmol), and
benzene (250 mL) was prepared in a volumetric flask. Thick-walled
glass Fisher-Porter reactors were charged with stock solution (10
(
O < 15 ppm for all reactions). Unless otherwise noted (e.g., for
2
aerobic reactions), dry, oxygen-free solvents were employed
throughout and stored over molecular sieves. Benzene was dried by
passage through columns of activated alumina. Pentane was dried over
sodium benzophenone ketyl. GC/MS was performed using a
Shimadzu GCMS-QP2010 Plus system with a 30 m × 0.25 mm
SHRXI-5MS column with a 0.25 μm film thickness using electron
impact (EI) ionization. GC/FID was performed using a Shimadzu
GC-2014 system with a 30 m × 90.25 mm SHRXI-5MS/HP5 column
mL), pivalic acid (0.0550 g, 0.539 mmol), and Cu(OPiv) (0.0716 g,
2
0.269 mmol). The vessels were sealed, pressurized with ethylene (50
psig), and subsequently stirred and heated to 150 °C. The color of the
reaction mixture is deep blue at the beginning of the reaction and
turns colorless when all Cu oxidant is consumed. The reaction was
sampled when the solution turned colorless. The reactors were cooled
to room temperature, and aliquots of the reaction mixture (<200 μL)
were analyzed by GC/FID using relative peak areas versus the internal
standard (hexamethylbenzene).
1
with a 0.25 μm film thickness. H NMR spectra were recorded on a
1
3
Varian Inova or Bruker Avance III 600 MHz spectrometer, and
C
NMR spectra were recorded on a Varian Inova 600 MHz
1
9
spectrometer (operating frequency 150 MHz). F NMR spectra
were recorded on a Varian Inova 600 MHz spectrometer (operating
Catalytic Oxidative Hydrophenylation of Propylene. A
representative catalytic reaction is described (Table 2, entry 1). A
I
DOI: 10.1021/jacs.8b07728
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
stock solution containing 1 (0.0156 g, 0.0280 mmol, 0.001 mol % of
rhodium catalyst), hexamethylbenzene (0.0911 g, 0.561 mmol), and
benzene (250 mL) was prepared in a volumetric flask. Thick-walled
glass Fisher-Porter reactors were charged with stock solution (10
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.8b07728.
■
*
S
mL), pivalic acid (0.0550 g, 0.539 mmol), and Cu(OPiv) (0.0716 g,
2
0
.269 mmol). The vessels were sealed, pressurized with propylene (30
Detailed information about the X-ray structure determi-
nation and computational modeling (PDF)
X-ray crystallographic data (CIF)
psig), and subsequently stirred and heated to 150 °C. The color of the
reaction mixture is deep blue at the beginning of the reaction and
turns colorless when all Cu oxidant is consumed. The reaction was
sampled when the solution turned colorless. The reactors were cooled
to room temperature, and aliquots of the reaction (<200 μL) mixture
were analyzed by GC/FID using relative peak areas versus the internal
standard (hexamethylbenzene).
Catalytic Oxidative Hydrophenylation of Propylene with
Cu(II) Oxidant Using in Situ Regeneration. A representative
catalytic reaction is described (using 1 as the catalyst). A stock
solution containing 1 (0.0156 g, 0.0280 mmol, 0.001 mol % of
rhodium catalyst), hexamethylbenzene (0.0911 g, 0.561 mmol), and
benzene (250 mL) was prepared in a volumetric flask. Thick-walled
glass Fisher-Porter reactors were charged with stock solution (10
AUTHOR INFORMATION
Corresponding Authors
robert.smith.nielsen@gmail.com
tbg7h@virginia.edu
ORCID
Robert J. Nielsen: 0000-0002-7962-0186
William A. Goddard, III: 0000-0003-0097-5716
Diane A. Dickie: 0000-0003-0939-3309
T. Brent Gunnoe: 0000-0001-5714-3887
■
*
*
mL), pivalic acid (0.0550 g, 0.539 mmol), and Cu(OPiv) (0.0716 g,
Notes
2
0
.269 mmol). The vessels were sealed, pressurized with propylene (30
The authors declare no competing financial interest.
psig), and subsequently stirred and heated to 150 °C. The color of the
reaction mixture is deep blue at the beginning of the reaction and
turns colorless when all Cu oxidant is consumed. The reaction was
sampled when the solution turned colorless. The reactors were cooled
to room temperature, and aliquots of the reaction mixture (<200 μL)
were analyzed by GC/FID using relative peak areas versus the internal
standard (hexamethylbenzene). After the sampling, 1 atm of fresh air
was purged into the reactor. Then the reactor was pressurized with 50
ACKNOWLEDGMENTS
■
T.B.G. was supported by the U.S. Department of Energy,
Office of Basic Energy Sciences, Chemical Sciences, Geo-
sciences, and Biosciences Division (DE-SC0000776). R.J.N.
and W.A.G. were supported by NSF CBET-15127509. The
quantum mechanic calculations used the resources of the
Extreme Science and Engineering Discovery Environment
psig of N . The reactor was heated to 120 °C for 15 min for in situ
2
(
XSEDE), which is supported by National Science Foundation
grant number ACI-1053575.
Cu(II) oxidant regeneration. After the regeneration, the reactors were
cooled to room temperature and purged and pressurized with
propylene (30 psig). The vessels were subsequently stirred and heated
to 150 °C for the reaction.
Air in Situ Catalytic Oxidative Hydrophenylation of
Propylene. A representative catalytic reaction is described (Figure
REFERENCES
■
(
1) Caro, J.; Noack, M. Chapter 1 - Zeolite Membranes − Status and
Prospective. In Advances in Nanoporous Materials; Ernst, S., Ed.;
Elsevier, 2010; Vol. 1, pp 1−96.
(2) Chen, S.-S. Styrene. Kirk-Othmer Encyclopedia of Chemical
Technology; John Wiley & Sons, Inc: Hoboken, NJ, 2006.
(3) Wittcoff, H. A., Reuben, B. G.; Plotkin, J. S. Chemicals and
Polymers from Ethylene. Industrial Organic Chemicals; John Wiley &
Sons, Inc: Hoboken, NJ, 2013.
4A). A stock solution containing 1 (0.0156 g, 0.0280 mmol, 0.001
mol % of rhodium catalyst), hexamethylbenzene (0.455 g, 2.24
mmol), and benzene (250 mL) was prepared in a volumetric flask.
Thick-walled glass Fisher-Porter reactors were charged with stock
solution (1 mL) and benzene (9 mL), pivalic acid (0.550 g, 5.38
mmol), and Cu(OPiv) (0.0716 g, 0.269 mmol). The vessels were
2
sealed and purged with 1 atm of air, pressurized with propylene (30
psig), and subsequently stirred and heated to 150 °C. After every 24
h, the reactors were cooled to room temperature first and to 0 °C with
an ice bath. Then the reactor was purged with air (1 atm), pressurized
with propylene (30 psig), and reheated. The reaction mixture was
sampled every 48 h. Aliquots of the reaction (<200 μL) mixture were
analyzed by GC/FID using relative peak areas versus the internal
standard (hexamethylbenzene). Figure S7 shows the L:B ratio as a
function of reaction time. The increase in L:B is due to the
consumption of branched products during the reaction (see
Supporting Information).
(4) Kocal, J. A.; Vora, B. V.; Imai, T. Production of Linear
Alkylbenzenes. Appl. Catal., A 2001, 221, 295−301.
(
5) Olah, G. A.; Prakash, G. S. Hydrocarbon Chemistry; John Wiley &
Sons, Inc: Hoboken, NJ, 2017.
6) Perego, C.; Ingallina, P. Recent Advances in the Industrial
(
Alkylation of Aromatics: New Catalysts and New Processes. Catal.
Today 2002, 73, 3−22.
(7) Gerzeliev, I.; Khadzhiev, S.; Sakharova, I. Ethylbenzene Synthesis
and Benzene Transalkylation with Diethylbenzenes on Zeolite
Catalysts. Pet. Chem. 2011, 51, 39−48.
̌
8) Cejka, J.; Wichterlova, B. Acid-catalyzed Synthesis of Mono- and
́
(
dialkyl Benzenes over Zeolites: Active sites, Zeolite Topology, and
Catalytic Oxidative Hydrophenylation of Propylene Using
Air as Oxidant. A stock solution containing 1 (0.0156 g, 0.0280
mmol, 0.001 mol % of rhodium catalyst), hexamethylbenzene (0.0911
g, 0.561 mmol), and benzene (250 mL) was prepared in a volumetric
flask. Thick-walled glass Fisher-Porter reactors were charged with
stock solution (10 mL) and pivalic acid (0.5500 g, 5.39 mmol). The
vessels were sealed and purged with 1 atm of air, pressurized with
propylene (30 psig), and subsequently stirred and heated to 150 °C.
The reaction was sampled every 48 h. At each time point, the reactors
were cooled to room temperature, sampled, purged with air (1 atm),
pressurized with propylene (30 psig), and reheated. Aliquots of the
reaction mixture (<200 μL) were analyzed by GC/FID using relative
peak areas versus the internal standard (hexamethylbenzene).
Reaction Mechanisms. Catal. Rev.: Sci. Eng. 2002, 44, 375−421.
(
9) Perego, C.; Ingallina, P. Combining Alkylation and Trans-
alkylation for Alkylaromatic Production. Green Chem. 2004, 6, 274−
79.
10) Wittcoff, H. A.; Reuben, B. G.; Plotkin, J. S. Industrial Organic
Chemicals; John Wiley & Sons, Inc: Hoboken, NJ, 2012.
11) De Almeida, J.; Dufaux, M.; Taarit, Y. B.; Naccache, C. Linear
Alkylbenzene. J. Am. Oil Chem. Soc. 1994, 71, 675−694.
12) Kocal, J. A.; Vora, B. V.; Imai, T. Production of Linear
2
(
(
(
Alkylbenzenes. Appl. Catal., A 2001, 221, 295−301.
(13) Matsumoto, T.; Taube, D. J.; Periana, R. A.; Taube, H.;
Yoshida, H. Anti-Markovnikov Olefin Arylation Catalyzed by an
Iridium Complex. J. Am. Chem. Soc. 2000, 122, 7414−7415.
J
DOI: 10.1021/jacs.8b07728
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(
14) Andreatta, J. R.; McKeown, B. A.; Gunnoe, T. B. Transition
(30) Lail, M.; Bell, C. M.; Conner, D.; Cundari, T. R.; Gunnoe, T.
B.; Petersen, J. L. Experimental and Computational Studies of
Ruthenium (II)-Catalyzed Addition of Arene C−H Bonds to Olefins.
Organometallics 2004, 23, 5007−5020.
Metal Catalyzed Hydroarylation of Olefins Using Unactivated
Substrates: Recent Developments and Challenges. J. Organomet.
Chem. 2011, 696, 305−315.
(
15) Foley, N. A.; Lee, J. P.; Ke, Z.; Gunnoe, T. B.; Cundari, T. R.
(31) Luedtke, A. T.; Goldberg, K. I. Intermolecular Hydroarylation
of Unactivated Olefins Catalyzed by Homogeneous Platinum
Complexes. Angew. Chem., Int. Ed. 2008, 47, 7694−7696.
Ru (II) Catalysts Supported by Hydridotris(pyrazolyl)borate for the
Hydroarylation of Olefins: Reaction Scope, Mechanistic Studies, and
Guides for the Development of Improved Catalysts. Acc. Chem. Res.
(32) Clement, M. L.; Grice, K. A.; Luedtke, A. T.; Kaminsky, W.;
Goldberg, K. I. Platinum(II) Olefin Hydroarylation Catalysts: Tuning
Selectivity for the anti-Markovnikov Product. Chem. - Eur. J. 2014, 20,
2
(
009, 42, 585−597.
16) Oxgaard, J.; Muller, R. P.; Goddard, W. A.; Periana, R. A.
Mechanism of Homogeneous Ir (III) Catalyzed Regioselective
Arylation of Olefins. J. Am. Chem. Soc. 2004, 126, 352−363.
(
1
(
7287−17291.
33) McKeown, B. A.; Foley, N. A.; Lee, J. P.; Gunnoe, T. B.
17) Periana, R. A.; Liu, X. Y.; Bhalla, G. Novel bis-acac-O, O−Ir
Hydroarylation of Unactivated Olefins Catalyzed by Platinum(II)
Complexes. Organometallics 2008, 27, 4031−4033.
(
III) Catalyst for anti-Markovnikov, Hydroarylation of Olefins
Operates by Arene CH Activation. Chem. Commun. 2002, 3000−
001.
18) Bhalla, G.; Oxgaard, J.; Goddard, W. A.; Periana, R. A. Anti-
(
34) McKeown, B. A.; Prince, B. M.; Ramiro, Z.; Gunnoe, T. B.;
3
(
II
Cundari, T. R. Pt -catalyzed Hydrophenylation of α-Olefins:
Variation of Linear/Branched Products as a Function of Ligand
Donor Ability. ACS Catal. 2014, 4, 1607−1615.
Markovnikov Hydroarylation of Unactivated Olefins Catalyzed by a
Bis-tropolonato Iridium (III) Organometallic Complex. Organo-
metallics 2005, 24, 3229−3232.
(35) Bair, J. S.; Schramm, Y.; Sergeev, A. G.; Clot, E.; Eisenstein, O.;
Hartwig, J. F. Linear-Selective Hydroarylation of Unactivated
Terminal and Internal Olefins with Trifluoromethyl-Substituted
Arenes. J. Am. Chem. Soc. 2014, 136, 13098−13101.
(
19) Oxgaard, J.; Periana, R. A.; Goddard, W. A. Mechanistic
Analysis of Hydroarylation Catalysts. J. Am. Chem. Soc. 2004, 126,
1658−11665.
20) Bhalla, G.; Bischof, S. M.; Ganesh, S. K.; Liu, X. Y.; Jones, C.;
1
(
(36) Ahuja, R.; Punji, B.; Findlater, M.; Supplee, C.; Schinski, W.;
Brookhart, M.; Goldman, A. S. Catalytic Dehydroaromatization of n-
Alkanes by Pincer-Ligated Iridium Complexes. Nat. Chem. 2011, 3,
Borzenko, A.; Tenn, W. J., III; Ess, D. H.; Hashiguchi, B. G.; Lokare,
K. S. Mechanism of Efficient anti-Markovnikov Olefin Hydroarylation
Catalyzed by Homogeneous Ir (III) Complexes. Green Chem. 2011,
1
(
67−171.
37) Dobereiner, G. E.; Yuan, J.; Schrock, R. R.; Goldman, A. S.;
1
(
3, 69−81.
Hackenberg, J. D. Catalytic Synthesis of n-Alkyl Arenes through
21) McKeown, B. A.; Gonzalez, H. E.; Michaelos, T.; Gunnoe, T.
aAlkyl Group Cross-Metathesis. J. Am. Chem. Soc. 2013, 135, 12572−
B.; Cundari, T. R.; Crabtree, R. H.; Sabat, M. Control of Olefin
Hydroarylation Catalysis via a Sterically and Electronically Flexible
Platinum (II) Catalyst Scaffold. Organometallics 2013, 32, 3903−
1
(
2575.
38) Lim, Y. Regioselective Alkylation of 2-Phenylpyridines with
Terminal Alkenes via C−H Bond Activation by a Rhodium Catalyst. J.
3
(
913.
22) Burgess, S. A.; Joslin, E. E.; Gunnoe, T. B.; Cundari, T. R.;
Chem. Soc., Perkin Trans. 1 1996, 1, 2201−2206.
(39) Vaughan, B. A.; Webster-Gardiner, M. S.; Cundari, T. R.;
Sabat, M.; Myers, W. H. Hydrophenylation of Ethylene Using a
Cationic Ru (II) Catalyst: Comparison to a Neutral Ru (II) Catalyst.
Chem. Sci. 2014, 5, 4355−4366.
Gunnoe, T. B. A Rhodium Catalyst for Single-Step Styrene
Production from Benzene and Ethylene. Science 2015, 348, 421−424.
(40) Vaughan, B. A.; Khani, S. K.; Gary, J. B.; Kammert, J. D.;
(
23) McKeown, B. A.; Gonzalez, H. E.; Friedfeld, M. R.; Gunnoe, T.
Webster-Gardiner, M. S.; McKeown, B. A.; Davis, R. J.; Cundari, T.
R.; Gunnoe, T. B. Mechanistic Studies of Single-Step Styrene
Production Using a Rhodium (I) Catalyst. J. Am. Chem. Soc. 2017,
B.; Cundari, T. R.; Sabat, M. Mechanistic Studies of Ethylene
Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes. J. Am.
Chem. Soc. 2011, 133, 19131−19152.
139, 1485−1498.
(
24) McKeown, B. A.; Gonzalez, H. E.; Gunnoe, T. B.; Cundari, T.
II
(41) Webster-Gardiner, M. S.; Chen, J.; Vaughan, B. A.; McKeown,
B. A.; Schinski, W.; Gunnoe, T. B. Catalytic Synthesis of “Super”
Linear Alkenyl Arenes Using an Easily Prepared Rh (I) Catalyst. J.
Am. Chem. Soc. 2017, 139, 5474−5480.
R.; Sabat, M. Pt -atalyzed Ethylene Hydrophenylation: Influence of
Dipyridyl Chelate Ring Size on Catalyst Activity and Longevity. ACS
Catal. 2013, 3, 1165−1171.
(
25) Foley, N. A.; Lail, M.; Gunnoe, T. B.; Cundari, T. R.; Boyle, P.
(
42) Jira, R. Acetaldehyde from ethyleneA Retrospective on the
Discovery of the Wacker Process. Angew. Chem., Int. Ed. 2009, 48,
034−9037.
43) Smidt, J.; Hafner, W.; Jira, R.; Sieber, R.; Sedlmeier, J.; Sabel, A.
D.; Petersen, J. L. Combined Experimental and Computational Study
of TpRu{P(pyr) }(NCMe)Me (pyr = N-pyrrolyl): Inter-and Intra-
3
9
(
molecular Activation of C−H Bonds and the Impact of Sterics on
Catalytic Hydroarylation of Olefins. Organometallics 2007, 26, 5507−
The Oxidation of Olefins with Palladium Chloride Catalysts. Angew.
Chem., Int. Ed. Engl. 1962, 1, 80−88.
5
(
516.
26) Foley, N. A.; Ke, Z.; Gunnoe, T. B.; Cundari, T. R.; Petersen, J.
̈
(44) Hafner, W.; Jira, R.; Sedlmeier, J.; Smidt, J. Uber die
L. Aromatic C−H Activation and Catalytic Hydrophenylation of
Ethylene by TpRu{P(OCH ) CEt}(NCMe)Ph. Organometallics
Reaktionen von Olefinen mit waß
zen. Chem. Ber. 1962, 95, 1575−1581.
45) Smidt, J.; Hafner, W.; Jira, R.; Sedlmeier, J.; Sieber, R.;
Ruttinger, R.; Kojer, H. Katalytische Umsetzungen von Olefinen an
rigen Losungen von Palladiumsal-
̈ ̈
2
3
2
(
008, 27, 3007−3017.
(
27) Joslin, E. E.; McMullin, C. L.; Gunnoe, T. B.; Cundari, T. R.;
̈
Sabat, M.; Myers, W. H. Catalytic Hydroarylation of Ethylene Using
TpRu(L)(NCMe)Ph (L = 2,6,7-Trioxa-1-phosphabicyclo[2,2,1]-
heptane): Comparison to TpRu(L′)(NCMe)Ph systems (L′ = CO,
PMe , P(pyr) , or P(OCH ) CEt). Organometallics 2012, 31, 6851−
Platinmetall-Verbindungen Das Consortium-Verfahren zur Herstel-
lung von Acetaldehyd. Angew. Chem. 1959, 71, 176−182.
(
46) Vilella-Arribas, L.; García-Melchor, M.; Balcells, D.; Lledos, A.;
́
3
3
2 3
Lopez, J. A.; Sancho, S.; Villarroya, B. E.; del Río, M. P.; Ciriano, M.
́
6
860.
(
28) McKeown, B. A.; Gonzalez, H. E.; Friedfeld, M. R.; Brosnahan,
A.; Tejel, C. Rhodium Complexes Promoting C− O Bond Formation
in Reactions with Oxygen: The Role of Superoxo Species. Chem. - Eur.
J. 2017, 23, 5232−5243.
(47) Ahijado, M.; Braun, T.; Noveski, D.; Kocher, N.; Neumann, B.;
Stalke, D.; Stammler, H. G. Rhodium-Mediated Formation of
Peroxides from Dioxygen: Isolation of Hydroperoxo, Silylperoxo,
and Methylperoxo Intermediates. Angew. Chem., Int. Ed. 2005, 44,
6947−6951.
A. M.; Gunnoe, T. B.; Cundari, T. R.; Sabat, M. Platinum(II)-
Catalyzed Ethylene Hydrophenylation: Switching Selectivity between
Alkyl-and Vinylbenzene Production. Organometallics 2013, 32, 2857−
2
(
865.
29) Lail, M.; Arrowood, B. N.; Gunnoe, T. B. Addition of Arenes to
Ethylene and Propene Catalyzed by Ruthenium. J. Am. Chem. Soc.
003, 125, 7506−7507.
2
K
DOI: 10.1021/jacs.8b07728
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(
48) Praetorius, J. M.; Allen, D. P.; Wang, R.; Webb, J. D.; Grein, F.;
(68) Werner, H.; Poelsma, S.; Schneider, M. E.; Windmuller, B.;
̈
Kennepohl, P.; Crudden, C. M. N-Heterocyclic Carbene Complexes
of Rh: Reaction with Dioxygen without Oxidation. J. Am. Chem. Soc.
Barth, D. Synthesis and Reactivity of Bis(ethene) Rhodium (I) and
Iridium (I) Carboxylato Complexes. Chem. Ber. 1996, 129, 647−652.
(69) M06/6-311G**++ including implicit solvation, see Supporting
2
(
008, 130, 3724−3725.
Information for details.
70) Lapointe, D.; Fagnou, K. Overview of the Mechanistic Work on
the Concerted Metallation-Deprotonation Pathway. Chem. Lett. 2010,
9, 1119−1126.
71) Keith, J. M.; Nielsen, R. J.; Oxgaard, J.; Goddard, W. A. Pd-
49) Fu, R.; O’Reilly, M. E.; Nielsen, R. J.; Goddard, W. A., III;
(
Gunnoe, T. B. Rhodium Bis(quinolinyl)benzene Complexes for
Methane Activation and Functionalization. Chem. - Eur. J. 2015, 21,
3
(
1
(
286−1293.
50) O’Reilly, M. E.; Fu, R.; Nielsen, R. J.; Sabat, M.; Goddard, W.
III
Mediated Activation of Molecular Oxygen in a Nonpolar Medium. J.
Am. Chem. Soc. 2005, 127, 13172−13179.
A., III; Gunnoe, T. B. Long-Range C−H Bond Activation by Rh -
Carboxylates. J. Am. Chem. Soc. 2014, 136, 14690−14693.
(72) Keith, J. M.; Muller, R. P.; Kemp, R. A.; Goldberg, K. I.;
(
51) O’Reilly, M. E.; Johnson, S. I.; Nielsen, R. J.; Goddard, W. A.,
Goddard, W. A.; Oxgaard, J. Mechanism of Direct Molecular Oxygen
Insertion in a Palladium(II)−Hydride Bond. Inorg. Chem. 2006, 45,
III; Gunnoe, T. B. Transition-Metal-Mediated Nucleophilic Aromatic
Substitution with Acids. Organometallics 2016, 35, 2053−2056.
9
(
631−9633.
(
52) Zhao, S.-B.; Song, D.; Jia, W.-L.; Wang, S. Regioselective C−H
73) Decharin, N.; Popp, B. V.; Stahl, S. S. Reaction of O with
2
Activation of Toluene with a 1, 2-Bis(N-7-azaindolyl)benzene
+
[
“
(−)-Sparteine]Pd(H)Cl: Evidence for an Intramolecular [H−L]
Reductive Elimination” Pathway. J. Am. Chem. Soc. 2011, 133,
3268−13271.
74) Gaggioli, C. A.; Belpassi, L.; Tarantelli, F.; Zuccaccia, D.;
Platinum(II) Complex. Organometallics 2005, 24, 3290−3296.
(
53) Vierkoetter, S. A.; Barnes, C. E.; Garner, G. L.; Butler, L. G.
1
(
1
3
1
2
Olefin Rotation in the Solid State: A C, H, and H NMR Study of
Rh(acac)(C H ) . J. Am. Chem. Soc. 1994, 116, 7445−7446.
2
4
2
Harvey, J. N.; Belanzoni, P. Dioxygen Insertion into the Gold(I)−
Hydride Bond: Spin Orbit Coupling Effects in the Spotlight for
Oxidative Addition. Chem. Sci. 2016, 7, 7034−7039.
(75) Keith, J. M.; Goddard, W. A.; Oxgaard, J. Pd-Mediated
Activation of Molecular Oxygen: Pd(0) versus Direct Insertion. J. Am.
Chem. Soc. 2007, 129, 10361−10369.
(76) Look, J. L.; Wick, D. D.; Mayer, J. M.; Goldberg, K. I.
Autoxidation of Platinum(IV) Hydrocarbyl Hydride Complexes To
Form Platinum(IV) Hydrocarbyl Hydroperoxide Complexes. Inorg.
Chem. 2009, 48, 1356−1369.
(
54) Liu, Z.; Yamamichi, H.; Madrahimov, S. T.; Hartwig, J. F.
Rhodium Phosphine−π-Arene Intermediates in the Hydroamination
of Alkenes. J. Am. Chem. Soc. 2011, 133, 2772−2782.
(
55) Caddy, P.; Green, M.; Smart, L. E.; White, N. Evidence for the
Ready Insertion of Rhodium into a Carbon−Hydrogen Bond of
Ethylene; Crystal and Molecular Structure of μ-But-2-ene-μ-ethylene-
bis-(η -1-methylindenyl)dirhodium. J. Chem. Soc., Chem. Commun.
5
1
(
978, 839−841.
56) Guggenberger, L. J.; Cramer, R. Crystal Structure of
C H )Rh(C F )(C H ). A Comparison of Rhodium-Ethylene and
(
5
5
2
4
2
4
Rhodium-Tetrafluoroethylene Bonding. J. Am. Chem. Soc. 1972, 94,
779−3786.
57) Mlekuz, M.; Bougeard, P.; Sayer, B. G.; McGlinchey, M. J.;
3
(
Rodger, C. A.; Churchill, M. R.; Ziller, J. W.; Kang, S. K.; Albright, T.
A. X-ray Crystal Structure and Molecular Dynamics of (Indenyl)bis-
(
ethylene)rhodium(I): 500-MHz NMR Spectra and EHMO Calcu-
lations. Organometallics 1986, 5, 1656−1663.
58) Cramer, R. Olefin Coordination Compounds of Rhodium: The
(
Barrier to Rotation of Coordinated Ethylene and the Mechanism of
Olefin Exchange. J. Am. Chem. Soc. 1964, 86, 217−222.
(
59) Kanas, D. A.; Geier, S. J.; Vogels, C. M.; Decken, A.; Westcott,
S. A. Synthesis, Characterization, and Reactivity of Rhodium(I)
Acetylacetonato Complexes Containing Pyridinecarboxaldimine
Ligands. Inorg. Chem. 2008, 47, 8727−8735.
(
60) Geier, S. J.; Chapman, E. E.; McIsaac, D. I.; Vogels, C. M.;
Decken, A.; Westcott, S. A. Bulky Rhodium Diimine Complexes for
the Catalyzed Borylation of Vinylarenes. Inorg. Chem. Commun. 2006,
9
(
, 788−791.
61) Budzelaar, P. H.; Moonen, N. N.; de Gelder, R.; Smits, J. M.;
Gal, A. W. Steric Control over Arene Coordination to β-Diiminate
Rhodium (I) Fragments. Chem. - Eur. J. 2000, 6, 2740−2747.
(
62) The reaction gives 1.6 TOs of PhOPiv at 150 °C, and a similar
amount PhOPiv was produced when heating the same amount of
Cu(OPiv) in benzene without Rh complex.
2
(
1
(
63) The reaction gives 1.6 TOs and 6.8 TOs of PhOPiv at 150 and
80 °C respectively.
64) Parish, E. J.; Kizito, S. A. Copper(I) Acetate e-EROS Encyclopedia
of Reagents for Organic Synthesis; John Wiley & Sons, Inc: Hoboken,
NJ, 2001.
(
65) Matsumoto, T.; Periana, R. A.; Taube, D. J.; Yoshida, H. Direct
Synthesis of Styrene by Rhodium-catalyzed Oxidative Arylation of
Ethylene with Benzene. J. Catal. 2002, 206, 272−280.
(
66) Matsumoto, T.; Yoshida, H. Oxidative Arylation of Ethylene
with Benzene to Produce Styrene. Chem. Lett. 2000, 29, 1064−1065.
67) Weissman, H.; Song, X.; Milstein, D. Ru-Catalyzed Oxidative
coupling of Arenes with Olefins Using O . J. Am. Chem. Soc. 2001,
(
2
1
23, 337−338.
L
DOI: 10.1021/jacs.8b07728
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX