Catalytic Synthesis of “Super” Linear Alkenyl Arenes Using an Easily Prepared Rh(I) Catalyst

Article abstract of DOI:10.1021/jacs.7b01165

Linear alkyl benzenes (LAB) are global chemicals that are produced by acid-catalyzed reactions that involve the formation of carbocationic intermediates. One outcome of the acid-based catalysis is that 1-phenylalkanes cannot be produced. Herein, it is reported that [Rh(μ-OAc)(η²-C₂H₄)₂]₂ catalyzes production of 1-phenyl substituted alkene products via oxidative arene vinylation. Since C C bonds can be used for many chemical transformations, the formation of unsaturated products provides a potential advantage over current processes that produce saturated alkyl arenes. Conditions that provide up to a 10:1 linear:branched ratio have been achieved, and catalytic turnovers >1470 have been demonstrated. In addition, electron-deficient and electron-rich substituted benzenes are successfully alkylated. The Rh catalysis provides ortho:meta:para selectivity that is opposite to traditional acid-based catalysis.

Full text of DOI:10.1021/jacs.7b01165

Article
pubs.acs.org/JACS
Catalytic Synthesis of “Super” Linear Alkenyl Arenes Using an Easily
Prepared Rh(I) Catalyst
Michael S. Webster-Gardiner,† Junqi Chen,† Benjamin A. Vaughan,† Bradley A. McKeown,†
William Schinski,§ and T. Brent Gunnoe*,†
†Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States
S
* Supporting Information
ABSTRACT: Linear alkyl benzenes (LAB) are global chemicals that
are produced by acid-catalyzed reactions that involve the formation
of carbocationic intermediates. One outcome of the acid-based
catalysis is that 1-phenylalkanes cannot be produced. Herein, it is
reported that [Rh(μ-OAc)(η2-C2H4)2]2 catalyzes production of 1-
phenyl substituted alkene products via oxidative arene vinylation.
Since CC bonds can be used for many chemical transformations,
the formation of unsaturated products provides a potential advantage
over current processes that produce saturated alkyl arenes.
Conditions that provide up to a 10:1 linear:branched ratio have been achieved, and catalytic turnovers >1470 have been
demonstrated. In addition, electron-deficient and electron-rich substituted benzenes are successfully alkylated. The Rh catalysis
provides ortho:meta:para selectivity that is opposite to traditional acid-based catalysis.
Scheme 1. Current Route for the Synthesis of LAB and
Proposed Route for the Synthesis of Straight-Chain Vinyl/
Allyl and Alkyl Benzenes (SLAB)
INTRODUCTION
■
Alkyl arenes are used in a wide range of products including
plastics, detergents, fuels, and fine chemicals and as a liquid
scintillator. Since the development of a commercially viable
route to produce alkyl benzenesulfonates on a large scale in the
1940s, they have formed the basis of the detergent industry.1−3
Initial synthetic routes to make alkyl benzenes produced highly
branched alkyl groups, and, thus, the products were called
branched alkyl benzenes (BAB). The highly branched alkyl
chain of BAB rendered them resistant to biodegradation and
resulted in pollution of lakes and streams.4,5 Linear alkyl
benzenes (LAB), which, in contrast to their moniker, are
primarily composed 2- and 3-phenyl alkanes,2 are more readily
biodegraded than BAB, and constitute the majority of alkyl
benzenesulfonates produced today.4 Currently, LAB are
produced from benzene and α-olefins using acid-based
catalysts, typically either a solid acid catalyst, HF or AlCl3 in
combination with a Brønsted acid (Scheme 1).1,6,7 These acid-
based catalytic processes generate carbocationic intermediates
and, as a result, are not able to produce truly linear 1-
phenylalkanes, which we label as “super” linear alkyl benzenes
(SLAB) to differentiate them from LAB that do not contain 1-
phenylalkanes. Even when utilizing shape- and size-selective
zeolite catalysts, to our knowledge, the generation of SLAB is
not possible.8 Thus, similar to other alkyl arenes, SLAB are a
potentially useful class of compounds. The discussion of anti-
Markovnikov products here is specific to nonheterofunctional-
ized olefins, as Michael acceptors are readily functionalized in
the position that is β to the electron-withdrawing group. Much
as the development of a route to BAB and then LAB led to new
commercial products, a route for the production of SLAB could
provide the foundation for valuable new materials. For example,
SLAB achieve the linearity of the alkyl chain typically found in
natural soaps and detergent precursors such as stearic acid and
lauric acid.9 In addition, straight-chain vinyl and allyl anisole
derivatives are used in the flavor and fragrance industries.10
Also, it is anticipated that SLAB will have enhanced stability
against dealkylation degradation compared to LAB and BAB.
Further, 1-phenylalkanes have increased detersive power at low
concentrations.11 Currently, the preparation of SLAB involves a
Friedel−Crafts acylation followed by a Clemmensen reduction,
which is not viable for large-scale processes.2
Received: February 2, 2017
© XXXX American Chemical Society
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An alternative to the acid-based catalytic synthesis of alkyl
arenes is the utilization of a transition-metal-mediated catalytic
reaction that operates via C−H activation of the arene and
olefin insertion into metal−aryl bonds.12,13 For this proposed
process, the linear to branched (L:B) ratio of alkyl (or
unsaturated) arene product depends on the regioselectivity of
olefin insertion (i.e., 2,1- versus 1,2-insertion) and, potentially,
the relative rates of product formation following insertion (i.e.,
Curtin−Hammett conditions). This mode of control over the
position of the phenyl group in the alkyl chain is not possible
with acid-based methods. The intermediate following olefin
insertion into the metal-aryl bond can undergo an arene C−H
activation reaction to release a saturated alkyl arene, or, perhaps
more desirably, it can undergo a β-hydride elimination reaction
and net dissociation of the olefin to produce a “straight-chain”
vinyl or allyl arene.
To our knowledge, a catalyst that converts simple arenes
from petrochemical feedstocks (e.g., benzene or toluene) and
simple α-olefins (e.g., propylene, 1-hexene and longer chain α-
olefins) to straight-chain alkyl or vinyl/allyl arenes with high
selectivity has not been reported. Such selectivity has been
limited to heterofunctionalized aromatics and/or heterofunc-
tionalized olefins with the regioselectivity often being directed
by the use of electrophilic Michael acceptors.14 Figure 1 shows
selected examples of previously reported transition-metal
catalysts for propylene hydrophenylation with observed n-
propylbenzene (anti-Markovnikov product) to cumene (Mar-
kovnikov product) ratios and turnover numbers (TON).15−21
To date, the highest L:B ratio is 1.6:1, barely selective for linear
product. Notably, Goldman, Schrock, and co-workers have
developed alkyl group cross-metathesis reactions that convert
alkyl benzenes and alkanes to n-alkyl benzenes.22 Hartwig and
co-workers reported a nickel complex that generates
octylbenzene with a L:B ratio of 19:1; however, the Ni-
mediated process is stoichiometric in nickel.23 Wang and co-
workers showed that [Rh(μ-Cl)(COD)2]2 (cod =1,5-cyclo-
octadiene) and an equivalent of acid produce linear selectivity
when using Michael acceptors as the olefin; however, with
neohexene this catalytic process favors the branched
Markovnikov product.24 Milstein and co-workers showed that
Ru complexes with oxygen and carbon monoxide can produce
alkenyl arenes.25 Further, Ackermann and co-workers showed
that Ru carboxylates could generate linear products for arenes
that contain heterodirecting groups.26
Recently, we reported that (FlDAB)Rh(TFA)(η2-C2H4)
[FlDAB = N,N′-bis(pentafluorophenyl)-2,3-dimethyl-1,4-diaza-
1,3-butadiene; TFA = trifluoroacetate] catalyzes the conversion
of benzene and ethylene directly to styrene using Cu(II) salts as
the in situ oxidant in a process similar to the commercialized
Wacker−Hoechst process for ethylene oxidation.27,28 Scheme 2
Scheme 2. General Catalytic Cycle for the Production of
Anti-Markovnikov Vinyl Arenes from Benzene and α-Olefins
shows a likely catalytic cycle for this process. Although in this
paper the reoxidation of Cu(I) to Cu(II) has not been
demonstrated, separation of Cu(I) and reoxidation are known
for the Wacker process. Herein, we report that [Rh(μ-OAc)(η2-
C2H4)2]2 (1) serves as a catalyst precursor to convert arenes
and α-olefins to vinyl and allyl arenes, which upon hydro-
genation yield straight-chain alkyl benzenes (SLAB) in ratios as
high as 10:1 with a turnover (TO) > 1470 including extension
of the catalysis to both electron-rich and electron-poor arenes
(toluene and anisole are used as examples of electron-rich
arenes, and chlorobenzene is used as a representative electron-
deficient arene).
RESULTS AND DISCUSSION
■
Heating a 10 mL of benzene solution of complex 1 (0.001 mol
% relative to benzene) to 150 °C under 25 psi of propylene
with Cu(OAc)2 (240 equiv relative to 1) over 48 h affords
functionalized benzene products with a L:B ratio of 8:1
Figure 1. Selected catalysts that have been reported to catalyze the
hydrophenylation of propylene with observed selectivities and TON.
Linear:branched (L:B) refers to ratio of n-propylbenzene to
cumene.15−21
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(Scheme 3 and Figure S1). Unless stated otherwise, complex 1
mol % is given relative to benzene, and Cu(II) equivalents are
at 48 h were 73(7). Since 2 equiv of Cu(II) are used per equiv
of vinyl or allyl arene product, the maximum yield is 50% of the
amount of Cu(II). Thus, 73(7) TOs using 240 equiv of
Cu(OAc)2 corresponds to ∼60% yield. Aliquots of the reaction
mixture were analyzed by GC/FID using relative peak areas
versus an internal standard (hexamethylbenzene). Detection
limits for the instruments were equivalent to ∼1 TO of
product.
Scheme 3. Oxidative Coupling of Benzene and Propylene to
cis-β-Methylstyrene, Allylbenzene, trans-β-Methystryene, and
α-Methylstyrene
For the reaction of benzene and propylene catalyzed by 1,
four products were observed (TOs after 48 h are given for
each): allylbenzene: 31(2), cis-β-methylstyrene: 5(1), trans-β-
methystryene: 28(4), and α-methylstyrene: 9(1). The 1.1:1
ratio of allylbenzene to β-methystryene could suggest that there
is negligible difference between β-hydride elimination from the
terminal CH3 group or the benzylic position of the putative
{Rh−C(CH3)HCH2Ph} intermediate. In contrast to Pt(II)
catalysts for olefin hydroarylation,16 these Rh-catalyzed
reactions do not give difunctionalized products.
relative to 1. The L:B ratio for the products is determined based
on straight-chain products that would result from hydro-
genations (i.e., allylbenzene, cis-β-methylstyrene, and trans-β-
methylstryene) compared to branched products from hydro-
genations (i.e., α-methylstyrene). The TOs of alkenyl benzenes
Using CuCl2 or CuO at 150 or 180 °C over 48 h afforded no
vinyl or allyl benzene products. In contrast, use of 240 equiv of
a
Table 1. Comparison of Arene Alkylation Using AlCl3 as the Primary Catalyst versus [Rh(μ-OAc)(η2-C2H4)2]2 (1)
a
L:B ratios and total TO of alkylated products determined after hydrogenation of unsaturated products. Unless otherwise noted, conditions are 0.01
mol % 1 relative to arene, 25 psig gaseous olefin or 2000 equiv of olefin, 150 °C, 48 h, 240 equiv Cu(OAc) relative to 1. Hydrogenation was
2
b
c
achieved through 5%Pt on carbon under hydrogen atmosphere. n.r. = not reported. Data and conditions are from ref 31. Data and conditions are
from ref 35. Data and conditions are from ref 36. Data and conditions are from ref 19. 72 h. Data and conditions are from ref 32.
d
e
f
g
C
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Cu(OPiv)2 (OPiv = pivalate) gave 72(3) TO after 12 h with a
L:B ratio of 6:1. The total TO using Cu(OPiv)2 (∼100 TO, ∼
83% yield) is improved relative to Cu(OAc)2 (∼80 TO). These
experiments revealed the potential importance of having a
carboxylate group to achieve catalysis, as only Cu(II) salts with
carboxylate groups are effective (see Figure S2). Transition-
metal carboxylates have been implicated in efficient arene C−H
activation, and thus we tentatively conclude that the apparent
necessity of Cu(II) carboxylate salts is partially due to their role
in regenerating Rh carboxylates, which are likely integral for
arene C−H activation.29 Furthermore, the effect of oxidant
loading was investigated and revealed that increasing the
oxidant:catalyst ratio increases rate of catalysis in a manner that
is approximately first-order in oxidant, revealing higher TO
with increasing Cu(II) amounts (Figure S3).
The effect of temperature was examined (Figure S1). At
temperatures lower than 150 °C, a negligible amount of
catalytic activity is observed. The catalyst is stable and
demonstrates increased catalytic activity at temperatures higher
than 150 °C, however, undesired side reactions become
accessible. For example, small quantities of biphenyl (11(1)
TO) and phenyl acetate (8(1) TO) are produced at 180 °C, in
addition to 84(12) TO of vinylbenzenes. The generation of
phenyl acetate is a side reaction mediated by the Cu oxidant, as
it is also observed when benzene and Cu(OAc)2 are heated at
180 °C in the absence of olefin and Rh catalyst.30 We probed
the impact of starting concentration of 1 on TO (Figure S4).
Performing catalysis over a concentration range from 0.01 to
0.001 mol % 1 (relative to benzene) revealed very little
difference in TO versus time plots, and similar L:B selectivities
were observed. The reaction appears to only be limited by the
amount of oxidant. Cu(OPiv)2 was investigated as the oxidant
due to its solubility in benzene, and thus experimental stirring
challenges are relieved. For example, catalysis using 0.001 mol
% of 1 with 2400 equiv of Cu(OPiv)2 led to 1148(133) TO of
alkylated products after 48 h, which corresponds to a 96% yield
based on the limiting oxidant (Figure S5). The addition of
more oxidant (1200 equiv to the reaction), before complete
consumption of the initial oxidant, resulted in continued
catalytic activity with a total of 1474(3) TO obtained after 96 h
for an overall 82% yield. Catalyst decomposition appears to
occur after consumption of oxidant.
We probed catalysis using other olefins and arenes (Table 1)
using a standard set of conditions {10 mL of benzene, 0.01 mol
% 1 (relative to benzene), 2000 equiv of olefin (relative to 1) or
25 psig for gaseous olefins, and 240 equiv of Cu(OAc)2
(relative to 1)}. The production of vinyl and allyl species was
observed for all olefins except neohexene, for which allyl arene
formation is not possible. Reported TOs in Table 1 are based
on analysis of hydrogenated products after the completion of
catalytic reactions. For neohexene, only the 100% linear
product (3,3-dimethylbutyl)benzene was observed [30(8) TO
after 72 h]. With isobutylene, the vinyl and allyl anti-
Markovnikov products were produced with 100% selectivity
and hydrogenated to give isobutylbenzene [100(2) TO after 72
h]. Hydrogenated samples from catalysis with 1-pentene gave
100% yield relative to oxidant and an ∼8:1 ratio of linear (n-
pentylbenzene, 110(10) TO) to branched (2-pentylbenzene,
12(3) TO) products after 72 h. It is important to note that the
reaction of 1-pentene can yield 1-pentyl or 2-pentyl products,
while reaction with 2-pentene can generate 2-pentyl or 3-pentyl
products. Therefore, the observation of minimal (<2 TO) 3-
pentylbenzene production indicates that isomerization of 1-
pentene to 2-pentene is slow, that reaction with 2-pentene is
likely selective for the 2-pentyl product, and/or that the catalyst
reacts more rapidly with 1-pentene than 2-pentene. Interest-
ingly, utilizing the internal olefin 2-pentene produces 22(4) TO
of n-pentylbenzene, 48(6) TO of 2-pentylbenzene with 27(5)
TO of 3-pentylbenzene after hydrogenation. Starting with 2-
pentene, the amount of 1-pentene is likely low since it is
thermodynamically disfavored. Thus, the formation of n-
pentylbenzene starting from 2-pentene suggests that reaction
of the catalyst with 1-pentene is likely more rapid than reaction
with 2-pentene.
In the absence of copper oxidant, complex 1 catalyzes rapid
isomerization of 1-pentene to 2-pentene as conversion of 1-
pentene to 2-pentene achieved equilibrium after at least 48 h
(see Table S1). In contrast, in the presence of Cu(OAc)2,
Cu(OTFA)2, or Cu(OPiv)2, the rate of isomerization of 1-
pentene to 2-pentene by 1 is much slower. For example,
without added Cu(OAc)2 after 72 h, only 12% 1-pentene
remains, whereas with 240 equiv of Cu(OAc)2 (relative to 1),
78% remains as 1-pentene after 72 h. The different rate of 1-
pentene isomerization in the presence (slower) and absence
(faster) of Cu(OAc)2 suggests the possibility that a Rh−H
intermediate could play a key role in the olefin isomerization.
Cu(OAc)2 might rapidly react with the Rh−H intermediate,
which would compete with its ability to isomerize 1-pentene. It
is interesting to note that catalysis with benzene and 2-pentene
produces 22 TO of n-pentylbenzene, but 1-pentene likely
remains approximately 1% of the total pentene throughout
catalysis. Thus, 1 appears to react more rapidly with 1-pentene
than 2-pentene.
The efficacy and selectivity of acid-based arene alkylation
varies dramatically with arene substrate, and selectivity is
typically dictated by directing group effects. But, catalysis using
1 appears to be broadly applicable to different types of arenes
with selectivity appearing to be catalyst driven. Table 1 shows
the results using AlCl3 and complex 1 for several arenes and
olefins (note: the selectivities in Table 1 are determined after
hydrogenation using 5% Pt on carbon to yield saturated alkyl
substituents). For AlCl3-catalyzed alkylation, electron-deficient
arenes react substantially slower, often not at all. For example,
the rate of chlorobenzene alkylation (AlCl3 with propylene in
nitromethane at 25 °C) is approximately 10 times slower
relative to benzene with a product distribution of ortho:meta:-
para (o:m:p) of ∼11:1:8.6,31,32 In contrast, using AlCl3,
electron-rich arenes react faster relative to benzene.33 Using
AlCl3 as a catalyst, toluene alkylation with propylene (AlCl3 in
nitromethane at 25 °C) produces an unselective o:m:p ratio of
3:1:2.6.31 Using standard conditions with 1, catalysis with
chlorobenzene gave 116(3) TO (97% yield) of alkyl
chlorobenzene with a L:B ratio of 10:1 and o:m:p ratio of
1:11:7. Toluene and anisole were investigated as representative
electron-rich arenes to evaluate catalytic activity. The reaction
of toluene and propylene gave 86(17) TO (72% yield) of alkyl
toluene, L:B ratio of 9.4:1, and an o:m:p ratio of 1:8.9:9.3.
Anisole gave a 92(7) TO (77% yield) of alkyl anisole, L:B of
7.8:1, and an o:m:p ratio of 1:2.4:6.4. Catalysis using 1 reveals
an unusually high selectivity toward meta products, which could
provide routes to new or difficult to access compounds.
Further, the anisole results are particularly intriguing because
the para products are estragole (allyl) and anethole (vinyl),
which are common materials in the flavors and fragrance
industries.10
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sole, 3-propylanisole, 4-propylanisole, and 4-isopropylanisole was
estimated using the slope and correlation coefficient of the regression
lines for 4-ethylanisole. Quantification of 2-propylchlorobenzene, 3-
propylchlorobenzene, 4-propylchlorobenzene, 3-isopropylchloroben-
zene, and 4-isopropylchlorobenzene was estimated using the slope and
correlation coefficient of the regression lines for 3-ethylchlorobenzene
and 4-ethylchlorobenzene. Quantification of 2-propyltoluene, 3-
propyltoluene, 4-propyltoluene, 3-isopropyltoluene, 4-isopropylto-
luene, and isobutyl benzene was estimated using the slope and
correlation coefficient of the regression lines for cumene. Quantifica-
tion of 3,3-dimethylbutylbenzene was estimated using the slope and
correlation coefficient of the regression lines for n-pentylbenzene.
Identification of peaks due to linear versus branched products was
determined by studying the mass fragmentation patterns. Branched
products have substantially larger peak 15 m/z units less than the
molecular ion peak relative to linear products. Linear products reveal
loss of alkyl chain up to the benzylic position.
For all the aromatic substrates evaluated, catalysis using 1
provides alternative selectivity to traditional acid-based
methods. The arene electronics have a negligible impact on
TO and the rate of reaction. Table 1 shows comparisons
between acid-based catalysis (AlCl3) and the results using Rh
precatalyst 1. The o:m:p ratios highlight the differences between
rhodium-mediated catalysis and acid-based catalysis. For
rhodium-mediated catalysis using 1, the relative TO is
comparable to benzene reactivity, and the o:m:p ratio favors
meta and para functionalization, regardless of benzene
functionality, presumably based on the regioselectivity of Rh-
mediated C−H activation.34 In addition, the rhodium
precatalyst 1 generates the vinyl and allyl products for each
product in an approximate 1:1 ratio. In contrast, AlCl3, a typical
Friedel−Crafts catalyst, generates only saturated products and
is highly selective in all cases for branched products.
Propylene and isobutylene were purchased in gas cylinders from
GTS-Welco and used as received. All other reagents were purchased
from commercial sources and used as received. [Rh(η2-C2H4)2(μ-
OAc)]2 (1) was prepared according to literature procedures.38
Catalytic Oxidative Hydrophenylation of Propylene. A
representative catalytic reaction is described. A stock solution
containing 1 (0.005 g, 0.012 mmol, 0.001 mol % of rhodium),
hexamethylbenzene (0.075 g, 0.45 mmol), and benzene (200 mL) was
prepared in a volumetric flask. Glass Fisher-Porter reactors were
charged with stock solution (10 mL) and Cu(OAc)2 (0.050 g, 0.28
mmol). The vessels were sealed, pressurized with propylene (25 psig),
and subsequently stirred and heated to 150 °C. The reaction was
sampled every 4 h for the first 12 h, then at the 24 h time point, and
then every 24 h subsequently. At each time point, the reactors were
cooled to room temperature, sampled, recharged with propylene, and
reheated. Aliquots of the reaction (<200 μL) mixture were analyzed by
GC/FID using relative peak areas versus the internal standard
(hexamethylbenzene).
To our knowledge, there are no previous examples of
catalytic conversion of simple arenes and α-olefins, such as
propylene, 1-pentene, 1-hexene, etc., to alkyl or vinyl/allyl
products with high selectivity for anti-Markovnikov products.
Herein, we have reported that a simple Rh(I) catalyst
precursor, easily generated from commercially available
materials,37,38 achieves such transformations. Such catalysis
opens the door to a range of previously inaccessible products
using common petrochemicals. Furthermore, the catalytic
process is effective for benzene substituted with electron-
donating or -withdrawing groups with ortho/meta/para
selectivity that is unique from acid-based catalysis. The range
of arene and olefin scope allows for the generation of previously
synthetically challenging materials using air-recyclable Cu(II)
oxidants.
Hydrogenation General Procedure. To a glass Fischer-Porter
reactor, an aliquot of reaction sample was mixed in a 1:1 v:v mix with
absolute ethanol, and approximately 50 mg of 5% Pt on carbon, and a
stir bar were added. The reactor was then pressurized with hydrogen
and released (3 × 70 psi) before being placed under 150 psig of
hydrogen while stirring for 17 h. The reaction was then degassed, and
the mixture was analyzed by GC/MS or GC/FID.
Olefin Identity Experiments. A stock solution containing 1 (0.01
mol % relative to benzene), hexamethylbenzene (20 equiv relative to
1), and benzene (200 mL) was prepared in a volumetric flask. When
using liquid olefins, 2000 equiv (relative to 1) of olefin was added to
the stock solution. Glass Fisher-Porter reactors were charged with
stock solution (10 mL) and oxidant (240 equiv relative to 1). The
vessels were sealed, charged with olefin or N2 (25 psig), and
subsequently stirred and heated to 150 °C. The reaction was sampled
after 24 h, 48 h, and 72 h. At each time point, the reactors were cooled
to room temperature, sampled, recharged with gaseous olefin or N2,
and reheated. Aliquots of the reaction (<200 μL) mixture were
analyzed by GC/MS using relative peak areas versus the internal
standard (hexamethylbenzene). Using neohexene, 30(8) TO of 100%
linear product (3,3-dimethylbutyl)benzene and 27(6) TO of the olefin
coupled product 2,2,4,6,6-pentamethylheptane were observed after 72
h.
Oxidant Loading Experiments. A stock solution containing 1
(0.001 mol % relative to benzene), hexamethylbenzene (20 equiv
relative to Rh), and benzene (200 mL) was prepared in a volumetric
flask. Glass Fisher-Porter reactors were charged with stock solution
(10 mL) and Cu(OAc)2 (60, 120, or 240 equiv relative to 1). The
vessels were sealed, charged with propylene (25 psig), and
subsequently stirred and heated to 150 °C. The reaction was sampled
every 4 h for the first 12 h, then at the 24 h time point, and then every
24 h subsequently. At each time point, the reactors were cooled to
room temperature, sampled, recharged with propylene, and reheated.
Aliquots of the reaction (<100 μL) mixture were analyzed by GC/FID
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were performed
under an atmosphere of dry nitrogen using standard Schlenk or
high-vacuum techniques and/or in a glovebox. Glovebox purity was
maintained by periodic nitrogen purges and was monitored by an
oxygen analyzer (O2 < 15 ppm for all 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 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 HP5 column
with 0.25 μm film thickness.
Phenyl acetate, 3-pentylbenzene, 2-pentylbenzene, n-pentylbenzene,
cumene, n-propylbenzene, α-methylstyrene, trans-β-methylstyrene,
and biphenyl production was quantified using linear regression
analysis of gas chromatograms of standard samples of authentic
product. A plot of peak area ratios versus molar ratios gave a regression
line. For the GC/FID instrument, the slope and correlation coefficient
of the regression lines were 2.51 and 0.97 (phenyl acetate), 1.78 and
0.98 (3-pentylbenzene), 1.82 and 0.98 (2-pentylbenzene), 2.09 and
0.98 (n-pentylbenzene), 0.68 and 0.99 (cumene), 0.73 and 0.99 (n-
propylbenzene), 0.74 and 0.99 (α-methylstyrene), 0.72 and 0.99
(trans-β-methylstyrene), 1.55 and 0.98 (biphenyl), 2.78 and 0.99 (1-
pentene), 2.9 and 0.99 (2-pentene), respectively. Quantification of allyl
benzene was estimated using the slope and correlation coefficient of
the regression lines for cumene. Quantification of cis-β-methylstyrene
was estimated using the slope and correlation coefficient of the
regression lines for trans-β-methylstyrene. For the GC/MS instrument,
the slope and correlation coefficient of the regression lines were 0.63
and 0.99 (4-ethylanisole), 0.56 and 0.99 (4-ethylchlorobenzene), 0.29
and 0.99 (3-ethylchlorobenzene), 0.55 and 0.99 (n-propylbenzene),
and 0.55 and 0.99 (n-pentylbenzene). Quantification of 2-propylani-
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using relative peak areas versus the internal standard (hexamethyl-
benzene).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b01165.
■
S
Temperature Variation Experiments. A stock solution contain-
ing 1 (0.001 mol % relative to benzene), hexamethylbenzene (20 equiv
relative to Rh), and benzene (200 mL) was prepared in a volumetric
flask. Glass Fisher-Porter reactors were charged with stock solution
(10 mL) and Cu(OAc)2 (240 equiv relative to 1). The vessels were
sealed, charged with propylene (25 psig), and subsequently stirred and
heated to 120, 150, or 180 °C. The reaction was sampled every 4 h for
the first 12 h, then at the 24 h time point, and then every 24 h after
that. At each time point, the reactors were cooled to room
temperature, sampled, recharged with propylene, and reheated.
Aliquots of the reaction (<100 μL) mixture were analyzed by GC/
FID using relative peak areas versus an internal standard
(hexamethylbenzene).
Rhodium Loading Experiments. Three stock solutions contain-
ing 1 (0.001 mol % relative to benzene, 0.005 mol % relative to
benzene, or 0.01 mol % relative to benzene), hexamethylbenzene (20
equiv relative to Rh), and benzene (200 mL) were prepared in
volumetric flasks. Glass Fisher-Porter reactors were charged with stock
solution (10 mL) and Cu(OAc)2 (240 equiv relative to 1). The vessels
were sealed, charged with propylene (25 psig), and subsequently
stirred and heated to 150 °C. The reaction was sampled every 4 h for
the first 12 h, then at the 24 h time point, and then every 24 h
subsequently. At each time point, the reactors were cooled to room
temperature, sampled, recharged with propylene, and reheated.
Aliquots of the reaction (<100 μL) mixture were analyzed by GC/
FID using relative peak areas versus an internal standard
(hexamethylbenzene).
Effect of temperature, copper oxidant identity, oxidant
concentration, rhodium loading, high-TO experiment,
isomerization, representative mass spectrum (PDF)
AUTHOR INFORMATION
Corresponding Author
*tbg7h@virginia.edu
ORCID
Benjamin A. Vaughan: 0000-0001-9607-5696
T. Brent Gunnoe: 0000-0001-5714-3887
Notes
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The authors declare no competing financial interest.
§Retired. Consulting scientist, San Rafael, CA 94903.
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), an AES
Graduate Fellowship in Energy Research (M.S.W.-G.), and a
University of Virginia Jefferson Arts & Sciences Dissertation
Year Fellowship (M.S.W.-G.)
High-Turnover Experiment. A stock solution containing 1 (0.001
mol % relative to benzene), hexamethylbenzene (20 equiv relative to
1), and benzene (200 mL) was prepared in a volumetric flask. Glass
Fisher-Porter reactors were charged with stock solution (10 mL) and
Cu(OPiv)2 (2400 equiv relative to 1). The vessels were sealed,
charged with propylene (50 psig), and subsequently stirred and heated
to 150 °C. The reaction was sampled at 12 h, 24 h, 48 h, 72 h and 96
h. At each time point, the reactors were cooled to room temperature,
sampled, recharged with propylene, and reheated. Aliquots of the
reaction (<200 μL) mixture were analyzed by GC/FID using relative
peak areas versus the internal standard (hexamethylbenzene). After the
48 h sampling, an additional 1200 equiv Cu(OPiv)2 was added.
Arene Identity Experiments. A stock solution containing 1 (0.01
mol % relative to arene), hexamethylbenzene (20 equiv relative to Rh),
and arene (100 mL) was prepared in a volumetric flask. Glass Fisher-
Porter reactors were charged with stock solution (10 mL) and oxidant
(240 equiv relative to 1). The vessels were sealed, charged with
propylene (25 psig), and subsequently stirred and heated to 150 °C.
The reaction was sampled after 24 h, 48 h, and 72 h. At each time
point, the reactors were cooled to room temperature, sampled,
recharged with propylene, and reheated. Aliquots of the reaction
(<100 μL) mixture were analyzed by GC/MS using relative peak areas
versus the internal standard (hexamethylbenzene).
Isomerization of 1-Pentene. A stock solution containing 1 (0.01
mol % relative to benzene), hexamethylbenzene (20 equiv relative to
1), and benzene (200 mL) was prepared in a volumetric flask. Either 1-
pentene or 2-pentene (2000 equiv relative to 1) was added to the
stock solution. Glass Fisher-Porter reactors were charged with stock
solution (10 mL), and, if required, Cu(OAc)2 (240 equiv relative to 1)
was added. An initial sample (100 μL) was taken before heating (t = 0
h) and analyzed by GC/FID to determine the ratio of 1-pentene and
2-pentene. The vessels were sealed, stirred, and heated to 150 °C. The
reaction was sampled at 48 h and 72 h. At each time point, the reactors
were cooled to room temperature, sampled at 0 °C and reheated.
Aliquots of the reaction (<100 μL) mixture were analyzed by GC/FID
using relative peak areas versus the internal standard (hexamethyl-
benzene). The total concentration of pentenes decreases over time due
to the high volatility.
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