Rhodium-Catalyzed Alkenylation of Toluene Using 1-Pentene: Regioselectivity to Generate Precursors for Bicyclic Compounds

Article abstract of DOI:10.1021/acs.organomet.9b00535

Rhodium catalysts for arene alkenylation reported by our group (e.g., Science 2015, 348, 421; J. Am. Chem. Soc. 2017, 139, 5474; J. Am. Chem. Soc. 2018, 140, 17007) have demonstrated selectivity for 1-aryl alkenes over y-aryl alkenes (y > 1). This selectivity is notable because 1-aryl alkenes or 1-aryl alkanes cannot be generated using acid-based Friedel-Crafts arene alkylation or acidic zeolite catalysts. Herein, we report the extension of Rh arene alkenylation catalysis to generate 1-tolyl-1-pentenes, which are potential precursors for bicyclic compounds. The olefin concentration, copper(II) oxidant identity and concentration, reaction temperature, and rhodium concentration for the alkenylation of toluene with 1-pentene have been optimized using [Rh(Η²-C₂H₄)₂(μ-OAc)]₂ as the catalyst precursor. The rhodium-based catalysis achieves up to 12(1):1 anti-Markovnikov selectivity for 1-tolyl-1-pentenes over 2-tolyl-2-pentenes and is selective for alkenylation in the meta and para positions.

Full text of DOI:10.1021/acs.organomet.9b00535

Article
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Cite This: Organometallics XXXX, XXX, XXX−XXX
Rhodium-Catalyzed Alkenylation of Toluene Using 1‑Pentene:
Regioselectivity To Generate Precursors for Bicyclic Compounds
Nichole S. Liebov,† Weihao Zhu,† Junqi Chen,† Michael S. Webster-Gardiner,† William L. Schinski,‡
and T. Brent Gunnoe*,†
†Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States
S
* Supporting Information
ABSTRACT: Rhodium catalysts for arene alkenylation re-
ported by our group (e.g., Science 2015, 348, 421; J. Am. Chem.
Soc. 2017, 139, 5474; J. Am. Chem. Soc. 2018, 140, 17007) have
demonstrated selectivity for 1-aryl alkenes over y-aryl alkenes (y
> 1). This selectivity is notable because 1-aryl alkenes or 1-aryl
alkanes cannot be generated using acid-based Friedel−Crafts
arene alkylation or acidic zeolite catalysts. Herein, we report the
extension of Rh arene alkenylation catalysis to generate 1-tolyl-
1-pentenes, which are potential precursors for bicyclic
compounds. The olefin concentration, copper(II) oxidant identity and concentration, reaction temperature, and rhodium
concentration for the alkenylation of toluene with 1-pentene have been optimized using [Rh(η2-C2H4)2(μ-OAc)]2 as the
catalyst precursor. The rhodium-based catalysis achieves up to 12(1):1 anti-Markovnikov selectivity for 1-tolyl-1-pentenes over
2-tolyl-2-pentenes and is selective for alkenylation in the meta and para positions.
acid based pathway that involves olefin insertion into metal-
aryl bonds and metal-mediated arene C−H activation, offer the
potential for different selectivity compared with the traditional
acid-based catalysis.8,9 For example, as discussed above, acid-
catalyzed alkylation using α-olefins forms branched products
(i.e., y-aryl alkanes with y ≥ 2) selectively due to the formation
of a carbocationic intermediate upon proton addition to the
olefin. In contrast, SLABs can be generated from transition
metal mediated processes that involve olefin insertion into
metal-aryl bonds (Scheme 2).10 Despite the opportunity to
produce SLABs, the selective generation of linear alkyl arenes
or alkenyl variants from unactivated hydrocarbon substrates
has been a challenge.10−17
INTRODUCTION
■
Alkyl and alkenyl arenes are produced on a large scale and used
as precursors for, inter alia, detergents, plastics, and
elastomers.1−4 One industrial method for alkyl arene synthesis
from arenes and alkenes uses Friedel−Crafts alkylation
catalysis with Lewis and Brønsted acids (e.g., HF and
AlCl3).1,5 Zeolites2 can also catalyze arene alkylation and
offer the advantage of recyclability.6 However, because zeolite
catalysts also operate through an acid-catalyzed mechanism,
many features of Friedel−Crafts catalysis are also observed for
zeolite-catalyzed arene alkylation. For example, for arene
alkylation using α-olefins, acid catalysts do not produce 1-aryl
alkanes. In addition, carbonium intermediates from α-olefins
can lead to products from oligomerization. This is a result of
the formation of carbocationic intermediates upon electro-
philic addition to the olefin that results in preferential reactivity
at the more substituted carbon to give the Markovnikov
product (Scheme 1). Thus, for arene alkylation with long-chain
α-olefins, y-aryl alkanes (where y > 1) are produced. In
industry, y-aryl alkanes (where y > 1) with linear alkane
moieties are known as linear alkylbenzenes (LABs).7 LABs are
primarily comprised of 2- and 3-phenyl alkanes and do not
include 1-phenyl alkanes. To differentiate LABs from 1-phenyl
alkanes, we have termed the latter super linear alkyl benzenes
(SLABs). Additionally, the regioselectivity of acid-based arene
alkylation is dictated by the donor properties of the substituent
on the arene ring. For electron-donating substituents, ortho/
para selectivity is observed while electron-poor arenes are
generally unreactive or undergo slow reaction.
Our group has studied ruthenium9,18−26 and platinum27−32
catalysts for olefin hydroarylation, but selectivity for the anti-
Markovnikov SLAB products when using α-olefins was either
modest or the catalysts were selective for the branched
Markovnikov products. A ruthenium catalyst from our group
achieved 1.6:1 linear:branched (L:B) ratio for propylbenze-
ne:cumene from benzene and propylene,18 as did iridium
catalysts studied by the Periana and Goddard groups.14,15
Goldberg and co-workers obtained a L:B ratio of up to ∼1.4:1
for the production of hexylbenzene using 1-hexene and
benzene with a Pt pre-catalyst,17,33 and the Hartwig group
reported a 19:1 L:B ratio for the generation of octylbenzene
from 1-octene and benzene using stoichiometric amounts of a
Ni complex.13
Transition metal-mediated olefin hydroarylation and oxida-
tive arene alkenylation reactions, which often operate by a non-
Received: August 6, 2019
© XXXX American Chemical Society
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a
Scheme 1. Mechanism of Friedel−Crafts Alkylation of Benzene with Propylene
a
The generation of a carbocationic intermediate dictates the selective formation of cumene, the branched Markovnikov product.
production of meta- and para-substituted alkenyl arenes,
whereas acid-based catalysis typically favors the production
of ortho- and para-substituted alkenyl arenes. Thus, the arene
regioselectivity for the Rh-catalyzed process is likely due to
steric effects rather than electronic effects.
Scheme 2. Comparison of Products That Have Been
Observed Using Transition Metal (TM) Catalyzed and
Acid-Catalyzed Arene Alkenylation/Alkylation Methods
Using Benzene and Propylene
Although there is precedent for 1-tolyl-1-pentenes and 2-
tolyl-2-pentenes serving as intermediates to form bicyclic
dimethylnaphthalenes,38−40 acid-catalyzed arene alkylation
reactions do not selectively produce 1-tolyl-1-pentenes and
2-tolyl-2-pentenes. Despite its dimensional stability, resistance
to heat, and barrier to gas permeability relative to the current
polyester market leader polyethylene terephthalate,41−43 the
production of polyethylenenaphthalate (PEN) polymer has
been limited since the methods for 2,6-dimethylnaphthalene
(2,6-DMN) production typically require multiple steps and
harsh reaction conditions (see Scheme 3 for one example of a
route to 2,6-DMN).41,44 Thus, the development of a more
economical method to produce 2,6-DMN is highly desirable as
it is a precursor for PEN polymer.42
We speculated that rhodium-catalyzed alkenylation of
toluene with 1-pentene could provide a strategy to selectively
generate 1-tolyl-1-pentenes (Schemes 4 and 5) using readily
available starting materials such as toluene and 1-pentene.
Thus, this rhodium-based catalysis could be advantageous in
terms of selectivity relative to acid-based catalysis, which favor
the formation of the 2-tolyl-2-pentanes and 3-tolyl-2-pentanes
(Scheme 6). As 2,6-DMN can be generated from isomerization
of other regioisomers of dimethylnaphthalene, meta, para, and
ortho isomers are not specified in Scheme 5. Herein, we report
on our results from rhodium-catalyzed toluene alkenylation
with 1-pentene to selectively generate 1-tolyl-1-pentenes that
are potential precursors for 2,6-DMN.
Recently, our group reported Rh-catalyzed arene alkenyla-
tion utilizing the Rh(I) catalyst precursor (FlDAB)Rh(TFA)-
(η2-C2H4) (FlDAB = N,N′-bis(pentafluorophenyl)-2,3-dimeth-
yl-1,4-diaza-1,3-butadiene) with CuX2 {X = acetate (OAc),
trifluoroacetate (TFA), pivalate (OPiv) or 2-ethyl hexanoate
(OHex)} as in situ oxidant.11 Using (FlDAB)Rh(TFA)(η2-
C2H4) as a catalyst precursor results in the conversion of
benzene, ethylene, and CuX2 to styrene, CuX and HX with
yields of ≥95% with high selectivity for styrene.11 Given the
success of (FlDAB)Rh(TFA)(η2-C2H4) for arene alkenyla-
tion,11,34 the catalysis was extended to α-olefins in order to
study L:B selectivity.10 Herein, 1-aryl alkanes are considered to
be linear, and y-aryl alkanes (where y > 1) are considered to be
branched (traditional linear alkylbenzenes, LABs, are y-aryl
alkanes where y ≥ 2). With the rhodium precursor [Rh(η2-
C2H4)2(μ-OAc)]2, products that give 1-aryl alkanes upon
hydrogenation are favored (Table 1).10,35 The lowest observed
L:B ratio in our previous study was ∼8:1. Additionally, for
monosubstituted arenes the rhodium catalysis favors the
a
Table 1. Examples of Arene Alkenylation Using [Rh(η2-C2H4)2(μ-OAc)]2 and Cu(OAc)2 versus AlCl3
a
L:B ratios with [Rh(η2-C2H4)2(μ-OAc)]2 were determined after hydrogenation of the unsaturated products. NR = not reported. Conditions using
[Rh(η2-C2H4)2(μ-OAc)]2: 0.01 mol % or Rh relative to arene, 25 psi gaseous olefin or 2000 equiv of olefin, 150 °C, 48 h, 240 equiv Cu(OAc)2
b
c
relative to Rh. Hydrogenation was performed using 5% Pt/C under hydrogen atmosphere. Data and conditions from ref 14. Data and conditions
from ref 36. Data and conditions from ref 37. 72 h. Reprinted (adapted) with permission from J. Am. Chem. Soc. 2017, 139, 5474−5480.10
d
e
Copyright 2017 American Chemical Society.
B
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a
Scheme 3. Overview of an Industrial Route for the Synthesis of 2,6-Dimethylnaphthalene from o-Xylene and Butadiene
a
K catalyst = calcined CaO stirred with potassium metal at 150 °C. Adapted from refs 41, 44, 45. 2,6-DMN can also be produced through di-
isopropylation of naphthalene using zeolite catalysts.41
Scheme 4. Envisioned Process for a Possible Route to 2,6-DMN from Toluene and 1-Pentene Using a Rh Catalyst with a CuX2
a
Oxidant
a
A hydrogenation step is included for clarity (vide infra).
Scheme 5. Proposed Catalytic Cycle for Transition Metal-Catalyzed Toluene Alkenylation with 1-Pentene To Give
a
Tolylpentenes
a
Linear versus branched selectivity is influenced by the olefin insertion step (Tol = tolyl).
Scheme 6. Comparison of Tolylpentane Products from Acid- and Rh-Based Functionalization and Implications for 2,6-DMN
a
Production
a
Products from Rh catalysis are shown following a hydrogenation step. Meta, para, and ortho isomers are not shown, as 2,6-dimethylnaphthalene
can be produced from isomerization.
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Scheme 7. Examples of Possible Products and Side Products from the Rh-Catalyzed Toluene Alkenylation Reaction
a
Table 2. Effect of 1-Pentene Amount on Tolylpentane and Decane Production
equiv 1-pentene
m:p
L:B
% yield tolylpentanes {TO}
equiv decane produced
250
500
1000
2000
2.4:1
1.8:1
1.9:1
1.7:1
7.2(7):1
10(1):1
11.8(4):1
15(1):1
60(4)% {71(4)}
72(5)% {87(6)}
64(4)% {77(5)}
47(4)% {57(5)}
ND
2.6(7)
7.81(5)
b
18(1)
a
Conditions: 0.01 mol % Rh {Rh source = [Rh(η2-C2H4)2(μ-OAc)]2} relative to toluene, 10 equiv of cyclooctane relative to Rh, toluene, 240 equiv
Cu(OPiv)2 relative to Rh, 250−2000 equiv 1-pentene relative to Rh, 150 °C, 1−13 h. Hydrogenation was achieved using 10% Pd/C under
hydrogen atmosphere. Yields represent the total percent yield of tolylpentanes based on the amount of the copper(II) oxidant. Percent yields are
b
the average of at least three experiments with standard deviations given in parentheses. ND = not detected. Decane was not fully hydrogenated in
one of the three trials; thus, data for decane formation are from two experiments.
Scheme 8. Examples of Possible Tolylpentenes Funneling to 1- or 2-Tolylpentanes on Hydrogenation
coupling, and pentenyl or tolyl pivalates from reaction with
Cu(OPiv)2. Since our preliminary results indicated that
tolylpentenes likely could be generated with both meta/para
and anti-Markovnikov selectivity using the rhodium catalyst
precursor [Rh(η2-C2H4)2(μ-OAc)]2 and Cu(OPiv)2 as the
oxidant, we sought to elucidate the details of toluene/1-
pentene alkenylation. The desired products for the oxidative
alkenylation of toluene with 1-pentene are 1-tolyl-1-pentenes
and 2-tolyl-2-pentenes, since they can be readily ring closed to
yield 2,6-DMN.38−40 Given the potential complexity of
product analysis, to simplify quantification we hydrogenated
the product mixtures to give tolylpentanes prior to analysis by
GC−MS.
RESULTS AND DISCUSSION
■
Previous results using the rhodium catalyst precursor [Rh(η2-
C2H4)2(μ-OAc)]2 indicated that toluene and 1-pentene were
reactive toward the generation of linear alkenyl arenes via
oxidative alkenylation with other coupling partners (e.g.,
benzene and 1-pentene, toluene and propylene).10 However,
due to the complexity of the resultant product mixture with
toluene and 1-pentene (vide infra), the alkenylation of toluene
with 1-pentene was not pursued in this previous study. Some
possible products from the Rh-catalyzed toluene alkenylation
with 1-pentene are shown in Scheme 7 and include ortho/
meta/para regioisomers, different products from alkenylation
at the 1-, 2-, or 3-position of pentene (isomerization of 1-
pentene to 2-pentene provides a potential pathway for the
formation of arylation at the 3-position of pentene), cis/trans
isomers for all alkenyl products, C10 compounds resulting from
1-pentene coupling, bis-aryl products resulting from toluene
Using [Rh(η2-C2H4)2(μ-OAc)]2 as a pre-catalyst, toluene
and 1-pentene are converted to pentenyltoluenes with Cu(II)
oxidants in neat toluene at 150 °C (Table 2). The production
of 3-tolyl-2-pentenes is not observed under the reaction
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Scheme 9. Possible Explanation of the Influence of 1-Pentene Concentration on the Ratio of 1-Tolyl-1-pentene to 2-Tolyl-2-
a
pentene (i.e., L:B Ratio)
a
Higher 1-pentene concentrations might inhibit olefin isomerization that would generate 2-tolyl-2-pentene from 1-tolyl-1-pentene.
conditions reported herein. When using (5-FP)Rh(TFA)(η2-
C2H4) (5-FP = 1,2-bis(N-7-azaindolyl)benzene) as pre-catalyst
(see below) only trace amounts of 3-tolyl-2-pentenes are
observed. Therefore, after hydrogenation the observed toluene
alkenylation products include ortho, meta, and para isomers of
1-tolylpentanes and 2-tolylpentanes (Scheme 8). High L:B
ratios (linear:branched ratios) are calculated as the ratio of all
ortho, meta, and para isomers of 1-tolyl-1-pentenes (i.e., linear)
to all 2-tolyl-2-pentenes (i.e., branched) after hydrogenation.
The L:B ratios are typically >7:1 with higher ratios of ∼15:1
under certain conditions (Table 2). After hydrogenation, C10
products, which are likely formed via 1-pentene coupling
reactions, are observed as decane. Cyclooctane, which was
found to be inert under the reaction conditions (see
Supporting Information Table S1), was used as an internal
standard for the catalytic reactions.
The effects of the concentration of 1-pentene, concentration
of Cu(II) oxidant, identity of Cu(II) oxidant, reaction
temperature, and rhodium concentration have been examined.
The amount of 1-pentene for the toluene alkenylation reaction
was varied from 250 to 2000 equiv (relative to Rh), with 0.01
mol % Rh and 240 equiv of Cu(OPiv)2 at 150 °C. The L:B
ratio (i.e., the ratio of 1-tolyl-1-pentenes to 2-tolyl-2-pentenes)
increases from 7.2(7):1 using 250 equiv of 1-pentene to
15(1):1 using 2000 equiv of 1-pentene. We propose that the
higher olefin concentrations might enable more rapid
dissociation of pentenyltoluenes from the Rh center, thus
reducing the extent of isomerization and in turn, the amount of
branched products (Scheme 9). In addition, 2-tolyl-2-pentenes
can potentially form from 2-pentene, which is formed from
isomerization of 1-pentene. Further, although the L:B ratio
varies with olefin concentration, the yields of tolylpentanes,
∼65% based on Cu(OPiv)2 as the limiting reagent, are
consistent as the 1-pentene concentration is varied with the
exception of the reaction with 2000 equiv of 1-pentene where
the percent yield dropped to 47(4)%. Using 1-pentene
amounts ≥500 equiv (relative to Rh), several C10 products
that likely result from 1-pentene coupling are observed. To
simplify quantification of 1-pentene coupling products, these
C10 products were converted to decane by hydrogenation (see
Table 2). Following hydrogenation, 4-methylnonane and
pentyl pivalate were also observed by GC−MS in minor
amounts but were not quantified. Control reactions in the
absence of the rhodium catalyst precursor [Rh(η2-C2H4)2(μ-
OAc)]2 did not result in the production of decane, indicating
that the Rh complex is likely involved in the generation of C10
products. The preference for formation of the linear 1-
tolylpentanes, similar to the preference for n-pentylbenzenes
over 2-pentylbenzenes in the initial report for the alkenylation
of benzene with 1-pentene, indicates that isomerization of 1-
pentene to 2-pentene is slow under the reaction conditions or
that the catalyst reacts more slowly with 2-pentene than 1-
pentene.10
In addition to the selectivity for 1-tolyl-1-pentene, 2-tolyl-2-
pentene, and 3-tolyl-2-pentene, we elucidated the ortho, meta,
and para regioselectivity. The generation of meta and para
products is favored, accounting for ∼60−67% and ∼29−34%,
respectively, of the tolylpentene products with ortho products
accounting for <5% of the total (Figure 1). Thus, the meta/
para ratio for the Rh catalyzed toluene alkenylation with 1-
pentene is ∼2:1, which demonstrates nearly identical
selectivity for C−H activation of the meta and para positions
of toluene after adjustment for the statistical ratio of meta and
para hydrogen atoms. We presume that the ortho position is
sterically protected by the methyl group, which likely raises the
barrier for either access to Rh coordination of the ortho C−H
bond or Rh-mediated breaking of the ortho C−H bond. Similar
selectivity has been observed previously for organometallic C−
H activation of toluene.10,33,46,47
To examine the effect of varying the amount of Cu(II)
oxidant, 120−360 equiv of Cu(OPiv)2 (relative to Rh) were
used for toluene alkenylation with 1000 equiv of 1-pentene
(relative to Rh). The L:B ratios are similar for each condition,
ranging from ∼12:1 to 14:1. The yield of tolylpentanes, based
on Cu(II), is statistically identical for each reaction, which
demonstrates that tolylpentane generation increases propor-
tionately as a function of Cu(OPiv)2 (Table 3). Therefore,
under the conditions studied conversions of 1-pentene are
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Figure 2. Tolylpentane product distribution as a function of
Cu(OPiv)2 amount. Error bars denote standard deviations based on
three experiments. Conditions: 0.01 mol % Rh {Rh source = [Rh(η2-
C2H4)2(μ-OAc)]2} relative to toluene, 10 equiv cyclooctane relative
to Rh, toluene, 120−360 equiv Cu(OPiv)2 relative to Rh, 1000 equiv
1-pentene relative to Rh, 150 °C, 1−2 h. Hydrogenation was achieved
using 10% Pd/C under hydrogen atmosphere.
Figure 1. Tolylpentane product distribution as a function of 1-
pentene amount (relative to Rh). Error bars denote standard
deviations based on at least three experiments. Conditions: 0.01
mol % Rh {Rh source = [Rh(η2-C2H4)2(μ-OAc)]2} relative to
toluene, 10 equiv of cyclooctane relative to Rh, toluene, 240 equiv
Cu(OPiv)2 relative to Rh, 250−2000 equiv 1-pentene relative to Rh,
150 °C, 1−13 h. Hydrogenation was achieved using 10% Pd/C under
hydrogen atmosphere.
preference for meta- and para-tolylpentanes does not vary with
identity of copper(II) oxidant (Figure S14). The exact reason
for the variation in the L:B ratio as a function of the identity of
the Cu(II) oxidant is uncertain, but the carboxylate (i.e., OPiv,
OHex, or Ole) of the Cu(II) oxidant is likely coordinated to
Rh during all steps of catalysis, and thus it is not surprising that
the identity of the carboxylation influences the selectivity of
olefin insertion. These results suggest that the L:B selectivity
might be more influenced by the Rh coordination sphere than
the regioselectivity for C−H activation of monosubstituted
arenes. CuCl2 was also examined as a potential oxidant for the
toluene alkenylation reaction, but only trace functionalization
was observed under the reaction conditions shown in Table 4
(see Supporting Information, Figure S15). We speculate that
the carboxylate ligand, which originates from the Cu(II)
oxidant, is necessary for toluene C−H activation. Further,
using CuCl2 the selectivity is for branched products with ortho
and para selectivity, which indicates a Friedel−Crafts type
reaction. It seems likely that HCl is generated in situ from the
CuCl2, and the HCl/CuCl2 mixture results in some arene
alkylation. From 120 to 180 °C, the most pronounced L:B
ratio observed is ∼19:1 (Table 5). Yields of tolylpentanes are
similar across this temperature range as is the production of
decane (Table 5). As indicated above, the formation of meta-
apparently limited only by the amount of Cu(II). This finding
is consistent our previous study regarding benzene alkenyla-
tion, in which we demonstrated that the decomposition of the
Rh catalyst [Rh(η2-C2H4)2(μ-OAc)]2 at 150 °C could be
suppressed through the addition of soluble Cu(II) oxidants
such as Cu(OPiv)2.35 Under conditions with soluble oxidants,
no evidence for the formation of elemental Rh is obtained.
Similar to the conditions described above, the production of
meta- and para-pentenyltoluenes is favored (Figure 2). Arene
alkenylation is not observed in the absence of Rh using
Cu(OPiv)2 under the reaction conditions shown in Table 3.
In order to determine if another soluble copper(II) oxidant
source could improve the L:B ratio or the yield of
tolylpentanes, Cu(OHex)2 and Cu(Ole)2 (OHex = 2-ethyl-
hexanoate, Ole = oleate) were tested for toluene alkenylation
under identical reaction conditions shown in Table 3. The L:B
ratio varies as a function of the copper(II) oxidant, from
11.8(4):1 with Cu(OPiv)2 to 7.4(5):1 with Cu(OHex)2 to
14.7(4):1 with Cu(Ole)2. The yields of tolylpentanes are
similar for Cu(OPiv)2 and Cu(OHex)2, approximately 70%,
but are significantly lower with Cu(Ole)2 (Table 4). The
a
Table 3. Variation of Copper(II) Oxidant Amount on Tolylpentane and Decane Production
equiv Cu(OPiv)2
m:p
L:B
% yield tolylpentanes {TO}
equiv decane produced
120
240
360
1.7:1
1.9:1
1.9:1
14(2):1
11.8(4):1
14.2(4):1
65(5)% {39(3)}
64(4)% {77(5)}
65(3)% {117(5)}
6.6(6)
7.81(5)
13(2)
a
Conditions: 0.01 mol % Rh {Rh source = [Rh(η2-C2H4)2(μ-OAc)]2} relative to toluene, 10 equiv of cyclooctane relative to Rh, toluene, 120−360
equiv Cu(OPiv)2 relative to Rh, 1000 equiv 1-pentene relative to Rh, 150 °C, 1−2 h. Hydrogenation was achieved using 10% Pd/C under
hydrogen atmosphere. % Yield represents the total % yield of tolylpentanes based on the amount of the copper(II) oxidant. Percent yields are the
average of at least three experiments with standard deviations given in parentheses.
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a
Table 4. Effect of Copper(II) Identity on Tolylpentane and Decane Production
Cu(II) oxidant
m:p
L:B
% yield tolylpentanes {TO}
equiv decane produced
Cu(OPiv)2
Cu(OHex)2
Cu(Ole)2
1.9:1
1.9:1
1.4:1
11.8(4):1
7.4(5):1
14.7(4):1
64(4)% {77(5)}
70(5)% {84(6)}
17(2)% {21(2)}
7.81(5)
11(1)
3.3(2)
a
Conditions: 0.01 mol % Rh {Rh source = [Rh(η2-C2H4)2(μ-OAc)]2} relative to toluene, 10 equiv of cyclooctane relative to Rh, toluene, 240 equiv
CuX2 relative to Rh, 1000 equiv 1-pentene relative to Rh, 150 °C, 2 h. Hydrogenation was achieved using 10% Pd/C under hydrogen atmosphere.
% Yield represents the total % yield of tolylpentanes based on the amount of the copper(II) oxidant. Percent yields are the average of at least three
experiments with standard deviations given in parentheses.
a
Table 5. Effect of Reaction Temperature on Tolylpentane and Decane Production
temperature (°C)
m:p
L:B
% yield tolylpentanes {TO}
equiv decane produced
120
150
180
1.6:1
1.7:1
1.4:1
19(1):1
14(2):1
16.2(7):1
66(2)% {40(1)}
65(5)% {39(3)}
66.7(9)% {40(1)}
7.2(2)
6.6(6)
6.5(4)
a
Conditions: 0.01 mol % Rh {Rh source = [Rh(η2-C2H4)2(μ-OAc)]2} relative to toluene, 10 equiv of cyclooctane relative to Rh, toluene, 120 equiv
Cu(OPiv)2 relative to Rh, 1000 equiv 1-pentene relative to Rh, 120−180 °C, 30−60 min. Hydrogenation was achieved using 10% Pd/C under
hydrogen atmosphere. % Yield represents the total % yield of tolylpentanes based on the amount of the copper(II) oxidant. Percent yields are the
average of at least three experiments with standard deviations given in parentheses.
a
Table 6. Effect of Rh Concentration on Tolylpentane and Decane Production
mol % Rh
m:p
L:B
% yield tolylpentanes {TO}
equiv decane produced
0.001
0.005
0.01
1.8:1
1.8:1
1.8:1
12(1):1
15.4(7):1
10(1):1
77(3)% {93(3)}
84(3)% {101(4)}
72(5)% {87(6)}
<1
2.9(4)
2.6(7)
a
Conditions: 0.001−0.01 mol % Rh {Rh source = [Rh(η2-C2H4)2(μ-OAc)]2} relative to toluene, 10 equiv of cyclooctane relative to Rh, toluene,
240 equiv Cu(OPiv)2 relative to Rh, 500 equiv 1-pentene relative to Rh, 150 °C, 3−30 h. Hydrogenation was achieved using 10% Pd/C under
hydrogen atmosphere. % Yield represents the total percent yield of tolylpentanes based on the amount of the copper(II) oxidant. Percent yields are
the average of at least three experiments with standard deviations given in parentheses.
a
Table 7. Effect of Rh Sources and Added HOPiv on Tolylpentane and Decane Production
Rh source
HOPiv (equiv)
m:p
L:B
% yield tolylpentanes {TO}
equiv decane produced
[Rh(η2-C2H4)2(μ-OAc)]2
(5-FP)Rh(TFA)(η2-C2H4)
(5-FP)Rh(TFA)(η2-C2H4)
0
0
2400
1.8:1
1.7:1
2.5:1
15.4(7):1
17(1):1
18(1):1
84(3)% {101(4)}
79(2)% {95(2)}
76(9)% {91(11)}
2.9(4)
2.3(5)
5(1)
a
Conditions: 0.005 mol % Rh {Rh source = [Rh(η2-C2H4)2(μ-OAc)]2 or (5-FP)Rh(TFA)(η2-C2H4)} relative to toluene, 10 equiv of cyclooctane
relative to Rh, toluene, 240 equiv Cu(OPiv)2 relative to Rh, 500 equiv 1-pentene relative to Rh, 0 or 2400 equiv HOPiv relative to Rh, 150 °C, 3−
30 h. Hydrogenation was achieved using 10% Pd/C under hydrogen atmosphere. % Yield represents the total percent yield of tolylpentanes based
on the amount of the copper(II) oxidant. Percent yields are the average of at least three experiments with standard deviations given in parentheses.
*3-Tolyl-3-pentane was detected in trace amount for all trials with (5-FP)Rh(TFA)(η2-C2H4), but was not quantified.
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a
Table 8. Catalytic Toluene Alkenylation Using 1-Pentene under Aerobic Conditions
Rh source
m:p
L:B
% yield tolylpentanes {TO}
equiv decane produced
[Rh(η2-C2H4)2(μ-OAc)]2
(5-FP)Rh(TFA)(η2-C2H4)
2.4:1
2.9:1
9.2(2):1
10(1):1
84(6)% {101(7)}
93(8)% {112(9)}
5.3(2)
3.9(5)
a
Conditions: 0.005 mol % Rh {Rh source = [Rh(η2-C2H4)2(μ-OAc)]2 or (5-FP)Rh(TFA)(η2-C2H4)} relative to toluene, 10 equiv of cyclooctane
relative to Rh, toluene, 240 equiv Cu(OPiv)2 relative to Rh, 500 equiv 1-pentene relative to Rh, 2400 equiv HOPiv relative to Rh, air (1 atm), 150
°C, 24 h. Hydrogenation was achieved using 10% Pd/C under hydrogen atmosphere. % Yield represents the total percent yield of tolylpentanes
based on the amount of the copper(II) oxidant. Percent yields are the average of at least three experiments with standard deviations given in
parentheses.
a
Table 9. Study of Product Inhibition for Aerobic Toluene Alkenylation
equiv added tolylpentanes
0
m:p
L:B
% yield tolylpentanes {TO}
equiv decane produced
2.9:1
2.4:1
2.4:1
10(1):1
9.9(2):1
9.2(1):1
93(8)% {112(9)}
47(5)% {56(6)}
39(7)% {47(9)}
3.9(5)
1.0(1)
0.8(1)
b
160
c
160
a
Conditions: 0.005 mol % Rh {Rh source = (5-FP)Rh(TFA)(η2-C2H4)} relative to toluene, 10 equiv of cyclooctane relative to Rh, toluene, 240
equiv Cu(OPiv)2 relative to Rh, 500 equiv 1-pentene relative to Rh, 2400 equiv HOPiv relative to Rh, 0 or 160 equiv added tolylpentenes, air (1
b
c
atm), 150 °C, 24 h. Hydrogenation was achieved using 10% Pd/C under hydrogen atmosphere. Using meta-tolyl-1-pentene. Using para-tolyl-1-
pentene. % Yield represents the total percent yield of tolylpentanes based on the amount of the copper(II) oxidant. Percent yields are the average of
at least three experiments with standard deviations given in parentheses.
and para-tolylpentanes is favored at each temperature (Figure
S16).
(Table 7). The addition of HOPiv did not inhibit the synthesis
of tolylpentanes, which is significant as the presence of added
acid is proposed to enable more efficient recycling of the
Cu(II) oxidant by minimizing the formation of Cu oxides from
Cu(OPiv) and H2O.12 Under all reaction conditions, the
formation of meta- and para-tolylpentanes was favored (Figure
S18). The ratio of L:B products does not change with time
(see Supporting Information, Table S2).
To elucidate the influence of the rhodium concentration on
the yield and selectivity of the toluene alkenylation reaction,
we examined the use of a solution with 0.001 mol % of Rh
relative to toluene. With 0.001 mol % of Rh, the L:B ratio,
12(1):1, is statistically identical to the L:B ratio (10(1):1) with
the higher Rh concentration of 0.01 mol % (Table 6). Also, the
production of C10 products is minimized when using the lower
concentration of Rh. The use of 0.005 mol % Rh results in a
slightly higher L:B ratio of 15.4(7):1 with >80% yield of
tolylpentanes relative to the Cu(II) oxidant. Under each
reaction condition, the generation of meta- and para-
tolylpentanes is favored (Figure S17).
We also tested the ability of [Rh(η2-C2H4)2(μ-OAc)]2 and
(5-FP)Rh(TFA)(η2-C2H4) to catalyze the toluene alkenylation
reaction under aerobic conditions. Aerobic catalysis was
performed in the presence of air and excess HOPiv in order
to regenerate the Cu(II) oxidant in situ. Excess carboxylic acid
has been shown to stabilize Cu(I) and promote its aerobic
regeneration to Cu(II) by preventing reaction with water to
generate insoluble and less easily oxidized Cu(I) hydroxide.35
The 93(8)% yield of tolylpentenes using (5-FP)Rh(TFA)(η2-
C2H4) after 24 h is statistically identical to the 84(6)%
observed using [Rh(η2-C2H4)2(μ-OAc)]2 (Table 8). The L:B
ratio under aerobic conditions is lower than typical L:B ratios
observed under anaerobic conditions. Similar to the anaerobic
conditions, aerobic catalysis with [Rh(η2-C2H4)2(μ-OAc)]2
and (5-FP)Rh(TFA)(η2-C2H4) exhibits selectivity toward
meta- and para-tolylpentanes (Figure S19).
Previously, we reported the use of the complex (5-
FP)Rh(TFA)(η2-C2H4) as an air-stable Rh(I) catalyst for
benzene alkenylation.12 Whereas under previously reported
conditions [Rh(η2-C2H4)2(μ-OAc)]2 is not an effective catalyst
for arene alkenylation under aerobic conditions, (5-FP)Rh-
(TFA)(η2-C2H4) was demonstrated to efficiently catalyze
arene alkenylation under aerobic conditions, enabling in situ
Cu(II) regeneration.
Similar to [Rh(η2-C2H4)2(μ-OAc)]2, (5-FP)Rh(TFA)(η2-
C2H4) catalyzes the synthesis of tolylpentanes from toluene
and pentane (Table 7). Both catalysts were tested under inert
atmosphere to obtain an initial direct comparison. Yields of
tolylpentanes were approximately 80% with each catalyst, with
L:B ratios of up to 18:1 with (5-FP)Rh(TFA)(η2-C2H4)
Under aerobic conditions we anticipated an increase in
percent yield of pentenyltoluenes relative to that observed
under anaerobic conditions. However, the increase in yield
under aerobic conditions (93(8)%) was statistically insignif-
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a
Scheme 10. Isolation of Tolylpentenes from Representative Catalytic Reaction
a
Conditions: 0.01 mol % Rh {Rh source = [Rh(η2-C2H4)2(μ-OAc)]2} relative to toluene, 10 equiv of cyclooctane relative to Rh, toluene, 360 equiv
Cu(OPiv)2 relative to Rh, 1000 equiv 1-pentene relative to Rh, 150 °C, 2 h.
icant compared to anaerobic conditions (76(9)%) (Tables 7
and 8). We have previously reported that the hydrophenylation
of ethylene catalyzed by (5-FP)Rh(TFA)(η2-C2H4) is
inhibited by the styrene product.12 In order to study potential
product inhibition for the aerobic toluene alkenylation, we
performed aerobic catalysis using (5-FP)Rh(TFA)(η2-C2H4)
with the addition of 160 equiv of meta-tolyl-1-pentene at the
beginning of the reaction under the reaction conditions shown
in Table 8. While the addition of the tolylpentene does not
affect the L:B ratio or ortho/meta/para selectivity (Figure
S20), a significant inhibition of the rate of catalysis was
observed. The yield of tolylpentenes decreased from 93%
without added tolyl-1-pentene to 47% in the presence of 160
equiv of meta-tolyl-1-pentene and 39% in the presence of 160
equiv of para-tolyl-1-pentene (Table 9). Thus, the limited
efficiency for the aerobic toluene alkenylation (i.e., less than
100% yield relative to Cu(II) oxidant) is likely related to
product inhibition. Addressing this issue would likely require a
continuous product distillation or a biphasic reaction analogue.
We have also demonstrated isolation of tolylpentene
products from a representative toluene alkenylation reaction.
Using the conditions in Table 3 with 360 equiv of Cu(OPiv)2,
after workup (see Figure S13), 88 mg of tolylpentenes were
isolated, a 32% yield based on Cu(OPiv)2 (Scheme 10). This
shows that precursors to bicyclic compounds such as 2,6-DMN
can be obtained from the rhodium-catalyzed alkenylation of
toluene with 1-pentene.
products using acid-based catalysis is dictated by the electronic
properties of the arene substituent. However, since the
rhodium-mediated arene alkenylation reactions do not likely
operate through electrophilic aromatic substitution, it becomes
possible to access unique regioselectivity for arene C−H
activation and functionalization that is catalyst directed rather
than substrate directed. While acid-based catalysis favors either
ortho/para selectivity or meta selectivity depending on the
substituents on the arene, the Rh-catalyzed arene alkenylation
consistently favors meta/para selectivity. Thus, the Rh
catalyzed arene alkenylation reactions offer access to products
with new substitution patterns.
EXPERIMENTAL SECTION
■
General Considerations. All alkenylation reactions were set up
under inert atmosphere in a glovebox. The purity of the glovebox
atmosphere was maintained by nitrogen purges and was monitored
using an oxygen analyzer (O2 < 15 ppm for all reactions).
Alkenylation and hydrogenation reactions were performed in
custom-built glass Fischer−Porter reactors (see Supporting Informa-
tion). Toluene was distilled over calcium hydride prior to use and was
stored over molecular sieves. Cu(OPiv)2 and [Rh(η-C2H4)2(μ-
OAc)]2 were synthesized according to literature procedures.48,49 1-
Tolylpentanes (ortho-, meta-, and para-) were independently
synthesized according to an adapted literature procedure and were
used as authentic standards for quantification (see Supporting
Information).50 The complex (5-FP)Rh(TFA)(η2-C2H4) [5-FP =
1,2-bis(N-7-azaindolyl)benzene] was synthesized according to a
literature procedure.12 Cu(Ole)2 (Ole = oleate) was synthesized
according to an adapted literature procedure (see Supporting
Information).51 All other chemicals were purchased from commercial
sources and used as received. Hydrogen was purchased from GTS-
Welco and used as received.
CONCLUSIONS
■
Oxidative alkenylation of toluene with 1-pentene to generate 1-
tolyl-1-pentenes and 2-tolyl-2-pentenes has been achieved
using the rhodium catalyst precursor [Rh(η-C2H4)2(μ-OAc)]2.
1-Tolyl-1-pentenes, possible precursors to the industrially
relevant bicyclic compound 2,6-dimethylnaphthalene, are the
major products of the arene alkenylation reaction. In contrast,
3-tolyl-2-pentenes, the branched products that do not undergo
the ring closure reaction to form dimethylnaphthalenes, are not
observed. The arene alkylation reaction gives high L:B ratios
(∼15:1) with a variety of olefin, oxidant, and rhodium
concentrations and operates over a range of temperatures
(120−180 °C). With the air-stable capping arene ligated Rh
pre-catalyst, (5-FP)Rh(TFA)(η2-C2H4), similarly high yields of
tolylpentanes and an optimal L:B ratio of 18:1 are observed,
indicating the potential for in situ regeneration of the Cu(II)
oxidant. Catalytic toluene pentylation using the Rh catalysts
provides high yields based on the Cu(II) oxidant; however,
product inhibition under current conditions likely inhibits
access to high turnovers.
GC−MS was performed using a Shimadzu GCMS-QP2010 Plus
instrument with a 30 m × 0.25 mm SHRXI-5MS column with a 0.25
μm thickness. Electron impact (EI) ionization was used. Products
were quantified using a linear regression analysis of gas chromato-
grams relative to standard samples. Plots of peak areas versus molar
ratios gave regression lines using cyclooctane as the internal standard
for tolylpentane and decane formation (see Supporting Information).
For the 1-tolyl-1-pentane products, the slope and correlation
coefficient of the regression lines were 1.34 and 0.998 (1-m-tolyl-1-
pentane), 1.85 and 0.997 (1-p-tolyl-1-pentane) and 1.84 and 0.998
(1-o-tolyl-1-pentane), respectively. For decane, the slope and
correlation coefficient were 2.08 and 0.998, respectively. The
production of 2-tolyl-2-pentenes was quantified using the slope and
correlation coefficient for a fit of cumene:n-propylbenzene, which
enabled an approximation of the ratio of 1-tolyl-1-pentenes to 2-tolyl-
2-pentenes. The slope and correlation coefficient of the regression line
were 1.24 and 0.98 for cumene:n-propylbenzene, respectively.
Catalytic Alkenylation of Toluene with 1-Pentene. In a
representative catalytic alkenylation reaction, a glass Fischer−Porter
reactor was charged with a stir bar, 10 mL of a 0.01 mol % Rh stock
solution {Rh source = [Rh(η2-C2H4)2(μ-OAc)]2, cyclooctane (63 μL,
10 equiv relative to Rh), and toluene (50 mL)}, 1-pentene and CuX2
oxidant (X = OPiv, OHex, or Ole). The reactor was sealed, charged
with 50 psi of N2 and heated and stirred in an oil bath. The reactor
The selectivity for meta- and para-tolylpentane products
using Rh catalysis further demonstrates the potential utility of
transition metal mediated arene alkenylation and alkylation
relative to acid-based catalytic alkylation. The selectivity of
I
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was removed from the oil bath when all of the copper(II) oxidant was
consumed, as indicated by a color change from blue to brown (see
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temperature, the reaction was analyzed by GC−MS and then
hydrogenated.
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Petersen, J. L. Combined experimental and computational studies on
the nature of aromatic C-H activation by octahedral ruthenium(II)
complexes: Evidence for σ-bond metathesis from Hammett studies.
Organometallics 2007, 26, 6604−6611.
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D.; Petersen, J. L. Combined experimental and computational study
of TpRu{P(pyr)3}(NCMe)Me (pyr)N-pyrrolyl(): inter- and intra-
molecular activation of C-H bonds and the impact of sterics on
catalytic hydroarylation of olefins. Organometallics 2007, 26, 5507−
5516.
(22) Foley, N. A.; Lail, M.; Lee, J. P.; Gunnoe, T. B.; Cundari, T. R.;
Petersen, J. L. Comparative reactivity of TpRu(L)(NCMe)Ph (L =
CO or PMe3): Impact of ancillary ligand L on activation of carbon-
hydrogen bonds including catalytic hydroarylation and hydro-
General Hydrogenation Procedure. To hydrogenate the
product mixture, a 0.1 mL aliquot was removed and added to a
glass Fischer−Porter reactor charged with a stir bar, 8 mg of 10% Pd/
C and 4 mL of ethanol. The reactor was sealed, and the headspace
was evacuated prior to pressurization with 100 psi of H2. The reaction
was stirred at room temperature overnight. After venting the reactor,
GC−MS was used to analyze the resulting product mixture.
Pentenyltoluene Isolation Procedure. Following a standard
procedure for toluene alkenylation (see above), the reactor was
allowed to cool to room temperature. Next, 40 mL of EtOAc, 10 mL
of 10% aqueous NaOH solution and 50 mL of DI water were added
to the reaction mixture. The layers were separated, and the aqueous
layer was extracted with 3 × 40 mL of EtOAc. The organic fractions
were combined and washed with 3 × 40 mL of DI water, dried over
MgSO4 and filtered. The filtrate was concentrated in vacuo and
purified using column chromatography with cyclohexane/EtOAc as
the eluent and silica as the solid phase. The eluent was collected and
concentrated under vacuum. The yellow product was further purified
on a plug of alumina using cyclohexane as the eluent and purified by
additional column chromatography using a silica column and eluting
with pentane. The pentenyltoluene mixture was characterized by
GC−MS following concentration in vacuo, to give 88 mg of
pentenyltoluenes, a 32% isolated yield based on Cu(OPiv)2.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00535.
1
Experimental procedures; H NMR spectra; calibration
curves for quantification; representative gas chromato-
grams (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: tbg7h@virginia.edu.
■
ORCID
T. Brent Gunnoe: 0000-0001-5714-3887
Present Address
‡Retired. Consulting scientist, San Rafael, California 94903,
United States.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the U.S. Department of Energy,
Office of Basic Energy Sciences, Chemical Sciences, Geo-
sciences, and Biosciences Division (DE-SC0000776).
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DOI: 10.1021/acs.organomet.9b00535
Organometallics XXXX, XXX, XXX−XXX