Synthesis of piperylene and toluene via transfer dehydrogenation of pentane and pentene

Article abstract of DOI:10.1021/cs400398w

The highly thermally stable anthraphos-based iridium pincer complex ( ^iPr4Anthraphos)Ir(C₂H₄) 3, was shown to catalyze transfer dehydrogenation of pentene and pentane using various olefins as acceptors in the temperature range of 160-250 C. Using pentene itself as an acceptor, disproportionation of pentene to pentane and pentadiene was observed, but yields of (E)- and (Z)-1,3-pentadienes (piperylenes) were limited to ～25% as a result of a self-Diels-Alder reaction of the 1,3-dienes to produce isomeric mixtures of the C₁₀ dimers. Using propylene as the acceptor, higher yields of piperylenes were obtained (～40%), but self-Diels-Alder adducts were again observed along with low fractions of propylene/pentadiene Diels-Alder adducts. Using ethylene as the acceptor, the pentadienes produced via hydrogen transfer undergo an in situ Diels-Alder reaction with ethylene to produce 3-methylcyclohexene (along with toluene from further dehydrogenation) in good yields (65%). 3-Methylcylohexene was quantitatively dehydrogenated to toluene over a heterogeneous Pd/C catalyst.

Full text of DOI:10.1021/cs400398w

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Article
Synthesis of Piperylene and Toluene via Transfer
Dehydrogenation of Pentane and Pentene
Sabuj Kundu, Thomas W Lyons, and Maurice Brookhart
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs400398w • Publication Date (Web): 26 Jun 2013
Downloaded from http://pubs.acs.org on July 2, 2013
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ACS Catalysis
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Synthesis of Piperylene and Toluene via Transfer Dehydrogenation
of Pentane and Pentene
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Sabuj Kundu, Thomas W. Lyons, and Maurice Brookhart*
Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United
States.
KEYWORDS: iridium pincer, catalysis, dehydrogenation, piperylene, toluene.
iPr4
ABSTRACT: The highly thermally stable anthraphos-based iridium pincer complex ( Anthraphos)Ir(C H ) 3, was shown
2
4
to catalyze transfer dehydrogenation of pentene and pentane using various olefins as acceptors in the temperature range
ο
of 160-250 C. Using pentene itself as an acceptor, disproportionation of pentene to pentane and pentadiene was ob-
served, but yields of E- and Z-1,3-pentadienes (piperylenes) were limited to ca. 25% due to a self- Diels-Alder reaction of
the 1,3-dienes to produce isomeric mixtures of the C10 dimers. Using propylene as the acceptor, higher yields of
piperylenes were obtained (ca. 40%), but self-Diels-Alder adducts were again observed along with low fractions of propyl-
ene/pentadiene Diels-Alder adducts. Using ethylene as the acceptor, the pentadienes produced via hydrogen transfer un-
dergo an in situ Diels-Alder reaction with ethylene to produce 3-methylcyclohexene (along with toluene from further de-
hydrogenation) in good yields ( 65%). 3-Methycylohexene was quantitatively dehydrogenated to toluene over a heteroge-
neous Pd/C catalyst.
Introduction
gard involves their use, in tandem with olefin metathesis
catalysts, to achieve alkane metathesis in which an alkane
Catalytic transfer dehydrogenation of alkanes to pro-
duce alkenes employing iridium complexes has seen ex-
of carbon number n (C ) is converted to an alkane of
n
1,2
higher carbon number (Cn+x) and one of lower carbon
number (Cn-x). One potential application of this process is
the conversion of less valuble lower carbon number al-
kanes produced by the Fischer-Tropsch process to higher
tensive development since the early reports of Crabtree
3-5
and Felkin. The most robust and productive complexes
discovered to date are based on iridium pincer complexes
bearing bulky substituents. The most common accep-
tors used in these transfer dehydrogenation reactions are
t-butyl ethylene and the strained alkene, norbornene.
6,7
1
0,16-18
carbon-number alkanes in the Diesel range (C - C ).
9
18
8
Aromatics constitute an important class of basic build-
ing blocks currently derived from crude oil. BTX (a
mixure of benzene, toluene and isomeric xylenes) is cur-
rently produced by dehydrogenating petroleum fractions
using a heterogeneous “reforming” catalyst at high tem-
peratures (> 500 °C). In 2011 we reported conversion of
linear alkanes to aromatics in homogeneous solution at
165 °C through transfer dehydrogenation to trienes, fol-
lowed by cyclization to cyclohexadienes and further
Extensive mechanistic studies have been carried out using
tBu4
iridium pincer complexes derived from ( PCP) and
tBu4
tBu4
3
t
(
(
POCOP) [( PCP = κ -C H -2,6-(CH P Bu ) ) and
6
3
2
2 2
tBu4
3
t
POCOP = κ -C H -2,6-(OP Bu ) )] with cyclooctane
6
3
2 2
9-15
and the substrate and t-butylethylene as the acceptor.
Key steps involve the oxidative addition of the C-H bond
of cyclooctane to a 14-electron iridium (Ι) center to yield a
cyclooctyl hydride which undergoes β-hydride elimina-
tion and loss of cyclooctene to produce an Ir(III) dihy-
dride. Hydrogenation of t-butylethylene closes the cata-
lytic cycle.
19
transfer dehydrogenation to yield aromatics. For exam-
ple, heptane yields toluene and octane leads to a mix of
ortho-xylene and ethylbenzene. Four equivalents of ac-
ceptor olefin are required which limits the practicality of
this process. Building on this work, we recently reported
that using an anthraphos-based iridium catalyst and eth-
ylene as acceptor, hexene could be converted to a mix of
Application of these catalysts to hydrocarbon conver-
sions beyond these model systems has received increasing
attention. As oil supplies diminish, new methods to trans-
form new or less-valued carbon feedstocks into useful
materials and chemical intermediates will be needed.
Such feedstocks include volatile fractions of petroleum
not readily used for fuels, low carbon number alkanes
from the Fischer-Tropsch process and shale gas-derived
hydrocarbons (methane, ethane, and ethylene). A promi-
nent application of iridium pincer complexes in this re-
2,4-hexadienes and 1,3-hexadienes which, in the presence
of ethylene in the same reactor, undergo Diels-Alder cy-
cloadditions to produce the corresponding cyclohexenes.
Dehydrogenation of these olefins readily occurs when
passed over Pd or Pt heterogeneous catalysts to yield pa-
ra-xylene (major product), ethylbenzene (minor product)
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Page 2 of 8
2
0
and hydrogen. Since hexene is commercially prepared
via trimerization of ethylene, this process provides a route
to para-xylene with ethylene as the sole feedstock. Ethane
produced in the transfer dehydrogenation process can be
readily cracked back to ethylene.
distribution of products (180 °C run) as a function of time
is shown in Figure 1.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
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2
2
2
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5
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5
5
5
5
5
5
5
5
6
3 (0.15 mol%)
160 or 180 °C
(
1)
E + Z
E + Z
(
+ isomers)
In an effort to expand this chemistry to other important
basic chemical building blocks, we describe here our ef-
forts directed towards the dehydrogenation of pentane
and pentene to 1,3-pentadiene (piperylene) and the con-
version of piperylene to toluene using Diels-
Alder/dehydrogenation chemistry similar to that de-
scribed above. Piperylene is an important intermediate in
the production of resins, adhesives and plastics and is
primarily derived from cracking of various petroleum
fractions. Toluene is an important solvent and fuel addi-
tive and is used as a precursor to a wide array of other
aromatic compounds. It is derived by separation from the
BTX mixture described above.
Self-DA
Table 1. 1-Pentene disproportionation catalyzed by 3
a
at 160 °C and 180 °C
Temp
Tim
Pentane
1-
2-
1,3-
Pentadi-
enes
Self-
DA
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(°C)
e (h)
Pentene
Pentenes
1
60
180
60
180
80
2
2
5.3%
24.2%
26.1%
33.7%
45.5%
17.7%
3.2%
2.7%
2.6%
1.9%
71.8%
49.0%
47.1%
37.1%
27.9%
5.2%
24.2%
23.4%
21.0%
4.4%
0%
0.4%
0.8%
5.6%
20.3%
1
19
17
130
Results and Discussion
1
Dehydrogenation of 1-Pentene to 1,3-Pentadiene.
Our initial approach to the synthesis of 1,3-pentadiene
from pentene using transfer dehydrogenation was to
simply employ pentene itself as the acceptor, thus effect-
ing disproportionation of pentene to pentane and penta-
diene. Olefin isomerization catalyzed by 1-3 is far more
rapid than transfer dehydrogenation under these condi-
tions, so, although 1-pentene is initially added to the reac-
tor, 1-pentene rapidly converts to a thermodynamic mix-
ture of 1-pentene and E- and Z-2-pentenes. Three differ-
ent iridium pincer catalysts shown in Scheme 1 were
screened for the disproportionation reaction. Reactions
were conducted using dry, degassed pentene heated at
a
Reaction conditions: 0.6 mL pentene and 3 (0.15 mol%).
Reactions were monitored by GC. Numbers shown are in mol
% of solution.
Figure 1. 1-Pentene dehydrogenation profile observed using
catalyst 3 at 180 °C
1
-Pentene Dehydrogenation
100
90
1-Pentene
2-Pentene
Pentadiene
Self-DA
80
70
60
50
40
Pentane
1
60 °C for 2 h in a closed, glass vessel together with 0.17
mol% Ir catalyst. Product concentrations were monitored
by gas chromatography and the turnover frequencies
30
20
10
(
TOFs) shown in Scheme 1 were calculated based on the 2
0
h heating time.
0
20
40
60
Time (h)
80
100
120
140
Scheme 1. Transfer dehydrogenation of 1-pentene
using pincer iridium catalysts.
Several features of these results should be noted. First,
as stated above, 1-pentene isomerization to 2-pentenes
occurs at a much faster rate relative to dehydrogenation
to pentadiene. Second, a significant amount of pentadi-
ene (~24%) is produced within 2 h of heating at 180 °C
while at 160 °C, the reaction is much slower with only 5%
diene produced. Third, as pentadiene concentrations
approach ~23%, new signals appear in the C region of the
GC trace and increase over time. These C10 products are
ascribed to “self-Diels-Alder” dimers and associated iso-
mers from Ir-catalyzed olefin isomerization (eq. 2). To
confirm these as self Diels-Alder products, pure samples
and mixtures of trans-1,3-pentadiene and cis-1,3-
pentadiene were heated at 180 °C for 3 days. GC analysis
showed the major products from these reactions matched
the signals of products from the pentene disproportiona-
[Ir] 0.17 mol%
+
+
+
1
60 °C, 2 h
E + Z
E + Z
1
0
20
As previously observed with 1-hexene, the anthraphos-
21
based catalyst 3 exhibited higher catalytic activity in
comparison to the more bulky tert-butyl-substituted cata-
lyst, 1 or hybrid PCOP system, 2. Complex 3 was selected
as the optimum catalyst and disproportionation of 1-
pentene with 3 was further investigated at temperatures
of 160 °C and 180 °C at extended time intervals. These
results are summarized in Equation 1 and Table 1. The
22
tion reactions. Attempts to recover piperylene via a ret-
ro-Diels-Alder reaction of the dimers at 250 °C were un-
2
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Here we attempted to dehydrogenate not only 1-
successful. From the data in Table 1 it’s clear that when
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
4
4
4
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4
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5
5
5
5
5
5
5
5
5
5
6
piperylene exceeds ca. 25 mol%, the rate of the second-
order self-Diels-Alder reaction begins to compete with
the transfer dehydrogenation, limiting the yield of
piperylene to ca. 25%.
pentene but also n-pentane using propene as an acceptor
(eq. 3 and 4 respectively). Propene was liquefied in a glass
tube at -78 °C and was cannula transferred to a Parr reac-
tor under air free conditions. Using a 2:1 ratio of propene
and pentene significant amounts of pentadiene (23%)
were generated after 2 d heating at 180 °C, with 12% pen-
tane also formed (Entry 1, Table 2). The fraction of pen-
tane was significantly decreased when 5 mL propene was
used, but the reaction rate decreased, most likely due to
an equilibrium favoring the Ir-propene complex (Entry 2,
Table 2). Optimized conditions for pentadiene formation,
summarized in entry 3, resulted in 79% conversion of
pentene with 41% pentadiene formed. The self Diels-Alder
reaction again reduces the yield of diene monomer.
Propene as Hydrogen Acceptor. We, together with
the Goldman group, previously demonstrated that pro-
pene can be used as a hydrogen acceptor in dehydroaro-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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2
3
4
5
6
7
8
9
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3
4
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7
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2
3
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7
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9
0
19
matization of n-octane.
3
(0.08 mol%)
80 °C
(
3)
1
Table 2. Propene as hydrogen acceptor for 1-pentene dehydrogenation catalyzed by 3.
Entry
Propene
used
Time
1-Pentene
2-Pentenes
1,3-Pentadienes
Pentane
Propene-
DA
Self-DA
a
1
2 mL
5 mL
3 mL
2 d
3.5 d
2 d
4.4%
4.7%
2.1%
60.8%
76.6%
24.4%
23.0%
18.2%
40.9%
12.3%
0.7%
4.1%
a
0%
0%
0.3%
0%
a
2
b
3
1.4%
27.4%
b
Reaction conditions: 1 mL pentene (9.1 mmol), propene (14.6 mmol) and 2 mL mesitylene. 3 (0.08 mol%), 180 °C. 3 (0.3 mol%),
200 °C. Numbers shown are in mol % of solution.
Propene also functions as a moderately efficient hy-
jor component along with a small amount of Ir-CO (δ
71.6 ppm). AnthraphosIr-CO was confirmed by compari-
son to an authentic sample prepared by passing CO
through a solution of 3 in C D at room temperature.
drogen acceptor in n-pentane dehydrogenation (eq. 4,
22
Table 3). 74% conversion of n-pentane with 40% yield
of pentadienes was observed with 0.6 mol% 3 after 5.5 d
6
6
ο
at 200 C (Entry 2, Table 3); although the dehydrogena-
When using propene as a hydrogen acceptor, four
previously unobserved peaks were detected in quite
small amounts in the C-8 region of the GC trace. We
reasoned these were most likely Diels-Alder adducts of
propene and pentadiene. To investigate this, 1 mL pen-
tadiene, 10 mL propene and 1 mL mesitylene were heat-
ed at 250 °C for 2.5 d. By GC, propene-DA and self-DA
products were detected (eq. 5). The four peaks associat-
ed with propene-DA products perfectly matched those
in the original transfer dehydrogenation reaction mix-
ture.
tion rate is slightly slower than that for 1-pentene. The
self Diels-Alder reaction again limits the yield of diene.
3
(4)
Table 3. Propene as hydrogen acceptor for n-
pentane dehydrogenation catalyzed by 3.
En-
try
1-
Pen-
tene
2-
Pen-
tenes
1,3-
Pentadi-
enes
Pen-
tane
Pro-
pene-
DA
Self-
DA
a
1
1.6%
24.5%
26.4%
42.1%
26.5%
0%
5.6
%
250
°
C
b
2
0.9%
11.1%
40.3%
1.9%
9.8
%
(5)
2.5 d
1 mL
10 mL
Reaction conditions: 1 mL pentane (9.1 mmol), 2 mL pro-
Propene-DA (18.7%)
Self-DA (21.3%)
a
pene (29.2 mmol), 4 mL mesitylene 3 (0.3 mol%), 200
Ethylene as Hydrogen Acceptor. Synthesis of Tol-
uene. Similar to propene, ethylene is also an attractive
choice as the hydrogen acceptor. Previously we demon-
strated that ethylene can be effectively used as the hy-
drogen acceptor for dehydrogenation of 1- hexene to 2,4-
hexadiene and as noted earlier, an in situ Diels-Alder
ο
b
C,5.5 d. 3(0.6mol%), 200 °C, 5.5 d. Reactions were moni-
tored by GC and numbers shown are in mol % of solution.
The anthraphos catalyst 3 shows remarkably high
31
thermal stability; after 2 days at 180 °C, a P NMR spec-
trum of the reaction mixture revealed an (anthra-
phos)Ir-olefin complex (δ 49.2 and 51.1 ppm) as the ma-
20
reaction can lead ultimately to para-xylene. Heating 1-
3
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pentene at 250 °C for 2.5 d with 250 psi of ethylene in Ethylene was also effective as a hydrogen acceptor for
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
the presence of 0.08 mol% 3 resulted in the formation of
pentadiene, 3-methyl cyclohexene (ethylene-DA), and
self-DA products (eq. 6, Entry 1, Table 4). Pentadienes
produced in the reaction, undergo an in situ Diels-Alder
the dehydrogenation of n-pentane (eq. 7, Table 5). Heat-
ing n-pentane at 250 °C for 6.5 days with 200 psi eth-
ylene in the presence of 0.18 mol% 3 resulted in 35%
conversion of n-pentane (Entry 2, Table 5). With 0.32
mol% 3, 74% conversion of n-pentane to dehydrogenat-
ed products was observed (Entry 3, Table 5). In the case
of 1-pentene under 600 psi ethylene in the presence of
0.3 mol% 3 a significantly higher amount of dehydro-
genation products was observed, but under similar con-
ditions with n-pentane only 4% dehydrogenation prod-
ucts were detected after one day (Entry 4, Table 5). This
is most likely the result of either the stronger binding
affinity for alkenes to the Ir catalyst compared to al-
kanes or the lower barrier for allylic C–H bond activa-
tion.
reaction
with
ethylene,
providing
3-methyl-1-
cyclohexene (ethylene-DA).
3
(6)
250 °C
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Remarkably, good conversion of pentene to 3-methyl-
-cyclohexene was observed using 600 psi of ethylene
1
and 0.3 mol% 3 (Entry 3, Table 4). At this high ethylene
pressure, formation of the self-DA dimers was minimal
and the ethylene-DA adduct was the major product. The
anthraphos catalyst showed remarkably high thermal
stability under these conditions. A 1:1 ratio of 3 and Ir-
31
CO was detected in the P NMR spectrum of the reac-
tion mixture after 5.5 days of heating at 250 °C.
Table 4. Ethylene as hydrogen acceptor for dehydrogenation of pentene catalyzed by 3 at 250 °C
Entry
Time
2.5 d
5.5 d
1 d
1-Pentene
8.8%
2-Pentenes
63.3%
1,3-Pentadienes
5.9%
Pentane
3.6%
3.3%
0%
Ethylene-DA
8.9%
Toluene
0%
Self-DA
9.5%
a
1
b
2
6.2%
66.2%
1.4%
13.8%
0%
9.2%
c
3
9.4%
62.0%
16.3%
11.8%
0.3%
2.1%
6.9%
b
0.4%
2 d
4.5%
46.0%
13.5%
0%
26.6%
4.3%
5
.5 d
1.5%
19.0%
2.5%
0%
57.9%
10.6%
a
Reaction conditions: 1 mL pentene (9.1 mmol), 3 mL mesitylene, 250 psi ethylene, 3 (0.08 mol%). 3 mL pentene, 3 mL mesity-
lene, 250 psi ethylene, 3 (0.08 mol%). 1 mL pentene (9.1 mmol), 3 mL mesitylene, 600 psi ethylene, 3 (0.3 mol%). Reactions were
c
monitored by GC and numbers shown are in mol % of solution.
3
(7)
250 C
°
Table 5. Ethylene as hydrogen acceptor for n-pentane dehydrogenation at 250 °C
Entry
[3] mol%
0.09
0.18
Time
9 d
Pentane
80.4%
65.0%
35.4%
95.6%
79.6%
Pentenes
12.6%
18.8%
17.7%
3.0%
1,3-Pentadienes
Ethylene-DA
Toluene
0.7%
Self-DA
0.5%
0.1%
a
1
0.7%
5.2%
2.5%
0%
5.7%
10%
4.5%
0%
a
2
6.5 d
6 d
1.5%
b
3
0.32
0.30
0.30
a
2.0%
36.5%
0%
c
4
1 d
0.4%
0.7%
c
5
10 d
9.4%
0%
4.4%
2.3%
b
Reaction conditions: 1 mL pentane (8.7 mmol) , 3 mL mesitylene, 200 psi ethylene. 1 mL pentane (8.7 mmol), 2 mL mesitylene,
250 psi ethylene. 1 mL pentane, 2 mL mesitylene, 600 psi ethylene. Reactions were monitored by GC and numbers shown are in
c
mol % of solution.
Toluene via Dehydrogenation of 3-methyl-1-
cyclohexene. Toluene was synthesized by dehydrogen-
ating 3-methyl-1-cyclohexene (MCH) under both homo-
geneous (eq. 7) and heterogeneous (eq. 8) conditions.
Table 6 summarizes results for four iridium pincer cata-
lysts that were screened for dehydrogenation of the 3-
methyl-1-cyclohexene to toluene using TBE as hydrogen
acceptor. Degassed 3-methyl-1-cyclohexene and TBE
4
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ACS Catalysis
°C significantly improved the yield of toluene and min-
were heated at 180 °C using 0.19 mol% catalyst in a
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
closed glass vessel and the reaction was monitored by
GC. The more bulky tert-butyl-substituted catalysts, 1
and 4, exhibited poor catalytic activities. The less bulky
isopropyl-substituted catalysts, 2 and 3, showed good
activities but 3 was found to be more efficient than 2.
imized the amount of benzene produced. Under these
conditions both Pt and Pd catalysts gave good conver-
sion of the starting material to toluene (Entry 4-7, Table
ο
7). Pt/C at 300 C was observed to give quantitative con-
version of the starting material to toluene without pro-
duction of side products (Entry 5, Table 7).
[
Ir] (0.19 mol%)
80 °C
isomers
1
Heterogeneous
Catalyst
methyl cylcohexene
(MCH)
Dienes
2
H
2
(9)
ꢀ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(MCH)
Dienes
(8)
Table 7. Dehydrogenation of 3-methylcyclohexene
a
using heterogeneous catalysts
Entry
Catalyst
Temp
MCH
Dienes
Benzene
Toluene
Table 6. Dehydrogenation of 3-methylcyclohexene
(°C)
a
using tert-butyl-ethylene (TBE) as acceptor
1
Pt/Al
2
O
O
3
3
400
450
400
300
300
300
300
0%
0%
2.7%
0.8%
1.0%
12.5%
0%
27.6%
53.6%
1.3%
69.7%
45.6%
97.7%
85.9%
100%
b
Catalyst
Time
MCH
Dienes
Toluene
2
3
4
5
6
7
Pt/Al
2
(h)
Pt/C
0%
iPr4
3 ( AN)Ir
1
0.6%
50.6%
30.7%
25%
1.4%
4.7%
3.8%
2.8%
9.9%
11.9%
9.5%
7.4%
7.9%
8.4%
98%
44.7%
65.5%
72.2%
10.3%
10.9%
13.8%
0%
Pt/Al
2
O
3
0.1%
0%
1.5%
0%
iPr4
2 ( PCOP)Ir
1
Pt/C
2
Pd/Al
2
O
3
0.3%
0%
18.3%
3.9%
2.4%
3.9%
79.0%
92.3%
5
.5
Pd/C
tBu4
PCP)Ir
1
(
1
80.1%
77.2%
76.5%
92.6%
91.9%
91.2%
a
0.6 g MCH passed over the hot catalyst (5 g, 5% metal by
2
wt) using N as carrier gas over the course of 2 hours. Reac-
2
5
.5
tions were monitored by GC. Numbers shown are in mol %
of solution.
tBu4
POCOP)Ir
4
(
1
2
0.3%
Conclusions. The highly thermally stable anthra-
phos-based pincer iridium complex, 3, efficiently cata-
lyzes transfer dehydrogenation of 1-pentene and n-
pentane using propene or ethylene as the hydrogen ac-
ceptor. This work highlights the unique reactivity that
can be achieved using these two readily available and
recyclable hydrogen acceptors. Using propene as an ac-
ceptor, pentene/pentane dehydrogenation led to a high
concentration of 1,3-pentadienes as the major products
together with significant quantities of C10 dimers from
the self-Diels-Alder reaction of the pentadienes. In con-
trast, when ethylene was used as an acceptor the 1,3 pen-
tadienes produced undergo in situ Diels-Alder reactions
with ethylene, providing 3-methyl-1-cyclohexene that
can be quantitatively dehydrogenated to toluene and
hydrogen via a common heterogeneous Pd/C catalyst.
5
.5
0.4%
a
Reactions were monitored by GC. Numbers shown are in
b
mol % of solution. Isomer mix of methylcyclohexadienes.
Dehydrogenation of cyclohexene to benzene using het-
23-25
erogeneous catalysts has been extensively studied.
Recently we reported the synthesis of p-xylene by dehy-
drogenation of cis-3,6-dimethylcyclohexene using com-
mercial heterogeneous catalysts such as Pt/C, Pt/Al O
2
3
and Pd/C. In this study, 3-methyl-1-cyclohexene was
passed over a heterogeneous catalyst bed at tempera-
tures between 300 and 400 °C in the vapor phase using
nitrogen as the carrier gas (eq. 9). The products were
trapped using liquid nitrogen and analyzed by GC. At
400 °C both Pt/C and Pt/Al O gave good conversions of
2
3
While applications of homogeneous iridium cata-
lysts are not practical on a large scale, the high stablility
3-methyl-1-cyclohexene (Entry 1 and 3, Table 7). Surpris-
ingly, benzene was detected as a side product in this
reaction, with concentrations as high as 26.5% using
Pt/Al O . Increased temperature led to a dramatic in-
(
up to 250 °C) of 3 and the ability to use ethylene both
as an acceptor molecule and a Diels-Alder trap (espe-
2
3
cially in view of the expected vast increase in ethylene
crease in benzene production, 53.6% at 450 °C using
26-30
production from shale gas
) suggests that supported
Pt/Al O (Entry 2, Table 7).
2
3
catalysts based on 3, working in tandem with standard
heterogeneous dehydrogeantion catalysts, could be
We suspected that cracking of the 3-methyl-1-
cyclohexene in preference to toluene generated ben-
zene. To probe this possibility, toluene was passed over
Pt/Al O at 400 °C and only 4.5% benzene was detected,
practical for converting C feedstocks plus ethylene di-
5
rectly and selectively to toluene and hydrogen. This will
be a focus of our future investigations in this area.
2
3
which is considerably lower than the fraction of benzene
26.5%) detected from dehydrogenation of pure 3-
methyl-1-cyclohexene. Lowering the temperature to 300
Experimental Section
(
General Procedures. All operations were carried out
under an argon atmosphere using standard Schlenk
5
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Page 6 of 8
techniques or in an argon-filled dry box. 1-Pentene, n- glovebox, then pressurized with ethylene to the speci-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
pentane and TBE (3, 3-dimethyl-1-butene) were pur-
chased from Aldrich and were dried over Na/K alloy and
collected by vacuum transfer. 3-methyl-1-cyclohexene
was purchased from TCI America Fine Chemicals and
dried over Na/K alloy and collected by vacuum transfer.
fied value and left at room temperature for 30 minutes
in order for ethylene to saturate the solution. Once sat-
urated, the vessel was sealed at the specified pressure
and heated to 250 °C. For GC analysis, the vessel was
cooled to room temperature and the pressure was care-
fully released. It was then opened in an argon filled
glovebox and an aliquot was taken via syringe. The ves-
sel was then recharged with ethylene as described above
and heating continued at 250 °C.
Propene and ethylene (polymer grade purity) were
purchased from Airgas National Welders and used as
31
32
19
19
received. Pincer complexes 1 , 2 , 3 and 4 were syn-
thesized according to literature methods. An Agilent
Technologies 6850 gas chromatograph having a fused
silica capillary column (100 m length × 0.25 mm ID ×
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Typical procedure for homogeneous dehydro-
genation of 3-methyl-1-cyclohexene using TBE. In an
argon-filled glovebox, the appropriate iridium complex
(0.19 mol%), 3-methyl-1-cyclohexene (0.4 mL), TBE
0
.50 μm film thicknesses) was used for gas chromato-
graphic analysis using the following settings.
(
0.86 mL) and 40 μL mesitylene were added in a 1.5 mL
FID detector: Temp = 300 °C
Kontes vial equipped with a Teflon screw-cap. The flask
was sealed tightly under an argon atmosphere, and the
solution was removed from the glovebox and stirred in a
sand bath at 180 °C. Periodically, the flask was removed
from the bath and cooled in an ice bath. An aliquot was
removed from the flask in the glovebox with a 1 μL GC
syringe and analyzed by GC. Mesitylene was used as an
internal standard.
Initial Temperature: 40 °C
Final Temperature: 250 °C
Oven Program: 40 °C, hold for 20 min
Ramp 1: 85 °C/min to 150 °C, hold for 5 min
Ramp 2: 10 °C/min to 250 °C, hold for 20 min
Products were confirmed using authentic samples and
mesitylene was used for an internal standard.
Dehydrogenation of 3-methyl-1-cyclohexene us-
ing heterogeneous catalysts. A Carbolite furnace with
a quartz tube was filled with 5g, 5 wt. % metal loading
either on Al O or C. To one end of the furnace tube was
Typical procedure for dehydrogenation of 1-
pentene. In an argon-filled glovebox, the appropriate
iridium complex, 1-pentene (0.6 mL), and 50 μL mesity-
lene were added to a 1.5 mL Kontes vial equipped with a
Teflon screw-cap and Teflon stir bar. The flask was
sealed tightly under an argon atmosphere, and the solu-
tion was removed from the glovebox and stirred in a
sand bath at the specified temperature. Periodically, the
flask was removed from the bath and cooled in an ice
bath. An aliquot was removed from the flask in the
glovebox with a 1 μL GC syringe and analyzed by GC.
Mesitylene was used as an internal standard.
2
3
attached a glass trap with an oil bubbler. The other end
was fitted with a 14/20 ground glass joint and connected
to a two-neck 50 mL round bottom flask. The second
neck of the flask was fitted with a septum and N was
2
admitted to the system via a needle through this sep-
tum. The furnace was warmed to the specified tempera-
ture (300 or 400 °C) under an atmosphere of N for 30
2
minutes to dry the catalyst. Following this preactivation,
the glass collection trap was cooled with liquid N and 1
2
Procedure for dehydrogenation of pentane and
pentene using propene as hydrogen acceptor. In an
argon filled glovebox, catalyst 3, 1-pentene or n-pentane
and mesitylene were added to a 22 mL stainless steel
Parr reactor. The vessel was sealed and taken out of the
glovebox. Propene was liquefied under air free condi-
tions in a graduated glass tube equipped with a Teflon
screw-cap at -78 °C. The specified amount was carefully
cannula transferred to the precooled Parr-reactor (-78
mL 3-methyl-1-cyclohexene added to the two neck flask.
N was purged through the solution at a rate of ca. 0.5
2
mL/second (1 bubble). After 2 hours, the contents of the
trap were warmed and collected. Standard (mesitylene)
was added and the solution analyzed by GC.
Generation and analysis of (Anthraphos)Ir(CO):
A C D solution of 5 mg of (Anthraphos)Ir(C H ) (3; 8.1
6
6
2
4
μmol) in a J-Young tube was exposed to 1 atm of CO. An
immediate color change from dark brown to light red
was observed. The solvent was removed under vacuum
and the resulting complex was characterized by NMR
°
C) under air free conditions. The Parr-reactor was then
kept at room temperature for 30 minutes and heated in
a sand bath at the specified temperature. For GC analy-
sis, the vessel was cooled to room temperature and the
pressure carefully released under a hood before it was
opened in an argon filled glovebox and an aliquot re-
moved via syringe. The vessel was then recharged with
propene as described above.
1
spectroscopy. H NMR (600 MHz, C D ): δ 7.98 (s, 1H),
6
6
3
7
.85 (d, J = 6.0 Hz, 2H), 7.33 (m, 2H), 7.27 (app t, JHH
HH
=
8.1 Hz, 2H), 2.51 (m, -CH(CH ) , 4H), 1.20 (m, -
3 2
3
1
1
CH(CH ) , 12H), 0.98 (m, -CH(CH ) , 12H). P{ H}
NMR(243 MHz, C D ): δ 71.6 (s). C{ H} NMR (151 MHz,
3
2
3 2
1
3
1
6
6
C D ): 197.3 (t, J = 6.8 Hz, Ir-CO), 188.9 (t, JPC = 5.3
6
6
PC
Procedure for dehydrogenation of pentane and
pentene using ethylene as hydrogen acceptor. In an
argon filled glovebox, catalyst 3, 1-pentene or n-pentane
and mesitylene were added to a 22 mL stainless steel
Parr reactor. The vessel was sealed, taken out of the
Hz), 151.7 (t, JPC = 10.6 Hz), 143.8 (t, JPC = 21.9 Hz), 132.3
(t, JPC = 6.8 Hz), 130.8 (s), 127.1 (s), 124.9 (t, JPC = 3.8 Hz),
121.9 (s), 27.4 (t, JPC = 14.3 Hz, - CH(CH ) ), 19.7 (-
3 2
CH(CH ), 18.8 (-CH(CH ).
)
2
)
3 2
3
6
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ASSOCIATED CONTENT
ACS Catalysis
(
17)
Bailey, B. C.; Schrock, R. R.; Kundu, S.;
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Goldman, A. S.; Huang, Z.; Brookhart, M. Organometallics
2008, 28, 355.
Supporting Information
Selected gas chromatograms illustrating analysis of reac-
tion products. “This material is available free of charge via
the Internet at http://pubs.acs.org.”
(18)
Huang, Z.; Rolfe, E.; Carson, E. C.;
Brookhart, M.; Goldman, A. S.; El-Khalafy, S. H.; MacArthur, A.
H. R. Adv. Synth. Catal. 2010, 352, 125.
(
19)
C.; Schinski, W.; Brookhart, M.; Goldman, A. S. Nat. Chem.
011, 3, 167.
20)
Brookhart, M. J. Am. Chem. Soc. 2012, 134, 15708.
Ahuja, R.; Punji, B.; Findlater, M.; Supplee,
AUTHOR INFORMATION
2
Corresponding Author
(
Lyons, T. W.; Guironnet, D.; Findlater, M.;
* E-mail: mbrookhart@unc.edu
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(21)
Romero, P. E.; Whited, M. T.; Grubbs, R. H.
Notes
Organometallics 2008, 27, 3422.
(22)
In
a
preliminary
report
transfer
A patent application has been submitted covering technol-
ogy described here.
dehydrogenation of light alkanes, including pentane, has been
investigated using various acceptor and other pincer-based
catalysts: Kumar, A.; Goldman A. S. Abstracts of papers, 245th
ACS national meeting, New Orleans, LA, United States, 2013,
April 7-11, INOR.
(23)
J. Am. Chem. Soc. 1982, 104, 2539.
(24) Aramendıa, M. A.; Boráu, V.; Garcı ́a , I. M.;
ACKNOWLEDGMENT
This work was supported by NSF under the CCI Center for
Enabling New Technologies Through Catalysis (CENTC)
Phase II Renewal, CHE-1205189 and by the Eastman Chemi-
cal Company. T.W.L. acknowledges support from an NSF-
American Competitveness in Chemistry postdoctoral fel-
lowship (CHE-1136951). We thank Dr. Robert T. Hembre
and Dr. Kelley M. Moran of Eastman Chemical for helpful
discussions.
Tsai, M. C.; Friend, C. M.; Muetterties, E. L.
́
Jiménez, C.; Marinas, A.; Marinas, J. M.; Urbano, F. J. J. Mol.
Catal. A: Chem. 2000, 151, 261.
(25)
Borade, R. B.; Zhang, B.; Clearfield, A.
Catal. Lett. 1997, 45, 233.
(
(
26)
27)
Bewley, L. Chem. Week 2012, 174, 19.
Gilbert, D. Chemical Makers Ride Gas
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Boom. Wall Street Journal, April 19, 2012: B1.
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29) Dow press release. April 21, 2012.
http://www.dow.com/news (accessed April 30, 2012).
30) Chevron Phillips press release. April 30,
012 http://www.cpchem.com (Accessed May 3, 2012).
31) Goldman, A. S.; Ghosh, R. Handbook of
(
1)
J. Chem. Soc., Chem. Commun. 1985, 1829.
2) Burk, M. J.; Crabtree, R. H.; Parnell, C. P.;
Uriarte, R. J. Organometallics 1984, 3, 816.
Burk, M. J.; Crabtree, R. H.; McGrath, D. V.
(
(
(
(3)
Baudry, D.; Ephritikhine, M.; Felkin, H.;
2
Holmes-Smith, R. J. Chem. Soc., Chem. Commun. 1983, 788.
(
(
4)
Holmes-Smith, R.; Zakrzewski, J. Tetrahedron Lett. 1984, 25,
279.
Felkin, H.; Fillebeen-Khan, T.; Gault, Y.;
C−H transformations – Applications in Organic Synthesis;
Dyker, G., Ed., Wiley-VCH: New York, 2005, pp 616-622.
1
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Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc. 2004, 126,
3044.
Zhu, K.; Achord, P. D.; Zhang, X.; Krogh-
(
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Smith, R.; Yingrui, L. Tetrahedron Lett. 1985, 26, 1999.
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Cramer, R. E.; Jensen, C. M. J. Am. Chem. Soc. 1997, 119, 840.
(7) Gupta, M.; C. Kaska, W.; M. Jensen, C.
Chem. Commun. 1997, 461.
8) Choi, J.; MacArthur, A. H. R.; Brookhart,
M.; Goldman, A. S. Chem. Rev. 2011, 111, 1761.
9) Biswas, S.; Huang, Z.; Choliy, Y.; Wang, D.
Felkin, H.; Fillebeen-khan, T.; Holmes-
1
(
(
(
Y.; Brookhart, M.; Krogh-Jespersen, K.; Goldman, A. S. J. Am.
Chem. Soc. 2012, 134, 13276.
(10)
Haibach, M. C.; Kundu, S.; Brookhart, M.;
Goldman, A. S. Acc. Chem. Res. 2012, 45, 947.
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Renkema, K. B.; Kissin, Y. V.; Goldman, A.
S. J. Am. Chem. Soc. 2003, 125, 7770.
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22, 11017.
Kanzelberger, M.; Singh, B.; Czerw, M.;
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Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804.
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Am. Chem. Soc. 2004, 126, 9330.
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I.;
White,
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Findlater, M.; Choi, J.; Goldman, A.;
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Catalysis; Pérez, P. J., Ed.; Springer Netherlands: 2012; Vol. 38,
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(16)
Goldman, A. S.; Roy, A. H.; Huang, Z.;
Ahuja, R.; Schinski, W.; Brookhart, M. Science 2006, 312, 257.
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Table of Contents
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Pt/C
or
"Ir"
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
8
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