Catalytic dehydroaromatization of n-alkanes by pincer-ligated iridium complexes

Article abstract of DOI:10.1038/nchem.946

Aromatic hydrocarbons are among the most important building blocks in the chemical industry. Benzene, toluene and xylenes are obtained from the high temperature thermolysis of alkanes. Higher alkylaromatics are generally derived from arene-olefin coupling, which gives branched products-that is, secondary alkyl arenes-with olefins higher than ethylene. The dehydrogenation of acyclic alkanes to give alkylaromatics can be achieved using heterogeneous catalysts at high temperatures, but with low yields and low selectivity. We present here the first catalytic conversion of n-alkanes to alkylaromatics using homogeneous or molecular catalysts-specifically 'pincerg'-ligated iridium complexes-and olefinic hydrogen acceptors. For example, the reaction of n-octane affords up to 86% yield of aromatic product, primarily o-xylene and secondarily ethylbenzene. In the case of n-decane and n-dodecane, the resulting alkylarenes are exclusively unbranched (that is, n-alkyl-substituted), with selectivity for the corresponding o-(n-alkyl)toluene.

Full text of DOI:10.1038/nchem.946

ARTICLES
PUBLISHED ONLINE: 19 DECEMBER 2010 | DOI: 10.1038/NCHEM.946
Catalytic dehydroaromatization of n-alkanes
by pincer-ligated iridium complexes
Ritu Ahuja1, Benudhar Punji1, Michael Findlater2, Carolyn Supplee3, William Schinski4,
Maurice Brookhart2 and Alan S. Goldman1
*
*
Aromatic hydrocarbons are among the most important building blocks in the chemical industry. Benzene, toluene and
xylenes are obtained from the high temperature thermolysis of alkanes. Higher alkylaromatics are generally derived from
arene–olefin coupling, which gives branched products—that is, secondary alkyl arenes—with olefins higher than ethylene.
The dehydrogenation of acyclic alkanes to give alkylaromatics can be achieved using heterogeneous catalysts at high
temperatures, but with low yields and low selectivity. We present here the first catalytic conversion of n-alkanes to
alkylaromatics using homogeneous or molecular catalysts—specifically ‘pincer’-ligated iridium complexes—and olefinic
hydrogen acceptors. For example, the reaction of n-octane affords up to 86% yield of aromatic product, primarily o-xylene
and secondarily ethylbenzene. In the case of n-decane and n-dodecane, the resulting alkylarenes are exclusively
unbranched (that is, n-alkyl-substituted), with selectivity for the corresponding o-(n-alkyl)toluene.
hree of the ‘seven basic building blocks of the chemical
Functionalized products derived from n-alkyl arenes could have
industry’ are aromatic molecules—the so-called BTX family: advantages over branched species, such as superior properties for
T
benzene, toluene and xylene1. These compounds are currently micelle formation as well as greater thermal stability owing to the
obtained by reforming petroleum feedstocks. As oil reserves dimin- fact that the benzylic position is less substituted29,30
.
ish and the world’s fuel supply shifts from gasoline to diesel2, aro-
In the present work we describe the first examples of dehydro-
matics will be in increasingly short supply. Higher alkyl benzenes aromatization of acyclic alkanes using molecular transition
are commercially produced on a scale of around 60 billion metal-based catalysts. The catalysts operate under relatively mild
pounds a year by arene–olefin coupling3–8. n-alkanes represent a conditions to give aromatics including n-alkyl arenes in the case
potential feedstock that is economically attractive compared with of n-alkanes of eight or more carbons—a conversion that is unpre-
both arenes and olefins, and will probably become more so with cedented for higher n-alkanes. In contrast to heterogeneous systems,
the increasing use of the Fischer–Tropsch process (reductive oligo- molecular (or single-site) catalysts afford opportunities for catalyst
merization of CO and H2)9,10
.
tuning to affect activity or selectivity through either empirical or
The catalytic dehydroaromatization of higher n-alkanes (carbon mechanism-based approaches. The disadvantage of molecular cata-
number . 6) has been reported using a variety of heterogeneous cat- lysts is generally the problem of catalyst/product separation, but we
alysts including metals, metal oxides and zeolites at high temperatures have recently reported that catalysts of this type can be efficiently
(typically 500–700 8C). Such reactions generally give low yields of aro- supported on solids such as alumina31,32; this should mitigate
matics and substantial quantities of non-aromatic compounds with separation problems.
fewer than six carbons owing to ‘cracking’. The aromatics themselves
generally consist of a complex mixture, including a significant fraction Results and discussion
with carbon numbers lower than the alkane feedstock. Toluene— The use of pincer-ligated iridium catalysts for alkane dehydrogena-
generally the member of the BTX triad in least demand—is frequently tion33 has been extensively developed and studied since the initial
the most abundant product of such mixtures, presumably reflecting report of transfer-dehydrogenation by Kaska and Jensen34–36. It
facile cleavage of the weak benzylic C–C bond11–23
.
has been found that hydrogen acceptors are not necessarily
The products of aromatic alkylation with linear olefins, although required, although alkene yields are limited in the absence of accep-
often referred to as ‘linear alkylbenzenes’, are not ‘linear’ in the usual tors37. The substitution of phosphino-t-butyl groups by i-propyl
sense; that is, they are not n-alkyl derivatives. The aryl group is a groups has been reported to increase activity38,39. In the course of
substituent at an internal position of an n-alkane chain (for investigating the dehydrogenation of n-alkanes catalysed by i-pro-
example, the 2-position when 1-alkenes are used as the olefinic pylphosphino-substituted pincer complexes, we have unexpectedly
reagent in the absence of prior isomerization); in this sense these observed the formation of small quantities of benzene (2.3%
molecules are branched. To our knowledge there are no existing yield) from n-hexane when using norbornene as the acceptor40.
atom-efficient large-scale routes to n-alkyl arenes. Current routes We now report a much more extensive and general investigation
use coupling methods that typically start with aryl halides, or of n-alkane aromatization using pincer-iridium catalysts.
Friedel–Crafts acylation and Clemenson reduction24, although the
Attempts to achieve dehydroaromatization using the t-butyl-
development of methods for anti-Markovnikov alkylation of non- substituted catalyst precursors (tBuPCP)IrHn (n ¼ 2, 4 or a
functionalized arenes is currently a goal under intense pursuit24–28
.
mixture thereof) gave negligible yields of aromatic product. Initial
1Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey 08854, USA, 2Department of Chemistry, University of North
Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA, 3Department of Chemistry, Monmouth University, West Long Branch, New Jersey
07764, USA, 4Chevron Research and Technology Co., 100 Chevron Way, Richmond, California 948023, USA. e-mail: brookhar@email.unc.edu;
*
alan.goldman@rutgers.edu
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.946
ARTICLES
hydrogen acceptor. Attempts to achieve dehydroaromatization
with the same catalysts, but without any acceptor (by allowing
H2 to escape from a refluxing solution37), have thus far
been unsuccessful.
a
2, 4 or 6
4
4
tBu
tBu
(TBE)
(TBA)
PiPr2
Ir
O
PtBu2
IrH4
PiPr2
IrH4
700
600
500
400
300
200
100
0
PiPr2
PtBu2
PiPr2
Hexadienes
Monoenes
Others
O
(
tBuPCP)IrH4
(
iPrPCP)IrH4
(
iPrPOCOP)Ir(C2H4)
1
2
3
Benzene
PtBu2
IrH4
PiPr2
PiPr2
Ir
O
Ir
PiPr2
PiPr2
PtBu2
(
iPrPCOP)Ir(C2H4)
(
tBuanthraphos)IrH4
(
iPranthraphos)Ir(C2H4)
0
25
50
75
100
125
4
5
6
Time (h)
b
2, 4 or 6
Aromatization of n-hexane. Heating solutions of n-hexane (1.53 M),
t-butylethylene (TBE; 6.13 M) and catalyst 2, 4 or 6 (5 mM) at 165 8C
initially resulted in the formation of acyclic C6 olefins (hexenes and
hexadienes) and commensurate quantities of hydrogenated TBE
(2,2-dimethylbutane or ‘t-butylethane’, TBA). Continued heating
resulted in the formation of benzene (Fig. 1a). After 120 h,
catalyst 4 gave the highest yield of benzene, 670 mM (44%). No
products lighter than C6 were observed by gas chromatography
(see Supplementary Information for full details).
tBu
tBu
3
3
2, 4 or 6
We propose that the formation of benzene from n-hexane pro-
ceeds via extensive and reversible dehydrogenation and olefin iso-
merization (probably including both double bond migration and
cis–trans isomerization) to give 1,3,5-cis-hexatriene. This triene
undergoes a classic thermal electrocyclization, with a first-order
tBu
tBu
Figure 1 | Dehydroaromatization of n-hexane catalysed by iridium pincer
complexes 2, 4 or 6. a, Overall reaction stoichiometry and products
versus time for dehydroaromatization catalysed by 4. Conditions:
[4] ¼5 mM; [n-hexane] ¼ 1.53 M; [TBE] ¼ 6.13 M; 165 8C. See
Supplementary Information for full data for product and reactant
concentrations obtained with catalysts 2, 4 and 6. b, Proposed pathway for
the dehydroaromatization.
2, 4 or 6
n-C8H18
+
4 TBE
4 TBA
(75%)
(11%)
attempts to catalyse dehydroaromatization using the isopropyl ana-
logue (iPrPCP)IrH4 (iPrPCP ¼ k3-C6H3-2,6-[CH2P(i-C3H7)2]2) (2)
gave results far superior to (tBuPCP)IrHn. As this difference was
presumably attributable to steric factors we investigated the corre-
sponding POCOP (iPrPOCOP ¼ k3-C6H3-2,6-(OP(i-Pr)2)2) deriva-
tive. Note that computationally generated models (using density
functional theory) reveal that POCOP complexes are sterically
less demanding than the PCP analogues41. (iPrPOCOP)Ir precursors
(for example, (iPrPOCOP)Ir(h2-C2H4)) were found to be less effec-
tive for dehydroaromatization than (iPrPCP)IrH4. However, the
‘hybrid’ phosphine/phosphinite catalyst (iPrPCOP)Ir(h2-C2H4) (4)
gave very good initial results and was used, along with
1,200
1,000
800
600
400
200
0
Total C8 aromatics
o-xylene
Ethylbenzene
1,2-dimethylcyclohexane
Benzene
(
iPrPCP)IrH4, throughout this investigation.
Haenel has reported that the ‘anthraphos’ catalyst
(
tBuanthraphos)IrH4 is thermally more robust than the simple
tBuPCP derivative42. With this in mind we have also studied the
0
25
50
75
100
125
(
iPranthraphos)Ir-derived catalyst43 for dehydroaromatization. In
Time (h)
this case the ethylene complex (iPranthraphos)Ir(h2-C2H4) (6) was
used as the catalyst precursor in analogy with our dehydrogenation Figure 2 | Dehydroaromatization of n-octane catalysed by 2, 4 or 6
studies using (tBuPOCOP)Ir(h2-C2H4) and related (pincer)Ir (with TBE as hydrogen-acceptor). Conditions: [4] ¼ 5 mM; [n-octane] ¼
(h2-C2H4) complexes.
1.46 M; [TBE] ¼ 5.84 M; 165 8C. Percentages in parentheses are yields
The stoichiometry of n-alkane aromatization requires the loss of based on n-octane obtained after 118 h with catalyst 4. See Supplementary
four equivalents of H2. Accordingly, most of the experiments con- Information for full data for product and reactant concentrations obtained
ducted in this study made use of four equivalents of an olefinic with catalysts 2, 4 and 6 (2 mM and 5 mM catalyst concentrations).
168
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.946
ARTICLES
rate constant of 2.5 × 1023 s21 at 165 8C (ref. 44), followed by
iridium-catalysed transfer-dehydrogenation (Fig. 1b). Very approxi-
mately, the maximum observed rate of benzene formation with cat-
alysts 2 and 4 is appoximately 10 mM h21. Assuming the
cyclization rate of 2.5 × 1023 s21, a maximum steady-state concen-
tration of the cis-hexatriene of approximately 1.1 mM is implied.
This does not seem to be a particularly high value for a triene
given that the steady-state concentration of monoenes and dienes
is on the order of 0.2 M and 0.6 M, respectively.
+
2 or 4
n-C10H22
4 TBE
(45%)
(2%)
(5%)
(2%)
4 TBA
+
700
600
500
400
300
200
100
0
Aromatization of higher n-alkanes. The reaction of n-octane
(1.46 M) with four equivalents of TBE (5.84 M) catalysed by 2, 4
or 6 (5 mM catalyst; 165 8C) gave substantial conversion to C8
mono-, di- and trienes45 at early reaction times (within about 2 h;
see Supplementary Information for full details). After 118 h,
95–99% of n-octane was consumed and the total yields of C8
aromatics (o-xylene and ethylbenzene) were 48% and 86% with
catalysts 2 and 4, respectively (Fig. 2).
Yields of aromatic product are thus significantly higher than
those obtained from n-hexane. All three catalysts are selective for
formation of o-xylene over ethylbenzene. In particular, catalyst 4
gives a 7:1 ratio of o-xylene:ethylbenzene after 118 h, although at
early reaction times the ratio is much lower. m-xylene, p-xylene
and toluene were not observed in any of these catalytic reactions.
Only small amounts of benzene (,2% of total aromatics, 0.5% in
the case of catalyst 4) were formed.
Total C10 aromatics
o-propyltoluene
1,2-diethylbenzene
n-butylbenzene
Benzene
0
25
50
75
100
125
Time (h)
The dehydrogenation of n-octane to olefins is catalysed more Figure 4 | Dehydroaromatization of n-decane catalysed by 4. Conditions:
rapidly by 2 than by 4; however, catalyst 4 affords higher yields of [4] ¼ 5 mM; [n-decane] ¼ 1.39 M; [TBE] ¼ 5.57 M; 165 8C. Percentages
aromatics. This may suggest that catalyst 4 is more active for isomer- in parentheses are yields based on n-decane obtained after 120 h with
ization, or that the dehydrogenation of 1,2-dimethylcyclohexadiene catalyst 4. See Supplementary Information for full data for product and
(the product of cyclization of 2,4,6-octatrienes) is faster with catalyst 4. reactant concentrations obtained with catalysts 2 and 4.
The latter scenario seems plausible as significantly higher formation
of cis- and trans-dimethylcyclohexane (presumably resulting from of catalyst. The turnover numbers (moles of aromatic product per
the hydrogenation of dimethylcyclohexadiene) is observed during moles of catalyst) obtained for total aromatics were 330 and 580
the reaction with catalyst 2 (see Supplementary Information for after 118 h, for catalysts 2 and 4 respectively.
full details).
On the laboratory scale, TBE is a convenient hydrogen acceptor
The use of 2 mM of catalyst 2 or 4 gave yields of o-xylene and as it is a liquid and is resistant to isomerization. Practical large-scale
ethylbenzene only slightly lower than those obtained with 5 mM applications would obviously require the use of acceptors that are
low-cost and/or readily recyclable. In this context, it is worth
2, 4 or 6
noting that propene can be used as a hydrogen acceptor for the
dehydroaromatization of n-octane (Fig. 3). After heating for 5 days
n-C8H18
+
under 1 atm propene at 165 8C, 38% and 18% conversions to
4
4
(75%)
(11%)
aromatics were obtained with catalysts 2 and 4, respectively.
Considering the very different conditions of these reactions, includ-
ing the large differences in concentrations of n-octane and olefin
acceptor in solution, and greatly different volumes of the reaction
vessels (60 ml for propene versus 0.2 ml for TBE), as well as the
presumably large differences in both the hydrogen-accepting and
coordinating abilities of propene versus TBE, a close comparison
of reaction yields is probably not meaningful.
500
400
300
200
100
0
Total C8 aromatics
o-xylene
Ethylbenzene
1,2-dimethylcyclohexane
Benzene
The dehydroaromatization of n-decane with four equivalents of
TBE resulted in surprisingly high selectivity for o-propyltoluene
(Fig. 4). Using catalyst 2 (120 h at 165 8C), 26% o-propyltoluene
was obtained with approximately 2% each of n-butyl benzene and
1,2-diethylbenzene, totalling 30% aromatic products. With catalyst
4, 52% conversion to total aromatics is obtained including 45%
o-propyltoluene as the major dehydroaromatization product. With
both catalysts, a small amount of benzene was produced—5% and
2% conversion with catalysts 2 and 4, respectively.
0
25
50
75
100
125
The reaction of n-dodecane with four equivalents of TBE
(Fig. 5a) yielded o-pentyltoluene as the major aromatic product.
With catalysts 2 and 4, 14% and 21% conversion to o-pentyltoluene
was obtained after 120 h, respectively. In addition to o-pentylto-
luene, n-hexylbenzene, 2-n-butylethylbenzene and 1,2-di-n-propyl
benzene were also formed totalling 23% and 35% conversion to
C12-alkyl-aromatics with 2 and 4, respectively. Alkenylaromatics
Time (h)
Figure 3 | Dehydroaromatization of n-octane catalysed by 2 with propene
as hydrogen acceptor. Conditions: [2] ¼ 5 mM; [n-octane] ¼ 1.23 M; in
0.39 ml mesitylene; Ppropene ¼ 1 atm (four equivalents in 60-ml reaction
vessel); 165 8C. See Supplementary Information for full data for product and
reactant concentrations obtained with catalysts 2 and 4.
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ARTICLES
n-C5H11
n-C4H9
used as the substrate for dehydrogenation under the same reaction
conditions, benzene formation was not observed. Moreover, the
use of n-hexylcyclohexane as a substrate gave only 22 mM
benzene, much less than that obtained from n-dodecane. These
observations suggest that C–C bond cleavage is not occurring
after cyclization. Nevertheless it seems highly unlikely that C–C
cleavage would occur selectively at the C6–C7 bond prior to cycliza-
tion. Therefore we suspect that the C–C bond cleavage occurs syn-
chronously with formation of a six-membered ring; however any
detailed mechanistic proposal would be highly speculative at this
point. The mechanism of this very surprising reaction is currently
under investigation.
a
+
(21%)
+
(6%)
2 or 4
n-C12H26
4 TBE
(17%)
4 TBA
n-C3H7
n-C6H13
+
n-C3H7
(1%)
(7%)
Total C12 aromatics
o-pentyltoluene
Summary
500
400
300
200
100
0
The dehydroaromatization of n-alkanes has been achieved using
pincer-ligated iridium catalysts with hydrogen acceptors at relatively
low temperatures. This use of molecular or solution-phase catalysts
for the aromatization of acyclic olefins is unprecedented. The result-
ing selectivity for the formation of arenes with the same carbon
number as the n-alkane reactant is very high for n-hexane and n-
octane. For n-decane, the aromatic products are predominantly
C10 but significant quantities of benzene are also formed (up to
around 15% of total aromatics at high overall conversion, with
much higher relative amounts at low conversion). In the reaction
of n-dodecane, benzene is the major arene observed at low conver-
sion whereas at high conversion the concentrations of benzene and
total C12 arenes are comparable. For the three Cn n-alkanes studied
with n . 6, the aromatic products with Cn conserved are either the
corresponding n-alkylbenzene or an ortho-substituted dialkylben-
zene, consistent with no skeletal isomerization of the linear
carbon chain during reaction. In particular, in all three cases the
major aromatic product with Cn conserved is the corresponding
ortho-alkyltoluene (alkyl ¼ methyl, n-propyl or n-pentyl for C8,
C10 and C12 respectively).
2-n-butylethylbenzene
1,2-di-n-propylbenzene
n-hexylbenzene
Benzene
0
25
50
75
100
125
Time (h)
90
80
70
60
50
40
30
20
10
0
b
Total C12 aromatics
o-pentyltoluene
2-n-butylethylbenzene
1,2-di-n-propylbenzene
n-hexylbenzene
Benzene
The carbon-number selectivity (affording exclusively benzene
and products with the carbon number of the reactant) has not
been observed with previously reported (heterogeneous) dehydroar-
omatization systems. The selectivity for production of n-alkyl-
toluenes is of particular interest in view of the current lack of
industrial methods for the production of n-alkylarenes.
Methods
All alkanes, mesitylene, and para-xylene were distilled under vacuum from Na/K
alloy after several freeze-pump-thaw cycles and stored in an argon-atmosphere glove
box. (iPrPCP)IrH4 (2) was synthesized in analogy with the procedure reported for the
para-methoxy derivative (p-MeO- iPrPCP)IrH4 (ref. 40). (iPranthraphos)Ir(H)Cl was
synthesized as previously reported by Grubbs and co-workers43. The synthesis of
0
1
2
3
4
Time (h)
5
6
7
(
iPrPCOP)Ir(h2-C2H4) is given in the Supplementary Information. Authentic
samples of 2,4,6-octatriene45, o-pentyltoluene46 and di-n-propylbenzene46 were
prepared according to literature procedures. Gas chromatography-mass
spectrometry (GC-MS) measurements were performed with a Varian 3900 Saturn
2100T instrument fitted with a capillary column (25 m × 0.2 mm inner diameter ×
0.5 mm film thickness). Details of the gas chromatography analyses are given in the
Supplementary Information.
Figure 5 | Dehydroaromatization of n-dodecane. a, Conditions: [4] ¼ 5 mM;
[n-dodecane] ¼ 1.33 M; [TBE] ¼ 5.33 M; 165 8C. Percentages in parentheses
are yields based on n-dodecane obtained after 120 h with catalyst 4. The
graph shows aromatic product concentrations over the course of 120 h.
b, Aromatic product concentrations over the first 7 h. See Supplementary
Information for full data for product and reactant concentrations obtained
with catalysts 2 and 4.
Representative procedure for dehydroaromatization of n-alkanes using
tert-butylethylene (TBE) as the H2 acceptor. In an argon-filled glovebox,
(
iPrPCP)IrH4 (2.7 mg, 0.005 mmol) or (iPrPCOP)Ir(h2-C2H4) (2.8 mg, 0.005 mmol)
was dissolved in n-alkane (for example, hexane: 0.2 ml, 1.53 mmol); TBE
(four equivalents with respect to n-hexane, 0.79 ml, 6.13 mmol) and mesitylene
(0.01 ml, internal standard) were then added to the solution. The total solution
(n-alkane, TBE and mesitylene) volume was 1.0 ml. Aliquots of this solution (0.1 ml
each) were transferred to multiple 5 mm glass tubes. The contents were cooled
under liquid nitrogen and then sealed under vacuum. These sealed tubes were
heated (in parallel) in a preheated oven at 165 8C. At regular intervals, a tube
was brought to room temperature and the sample was analysed by gas
chromatography in comparison with authentic products. The assignments of
the major products were confirmed with GC-MS.
were not observed when four equivalents of TBE were used.
However, when eight equivalents of TBE were used the major
product obtained from the reaction catalysed by 4 (120 h at
165 8C) was ortho-(penta-di-1,3-enyl)toluene (187 mM) with a
small amount of 1,5-dimethylnaphthalene (11 mM).
In addition to the C12 aromatic compounds, significant conver-
sion to benzene was obtained with catalysts 2 and 4—22% and 17%
respectively after 120 h. At early reaction times (ꢀ24 to 48 h)
benzene was the major aromatic product (Fig. 5b). We initially sus-
pected that the benzene formation might result from a C–C cleavage
Representative procedure for dehydroaromatization of n-alkanes using
propylene as the H2 acceptor. In an argon-filled glovebox, (iPrPCP)IrH4 (2.7 mg,
reaction of n-hexylbenzene. However, when n-hexylbenzene was 0.005 mmol) or (iPrPCOP)Ir(h2-C2H4) (2.8 mg, 0.005 mmol) was dissolved in
170
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ARTICLES
n-octane (0.1 ml, 0.615 mmol) and mesitylene (0.39 ml, 6.47 mmol). p-xylene
(0.01 ml, internal standard) was added. This solution (0.5 ml) was then transferred
to a Schlenk flask (approximately 60 ml total volume) containing a micro stir bar
fitted with a Kontes valve and an o-ring joint. The contents were cooled under liquid
nitrogen and the solution was degassed via freeze-pump-thaw cycles and then
brought to room temperature and charged with propylene (800 torr). The flask was
then immersed in an oil bath maintained at 165 8C while the solution was stirred.
After heating for 24 h, the flask was removed from the oil bath and brought to room
temperature. The solution was then cooled in liquid nitrogen and the flask was
evacuated and transferred to the glove box. The reaction mixture was analysed by gas
chromatography. Fresh propylene was charged into the flask as above. The cycle
was repeated until there was no significant change in the concentration of the
products (as determined by gas chromatography).
26. Foley, N. A., Lee, J. P., Ke, Z., Gunnoe, T. B. & Cundari, T. R. Ru(II) catalysts
supported by hydridotris(pyrazolyl)borate for the hydroarylation of olefins:
reaction scope, mechanistic studies, and guides for the development of improved
catalysts. Acc. Chem. Res. 42, 585–597 (2009).
27. McKeown, B. A., Foley, N. A., Lee, J. P. & Gunnoe, T. B. Hydroarylation of
unactivated olefins catalyzed by platinum(II) complexes. Organometallics 27,
4031–4033 (2008).
28. Luedtke, A. T. & Goldberg, K. I. Intermolecular hydroarylation of unactivated
olefins catalyzed by homogeneous platinum complexes. Angew. Chem. Int. Ed.
47, 7694–7696 (2008).
29. Peng, Y., Ma, X. & Schobert, H. H. Thermopyrolysis mechanism of
n-alkylbenzene: experiment and molecular simulation. Prepr. Am. Chem. Soc.
Div. Pet. Chem. 43, 368–372 (1998).
30. Eapen, K. C., Snyder, C. E., Jr., Gschwender, L., Dua, S. S. & Tamborski, C.
Poly-n-alkylbenzene compounds. A class of thermally stable and wide liquid
range fluids. Prepr. Am. Chem. Soc. Div. Pet. Chem. 29, 1053–1058 (1984).
31. Huang, Z. et al. Efficient heterogeneous dual catalyst systems for alkane
metathesis. Adv. Synth. Catal. 352, 125–135 (2010).
32. Huang, Z. et al. Highly active and recyclable heterogeneous iridium pincer
catalysts for transfer dehydrogenation of alkanes. Adv. Synth. Catal. 351,
188–206 (2009).
33. Dobereiner, G. E. & Crabtree, R. H. Dehydrogenation as a substrate-activating
strategy in homogeneous transition-metal catalysis. Chem. Rev. 110,
681–703 (2010).
34. Gupta, M., Hagen, C., Flesher, R. J., Kaska, W. C. & Jensen, C. M. A highly active
alkane dehydrogenation catalyst: stabilization of dihydrido Rh and Ir complexes
by a P-C-P pincer ligand. Chem. Commun. 2083–2084 (1996).
35. Gupta, M., Hagen, C., Kaska, W. C., Cramer, R. E. & Jensen, C. M. Catalytic
dehydrogenation of cycloalkanes to arenes by a dihydrido iridium P-C-P pincer
complex. J. Am. Chem. Soc. 119, 840–841 (1997).
36. Gupta, M., Kaska, W. C. & Jensen, C. M. Catalytic dehydrogenation of
ethylbenzene and THF by a dihydrido iridium P-C-P pincer complex. Chem.
Commun. 461–462 (1997).
37. Xu, W. et al. Thermochemical alkane dehydrogenation catalyzed in solution
without the use of a hydrogen acceptor. Chem. Commun. 2273–2274 (1997).
38. Liu, F., Pak, E. B., Singh, B., Jensen, C. M. & Goldman, A. S. Dehydrogenation of
n-alkanes catalyzed by iridium ‘pincer’ complexes. regioselective formation of
alpha-olefins. J. Am. Chem. Soc. 121, 4086–4087 (1999).
39. Liu, F. & Goldman, A. S. Efficient thermochemical alkane dehydrogenation and
isomerization catalyzed by an iridium pincer complex. Chem. Commun.
655–656 (1999).
40. Zhu, K., Achord, P. D., Zhang, X., Krogh-Jespersen, K. & Goldman, A. S. Highly
effective pincer-ligated iridium catalysts for alkane dehydrogenation. DFT
calculations of relevant thermodynamic, kinetic, and spectroscopic properties.
J. Am. Chem. Soc. 126, 13044–13053 (2004).
Received 8 July 2010; accepted 11 November 2010;
published online 19 December 2010
References
1. Wittcoff, H. A., Reuben, B. G. & Plotkin, J. S. Industrial Organic Chemicals
2nd edn (Wiley–IEEE, 2005).
2. Annual Energy Outlook 2010 With Projections to 2035 (US Energy Information
Administration, 2010); see http://www.eia.doe.gov/oiaf/aeo/pdf/0383(2010).pdf
3. Rueping, M. & Nachtsheim, B. J. A review of new developments in the Friedel-
Crafts alkylation. From green chemistry to asymmetric catalysis. Beilstein J. Org.
Chem. 6, No 6 (2010).
4. Olah, G. A., Reddy, V. P. & Prakash, G. K. S. in Kirk-Othmer Encyclopedia of
Chemical Technology 5th edn, vol 12, 159–199 (Wiley, 2005).
5. Kocal, J. A., Vora, B. V. & Imai, T. Production of linear alkylbenzenes. Appl.
Catal. A 221, 295–301 (2001).
6. Vora, B. V., Pujado, P. R., Imai, T. & Fritsch, T. R. Recent advances in the
production of detergent olefins and linear alkylbenzenes. Tenside Surfact. Det.
28, 287–294 (1991).
7. Perego, C. & Ingallina, P. Recent advances in the industrial alkylation of
aromatics: new catalysts and new processes. Catal.Today 73, 3–22 (2002).
8. Perego, C. & Ingallina, P. Combining alkylation and transalkylation for alkyl
aromatic production. Green Chem. 6, 274–279 (2004).
9. Dry, M. E. Present and future applications of the Fischer-Tropsch process. Appl.
Catal. A 276, 1–3 (2004).
10. Dry, M. E. The Fischer-Tropsch process: 1950–2000 Catal. Today 71,
227–241 (2002).
ˇ
ˇ
´
´
11. Smieskova, A., Rojasova, E., Hudec, P. & Sabo, L. Aromatization of light alkanes
over ZSM-5 catalysts. Influence of the particle properties of the zeolite. Appl.
Catal. A 268, 235–240 (2004).
12. Davis, B. H. Alkane dehydrocyclization mechanism. Catal. Today 53,
443–516 (1999).
13. Meriaudeau, P. & Naccache, C. Dehydrocyclization of alkanes over zeolite-
supported metal catalysts: monofunctional or bifunctional route. Catal. Rev. Sci.
Eng. 39, 5–48 (1997).
14. Davis, R. J. Aromatization on zeolite L-supported Pt clusters. Heterogen. Chem.
Rev. 1, 41–53 (1994).
15. Arata, K., Hino, M. & Matsuhashi, H. Solid catalysts treated with anions. XXI.
Zirconia-supported chromium catalyst for dehydrocyclization of hexane to
benzene. Appl. Catal. A 100, 19–26 (1993).
16. Spitsyn, V. I., Pirogova, G. N., Korosteleva, R. I. & Kalinina, G. E. Aromatization
of hexane and heptane on technetium catalysts. Doklady Akademii Nauk SSSR
298, 149–151 [Phys. Chem.] (1988).
17. Hino, M. & Arata, K. Solid catalysts treated with anions. Dehydrocyclization of
hexane to benzene over zirconia-supported chromia. J. Chem. Soc. Chem.
Commun. 1355–1356 (1987).
18. Isagulyants, G. V., Sterligov, O. D., Barkova, A. P., Mashinskii, V. I. &
Kugucheva, E. E. Effect of modification of alumina-platinum catalysts on the
composition of arenes formed in the process of dehydrogenation of higher
n-paraffins. Neftekhimiya 27, 357–362 (1987).
19. Szebenyi, I. & Szechy, G. Acta. Chim. Hung. 98, 115 (1978).
20. Gairbekov, T. M., Takaeva, M. I., Khadzhiev, S. N. & Manovyan, A. K. Cracking
and aromatization of C6–10 n-alkanes and n-alkenes by a zeolite-containing
catalyst. J. Appl. Chem. USSR 64, 2396–2400 (1991).
41. Biswas, S. et al. in Abstracts of Papers, 235th ACS National Meeting, New Orleans,
LA, United States INOR-302 (2008).
42. Haenel, M. W. et al. Thermally stable homogeneous catalysts for alkane
dehydrogenation. Angew. Chem. Int. Ed. 40, 3596–3600 (2001).
43. Romero, P. E., Whited, M. T. & Grubbs, R. H. Multiple C-H activations of
methyl tert-butyl ether at pincer iridium complexes: synthesis and thermolysis of
Ir(I) Fischer carbenes. Organometallics 27, 3422–3429 (2008).
44. Doledec, G. & Commereuc, D. Synthesis and properties of homogeneous
models of the Re2O7/Al2O3 metathesis catalyst. J. Mol. Cat. A 161,
125–140 (2000).
45. Jacobson, B. M., Arvanitis, G. M., Eliasen, C. A. & Mitelman, R. Ene reactions of
conjugated dienes. 2. Dependence of rate on degree of hydrogen removed and s-
cis or s-trans diene character. J. Org. Chem. 50, 194–201 (1985).
46. Garg, N. & Lee, T. R. Regioselective bromomethylation of 1,2-dialkylbenzenes.
Synlett 310–312 (1998).
Acknowledgements
The authors are grateful to the National Science Foundation (grant no. CHE-0650456) for
financial support for this work through the Center for Enabling New Technologies
through Catalysis (CENTC).
Author contributions
R.A., B.P., M.F., W.S., M.B. and A.S.G. conceived and designed the experiments; R.A.,
B.P., M.F. and C.S. performed the experiments; R.A., B.P., M.F., C.S., M.B. and A.S.G.
co-wrote the paper.
21. Komarewsky, V. I. & Riesz, C. H. Aromatization of octane and decane in the
presence of nickel-alumina catalyst. J. Am. Chem. Soc. 61, 2524–2525 (1939).
22. Vaisberg, K. S., Zhorov, Y. M., Panchenkov, G. M. & Rudyk, L. G. Formation of
isomers of aromatic hydrocarbons during the dehydrocyclization of n-decane.
Zh. Fiz. Khim. 44, 2630 (1970).
23. Shell. Dehydrocyclization of paraffins. NL patent 6715757 (1968).
24. Matsumoto, T., Taube, D. J., Periana, R. A., Taube, H. & Yoshida, H. Anti-
Markovnikov olefin arylation catalyzed by an iridium complex. J. Am. Chem. Soc.
122, 7414–7415 (2000).
25. Oxgaard, J., Periana, R. A. & Goddard, W. A., III. Mechanistic analysis of
hydroarylation catalysts. J. Am. Chem. Soc. 126, 11658–11665 (2004).
Additional information
The authors declare no competing financial interests. Supplementary information and
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naturechemistry. Reprints and permission information is available online at http://npg.nature.
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to M.B. and A.S.G.
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