Selective and unexpected transformations of 2-methylpropane to 2,3-dimethylbutane and 2-methylpropene to 2,3-dimethylbutene catalyzed by an alumina-supported tungsten hydride

Article abstract of DOI:10.1039/b823226a

2-Methylpropane and 2-methylpropene, in the presence of the W(H) ₃/Al₂O₃ catalyst, are unexpectedly transformed to 2,3-dimethylbutane and 2,3-dimethylbutenes, respectively, with high selectivity; in case of 2-methylpropane

Full text of DOI:10.1039/b823226a

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Selective and unexpected transformations of 2-methylpropane to
2,3-dimethylbutane and 2-methylpropene to 2,3-dimethylbutene
catalyzed by an alumina-supported tungsten hydridew
Nicolas Merle, Franc¸ ois Stoffelbach, Mostafa Taoufik,* Erwan Le Roux, Jean Thivolle-Cazat
and Jean-Marie Basset*
Received (in Cambridge, UK) 24th December 2008, Accepted 30th January 2009
First published as an Advance Article on the web 16th March 2009
DOI: 10.1039/b823226a
2-Methylpropane and 2-methylpropene, in the presence of
the W(H)3/Al2O3 catalyst, are unexpectedly transformed to
2,3-dimethylbutane and 2,3-dimethylbutenes, respectively, with
high selectivity; in case of 2-methylpropane, this reaction
represents the first example of the transfer of two carbons in
alkane metathesis.
in parallel via a-H elimination from the metal-alkyl; and
(iii) hydrogenation of the new olefins on the metal-hydride.5
Recently, we have developed a W(H)3/Al2O3–500 catalyst,
1b, giving higher turnover numbers (TON) and having a much
greater stability than Ta hydrides.6–9 The peculiarity of
both systems lies in the fact that Ta or W hydrides are
‘‘multifunctional–single site’’ catalysts (alkane dehydrogenation,
olefin metathesis and olefin hydrogenation). Herein, we report
that 2-methylpropane, in the presence of the W(H)3/Al2O3–500
catalyst, 1b, is unexpectedly transformed to 2,3-dimethylbutane
(2,3-DMB) with a high selectivity.10 This metathesis reaction
of an iso-Cn paraffin leading to an iso-Cn+2 product instead
of an iso-Cn+1, involves formally the selective transfer of
two carbons, which is unexpected in view of the classical
mechanism previously proposed for this reaction (see ESI
Scheme S1w).5 Interestingly, 2,3-DMB exhibits the highest
octane number (RON = 104) among the hexane isomers.11
Similarly, the W(H)3/Al2O3–500 catalyst, 1b, also unexpectedly
transforms 2-methylpropene to 2,3-dimethylbutenes (2,3-DMB=)
with a good selectivity which is the first case of productive
metathesis of 2-methylpropene12,13 and again fits with the role
of 2-methylpropene as a primary product for the metathesis of
2-methylpropane.
The transformation of lower alkanes into higher homologues
remains a great challenge in chemistry, in particular for
branched alkanes in view of fuel needs.1–3 Using a ‘‘single site
approach’’, we reported in 1997 the discovery of a new
reaction, the alkane metathesis where a highly electrophilic
tantalum hydride supported on silica, [(RSiO)2Ta–H] 1a,
could catalytically transform at moderate temperatures any
light alkane into its lower and higher homologues.2 This
reaction consists mainly in the transfer of a single carbon
from one chain to another, by both cleavage and formation of
C–H and C–C bonds (Scheme 1).
Scheme 1
Typically, when 2-methylpropane was passed over the
alumina-supported tungsten hydride, W(H)3/Al2O3–500, 1b,
(Pi-C4H10 = 1 bar, T = 150 1C, flow rate 4 mL minꢀ1 or
volume hourly space velocity (VHSV) 260 hꢀ1) its conversion
reached a maximum at 8% and approximately 37 TON were
achieved over 43 h (Fig. 1(a)). At the pseudo-steady state, the
observed selectivities in 2-methylpropane metathesis are
2,3-DMB (41.7%), ethane (41.3%), 2-methylbutane (5.3%),
propane (5.2%), methane (1.5%), heptanes (1.0%) and
2-methylpropene (3.1%) (Fig. 1(b)). The higher homo-
Typically propane gives mainly ethane (45–50%) and
butanes (30–35%) along with methane and higher C5+
alkanes. Ethylbenzene and methane can also be obtained
by cross-metathesis between toluene and ethane.4 In all
experiments carried out so far, the selectivity for the formation
of the Cn+1 product proved to be higher than that of the Cn+2
one, in agreement with the previously proposed mechanism.5
In fact, kinetic studies on propane metathesis, carried out at
extremely low contact time in a continuous flow reactor,
revealed the primary products of the reaction, namely olefins
and H2. This observation as well as elementary steps known in
tantalum organometallic chemistry led us to propose a
mechanism based on the following key steps: (i) paraffin
dehydrogenation via C–H bond activation leading to a
metal-alkyl with subsequent formation of an olefin and a
metal-hydride; (ii) olefin metathesis on a metallocarbene formed
logues can be ordered as follows: Cn+2 c Cn+1 4 Cn+3
These results are quite different from those obtained with
.
W(H)3/Al2O3–500 in metathesis of linear alkanes, where the
6–8
selectivities always follow the order: Cn+1 4 Cn+2 c Cn+3
Interestingly the ratio of selectivities of 2,3-DMB/ethane and
.
2-methylbutane/propane are close to unity.
2-Methylpropene was observed (up to 3.1%) among the
products of 2-methylpropane metathesis and therefore, its
metathesis was also checked over 1b in a continuous flow
reactor (Pi-C4H8 = 1 bar, T = 150 1C, flow rate 4 mL minꢀ1 or
volume hourly space velocity (VHSV) 260 hꢀ1) (Fig. 2). The
reaction presented an initial maximal conversion rate of
0.61 moli-C4H8 molWꢀ1 minꢀ1 before reaching a pseudo-plateau
´
Universite Lyon 1, Institut de Chimie de Lyon, CPE Lyon, CNRS,
UMR 5265 C2P2, LCOMS, Baˆtiment 308 F 43 Blvd du 11 Novembre
1918, F-69616, Villeurbanne Cedex, France. E-mail: taoufik@cpe.fr,
basset@cpe.fr; Fax: +33(0)472431795; Tel: +33(0)472431798
w Electronic supplementary information (ESI) available: Experimental
details, Schemes S1 and S2. See DOI: 10.1039/b823226a
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tris(isobutyl)tungsten intermediate 2 (Scheme 2); in the first
case hydrogen is also evolved which is likely consumed via
hydrogenolysis of 2-methylpropane leading to the sharp initial
peaks of methane and propane (Fig. 1(b)), as 1b is known to
catalyze such a reaction.6–8
The trisisobutyl tungsten complex 2, [W(CH2CH(CH3)2)3]/
Al2O3–500, resulting from the initiation steps, can undergo an
a-H
transfer
to
form
3,
[W(CH2CH(CH3)2)-
(QCHCH(CH3)2)]/Al2O3–500, which further leads to the
carbene-hydride 4, [W(H)(QCHCH(CH3)2)]/Al2O3–500 by
b-H elimination (Scheme 3).9
Fig. 1 Metathesis of 2-methylpropane catalyzed by (W(H)3/Al2O3,
1b (3.86 wt% W): (a) conversion of 2-methylpropane (K) and TON
(m); (b) selectivities: (E) methane, (J) ethane, (’) propane; (*)
2-methylpropene; (&) 2-methylbutane; (K) 2,3-dimethylbutane
(2,3-DMB); (+) 2,4-dimethylpentane; (ꢀ) octanes.
Scheme 3
As known in the classical olefin metathesis mechanism15 and
also proposed for alkane metathesis,5 the 2-methylpropene used
as a reactant (2-methylpropene metathesis) or released from any
isobutyl tungsten species by b-H elimination (2-methylpropane
metathesis), can react with the hydrido-tungsta-carbene 4.
Depending on the two coordinative approaches of
2-methylpropene, two possible tungstacyclobutanes 5a and 5b,
are obtained, which have gem-methyl and -isopropyl substituents
in either the [1,2] or [1,3]-positions (Scheme 4a and c). These
two tungstacycles will further undergo metathetical cleavages:
(i) 5a giving 3-methyl-1-butene and the new hydrido-
tungsta-carbene 6a, [W(H)(=C(CH3)2)]/Al2O3–500, (ii) 5b giving
2,4-dimethyl-2-pentene and the hydrido-tungsta-carbene 6b,
Fig. 2 Metathesis of 2-methylpropene catalyzed by (W(H)3/Al2O3, 1b
(3.86 wt% W): (a) conversion of 2-methylpropene (E) and TON (m);
(b) selectivities: (J) ethylene, (’) propene; (m) 2-methylbutene; (&)
neohexene, (K) 2,3-dimethylbutene (2,3-DMB=); (E) 2,4,4-trimethyl-
1-pentene and 2,4,4-trimethyl-2-pentene.
ꢀ1
of 0.22 moli-C4H8 molW minꢀ1, with an overall turnover
number (TON) of 235 after 16 h (Fig. 2(a)). At the pseudo-
steady state, the product selectivities were 2,3-dimethylbutenes
50%, ethylene 30%; 2,4,4-trimethylpentenes 12%, neohexene
9%, isopentene 3% and propene 1% (Fig. 2(b)).
[W(H)(QCH2)]/Al2O3–500
.
2,4,4-Trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene
are likely formed by dimerisation of 2-methylpropene on
the acidic sites of alumina dehydroxylated at 500 1C13,14
while the presence of neohexene can be explained by a
cross-metathesis between 2,4,4-trimethylpentenes and ethylene.
In summary, whereas the metathesis of 2-methylpropene is
usually known as a degenerate process with the classical olefin
metathesis catalysts,12,13 W(H)3/Al2O3–500 also proves to be
the first compound capable of catalyzing the productive
self-metathesis of 2-methylpropene into 2,3-dimethylbutene.
Interestingly, 2,3-dimethylbutene has the same carbon
skeleton as the major product formed during the metathesis
of 2-methylpropane i.e. 2,3-DMB. These observations confirm
that 2-methylpropene is a primary product which leads to a
specific mechanistic pathway for 2-methylpropane metathesis
as explicated hereafter.
The hydrido-tungsta-carbenes 6a and 6b can also react
further with 2-methylpropene giving the corresponding
tungstacyclobutanes intermediates 7a and 7b (Scheme 4b).
Again, these two tungstacycles can undergo metathetical
cleavage, 7a giving 2,3-dimethyl-2-butene and the
hydrido-tungsta-carbene 6b, 7b giving ethylene and the hydrido-
tungsta-carbene 6a (Scheme 4b).
Paradoxically, 7a is a priori the less thermodynamically
favored intermediate, compared to 5b and particularly 5a, since
it has gem-methyl substituents in the [1,2]-positions generating a
sterically encumbered tungstacyclobutane intermediate.5,16 Two
other intermediates 7a0 and 7b0 could be formed from the
reverse favored approach of 2-methylpropene on 6a and
6b but they lead to degenerate metathesis (Scheme S2w).
Productive metathesis of 2-methylpropene involving 7a is thus
disfavored for steric reasons which accounts for the fact that it
was not reported previously12,13 and a moderate activity is
observed with W(H)3/Al2O3–500 by comparison with metathesis
of linear alkenes, such as propylene.17 From the various
catalytic cycles (a), (b), (c), the 2-methylpropane metathesis
process proceeds with the insertion of all the released olefins
into tungsten-hydride species such as 4, 6a or 6b. The resulting
tungsten alkyl groups can be further cleaved by H2
(Scheme S1w), affording the liberation of ethane, 2-methylbutane,
2,3-DMB and 2,4-dimethylpentane.5 The alkyl groups could
The initiation steps of both 2-methylpropane and 2-methyl-
propene, respectively by C–H bond activation or CQC
double bond insertion in 1b are expected to lead to the same
Scheme 2
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2,3-DMB. A new mechanistic pathway is proposed based on
two key hydrido-tungsta-carbenes 4 and 6a which accounts for
the formation and the selectivities of all the products observed.
Notes and references
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J. Thivolle-Cazat and J. M. Basset, Chem. Commun., 2004,
1434–1435.
Scheme 4 (a) and (c) classical and (b) unusual mechanistic pathways
in the metathesis of 2-methylpropane and 2-methylpropene (M = W).
5 J. M. Basset, C. Coperet, L. Lefort, B. M. Maunders, O. Maury,
E. Le Roux, G. Saggio, S. Soignier, D. Soulivong, G. J. Sunley,
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8604–8605.
6 E. Le Roux, M. Taoufik, C. Coperet, A. de Mallmann, J. Thivolle-
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´
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also be displaced via s-bond metathesis, by the incoming
2-methylpropane present in excess. The excess of 2-methylbutane
(5%) over 2,4-dimethylpentane (0.5%) can be understood on
the basis of the stability of the key metallacyclobutanes or of
the facility of their formation (5a 4 5b), which both depends
on the interaction between the substituents in the [1,2] or
[1,3]-positions as proposed for olefin metathesis.16
Alternatively, although unfavorably, the hydrido-tungsta-
carbenes 6a and 6b could undergo the migration of the hydride
onto the carbene ligands involving the formation of
WIV(–CH(CH3)2) 8a and WIV(–CH3) 8b but also a concomitant
disfavored reduction of WVI to WIV 18
The resulting alkyl
.
groups could again be displaced by H2 or the incoming alkane
reactant via s-bond metathesis giving propane and methane in
minor amounts (Scheme 4a and 1c). Finally, it should be
mentioned here that 2,3-DMB, which is of interest for its high
octane number (RON = 104),11 is currently obtained by
paraffin isomerisation with the use of two major commercial
bifunctional catalysts: Pt on highly chlorinated alumina, and
Pt on mordenite; however, the selectivity remains low around
8% and both catalysts are deactivated by coke formation.19
In conclusion, tungsten hydride supported on alumina
W(H)3/Al2O3–500 catalyzes both 2-methylpropene and
2-methylpropane metathesis with a common core mechanistic
scheme affording the main formation of 2,3-DMB= and
16 J. L. Bilhou, J. M. Basset, R. Mutin and W. F. Graydon, J. Am.
Chem. Soc., 1977, 99, 4083–4090.
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C. Coperet, CPE Lyon FCR, France, WO 2006013263, 2006.
18 S. Y. S. Wang, D. D. VanderLende, K. A. Abboud and
J. M. Boncella, Organometallics, 1998, 17, 2628–2635.
19 M. Stoecker, Microporous Mesoporous Mater., 2005, 82, 257–292.
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