Primary products and mechanistic considerations in alkane metathesis

Article abstract of DOI:10.1021/ja051679f

Alkane metathesis, a reaction catalyzed by the silica-supported tantalum hydride [(≡SiO)₂Ta-H], 1, which transforms acyclic alkanes into their higher and lower homologues, was reported in 1997. New studies conducted in a continuous flow reactor in the case of propane indicate that, by varying the contact time, hydrogen and olefins are primary products. This crucial observation, as well as the known properties of tantalum alkyls to perform α-H or β-H eliminations, supports the proposition of a new mechanism involving metallacyclobutane intermediates just like in olefin metathesis. The observed selectivities for linear and branched C_n+1 and C_n+2 products as well as the linear/branched ratio can be well-explained on the basis of the minimization of steric interactions between 1,2- or 1,3-substituents in the various tantallacyclobutane intermediates or during their formation. Hydrogen plays a specific role in the cleavage of metal alkyls to complete the catalytic cycle. Copyright

Full text of DOI:10.1021/ja051679f

Published on Web 05/25/2005
Primary Products and Mechanistic Considerations in Alkane Metathesis
Jean Marie Basset,*,† Christophe Cope´ret,† Laurent Lefort,† Barry M. Maunders,‡ Olivier Maury,†
Erwan Le Roux,† Guillaume Saggio,† Sophie Soignier,† Daravong Soulivong,† Glenn J. Sunley,§
Mostafa Taoufik,† and Jean Thivolle-Cazat*,†
Laboratoire de Chimie Organome´tallique de Surface (UMR 9986 CNRS-ESCPE Lyon), ESCPE Lyon,
43 Bd du 11 NoVembre 1918, 69616 Villeurbanne, Ce´dex, France, BP Chemicals Ltd., Chertsey Road,
Sunbury-On-Thames, Middlesex, TW167LN, U.K., and BP Chemicals Ltd., Hull Research and Technology Center,
Saltend, Hull HU128DS, U.K.
Received March 16, 2005; E-mail: basset@cpe.fr; thivolle@cpe.fr
We reported in 1997 the discovery of a new catalytic reaction:
the metathesis of alkanes.1,2 Acyclic alkanes, except methane, when
contacted with the silica-supported tantalum hydride [(tSiO)2Ta-H],
1, are transformed at moderate temperatures into their lower and
higher homologues. For example, ethane is transformed into
methane and propane along with traces of butanes. Interestingly, 1
is also able to achieve the reverse reaction, that is, the cross-
metathesis of methane and propane, giving two ethanes and thereby
incorporating methane into higher alkanes.3 It is also possible to
perform the cross-metathesis of ethane and toluene to give xylenes
and ethylbenzene.4 When the reaction was discovered, two mecha-
nistic hypotheses were formulated on the basis of the d2 configu-
ration and the highly electrophilic character of (tSiO)2TaIII-H,
formally an eight-electron species.5 The first one was based on
successive oxidative addition and reductive elimination steps
(Scheme 1a). To our knowledge, such oxidative addition of a C-C
bond on early transition metals is not known at the moment.6,7 The
second one involved exchange of alkyl fragment through a four-
centered transition state in which there would be a concomitant
cleavage and formation of C-C and M-C bonds, a σ-bond
metathesis type mechanism (Scheme 1b). Although many examples
of σ-bond metathesis have been observed between the C-H bond
of alkanes or aromatics and various d0 early transition metal
hydrides, alkyls or aryls, in each case, the four-centered transition
state always bears a hydrogen atom in the middle position and never
a sp3 carbon (Scheme 1c).8-13 To summarize, these mechanisms
were not especially satisfactory without experimental evidence.
We wish to report a kinetic study performed in a continuous
flow reactor, which shows unambiguously that olefins and H2 are
primary products and which is consistent with a mechanism based
on elementary steps known in tantalum organometallic chemistry
and related to that of olefin metathesis.
When propane metathesis is carried out in a continuous flow
reactor, conversion goes through a maximum during time on stream,
and both selectivity and turnover number (TON) after 100 h reach
values comparable to those obtained under batch conditions (Figure
1a,b, Table S1, and Figure S1).1,2,14,15 The higher selectivity in
methane at short time on stream is due to hydrogenolysis of the
alkane since 1 generates H2 upon activation and catalyzes the
hydrogenolysis of alkanes.16
After 100 h, the observed selectivities in the metathesis of
propane (Cn) are: 30.5 (butane), 3.7 (isobutane), 7.3 (pentane), 1.3
(isopentane), and 2.0 (hexanes), and they can be ordered as
follows: Cn+1 > Cn+2 . Cn+3. Moreover, the linear products are
always favored over the branched ones. These observations are quite
Figure 1. (a) Conversion and (b) selectivities obtained during the metathesis
of propane catalyzed by [(tSiO)2Ta-H], 1, (5.33% wt Ta) in a continuous
flow reactor (150 °C, 1 atm, 1 mL/min, VHSV ) 38 h-1).
Scheme 1
general if we look at the selectivities obtained during ethane,
propane, butane, or isobutane metatheses (Tables S1 and S2).1
The effect of inverse space velocity (proportional to contact time)
on selectivity was studied using flow rates varying between 1 and
100 mL/min (or volume hourly space velocity (VHSV) of 38 to
3807 h-1). During this study, the conversion increased linearly with
the inverse space velocity, showing that the data determine reaction
rates (Figure S2). The extrapolated selectivities at zero inverse space
velocity allow a reliable determination of real primary products.
Upon decreasing inverse space velocity, a strong increase in the
proportion of H2 and alkenes (from C2 to C5) and a parallel decrease
in the production of alkanes are observed (Figure 2a-c). Interest-
ingly the alkanes/alkenes ratio decreases when the inverse space
velocity decreases and clearly reaches 0 at extrapolated zero inverse
space velocity (Figure 2d). These data imply that olefins and H2
are primary products.
These data and the known elementary steps of organometallic
chemistry lead us to propose the following mechanism. The first
step is the C-H bond activation of the alkane, as observed when
1 reacts with cycloheptane or methane to form the corresponding
tantalum-alkyl complexes 2-R and H2.5 In the case of propane,
this step probably leads to linear and branched propyl fragments,
2-Pr and 2-iPr, with liberation of H2 as a primary product (Scheme
2a). These Ta-propyls can then undergo R-H transfer to give
Ta(H)(dC(CH3)2) and Ta(H)(dCH-CH2-CH3),17,18 3, or â-H
transfer leading to Ta(H)(η2-CH2dCH-CH3).18,19 Propene resulting
† Laboratoire de Chimie Organome´tallique de Surface.
‡ BP Chemicals Ltd.
§ BP Chemicals Ltd., Hull Research and Technology Center.
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10.1021/ja051679f CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
the incoming alkane reactant via σ-bond metathesis, but it is usually
much less favored. The observed product selectivities can be
understood on the basis of the stability of the key metallacyclobu-
tane or of the facility of their formation (4a > 4b; 4c > 4d; 4c >
4a; 4d > 4b), both of which depend on the interaction between
substituents in [1,2]- or [1,3]-positions as it has been proposed in
olefin metathesis.25
In conclusion, kinetic studies on propane metathesis, in a
continuous flow reactor, have allowed primary products to be
detected, namely olefins and H2. A mechanism is proposed on the
basis of known elementary steps of organometallic chemistry, which
is in full agreement with all the observed selectivities. We are
currently developing better catalysts on the basis of these insights.26
Acknowledgment. This work was supported by CNRS, CPE-
Lyon, and BP chemicals.
Supporting Information Available: Experimental details, Tables
S1 and S2, and Figures S1-S3. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 2. Metathesis of propane in a continuous flow reactor (150 °C, 1
atm, 520 mg of (tSiO)2TaH, 5.33% wt Ta). Selectivity vs inverse space
velocity expressed in [(min)(volume of catalyst)/(volume of propane)] for
(a) alkanes, (b) alkenes, and (c) H2. (d) Alkanes/alkenes ratio vs inverse
space velocity.
References
(1) Vidal, V.; Theolier, A.; Thivolle-Cazat, J.; Basset, J.-M. Science 1997,
276, 99.
(2) Maury, O.; Lefort, L.; Vidal, V.; Thivolle-Cazat, J.; Basset, J.-M. Angew.
Chem., Int. Ed. 1999, 38, 1952.
Scheme 2
(3) Soulivong, D.; Cope´ret, C.; Thivolle-Cazat, J.; Basset, J.-M.; Maunders,
B. M.; Pardy, R. B. A.; Sunley, G. J. Angew. Chem., Int. Ed. 2004, 43,
5366.
(4) Taoufik, M.; Schwab, E.; Schultz, M.; Vanoppen, D.; Walter, M.; Thivolle-
Cazat, J.; Basset, J.-M. Chem. Commun. 2004, 1434.
(5) Vidal, V.; Theolier, A.; Thivolle-Cazat, J.; Basset, J.-M.; Corker, J. J.
Am. Chem. Soc. 1996, 118, 4595.
(6) Rybtchinski, B.; Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 871.
(7) Crabtree, R. H. J. Chem. Soc., Dalton Trans. 2001, 2437.
(8) Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6491.
(9) Watson, P. L. J. Chem. Soc., Chem. Commun. 1983, 276.
(10) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M.
C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc.
1987, 109, 203.
(11) Deelman, B.-J.; Teuben, J. H.; MacGregor, S. A.; Eisenstein, O. New. J.
Chem. 1995, 19, 691.
(12) Maron, L.; Perrin, L.; Eisenstein, O. J. Chem. Soc., Dalton Trans. 2002,
534.
(13) Cope´ret, C.; Grouiller, A.; Basset, M.; Chermette, H. ChemPhysChem
2003, 4, 608.
(14) Cope´ret, C.; Maury, O.; Thivolle-Cazat, J.; Basset, J.-M. Angew. Chem.,
Int. Ed. 2001, 40, 2331.
(15) Le Roux, E. L.; Chabanas, M.; Baudouin, A.; de Mallmann, A.; Cope´ret,
C.; Quadrelli, E. A.; Thivolle-Cazat, J.; Basset, J.-M.; Lukens, W.; Lesage,
A.; Emsley, L.; Sunley, G. J. J. Am. Chem. Soc. 2004, 126, 13391.
(16) Chabanas, M.; Vidal, V.; Cope´ret, C.; Thivolle-Cazat, J.; Basset, J.-M.
Angew. Chem., Int. Ed. 2000, 39, 1962.
(17) Van Asselt, A.; Burger, B. J.; Gibson, V. C.; Bercaw, J. E. J. Am. Chem.
Soc. 1986, 108, 5347.
(18) Parkin, G.; Bunel, E.; Burger, B. J.; Trimmer, M. S.; Van Asselt, A.;
Bercaw, J. E. J. Mol. Catal. 1987, 41, 21.
(19) Sharp, P. R.; Schrock, R. R. J. Organomet. Chem. 1979, 171, 43.
(20) Herisson, J. L.; Chauvin, Y. Makromol. Chem. 1971, 141, 161.
(21) McLain, S. J.; Sancho, J.; Schrock, R. R. J. Am. Chem. Soc. 1979, 101,
5451.
(22) Turner, H. W.; Schrock, R. R. J. Am. Chem. Soc. 1982, 104, 2331.
(23) Wallace, K. C.; Dewan, J. C.; Schrock, R. R. Organometallics 1986, 5,
2162.
(24) Wallace, K. C.; Liu, A. H.; Dewan, J. C.; Schrock, R. R. J. Am. Chem.
Soc. 1988, 110, 4964.
from â-H transfer decoordinates (primary product) and can react
with either of the hydrido-tantallacarbenes, 3, giving four possible
tantallacyclobutanes, 4, having methyl or ethyl substituents in the
[1,2]- or [1,3]-positions.14,15,20-24 These four tantallacycles can
undergo metathetical cleavage, giving back propene and carbenes
or new olefins and hydrido-tantallacarbenes (Scheme 2b). The
catalytic cycle ends with the insertion of the olefins into the tantalum
hydride or the migration of the hydride onto the new carbene
ligands. The resulting alkyl groups can be cleaved by H2 (a primary
product), a known step already observed during the hydrogenolysis
of alkanes with 1.16 The alkyl groups could also be displaced by
(25) Bilhou, J. L.; Basset, J. M.; Mutin, R.; Graydon, W. F. J. Am. Chem.
Soc. 1977, 99, 4083.
(26) Cope´ret, C.; Chabanas, M.; Petroff Saint-Arroman, R.; Basset, J.-M.
Angew. Chem., Int. Ed. 2003, 42, 156.
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