σ-bond metathesis of alkanes on a silica-supported tantalum(v) alkyl alkylidene complex: First evidence for alkane cross-metathesis

Article abstract of DOI:10.1002/1521-3773(20010618)40:12<2331::AID-ANIE2331>3.0.CO;2-P

C-C activation of unactivated alkanes on a silica-supported neopentyl neopentylidene Ta^v complex [Eq. (1)] affords the alkane cross-metathesis products tBuCH₂R and an alkane-metathesis catalyst. Since the activity and product distributions are similar to those obtained with a silica-supported Ta^III hydride, these results are a first step in understanding alkane ?-bond metathesis on metal hydrides.

Full text of DOI:10.1002/1521-3773(20010618)40:12<2331::AID-ANIE2331>3.0.CO;2-P

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aqueous media (V. Bütün, M. Vamvakaki, S. P. Armes, N. C. Bill-
ingham, Polymer 2000, 41, 3173). However, in this case we were
unable to obtain any NMR evidence to support this hypothesis, which
remains unverified at present.
In summary, we report the second fully verified example of
a ªschizophrenicº diblock copolymer which can self-assemble
to form either micelles or reverse micelles in aqueous
solution. This new class of diblock copolymer surfactant is
particularly easy to synthesize and its two micellar phase
transitions are dictated solely by the solution temperature and
the solution pH value. In view of this, we expect that the rich
phase behavior of this new diblock copolymer will be of
particular interest to theoreticians for testing the validity of
the various theories of micellization.[10]
[5] J. S. Wang, K. Matyjaszewski, J. Am. Chem. Soc. 1995, 117, 5614.
[6] T. E. Patten, K. Matyjaszewski, Adv. Mater. 1998, 10, 901.
[7] a) M. Kato, M. Kamagaito, M. Sawamoto, H. Higashimura, Macro-
molecules 1995, 28, 1721; b) D. M. Haddleton, C. Waterson, P. J.
Derrick, C. B. Jasieczek, A. J. Shooter, Chem. Commun. 1997, 683;
c) M. Sawamoto, M. Kamagaito. Trends Polym. Sci. 1996, 4, 371.
[8] P. Alexandridis, T. A. Hatton. Colloids Surf. 1995, 96, 1.
[9] a) V. Bütün, N. C. Billingham, S. P. Armes. Chem. Commun. 1997, 671;
b) A. S. Lee, A. P. Gast, V. Bütün, S. P. Armes, Macromolecules 1999,
32, 4302; c) V. Bütün, S. P. Armes, N. C. Billingham, Z. Tuzar, A.
Rankin, J. Eastoe, R. K. Heenan, Macromolecules, in press.
[10] a) A. Halperin, M. Tirrell, T. P. Lodge, Adv. Polym. Sci. 1992, 100, 31;
b) M. Moffit, K. Khougaz, A. Eisenberg. Acc. Chem. Res. 1996, 29, 95;
c) A. P. Gast, Curr. Opin. Colloid Interface Sci. 1997, 2, 258; d) G. E.
Yu, Z. Yang, M. Ameri, D. Attwood, J. H. Collett, C. Price, C. Booth,
J. Phys. Chem. B 1997, 101, 4394.
Experimental Section
Synthesis of the PPO ± DEA diblock copolymer: The diblock copolymer
was synthesized by ATRP using a macro-initiator approach. A monohy-
droxy-capped poly(propylene oxide) (PPO-OH; mean degree of polymer-
ization, Dp ˆ 33 by 1H NMR; Mw/Mn ˆ 1.06 by gas-phase chromatography
(GPC) with THF eluent) was kindly donated by Laporte Performance
Chemicals (Hythe, UK). This PPO-OH was converted into an ATRP
macro-initiator, PPO-Br, by allowing the terminal hydroxy group to react
with 2-bromoisobutyryl bromide in the presence of triethylamine in
toluene. 1H NMR spectroscopy confirmed that the degree of chain-end
functionalization was 100%. The PPO macro-initiator (1.5 g, 0.75 mmol,
1 equiv), DEA (5.55 g, 30 mmol, 40 equiv), 1,1,4,7,10,10-hexamethyltri-
ethylenetetramine (HMTETA, 172 mg, 0.75 mmol, 1 equiv) and methanol
(7 mL) were added to the reaction flask; the solution was degassed by two
freeze ± thaw cycles. After the solution temperature was increased to 558C,
CuCl (74 mg, 0.75 mmol, 1 equiv) was introduced as a solid into the
reaction flask to start polymerization at this temperature. The reaction
solution became dark green and more viscous as polymerization proceed-
ed. After about 3 h the conversion was close to 100% as judged by 1H NMR
spectroscopy; the reaction mixture was diluted with methanol and passed
through a silica column to remove residual ATRP catalyst. After solvent
evaporation, the products were extracted with ice-cold water (08C, pH 9)
several times to remove any traces of unchanged PPO macro-initiator and
then dried under vacuum at room temperature. The mean Dp of the DEA
block was calculated to be 42 using 1H NMR spectroscopy. The GPC
studies (THF eluent, PMMA standards, refractive index detector) of the
diblock copolymer indicated a relatively narrow polydispersity (Mw/Mn) of
1.20.
s-Bond Metathesis of Alkanes on a
Silica-Supported Tantalum(v) Alkyl Alkylidene
Complex: First Evidence for Alkane
Cross-Metathesis**
Â
Christophe Coperet, Olivier Maury,
Jean Thivolle-Cazat,* and Jean-Marie Basset*
Here we report on direct evidence for the stoichiometric
cross-metathesis of several alkanes (ethane, propane, and
butanes) with the hydrocarbyl ligands of the surface com-
ꢀ
ˆ
plexes [( Si O)xTa( CHCMe3)(CH2CMe3)(3 x)] (1: x ˆ 1, 2:
x ˆ 2), and the catalytic activity of 1‡2 in alkane metathesis.[1]
We have shown that the silica-supported tantalum hydride
Dynamic light scattering (DLS) studies were conducted with a Brookhaven
model BI-200SM and 9000AT correlator using a solid-state laser (50 mW,
l ˆ 532 nm) at a fixed scattering angle (q) of 15o with both cumulants and
CONTIN software. The dn/dc of the PPO33 ± DEA42 diblock copolymer was
determined to be 0.128 in aqueous solution at pH 6.5 and 208C, using an
Optokem differential refractometer operating at l ˆ 632.8 nm. Static light
scattering studies (SLS) were conducted at 58C (DEA cores) as well as
408C and 708C (PPO cores) using the same instrument at scattering angles
ranging from 158 to 1358. The Mw and Rg data were obtained using standard
Zimm plot analyses, assuming that the effects of temperature and pH on
the dn/dc were negligible. Variable-temperature 1H NMR spectra were
recorded on 1.0 w/v% copolymer solutions in D2O using a Bruker Avance
DPX 300-MHz spectrometer. Transmission electron microscopy studies
were conducted using a Hitachi 7100 instrument operating at 75 kV and
employing OsO4 as a staining agent.
ꢀ
[( SiO)2TaH] (3) catalyzes the metathesis of alkanes
(Scheme 1).[2, 3] The key steps proposed for the reaction
mechanism are: a) activation of the C C bond of an incoming
alkane on the surface Ta alkyl complex 4 leading to the
evolution of an alkane and the formation of a new surface
complex 5; and b) regeneration of 4 in an alkyl-exchange
reaction (C H bond activation).[4]
The related structures of 1‡2 and 4/5, the two key
intermediates of the proposed mechanism for alkane meta-
thesis, is noteworthy. This analogy led us to investigate the
relative reactivity towards alkanes of 1‡2, a formal d0 10-e
metal center, and 3, (the precursor of 4/5) a formal d2 8-e
metal center.
Received: December 27, 2000
Revised: March 30, 2001 [Z16330]
Â
[*] Dr. J. Thivolle-Cazat, Dr. J.-M. Basset, Dr. C. Coperet, Dr. O. Maury
Â
Laboratoire de Chimie Organometallique de Surface
UMR 9986 CNRS ± ESCPE Lyon
43 bd du 11 Novembre 1918
69626 Villeurbanne Cedex (France)
Fax : (‡33)4-72-43-18-11
[1] V. Bütün, N. C. Billingham, S. P. Armes, J. Am. Chem. Soc. 1998, 120,
11818.
[2] O. W. Webster, W. R. Hertler, D. Y. Sogah, W. B. Farnham, T. V.
RanjanBabu, J. Am. Chem. Soc. 1983, 105, 5706.
[3] I. B. Dicker, G. M. Cohen, W. B. Farnham, W. R. Hertler, E. D.
Langanis, D. Y. Sogah, Macromolecules 1990, 23, 4034.
E-mail: basset@mulliken.cpe.fr
[**] We are grateful to Dr. L. Lefort (LCOMS) and Dr. B. Maunders (BP
Chemicals) for fruitful discussions. We also wish to thank the C.N.R.S.,
C.P.E. Lyon, and BP Chemicals for financial support.
[4] We recently reported another apparent example of
a diblock
copolymer which can form both micelles and reverse micelles in
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the C C bond by a Ta C bond (s-bond meta-
ˆ
thesis); b) a two-step process involving the Ta C
ˆ
bond, that is, addition of a C C bond to a Ta C
bond[7] followed by an a-H abstraction,[8] which is
overall an apparent s-bond metathesis process.
Both processes can explain the formation of the
observed products.
Scheme 1. Alkane metathesis by a s-bond metathesis mechanism.
The reaction of 1‡2 with ethane also gives
neopentane, the formation of which does not
After reaction of ethane with 1‡2 (see Experimental
Section), analysis of the gas phase showed the formation of
methane, propane, and butanes, that is, the usual metathesis
products of ethane. This demonstrates that TaV complexes can
also act as precursors to the active species in alkane meta-
thesis (Figure 1 and Table 1).[5] In addition, other products
that have so far not been observed in the metathesis of ethane
in the presence of 3 were also obtained, namely, neopentane
(NpH) and 2,2-dimethylbutane (NpMe), which are presum-
ably derived from the neopentyl groups of 1‡2 (Table 1,
entry 1). The corresponding reaction of 13C-monolabeled
ethane with 1‡2 gave monolabeled NpMe[6] and unlabeled
neopentane, and thus provided clear evidence for a cross-
metathesis process. Since 1‡2 contains both neopentyl and
neopentylidene ligands, two pathways can be considered
Table 1. Stoichiometric cross-metathesis of alkanes with silica-supported tantalum
complexes 1‡2. Selectivity for neopentyl-containing compounds and catalytic
activity in alkane metathesis.
Entry Alkane
NpH NpMe NpEt NpPr
NpiPr Total NpR TON[b]
in gas
phase[a] [%]
[d]
[d]
1
2
3
4
ethane
propane 70
butane 70
isobutane 71
76
24
26
15
29
< 1[c]
4
±
±
±
±
37
48
48
51
25 (46)
58 (60)
55 (55)
33 (40)
< 1[c]
[d]
[d]
11
3
±
[d]
[d]
±
< 1[c]
[a] Yield with respect to the amount of neopentyl and neopentylidene groups on the
surface before reaction. [b] TON is defined as the number of cycles. Data in
parentheses are for 3. [c] These compounds were detected in amounts of about
(Scheme 2): a) a one-step mechanism involving activation of 0.1% after concentration of the gas phase. [d] Not detected (<0.1% if any).
Figure 1. Percentage product distributions and turnover numbers (TONs) for the catalytic metathesis of various alkanes on 1‡2 (light gray) and 3 (dark
gray). Products of cross-metathesis in the case of 1‡2 have been excluded for the sake of clarity; see Table 1 for details.
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involve a C C bond cleavage, but rather the activation of a
C H bond (Scheme 2). Since neopentane is the major
product, C H activation is kinetically favored over C C
activation. A C H bond is in fact about three times easier to
In conclusion, the reaction of C C s bonds with a surface
Ta complex containing both neopentyl and neopentylidene
groups yields cross-metathesis products that were clearly
identified by the tBu ªmarkersº and an active catalyst for s-
bond metathesis of alkanes. The surface complex 1‡2 (TaV d0
system) gives turnovers and selectivities comparable to those
obtained with 3 (TaIII d2 system). This suggests that both
systems probably lead to the same active catalyst. Since s-
bond metathesis has been mainly found for d0 transition
metals, that is, metals in their highest oxidation state, it
suggests that the active species could be a TaV compound in
both cases.[10] However, we do not yet know which of the two
oxidation states, if not both, is responsible for this unusual
reactivity towards alkanes. Further studies are currently
underway to clarify this matter.
Experimental Section
The reactions were carried out with the same batch of catalyst in the strict
absence of oxygen and water. 1‡2 (50 mg, 0.013 mmol, 4.8% Ta) was
introduced into a Pyrex batch reactor (300 mL) followed by an alkane
(500 ± 600 equiv), dried and deoxygenated over freshly regenerated mo-
lecular sieves (3 Š) and deoxo. The reaction mixture was then heated at
1508C. Small-volume aliquots were removed, brought to atmospheric
pressure, and analyzed by gas chromatography (HP 5890 apparatus, Al2O3/
KCl on fused silica column, 50 m  0.32 mm) and GC-MS.
Scheme 2. Carbon ± carbon bond activation on 1‡2.
activate than a C C bond of ethane under these conditions.[9]
Since the neopentyl and neopentylidene ligands were trans-
formed by ethane into NpH and NpMe, the surface Ta
complex now bears methyl (or methylidene) and ethyl
(ethylidene) ligands;[5] it is therefore similar to 4/5 or 1‡2
and results in the subsequent catalytic metathesis of ethane
(Table 1).
The reaction of other alkanes with 1‡2 gave similar results,
that is, catalytic alkane metathesis and formation of NpH and
other neopentyl-containing products NpR (Table 1 and Fig-
ure 1). Propane gave NpMe and 2,2-dimethylpentane (NpEt,
entry 2). Butane afforded NpMe, NpEt, and 2,2-dimethylhex-
ane (NpPr), in contrast to isobutane, for which NpMe and
only traces of 2,2,4-trimethylpentane (NpiPr) were detected
(entries 3 and 4). This clearly shows that each alkane gives
specific products that depend on both its structure and the
structure of the surface Ta alkyl complex (in the stoichio-
metric case: neopentyl). Moreover, the cross-metathesis
leading to the transfer of one carbon atom from the incoming
alkane to the surface alkyl fragment (NpMe) is favored over
that of larger carbon fragments, as previously suggested by the
observed selectivities for higher hydrocarbons in the catalytic
reaction.[2] The absence of reverse C H activation products
such as NpR for the reaction of RH on 1‡2 is noteworthy.
Hence, an order of reactivity can be proposed (Figure 2).
Received: November 6, 2000
Revised: February 26, 2001 [Z16048]
[1] The surface complexes 1 and 2 are obtained in an 1/1 ratio by
sublimation of the tantalum tris(neopentyl)neopentylidene complex
onto a silica pellet. For their synthesis and characterization, see V.
Dufaud, G. P. Niccolaï, J. Thivolle-Cazat, J.-M. Basset, J. Am. Chem.
Soc. 1995, 117, 4288. This mixture of surface complexes is denoted
1‡2. For a full description of the preparation of 1‡2 by impregnation,
Â
see L. Lefort, M. Chabanas, O. Maury, D. Meunier, C. Coperet, J.
Thivolle-Cazat, J.-M. Basset, J. Organomet. Chem. 2000, 593 ± 594, 96.
Â
[2] a) V. Vidal, A. Theolier, J. Thivolle-Cazat, J.-M. Basset, Science 1997,
276, 99; b) O. Maury, L. Lefort, V. Vidal, J. Thivolle-Cazat, J.-M.
Basset, Angew. Chem. 1999, 111, 2121; Angew. Chem. Int. Ed. 1999, 38,
1952.
[3] a) For the synthesis and characterization of 3, see V. Vidal, A.
Â
Theolier, J. Thivolle-Cazat, J.-M. Basset, J. Corker, J. Am. Chem. Soc.
1996, 118, 4595; b) for the C H activation of alkanes with 3, see V.
Â
Vidal, A. Theolier, J. Thivolle-Cazat, J.-M. Basset, J. Chem. Soc.
Chem. Commun. 1995, 991; c) for its activity in the hydrogenolysis of
Â
alkanes, see M. Chabanas, V. Vidal, C. Coperet, J. Thivolle-Cazat, J.-
M. Basset, Angew. Chem. 2000, 112, 2038; Angew. Chem. Int. Ed. 2000,
39, 1962; d) for its activity in the H/D exchange reaction, see: L.
Â
Lefort, C. Coperet, M. Taoufik, J. Thivolle-Cazat, J.-M. Basset, Chem.
Commun. 2000, 663.
[4] For reviews on carbon ± carbon bond activation, see R. H. Crabtree,
Chem. Rev. 1985, 85, 245; B. Rybtchinski, D. Milstein, Angew. Chem.
1999, 111, 918; Angew. Chem. Int. Ed. 1999, 38, 871.
[5] Hydrolysis of the silica pellet after
reaction gave a mixture of methane,
ethane, and neopentane. No evolu-
tion of H2 was observed, in agreement
with a TaV complex as the major
surface species. After metathesis of
ethane on 1‡2 for 12 h at 1508C and
evacuation of the gas phase at room
temperature, acetone was added to
the catalyst, which yielded isobutene
as the major product. This is in agree-
Figure 2. Order of reactivity in the reaction of alkanes with surface Ta hydrocarbyl complexes. C1 and C2
represent different alkyl groups.
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ment with the presence of methylidene tantalum species. Determi-
nation of the exact structure of these surface complexes is underway.
[6] The reaction of 13C-monolabeled ethane on 1‡2 gave the partially
scrambled metathesis products of ethane along with unlabeled NpH
and partially labeled NpMe (35% 13C incorporated). This shows that
at least 70% of this compound arises from cross-metathesis. The
excess of unlabeled compound probably arises from rearrangement of
1‡2. Heating 1‡2 under Ar at 1508C for 15 h gives 0.25 equiv of NpH
and 0.01 equiv of NpMe.
R
R
R
R
R
R
1
2
(±)-3
5
6
ˆ
[7] While the addition of a C C bond to a M C bond has never been
observed before in organometallic chemistry, there are examples of
additions of C H bonds: L. R. Chamberlain, I. P. Rothwell, J. C.
Huffman, J. Am. Chem. Soc. 1986, 108, 1502; H. van der Heijden, B.
Hessen, Chem. Commun. 1995, 145.
4
Scheme 1. Diethynyl- and tetraethynyl[n]cumulenes (n ˆ 1 ± 3) as carbon-
rich modules for acetylenic scaffolding.
[8] a-H abstraction is commonly observed for tantalum alkyl complexes:
R. R. Schrock, J. D. Fellmann, J. Am. Chem. Soc. 1978, 100, 3359.
[9] This is an upper limit, since neopentane can also be formed by direct
display low-energy electronic absorption bands with remark-
ably high molar extinction coefficients,[5] and to photochromic
molecules for photochemical switching without competing
thermal isomerization pathways.[6] Expansion of the central
olefinic fragment in DEEs and TEEs leads to the di- and
tetraethynylated allenes (Æ)-3 and 4, and butatrienes 5 and 6
(Scheme 1). Whereas silylated derivatives of peralkynylated
butatriene 6 have been reported,[7] 1,3-diethynyl- and tetra-
ethynylallenes have remained elusive, despite intensive ef-
forts aimed at their preparation.[8] The major problems
encountered in previous attempts to synthesize these novel
building blocks for three-dimensional acetylenic scaffolding
were their high tendency for rearrangement and facile [2‡2]
cycloaddition, which occurred readily even at room temper-
ature or below.[9]
a-H abstraction of
a neighboring ligand without involving an
intermolecular C H bond. For the reference experiment under Ar,
see ref. [6].
[10] For examples of s-bond metathesis on electrophilic metal centers, see
P. L. Watson, J. Am. Chem. Soc. 1983, 105, 6491; C. M. Fendrick, T. J.
Marks, J. Am. Chem. Soc. 1984, 106, 2214; P. L. Watson, G. W.
Parshall, Acc. Chem. Res. 1985, 18, 51; C. M. Fendrick, T. J. Marks, J.
Am. Chem. Soc. 1986, 108, 425; M. E. Thompson, S. M. Baxter, A. R.
Bulls, B. J. Burger, M. C. Nolan, B. D. Santarsiero, W. P. Schaefer, J. E.
Bercaw, J. Am. Chem. Soc. 1987, 109, 203; I. P. Rothwell, Acc. Chem.
Res. 1988, 21, 153; R. H. Crabtree, Chem. Rev. 1995, 95, 987.
Herein we describe the synthesis of the first 1,3-diethynyl-
allenes (Æ)-7a ± d. As a result of the inherent 908 twist of the
1,3-Diethynylallenes: New Modules for Three-
Dimensional Acetylenic Scaffolding**
SiiPr3
R
Robert C. Livingston, Liam R. Cox, Volker Gramlich,
and François Diederich*
R = CH2OSitBuMe2
R = n-C6H13
R = p-CH2C4H6OMe
R = C6H5
(±)-7a:
(±)-7b:
(±)-7c:
(±)-7d:
R
Oxidative coupling[1] of multiply ethynylated building
blocks has facilitated the assembly of well-defined molecular
architecture that displays unusual electronic and optical
properties.[2] In particular, derivatives of (E)-1,2-diethynyl-
ethene (1, (E)-DEE, (E)-hex-3-ene-1,5-diyne) and tetraethy-
nylethene (2, TEE, 3,4-diethynylhex-3-ene-1,5-diyne) have
provided a unique class of p-conjugated precursors for the
modular construction of one- and two-dimensional carbon-
rich scaffolds (Scheme 1).[3] They have provided access to
monodisperse, linearly p-conjugated polytriacetylene (PTA)
oligomers that extend up to 18 nm in length,[4] to perethyny-
lated expanded radialenes that feature all-carbon cores (C30 to
C60), which undergo facile electrochemical reduction and
iPr3Si
allene moiety, alkyne deprotection and oxidative acetylenic
coupling of these compounds in enantiomerically pure form
promises access to a fascinating new class of three-dimen-
sional helical oligomers and polymers.[10] Optically active
helical polymers[11] are attracting increasing interest as
materials that exhibit circularly polarized electrolumines-
cence[12] or as dopants for cholesteric liquid-crystalline
phases.[13]
Palladium-catalyzed cross-coupling reactions have proven
valuable in the preparation of alkynylallenes from substrates
that bear propargylic leaving groups such as halides, epoxides,
acetates, and carbonates.[14] Given an appropriate regiochem-
ical bias, application of these conditions to bispropargylic
precursors should yield the corresponding 1,3-diethynylal-
lenes. Thus, target compound (Æ)-7a was obtained from
bispropargylic epoxide precursor (Æ)-8[15] (prepared in five
[*] Prof. Dr. F. Diederich, Dr. R. C. Livingston, Dr. L. R. Cox
Laboratorium für Organische Chemie, ETH-Zentrum
Universitätstrasse 16, 8092 Zürich (Switzerland)
Fax : (‡41)1-632-1109
E-mail: diederich@org.chem.ethz.ch
ꢀ
steps from but-2-yne-1,4-diol, Scheme 2) and iPr3SiC CH
Prof. Dr. V. Gramlich
Laboratorium für Kristallographie, ETH-Zentrum
Sonneggstrasse 5, 8092 Zürich (Switzerland)
under standard palladium-catalyzed cross-coupling conditions
([Pd(PPh3)4], CuI, HNiPr2, CH2Cl2). In situ protection of the
primary alcohol that results from epoxide ring opening as the
SitBuMe2 ether gave 1,3-diethynylallene (Æ)-7a (Table 1)
[**] We thank the ETH Research Council and the Fonds der Chemischen
Industrie for their support of this work.
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