Low-temperature hydrogenolysis of alkanes catalyzed by a silica- supported tantalum hydride complex, and evidence for a mechanistic switch from group IV to group V metal surface hydride complexes

Article abstract of DOI:10.1002/1521-3773(20000602)39:11<1962::AID-ANIE1962>3.0.CO;2-1

Carbon-carbon bond cleavage by a new mechanism, probably based on o-bond metathesis, is reported for a silica-supported tantalum hydride, (≡ SiO)₂TaH (1). This complex, which is also able to realize alkane metathesis, catalytically cleaves the carbon-carbon bond of alkanes under mild conditions.

Full text of DOI:10.1002/1521-3773(20000602)39:11<1962::AID-ANIE1962>3.0.CO;2-1

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[7] Traces of lithium can be present. See L. Lochmann, J. Pospisil, D. Lim,
Tetrahedron Lett. 1966, 256.
substituent intact. Conventional homometallic bases would
conversely be expected to attack the latter group to generate
resonance-stabilized benzyl carbanions.[11] Presumably oper-
ating through a special ring templating effect,[12] the active
base(s) involved in the formation of 2 and 3 is a product of the
heterodimetallic synergism taking place within the reaction
mixture. In this respect, a parallel can be drawn with the
chemistry of LiR ´ KOR' superbases,[13] the mixed-metal
reagents frequently utilized by synthetic organic chemists.
More than thirty years since the first such application of
superbases,[14] their mechanistic intricacies still remain to be
fully unravelled, although it is established that the participa-
tion of the two distinct metal atoms is essential to the
enhanced performance of superbases over that of analogous
homometallic bases. Logic dictates that the mixed-metal
templating ring involved in the formation of 2 and 3 must be
larger and have more breadth than its counterpart in II given
that only one end of the arene molecule experiences
deprotonation, implying that the ªother endº lies distant
from a metal center.
[8] P. E. Eaton, C.-H. Lee, Y. Xiong, J. Am. Chem. Soc. 1989, 111, 8016.
[9] Crystal structure data for 2: A colourless rod of dimensions 0.4 Â
Å
0.2  0.1 mm, trigonal, space group R3 (No. 148), a ˆ b ˆ 37.2805(9),
c ˆ 11.8346(9) Š, Vˆ 14244(1) Š3, Tˆ 123 K, Z ˆ 3, 1calcd
ˆ
1.022 MgmÀ3, 2qmax ˆ 468, MoKa (l ˆ 0.71073 Š). The structure was
solved, and refined on F2, using programs of the Shelx family to
convergence at R1 ˆ 0.0862 (for 3240 reflections with I > 2s(I)),
wR2 ˆ 0.2653, and S ˆ 1.062 for 287 parameters and 4384 unique
reflections. Max. residual electron density 0.685 eŠÀ3. Hydrogen
atoms were placed in calculated positions and the benzene solvent of
crystallization was treated as rigid C6H6 groups. Compound 3 is
isomorphous with 2 (a ˆ b ˆ 37.4442(6), c ˆ 11.9489(6) Š). Crystallo-
graphic data (excluding structure factors) for the structures reported
in this paper have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication no. CCDC-140112
and -140113. Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax:
(‡44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
[10] M. M. Olmstead, W. J. Grigsby, D. R. Chacon, T. Hascall, P. P. Power,
Inorg. Chim. Acta 1996, 251, 273.
[11] D. Hoffmann, W. Bauer, F. Hampel, N. J. R. von E. Hommes, P. van R.
Schleyer, P. Otto, U. Pieper, D. Stalke, D. S. Wright, R. Snaith, J. Am.
Chem. Soc. 1994, 116, 528.
[12] R. W. Saalfrank, I. Bernt, E. Uller, F. Hampel, Angew. Chem. 1997,
109, 2596; Angew. Chem. Int. Ed. 1997, 36, 2482.
[13] A. Mordini, Advances in Carbanion Chemistry, Vol. 1 (Ed.: V.
Snieckus), JAI Press, London, 1992, p. 1.
Experimental Section
2 or 3: Preparations were carried out in Schlenk tubes under a protective
atmosphere of dry, oxygen-free argon. Freshly prepared but unrefined
nBuK (15.1 or 12.3 mmol) was suspended in hexane (30 mL) and subjected
to ultrasound for 5 minutes until a fine brown suspension formed. To this
was added an equimolar amount of dibutylmagnesium (15.1 or 12.3 mmol,
in heptane) resulting in the formation of a congealed brown/cream mass.
Three molar equivalents of 2,2,6,6-tetramethylpiperidine (45.3 or
36.9 mmol) were then added. A slightly exothermic reaction ensued as
most of the solid dissolved in the solvent mixture. Fine particulates were
removed by filtration through Celite and the orange-brown filtrate was
concentrated in vacuo to half of its original volume. At this stage 15 mL of
the arene (2: benzene; 3: toluene) was introduced and the solution was
heated to 608C and surrounded by a Dewar water bath. The solution cooled
slowly over several hours to deposit rodlike orange/brown (2) or yellow (3)
crystals. Yields 2: 72.2%; 3: 80.5%. Satisfactory C,H,N elemental analyses
were obtained for both compounds. 1H NMR (300 MHz, [D8]THF, 258C,
TMS): 2: d ˆ 7.94 (m, 2H, o-C6H5), 7.31 (s, free C6H6), 6.87 (m, 2H, m-
C6H5), 6.72 (m, 1H, p-C6H5), 1.68(m, 2H, a-H), 1.25 (m, 4H, b-H), 1.06 (s,
12H, g-H; 3: d ˆ 7.83 ± 7.65 (m, 2H, bound C6H4CH3), 7.18± 7.10 (m, free
C6H5CH3), 6.75 ± 6.63 (m, 1H, bound C6H4CH3), 6.08(m, 1H, bound
C6H4CH3), 2.30 (s, free C6H5CH3), 2.16 (s, 3H, bound C6H4CH3), 1.65 (m,
2H, a-H), 1.25 (m, 4H, b-H) and 1.09 (s, 12H, g-H); 13C NMR (75 MHz,
[D8]THF, 258C, TMS): 2: d ˆ 181.57 (ipso-C6H5), 142.28( o-C6H5), 128.84
(free C6H6), 125.10 (m-C6H5), 122.61 (p-C6H5), 52.90 (tmp g-C), 42.47 (tmp
a-C), 36.13 (tmp b-C), 21.62 (tmp Me); 3: d ˆ 158.64 (bound C6H4CH3),
143.56 (bound C6H4CH3), 142.13 (bound C6H4CH3), 137.94 (free C6H5CH3),
129.48(free C 6H5CH3), 128.71 (free C6H5CH3), 127.61 (bound C6H4CH3),
125.70 (free C6H5CH3), 123.10 (bound C6H4CH3), 112.63 (bound C6H4CH3),
52.75 (tmp g-C), 42.44 (tmp a-C), 36.11 (tmp b-C), 24.64 (free C6H5CH3),
[14] M. Schlosser, J. Organomet. Chem. 1967, 8, 9.
Low-Temperature Hydrogenolysis of Alkanes
Catalyzed by a Silica-Supported Tantalum
Hydride Complex,and Evidence for a
Mechanistic Switch from Group IV to Group V
Metal Surface Hydride Complexes**
Â
Mathieu Chabanas, Veronique Vidal,
Â
Christophe Coperet, Jean Thivolle-Cazat,* and
Jean-Marie Basset*
We report herein a low-temperature hydrogenolysis of
alkanes on a silica-supported tantalum hydride and the
striking difference in reactivity compared to similar reactions
[*] Dr. J. Thivolle-Cazat, Dr. J.-M. Basset, M. Chabanas, V. Vidal,
Â
Dr. C. Coperet
21.14 (tmp Me), 19.98(bound C H4CH3).
6
Â
Laboratoire de Chimie Organometallique de Surface
UMR 9986 CNRS ± ESCPE Lyon
43 bd du 11 Novembre 1918
Received: February 7, 2000 [Z14665]
69626 Villeurbanne Cedex (France)
Fax : (‡33)4-7243-1793/95
[1] A. R. Kennedy, R. E. Mulvey, R. B. Rowlings, Angew. Chem. 1998,
110, 3321; Angew. Chem. Int. Ed. 1998, 37, 3180.
[2] D. R. Armstrong, A. R. Kennedy, R. E. Mulvey, R. B. Rowlings,
Angew. Chem. 1999, 111, 231; Angew. Chem. Int. Ed. 1999, 38, 131.
[3] A. R. Kennedy, R. E. Mulvey, C. L. Raston, B. A. Roberts, R. B.
Rowlings, Chem. Commun. 1999, 353.
[4] W. Clegg, K. W. Henderson, R. E. Mulvey, P. A. OꢁNeil, J. Chem. Soc.
Chem. Commun. 1994, 769.
[5] A. R. Kennedy, R. E. Mulvey, R. B. Rowlings, J. Am. Chem. Soc. 1998,
120, 7816.
E-mail: basset@cpe.fr
[**] We are grateful to Dr. L. Lefort, Dr. O. Maury (LCOMS), Dr. C.
Meric de Bellefon, and Dr. D. Schweich from the laboratory LGPC
(ESCPE Lyon) for fruitful discussions. We wish to thank the CNRS,
and the ESCPE-Lyon for financial support. M.C. is also grateful to the
French Ministry for Education, Research, and Technology (MENRT)
for a predoctoral fellowship.
Supporting information for this article is available on the WWW under
http://www.wiley-vch.de/home/angewandte/ or from the author.
[6] K. W. Klinkhammer, Chem. Eur. J. 1997, 3, 1418.
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Angew. Chem. Int. Ed. 2000, 39, No. 11

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of silica-supported Group IV hydrides, for example in the
hydrogenolysis of ethane.[1]
Our group has shown that the silica-supported Group IV
metal hydrides 1, ( SiO)3M H (M ˆ Ti,[1c], Zr,[1a] or Hf[1b]),
readily catalyze the hydrogenolysis of simple alkanes, such as
propane, butanes, and pentanes (but with the exception of
ethane), into mixtures of methane and ethane at low temper-
atures (25 to 1508C). It even transforms polyethylene into
lower molecular weight alkanes (all the way from diesels to
mixtures of methane and ethane).[2] These catalytic reactions
are readily explained by a mechanism which involves a b-alkyl
transfer[3] as the key step for carbon ± carbon bond cleavage.
Interestingly, this elementary step corresponds to the micro-
scopic reverse of insertion of an olefin into a metal ± carbon
bond, the carbon ± carbon bond formation step (propagation)
involved in Ziegler± Natta polymerization.[4]
ꢀ
À
Scheme 1. The metals are attached to the silica suface by siloxy groups
ꢀ
shown as SiO.
mixture of ethane and methane could be expected at low
conversion in the hydrogenolysis of propane, yet a 43:57
mixture is obtained at various temperatures, showing that
À
The surface complex 1 is usually obtained by selective
ethane resulting from the C C bond cleavage of propane is
partially cleaved before being desorbed (Table 1, entry 2).
ꢀ
hydrogenolysis of ( SiO)M(Np)3 with molecular H2 at 1508C
(Np ˆ neopentyl). During this hydrogenoly-
sis, there is formation of a 3:1 mixture of
methane and ethane, a product distribution
which is in agreement with the b-alkyl
transfer mechanism (Scheme 1).[1a]
In sharp contrast to the behavior of group
IV metal hydrides, the hydrogenolysis of the
tantalum surface complexes 3 and/or 4 leads
to the formation of methane as the only final
product, thus showing a dramatic difference
of reactivity (Scheme 1). This hydrogenoly-
sis also yields a tantalum hydride complex
ꢀ
À
( SiO)2Ta H, 2, which is a remarkable
catalyst for the metathesis of paraffins.[5, 6]
We have been puzzled by the striking differ-
ences of behavior between 1 and 2 for
the stoichiometric formation of the surface
hydrides. This led us to further investigate
the catalytic activity of 2 for the hydro-
genolysis of various alkanes, including
ethane.
In a glass batch reactor, a mixture of
hydrogen and acyclic alkanes (CnH2n‡2
;
n > 1) in the presence of 2 are typically
converted into their lower homologues, that
is, CnÀ1H2n to CH4, and only CH4 is detected
at full conversion (Figure 1). The product
distribution is typical of successive cleavage
of the carbon ± carbon bonds, for example,
the successive formation and disappearance
of propane and ethane in the hydrogenolysis
of 2-methylpropane. The product selectivity
is constant at conversions of 50 ± 80% (Ta-
ble 1), depending on the starting material;
these constant selectivities are consistent
with the observation of primary products,
and can therefore provide potential mecha-
nistic information regarding the carbon ±
carbon bond cleavage step. Moreover, the
observed selectivities at low conversions are
quite informative. For example, a 50:50
Figure 1. Hydrogenolysis of alkanes over a silica-supported tantalum hydride surface complex 2.
The y axis shows the equivalents of products formed relative to one equivalent of 2. The x axis
shows time in hours. a) Ethane hydrogenolysis; b) propane hydrogenolysis; c) butane hydro-
genolysis; d) 2-methylpropane hydrogenolysis; e) neopentane hydrogenolysis; f) 2,2-dimethylbu-
~
&
tane hydrogenolysis. ˆ Methane, ˆ ethane, ‡ ˆ propane,  ˆ butane, ˆ 2-methylpropane,
*
*
ˆ neopentane, - ˆ 2,2-dimethylbutane.
Angew. Chem. Int. Ed. 2000, 39, No. 11
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[a]
ꢀ
À
Table 1. Initial rate constants (k) and selectivities at low conversions for the hydrogenolysis of alkanes over ( SiO)2Ta H at 1608C.
Entry
alkane
k [hÀ1
]
pentanes
butanes
propane
ethane
methane
1
2
3
4
5
6
7
ethane
1.0
±
±
±
±
±
±
±
±
±
100
propane
2-methylpropane
butane
Np-H
n-C5H12
0.36
0.74
0.65
0.08±
0.33
0.07
43 (50)
9.2 (0)
36.3 (33)
57 (50)
53.9 (50)
42.4 (33)
1.6[b]
35.2 (50)
19.5 (33)
±
20.0 (25)
11.7 (0)
1.8[b]
[c]
[c]
[c]
[c]
±
±
±
2.2[d]
8.5[f,g] (10)
7.1[e] (25)
7.5[f,h] (10)
37.5 (25)
29.8(10)
33.0 (25)
40.8(40)
Np-Me
[a] The numbers in parenthesis correspond to the expected selectivities if the first carbon ± carbon bond cleavage were statistical. [b] This corresponds to the
selectivity in the other isomer. [c] Selectivies are not constant even at low conversion since neopentane reacts slower than the products formed. [d] Selectivity
in 2-methylbutane. [e] Selectivity in butane; the selectivity in 2-methylpropane is 0.2%. [f] The selectivities decrease slowly with conversion. [g] Selectivity in
neopentane; the selectivities in 2-methylbutane and pentane are 1.0%(30), and 0%(0), respectively. [h] Selectivity in 2-methylpentane only, the selectivity in
butane is 0.7%(0).
The same observation applies to higher alkanes. In fact, it is
even more dramatic in the case of 2-methylpropane in which
the selectivity in propane is only 35% compared with an
expected 50% (Table 1, entry 3). This shows that the process
does not involve purely successive reactions and, also, that
there is a greater probability for a higher alkane to react
further rather than desorb (Scheme 2, physisorption and/or
chemisorption).
reasons and also suggests that the actual carbon±carbon bond
cleavage step might also be different from
transfer.
a b-alkyl
In the light of the specific abilities of tantalum hydride to
also catalyze alkane metathesis,[6] which transforms an alkane
into its higher and lower homologues, we suggest that the
mechanism for carbon ± carbon bond cleavage could be
closely related to that of alkane metathesis. The key step of
the proposed mechanism involves the formation and cleavage
of a carbon ± carbon bond by interaction of an alkane
molecule with a tantalum ± carbon bond by s-bond metathesis
(Scheme 3). In the case of the hydrogenolysis of ethane (R ˆ
À
H), the interaction of the Ta H bond directly with a carbon ±
carbon bond, via a four-centered transition state, leads to
À
methane and a Ta Me surface species; the latter, which can
Scheme 2.
readily give methane through a related s-bond metathesis
mechanism under H2, is regenerated into the catalyst, 2.[8] The
same mechanism can be also extended to higher alkanes in the
case of tantalum (see above, for example, neopentane and 2,2-
dimethylbutane). This is dramatically different from group IV
metals in which b-alkyl transfer is the only carbon ± carbon
bond cleavage process.
Three molecules deserve a special interest. Firstly, ethane is
catalytically cleaved by 2 into methane (Table 1, entry 1),
which obviously cannot be explained by a b-alkyl transfer
mechanism since the corresponding ethyl ± tantalum surface
ꢀ
À
complex, ( SiO)2Ta CH2CH3, does not have an alkyl group
in the b-position. This shows that a one-carbon transfer
process is possible with this catalyst, in contrast to what has
been observed for Group IV metals (see above). However, it
is difficult to discriminate between one- or two-atom transfer
processes (like b-alkyl transfer) for simple linear or branched
alkanes such as butane or 2-methylpropane. This led us to
investigate the hydrogenolysis of 2,2-dimethylbu-
tane (Np-Me, Table 1, entry 7). Under identical
conditions, this molecule gave neopentane as the
major product of higher alkanes (>C3). In this case,
neopentane cannot be produced by a b-alkyl trans-
fer, which clearly shows that another process for
carbon ± carbon bond cleavage takes place even for
higher alkanes. Finally, in the case of neopentane
(Np-H, Table 1, entry 5), there is no build-up of
intermediary products (like 2-methylpropane or
propane), thus showing its slower reactivity relative
to the lighter alkanes formed during its hydro-
genolysis. This is in contrast to what has been
observed for other alkanes including pentane
In conclusion, the tantalum hydride 2 catalyzes the low-
temperature hydrogenolysis of alkanes, including that of
ethane, in sharp contrast with Group IV metal hydrides. This
surface complex also catalyzes alkane metathesis, while
surface complexes like 1 do not. We are currently further
investigating these two reactions.
(see above) and also for its hydrogenolysis in the
presence of 1.[1a, 7] This is probably due to steric
Scheme 3. The tantalum is attached to the silica suface by siloxy groups shown as
ꢀ
SiO.
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Experimental Section
Unprecedented Formation of Five-,Six-,and
Seven-Membered Metallacycles by Single and
Double Insertion of Mono- and Disubstituted
The reactions were carried out with the same batch of catalyst in the
ꢀ
À
absence of oxygen and water under the following conditions: ( SiO)2Ta H
(prepared by impregnation, 18.8 Â 10À6 mol of Ta, 1 equiv), alkane
(40 Torr, 43 equiv), and H2 (630 Torr, 710 equiv) were heated with an oil
bath to the desired temperature (Æ18C) in a glass batch reactor (376 mL).
During the reaction, aliquots were expended, brought to atmospheric
pressure, and analyzed by gas chromatography (HP 5890 apparatus, Al2O3/
KCl on fused silica column, 50m  0.32mm). Alkanes and H2 were dried
over freshly regenerated molecular sieves (3Š) and deoxo traps before
addition.
À
Alkynes into an Rh O Bond**
Yasuhiro Yamamoto,* Xiao-Hong Han, and
Jian-Fang Ma
Metal alkynyl complexes are currently of great interest[1, 2]
since they are valuable for constructing vinylidene or carbene
complexes for use in organic synthesis.[3±8] Reactions of
organotransition metal halides with 1-alkynes in the presence
Received: December 20, 1999 [Z14423]
À
À
of anions such as PF6 , BF4 , and CF3SO3À are representative
methods for the preparation of vinylidene complexes.[9] One
ortho-methoxy group in (2,6-dimethoxyphenyl)diphenylphos-
phane (mdmpp) was demethylated in the reaction with the
isoelectronic complexes [(h6-arene)RuCl2]2 (arene ˆ C6Me6,
p-cymene, C6H3Me3) and [Cp*MCl2]2 (M ˆ Rh, Ir) to give
metal complexes with a (P,O)-chelating phosphane: [(h6-
arene)RuCl(mdmpp-kP,kO)][10, 11] and [Cp*MCl(mdmpp-
kP,kO)][12, 13] (Cp* ˆ C5Me5). We recently reported the un-
precedented insertion of tetracyanoethylene (tcne) and
[1] For the low-temperature hydrogenolysis of alkanes on group IV silica
ꢀ
À
supported complexes, see: a) ( SiO)3Zr H: C. Lecuyer, F. Quignard,
A. Choplin, D. Olivier, J.-M. Basset, Angew. Chem. 1991, 103, 1692;
Angew. Chem. Int. Ed. Engl. 1991, 30, 1660; J. Corker, F. Lefebvre, C.
Á
Â
Lecuyer, V. Dufaud, F. Quignard, A. Choplin, J. Evans, J.-M. Basset,
ꢀ
À
Science 1996, 271, 966; b) ( SiO)3Hf H: L. DꢁOrnelas, S. Reyes, F.
Quignard, A. Choplin, J.-M. Basset, Chem. Lett. 1993, 1931;
ꢀ
À
c) ( SiO)3Ti H: C. Rozier, G. P. Niccolaï, J.-M. Basset, J. Am. Chem.
Soc. 1997, 110, 848.
[2] V. Dufaud, J.-M. Basset, Angew. Chem. 1998, 110, 848; Angew. Chem.
Int. Ed. 1998, 37, 806; V. Dufaud, C. Marangelli, J.-M. Basset,
unpublished results.
[3] For some examples of b-alkyl transfer, see: P. Watson, C. Roe, J. Am.
Chem. Soc. 1982, 104, 6471; P. L. Watson, G. W. Parshall, Acc. Chem.
Res. 1985, 18, 51; E. Bunel, B. Burger, J. Bercaw, J. Am. Chem. Soc.
1988, 110, 976; L. Schock, T. Marks, J. Am. Chem. Soc. 1990, 110, 7701;
B. Burger, M. Thompson, D. Cotter, J. Bercaw, J. Am. Chem. Soc. 1990,
112, 1566; K. McNeill, R. A. Andersen, R. Bergman, J. Am. Chem. Soc.
1995, 117, 3625.
[4] For leading references on the insertion of an olefin in metal ± alkyl
bond, see: P. Cossee, J. Catal. 1964, 3, 80; H. Sinn, W. Kaminsky, Adv.
Organomet. Chem. 1980, 18, 99; H. Sinn, W. Kaminsky, H.-J. Vollmer,
R. Woldt, Angew. Chem. 1980, 92, 396; Angew. Chem. Int. Ed. Engl.
1980, 19, 390; R. F. Jordan, Adv. Organomet. Chem. 1991, 32, 325; T. J.
Marks, Acc. Chem. Res. 1992, 25, 57.
À
tetracyanoquinodimethane into the C H bond adjacent to
À
the M O bonds of the above rhodium(iii) and iridium(iii)
complexes to produce (P,O)-chelated [Cp*MCl[PPh2{2-O-6-
MeO-3-(CH(CN)2C(CN)2)C6H2}] in the case of tcne.[14]
ꢀ
Treatment of the ruthenium(iii) complex with PhC CH in
the presence of NaPF6 in acetone/CH2Cl2 afforded the vinylide-
6
ˆ ˆ
ne complex [(h -arene)Ru(mdmpp-kP,kO)( C CHPh)]-
PF6.[15] However, treatment of [Cp*RhCl(mdmpp-kP,kO)]
(1)[12] with 1-alkynes such as HC CCOOMe, PhC CH, and
nBuC CH, and the disubstituted alkyne C2(CO2Et)2 in the
presence of NaPF6 or KPF6 resulted in unusual reactions
ꢀ
ꢀ
ꢀ
ꢀ
(Scheme 1). In the reaction with HC CCOOMe, extraction of
CO from the ester group of one and insertion of another
ꢀ
À
[5] For the synthesis and characterization of ( SiO)2Ta H, see: V. Vidal,
À
1-alkyne molecule into the Rh O s bond occurred to produce
Â
A. Theolier, J. Thivolle-Cazat, J.-M. Basset, J. Corker, J. Am. Chem.
ꢀ
Soc. 1996, 118, 4595.
a seven-membered metallacycle, and reactions with HC CR
Â
[6] V. Vidal, A. Theolier, J. Thivolle-Cazat, J.-M. Basset, Science 1997, 276,
led to the formation of complexes with five- and six-membered
99; O. Maury, L. Lefort, V. Vidal, J. Thivolle-Cazat, J.-M. Basset,
Angew. Chem. 1999, 111, 2121; Angew. Chem. Int. Ed. 1999, 38, 1952.
[7] F. Quignard, C. Lecuyer, A. Choplin, D. Olivier, J.-M. Basset, J. Mol.
Cat. 1992, 74, 353.
À
rings by double insertion of 1-alkynes into the Rh O bond.
The disubstituted alkyne also underwent single insertion into
À
the Rh O bond. Similar insertion of unsaturated molecules
2
À
[8] Nevertheless, since Ta H is a d surface complex, it is not possible to
into the metal ± oxygen bonds of metal alkoxides was ach-
exclude processes that involve an increase in oxidation state, such as a
direct oxidative addition of ethane or an a-alkyl transfer from the
tantalum ± ethyl fragment; the reverse step has been documented, see:
P. R. Sharp, R. R. Schrock, J. Organomet. Chem. 1979, 171, 43; J. C.
Hayes, G. D. N. Pearson, M. J. Cooper, J. Am. Chem. Soc. 1981, 103,
4648.
ieved with cyclooctadiene[16] and perfluoroolefins[17] such as
ˆ
F2C CF2 and hexafluorocyclobutene. The insertion of alkynes
into the transition metal to oxygen s bonds was achieved here
for the first time.[18] Interestingly, these unprecedented reactions
À
allow single and double insertion of alkynes into the Rh O
bond of 1 to be controlled by means of the alkyne substituents.
[*] Prof. Y. Yamamoto, X.-H. Han, J.-F. Ma[‡]
Department of Chemistry, Faculty of Science
Toho University, Miyama
Funabashi, Chiba 274-8510 (Japan)
Fax : (‡81)474-75-1855
E-mail: yamamoto@chem.sci.toho-u.ac.jp
‡
[ ] On leave from Changchun Institute of Applied Chemistry, Chinese
Academy of Science
[**] We thank Prof. Shigetoshi Takahashi and Dr. Fumie Takei at The
Institute of Scientific and Industrial Research, Osaka University for
the measurement of FAB mass spectra.
Angew. Chem. Int. Ed. 2000, 39, No. 11
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
0570-0833/00/3911-1965 $ 17.50+.50/0
1965