Note
Organometallics, Vol. 29, No. 23, 2010 6613
Scheme 1
Table 1. Theoretically Expected Isotopomeric Distribution of
Methane and Propane According to the Isotopomeric Composi-
tions of the TadC2 Carbene Species and the C2 Alkene
the expected statistical distribution (12.5:37.5:37.5:12.5 or
1:3:3:1) (Figure 1).
ꢀ
4 CH3- CH3 f CH3- CH3
starting
TadC2 carbene
ending
ꢀ
ꢀ
ꢀ
þ 2 CH3- CH3 þ CH3- CH3
ð2Þ
ethylene
methane
propane
Therefore, it is worth emphasizing that if 13C-monolabeled
ethane is progressively scrambled into unlabeled ethane and
13C-dilabeled ethane (Figure S1), methane and propane are
liberated totally scrambled in the gas phase and their iso-
topomeric distributions remain constant during the entire
reaction time. If the previously reported alkane metathesis
mechanism8 is applied directly to the initially pure 13C-
monolabeled ethane, a 1:1 distribution of CH4 and *CH4
would be effectively obtained but for propane only a mixture
of mono- and dilabeled compounds would be generated: that
is, a 0:1:1:0 distribution instead of the observed 1:3:3:1
(Scheme 1 and Scheme S2 (Supporting Information)).
This mechanistic scheme indicates in fact that the propane
formation involves an interaction between the TadC2 car-
bene species 2a,b (tantalum ethylidene) and the C2 alkene
ethylene, leading to a tantalacyclobutane species,10-13 as it is
known in alkene metathesis;14 the tantalum ethylidene spe-
cies 2a,b ensue from the C-H bond activation of ethane on
the tantalum hydride 1, followed by R-H abstraction,
whereas a β-H elimination affords the liberation of ethylene
(Scheme S2).8 The combinatorial examination of all the
possible interactions between the TadC2 carbene species
and the C2 alkene shows that the observed 1:3:3:1 propane
distribution can be reached only if the TadC2 carbene
species has undergone a total scrambling, that is, if it con-
tains a 1:2:1 distribution of nonlabeled and mono- and
dilabeled C2 fragments, whatever the isotopomeric distribu-
tion of ethylene (Table 1).
0:1:0
0:1:0
1:2:1
1:2:1
0:1:0
1:2:1
0:1:0
1;2:1
1:1
1:1
1:1
1:1
0;1:1:0
0:1:1:0
1;3:3:1
1:3:3:1
of propane would be produced (Scheme 1 and Scheme S2).
Afterward, the tantalum methylidene hydrides 3a,b could
also interact with 13C-monolabeled ethylene to form the new
tantalacyclobutane species 4a,b, from which nonlabeled and
dilabeled ethane can be formed through various known
elementary steps depicted in Scheme 2b; note that in addition
to the formation of the productive species 4a,b, the unmen-
tioned unproductive species 4a0,b0 can be considered (Scheme
S3 in the Supporting Information).
In particular, the tantalacyclobutanes 4a,b can be formed
from or evolve to the tantalum methylidene (ethylene)
hydrides 5a-d. These latter species can undergo ethylene
insertion into the Ta-H bond to form the four tantalum
methylidene ethyl species 6a-d variously labeled on carbons.
From complexes 6a,c, non- and dilabeled ethane can be
liberated via σ-bond metathesis by the 13C-monolabeled
ethane reagent,15,16 leading to the progressive scrambling
of the gas phase. It is worth noting that the population of
species 6a-d responds to the 1:2:1 distribution of a total
scrambling of C2-alkyl fragments or the stoichiometry of
eq 2. Otherwise, the same complexes 6a,c can enter into a pro-
ductive but slower metathesis cycle as shown in Scheme 2a,c
(clockwise mode), ensuring the formation of nonlabeled and
13C-trilabeled propane. This productive cycle involves in
particular the C-H bond activation of the 13C-monolabeled
ethane reagent on the tantalum-methylidene double bond,
as previously reported by Rothwell,15,16 to form the tan-
talum trialkyl species 7a-d. The activation of a C-H bond
by a carbene is also consistent with the observation of
initiation products during the metathesis of propane with
tantalum neopentyl neopentylidene supported on silica.17,18
Complexes 6b,d can behave in the same way as 6a,c,
leading to mono- and dilabeled propane (not represented
in Scheme 2).
Therefore, a mechanistic scheme has to be considered
combining the scrambling process of ethane and the produc-
tive formation of methane and propane. The scrambling
process can be explained, when starting from the two labeled
and nonlabeled tantalum methylidene hydride intermediates
3a,b; these latter species result from the liberation of propyl-
ene molecules from the tantalacyclobutane species formed
by the interaction of the tantalum ethylidenes 2a,b with
ethylene (Scheme 1 and Scheme S2). This corresponds to
the first initiating cycle during which the 0:1:1:0 distribution
(15) Chamberlain, L. R.; Rothwell, A. P.; Rothwell, I. P. J. Am.
Chem. Soc. 1984, 106, 1847–1848.
(16) Chamberlain, L. R.; Rothwell, I. P.; Huffman, J. C. J. Am.
(10) McLain, S. J.; Sancho, J.; Schrock, R. R. J. Am. Chem. Soc.
1979, 101, 5451–5453.
(11) Turner, H. W.; Schrock, R. R. J. Am. Chem. Soc. 1982, 104,
2331–2333.
(12) Wallace, K. C.; Dewan, J. C.; Schrock, R. R. Organometallics
Chem. Soc. 1986, 108, 1502–1509.
(17) Coperet, C.; Maury, O.; Thivolle-Cazat, J.; Basset, J. M. Angew.
Chem., Int. Ed. 2001, 40, 2331.
1986, 5, 2162–2164.
(18) Le Roux, E.; Chabanas, M.; Baudouin, A.; de Mallmann, A.;
Coperet, 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,
13391–13399.
(13) Wallace, K. C.; Liu, A. H.; Dewan, J. C.; Schrock, R. R. J. Am.
Chem. Soc. 1988, 110, 4964–4977.
ꢀ
(14) Herisson, J. L.; Chauvin, Y. Markromol. Chem. 1970, 141, 161.
Maury, Olivier
