Journal of the American Chemical Society
Communication
Supporting Information [SI]) displays a shoulder at 7.6 ppm,
tentatively assigned to the metal alkylidene proton by
comparison of C6D6 solutions containing the molecular
precursor 1. The 13C cross-polarization magic angle spinning
(CP-MAS) spectrum also contains the expected signals for the
ArO fragment and the tBu unit of the alkylidene, though the
resonance of the alkylidene carbon is not observed in this
spectrum. The missing alkylidene signal in the solid state NMR
spectrum is a common feature in silica-grafted materials
because of the small number of sites and large chemical shift
anisotropy, making carbon-13 labeling necessary for a direct
evidence of this ligand.
Table 1. Catalytic Activity of Complex 1@SiO2‑700 in
Toluene at 30°C
a
a
b
substrate
mol %
TOF
time to equilibrium conversion
c
cis-4-nonene
cis-4-nonene
1-nonene
0.1
170
3 min (500)
0.02
0.1
280 (17%)
13 (4%)
<60 min (2500)
66% after 8 h (660)
<3 h (250)
ethyl oleate
ethyl oleate
0.2
44 (29%)
143 (19%)
0.05
46% after 4 h (920)
a
TOF at 3 min, given in min−1 with the corresponding conversions
b
c
given in brackets. TON given in parentheses. Calculated at full
conversion.
We synthesized 13C-labeled analogue of 1 with 15% isotopic
enrichment at the alkylidene carbon (1*). Grafting 1* on
SiO2‑700 gives 1*@SiO2‑700 with identical IR properties, and
mass balance again infers that ∼40% of the silanols on the
surface react with 1a*. The 13C CPMAS spectrum (Figures 2
activity in alkene metathesis at 30 °C for all three substrates
(Table 1, see the SI for details). With 0.1 mol % 1@SiO2, cis-4-
nonene is equilibrated to cis/trans mixtures of 4-octene and 5-
decene in less than 3 min, and the catalyst can be recovered and
recycled five times without significant loss of activity. Note that
the corresponding W imido [(SiO)W(NAr)(CHtBu)-
(2,5-diMePy)]14 has a much lower activity (TOF = 4 min−1)
and stability (deactivation after 40%) under the same reaction
conditions. 1@SiO2‑700 is also 1 order of magnitude more active
than one of the best silica-supported Mo catalysts [(
SiO)Mo(NAr)(CHtBu)(OtBuF6)] under the same condi-
tions (TOF = 46; equilibrium conversion reached in 30 min).14
At lower loadings of [(SiO)W(O)(CHtBu)(OAr)]
(0.02 mol %) cis-4-nonene was converted to 4-octenes and 5-
decenes with an initial turnover frequency (TOF) of 280 min−1.
This value is significantly higher than for the molecular complex
1 (TOF at 3 min = 5 min−1; 35% conversion after 24 h). The
higher activity of 1@SiO2 is probably due to the substitution of
one aryloxy group by a smaller siloxy ligand. The high activity
and stability of 1@SiO2‑700 allowed us to decrease catalyst
loading to 50 ppm in cis-4-nonene metathesis with full
conversion (TON ∼10000) in less than 4 h at 30 °C. Under
the same reaction conditions, 1-nonene reacts more slowly than
internal alkenes, as previously observed with molecular
complexes,7 probably as a result of ethylene buildup in the
reaction mixture and the formation of stable metallocyclobu-
tane intermediates. In contrast, ethyl oleate, an alkene
containing an ester, is converted with a fast TOF of 44
min−1 and reaches equilibrium in less than 3 h with 0.5 mol %
catalyst. Decreasing the loading to 0.02 mol % leads to an
increase of the TOF to 143 min−1. These results are
noteworthy considering that the classical WO3/SiO2 catalyst
operates only at high temperatures and is not compatible for
functionalized alkenes.
In conclusion, [(SiO)W(O)(C*HtBu)(OAr)], a
well-defined silica-supported tungsten oxo alkylidene surface
complex, prepared by Surface Organometallic Chemistry,
displays unprecedented activity at room temperature in the
metathesis of alkenes. In sharp contrast to the industrial
catalysts based on WO3/SiO2, [(SiO)W(O)(C*HtBu)-
(OAr)] tolerates oleic acid esters. These results show that the
low activity and the lack of compatibility with functional groups
of the industrial catalyst is not intrinsic to silica-supported W
catalysts or to oxo alkylidene surface species, but probably is a
consequence of the high temperatures employed to generate
the small number of active sites in the classical, ill-defined
systems. We will continue to explore new ways of generating
highly active W-based heterogeneous (silica-supported) alkene
metathesis catalysts.
Figure 2. Two-dimensional 1H−13C HETCOR spectrum of 1*@SiO2
recorded with a CP contact time of 200 μs. Other experimental details
are given in the SI. The one-dimensional carbon-13 CPMAS spectrum
is shown above the 2D plots together with the assignment of the
resonances.
and S4SI) contains signals associated with the ArO ligand at
139 and 130 ppm (respectively assigned to the tertiary and
secondary carbons from the aryloxy ligand) as well as at 21
ppm for the methyl groups. The tBu methyl groups appear at
34 ppm, with the closely overlapping tertiary carbon from the
tBu group at 36 ppm. In addition, the spectrum contains a
resonance at 260 ppm that is assigned to the alkylidene carbon
of [(SiO)W(O)(C*HtBu)(OAr)] (2*). To confirm
1
this assignment we performed H−13C two-dimensional (2D)
heteronuclear correlation (HETCOR) experiments that
showed a strong correlation between the alkylidene carbon
resonance and the proton resonance at 7.6 ppm (Figure 2).
These results unambiguously confirm the presence of the
alkylidene ligand in 1@SiO2‑700. The relatively broad 13C
resonance at 85 ppm that correlates with proton chemical shifts
at ∼1.5 ppm clearly establishes the identity of the second
species as [(SiO)W(O)(*CH2tBu)(OAr)2] (3*), which
arises from addition of the silanol to the alkylidene carbon.
The catalytic activities of the [(SiO)W(O)(CHtBu)-
(OAr)] in the self-metathesis of cis-4-nonene, 1-nonene, and
ethyl oleate as prototypical substrates were investigated (Table
1). [(SiO)W(O)(CHtBu)(OAr)] displays very high
B
dx.doi.org/10.1021/ja410052u | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Conley, Matthew P.
