Angewandte
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The rate order with respect to the silane was established
Our calculations (Figure 4) show that [Cp2Ti-OEt] forms
by varying [PhSiH3] and monitoring [Cp2TiH] through in situ
Vis spectroscopy at 601 nm because of interfering silane
bands in the IR spectrum. The overlap of the graphs was
consistent with a rate order of zero with respect to the silane.
To confirm these findings, two further sets of experiments
were carried out. First, the rate order with respect to silane
was additionally determined using “different excess” experi-
ments (Supporting Information). The experiment examining
the impact of [PhSiH3] confirmed that the rate order with
respect to the silane is indeed zero. Second, an experiment on
the hydrosilylation of 7 (Scheme 3) also established an inverse
rate order with respect to the epoxide (Table 1).
a more stable epoxide complex than [Cp2Ti-H]. The opening
of the [Cp2Ti-OEt] epoxide complex to the b-metaloxy radical
is distinctly slower. In agreement with this prediction, we
found that [Cp2TiOEt]2 does not open 4 (see the Supporting
Information). Therefore, it seems that an epoxide complex of
titanocene(III) alkoxides is indeed the resting state of this
catalyst.
Clearly, the mechanism of epoxide hydrosilylation is
rather intricate. Neither epoxide opening, nor the intra-
molecular hydrogen atom transfer (HAT), nor s-bond meta-
thesis can be the rate-determining step. The formation of
a resting state of the catalyst that reversibly binds the epoxide
substrate without inducing ring opening must thus be
responsible for the observed reaction orders. A plausible
scenario is epoxide binding by Cp2TiOR (OR is formed
through epoxide opening followed by HAT, Scheme 1).
Figure 4. Computational analysis of epoxide complexation and open-
ing by [Cp2Ti-H] and [Cp2Ti-OEt]. Free energies are given in kcalmolÀ1
.
To validate this conclusion, a solution of Cp2TiAllyl was
mixed with PhSiH3 and 4 and immediately shock-frozen with
liquid N2 (Figure 5, black lines). Sample preparation was
repeated using PhSiH3 and 2,2-D2-4 (red lines), PhSiD3 and 4
(green lines), and PhSiD3 and 2,2-D2-4 (blue lines). The
frozen solutions were subjected to pulsed Q-band EPR and
electron nuclear double resonance (ENDOR) spectroscopy
(Figure 5a).[12] The EPR spectra of the different samples are
all similar and cover a field range of approximately 30 mT,
with intense maxima at g-values of 1.9857 and 1.9651 (see the
Supporting Information). The values are in the typical range
for titanocenes.[4,13] The spectra are the result of the presence
of several paramagnetic species.
Scheme 3. Titanocene-catalyzed epoxide hydrosilylation with 3 after
“allyl activation” in the presence of 1.5 equiv (second example
2.3 equiv) PhSiH3.
1
ENDOR spectroscopy of these samples yielded the H
hyperfine coupling patterns shown in Figure 5a. When 2,2-
D2-4 was used instead of 4, the proton resonance at an
approximate hfc constant of 2.5 MHz was no longer observed.
Similarly, resonances at an approximate hfc constant of
1.0 MHz were not detected with PhSiD3, although the
magnitude of the observed effect appeared to be weaker.
The smaller hfc constant indicates a larger electron–nucleus
separation. Finally, both resonances are suppressed when
both reagents are deuterated. Additionally, deuteron reso-
nances at corresponding hfc constants were observed when
deuterated reagents were used (Figure 5b). The observed
effects in the ENDOR spectra indicate that one of the ligands
at the TiIII center is the alkoxide formed after epoxide opening
and HAT. Whether or not an additional epoxide is also bound
to the TiIII center cannot conclusively be answered by using
ENDOR spectroscopy. However, the obtained spectra do not
Table 1: Rate orders with respect to the different reactants for the
titanocene-catalyzed hydrosilylation of 4 after “allyl activation”. Values in
parentheses refer to 7.
Reaction component
Rate order
[Ti]
PhSiH3
Epoxide 4 (and 7)
1.2Æ0.3[a]
0.1Æ0.4[b]
À1.4Æ0.3[c] (À1.3Æ0.2)[d]
[a] Conditions: Initial rate method. [Ti] 3.3–13 mm, 67 mm epoxide 4,
103 mm PhSiH3. [b] Owing to interfering silane bands in the IR spectrum,
this rate order was determined by in situ monitoring of the [Ti] species
through Vis spectroscopy. Conditions: Initial rate method. [Ti] 2 mm,
37.5 mm epoxide 4, 38.0–47.6 mm PhSiH3. [c] Conditions: Initial rate
method. [Ti] 6.7 mm, 67–267 mm epoxide 4, 103 mm PhSiH3. [d] Con-
ditions: Initial rate method. [Ti] 6.7 mm, 67–183 mm epoxide 7, 103 mm
PhSiH3.
Angew. Chem. Int. Ed. 2016, 55, 7671 –7675
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