Journal of the American Chemical Society
Communication
free-energy change ΔG = −14.6 kcal/mol for 3a), proceeding
with an exceedingly low activation barrier of <2.0 kcal/mol (for
the potential energy curve, see Figure S3). Moreover, in
complete accord with our initial expectations, computations
showed that as the silylene complex and the alkyne approach
each other, the d(Ti)−π*(CC) interaction between the
transition metal and the alkyne begins to develop as the initial
step of the reaction (visualized in the HOMOs of the reaction
system formed upon the interaction of the silylene complex 2
(without ligand L) and trimethylsilylacetylene, forming metal-
lacycle 3a, see Figure S4). Such coordination of the alkyne at
the transition metal is enabled by the preliminary elimination of
the Lewis base ligand L from 2, which provides a coordination
site at the titanium center. Indeed, the [2 + 2] cycloaddition
rate agrees well with the strength of the ligand L-to-titanium
bonding: Complexes 2a,b with the very loosely bound THF
and moderately bound phosphine ligands react with alkynes
instantly even at temperatures as low as −78 °C, whereas 2c
with the most strongly coordinating isocyanide ligand needs a
couple of days at room temperature to complete the reaction.
Interesting to note that in all cases of the cycloaddition of
terminal alkynes, only one regiosisomer of the silatitanacyclo-
butene is exclusively formed with the substituted fragment of
the CC bond bound to Ti and the unsubstituted fragment
bound to Si. Computations revealed a clear free energy
stabilization of 21.8 kcal/mol for the cycloadduct 3a, compared
with its regioisomer 3a1 (Chart 1). Such thermodynamic
Figure 3. ORTEP view of the [2 + 2] cycloadduct 3a synthesized by
the reaction of the titanium−silylene complex 2 with trimethylsilyla-
cetylene (thermal ellipsoids are given at the 30% probability level,
hydrogen atoms are not shown). Selected bond lengths (Å): Ti1−Si1
= 2.4868(8), Si1−Si2 = 2.3427(10), Si1−Si3 = 2.3445(10), Si2−Si4 =
2.3795(10), Si2−Si3 = 2.3880(10), Si3−Si4 = 2.3789(10), Ti1−C38 =
2.110(3), Si1−C37 = 2.030(3), C37−C38 = 1.324(4), Ti1···C37 =
2.319(3).
configuration. The silatitanacyclobutene ring in 3a is nearly
planar, with the skeletal bond lengths shown in Figure 3.
Although the Ti1−C38 bond length of 2.110(3) Å is in the
normal range, the trend in the other bond distances is notable
(geometrical parameters of the calculated real molecule agree
very well with the experimental data). Thus, the Si1−C37 bond
of 2.030(3) Å is slightly stretched, whereas the Ti1−Si1 bond
of 2.4868(8) Å and the C37−C38 bond of 1.324(4) Å are
slightly shortened. In fact, the Ti−Si bond distance in the
cycloadduct 3a is even marginally shorter than the TiSi
double bond of its precursor silylene complex 2c: 2.4868(8) vs
2.5039(6) Å. All of these structural peculiarities (along with the
unusually strongly deshielded Ti-bound Si1 atom, see above)
indicate the equally important (if not predominant) contribu-
tion of another structure to the overall composition of the [2 +
2] cycloadduct 3a, which has the character of a titanium
silylidene−alkyne π-complex (for representation of the two
major contributions to the overall structure of 3a, see Scheme
S1). Moreover, the shape of the four-membered silatitanacy-
clobutene SiTiC2 in 3a resembles a distorted trapeze with a
relatively short diagonal Ti1−C37 interatomic distance of
2.319(3) Å, indicative of their distant interaction and
supporting the contribution of the π-complex form.
The cycloadducts 3 are room temperature stable and even on
heating up to 100 °C, the expected cycloreversion products
TiC−CSi were not observed but only partial decom-
position of 3. The search for other unsaturated substrates
(including alkenes) that may form cycloadducts capable of the
subsequent metathetical cycloreversion is our current focus.
In summary, in this contribution we presented our method
for the synthesis of novel group 4 metal silylene complexes with
loosely bound and readily removable Lewis base ligands that
are reliably classified as the Schrock-type silylidenes, based on
their structural, computational, and reactivity studies. More-
over, it was demonstrated that these titanium silylidenes
smoothly react with the terminal alkynes forming unprece-
dented silatitanacyclobutenes as the [2 + 2] cycloaddition
products, which may lead to the development of an alternative
synthetic methodology for the design of novel organosilicon
materials.
Chart 1. Energetic Preferences for the Regioisomers 3a and
3a1
preference can be attributed to the unfavorable steric repulsive
interaction between the Me3Si substituent on the silicon-bound
sp2-C atom and voluminous R3Si groups at the bridgehead Si
atoms of the tetrasilabicyclo[1.1.0]butane fragment in the
regioisomer 3a1.
The peculiar spectral and structural features of 3a are
noteworthy. Thus, a characteristic low-field resonance of
+125.4 ppm was observed for the Ti-bound spiro-Si atom
(calculated value 119.9 ppm). In the 13C NMR spectrum of 3a,
the olefinic carbons were observed at +104.7 ppm (Si-bound
olefinic C) and +221.1 ppm (Ti-bound olefinic C), with the
latter signal being a diagnostic feature of the titanacyclobutene
derivatives21 (calculated values 103.9 and 218.5 ppm,
respectively).
The structure of the tricyclic cycloadduct 3a is rather
interesting as it has a number of unusual features (X-ray
analysis of metallacyclobutene 3b revealed similar structural
trends) (Figure 3). The Ti-bound Si1 atom manifests a
remarkable inverted sp3 geometry (so-called “umbrella”
configuration), dictated by its peculiar position as the spiro-
atom joining together cyclobutene and bicyclo[1.1.0]butane
fragments. Moreover, the sum of the bond angles around Si1
(ignoring the C37 atom) comes to 357.1°, which is markedly
closer to the sp2 geometry, rather than the anticipated sp3
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dx.doi.org/10.1021/ja401072j | J. Am. Chem. Soc. 2013, 135, 2987−2990