3.1 Mechanism of Stereochemical Control
Breaking away from the symmetry considerations (summarized in Fig. 18), Fink,
Angermund and coworkers [32] developed a uniform model that accurately
describes the experimental microstructures of the polymers by means of four
lowest-energy conformers of the metallocene species coordinating the prochiral
propylene (R re , S re , S si , R si ) and the positional changes that the polymer chain
undergoes during insertion (R and S relate to the configuration of the Zr center;
re and si relate to the different coordinations of the α-olefin).
The relative energy levels of the four diastereomers were determined by
molecular modeling calculations [32, 33]. Figure 19 demonstrates as examples
the R re -π complex conformers of four zirconocene(propylene)(isobutyl) species.
In Fig. 20, the relative energies of four diastereomers (I–IV) are compared for four
zirconocenes – each differently substituted at the Cp ring.
It becomes clear that it is not the symmetry of the catalysts that is decisive
for the stereospecificity, but the energy graduations, in particular the size of the
energy gap between the individual diastereomeric states. Thus, catalyst I operates
syndiotactically because R re ¼ S si < S re ¼ R si ; catalyst II hemiisotactically
because R re < S re , S si , R si ; catalyst III isotactically because R re , S re < S si , R si ; and
catalyst IV atactically because R re % S re , S si , R si . The “symmetry rule” still applies,
but is not the decisive factor and is only a subordinated aspect of the more uniform
model.
To find out whether the stereospecificity of a catalyst should ultimately be
determined at the transition state of the rate-determining reaction step, Angermund,
Jensen and coworkers [34, 35] recalculated the system using density functional
theory (DFT). This computational approach is based on the simplest possible
combination of quantum chemistry (QM) and molecular mechanics (MM). The
geometry of the central part, where bond breaking and bond forming takes place
(termed aggregate), was first optimized in a separate DFT calculation and then
later used “as is” in a series of force field-based calculations. As an example,
Fig. 21, left, shows the structure of a transition state optimized in this way for the
insertion of propylene in the zirconocene cation [iPr(3-Me-Cp)(Flu)Zr(propylene)
(isobutyl)]
+ with the four-membered ring as central part (aggregate) [36]. For
an exact description of the course of stereospecificity, four different transition
state conformers are also necessary here (shown in Fig. 21, right).
Furthermore, supposing that the single insertions occur independently of
each other, these relative transition state energies are converted via a suitable
averaged Boltzmann statistic into the corresponding pentad distributions, which are
representative for the sequence of five transition states. These theoretical pentads
are in very good agreement with the experimental pentads at low and intermediate
temperatures, which proves that stereoregulation (i.e., the stereosequence of a
polymer chain and the stereo-errors) can be calculated and predicted with this
model. Stereo-errors occur as a result of insertion with the wrong enantiofacial
orientation of propylene (triad errors), and this route of insertion becomes more
important with rising temperature. At temperatures >50
C discrepancies occur
22
G. Fink
Breaking away from the symmetry considerations (summarized in Fig. 18), Fink,
Angermund and coworkers [32] developed a uniform model that accurately
describes the experimental microstructures of the polymers by means of four
lowest-energy conformers of the metallocene species coordinating the prochiral
propylene (R re , S re , S si , R si ) and the positional changes that the polymer chain
undergoes during insertion (R and S relate to the configuration of the Zr center;
re and si relate to the different coordinations of the α-olefin).
The relative energy levels of the four diastereomers were determined by
molecular modeling calculations [32, 33]. Figure 19 demonstrates as examples
the R re -π complex conformers of four zirconocene(propylene)(isobutyl) species.
In Fig. 20, the relative energies of four diastereomers (I–IV) are compared for four
zirconocenes – each differently substituted at the Cp ring.
It becomes clear that it is not the symmetry of the catalysts that is decisive
for the stereospecificity, but the energy graduations, in particular the size of the
energy gap between the individual diastereomeric states. Thus, catalyst I operates
syndiotactically because R re ¼ S si < S re ¼ R si ; catalyst II hemiisotactically
because R re < S re , S si , R si ; catalyst III isotactically because R re , S re < S si , R si ; and
catalyst IV atactically because R re % S re , S si , R si . The “symmetry rule” still applies,
but is not the decisive factor and is only a subordinated aspect of the more uniform
model.
To find out whether the stereospecificity of a catalyst should ultimately be
determined at the transition state of the rate-determining reaction step, Angermund,
Jensen and coworkers [34, 35] recalculated the system using density functional
theory (DFT). This computational approach is based on the simplest possible
combination of quantum chemistry (QM) and molecular mechanics (MM). The
geometry of the central part, where bond breaking and bond forming takes place
(termed aggregate), was first optimized in a separate DFT calculation and then
later used “as is” in a series of force field-based calculations. As an example,
Fig. 21, left, shows the structure of a transition state optimized in this way for the
insertion of propylene in the zirconocene cation [iPr(3-Me-Cp)(Flu)Zr(propylene)
(isobutyl)]
+ with the four-membered ring as central part (aggregate) [36]. For
an exact description of the course of stereospecificity, four different transition
state conformers are also necessary here (shown in Fig. 21, right).
Furthermore, supposing that the single insertions occur independently of
each other, these relative transition state energies are converted via a suitable
averaged Boltzmann statistic into the corresponding pentad distributions, which are
representative for the sequence of five transition states. These theoretical pentads
are in very good agreement with the experimental pentads at low and intermediate
temperatures, which proves that stereoregulation (i.e., the stereosequence of a
polymer chain and the stereo-errors) can be calculated and predicted with this
model. Stereo-errors occur as a result of insertion with the wrong enantiofacial
orientation of propylene (triad errors), and this route of insertion becomes more
important with rising temperature. At temperatures >50
C discrepancies occur
22
G. Fink