Propene insertion

Figure 1.8 Approximated transition states for primary propene insertion for model complexes with (a) isopropyl-bis(l-indenyl) ligand (isospecific) for the (R, R) coordination and (b) iso-propyl(cyclopentadienyl-9-fluorenyl) ligand (syndiospecific) for the R chirality at metal atom. Corresponding preinsertion intermediates, labeled a in Figures 1.7a,b, are sketched in Figures 1.4 and 1.6a, respectively.

Figure 1.10 Preinsertion intermediates for secondary propene insertion into primary polypropylene chain for (a) isospecific model complex based on (R, R)-coordinatedisopropyl-bis(l-indenyl) ligand and (b) syndiospecific model complex based on isopropyl(cyclopentadienyl-9-fluorenyl) ligand for R chirality at metal atom. Stereoselectivity of isospecific model site is in favor of opposite monomer prochiral faces for primary and secondary insertions (cf. Figures 1.4 and 1.10a). Stereoselectivity of syndiospecific model site is in favor of same monomer prochiral face for primary and secondary insertions (cf. Figures 1.6a and 1.1 Ob).

For the case of catalytic model sites based on metallocenes 23, a detailed molecular mechanics analysis has also been conducted for the case of primary or secondary propene insertions on secondary polypropylene chains... 

Syndiospecific catalytic systems based on metallocenes are highly regioreg-ular. As a consequence, their stereoselectivity in possible regioirregular insertions has been experimentally investigated for propene copolymers only.78,79 However, an analysis of the stereoselectivity of possible secondary propene insertions on syndiospecific catalytic models based on -symmetric metallocenes is reported here, also due to its relevance to the rationalization of the dependence of regiospecificity on the type of stereospecificity (see Section 3.1.4.1).80... 

Dependence on Metallocene Symmetry of E-Z Selectivity for 2-Butene Copolymerizations. We have seen in the Section 3.1.3 that opposite enantiofaces are favored for primary and secondary propene insertion on C2-symmetric metallocenes, whereas the same enantioface is favored for primary and secondary insertion on Cv-symmetric metallocenes. In this framework, if the same steric interactions which rule the enantioselectivity of primary and secondary propene insertions hold for 2-butene, the insertion of... 

As an example, C2-symmetric isospecific models for homogeneous catalytic systems based on the (R, / -coordinated isopropyl-bis(l-indenyl) ligand and for heterogeneous catalytic systems based on TiCLt supported on MgCl2 are compared in Figure 1.19. These models correspond to minimum-energy preinsertion intermediates calculated to be suitable for primary propene insertion... 

Since the 1960s the syndiospecific chain-end controlled polymerization of propene in the presence of homogeneous vanadium-based catalytic systems has been known. For these systems, it has been well established by the work of Zambelli and co-workers that the polymerization is poorly regioselective and the stereoselective (and possibly syndiospecific) step is propene insertion into the metal secondary carbon bond with formation of a new secondary metal-carbon bond.133134... 

Possible mechanisms for chain-end stereocontrol for catalytic systems presenting primary and secondary 1-alkene (mainly propene) insertion will be described in Sections 4.1.1 and 4.1.2, respectively. 

Diastereoisomeric transition states calculated for propene insertion in a model for a Brookhart-type Ni(II) catalyst, based on diacetylbis(2,6-diisopro-pylphenylimine)nickel derivative,143,144 are shown in Figure 1.21. Diastereomeric transition states for si (Figure 1.21a) and re (Figures 1.21b,c) monomer insertions into a si chain correspond to like (isotactic) and unlike (syndiotactic) propagations, respectively.144,143... 

Figure 1.21 Minimum-energy transition states for primary propene insertion on Brookhart-type Ni(II) catalyst for the model including si growing chain (whose last monomeric unit was generated by insertion of -coordinated monomer), (a) There is only minimum-energy transition state for insertion of, v(-propene (like insertion) while (b,c) there are two nearly energetically equivalent minimum-energy transition states for insertion of re-propene (unlike insertion).

A clear experimental estimate of the intrinsic reaction barrier to olefin insertion is still missing. The NMR analysis of Erker and co-workers estimated the intrinsic activation barrier for 1-olefin insertion into the Zr-C bond of the (MeCp)2Zr( j.-C4H6-borate betaine) to be about 10-11 kcal/mol [69, 77]. Very recent NMR experiments of Casey and co-workers on the propene insertion into the Y-C c-bond of the neutral group 3 (C5Me5)YCH2CH2CH(CH3)2 system resulted in a AG of 11.5 kcal/mol [70],... 

The polymerization of 1-olefins introduces the problems of regioselectivity and stereoselectivity. In this section we will focus on the origin of the regioselectivity of the insertion reaction (primary vs. secondary 1-propene insertion, see Figure 9), while the origin of the stereoselectivity will be discussed in the next section. 

Before continuing, it has to be noted that the energy difference between the secondary and primary propene insertion, AEK 0, can be considered composed by two main contributions, electronic and steric. The steric contribution to AE po, due to steric interaction between the monomer, the growing chain and the ligand skeleton, was modeled successfully through simple molecular mechanics calculations [78-80], and was reviewed recently [11,24], For this reason in the following we will focus only on the electronic contribution to AEKgio. 

Figure 9. Representation of primary (regioregular) and secondary (regioirregular) propene insertions into the Mt-C bond of group 4 polymerization catalysts, parts a and b, respectively.

To confirm these conclusions with more realistic models, we performed BP86 calculations on the primary and secondary propene insertion into the Zr-CH3+ o-bond of the H2Si(Cp)2ZrCH3(propene)+ system. The transition states for primary and secondary propene insertion are reported in Figure 10. 

Figure 13. Transition states for propene insertion into the Zr-isobutyl bond of the racemic-dimethylsilyl-bis-l-indenyl zirconocene with a (RJt) coordination of the aromatic ligand. C2 is the overall symmetry of the metallocene, while re and si is the chirality of coordination of the propene molecule in the transition states of parts a and b, respectively.

Table 2. Propene insertion for the benzoxantphos system, energies and geometrical parameters of transition states. Energies in kJ.mol"1, distances in A and angles in degrees, (a) Dihedral angle Hhvdndc-Rh-Cali.mc-C llktr . for defining alkene rotation.

Figure 12. Benzoxantphos CW B in transition state for propene insertion...

An analysis of polymer end groups provided insight into the mechanism of stereo-control in such catalysts. The first polymerisation step, where propene inserts into a Zr-Me bond, is in fact not stereoselective, while the insertion into a Zr-iso-butyl bond proceeds with high enantioselectivity. Ligand stereo-control operates therefore by an indirect mechanism the ligand determines the conformation of the polymery] chain, and this in turn influences the preferred orientation of the incoming alkene [127], as illustrated in structure 89 for a syndiospecific case. 

Sen reported that (C6F5)2AlR (2) (generated in situ) is an ethene polymerization catalyst (precursor) [13]. Moreover, the system also catalyzes copolymerization of ethene and propene. This latter fact, in particular, is remarkable, since a j5-branched alkyl (formed after propene insertion) should undergo very easy jS-elimination and hydrogen transfer to ethene, as discussed above. Thus, for this system to work the intrinsic chain transfer barriers of (QF5)2AlR should be much higher than those of trialkylaluminium. 

Absolute configuration of the chiral centers in the isotactic copolymer main chain can be determined by comparing the CD spectrum of the copolymer that is a polyketone with that of (S)-3-methylpentan-2-one [126]. A recent model study has confirmed this assignment, showing that the copolymer with ( -configuration in the main chain exhibits plus optical rotation in (CF3)2CHOH and minus in CHC13 [128-130], The study has also revealed that the enantiose-lectivity for the propene insertion is at least 95% ee. 

To calculate the insertion transition states from each propene adduct, the authors considered the olefin rotation in clockwise and counterclockwise fashion for these two intermediates, as previously described by Carbo et al [115]. For the equatorial-axial adduct, the barrier to propene insertion leading to the linear insertion product was predicted to be 2.8 kcal/mol smaller than the barrier for the insertion reaction leading to the branched product. For the equatorial-equatorial adduct, the barrier for the insertion leading to the branched product was predicted to be 1.4 kcal/mol lower in energy than the barrier for the reaction leading to the linear product. Therefore, it appears that for this type of catalyst there are two separate propene insertion reaction channels, one generating almost exclusively the linear product, and the other producing primarily the branched product. 

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