A model for the homogeneous isospecific Ziegler–Natta polymerization of olefins: Enantioselectivity in the deuteration and deuteriooligomerization of 1‐alkenes

Abstract

The enantioselectivity found in homogeneous isospecific ZieglerNatta catalysts for the insertion of 1-alkenes in metal-deuterium or in metalisobutyl bonds is discussed from a theoretical point of view. Nonbonded energy calculations, based on a model of the catalytic site previously proposed by us, indicate that the strong enantioselectivity found in the insertion of 1-alkenes in a metal-isobutyl bond is drastically reduced in the presence of a metal-deuterium bond. In particular, a weak enantioselectivity in favour of a monomer coordinated with the opposite chirality (lower for the case of 1-butene, higher for the case of styrene) is shown to occur in the latter case.

Mechanism ofUnlikeStereoselectivity in 1-Alkene Primary Insertions: Syndiospecific Propene Polymerization by Brookhart-Type Nickel(II) Catalysts

“…According to this mechanism of chiral orientation of the growing chain , the stereoselectivity of these models is due to interactions of the π-ligands with the growing chain, determining its chiral orientation, which, in turn, discriminates between the two prochiral faces of the monomer, and not to direct interactions of the π-ligands with the monomer. The latter can only reinforce or diminish the extent of steric control in certain cases. , This mechanism of stereoselectivity is in agreement 14 with several experimental data relative to the stereostructures of polymer chain ends , as well as in agreement with the absolute configurations of products of deuteration and deuteriooligomerizations of 1-alkenes. − It is worth noting that also fine details of these stereospecific behavior have been rationalized, e.g., the dependence of stereoselectivity and regioselectivity on alkyl substituents of metallocenes, the stereoselectivities relative to occasional secondary monomer insertions or relative to primary insertions following these secondary insertions, and the relationship between regiospecificity and type of stereospecificity …”

“…The latter can only reinforce or diminish the extent of steric control in certain cases. 26,27 This mechanism of stereoselectivity is in agreement 14 with several experimental data relative to the stereostructures of polymer chain ends 28,29 as well as in agreement 30 with the absolute configurations of products of deuteration and deuteriooligomerizations of 1-alkenes. [31][32][33] It is worth noting that also fine details of these stereospecific behavior have been rationalized, e.g., the dependence of stereoselectivity and regioselectivity on alkyl substituents of metallocenes, 34 the stereoselectivities relative to occasional secondary monomer insertions or relative to primary insertions following these secondary insertions, 22 and the relationship between regiospecificity and type of stereospecificity.…”

Chirality of Catalysts for Stereospecific Polymerizations

“…On the other hand, the presence of opposite stereoselectivities in deuteration and oligomerization has also been rationalized by molecular mechanics calculations analogous to those previously described, thus constituting a further valuable support to the proposed catalytic models and stereoselective mechanism. 46 More recent results by Bercaw and co-workers relative to deuteration and deuterodimerization experiments on isotopically chiral 1-pentene are also in agreement with a mechanism involving a chiral orientation of the growing chain. 47 It is worth noting that the proposed preinsertion intermediates, like those of Figures 1.4 and 1.6, are suitable for the formation of an α-agostic bond between the metal and the hydrogen atom on the opposite side with respect to the incoming monomer.…”

Reactivity ofZandEIsomers, Growing Chain Isomerization, and Chain Transfer Reactions in Ethene/2-Butene Copolymerization by Metallocene-Based Catalysts

“…on the saturated propene oligomers obtained, under suitable conditions, with this kind of catalyst, proving that the re insertion of the monomer is favored in the case of ( R , R ) chirality of coordination of the bis(indenyl) ligand. Moreover, also the presence of opposite stereoselectivities in deuteration and oligomerization have been rationalized in terms of the proposed stereoselective mechanism. − …”

“…Moreover, also the presence of opposite stereoselectivities in deuteration and oligomerization 18 have been rationalized in terms of the proposed stereoselective mechanism. [17][18][19] For a given catalytic model, the stereoselectivity of each insertion step does not ensure its stereospecificity. In fact, the possible presence, as well as the kind, of stereospecificity depends on possible differences between stereostructures of transition states of two successive insertion steps.…”