Propagation at Carbon-Transition Metal Bond

The group I El metal component increases activity and stereoselectivity by alkylation (oflen together with reduction) (Eqs. 8-28 through 8-32) of the transition metal component to form more active and stereoselective reaction sites. The group I-EI metal component may also be involved in stabilizing the active transition metal sites by complexation and may be part of the counterion strucmre. 

A variety of stmctures have been proposed for the active species in Ziegler-Natta initiator systems [Allegra, 1971 Arlman and Cossee, 1964 Corradini et al., 1989 Natta, l%0a,b Patat and Sinn, 1958 Rodriguez and van Looy, 1%6 Tait and Watkins, 1989]. Structure 

XX is generally considered as the active species formed from titanium chloride and alkyla-luminum components. The in the structure represents an unoccupied (vacant) site of the octahedral titanium complex. XX represents an active titanium site at the surface of a TiCls crystal after modification by reaction with the alkylaluminum component. The titanium atom shares four chloride ligands with its neighboring titanium atoms and has an alkyl ligand (incorporated via exchange of alkyl from the alkylaluminum for chloride) and a vacant orbital. There are molecular mechanics calculations that indicate dimeric Ti2Cl6 may be the active species instead of monomeric TiCls [Monaco et al., 2000]. Other proposals for the active species include bimetallic species that contain both titanium and aluminum [Liu et al., 2002]. To simplify matters, our discussions will center on the monomeric and monometallic titanium species, especially since the mechanistic details of stereoselectivity and activity are essentially the same for both monomeric and dimeric titanium species as well as titanium-aluminum species. 

The high-mileage supported initiators previously mentioned involve a species similar to XX as part of a mixed titanium-magnesium-chloride lattice, usually involving Mg—Cl—Ti bonds. Ti and Mg are interchangeable within the metal lattice. 

Cossee-Arlman mechanism, is based on the observed stereoselectivity and molecular modeling studies [Arbnan and Cossee, 1964 Ewen, 1999 Rappe et al., 2000 Resconi et al., 20(X)]. A variation on this mechanism involves a lowering of the transition state barrier to insertion by an a-agostic interaction, specifically, an attractive interaction between titanium and a hydrogen on the first carbon attached to Ti. 

Neighboring metal atoms (bridged by two chlorines) have opposite chirality [Allegra, 1971]. The two enantiomeric titaniums can be represented as XXI and XXII where one of 

---