1-Alkenes primary insertions

Understanding the factors controlling primary versus secondary insertion of alkenes is critically important to practical applications, because regioerrors (i.e., occasional secondary insertions in a polymer predominantly formed by primary insertions) can adversely affect relative molecular masses, responsivities to hydrogen, and melting points of polymers. 

The models considered in this section refer to catalytic systems for which the polymerization reaction occurs by primary insertion of the 1-alkene and neither chirality of coordination of the aromatic ligands nor chirality at the... 

S-Hydride transfer after a primary insertion (Scheme 16) produces vinylidene-terminated, n-pro-pyl-initiated PP and follows the rate law Rt = (/5-h)q[Q. when it is unimolecular (yS-hydride transfer to the metal), and Rt = /C( -h)i[D] [M], when it is bimolecular (either by concerted /1-hydride transfer to the coordinated monomer or by associative displacement). Both neutral group 3 and cationic group 4 metallocene alkyl complexes readily undergo spontaneous /S-hydride transfer to the corresponding metal hydrides and alkenes, as shown by Ber-... 

As shown in Table 4.38, three major reaction pathways are available to hypova-lent metals in the presence of an alkene (A) and (C) dative and synergistic coordination (B) carbocation formation and (D) and (E) metallacyclic and migratory insertions. The latter types are of particular importance in metal-catalyzed alkene polymerizations and will be given primary attention in the discussion that follows. 

Schematic plots of the internal energy versus the reaction coordinate for both primary and secondary insertions and for generic aspecific, syndiospecific, and isospecific model complexes are sketched in Figures 1.11 a,b, and c, respectively. The minima at the centers and at the ends of the energy curves correspond to alkene-free intermediates, including a growing chain with n and n + 1 monomeric units, respectively. Movements from the central minima toward the left and the right correspond to possible reaction pathways leading to primary and secondary insertions, respectively. For the enantioselective complexes the reaction pathways for monomer enantiofaces being...

Chain-end controlled isospecificity and syndiospecificity for 1-alkene polymerizations at low temperatures with achiral metallocenes have also been reported.2,163 81131135 The polymerization with these catalysts is highly regio-specific in favor of primary monomer insertion. 

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. 

To date, the most frequently used ligand for combinatorial approaches to catalyst development have been imine-type ligands. From a synthetic point of view this is logical, since imines are readily accessible from the reaction of aldehydes with primary or secondary amines. Since there are large numbers of aldehydes and amines that are commercially available the synthesis of a variety of imine ligands with different electronic and steric properties is easily achieved. Additionally, catalysts based on imine ligands are useful in a number of different catalytic processes. Libraries of imine ligands have been used in catalysts of the Strecker reaction, the aza-Diels-Alder reaction, diethylzinc addition, epoxidation, carbene insertions, and alkene polymerizations. 

Although carbene type insertions have been discussed as displacements and their reactions with alkenes could be treated as cycloadditions, both could just as easily fit into this section. Typical of the latter is the addition (145). Apart from a primary syn stereospecificity, there is a... 

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