Clare Bakewell

Metal‐Size Influence in Iso‐Selective Lactide Polymerization

Iso-selective initiators for the ring-opening polymerization (ROP) of rac-lactide are rare outside of Group 13. We describe the first examples of highly iso-selective lutetium initiators. The phosphasalen lutetium ethoxide complex shows excellent iso-selectivity, with a Pi value of 0.81–0.84 at 298 K, excellent rates, and high degrees of polymerization control. Conversely, the corresponding La derivative exhibits moderate heteroselectivity (Ps=0.74, 298 K). Thus, the choice of metal center is shown to be crucial in determining the level and mode of stereocontrol. The relative order of rates for the series of complexes is inversely related to metallic covalent radius: that is, La>Y>Lu.

8-quinolinolato gallium complexes: iso-selective initiators for rac-lactide polymerization

The synthesis and characterization of a series of 8-quinolinolato gallium complexes is presented, and the complexes are analogous to a series of aluminum complexes previously reported. The complexes have been shown to be active initiators for the ring-opening polymerization of rac-lactide. High degrees of polymerization control are demonstrated, as exemplified by the linear evolution of molecular weight as the polymerization progresses, narrow polydispersity indices, and molecular weights corresponding to those predicted on the basis of initiator concentration. Some of the initiators show iso-selective polymerization of rac-lactide, with Pi = 0.70. The polymerization rates have been monitored, and the pseudo first-order rate constants are compared to those of analogous aluminum compounds. The 8-quinolinolato gallium initiators show rates approximately 3 times higher than those of the series of aluminum compounds, while maintaining equivalently high iso-selectivity (Pi = 0.70) and polymerization control.

Scandium and yttrium phosphasalen complexes as initiators for ring-opening polymerization of cyclic esters.

The synthesis and characterization of novel scandium and yttrium phosphasalen complexes is reported, where phosphasalen refers to two different bis(iminophosphorane) derivatives of the more ubiquitous salen ligands. The activity of the complexes as initiators for the ring-opening polymerization of cyclic esters is presented. The scandium complexes are inactive for lactide polymerization but slow and controlled initiators for ε-caprolactone polymerization. The lack of activity toward lactide exhibited by these compounds is probed, and a rare example of single-monomer insertion product, unable to undergo further reactions with lactide, is identified. In contrast, the analogous yttrium phosphasalen complex is a very active initiator for the ring-opening polymerization of rac-lactide (kobs = 1.5 × 10(-3) s(-1) at 1:500 [yttrium initiator]:[rac-lactide], 1 M overall concentration of lactide in THF at 298 K). In addition to being a very fast initiator, the yttrium complex also maintains excellent levels of polymerization control and a high degree of isoselectivity, with the probability of isotactic enchainment being Pi = 0.78 at 298 K.

Recent Developments in Catalytic Activation of Renewable Resources for Polymer Synthesis

This review describes the application of organometallic and inorganic complexes as initiators and catalysts for polymerizations using renewable resources. It focuses on the ring-opening polymerization of lactide and the alternating copolymerization of carbon dioxide and epoxides. For lactide ring-opening polymerization, a general background to the reaction mechanism, kinetics, stereochemical control and polymerization control is presented. This is followed by reviews of the use of groups 3 and 13 complexes as initiators. Group 3 complexes show excellent rates, amongst the fastest reported for this polymerization, and in some cases stereocontrol. The group 13 complexes have good precedent for stereocontrol; recent advances using heavier group 13 elements, In and Ga, are highlighted. For the alternating copolymerization, an overview of the reaction kinetics, mechanism and control is presented. Recent advances in the use of catalyst operating at low pressure and dinuclear catalysts are presented.

Reactions of Fluoroalkenes with an Aluminium(I) Complex

A series of industrially relevant fluoroalkenes react with a monomeric AlI complex. These reactions break strong sp2 and sp3 C-F bonds, and result in the formation of a diverse array of organoaluminium compounds. Mechanistic studies show that two mechanisms are likely in operation: 1) direct oxidative addition of the C-F bond to AlI occurs with retention of alkene stereochemistry, and 2) stepwise formation and decomposition of a metallocyclopropane intermediate occurs with inversion of alkene stereochemistry. As part of this mechanistic analysis, we have isolated the first aluminium metallocyclopropane complex from oxidative addition of an alkene to AlI . Remarkably this reaction is reversible and reductive elimination of the alkene occurs at higher temperature reforming AlI . Furthermore, in selected cases the organoaluminium products are susceptible toward β-fluoride elimination to yield a double C-F activation pathway.

Reactions of Fluoroalkanes with Mg−Mg Bonds: Scope, sp3C−F/sp2C−F Coupling and Mechanism

Abstract sp3C−F Bonds of fluoroalkanes (7 examples; 1°, 2° and 3°) undergo addition to a low‐valent Mg−Mg species generating reactive organomagnesium reagents. Further reactions with a series of electrophiles results in a net C−F to C−B, C−Si, C−Sn or C−C bond transformation (11 examples, diversity). The new reactivity has been exploited in an unprecedented one‐pot magnesium‐mediated coupling of sp3C−F and sp2C−F bonds. Calculations suggest that the sp3C−F bond activation step occurs by frontside nucleophilic attack of the Mg−Mg reagent on the fluoroalkane.