Studies of the kinetics of polymerization of ε-caprolactone (CL) by salen-aluminum catalysts comprising ligands with similar steric profiles but different electron donating characteristics (R = OMe, Br, or NO2) were performed using high initial monomer concentrations (2 M < [CL]0 < 2.6 M) in toluene-d8 at temperatures ranging from 20 to 90 °C. Saturation behavior was observed, enabling determination of monomer equilibrium constants (Keq) and catalytic rate constants (k2) as a function of R and temperature. While Keq varied only slightly with the electron donating properties of R (Hammett ρ = +0.16(8)), k2 showed a more significant dependence reflected by ρ = +1.4(1). Thermodynamic parameters ΔG° (associated with Keq) and ΔG(‡) (associated with k2) were determined, with the former being ∼0 kcal/mol for all catalysts and the latter exhibiting the trend R = OMe > Br > NO2. Density functional theory (DFT) calculations were performed to characterize mechanistic pathways at a microscopic level of detail. Lowest energy transition-state structures feature incipient bonding of the nucleophile to the lactone carbonyl that is approaching the metal ion, but a distinct CL adduct is not an energy minimum on the reaction pathway, arguing against Keq being associated with coordination of monomer according to the typical coordination-insertion mechanism. An alternative hypothesis is presented associating Keq with "nonproductive" coordination of substrate in a manner that inhibits the polymerization reaction at high substrate concentrations.

The kinetics of polymerization of ε-caprolactone (CL) initiated by aluminum-alkoxide complexes supported by the dianionic forms of N,N-bis[methyl-(2-hydroxy-3-tert-butyl-5-R-phenyl)]-N,N-dimethylethylenediamines, (LR)Al(Oi-Pr) (R = OMe, Br, NO2) were studied. The ligands are sterically similar but have variable electron donating characteristics due to the differing remote (para) ligand substituents R. Saturation kinetics were observed using [CL]0 = 2–2.6 M and [complex]0 = 7 mM, enabling independent determination of the substrate coordination (K eq) and insertion (k 2) events in the ring-opening polymerization process. Analysis of the effects of the substituent R as a function of temperature on both K eq and k 2 yielded thermodynamic parameters for these steps. The rate constant k 2, related to alkoxide nucleophilicity, was strongly enhanced by electron-donating R substituents, but the binding parameter K eq is invariant as a function of ligand electronic properties. Density functional calculations provide atomic-level detail for the structures of key reaction intermediates and their associated thermochemistries.

The palladium-catalyzed decarbonylation of bio-derived carboxylic acids to alkyl acrylates, styrene, and acrylonitrile is reported. The olefins were isolated by continuous distillation in yields up to 87% by heating a neat, equimolar mixture of pivalic anhydride with the appropriate carboxylic acid to 190 • C in the presence of a Pd-phosphine catalyst.

Five new homoleptic [NiX(4)](2-) compounds have been prepared with the fluorinated ligands OC(6)F(5) (OAr(F)), OC(6)H(3)(CF(3))(2) (OAr'), and OC(4)F(9) (OR(F)) and characterized with X-ray crystallography, magnetic susceptibility, and elemental analysis. Electronic spectral studies show that these ligands engender a ligand-field environment similar to that of fluoride and thus act electronically like fluoride, but with none of the drawbacks of F(-) as a transition-metal ligand.