Mechanistic Studies on the Insertion of Carbonyl Substrates into Cu-H: Different Rate-Limiting Steps as a Function of Electrophilicity.

We report mechanistic studies on the insertion reactions of [(NHC)Cu(μ-H)] complexes with carbonyl substrates by UV-vis and H NMR spectroscopic kinetic studies, H/D isotopic labelling, and X-ray crystallography. The results of these comprehensive studies show that the insertion of Cu-H with an aldehyde, ketone, activated ester/amide, and unactivated amide consist of two different rate limiting steps: the formation of Cu-H monomer from Cu-H dimer for more electrophilic substrates, and hydride transfer from a transient Cu-H monomer for less electrophilic substrates. We also report spectroscopic and crystallographic characterization of rare Cu-hemiacetalate and Cu-hemiaminalate moieties from the insertion of an ester or amide into the Cu-H bond.

Copper Complexes of Multidentate Carboxamide Ligands.

The copper coordination chemistry of two multidentate carboxamido ligands derived from HL (offering two quinolyl and one carboxamide donor) and HL (with two pyridine(dicarboxamido) units linked by naphthalene spacers) was explored. The former was chosen because upon deprotonation it would provide a monoanionic -coordinating N-donor set that would model the putative deprotonated form of the His-brace in copper monooxygenases, while the latter was designed to bind two copper ions and enable comparisons to other systems with different ligand spacers. Upon reaction with Cu(I)-mesityl, HL yielded a symmetric dimer in which each bis(quinolyl)amide ligand binds via two N-donors to one Cu(I) ion and via the third to the other Cu(I) center.

Sterically Induced Ligand Framework Distortion Effects on Catalytic Cyclic Ester Polymerizations.

Aluminum alkoxide complexes supported by salen ligands [salen = N, N'-bis(salicylaldimine)-2-methylpropane-1,2-diamine or N, N'-bis(salicylaldimine)-2,2-dimethylpropane-1,3-diamine] with o-adamantyl substituents have been synthesized and investigated for the polymerization of ε-caprolactone. Geometric analysis of the catalysts used for the reaction reveals the metal coordination geometries to be intermediate between square-pyramidal and trigonal-bipyramidal. A detailed kinetic study accompanied by density functional theory modeling of key mechanistic steps of the reaction suggest that, in addition to the length of the backbone linker, the o-aryl substituents have a significant impact on the catalyst's reactivity.

Mechanism of the Polymerization of rac-Lactide by Fast Zinc Alkoxide Catalysts.

The ring-opening transesterification polymerization (ROTEP) of rac-lactide (rac-LA) using LZn catalysts (L = ligand having phenolate, amine, and pyridine donors with variable para substituents X on the bound phenolate donor; X = NO, Br, t-Bu, OMe) was evaluated through kinetics experiments and density functional theory, with the aim of determining how electronic modulation of the ligand framework influences polymerization rate, selectivity, and control. After determination that zinc-ethyl precatalysts required 24 h of reaction with benzyl alcohol to convert to active alkoxide complexes, the subsequently formed species proved to be active and fairly selective, polymerizing up to 300 equiv of rac-LA in 6-10 min while yielding isotactic (P = 0.72-0.

Formally Copper(III)-Alkylperoxo Complexes as Models of Possible Intermediates in Monooxygenase Enzymes.

Reaction of [NBu][LCuOH] with excess ROOH (R = cumyl or tBu) yielded [NBu][LCuOOR], the reversible one-electron oxidation of which generated novel species with [CuOOR] cores (formally CuOOR), identified by spectroscopy and theory for the case R = cumyl. This species reacts with weak O-H bonds in TEMPO-H and 4-dimethylaminophenol (PhOH), the latter yielding LCu(OPh), which was also prepared independently. With the identification of [CuOOR] complexes, the first precedent for this core in enzymes is provided, with implications for copper monooxygenase mechanisms.

Copper-Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity.

A longstanding research goal has been to understand the nature and role of copper-oxygen intermediates within copper-containing enzymes and abiological catalysts. Synthetic chemistry has played a pivotal role in highlighting the viability of proposed intermediates and expanding the library of known copper-oxygen cores. In addition to the number of new complexes that have been synthesized since the previous reviews on this topic in this journal (Mirica, L.

Why So Slow? Mechanistic Insights from Studies of a Poor Catalyst for Polymerization of ε-Caprolactone.

Polymerization of ε-caprolactone (CL) using an aluminum alkoxide catalyst (1) designed to prevent unproductive trans binding was monitored at 110 °C in toluene-d by H NMR and the concentration versus time data fit to a first-order rate expression. A comparison of t for 1 to values for many other aluminum alkyl and alkoxide complexes shows much lower activity of 1 toward polymerization of CL. Density functional theory calculations were used to understand the basis for the slow kinetics.

Secondary Sphere Hydrogen Bonding in Monocopper Complexes of Potentially Dinucleating Bis(carboxamide) Ligands.

Reaction of a macrocyclic ligand precursor comprising two bis(carboxamido)pyridine units () connected by ethylene linkers with NMeOH and CuX (X = Cl, OAc, or OTf) yielded monocopper complexes [NMe][(HL4)Cu(X)] (X = Cl (), OAc (), or OH ()), in contrast to previous work using a related ligand with ortho-phenylene linkers wherein dicopper compounds were isolated. X-ray structures of the complexes revealed hydrogen bonding from the free carboxamide N-H groups in the doubly protonated form of the ligand ( ) to the monodentate fourth ligand coordinated to the Cu(II) ion. Similar secondary sphere hydrogen bonding interactions were identified in multinuclear compounds [NMe][((HL4)Cu)(CO)] (n = 2 or 3) that were isolated from exposure of to air.

Mechanistic Studies of ε-Caprolactone Polymerization by (salen)AlOR Complexes and a Predictive Model for Cyclic Ester Polymerizations.

Aluminum alkoxide complexes () of salen ligands with a three-carbon linker and para substituents having variable electron-withdrawing capabilities (X = NO, Br, OMe) were prepared, and the kinetics of their ring-opening polymerization (ROP) of ε-caprolactone (CL) were investigated as a function of temperature, with the aim of drawing comparisons to similar systems with two-carbon linkers investigated previously (). While and exhibit saturation kinetics and similar dependences of their ROP rates on substituents X (invariant , similar Hammett ρ = +1.4(1) and 1.