The development of processes for selective hydrocarbon oxidation is a goal that has long been pursued. An additional challenge is to make such processes environmentally friendly, for example by using non-toxic reagents and energy-efficient catalytic methods. Excellent examples are naturally occurring iron- or copper-containing metalloenzymes, and extensive studies have revealed the key chemical principles that underlie their efficacy as catalysts for aerobic oxidations. Important inroads have been made in applying this knowledge to the development of synthetic catalysts that model enzyme function. Such biologically inspired hydrocarbon oxidation catalysts hold great promise for wide-ranging synthetic applications.

No abstract

A key step in dioxygen evolution during photosynthesis is the oxidative generation of the O-O bond from water by a manganese cluster consisting of M2(mu-O)2 units (where M is manganese). The reverse reaction, reductive cleavage of the dioxygen O-O bond, is performed at a variety of dicopper and di-iron active sites in enzymes that catalyze important organic oxidations. Both processes can be envisioned to involve the interconversion of dimetal-dioxygen adducts, M2(O2), and isomers having M2(mu-O)2 cores. The viability of this notion has been demonstrated by the identification of an equilibrium between synthetic complexes having [Cu2(mu-eta2:eta2-O2)]2+ and [Cu2(mu-O)2]2+ cores through kinetic, spectroscopic, and crystallographic studies.

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. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. Rev. 2004, 104, 1013–1046 and Lewis, E. A.; Tolman, W. B. Chem. Rev. 2004, 104, 1047–1076), the field has seen significant expansion in the (1) range of cores synthesized and characterized, (2) amount of mechanistic work performed, particularly in the area of organic substrate oxidation, and (3) use of computational methods for both the corroboration and prediction of proposed intermediates. The scope of this review has been limited to well-characterized examples of copper–oxygen species but seeks to provide a thorough picture of the spectroscopic characteristics and reactivity trends of the copper–oxygen cores discussed.

No abstract

Nearly all polymers are derived from nonrenewable fossil resources, and their disposal at their end of use presents significant environmental problems. Nonetheless, polymers are ubiquitous, key components in myriad technologies and are simply indispensible for modern society. An important overarching goal in contemporary polymer research is to develop sustainable alternatives to "petro-polymers" that have competitive performance properties and price, are derived from renewable resources, and may be easily and safely recycled or degraded. Aliphatic polyesters are particularly attractive targets that may be prepared in highly controlled fashion by ring-opening polymerization of bioderived lactones. However, property profiles of polyesters derived from single monomers (homopolymers) can limit their applications, thus demanding alternative strategies. One such strategy is to link distinct polymeric segments in an A-B-A fashion, with A and B chosen to be thermodynamically incompatible so that they can self-organize on a nanometer-length scale and adopt morphologies that endow them with tunable properties. For example, such triblock copolymers can be useful as thermoplastic elastomers, in pressure sensitive adhesive formulations, and as toughening modifiers. Inspired by the tremendous utility of petroleum-derived styrenic triblock copolymers, we aimed to develop syntheses and understand the structure-property profiles of sustainable alternatives, focusing on all renewable and all readily degradable aliphatic polyester triblocks as targets. Building upon oxidation chemistry reported more than a century ago, a constituent of the peppermint plant, (-)-menthol, was converted to the ε-caprolactone derivative menthide. Using a diol initiator and controlled catalysis, menthide was polymerized to yield a low glass transition temperature telechelic polymer (PM) that was then further functionalized using the biomass-derived monomer lactide (LA) to yield fully renewable PLA-PM-PLA triblock copolymers. These new materials were microphase-separated and could be fashioned as high-performing thermoplastic elastomers, with properties comparable to commercial styrenic triblock copolymers. Examination of their hydrolytic degradation (pH 7.4, 37 °C) revealed retention of properties over a significant period, indicating potential utility in biomedical devices. In addition, they were shown to be useful in pressure-sensitive adhesives formulations and as nucleating agents for crystallization of commercially relevant PLA. More recently, new triblocks have been prepared through variation of each of the segments. The natural product α-methylene-γ-butyrolactone (MBL) was used to prepare triblocks with poly(α-methylene-γ-butyrolactone) (PMBL) end blocks, PMBL-PM-PMBL. These materials exibited impressive mechanical properties that were largely retained at 100 °C, thus offering application advantages over triblock copolymers comprising poly(styrene) end blocks. In addition, replacements for PM were explored, including the polymer derived from 6-me...

We report the preparation, structural characterization, and detailed lactide polymerization behavior of a new Zn(II) alkoxide complex, (L(1)ZnOEt)(2) (L(1) = 2,4-di-tert-butyl-6-{[(2'-dimethylaminoethyl)methylamino]methyl}phenolate). While an X-ray crystal structure revealed the complex to be dimeric in the solid state, nuclear magnetic resonance and mass spectrometric analyses showed that the monomeric form L(1)ZnOEt predominates in solution. The polymerization of lactide using this complex proceeded with good molecular weight control and gave relatively narrow molecular weight distribution polylactide, even at catalyst loadings of <0.1% that yielded M(n) as high as 130 kg mol(-)(1). The effect of impurities on the molecular weight of the product polymers was accounted for using a simple model. Detailed kinetic studies of the polymerization reaction enabled integral and nonintegral orders in L(1)ZnOEt to be distinguished and the empirical rate law to be elucidated, -d[LA]/dt = k(p)[L(1)ZnOEt][LA]. These studies also showed that L(1)ZnOEt polymerizes lactide at a rate faster than any other Zn-containing system reported previously. This work provides important mechanistic information pertaining to the polymerization of lactide and other cyclic esters by discrete metal alkoxide complexes.

A description of the structure and bonding of novel bis(μ-oxo)dicopper complexes and their bis(μ-hydroxo)dicopper decomposition products was derived from combined X-ray crystallographic, spectroscopic, and ab initio theoretical studies. The compounds [(LCu)2(μ-O)2]X2 were generated from the reaction of solutions of [LCu(CH3CN)]X with O2 at −80 °C (L = 1,4,7-tribenzyl-1,4,7-triazacyclononane, LBn 3 ; 1,4,7-triisopropyl-1,4,7-triazacyclononane, LiPr 3 ; or 1-benzyl-4,7-diisopropyl-1,4,7-triazacyclononane, LiPr 2 Bn; X = variety of anions). The geometry of the [Cu2(μ-O)2]2+ core was defined by X-ray crystallography for [(d 21-LBn 3 Cu)2(μ-O)2](SbF6)2 and by EXAFS spectroscopy for the complexes capped by LBn 3 and LiPr 3 ; notable dimensions include short Cu−O (∼1.80 Å) and Cu···Cu (∼2.80 Å) distances like those reported for analogous M2(μ-O)2 (M = Fe or Mn) rhombs. The core geometry is contracted compared to those of the bis(μ-hydroxo)dicopper(II) compounds that result from decomposition of the bis(μ-oxo) complexes upon warming. X-ray structures of the decomposition products [(LBn 3 Cu)(LBn 2 HCu)(μ-OH)2](O3SCF3)2·2CH3CO, [(LiPr 2 HCu)2(μ-OH)2](BPh4)2·2THF, and [(LiPr 2 BnCu)2(μ-OH)2](O3SCF3)2 showed that they arise from N-dealkylation of the original capping macrocycles. Manometric, electrospray mass spectrometric, and UV−vis, EPR, NMR, and resonance Raman spectroscopic data for the bis(μ-oxo)dicopper complexes in solution revealed important topological and electronic structural features of the intact [Cu2(μ-O)2]2+ core. The bis(μ-oxo)dicopper unit is diamagnetic, undergoes a rapid fluxional process involving interchange of equatorial and axial N-donor ligand environments, and exhibits a diagnostic ∼600 cm-1 18O-sensitive feature in Raman spectra. Ab initio calculations on a model system, {[(NH3)3Cu]2(μ-O)2}2+, predicted a closed-shell singlet ground-state structure that agrees well with the bis(μ-oxo)dicopper geometry determined by experiment and helps to rationalize many of its physicochemical properties. On the basis of an analysis of the theoretical and experimental results (including a bond valence sum analysis), a formal oxidation level assignment for the core is suggested to be [CuIII 2(μ-O2-)2]2+, although a more complete molecular orbital description indicates that the oxygen and copper fragment orbitals are significantly mixed (i.e., there is a high degree of covalency).