William B. Tolman

Biologically inspired oxidation catalysis.

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.

Reversible cleavage and formation of the dioxygen O-O bond within a dicopper complex

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(μ-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(μ-O)2 cores. The viability of this notion has been demonstrated by the identification of an equilibrium between synthetic complexes having [Cu2(μ-η2:η2-O2)]2+ and [Cu2(μ-O)2]2+ cores through kinetic, spectroscopic, and crystallographic studies.

Bis(μ‐oxo)dimetal “Diamond” Cores in Copper and Iron Complexes Relevant to Biocatalysis

Although quite a familiar feature in high-valent manganese chemistry, the M(2)(mu-O)(2) diamond core motif has only recently been found in synthetic complexes for M=Cu or Fe. Structural and spectroscopic characterization of these more reactive Cu(2)(mu-O)(2) and Fe(2)(mu-O)(2) compounds has been possible through use of appropriately designed supporting ligands, low-temperature handling methods, and techniques such as electrospray ionization mass spectrometry and X-ray crystallography with area detector instrumentation for rapid data collection. Despite differences in electronic structures that have been revealed through experimental and theoretical studies, Cu(2)(mu-O)(2) and Fe(2)(mu-O)(2) cores exhibit analogously covalent metal-oxo bonding, remarkably congruent Raman and extended X-ray absorption fine structure (EXAFS) signatures, and similar tendencies to abstract hydrogen atoms from substrates. Core isomerization is another common reaction attribute, although different pathways are traversed; for Fe, bridge-to-terminal oxo migration has been discovered, while for Cu, reversible formation of an O-O bond to yield a peroxo isomer has been identified. Our understanding of biocatalysis has been enhanced significantly through the isolation and comprehensive characterization of the Cu(2)(mu-O)(2) and Fe(2)(mu-O)(2) complexes. In particular, it has led to the development of new mechanistic notions about how non-heme multimetal enzymes, such as methane monooxygenases, fatty acid desaturase, and tyrosinase, may function in the activation of dioxygen to catalyze a diverse array of organic transformations.

Understanding the copper–phenoxyl radical array in galactose oxidase: contributions from synthetic modeling studies

Abstract The two-electron oxidation of primary alcohols with dioxygen to yield aldehyde and hydrogen peroxide that is catalyzed by galactose oxidase (GAO) occurs at an intriguing active site comprising of a copper ion ligated by an unusual cysteine-modified tyrosine group. Both the metal ion and the tyrosinate undergo 1-electron redox interconversions during catalysis, the Cu(II)–tyrosyl radical form being a critical species. Due to the novelty of this coupled metal–radical cofactor unit in chemistry and biology and its importance within the more general context of radical–enzyme biochemistry, chemists have attempted to prepare model complexes for this and other redox-related states of GAO. The primary goals of such research are to better understand the enzyme active site spectral properties, structural attributes, and reactivity. In this review article, progress toward these goals is surveyed, beginning with a discussion of the synthesis and structural and spectroscopic characterization of model complexes of the GAO active site and ending with a description of more recent discoveries of catalytic reactivity by Cu(II)–phenoxyl radical species that replicate and provide insights into GAO function.

Variable character of O—O and M—O bonding in side-on (η2) 1:1 metal complexes of O2

The structures and the O—O and M—O bonding characters of a series of reported side-on (η2) 1:1 metal complexes of O2 are analyzed by using density functional theory calculations. Comparison of the calculated and experimental systems with respect to O—O bond distance, O—O stretching frequency, and O—O and M—O bond orders provides new insights into subtle influences relevant to O2 activation processes in biology and catalysis. The degree of charge transfer from the generally electron-rich metals to the dioxygen fragment is found to be variable, such that there are species well described as superoxides, others well described as peroxides, and several cases having intermediate character. Increased charge transfer to dioxygen takes place via overlap of the metal dxy orbital with the in-plane π* orbital of O2 and results in increased M—O bond orders and decreased O—O bond orders. Comparison of theory and experiment over the full range of compounds studied suggests that reevaluation of the O—O bond lengths determined from certain x-ray crystal structures is warranted; in one instance, an x-ray crystal structure redetermination was performed at low temperature, confirming the theoretical prediction. Librational motion of the coordinated O2 is identified as a basis for significant underestimation of the O—O distance at high temperature.

Polymerization of lactide and related cyclic esters by discrete metal complexes

This perspective highlights recent research on the preparation of polyesters by the ring-opening polymerization of cyclic esters employing well-characterized metal complexes. Particular focus is placed on the preparation of polylactide because of environmental advantages: it is biodegradable and its feedstock, lactide, is a renewable resource. A recurring theme is the correlation of precatalyst structure, often by X-ray crystallography, with polymerization activity and selectivity. Through this systematic approach to the deconvolution of catalyst structure/reactivity relationships, improved mechanistic understanding has been attained and key design criteria required for the development of new catalysts that exert control over the molecular parameters of polyesters and related copolymers have been revealed.

Binding and Activation of N2O at Transition Metal Centers: Recent Mechanistic Insights

No laughing matter, nitrous oxides role in stratospheric ozone depletion and as a greenhouse gas has stimulated great interest in developing and understanding its decomposition, particularly through the use of transition-metal promoters. Recent advances in our understanding of the reaction pathways for N(2)O reduction by metal ions in the gas phase and in heterogeneous, homogeneous, and biological catalytic systems have provided provocative ideas about the structure and properties of metal N(2)O adducts and derived intermediates. These ideas are likely to inform efforts to design more effective catalysts for N(2)O remediation.

Mechanistic study of the stereoselective polymerization of D,L-lactide using indium(III) halides.

We report the results of a comprehensive investigation of the recently discovered stereoselective and controlled polymerization of racemic lactide (D,L-LA) using an initiator prepared in situ from indium(III) chloride (InCl(3)), benzyl alcohol (BnOH), and triethylamine (NEt(3)). Linear relationships between number-average molecular weight (M(n)) and both monomer to alcohol concentration ratio and monomer conversion are consistent with a well-controlled polymerization. Studies on polymerization kinetics show the process to be first-order in [InCl(3)](0) and zero-order in both [BnOH](0) and [NEt(3)](0). The rate of D,L-LA conversion is also dependent on the indium(III) halide (i.e., t(1/2)(InCl(3)) approximately = 43 min versus t(1/2)(InBr(3)) approximately = 7.5 h, 21 degrees C, CD(2)Cl(2), [D,L-LA](0)/[BnOH](0) approximately = 100, [D,L-LA](0) = 0.84 M, [InX(3)](0)/[BnOH](0) = 1) and lactide stereoisomer (i.e., k(obs)(D,L-LA) approximately = k(obs)(meso-LA) > k(obs)(L-LA)). A model system that polymerizes D,L-LA with the same high degree of stereoselectivity was developed using 3-diethylamino-1-propanol (deapH) in lieu of BnOH and NEt(3). The product of the reaction of deapH with InCl(3) was identified as [InCl(3)(deapH)(H(2)O)](2) by elemental analysis, X-ray crystallography, and NMR and FTIR spectroscopies. An anhydrous version of the complex was also isolated when care was taken to avoid adventitious water, and was shown by pulsed gradient spin-echo (PGSE) NMR experiments to adopt a dinuclear structure in CD(2)Cl(2) solution under conditions identical to those used in its stereoselective polymerization of D,L-LA. The combined data suggest that the initiating species for the InCl(3)/BnOH/NEt(3) system is similar to [InCl(3)(deapH)(H(2)O)](2) and of the type [InCl((3-n))(OBn)(n)](m). With this information we propose a mechanism that rationalizes the observed stereocontrol in D,L-LA polymerizations. Finally, in an exploration of the scope of the InCl(3)/BnOH/NEt(3) system, we found this system to be effective for the polymerization of other cyclic esters, including epsilon-caprolactone and several substituted derivatives.

Comparison of structurally analogous Zn2, Co2, and Mg2 catalysts for the polymerization of cyclic esters

Three dimetallic monoethoxide complexes supported by a binucleating phenoxide ligand, LM2Cl2OEt (M = Zn, Co, or Mg), were prepared and shown by X-ray crystallography to be structurally analogous. Comparative studies of their cyclic ester polymerization reactivity revealed different trends for reactions with epsilon-caprolactone and lactide, however, implicating complicated effects of metal ion variation in these polymerizations.

Electronic influence of ligand substituents on the rate of polymerization of ε-caprolactone by single-site aluminium alkoxide catalysts

A series of novel five-coordinate aluminium mono alkoxide complexes supported by R1,R2BPBA (bis-3-R1-5-R2-phenoxymethyl-bisamine) ligands were synthesized to probe the effect of electronic variation in the supporting ligand on the rate of e-caprolactone polymerization. Substitution on the aromatic position para to the phenoxide donor oxygen by tert-butyl, methoxy and bromo substituents furnished aluminium complexes that catalyzed the polymerization of e-caprolactone at different rates. We propose that a subtle interplay between complex Lewis acidity and alkoxide nucleophilicity determines the overall rate of polymerization in these systems.