A new class of chelating agents

The Role of Through-Bond Stereoelectronic Effects in the Reactivity of 3-Azabicyclo[3.3.1]nonanes

Hyperconjugation/conjugation through-bond stereoelectronic effects were studied with density functional theory in the context of 3-azabicyclo[3.3.1]nonanes to unravel puzzling differences in reactivity between a vinylogous chloride (4) and a vinylogous ester (5). These compounds─whose structures differ only by one substituent─were found to display strikingly different reactivities in hydrochloric acid by Risch and co-workers ( J. Am. Chem. Soc., 1991, 113, 9411-9412). Computational analyses of substituent effects, noncovalent interactions, natural bond orbitals, isodesmic reactions, and hydration propensities lead to a model for which the role of remote, through-bond stereoelectronic effects is the key to explaining 4 and 5's diverging reactivity.

Stereoelectronic power of oxygen in control of chemical reactivity: the anomeric effect is not alone

Although carbon is the central element of organic chemistry, oxygen is the central element of stereoelectronic control in organic chemistry. Generally, a molecule with a C-O bond has both a strong donor (a lone pair) and a strong acceptor (e.g., a σ*_C-O orbital), a combination that provides opportunities to influence chemical transformations at both ends of the electron demand spectrum. Oxygen is a stereoelectronic chameleon that adapts to the varying situations in radical, cationic, anionic, and metal-mediated transformations. Arguably, the most historically important stereoelectronic effect is the anomeric effect (AE), i.e., the axial preference of acceptor groups at the anomeric position of sugars. Although AE is generally attributed to hyperconjugative interactions of σ-acceptors with a lone pair at oxygen (negative hyperconjugation), recent literature reports suggested alternative explanations. In this context, it is timely to evaluate the fundamental connections between the AE and a broad variety of O-functional groups. Such connections illustrate the general role of hyperconjugation with oxygen lone pairs in reactivity. Lessons from the AE can be used as the conceptual framework for organizing disjointed observations into a logical body of knowledge. In contrast, neglect of hyperconjugation can be deeply misleading as it removes the stereoelectronic cornerstone on which, as we show in this review, the chemistry of organic oxygen functionalities is largely based. As negative hyperconjugation releases the "underutilized" stereoelectronic power of unshared electrons (the lone pairs) for the stabilization of a developing positive charge, the role of orbital interactions increases when the electronic demand is high and molecules distort from their equilibrium geometries. From this perspective, hyperconjugative anomeric interactions play a unique role in guiding reaction design. In this manuscript, we discuss the reactivity of organic O-functionalities, outline variations in the possible hyperconjugative patterns, and showcase the vast implications of AE for the structure and reactivity. On our journey through a variety of O-containing organic functional groups, from textbook to exotic, we will illustrate how this knowledge can predict chemical reactivity and unlock new useful synthetic transformations.

Pb3[C6(CH3)3(CO2)3H6]2[DMF]3: first layered Pb-Kemp's triacid complex

The first layered Pb-Kemp's triacid material, Pb₃[C₆(CH₃)₃(CO₂)₃H₆]₂[DMF]₃, reveals reversible solvent coordination, intercalative addition, and higher selectivity of Cd²⁺ over Pb²⁺.

Evolution of strategies to prepare synthetic mimics of carboxylate-bridged diiron protein active sites

We present a comprehensive review of research conducted in our laboratory in pursuit of the long-term goal of reproducing the structures and reactivity of carboxylate-bridged diiron centers used in biology to activate dioxygen for the conversion of hydrocarbons to alcohols and related products. This article describes the evolution of strategies devised to achieve these goals and illustrates the challenges in getting there. Particular emphasis is placed on controlling the geometry and coordination environment of the diiron core, preventing formation of polynuclear iron clusters, maintaining the structural integrity of model complexes during reactions with dioxygen, and tuning the ligand framework to stabilize desired oxygenated diiron species. Studies of the various model systems have improved our understanding of the electronic and physical characteristics of carboxylate-bridged diiron units and their reactivity toward molecular oxygen and organic moieties. The principles and lessons that have emerged from these investigations will guide future efforts to develop more sophisticated diiron protein model complexes.

Some Got Away, but Others Didn't

A history of research projects undertaken in the group-ranging from mechanistic investigations to biomimetic chemistry to molecular devices and synthesis-is presented.

A Trapped Intermediate in the Copper(II)-Mediated Template Synthesis of an Amino Acid-Containing Ligand

A transition metal ion-templating reaction that has been widely exploited for the synthesis of nitrogen macrocycles, sepulchrates, and linear tetradentate ligands is the template-directed Mannich condensation of carbon acid and aldehyde equivalents with amines. In the course of investigating the copper(II) template-directed synthesis of linear tetradentate amino acid-containing ligands for the design and synthesis of double-strand DNA cleavage agents, we have trapped and characterized an intermediate in the templating reaction. Condensation of bis(phenylalaninato)copper(II) with formaldehyde and nitroethane led to a nitro-substituent-bearing precursor complex that upon reduction is crystallographically characterized as ((2S)-5-amino-2-benzyl-6-hydroxy-5-methyl-3-azahexanoate)copper(II) (space group P2(1)2(1)2(1), a = 9.0066(5) Å, b = 11.1040(6) Å, c = 16.009(2) Å, alpha = beta = gamma = 90.0 degrees, Z = 4, R(1) = 0.0265, wR(2) = 0.0612). NMR investigation of the precursor complex and the precursor and product ligands shows that the templated ligands each contain only a single phenylalanine unit, not the two phenylalanine units expected on the basis of similar synthetic procedures. The structure of the reduced product confirms this conclusion and provides structural characterization of a template intermediate trapped during ligand assembly.

Molecular tuning of ion binding to calcium signaling proteins

Intracellular calcium plays an essential role in the transduction of most hormonal, neuronal, visual, and muscle stimuli. (Recent reviews include Putney, 1993; Berridge, 1993a,b; Tsunoda, 1993; Gnegy, 1993; Bachset al.1992; Hanson & Schulman, 1992; Villereal & Byron, 1992; Premack & Gardner, 1992; Meanset al.1991).