A bis(oxazoline) (box) copper(II) complex and its hydrated counterpart (1 and 2) function as enantioselective Lewis acid catalysts for carbocyclic and hetero Diels-Alder, aldol, Michael, ene, and amination reactions with substrates capable of chelation through six- and five-membered rings. X-ray crystallography of the chiral complexes reveals a propensity for the formation of distorted square planar or square pyramidal geometries. The sense of asymmetric induction is identical for all the processes catalyzed by [Cu((S,S)-t-Bu-box)](X)(2) complexes 1 and 2 (X = OTf and SbF) resulting from the intervention of a distorted square planar catalyst-substrate binary complex. These catalyzed processes exhibit excellent temperature-selectivity profiles. Reactions catalyzed by [Cu(S,S-Ph-pybox)](SbF(6))(2) and their derived chelation complexes are also discussed.

This study examined the role of attention in maintaining feature bindings in visual short-term memory (VSTM). In a change-detection paradigm, participants attempted to detect changes in the colors and orientations of multiple objects; the changes consisted of new feature values in a feature-memory condition and changes in how existing feature values were combined in a binding-memory condition. In the critical experiment, a demanding visual search task requiring sequential shifts of spatial attention was interposed during the delay interval of the change-detection task. If attention is more important for the maintenance of feature bindings than for the maintenance of unbound feature values, the attention-requiring search task should specifically disrupt performance in the binding-memory task. Contrary to this proposal, we found that memory for bindings and memory for features were equally impaired by the search task.

The addition of an N-H bond across a carbon-carbon double or triple bond is potentially one of the most direct and efficient methods of alkylating an amine. Considerable effort has been expended in the search for transition metal complexes that will catalyze this desirable transformation, and while notable successes have been realized, general solutions to the problem have proven elusive. 1 As a consequence, the discovery and development of hydroamination reactions remain important goals.Relatively few early transition metal complexes have been shown to be hydroamination catalyst precursors. Among these are the class of bis(cyclopentadienyl)zirconium and titanium alkyne hydroamination catalysts studied earlier by our group 2 and by Doye's 3,4 (eq 1). In both systems it has been assumed that the critical intermediates are the 16-electron imido complexes Cp 2 M=NR. The purpose of this communication is to report titanium-based imido complexes 5,6 that function as catalyst precursors for both allene and alkyne hydroamination reactions. 7-9 A detailed mechanistic investigation has revealed an unexpected cyclopentadienide/amide ligand exchange reaction that transforms the biscyclopentadienyltitanium precursors (Cp 2 TiL n ) into a monocyclopentadienyl titanium (amido) complex (Cp(ArNH)TiL n ) 10 exhibiting substantially higher reactivity than the Cp 2 TiL n precursor.(1).We became interested in catalytic allene hydroamination with the discovery that enantiopure (ebthi)zirconium imido complexes kinetically resolve chiral racemic allenes under stoichiometric conditions (ebthi = ethylene bis(tetrahydroindenyl)). 11 Experiments to explore the feasibility of metal-catalyzed allene hydroamination were undertaken with the eventual goal of developing a catalytic variant of this kinetic resolution. A brief screen of catalysts revealed that titanium complexes were more reactive than their zirconium counterparts, and that Cp 2 TiMe 2 12 was a convenient precursor to an undefined, but competent, catalyst species that hydroaminated 1,2-propadiene in good yields (eq 2). Aryl and alkylamines are both tolerated, as are hydrazines; bulky amines appear to react with greater facility than those with lessened steric demand.

The X-ray crystal structure of a 19 kDa active fragment of human fibroblast collagenase has been determined by the multiple isomorphous replacement method and refined at 1.56 A resolution to an R-factor of 17.4%. The current structure includes a bound hydroxamate inhibitor, 88 waters and three metal atoms (two zincs and a calcium). The overall topology of the enzyme, comprised of a five stranded beta-sheet and three alpha-helices, is similar to the thermolysin-like metalloproteinases. There are some important differences between the collagenase and thermolysin families of enzymes. The active site zinc ligands are all histidines (His-218, His-222, and His-228). The presence of a second zinc ion in a structural role is a unique feature of the matrix metalloproteinases. The binding properties of the active site cleft are more dependent on the main chain conformation of the enzyme (and substrate) compared with thermolysin. A mechanism of action for peptide cleavage similar to that of thermolysin is proposed for fibroblast collagenase.

Many tasks rely on our ability to hold information about a stimulus in mind after it is no longer visible and to compare this information with incoming perceptual information. This ability relies on a short-term form of memory known as visual working memory. Research and theory at the behavioral and neural levels has begun to provide important insights into the basic properties of the neuro-cognitive systems underlying this form of memory. However, to date, no neurally-plausible theory has been proposed that addresses both the storage of information in working memory and the comparison process in a single framework. To address these limitations, I have developed a new model where working memory is realized via peaks of activation in dynamic neural fields, and comparison emerges as a result of interactions among the model's layers.In a series of simulations, I show how the model can be used to capture each of the components underlying performance in simple visual comparison tasks-from the encoding, consolidation, and maintenance of information in working memory, to comparison and updating in response to changed inputs. Importantly, the proposed model demonstrates how these elementary perceptual and cognitive functions emerge from the coordinated activity of an integrated, dynamic neural system. The model also makes novel predictions that were tested in a series of behavioral experiments. Specifically, when similar items are stored, shared lateral inhibition produces a sharpening of the peaks of activation associated with each item in memory. In the context of the model, this leads to the prediction that change detection will be enhanced for similar versus dissimilar features. This prediction was confirmed in a series of change detection experiments exploring memory for both color and orientation.In addition to sharpening, shared lateral inhibition among similar items produces mutual repulsion between nearby peaks. This leads to the prediction that when similar features are held, they will be systematically biased away from each other over delays. 2This prediction was confirmed in a cued color recall experiment comparing memory for a "far" color with memory for two "close" colors.Abstract Approved: ____________________________________ Thesis Supervisor ____________________________________