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During the second year of the grant, progress has been made on two major aspects of the project including work focused on the binding of various functional groups to the Jacobsen catalyst and an expansion to a broader set of macrocyclic ligand systems.  We are finishing off the last aspects of some studies during the added grant year.

Ligand Binding Scale

We decided that it would be useful to have a general measure of the ligand binding of various functional groups to the Jacobsen catalyst.  This will be the first general gas phase binding study to a metal with a realistic macrocyclic ligand.  The results shed light on steric effects and their impact on the binding. To this end, we have completed a series of equilibrium experiments in which we compare binding of two potential ligands (achiral).  By doing pair-wise comparisons, a scale of ligand affinities can be created. The scale contains 11 species spanning a binding range of nearly 6 kcal/mol.  The list contains 16 ethers, esters, ketones, alcohols, and amines.  Steric effects are significant and THF binds nearly 4 kcal/mol stronger than diethyl ether.  The most unexpected result is that triethylamine is one of the weakest binders whereas diethylamine is one of the strongest.   Not surprisingly, the results are quite different form those obtained with bare metals or those with simple ligands.  More importantly, they suggest a delicate balance between tight binding and steric repulsion that can be useful in choosing ligands for condensed-phase applications.  A manuscript is in the final stages of preparation and will be submitted to Organic Letters in the next few weeks.

Other Catalyst Systems

As an extension of these studies, we have begun work on a group of chiral macrocyles related to salen.  Namely, diimines derived from the condensation of aldehydes (rather that salicylaldehydes) with ethylenediamine derivatives.  They are well suited for binding copper cations and are used in a variety of synthetic applications.  Using our same approach of binding chiral alcohols to them in the gas phase, we have seen very high selectivity (R/S preferences nearing 5:1).  To date, we have only examined a few lead species.  The work is promising and easy access to a wide range of substituted species will allow for a thorough study of the features that influence gas-phase stereoselectivity.  These species seem better suited than the salens for this task.

Studies Underway/Planned

In the extension period, we plan to finish the work on the binding scale of ligands to the Jacobsen catalyst.  We wish to extend the scale to more powerful ligands and this will require new methodology because the equilibration rate becomes too slow for our instrumentation with stronger binders.  We will switch to the Cooks kinetic method, which approximates the equilibrium by competitive dissociation of a ternary complex (catalyst and two ligands).  The main focus will be amines, nitriles, phosphine oxides, and other common ligands.  Preliminary work with amines is underway.  In addition, we will complete the studies that we have started with the copper catalysts and explore substituent effects.  Synthesis of new macrocycles is underway.  These studies will bring the originally proposed studies to a logical endpoint and put us in a position for pursuing continuing funding from another agency.

Participation

During the year, three PhD students and three undergraduates participated in studies related to the project. A new undergraduate has joined the group and will continue the projects.  The project will go forward with one PhD student and an undergraduate.