46030-GB3
Metal-Organic Network Materials For Asymmetric Catalysis
This report outlines our achievements in the field of asymmetric catalysis with the support of a new faculty undergraduate start-up grant, and discusses the impact of the research on the field of study, the career of the principal investigator, and the research training and careers of the undergraduate students who carried out the work. In particular, an unexpected result is described that opened up a new line of research that has captured the interest of the P.I.’s student researchers, led to a publication, and to the solidification of a new and fruitful ongoing area of study for the P.I. Alongside the goal of preparing heterogeneous asymmetric titanium Lewis acid catalysts, we began a study of soluble, chiral titanium alkoxides as homogeneous catalysts, in order to make comparisons between the heterogeneous and homogeneous systems. When a student researcher investigating the methylation of benzaldehyde with dimethyl zinc catalyzed by resolved titanium(IV) sec-butoxide discovered an unexpected result, a new line of research began to take hold in our laboratory. In an interesting example of a "matched pair" asymmetric activation phenomena that allows us to achieve higher enantiomeric excess than previously reported in the literature for this catalysis, we observed that resolved R- or S- titanium sec-butoxide, combined with resolved BINOL (BINOL = 1,1'-bi-2-naphthol) of the opposite configuration designation, yields a dichiral matched pair that mediates the addition of dimethyl zinc to benzaldehyde with higher enantioselectivity than resolved BINOL with achiral Ti(OiPr)4 or the mismatched dichiral pair.
The impact of this research on the field of catalysis is beginning to take shape. Whereas the titanium mediated reduction of arylaldehydes with diethyl zinc to transfer an ethyl group is well studied, the transfer of a methyl group with dimethyl zinc has yet to be fully addressed. While we have achieved the best results yet in this system, there is more room for improvement. Currently, we are investigating other ligand systems besides BINOL, such as Salcean (Salcean = N,N '-bis(o-hydroxybenzyl)-trans-1,2-diaminocyclohexane) and several chiral bis(phosphine oxide) ligands, in combination with resolved titanium sec-butoxide as asymmetric Lewis acid catalysts. The results obtained have; i) seeded the scholarship, and therefore career development, of the P.I. in a new area of investigation, ii) provided research training for several undergraduate students with novel, publishable research, and iii) allowed for the dissemination of the interesting new results at conferences and in publications. Two former undergraduate students are the lead authors on a publication in 2008 that describes their work on the synthesis of the chiral alkoxides and the catalysis screening with BINOL; both students are now in graduate school in chemistry.
As explained in the original proposal, a series of three goals described the fitting a selected catalyst molecule with a “tether” arm, such that it could be anchored inside a porous lattice, effectively making the ensuing catalyst heterogeneous. While we continue to pursue these goals, our attention is also drawn to homogeneous catalysis. These parallel lines of work will allow us to make comparisons between the homogeneous and heterogeneous catalytic systems. In support of the heterogeneous work, the chiral, bis(oxazoline) with a hydroxyethyl tether arm that was originally proposed has been successfully prepared as a methoxybenzyl protected ether. Subsequent deprotection of the ligand precursor will make it available for incorporation into the proposed metal-organic network materials for asymmetric heterogeneous catalysis. Together with this work, several new metal-organic network materials were synthesized and structurally characterized by X-ray crystallography. Heating the bisphenolic ligand spacer precursor 4,4’-bis(hydroxyphenyl)sulfide and metal precursor titanium(IV) isopropoxide, Ti(OiPr)4, in various solvents yielded several coordination polymers, whose structure depends on the solvent used. A one-dimensional coordination polymer chain, {Ti(OC6H4SC6H4O)2(C5H5N)2}n was obtained from pyridine; a two-dimensional sheet, {[Ti(OC6H4SC6H4O)2(C4H8O)]2}n from tetrahydrofuran, and a three-dimensional network, {Ti(OC6H4SC6H4O)2}n from benzene. In this system, the coordinating ability of the solvent helps control the dimensionality of the network material.
In summary, we report that the PRF funded work described here has led to publishable fundamental research in the field of asymmetric catalysis and metal-organic network materials that have positively impacted the scholarship and careers of the P.I. and students who have worked on the project. With four current undergraduate students, we continue to work on incorporating a chiral bidentate ligand inside of a porous network such that, when combined with resolved titanium sec-butoxide, the material will serve as an insoluble, recyclable, heterogeneous version of our dichiral homogeneous catalyst system that exhibits a matched pair asymmetric activation phenomenon.
